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Adsorption of oleate on magnesite and its influence on flotation Branda̧o, Paulo Roberto Gomes 1982

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ADSORPTION OF OLEATE ON MAGNESITE"AND ITS INFLUENCE ON FLOTATION. . by PAULO ROBERTO GOMES BRANDAO B. Eng., U.F. Minas G e r a i s , B r a z i l , 1968 M. Eng., The U n i v e r s i t y of B r i t i s h Columbia, 1980 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY i n THE FACULTY OF GRADUATE STUDIES (Department of Mining and M i n e r a l Process Engineering) We accept t h i s t h e s i s as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA October 1982 (c) Paulo Roberto Gomes Brandao, 1982 In p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t o f the r e q u i r e m e n t s f o r an advanced degree a t the U n i v e r s i t y o f B r i t i s h C o l u m b i a , I agree t h a t the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and s t u d y . I f u r t h e r a g r e e t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u rposes may be g r a n t e d by the Head o f my Department o r by h i s r e p r e s e n t a t i v e s . I t i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l not be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . Paulo Roberto Gomes Brandao Department Of Mining & M i n e r a l Process Engineering The U n i v e r s i t y o f B r i t i s h Columbia 2075 Wesbrook P l a c e V a n c o u v e r , Canada V6T 1W5 Date November 9 r 1982 D E - 6 B P 75-51 1 E i i Research Supervisor George W. P o l i n g , P r o f e s s o r and Head Dept. of Mining and M i n e r a l Process Engineering Abstract A h i g h l y s e n s i t i v e r e f l e c t i o n - a b s o r p t i o n i n f r a r e d spectrophotometric technique was used t o i n v e s t i g a t e the adsorption of sodium o l e a t e from aqueous s o l u t i o n s onto magnesium carbonate (magnesite) t h i n f i l m s u bstrates. Valuable q u a l i t a t i v e r e s u l t s were obtained, regarding the nature of the adsorbed species and t h e i r molecular o r i e n t a t i o n . However, t h i s o r i e n t a t i o n e f f e c t s t r o n g l y i n f l u e n c e d the r e l a t i v e i n t e n s i t i e s of the i n f r a r e d a bsorption bands; t h i s prevented p r e c i s e q u a n t i t a t i v e determinations. Under moderately a c i d i c c o n d i t i o n s , the o r i g i n a l surface charge of magnesite was p o s i t i v e and near zero. The a n i o n i c c o l l e c t o r was chemisorbed, forming only one type of complex, very s i m i l a r t o the bulk magnesium o l e a t e . The hydrocarbon chains were o r i e n t e d n e a r l y p a r a l l e l t o the s u b s t r a t e , forming a homogeneous p a r t i a l l y polymerized epoxide-polyether l a y e r , s t r o n g l y l a t e r a l l y bound, at the s i t e s of the former C=C double bonds. The C-0 bonds were formed f o l l o w i n g a p a r t i a l o x i d a t i o n of the o l e a t e i n s o l u t i o n , by oxygen from the a i r . This adsorbed f i l m converted the carbonate surface t o a s t r o n g l y hydrophobic c o n d i t i o n , which was r e s p o n s i b l e f o r the high f l o t a t i o n r e c o v e r i e s obtained. Under ba s i c c o n d i t i o n s , the o r i g i n a l magnesite surface was n e g a t i v e l y charged. This l i m i t e d the formation of the chemisorbed c o l l e c t o r complex to a lower amount; d i f f e r e n t complexes, probably p h y s i c a l l y adsorbed, j o i n e d i n forming a f i l m . However, t h i s was a more heterogenous f i l m , showing i i i r d i f f e r e n t molecular o r i e n t a t i o n s - - from nearly p a r a l l e l , to c l o s e r to the normal to the adsorbent's surface. These aspects caused a net decrease i n the degree of hydrophobicity, i n comparison to the a c i d i c c o n d i t i o n s . The same o x i d a t i o n and po l y m e r i z a t i o n e f f e c t s were observed, although to a lower extent. The f i l m was a l s o very s t a b l e . These p r o p e r t i e s of the adsorbed f i l m were c o n s i s t e n t w i t h the lower f l o t a t i o n response observed under ba s i c c o n d i t i o n s . Therefore, more concentrated c o l l e c t o r s o l u t i o n s were needed, to reach the same high recoveries obtained under moderately a c i d i c c o n d i t i o n s . i v TABLE OF CONTENTS Page ABSTRACT i i TABLE OF CONTENTS i v LIST OF TABLES v i i LIST OF FIGURES v i i i ACKNOWLEDGEMENTS x i CHAPTER 1 - INTRODUCTION 1 CHAPTER 2 - LITERATURE REVIEW 3 2.1 - C r y s t a l Chemistry and Mineralogy 3 2.2 - Water Chemistry of Magnesium Carbonate 5 2.2.1 - S o l u b i l i t y 5, 2.2.2 - E l e c t r o k i n e t i c P r o p e r t i e s 9 2.3 - Fundamental F l o t a t i o n Studies 11 2.3.1 - A n i o n i c C o l l e c t o r s 13 2.3.2 - M i c r o f l o t a t i o n Studies 15 2.3.3 - I n f r a r e d Spectrophotometry Studies 17 2.3.4 - Concluding Remarks 22 CHAPTER 3 - SCOPE AND APPROACH OF THE STUDY 23 CHAPTER 4 - INFRARED SPECTROSCOPY IN THE STUDY OF ADSORBED COLLECTOR SPECIES 25 4.1 - Transmission Technique 25 4.1.1 - Absorption of R a d i a t i o n 26 4.1.2 - Adsorbed Species 27 4.2 - M u l t i p l e Specular, R e f l e c t i o n - A b s o r p t i o n Technique (MSRA) 28 4.2.1 - Molecular O r i e n t a t i o n 32 4.2.2 - Uses of the M u l t i p l e Specular R e f l e c t i o n - A b s o r p t i o n Technique 35 V Page CHAPTER 5 - EXPERIMENTAL METHODS 36 5.1 - M i c r o f l o t a t i o n Tests 36 5.1.1 - M a t e r i a l s and Apparatus 38 5.2 - Poi n t of Zero Charge 39 5.3 - I n f r a r e d Spectroscopy AO 5.3.1 - Experimental Techniques 41 5.3.2 - Equipment 43 5.3.3 - M a t e r i a l s 46 5.3.4 - Sample P r e p a r a t i o n 47 a) Transmission Technique 47 b) MSRA Technique 48 5.4 - - Contact Angles 53 CHAPTER 6 - RESULTS AND DISCUSSION 55 6.1 - Point of Zero Charge 55 6.2 - M i c r o f l o t a t i o n Tests 55 6.2.1 - Sodium o l e a t e 55 6.2.2 - Sodium dodecylsulphonate 58 6.2.3 - Dodecylammonium c h l o r i d e 61 6.2.4 - Concluding Remarks 63 6.3 - I n f r a r e d Spectrophotometry Studies 65 6.3.1. - Reference Spectra ( t r a n s m i s s i o n ) - Band 65 Assignments 6.3.2 - Thin Films - C h a r a c t e r i z a t i o n 71 a) Magnesium oxide f i l m s 73 b) Amorphous magnesium carbonate f i l m s 73 c) Magnesium carbonate f i l m s s i m i l a r to magnesite 75 6.3.3 6.4 7 Adsorption of sodium ol e a t e ( o l e i c a c i d ) onto magnesium carbonate (magnesite) t h i n f i l m s a) A d s o r p t i o n of aqueous sodium o l e a t e under moderately a c i d i c c o n d i t i o n s b) Adsorption of aqueous sodium ol e a t e under ba s i c c o n d i t i o n s c) Bonding str e n g t h of the adsorbed species d) Adsorption of aqueous sodium ol e a t e under an i n e r t atmosphere e) Adsorption of evaporated o l e i c a c i d f ) C o r r e l a t i o n between the adsorption mechanisms and the degree of hydrophobicity O x i d a t i o n of o l e i c a c i d and soaps SUMMARY AND CONCLUSIONS RECOMMENDATIONS FOR FURTHER WORK CHAPTER CHAPTER 8 REFERENCES APPENDICES I . S o l u b i l i t y Data on A n i o n i c S u r f a c t a n t s I I . C r i t i c a l M i c e l l e Concentrations of An i o n i c Surfactants I I I . Magnesite M i c r o f l o t a t i o n Tests - Confidence I n t e r v a l s IV. E l e c t r o n D i f f r a c t i o n P a t t e r n s of magnesium carbonate Films - I n t e r p r e t a t i o n v i Page 87 88 105 117 127 128 137 139 150 152 153 163 164 165 171 LIST OF TABLES I) Thermodynamical data i n the system magnesite - carbon d i o x i d e water I I ) I s o e l e c t r i c p o i n t of magnesite I I I ) Chemical composition of B r a z i l i a n magnesite sample IV) Size d i s t r i b u t i o n of the f i n e magnesite sample V) I n f r a r e d a b s o r p t i o n r a t i o f o r the bands at 890 and 1500 cm"1 VI) C o r r e l a t i o n between f i l m thickness and IR absorption v i i i LIST OF FIGURES Page Figure 1) C r y s t a l s t r u c t u r e of magnesite 4 Figure 2) Magnesite-water -CO2 e q u i l i b r i u m 8 Figure 3) General d i s s o c i a t i o n curve f o r the f a t t y a c i d s 14 Figure 4) M i c r o f l o t a t i o n c e l l (modified Hallimond tube) 16 Figure 5) I n f r a r e d absorption (transmission) spectra of 19 l a u r i c a c i d and soaps Figure 6) I n f r a r e d a b s o r p t i o n ( t r a n s m i s s i o n ) spectrum of magnesite, before and a f t e r treatment i n a Na l a u r a t e s o l u t i o n 20 Figure 7) Specular r e f l e c t i o n on a g o l d m i r r o r , through an absorbing t h i n f i l m 29 Figure 8) O p t i c a l arrangement f o r r e f l e c t i o n - a b s o r p t i o n spectroscopy 31 Figure 9) R e f l e c t i v i t y band 31 Figure 10) Multiple-beam in t e r f e r o g r a m of a f l a t Au m i r r o r . The spacing between the f r i n g e s i s 2945X. H o r i z o n t a l m a g n i f i c a t i o n = 15x 49 Figure 11) Determination of the p o i n t of zero charge of magnesite 56 Figure 12) F l o t a t i o n of magnesite w i t h Na oleate 57 Figure 13) F l o t a t i o n of magnesite w i t h Na dodecylsulphonate, alone, and w i t h the a d d i t i o n of dodecanol 59 Figure 14) F l o t a t i o n of magnesite w i t h dodecylammonium c h l o r i d e 62 Figure 15) Absorption (transmission) spectrum of o l e i c a c i d 67 Figure 16) A b s o r p t i o n (transmission) spectrum of Na o l e a t e 67 Figure 17) Absorption (transmission) spectrum of Mg oleate 68 Figure 18) Absorption (transmission) spectrum of magnesite 70 Figure 19) Absorption (transmission) spectrum of hydromagnesite 70 Figure 20) Absorption (transmission) spectrum of b r u c i t e 72 Figure 21) A b s o r p t i o n (transmission) spectrum of p e r i c l a s e 72 I X Page Figure 22) MSRA spectrum of Mg oxide t h i n f i l m 74 Figure 23) MSRA spectrum of an amorphous Mg carbonate t h i n f i l m 74 Figure 24) MSRA spectrum of a c r y s t a l l i n e Mg carbonate (magnesite) t h i n f i l m . The f i l m i s about 850$ t h i c k . 76 Figure 25) MSRA spectrum of a c r y s t a l l i n e Mg carbonate (magnesite) t h i n f i l m (rough s u b s t r a t e ) . The f i l m i s about 850A" t h i c k . 76 Figure 26) Mg carbonate (magnesite) f i l m thickness versus i n t e g r a t e d area (absorbances) of the IR MSRA band i n the 1500 cm - 1 region. 82 Figure 27) Mg carbonate (magnesite) f i l m thickness versus r e f l e c t i v i t y change of the IR MSRA band around i 890 cm - 1 83 Figure 28) E l e c t r o n d i f f r a c t i o n p a t t e r n of Mg carbonate (magnesite). The zone a x i s i s [110]. 85 Figure 29) E l e c t r o n d i f f r a c t i o n p a t t e r n of Mg carbonate (magnesite). The zone a x i s i s [001]. 85 Figure 30) MSRA spectra of a c r y s t a l l i n e Mg carbonate t h i n f i l m , before and a f t e r the adsorption of aqueous Na o l e a t e , f o r 2 min ( a ) , 6 min ( b ) , and 15 min (c) exposure times; pH=6.0; s o l u t i o n c o n c e n t r a t i o n = 10~^M. 90 Figure 31) Spectra obtained by the s u b t r a c t i o n from each spectrum a f t e r the adsorption of aqueous Na o l e a t e , of the spectrum of the adsorbent alone. The o r i g i n a l spectra are shown i n F i g . 30: (a) 2 min exposure, (b) 6 min, (c) 15 min. 92 Figure 32) Molecular model of o l e i c a c i d . 102 Figure 33) MSRA spectra of a c r y s t a l l i n e Mg carbonate t h i n f i l m , before and a f t e r the adsor p t i o n of aqueous Na o l e a t e , f o r 2 min ( a ) , 6 min (b ) , and 15 min ( c ) . S o l u t i o n concentration = 10~4M; pH = 10.0. 107 Figure 34) Spectra detained by the s u b t r a c t i o n from each spectrum a f t e r the absorption of aqueous Na o l e a t e , of the spectrum of the adsorbent alone. The o r i g i n a l spectra are shown i n F i g . 33: 2 min exposure ( a ) , 6 min ( b ) , and 15 min ( c ) . 109 X Page Figure 35) Region of the hydrocarbon s t r e t c h i n g v i b r a t i o n s of the tra n s m i s s i o n reference spectrum of Na o l e a t e , and the MSRA spectra of adsorbed compounds. I l l Figure 36) MSRA spectrum of the m a t e r i a l removed from the Mg carbonate front-covered m i r r o r s , t r e a t e d i n Na oleate s o l u t i o n at pH = 10, by washing w i t h CCI4. 120 Figure 37) As i n F i g . 36, but transmission spectrum. 120 Figure 38) MSRA spectrum of the Mg carbonate t h i n f i l m , t r e a t e d i n Na ol e a t e s o l u t i o n at pH = 10, a f t e r washing w i t h CCI4. 121 Figure 39) Spectrum obtained by s u b t r a c t i o n of the spectrum of Mg carbonate alone, from the spectrum i n F i g . 38. 121 Figure 40) MSRA spectra of a c r y s t a l l i n e Mg carbonate t h i n f i l m , before and a f t e r the adsor p t i o n of evaporated o l e i c a c i d , a f t e r lh30 ( a ) , 1 day ( b ) , and 10 days ( c ) . 130 Figure 41) Spectra obtained by the s u b t r a c t i o n of the spectrum of Mg carbonate alone, from the spectra recorded a f t e r the adsor p t i o n of evaporated o l e i c a c i d . The o r i g i n a l spectra are shown i n F i g . 40: a f t e r lh30 ( a ) , 1 day ( b ) , and 10 days ( c ) . 132 Figure 42) Absorption (transmission) spectrum of Na o l e a t e , removed from a 10~^M s o l u t i o n , pH = 10.0, a f t e r 2 hours, and evaporated. 143 x i ACKNOWLEDGEMENTS The author expresses h i s sin c e r e g r a t i t u d e to h i s research s u p e r v i s o r , Dr. George W. P o l i n g , f o r h i s constant guidance, patience, support and encouragement. Thanks are a l s o due t o other p r o f e s s o r s of the Department of Mining and M i n e r a l Process Engineering f o r t h e i r v a l u a b l e a s s i s t a n c e , mainly t o P r o f e s s o r s J . L e j a , A.L. Mular and J . Laskowski, and a l s o to Profess o r E. Peters of the Dept. of M e t a l l u r g i c a l Engineering. Extensive help from a l l the s t a f f members of the D.M.M.P.E." i s g r a t e f u l l y acknowledged - - mostly from S. F i n o r a , E. J i c k e l s , F. Schmidiger, O.M. Weber and P. Kempe, to name a few, and a l s o from the v a r i o u s research a s s i s t a n t s and graduate students, p a r t i c u l a r l y Dr. D.T.B. Hornsby, who contibuted h e l p f u l d i s c u s s i o n . The author f e e l s deeply indebted t o Magnesita S.A., of Belo Horizonte, MG, B r a z i l , f o r i t s continuous support and confidence, e s p e c i a l l y t o Mr. P.R.H.M. B i t t e n c o u r t and Dr. G.E. Goncalves. The f i n a n c i a l support of the N a t i o n a l Science and Engineering Research C o u n c i l of Canada i s a l s o appreciated. This t h e s i s i s dedicated t o my w i f e E l i z a b e t e , f o r her continu i n g pa t i e n c e , understanding and a i d , and to my young daughters B e a t r i z and I s a b e l ; a l s o t o my parents, Geraldo and Geni, f o r t h e i r e n t h u s i a s t i c support. CHAPTER 1 INTRODUCTION The carbonates comprise one of the most important groups of minerals, with a widespread occurrence i n a variety of geological environments. They are very important petrographic minerals, being present i n magmatic, sedimentary and metamorphic rocks. The calcium/magnesium carbonates are notable components of economic mineral deposits. Most important are: magnesite: MgCO^  calcite: C a C 0 3 dolomite: CaMg (C0 3 > 2 Magnesite i s an important economic nonmetallic mineral, since i t i s the main source of magnesium oxide which i s widely used as a refractory. Calcite and dolomite often occur as gangue minerals i n a variety of either sulphide or nonmetallic ores, although they can also be economic minerals i n a few important cases. Magnesite almost never occurs i n any significant deposit pure enough to be used directly. Its ores contain a variety of gangue minerals, mostly other carbonates, silicates and oxides. Therefore, some kind of ore benefication i s mandatory. Many techniques have been used i n research situations, and a few i n industrial operations. Among these techniques, two are outstanding: heavy-medium separation and f l o t a t i o n ^ 1 ^ . In recent years, flotation has become an increasingly important technique for the treatment of magnesite ores. Flotation i s applicable to most of the ores and can meet more stringent concentrate specifications. For ores having essentially a s i l i c a t e gangue, cationic flotation i s (2 3) frequently the most suitable technique. ' ' The silicates are floated away, and the depressed fraction becomes the magnesite concentrate. This 2 l a t t e r f r a c t i o n u s u a l l y comprises the m a j o r i t y of the o r i g i n a l ore. When dolomite and/or c a l c i t e i s a major gangue component, a n i o n i c reagents (2) seem to be the only ones capable of achieving s e l e c t i v i t y . F a t t y a c i d s that have from 12 to 18 carbon atoms are the most f r e q u e n t l y used c o l l e c t o r s . Magnesite i s normally the f l o a t e d m i n e r a l , w h i l e dolomite i s kept depressed by a v a r i e t y of organic and/or in o r g a n i c modifying i- (5,6,7) agents. ' ' The surface chemistry and f l o t a t i o n fundamentals of magnesite and r e l a t e d minerals have not so f a r been studied e x t e n s i v e l y , although some outstanding c o n t r i b u t i o n s e x i s t , such as the work of P r e d a l i et (8,9,10,11) a.JL • v 3 CHAPTER 2 LITERATURE REVIEW 2.1 C r y s t a l Chemistry and Mineralogy Magnesite, MgCO^, belongs to the c a l c i t e group of minerals, along w i t h s i d e r i t e , rhodochrosite, smithsonite and a few others. These minerals c r y s t a l i z e i n the rhombohedral system, d i t r i g o n a l scalenohedral (3m) (12) c l a s s . The c r y s t a l s t r u c t u r e i s considered to be derived from that of 2-NaCl, i n which the s i t e s of the CI ions are taken by planar CO^ groups (13) ( F i g . 1) ' The o r i g i n a l cube i s thus d i s t o r t e d i n t o a rhombohedron, (13) being compressed along a t e r t i a r y a x i s (cubic diagonal) ; the planes of 2-the CO^ are a l l perpendicular to t h i s v e r t i c a l t h r e e - f o l d a x i s . The place + 2+ of the o r i g i n a l Na ions are occupied by Mg . Bonding between the cat i o n s and the anion r a d i c a l i s i o n i c , while bonding w i t h i n each complex CO^ i o n i s covalent. Magnesite i s the only anhydrous Mg carbonate. However, some hydrated and/or hydroxylated minerals a l s o occur, (13>15,16,17) ^ t h o u g h n Q n e i s as abundant as magnesite. The important ones are as f o l l o w s : nesquehonite Mg(HCO.j) (OH) .2^0 monoclinic MgCO^.S^O monoclinic l a n s f o r d i t e a r t i n i t e Mg2CO.j(OH)2.3^0 monoclinic hydromagnesite Mg^CO^-^OH^-S^O monoclinic b r u c i t e MgCOH)^ hexagonal The carbonates and other oxo anion compounds are c a l l e d " s a l t - t y p e minerals" ^^'^-^ Some very important economic non-sulphide minerals belong to t h i s c l a s s . Examples includ e b a r i t e , a p a t i t e and s c h e e l i t e . T y p i c a l l y , • Mg Oo F i g . 1. C r y s t a l s t r u c t u r e of magnesite. A f t e r reference 13. 5 / the minerals belonging t o t h i s group have s o l u b i l i t i e s t hat are intermediate between those of the simple s a l t s such as h a l i t e , and the much more i n s o l u b l e (19) minerals l i k e most oxides and s i l i c a t e s . A l l the s a l t - t y p e m i n erals present s t r o n g l y h y d r o p h i l i c s u r f a c e s , since any f r a c t u r e breaks bonds that (4 14 18) have t o t a l or at l e a s t p a r t i a l i o n i c c h a r a c t e r . ' ' A H occurrences of magnesite ores can be c l e a r l y d i v i d e d i n t o two main groups (a) c o a r s e - c r y s t a l l i n e - c h a r a c t e r i z e d by g r a i n s i z e s v arying from about 0.5 mm to a few centimeters; gangue minerals are t y p i c a l l y : dolomite, c a l c i t e , t a l c , c h l o r i t e , quartz, hematite, e n s t a t i t e , d i o p s i d e , t r e m o l i t e , serpentine. (b) c r y p t o c r y s t a l l i n e - which has a very small g r a i n s i z e , u s u a l l y around a few micrometers, or even s m a l l e r ; usual gangue minerals are serpentine, s i l i c e o u s minerals ( q u a r t z , chalcedony, o p a l ) , hydrated i r o n oxides, c l a y m i n erals, and l e s s f r e q u e n t l y , dolomite, c a l c i t e . 2.2 Water chemistry of magnesium carbonate 2.2.1 S o l u b i l i t y Magnesite (as w e l l as the other Ca/Mg carbonates) has considerable s o l u b i l i t y i n water, p a r t i c u l a r l y under a c i d i c c o n d i t i o n s . The s o l u b i l i t y r e l a t i o n s h i p s i n the heterogeneous system mineral-water are very important t o f l o t a t i o n , since they c o n t r o l most of the chemical composition of the aqueous (19) phase, as w e l l as the charge c h a r a c t e r i s t i c s of the i n t e r f a c e . The computation of a s o l u b i l i t y diagram f o r the magnesite-water system i s subject to u n c e r t a i n t y i n the s o l u b i l i t y product of the m i n e r a l . Two values are found i n l i t e r a t u r e on the subject. A higher pK value (7.91) i s based on S O experimental works done i n 1913 and 1955^"^ and i s used i n G a r r e l s and 6 C h r i s t ' s book. This value i s a l s o used i n the only thorough thermodynamic c a l c u l a t i o n of the s o l u b i l i t y of magnesite so f a r published, by P r e d a l i . A lower value (pK = 4.90), which comes from more recent so ' experimental work (1965), seems more c r e d i b l e .(16,21) This l a t t e r value was used f o r the c a l c u l a t i o n of the magnesite - H^ O - CO2 e q u i l i b r i u m diagram shown i n F i g . 2. The thermodynamic data used are shown i n Table I . - A l l Mg complexes ( F i g . 2 ) , except Mg(OH) (aq), are i o n - p a i r s , e i t h e r p o s i t i v e l y charged or n e u t r a l . The absence of n e g a t i v e l y charged Mg (23) complexes e x p l a i n s why the s o l u b i l i t y does not in c r e a s e at hig h pH val u e s . The t o t a l s o l u b i l i t y of magnesite i s defined b y : ^ ^ s = [Mg 2 +]+[MgHC03]+[MgC0 3(aq)]+[MgOH +]+[Mg(OH) 2(aq)]* However, only two Mg species are c r i t i c a l to the s o l u b i l i t y ( F i g . 2 ) : f o r pH 2+ values below 9.08, only the Mg concentration i s s i g n i f i c a n t ; above t h i s pH, MgCO^(aq) i s the s o l u b i l i t y - c o n t r o l l i n g species. (24) The i s o e l e c t r i c p o i n t of the s o l u t i o n i s defined by the equation g i v i n g the e l e c t r o n e u t r a l i t y c o n d i t i o n (see the f o l l o w i n g s e c t i o n ) : 2[Mg 2 +]+[MgHC02]+[Mg0H +]+[H +]=2[C0 3"]+[HC0~]+[0H"] I t s exact value can be computed from the data i n Table I and F i g . 2. A very c l o s e approximation can be obtained by considering only the highest 2+ -equivalence point i n F i g . 2, i . e . the one f o r Mg and HCO^, thereby achieving the r e l a t i o n : * The brackets [ ] i n d i c a t e molar concentrations of the r e s p e c t i v e species, i . e . the amount of moles, formula-grams or ion-grams, per l i t e r of the aqueous s o l u t i o n . 7 TABLE I THERMODYNAMIC.•• DATA IN THE SYSTEM MAGNESITE-CO7-WATER* Reaction pK** Reference 1) MgC0 3( s) t Mg + + C0 3 pK c so = = 4.90 (16) 2) C0 2(g) Z C0 2(aq) [C0 2(aq) J-KgP^ PK R = 1.5 (21) 3) C0 2(aq) + H 20 Z H + + HCO~ pK 1 = 6.3 (21) 4) HCO~ Z H + + CO?" 3 r , 3 PK 2 = 10.25 (21) 5) MgHC03 j M g 2 + + HC0 3 P RA = 1.16 (16) 6) MgC0 3(aq) Z M g 2 + + CO 2" PK B = 3.4 (21) 7) M g 2 + + 0H~ jMgOH + PK C = -2.58 (21) 8) MgOH+ + OH~ Z. Mg(OH) 2(aq) PK D = -5.04*** Remarks: * C a l c u l a t e d f o l l o w i n g a modified approach, a f t e r r e f . 22. ** The e q u i l i b r i u m constants K, and obviously t h e i r pK values (negative logarithms of K) are defined f o r a constant i o n i c medium a c t i v i t y s c a l e . ( ^ 1 ) *** C a l c u l a t e d f o l l o w i n g a modified approach, a f t e r r e f . 11. 8 0 2 4 6 8 10 12 14 P H F i g . 2. Magnesite-water-CCv, e q u i l i b r i u m , pK s Q=4.90. System open to the atmosphere, ii ' e . p . s . : pH=9.42. s = s o l u b i l i t y . 9 2[Hg*T] = [HC0 3] or [Mg^"] =.1/2[HC0 3] The computed value i s pH = 9.42 S o l u b i l i t y c a l c u l a t e d from thermodynamic data i s very high, as shown i n F i g . 2. However, i t appears that from the k i n e t i c point of view, magnesite i s not so s o l u b l e . Several researchers have reported very long periods ( i . e . / s e v e r a l days) needed i n order t o achieve e q u i l i b r i u m i n s i m i l a r aqueous systems. ^  During t y p i c a l times i n v o l v e d i n f l o t a t i o n , these very high s o l u b i l i t i e s would not be achieved. The trends shown i n F i g . 2 are expected t o occur, n e v e r t h e l e s s , even i n s h o r t , non-equilibrium c o n d i t i o n s , i . e . the s o l u b i l i t y would be higher i n a c i d i c c o n d i t i o n s decreasing around the n e u t r a l range, and would l e v e l t o low values i n the basic region. 2.2.2 E l e c t r o k i n e t i c P r o p e r t i e s Published data show considerable v a r i a t i o n > concerning the e l e c t r o k i n e t i c p r o p e r t i e s of magnesite, and the other Mg/Ca carbonates as w e l l . The main reason i s thought t o be the h i g h s o l u b i l i t i e s of the m i n e r a l s , mostly i n a c i d i c c o n d i t i o n s . ^ ^ Another reason f o r t h i s v a r i a t i o n i s the d i f f e r e n c e i n surface p r o p e r t i e s that the same mineral may (19) show, depending on i t s o r i g i n and type of d e p o s i t . The main changes concerning the minerals are as f o l l o w s : d i f f e r e n c e s i n chemical composition, l i k e s o l i d s o l u t i o n s and t r a c e i m p u r i t i e s ; - v a r i a t i o n s i n the degree of c r y s t a l l i n i t y ; - d i f f e r e n c e s i n the g r a i n s i z e and t exture of the minerals and t h e i r a s s o c i a t i o n s . 10 Other reasons f o r the i n c o n s i s t e n t e l e c t r o k i n e t i c data are the f o l l o w i n g : - use of d i f f e r e n t experimental techniques and methods; - d i f f e r e n c e s i n sample p r e p a r a t i o n , notably g r i n d i n g ; - previous treatments on the sample, l i k e aging, storage c o n d i t i o n s and heat treatment. The important p r o p e r t i e s regarding the s o l u t i o n are the presence of (25) p o t e n t i a l - d e t e r m i n i n g i o n s , and i o n i c s t r e n g t h . A l s o , the degree of p u r i t y of the reagent water used i s c r i t i c a l . Since there i s some confusion and controversy i n the l i t e r a t u r e about the terms i s o e l e c t r i c p o i n t , and point of zero charge, the f o l l o w i n g f 2 2 25 26) d e f i n i t i o n s w i l l be fo l l o w e d i n t h i s work: ' ' when a s o l i d d i s s o l v e s i n water, both negative and p o s i t i v e species are present i n the bulk s o l u t i o n ; t h e r e f o r e there e x i s t s a p a r t i c u l a r c o n d i t i o n c a l l e d the i s o e l e c t r i c p o i n t of the s o l u t i o n ( i . e . p . s ) ^ ^ at which the two charges are equivalent e l e c t r i c a l l y . A s i m i l a r s i t u a t i o n p r e v a i l s at the s o l i d - l i q u i d i n t e r f a c e , f o r which H + and OH mainly, but a l s o other ions l i k e the components of the c r y s t a l l i n e l a t t i c e , have been shown to be po t e n t i a l - d e t e r m i n i n g f o r most mi n e r a l s . At some p a r t i c u l a r s o l u t i o n c o n d i t i o n , the surface w i l l c a r r y no f i x e d charge: such c o n d i t i o n i s c a l l e d ( 2 6") the p o i n t of zero charge ( p . z . c . ) ^ . The z e t a - p o t e n t i a l , or the p o t e n t i a l at the shear plane i n the e l e c t r i c a l double-layer can be i n d i r e c t l y measured by the various e l e c t r o k i n e t i c methods - - such as electro-osmosis and e l e c t r o p h o r e s i s - - i n order t o c h a r a c t e r i z e the e l e c t r i c a l charge and i t s i n t e n s i t y at the i n t e r f a c e . A surface which shows no electro-osmosis, or a p a r t i c l e showing no e l e c t r o p h o r e s i s ( i . e . having e l e c t r o p h o r e t i c m o b i l i t y equal t o zero) i s ( 26 ) s a i d to be at the i s o e l e c t r i c point ( i . e . p . ) . 11 The i s o e l e c t r i c p o i n t s of magnesite reported i n the l i t e r a t u r e are shown i n Table I I . The two values f o r coarse samples, determined by the streaming p o t e n t i a l technique, show pH's between 5 and 6, i . e . i n the moderately a c i d i c range. These r e s u l t s of the i . e . p . (near pH=6) do not c o i n c i d e w i t h the c a l c u l a t e d i . e . p . s . (pH=9.42; see S e c t i o n 2.2.1 and F i g . 2 ) . This i s not s u r p r i s i n g , because of the f o l l o w i n g reasons: - the i . e . p . s . can only be expected t o c o i n c i d e w i t h the i.e.p. (or the p.z.c.) f o r h i g h l y i n s o l u b l e compounds l i k e most oxides (e.g. A l 20^), and s i l i c a t e s , not n e c e s s a r i l y f o r s l i g h t l y s o l u b l e compounds l i k e the c a r b o n a t e s ^ a n d other s a l t - t y p e m i n e r a l s ; - the i . e . p . s . i s c a l c u l a t e d e n t i r e l y from thermodynamic data, t h e r e f o r e under the assumption that f u l l e q u i l i b r i u m has been reached; on the other hand, the i . e . p . and the p.z.c. are u s u a l l y the r e s u l t s of p r a c t i c a l determinations, where the e q u i l i b r i u m i s not n e c e s s a r i l y achieved; - even from an e n t i r e l y thermodynamical po i n t of view, the i . e . p . s . can only be" expected t o agree w i t h the p.z.c. when the former c o i n c i d e s w i t h the minimum i n the s o l u b i l i t y c u r v e ; ^ ^ t h i s i s not the case f o r magnesite, where the s o l u b i l i t y curve does not show any minimum point ( F i g . 2 ) . The p o t e n t i a l - d e t e r m i n i n g ions f o r magnesite are mostly H + and OH . M g 2 + has a m i l d e f f e c t only above pH=10, thought to be due t o the p r e c i p i t a t i o n of p o s i t i v e l y charged Mg(0H>2 onto the carbonate surf ace. Carbonate ions (both C0^ and HCO^) were found experimentally t o be not p o t e n t i a l - d e t e r m i n i n g f o r magnesite.^"'""'"^ 12 TABLE I I ISOELECTRIC POINT OF MAGNESITE i.e.p. (pH) Method* Remarks Reference r 6.0 SP pH 2 to 11 (8) 5.2 SP aged 30 min; pH 6 to 11; (10,11) KC1: 10~ 2M, 10~ 3M, no a d d i t i o n ; p a r t i c l e s i z e : 100 to 200 pm 3.2 ? very c l o s e to zero at pH 5.5 (28) 2.0 ME pH 1.5 to 12 - (29) 11.5 HS s i z e : 95% <20 um (30) only + values ME H C 1 ° 4 ; NaOH; pH 6 to 12.5 (27) Methods:-SP: Streaming p o t e n t i a l ME: M i c r o e l e c t r o p h o r e s i s HS: Hindered s e t t l i n g 13 2.3 Fundamental F l o t a t i o n Studies A v a i l a b l e i n f o r m a t i o n on the fundamental aspects of magnesite f l o t a t i o n i s very scarce. The only t o p i c s that have been studied i n some d e t a i l are the f l o t a t i o n response under i d e a l c o n d i t i o n s using f a t t y a c i d and soaps as c o l l e c t o r s , and i n f r a r e d spectrophotometric i n v e s t i g a t i o n of the adsorp t i o n of these c o l l e c t o r s on magnesite. No fundamental s t u d i e s were found on the c a t i o n i c f l o t a t i o n of t h i s m i n e r a l . A d e s c r i p t i o n of the more p r a c t i c a l aspects of the s e l e c t i v e f l o t a t i o n of the magnesite ores, as w e l l as of the other Mg/Ca carbonates, i s presented i u (31) elsewhere 2.3.1 A n i o n i c C o l l e c t o r s Long-chain n - a l k y l c a r b o x y l i c a c i d s ( o f t e n r e f e r r e d t o as f a t t y a c i d s ) and t h e i r Na soaps are important c o l l e c t o r s f o r non-sulphide minerals, i n c l u d i n g magnesite and other carbonates. Most of the present p r a c t i c a l a p p l i c a t i o n s of f l o t a t i o n of magnesite ores use these reagents as c o l l e c t o r s . One of the most important p r o p e r t i e s of the long-chain c a r b o x y l i c a c i d s i s t h e i r d i s s o c i a t i o n behaviour. F i g . 3 shows a s i m p l i f i e d approach to t h i s behaviour. In t h i s f i g u r e , only the s i n g l e n e u t r a l species RCOOH, and the carboxylate anion RCOO , are taken i n t o account. Below pH 4, the n e u t r a l species i s dominant; between pH 4 to 8, both species are present i n comparable c o n c e n t r a t i o n s , and above pH 8, only the anion i s abundant i n (32) s o l u t i o n . ' A more thorough approach should a l s o consider the i n f l u e n c e (33) of other species, mainly the f o l l o w i n g : the ionomolecular dimer, (RCOO^H the a n i o n i c dimer, (RCOO)|~ the a c i d molecular dimer, (RCOOH)2 the acid-soap s a l t , (RCOO)2HNa 4 6 8 PH F i g . 3. General d i s s o c i a t i o n curve f o r the f a t t y a c i d s i n aqueous s o l u t i o n . The shaded area shows the region where both n e u t r a l and i o n i z e d species have s i g n i f i c a n t c o n c e n t r a t i o n s . A f t e r reference 32. 15 However, an assessment of the i n f l u e n c e of these other species on f l o t a t i o n (33) has not been made y e t , except f o r a few very i n s o l u b l e m i n e r a l s . ' The most important commercial a n i o n i c c o l l e c t o r s are composed of (34) mixtures of f a t t y a c i d s . T a l l o i l , one of the most widely used, c o n s i s t s mainly of unsaturated f a t t y a c i d s , the most abundant ones being o l e i c ( c i s ) and l i n o l e i c ( c i s , c i s ) a c i d s , followed by l i n o l e n i c a c i d ( c i s , c i s , c i s ) . T a l l o i l a l s o contains minor amounts of saturated acids such as s t e a r i c and p a l m i t i c . Crude t a l l o i l contains from 38 t o 58% of r e s i n a c i d s , w i t h a b i e t i c and levopimaric being the most abundant. In r e f i n e d t a l l o i l , (34) the r e s i n a c i d s content i s lowered t o about 5%. ' A c o m p i l a t i o n of recent s o l u b i l i t y data on these pure compounds and some of t h e i r d e r i v a t i v e s i s presented i n Appendix I ; the data should be regarded w i t h some c a u t i o n , s i n c e many of the references do not s p e c i f y a l l the experimental c o n d i t i o n s , e.g. even pH. Appendix I I d i s p l a y s the c r i t i c a l m i c e l l e concentrations of these same s u r f a c t a n t s and/or some of t h e i r derived compounds. Sodium dodecylsulphonate, an important non-carboxylic a n i o n i c s u r f a c t a n t i s a l s o i n c l u d e d i n the same t a b l e s , f o r comparison purposes. 2.3.2 M i c r o f l o t a t i o n Studies M i c r o f l o t a t i o n t e s t s are experiments c a r r i e d out i n r e l a t i v e l y small g l a s s devices l i k e the Hallimond tube ( F i g . 4 ) , described i n r e f . 35. T y p i c a l l y , only 1 t o 3 grams of the pure s i n g l e minerals are te s t e d i n 100 t o 200 ml of s o l u t i o n s of pure c o l l e c t o r s . U s u a l l y , no f r o t h e r s are added. A few m i c r o f l o t a t i o n t e s t s w i t h magnesite, using a n i o n i c c o l l e c t o r s , have been described i n the l i t e r a t u r e . A sample of c o a r s e - c r y s t a l l i n e magnesite from Czechoslovakia was f l o a t e d w i t h Na ole a t e f o r a con c e n t r a t i o n of 5 x 10 ^M, there was a recovery maximum at a c i d pH, and another one at bas i c values; a minimum was found around the n e u t r a l r e g i o n , and the recovery dropped sharply at pH's I stopper I Magnetic s t i r r e r M i c r o f l o t a t i o n c e l l (modified Hallimond tube). 17 lower than 3. For a higher o l e a t e c o n c e n t r a t i o n (3 x 10 Si) the f l o t a t i o n response was again lower below pH 4, but increased t o 100% recovery from t h i s pH value, up to pH=10.5. A c r y p t o c r y s t a l l i n e magnesite from Yugoslavia showed a roughly s i m i l a r behaviour. Another important study was made w i t h the same c o a r s e - c r y s t a l l i n e magnesite (and a dolomite sample, as w e l l ) , using s h o r t e r - c h a i n f a t t y a c i d s . ^ ' 1 ^ ^ For the Na soaps of decanoic, l a u r i c , and m y r i s t i c a c i d s , r e c o v e r i e s of both magnesite and dolomite were higher i n a c i d i c c o n d i t i o n s . As the c o l l e c t o r c o n c e n t r a t i o n i n c r e a s e d , f l o t a t i o n was achieved at b a s i c v a l u e s , but the re c o v e r i e s were always higher or equal i n the medium a c i d i c c o n d i t i o n s . Again, a sharp drop i n recovery, or a minimum, was found around" ( 9 ) the n e u t r a l r e g i o n . The general f l o t a t i o n behaviour f o r magnesite w i t h s a l t s of f a t t y a c i d s seemed t o be the f o l l o w i n g : a) at high c o l l e c t o r concentrations (about 10 ^M), near 100% recovery was found f o r most of the pH range, except i n the very a c i d i c regions; b) at lower c o l l e c t o r c o n c e n t r a t i o n s , f l o t a t i o n response was always g r e a t e r i n moderately a c i d i c c o n d i t i o n s , decreasing towards both the n e u t r a l and the more a c i d i c pH values. 2.3.3 I n f r a r e d Spectrophotometric Studies i Only the a v a i l a b l e i n f o r m a t i o n on i n f r a r e d spectroscopy of magnesite i s described i n t h i s s e c t i o n . D e t a i l s on methods and band assignments are gi v e n i n S e c t i o n 6.3.1. An i n f r a r e d spectrophotometric , study was c a r r i e d out by P r e d a l i , using a magnesite of the c o a r s e - c r y s t a l l i n e v a r i e t y (and a l s o dolomite) as adsorbent. Aqueous s o l u t i o n s of Na l a u r a t e and m y r i s t a t e at 18 d i f f e r e n t concentrations were s t u d i e d , at pH values around 3 and 9. Only trans m i s s i o n s p e c t r a were recorded, using the KBr p e l l e t technique. In order t o achieve a b e t t e r d i s t i n c t i o n between the absorption bands, and to minimize the e f f e c t s of the strong carbonate band at 1400-1600 cm-"'", d i f f e r e n t i a l s pectra were recorded i n some cases. F i g . 5 shows the reference s p e c t r a of the p o t e n t i a l l y important l a u r a t e c o l l e c t o r species; the spectra of magnesite before and a f t e r the adsorption t e s t s are shown i n F i g . 6. The main c o n c l u s i o n of t h i s study was that i n both a c i d i c and ba s i c c o n d i t i o n s , evidence was found that the adsorption i s mainly of chemical o r i g i n . This would be v a l i d f o r magnesite and dolomite, t r e a t e d w i t h both l a u r a t e and m y r i s t a t e . Under b a s i c (pH=10) c o n d i t i o n s , the f o l l o w i n g observations were considered r e l e v a n t f o r magnesite^"^ ( F i g . 6-b): a) the presence of Mg carboxylate was i n d i c a t e d by a band of 1580 cm "*" (the same as f o r the bulk Mg c a r b o x y l a t e ) , due to the asymmetrical s t r e t c h i n g v i b r a t i o n of the i o n i z e d c a r b o x y l i c group < C 0 " ) ; b) c a r b o x y l i c a c i d molecules were absent; c) no evidence was found on the s t r u c t u r e of the adsorbed l a y e r ( i . e . molecular o r i e n t a t i o n and presence of monolayers). At a c i d i c pH (around 3 ) , the important p o i n t s were ( F i g . 6-c): a) Mg carboxylate was present as i n d i c a t e d by the same band at 1580 cm ^; contrary t o the case of b a s i c c o n d i t i o n s , t h i s band could only be detected at a c i d pH i n the d i f f e r e n t i a l s pectra; b) evidence of the presence of very s m a l l amounts of n e u t r a l molecules of the f a t t y a c i d s was claimed, mostly from a hardly n o t i c e a b l e shoulder around 1700 cm "*" ( F i g . 6-c); 1'9 co o c ca 4-1 4J •H e 05 C n! u H Wavenumber (cm~l) F i g . 5. I n f r a r e d absorption (transmission) spectra of l a u r i c a c i d and soaps (KBr d i s c s ) . A f t e r reference 10. 2,0 S~8 CL) CJ c cd 4-1 •H B CO c ca t-i H 4000 F i g . 6. 3000 2000 1500 1000 _700 Wavenumber (cm ^) I n f r a r e d a b s o r p t i o n (transmission) spectra of magnesite, before and a f t e r treatment i n Na l a u r a t e s o l u t i o n (KBr d i s c s ) . A f t e r reference 10. 21 c) No evidence was found of formation of an organized c o l l e c t o r l a y e r on magnesite; however t h i s apparently was proved f o r dolomite ( p r o g r e s s i o n of bands between 1180 and 1340 cm \ due to the d i s t u r b e d wagging v i b r a t i o n of CH^ groups). This work by P r e d a l i ^ ^ was important, due not only t o i t s pioneer nature, but mainly because i t showed strong evidence of the presence of Mg carboxylate on the mineral s u r f a c e , under b a s i c and a c i d c o n d i t i o n s . This compound was probably i d e n t i c a l t o the bulk Mg c a r b o x y l a t e , MgCRCOO^, due t o the coincidence of the band at 1580 cm ^. However, no evidence was presented t h a t t h i s compound was the one d i r e c t l y adsorbed on the mineral s u r f a c e . The author himself admitted t h a t no d i r e c t c o n c l u s i o n could be drawn regarding the i n i t i a l adsorbed species. I t may w e l l be p o s s i b l e that the i d e n t i f i e d Mg carboxylate was indeed present as a l a y e r , or m u l t i l a y e r s , adsorbed on top of an o r i g i n a l monolayer. An argument i n favour of t h i s p o s s i b i l i t y i s t h a t , i n t h i s i n f r a r e d a d s o r p t i o n study, much higher c o l l e c t o r concentrations and longer c o n d i t i o n i n g times were used, as compared to the microf l o t a t i o n t e s t s . I t i s a l s o obvious that no s i g n i f i c a n t evidence was found i n d i c a t i n g the presence of adsorbed n e u t r a l a c i d molecules, i n a c i d i c c o n d i t i o n s , although t h i s i s a v a l i d hypothesis. The presence of a metal carboxylate at low pH i s apparently abnormal, s i n c e the non-dissociated a c i d i s the t o t a l l y dominant adsorbate species i n these c o n d i t i o n s . However, when ca t i o n s f o r which the carboxylate ions show high r e a c t i v i t y are present i n aqueous s o l u t i o n s , i o n i z a t i o n can take place at f a i r l y low pH v a l u e s ^ ^ . The same process occurs i n the presence of s o l i d p a r t i c l e s having r e a c t i v e surface s p e c i e s . A s i m i l a r chemisorption i n the a c i d i c region (down t o pH = 3) was a l s o ( 36 ) proven f o r f l u o r i t e , w i t h the formation of surface Ca o l e a t e . 22 Another example was i n the adsorption of Na l a u r a t e on o x i d i z e d Cu 37 38) subst r a t e s . ' M u l t i l a y e r s of Cu l a u r a t e were found, p h y s i c a l l y adsorbed on top of a i n i t i a l l y chemisorbed monolayer, at pH 5. 2.3.4 Concluding Remarks A v a i l a b l e i n f o r m a t i o n i n the l i t e r a t u r e shows some agreement between the m i c r o f l o t a t i o n behaviour of magnesite (and s i m i l a r m i n e r a l s ) , and some other fundamental surface p h y s i c a l - c h e m i c a l aspects. Chemisorption, w i t h the formation of a surface Mg carboxylate (or sulphonate, etc.) was g e n e r a l l y thought t o be the i n i t i a l a d s o r p t i o n mechanism. Evidence of t h i s was apparently found i n the f o l l o w i n g systems: a) magnesite and dolomite, w i t h s h o r t - c h a i n c a r b o x y l i c a c i d s ; (39) b) c a l c i t e w i t h s h o r t - c h a i n carboxylates and sulphonates; v c) c a l c i t e and o l e a t e . Other secondary adsorption mechanisms have been p o s t u l a t e d , i n a d d i t i o n t o chemisorption, i n order t o e x p l a i n the f l o t a t i o n behaviour more adequately. The p h y s i c a l co-adsorption of n e u t r a l compounds was the most f r e q u e n t l y mentioned mechanism. These n e u t r a l species were the c a r b o x y l i c a c i d molecules, and/or the a l k a l i n e - e a r t h soaps precipated from the s o l u t i o n . However, except f o r the already-mentioned Mg ca r b o x y l a t e , no d e f i n i t e evidence has been found of t h e i r presence. r 23 CHAPTER 3 SCOPE AND APPROACH OF THE STUDY I t i s evident from the review of the l i t e r a t u r e that knowledge of the more fundamental aspects of magnesite f l o t a t i o n i s scarce. Information on adsorption mechanisms i s somewhat i n d i r e c t , and the methods employed leave room f o r other i n t e r p r e t a t i o n s . A high degree of i n f e r e n c e was i n v o l v e d i n most of the published c o n t r i b u t i o n s . Very l i t t l e has been studied on the f l o t a t i o n behaviour of magnesite w i t h commercially important c o l l e c t o r s and t h e i r components. Since magnesite i s a n a t u r a l l y h i g h l y h y d r o p h i l i c m i n e r a l , i t can only be recovered as a f r o t h product i n the f l o t a t i o n process i f a c o l l e c t o r i s adsorbed onto the mineral's s u r f a c e . Therefore, any study of the b a s i c p r i n c i p l e s of f l o t a t i o n has t o i n c l u d e c o n s i d e r a t i o n of c o l l e c t o r adsorption as a main concern.. Consequently, the main purpose of the present i n v e s t i g a t i o n was t o study the mechanisms of the adsorption of a n i o n i c c o l l e c t o r s onto magnesium carbonate s u b s t r a t e s , and the r e l a t i o n s h i p between adsor p t i o n and f l o a t a b i l i t y of pure magnesite samples. P r a c t i c a l f l o t a t i o n i s a very complex phenomenon, i n which a l a r g e (32) number of v a r i a b l e s p l a y important r o l e s . I t was t h e r e f o r e d e s i r a b l e that a s i m p l i f i e d , more i d e a l approach be adopted. In t h i s way, the t r u e important v a r i a b l e s regarding the a d s o r p t i o n of the c o l l e c t o r could be s e l e c t e d , and t h e i r i n f l u e n c e b e t t e r evaluated. For t h i s reason, a l l f l o a t a b i l i t y experiments i n t h i s study were c a r r i e d out using m i c r o f l o t a t i o n techniques. 24 E x p e r i m e n t a l l y , the p r o j e c t c o n s i s t e d of two main phases: ( i ) m i c r o f l o t a t i o n s t u d i e s , using sodium oleate as the main c o l l e c t o r ; other s u r f a c t a n t s were used only f o r comparison purposes. ( i i ) i n f r a r e d spectroscopic s t u d i e s , using mainly the m u l t i p l e specular r e f l e c t i o n - a b s o r p t i o n (MSRA) and the t r a n s m i s s i o n techniques. 25 CHAPTER 4 INFRARED SPECTROSCOPY IN THE STUDY OF ADSORBED COLLECTOR SPECIES A l a r g e p o r t i o n of t h i s i n v e s t i g a t i o n was concerned w i t h the study of adsorbed a n i o n i c c o l l e c t o r s ( o l e i c a c i d and i t s Na soap) on the magnesite surface. For the i n f r a r e d r e f l e c t i o n - a b s o r p t i o n s t u d i e s , t h i n f i l m s of Mg carbonate were formed on top of s u i t a b l e s u b s t r a t e s . These t h i n f i l m s were then contacted w i t h aqueous s o l u t i o n s of the c o l l e c t o r , under c a r e f u l l y c o n t r o l l e d c o n d i t i o n s of pH and con c e n t r a t i o n . When using the m u l t i p l e specular r e f l e c t i o n - a b s o r p t i o n technique, the f i l m s w i t h the coating of the adsorbed c o l l e c t o r were removed from the s o l u t i o n , d r i e d , and t h e i r spectra recorded. The MSRA technique was chosen mainly because of i t s very h i g h s e n s i t i v i t y , and the p o s s i b i l i t y of gathering i n f o r m a t i o n on the o r i e n t a t i o n of the adsorbed species ( S e c t i o n 4.2). The conventional t r a n s m i s s i o n technique was used i n t h i s study p r i m a r i l y f o r recording the reference spectra of magnesite (and other r e l a t e d compounds), and of the c o l l e c t o r species which could p o s s i b l y occur as adsorbates, l i k e c a r b o x y l i c a c i d s and t h e i r sodium and magnesium soaps. 4.1 Transmission Technique Imfrared l i g h t covers the wavelength range of 0.8 to 2000 jam. The i n f r a r e d spectrum i s d i v i d e d i n t o the f o l l o w i n g regions: the "near i n f r a r e d " , which i s the region between 0.8 and 2.5 um (12500 to 4000 cm" 1); the " m i d - i n f r a r e d " , between 2.5 and 50 um (4000 to 200 cm" 1); 26 - the " f a r i n f r a r e d " , which l i e s between 50 and 100 pm (200 to 100 -1. cm ;. The term i n f r a r e d spectroscopy c o n v e n t i o n a l l y denotes the study of absorption spectra i n the m i d - i n f r a r e d region. I n f r a r e d l i g h t i s a l s o described i n terms of wavenumbers: v .=: 1/X , X = wavelength i n centimeters The wavenumber ( v ) i s always expressed i n cm ^. The conventional i n f r a r e d spectroscopy range i s then between 4000 and 200 cm ^ (2.5 to 50 pm). The recording of the i n t e n s i t y of the IR r a d i a t i o n transmitted through (or absorbed by) a sample versus frequency y i e l d s a curve showing absorption bands c h a r a c t e r i s t i c of the atoms and chemicals bonds present i n the sample. This i s the a b s o r p t i o n i n f r a r e d spectrum as obtained by conventional t r a n s m i s s i o n spectroscopy. Absorption bands that appear i n a range c h a r a c t e r i s t i c f o r a c e r t a i n group and that are u s e f u l f o r i d e n t i f i c a t i o n of t h i s group are c a l l e d c h a r a c t e r i s t i c frequencies. One of the most d i s t i n c t i v e p r o p e r t i e s of a compound i s i t s i n f r a r e d spectrum. 4.1.1 Absorption of R a d i a t i o n Absorption band i n t e n s i t i e s are r e l a t e d to the c o n c e n t r a t i o n of the absorbing species. According t o Beer's law, the i n t e n s i t y of l i g h t ( I ) , t r a n s m i t t e d at a wavenumber ( v ) by an absorbing medium i s g i v e n by: x - k e l I = I o e where: I = i n t e n s i t y of i n c i d e n t r a d i a t i o n o J c = c o n c e n t r a t i o n of absorbing 'species i n the sample 1 = path l e n g t h of l i g h t i n the sample 27 k = absorption c o e f f i c i e n t , c h a r a c t e r i s t i c of the absorbing species i n the sample Two important parameters are defined as f o l l o w s : - transmittance: T = I / I , a l s o expressed as: %T = 100I/I o o - absorbance: A = e e l -1 -1 where e i s an " e x t i n c t i o n c o e f f i c i e n t expressed i n mole . l i t e r . cm-(molar e x t i n c t i o n c o e f f i c i e n t = e' ), or i n molecule ^. cm "'"(molecular e x t i n c t i o n c o e f f i c i e n t = e ). Hence, A = l o g (I o/I)=log(l/T)=log(100/%T) 4.1.2 Adsorbed Species Conventional t r a n s m i s s i o n IR spectroscopy i s of l i m i t e d a p p l i c a t i o n to s t u d i e s of adsorbed species, mainly f o r the f o l l o w i n g reasons: a) the IR l i g h t i s s c a t t e r e d by the s o l i d p a r t i c l e s , thus reducing instrument performance, b) the absorption of r a d i a t i o n by t h i n surface l a y e r s i s u s u a l l y very weak. One way t o counteract these e f f e c t s i s by the use of extremely f i n e (41 42) p a r t i c l e s , w i t h very high surface areas. ' The most popular powder method, i n ads o r p t i o n s t u d i e s , i s the KBr p e l l e t , i n which the powder i s dispersed i n a c l e a r d i s k of t h i s IR transparent m a t e r i a l . Sometimes, a v a r i a n t technique i s used, the " d i f f e r e n t i a l spectrum". In t h i s method, (41 43) a KBr d i s c c o n taining only the adsorbent i s placed i n the reference beam of a double beam IR spectrophotometer, while a s i m i l a r d i s c , but having a l s o an adsorbed compound, i s placed i n the sample beam. In t h i s way the spectrum of the adsorbent i s mostly c a n c e l l e d out, l e a v i n g a d i f f e r e n c e spectrum due mainly to adsorbed species. 28 Despite being l a r g e l y used i n adsorption s t u d i e s , the s a l t p e l l e t technique presents some s i g n i f i c a n t shortcomings, of which the f o l l o w i n g are most important: i ) changes i n the character of the surface may be produced by the n f f . (44) p e l l e t forming process, i i ) i o n exchange r e a c t i o n s between the surface species and the s a l t <- A (45) m a t r i x may occur; i i i ) IR transparent s a l t s u s u a l l y absorb a f a i r amount of water from the atmosphere, which causes p a r t i a l , or t o t a l b l o c king of some s p e c t r a l regions. The KBr t r a n s m i s s i o n method was not very s u c c e s s f u l i n e x p l a i n i n g the d e t a i l s of the a d s o r p t i o n mechanism of a n i o n i c c o l l e c t o r s on the Mg/Ca carbonates, as described i n S e c t i o n 2.3.3. 4.2 M u l t i p l e Specular R e f l e c t i o n - A b s o r p t i o n Technique I f r a d i a t i o n passes through a t h i n l a y e r of a l i g h t - a b s o r b i n g m a t e r i a l l o c a t e d on a h i g h l y r e f l e c t i n g metal surface l i k e g o l d ( F i g . 7 ) , a p l o t of the r e f l e c t a n c e against the r a d i a t i o n c o n s t i t u t e s a r e f l e c t i o n - a b s o r p t i o n (46 47) spectrum of the f i l m m a t e r i a l . ' A r e f l e c t a n c e band, i . e . a removal of energy from the r e f l e c t e d r a d i a t i o n i s produced by two overlapping phenomena:. a) an increase i n the a b s o r p t i o n c o e f f i c i e n t (k) of the absorbing medium; b) a decrease i n the r e f r a c t i v e index (n) of the medium at the frequencies near the absorption band. For a moderately absorbing f i l m ( l i k e organic m a t e r i a l s ) , the d i s p e r s i o n i n t h e 1 value of n i s s m a l l . In t h i s i n c i d e n t beam Specular r e f l e c t i o n on a gold m i r r o r , through an absorbing t h i n f i l m . 30 s i t u a t i o n , the increase i n the absorption c o e f f i c i e n t of the f i l m i s the main cause f o r the formation of the r e f l e c t a n c e band, which, i n t h i s case, has shape and p o s i t i o n c l o s e l y c o i n c i d i n g w i t h those of a reg u l a r absorption (47) band, as obtained by tran s m i s s i o n spectroscopy. In r e f l e c t i o n - a b s o r p t i o n spectra, other important aspects have been confirmed as f o l l o w s : ( 4 8 ) i ) recording spectra of f i l m s as t h i n s as one monolayer can be achieved w i t h the use of commercial d i s p e r s i v e - t y p e spectrophotometers; i i ) i n f o r m a t i o n on molecular o r i e n t a t i o n s i n a n i s o t r o p i c f i l m s can be gained from the spectra; i i i ) band i n t e n s i t i e s can o f t e n be r e l a t e d d i r e c t l y to average f i l m t h i c k n e s s . In the m u l t i p l e specular r e f l e c t i o n - a b s o r p t i o n technique, u s u a l l y a p a i r of m i r r o r s covered w i t h the f i l m to be analysed i s employed and an o p t i c a l arrangement f o r c e s the r a d i a t i o n to pass through the f i l m s e v e r a l times, f o l l o w i n g a s e r i e s of r e f l e c t i o n s ( F i g . 8 ) . Therefore, a s i g n i f i c a n t f i n a l enhancement of the a b s o r p t i o n bands can be achieved. This technique i s of p a r t i c u l a r i n t e r e s t i n st u d i e s of p h y s i c a l and chemical i n t e r a c t i o n s of re a c t i n g species w i t h s o l i d s urfaces. x In r e f l e c t i o n s t u d i e s , band i n t e n s i t i e s are f r e q u e n t l y measured i n terms of the " f r a c t i o n a l change i n r e f l e c t i v i t y at a band maximum", AR, as defined b y : ( 4 9 ^ AR = (R - R) / R o o \ 31 Sample beam to the detector sample m i r r o r s from source F i g . 8. O p t i c a l arrangement f o r r e f l e c t i o n - a b s o r p t i o n spectroscopy. A f t e r reference 46. 0) CJ c cfl 4J CJ 0) P i R v Wavenumber (cm ^) o F i g . 9. R e f l e c t i v i t y band. A f t e r reference 48, 32 where: R q = r e f l e c t i v i t y i n the absence of an absorbing f i l m ; R = r e f l e c t i v i t y at band maximum ( F i g . 9). In the case of m u l t i p l e r e f l e c t i o n s , a spectrophotometer, l i n e a r i n transmittance, w i l l record T and T .: ' o N T = R T = tfansm'ittance. or. reflectance at band maximum; T = R N T = transmittance or r e f l e c t a n c e i n the o o o absence of an absorbing f i l m j N = number tof, r e f l e c t ion's.-AR values can be c a l c u l a t e d from these transmittance data: AR = ( T 1 / N - T 1 / N ) / T 1 / N o . o The number of r e f l e c t i o n s determines the minimum AR value which can be measured. Values as low as 0.0005 have been reported to be measured (49) r e p r o d u c i b l y , , f o r N=7 r e f l e c t i o n s . ' Even lower AR values can be achieved, f o r higher number of r e f l e c t i o n s . However, there i s an optimum N_ value f o r each incidence angle, varying from N=8 to N=25, f o r angles from 88 to 70°, r e s p e c t i v e l y . There i s a decrease i n s e n s i t i v i t y of about. 20 to 30% when h a l f of the optimum number of r e f l e c t i o n s i s used. 4.2.1 Molecular O r i e n t a t i o n Among the various t h e o r e t i c a l approaches to i n f r a r e d r e f l e c t i o n - a b s o r p t i o n phenomena of t h i n f i l m s on metal s u r f a c e s , the work of Fr a n c i s and E l l i s o n i s o u t s t a n d i n g / 5 1 ^ This theory describes the absorption of r a d i a t i o n by a t h i n f i l m on a r e f l e c t i n g surface. I t p r e d i c t s 33 that the abso r p t i o n of the component of the r a d i a t i o n perpendicular to the plane of incidence cannot be detected. The f r a c t i o n a l change i n r e f l e c t i v i t y of the p a r a l l e l component (( AR^ y ) would be given by the f o l l o w i n g equation, f o r metals m i r r o r s having high r e f l e c t i v i t y and incidence angles between 0 and 80° ( F i g . 7 ): AR^ = (16 ir s i n 2 0 k x d) / (AcosGn^) where: 0 = angle of incidence v n^ = r e f r a c t i v e index of the f i l m = a b s o r p t i o n constant of the f i l m X = wavelength of the r a d i a t i o n d = f i l m thickness For a sharp r e f l e c t a n c e peak, the equation above can be f u r t h e r s i m p l i f i e d t o : AR = Kd This equation proves t h a t , under the assumptions considered, band i n t e n s i t y i s p r o p o r t i o n a l to f i l m t h i c k n e s s . A t h i n f i l m cannot absorb the component of the r a d i a t i o n perpendicular to the plane of incidence because t h i s component undergoes an approximately 180° phase s h i f t during r e f l e c t i o n , f o r a l l angles of incidence. A standing wave i s then formed, having zero amplitude at the surface. Therefore, no i n t e r a c t i o n s w i t h the o s c i l l a t i n g d i p o l e s of the absorbing species at the surface can take p l a c e . However, t h i s i s not true f o r the p a r a l l e l component, which undergoes a f i n i t e change on r e f l e c t i o n , but only becomes 180 u at grazing (90°) incidence angle. The p a r a l l e l component gives r i s e to a standing wave f i e l d d i r e c t e d mainly normal to the plane of 34 the r e f l e c t i n g s u r f a c e . Since only t h i s component can i n t e r a c t w i t h , and be absorbed by the f i l m , i t i s very e f f e c t i v e p r a c t i c e to i n s t a l l an IR p o l a r i z e r , o r i e n t e d to e l i m i n a t e the perpendicular component of the r a d i a t i o n from reaching the detector of the instrument. Thus, the AR, or percentage absorption of the remaining p a r a l l e l component i s s i g n i f i c a n t l y enhanced, and the o v e r a l l s e n s i t i v i t y increased. Another important reason f o r the e l i m i n a t i o n of the perpendicular component i s t h a t , on specular r e f l e c t i o n , t h i s component i s the dominant one, showing r e f l e c t a n c e s higher (52) than the p a r a l l e l component. A very important consequence of the F r a n c i s and E l l i s o n theory i s that i n f o r m a t i o n on molecular o r i e n t a t i o n can be e a s i l y achieved using r e f l e c t i o n - a b s o r p t i o n spectroscopy. This i s due to the f a c t that only the v i b r a t i o n s having t r a n s i t i o n moments approximately normal to the r e f l e c t i n g metal surface can e f f e c t i v e l y absorb r a d i a t i o n (of the p a r a l l e l component). In t h e i r o r i g i n a l work,^ 5 1^ these authors proved t h e i r t h e o r e t i c a l d e r i v a t i o n by a p r a c t i c a l example. They recorded the spectra of metal st e a r a t e s deposited as s o l i d i f i e d monolayers on metal m i r r o r s using the Langmuir-Blodgett technique. S e n s i t i v i t y f o r recording spectra of f i l m s one monolayer t h i c k was obtained. In the deposited f i l m s , the metal carboxylates were or i e n t e d w i t h t h e i r molecular axes n e a r l y perpendicular t o the surface of the m i r r o r s . In t h i s s i t u a t i o n , the methylene groups of the hydrocarbon chains l a y i n planes nearly p a r a l l e l to the surface. Thus, both symmetric and asymmectric s t r e t c h i n g v i b r a t i o n s (see Secti o n 6.3.3a) had t r a n s i t i o n moments p a r a l l e l to the surface.~ The bands assigned to them were weak, as expected. S i m i l a r l y , good c o r r e l a t i o n s were found between the r e l a t i v e 35 i n t e n s i t i e s of the bands due to the i o n i z e d carboxylate group and t h e i r expected o r i e n t a t i o n s . The specular r e f l e c t i o n - a b s o r p t i o n technique can be extremely u s e f u l f o r molecular o r i e n t a t i o n determination. However, i t has one l i m i t a t i o n : i t can y i e l d only q u a l i t a t i v e i n f o r m a t i o n about the o r i e n t a t i o n , mostly because only one of the t r a n s i t i o n moment , components can be observed. As a consequence, the a c t u a l angles that a molecule makes w i t h the r e f l e c t i n g surface cannot be c a l c u l a t e d w i t h data from the specular r e f l e c t i o n - a b s o r p t i o n technique alone, i n a general s i t u a t i o n . O n l y i n very s p e c i a l cases, or by making assumptions about the molecular a t t i t u d e s , can some of the angles be c a l c u l a t e d . 4.2.2 Uses of the M u l t i p l e R e f l e c t i o n - A b s o r p t i o n Technique This technique has been used i n a number of s t u d i e s of metal s u r f a c e s , mostly r e l a t e d to o x i d a t i o n , i n t e r a c t i o n w i t h l u b r i c a n t s , c o r r o s i o n (48 49) i n h i b i t o r s and c a t a l y s i s . ' In the f i e l d of the adsorption of f l o t a t i o n reagents, the work of L e j a , P o l i n g , et a l . on adsorption of (38 55) xanthates on sulphide and m e t a l l i c surfaces i s outstanding. ' Adsorption of carboxylates on metal surfaces has a l s o been stu d i e d , ^ ) a g w e i ^ a s t h e i n t e r a c t i o n of these s u r f a c t a n t s w i t h copper oxide surf aces. No a p p l i c a t i o n s of t h i s technique to carbonate compounds have been found i n the l i t e r a t u r e . As most of the above mentioned references p o i n t out, the m u l t i p l e specular r e f l e c t i o n - a b s o r p t i o n technique can be extremely u s e f u l i n studying a d s o r p t i o n phenomena. Very high s e n s i t i v i t y , reaching surface concentrations as low as f r a c t i o n s of a monolayer can be achieved. A l s o , i t i s p o s s i b l e that i n f o r m a t i o n on q u a l i t a t i v e and even q u a n t i t a t i v e aspects of the complexes formed at the i n t e r f a c e can be gained.^4^» 36 CHAPTER 5 EXPERIMENTAL METHODS. From an experimental point of view, m i c r o f l o t a t i o n t e s t s using a r e l a t i v e l y pure, n a t u r a l sample of magnesite and adsorption s t u d i e s w i t h s y n t h e t i c t h i n f i l m s of Mg carbonate were the main techniques used i n t h i s i n v e s t i g a t i o n . However, due to the extreme v a r i a t i o n i n the reported values of the i s o e l e c t r i c point of magnesite (see Table I I ) , determination of the surface-charge behaviour was a l s o c a r r i e d out f o r the magnesite sample used i n the microf l o t a t i o n experiments. This was accomplished by the use of the ( 58) Mular & Roberts method, ' a f a s t and accurate technique, i n which both coarse and f i n e samples can be used. The method i s based on the change of the pH value, when the i o n i c s t r e n g t h of the s o l u t i o n i s suddenly v a r i e d i n a co n s i s t e n t way, by the a d d i t i o n of an i n e r t solid"" s o l u b l e e l e c t r o l y t e . In t h i s method, only the pH of the point of zero charge i s determined. 5.1 M i c r o f l o t a t i o n Tests A l l the microf l o t a t i o n t e s t s were c a r r i e d out i n a Hallimond tube s i m i l a r t o that described i n r e f . 35 but which had - - i n s t e a d of a c a p i l a r y tube - - a f r i t t e d g l a s s bottom ( F i g . 4 ) . This c e l l was s e l e c t e d because of i t s widespread use i n t h i s type of i n v e s t i g a t i o n , thus f a c i l i t a t i n g comparison w i t h other s t u d i e s . Pure a i r (Linde Medical A i r , breathing grade) was the gas used f o r generating bubbles throughout the experiment. I t s s p e c i f i c a t i o n s were as f o l l o w s : - CO2: approx. 300 ppm CO: 5 ppm max. H^O: 25 ppm max. - hydrocarbons: 25 ppm max. 37 Other c o n d i t i o n s p r e v a i l i n g during the m i c r o f l o t a t i o n t e s t s were: - 1 g of mineral i n 125 ml of s o l u t i o n ; - gas flow: 60 ml/min - c o n d i t i o n i n g time: 5 min - f l o t a t i o n time: 1 min The pH was checked before c o n d i t i o n i n g and immediately a f t e r f l o t a t i o n . Very seldom was there any s i g n i f i c a n t change; the readings u s u a l l y d i f f e r e d by l e s s than 0.3 pH u n i t s . In t h i s work, the reported pH value i s always the one a f t e r f l o t a t i o n . O l e i c a c i d (cis-9-octadecenoic a c i d ) was s e l e c t e d as the main c o l l e c t o r because i t i s the most important s i n g l e a n i o n i c c o l l e c t o r compound f o r n onmetallic (non-sulphide) m i n e r a l s . In most of the commercial f a t t y a c i d c o l l e c t o r s , o l e i c a c i d i s most a c t i v e compound, or at l e a s t i t i s one of the most abundant compounds among t h e i r a c t i v e components. In many m i c r o f l o t a t i o n s t u d i e s of nonmetallic m i n e r a l s , pure o l e i c a c i d has been used as one of the c o l l e c t o r s . In t h i s study, the sodium soap of o l e i c a c i d was always used. Na dodecylsulphonate was chosen f o r comparison purposes. The sulphonates stay d i s s o c i a t e d over most of the pH range, as opposed t o the f a t t y a c i d s . Therefore, the use of both reagents allowed a comparison of the r o l e played by the h y d r o l y s i s of the c o l l e c t o r , and by the molecular a s s o c i a t i o n s between the species formed. ». A c a t i o n i c c o l l e c t o r - - dodecylammonium c h l o r i d e - - was a l s o i n c l u d e d , mainly t o help i n the e v a l u a t i o n of the e f f e c t of the e l e c t r i c charge at the m i n e r a l - s o l u t i o n i n t e r f a c e . Hallimond tube f l o t a t i o n t e s t s were c a r r i e d out at s e v e r a l concentrations of a l l these c o l l e c t o r s , over a wide range of pH values. 38 5.1.1 M a t e r i a l s and Apparatus The magnesite sample used was of high p u r i t y (Table I I I ) and co n s i s t e d of c l e a r , s p a t h i c s i n g l e c r y s t a l s of centimeter s i z e that occur near the town (59) of Brumado, Bahia S t a t e , B r a z i l . This sample was c h a r a c t e r i z e d a l s o by X-ray d i f f r a c t i o n and o p t i c a l microscopy. The sample was crushed, and dry-ground i n a p o r c e l a i n m i l l . Then i t was si e v e d , deslimed and washed i n pure water many times. The s i z e range used i n a l l m i c r o f l o t a t i o n experiments was from 210 t o 105 pm. TABLE I I I CHEMICAL COMPOSITION OF BRAZILIAN MAGNESITE SAMPLE^ Crude Basis Calcined 1 I g n i t i o n l o s s 48.5 — sioO» N i l N i l F e 2 0 3 0.64 1.24 MnO ^ 0.07 0.14 A 1 2 0 3 N i l N i l CaO 0.22 0.43 Mg0 ( c ) 50.57 98.19 (a) A l l elements determined by atomic absorption, w i t h the exception of Mg and S i . (b) •^'-^ 2 a s s a v e d by g r a v i m e t r i c method. (c) MgO determined by d i f f e r e n c e . The Na ole a t e sample, w i t h a p u r i t y of about 99%, was supplied by Sigma Chemical Co. The Na dodecylsulphonate supplied by A l d r i c h Chemical Corp. was more than 99% pure. Eastman Kodak Co. was the s u p p l i e r of the 39 dodecylammonium c h l o r i d e , and a l s o of a dodecanol sample (used i n some t e s t s w i t h the sulphonate). Both m a t e r i a l s were of a n a l y t i c a l - r e a g e n t grade. H y d r o c h l o r i c a c i d and sodium hydroxide were used f o r a l l pH adjustments; both were of a n a l y t i c a l - r e a g e n t grade. The water used throughout t h i s i n v e s t i g a t i o n was f i l t e r e d through a Pyrex f i b r e bed (produced by Corning G.W.), and then' d o u b l e - d i s t i l l e d ; the f i r s t - s t a g e s t i l l was a t i n - c o a t e d metal unit,, and the second-stage d i s t i l l a t i o n was c a r r i e d out i n a Pyrex g l a s s model. The e l e c t r i c a l c o n d u c t i v i t y was checked on a re g u l a r basis and stayed always below 1.5 pmho/cm (at 25°C). A Radiometer A/S Type CDM2d c o n d u c t i v i t y meter w i t h a type CDC 104 c o n d u c t i v i t y c e l l was used. The pH values were measured w i t h a F i s h e r Accumet Model 230 pH/ion meter. The ele c t r o d e s used were the Standard Combination and Microprobe Combination types, s u p p l i e d by F i s h e r S c i e n t i f i c Co. The pH measurements were c a l i b r a t e d w i t h f r e s h standard b u f f e r s o l u t i o n s (pH 4.01, 7.41, 9.18 and 10.4 at 25°C) a l s o s u p p l i e d by F i s h e r . This same equipment and procedure were used i n the other p a r t s of t h i s study. The temperature of the s o l u t i o n s during the t e s t s was hel d between 22 and 25°C. 5.2 Point of Zero Charge The c o n d i t i o n s during the t e s t s t o determine the poin t of zero charge ( 5 8 ) of magnesite by the Mular & Roberts v ' method were kept as c l o s e as p o s s i b l e to those i n the m i c r o f l o t a t i o n t e s t s . The i n e r t e l e c t r o l y t e used f o r changing the i o n i c s t r e n g t h of the s o l u t i o n was KC1. The f o l l o w i n g c o n d i t i o n s set up: - amount of magnesite sample: 1 g - volume of the s o l u t i o n : 125 ml 40 i n i t i a l KC1 c o n c e n t r a t i o n : 10 ZM f i n a l KC1 c o n c e n t r a t i o n : 10 _ 1M - pH adjustments: h y d r o c h l o r i c a c i d and Na hydroxide c o n d i t i o n i n g (aging) time, i . e . before changing the i o n i c s t r e n g t h : 20 min. The magnesite sample was used i n two s i z e ranges f o r these t e s t s . One had the same s i z e range used i n the microf l o t a t i o n t e s t s , i . e . from 210 t o 105 pm. The second one was f i n e r than 105 y.m; i t s s i z e d i s t r i b u t i o n i s shown i n Table IV. The composition i s the same as that shown i n Table I I I , since the o r i g i n a l sample f o r both the m i c r o f l o t a t i o n and p o i n t of zero charge t e s t s was the same. TABLE IV SIZE DISTRIBUTION OF THE FINE MAGNESITE SAMPLE* Si z e range, fum % Passing % Cumulative +105 0 — -105 + 44 44 100 - 44 + 32 17 56 - 32 + 23 13 39 - 23 + 16 8 26 - 16 + 12 5 18 - 12 13 - 13 * S i z e a n a l y i s by the C y c l o s i z e r method. 5.3 I n f r a r e d Spectroscopy The m u l t i p l e specular r e f l e c t i o n - a b s o r p t i o n technique was the main a n a l y t i c a l t o o l used throughout t h i s part of the i n v e s t i g a t i o n ; the 41 conventional t r a n s m i s s i o n method was used mostly f o r the recording of reference s p e c t r a , and as an a n c i l l a r y technique f o r the main method. 5.3.1 Experimental Techniques Transmission Method A l l the samples were analysed by the KBr p e l l e t method, a s described i n S e c t i o n 4.1.2. M u l t i p l e Specular R e f l e c t i o n - A b s o r p t i o n Method In t h i s technique, t h i n f i l m s of Mg compounds were formed on top of f r o n t - s u r f a c e d Au m i r r o r s . The Au f i l m s were evaporated under hig h vacuum onto o p t i c a l l y f l a t v i t r e o u s s i l i c a s u b s t r a t e s . Mg metal f i l m s , w i t h v a r i a b l e t h icknesses, were then deposited on top of the Au l a y e r s . The Mg f i l m s were l a t e r converted to Mg carbonate by r e a c t i o n w i t h CO^. The conversion t o carbonate was checked by IR spectrophotometry and a l s o confirmed by e l e c t r o n d i f f r a c t i o n s t u d i e s . Gold was used as the r e f l e c t i n g l a y e r f o r the f o l l o w i n g reasons: - to provide a h i g h l y r e f l e c t i v e l a y e r f o r the i n f r a r e d r a d i a t i o n ; to achieve t h i s g o a l , the Au f i l m s needed to be at l e a s t 350A* t h i c k . - to provide a h i g h l y i n e r t substrate f o r the Mg f i l m s ; i n t h i s way, the substrate would not i n t e r f e r e during the subsequent thermal and chemical treatment needed to convert m e t a l l i c Mg, i n t o the o x i d i z e d compounds, and a l s o during the adsorption t e s t s . Adsorption s t u d i e s were performed by immersion of the Au m i r r o r s , covered w i t h MgC0 Q f i l m s , i n s o l u t i o n s of Na o l e a t e , under c a r e f u l l y 42 c o n t r o l l e d c o n d i t i o n s of co n c e n t r a t i o n , pH, time and temperature. A f t e r removal from the adsorbate s o l u t i o n , the m i r r o r s were washed w i t h d o u b l e - d i s t i l l e d water at the same pH, d r i e d under an argon j e t , and t h e i r spectra recorded. In s t u d i e s using the MSRA technique, the i n t e n s i t y of the IR r a d i a t i o n f a l l s c o n s i d e r a b l y , due to the many r e f l e c t i o n s and extended o p t i c a l path (see F i g . 8 ) . To compensate f o r t h i s , u s u a l l y a s i m i l a r attachment i s placed i n the reference beam of the spectrophotometer, i n c l u d i n g a p a i r of Au m i r r o r s / " ' ^ In t h i s procedure, most of the IR absor p t i o n due to atmospheric water and CO^ i s a l s o c a n c e l l e d out. I f d i f f e r e n t i a l spectra are to be recorded, s i m i l a r t h i n f i l m s of the adsorbent are deposited on the front-covered Au m i r r o r s and placed i n the reference beam.^^ Thus, the only d i f f e r e n c e between the attachments i n the two beams i s the presence of - the adsorbed c o l l e c t o r l a y e r i n the sample beam. With t h i s technique, even higher s e n s i t i v i t y can be achieved. However, i t i s a l s o d i f f i c u l t and time-consuming, due to the p r e c i s e o p t i c a l alignments needed. In t h i s i n v e s t i g a t i o n , we avoided t h i s step by using the technique of scaled s p e c t r a s u b t r a c t i o n , ' employing a d i g i t a l computer. In t h i s ' procedure, the spectra were recorded w i t h the MSRA attachment placed only i n the sample beam, and using normal a t t e n u a t i o n i n the reference beam. T y p i c a l l y , the f o l l o w i n g raw spectra were recorded: a) the Au m i r r o r s only; b) the adsorbent f i l m (Mg carbonate or s i m i l a r compound) on top of the Au m i r r o r s ; c) a l a y e r of adsorbed c o l l e c t o r species on the MgCO^ substrate. The raw spectra were stored i n the memory of a d i g i t a l computer (using magnetic d i s c s ) without any attempt to e s t a b l i s h a b e t t e r compensation 43 between the two beams of the spectrophotometer. Then, by s u b t r a c t i o n of the s u i t a b l e raw data, the d i f f e r e n c e - c o r r e c t e d spectra were achieved. Following t h i s procedure, the spectrum of the adsorbent f i l m was obtained by su b t r a c t i n g spectrum a_ from spectum _b; i n the same way, the spectrum of the adsorbed species would be: spectrum c_ minus spectrum _b. 5.3.2 Equipment The m e t a l l i c t h i n f i l m s were made i n an ultra-high-vacuum system, Model TBK, produced by Perkin-Elmer, U l t e k D i v i s i o n , w i t h a 60 1 chamber. - 9 Pressures as low as 10 t o r r were achieved. The system used two adsor p t i o n pumps i n the roughing stage; hence no o i l s were i n v o l v e d , and a contaminant-free environment was provided. Attached t o the vacuum chamber there was a r e s i d u a l gas a n a l y s e r , Model SPI-10, made by Veeco Instruments, Inc.; t h i s was a monopole mass spectrometer, capable of performing mass a n a l y s i s from zero to 200 atomic mass u n i t s . The evaporation of metals was (62) c a r r i e d out by the r e s i s t i v e heating technique, using molybdenum boats. The power supply u n i t , a l s o made by P-E U l t e k , was capable of the f o l l o w i n g output: c u r r e n t : 0 t o 400A; v o l t a g e : 0 to 40V. The m e t a l l i c f i l m s were deposited on v i t r e o u s s i l i c a s l i d e s , having the f o l l o w i n g dimensions: 57x20x3 mm and 44x20x3 mm. These s l i d e s had o p t i c a l l y f l a t s u r f a c e s , and were supplied by U.S. Fused Quartz Co. F i l m thicknesses were measured using a m u l t i p l e beam (62 63) in t e r f e r o m e t e r , ' ' Model M-100 Angstrometer, marketed by Sloan Instrument Corp. This apparatus had a p r e c i s i o n of + 30 £. The m e t a l l i c Mg f i l m s were converged t o Mg carbonate and r e l a t e d compounds by s u i t a b l e treatment i n a tube furnace ( S o l a B a s i c - Lindberg), V 44 \ capable of reaching 1400°C; t h i s furnace used v i t r e o u s s i l i c a tubes w i t h 21 mm I.D. The IR s p e c t r a were recorded i n a Perkin-Elmer Model 283 B double-beam, two-grating, spectrophotometer. This instrument was connected to a P-E Model 3500 Data S t a t i o n , whose bas i c u n i t was a minicomputer, having a 64 k-bytes memory c a p a c i t y and using microfloppy d i s c s (5.25 i n ) . This system allowed storage of the spectra i n the d i s c s , and the d i g i t a l processing of t h e i r data. In t h i s way, s e v e r a l numerical operations w i t h the s p e c t r a , such as accumulation, averaging,' s u b t r a c t i o n , smoothing, m u l t i p l i c a t i o n and d i v i s i o n by numerical f a c t o r s , l i n e a r i z a t i o n and n o r m a l i z a t i o n of the b a s e l i n e , (^ly^^) coun t, e performed. As already d e s c r i b e d , these operations can s i g n i f i c a n t l y increase the s i g n a l - t o - n o i s e r a t i o . A l s o , they can s i g n i f i c a n t l y reduce negative aspects l i k e a b s o r p t i o n from a solvent or atmospheric water and C0^, l a c k of balance between the sample and reference beams and s c a t t e r i n g of r a d i a t i o n . A Model 9 s i n g l e beam m u l t i p l e r e f l e c t i o n attachment, supplied by Wilks S c i e n t i f i c Corp. and sample h o l d e r s , were used i n a l l the experiments (see o p t i c a l layout i n F i g . 8 ) . The m i r r o r s were kept p a r a l l e l , f a c i n g each other, at a f i x e d d i s t a n c e apart (2 mm), by spacers. The incidence angle was approximately 60°. The t o t a l number of r e f l e c t i o n s was 13. P a r a l l e l p o l a r i z e d l i g h t was used f o r recording, a l l the spectra of the MSRA experiments. A gold w i r e - g r i d p o l a r i z e r (Perkin-Elmer Model 186-0241) was used throughout. During the recording of a l l c r i t i c a l s p e c t r a , the o p t i c a l u n i t of the IR spectrophotometer was purged w i t h dry a i r , to decrease the i n t e n s i t y of the bands due to atmospheric water vapour. The dry a i r was supplied by an o i l l e s s compressor f i t t e d w i t h water-absorbing z e o l i t e columns. This 45 compressor was manufactured by Puregas Equipment Corp. In a few cases, a b o t t l e d gas, E x t r a Dry A i r , s u p p l i e d by Linde-Union Carbide was used f o r purging the spectrophotometer. With the l a t t e r , CC^ - - as w e l l as water -- was e l i m i n a t e d from the o p t i c a l path. The spectrophotometer was set at the #7 (wider) s l i t program f o r recording a l l the MSRA spec t r a ; t h i s allowed a r e s o l u t i o n of approximately 2.3 cm ^ at 1000 cm \ and 4.5 cm ^ at 3000 cm "*". The adopted scan rates were 250 cm ''"/min (from 2000 t o 601 cm "*") and 500 cm "'"/min (from 4000 t o 2000 cm An average of 9 i n d i v i d u a l scans were always recorded f o r each spectrum. For the t r a n s m i s s i o n s p e c t r a , the c o n d i t i o n s were g e n e r a l l y the same, w i t h the f o l l o w i n g exceptions: - s l i t program: #6 (normal) - average of 4 i n d i v i d u a l scans per spectrum The a b s c i s s a s c a l e was always c a l i b r a t e d w i t h a polystyrene f i l m i standard before every recording s e s s i o n . The ads o r p t i o n - f r o m - s o l u t i o n experiments were c a r r i e d out i n s i d e a T e f l o n beaker. A l l the l a b o r a t o r y ware used i n contact w i t h the m i r r o r s and the Na ole a t e s o l u t i o n s , such as beakers, washing b o t t l e s , s t i r r i n g rods and tweezers, were a l l a l s o made of Te f l o n . No contact between the Na oleate s o l u t i o n s and the pH-measuring e l e c t r o d e s was allowed, i n order t o avoid contamination by the ele c t r o d e s o l u t i o n . Instead, samples were taken from the s o l u t i o n s i n t o very small (5 ml) T e f l o n beakers, and the pH determined with the Microprobe Combination e l e c t r o d e . E l e c t r o n d i f f r a c t i o n p a t t e r n photographs from c r y s t a l s i n the Mg carbonate f i l m s were taken using a t r a n s m i s s i o n e l e c t r o n microscope, H i t a c h i Model HU-11A. 46 5.3.3 M a t e r i a l s Samples of magnesite and other p o t e n t i a l l y important i n o r g a n i c compounds were used f o r recording the reference spectra by the tr a n s m i s s i o n technique (KBr p e l l e t ) . These samples were the f o l l o w i n g : 1) magnesite: mineral specimen from Brumado, Bahia S t a t e , B r a z i l . This i s the same m a t e r i a l used i n the microf l o t a t i o n t e s t s and PZC determinations. I t s chemical assay i s shown i n Table I I I . 2) b r u c i t e : mineral specimen from Gabbs, Nev., U.S.A. 3) hydromagnesite: m i n e r a l , a l s o from Gabbs, Nev., U.S.A. i 4) magnesium carbonate, b a s i c : a c e r t i f i e d grade reagent, supplied by F i s h e r S c i e n t i f i c Co. This compound i s chemi c a l l y (and probably c r y s t a l l o g r a p h i c a l l y ) very s i m i l a r to n a t u r a l hy d romag ne s i t e. 5) magnesium oxide: a l s o a c e r t i f i e d grade reagent, supplied by F i s h e r S c i . Co. O l e i c a c i d and Na o l e a t e were supplied by Sigma Chemical Cot , w i t h a p u r i t y of approximately 99%. -3 Mg ole a t e was prepared by p r e c i p i t a t i o n from 10 M s o l u t i o n s of Na olea t e and magnesium c h l o r i d e ( c e r t i f i e d grade reagent, J.T. Baker Co.), at 22°C. Both s o l u t i o n s had t h e i r pH's adjusted to rJ 9.0 p r i o r to p r e c i p i t a t i o n . The p r e c i p i t a t e was f i l t e r e d , washed i n d o u b l e - d i s t i l l e d water w i t h the same pH, washed i n iso p r o p a n o l , and r e c r y s t a l l i z e d from benzene. I t was then d r i e d overnight at 60°C, and stored i n a vacuum d e s i c c a t o r . Magnesium metal was acquired as a 0.1 mm t h i c k sheet, having 99.9% p u r i t y , from Goodfellow Metals. Gold wire (0.5 mm i n diameter), w i t h a p u r i t y b e t t e r than 99.99%, was supplied by Johnson Matthey & M a l l o r y L t d . 47 The evaporation boats were made from a 0.05 mm t h i c k pure molybdenum sheet (from A.D. MacKay, I n c . ) . Organic s o l v e n t s , mostly f o r use i n washing the absorbed c o l l e c t o r f i l m s , were a l l spectroscopic grade reagents, acquired from F i s h e r S c i . Co. These solvents- were the f o l l o w i n g : acetone, carbon t e t r a c h l o r i d e , i s o p r o p a n o l , benzene and hexane (mixture of isomers). H y d r o c h l o r i c a c i d and sodium hydroxide were used f o r a l l pH adjustments. Several compressed gases were used i n t h i s study, e i t h e r d i r e c t l y as A-. reagents, or f o r secondary purposes such as c o n d i t i o n i n g , d r y i n g , purging, f i l l i n g up the vacuum systems a f t e r use, lea k d e t e c t i o n , storage of r e a c t i v e samples i n i n e r t atmospheres, e t c . Nitro g e n , oxygen, helium, p u r i f i e d a i r , e x t r a dry a i r and argon were su p p l i e d by Linde - Union Carbide Canada L t d . Carbon d i o x i d e , s u p p l i e d by Matheson, was at l e a s t 99.99% pure. The water used i n the a d s o r p t i o n experiments was always f r e s h l y d i s t i l l e d , and stored only i n T e f l o n b o t t l e s . I t had the same c h a r a c t e r i s t i c s a l r e ady-described. 5.3.4 Sample Pre p a r a t i o n A) Transmission Technique The s o l i d samples were analysed by the KBr p e l l e t method. The powders, i n amounts from 1.0 to 3.0 mg, were ground very f i n e l y i n an agate mortar and then thoroughly mixed w i t h 300 mg of KBr. The mixture was t r a n s f e r r e d to a d i e (Perkin-Elmer Model 186-00251), evacuated f o r 8 min, and pressed at approximately 30000 kPa, f o r 1 min. Only uniformly transparent p e l l e t s were accepted. They were 13 mm i n diameter and about 1 mm t h i c k . 48 Samples which were l i q u i d at room temperature ( o l e i c a c i d ) were analysed by smearing a small amount on the face of pure KBr p e l l e t . ( ^ 5 ) B) M u l t i p l e Specular R e f l e c t i o n - A b s o r p t i o n Technique The p r e p a r a t i o n of the samples f o r the MSRA st u d i e s was a complex and time-consuming operation. The f i r s t stage was the p r e p a r a t i o n of the front-covered Au m i r r o r s . The o p t i c a l l y f l a t v i t r e o u s s i l i c a s l i d e s were c a r e f u l l y washed i n detergent s o l u t i o n s and r i n s e d ; then tr e a t e d i n hot (80°C) f r e s h chromic a c i d , and immersed i n t h i s s o l u t i o n overnight; then thoroughly r i n s e d i n d i s t i l l e d water; d r i e d under vacuum and stored i n a d e s i c c a t o r . ^ —8 The Au f i l m s were deposited under a vacuum i n the 10 t o r r range on top of the v i t r e o u s s i l i c a s u b s t r a t e s . The thickness v a r i e d from 350 to 1000 o ! o A; t y p i c a l l y i t was kept around 700A. Long evaporation periods (10 min) were maintained, i n order to achieve uniform and continuous f i l m s . F i g . 10 shows a photograph taken through the multiple-beam i n t e r f e r o m e t e r used f o r checking the u n i f o r m i t y and thickness of the t h i n f i l m s ; the p a r a l l e l and almost s t r a i g h t i n t e r f e r e n c e f r i n g e s c l e a r l y show the extreme smoothness and evenness of the f i l m s . In some cases, a rough substrate was d e s i r a b l e . This was achieved by t r e a t i n g the v i t r e o u s s i l i c a s l i d e s i n a concentrated s o l u t i o n (49%) of h y d r o f l u o r i c a c i d f o r two minutes. The Au f i l m s were then deposited on these rough s u r f a c e s . The a c t u a l evaporation of gold d i d not present any p a r t i c u l a r d i f f i c u l t y . The c o n d i t i o n s f o r a t y p i c a l f i l m (600 - 70oR) were the f o l l o w i n g : F i g . 10. Multiple-beam interferogram of a f l a t gold m i r r o r . The spacing between the f r i n g e s i s 2945 R. H o r i z o n t a l m a g n i f i c a t i o n : 15x. 50 - mass of Au w i r e : 230 mg - di s t a n c e between evaporation source and s u b s t r a t e s : 20 cm - c u r r e n t : 95 A -9 ' i n i t i a l pressure: 2 x 10 t o r r f i n a l pressure: 7 x 10 t o r r - evaporation time ( t o t a l ) : 10 min. Although the t o t a l evaporation time was around 10 min, the o p t i c a l b a f f l e of the evaporation system was kept shut f o r the f i r s t minute, i n order to prevent any i m p u r i t i e s generated during the e a r l y stages of the evaporation from reaching the s u b s t r a t e s . The Mg f i l m s were deposited on top of the Au f i l m s . Magnesium proved to be a d i f f i c u l t metal to evaporate under reproducible c o n d i t i o n s , because i t sublimes i n s t e a d of melting; thus i t tended to produce very narrow vapour beams. Larger metal chips were needed to assure a wider vapour beam during evaporation. A l s o , the l a r g e r mass helped i n preventing the ch i p from . t i l t i n g during the evaporation, which caused changes i n the beam d i r e c t i o n . The only p r a c t i c a l v a r i a b l e i n c o n t r o l l i n g f i l m thickness was the time during which the substrates were exposed to the metal vapour beam. The current had to be c a r e f u l l y c o n t r o l l e d and changed to avoid instantaneous (even e x p l o s i v e ) evaporation. T y p i c a l evaporation c o n d i t i o n s were: mass of Mg f o i l : 20+1 mg - distance between evaporation source and s u b s t r a t e s : 25cm - c u r r e n t : 35 to 38 A -9 i n i t i a l pressure: 2 x 10 t o r r _ Q f i n a l pressure: 3 x 10 t o r r 51 exposure time: 10 to 60s ( f i l m s approximately 50 to 500 A* t h i c k ) . Again, the o p t i c a l b a f f l e was kept shut f o r at l e a s t one minute a f t e r evaporation s t a r t e d . Conversion to magnesium carbonate t h i n f i l m s The problems of determining the c o n d i t i o n s f o r making t h i n f i l m s of Mg carbonate i n a reproducible way were extremely d i f f i c u l t and time-consuming. The m e t a l l i c Mg t h i n f i l m s were tested i n s i d e a v i t r e o u s s i l i c a tube furnace, under a continuous f l o w of always at approximately atmospheric pressure;. The f i r s t r e a c t i o n was simply o x i d a t i o n of Mg: Mg + C0 2 f MgO + CO In a second stage, the carbonate was formed: MgO + C0 2 % MgC03 I t was found t h a t the presence of moisture was c r i t i c a l to the synthe s i s of the Mg carbonate, although the f i n a l r e a c t i o n product was always anhydrous. Therefore, the water vapour was needed only as a m i n e r a l i z e r . When t o t a l l y dry c o n d i t i o n s were maintained, only the oxide was formed. High moisture content was d e t r i m e n t a l , too. In the l a t t e r case, a t o t a l l y amorphous Mg carbonate f i l m was formed. This compound turned out to be very water-soluble, even under b a s i c c o n d i t i o n s , i . e . pH = 10. The best c o n d i t i o n s were found when moderately high moisture was maintained during the e a r l i e r stages of the o x i d a t i o n of the metal but removed a f t e r the completion of t h i s stage. P r a c t i c a l l y , t h i s was accomplished by i n t r o d u c i n g a small ceramic boat, f i l l e d w i t h water i n t o 1 the, tube furnace, i n a region where some evaporation was allowed. In t h i s way the C0 2 gas picked up some water vapour and t h i s mixture reacted upon the Mg f i l m s . 52 The f i l m s formed under these i d e a l c o n d i t i o n s were completely w a t e r - i n s o l u b l e at ba s i c and n e u t r a l c o n d i t i o n s . Even under a c i d i c c o n d i t i o n s they showed high r e s i s t a n c e to d i s s o l u t i o n . At pH = 2, no decrease i n thickness was n o t i c e d a f t e r 30 min (monitored by IR spectrophotometry); only at pH = 1 was a slow d i s s o l u t i o n observed. These f i l m s proved extremely s i m i l a r to the bulk magnesite, both by IR spectrophotometric and e l e c t r o n d i f f r a c t i o n methods ( S e c t i o n 6.3.2.c) The heat treatment was a l s o c r i t i c a l . I d e a l l y , the temperature was slowly r a i s e d ( i n about 3 hours) to 400°C, maintained f o r \2 hours, and slowly decreased to room temperature, under a constant CO^ f l o w (0.5 1/min); thus an e n t i r e heat treatment c y c l e took about 7 hours. I f temperatures lower than 400°C were used, u s u a l l y a mixture of the two carbonate types was obtained, even under i d e a l moisture c o n d i t i o n s . No references were found i n the l i t e r a t u r e on the synt h e s i s of t h i n f i l m s of Mg carbonate. However, s t u d i e s on the ads o r p t i o n of on MgO powders^^^ showed that even tr a c e s of water vapour had a dramatic e f f e c t on the k i n e t i c s of the adsorption. A l s o , they proved that surface complexes s i m i l a r to the bulk magnesite could only be obtained when water vapour was present. A l l the f i l m s were stored i n vacuum d e s i c c a t o r s . In some c r i t i c a l cases they were kept under argon atmospheres. Adsorption Tests The procedures during the ads o r p t i o n experiments w i t h Na o l e a t e s o l u t i o n s have already been described. The s o l u t i o n s were prepared by d i s s o l v i n g the weighed amounts of Na o l e a t e i n water j u s t before the experiments. They were kept i n covered T e f l o n beakers f o r 30 min, and 53 s t i r r e d . Then the pH was adjusted to the s p e c i f i c value of the t e s t . Again the beaker was covered during s t i r r i n g f o r another 30 min, a f t e r which the pH was checked, c o r r e c t e d i f necessary, and the a c t u a l adsorption t e s t s t a r t e d . Most t e s t s were c a r r i e d out from 1 to 2 hours f o l l o w i n g the d i s s o l u t i o n of Na o l e a t e . The temperature of the s o l u t i o n was always kept at 21 + 1°C. Contact w i t h the a i r was maintained during the e n t i r e experiment, through a vent i n the T e f l o n cover. This was because of the strong i n f l u e n c e of atmosphere CO^ on the water chemistry of the carbonates ( S e c t i o n 2.2.1). This technique was a l s o c o n s i s t e n t w i t h the m i c r o f l o t a t i o n t e s t s . U s u a l l y , i n e r t atmospheres were not kept i n contact w i t h the adsorbate s o l u t i o n . O l e i c a c i d from the vapour phase was deposited onto MgCO^ t h i n f i l m s , by evaporation, i n a i r . The m i r r o r s were placed 10 cm away from, and f a c i n g , the a c i d c o n t a i n e r , which was heated u n t i l the emission of vapours. The evaporation was then maintained f o r 5 min. 5.4 Contact Angles Contact angles were measured on the Mg carbonate f i l m s before and . a f t e r the adsorption t e s t s , i n order to evaluate t h e i r degree of hydrophobicity. D o u b l e - d i s t i l l e d water was' used i n a l l cases. In a l l t e s t s , very small water drops (about 6.0 p l ) were deposited onto the surface of the f i l m s . An A g l a micrometer s y r i n g e , manufactured by Burroughs Wellcome was employed. I t had an accuracy of + 0.05 p l . For angles s m a l l e r than 90°, the diameter m e t h o d ^ ^ was used. The volume of the drop (V) was measured d i r e c t l y from the c a l i b r a t e d s y r i n g e , and the drop base diameter (d) was determined on the s u r f a c e , w i t h the a i d of 54 a c a l i b r a t e d low m a g n i f i c a t i o n microscope. The contact angles (0) were (68) c a l c u l a t e d using the f o l l o w i n g equation:^ 3 3 d 24 . s i n 9 IT ' i 3 2 - 3cos6 + cos 6 For angles ^ 90 , the conventional d i r e c t p r o f i l e angle measurements (68) were performed, using a goniometer eyepiece. 55 CHAPTER 6  RESULTS AND DISCUSSION 6.1 Point of Zero Charge F i g . 11 shows the r e s u l t s of the determination of the point of zero (58) charge (p.z.c.) of magnesite by the Mular and Roberts method. The pH of the p.z.c. was found to be 6.05. Apparently t h i s was the f i r s t record of t h i s method being a p p l i e d to a carbonate m i n e r a l . The i.e.p.'s of magnesite reported i n the l i t e r a t u r e are d i s p l a y e d i n Table I I . They vary from pH 2.0 to 11.5. However, the only two i . e . p . values f o r coarse samples determined by the streaming p o t e n t i a l technique agree w i t h the p.z.c. value of 6.0, determined i n t h i s work. There was a very p r e c i s e match f o r the p.z.c.'s of both the coarse and f i n e powdered samples i n t h i s i n v e s t i g a t i o n . 6.2 M i c r o f l o t a t i o n Tests The r e s u l t s of the m i c r o f l o t a t i o n t e s t s w i t h magnesite and the d i f f e r e n t c o l l e c t o r s , over the e n t i r e pH range, are presented i n F i g s . 12, 13 and 14. The p l o t t e d values are the averages of the recovery r e s u l t s , u s u a l l y rounded t o the nearest m u l t i p l e of 5%. The exact v a l u e s , p l u s the s t a t i s t i c a l data, i n c l u d i n g the confidence i n t e r v a l s , are shown i n Appendix I I I . 6.2.1 Sodium Oleate ( F i g . 12) For the 10 M^ c o n c e n t r a t i o n , f l o t a t i o n was not achieved over the -4 e n t i r e pH range; at 10 M, recovery was always 100% (or very c l o s e ) , except f o r the low pH region (pH 3.0). At the intermediate c o l l e c t o r T 1 1 1 1 1 r J I ! 1 l d I L I 2 3 4 5 6 7 8 9 10 Initial p H F i g . 11. Determination of the point of zero charge of magnesite by the Mular & Roberts method. 58 conce n t r a t i o n s , a modulation i n the f l o t a t i o n response, as a f u n c t i o n of pH, was always observed. The r e c o v e r i e s were c o n s i s t e n t l y higher i n the moderately a c i d i c range, decreasing towards both the more a c i d i c and the n e u t r a l regions. As the Na ole a t e c o n c e n t r a t i o n increased, the region of 100% recovery a l s o expanded towards these pH's ranges. However, a minimum i n the f l o t a t i o n response was observed around pH 10 f o r concentrations as high as 4x10" 5M. The behaviour j u s t described was c o n s i s t e n t w i t h the reported m i c r o f l o t a t i o n behaviour of magnesite, and a l s o of dolomite, w i t h o l e a t e , decanoate, l a u r a t e and m y r i s t a t e . A b e t t e r f l o t a t i o n of c a l c i t e i n the a c i d i c range was a l s o reported w i t h o l e a t e . The f l o t a t i o n behaviour of magnesite, under the i d e a l c o n d i t i o n s of the Hallimond tube t e s t s , could t e n t a t i v e l y be explained by the c u r r e n t l y accepted t h e o r i e s of c o l l e c t o r adsorption. However, t h i s w i l l be discussed l a t e r - i n the s e c t i o n on i n f r a r e d s t u d i e s , since more d i r e c t evidence f o r the adsorption mechanism i s p o s s i b l e . The m i c r o f l o t a t i o n t e s t s w i t h Na ole a t e i n d i c a t e d that two pH regions are of s p e c i a l i n t e r e s t f o r the more in-depth i n v e s t i g a t i o n using i n f r a r e d spectroscopy: a) w i t h moderately a c i d i c c o n d i t i o n s , around pH = 6, where f l o t a t i o n c o n d i t i o n s seemed to be optimal; b) the region around pH = 10, because the f l o t a t i o n response was always comparatively low. The second value i s important due to the f a c t t h a t the n a t u r a l pH of most of the magnesite ores occurs i n moderately a l k a l i n e c o n d i t i o n s (pH 8 to 10). 6.2.2 Sodium Dodecylsulphonate ( F i g . 13) The f l o t a t i o n response of Na dodecylsulphonate was s i m i l a r to that of the c a r b o x y l a t e , w i t h one major exception: recovery d i d not drop i n the i r > o o 100 A A L 80 60 40 20 0 5 *I0~ 4M — I0"4M • 5*I0~ 5M dodecanol \ I0"4M \ \ \ \ \ \ \ \ \ \ \ 7 / \ I / t \ \ I0"5M I0" 4M + IO" 5M dodecanol / / / / - o A , 7 P H 8 10 II 12 13 F i g . 13. F l o t a t i o n of magnesite with sodium dodecylsulphonate alone, and with the addition of dodecanol. 60 very low pH range. Instead, i t remained 100%, even at values as low as pH -4 1.0, f o r 10 M and higher concentrations. Recovery was low from pH 5.5 up -4 to 11.5, at 10 M, but showed a sharp increase above t h i s pH value. Only at 5x10 sulphonate c o n c e n t r a t i o n , was the recovery c l o s e t o 100% f o r most of the pH range. S i m i l a r optimal f l o t a t i o n response i n the a c i d i c region has already been reported f o r c a l c i t e , i n f l o t a t i o n w i t h Cg to s u l p h o n a t e s ( 6 ' 3 9 , 4 0 ) . The o v e r a l l i n f e r i o r c o l l e c t i o n a b i l i t y of dodecylsulphonate, as compared to o l e a t e i s b e l i e v e d to be due to the f o l l o w i n g f a c t o r s : - the s h o r t e r hydrocarbon chain of the sulphonate, which r e s u l t e d i n decreasing the t o t a l s t r e n g t h of the Van der Waals bonds (14) between the chains, f o r the same c o l l e c t o r c o n c e n t r a t i o n ; - . the higher s o l u b i l i t y of Mg dodecylsulphonate, as compared to Mg ol e a t e (see Appendix I ) ; - the absence of n e u t r a l molecules - - the sulphonates are f u l l y d i s s o c i a t e d even at low p H . ^ 9 ^ Ad d i t i o n s of a non-ionic compound l i k e dodecanol caused a dramatic increase i n the recovery ( F i g . 13). Dodecanol was so e f f e c t i v e i n enhancing sulphonate f l o t a t i o n over the pH range from 5.5 t o 11.5, that f u r t h e r a d d i t i o n s of the c o l l e c t o r alone to achieve the same e f f e c t had to be e i g h t times l a r g e r than the a d d i t i o n of the a l c o h o l . These n e u t r a l molecules are e f f i c i e n t i n enhancing f l o t a t i o n because they can co-adsorb more e a s i l y w i t h the c o l l e c t o r onto the mineral surface, since there i s no e l e c t r o s t a t i c r e p u l s i o n between the p o l a r heads. Therefore, the van der Waals a t t r a c t i v e f o r c e s between the hydrocarbon chains are t o t a l l y e f f e c t i v e i n promoting the i o n - n e u t r a l molecule a s s o c i a t i o n s . The formation of hemimicelles i s thus 61 achieved at lower o v e r a l l ( c o l l e c t o r plus non-ionic s u r f a c t a n t ) (14) concentrations than i n the case of the c o l l e c t o r ions alone. The e l e c t r i c charge at the magnesite-solution i n t e r f a c e i s a l s o be l i e v e d t o play an important r o l e i n the adsorption of the sulphonate i o n . The p o s i t i v e zeta p o t e n t i a l s were probably the main cause of the high r e c o v e r i e s i n a c i d i c c o n d i t i o n s (at 10 4M c o n c e n t r a t i o n ) . The e l e c t r o s t a t i c r e p u l s i o n above the poi n t of zero charge (pH=6.0) expl a i n s the lower f l o t a t i o n response f o r the n e u t r a l and b a s i c regions at medium c o l l e c t o r concentrations. Therefore, higher sulphonate concentrations and/or the presence of n e u t r a l molecules were necessary t o achieve higher adsorption d e n s i t i e s and thus a higher degree of hydrophobicity. The e f f e c t of the surface charge would s t i l l be v a l i d i f the adsorption mechanism were of a (39) chemical nature, as evidenced i n the system c a l c i t e - a l k y l sulphonate. The e l e c t r o s t a t i c r e p u l s i o n above the PZC creates an energy b a r r i e r t o be overcome before chemisorption can take p l a c e , t h e r e f o r e decreasing the adsorption d e n s i t y but not preventing i t . Obviously, a p o s i t i v e charge at the i n t e r f a c e (below the PZC) would enhance the chemisorption' of the c o l l e c t o r . 6.2.3 Dodecylammonium C h l o r i d e ( F i g . 14) Tests w i t h dodecylammonium c h l o r i d e showed the f o l l o w i n g important r e s u l t s : r e c o v e r i e s v a r i e d from zero to approximately 100%, depending on c o l l e c t o r c oncentration and pH; - f l o t a b i l i t y was only achieved above the point of zero charge of the mineral (pH = 6.0); below t h i s point recoveries were always zero, even f o r the highest c o l l e c t o r c o n c e n t r a t i o n used; 63 h i g h recoveries were only found i n the basic r e g i o n , above pH=9. Evidence from the m i c r o f l o t a t i o n t e s t s w i t h the amine showed that the adsorp t i o n on the magnesite surface was of a p h y s i c a l nature - - by e l e c t r o s t a t i c a t t r a c t i o n between the aminium ions and the n e g a t i v e l y charged su r f a c e . The f a c t that high recoveries could be achieved only at pH values w e l l above the point of zero charge suggests that the surface must not only be o p p o s i t e l y charged, but a l s o have a s i g n i f i c a n t l y high net charge i n order t o reach the c o n d i t i o n s f o r hemimicelle formation. Another f a c t o r c o n t r i b u t i n g t o the high f l o t a t i o n response of magnesite above pH 9, was the higher c o n c e n t r a t i o n of n e u t r a l amine molecules present i n the s o l u t i o n at t h i s pH region. The r e g i o n around pH 10.5 was p a r t i c u l a r l y favourable f o r f l o t a t i o n , since i t was where the 1:1 amine/aminium complex i o n reached i t s maximum con c e n t r a t i o n . This species has a h i g h degree of s u r f a c e - a c t i v i t y and i s supposed t o promote f l o t a t i o n i (33) s t r o n g l y . 6.2.4 Concluding Remarks Magnesite f l o t a t i o n showed good r e c o v e r i e s i n the re g i o n of very high pH f o r a l l c o l l e c t o r s t e s t e d . For dodecylsulphonate the f l o t a t i o n response was maximum at both extremes of the pH range ( F i g . 13). Because f l o t a b i l i t y i n these ranges i s of l i t t l e p r a c t i c a l importance, no adsorption t e s t s were c a r r i e d out under these c o n d i t i o n s . However, i t i s b e l i e v e d t h a t these i n c r e a s e s i n the f l o t a t i o n response were at l e a s t p a r t i a l l y caused by the higher i o n i c s t r e n g t h , through some mechanisms that c o n t r i b u t e d t o increase the a d s o r p t i o n of the c o l l e c t o r . The f i r s t e f f e c t i s s a l t i n g - o u t of the surf actant; i t i s supposed t o be e f f e c t i v e i n f l o t a t i o n enhancement only when the c o l l e c t o r i s 64 i n i o n i c form. The favourable i n f l u e n c e of s a l t i n g - o u t was studied on the system hematite-oleate. A second e f f e c t of the higher i o n i c s t r e n g t h i s a decrease i n the r e p u l s i o n between the p o l a r heads of the s u r f a c t a n t i o n s , due to the increased c o n c e n t r a t i o n of counterions. This i s the main f a c t o r i n the decrease i n the CMC observed under these c o n d i t i o n s . In t h i s s i t u a t i o n , as w e l l as i n s a l t i n g - o u t , an enhancement i n the adsorption onto the m i n e r a l surface i s expected, s i n c e increased i n t e r m o l e c u l a r a s s o c i a t i o n s favour t h i s a d s o r p t i o n , v i a the hemimicelle mechanism. However, i t i s necessary that the c o l l e c t o r c o n c e n t r a t i o n remain below the new CMC value; otherwise a b e n e f i c i a l e f f e c t i n f l o t a t i o n cannot be expected. A t h i r d favourable consequence of a higher i o n i c s t r e n g t h i s a r e d u c t i o n i n the e l e c t r o s t a t i c r e p u l s i o n between the n e g a t i v e l y charged mi n e r a l surface and the c o l l e c t o r anions i n the b a s i c pH range. These three mechanisms are expected t o p l a y s i g n i f i c a n t r o l e s i n i n c r e a s i n g the f l o t a t i o n responses f o r both a n i o n i c c o l l e c t o r s t e s t e d , at high pH regions. Under s t r o n g l y a c i d i c c o n d i t i o n s , the f i r s t mechanism would be e f f i c i e n t o n ly f o r the sulphonate, because of i t s f u l l y d i s s o c i a t e d character and p o s i t i v e charge at the i n t e r f a c e . The decrease i n the r e p u l s i o n between the p o l a r heads of the s u r f a c t a n t at increased - i o n i c s t r e n g t h i s the only mechanism expected t o enhance the a d s o r p t i o n of the amine at s t r o n g l y b a s i c c o n d i t i o n s i n a s i g n i f i c a n t way. Not only H + and OH c o n t r i b u t e d t o the h i g h i o n i c strengths. Mg -2 - + complexes a l s o c o n t r i b u t e d at low pH, and CO^ , HCO-j a n ^ Na i n regions of strong b a s i c i t y ( F i g . 2 ) . 65 In a l l the c o n d i t i o n s during the m i c r o f l o t a t i o n t e s t s , the concentrations of the c o l l e c t o r s remained below the r e s p e c t i v e CMC's (see Appendix I I and r e f . 72). 6.3 I n f r a r e d Spectrophotometric Studies 6.3.1 Reference Spectra Transmission - Band Assignments The a b s o r p t i o n spectra of a l l the organic and i n o r g a n i c compounds thought to be of i n t e r e s t - d i r e c t or p o t e n t i a l - t o t h i s i n v e s t i g a t i o n were recorded by the t r a n s m i s s i o n method (KBr p e l l e t , m o s t l y ) , as o u t l i n e d i n S e c t i o n 5.3.4. These spectra are shown i n F i g s . 15 to 21. Only the d i a g n o s t i c or very important bands are described here. The other bands are f u l l y explained i n references 73 and 85. 1) o l e i c a c i d ( F i g . 15):- The most important band i s the strong one at 1710 cm \ assigned t o the s t r e t c h i n g v i b r a t i o n (symbol v ) of the carbonyl group. I t i s d i a g n o s t i c of the s t r o n g l y hydrogen bonded a c i d dimer, C 7 3 7 A 75} (RC00H) 2. v / J» Another d i a g n o s t i c band of the dimer i s the one which I s s m a l l e r and broader at 2669 cm due to the s t r e t c h i n g v i b r a t i o n of the OH group. 2) sodium ol e a t e ( F i g . 16):- In the carboxylate i o n , the two oxygen atoms become i d e n t i c a l . Therefore, two CO" s t r e t c h i n g bands are o r i g i n a t e d . The most important one i s the asymmetric v i b r a t i o n (v ), that 3. occurred at 1560 cm The symmetric v i b r a t i o n ( v ) i s not as s d i a g n o s t i c , due to s u p e r p o s i t i o n w i t h hydrocarbon v i b r a t i o n s / 7 " ^ This composite band appeared between 1470 and 1415 cm 1 . The a c t u a l v band -1 ' occurred around 1425 cm The band progression between 1350 and 1180 cm \ which i s associated w i t h the d i s t u r b e d deformation ( 8) v i b r a t i o n s (wagging, t w i s t i n g etc.) of the 66 CH^ groups i n the hydrocarbon chains, i s a l s o important. Although these bands are not d i r e c t l y r e l a t e d to the c a r b o x y l i c or carboxylate groups, they are p o t e n t i a l l y r e l e v a n t because they can occur only i n s o l i d or other o r d e r l y - o r i e n t e d molecular a s s o c i a t i o n s , as i n s o l i d i f i e d monolayers. 3) magnesium o l e a t e ( F i g . 17):- This compound, having the formula ^^18^33^2^2» had an IR spectrum s i m i l a r to Na o l e a t e . The main d i f f e r e n c e was the l o c a t i o n of the asymmetric s t r e t c h i n g v i b r a t i o n of the carboxylate group, at 1582 cm \ which i s the only r e a l d i a g n o s t i c band. This value agreed, w i t h i n the experimental accuracy, w i t h p r e v i o u s l y reported A . • (78) determinations. 4) hydrocarbon chains Several bands due to the hydrocarbon chains, appeared i n most of the spectra of the organic compounds and adsorbates i n t h i s work. The most (79) important ones were the f o l l o w i n g , at t h e i r approximate p o s i t i o n s : v v CH, : 2960 cm"1 a 3 v CH : 2870 cm"1 s j 6 CH, : 1460 cm"1 a 3 <5 CH, : 1380 cm" 1 s 3 v CH„ : 2920 cm"1 a 2 ; CH, s 2 v 0 : 2850 cm 1 s c i s s o r i n g , CH 2 : 1470 cm 1 r o c k i n g , ( C H ^ : 725 - 720 cm"1 v =CH : 3010 - 3005 cm"1 v C=C : 1680 - 1600 cm" 1 <5 =CH : 695 - 690 cm"1 ( c i s ) op v ' 5) magnesite - ( F i g . 18):- The i n f r a r e d spectra of magnesite and the other c a l c i t e - g r o u p carbonates have been p r e v i o u s l y studied i n CU a c d rt u H O II o U rH CO •H O CO 4000 3500 3000 2500 2000 1500 1000 -1 600 Wavenumber (cm ) F i g . 15. Absorption (transmission) spectrum of o l e i c a c i d ( f i l m on KBr). 6-8 <D O a rt rt M H 4000 3500 3000 2000 600 Wavenumber (cm ) F i g . 16. Absorption (transmission) spectrum of Na o l e a t e (KBr d i s c ) Transmittance % 4> 89 69 C 80 81) d e t a i l . ' The determinations i n t h i s work agree w i t h these s t u d i e s . The important bands were the f o l l o w i n g : the sharp band at 744 cm 1 corresponds to the in-plane bending v i b r a t i o n ( ^ ) of the carbonate group; the sharp and stronger band at 882 cm 1 i s ass o c i a t e d w i t h the out-of-plane -bending ( ) of the carbonate group; t h i s band i s f r e q u e n t l y s p l i t ; i t shows two small bands or s h o u l d e r s / " ^ ' 82) —1 at 1030 and 855 cm i n the spectrum i n F i g . 18. the broad and very strong band showing maximum absorption between 1520 and 1430 cm 1 i s assigned to the asymmetric v i b r a t i o n of the carbonate i o n ; t h i s band a l s o sometimes shows s p l i t t i n g . , e i t h e r as d i s t i n c t bands or shoulders. Some smaller bands are a l s o worth n o t i n g , i . e . , at 1087cm \ corresponding to the symmetric s t r e t c h i n g 2-v i b r a t i o n of the CO^ group; t h i s band i s t y p i c a l l y small i n the i v (81) c a l c i t e - g r o u p carbonates. - other bands were i n d i s t i n c t l y assigned to the carbonate i o n / * * ^ such as the ones at 2538 and 1822 cm ^. The other more or l e s s prominent bands were due to water contamination i n the KBr p e l l e t s . 6) hydromagnesite ( F i g . 19):- This mineral has the f o l l o w i n g approximate f o r m u l a / 1 7 )Mg 4(C0 3) 3(OH) 2.3H 20 I t s spectrum showed bands t h a t are t y p i c a l of the carbonates, hydroxides and hydrates. ^ 3 ' * ^ The sharp bands between 3700-3600 cm 1 are due to the l a t t i c e s t r e t c h i n g v i b r a t i o n s of the hydroxyl group. The broad band between 3550 and 2900 cm 1 i s as s o c i a t e d w i t h the symmetric and asymmetric s t r e t c h i n g QJ O c cfl 4J 4-1 •H I c cfl U H 00 CO m CN \ 1 / 00 CN CN \ 1 cn 1 O 00 \ o / t—i IT) i—1 \ u / vO 1 cn 1 cn cfl / cn o 1 \ ? / 1 O a I cn 1 CO CN O O a U CJ \ r-» O / CO \ ^ <r 1 > \ m -<r / \ i— i i — i / I cn u O CN 00 00 <r O •H 4000 3500 3000 2500 2000 1500 1000 -1 600 Wavenumber (cm ) F i g . 18. Absorption (transmission) spectrum of magnesite (KBr d i s c ) . cu CJ c cfl C cfl U H O f—11 CN /— CT\ 1 cu a cn > •H 4-1 4-> cfl i-H o cn —y. o CN r e o ? CN o <r / o . (•"- vO / t—1 00 cn -d CO VyT m cn 8 00 vO 00 00 1 v o 1 1 60 1 co S to o 1 cn o > CO O CO a. •H 4000 3500 3000 2500 2000 1500 1000 -1 600 Wavenumber (cm ) F i g . 19. Absorption (transmission) spectrum of hydromagnesite (KBr d i s c ) . o 71 v i b r a t i o n s of water, e i t h e r when i t i s l o c a t e d w i t h i n the c r y s t a l l i n e l a t t i c e , or when H^O i s only p h y s i c a l l y bonded. The l a c k of sharper r e s o l u t i o n i s due t o hydrogen bonding. The broad and strong band i n the region 1700-1200 cm 1 i s mostly 2-associated w i t h a CO^, w i t h a minor c o n t r i b u t i o n from ^ 0 . The group of. bands from 1100 to 800 cm 1 corresponds to s e v e r a l carbonate and hydroxyl v i b r a t i o n s . The stronger band i n the region 800-600 cm 1 i s due to carbonate and a l s o t o bending v i b r a t i o n s of the Mg-OH (84) group. 7) b r u c i t e ( F i g . 20):- Pure Mg hydroxide should show only a few bands. However, carbonate i s a common imp u r i t y i n n a t u r a l b r u c i t e . Hence, some of the peaks i n i t s spectrum were e i t h e r t o t a l l y or p a r t i a l l y due t o carbonate (83 85) absorption. ' A l s o , some molecular water was present. The t y p i c a l b r u c i t e bands are the f o l l o w i n g : - v OH ( l a t t i c e ) , at 3700-3600 cm"1 -, <5 Mg-OH, at 750-600 cm"1 v Mg-0, at 550-400 cm"1 8) p e r i c l a s e , MgO ( F i g . 21):- Pure magnesium oxide should show only a broad band w i t h a maximum i n the range 550 t o 400 cm 1.^ 8^^ This band i s due to the s t r e t c h i n g v i b r a t i o n of the Mg-0 bond. A l l the other bands shown i n F i g . 21 were caused by carbonate and water contaminations. 6.3.2. Thin Films - C h a r a c t e r i z a t i o n The formation of t h i n f i l m s of Mg carbonate s i m i l a r t o bulk magnesite i s d i f f i c u l t , as mentioned i n S e c t i o n 5.3.4. Therefore, a thorough study was c a r r i e d out on the d i f f e r e n t types of f i l m s formed. The f i l m s were test e d by Transmittance % Transmittance % ZL 73 i n f r a r e d m u l t i p l e specular r e f l e c t i o n - absorption (MSRA) spectroscopy, d i s s o l u t i o n behaviour i n water and e l e c t r o n d i f f r a c t i o n . a) Magnesium oxide f i l m s A t y p i c a l MSRA spectrum of an oxide f i l m i s shown i n F i g . 22. Films such as t h i s were obtained by treatment of m e t a l l i c Mg f i l m s by CO^ under t o t a l l y anhydrous c o n d i t i o n s , or by r e a c t i o n w i t h 0^. The only prominent band i s the one near 700 cm \ which i s assigned to the s t r e t c h i n g v i b r a t i o n of the Mg-0 bond. In the reference (transmission) spectrum of MgO ( F i g . 21) t h i s band appeared as a broad one, showing a maximum around 550-400 cm 1 . However i t was d e t e r m i n e d ^ 8 ^ that i n the r e g i o n between 732 and .368 cm 1 more than one fundamental frequency e x i s t s . One of the coupling v i b r a t i o n s of the fundamental frequencies was c a l c u l a t e d to occur at 704 cm 1 and to be i n f r a r e d a c t i v e . T h e r e f o r e , the presence of the strong band i n the MSRA spectra i s j u s t i f i e d . In t r a n s m i s s i o n work, a maximum around 695 cm 1 was reported; a l s o , very h i g h absorptions at —1 (85 88) a 700 cm , c l o s e to the maximum, were determined f o r Mg.v ' The oxide f i l m i n F i g . 22 a l s o showed a broad but weak carbonate band, from 1700 to 1300 cm This was probably due to r e a c t i o n w i t h atmospheric C0 2. However, exposure to the atmosphere, even f o r s e v e r a l months, d i d not convert these oxide f i l m s to Mg carbonate. The Mg oxide f i l m s were completely d i s s o l v e d by water, even at b a s i c c o n d i t i o n s (pH 10), a f t e r 1 min exposure. No attempts were made to determine the degree of c r y s t a l l i n i t y and the accurate thickness of the oxide f i l m s . b) Amorphous magnesium carbonate f i l m s F i g . 23 shows the MSRA spectrum of a t y p i c a l Mg carbonate f i l m of the amorphous k i n d . The band i n the range 1700-1300 cm 1 was f r e q u e n t l y q u i t e F i g . 22. MSRA spectrum of Mg oxide t h i n f i l m . Ordinate expansion: 1 .52-x. 75 strong. I t was assigned t o the asymmetric s t r e t c h i n g v i b r a t i o n of the carbonate group, by comparison t o the reference magnesite spectrum ( F i g . 18). More t y p i c a l of t h i s type of carbonate was the band at 860 cm 1 . I t was always broader, s m a l l e r , and s h i f t e d t o a lower frequency, as compared to the equivalent one i n magnesite: the out-of-plane bending v i b r a t i o n of 2- -1 x C0.j , at 882 cm . Another major d i f f e r e n c e was the absence of a band around 744 cm 1 i n the spectrum of the t h i n f i l m carbonate. The small and broad bands around 1100 cm were assigned to the symmetric s t r e t c h i n g v i b r a t i o n of the carbonate group. Some hydration was evidenced by the broad band centred about 3400 cm 1 (hydrogen-bonded s t r e t c h i n g v i b r a t i o n s of water). The Mg carbonate f i l m s d i s s o l v e d i n water even f a s t e r than the oxide f i l m s , i n comparable c o n d i t i o n s (pH = 10, 1 min). In some cases, r e l a t i v e l y t h i c k f i l m s disappeared i n only 15 sec. This type of Mg carbonate d i d not show any s i g n i f i c a n t degree of c r y s t a l l i n i t y , when tested by e l e c t r o n d i f f r a c t i o n . They were a l s o q u i t e f e a t u r e l e s s . M a g n i f i c a t i o n s up to 85000x i n the t r a n s m i s s i o n e l e c t r o n microscope f a i l e d to show d i s t i n c t signs of g r a i n s and other f i n e d e t a i l s t r u c t u r e s . c) Magnesium carbonate f i l m s s i m i l a r to magnesite The i d e a l type of t h i n f i l m was obtained when the o r i g i n a l Mg f i l m was heat-treated under CO^ flow, having a moderate p a r t i a l pressure of t^O vapour ( r e f e r to S e c t i o n 5.3.4). A t y p i c a l MSRA i n f r a r e d spectrum of a 850A" t h i c k Mg carbonate f i l m i s shown i n F i g . 24. The spectrum of another f i l m of s i m i l a r thickness i s d i s p l a y e d i n F i g . 25. The l a t t e r f i l m was made on top of a rough s u b s t r a t e , w h ile the former one ( l i k e the vast m a j o r i t y of the f i l m s used i n t h i s i n v e s t i g a t i o n ) was formed on an o p t i c a l l y f l a t cu CJ C rd 4-1 o CO co Pi 100 4000 3500 F i g . 24. 3000 2500 2000 1500 1000 _^  600 Wavenumber (cm ) MSRA spectrum of a c r y s t a l l i n e Mg carbonate (magnesite) t h i n f i l m . The f i l m i s 850 7A t h i c k . Ordinate expansion: 0.46x. 6-S CU O c CO 4-) o 0) CO 100 4000 3500 F i g . 25. 3000 2500 2000 1500 1000 _ x 600 Wavenumber (cm ) MSRA spectrum of a c r y s t a l l i n e Mg carbonate (magnesite) t h i n f i l m (rough sub-s t r a t e ) . The f i l m i s 850 & t h i c k . Ordinate expansion: 0.50x. I 77 s u b s t r a t e . The s i m i l a r i t y between these two spectra and the magnesite reference spectrum ( F i g . 18) i s s t r i k i n g . This matching was not r e s t r i c t e d only t o the major bands; the small ones f i t t e d even b e t t e r w i t h the reference spectrum. This was the case f o r the bands at 2540 and 1825 cm \ assigned to v i b r a t i o n s of the carbonate group, and the one near 1090 cm 1 , due s p e c i f i c a l l y to the symmetric s t r e t c h i n g of the same group. Among the major bands, the best matching was f o r the in-plane bending of the carbonate i o n , around 745 cm The two strongest bands were s h i f t e d to higher frequencies. This i s to be expected i n r e f l e c t i o n - a b s o r p t i o n spectra i n the v i c i n i t y of st r o n g l y absorbing bands. The combined e f f e c t of an increase i n the absorption c o e f f i c i e n t (k) and a decrease i n the r e f r a c t i v e index of the absorbing (47) m a t e r i a l were the causes of t h i s s h i f t . ' The band assigned to the 2- -1 out-of-plane bending of CO^ showed a maximum s h i f t of 11 cm (from 882 to 893 cm"1) f o r the f i l m i n F i g . 24. A small band at 855 cm"1 ( F i g . (10 82) ' 18), due to the s p l i t t i n g of t h i s major band, v ' ' was a l s o present i n bothMSRA spectra i n F i g s . 24 and 25, at almost the same p o s i t i o n . The s h i f t i n g of the strong, broad band i n the 1600-1400 cm 1 region (asymmetric s t r e t c h i n g of the carbonate ion) was noticed more as a change i n the r e g i o n of maximum absorption than as a displacement of the whole band. I t a l s o appeared asymmetric i n the MSRA spectra. However, t h i s i s a normal feat u r e of t h i s band i n the trans m i s s i o n s p e c t r a , where s p l i t t i n g i s a l s o common ( S e c t i o n 6.3.1). • The IR spec t r a of the r e l a t i v e l y t h i c k Mg carbonate f i l m s ( l i k e those i n F i g s . 24 and 25) showed a l l the same bands as i n the transmission spectra of the bulk magnesite. However, t h i n n e r f i l m s , from 60 to 100 X, , showed only two strong bands, those around 1500 and 890 cm 1 ; a l s o , the band near 78 i 745 cm 1 was almost i n v a r i a b l y present. A l l the other small ones were u s u a l l y absent i n the very t h i n f i l m s (e.g. F i g s . 30, 33, 40). This was obviously due to the f a c t that t h e i r a b s o r p t i o n became so small because of the very short path length, that t h e i r s i g n a l was t o t a l l y l o s t i n the o v e r a l l noise. There was a tendency f o r the band of 890 cm 1 to show a r e f l e c t i v i t y that was n e a r l y the same, or even higher, than the l a r g e band at 1500 cm ^, f o r almost a l l o p t i c a l l y f l a t f i l m s . However, i n the t r a n s m i s s i o n spectra of the bulk magnesite, the i n v e r s e always happened, i . e . the absorption of the broad band at 1500 cm 1 was always g r e a t e r than the sharp one at 880-890 cm "*". And t h i s was the r u l e f o r the f i l m s on rough s u b s t r a t e , too. Table V summarizes t h i s t r e n d . The a b s o r p t i o n band at 890 cm 1 became stronger (as compared to the one at 1500 cm "*") f o r the rough su b s t r a t e f i l m s , but much stronger f o r the f l a t substrate f i l m s . A l o g i c a l e x p l a nation f o r t h i s trend i s that the Mg carbonate c r y s t a l l i t e s i n the f i l m s showed some degree of o r i e n t a t i o n i n r e l a t i o n to the s u b s t r a t e . This o r i e n t a t i o n was such that the v e r t i c a l (perpendicular to the substrate) component of the d i p o l e moment f o r the cS v i b r a t i o n " i s r e l a t i v e l y greater than that f o r the v v i b r a t i o n op 3 & a (see S e c t i o n 4.2.1). However, t h i s degree of o r i e n t a t i o n should not be extensive, due to the r e l a t i v e l y s m a ll i n c r e a se i n the r a t i o s of the absorption of the two bands (Table V). A l l the f l a t s u bstrate f i l m s showed a small band near 1267 cm 1 ( F i g s . 24, 30, 33), which i s not observed e i t h e r i n the rough substrate f i l m s o r - i n the reference spectrum of magnesite. I t was not observed e i t h e r i n the t r a n s m i s s i o n spectra of other r e l a t e d compounds, such as basic 79 TABLE V INFRARED ABSORPTION RATIOS FOR THE BANDS AT 890 AND 1500 cm AR (890) AA (890) S (890) S a m p i e AR(1500) AA(1500) S(1500) magnesite - 0.67 0.08 (transmission) rough substrate 0.70-0.87 0.70-0.87 0.11 f i l m s f l a t substrate 0.95-1.14 0.94-1.14 0.13-0.18 f i l m s AR = f r a c t i o n a l change i n r e f l e c t i v i t y at the peak AA = absorbance d i f f e r e n c e , between the peak maximum and the b a s e l i n e S = i n t e g r a t e d area under the peak, i n absorbance u n i t s 80 (89) (90) •" " carbonates, ; bicarbonates, and the aragonite group of anhydrous (81) carbonates. Since t h i s band could not be assigned to a bulk compound v i b r a t i o n , the only other p o s s i b i l i t y would be a surface complex species. Indeed, a band i n the range 1270-1260 cm 1 was assigned to the asymmetric (41) s t r e t c h i n g v i b r a t i o n of the bidentate carbonate complex: ,,-0\^ (surface) Mg' > C = 0 The band at 1270-1260 cm 1 was the only d i a g n o s t i c one f o r t h i s complex, because a l l the other peaks could be covered by or confused with the normal carbonate bands. ( 4 1»^) The f a c t that the band at 1267 cm 1 was observed only i n the MSRA spectra of the f l a t substrate f i l m s was evidence that the surface species was at l e a s t p a r t i a l l y o r i e n t e d r e l a t i v e to the mir r o r s u rface. Therefore, when t h i s o r i e n t a t i o n was l o s t i n large areas, as i n the rough substrate f i l m s , the band p r a c t i c a l l y disappeared. C o r r e l a t i o n between i n f r a r e d absorption data and f i l m thickness According to the equations presented i n S e c t i o n 4.2.1, the f i l m t hickness should be approximately p r o p o r t i o n a l to the absorption of IR r a d i a t i o n . Since the exact o p t i c a l constants of Mg carbonate could not be found i n the l i t e r a t u r e , t h i s r e l a t i o n s h i p had to be checked e m p i r i c a l l y . Table VI presents the r e s u l t s of the l i n e a r c o r r e l a t i o n s f o r f i l m s varying from 60 to 870 7A i n t h i c k n e s s , and the IR absorption parameters, at the two most important bands. The best o v e r a l l f i t was f o r the in t e g r a t e d area of the l a r g e band around 1500 cm ^. However, the f i t f o r the r e f l e c t i v i t y change at 890 cm 1 was almost as good. When the a c t u a l data were p l o t t e d ( F i g . 26 and 27), i t became evident that the AR data f i t t e d b e t t e r f o r 81 TABLE VI CORRELATION BETWEEN FILM THICKNESS AND INFRARED ABSORPTION Parameters 890 cm~l 1500 cm 1  AR AA S AR AA S a 4.25 9.23 21.99 1.98 6.64 29.67 o a 1 8058.23 1342.00 67.71 7771.57 1291.11 8.10 r 2 0.9963 0.9962 0.9821 0.9900 0.9893 0.9974 AC , ± 15.7 16.1 34.6 25.8 26.8 13.2 Linear r e g r e s s i o n , by the l e a s t squares f i t t i n g : Y = a^x + a Q Y = f i l m t h i c k n e s s , i n A x = any of the f o l l o w i n g i n f r a r e d absorption parameters: AR = f r a c t i o n a l change i n r e f l e c t i v i t y at the peak AA = absorbance d i f f e r e n c e between the peak maximum and the ba s e l i n e S = i n t e g r a t e d area under the peak, i n absorbances 2 r = c o e f f i c i e n t of determination AC = confidence i n t e r v a l , at 90% l e v e l Number of determinations: 8 S (1500), abs.cm-' F i g . 26. Mg carbonate (magnesite) f i l m t h i c k n e s s versus i n t e g r a t e d area (absorbances) of the IR MSRA band i n the 1500 cm 1 r e g i o n . r 2 = 0.9974; AC = ±13 &. CO h o R (890 cm-') F i g . 27. Mg carbonate (magnesite) f i l m t h i c k n e s s versus r e f l e c t i v i t y change of the IR MSRA band around 890 cm *. r 2 = 0.9963; A C = ±16 A. CO CO \ 84 thinner f i l m s (below 20o£), and the area data f o r the t h i c k e r ones. The c o r r e l a t i o n s f o r the area at 890 cm \ and the r e f l e c t i v i t y at 1500 cm 1 were not as good (see Table V I ) . In a l l cases, the r e s u l t s f o r the s i n g l e absorbance values were very c l o s e to the r e f l e c t i v i t y ones, as expected. E l e c t r o n d i f f r a c t i o n The c r y s t a l l i n e Mg carbonate f i l m s showed i n f r a r e d a bsorption spectra extremely s i m i l a r to the bulk magnesite. • The d i s s o l u t i o n behaviour i n water was a l s o s i m i l a r ( S e c t i o n 5 . 3 . 4 b ) . However, a c o n f i r m a t i o n of the c r y s t a l l o g r a p h i c s t r u c t u r e of the t h i n f i l m m a t e r i a l was considered as a c r i t i c a l stage: t h i s was accomplished by e l e c t r o n d i f f r a c t i o n s t u d i e s using a tra n s m i s s i o n e l e c t r o n microscope. The Mg carbonate f i l m s were composed of subhedral t o euhedral g r a i n s , varying- i n diameter from 0.2 to 0.7 pm. Because of the r e l a t i v e coarseness of the f i l m g r a i n s , spot patterns were always recorded. F i g s . 28 and 29 show (91) two p a r t i a l l y indexed spot d i f f r a c t i o n p a t t e r n s , ' that belong t o two d i f f e r e n t c r y s t a l l o g r a p h i c zones. This i s a necessary and s u f f i c i e n t c o n d i t i o n to i d e n t i f y unambiguously a c r y s t a l l i n e s t r u c t u r e by spot p a t t e r n s . The d e t a i l e d i n t e r p r e t a t i o n and complete indexing of the corresponding r e c i p r o c a l l a t t i c e s i s given i n Appendix 4. The compound that made up the f i l m matched w e l l w i t h the magnesite standard i n both c r y s t a l l i n e i n t e r p l a n a r spacings and angles. Therefore, i t can be concluded that the t h i n f i l m s were composed of a Mg carbonate that had the same c r y s t a l l i n e s t r u c t u r e as the bulk magnesite. 220 F i g . 28. E l e c t r o n d i f f r a c t i o n p a t t e r n of magnesium carbonate (magnesite). The zone a x i s i s [ l i o ] F i g . 29. E l e c t r o n d i f f r a c t i o n p a t t e r n of magnesium carbonate (magnesite). The zone a x i s i s [ 0 0 l ] 86 Some f a i n t rings, are al s o seen i n F i g s . 28 and 29, superimposed on the spot patt e r n s of magnesite. The r i n g s i n F i g . 28 can be t e n t a t i v e l y i n t e r p r e t e d as belonging to Mg oxide. A s p e c u l a t i v e i n t e r p r e t a t i o n of these r i n g s i s that the carbonate slowly decomposed under the vacuum i n s i d e the e l e c t i o n microscope (approx. 5x10 t o r r ) , and under the heating e f f e c t of the e l e c t r o n beam. Therefore, very small Mg oxide c r y s t a l l i t e s could have formed at the surfaces of the l a r g e r Mg carbonate c r y s t a l s , w i t h the emission of The decomposition of MgCO^ i s q u i t e p o s s i b l e , s i n c e i t s d i s s o c i a t i o n pressure i s o n l y 0.1 t o r r at 400°C.^ 2^ A c t u a l l y , some minor changes were noticed on the f i l m , when observed under the e l e c t r o n microscope. The f a i n t r i n g s i n F i g . 29 do not belong to Mg oxide, but can t e n t a t i v e l y be assigned to a Mg carbonate, w i t h a lower degree of c r y s t a l l i n i t y than magnesite. The i n t e r p r e t a t i o n i s q u i t e u n c e r t a i n because the r i n g s are very f a i n t and d i f f u s e . They may be due to extremely small Mg carbonate c r y s t a l s present i n the f i l m , along w i t h the l a r g e r and b e t t e r c r y s t a l l i z e d magnesite c r y s t a l s . Some of the f i l m s a c t u a l l y showed IR spectra i n d i c a t i n g the presence of both the magnesite-like and the amorphous or poorly c r y s t a l l i n e carbonates. The f a i n t r i n g s observed i n d i c a t e that other minor phases could be present under the c o n d i t i o n s of vacuum and higher temperatures p r e v a i l i n g i n s i d e the tra n s m i s s i o n e l e c t r o n microscope. However, t h i s p o s s i b i l i t y by no means decreases the evidence that a Mg carbonate p r a c t i c a l l y analogous to magnesite was the dominant component of the t h i n f i l m s . Contact angles The magnesium carbonate (magnesite) front-coated g o l d m i r r o r s showed a nea r l y h y d r o p h i l i c surface (or a surface which showed a q u i t e low degree of 87 h y d r o p h o b i c i t y ) , as test e d by the contact angles ( S e c t i o n 5.4). Before the determination of the contact angles, the f i l m s were exposed t o double d i s t i l l e d water f o r 2 min, i n order t o e l i m i n a t e any s o l u b l e component (Mg oxide and/or amorphous Mg carbonate). The samples were then heated at 110°C f o r 2 hours, and kept overnight i n a vacuum d e s i c c a t o r . An i n f r a r e d MSRA spectrum was always run before the t e s t , to check f o r any anomalous f e a t u r e . The average contact angles u s u a l l y stayed between 2° and 4°. A t y p i c a l m i r r o r presented the f o l l o w i n g s t a t i s t i c a l data: average contact angle (0) : 3° y - number of determinations (n) : 7 standard d e v i a t i o n (s) : 1.1° confidence i n t e r v a l (at 90% l e v e l ) : +0.8 6.3.3 Adsorption of sodium ol e a t e and o l e i c a c i d onto magnesium carbonate  (magnesite) t h i n f i l m s The ads o r p t i o n of Na ole a t e from aqueous s o l u t i o n s was c a r r i e d out at moderately a c i d i c (pH = 6) and ba s i c (pH = 10) c o n d i t i o n s . The choice of these values was based upon the r e s u l t s of the m i c r o f l o t a t i o n t e s t s ( S e c t i o n -4 6.2.1). A co n c e n t r a t i o n of 10 M was used i n the experiments. P r e l i m i n a r y t e s t s proved that lower concentrations (10 5M t o 5x10 5M) were not s u f f i c i e n t to achieve s e n s i t i v i t y i n the recorded spectra. The high a b s o r p t i o n r e g i o n of the carbonates (1600 t o 1400 cm "*") was the reason f o r t h i s : i t tended to cover the c h a r a c t e r i s t i c bands of the carboxylates occurring i n part of the same range. For the same reason, the thickness of 88 the Mg carbonate f i l m s used i n most of the adsorption t e s t s had to be l i m i t e d to a maximum of approximately 100 %. Under these c o n d i t i o n s , good s e n s i t i v i t y i n that c r i t i c a l s p e c t r a l range was achieved. Exposure times t o the adsorbate s o l u t i o n s of 2, 6 and 15 min were adopted i n both a c i d i c and b a s i c c o n d i t i o n s . Before the adsorption t e s t s , a l l the m i r r o r s were immersed i n d o u b l e - d i s t i l l e d /water, which had i t s pH adjusted t o the same pH as the adsorbate s o l u t i o n . They were kept i n contact w i t h the water u n t i l any e x i s t i n g s o l u b l e compound (Mg oxide and/or amorphous' Mg carbonate) was removed, as checked by IR spectrophotometry. The m i r r o r s were then thoroughly d r i e d under an argon j e t , before the i n f r a r e d MSRA spectrum of the o r i g i n a l f i l m was recorded. The spectra corresponding t o the f i l m a f t e r each exposure t o the adsorbate s o l u t i o n were recorded i n c o n d i t i o n s as s i m i l a r as p o s s i b l e t o the o r i g i n a l Mg carbonate f i l m . a) Adsorption of aqueous sodium ol e a t e under moderately a c i d i c c o n d i t i o n s (pH=6) The spectrum of the Mg carbonate (magnesite) t h i n f i l m before the adsorption of Na oleate i s shown i n F i g . 30; i t d i s p l a y s the normal features already described i n S e c t i o n 6.3.2 .C. A s i g n i f i c a n t aspect of t h i s spectrum i s the strong and broad band around 3600-3200 cm \ which i s assigned t o the symmetric and asymmetric s t r e t c h i n g v i b r a t i o n s of water, modified by strong hydrogen bonding. A broad shoulder i n the 1700-1600 cm 1 corresponds t o the bending v i b r a t i o n of 1^0. This i n d i c a t e s that a h y d r a t i o n l a y e r e x i s t e d at the Mg carbonate surface. This l a y e r was a l s o q u i t e s t a b l e , considering t h a t the sample was removed from water and d r i e d (see S e c t i o n 5.3.1). In other f i l m s , such h y d r a t i o n l a y e r s were s t i l l 89 p e r s i s t e n t , even a f t e r the m i r r o r s had been kept overnight at 110"c, or f o r s e v e r a l days i n a d e s s i c a t o r . The i n f r a r e d MSRA s p e c t r a of the same m i r r o r s a f t e r exposures t o the Na ol e a t e s o l u t i o n are d i s p l a y e d i n F i g s . 30a, b, and c. Several new absorption bands, due t o an adsorbed c o l l e c t o r l a y e r are present. The bands are b e t t e r observed i n the corresponding d i f f e r e n c e s p e c t r a , i . e . the spectrum a f t e r exposure t o the s o l u t i o n , from which the spectrum of the o r i g i n a l Mg carbonate substrate was scaled subtracted. This i s e s p e c i a l l y u s e f u l i n the c r i t i c a l r e g i o n between 1600 and 1350 cm where the carbonate ab s o r p t i o n i s very h i g h , and the most d i a g n o s t i c bands of the adsorbed species are l o c a t e d . These d i f f e r e n t s p e c t r a are shown i n F i g . 31a, b, and c. The s p e c t r a of the adsorbed species agree w e l l w i t h t h a t of a long-chain carboxylate from the higher frequencies down t o around 1300 cm 1 . Below t h i s p o i n t , at l e a s t f o u r strong bands i n d i c a t e the presence of d i f f e r e n t f u n c t i o n a l groups. The r e g i o n between 1750 and 1300 cm 1 The band corresponding t o the asymmetric v i b r a t i o n of the carboxylate i o n i s l o c a t e d from 1583 t o 1585 cm 1 i n the sp e c t r a of the adsorbate. I t d i d not change s i g n i f i c a n t l y i n frequency when exposure time was increased from 2 min up to 15 min, thereby presenting evidence that there was only one adsorbate compound, regardless of the i n t i a l and f i n a l stages of the adsorption. The frequency of t h i s band i s very c l o s e t o that of the bulk Mg ol e a t e , at 1582 cm 1 ( F i g . 17). Th i s matching i s even b e t t e r when one considers t h a t the r e s o l u t i o n f o r the MSRA s p e c t r a i s 3.3 cm 1 near 1600 cm 1 . A l s o , a s l i g h t s h i f t to higher frequency i s expected i n (47) a b s o r p t i o n - r e f l e c t i o n s p e c t r a . 100 ai o c ca 4-1 o cu rH m cu Pi 80 1 ' ' ' ' j ^ A ^ v J ^ ^ ^ ^ 1 1 ^ 00 u~i ^ > <N r H 00 i - H r H o r-» 1 'O" / 1 ° / o < 00 1 1 1 1 L 1 ^  / 1 '—1 / 1 4000 F i g . 3500 3000 2500 -1, 30. 2000 1500 Wavenumber (cm *) MSRA spectrum of a c r y s t a l l i n e Mg carbonate (magnesite) t h i n f i l m , before the adsorption of aqueous Na o l e a t e . Ordinate expansion: 1.54x. 100 o a ca U u 0) rH 4H cu Pi 4000 F i g . 3500 30a 3000 2500 1500 1000 _ x 600 Wavenumber (cm ) MSRA spectrum of the same Mg carbonate t h i n f i l m , a f t e r exposure to an aqueous s o l u t i o n of Na ol e a t e (10 M),at pH=6.0, f o r 2 min. Ordinate expansion: 1.54x. o R e f l e c t a n c e % R e f l e c t a n c e % 100 cu o c CTJ o cu cu P i 00 <f 00 CN v O l O 1/1 ' CN CTi G\ CN CN 80 4 0 0 0 F i g . 31a 3500 3~0~0~0 2 5 0 0 2 0 0 0 1 5 0 0 1 0 0 0 600 Wavenumber (cm ) Spectrum obtained by the s u b t r a c t i o n of the spectrum of the adsorbent from the spectrum a f t e r the adsorption of aqueous Na o l e a t e . The o r i g i n a l spectra are shown i n F i g s . 30. Exposure: 2min. Ordinate expansion: 1.54x. 100 cu o a a cu cu 80 4 0 0 0 3 5 0 0 3 0 0 0 2 5 0 0 2 0 0 0 F i g . 31b. Same as i n F i g . 31a, but t o t a l exposure: Wavenumber (cm ) 6 min. Ordinate expansion: 1.54x. Reflectance % 94 The evidence i n d i c a t e s t h e r e f o r e that the o l e a t e was chemisorbed on the Mg carbonate s u b s t r a t e . The compound formed had a p o l a r group that was very s i m i l a r t o the bulk Mg o l e a t e , which i s a 1:2 complex, i . e . having one 2+ Mg i o n f o r two o l e a t e anions. Other workers suggested the f o l l o w i n g r e a c t i o n , f o r the adsorption of saturated f a t t y a c i d s a l t s on c a l c i t e , using (39) d i f f e r e n t techniques: CaC0 3(s) + 2 RCOO~ % Ca(RCOO) 2(s) + CO 2" The r e s u l t s of t h i s work agree w i t h a s i m i l a r mechanism f o r magnesite, at l e a s t f o r the i n i t i a l a d s o r p t i o n stages. This i s suggested not only by the formation of a surface 1:2 complex, but a l s o by a decrease i n the carbonate bands f o l l o w i n g the exposure t o the o l e a t e s o l u t i o n (e.g. F i g s . 30 and 30a, and the r e s p e c t i v e r e f l e c t i v i t i e s ) . This i n d i c a t e s t h a t some of the carbonate was removed from the f i l m . T his removal was not due t o d i s s o l u t i o n of the f i l m i n water, since exposure of the same f i l m to water at the same pH, p r i o r t o the adsorption t e s t , d i d not cause any decrease i n the carbonate bands. A d e f i n i t e i n c r e a s e i n the i n t e n s i t y of the carboxylate bands r e s u l t e d from i n c r e a s i n g exposure time from 2 to 6 min. The same was t r u e f o r the s t r e t c h i n g v i b r a t i o n s of the hydrocarbon chains between 3000 and 2800 cm ^. This i n d i c a t e s that a d s o r p t i o n was s t i l l proceeding during t h i s i n t e r v a l . F urther exposure to the s o l u t i o n d i d not cause any s u b s t a n t i a l increase i n the a b s o r p t i o n bands, thus i n d i c a t i n g that the adsorption process reached a s a t u r a t i o n l i m i t at l e s s than 6 min contact time, at the corresponding c o n d i t i o n s of pH and Na o l e a t e c o n c e n t r a t i o n . The compound band i n the 1480-1380 cm 1 range i s assigned mostly to the symmetric s t r e t c h i n g v i b r a t i o n of the carboxylate i o n , p l u s some 95 hydrocarbon v i b r a t i o n s . This band a s s i s t s only i n proving that a f u l l y d i s s o c i a t e d carboxylate was the main component of the adsorbed l a y e r . No i n f o r m a t i o n can be gathered regarding the associated c a t i o n since t h i s i s a non-diagnostic band ( S e c t i o n 6.3.1). The shoulder near 1410 cm 1 i s not a usual f e a t u r e of t h i s band; i t i s discussed i n the s e c t i o n that f o l l o w s . A small broad band i s n o t i c e a b l e i n the d i f f e r e n c e s p e c t r a a f t e r the 6 min and 15 min exposures, between 1730 and 1700 cm ^; i t i n d i c a t e s a minor presence of undissociated o l e i c a c i d i n the adsorbed f i l m . Considering the s t i l l s i g n i f i c a n t c o n c e n t r a t i o n of n e u t r a l a c i d molecules i n the s o l u t i o n at t h i s pH, i t seems that a higher c o n t r i b u t i o n of the a c i d species to the adsorbate l a y e r should be expected. However, i t i s b e l i e v e d that the n e u t r a l molecules d i d approach the surface e x t e n s i v e l y , e i t h e r as a c i d dimers or (33) ionomolecular complexes. When c l o s e enough t o the s u r f a c e , they 2+ promptly reacted w i t h the Mg , thus forming the chemisorbed Mg carboxylate compound. Therefore, only a very small f r a c t i o n of n e u t r a l a c i d molecules remained unreacted i n the adsorbed c o l l e c t o r l a y e r . Thus, the f i n a l compound making up the most of the adsorbate f i l m was the same, regardless of the o r g i n a l adsorbing species, e i t h e r d i s s o c i a t e d species (the most abundant ones at pH 6) or n e u t r a l c a r b o x y l i c a c i d molecules. The region between 1300 and 600 cm 1 Strong bands occur i n t h i s r e gion ( F i g s . 31a, b, c) i n the spectra of the adsorbed m a t e r i a l . They cannot be assigned to any of the p o t e n t i a l l y important compounds such as o l e i c a c i d , Na o l e a t e or Mg o l e a t e . They are not r e l a t e d d i r e c t l y to any hydrated or hydroxylated species e i t h e r , since no s i g n i f i c a n t band i s found i n the 3600-3200 cm 1 range (although the Mg carbonate substrate i t s e l f i s hydrated). 96 Strong bands i n t h i s r e g i o n (1300-750 cm are h i g h l y suggestive of C-0 b o n d s , ^ 7 5 ' 7 ^ under the c o n d i t i o n s of t h i s work. No major changes are evident or expected regarding the p o l a r groups of the carboxylates that could l e a d t o the formation of s i n g l e C-0 bonds. The only remaining p o s s i b i l i t y would be the hydrocarbon chain of the c o l l e c t o r . The s i t e showing the highest r e a c t i v i t y p o t e n t i a l f o r such change i s the double bond, between the 9th and 10th carbon atoms, i n the c i s - c o n f i g u r a t i o n of the o l e i c a c i d molecule. Indeed, these bands can be c l e a r l y explained f o l l o w i n g t h i s assumption. The bands at 1260 and 816 cm 1 are c h a r a c t e r i s t i c of an epoxide (73 93 94 95 96) compound. > > » > ' The f i r s t one i s the most r e l i a b l e band of the epoxides ( l o c a t e d from 1260 t o 1240 cm and i s assigned t o the C-0 (73) s t r e t c h i n g v i b r a t i o n s of the oxirane r i n g : H H -C-C-0 The p a r t i a l l y broad band at 816 cm 1 i s assigned t o r i n g (73) v i b r a t i o n s . ' I t s frequency i s q u i t e s e n s i t i v e t o the p o s i t i o n of the oxirane r i n g i n the molecule, whether t e r m i n a l or l o c a t e d nearer to the centre of the molecule. I t i s a l s o s e n s i t i v e t o a c i s - or t r a n s - c o n f i g u r a t i o n , and t o the number and type of s u b s t i t u t i o n s i n v o l v i n g the r i n g . In the spectra i n t h i s work, t h i s band was d e f i n i t e l y c h a r a c t e r i s t i c of an a l i p h a t i c , epoxide compound, where the oxirane r i n g was (73 93) l o c a t e d i n a c e n t r a l p o s i t i o n of a c i s - s t r u c t u r e d molecule. v * ' A c a r b o x y l i c a c i d having such a hydrocarbon c h a i n would be, i n the present (97) s i t u a t i o n , the cis-9-epoxyoctadecanoic a c i d or c i s - 9 - e p o x y s t e a r i c a c i d , w i t h the formula: 97 H H CH 3-(CH 2) 7-C-C-(CH 2) 7-COOH ' A compound l i k e t h i s , or the corresponding carboxylate, could e x p l a i n a l l the s i g n i f i c a n t bands i n the spectra i n F i g s . 31, except f o r the strong and broad band showing maximum at 1106 cm and a prominent shoulder near 1035 cm 1 . Obviously, a complete band assignment f o r these spectra i n v o l v e s more complicated s t r u c t u r e s . A broad and s t r o n g , f r e q u e n t l y s p l i t band i n the 1200-950 cm 1 range suggests a p a r t i a l l y or t o t a l l y polymerized compound showing C-0-C bonds, or a polyether. The epoxides can show v a r i a b l e bands i n t h i s r e g i o n , but (79) they tend to be much smaller. However, the bands i n t h i s range can be completely explained by assuming a r e a c t i o n of the oxygen from an oxirane r i n g of a hydrocarbon chain w i t h a neighbour c h a i n , thus forming the s t r u c t u r e : H H 1 H - C - C - C - C -H 1 H H 0 H 1 H H - C - c - C - C -H H 1 H This i s e x a c t l y the p o l y m e r i z a t i o n mechanism f o r the epoxides, p a r t i c u l a r l y f o r the a l i p h a t i c ones. The band assignment f o r the e t h e r e a l s t r u c t u r e formed can thus be completed. The strong band near 1106 cm 1 corresponds to the asymmetric s t r e t c h i n g v i b r a t i o n of the C-0-C ' (73 79) ' -1 group, ' and the shoulder at 1035 cm to the symmetric s t r e t c h i n g of the same group, f o r the a l i p h a t i c ethers. The l a t t e r band i s u s u a l l y weaker than the asymmetric s t r e t c h i n g mode. This i s p a r t i c u l a r l y true when the two chains are attached to the e t h e r e a l oxygen i n a symmetrical c o n f i g u r a t i o n . In t h i s case, the asymmetric s t r e t c h i n g band becomes stronger, and occurs at about 1110 cm ^; the symmetric v i b r a t i o n i s then 98 weaker or may even be a b s e n t . w J > This i s due to coupling between the two o r i g i n a l f r e q u e n c i e s . Therefore, the c o r r e l a t i o n between the proposed s t r u c t u r e and the p o s i t i o n and i n t e n s i t y of the two bands i n t h i s study was q u i t e c o n s i s t e n t . In order t o summarize the proposed mechanism oc c u r r i n g at the t hydrocarbon chains of the adsorbed o r i g i n a l o l e a t e , two overlapping r e a c t i o n s vare considered: i ) the o x i d a t i o n of the double bonds at the middle of the chain, w i t h the formation of an oxirane r i n g ; i i ) the p a r t i a l p o l y m e r i z a t i o n of the epoxide compound thus forming a polyether s t r u c t u r e . The s p e c t r a l evidence gathered from adsorption t e s t s at moderately a c i d i c c o n d i t i o n s made i t p o s s i b l e t o ob t a i n and evaluate a f a i r l y d e t a i l e d p i c t u r e of the i n t e r a c t i o n o c c u r r i n g at or near the m i n e r a l / s o l u t i o n i n t e r f a c e . The evidence i s that the c o l l e c t o r was chemisorbed on the surface by i t s p o l a r group, and the hydrocarbon chains s t r o n g l y i n t e r a c t e d between themselves w i t h the formation of a p a r t i a l l y polymerized epoxide-polyether l a y e r . Following t h i s i n t e r p r e t a t i o n , not only a l l the major bands i n the r e s p e c t i v e spectra were c o n s i s t e n t l y explained, but a l s o smaller ones outside the 1300-600 cm 1 range. This i s the case of the n o t i c e a b l e shoulder at about 1410 cm 1 ( F i g . 31), assigned to the methylene groups adjacent t o the oxirane r i n g , ^ 9 ^ reported to occur at 1412 cm 1 . L i k e the carboxylate bands, a l l the bands r e l a t e d t o the organic oxide groups showed an increase i n i n t e n s i t y between the 2 min and the 6 min exposure, but not a f t e r the 15 min adsorption time. 9 9 The region between 3050 and 2800 cm x This i s an important s p e c t r a l r e gion, because the hydrocarbon s t r e t c h i n g v i b r a t i o n s occur here. These v i b r a t i o n s do not provide any i n f o r m a t i o n on the nature and changes i n the p o l a r group of the c a r b o x y l i c a c i d s . However, they are q u i t e u s e f u l i n regard to such f a c t o r s as the r e l a t i v e amount of the adsorbed c o l l e c t o r , i t s molecular o r i e n t a t i o n and the degree of u n s a t u r a t i o n of the hydrocarbon chain. The l a s t f a c t o r i s q u i t e important, s i n c e the occurrence of o x i d a t i o n at the double bonds s i t e s had to be associated w i t h a decrease or disappearance of the bands r e l a t e d to these double bonds. The best band f o r a q u a l i t a t i v e e v a l u a t i o n of the extension of u n s a t u r a t i o n i s the medium but sharp peak due t o the s t r e t c h i n g v i b r a t i o n of CH neighbour to the double bond. I t i s normally present between 3010 and 3005 cm 1 i n mono-unsaturated f a t t y a c i d s and t h e i r d e r i v a t i v e s (see reference s p e c t r a , F i g s . 15, 16 and 17). In the s p e c t r a of the adsorbed c o l l e c t o r at pH=6, small bands or shoulders are present near 3000 cm 1 ( F i g s . 30 and 31, a, b, c ) . However, these bands have approximately the same i n t e n s i t y as the noise background i n that s p e c t r a l region. Therefore, they cannot be considered as t r u e bands, even though they occur i n the appropriate frequency region. The other neighbour hydrocarbon s t r e t c h i n g v i b r a t i o n s , r e l a t e d t o the CH^ and CH^ groups, are very conspicuous, even f o r the s h o r t e s t exposure time. This can be b e t t e r evaluated at higher s c a l e expansion, as shown i n F i g . 35. By comparison w i t h the reference s p e c t r a , the band as s o c i a t e d w i t h the double bond should s t i l l be n o t i c e a b l e , although s m a l l . Thus there i s evidence t o conclude that the degree of u n s a t u r a t i o n of the hydrocarbon chains of the c o l l e c t o r had decreased s u b s t a n t i a l l y i n the adsorbed f i l m . U n f o r t u n a t e l y , 100 however, no co n c l u s i o n can be drawn, at t h i s point on whether or not r e s i d u a l double-bonds were present i n the adsorbed f i l m or on the r e l a t i v e amount of unreacted molecules. The s t r e t c h i n g modes as s o c i a t e d w i t h the groups are c l e a r l y observed i n a l l the MSRA spectra i n F i g s . 30 and 31. No s i g n i f i c a n t s h i f t s are observed i n t h e i r f r e q u e n c i e s , the asymmetric mode near 2920 cm 1 and the symmetric one at about 2850-2945 cm I t i s s i g n i f i c a n t that t h e i r r e l a t i v e i n t e n s i t y i s not f a r from the one shown i n the reference transmission spectra (compare'Figs. 16, 31 and a l s o F i g . 35). The i n t e n s i t y of the band as s o c i a t e d w i t h the asymmetric s t r e t c h i n g v i b r a t i o n of the CH^ groups (at 2955 cm 1 ) i s much g r e a t e r i n the MSRA, than i n the tra n s m i s s i o n spectra. This f a c t has important i m p l i c a t i o n s f o r molecular o r i e n t a t i o n , as described below. The l a s t band i n the region, the symmetric s t r e t c h i n g mode of the methyl groups, i s not conspicuous i n the MSRA. However, t h i s band i s the l e a s t prominent among t h i s group of f o u r , and i s d i f f i c u l t to observe even i n regul a r t r a n s m i s s i o n spectra. No s i g n i f i c a n t i n c r e a s e occurred i n the o v e r a l l i n t e n s i t y of the bands i n t h i s s p e c t r a l region, f o l l o w i n g the 6 min exposure, which i s true a l s o f o r a l l the other p a r t s of the spec t r a . Molecular o r i e n t a t i o n s One of the best f e a t u r e s of the r e f l e c t i o n - a b s o r p t i o n method i s the q u a l i t a t i v e i n f o r m a t i o n that can be gathered i n many cases on the o r i e n t a t i o n of the molecules i n r e l a t i o n to the r e f l e c t i n g s u r f ace. This i s due t o the f a c t that only the components of the t r a n s i t i o n moments of the v i b r a t i n g bonds that are perpendicular t o the surface can i n t e r a c t e f f e c t i v e l y w i t h the i n f r a r e d r a d i a t i o n , and there f o r e absorb energy ( S e c t i o n 4.2.1). 101 O l e i c a c i d and other oleates have a bent hydrocarbon c h a i n , due t o the c i s s t r u c t u r e at the double bond (see molecular model, F i g . 32). The formation of an oxirane r i n g at the s i t e of the former double bond, does not appreciably change the conformation of the cha i n , since c i s - and trans-isomerism are s t i l l p o s s i b l e . Consequently, the determination of the o r i e n t a t i o n of molecules that show d i f f e r e n t s t r u c t u r a l d i r e c t i o n s i n the same ch a i n even when f u l l y s t r e t c h e d , can only be of approximate nature, i . e . i n a broad q u a l i t a t i v e approach. Most of the s t u d i e s on molecular o r i e n t a t i o n using IR ab s o r p t i o n methods are concerned w i t h saturated compounds, which present a l e s s complicated s i t u a t i o n than the unsaturated ones l i k e o l e i c a c i d . Of a l l the v i b r a t i n g modes occurr i n g i n the adsorbate species the only ones t h a t are s u i t a b l e f o r a treatment r e l a t e d t o molecular o r i e n t a t i o n , are the s t r e t c h i n g v i b r a t i o n s of the methylene and methyl groups, and the same v i b r a t i o n s of the carbo x y l a t e group. The t r a n s i t i o n moments of these modes show the f o l l o w i n g o r i e n t a t i o n s : ^ 4 6 , 4 8 , 5 1 , 5 3 ' 5 4 ^ the methylene s t r e t c h i n g v i b r a t i o n s are both o r i e n t e d i n a plane perpendicular t o the c h a i n a x i s ; t h i s plane contains one carbon and the two hydrogen atoms. Another element of reference i s the plane that contains the zigzag s k e l e t a l s t r u c t u r e of the carbon atoms along (53) the chain; ' i t i s a l s o a plane of symmetry of a l l the hydrocarbon c h a i n of the o l e a t e , and a l s o of the carboxylate group. The symmetric v i b r a t i o n i s p a r a l l e l t o the zigzag plane, while the asymmetric one i s perpendicular t o i t . These o r i e n t a t i o n s can a l s o be r e l a t e d t o the CH^ t r i a n g l e s : 102 103 H H c T c +-» H H asymmetric s t r e t c h i n g symmetric s t r e t c h i n g the methyl s t r e t c h i n g modes are r e l a t e d t o the l i n e connecting the two l a s t carbon atoms of the chain ( c 1 7 ~ c 1 8 d i r e c t i o n ) ; t h i s l i n e makes an angle of about 145° w i t h the a x i s of the second branch of the oleate c h a i n (opposed to the p o l a r group). The symmetric mode i s (53 99) p o l a r i z e d p a r a l l e l to the ^\7~^IS d i r e c t i o n . ' The asymmetric s t r e t c h i n g has a more complicated o r i e n t a t i o n . The main components are doubly-degenerated i n the plane perpendicular to the • C17~ C18 d i r e c t i o n - ( 5 3 ' 9 9 ) - the symmetric and asymmetric s t r e t c h i n g modes of the carboxylate group have t r a n s i t i o n moments o r i e n t e d i n the COO plane, and are perpendicular t o each other. The symmetric mode i s p a r a l l e l t o the zigzag plane of the chain. As mentioned i n the l a s t s e c t i o n , the r e l a t i v e i n t e n s i t y of the hydrocarbon s t r e t c h i n g v i b r a t i o n s i n F i g s . 31 and 35 are not f a r from the i n t e n s i t i e s i n the reference spectra. The only exception i s the pronounced increase i n the asymmetric s t r e t c h i n g mode of the CH^ group, i n the MSRA spectra. An o r i e n t a t i o n that could e x p l a i n such behaviour would be the chains l y i n g approximately f l a t along the su r f a c e , i n a more or l e s s contorted a t t i t u d e , i . e . w i t h the zigzag plane n e i t h e r v e r t i c a l nor h o r i z o n t a l . Both methylene v i b r a t i o n modes would be enhanced by approximately the same p r o p o r t i o n , f o l l o w i n g t h i s assumption. The methyl asymmetric s t r e t c h i n g would be g r e a t l y enhanced, since the plane perpendicular to the ^y-C^g d i r e c t i o n would always be near to the 104 v e r t i c a l , t h e r e f o r e presenting a component p a r a l l e l to the only d i r e c t i o n of p o s s i b l e energy absorption (perpendicular to the s u r f a c e ) . On the other hand, the symmetric mode of the methyl group would not be. expected to be increased i n such o r i e n t a t i o n . Indeed, i t i s not prominent i n the MSRA spectra. Another important aspect i n the spectra i n F i g . 31 i s that the o v e r a l l i n t e n s i t y of the hydrocarbon s t r e t c h i n g bands (3050 to 2800 cm "*") i s s u b s t a n t i a l l y increased i n r e l a t i o n to both carboxylate bands (compare w i t h reference s p e c t r a ) . This i s a l s o c o n s i s t e n t w i t h an o r i e n t a t i o n of the chain nearly p a r a l l e l to the surface. Both carboxylate v i b r a t i o n s are expected to decrease i n i n t e n s i t y , when the COO plane has a r e l a t i v e l y small angle w i t h the sur f a c e . Furthermore, the asymmetric mode should be the most reduced under t h i s assumption, since i t s t r a n s i t i o n moment would be c l o s e r to the h o r i z o n t a l . Indeed, i t s i n t e n s i t y i s approximately the same as compared to the symmetric carboxylate mode I n the MSRA sp e c t r a . In the absence of o r i e n t a t i o n , the l a t t e r i s always smaller ( F i g s . 16 and 17). Therefore, a l l the r e l a t i v e i n t e n s i t i e s of the most c h a r a c t e r i s t i c bands of the spectra of the adsorbed c o l l e c t o r , at moderately a c i d i c c o n d i t i o n , suggest that the adsorbed species l a y approximately f l a t on the surface of Mg carbonate. Contact angle The degree of hydrophobicity of the c o l l e c t o r - c o v e r e d Mg carbonate surface was evaluated by measuring the contact angles w i t h s e s s i l e drops of pure water ( S e c t i o n 5.4), a f t e r the 15 min exposure. The r e s u l t s were as f o l l o w s : 105 average contact angle: 9 8 u - number of determinations: 8 standard d e v i a t i o n : 6.9° confidence i n t e r v a l (at 90% l e v e l ) : + 5° Therefore, a h i g h l y hydrophobic surface was achieved f o l l o w i n g the -4 adsorption of Na o l e a t e from a 10 M s o l u t i o n at pH = 6.0. b) Adsorption of aqueous sodium oleate under basic c o n d i t i o n s (pH = 10) T y p i c a l Mg carbonate (magnesite) t h i n f i l m s were used f o r the a d s o r p t i o n t e s t s at basic c o n d i t i o n s . F i g . 33 shows the i n f r a r e d MSRA spectrum of such a f i l m , before any adsorption of c o l l e c t o r . L i k e the one i n F i g . 30 (used f o r a c i d i c c o n d i t i o n s ) , i t shows a l l the most c h a r a c t e r i s t i c f e a t u r e s of the magnesite-like t h i n f i l m s . A l s o , i t shows a very strong and broad band i n the 3600-3200 cm \ i n d i c a t i n g a h y d r a t i o n l a y e r at the Mg carbonate surface. As i n the previous case, t h i s h y d r a t i o n was s t i l l present, even a f t e r the m i r r o r had been thoroughly d r i e d . The i n f r a r e d s p e c t r a , f o l l o w i n g exposures to the Na oleate s o l u t i o n , are shown vin F i g s . 33a, b, and c. The corresponding d i f f e r e n c e s p e c t r a are presented i n F i g s . 34a, b, and c. Band assignments The s p e c t r a of the adsorbed c o l l e c t o r shows the same general aspects as compared to the adsorption i n moderately a c i d i c c o n d i t i o n s . The adsorbate i c o n s i s t e d of a carboxylate, showing appreciable e p o x i d i z a t i o n and p o l y m e r i z a t i o n , o c c u r r i n g at the s i t e s of the o r i g i n a l double bonds of the olea t e . However, important q u a l i t a t i v e and q u a n t i t a t i v e d i f f e r e n c e s are found between the adsorbed species at these d i f f e r e n t pH c o n d i t i o n s . 106 An important d i f f e r e n c e i s the s h i f t i n frequency i n the asymmetric s t r e t c h i n g mode of the carboxylate group, a f t e r i n c r e a s i n g exposure time at b a s i c c o n d i t i o n s . The frequency a f t e r the 2 min exposure was 1583 cm 1 ( F i g . 34a), t h e r e f o r e very near the only value at a c i d i c c o n d i t i o n s . However, the band maxima were s h i f t e d to lower frequencies (1573-1570 cm ^) a f t e r increased exposures to the s o l u t i o n . S i m i l a r s h i f t s were no t i c e d i n the non-diagnostic symmetric mode, too. Consequently, there i s evidence that a d i f f e r e n t complex formed at the s o l i d - s o l u t i o n i n t e r f a c e , a f t e r the adsorption of a f i r s t s u rface compound. The f i r s t one was probably chemisorbed, and q u i t e s i m i l a r to the bulk Mg oleate soap (at l e a s t regarding the p o l a r group), as i n the adsorption at a c i d i c conditions.. The reference spectra of Mg o l e a t e shows a maximum f o r t h i s band at 1582 cm 1 ( F i g . 17). i A s h i f t of t h i s p a r t i c u l a r band i s s i g n i f i c a n t i n s e v e r a l ways: the r e s o l u t i o n f o r the MSRA s p e c t r a near 1600 cm 1 i s about 3.3 cm 1; - the s h i f t due t o the d i s p e r s i o n of the r e f r a c t i v e index during r e f l e c t i o n - a b s o r p t i o n (see S e c t i o n 4.2) would be toward higher frequencies, i . e . i n the opposite d i r e c t i o n of the observed one; - the s h i f t t o lower frequency i s r e p r o d u c i b l e . This was confirmed not only concerning the 6 min and 15 min exposures, but a l s o when the whole experiment was repeated at l e a s t two times. Considering that the s h i f t observed a f t e r the 6 min exposure a l s o i n c l u d e s the one corresponding t o the 2 min exposure, the a c t u a l s h i f t due to the compounds adsorbed between 2 and 6 min would be towards frequencies even lower than 1570 cm This means that the observed peak at 1570 cm 1 i s a c t u a l l y the weighted average between the peak at 1582 cm \ and another peak, n e c e s s a r i l y l o c a t e d below 1570 cm This suggests that the second 100 QJ o c CO 4-1 o CU I—I I M CU Pi 4000 3500 3000 2500 2000 1500 1000 ^ 600 Wavenumber (cm ) F i g . 33. MSRA spectrum of a c r y s t a l l i n e Mg carbonate (magnesite) t h i n f i l m , before the adsorption of aqueous Na o l e a t e . Ordinate expansion: 1.70x. R e f l e c t a n c e % R e f l e c t a n c e % 100 4000 3500 3000 2500 2000 1500 1000 _ x 600 Wavenumber (cm ) F i g . 34a. Spectrum obtained by the s u b t r a c t i o n of the spectrum of the adsorbent from the spectrum a f t e r the adsorption of aqueous Na o l e a t e . The o r i g i n a l spectra are shown i n F i g s . 33. Exposure: 2 min. Ordinate expansion: 2.15x. 100 4000 2000 600 Wavenumber (cm ) F i g . 34b. Same as i n F i g . 34a, but t o t a l exposure: 6 min. Ordinate expansion: 2.15x. I l l 00 o m CM C ^ 00 CM CM ! I ! ! I I 3100 3000 2900 2800 2700 _ 1 Wavenumber (cm ) Region of the hydrocarbon s t r e t c h i n g v i b r a t i o n s of the r e f -erence t r a n s m i s s i o n spectrum of Na oleate ( a ) , and the MSRA spectra of compounds adsorbed from 10 M s o l u t i o n s of Na o l e a t e onto Mg carbonate t h i n f i l m s , a f t e r 6 min exposures: (b) pH=6.0, rough s u b s t r a t e ; (c) pH=6.0, f l a t s u b s t r a t e ; (d) pH=10.0, f l a t s u b s t r a t e . Abscissa expansion: 3.85x; o r d i n a t e expansion: v a r i a b l e . F i g . 35. 112 adsorbing species could be a p h y s i c a l l y adsorbed oleate i o n , since the frequency f o r the asymmetric s t r e t c h i n g mode of the carboxylate i o n i n the reference s p e c t r a of Na oleate i s 1560 cm ^. To t e s t the assumption that the peak at 1570 cm 1 could be the an average of the absorption due to Mg oleate and Na o l e a t e , the f o l l o w i n g c a l c u l a t i o n s were performed: a technique based on the band contour a n a l y s i s ^ 4 ' w a s c a r r i e d out, by summing up the or d i n a t e values ( i n absorbances) of . the a c t u a l bands due to the asymmetric s t r e t c h i n g mode of the carboxylate i o n , f o r the two oleate species. The r e l a t i v e p r o p o r t i o n of each compound i n t h i s summation could be v a r i e d from 0% t o 100% each, i . e . from 100% Na olea t e (0% Mg oleate) to 0% Na oleate (100% Mg o l e a t e ) . The c a l c u l a t i o n s were made using the computing f a c i l i t i e s provided i n the Perkin-Elmer Model 3500 Data S t a t i o n ( S e c t i o n 5.3.2). Following t h i s approach, a peak showing maximum absorption a t 1570 cm 1 was found corresponding t o the composition: 47% Na o l e a t e , 53% Mg o l e a t e . This does not prove that the above composition i s the t r u e one of the adsorbed species a f t e r 6 min exposure. I t i n d i c a t e s that an unresolved peak at 1570 cm 1 could be explained by the j o i n t absorptions of complexes s i m i l a r t o the reference ones, at the c a l c u l a t e d percentages. However, t h i s i n t e r p r e t a t i o n along w i t h other evidence from the MSRA, gi v e s strong support f o r the f o l l o w i n g adsorption model, under ba s i c s o l u t i o n c o n d i t i o n s : - under the c o n d i t i o n s of negative charge at the i n t e r f a c e , the oleat e anions from the s o l u t i o n would be able to adsorb only at the most r e a c t i v e s i t e s , probably some carbonato-hydroxo Mg complex. The chemisorbed species, responsible f o r the peak at 1582 cm \ would have a p o l a r group s t r u c t u r e s i m i l a r to the bulk Mg o l e a t e . 113 - a f t e r the r e a c t i o n w i t h a l l a v a i l a b l e a c t i v e s i t e s on the Mg carbonate surface, the o l e a t e ions would not be able t o cross the energy b a r r i e r at the surface to react w i t h other l e s s a c t i v e s i t e s . Therefore, they could be only p h y s i c a l l y adsorbed. The o l e a t e ions would be maintained i n i t i a l l y only by the Van der Waals f o r c e s between the hydrocarbon chains. A f t e r a very short time, maybe even simultaneously w i t h the p h y s i s o r p t i o n , the o x i d a t i o n of the chains at the double-bonds would take p l a c e , followed by the p a r t i a l p o l y m e r i z a t i o n . Therefore, very strong, covalent, l a t e r a l bonds would develop between the hydrocarbon chains. I n t h i s way, a l l the adsorbed species would become st r o n g l y anchored t o the surface, thus forming a r e s i l i e n t f i l m . In the spectra of the adsorbed compound, other aspects are a l s o c o n s i s t e n t w i t h t h i s proposed model. Both carboxylate s t r e t c h i n g modes show pronounced i n t e n s i t y increases f o l l o w i n g a l l exposures. This i s opposed to the observed behaviour at a c i d i c c o n d i t i o n s , where the adsorption p r a c t i c a l l y ceased a f t e r 6 min. This i n d i c a t e s that c o l l e c t o r species were continuously adsorbing, mostly f o l l o w i n g the second mode ( p h y s i s o r p t i o n ) , under b a s i c c o n d i t i o n s . This adsorption mode, that i s not p r i m a r i l y s i t e - r e l a t e d , would be expected to take longer t o reach s a t u r a t i o n , f o r the same c o l l e c t o r c o n c e n t r a t i o n i n s o l u t i o n . There was a small decrease i n the i n t e n s i t y of the carbonate bands f o l l o w i n g the adsorption of the c o l l e c t o r . This e f f e c t i s more n o t i c e a b l e on the l a r g e carbonate band near 1480 cm \ f o l l o w i n g the 2 min exposure ( F i g s . 30 and 30a). However, t h i s decrease i n i n t e n s i t y i s much l e s s pronounced than at pH = 6. This i s f u r t h e r evidence f o r the l e s s e r occurrence of chemisorption at b a s i c c o n d i t i o n s . 114 No s i g n i f i c a n t absorption bands can be found i n the range 1740-1700 cm 1 ( F i g . 34), which i n d i c a t e s the absence of n e u t r a l c a r b o x y l i c a c i d molecules i n the adsorbed l a y e r . This i s expected, i n view of the n e g l i g i b l e concentration of n e u t r a l species i n s o l u t i o n , under ba s i c c o n d i t i o n s . The same major bands and shoulder are present i n the region between 1300 and 750 cm \ I n ba s i c c o n d i t i o n s , as compared t o moderately a c i d i c r e s u l t s . The former are nevertheless s m a l l e r , thus showing evidence that the o x i d a t i o n and po l y m e r i z a t i o n of the ol e a t e hydrocarbon chain at the double-bond s i t e s was l e s s accentuated under basic c o n d i t i o n s , although s t i l l o ccurring widely. The assignments of these bands remain the same as f o r the spec t r a corresponding to a c i d i c c o n d i t i o n s . Some minor s h i f t s are n o t i c e a b l e f o r the bands at ro 1100 and ^ 8 0 0 cm"1, as w e l l as f o r the shoulder at 1040-1030 cm However, these C-0 and r i n g bands are notorious f o r showing such v a r i a b i l i t y without l o s s of t h e i r j o i n t d i a g n o s t i c value.(^3,79) T h g C _ Q band at 1260 cm 1 i s much more constant, concerning the frequency v a r i a t i o n s , as described i n the l i t e r a t u r e . egion 715-710 cm 1 i s a s s i (75) The~ small and broad band i n the r s assigned to the rocking v i b r a t i o n of the sequence of methylene groups i n the chain. The s p e c t r a l region c o n t a i n i n g the hydrocarbon s t r e t c h i n g v i b r a t i o n s (3050-2800 cm - 1) i n the MSRA. spec t r a of the adsorbed c o l l e c t o r under b a s i c c o n d i t i o n s presents some i n t e r e s t i n g f e a t u r e s . As most of these aspects are r e l a t e d t o molecular o r i e n t a t i o n , they w i l l be discussed i n the next s e c t i o n . There i s no sure i n d i c a t i o n of the presence or absence of double bonds i n the adsorbed c o l l e c t o r l a y e r , as judged by the band i n the 3010-3000 cm 1 range. However, i n t h i s case, a stronger band i s present at the expected l o c a t i o n (3003 cm 1 ) , e.g. i n Fig.' 35d, as compared to F i g . 35c. 115 The l a t t e r f i g u r e r e f e r s to moderately a c i d i c c o n d i t i o n s . In basic c o n d i t i o n s , 6 min exposure ( F i g . 35d), t h i s band i s even stronger than the one at 2955 cm 1 (due t o the asymmetric s t r e t c h i n g of CH^). Unfortunately, however, the noise l e v e l i s s t i l l high i n t h i s s p e c t r a l region. Molecular o r i e n t a t i o n There i s evidence from the MSRA spectra that changes i n the molecular o r i e n t a t i o n of the adsorbed species occurred under basic c o n d i t i o n s during the exposure time, as opposed t o the adsorption at pH = 6. The . spectrum corresponding to the 2 min exposure time shows hydrocarbon s t r e t c h i n g bands (3050-2800 cm 1 region) having approximately the same i n t e n s i t i e s as the s t r e t c h i n g ones assigned t o the carboxylate group (1650-1350 cm 1 r e g i o n ) , i n F i g . 34. About the l a t t e r , the asymmetric mode i s conspicuously smaller than the symmetric one. Moreover, the r e l a t i v e i n t e n s i t i e s of the symmetric and the asymmetric s t r e t c h i n g v i b r a t i o n s of the , methylene groups (2842 and 2918 cm \ r e s p e c t i v e l y ) are not s i g n i f i c a n t l y changed. A l s o , the r e l a t i v e i n t e n s i t y of the asymmetric s t r e t c h i n g mode of the methyl group (at 2954 cm i s s u b s t a n t i a l l y i n c r e a s e d , although not as much as under a c i d i c c o n d i t i o n s . As discussed p r e v i o u s l y i n the case of adsorption at pH = 6, the present set of i n t e n s i t y r a t i o s suggests an o r i e n t a t i o n i n which the hydrocarbon chains would be ne a r l y p a r a l l e l t o the substrate surface. However, i n t h i s case the angle w i t h that surface was probably g r e a t e r than under a c i d i c c o n d i t i o n s , i . e . the chain would show an average i n c l i n a t i o n t o the surface, but c l o s e r t o the p a r a l l e l than to the v e r t i c a l t o the sub s t r a t e . In the spectrum corresponding t o the 6 min contact time ( F i g . 34b), the i n t e n s i t y of the hydrocarbon s t r e t c h i n g bands i s d e f i n i t e l y s m a l l e r , i n r e l a t i o n t o the carboxylate bands. The i n t e n s i t y r a t i o between the symmetric 116 and asymmetric s t r e t c h i n g modes of CE^ i s s t i l l the normal one. These i n t e n s i t y r a t i o s suggest an o r i e n t a t i o n that i s c l o s e r t o the v e r t i c a l than t o the p a r a l l e l t o the surface. The planes containing the CR^ groups would then be o r i e n t e d c l o s e r t o the p a r a l l e l t o the surface. Therefore, the v e r t i c a l components of the t r a n s i t i o n moments of both s t r e t c h i n g v i b r a t i o n s would be s m a l l . The absorption of i n f r a r e d r a d i a t i o n would be s u b s t a n t i a l l y lowered, thus reducing the i n t e n s i t y of both bands. Furthermore, the frequency of the asymmetric CH^ s t r e t c h i n g i s abnormally s h i f t e d to a lower frequency (from 2920 t o 2910 cm \ F i g ; 35). In one example i n the (53) l i t e r a t u r e a s i m i l a r s h i f t of the same band was as s o c i a t e d w i t h an angle of the hydrocarbon chain of about 70° w i t h the sub s t r a t e . The spectrum f o l l o w i n g the 15 min exposure shows s i m i l a r , but even more accentuated b a n d - i n t e n s i t y r a t i o s ( F i g . 34c). The co n c l u s i o n favours again an average o r i e n t a t i o n nearer t o the normal than t o the p a r a l l e l t o the surface. Probably the average angle w i t h the plane of the sub s t r a t e was gre a t e r i n the present case than i n the 6 min contact time f o r the f o l l o w i n g reasons: a much increased r e l a t i v e i n t e n s i t y of the carboxylate s t r e t c h i n g bands; - an increase i n the r a t i o between the symmetric and the asymmetric methylene s t r e t c h i n g modes. The i n f o r m a t i o n from the molecular o r i e n t a t i o n s t u d i e s provided some evidence t h a t the c o l l e c t o r species had d i f f e r e n t a t t i t u d e s i n r e l a t i o n to the s u r f a c e , according to the adsorption mode, under b a s i c c o n d i t i o n s as f o l l o w s : \ 117 - during the f i r s t stages of the adsorption, when the mechanism was mostly of chemical nature and the surface compound was s i m i l a r to the bulk Mg o l e a t e , the adsorbed molecules o r i e n t e d themselves mostly p a r a l l e l t o the surface of the s u b s t r a t e . Under moderately a c i d i c c o n d i t i o n s , the c o l l e c t o r molecules maintained t h i s o r i e n t a t i o n during a l l the adso r p t i o n process u n t i l a s a t u r a t i o n l e v e l was achieved, when the adsorption p r a c t i c a l l y ceased. - during the second stage of the adsorption, when the process was probably more of p h y s i c a l nature, the adsorbed molecules tended to an i n c l i n e d a t t i t u d e , c l o s e r t o the v e r t i c a l than t o the p a r a l l e l to the substrate surface. Contact angle Contact angles were measured, by the s e s s i l e drop method, f o l l o w i n g -4' the procedures o u t l i n e d i n S e c t i o n 5.4, a f t e r 15 min exposure t o the 10 M Na ole a t e s o l u t i o n , at pH=10. The r e s u l t s were the f o l l o w i n g : - average contact angle: 77° - number of determinations: 8^ - standard d e v i a t i o n : 7.3° confidence i n t e r v a l (at 90% l e v e l ) : + 5° Thus, the surface had a f a i r l y high hydrophobic c h a r a c t e r , a f t e r the ads o r p t i o n under a l k a l i n e c o n d i t i o n s . However, the degree of hydrophobicity was lower than under moderately a c i d i c c o n d i t i o n s , c) Bonding s t r e n g t h of the adsorbed species The o v e r a l l bonding between the Mg carbonate substrate and the f i l m of adsorbed c o l l e c t o r seemed t o be very s t r o n g , i n both a c i d i c and b a s i c adsorbing c o n d i t i o n s . Storage i n contact w i t h dry a i r f o r periods as long as 118 one month d i d not s i g n i f i c a n t l y change the spectra of the m i r r o r s . Exposures t o water up t o one hour a l s o f a i l e d t o cause any n o t i c e a b l e change. The s t r e n g t h of the bonding of the adsorbed f i l m was f u r t h e r t e s t e d by washing the m i r r o r s w i t h s o l v e n t s f o r organic m a t e r i a l s . Isopropanol and carbon t e t r a c h l o r i d e were used. The l a t t e r proved t o be a stronger solvent f o r the adsorbed m a t e r i a l . Therefore, i t was s e l e c t e d f o r use i n most cases, a l s o because C C l ^ does not have i n t e r f e r i n g IR absorption bands w i t h the hydrocarbons. The m a t e r i a l that was removed from the adsorbed f i l m s was a l s o studied by IR spectrophotometry, by r e f l e c t i o n - a b s o r p t i o n and tran s m i s s i o n techniques. In the former case, the m i r r o r s containing the Mg carbonate substrate plus the adsorbed f i l m were c a r e f u l l y washed w i t h C Cl^, drop by drop. The. s o l u t i o n was c o l l e c t e d onto cl e a n gold m i r r o r s , where the solvent was allowed t o evaporate, thus l e a v i n g a f i l m of the s o l u t e . The procedure was prolonged u n t i l the amount of the d i s s o l v e d m a t e r i a l c o l l e c t e d onto the m i r r o r s was enough to a l l o w ,the recording of spectra by the MSRA technique. For the t r a n s m i s s i o n s p e c t r a , a s i m i l a r procedure was used, but the C C l ^ s o l u t i o n was c o l l e c t e d onto a KBr p l a t e , i n s t e a d . The e x t r a c t e d m a t e r i a l c o n s i s t e d of a c l e a r , viscous l i q u i d at room temperature, s t r o n g l y resembling a p a r t i a l l y polymerized epoxy r e s i n . I t d i f f e r e d markedly i n appearance from e i t h e r o l e i c a c i d or a soap (Mg or Na o l e a t e ) • The s p e c t r a showed i n t h i s work are f o r the f i l m s adsorbed under b a s i c c o n d i t i o n s ; however, the f i l m s obtained under moderately a c i d i c c o n d i t i o n s showed the same q u a l i t a t i v e aspects. The spectrum of a s t a r t i n g p a i r of m i r r o r s i n a des o r p t i o n experiment i s shown i n F i g . 33c. I t r e f e r s t o a Mg carbonate t h i n f i l m t r e a t e d i n a 119 10 M Na o l e a t e s o l u t i o n , at pH=10. The d i f f e r e n c e spectrum i n F i g . 34c represents the adsorbed c o l l e c t o r - d e r i v e d f i l m , as discussed i n the l a t t e r t o p i c . The MSRA and tr a n s m i s s i o n IR spec t r a of the m a t e r i a l removed by carbon t e t r a c h l o r i d e are di s p l a y e d i n F i g s . 36 and 37, r e s p e c t i v e l y . These spectra i n d i c a t e that the compound d i s s o l v e d by C C l ^ from the adsorbed f i l m was e s s e n t i a l l y an a l i p h a t i c epoxide compound, p a r t i a l l y polymerized t o a polyether s t r u c t u r e . This agrees p e r f e c t l y w i t h the previous conclusions from the spectra of the adsorbed compounds alone, regarding the changes i n the hydrocarbon chains. Some aspects are c l e a r e r now, s i n c e the transm i s s i o n spectrum i n F i g . 37 i s f r e e of any o r i e n t a t i o n e f f e c t . The MSRA spectrum ( F i g . 36) a l s o shows the bands w i t h r e l a t i v e i n t e n s i t i e s that are c l o s e r t o those i n a much more random o r i e n t a t i o n . This e f f e c t i s due mostly t o a more disorganized a t t i t u d e of the m a t e r i a l deposited from the C C l ^ s o l u t i o n . A l s o , the chemical changes associated w i t h the removal from the adsorbed f i l m s have c o n t r i b u t e d t o the more random o r i e n t a t i o n . The most i n t e r e s t i n g f e a t u r e i n the spectra of the removed m a t e r i a l i s that n e i t h e r c a r b o x y l i c a c i d nor carboxylate bands are present. A l l the major bands i n F i g s . 36 and 37 are associated w i t h hydrocarbon, epoxide and ether groups. The bands at 1457 and 1374 cm 1 are assigned to the s c i s s o r i n g of the CH^, and. symmetric bending of the CH^ groups, (73 79) -1 r e s p e c t i v e l y . ' ' The s m a l l bands, i n the 1740-1720 cm range are probably a s s o c i a t e d w i t h carbonyl groups, o r i g i n a t e d by f u r t h e r o x i d a t i o n of the e p o x i d e s / 1 ^ 7 ^ The absence of a p o l a r group i n the m a t e r i a l removed from the adsorbed f i l m i m p l i e s that the hydrocarbon chain was cleaved along a C-C bond, most probably next to the p o l a r group. Evidence f o r t h i s mechanism i s a l s o found 100 cu a c cd 4-1 O CL) CD 4000 F i g . 36 3500 3000 2500 2000 1500 1000 _ x 600 Wavenumber (cm ) . MSRA spectrum of the m a t e r i a l removed from the Mg carbonate front-covered m i r r o r s , p r e v i o u s l y treated i n Na o l e a t e s o l u t i o n at pH=10, by washing w i t h CC1,. Ordinate expansion: 1.53x. 100 cu u c cfl 4 J 4 J • H a CO C cd u H 4000 2500 600 Wavenumber (cm ) F i g . 37. Absorption (transmission) spectrum of the m a t e r i a l removed from the Mg carbon-ate front-covered m i r r o r s , p r e v i o u s l y t r e a t e d i n Na o l e a t e s o l u t i o n at pH-10, by washing with CCl^. Ordinate expansion: 0.55x. o I I I I I I I I 4000 3500 3000 2500 2000 1500 1000 , 600 Wavenumber (cm ) F i g . 38. MSRA spectrum of a Mg carbonate t h i n f i l m , t r e a t e d i n Na o l e a t e s o l u t i o n at pH=10, a f t e r washing w i t h CC1,. Ordinate expansion: 1.7Ox. 100 cu o c cd 4 J o cu r H 4-1 CU P4 4000 3500 3000 2500 2000 1500 1000 600 Wavenumber (cm ) Fig.39. Spectrum obtained by s u b t r a c t i o n of the spectrum of the Mg carbonate substrate ( F i g . 33), from the spectrum i n F i g . 38. Ordinate expansion: 2.15x. 122 i n the spectrum of the adsorbed f i l m that remained attached to the substrate. This i n f r a r e d MSRA spectrum i s d i s p l a y e d i n F i g . 38. The corresponding d i f f e r e n c e spectrum i s shown i n F i g . 39; i t was obtained by s u b t r a c t i n g the spectrum of the Mg carbonate substrate alone from the spectrum corresponding t o t h i s s ubstrate w i t h the adsorbed c o l l e c t o r , i . e . the spectrum i n F i g . 38 minus the spectrum i n F i g . 33. Those spectra prove that most of the carboxylate - compound remained adsorbed. The asymmetric s t r e t c h i n g frequency of t h i s group shows some changes ( F i g . 39) mostly due to the i n t e r a c t i o n w i t h the solvent and the p a r t i a l removal of m a t e r i a l ; t h i s i peak i s broader now, showing higher absorption i n the 1592-1570 cm range. No s i g n i f i c a n t changes are observed regarding the q u a l i t a t i v e f eatures of the hydrocarbon, epoxide and ether bands. This means that the p a r t i a l l y polymerized compound l e f t adsorbed on the surface has kept a l l i t s s i g n i f i c a n t f e a t u r e s . The only c r i t i c a l q u a l i t a t i v e change i n the spectrum of the remaining adsorbed f i l m i s the h i g h i n c r e a s e i n absorption i n the 1670-1600 cm 1 range. I t appears as a s e r i e s of broad shoulders on the asymmetric s t r e t c h i n g band of the carboxylate group i n F i g . 38 and e s p e c i a l l y i n F i g . 39. These shoulders are n e a r l y as strong as the l a t t e r band. The only compounds, which s t r o n g l y absorb i n t h i s r e g i o n , and are s t i l l compatible w i t h the present system, are carbonate complexes, probably of the bidentate t y p e / 4 1 ^ These complexes have maximum absorption i n the 1630-1590 cm 1 r e g i o n . A l l t h e i r other bands are not expected to be observable under the (41") present c o n d i t i o n s : v ' they would be e i t h e r covered by the t h i n f i l m carbonate and the organic m a t e r i a l , or too small t o be detected above the noise l e v e l . Some other modes that a l s o absorb near t h i s r e gion cannot i p o s s i b l y e x p l a i n the bands i n the 1670-1600 cm 1 range considering t h a t : 123 a strong band i s present near t h i s region i n the spectrum of C C l ^ ; ^ ^ however, i t s maximum i s at 1550 cm \ and i t f a l l s to near zero at 1600 cm 1 ; the d i f f e r e n t organic carbonyl v i b r a t i o n s are l o c a t e d at frequencies higher than 1700 cm \ mainly f o r a l i p h a t i c (73 79) compounds;v ' ' the s c i s s o r i n g v i b r a t i o n ^of water occurs i n t h i s region; however, t h i s mode must be accompanied by the s t r e t c h i n g modes, i n the 3600-3200 cm 1 range, which are u s u a l l y much stronger: t h i s i s not the case, s i n c e t h i s region does not show any s i g n i f i c a n t i n c r e a s e i n absorption, . when compared w i t h the spectra before the treatment w i t h C C l ^ ( F i g s . 34c and 39); t h e r e f o r e , water cannot p o s s i b l y be r e s p o n s i b l e f o r the bands i n the 1670-1600 cm \ e i t h e r . The assumption that the hydrocarbon c h a i n i s cleaved near the p o l a r group, upon d i s s o l u t i o n by CCl^, i s thus supported by s p e c t r a l data. The remaining p o r t i o n of the p o l a r group would be l e f t attached t o the s u b s t r a t e , as a bidentate carbonate complex. Further evidence i s provided by the spectra of the detached m a t e r i a l ( F i g s . 36 and 37): from the p o s i t i o n of the band near 800 cm \ i t can be concluded that i n the epoxide compound (of c i s - type) the oxirane r i n g was l o c a t e d near the centre of an a l i p h a t i c (73) chain. I f the cleavage had occurred away from the p o l a r group, the p o s i t i o n of the oxirane r i n g would be nearer to one of the ends of the detached chain. This would have t o be a s s o c i a t e d w i t h an observable s h i f t of (73) the r i n g v i b r a t i o n toward higher f r e q u e n c i e s . v ' 124 A l i p h a t i c c a r b o x y l i c a c i d s can be decarboxydized, y i e l d i n g CO^ plus hydrocarbons, i n homogeneous r e a c t i o n s . In the present case, however, due to the adsorbed s t a t e , the remnant p o l a r group would stay attached t o the substrate. The s p e c t r a of the m a t e r i a l e x t r a c t e d by C C l ^ from the adsorbed c o l l e c t o r f i l m are not a c r i t i c a l part of t h i s i n v e s t i g a t i o n , because removal of t h i s f i l m i s not u s u a l l y a s i g n i f i c a n t aspect of p r a c t i c a l nonmetallic f l o t a t i o n . However, these s p e c t r a l data can provide very u s e f u l i n d i r e c t evidence about the o r i g i n a l adsorbed l a y e r . A l s o , they present some very i n t e r e s t i n g f e a t u r e s of t h e i r own. The f o l l o w i n g f e a t u r e s are worth mentioning here: " -1 the s t r e t c h i n g v i b r a t i o n of the =C-H group (3010-3000 cm ) i s t o t a l l y absent i n these s p e c t r a : t h i s i s d e f i n i t e evidence that no double-bonds were present i n the removed m a t e r i a l ; although t h i s c o n c l u s i o n cannot be extended t o t a l l y to a l l the c o l l e c t o r - d e r i v e d adsorbed f i l m , i t c o n s t i t u t e s good evidence that the o x i d a t i o n of the double-bonds was of considerable extent; - the true band i n t e n s i t y r a t i o s can be evaluated i n a c o n d i t i o n of t o t a l l a c k of o r i e n t a t i o n e f f e c t s , mostly i n the tr a n s m i s s i o n spectrum, w i t h unpolarized r a d i a t i o n ( F i g . 37): one can thus see how the s t r e t c h i n g modes of the hydrocarbon (3000-2750 cm "*") bands are much stronger than the C-0 bands (1270 to 750 cm "*"); a dramatic demonstration of the molecular o r i e n t a t i o n e f f e c t s i n the r e f l e c t i o n - a b s o r p t i o n s p e c t r a i s evidenced, by comparing w i t h , say, F i g . 34c: i n t h i s spectrum, the hydrocarbon s t r e t c h i n g bands have approximately the same height as the C-0 ones. 125 as discussed i n Secti o n 4.2, i n r e f l e c t i o n - a b s o r p t i o n s p e c t r a , the bands due to organic m a t e r i a l s are not expected t o be s i g n i f i c a n t l y s h i f t e d i n r e l a t i o n to the r e s p e c t i v e t r a n s m i s s i o n reference s p e c t r a . An e x c e l l e n t c o n f i r m a t i o n of t h i s i s presented i n the spectra i n F i g s . 36 and 37. They represent the same m a t e r i a l , recorded under those two d i f f e r e n t techniques. The frequencies associated w i t h the same bands match very n e a r l y i n the two sp e c t r a . The only ones showing some degree of s h i f t i n g are the strong C-0 compound band between 1160 and 950 cm \ and the r i n g v i b r a t i o n of the epoxide around 800 cm ^. However, as p r e v i o u s l y mentioned, these bands are always notorious f o r showing s h i f t s i n the observed extension; the r e l a t i v e I n t e n s i t i e s between the organic oxide bands are not much d i f f e r e n t i n the o r i g i n a l adsorbed f i l m and i n the ex t r a c t e d m a t e r i a l . This i s important i n r e l a t i o n to the i n t e r p r e t a t i o n concerning molecular o r i e n t a t i o n i n the o r i g i n a l adsorbed f i l m , as p r e v i o u s l y discussed. In that d i s c u s s i o n , no o r i e n t a t i o n e f f e c t i n r e l a t i o n t o the r e l a t i v e heights of these bands between themselves was considered, although the r a t i o s between them and other bands ( l i k e the hydrocarbon bands) was f r e q u e n t l y considered. The evidence from F i g s . 36 and 37 i s a co n f i r m a t i o n that the organic oxide bands are not s e n s i t i v e to changes i n molecular o r i e n t a t i o n i n r e f l e c t i o n - a b s o r p t i o n spectra. The only exception i s that of the change i n i n t e n s i t y between the peaks i n the compound bands i n the 1160-950 cm 1 range, i n the tra n s m i s s i o n and MSRA spec t r a . F i g . 36 shows that o r i e n t a t i o n e f f e c t s , although small,, are s t i l l present i n the spectrum. This 126 i s confirmed by the d i f f e r e n c e i n the i n t e n s i t y r a t i o between the symmetric (2848 cm 1 ) and the asymmetric (2920 cm 1 ) s t r e t c h i n g v i b r a t i o n s of the methylene groups. However, the t o t a l ( i n t e g r a t e d ) i n t e n s i t y of the compound band i n the 1160-950 cm 1 has not changed much. Another dramatic example of the o r i e n t a t i o n e f f e c t s i n r e f l e c t i o n - a b s o r p t i o n spectra i s " the d i f f e r e n c e i n height between the s t r e t c h i n g hydrocarbon bands of the o r i g i n a l adsorbed f i l m ( F i g . 34c), and the remaining f i l m , a f t e r the removal of some m a t e r i a l by d i s s o l u t i o n i n C C l ^ ( F i g . 39). Although the c o n c e n t r a t i o n of organic m a t e r i a l was o b v i o u s l y g r e a t e r i n the untreated f i l m , the hydrocarbon bands are s u b s t a n t i a l l y smaller i n i t s spectrum, than i n the spectrum of the remaining f i l m . I t should be emphasized that both are d i f f e r e n c e spectra and p l o t t e d i n the same s c a l e , t h e r e f o r e under t o t a l l y comparable c o n d i t i o n s . The most important aspect of the studies on the treatment of the c o l l e c t o r - d e r i v e d adsorbed f i l m w i t h s o l v e n t s f o r organic m a t e r i a l s i s that they a l l o w an e v a l u a t i o n of the s t r e n g t h of the bonds developed between the substrate and the adsorbate. This e v a l u a t i o n should not be r e s t r i c t e d t o the ..(9) s o - c a l l e d "normal i n t e r a c t i o n " between the substrate and the p o l a r group of"the c o l l e c t o r . Information on the s t r e n g t h of the l a t e r a l bonds developed between the hydrocarbon chains of the adsorbed c o l l e c t o r i s a l s o h i g h l y d e s i r a b l e . In the present study, important i n f o r m a t i o n was "gained on both kinds of i n t e r a c t i o n s . The evidence t h a t the hydrocarbon c h a i n was cleaved near the p o l a r group, along a C-C bond, i s c l e a r i n d i c a t i o n that both normal and l a t e r a l bonds were very strong i n the adsorbed f i l m . This i s not s u r p r i s i n g c o n s i d e r i n g the chemisorption of the p o l a r groups i n the former case. A l s o , 127 the strong, covalent C-0-C bonds (p l u s the Van der Waals bonds) i n the semi-polymerized hydrocarbon chains were expected t o be very r e s i s t a n t t o d i s s o l u t i o n . However, p h y s i c a l adsorption was a l s o a prominent f e a t u r e i n the o r i g i n a l l y adsorbed m a t e r i a l under b a s i c c o n d i t i o n s (as discussed i n the l a t t e r t o p i c ) . So, some carboxylate would be expected t o have been removed along w i t h the d i s s o l v e d c o l l e c t o r , during washing w i t h CCl^,, by breaking of the weaker adsorption bonds. An explanation could be that the p h y s i c a l l y adsorbed p o l a r groups have developed a l a t e r a l bonding between themselves and the neighbour chemisorbed groups. According to t h i s assumption, a k i n d of two-dimensional network would be e s t a b l i s h e d on the surface of the Mg carbonate s u b s t r a t e , i n v o l v i n g chemisorbed complexes, physisorbed p o l a r groups and i n o r g a n i c surface complexes ( l i k e the already mentioned bidentate carbonate, S e c t i o n 6.3.2.c). Such networks have been proposed t o e x p l a i n the surface i m m o b i l i t y of physisorbed c o l l e c t o r f i l m s . ^ 9 ) This i m m o b i l i t y i s a c r i t i c a l step i n the a b i l i t y of the adsorbed f i l m to withstand strong d i s j o i n i n g h y d r a u l i c f o r c e s during the l i q u i d f i l m t h i n n i n g stage, f o r so l i d - b u b b l e attachment, l e a d i n g to the f l o t a t i o n process. The strength along t h i s surface network could reach such l e v e l s t h a t , under a detaching f o r c e such as i n the d i s s o l u t i o n by CCl^, small patches of the p a r t i a l l y - p o l y m e r i z e d hydrocarbon p o r t i o n would break way by cleavage of the C-C bonds neighbour t o the p o l a r groups. d) Adsorption of aqueous sodium oleate under an i n e r t atmosphere E x p l o r a t o r y a d s o r p t i o n t e s t s were c a r r i e d out under an Argon atmosphere, f o r purposes of comparison w i t h normal c o n d i t i o n s , i n which contact w i t h a i r was maintained throughout the experiments. The same -4 conc e n t r a t i o n of Na o l e a t e (10 M) was used; the pH was kept near 6.0; the contact time between the s o l u t i o n and the Mg carbonate substrate was 6 min. 128 The spectra obtained were very s i m i l a r to those obtained when contact w i t h a i r was allowed ( F i g s . 30 and 31), f o r the carboxylate bands as w e l l as f o r the hydrocarbon bands. However, no s i g n i f i c a n t bands were observed at the c h a r a c t e r i s t i c frequencies of the C-0 and epoxide modes. Thus, the c r i t i c a l i n f l u e n c e of contact w i t h a i r and the r e l a t e d o x i d a t i o n of the hydrocarbon chains at the C=C double bonds were f u r t h e r demonstrated. Due t o the e x p l o r a t o r y nature of these experiments, as w e l l as t o the molecular o r i e n t a t i o n e f f e c t s already-mentioned, no s i g n i f i c a n t q u a n t i t a t i v e r e l a t i o n s h i p s could be e s t a b l i s h e d , e) Adsorption of evaporated o l e i c a c i d Adsorption experiments were a l s o c a r r i e d out under t o t a l l y dry c o n d i t i o n s . The main purpose was t o compare the r e s u l t s that occurred at the s o l i d - l i q u i d i n t e r f a c e w i t h the adsorbate present as an aqueous s o l u t e , w i t h the a d s o r p t i o n under c o n d i t i o n s where water was p r a c t i c a l l y t o t a l l y absent. Another important c o n s i d e r a t i o n was that some in f o r m a t i o n could be obtained on the e f f e c t s of contact w i t h a i r only, during the adsorption and subsequent re a c t i o n s p e r i o d . This i s s i g n i f i c a n t i n r e l a t i o n t o the previous adsorption experiments at the s o l i d - l i q u i d i n t e r f a c e , when the f i l m s needed t o be removed from the s o l u t i o n , and d r i e d , before t h e i r spectra could be recorded. The consequences of d r y i n g and exposing the adsorbed f i l m t o the a i r could thus be at l e a s t p a r t i a l l y assessed. The spectrum of one of the Mg carbonate (magnesite) t h i n f i l m s used i n the experiment i s shown i n F i g . 40. This f i l m had an IR MSRA spectrum s i m i l a r t o others used i n the adsorption experiments ( F i g s . 30 and 33). The only major d i f f e r e n c e was that the f i l m used i n the adsorption of evaporated o l e i c a c i d d i d not show the t y p i c a l h y d r a t i o n band i n the 3600-3200 cm 1 range, u n l i k e the other t h i n f i l m s . This was because the sample had been 129 kept under dry c o n d i t i o n s always and was never allowed t o contact water, i n order t o assure not only a dry s u b s t r a t e s u r f a c e , but a l s o t o prevent any s i g n i f i c a n t h y d r a t i o n and/or h y d r o x y l a t i o n . O l e i c a c i d was evaporated and condensed onto the surface of the Mg carbonate f r o n t - c o a t e d Au m i r r o r s , f o l l o w i n g the procedure o u t l i n e d i n S e c t i o n 5.3.4. I n f r a r e d MSRA spec t r a were recorded f o l l o w i n g the d e p o s i t i o n , u n t i l s e v e r a l days a f t e r . The important spectra are shown i n F i g s . 40a, b, and c. The r e s p e c t i v e d i f f e r e n c e spectra are presented i n F i g . 41. The r e a c t i o n of o l e i c a c i d w i t h Mg carbonate under dry c o n d i t i o n s was much slower than under an aqueous environment. The spectrum i n F i g s . 40a and 41a was recorded approximately 1.5 hours a f t e r the d e p o s i t i o n . A conspicuous band i n the 1740-1680 cm 1 range shows that a s i g n i f i c a n t p r o p o r t i o n of the adsorbate was s t i l l present as unreacted n e u t r a l o l e i c a c i d . I t i s assigned t o the carbonyl group of the p o l a r head. The f i n e s t r u c t u r e of t h i s band shows s p l i t t i n g t o at l e a s t t r i p l e t s ( b e t t e r observed i n F i g . 4 l a ) . This means that the unreacted o l e i c a c i d was adsorbed under a v a r i e t y of polymeric s t r u c t u r e s regarding the a s s o c i a t i o n s of the p o l a r groups. The subpeaks at (73 79) the lower frequencies are r e l a t e d t o the dimer s t r u c t u r e , ' the one near 1720 cm 1 probably i n d i c a t e s a s i n g l e - b r i d g e , hydrogen-bonded , (46) a s s o c i a t i o n : R R I I - H 0 / C % — H 0 ^ C ^ 0 -The s m a l l shoulder at 1735 cm 1 may i n d i c a t e minor amounts of the monomer (74) f a t t y a c i d s . K ' The same spec t r a a f t e r 1.5 hours show that much of the o r i g i n a l o l e i c a c i d had already c h e m i c a l l y i n t e r a c t e d w i t h the s u b s t r a t e , forming a Mg 90 r -oo _ _ i L 4000 3500 3000 2500 2000 1500 1000 ^ 600 Wavenumber (cm ) F i g . 40. MSRA spectrum of a c r y s t a l l i n e Mg carbonate (magnesite) t h i n f i l m , before the adsorption of evaporated o l e i c a c i d . Ordinate expansion: 2.20x. Wavenumber (cm ) F i g . 40a. MSRA spectrum of the same Mg carbonate t h i n f i l m , lh30 a f t e r the adsorption of evaporated o l e i c a c i d . Ordinate expansion: 2.20x. R e f l e c t a n c e % R e f l e c t a n c e % IEI 100 6^8 CD C J C cfl •u o CJ CJ Pi 0) C J c cfl a CJ 4-1 CJ Pi 4000 3500 3000 2500 2000 1500 1000 600 Wavenumber (cm ) F i g . 41a. Spectrum obtained by the s u b t r a c t i o n of the spectrum of the adsorbent from the spectrum a f t e r the adsorption of evaporated o l e i c a c i d . The o r i g i n a l spectra are shown i n F i g s . 40. A f t e r lh30. Ordinate expansion: 3.15x. 100 4000 3500 3000 2500 2000 1500 1000 -1 600 Wavenumber (cm ) F i g . 41b. Same as i n F i g . 41a,but a f t e r 1 day. Ordinate expansion: 3.15x. R e f l e c t a n c e % 134 carboxylate. The c h a r a c t e r i s t i c asymmetric s t r e t c h i n g v i b r a t i o n of the carboxylate group (1595 cm 1 ) i s much stronger than the compound band at 1740-1680 cm 1 , d i a g n o s t i c of the unreacted o l e i c a c i d . A l s o , the symmetric mode of the carboxylate group (1455-1430 cm 1 ) i s a proof of the chemical r e a c t i o n , although i t i s not d i a g n o s t i c of the species formed. The chemisorbed Mg carboxylate complex formed on the sur f a c e at dry c o n d i t i o n s was of a d i f f e r e n t k ind as compared w i t h the other two types found a f t e r a d s o r p t i o n from aqueous s o l u t i o n s . The asymmetric s t r e t c h i n g v i b r a t i o n of the dry complex, at 1595 cm \ i s s i g n i f i c a n t l y s h i f t e d i n comparison t o the other v i b r a t i o n s of the Mg carboxylate complexes at 1582 and 1570 cm The r e a c t i o n of o l e i c a c i d w i t h Mg carbonate evolved s l o w l y , as shown by the spectrum 24 hours a f t e r the d e p o s i t i o n ( F i g . 41b). The carboxylate band at 1595 cm 1 has increased even f u r t h e r , w h i l e the c a r b o x y l i c a c i d carbonyl v i b r a t i o n s (1740-1680 cm - 1) are n o t i c e d only as small but sharp peaks at the s i d e of the former band. Ten days a f t e r i n i t i a t i o n the r e a c t i o n had not yet reached completion. However, the corresponding spectrum ( F i g . 4 l c ) shows th a t only very s m a l l amounts of unreacted c a r b o x y l i c a c i d were l e f t . The carbonyl bands are reduced t o sm a l l stepped shoulders i n the 1740-1680 cm 1 range. I t i s remarkable that even at such t r a c e l e v e l s , the same molecular a s s o c i a t i o n s of the n e u t r a l f a t t y a c i d were preserved, i . e . dimers and s i n g l e - b r i d g e s t r u c t u r e s . The same Mg- o l e a t e complex was formed at the surface during the 10-day p e r i o d , since the maximum of the asymmetric s t r e t c h i n g of the carboxylate group stayed near 1595 cm - 1. However, i n the 10-day spectrum, a shoulder appeared at 1577 cm 1 . This may mean that a d i f f e r e n t complex was s t a r t i n g 135 t o develop. The reason could be that the f i l m had absorbed some water from the a i r during the short periods i t was removed from the d e s i c c a t o r , where i t was kept, i n order t o record the spe c t r a . A small increase i n the absor p t i o n at 3500-3100 cm 1 could i n d i c a t e such minor hy d r a t i o n . The r e l a t i v e i n t e n s i t i e s of the s t r e t c h i n g v i b r a t i o n s of the hydrocarbon bands (3000-2800 cm ^) remained^ p r a c t i c a l l y unchanged, as compared to the reference t r a n s m i s s i o n spectra ( F i g s . 15 to 17). The r a t i o of the i n t e n s i t i e s between t h i s group of bands, and the s t r e t c h i n g v i b r a t i o n of the carboxylate group (1595 and 1440 cm ^) was a l s o unchanged. Therefore, no s i g n i f i c a n t o r i e n t a t i o n e f f e c t s were observed i n the adsorbed oleate l a y e r . . This random o r i e n t a t i o n c o n s t i t u e s a t o t a l l y d i f f e r e n t f e a t u r e , as contrasted w i t h the h i g h l y o r i e n t e d f i l m s formed by adsorption from an aqueous s o l u t i o n of the c o l l e c t o r . Another important c o n c l u s i o n , when adsorption under dry conditions 1 i s compared w i t h t h a t i n an aqueous environment, i s the l a c k of s i g n i f i c a n t o x i d a t i o n i n v o l v i n g the hydrocarbon chains. Some small bands occur i n the spect r a i n F i g . 41, that might i n d i c a t e minor o x i d a t i o n (1120 to 1040 cm 1 ) . However, they are not s i g n i f i c a n t l y above the noise l e v e l . The same occurs i n the r e l a t i o n t o the band a t 3020-3000 cm 1 ( s t r e t c h i n g v i b r a t i o n of the =C-H group). Since there was no important o x i d a t i o n , t h i s band d i a g n o s t i c of the double-bonds should be observable, but the noise l e v e l i s again too high i n that region, although a shoulder i s present i n the expected frequency. This confirms the co n c l u s i o n drawn i n previous s e c t i o n s that t h i s band, u s u a l l y very u s e f u l i n transmission work, i s not meaningful i n MSRA spe c t r a . The l a c k of o x i d a t i o n i n dr y - c o n d i t i o n s experiments, where t o t a l contact w i t h oxygen ( i n a i r ) was maintained c o n s t a n t l y , c o n s t i t u t e s extremely 136 val u a b l e i n f o r m a t i o n on the mechanism i n v o l v e d during the adsorption' from aqueous s o l u t i o n s , as discussed i n the f o l l o w i n g s e c t i o n . The experiments w i t h evaporated o l e i c a c i d were a l s o u s e f u l i n eva l u a t i n g the e f f e c t s of drying and exposure t o the a i r , f o l l o w i n g the removal from the c o l l e c t o r s o l u t i o n , i n the previous set of experiments. The adsorbed f i l m s formed under dry c o n d i t i o n s markedly d i f f e r e d from the ones formed at the s u b s t r a t e - s o l u t i o n i n t e r f a c e . These d i f f e r e n c e s were r e l a t e d to both the p o l a r group and the hydrocarbon ch a i n , i n c l u d i n g o r i e n t a t i o n e f f e c t s and chemical r e a c t i o n s regarding the l a t t e r . The f a c t that the same general type of compound was formed upon adsorption, i . e . a Mg carboxylate complex, emphasizes these d i f f e r e n c e s . Therefore, i t can be concluded that the c r i t i c a l c h a r a c t e r i s t i c s of the adsorbed f i l m s formed at the s u b s t r a t e - s o l u t i o n i n t e r f a c e were probably c o n t r o l l e d only by the c o n d i t i o n s p r e v a i l i n g i n t h i s environment. The subsequent removal from the s o l u t i o n , d r y i n g , and exposure t o the a i r are not expected to have changed s i g n i f i c a n t l y any of t h e i r c h a r a c t e r i s t i c s . Contact angle As i n the other cases, the contact angle between pure water and the c o l l e c t o r - c o a t e d Mg carbonate t h i n f i l m s was determined, 10 days a f t e r the evaporation of o l e i c a c i d . The method was e x a c t l y the same, and the c o n d i t i o n s t o t a l l y comparable. The r e s u l t s were as f o l l o w s : average contact angle: 61° - number of determinations: 8 ^ - standard d e v i a t i o n : 4.3° - confidence i n t e r v a l (at 90% l e v e l ) : +3° The surface was thus s t i l l hydrophobic, but at a lower l e v e l than any of the surface exposed t o the Na ol e a t e aqueous s o l u t i o n s . 137 f ) C o r r e l a t i o n between the adsorption mechanisms and the degree of hydrophobicity The r e s u l t s discussed i n the l a s t s e c t i o n s showed that a higher degree of hydrophobicity (as measured by the contact angles) was a s s o c i a t e d w i t h the Mg carbonate surface a f t e r the a d s o r p t i o n of Na o l e a t e under moderately a c i d i c c o n d i t i o n s , than at b a s i c pH values under comparable c o n d i t i o n s . This agrees w e l l w i t h the m i c r o f l o t a t i o n r e s u l t s that showed b e t t e r f l o t a t i o n r e c o v e r i e s i n the moderately a c i d i c range, f o r the same c o l l e c t o r c oncentrations ( S e c t i o n 6.2.1). This f l o t a t i o n behaviour of magnesite w i t h Na o l e a t e (and other a n i o n i c c o l l e c t o r s ) i s c o n s i s t e n t w i t h the c u r r e n t l y accepted t h e o r i e s on the f l o t a t i o n fundamentals of n o n m e t a l l i c m i n e r a l s , as discussed e l s e w h e r e Z ^ 2 ^ Here, only the aspects r e l a t e d t o the i n f r a r e d a d s o r p t i o n studies are discussed. The IR spectroscopic r e s u l t s have shown an o r i e n t a t i o n of the adsorbed c o l l e c t o r - d e r i v e d species mostly p a r a l l e l to the surface at a c i d i c c o n d i t i o n s ; the p r e f e r r e d molecular o r i e n t a t i o n at b a s i c values was nearer t o the normal t o the surface, although i n a l e s s organized way. I n the former case, the adsorbed l a y e r would contact the s o l u t i o n mostly by i t s CH 2 groups, and a l s o p o s s i b l y some of the C-0 groups. , An o r i e n t a t i o n nearer t o the v e r t i c a l would be expected t o expose mostly the t e r m i n a l CH^ groups of the hydrocarbon chains. Therefore, an apparent c o n t r a d i c t i o n e x i s t s w i t h the i d e a that CH^ -type monolayers would r e s u l t i n more hydrophobic surfaces than C ^ - t y p e monolayers. ( 1 ^ ) However, even i f s p e c i f i c patches on the surface t r e a t e d under ba s i c c o n d i t i o n s were more hydrophobic, t h i s does not mean that a l l the surface would be e q u a l l y hydrophobic. This f i l m , being l e s s uniform, would present patches where the Mg carbonate would be only p a r t i a l l y covered by the c o l l e c t o r , or even not coated at a l l . Therefore, on the average, t h i s 138 surface could s t i l l be l e s s hydrophobic than the one f o l l o w i n g the adsorption at a c i d i c c o n d i t i o n s . Another important observation i s that a r e c l i n i n g molecule of oleate presents a much l a r g e r p r o j e c t e d area than one standing near the v e r t i c a l ; f o r the same c o l l e c t o r c o n c e n t r a t i o n i n s o l u t i o n , a l a r g e r surface coverage would be a t t a i n e d by the h o r i z o n t a l molecules than by the v e r t i c a l ones. Therefore, a more hydrophobic surface could be formed by the h o r i z o n t a l - l y i n g species, even i f t h e i r i n t r i n s i c a t t r a c t i o n f o r water were higher at t h i s p o s i t i o n . The occurrence of o x i d a t i o n and p o l y m e r i z a t i o n was lower i n the f i l m formed under ba s i c c o n d i t i o n s . The degree of l a t e r a l bonding between the hydrocarbon chains was thus smaller, r e s u l t i n g i n a l e s s compact and uniform l a y e r . This a l s o c o n t r i b u t e d t o a net lower hydrophobicity, even considering that the s p e c i f i c a t t r a c t i o n t o water would probably be lower f o r a non-oxidized o l e a t e molecule. Strong evidence on the c o n f l i c t i n g i n f l u e n c e of the degree of molecular o r i e n t a t i o n and the occurrence of o x i d a t i o n was presented by the adsorption t e s t w i t h o l e i c a c i d from the vapour phase. The r e s u l t i n g f i l m d i d not show any s i g n i f i c a n t o x i d a t i o n and a s s o c i a t e d p o l y m e r i z a t i o n ( S e c t i o n 6.3.3d). The adsorbed molecules a t t a i n e d mostly a random a t t i t u d e . This f i l m showed a smaller contact angle than any of the f i l m s formed from aqueous s o l u t i o n s , where o x i d a t i o n and p o l y m e r i z a t i o n were ext e n s i v e . Therefore, t h i s study showed evidence that the highest degrees of hydrophobicity i n the system Mg carbonate-aqueous Na o l e a t e were associated w i t h the formation of uniform, compact and s t a b l e adsorbed l a y e r s , w i t h the molecules s t r o n g l y o r i e n t e d n e a r l y p a r a l l e l t o the s o l i d s u b s t r a t e . The c o n c l u s i o n that a higher degree of hydrophobicity was associated w i t h an o r i e n t a t i o n of the adsorbed c o l l e c t o r species p a r a l l e l to the s o l i d 139 surface has been proposed a l s o by other authors. For example, i n the b a r i t e - f a t t y a c i d s system ( i n c l u d i n g Na o l e a t e ) , t h i s o r i e n t a t i o n was i n f e r r e d from c a l c u l a t i o n s using the adsorption d e n s i t i e s and the l i m i t i n g areas of the c o l l e c t o r species. ^ "^^ 6.4 Oxi d a t i o n of O l e i c A c i d and Soaps Outstanding e f f e c t s due t o the o x i d a t i o n of the unsaturated hydrocarbon chains of adsorbed oleate were discussed i n the previous s e c t i o n s . The o x i d a t i o n of o l e i c a c i d and other unsaturated f a t t y a c i d s and t h e i r soaps has not received much a t t e n t i o n as f a r as t h e i r a b i l i t y as c o l l e c t o r s i s concerned, d e s p i t e the enormous importance of these compounds i n the processing of nonmetallic m i n e r a l s . I t has long been recognized that the unsaturated f a t t y a c i d s are f a r b e t t e r c o l l e c t o r s than t h e i r saturated homologues, even the ones w i t h s h o r t e r hydrocarbon chains. A great d i f f e r e n c e i n water s o l u b i l i t y at room temperature has been t r a d i t i o n a l l y considered as the major reason f o r t h i s behaviour. (^5) There i s no doubt that the much higher s o l u b i l i t y of the unsaturated f a t t y a c i d soaps i s a d e c i s i v e f a c t o r i n t h e i r b e t t e r performance. However, other aspects are important t o e x p l a i n the d i f f e r e n t a b i l i t y of the f a t t y a c i d s as c o l l e c t o r s , even among the s e v e r a l unsaturated ones, such as l i n o l e i c and l i n o l e n i c a c i d s . The d i f f e r e n t c r o s s - s e c t i o n a l areas of t h e i r molecules were considered important, s i n c e they would i n f l u e n c e the formation of compact monolayers upon adsorption, on the same mineral surface and comparable A.„. (102) c o n d i t i o n s . Another important f a c t o r may be a tendency toward o x i d a t i o n and subsequent p o l y m e r i z a t i o n of the unsaturated c a r b o x y l i c a c i d s . As e a r l y as 1954, t h i s has been suggested as a major reason t o e x p l a i n the outstanding c o l l e c t o r p r o p e r t i e s of o l e i c a c i d , as compared to the saturated f a t t y a c i d s . This reasoning was based on the disappearance of i n f r a r e d bands 140 a s s o c i a t e d w i t h the double-bonds, a f t e r adsorption of oleate on f l u o r i t e . H o w e v e r , l i t t l e a t t e n t i o n has been g i v e n t o t h i s chemical aspect of these reagents i n the l a s t decades. Most of the research on the o x i d a t i o n of the unsaturated f a t t y a c i d s has been done by workers r e l a t e d t o the f a t s and o i l s i n d u s t r y . Therefore, the vast m a j o r i t y of the a v a i l a b l e l i t e r a t u r e i s concerned w i t h the n e u t r a l f a t t y a c i d s and t h e i r e s t e r s , i n non-aqueous environments. The o x i d a t i o n of chemical reagents i n g e n e r a l , ^ ^ 8 ^ and p a r t i c u l a r l y by atmospheric oxygen, a u t o x i d a t i o n , h a v e been e x t e n s i v e l y s t u d i e d , under these c o n d i t i o n s . However, the r e a c t i o n s i n v o l v i n g the a l k a l i n e soaps, and even the n e u t r a l a c i d s , i n aqueous s o l u t i o n or emulsions have not been the subject of extensive research, despite some outstanding c o n t r i b u t i o n s . The o x i d a t i o n of the unsaturated f a t t y a c i d s i s u s u a l l y a r a t h e r complex phenomenon, e s p e c i a l l y i n the case of a u t o x i d a t i o n . Many intermediary compounds are present. These r e s u l t i n a m u l t i p l i c i t y of daughter products. The nature and r e l a t i v e p roportions of these products are h i g h l y s e n s i t i v e t o even s l i g h t changes i n the c o n d i t i o n s during the pr o c e s s . ^ ^ 8 ' 1 0 9 ) However, the i n i t i a l stages tend t o be r e l a t i v e l y simple. The most accepted theory e x p l a i n i n g the i n i t i a l phase i s r e f e r r e d t o as the "hydroperoxide theory". E s s e n t i a l l y , i t proposes t h a t the f i r s t step i n the o x i d a t i o n process i s not the rupture of the double-bonds, but the r e a c t i o n w i t h one of the hydrogens of the neighbour methylene groups, i n each molecule. Therefore, f o u r isomers of monohydroperoxides would be formed, s t i l l unsaturated. One of them would b e : ^ 1 ^ ^ H H H H H H - c - c - c = c - c - c -I I I I H I H H 0-OH 141 These hydroperoxides are l a b i l e compounds, r e a d i l y undergoing f u r t h e r r e a c t i o n , that r e s u l t s i n a v a r i e t y of compounds i n which the double-bonds are t o t a l l y or p a r t i a l l y consumed. One of the most important secondary compounds i s an epoxide, w i t h the oxirane r i n g p o s i t i o n e d at the s i t e of the former double bond; i n the case of o l e i c a c i d , . 9-epoxystearic a c i d i s f o r m e d / 1 0 8 ' 1 0 9 ) The few references on the a u t o x i d a t i o n of f a t t y a c i d s i n aqueous systems show that water has a tendency t o enhance the r e a c t i o n . ^ 1 0 8 ^ Water contents as low as 0.1% are s a i d to g r e a t l y increase the r e a c t i o n of o l e i c . ^ L J C • ^ • « , , j (109) a c i d w i t h oxygen, w i t h the formation of epoxide and peroxide groups. Another important evidence on the i n f l u e n c e of an aqueous s o l u t i o n as opposed t o a dry c o n d i t i o n i n the a u t o x i d a t i o n of o l e i c a c i d i s presented i n the a d s o r p t i o n t e s t s from the vapour phase, i n t h i s i n v e s t i g a t i o n . No s i g n i f i c a n t o x i d a t i o n was observed under these c o n d i t i o n s , c o n t r a r y t o the a d s o r p t i o n from aqueous s o l u t i o n s , where o x i d a t i o n was extensive. I t should be emphasized that the c o n d i t i o n s during the evaporation of o l e i c a c i d would tend to enhance the r e a c t i o n w i t h oxygen, as f o l l o w s : - - higher contact between the o l e i c a c i d molecules and a i r , during the evaporation stage, and f o l l o w i n g the adsorption; - the r e l a t i v e l y h i g h temperature necessary f o r evaporating o l e i c a c i d at atmospheric pressure: i t s b o i l i n g p o i n t i s 223°C. The evidence i s that i n the present study where contact w i t h the a i r was allowed during a l l the stages of the a d s o r p t i o n t e s t s , a u t o x i d a t i o n was the dominant process. I t seems that i t occurred i n c o n d i t i o n s t h a t were mil d e r and simpler than i n the general cases r e f e r r e d t o i n the l i t e r a t u r e . This i s mainly because - a r e l a t i v e l y d i l u t e d s o l u t i o n of the soap was used. A l s o , the s o l u t i o n s were used a f t e r r e l a t i v e l y short times a f t e r being 142 f r e s h l y prepared (1 t o 3 hours). The f a c t that only one o x i d i z e d compound was found i n the adsorbed f i l m s i s proof of these m i l d e r o x i d i z i n g c o n d i t i o n s . (A second compound, the polyether, i s not a f u r t h e r o x i d a t i o n stage, but a product of the p o l y m e r i z a t i o n of the epoxystearate). An important question i n the present i n v e s t i g a t i o n that could not be explained unambigously by the study of the adsorbed c o l l e c t o r f i l m alone i s : has the epoxystearic compound formed only at the i n t e r f a c e , upon the adsorption; or d i d i t already e x i s t i n s o l u t i o n before the adsorption? To answer these questions experiments were c a r r i e d out, t o t e s t the extension of the a u t o x i d a t i o n of the c o l l e c t o r i n s o l u t i o n . Following a reg u l a r adsorption t e s t at pH=10, a sample of the s o l u t i o n was taken and evaporated under vacuum at room temperature. I t was then thoroughly d r i e d f o r 5 hours a t approximately 60°C. The s o l i d residue was s t u d i e d by i n f r a r e d t r a n s m i s s i o n spectroscopy (KBr p e l l e t ) . The same procedure was repeated f o r Na ole a t e s o l u t i o n s of the same conc e n t r a t i o n and pH, but no adsorption was c a r r i e d out, i . e . no contact w i t h any s o l i d substrates was allowed, aside from the T e f l o n beakers used. Samples were taken from these s o l u t i o n s at various times a f t e r the p r e p a r a t i o n , from 2 to 24 hours; contact w i t h the r a i r and slow a g i t a t i o n were maintained throughout. The spectrum i n F i g . 42 corresponds to the sample removed from the s o l u t i o n a f t e r 2 hours (the most r e p r e s e n t a t i v e of the s o l u t i o n c o n d i t i o n s during a d s o r p t i o n experiments). A l l the other samples showed the same c r i t i c a l q u a l i t a t i v e aspects. The important features are as f o l l o w s : no band c h a r a c t e r i s t i c of the epoxide compound i s present i n the spectrum; the same i s tr u e i n r e l a t i o n to the polyether product; 4000 3000 600 Wavenumber (cm ) ,Fig. 42. Absorption (transmission) spectrum of Na o l e a t e , removed from a 10 M s o l u t i o n , at pH=10.0, a f t e r 2 hours, and evaporated. Ordinate expansion: 0.45x. 4> 144 a s m a l l , but s u b s t a n t i a l amount of hydroperoxide was already formed: i t s most c h a r a c t e r i s t i c band occurs at 846 cm 1 (850 t o 830 cm "*"), due t o v i b r a t i o n of the 0-0 group; the band at 3460 cm 1 i s assigned to the hydroxyl s t r e t c h i n g v i b r a t i o n of the 0-0H group. - the double-bonds were p r a c t i c a l l y u n a l t e r e d , as derived from the band at 3005 cm ^; - the p o l a r group was a l s o unchanged, showing the asymmetric s t r e t c h i n g v i b r a t i o n of the carboxylate at e x a c t l y the same ! frequency as the reference anhydrous Na o l e a t e , 1560 cm ^; an unimportant aspect of the spectrum i n F i g . 42 was the presence of n e u t r a l c a r b o x y l i c a c i d , i n d i c a t e d by the broad band at' 1720-2680 cm ^; i t was probably present as an acid-soap compound,^ 1 1 2^ a common occurrence i n p r e c i p i t a t i o n from soap s o l u t i o n s at n e u t r a l to medium a l k a l i n e c o n d i t i o n s . The only s i g n i f i c a n t d i f f e r e n c e between the s p e c t r a of the samples removed from the Na oleate s o l u t i o n a f t e r longer periods was an increase i n the i n t e n s i t y of the hydroperoxide bands. The sample taken a f t e r 24 hours showed an i n c r e a s e of 2.8x i n r e l a t i o n to the 2 hours sample, measured by the i n t e g r a t e d areas ( i n absorbances) under the band at 846 cm 1 . The same method was a p p l i e d t o evaluate the p o s s i b l e changes i n the i n t e n s i t y of the band at 3005 cm 1 , c h a r a c t e r i s t i c of the double-bond; i t s i n t e g r a t e d area was compared to the one of a s t a b l e and r e l i a b l e band, the symmetric s t r e t c h i n g of the methylene, at 2850 cm \ f o r each spectrum, i n c l u d i n g a standard anhydrous sample of Na ol e a t e ( l i k e the spectrum i n F i g . 16). The r e s u l t s showed no s i g n i f i c a n t changes f o r the samples at 2 and 6 hours; f o r 145 the sample a f t e r 24 hours, some minor decrease occurred, but at l e a s t 96% of the o r i g i n a l double-bonds, were s t i l l preserved. No s i g n i f i c a n t changes were found between the s o l u t i o n contacted w i t h the m i r r o r s , and the ones where no contact was allowed. This was tr u e f o r both q u a l i t a t i v e and q u a n t i t a t i v e aspects. The most important c o n c l u s i o n of t h i s study on the aqueous s o l u t i o n s of Na ol e a t e i s that the only appreciable change i n the c o l l e c t o r ions was a small formation of hydroperoxide, under the c o n d i t i o n s set up during the adsor p t i o n t e s t s . This i s an expected r e s u l t , according to the accepted t h e o r i e s on a u t o x i d a t i o n of f a t t y a c i d s , and s t r o n g l y confirms the m i l d o x i d i z i n g c o n d i t i o n s during the experiments. A very important i m p l i c a t i o n of the study of the Na ole a t e s o l u t i o n s i s that the e p o x i d i z i n g r e a c t i o n must have taken place at the surface, since i t d i d not occur i n the bulk s o l u t i o n . This provides strong evidence that t h i s r e a c t i o n followed a heterogeneous c a t a l y t i c mechanism i n t h i s case. C a t a l y t i c e f f e c t s of surfaces have already been observed, as i n the case of s i l i c a powders and unsaturated o i l s / ' ' " 0 9 ) A l s o , i t has been found that epoxystearic a c i d from o l e i c a c i d could only be' formed i n homogeneous aqueous systems when strong o x i d i z i n g agents l i k e potassium permanganate were p r e s e n t / 1 1 3 ) A few references were found i n the l i t e r a t u r e that d i r e c t l y or i n d i r e c t l y support the f i n d i n g s of t h i s i n v e s t i g a t i o n . In none of these works, however, i s the f u l l mechanism of a u t o x i d a t i o n , heterogeneous c a t a l y t i c e p o x i d i z a t i o n , and p a r t i a l p o l y m e r i z a t i o n of the adsorbed compound explained i n a more complete way. Epoxide and peroxide compounds, forming a quasipolymeric adsorbed f i l m , were i d e n t i f i e d by d i f f e r e n t i a l t r a n s m i s s i o n i n f r a r e d spectrophotometry 146 i n one s t u d y . ^ l l 4 ^ This f i l m was formed on the surface of oxide minerals, f o l l o w i n g the adsorption of o l e i c a c i d from aqueous s o l u t i o n s , i n the presence of i n o r g a n i c modifying agents. In another work, the a d s o r p t i o n of aqueous Na o l e a t e on o x i d i z e d aluminum t h i n f i l m s was studied by s i n g l e r e f l e c t i o n - a b s o r p t i o n IR spectroscopy. Formation of polymeric adsorbed f i l m s was proposed, based on the disappearance of the bands as s o c i a t e d w i t h the C=C double-bonds.^ 1 1"^ There was no mention of o x i d a t i o n of these bonds, however; the s p e c t r a l r e g i o n where the C-0 bonds would have appeared i s not shown nor r e f e r r e d to i n t hat work. Ad s o r p t i o n of o l e i c a c i d from C C l ^ s o l u t i o n s onto s i l i c a powder was studied by t r a n s m i s s i o n IR s p e c t r o s c o p y . ^ 1 1 6 ^ A marked decrease i n the i n t e n s i t y of the band at 3008 cm 1 (corresponding t o the C=C double-bond) was n o t i c e d i n the spectra of the adsorbed o l e i c a c i d . The authors i n t e r p r e t e d t h i s as a p e r t u r b a t i o n by the surface of the alkene groups w i t h i n the a l k y l chains of the adsorbed molecules. However, the s p e c t r a showed only the r e g i o n above 1600 cm 1 , i . e . where no C-0 bonds could be formed. The same d r a s t i c decrease i n the =C-H band near 3010 cm 1 was a l s o observed i n the f o l l o w i n g systems, a l l using organic s o l v e n t s : - l i n o l e i c a c i d on s i l i c a ; ^ 1 1 ^ - l i n o l e i c a c i d on alumina; ( H ^ ) ,- o l e i c a c i d on r u t i l e ; ^ 1 1 6 ^ The o x i d a t i o n of o l e i c a c i d and i t s soaps i s probably at l e a s t p a r t i a l l y r e s p o n s i b l e f o r the higher f l o t a t i o n r e c o v e r i e s f o r oxide minerals i n a i r , as opposed t o oxygen-free c o n d i t i o n s , as reported i n another study. ( H ^ ) S i m i l a r l y , i t can a l s o p a r t i a l l y e x p l a i n the observed increases i n the f l o t a t i o n r e c o v e r i e s of f l u o r i t e , c a l c i t e , a c t i v a t e d quartz and phosphorite a s s o c i a t e d w i t h increased concentrations of oxygen d i s s o l v e d 147 i n the water, when o l e i c a c i d and other unsaturated f a t t y acids are used as „ _ s (120, 121) c o l l e c t o r s . ' The only d e n i a l of the o x i d a t i o n of o l e i c a c i d f o l l o w i n g adsorption and f l o t a t i o n found i n he l i t e r a t u r e was a b r i e f t e c h n i c a l note published i n (122) 1953. In t h i s study, f l u o r i t e was f l o a t e d w i t h o l e i c a c i d , and the adsorbed c o l l e c t o r was removed by ethanol, and the i o d i n e number determined. The observed i o d i n e numbers were very c l o s e t o the ones of unreacted o l e i c a c i d , thereby i n d i c a t i n g that no s i g n i f i c a n t o x i d a t i o n had occurred. However, the authors acknowledged that the r e s u l t s of IR spectroscopy were i n c o n c l u s i v e i n proving or d i s p r o v i n g the o x i d a t i o n of the double-bonds. This i s not s u r p r i s i n g , c o n s i d e r i n g the r e l a t i v e l y i n a c c u r a t e instruments a v a i l a b l e at that time, and the technique used - - trans m i s s i o n spectroscopy. A l s o , the f a c t that the i o d i n e number d i d not decrease i s not proof that o x i d a t i o n d i d not occur. Compounds l i k e the epoxides - - common daughter products i n the a u t o x i d a t i o n of unsaturated f a t t y a c i d s - - a l s o (93) react w i t h i o d i n e . ' Furthermore, at the e a r l i e r stages of the a u t o x i d a t i o n mechanism, where hydroperoxides and epoxides are formed, the i o d i n e number decreases only s l o w l y / 1 0 7 ^ Therefore, the arguments (122) , presented i n the work c i t e d against the o x i d a t i o n of o l e i c a c i d are not j u s t i f i e d , e s p e c i a l l y i n view of more recent evidence. The r e s u l t s of t h i s i n v e s t i g a t i o n , plus the a v a i l a b l e i n f o r m a t i o n i n the l i t e r a t u r e , a l l o w the f o l l o w i n g mechanism to be proposed, f o r the o x i d a t i o n of o l e a t e under the c o n d i t i o n s e s t a b l i s h e d : Oleate ( a l s o o l e i c a c i d i n a c i d i c c o n d i t i o n s ) was p a r t i a l l y a u t o x i d i z e d t o hydroperoxide, w i t h p r a c t i c a l l y a l l the double-bonds preserved; a s i g n i f i c a n t p r o p o r t i o n of unreacted oleate s t i l l e x i s t e d i n s o l u t i o n . The p o l a r groups were t o t a l l y unaffected by the a u t o x i d a t i o n process. 148 The p a r t i a l l y o x i d i z e d oleate adsorbed onto the Mg carbonate s u b s t r a t e , w i t h the formation of chemisorbed complexes; depending on the c o n d i t i o n s , p h y s i s o r p t i o n a l s o occurred e x t e n s i v e l y . The hydroperoxides were changed completely t o epoxides by a c a t a l y t i c process t a k i n g place only at the m i n e r a l - s o l u t i o n i n t e r f a c e . The adsorbed species were thus converted to cis-9-epoxystearate complexes. This epoxide compound p a r t i a l l y polymerized to a polyether s t r u c t u r e w i t h the formation of C-O—C bonds between the hydrocarbon chains. This organic f i l m formed at the i n t e r f a c e had very strong bonds w i t h the substrate as w e l l as strong, c o v a l e n t , l a t e r a l bonds, complemented by Van der Waals bonds, between the hydrocarbon chains. These semipolymerized f i l m s changed the o r i g i n a l h y d r o p h i l i c surfaces t o h i g h l y hydrophobic ones. A u t o x i d a t i o n and the a s s o c i a t e d phenomena on adsorbed f i l m s are most probably i n v o l v e d i n the d i f f e r e n t f l o t a t i o n response of f a t t y a c i d (102 123) c o l l e c t o r s w i t h increased degrees of u n s a t u r a t i o n . ' ' In t h i s study, the Mg carbonate s u r f a c e , coated w i t h the o x i d i z e d and semipolimerized oleate c o l l e c t o r , was s t i l l h i g h l y hyrophobic. However, when more double-bonds are present i n the o r i g i n a l a c i d molecule (as i n l i n o l e i c and l i n o l e n i c a c i d s ) , the r e l a t i v e number of r e s u l t i n g oxygen bonds would be so h i g h t h a t a net decrease i n the degree of hydrophobicity could occur. This would be one e x p l a n a t i o n f o r the decrease i n the f l o t a t i o n response w i t h increased . j -i (102,123) unsat u r a t i o n , reported by some authors. F i n a l l y , the i n f l u e n c e of the o x i d a t i o n of unsaturated c o l l e c t o r s under p r a c t i c a l c o n d i t i o n s should be considered. Since the parameters a s s o c i a t e d w i t h t h i s o x i d a t i o n played such an important r o l e under the s t r i n g e n t c o n d i t i o n s employed i n t h i s i n v e s t i g a t i o n , the e f f e c t s under c o n d i t i o n s p r e v a i l i n g i n the a c t u a l f l o t a t i o n p r a c t i c e of nonmetallic 149 minerals should be i n c r e a s e d . The very high l e v e l of a e r a t i o n i n the f l o t a t i o n c e l l s , the i n t e n s e s t i r r i n g , the h i g h percentage of s o l i d s , the g r e a t e r surface area of s t r o n g l y o x i d i z e d minerals, and the more concentrated c o l l e c t o r s o l u t i o n s , i n c l u d i n g compounds w i t h m u l t i p l e double-bonds, a l l would c o n t r i b u t e t o i n c r e a s i n g the extension and the e f f e c t s of a u t o x i d a t i o n . The nature, c o n c e n t r a t i o n and combination of the r e g u l a t i n g reagents (such as a c t i v a t o r s and depressants), almost i n v a r i a b l y used i n nonmetallic f l o t a t i o n under p r a c t i c a l c o n d i t i o n s , should a l s o have a v a r i a b l e , strong i n f l u e n c e . 150 CHAPTER 7 SUMMARY AND CONCLUSIONS The i n f r a r e d m u l t i p l e specular r e f l e c t i o n - a b s o r p t i o n (MSRA) spectrophotometry technique was used t o i n v e s t i g a t e the i n t e r a c t i o n between magnesium carbonate (magnesite) s u b s t r a t e s , and aqueous s o l u t i o n s of sodium o l e a t e , i n contact w i t h a i r . This was complemented by the conventional tr a n s m i s s i o n technique. M i c r o f l o t a t i o n t e s t s were c a r r i e d out using a pure sample of magnesite, and sodium oleate as the main c o l l e c t o r . The MSRA method proved t o be extremely s e n s i t i v e , a c c u r a t e l y d e t e c t i n g c l o s e l y r e l a t e d surface complexes. A l s o , very u s e f u l i n f o r m a t i o n on the o r i e n t a t i o n of the adsorbed species was obtained. However, t h i s same o r i e n t a t i o n e f f e c t prevented the a c q u i s i t i o n of r e l i a b l e q u a n t i t a t i v e data, because the r e l a t i v e band i n t e n s i t i e s were a f f e c t e d s u b s t a n t i a l l y . The r e a c t i v i t y of the C=C double-bonds i n the hydrocarbon c h a i n of oleate proved to play an important r o l e i n determining the c h a r a c t e r i s t i c s of the adsorbed c o l l e c t o r f i l m s , both under b a s i c and moderately a c i d i c c o n d i t i o n s . Hydroperoxides formed i n s o l u t i o n upon o x i d a t i o n by atmospheric oxygen ( a u t o x i d a t i o n ) . These compounds changed to epoxides at the mi n e r a l surface, f o l l o w i n g a heterogeneous c a t a l y t i c mechanism. A p a r t i a l p o l y m e r i z a t i o n to a polyether s t r u c t u r e was the f i n a l stage. This epoxide-polyether adsorbed f i l m was s t r o n g l y bound by i n t e r c h a i n bonds (mostly covalent C-0, and a l s o Van der Waals bonds). The p o l a r groups of the o r i g i n a l o l e a t e were unaffected by these d r a s t i c changes i n the hydrocarbon ch a i n . Despite r e a c t i o n s i n v o l v i n g the hydrocarbon chains, the f i n a l p r o p e r t i e s of the adsorbed f i l m were c o n t r o l l e d mostly by the i n t e r a c t i o n 151 between the magnesium carbonate substrate and the p o l a r groups of the c o l l e c t o r . At moderately a c i d i c c o n d i t i o n s , the magnesium carbonate surface was p o s i t i v e l y charged, or had a n e a r l y zero charge. I t was a l s o covered by a s t a b l e , s t r o n g l y hydrogen-bonded hy d r a t i o n l a y e r . The i n t e r a c t i o n between the substrate and the p o l a r group of the c o l l e c t o r was of chemical nature only, w i t h the formation of a surface complex very s i m i l a r t o the bulk 2-magnesium ol e a t e . This r e a c t i o n d i s p l a c e d CO^ ions from the s u b s t r a t e . The adsorbed f i l m formed was extremely homogenous, w i t h the hydrocarbon chains a l i g n e d p r e f e r e n t i a l l y p a r a l l e l to the surface. A high degree of hydrophobicity was achieved, which was p r i m a r i l y responsible f o r -the high f l o t a t i o n r e c o v e r i e s reached at a c i d i c c o n d i t i o n s . At basic c o n d i t i o n s , the magnesite surface was n e g a t i v e l y charged and was a l s o covered by a s t r o n g l y hydrogen-bonded h y d r a t i o n l a y e r . Only a r e l a t i v e l y s m a ll amount of c o l l e c t o r was chemisorbed on the s u b s t r a t e , probably because of the charge b a r r i e r . A slower p h y s i c a l adsorption a l s o took place. However, the r e s u l t i n g adsorbed l a y e r was r e l a t i v e l y heterogeneous, showing d i f f e r e n t molecular o r i e n t a t i o n s i n i t s s t r u c t u r e . These o r i e n t a t i o n s v a r i e d from n e a r l y p a r a l l e l t o the s u r f a c e , to s t e e p l y i n c l i n e d , nearer t o the normal t o the s u b s t r a t e . This more diso r g a n i z e d s t r u c t u r e , plus a lower occurrence of e p o x i d i z a t i o n and p o l y m e r i z a t i o n , were the reasons f o r a s i g n i f i c a n t l y lower degree of hydrophobicity of the c o l l e c t o r - c o a t e d s u rface. This was c o n s i s t e n t w i t h the lower f l o t a t i o n responses at b a s i c c o n d i t i o n s , which needed stronger c o l l e c t o r s o l u t i o n s to reach the same recoveries obtained at moderately a c i d i c , c o n d i t i o n s . 152 CHAPTER 8 RECOMMENDATIONS FOR FURTHER WORK The va l u a b l e q u a l i t a t i v e i n f o r m a t i o n obtained w i t h the MSRA i n f r a r e d spectroscopy, should be complemented by t r a n s m i s s i o n techniques. Although t r a n s m i s s i o n methods are f a r l e s s s e n s i t i v e , they could provide important q u a n t i t a t i v e data on such aspects as the number of absorbed monolayers, and a d s o r p t i o n d e n s i t i e s under d i f f e r e n t c o n d i t i o n s . D i g i t a l computer techniques were used e x t e n s i v e l y i n t h i s work, and are p a r t i a l l y r e s p o n s i b l e f o r the h i g h q u a l i t y of the s p e c t r a . Other more s o p h i s t i c a t e d computer programs could be used t o i n c r e a s e the q u a l i t y and the amount of i n f o r m a t i o n t h a t could be obtained from e x i s t i n g s p e c t r a l data. Among these, the techniques of band contour a n a l y s i s and spectra deconvolution should be p a r t i c u l a r l y u s e f u l . The e f f e c t s of o x i d a t i o n on the c o l l e c t o r and s u r f a c t a n t p r o p e r t i e s of the unsaturated f a t t y a c i d s should be more e x t e n s i v e l y s t u d i e d a l s o : comparative experiments under c o n t r o l l e d d i f f e r e n t redox c o n d i t i o n s at various temperatures, i n c l u d i n g the a d s o r p t i o n on other nonmetallic m i n e r a l s , as w e l l , should be undertaken f o r t h i s purpose. 153 REFERENCES 1. WICKEN, O.M. and DUNCAN, L.R. "Magnesite and Related M i n e r a l s , i n I n d u s t r i a l M i n e r a l s and Rocks, New York, U.S.A. (1975), pp. 805-20. 2. 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I I I n f r a r e d Spectra and X-Ray D i f f r a c t i o n of 10- and 12- Hydroxystearic A c i d s and Methyl E s t e r s , and of 9, 10- Epoxystearic A c i d s " , Journal  Organic Chem., V o l . 18, (1953), pp. 693-701. 98. HUMMEL, D.O., "Polymer Spectroscopy", Verlag Chemie, Weinheim, W. Germ., (1974). 99. "The S a d t l e r Handbook of I n f r a r e d Spectra", Simons, W.W., Ed., S a d t l e r Res. Labs., P h i l . , U.S.A., (1978). 100. JACOBS, P.A. and UYTTERH0EVEN, J.B. "Assignment of the Hydroxyl Bands i n the I n f r a r e d Spectra of Z e o l i t e s X and Y", Jo u r n a l Chem.  S o c , Faraday Trans. I , V o l . 69, (1973), pp. 359-86. 101. MORRISON, R.T., and BOYD, R.N. "Organic Chemistry", 3rd Ed., A l l y n & Bacon, Inc., Boston, U.S.A., (1973). 102. BRANDAO, P.R.G. and POLING, G.W., "Anionic F l o t a t i o n of Magnesite", Canadian M e t a l l . Quart., i n Press. 103. SHAFRIN, E.G. and ZISMAN, W.A. " C o n s t i t u t i v e R e l a t i o n s i n the Wetting of Low Energy Surfaces and the Theory of the R e t r a c t i o n Method of Preparing Monolayers", J o u r n a l P h y s i c a l Chem., V o l . 64, (1960), pp. 519-24. 104. PLITT, L.R., and KIM, M.K., "Adsorption Mechanism of F a t t y A c i d C o l l e c t o r s on B a f i t e " , Trans. AIME-SME, V o l 256, (1974), pp. 188-93. 105. COOKE, S.R.B. et a l . " E f f e c t of Temperature on Soap F l o t a t i o n of I r o n Ore", Trans AIME-SME, V o l . 217, (1960), pp. 76-83. 106. FRENCH, R.D., et al.*'The Q u a n t i t a t i v e A p p l i c a t i o n of I n f r a r e d Spectroscopy t o Studies i n Surface Chemistry", Journa l  P h y s i c a l Chem., V o l . 58, (1954), pp. 805-11. 107. GUNSTONE, F.D., "An I n t r o d u c t i o n t o the Chemistry of Fats and F a t t y A c i d s " , Chapman & H a l l , L t d . , London, U.K., (1958). 108. SWERN, D., 'Chemical O x i d a t i o n " , i n "F a t t y A c i d s " , Part 2, Markley, K. S. Ed., I n t e r s c i e n c e P u b l . , New York, U.S.A., (1961), pp. 1307-85. 109. SWERN, D., "Oxidation by Atmospheric Oxygen ( A u t o x i d a t i o n ) " , i n " F a t t y A c i d s , Part 2, Markley, K.S., Ed. I n t e r s c i e n c e P u b l . , New York, U.S.A. (1961), pp. 1387-1444. 110. PLAKSIN, I.N., et a l , " A c t i o n of Oxygen on O l e i c A c i d " , Doklady Chemistry, Proc. Acad. S c i . USSR, V o l . 153, (1963), pp. 1048-50. 161 111. SHREVE, O.D. et a l . " I n f r a r e d Absorption Spectra of Some Hydroperoxides, Peroxides, and Related Compounds", A n a l y t i c a l Chem., V o l . 23, (1951), pp. 282-5. 112. MARKLEY, K.S. " S a l t s of F a t t y A c i d s " , i n " F a t t y A c i d s " , Part 2, Markley, K.S., Ed., I n t e r s c i e n c e Publ., New York, U.S.A., (1961), pp. 715-56. 113. IWAHASHI, M., et a l . "Radiotracer Study on Ox i d a t i o n of O l e i c A c i d Monolayer on Aqueous Permanganate S o l u t i o n s " , J o u r n a l  C o l l o i d I n t e r f a c e S c i . , V o l . 79, (1981), pp. 21-32. 114. KUZKIN, S.F. et a l . "Study of Forms of O l e i c A c i d Attachment and G r a i n Aggregation i n Oxidized M i n e r a l s by I n f r a r e d Spectroscopy", i n " F l o t a t i o n P r o p e r t i e s of Rare Metal M i n e r a l s " , P l a k s i n , I.N., Ed., Primary Sources, New York, U.S.A., (1967), pp. 83-91. 115. ROZENFELD, I.L. et a l . "Adsorption of Sodium Oleate and Water on an Oxidized Aluminum Surface" B u l l . Acad. S c i . USSR, Div. Chem. S c i . , V o l . 31, (1982). pp. 254-9. 116. MARSHALL, K. and ROCHESTER, C H . " I n f r a r e d Study of the Adsorption of O l e i c and L i n o l e i c Acids onto the Surface of S i l i c a Immersed i n Carbon T e t r a c h l o r i d e " , J o u r n a l Chem. S o c ,  Faraday Trans. I , V o l . 71, (1975), pp. 1754-61. 117. MARSHALL, K. and ROCHESTER, CH., " I n f r a r e d Studies of Adsorption at the S o l i d / L i q u i d I n t e r f a c e " , Faraday D i s c . Chem. S o c , No. 59, (1975), pp. 117-26. 118. CROSS, S.N.W., and ROCHESTER, CH., " I n f r a r e d Study of the Adsorption of L i n o l e i c A c i d on Alumina Immersed i n Carbon T e t r a c h l o r i d e " , J o u r n a l Chem. S o c , Faraday Trans. I , V o l . 74, (1978), pp. 2141-5. 119. LASKOWSKI, J . "Redox Conditions i n F l o t a t i o n : Oxides", t o be published. 120. KLASSEN, V . I . , and M0KR0US0V, V.A. "An I n t r o d u c t i o n t o the Theory of F l o t a t i o n " , Butterworths, London, U.K., (1963). 121. PLAKSIN, I . , " I n t e r a c t i o n of M i n e r a l s w i t h Gases and Reagents i n F l o t a t i o n " , Trans. AIME-SME, V o l . 214, (1959), pp. 319-24. 122. GAUDIN, A.M., and COLE, R.E., "Double-Bond R e a c t i v i t y of O l e i c A c i d During F l o t a t i o n " , Trans. AIME-SME, V o l . 196, (1953), pp. 418. 123. PURCELL, G. and SUN, S.C " S i g n i f i c a n c e of Double Bonds i n F a t t y A c i d F l o t a t i o n - A F l o t a t i o n Study", Trans. AIME-SME, V o l . 226, (1963), pp. 13-16. 162 124. DuRIETZ, C. "Chemisorption of C o l l e c t o r s i n F l o t a t i o n " , 11th I n t e r n ' 1 . M i n e r a l Process Congr., C a g l i a r i , I t a l y (1975) pp. 375-403. 125. DuRIETZ, C. " F a t t y A c i d s i n F l o t a t i o n " , 4th I n t e r n ' 1 . M i n e r a l Dressing Congr., Stockholm, Sweden (1957), pp. 417-33. 126. SINGLETON, W.S. " S o l u t i o n P r o p e r t i e s " i n F a t t y A c i d s - Part 5 Markley, K.S. Ed., I n t e r s c i e n c e P u b l . , New York, U.S.A. (1968). 127. FUERSTENAU, M.C. and PALMER, B.C. "Anionic F l o t a t i o n of Oxides and S i l i c a t e s " , i n Flotation-A.M. Gaudin Memorial Volume, V o l . I , Fuerstenau, M.C., Ed., AIME, New York, USA (1976) pp. 148-96. 128. STEPHEN, H. and STEPHEN, T. " S o l u b i l i t i e s of Inorganic and Organic Compounds", The MacMillan C o l . , New York, USA (1963). 129. LINKE, W.F. " S o l u b i l i t i e s : Inorganic and Metal-Organic Compounds", V o l . I I , 4th Ed., Amer. Chemical S o c , Wash. D.C, U.S.A. (1965). 130. POHLE, W.D. " S o l u b i l i t y of Calcium Soaps of Gum Rosin, Rosin Acids and F a t t y A c i d s " , O i l & Soap, V o l . 18, D e c (1941) pp. 244-5. 131. BUCKENHAM, M.H. and MACKENZIE, J.M.W. "Fa t t y A c i d as F l o t a t i o n C o l l e c t o r s f o r C a l c i t e " , Trans. AIME-SME, V o l . 220 (1961) pp. 450-4. 132. KLEVENS, H.B. "Structure and Aggregation i n D i l u t e S o l u t i o n s of Surface A c t i v e Agents", J o u r n a l Amer. O i l Chemists' S o c , V o l . 30, No. 2, Feb. (1953), pp. 74-80. 133. MULAR, A.L. and BULL, W.R. "Mineral Processes: T h e i r A n a l y s i s , O p t i m i z a t i o n and C o n t r o l " , Univ. B r i t i s h Columbia -Colorado School of Mines, (1969). 134 "X-Ray Powder Data F i l e " , P ubl. Amer. S o c T e s t i n g M a t e r i a l s , P h i l . , U.S.A. A P P E N D I X I S O L U B I L I T Y D A T A ON A N I O N I C S U R F A C T A N T S S o l u b i l i t y i n w a t e r ( s ) : ( M o l e s / 1 ) S u r f a c t a n t F o r m u l a • M o l e c . N e u t r a l s p e c i e s N a s a l t T e m p . ° C R e f . T e m p . ° C O l e i c a c i d L i n o l e i c a c i d C 1 8 H 3 4 ° 2 C 1 8 H 3 2 ° 2 L i n o l e n i c a c i d C 1 8 H 3 ( ) 0 2 E l a i d i c a c i d S t e a r i c a c i d C 1 8 H 3 4 ° 2 C 1 8 H 3 6 ° 2 R i c i n o l e i c a c i d C l g H 3 4 0 3 C 2 0 H 3 0 ° 2 C 1 2 H 2 6 S 0 3 A b i e t i c a c i d D o d e c y l s u l -p h o n i c a c i d 2 9 8 . 5 3 0 2 . 5 2 5 0 . 4 9 . 3 x 1 0 , - 3 2 5 R e f . 2 8 2 . 5 7 . 3 x 1 0 ~ 7 2 0 1 2 4 1 . 7 x 1 0 "1 2 0 1 2 8 4 . 3 x 1 0 " 6 2 0 1 2 5 2 8 0 . 5 3 . 7 x 1 0 ~ 6 2 0 1 2 5 5 . 7 x 1 0 " 4 6 . 7 1 2 5 2 7 8 . 4 1 . 9 x 1 0 " 6 2 0 1 2 5 2 8 2 . 5 2 . 8 x 1 0 " 6 2 0 1 2 5 2 8 4 . 5 1 . 3 x 1 0 " 7 2 0 1 2 4 8 . 4 x 1 0 "2 7 0 1 2 9 4 . 6 x 1 0 " 7 2 0 1 2 5 3 . 0 x 1 0 ~ 7 2 0 1 2 7 3 . 5 x 1 0 " 3 3 7 1 2 8 1 2 8 S o l u b i l i t y p r o d u c t ( K ) M g s a l t C a s a l t K s o T e m p . ° C R e f . K s o T e m p . ° C R e f . 6 . 3 x 1 0 " 1 6 1 2 7 2 . 5 x 1 0- 1 6 1 2 7 - 1 4 1 . 6 x 1 0 2 0 1 2 4 4 . 0 x 1 0- 1 6 2 0 1 2 4 1 . 6 x 1 0 " 1 1 2 0 1 2 5 - 1 3 4 . 0 x 1 0 2 0 1 2 5 - 1 3 4 . 0 x 1 0 2 0 1 2 5 1 . 3 x 1 0 " 1 1 2 0 1 2 5 - 1 3 6 . 3 x 1 0 1 3 2 0 1 2 5 5 . 0 x 1 0 " 1 5 2 0 1 2 5 3 . 2 x 1 0 " 1 6 2 0 1 2 5 4 . 0 x 1 0 "1 8 2 0 1 2 5 4 . 7 x 1 0 " 1 9 1 2 7 2 . 0 x l O "2 0 1 2 7 4 . 8 x 1 0 " 1 3 5 0 1 3 0 8 . 9 x 1 0 - 1 0 5 0 1 3 0 1 . 0 x 1 0 " 9 2 5 1 2 8 3 . 2 x 1 0 "1 1 2 5 1 2 8 A P P E N D I X I I C r i t i c a l m i c e l l e c o n c e n t r a t i o n s o f a n i o n i c s u r f a c t a n t s ( m o l e s / 1 ) N a s a l t K s a l t S u r f a c t a n t C . M . C . T e m p . ° c R e f . C . M . C . T e m p . ° c R e f . O l e i c a c i d 2 . 1 x 1 0 ~3 2 5 7 2 8 . 0 x 1 0 "4 2 5 7 2 3 . 0 x 1 0 " 3 4 0 7 2 3 . 5 x 1 0 ~ 3 5 0 7 2 1 . 2 x 1 0 "3 5 0 7 2 3 . 2 x 1 0 ~ 3 6 0 7 2 3 . 5 x 1 0 " 2 7 5 7 2 1 . 1 x 1 0 " 3 1 3 1 L i n o l e i c a c i d 5 . 3 x 1 0 ~4 1 3 1 L i n o l e n i c a c i d 7 . 2 x 1 0 ~ 4 1 3 1 E l a i d i c a c i d 1 . 4 x 1 0 "3 4 0 7 2 1 . 5 x 1 0 - 3 5 0 7 2 2 . 6 x 1 0 " 3 6 0 7 2 S t e a r i c a c i d 1 . 8 x 1 0 "3 5 0 7 2 4 . 5 x 1 0 " 4 5 5 7 2 7 . 5 x 1 0 ~ 5 7 0 7 2 5 . 0 x 1 0 - 4 6 0 7 2 R i c i n o l e i c a c i d 1 . 5 x 1 0 " 3 1 3 1 3 . 6 x 1 0 " 3 5 5 7 2 A b i e t i c a c i d 1 . 2 x 1 0 " 2 2 5 1 3 2 D o d e c y l s u l p h o n i c a c i d 9 . 8 x 1 0 ~ 3 2 5 7 2 9 . 0 x 1 0 " 3 2 5 7 2 1 . 1 x l O " 2 4 0 - 5 0 7 2 9 . 3 x 1 0 "3 4 0 7 2 1 . 2 x 1 0 - 2 6 0 7 2 9 . 7 x 1 0 "3 5 0 7 2 1 . 4 x l O " 2 8 0 7 2 1 . 0 x 1 0 "2 6 0 7 2 N e u t r a l s p e c i e s C . M . C . T e m p . ° C R e f . 4 . 5 x 1 0 2 7 7 2 165 Appendix I I I Magnesite m i c r o f l o t a t i o n t e s t s - Recoveries % Confidence I n t e r v a l s (133) L66' A. C o l l e c t o r : Na ol e a t e confidence l e v e l : 90% Concentration No. of Standard Mean Confidence Lower Upper (Molar) pH Tests D e v i a t i o n (x) I n t e r v a l L i m i t L i m i t (s) ± . 10~ 5 2.0 3 1.2 5.3 2.0 3 7 3.0 4 5.2 37.5 6.1 31 44 4.0 4 1.3 95.4 2.2 93 98 4.5 3 0.0 100.0 - - -5.0 3 0.0 100.0 - - -6.1 5 3.2 85.6 3.1 84 89 7.6 4 2.7 26.8 3.2 24 30 10.0 4 1.7 4.5 2.0 3 7 12.0 3 2.1 4.7 3.5 1 8 2 x 10~5 1 0 . o 5 2.4 21.0 2.3 19 23 2.3 x 10-5 2.0 4 3.0 20.8 3.5 17 24 3.0 3 3.1 94.7 5.2 90 100 4.0 2 0.0 100.0 - - -6.0 2 0.0 100.0 7.8 3 1.8 96.4 3.0 93 99 9.0 5 6.1 41.8 5.8 36 48 10.0 4 2.9 34.2 3.4 31 38 11.0 4 3.1 42.2 3.6 39 46 12.0 4 3.7 53.5 4.4 49 58 13.0 2 0.0 100.0 - - -3 x 10"5 2.0 4 4.0 19.8 4.7 15 25 9.0 3 3.0 94.0 5.1 89 99 10.0 5 5.8 66.3 5.5 61 72 167 C o l l e c t o r : Na ol e a t e confidence l e v e l : 90% Concentration (Molar) No. of Standard Mean Confidence pH Tests D e v i a t i o n (x) I n t e r v a l (s) ± Lower Upper L i m i t L i m i t 3 x 10 -5 4 x 10-5 10-4 11.0 12.0 13.0 10.0 1.0 2.0 3.0 4.5 6.0 7.5 10.0 12.0 3 3 2 5 4 3 s e v e r a l 3.1 3.4 0.0 6.2 5.1 3.5 0.0 93.7 94.4 100.0 82.4 67.9 93.7 100.0 5.2 5.7 5.9 6.0 5.9 89 89 99 100 77 88 62 74 88 100 \ 168 B. C o l l e c t o r : Na dodecylsulphonate confidence l e v e l : 90% Concentration (Molar) No. of Standard Mean Confidence pH Tests D e v i a t i o n (x) I n t e r v a l (s) ± Lower Upper L i m i t L i m i t 10-5 10-4 3x10-4 5x10-4 3.0 5.1 7.6 10.0 1.0 2.0 3.0 4.0 5.2 6.0 7.8 10.0 11.0 12.0 13.0 10.0 5.7 7.6 9.0 10.0 11.0 11.5 s e v e r a l s e v e r a l 4 5 4 3 s e v e r a l 5 2 4 s e v e r a l 4 4 s e v e r a l 0.0 0.0 3.5 3.4 2.8 2.5 0.0 3.8 0.0 3.9 0.0 2.7 2.4 b.'o 0.0 100.0 88.2 41.4 15.3 4.7 0.0 52.2 100.0 24.0 100.0 93.8 94.3 100.0 4.1 3.2 3.3 4.2 3.6 4.6 3.2 2.8 84 38 12 1 49 19 91 92 92 45 19 9 56 29 97 97 169 B.l Collector: Na dodecylsulphonate (A)+ dodecanol (B) confidence level: 90% Concentration No. of (Molar) pH Tests Standard Deviation (s) Mean (x) Confidence Interval + Lower Limit Upper Limit 10 - 4A+ 10 5 B 10_4A.+ 5x10- 5B 6.1 2 0.0 100.0 - - -7.8 4 5.6 69.1 6.6 63 76 10.0 4 4.9 28.8 5.8 23 35 11.0 4 4.7 46.3 4.7 42 51 12.0 2 0.0 100.0 - - -6.0 2 0.0 100.0 - - -7.6 2 0.0 100.0 - - -9.0 4 2.8 94.4 3.3 91 98 10.0 4 3.1 92.9 3.7 89 97 11.0 several 0.0 100.0 _ — 12.0 \ C. C o l l e c t o r : dodecylammonium c h l o r i d e confidence l e v e l : 90% Concentration No. of (Molar) pH Tests Standard D e v i a t i o n (s) Mean (x) Confidence I n t e r v a l ± Lower Upper L i m i t L i m i t 10-5 10-4 2 x 10-5 5 x 10-5 3.0 7.8 11.0 2.0 3.8 6.2 7.8 8.8 9.4 9.9 12.0 13.0 8.7 10.0 10.5 11.0 12.0 7.5 8.2 9.5 10.0 11.0 12.0 s e v e r a l s e v e r a l 4 5 4 s e v e r a l 2 3 5 s e v e r a l 2 3 4 4 s e v e r a l 0.0 0.0 2.8 3.7 4.3 0.0 0.0 2.7 4.3 0.0 0.0 2.1 2.6 4.0 0.0 0.0 0.0 10.3 31.4 80.0 100.0 0.0 5.7 51.4 100.0 0.0 6.3 34.8 76.3 100.0 3.3 3.5 5.1 4.. 6 4.1 3.5 3.1 4.7 7 28 75 1 47 3 32 72 14 35 85 10 56 7 38 81 171 Appendix IV Electron D i f f r a c t i o n Patterns of Magnesium Carbonate (91) Thin Films - Interpretation Figure 28 Indexing the spots (camera constant = 5.02) D d d' {hkl} D l 2.89 1.74 1.700 116 D2 2.86 1.76 1.700 116 D 3 3.98 1.26 1.252 0.0.12 4.08 1.23 1.158 200 D = double of the di s t a n c e between the o r i g i n and a s p e c i f i c spot i n the d i f f r a c t i o n p a t t e r n ( i n cm). d = c a l c u l a t e d c r y s t a l l o g r a p h i c i n t e r p l a n a r spacing ( i n &) , by the equation: D.d = camera constant d'= standard i n t e r p l a n a r spacing ( i n °0 corresponding to the c r y s t a l -l o g r a p h i c planes, defined by t h e i r M i l l e r i n d i c e s h , k , l , f o r magnesite, card no. 8-479 Checking the i n t e r p l a n a r angles Angle Measured Standard D j A D 3 46° 47.21° D 3 AD 4 90° 90.00° C r y s t a l l o g r a p h i c zone: [ l i o ] R e c i p r o c a l l a t t i c e corresponding to the e l e c t r o n d i f f r a c t i o n spot p a t t e r n i n F i g . 28. The s c a l e i s the double of the one i n F i g . 28. Figure 29 Indexing the spots (camera constant = 5.68) # D d ' d' {hkl} D l 2.44 2.33 2 .318 110 D 2 2.44 2.33 2 .317 210 D 3 2.44 2.33 2 .317 120 D4 4.32 1.31 1 .338 300 Checking the interpl. anar angles Angle Measured Standard A T>2 60° 60.00° D 2 A D 3 60° 60.00° T>. AD. 1 4 30° 30.00° D, AD. 90° 90.00° 3 4 Crystallographic zone: [OOl] Conclusion: the c r y s t a l l o g r a p h i c interplanar spacings matched with the magnesite standard within acceptable p r e c i s i o n ; the corresponding angles also checked w e l l . This was determined for two d i f f e r e n t c r y s t a l l o g r a p h i c d i r e c -t i o n s , defined by t h e i r zone axes, [llO] and [OOl] . Therefore, the c r y s t a l s making up the t h i n films were confirmed as magnesite. 175 R e c i p r o c a l l a t t i c e corresponding to the e l e c t r o n d i f f r a c t i o n spot p a t t e r n i n F i g . 29. The s c a l e i s the double of the one i n F i g . 29. 

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