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Flotation of oxidized copper minerals: an infrared spectroscopic study Coelho, Elcio Marques 1972

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H 1 a FLOTATION OF OXIDIZED COPPER MINERALS: AN INFRARED SPECTROSCOPIC STUDY BY ELCIO MARQUES COELHO B.Sc., Universidade Federal de Minas Gerais, Brazil, 1964 M.Sc, Stanford University, California, U.S.A., 1968 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in the Department of Mineral Engineering We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA Ap r i l , 1972 In presenting t h i s thes is i n p a r t i a l f u l f i l m e n t of the requirements f o r an advanced degree at the U n i v e r s i t y of B r i t i s h Columbia, I agree that the L ibrary 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 reference and study. I fur ther agree that permission f o r extensive copying of t h i s thesis f o r s c h o l a r l y purposes may be granted by the Head of my Department or by h i s representat ives . I t i s understood that copying or p u b l i c a t i o n of t h i s thesis f o r f i n a n c i a l gain s h a l l not be allowed without my wri t ten permission. Department of M itfSRSU hA/& /A/£f* !Af&-The U n i v e r s i t y of B r i t i s h Columbia Vancouver 8 , Canada Date AP/flZ - i i -Supervisor: P r o f e s s o r George W. P o l i n g ABSTRACT Chemical and p h y s i c a l p r o p e r t i e s of the copper oxide surface as modified by f l o t a t i o n c o l l e c t o r s have been i n v e s t i g a t e d . The f l o t a t i o n behaviour of copper oxide minerals i n the presence of a n i o n i c and c a t i o n i c c o l l e c t o r s has been stud i e d using the Hallimond tube method. An eleven r e f l e c t i o n , 70° in c i d e n c e angle, specular r e f l e c t i o n i n f r a r e d s p e c t r o s c o p i c technique has been employed to i n v e s t i g a t e the a d s o r p t i o n of c a r b o x y l i c a c i d onto copper oxide s u b s t r a t e s . An ATR " i n s i t u " technique has been developed to record the sp e c t r a of the adsorbates which were present at the copper oxide/aqueous s u r f a c t a n t s o l u t i o n i n t e r f a c e . M i c r o - f l o t a t i o n t e s t s i n d i c a t e d that m i n e r a l s u r f a c e charge played the most important r o l e i n f l o t a t i o n of copper oxide minerals w i t h i o n i c c o l l e c t o r s . T e n o r i t e was e a s i l y f l o a t e d w i t h an a n i o n i c c o l l e c t o r such as l a u r i c a c i d when the pH of the s o l u t i o n was lower than the IEP of the mi n e r a l (pH 9.4). On the other hand, l a u r y l amine, a c a t i o n i c c o l l e c t o r , y i e l d e d 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 at pH values g r e a t e r than the IEP. Adsorp t i o n experiments conducted using the h i g h l y s e n s i t i v e specular r e f l e c t i o n s p e c t r o s c o p i c technique suggested that m i n e r a l s u r f a c e charge was the major v a r i a b l e a f f e c t i n g a d s o r p t i o n of aqueous l a u r i c a c i d onto copper oxide s u b s t r a t e s . Therefore, a r e l a t i o n s h i p between f l o a t a b i l i t y , surface charge and a d s o r p t i o n was e s t a b l i s h e d . The i n f r a r e d s p e c t r o s c o p i c s t u d i e s showed that a d s o r p t i o n of c o l l e c t o r species d i d not occur at pH values above the IEP. Q u a l i t a t i v e and q u a n t i t a t analyses of the spec t r a i n d i c a t e d t h a t , at pH values below the IEP, chemi-- i i i -s o r p t i o n of l a u r a t e ions as counter ions i n the i n t e r n a l p a r t of the e l e c t r i c a l double l a y e r was the predominant a d s o r p t i o n mechanism r e s p o n s i b l e f o r c o l l e c t i o n . In a d d i t i o n , the a d s o r p t i o n of monomeric l a u r i c a c i d molecules, probably by hydrogen bonding to the s u b s t r a t e , was a l s o observed. In the presence of excess copper i o n s , c u p r i c l a u r a t e and monomeric l a u r i c acid-copper complexes were p h y s i c a l l y adsorbed on the chemisorbed f i l m . E f f e c t s of molecular o r i e n t a t i o n on specular r e f l e c t i o n s p e c t r a were analyzed on the b a s i s of e x i s t i n g o p t i c a l t h e o r i e s . The q u a l i t a t i v e i n t e r p r e t a t i o n of the i n f r a r e d r e f l e c t i o n s p e c t r a of c a r b o x y l i c s u r f a c e species r e q u i r e d the r e c o r d i n g of a s e r i e s of refe r e n c e s p e c t r a . These in c l u d e d specular r e f l e c t i o n s p e c t r a of f i l m s formed by condensing l a u r i c and s t e a r i c a c i d from the vapour phase onto copper oxide and gold s u b s t r a t e s . Standard q u a n t i t a t i v e references were obtained by t r a n s f e r r i n g s o l i d i f i e d monolayers of s t e a r i c a c i d from the a i r / w a t e r i n t e r f a c e to the substrates according to the Langmuir-Blodgett method. Remarkable e f f e c t s of molecular o r i e n t a t i o n were observed i n the r e f l e c t i o n s p e c t r a of the f i l m s mentioned above. These e f f e c t s were In agreement w i t h the p r e d i c t i o n s of the F r a n c i s and E l l i s o n theory. In a d d i t i o n , these s t u d i e s c o n t r i b u t e d to the understanding of the behaviour of organic monolayers on s o l i d and l i q u i d s u r f a c e s . A t h i n f i l m of cuprous oxide on an i n f r a r e d transparent ATR prism was used as adsorbent and i n f r a r e d spectra of the compounds present at the cuprous oxide/aqueous l a u r i c a c i d i n t e r f a c e were recorded " i n s i t u " . R e s u l t s of these t e s t s i n d i c a t e d that the composition of the adsorbed c a r b o x y l i c f i l m s was not a l t e r e d to any s i g n i f i c a n t extent by the removal of the s u b s t r a t e from the aqueous phase. - i v -TABLE OF CONTENTS Page ABSTRACT • i i LIST OF FIGURES v i i i LIST OF TABLES x i v ACKNOWLEDGEMENTS xv CHAPTER I . INTRODUCTION 1 1.1 Statement of the Problem 1 1.2 L i t e r a t u r e Review 2 (a) F l o t a t i o n of Ox i d i z e d Copper M i n e r a l s 2 (b) Mechanisms of C o l l e c t o r A d s o r p t i o n 8 (c) Charge Formation on Oxide Surfaces 14 1.3 Scope and Approach of the Study 18 CHAPTER I I . INFRARED SPECTRA OF SOLID SURFACES 23 2.1 Transmission Technique 23 2.2 M u l t i p l e Specular R e f l e c t i o n Technique 28 2.3 Attenuated T o t a l I n t e r n a l R e f l e c t i o n Technique ... 37 CHAPTER I I I . EXPERIMENTAL METHODS 41 3.1 F l o t a t i o n Tests 41 (a) M a t e r i a l s 42 (b) F l o t a t i o n Apparatus 43 (c) Operating Procedures 47 3.2 I n f r a r e d Spectroscopic Studies 49 3.2.1 M a t e r i a l s 50 3.2.2 Sample P r e p a r a t i o n 52 (a) Potassium Bromide Discs 52 ) - v -Page (b) Samples for Specular R e f l e c t i o n Spectroscopy 53 (c) ATR Samples 63 3.2.3 Recording the Spectra 64 (a) Transmission Spectra 64 (b) Specular R e f l e c t i o n Spectra 64 (c) ATR Spectra 66 CHAPTER IV. RESULTS AND DISCUSSION 69 4.1 F l o t a t i o n Experiments 69 (a) The Role of Mineral Surface Charge i n Tenorite • F l o t a t i o n 69 (b) Changes i n Pulp pH 72 (c) E f f e c t of Sulphide Ions 76 (d) E f f e c t of Added Copper Ions 79 (e) F l o t a t i o n of Tenorite with O l e i c A cid and Amyl Xanthate 79 4.2 Infrared Spectroscopic Studies 84 4.2.1 Q u a l i t a t i v e and Quantitative Analysis of the Spectra 84 (a) The Assignment of Bands 84 (b) E f f e c t s of Molecular O r i e n t a t i o n on IR Specular R e f l e c t i o n Spectra 108 (c) Langmuir-Blodgett Monolayers of S t e a r i c Acid Deposited onto Cupric Oxide Substrates 118 4.2.2 Adsorption of Aqueous L a u r i c Acid onto Copper Oxide Substrates 131 - v i -Page (I) Specular Reflection Studies 131 (a) Effects of pH on Adsorption of Laurie Acid onto Cupric Oxide Substrates 131 (b) Effect of Time of Exposure and Agitation 138 (c) Effect of Surfactant Concentration 139 (d) Effect of Added Copper Ions 139 (e) Effect of Dissolved Carbon Dioxide 143 (f) Adsorption of Aqueous Laurie Acid onto Cuprous Oxide Substrate ...... 144 (II) "In s i t u " ATR Studies 146 SUMMARY AND CONCLUSIONS 150 APPENDICES .• . 153 A. Solubility Data for Selected Organic Compounds .... 153 B. Equilibrium Diagrams 154 C. Characteristic Group Frequencies 158 D. Determination of the IEP of Tenorite 163 E. Calibration of the Torsion-Wire Langmuir Balance.. 165 F. Typical Behavior of Stearic Acid Films Spread at the Air-Water Interface 167 G. Calculation of AR „IT , 170 a S j C H ^ random H. Calculation of a Theoretical Value for AR r u i/AR , = r 172 as.CH^J- aSjCB^ random I. Measurement of Copper Oxide Film Thickness from AR Values 174 j - v i i -Page J . C a l c u l a t i o n of Angles Formed by Planes Containing CH2 Groups of S t e a r i c A c i d Monolayers w i t h the S o l i d Surface 176 REFERENCES 177 - v i i i -LIST OF FIGURES Figure Page 1 The e l e c t r i c a l double layer 13 2 Solubility diagram for CuO and Cu(OH)2 17 3 Optical arrangement for reflection spectroscopy ... 27 4 Variation of n and k through an absorption band ... 27 5 Francis and Ellison theory a - Reflection from a filmed metal 31 b - Phase shift during reflection. Component normal to the plane of incidence 31 c - Parallel component 31 6 Attenuated total internal reflection spectroscopy (ATR) a - Total internal reflection 38 b - Multiple ATR prism 38 c - "In s i t u " ATR 38 7 Modified Hal limond tube 44 8 Microflotation Apparatus 46 9 Surface topography of cupric oxide substrates a - Fizeau interference fringes 55 b - Transmission electron-micrograph 55 10 Surface topography of smooth gold substrates a - Fizeau interference fringes 57 b - Transmission electron-micrograph 57 11 Surface topography of rough gold substrates a - Fizeau interference fringes 58 b - Transmission electron-micrograph 58 - ix -Figure Page 12 Adsorption of aqueous lauric acid onto mirrors. A l l teflon reaction vessel a - Arrangement inside the vessel before reaction.. 60 b - Arrangement inside the vessel during reaction.. 60 13 Sampling area of the spectrophotometer covered for the dry-air purge 65 14 Multiple reflection attachment and sample holder .. 65 15 Flotation of tenorite with anionic and cationic collector (Procedure I) 70 16 Flotation of tenorite in the presence and in the absence of carbon dioxide contaminations 71 17 Cationic flotation of tenorite with and without sulphide ions in solution 77 18 Anionic flotation of tenorite with and without sulphide ions in solution 78 19 Flotation of tenorite with anionic and cationic collectors and in the presence of added copper ions 80 20 Flotation of tenorite with oleic acid 81 21 Flotation of tenorite with potassium amyl xanthate. 82 22 Absorption spectrum of cupric oxide 86 23 Absorption spectrum of cuprous oxide 86 24 Absorption spectrum of cupric hydroxide 87 25 Absorption spectrum of malachite 87 26 (A) Spectrum of gold mirrors (3700-2600 cm-1) 88 (B) Spectrum of cupric oxide covered front surface gold mirrors (3700-2600 cm"1) 88 - X -F i g u r e Page 27 (A) Spectrum of gold m i r r o r s (1800-700 cm"1) 89 (B) Spectrum of c u p r i c oxide covered f r o n t surface gold m i r r o r s (1800-700 cm - 1) 89 28 R e f l e c t i o n spectrum of cuprous oxide covered f r o n t s u r f a c e gold m i r r o r s 29 R e f l e c t i o n spectrum of c u p r i c oxide covered f r o n t surface g o l d m i r r o r s 91 30 ATR s p e c t r a of cuprous oxide (A) Alone 92 (B) In the presence of B^O 92 (C) I n the presence of D 20 92 31 Absorption spectrum of l a u r i c a c i d 94 32 A b s o r p t i o n spectrum of s t e a r i c a c i d 94 33 A b s o r p t i o n spectrum of sodium l a u r a t e 97 34 Absorption spectrum of c u p r i c l a u r a t e 97 35 Specular r e f l e c t i o n spectrum of l a u r i c a c i d evaporated onto gold s u b s t r a t e s 98 36 Specular r e f l e c t i o n spectrum of s t e a r i c a c i d evaporated onto gold s u b s t r a t e s 98 37 Spectra of monomeric c a r b o x y l i c a c i d formed on gold by: '(A) washing w i t h c a r b o n - t e t r a c h l o r i d e (evaporated s t e a r i c a c i d f i l m ) 100 (B) adsorption of l a u r i c a c i d from H^ O s o l u t i o n s . . . 100 (C) adsorption of l a u r i c a c i d from D^ O s o l u t i o n s . . . 100 38 Specular r e f l e c t i o n spectrum of s t e a r i c a c i d deposited on c u p r i c oxide s u b s t r a t e s from the vapour phase (30 minutes a f t e r d e p o s i t i o n ) 105 - x i -Figure Page 39 Specular reflection spectrum of stearic acid evaporated onto cupric oxide substrate (90 hours after deposition) 105 40 Spectrum of stearic acid film deposited by evaporation onto cupric oxide substrate (residual layer) 106 41 Difference spectrum of the residual layer 106 42 Spectrum of a monolayer of stearic acid deposited onto smooth surface, gold mirrors (10 minutes after deposition) .. ... 109 43 Spectrum of a monolayer of stearic acid deposited onto smooth surface, gold mirrors (24 hours after deposition) 109 44 Spectrum of a monolayer of stearic acid deposited onto rough surface, gold mirrors (10 minutes after deposition) 114 45 Spectrum of a monolayer of stearic acid deposited onto rough surface, gold mirrors (24 hours after deposition) 114 46 Orientation of stearic acid molecules (dimer) in films deposited from the vapour phase 116 47 Spectrum of a monolayer of stearic acid deposited on cupric oxide substrate (3700-2600 cm L) 119 48 Spectrum of a monolayer of stearic acid deposited on cupric oxide substrate (1800-700 cm " S (A) Actual trace . 120 (B) Difference spectrum 120 - x i i -Figure Page 49 (A) Spectrum of 3 l a y e r s of s t e a r i c a c i d deposited on c u p r i c oxide substrates 122 (B) D i f f e r e n c e spectrum of 2nd and 3rd l a y e r s 122 50 Spectrum of 9 l a y e r s of s t e a r i c a c i d deposited on c u p r i c oxide s u b s t r a t e 122 51 Measured AR values f o r the CH^ asymmetric s t r e t c h i n g band of s p e c t r a of s t e a r i c a c i d monolayers t r a n s f e r r e d to c u p r i c oxide s u b s t r a t e s 126 52 Surface topography of c u p r i c oxide s u b s t r a t e s -4 5 exposed to 10 * M H^SO^ s o l u t i o n f o r 30 minutes a - F i z e a u i n t e r f e r e n c e f r i n g e s 128 b - Transmission electron-micrograph ... 128 53 Adso r p t i o n of aqueous l a u r i c a c i d onto c u p r i c oxide s u b s t r a t e 133 54 R e f l e c t i o n s p e c t r a of c u p r i c oxide s u b s t r a t e s exposed to aqueous l a u r i c a c i d (A) pH 4.7 134 (B) pH 8.4 134 (C) pH 10.5 134 55 R e f l e c t i o n s p e c t r a of l a u r i c a c i d f i l m s adsorbed onto c u p r i c oxide s u b s t r a t e s from s o l u t i o n s c o n t a i n i n g added copper ions (A) pH 5.5 141 (B) 30 minutes immersion i n hexane 141 CC) pH 8.5 141 - x i i i -Figure Page 56 Reflection spectra of cuprous oxide substrates exposed to lauric acid solutions (A) pH 6.0 145 (B) pH 8.6 145 (C) pH 10.0 145 57 "In s i t u " ATR spectrum: cuprous oxide substrate i n 2 x 10 ^ M lauric acid solution i n R^ O 147 58 ATR spectrum of the cuprous oxide substrate after being exposed to R^ O solution of lauric acid ...... 147 59 "In s i t u " ATR spectrum: cuprous oxide substrate in 2 x 10 M lauric acid solution i n D^ O 149 60 ATR spectrum of the cuprous oxide substrate after being exposed to D^O solution of lauric acid 149 Bl Stability of copper compounds as functions of par t i a l pressures of oxygen and carbon dioxide 154 B2 Potential-pH equilibrium diagram for the system Cu-H20-02 155 B3 Potential-pH equilibrium diagram for the system Cu-H20-02-C02-S 156 B4 Logarithmic concentration diagrams for carbonate species in aqueous solution 157 D A pH versus pH^ for tenorite 164 E Torsion-wire Langmuir balance calibration curve.... 166 F Pressure-area curve for a typical stearic acid monolayer 168 I Relation between cupric oxide film thickness and intensity of the CuO reflection band at 570 cm 1... 175 - x i v -LIST OF TABLES Table Page 1 C l a s s i f i c a t i o n of the mechanisms of c o l l e c t o r a d s o r p t i o n 9 2 R e s u l t s of f l o t a t i o n t e s t s on t e n o r i t e (Procedure I) 73 3 R e s u l t s of f l o t a t i o n t e s t s on t e n o r i t e (Procedure I I ) 74 4 Average angle made by planes c o n t a i n i n g CH^ groups of s t e a r i c a c i d molecules i n the subsequent l a y e r s w i t h the s o l i d s u rface 130 C Selected c h a r a c t e r i s t i c group frequencies 158 F Langmuir trough data 169 - X V ACKNOWLEDGEMENTS The author wishes to express h i s g r a t i t u d e and s i n c e r e a p p r e c i a t i o n to the d i r e c t o r of t h i s r esearch, Dr. G.W. P o l i n g f o r h i s guidance and encouragement. Thanks are a l s o extended to the va r i o u s members of the Department of M i n e r a l Engineering, i n p a r t i c u l a r to Dr. I.C.G. Ogle and Mr. J . Scott f o r spending many hours i n meticulous proof reading of the manuscript; to Dr. J . L e j a and a l l the graduate students f o r t h e i r h e l p f u l d i s c u s s i o n s . The f i n a n c i a l a s s i s t a n c e of the N a t i o n a l Research C o u n c i l of Canada, without which t h i s study would not have been p o s s i b l e , i s g r e a t l y appreciated. The f o l l o w i n g s c h o l a r s h i p s are' g r a t e f u l l y acknowledged: B r a z i l i a n Government, CAPES Sch o l a r s h i p (1968-1970) U.B.C. Graduate F e l l o w s h i p (1971). To my w i f e , my g r a t i t u d e f o r her patience and understanding. - 1 -CHAPTER I INTRODUCTION 1.1 Statement of the Problem Non-sulphide copper ores are u s u a l l y r e f e r r e d to as " o x i d i z e d copper ores". Many copper prospects show the presence of o x i d i z e d copper minerals along w i t h u n d e r l y i n g s u l p h i d e s . Around the w o r l d , there are s e v e r a l l a r g e and s m a l l deposits of these m i n e r a l s . The two b a s i c carbonates, malachite (CuCO„(OH)„) and a z u r i t e (Cu,,(C0„)„(OH) ) are the most abundant o x i d i z e d copper m i n e r a l s . The oxides of copper, c u p r i t e (Cu^O) and t e n o r i t e (CuO), and one group of s i l i c a t e s , c h r y s o c o l l a s , are a l s o f r e q u e n t l y found. Phosphates, sulphates, c h l o r i d e s and other w a t e r - s o l u b l e copper s a l t s occur i n a s m a l l number of d e p o s i t s . A l a r g e m a j o r i t y of o x i d i z e d copper ores i s t r e a t e d by hydro-or p y r o m e t a l l u r g i c a l processes, along, or i n combination w i t h f l o t a t i o n . While d i r e c t f l o t a t i o n , r e q u i r i n g low c a p i t a l investment, would be an a t t r a c t i v e method, e s p e c i a l l y f o r s m a l l d e p o s i t s , i t has i n most cases been i n e f f e c t i v e and t h e r e f o r e uneconomic. F r o t h f l o t a t i o n i s a process to separate s o l i d p a r t i c l e s from each other i n an aqueous pulp by s e l e c t i v e attachment of p a r t i c l e s to gas bubbles. The bubbles r i s e to the surface c a r r y i n g the s e l e c t e d p a r t i c l e s which are removed 2 -i n a f r o t h w h i l e the other p a r t i c l e s remain suspended i n the pulp. With a few exceptions, minerals are completely wetted by the aqueous phase. In order to make them capable of adhering to the gas bubbles, i t i s necessary to change t h e i r s urface c h a r a c t e r i s t i c s from hydro-p h i l i c to hydrophobic. C o l l e c t o r s are used f o r t h i s purpose. They are organic species which, once attached to m i n e r a l s , provide them w i t h a non-polar, hydrocarbon-like (hydrophobic) s u r f a c e . The m i n e r a l i n d u s t r y today makes widespread use of t h i s process to concentrate metal s u l p h i d e s . On the other hand, only a s m a l l but i n c r e a s i n g f r a c t i o n of the non-sulphide minerals i s concentrated by f l o t a t i o n . Most of the e a r l i e r research on f l o t a t i o n of o x i d i z e d copper mi n e r a l s has been of an e m p i r i c a l nature. As a r e s u l t , the surface chemistry of the system f l o t a t i o n c o l l e c t o r / o x i d i z e d copper m i n e r a l i s f a r from w e l l understood. A b e t t e r knowledge of the b a s i c phenomena of a d s o r p t i o n of c o l l e c t o r s onto o x i d i z e d copper minerals i s the s t a r t i n g p o i n t f o r a more l o g i c a l approach to the study of recovery of these minerals by f l o t a t i o n . I t i s to t h i s end that the research a s s o c i a t e d w i t h t h i s t h e s i s i s d i r e c t e d . 1.2 L i t e r a t u r e Review (a) F l o t a t i o n of O x i d i z e d Copper M i n e r a l s The c o n c e n t r a t i o n of o x i d i z e d copper minerals by f l o t a t i o n has been the subject of much research i n recent years (1-59). A great v a r i e t y of methods has been proposed f o r f l o a t i n g these minerals but not many have advanced beyond the l a b o r a t o r y stage. F l o t a t i o n of most o x i d i z e d copper minerals i s d i f f i c u l t . There i s as yet no s u c c e s s f u l - 3 -process f o r i n d u s t r i a l f l o t a t i o n of c h r y s o c o l l a s . D i r e c t f l o t a t i o n of ores c o n t a i n i n g carbonaceous or ferruginous gangue gives poor r e s u l t s because of the n o n - s e l e c t i v i t y of c o l l e c t i o n . Clay mixed w i t h the m i n e r a l s prevents f l o t a t i o n e n t i r e l y . P r i o r d e sliming may permit s u c c e s s f u l f l o t a t i o n of malachite and a z u r i t e ores. In some cases, treatment of the pulp w i t h f l o c c u l a n t s such as a c r y l i c a c i d polymers may be used as a s u b s t i t u t e f o r the d e s l i m i n g step (43,56). Long chain c a r b o x y l i c c o l l e c t o r s such as o l e i c a c i d (CH 3(CH 2) 7CH=CH(CH 2) 7COOH) or t h e i r s a l t s (soaps), i n the pH range of 8 to 10.5, give good recovery of malachite and a z u r i t e but mediocre recovery of c u p r i t e and t e n o r i t e . S u l p h y d r y l compounds are a l s o e f f e c t i v e c o l l e c t o r s f o r the carbonates of copper but excessive amounts of these reagents are r e q u i r e d f o r good r e c o v e r i e s i f used alone. S u l p h i d i z a t i o n (using Na 2S, H^S or molten sulphur) can reduce c o l l e c t o r consumption. However, the amount of s u l p h i d i z i n g agent i s c r i t i c a l ; a s l i g h t excess l e a d i n g to l a c k of f l o a t a b i l i t y . Hence, stepwise s u l p h i d i z a t i o n i s becoming the standard procedure (53,55). Few i n v e s t i g a t o r s have attempted to e x p l a i n the p h y s i c a l and chemical mechanisms i n v o l v e d i n f l o t a t i o n of o x i d i z e d copper m i n e r a l s . Most of the work has been c a r r i e d out on copper carbonates and s i l i c a t e s , i n the presence of complex reagents l i k e unsaturated f a t t y a c i d s , xanthates and s u l p h i d e s . The s t u d i e s by Burkin and Halsey (25) and by I v a n o v s k i i (34) are exceptions. In the former, the adsorption of l a u r y l amine ( C ^ H ^ N H ^ on c u p r i c , n i c k e l and z i n c oxides was s t u d i e d and i t was found that the f ree energies of a d s o r p t i o n are of the same order of magnitude as those expected f o r the formation of the corresponding _ 4 -amine complexes i n s o l u t i o n . Therefore, formation of i n s o l u b l e metal-amine complexes at the min e r a l surfaces was p o s t u l a t e d . In the l a t t e r , i t was shown that organic compounds used i n chemical a n a l y s i s to p r e c i p i t a t e copper ions a l s o have d e f i n i t e c o l l e c t i n g p r o p e r t i e s on f l o t a t i o n of c u p r i t e and c h a l c o p y r i t e (CuFeS^). F l o a t a b i l i t y of these minerals was c o r r e l a t e d w i t h the s o l u b i l i t y of the c o p p e r - c o l l e c t o r complexes. The Use of C o l l e c t o r s Alone In 1935, Rey (5) s t u d i e d the f a t t y a c i d and soap f l o t a t i o n of malachite at pH's 8 to 9.5. He concluded that c o l l e c t i o n r e s u l t s from i n t e r a c t i o n s of the a c i d w i t h the mineral p a r t i c l e s ; the soaps act mainly as f r o t h e r s . Gaudin and Schuhmann (1936) (7) i n v e s t i g a t e d a d s o r p t i o n of xanthates on carbonate minerals and observed t h a t c o l l e c t o r f i l m s adsorbed on malachite were e a s i l y removed from the surface on t r e a t i n g the mineral, w i t h organic s o l v e n t s . They were the f i r s t to a s s o c i a t e the d i f f i c u l t y i n f l o a t i n g o x i d i z e d copper minerals w i t h the la c k of adhesion of the c o l l e c t o r c o a t i n g . Peterson and c o l l a b o r a t o r s (1965) (38) recommended the use of o c t y l hydroxamate as c o l l e c t o r f o r c h r y s o c o l l a and suggested pH 6 as the optimum f l o t a t i o n pH. An adsor p t i o n mechanism proposed by them shows the i n t e r a c t i o n of c o l l e c t o r w i t h the m i n e r a l surface as r e s u l t i n g from negative hydroxamate ions being adsorbed on p o s i t i v e copper hydroxide s i t e s (CuOH + formed at the surface of the m i n e r a l ) . C a l c u l a t e d b u l k c o n c e n t r a t i o n of CuOH+ ions y i e l d e d a maximum at pH 6.0 to 6.2. - 5 -The A c t i o n of S u l p h i d i z i n g Agents In 1938, Rehbinder and c o l l a b o r a t o r s (10) n o t i c e d that the treatment of malachite w i t h sulphides l e a d to a decrease of surface w e t t a b i l i t y by x^ater d r o p l e t s i n a i r . Shorsher (16), i n 1949, showed that s u l p h i d i z a t i o n reduced by s e v e r a l times the amount of xanthate necessary f o r good recovery of malachite but excessive r e s i d u a l amounts of sodium sulphide i n s o l u t i o n decreased the a d s o r p t i o n of c o l l e c t o r and suppressed f l o t a t i o n . M i t r o f a n o v and co-workers (1955) (17) s t u d i e d the k i n e t i c s of s u l p h i d i z a t i o n and a d s o r p t i o n of xanthates and dithiophosphates on o x i d i z e d l e a d and copper m i n e r a l s , by means of r a d i o m e t r i c methods. An i n c r e a s e d r a t e of c o l l e c t o r a d s o r p t i o n a f t e r s u l p h i d i z a t i o n was obtained. They observed a l s o t h a t sulphide f i l m s formed on malachite and c h r y s o c o l l a peeled o f f under the abrasive a c t i o n of p a r t i c l e s i n an a g i t a t e d f l o t a t i o n pulp. In a d d i t i o n , they e x p l a i n e d depression (decrease i n f l o a t a b i l i t y ) i n the presence of excessive concentrations of s u l p h i d e ions by competitive adsorption of c o l l e c t o r and hydro-su l p h i d e ions (HS ) at the s u r f a c e of the m i n e r a l s . In 1956, L e j a (19) i n v e s t i g a t e d the f l o a t a b i l i t y of malachite w i t h xanthates and c a r b o x y l a t e s , before and a f t e r s u l p h i d i z a t i o n , and n o t i c e d again the poor adhesion of c o l l e c t o r f i l m s . He suggested an e x p l a n a t i o n f o r t h i s phenomenon based on the assumption that the surface of malachite i s formed e s s e n t i a l l y by carbonate and h y d r o x y l groups, w i t h copper atoms u n d e r l y i n g them. When c o l l e c t o r or sulphide ions e s t a b l i s h a strong bond w i t h the u n d e r l y i n g copper atoms, copper d i f f u s e s out, weakening the s t r u c t u r e of the m i n e r a l . A l s o , on the b a s i s that - 6 -d i s s o l v e d copper ions r e a c t w i t h c o l l e c t o r o u t s i d e the i n t e r f a c e , prevent-i n g p a r t i c l e - c o l l e c t o r i n t e r a c t i o n , he proposed;that s u l p h i d e ions a c t i v a t e (improve f l o t a t i o n of) malachite not d i r e c t l y on the surface but r a t h e r i n the s o l u t i o n by p r e c i p i t a t i n g copper i o n s . In apparent c o n t r a d i c t i o n to t h i s l a t t e r h y p o t h e s i s , Pryor and Lowe (1956) (20) measured changes i n copper s o l u b i l i t y during f l o t a t i o n of malachite w i t h xanthates and observed that complete f l o t a t i o n occurred only when d i s s o l v e d copper exceeded a c e r t a i n minimum c o n c e n t r a t i o n . This minimum amount of d i s s o l v e d copper was much higher than the amount expected from the p u b l i s h e d values f o r the s o l u b i l i t y product of the copper xanthates. Abramov (1969) (52) e x p l a i n e d the weak adhesion of c o l l e c t o r s to the s u r f a c e of lead and copper carbonates as a r e s u l t of the h i g h l y h y d r o p h i l i c nature of the su r f a c e of these m i n e r a l s . The c o l l e c t o r c o a t i n g (metal xanthate and dixanthogen, i d e n t i f i e d by X-ray d i f f r a c t i o n and IR spectroscopy) i s porous and unable to prevent p e n e t r a t i o n of water underneath the hydrophobic f i l m s . Such weakly bonded f i l m s are e a s i l y destroyed by the a c t i o n of the a i r bubbles causing f l o t a t i o n f a i l u r e s . Abramov b e l i e v e s that the main e f f e c t of s u l p h i d i z a t i o n i s to i n c r e a s e the hydrophobic nature of the m i n e r a l s u b l a y e r , making p o s s i b l e stronger adhesion of c o l l e c t o r c o a t i n g s , and consequently, 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 . The E f f e c t of pH Bowdish and Chen (1963) (31) reported 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 of c h r y s o c o l l a w i t h xanthates i n the pH range of A t o 6. - 7 -In 1965, Wright and Prosser (37), using i n f r a r e d and u l t r a v i o l e t > sp e c t r o s c o p i c techniques, i d e n t i f i e d cuprous xanthate together w i t h dixanthogen and c u p r i c sulphide as the products of the r e a c t i o n s of chryso-c o l l a and malachite w i t h xanthate i n the presence of sodium s u l p h i d e . They expl a i n e d the f l o t a t i o n behavior of c h r y s o c o l l a on the b a s i s of the r e l a t i v e p o s i t i o n s of these products and a s i l i c a g e l l a y e r formed during the r e a c t i o n s . According to t h e i r model, f l o t a t i o n i s p o s s i b l e near pH 5 because only under t h i s c o n d i t i o n does p r e c i p i t a t i o n of the r e a c t i o n products occur at the sur f a c e of the mi n e r a l s . In 1970, Bowdish and P l o u f (54) observed that the negative surface charge of c h r y s o c o l l a shows an unexpected increase as pH decreases from 6.0 to 4.5. They explained t h i s behavior by assuming t h a t i n t h i s pH range the i n c i p i e n t l e a c h i n g of copper from the m i n e r a l i s higher which gives the surface a l e s s negative p o t e n t i a l . They c o r r e l a t e d t h i s phenomenon w i t h i n c r e a s i n g sulphide a d s o r p t i o n and f l o t a t i o n recovery. Summary I t i s apparent from the above review that no s a t i s f a c t o r y c o r r e l a t i o n e x i s t s among the v a r i o u s t h e o r i e s proposed. The author b e l i e v e s that i n order to understand such complex f l o t a t i o n systems, i t would be b e n e f i c i a l to examine the mechanisms of adso r p t i o n of simple c o l l e c t o r species on copper oxides. This r e q u i r e s a general d i s c u s s i o n on the t h e o r i e s of adso r p t i o n and charge formation on oxide s u r f a c e s . - 8 -(b) Mechanisms of C o l l e c t o r Adsorption Surface chemists c l a s s i f y a d s o r p t i o n as e i t h e r p h y s i c a l or chemical. This c l a s s i f i c a t i o n i s p u r e l y r e l a t i v e and does not c h a r a c t e r i z e the r e a l nature of the phenomena (60). " P h y s i c a l " a d s o r p t i o n can be caused by van der Waals f o r c e s (weak f o r c e s due p r i m a r i l y to induced p o l a r i z a t i o n r e s u l t i n g from instantaneous d i p o l e s created by e l e c t r o n i c motion) or by other weak f o r c e s such as long range coulombic a t t r a c t i o n s , hydrogen bonding,* charge-dipole, d i p o l e - d i p o l e and induced d i p o l e i n t e r a c t i o n s . I n . t h i s type of a d s o r p t i o n , the adsorbate maintains i t s chemical i d e n t i t y . "Chemical" a d s o r p t i o n (or "chemisorption") i s due to strong i n t e r a t o m i c f o r c e s (covalent or i o n i c ) . I t i s more s e l e c t i v e and leads to formation of monomolecular l a y e r s only. Chemisorbed monolayers are more p r o p e r l y c a l l e d " s u r f a c e compounds" (2,3). One i n t e r e s t i n g c h a r a c t e r i s t i c of the s u r f a c e compounds i s that t h e i r chemical and p h y s i c a l p r o p e r t i e s do not always c o i n c i d e w i t h the p r o p e r t i e s of e x i s t i n g bulk compounds. Peck and Wadsworth (61), obtained d i r e c t evidence of formation of surface compounds by means of i n f r a r e d spectroscopy. The mechanisms most o f t e n used to e x p l a i n a d s o r p t i o n of f l o t a t i o n c o l l e c t o r s on m i n e r a l surfaces (1-4, 62,63) are summarized i n Table 1. C o l l e c t o r a d s o r p t i o n phenomena f r e q u e n t l y i n v o l v e more than one mechanism. L a t e r a l van der Waals a t t r a c t i o n between non-polar chains of the c o l l e c t o r species may a l s o p l a y a r o l e i n the mechanism by strengthening a d s o r p t i o n . Somewhat strong hydrogen bonds may be formed between the adsorbent and the adsorbate. The phenomenon, i n t h i s case, i s more o f t e n c l a s s i f i e d as chemical a d s o r p t i o n . - 9 -Table 1. C l a s s i f i c a t i o n of the Mechanisms of C o l l e c t o r Adsorption. A. As n e u t r a l molecules: i . P h y s i c a l a d s o r p t i o n on top of p r e v i o u s l y adsorbed c o l l e c t o r species ( m u l t i l a y e r f o r m a t i o n ) . i i . P h y s i c a l adsorption at the m i n e r a l s u r f a c e (monolayer). i i i . Chemisorption due to strong hydrogen bonding. i v . Surface compound formation by chemical r e a c t i o n between c o l l e c t o r molecules and surface groups. The r e a c t i o n product can be: i v . a . a surface compound f o r which analogous species are known to e x i s t i n the bulk s t a t e , or i v . b . a surface compound f o r which no analogous compounds have ever been i s o l a t e d . B. As i o n i c s p e c i e s : i . P r e c i p i t a t i o n of surface compounds ( s o l u b i l i t y theory of f l o t a t i o n ) . i i . A d s orption as counter ions i n the double l a y e r ( i o n a d s o r p t i o n t h e o r y ) ! i i . a . p h y s i c a l a d s o r p t i o n i n the e x t e r n a l p a r t of the double l a y e r , i i . b . p h y s i c a l adsorption i n the i n t e r n a l p a r t of the double l a y e r , or i i . c . chemical a d s o r p t i o n i n the i n t e r n a l p a r t of the double l a y e r . - 10 -Adsorption of N e u t r a l Molecules > In Table 1, Class A mechanisms describe the i n t e r a c t i o n s of mi n e r a l surfaces w i t h uncharged c o l l e c t o r s p e c ies. F l o t a t i o n c o l l e c t o r s are i n most cases i o n i z a b l e compounds / B u t n e u t r a l molecules, i n the form of u n d i s s o c i a t e d c o l l e c t o r or n e u t r a l products of r e a c t i o n s i n v o l v i n g c o l l e c t o r s p e c i e s , are l i k e l y to be present i n the f l o t a t i o n systems and may be adsorbed at the min e r a l s u r f a c e . The r e a l s i g n i f i -cance of adsorption of n e u t r a l c o l l e c t o r species to the f l o t a t i o n process i s not known. Some i n v e s t i g a t o r s b e l i e v e that i n many cases chemisorption of c o l l e c t o r s i n t h e i r n e u t r a l forms (mechanisms A . i i i . and A.iv . ) i s the predominant phenomenon l e a d i n g to mi n e r a l c o l l e c t i o n . Although p h y s i c a l l y adsorbed molecules (mechanisms A . i . and A . i i . ) are commonly present i n c o l l e c t o r c o a t i n g s , t h e i r r o l e i n the f l o t a t i o n process has been assumed of secondary importance compared w i t h that of the chemisorbed species (61). S o l u b i l i t y Theory of F l o t a t i o n The " s o l u b i l i t y theory" (mechanism B . i . , Table 1) was suggested by Taggart and a s s o c i a t e s (64), i n 1930. This theory assumes th a t a d s o r p t i o n of c o l l e c t o r s on minerals i s due to chemical bonds which obey laws c l o s e l y r e l a t e d to those governing the p r e c i p i t a t i o n of substances of low s o l u b i l i t y . C r y s t a l s t r u c t u r e , s urface e l e c t r i c a l p r o p e r t i e s and p h y s i c a l a d s o r p t i o n phenomena are u s u a l l y disregarded by t h i s theory. The d i f f e r e n t i a t i o n between t h i s mechanism and mechanism A . i v . a . i s o f t e n d i f f i c u l t i n p r a c t i c e . E x i s t i n g a n a l y t i c a l t o o l s , u s u a l l y , do not enable one to decide whether ads o r p t i o n occurs by the r e a c t i o n of unioniz e d c o l l e c t o r s at the min e r a l surface or by an i o n - 11 -p r e c i p i t a t i o n mechanism. > Many examples e x i s t showing a c l o s e r e l a t i o n s h i p between s o l u b i l i t y of m e t a l - c o l l e c t o r complexes, adsorption of c o l l e c t o r on m inerals and f l o t a t i o n . Du R i e t z (65) p u b l i s h e d a s e r i e s of s o l u b i l i t y data f o r c a r b o x y l i c a c i d s and t h e i r s a l t s and s t r e s s e d i t s importance as a guide i n s e l e c t i n g adequate f l o t a t i o n c o n d i t i o n s . P a r t of h i s data i s reproduced i n Appendix A, together w i t h other s o l u b i l i t y data f o r c a r b o x y l i c compounds most p e r t i n e n t to the present work. Based on the s o l u b i l i t y theory and on the p o s s i b i l i t y of formation of very s t a b l e surface complexes, c h e l a t i n g agents were suggested as c o l l e c t o r s f o r the f l o t a t i o n process. C h e l a t i n g agents are molecules which have more than one a c t i v e group capable of c o o r d i n a t i n g w i t h a m e t a l l i c ±>n to form a r i n g - l i k e s t r u c t u r e , u s u a l l y very s t a b l e . The use of c h e l a t i n g agents i n f l o t a t i o n of o x i d i z e d copper minerals has been proposed by s e v e r a l i n v e s t i g a t o r s (11-13,15,34,38). Ion A d s o r p t i o n Theory In 1955, Gaudin and Fuerstenau (66,67) introduced the fundamentals of the s o - c a l l e d " i o n adsorption theory" (mechanisms B . i i . a . , B . i i . b . , and B . i i . c , Table 1). I t s c o n t r i b u t i o n to the understanding of the mechanisms of f l o t a t i o n , e s p e c i a l l y f l o t a t i o n of oxide and s i l i c a t e minerals has been outstanding. According to t h i s theory, the d r i v i n g f o r c e f o r a d s o r p t i o n of c o l l e c t o r s on minerals i s p r i m a r i l y couiombic i n nature; i . e . , c o l l e c t o r ions are a t t r a c t e d to o p p o s i t e l y charged surfaces by e l e c t r o s t a t i c f o r c e s . F u r t h e r d i s c u s s i o n of the i o n - 12 -ads o r p t i o n theory r e q u i r e s the knowledge of the s t r u c t u r e of the " e l e c t r i c a l double l a y e r s " . F i g u r e 1 i s a schematic r e p r e s e n t a t i o n (Stern model) of the e l e c t r i c a l double l a y e r (1,60). I t shows a s p h e r i c a l p a r t i c l e i n an aqueous s o l u t i o n and the p o t e n t i a l drop across the i n t e r f a c e . F i v e zones can be d i f f e r e n t i a t e d : the s o l i d p a r t i c l e zone (AB), the l a y e r of p o t e n t i a l - d e t e r m i n i n g ions (BC), the l a y e r of anchored counter i o n s , known a l s o as i n t e r n a l or Stern l a y e r (CD), the aqueous l a y e r c o n t a i n i n g counter i o n s , c a l l e d the e x t e r n a l or Gouy-Chapman l a y e r (DE) and the bu l k l i q u i d (EL). The p o t e n t i a l drop across the double l a y e r can be d i v i d e d i n t o two p a r t s : ^0~^>^» the l i n e a r drop across the Stern l a y e r and ijjg, the e x p o n e n t i a l drop across the d i f f u s e l a y e r . The p o t e n t i a l C - i/j~is c a l l e d the e l e c t r o k i n e t i c or z e t a p o t e n t i a l . I f a p a r t i c l e o * such as the one shown i n F i g . 1 i s plac e d i n an e l e c t r i c p o t e n t i a l g r a d i e n t , the s o l i d and i t s surroundings to the l e f t of plane P (shear plane) w i l l move i n one d i r e c t i o n and the l i q u i d to the r i g h t w i l l move i n the opposite d i r e c t i o n . The ze t a p o t e n t i a l i s the p o t e n t i a l at the shear plane. I t i s r e l a t e d to measureable parameters such as e l e c t r o p h o r e t i c m o b i l i t y , streaming p o t e n t i a l , sedimentation p o t e n t i a l and e l e c t r o o s m o t i c pressure. Techniques f o r e v a l u a t i n g t, are discussed elsehwere (1,60,68). The i o n ad s o r p t i o n theory s t a t e s that c o l l e c t o r ions f u n c t i o n as counter ions i n the double l a y e r . The ad s o r p t i o n of ions i n the d i f f u s e l a y e r (mechanism B . i . i . a . , Table 1) i s p h y s i c a l i n nature. A d s o r p t i o n i n the Stern l a y e r can be p h y s i c a l or chemical, according to the s t r e n g t h of the adsorbate/adsorbent bonds. Besides e l e c t r o s t a t i c - 13 -dis t a n c e F i g u r e 1. The e l e c t r i c a l double l a y e r . - 14 -a t t r a c t i o n s , e l e c t r o s t a t i c r e p u l s i o n s between i o n s , l a t e r a l van der Waals f o r c e s , hydrogen bonds and covalent f o r c e s are l i k e l y to p a r t i c i p a t e In the mechanisms of a d s o r p t i o n of f l o t a t i o n c o l l e c t o r s i n the Stern l a y e r . In the case of mechanism B . i i . b . , i . e . , p h y s i c a l a d s o r p t i o n i n the Stern l a y e r , the d i s t a n c e of approach of the adsorbed ions to the s u r f a c e i s r e l a t i v e l y l a r g e and consequently e l e c t r i c a l a t t r a c t i o n s are weaker. This mechanism i s h i g h l y n o n - s e l e c t i v e . A l s o , any decrease i n the c o n c e n t r a t i o n of c o l l e c t o r ions i n s o l u t i o n leads to a p r o p o r t i o n a l decrease i n the amount of c o l l e c t o r p h y s i c a l l y adsorbed at the mineral s u r f a c e . This i s not always true i n the case of chemisorption of ions (mechanism B . i i . c . ) which i n v o l v e s much c l o s e r d i s t a n c e s of approach of adsorbate to the s u r f a c e . Strong e l e c t r o -s t a t i c a t t r a c t i o n s are developed and depending on chemical a f f i n i t i e s , o ther bonds (covalent or hydrogen bonds) can r e i n f o r c e the c o l l e c t o r / s u r f a c e group i n t e r a c t i o n s . S o l u b i l i t y of the s u r f a c e compounds and s t e r i c e f f e c t s become important; the a d s o r p t i o n process i s more s e l e c t i v e . (c) Charge Formation on Oxide Surfaces The p o t e n t i a l - d e t e r m i n i n g ions have a major i n f l u e n c e i n the mechanisms of the i o n a d s o r p t i o n theory s i n c e they determine the s i g n and magnitude of the m i n e r a l s u r f a c e charge. In the case of i o n i c s o l i d s , the ions c o n s t i t u t i n g the c r y s t a l l a t t i c e are the most important p o t e n t i a l - d e t e r m i n i n g i o n s . The s u r f a c e charge of oxides i s h i g h l y dependent on the pH of the s o l u t i o n , and the r o l e of hydrogen and h y d r o x y l ions ( H + and OH ) as p o t e n t i a l - d e t e r m i n i n g ions i n the case of - 15 -oxide surfaces i s w e l l recognized. According to Parks (69) , s u r f a c e charge formation on oxides can be e x p l a i n e d by the f o l l o w i n g mechanisms: Oxide surfaces i n an aqueous environment are hydroxylated. Charge can be formed on these surfaces by d i s s o c i a t i o n of the hydroxide group; MOH >- M0~ + H + (I) MOH M + + OH ( I I ) M + + H o0 y MOH„ + ( I I I ) — 2 •* — 2 MOH + H + -—>- MOH 2 + (IV) (M represents a metal atom at the oxide s u r f a c e ) . A c i d d i s s o c i a t i o n ( r e a c t i o n I ) produces negative s i t e s . Reactions I I , I I I , and IV produce p o s i t i v e s i t e s . Also., charged hydroxo-complexes der i v e d from d i s s o l u t i o n of the s o l i d are expected to be s p e c i f i c a l l y adsorbed and act as p o t e n t i a l - d e t e r m i n i n g i o n s . Adsorbed p o s i t i v e complex ions form p o s i t i v e s urface s i t e s and negative complex ions form negative s i t e s . A d s o r p t i o n of these p o t e n t i a l - d e t e r m i n i n g ions i s p r o p o r t i o n a l to t h e i r c o n c e n t r a t i o n i n the s o l u t i o n which i n t u r n depends on pH. The pH at which the net surface charge of an oxide i s zero was designated as the i s o e l e c t r i c p o i n t (IEP) or the p o i n t of zero charge (PZC). L a t e r , Parks ( 7 0 ) d i f f e r e n t i a t e d these terms s t a t i n g t h a t : " I f charge i s e s t a b l i s h e d only by~H +, OH , and species capable of i n t e r a c t i n g w i t h H +, OH , and H^ O to form species present i n the s o l i d l a t t i c e , i . e . by p o t e n t i a l - d e t e r m i n i n g ions (PDI), then the PZC may be conveniently given the s p e c i a l name of I E P ^ " . - 16 -More r e c e n t l y Parks and Smith (1969) (71) r e d e f i n e d the terms PZC and IEP. The PZC was considered as the pH at which equal a d s o r p t i o n of H + and OH should l e a d to zero surface charge. The pH at which the observed zeta p o t e n t i a l i s equal to zero was named the IEP. PZC and IEP are i d e n t i c a l only when no s p e c i f i c a d s o r p t i o n of charged hydroxo-complexes occurs. (This l a s t nomenclature w i l l be f o l l o w e d throughout t h i s d i s s e r t a t i o n . ) IEP of Cupric Oxide The IEP of c u p r i c oxide has been measured at pH 9.4 +0.4 (69). The s o l u b i l i t y diagram f o r c u p r i c oxide ( F i g . 2) shows that the IEP i s approximately equal to the pH at p o i n t P where equal concentrations of Cu0H + and Cu(0H) 3 should be present i n the bulk s o l u t i o n . This seems to i n d i c a t e t h a t the e q u i l i b r i u m constant of the a c i d and b a s i c d i s s o c i a t i o n r e a c t i o n s of the hydroxylated copper s u r f a c e s i t e s are equal'to those of analogous bulk r e a c t i o n s l e a d i n g to the formation of the hydroxo-complexes CuOH + and Cu^H)^. The p o i n t P i s a l s o at the pH where the c a l c u l a t e d s o l u b i l i t y of c u p r i c oxide i s minimum. According to Parks and deBruyn (72), the pH of minimum s o l u b i l i t y should c o i n c i d e w i t h the IEP only i n the absence of s p e c i f i c a d s o r p t i o n of hydroxo-complexes. I f t h i s i s the case, OH and H*^  should be the main p o t e n t i a l - d e t e r m i n i n g ions of c u p r i c oxide and the c o n c e n t r a t i o n of copper ions i n s o l u t i o n should not a f f e c t the p o s i t i o n of the IEP of the system. - 18 -IEP of Cupric Hydroxide Yoon and Salman (73) s t u d i e d the e f f e c t of pH on the z e t a p o t e n t i a l of c u p r i c hydroxide (CutOH^, a metastable compound formed by b a s i c p r e c i p i t a t i o n of copper ions i n water ) and found the IEP o f f r e s h l y prepared samples at pH 7.7. The IEP of dry aged hydroxide c o n t a i n i n g s u p e r f i c i a l c u p r i c oxide was found at pH 7.3. The s o l u b i l i t y diagram f o r CuO and f o r Cu(0H)2 ( F i g . 2) shows the l i n e s f o r Cu i o n s d e r i v e d from the hydroxide and from the oxide i n t e r c e p t i n g the OH l i n e at pH's 7.8 and 7.4, r e s p e c t i v e l y ( p o i n t s P l and P2 i n the I | _ diagram). Based on t h i s f a c t , they concluded that Cu and OH are the major p o t e n t i a l - d e t e r m i n i n g ions of c u p r i c hydroxide p r e c i p i t a t e s . 1.3 Scope and Approach of the Study The purpose of the present work has been to i n v e s t i g a t e the mechanisms of a d s o r p t i o n of c o l l e c t o r s onto copper oxide s u b s t r a t e s and t h e i r r e l a t i o n s h i p w i t h the f l o a t a b i l i t y of the o x i d i z e d copper m i n e r a l s . E x p e r i m e n t a l l y , the p r o j e c t has c o n s i s t e d of two main phases: ( i ) f l o t a t i o n s t u d i e s and ( i i ) i n f r a r e d s p e c t r o s c o p i c s t u d i e s . The r e s u l t s of these experiments have been used to i n t e r p r e t the r o l e of m i n e r a l s u r f a c e charge i n the mechanisms of a d s o r p t i o n of c o l l e c t o r onto copper oxide s u b s t r a t e s . Diagrams of e q u i l i b r i a i n systems c o n t a i n i n g copper, water, oxygen, carbon d i o x i d e and sulphur are presented i n Appendix B. - 19 -The Role of M i n e r a l Surface Charge on Teno r i t e F l o t a t i o n A S i n g l e m i n e r a l m i c r o - f l o t a t i o n t e s t s have been c a r r i e d out on t e n o r i t e to i n v e s t i g a t e the s i g n i f i c a n c e of surface charge on the f l o a t a b i l i t y of t h i s m i n e r a l w i t h i o n i c c o l l e c t o r s . In so doing, the r o l e i n the f l o t a t i o n of t e n o r i t e played by H + and other ions which are l i k e l y to be s p e c i f i c a l l y adsorbed on o x i d i z e d copper surfaces has been examined. Since t h i s stage has e s t a b l i s h e d a c o r r e l a t i o n between su r f a c e charge and f l o t a t i o n recovery, i t was then of prime i n t e r e s t to i n v e s t i g a t e the e f f e c t of sur f a c e charge on the adsor p t i o n of c o l l e c t o r species onto o x i d i z e d s u b s t r a t e s . A d s o r p t i o n of Aqueous L a u r i c A c i d onto Copper Oxide Substrates In saarching f o r the r e l a t i o n s h i p between surface charge and ads o r p t i o n of c o l l e c t o r , the i n t e r f a c i a l r e a c t i o n s of copper oxides i n aqueous s o l u t i o n s of l a u r i c a c i d (C^^H^^COOH) have been s t u d i e d . The e f f e c t of v a r i a b l e s such as p H , c o l l e c t o r c o n c e n t r a t i o n , c u p r i c ions c o n c e n t r a t i o n , p a r t i a l pressure of carbon d i o x i d e , d i s s o l v e d oxygen c o n c e n t r a t i o n , time of exposure and c i r c u l a t i n g v e l o c i t y of the l i q u i d , on the nature and co n c e n t r a t i o n of the adsorbed l a u r a t e s p e c i e s , has been i n v e s t i g a t e d . To' avoid side r e a c t i o n s which could preclude the i n t e r p r e t a t i o n of the adsorption mechanisms, the systems s t u d i e d have been made as simple as p o s s i b l e . L a u r i c a c i d , a s t r a i g h t c h a i n , s a t u r a t e d c a r b o x y l i c ( f a t t y ) a c i d was the c o l l e c t o r s e l e c t e d . I t i s a powerful c o l l e c t o r w i t h r e l a t i v e l y simple chemistry. H i g h l y pure, a r t i f i c i a l oxides of copper ( c u p r i c and cuprous oxides) were used as Laboratory f l o t a t i o n t e s t s on the s c a l e of a few grams sample are commonly c a l l e d m i c r o - f l o t a t i o n t e s t s . - 20 -adsorbents. During r e a c t i o n , the l i q u i d phase c o n t a i n i n g known concentrations of reagents was kept i n an a l l t e f l o n v e s s e l under c l o s e l y c o n t r o l l e d gaseous environments. Surface r e a c t i o n products and adsorbed f i l m s have been analysed by i n f r a r e d (IR) spectroscopy. The r e c o r d i n g of IR s p e c t r a of t h i n surface f i l m s r e q u i r e s the use of unconventional, h i g h l y s e n s i t i v e , s p e c t r o s c o p i c techniques (74,75). " R e f l e c t i o n spectroscopy" (76) makes use of o p t i c a l arrangements to f o r c e the r a d i a t i o n to pass through one f i l m s e v e r a l times, i n c r e a s i n g s e n s i t i v i t y . The f o l l o w i n g r e f l e c t i o n s p e c t r o s c o p i c techniques have been used i n the present s t u d i e s : ( i ) M u l t i p l e Specular R e f l e c t i o n Technique An eleven r e f l e c t i o n , 70° in c i d e n c e angle technique has been a p p l i e d to the s t u d i e s of ad s o r p t i o n of l a u r i c a c i d onto copper oxide s u b s t r a t e s . This technique has made p o s s i b l e i d e n t i f i c a t i o n and q u a n t i t a t i v e a p p r a i s a l of adsorbed c o l l e c t o r species at sur f a c e concentra-t i o n l e v e l s as low as o n e - t h i r d of a monomolecular l a y e r ("monolayer"). Concentrations lower than one-tenth of a monolayer have been detected. P r e v i o u s l y , t h i s and s i m i l a r systems had never been analyzed w i t h such a high degree of s e n s i t i v i t y . Successive monolayers of s t e a r i c a c i d (C^H^COOH) were deposited onto copper oxide coated f r o n t s urface gold m i r r o r s according to the "Langmuir-Blodgett method" and t h e i r s pecular r e f l e c t i o n s p e c t r a were recorded. The purpose of these experiments has been to e s t a b l i s h A general d i s c u s s i o n of the t h e o r i e s and a p p l i c a t i o n s of these techniques i s presented i n Chapter I I . - 21 -standard reference s p e c t r a f o r q u a n t i t a t i v e a n a l y s i s of the c a r b o x y l a t e species adsorbed on copper oxide s u b s t r a t e s from aqueous s o l u t i o n s . R e f l e c t i o n spectroscopy s t i l l i s , e s s e n t i a l l y , a r e l a t i v e method of a n a l y s i s . Absolute measurement of adsorbate c o n c e n t r a t i o n or average f i l m t h i c k n e s s , d i r e c t l y from IR band i n t e n s i t y , i s not p o s s i b l e at t h i s stage of advance of the r e f l e c t a n c e t h e o r i e s . Valuable q u a n t i t a t i v e i n f o r m a t i o n can be gained by comparing sample s p e c t r a w i t h the s p e c t r a of f i l m s of known co n c e n t r a t i o n or known t h i c k n e s s . Films w i t h known conce n t r a t i o n s of f a t t y a c i d s can be b u i l t by t r a n s f e r i n g compact mono-l a y e r s of the a c i d from the l i q u i d / a i r i n t e r f a c e to the s o l i d s u rface (Langmuir-Blodgett method) (77). A s e r i e s of experiments have been conducted to i n v e s t i g a t e the e f f e c t s of molecular o r i e n t a t i o n on the IR s p e c t r a of s u r f a c e f i l m s * recorded by the s p e c u l a r r e f l e c t i o n technique. These have i n c l u d e d the r e c o r d i n g of s p e c t r a of s t e a r i c a c i d molecules deposited onto gold and copper oxide s u b s t r a t e s , from the gaseous phase and from the a i r / water i n t e r f a c e . Films w i t h molecules p r e s e n t i n g d i f f e r e n t degrees of o r i e n t a t i o n were formed by d e p o s i t i o n on smooth and rough s u b s t r a t e s . ( i i ) " I n s i t u " Attenuated T o t a l I n t e r n a l R e f l e c t i o n Technique "In s i t u " i n f r a r e d s p e c t r a of c a r b o x y l i c compounds adsorbed on copper oxide f i l m s have a l s o been recorded. Specular r e f l e c t i o n techniques r e q u i r e the e x t r a c t i o n of the adsorbent from the aqueous This subject may seem to bear l i t t l e r e l a t i o n s h i p to the main o b j e c t i v e s of the present work. However, the author b e l i e v e s that the r e s u l t s of these experiments may be.of s p e c i a l use to the development of the t h e o r i e s of r e f l e c t i o n spectroscopy and the understanding of the behavior of organic monolayers at s o l i d surfaces and at l i q u i d / g a s i n t e r f a c e s . For t h i s reason, these experiments w i l l be d e s c r i b e d and discussed at f u l l l e n g t h i n the present d i s s e r t a t i o n . - 22 -medium p r i o r to r e c o r d i n g of the s p e c t r a . Since there was a p o s s i b i l i t y t hat t h i s procedure might have a l t e r e d the composition of the i n t e r f a c e s , a technique f o r r e c o r d i n g " i n s i t u " IR s p e c t r a of A cuprous oxide surfaces has been developed. This technique was based on the p r i n c i p l e s of attenuated t o t a l i n t e r n a l r e f l e c t i o n spectroscopy (ATR) (78). Spectra of the s u b s t r a t e , the i n t e r f a c e and p o r t i o n s of the bulk aqueous medium were recorded simultaneously. Attempts to record " i n s i t u " s p e c t r a of c u p r i c oxide f i l m s f a i l e d because of l a c k of adhesion of the oxide to the IR transparent m a t e r i a l s . - 23 -CHAPTER I I INFRARED SPECTRA OF SOLID SURFACES 2.1 Transmission Technique Conventional Spectroscopy I t has been s a i d that one of the most c h a r a c t e r i s t i c 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. I n f r a r e d l i g h t covers the wavelength range of 0.8 to 2000 u (one -4 micron = 1 u = 10 cm). The i n f r a r e d spectrum i s d i v i d e d i n t o the r e g i o n s : the "near i n f r a r e d " which i s the r e g i o n between 0.8 and 2.5 u; the " m i d - i n f r a r e d " between 2.5 and 50 u; and the " f a r i n f r a r e d " which l i e s between 50 and 100 p. 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 a b s o r p t i o n s p e c t r a i n the m i d - i n f r a r e d r e g i o n . 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; i . e . , v = (cm 1 u n i t s ) where X i s wavelength i n centimeters. The A wavenumber range of conventional i n f r a r e d spectroscopy i s 4000 to 200 cm The r e c o r d i n g of i n t e n s i t y of the IR r a d i a t i o n t r a n s m i t t e d through (or absorbed by) a sample versus frequency y i e l d s a curve showing ab s o r p t i o n bands c h a r a c t e r i s t i c of the atoms and chemical 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 c o n v e n t i o n a l t r a n m i s s i o n i n f r a r e d spectroscopy. A b s o r p t i o n bands t h a t 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 - 24 -u s e f u l f o r i d e n t i f i c a t i o n of that group are c a l l e d c h a r a c t e r i s t i c f r e q u e n c i e s . Appendix C contains c h a r a c t e r i s t i c group frequencies f o r compounds most c l o s e l y r e l a t e d to the present work. Beer's Law and Absorption Parameters 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 s p e c i e s . According to 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 wavenumber v by an absorbing medium i s given by: -k c l 1 - v v [ 1 ] 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 ; c = c o n c e n t r a t i o n of o 3 ' absorbing molecules i n the sample; l=path length of l i g h t i n the sample; and 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). The transmittance T i s defined as \\J\\ . Another q u a n t i t y , the absorbance A, i s defined as: A = e ^ c l [2] 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 . c m 1 ("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 v e x t i n c t i o n c o e f f i c i e n t " = e ). v A "mass absorption c o e f f i c i e n t " a , a l s o used i n a s s o c i a t i o n w i t h v Beer's law, i s defined as: - 25 -a v = 2.303 j (1 i n cm) [3] or a v = 2.303 p x e (p = d e n s i t y i n molecule.cm )[4] The abso r p t i o n c h a r a c t e r i s t i c s of a compound can be expressed by s t i l l another constant K ( u n i t l e s s ) , c a l l e d " a b s o r p t i o n constant" and defined by the r e l a t i o n : where X i s the wavelength i n centimeters. D i f f i c u l t i e s ..in..Recording IR s p e c t r a ,of Thin .Surface Films 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 or t h i n f i l m s of r e a c t i o n products formed on s o l i d s u r f a c e s . This i s due to the f o l l o w i n g reasons: ( i ) Most s o l i d s present very strong a b s o r p t i o n bands i n the IR reg i o n which confines the abso r p t i o n bands belonging to surface species to a r a t h e r l i m i t e d s p e c t r a l range. ( i i ) S o l i d p a r t i c l e s may s c a t t e r a s i g n i f i c a n t amount of energy which i s then l o s t , reducing instrument performance. ( i i i ) Absorbance of t h i n s urface l a y e r s i s u s u a l l y extremely low. To counteract these e f f e c t s there are two p r a c t i c a l ways of i n c r e a s i n g absorbance by the surface species and decreasing r a d i a t i o n a b s o r p t i o n or s c a t t e r i n g by the s u b s t r a t e . One, the powder method, uses high area samples to i n c r e a s e the a v a i l a b l e surface f o r adsorption K = CtX/4TT [5] - 26 -and to decrease r a d i a t i o n s c a t t e r i n g by reducing the p a r t i c l e s i z e s , below the wavelength of the IR l i g h t . The other, the r e f l e c t i o n method, w i l l be the subject of Sections 2.2 and 2.3. Pressed S a l t Technique In order to r e c o r d the s p e c t r a of powders, one must deal w i t h the problem of supporting the sample i n the normal beam p o s i t i o n of the spectrophotometer. There are s e v e r a l techniques to accomplish t h i s . One of the most.: commonly used i s the "pressed s a l t technique" i n which the powder i s mixed w i t h f i n e c r y s t a l s of an IR transparent s a l t and pressed i n order to form a d i s c . The d i s c i s e a s i l y mounted on the spectrophotometer and i t s IR t r a n s m i s s i o n spectrum recorded. To a ,goo.d approximation,, .band i n t e n s i t i e s obey Beer's law. A few f l o t a t i o n i n v e s t i g a t o r s have used the pressed s a l t technique f o r a n a l y s i n g c o l l e c t o r f i l m s formed on o x i d i z e d copper substrates (37,52,79,80). However, care must be e x e r c i s e d i n the i n t e r p r e t a t i o n of s p e c t r a of surface f i l m s obtained by t h i s technique. Changes i n the character of the s u r f a c e may be produced by the p r e s s i n g process (81). Ion exchange r e a c t i o n s between the s u r f a c e species and the s a l t m a t r i x may occur (82). A l s o , IR transparent s a l t s u s u a l l y absorb a f a i r amount of atmospheric water. The presence of water i n the sample makes d i f f i c u l t the assignment of bands due to surface species i n the IR s p e c t r a l regions c l o s e to the c h a r a c t e r i s t i c a b s o r p t i o n bands of water (given i n Appendix C). For these reasons, the pressed s a l t technique was used i n the present work only f o r r e c o r d i n g s p e c t r a of b u l k s o l i d compounds (reference t r a n s m i s s i o n s p e c t r a ) . Surface f i l m s were analyzed by means of r e f l e c t i o n techniques. - 27 -to the detector Sample beam from source sample mirrors (or internal reflector plate) Figure 3. Optical arrangement for reflection spectroscopy. (Multiple reflection attachment manufactured by Wilts Scientific Corp. Model 9). Frequency Figure 4. Variation of n and k through an absorption band. 2.2 M u l t i p l e Specular R e f l e c t i o n Technique 1 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 absorbing 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 s u r f a c e , a p l o t of r e f l e c t a n c e against frequency c o n s t i t u t e s a specular r e f l e c t i o n spectrum of the m a t e r i a l . 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 analyzed i s employed and an o p t i c a l arrangement forces 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 (as i l l u s t r a t e d i n F i g . 3). M u l t i p l e r e f l e c t i o n spectroscopy i s of p a r t i c u l a r i n t e r e s t to s t 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 molecules w i t h the s o l i d s u r f a c e s . I n a recent review of the technique, P o l i n g concluded that although there are some d i s c r e p a n c i e s among the t h e o r e t i c a l analyses of the o p t i c a l phenomena i n v o l v e d , they a l l confirm t h a t : "1. .recording i n f r a r e d r e f l e c t i o n s p e c t r a of f i l m s as t h i n as one monolayer can be achieved w i t h the use of commercial d i s p e r s i v e type spectrophotometers; 2. r e f l e c t i o n s p e c t r a of moderately absorbing f i l m s such as organic f i l m s d i f f e r very l i t t l e i n appearance from t r a n s m i s s i o n s p e c t r a ; 3. i n f o r m a t i o n on molecular o r i e n t a t i o n i n a n i s o t r o p i c f i l m s can be gained from the s p e c t r a ; 4. 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 " (83). P o s i t i o n , Shape and I n t e n s i t y of Reflectance Bands 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 i n t e r r e l a t e d phenomena: (a) an i n c r e a s e i n the absorption 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 near the a b s o r p t i o n band (84). - 29 -T y p i c a l v a r i a t i o n s of n and k through an a b s o r p t i o n band are shown i n F i g . 4. The extent of the d i s p e r s i o n i n the r e f r a c t i v e index i s p r o p o r t i o n a l to the v a r i a t i o n i n the value o f k. For a moderately a b s o r b i n g f i l m (maximum k v a l u e s of approximately 0.1; e.g., org a n i c f i l m s ) the d i s p e r s i o n i n the v a l u e of n i s s m a l l . Phenomenon A i s mai n l y r e s p o n s i b l e f o r the fo r m a t i o n of the r e f l e c t a n c e band which, in t h i s case, has shape and p o s i t i o n c o i n c i d i n g w i t h those of an a b s o r p t i o n band (as obtained by t r a n s m i s s i o n s p e c t r o s c o p y ) . For the case o f very h i g h v a l u e s of k, the d i s p e r s i o n i n n i s a p p r e c i a b l e and band f o r m a t i o n i s governed by phenomenon B. R e f l e c t a n c e bands of s t r o n g l y absorbing f i l m s are expected to resemble the d i s p e r s i o n i n the r e f r a c t i v e index. Consequently t h e i r shape and p o s i t i o n should d i f f e r from a b s o r p t i o n bands. 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 u s u a l l y measured i n terms of 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 a band maximum: . R. .- R AR - -\ [6] o where, R q = r e f l e c t i v i t y i n the absence of a f i l m , and R = r e f l e c t i v i t y a t band maximum. For the case of m u l t i p l e r e f l e c t i o n s , a spectrophotometer, N N l i n e a r in t r a n s m i t t a n c e , w i l l r e c o r d T = R and T = R (N i s the ' o o number of r e f l e c t i o n s ) from which AR v a l u e s can be c a l c u l a t e d (T = t r a n s m i t t a n c e or r e f l e c t a n c e i n the absence of a f i l m , and T = tr a n s m i t t a n c e or r e f l e c t a n c e a t band maximum). The minimum £R v a l u e which can be measured depends on the number of r e f l e c t i o n s employed. Using seven r e f l e c t i o n s , AR v a l u e s as low - 30 -as 0.0005 have been measured r e p r o d u c i b l y (85). For a higher number > of r e f l e c t i o n s , even lower values can be obtained. However, Greenler (86) has shown that there i s an optimum number of r e f l e c t i o n s f o r each i n c i d e n c e angle. His r e s u l t s i n d i c a t e that the optimum number of r e f l e c t i o n s v a r i e s from 8, f o r 88° i n c i d e n c e , to 25, f o r 70° i n c i d e n c e . A decrease i n s e n s i t i v i t y of the order of 20 to 30 percent i s obtained by usi n g h a l f the optimum number of r e f l e c t i o n s . F r a n c i s and E l l i s o n Theory Among the va r i o u s attempts to e x p l a i n the IR r e f l e c t i o n s p e c t r a of t h i n f i l m s on metal s u r f a c e s , the work by F r a n c i s and E l l i s o n (87) deserves s p e c i a l mention. Based on Fry's treatment of the abso r p t i o n of r a d i a t i o n by a - t h i n - i s o t r o p i c f i l m on a r e f l e c t i n g s urface (88), t h e i r theory p r e d i c t s that a b s o r p t i o n of the component of r a d i a t i o n p e r p e n d i c u l a r to the plane of indicence would not be d e t e c t a b l e . 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//) would be given by the f o l l o w i n g expression: 2 16^d cose . " l K l S ± n 9 n 4 f ( n l ' 9 ) . // = — C - A — r i a — 4 ~ ] [ 7 ] n, cos 6 K. cos 6 , 1 4 where (see F i g . 5a), d i s the f i l m t h i c k n e s s , X i s the wavelength, 0 i s the angle of i n c i d e n c e , n^ and n^ are the r e f r a c t i v e i n d i c e s of the f i l m and of the metal, r e s p e c t i v e l y , and are abso r p t i o n constants, and f(n^,6) i s a f u n c t i o n of n^ and 6 which has a value of the order of magnitude of u n i t y . 6 = 180°; E 1 = 0 at s u r f a c e 5(b) Phase s h i f t (6) during r e f l e c t i o n component normal to the plane of in c i d e n c e 6 = s m a l l ; E 5(c) P a r a l l e l component. F i n i t e phase s h i f t . Standing wave f i e l d (E ) normal to the metal surface. / / » z F i g u r e 5. F r a n c i s and E l l i s o n Theory. - 32 -For good r e f l e c t o r s and 6 values between 0° and 80°, K^cos 0 » n^,1 n^ f ( n 1 , e ) f(n^,0) and the approximation —^ ^ - 0 can be made. Equation K. cos G 4 [7] can then be s i m p l i f i e d to a more p r a c t i c a l form: 2 . 16TT s i n 0 K.. AR i i = [ 3-^] x d [8] X cose n^ A t h i n f i l m cannot absorb the component of the r a d i a t i o n p e r p e n d i c u l a r to the plane of i n c i d e n c e because of the f a c t that t h i s component undergoes a near 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 i n c i d e n c e . As i l l u s t r a t e d i n F i g . 5b, a standing wave i s formed having zero amplitude at the s u r f a c e . Consequently, 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 . The same i s not t r u e f o r the p a r a l l e l component ( F i g . 5c) which undergoes a f i n i t e phase change on r e f l e c t i o n , but t h i s becomes 180° only at 90° i n c i d e n c e 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 (E,, ) d i r e c t e d mainly normal to the / / > z plane of 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 and be absorbed by the f i l m , i n s t a l l a t i o n of 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 " i n e f f e c t i v e " p e r p e n d i c u l a r component from the r a d i a t i o n reaching the detector of the instrument, markedly enhances the percentage a b s o r p t i o n of the p a r a l l e l component of the l i g h t . E f f e c t s of Molecular O r i e n t a t i o n The preceding d i s c u s s i o n a p p l i e s to f i l m s i n which there i s no molecular o r i e n t a t i o n . For the case of f i l m s having a n i s o t r o p i c o p t i c a l - 33 -p r o p e r t i e s , an a n a l y s i s of the behavior of t h e i r IR s p e c t r a , based on the aforementioned work by F r a n c i s arid E l l i s o n , i s presented below. The i n t e n s i t y of a s p e c t r o s c o p i c t r a n s i t i o n (as given by any of the a b s o r p t i o n parameters defined e a r l i e r ) i s d i r e c t l y r e l a t e d to the p r o b a b i l i t y P that a molecule (or group) w i l l undergo a t r a n s i t i o n i n u n i t time. For good r e f l e c t o r s at in c i d e n c e angles between 0° and 80°, P can be approximated by the f o l l o w i n g expres s i o n : where, h i s Planck's constant, u z i s the component (per p e n d i c u l a r to the metal surface) of the ma t r i x element of the d i p o l e moment f o r the t r a n s i t i o n , and E.. i s the " e f f e c t i v e " e l e c t r i c f i e l d at the molecule (or group). M o l e c u l a r o r i e n t a t i o n should not s t r o n g l y a f f e c t the component of the f i e l d p e r p e n d i c u l a r to the surface (the only component of the l i g h t which canbe absorbed by the f i l m ) . A n i s o t r o p y , however, should a f f e c t the components of the matrix element of the d i p o l e moment. I f the molecules (or groups) i n the f i l m are o r i e n t e d w i t h respect to the r e f l e c t i n g s u r f a c e , the d i p o l e moment changes w i l l have a s p e c i f i c d i r e c t i o n f o r each v i b r a t i o n a l mode. Since the f i e l d w i l l be predomin-a n t l y p e r p e n d i c u l a r to the s u r f a c e , only w i l l be e f f e c t i v e i n producing a b s o r p t i o n of r a d i a t i o n . V i b r a t i o n a l modes i n surface species producing d i p o l e moment changes p a r a l l e l to the surface w i l l appear abnormally weak i n the s p e c t r a of f i l m s on metal m i r r o r s . - 34 -F r a n c i s and E l l i s o n reported experimental r e s u l t s which confirmed 1 t h e i r t h e o r e t i c a l p r e d i c t i o n s . They recorded the s p e c t r a of metal s t 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. Four r e f l e c t i o n s and a 72° inc i d e n c e angle were employed. S e n s i t i v i t y f o r r e c o r d i n g s p e c t r a of f i l m s one monolayer t h i c k was obtained. In the deposited l a y e r s , the methylene groups l i e i n planes n e a r l y p a r a l l e l to the s u r f a c e . S t r e t c h i n g v i b r a t i o n bands assigned to these groups were unusually weak. Asymmetric CV> and symmetric £t s t r e t c h i n g v i b r a t i o n s of o r i e n t e d o o o . o carboxylate groups have d i p o l e moment changes n e a r l y p a r a l l e l and normal to the s u r f a c e , r e s p e c t i v e l y . Bands assigned to the former mode were abnormally weak; the l a t t e r produced r e l a t i v e l y i ntense bands. Spectra of Copper Oxide Films P o l i n g (85) recorded IR spe c u l a r r e f l e c t i o n s p e c t r a of copper m i r r o r s subsequent to o x i d a t i o n i n a i r , at moderately hi g h temperatures. Seven r e f l e c t i o n s and an incidence angle of 73° were used. The i n t e n s i t i e s of the copper oxide bands were d i r e c t l y p r o p o r t i o n a l to o f i l m thickness (0 to 900 A) as determined by an i n t e r f e r o m e t e r . For m i r r o r s o x i d i z e d at a temperature of 140°C, the only band detected was an int e n s e band at 640 cm 1 . Transmission s p e c t r a of bulk cuprous and c u p r i c oxides e x h i b i t s i n g l e bands at approximately 620 and 510 cm x , r e s p e c t i v e l y (see Appendix C). The oxide f i l m grown on these m i r r o r s * -1 was i d e n t i f i e d as r e l a t i v e l y pure cuprous oxide (Cu 0) . The +20 cm * Wieder and Czanderna (89) reported that i n s t e a d of Cu„0, an oxide phase of composition CuO , 7 i s formed i n t h i n copper f i l m s (200 to to 2400 A t h i c k ) by he a t i n g i n oxygen atmosphere. - 35 -s h i f t of band p o s i t i o n , due to d i s p e r s i o n of the r e f r a c t i v e index, 1 agreed w i t h that c a l c u l a t e d by means of equation [8] and using K and n values given by O'Keefe (90). M i r r o r s o x i d i z e d at temperatures above 250°C produced s p e c t r a c o n t a i n i n g a s i n g l e band at 560 cm \ thus the f i l m s were i d e n t i f i e d as c u p r i c oxide (CuO). The 560 cm X band maximum f o r c u p r i c oxide represents a +50 cm 1 s h i f t from the p o s i t i o n of the abs o r p t i o n band. Greenler and co-workers (84) reported the r e f l e c t a n c e band f o r C^O (or CuO ^ ) at 655 cm \ They employed an o p t i c a l accessory which produced one to three r e f l e c t i o n s and a spread of the in c i d e n c e angles from 84° to 90°. F i l m t h i c k n e s s of 450, 900, 2500, and 4200 A were analysed. Band i n t e n s i t y i n c r e a s e d w i t h f i l m t h i c k n e s s to a i o maximum -at a thic k n e s s of 2500 A then decreased on a f u r t h e r i n c r e a s e i n f i l m t h i c k n e s s . Spectra of C o l l e c t o r Films Adsorbed on Copper Substrates L e j a and a s s o c i a t e s (91) a p p l i e d the m u l t i p l e specular r e f l e c t i o n technique to a n a l y s i s of xanthate species adsorbed from aqueous s o l u t i o n s onto o x i d i z e d and s u l p h i d i z e d copper s u b s t r a t e s . They i d e n t i f i e d the sp e c t r a of the chemisorbed xanthate as that of cuprous xanthate. Formation of m u l t i l a y e r s of cuprous xanthate and dixanthogen was a l s o deduced. Low and co-workers (92) recorded specular r e f l e c t i o n and emission s p e c t r a of the products of r e a c t i o n of o l e i c a c i d w i t h copper p l a t e s (various degrees of o x i d a t i o n ) , i n a i r , and at temperatures v a r y i n g from 27 to 300°C. The i r r e s u l t s suggested that the main product of the - 36 -reaction is a binuclear copper oleate complex: [R-COO ]^Cu * ' " " C u A small band formed at 1735 cm 1 was assigned to an ester- l ike species: 0 Cu-O-C-R It 0 0 Cu 0-H Paterson and Salmon (80) also observed a very minor band at 1735cm ' during studies of absorption of aqueous sodium oleate on cupric hydroxide, using the pressed salt technique. Eischens (93) reported the presence of similar "ester" species during chemisorption of acetic acid (CH^-COOH) •on sili-ca and SiO^-MgO catalysts. His spectra showed the C=0 stretching band of the supposed "ester" at 1747 cm ' . A band at 1389 cm \ associated with the "ester" was attributed to bending vibrations of the CH^ groups. However, as pointed out by Eischens, "no attempt was made to confirm this interpretation by a study of chemisorbed dueterated acetic a c i d . " (93) Scowen and Leja (94) published IR specular reflect ion spectra of films adsorbed on oxidized copper substrates from aqueous sodium -4 -3 laurate (10 to 10 M solutions) at various pH's. Their spectra were recorded using a six ref lec t ion , 45° incidence angle technique. At neutral pH, they reported that sodium laurate was adsorbed on thickly oxidized copper; a mixture of sodium and cupric laurate was adsorbed on s l ight ly oxidized copper; and for a vacuum deposited f i l m , the species was cupric laurate. In acidic solutions, the adsorbed species was cupric laurate. At pH's>9, no adsorption was detected. - 37 -2.3 Attenuated T o t a l I n t e r n a l R e f l e c t i o n Technique Theory and a p p l i c a t i o n s of ATR spectroscopy have been summarized i n a book by H a r r i c k (78). The f a m i l i a r phenomenon of t o t a l i n t e r n a l r e f l e c t i o n i s shown i n F i g . 6a. The r a d i a t i o n i s t o t a l l y r e f l e c t e d from the i n t e r f a c e of a transparent d i e l e c t r i c prism ( r e f r a c t i v e index n^) and a medium of lower r e f r a c t i v e index (n^) at angles g r e a t e r than 0 c r i t i c a l ( 6 c = s i n " " ^ i ' n 2 1 = n 2 ^ n l ^ ' u u r i n g t o t a l i n t e r n a l r e f l e c -t i o n , the r a d i a t i o n penetrates s l i g h t l y i n t o the r a r e r medium. The p e n e t r a t i n g e l e c t r i c f i e l d (evanescent wave) can i n t e r a c t and be absorbed by the r a r e r medium or any t h i n f i l m of l i g h t absorbing m a t e r i a l p l a c e d on the face of the d i e l e c t r i c prism. The ATR s p e c t r a of the r a r e r medium and the f i l m ( i f present) are obtained by p l o t t i n g r e f l e c t a n c e versus frequency. The p o s i t i o n and shape of the ATR bands are s i m i l a r to those of t r a n s m i s s i o n a b s o r p t i o n bands. The s e n s i t i v i t y of the technique i s h i g h l y i n c r e a s e d by using m u l t i p l e r e f l e c t i o n s as i l l u s t r a t e d i n F i g . 6b. "In s i t u " ATR With the advent of ATR spectroscopy i n 1959 (95) , the hopes f o r r e c o r d i n g " i n s i t u " IR s p e c t r a of aqueous l i q u i d / s o l i d i n t e r f a c e s were r e v i v e d . The procedure normally f o l l o w e d during s p e c t r o s c o p i c ( t r a n s m i s s i o n or spe c u l a r r e f l e c t i o n ) s t u d i e s of surface f i l m s formed at w a t e r / s o l i d i n t e r f a c e s i s to record s p e c t r a of the s o l i d before and a f t e r exposure to the s o l u t i o n . This i s a consequence of the f a c t that the water IR spectrum u s u a l l y presents very broad a b s o r p t i o n bands. - 38 -p e n e t r a t i o n n^>r)2 displacement 6(a) T o t a l i n t e r n a l r e f l e c t i o n - A -V v — V s if ^ ^  f <» S V V " V 6(b) M u l t i p l e ATR prism IR beam sub s t r a t e (C^O) Figure 6. aqueous medium 6(c) "In s i t u " ATR v Attenuated t o t a l i n t e r n a l r e f l e c t i o n spectroscopy (ATR) adsorbate f i l m - 39 -Path lengths greater than a few microns in the aqueous medium are sufficient to block most of the IR spectral region. The extraction of the solid from the solution is undesirable because i t might alter the chemistry of the interfaces and intermediates might hot be observed. ' The extent of penetration of the evanescent wave into the rarer medium depends upon parameters such as the angle of incidence, the matching of refractive indices, the wavelength, and polarization. It can be made to vary from practically zero to several microns. It i s the possi b i l i t y of limiting the penetration of the radiation into an aqueous (rarer) medium, which makes ATR a potential technique for "in s i t u " IR spectroscopic studies. Previous "in s i t u " ATR Studies (i) Ultraviolet and Visible Region - Hansen and co-workers (96) and Srinivasan and Kuwana (97) used a glass-prism coated with tin oxide for recording "in s i t u " ultraviolet-visible ATR spectra of compounds formed during electrochemical reactions between the t i n oxide film and an electrolyte. A similar technique has been employed for simultaneous electrochemical and v i s i b l e spectro-scopic studies on thin films of platinum (98) and gold (99) deposited on glass. ( i i ) Infrared Region Mark and Pons (100) used an "in s i t u " ATR technique for observing IR spectra of organic species formed at a germanium prism electrode during electrolysis. In their technique the adsorbent was the IR transparent prism. The recording of "in s i t u " IR spectra of adsorbed - AO -species u s i n g t h i n f i l m s of l i g h t absorbing m a t e r i a l as the adsorbent (such -as a prism coated w i t h a cuprous oxide f i l m , employed i n the present work and shown s c h e m a t i c a l l y i n F i g . 6c) has not been reported. - 41 -CHAPTER I I I EXPERIMENTAL METHODS 3.1 F l o t a t i o n Tests  I n t r o d u c t i o n S i n g l e m i n e r a l 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 w i t h a mo d i f i e d v e r s i o n of the c e l l proposed by Hallimond (101,102) and known as the Hallimond tube. The s e l e c t i o n of t h i s m i c r o c e l l was made on the assumption that r e p r o d u c i b i l i t y of experimental parameters i s e a s i e r w i t h t h i s device than w i t h other known l a b o r a t o r y m i c r o c e l l s . The widespread use of the Hallimond tube and i t s m o d i f i c a t i o n s i n l a b o r a t o r y f l o t a t i o n i n v e s t i g a t i o n s would a l s o a l l o w more d i r e c t comparison w i t h r e p o r t e d data. T e n o r i t e was f l o a t e d using l a u r i c a c i d , l a u r y l amine, o l e i c a c i d and amyl xanthate, under d i f f e r e n t pH c o n d i t i o n s . The e f f e c t of ions which might be s p e c i f i c a l l y adsorbed by o x i d i z e d copper s u r f a c e s , e.g. carbonates, sulphides and c u p r i c i o n s , was i n v e s t i g a t e d . L a u r i c a c i d , - + + C 1 1 H 2 3 C 0 0 H ^ C l l H 2 3 C O ° + H ' a n d l a u r y 1 a m i n e > C i 2 H 2 5 N H 2 + H ^ 1 2 ^ 2 5 ^ 3 + ' W e r e s e-'- e c t e^ t o represent the a n i o n i c and c a t i o n i c f a m i l y of c o l l e c t o r s , r e s p e c t i v e l y . The main purpose of usi n g o l e i c a c i d and xanthate as c o l l e c t o r s was to o b t a i n m i c r o f l o t a t i o n data which could be compared w i t h p l a n t p r a c t i c e or w i t h previous l a b o r a t o r y r e s u l t s . - 42 -(a) Materials  Mineral The copper oxide used was analytical grade cupric oxide manufactured ' by Fisher Sci e n t i f i c , Co. (Cat. No. C-474). This material comes in the form of "wires"; e.g. cylindrical geometry of approximately 0.5 mm diameter and 5 mm long. These wires were stage ground in an agate mortar, then dry screened with the -48 to +100 mesh fraction being separated for the flotation tests. The flotation samples assayed 82.4% copper, compared with the theoretical values of 79.8% and 88.8% Cu for tenorite and cuprite, respectively. Minor impurities were specified as 0.002% carbonates and less than 0.004% sulphates. X-Ray diffraction showed that the samples contained tenorite as the predominant mineral and cuprite as -a secondary component. The diffraction pattern, showed no other lines besides those of the oxides. The IEP of this tenorite sample was measured at pH 9.4, using Mular and Roberts method (103) (see Appendix D). Collectors The lauric acid used was 99.5% pure (gas-liquid chromatography) supplied by K & K Laboratories, Inc. Lauryl amine and oleic acid were of a purified grade obtained from Chem. Service, Inc. The xanthate used was a laboratory grade potassium amyl xanthate, purified by successive dissolution in acetone and recrystallization i n ether. Modifiers Analytical grade sulphuric acid and sodium hydroxide were employed for pH control. The other reagents used; sodium sulphide and cupric - A3 -s u l p h a t e , were a l s o a n a l y t i c a l grade. > Water Throughout t h i s research s o l u t i o n s were prepared u s i n g double d i s t i l l e d water. This water, when f r e s h l y d i s t i l l e d showed: c o n d u c t i v i t y —6 —6 —1 —1 ranging from 1.5 x 10 to 1.9 x 10 ohms .cm ; surface t e n s i o n higher than 70.0 dynes.cm 1 and pH v a r y i n g from 6.5 to 6.9. Water c o n d u c t i v i t i e s were determined us i n g a c o n d u c t i v i t y meter manufactured by Radiometer Copenhagen (type CDM 2d). Surface t e n s i o n was measured by the drop-volume method (10A). A -Beckman Zeromatic SS3 pHmeter, provided w i t h a probe combination e l e c t r o d e (Beckman 39183) was used f o r pH determinations. Before each t e s t the e l e c t r o d e was standardized w i t h s o l u t i o n s b u f f e r e d at pH's 4.01, 7.00 and .9...1-8, f r e s h l y prepared at the beginning of each week. pH's were taken w i t h a p r e c i s i o n of + 0.05 pH u n i t s . Gases High p u r i t y , oxygen-free n i t r o g e n and argon were used. In order to reduce the carbon-dioxide content of these gases and consequently the pH changes r e s u l t i n g from i t s adsorption i n t o the pulp during f l o t a t i o n , the gases were f u r t h e r p u r i f i e d by passing them through a gas washing b o t t l e c o n t a i n i n g a s a t u r a t e d sodium hydroxide s o l u t i o n . (b) F l o t a t i o n Apparatus The Hallimond tube employed i s i l l u s t r a t e d i n F i g . 7. I t i s s i m i l a r to that described by Fuerstenau and co-workers (105) except that the glass f r i t i s used i n pl a c e of a c a p i l l a r y and the f l o t a t i o n - 44 -pH meter \ magnetic s t i r r e r F i g u r e 7. M o d i f i e d Hallimond Tube. - 45 -column i s modified to prevent mechanical carry-over of n o n - f l o a t a b l e m a t e r i a l aid "hold back" of m i n e r a l i z e d bubbles at the edge of the ground j o i n t . The rubber stopper serves to support a combination e l e c t r o d e . The probe type e l e c t r o d e has a s m a l l (10 mm) diameter which makes p o s s i b l e measurements of pH i n s i d e the f l o t a t i o n c e l l . An o u t l e t (open to a i r ) i s provided t o permit the escape of gases i n t r o d u c e d i n t o the c e l l . The gaseous atmosphere above the pulp can be c o n t r o l l e d by forming a p o s i t i v e p r essure, gas b a r r i e r i n s i d e the c e l l . S e l e c t e d gases are i n t r o d u c e d i n t o the c e l l through the gas i n l e t shown i n F i g . 7. The scoop i s used to p l a c e the m i n e r a l sample at the bottom of the f l o t a t i o n column without breaking t h i s b a r r i e r . The complete f l o t a t i o n apparatus i s shown s c h e m a t i c a l l y i n F i g . 8. I t i n c l u d e s a gas system designed to d e l i v e r known volumes of gas to the Hallimond tube at a measureable average flow r a t e . This system i s a s i m p l i f i c a t i o n of the one used by Fuerstenau (106) and c o n t a i n s : A - n i t r o g e n and argon c y l i n d e r s , provided w i t h pressure r e g u l a t o r s ; B - gas-washing b o t t l e c o n t a i n i n g s a t u r a t e d sodium hydroxide s o l u t i o n ; C - three-way stopcock to d i r e c t gas to D, or t o the f l o t a t i o n c e l l through the gas i n l e t supported by the rubber stopper; D - three-way stopcock to d i r e c t gas to the b u r e t t ( E ) , or to the c e l l through the g l a s s f r i t ; E - b u r e t t e , to measure volume of gas d e l i v e r e d to the c e l l ; F - constant water-head v e s s e l , t o m a i n t a i n the gas at a constant pressure; G - t r a p ; H - two-way stopcock. Figure 8. M i c r o f l o t a t i o n apparatus. - 47 -(c) Operating Procedures Two procedures were fo l l o w e d during the f l o t a t i o n t e s t s . In the k f i r s t procedure, no attempt was made to c o n t r o l the gas above the f l o t a t i o n pulp. In the second, an argon b a r r i e r was maintained above the l i q u i d to minimize d i f f u s i o n of atmospheric gases, c o n t a i n i n g carbon d i o x i d e , i n t o the pulp. Procedure I : No attempt to prevent d i f f u s i o n of atmospheric carbon d i o x i d e i n t o the pulp. The sequence of steps i s described below. 3 1. T h i r t y cm of n i t r o g e n gas was introduced i n t o the b u r e t t e and kept at a pressure of approximately 60 cm of water. This pressure 3 -1 provided an average gas flow through the c e l l of 0.5 cm .sec , as estimated i n separate t e s t s . 2. The upper h a l f of the Hallimond tube (part above the ground j o i n t ) was removed and the sample to be f l o a t e d (two grams of -48 to +100 mesh t e n o r i t e ) was plac e d i n t o the c e l l . 3. The two halves of the tube were then r e j o i n e d and clamped. 4. 100 ml of s o l u t i o n c o n t a i n i n g the f l o t a t i o n reagents were prepared i n a graduated c y l i n d e r . When r e q u i r e d , the s o l u t i o n was heated to a temperature s l i g h t l y below 50°C, to speed d i s s o l u t i o n of the c o l l e c t o r . The s o l u t i o n was brought to a temperature of 25 + 1°C and i t s e q u i l i b r i u m pH ( i n i t i a l pH = pH^) was measured. 5. The reagent s o l u t i o n was t r a n s f e r r e d from the graduated c y l i n d e r to the c e l l through the upper opening of the tube. The pH e l e c t r o d e was then immersed i n the l i q u i d by i n s e r t i n g the rubber stopper i n the opening. - 48 -6. The magnetic s t i r r e r was turned on. The pulp was conditioned f o r f i v e minutes a f t e r which the pH was again measured ( c o n d i t i o n i n g pH = pH c)• The s t i r r e r had been pre-set to the minimum v e l o c i t y necessary to keep the s o l i d s i n suspension. 3 7. The 30 cm of gas sto r e d i n the bu r e t t e was then f o r c e d to flow through the Hallimond tube. Bubbles were formed a t an average 3 -1 flow r a t e of 0.5 cm .sec (one minute f l o t a t i o n t i m e ) . 8. The f i n a l pH (pH^) was read. The average of pH^ and pH^ was considered as the f l o t a t i o n pH (pH ). r 9. The f l o t a t e d f r a c t i o n , c o l l e c t e d i n the "concentrate" stem, and the no n - f l o a t e d m a t e r i a l , remaining on top of the g l a s s f r i t , were i n d i v i d u a l l y removed from the c e l l , d r i e d at 80°C and f i n a l l y weighed on an a n a l y t i c a l balance. Procedure II; Minimization of the diffusion of carbon dioxide into the pulp. .This procedure i s essentially the same as that described i n Procedure I except for the use of argon gas instead of nitrogen, preparation of reagents and introduction of the mineral sample to be floated. The solution containing reagents was prepared in the following manner. Carbon dioxide content of the double d i s t i l l e d water was lowered by boiling and by bubbling argon in a narrow necked flask. The water was then cooled to approximately 50°C, the reagents were added, and this solution cooled further to 25°C. During this time, bubbling of argon was maintained. At this stage, 100 ml of the reagent solution was poured i n the - 49 -Hallimond tube as described p r e v i o u s l y . Argon was introduced i n t o the 1 c e l l through ftie g l a s s f r i t and a f u r t h e r argon purging of the s o l u t i o n s t a r t e d i n s i d e the f l o t a t i o n c e l l . The rubber stopper was placed on the tube. The scoop c o n t a i n i n g the m i n e r a l sample was kept above the l i q u i d . Argon was a l s o introduced i n t o the c e l l through the gas i n l e t of fie rubber stopper. The purpose of t h i s was t o m a i n t a i n a p o s i t i v e argon pressure to act as a b a r r i e r between the l i q u i d and the atmospheric gases. The argon b a r r i e r was maintained up to the end of step 8 (Procedure I ) . The purging of the s o l u t i o n l a s t e d approximately t h i r t y minutes a f t e r which pH\ was measured. Then, by s l i d i n g the scoop i n t o the l i q u i d , the sample was placed on the bottom of the f l o t a t i o n column. Steps 6 to 9 were c a r r i e d out as described i n the previous procedure. 3.2 I n f r a r e d Spectroscopic Studies  I n t r o d u c t i o n Transnission reference s p e c t r a of copper oxides, c u p r i c hydroxide, m a l a c h i t e , c u p r i c l a u r a t e , sodium l a u r a t e , l a u r i c a c i d and s t e a r i c a c i d were recorded using the potassium bromide d i s c ( p e l l e t ) technique (107). M i r r o r s f o r the specular r e f l e c t i o n s p e c t r o s c o p i c s t u d i e s were p a i r s of quartz s l i d e s coated w i t h a r e f l e c t i n g metal f i l m (vacuum-evaporated gold f i l m ) . A f i l m of copper oxide on top of the r e f l e c t i n g metal or the gold i t s e l f were used as s u b s t r a t e s f o r a d s o r p t i o n s t u d i e s . The r e f l e c t i o n s p e c t r a of c a r b o x y l i c surface f i l m s , formed by v a r i o u s methods, were analysed by comparison w i t h t r a n s m i s s i o n reference s p e c t r a of s i m i l a r bulk compounds or w i t h r e f l e c t i o n s p e c t r a of f i l m s of known - 50 -composition. Hence, some specular r e f l e c t i o n s t u d i e s were conducted w i t h the unique purpose of f a c i l i t a t i n g the assignment of IR bands a s s o c i a t e d w i t h c a r b o x y l i c s u r f a c e s p e c i e s . Samples prepared f o r " i n s i t u " ATR s t u d i e s of adso r p t i o n of l a u r i c a c i d onto cuprous oxide s u b s t r a t e s were composed of KRS-5 (t h a l l i u m bromide-iodide)prisms coated w i t h a t h i n f i l m of the oxide. The presence of strong water bands i n the IR s p e c t r a obtained using " i n s i t u " techniques makes rat h e r d i f f i c u l t the assignment of weak bands, a s s o c i a t e d w i t h adsorbed s u r f a c e species,and l o c a t e d i n the v i c i n i t y of the strong bands. In order to overcome t h i s problem, " i n s i t u " s p e c t r a were recorded using H^ O and D^ O as aqueous media. 3.. 2.1. M a t e r i a l s Evaporation sources Oxygen-free h i g h - c o n d u c t i v i t y (OFHC) copper c h i p s , contained i n a molybdenum dimpled boat were employed as the evaporation source during vacuum-deposition of copper f i l m s . Cominco gold b u l l i o n (99.999%) i n s p l a t t e r form, a l s o contained i n a molybdenum boat, was used as the source f o r evaporated gold s u b s t r a t e s . C a r b o x y l i c S u r f a c t a n t s High p u r i t y s t e a r i c a c i d manufactured by Chem S e r v i c e , Inc. was employed. Measurements of the sur f a c e pressure of monolayers of t h i s a c i d , spread at the a i r - w a t e r i n t e r f a c e of a Langmuir trough (see .Appendix F ) , gave pressure-area curves analogous to those given i n the _ The molecules of H 20 and DO absorb the IR l i g h t i n q u i t e d i s t i n c t s p e c t r a l regions (see t a b l e of c h a r a c t e r i s t i c frequencies given i n Appendix C). The purpose of the a l t e r n a t i v e use of these media was to expand the IR region a v a i l a b l e f o r r e c o r d i n g s p e c t r a of t h i n s urface f i l m s . - 51 -l i t e r a t u r e (77), confirming the high p u r i t y of the m a t e r i a l . L a u r i c , a c i d was of the same q u a l i t y as that used during the f l o t a t i o n t e s t s (already discussed i n S e c t i o n 3.1, p. 42 ). Sodium l a u r a t e was obtained by n e u t a l i z a t i o n of l a u r i c a c i d s o l u t i o n s w i t h sodium hydroxide and was p u r i f i e d by r e c r y s t a l l i z a t i o n from ethanol. Cupric l a u r a t e was prepared by p r e c i p i t a t i o n from s o l u t i o n s of l a u r i c a c i d i n the presence „ of c u p r i c sulphate, f o l l o w e d by r i n s i n g w i t h water, washing w i t h acetone, and d r y i n g under vacuum. Organic S o l u t i o n s Carbon t e t r a c h l o r i d e , normal hexane, acetone, and e t h a n o l were a l l spectrophotometric grade reagents. H 20 and D 20 The p r o p e r t i e s of the double d i s t i l l e d water used throughout t h i s work have already been described i n S e c t i o n 3.1, p. 43 , Deuterium oxide c o n t a i n i n g 99.82% D 20 was s u p p l i e d by Atomic Energy of Canada L t d . pH Regulators A n a l y t i c a l grade s u l p h u r i c a c i d , sodium carbonate, bicarbonate and hydroxide were employed f o r pH c o n t r o l . Gases High p u r i t y compressed a i r and argon were used a f t e r p a s sing them through a gas washing b o t t l e c o n t a i n i n g s a t u r a t e d sodium hydroxide s o l u t i o n to reduce carbon d i o x i d e contaminations. Analysed gas mixture - 52 -of carbon d i o x i d e i n n i t r o g e n (295 ppm CO^) was s u p p l i e d by Canadian L i q u i d A i r L t d . O x i d i z e d Copper M i n e r a l s M a l a c h i t e was an a n a l y t i c a l grade b a s i c c u p r i c carbonate 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. (C 453). Cupric oxide was obtained by heating t h i s m a lachite f o r 30 hours at 280°C, i n a i r . C e r t i f i e d grade cuprous oxide ( F i s h e r C 477) was used a f t e r i t had been washed w i t h acetone to remove the pine o i l p r e s e r v a t i v e . Cupric hydroxide was p r e c p i t a t e d from copper sulphate s o l u t i o n s by n e u t r a l i z a t i o n w i t h sodium hydroxide. The p r e c i p i t a t e was r i n s e d w i t h double d i s t i l l e d water, f i l t e r e d and d r i e d under vacuum. Misce l l a n e o u s Chemicals I n f r a r e d q u a l i t y potassium bromide c r y s t a l s were employed as the m a t r i x m a t e r i a l of the pressed d i s c s . A l l the other chemicals used were of a c e r t i f i e d grade. 3.2.2 Sample P r e p a r a t i o n (a) Potassium Bromide Discs . D i s c s f o r transmission spectroscopy were prepared by mixing 1 mg of the sample m a t e r i a l w i t h 500 mg of potassium bromide i n an agate mortar; t r a n s f e r r i n g the mixture to a die (Perkin-Elmer Model 186-00251); evacuating f o r 5 minutes; and p r e s s i n g f o r 3 minutes at 5000 p s i . The d i s c s were 16 mm i n diameter and approximately.. 1 mm t h i c k . (b) Samples f o r Specular R e f l e c t i o n Spectroscopy (I) P r e p a r a t i o n of Substrates  Copper Oxide Coated Front Surface Gold M i r r o r s Cupric aid cuprous oxide s u b s t r a t e s were prepared f o r the IR spec u l a r r e f l e c t i o n s p e c t r o s c o p i c s t u d i e s according to the f o l l o w i n g procedure: 1) The quartz s l i d e s (57 x 20 x 3 mm and 44 x 20 x 3 mm t h i c k ; o p t i c a l l y f l a t s urfaces) were c a r e f u l l y cleaned by soaking i n warm detergent s o l u t i o n s ; r i n s i n g w i t h b o i l i n g water; degreasing i n f r e s h l y prepared chromic a c i d (potassium dichromate added to concentrated s u l p h u r i c a c i d ) ; thoroughly r i n s i n g w i t h b o i l i n g water; and d r y i n g a t temperatures over 100°C. 2) Gold f i l m s (370 A t h i c k ) were vacuum deposited on a p a i r of s l i d e s using the r e s i s t i v e - h e a t i n g , thermal evaporation technique (108). Thickness of the f i l m s was a c r i t i c a l f a c t o r . Thicker f i l m s showed a tendency to p e e l o f f i n aqueous media e s p e c i a l l y a f t e r exposure to temperatures above 200°C. On the other hand }thinner f i l m s presented poor r e f l e c t i v i t y i n the long wavenumber IR region (4000 to 2500 cm ^ ) . For t h i s reason the f o l l o w i n g evaporation procedure had to be f o l l o w e d c l o s e l y : c l e a n s l i d e s were placed at a di s t a n c e of 20 cm —8 from the. source; the u n i t was evacuated to 10 t o r r ; and 260 + 10 mg of gold were evaporated by ap p l y i n g a f i x e d v o l t a g e to the source which permitted complete evaporation of the metal i n three minutes. A ° F i l m t hickness was measured w i t h i n + 30 A usinga m u l t i p l e beam i n t e r f e r o m e t e r (Angstrometer, SLOAN model M-100, marketed by Sloan Instrument Corp.). - 54 -o 3) A m e t a l l i c copper f i l m (200 to 250 A t h i c k ) was deposited on top of the g o l d f i l m . The evaporation procedure adopted was analogous to that described i n Step 2 above except that the distance from the s l i d e to the source was 25 cm i n s t e a d of 20 cm, and 65 + 5 mg of copper were evaporated. A f t e r the d e p o s i t i o n of the copper f i l m , the samples were removed from the vacuum chamber and s t o r e d i n an argon gas environment. 4.i) Cupric oxide coated f r o n t s u r f a c e gold m i r r o r s were formed by o x i d i z i n g the copper f i l m s , i n an oxygen atmosphere, at 240 + 5°C, f o r approximately 15 hours. The m i r r o r s were cooled to ambient temperature before t h e i r s p e c t r a were recorded. The t h i c k n e s s of the oxide f i l m s v a r i e d from 340 to 420 A (measured by i n t e r f e r o m e t r y ) . A n a l y s i s of the c u p r i c oxide f i l m s formed on top of g o l d by t r a n s m i s s i o n e l e c t r o n -d i f f r a c t i o n showed that the o x i d i z e d f i l m s were composed e s s e n t i a l l y of c u p r i c oxide. The 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 contained no other r i n g s besides those c h a r a c t e r i s t i c of CuO (and of the g o l d underneath). P i c t u r e s (Fizeau i n t e r f e r e n c e f r i n g e s and t r a n s m i s s i o n e l e c t r o n micrograph) showing the topography of the c u p r i c oxide f r o n t surface m i r r o r s are presented i n F i g . 9. 4 . i i ) Cuprous oxide (or CuOrt ,.,) f i l m s were formed by o x i d a t i o n of — U. o / the copper f i l m s at 140 + 5°C, i n the presence of oxygen, f o r 6 hours. o Films 320 to 400 A t h i c k were produced. Transmission 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 confirmed t h a t the o x i d i z e d f i l m s contained cuprous oxide as the predominant component. However, weak r i n g s assigned to c u p r i c oxide were a l s o observed. - 55 -9(a) Fizeau interference fringes of a t y p i c a l cupric oxide substrate (front surface gold mirrors; the oxide has been coated with a thin gold f i l m ) . Taken with the multiple beam interferometer: f r i n g e - t o - f r i n g e spacing = 2945 A° 15 x h o r i z o n t a l magnification. 9(b) Transmission electron-micrograph of a cupric oxide surface (acetate r e p l i c a ) . Figure 9. Surface topography of cupric oxide substrates. - 56 -Smooth and Rough Gold Substrates » Smooth gold surfaces were prepared by c a r r y i n g out only the f i r s t two steps of the procedure used f o r production of copper oxide f r o n t surface gold m i r r o r s given above. The gold m i r r o r s were removed from the vacuum chamber a f t e r completion of Step 2 and s t o r e d i n an argon environment. Transmission e l e c t r o n - d i f f r a c t i o n was used to analyse the gold f i l m s and confirmed t h e i r high degree of p u r i t y . I n t e r f e r o m e t r i c f r i n g e s ( F i g . 10a) and an e l e c t r o n micrograph ( F i g . 10b) are presented to i l l u s t r a t e the f l a t n e s s of the surface of these m i r r o r s . Rough gold s u b s t r a t e s were obtained by d e p o s i t i n g f i l m s of the metal on rough quartz s u r f a c e s . Immersion of the o p t i c a l l y f l a t quartz s l i d e s i n a 48% s o l u t i o n of h y d r o f l u o r i c a c i d f o r 15 minutes y i e l d e d surfaces w i t h the d e s i r e d degree of roughness (surface depressions o approximately 1000 A deep). The procedure f o l l o w e d f o r evaporation of these gold f i l m s was s i m i l a r to that used during p r e p a r a t i o n of smooth o gold m i r r o r s except that i n t h i s case t h i c k e r f i l m s (2000 A) were deposited. The formation of t h i c k e r f i l m s was r e q u i r e d to i n s u r e that surface depressions were completely f i l l e d and no quartz was exposed at the s u r f a c e . The degree of roughness of the surface of these m i r r o r s i s i l l u s t r a t e d i n F i g . 11. (I I ) D e p o s i t i o n of C a r b o x y l i c Films A d s o r p t i o n from Aqueous S o l u t i o n s Adsorption of l a u r i c a c i d from aqueous s o l u t i o n s onto copper oxide or g o l d s u b s t r a t e s was conducted i n an a l l - t e f l o n r e a c t i o n v e s s e l under - 57 -10(a) Fizeau interference fringes of a typical gold mirror (smooth surface) (fringe-to-fringe spacing = 2 9 4 5 A ° ; 15 x horizontal magnification). 10(b) Transmission electron micrograph of a smooth gold surface (acetate replica). Figure 1 0 . Surface topography of smooth gold substrates. - 58 -11(a) Fizeau interference fringes of a rough surface gold mirror, ( f r i n g e - t o - f r i n g e spacing = 2945 A°, 15 x h o r i z o n t a l magnification). 11(b) Transmission electron micrograph of a rough gold surface (acetate r e p l i c a ) . Figure 11. Surface topography of rough gold substrate. - 59 -c o n t r o l l e d gaseous atmosphere and at constant temperature (25 + 1°C). The procedure i n v o l v e d d i s s o l u t i o n of l a u r i c a c i d i n warm ( l e s s than 50°C) double d i s t i l l e d water, o u t s i d e the r e a c t i o n v e s s e l , i n a 500 ml t e f l o n b o t t l e . Approximately 300 ml of the s o l u t i o n was t r a n s f e r r e d to the r e a c t i o n v e s s e l which was then immersed i n a constant temperature bath. pH r e g u l a t o r s and other reagents were added. F i g . 12a shows s c h e m a t i c a l l y the arrangement i n s i d e the r e a c t i o n v e s s e l a t t h i s stage. The s e l e c t e d gas was con t i n u o u s l y bubbled through the s o l u t i o n to ensure s a t u r a t i o n . S o l u t i o n s of l a u r i c a c i d i n B^O were prepared s i m i l a r l y . In order to avoid p o s s i b l e contamination of the system, introduced by the pH meter combination e l e c t r o d e , pH measurements were made out s i d e the r e a c t i o n v e s s e l . With the help of a hypodermic s y r i n g e , s m a l l (10 ml) samples were taken from the s o l u t i o n a t r e g u l a r time i n t e r v a l s and placed i n a separate c o n t a i n e r f o r pH checks. When gases c o n t a i n i n g carbon d i o x i d e were used, bicarbonate and carbonate of sodium were added as pH r e g u l a t o r s i n order to a l l o w f a s t e r e q u i l i b r i a of the gas-s o l u t i o n system. A f t e r reaching the e q u i l i b r i u m pH, the l e v e l of the s o l u t i o n was adjusted to 200 ml (using the syringe) and the su b s t r a t e ( p a i r of m i r r o r s ) was immersed i n the l i q u i d . F i g . 12b shows the i n s i d e of the v e s s e l at t h i s new stage. A flo w of the gas was maintained above the s o l u t i o n d u ring the r e a c t i o n . A g i t a t i o n was provided by means of a magnetic s t i r r e r . At p r e - e s t a b l i s h e d time i n t e r v a l s , the m i r r o r s were removed from the l i q u i d , washed w i t h double d i s t i l l e d water which had been pre-adjusted - 60 -gas magnetic s t i r r i n g bar to atmosphere sample m i r r o r s reagent s o l u t i o n a. . Arrangement i n s i d e the v e s s e l before r e a c t i o n ( e q u i l i b r i u m pH not a t t a i n e d yet) gas magnetic s t i r r i n g bar to atmosphere m i r r o r s 200 ml of s o l u t i o n b. Arrangement i n s i d e the v e s s e l during r e a c t i o n . F i g ure 12. Adsorption of aqueous l a u r i c a c i d onto m i r r o r s . A l l t e f l o n r e a c t i o n v e s s e l . - 61 -to approximately the same pH value as the s o l u t i o n and d r i e d under an argon j e t . I n f r a r e d s p e c t r a of the samples were recorded immediately a f t e r the d r y i n g step. The m i r r o r s were again immersed i n the s o l u t i o n , f o l l o w i n g the reco r d i n g of the s p e c t r a , when a longer exposure to the reagents was r e q u i r e d . In t h i s case, the s o l u t i o n was kept under constant gas purging during the s p e c t r o s c o p i c measurements. Monolayers Deposited from the Langmuir Trough A t e f l o n coated Langmuir trough (77,109) (3 l i t e r s c a p a c i t y ) was used f o r the t r a n s f e r of s o l i d i f i e d s t e a r i c a c i d monolayers from the a i r - w a t e r i n t e r f a c e onto gold (smooth and rough) and c u p r i c oxide s u b s t r a t e s , according to the Langmuir-Blodgett method. The trough was provided w i t h a wax-coated aluminum f l o a t i n g b a r r i e r ( t e f l o n ribbon c l o s i n g the gap) and a mechanically operated, moveable t e f l o n b a r r i e r . A t o r s i o n w i r e balance was employed f o r the measurement of f o r c e s ; one degree r o t a t i o n of the t o r s i o n head corresponded to a surface _1 * pressure of about 0.21 dynes.cm -4 5 The trough was f i l l e d w i t h a 10 N H^SO^ s o l u t i o n (pH = 4.4 + 0.2). F o l l o w i n g c l e a n i n g of the l i q u i d s u rface by sweeping w i t h b a r r i e r s , and sucking up contaminants w i t h a s m a l l clean tube attached to a s u c t i o n pump, a p a i r of m i r r o r s ( s l i d e s f u l l y covered w i t h the su b s t r a t e m a t e r i a l ) were Immersed i n the water. A monolayer of s t e a r i c a c i d was then formed on the l i q u i d s u rface by spreading the a c i d w i t h a v o l a t i l e _ C a l i b r a t i o n curve f o r the t o r s i o n w i r e i s given i n Appendix E. - 62 -w a t e r - i n s o l u b l e s o l v e n t (hexane). This monolayer was compressed and the r e l a t i o n s h i p between the surface pressure and the area occupied by the molecules of the f i l m was e s t a b l i s h e d . A f t e r s o l i d i f i c a t i o n of the monolayer, the m i r r o r s were drawn up through the monolayer covered l i q u i d s u r f a c e w h i l e the surface pressure of the f i l m was kept constant at 27 + 1 dyne.cm \ m so doing, p a r t of the s o l i d i f i e d monolayer was t r a n s f e r r e d to the m i r r o r s u r f a c e s . For a l l the monolayers t r a n s f e r r e d throughout t h i s work the measured decrease i n the area of the f i l m remaining i n the trough (area of the f i l m t r a n s f e r r e d to the m i r r o r s ) was e q u a l , w i t h i n + 2% estimated e x p e r i -mental e r r o r , to the geometric area of the m i r r o r s ( t r a n s f e r r a t i o = 1.00 + 0 . 0 2 ) . In view of the f a c t that the t r a n s f e r r a t i o of s o l i d i f i e d monolayers f o r quartz may be d i f f e r e n t from the r a t i o s f o r g o l d and f o r copper oxide s u b s t r a t e s (110), m i r r o r s used i n connection w i t h t h i s method were covered on both s i d e s by s u b s t r a t e . The m i r r o r s were withdrawn from the water w i t h t h e i r longer edges i n the h o r i z o n t a l p o s i t i o n and t h e i r l a r g e r faces at an angle of 45° w i t h the l i q u i d s u r f a c e . The r a t e of w i t h d r a w l was 0 . 5 mm.min \ Once a f i r s t l a y e r was deposited and i t s spectrum recorded, m u l t i -l a y e r s were b u i l t up by lowering and r a i s i n g the m i r r o r s through the AA f i l m e d l i q u i d s u r f a c e . A l l the f i l m s were deposited at room temperature which v a r i e d from 20 to 25°C. * See Appendix F f o r t y p i c a l pressure-area curve obtained. AA M u l t i l a y e r s b u i l t i n t h i s way contained an odd number of " a l t e r n a t i v e l a y e r s " of the f a t t y a c i d molecules (111); the a c i d molecules i n l a y e r s of odd number were o r i e n t e d i n the opposite d i r e c t i o n to those i n l a y e r s of even number, as shown below: J J J J v * — odd number l a y e r s : ( X X A J hydro-(*carboxylic heads carbon even number l a y e r s : T Q Q Q Q ' t a i l s - 63 -Films Condensed from the Vapour Phase ' Lauric and stearic acid thin films were deposited onto gold and cupric oxide substrates by heating the acids in open a i r and condensing their vapour onto the mirrors. No attempt was made to quantize the deposition parameters. The substrates were located at approximately 10 cm above the source and were exposed to the vapour for a few minutes. The samples were stored under an argon environment and their spectra were recorded at pre-established intervals of time. After deposition of organic f i lms, some samples were treated with organic solvents i n order to investigate the s o l u b i l i t y and strength of adhesion of the adsorbed f i lms. This procedure was applied also i n the case of films formed i n aqueous solutions or deposited by the Langmuir-Blodgett method. (c) ATR Samples A copper f i lm was vacuum deposited onto the i rger face of a 52.5 x 20 x 2 mm thick, 45° face angles, KRS-5 prims (cleaned in acetone) and then oxidized to cuprous oxide. A procedure, analogous to steps 3 and 4 . i i used for formation of cuprous oxide front surface gold mirrors, was followed during evaporation and oxidation of the copper f i l m . Lauric acid solutions (2 x 10 ~* M, i n H^O or E^O) were prepared in a teflon bottle under argon purging and placed in a constant temperature bath. - 64 -3.2.3 Recording the Spectra  Instrument I n f r a r e d s p e c t r a were recorded on a Perkin-Elmer, model 521, double-beam, g r a t i n g spectrophotometer. This spectrophotometer permitted the r e c o r d i n g of d i f f e r e n t i a l s p e c t r a from 4000 to 250 cm ' on a percent t r a n s m i s s i o n mode, at expansions up to 20X. The whole o p t i c a l system was kept under a constant d r y - a i r purge. The sampling area was covered w i t h a p l e x i g l a s s box (see F i g . 13) provided w i t h s l i d i n g doors on top. This allowed easy manipulation of samples and r e f l e c t i o n a c c e s s o r i e s without exposing the u n i t to l a r g e volumes of atmospheric a i r c o n t a i n i n g water vapour. D r y - a i r purging reduced the i n t e n s i t y of i n t e r f e r i n g bands r e s u l t i n g from the presence of r e s i d u a l water vapour i n the unbalanced arms of the o p t i c a l system. I t was p a r t i c u l a r l y u s e f u l during expanded s c a l e o p e r a t i o n s . The d r y - a i r was s u p p l i e d by an o i l l e s s compressor f i t t e d w i t h z e o l i t e f i l l e d d r y i n g columns. Spectra were recorded at temperatures (measured i n the sampling area of the instrument) v a r y i n g from 25 to 33°C. (a) Transmission Spectra Spectra of sample d i s c s were recorded d i f f e r e n t i a l l y a g a i n s t pure potassium bromide reference d i s c s . (b) Specular R e f l e c t i o n Spectra M u l t i p l e s p e c u l a r r e f l e c t i o n s p e c t r a {eleven r e f l e c t i o n s , 70° i n c i d e n c e angle) of sample m i r r o r s were recorded a g a i n s t a reference beam at t e n u a t o r . The procedure was to record s p e c t r a of the m i r r o r s before - 65 -Figure 13. Sampling are of the spectrophotometer covered f o r the dry-air purge. I Figure 14. Mul t i p l e r e f l e c t i o n attachment (Wilks Model 9) and sample holder. - 66 -and a f t e r t h e i r exposure to aqueous media. « P a r a l l e l p o l a r i z a t i o n was used i n a l l the measurements. A p o l a r i z e r (Perkin-Elmer p a r t 186-0241), placed i n the common beam of the spectrophotometer, before the entrance s l i t of the monochromator, was employed to e l i m i n a t e the component of the l i g h t p e r p e n d i c u l a r to the r e f l e c t i n g s u r f a c e s . A 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 (see F i g . 14), marketed by Wilks S c i e n t i f i c Corp. (Model 9; o p t i c a l l a y - o u t shown i n F i g . 3) was used f o r t r a n s f e r r i n g l i g h t from the sample beam of the source o p t i c s to the m i r r o r s . The sample h o l d e r ( a l s o shown i n F i g . 14) was s p e c i a l l y constructed to give the m i r r o r s a f i x e d p o s i t i o n r e l a t i v e to the sample beam, and consequently, to provide a r e p r o d u c i b l e number .of..reflections and i n c i d e n c e angles. Spacers kept the m i r r o r s p a r a l l e l to each other, at a f i x e d , 2 mm, distance apart. Eleven r e f l e c t i o n s were obtained using i n c i d e n c e angles spreading from 64 to 76°. This arrangement made p o s s i b l e the d e t e c t i o n of f r a c t i o n a l changes i n r e f l e c t i v i t y (AR) of the order of 0.0001. AR values as low as 0.0003 were measured r e p r o d u c i b l y w i t h i n t o l e r a n c e l i m i t s of + 10%. The presence of the sample m i r r o r s and the r e f l e c t i o n attachment i n the path of the sample beam lowered the t r a n s m i t t e d power l e v e l of the instrument (as measured at 2100 cm "*") from 100% to 45-48%, f o r the case of copper oxide front surface gold m i r r o r s ; 48-52%, f o r smooth gold m i r r o r s , and 38-40%, f o r rough surface gold m i r r o r s . (c) ATR Spectra A t w e n t y - f i v e r e f l e c t i o n , 45° i n c i d e n c e angle, ATR technique was employed. D i f f e r e n t i a l s p e c t r a were recorded w i t h i n t e r n a l r e f l e c t o r - 67 -p l a t e s i n both beams of the instrument. The purpose was to o b t a i n b e t t e r compensation f o r the atmospheric a b s o r p t i o n and f l a t t e r base l i n e s . KRS-5 i n t e r n a l r e f l e c t o r p l a t e s were s u p p l i e d by Wilks S c i e n t i f i c Corp. ( c a t . # 9-3004-A). Spectra of the su b s t r a t e (cuprous oxide f i l m c o ating the l a r g e r face of the prism) were taken b e f o r e , during and a f t e r i t s exposure to aqueous s o l u t i o n s . During " i n s i t u " measurement there were only eleven r e f l e c t i o n s from the l i q u i d - s o l i d i n t e r f a c e because of the geometry of the c e l l c o n f i n i n g the l i q u i d . * Spectra were recorded without p o l a r i z a t i o n of the l i g h t . Model 9 (Wilks S c i e n t i f i c Corp.) m u l t i p l e r e f l e c t i o n attachments (same used f o r specular r e f l e c t i o n ) were i n s e r t e d i n the paths of the sample beam and the reference beam. The holder used f o r the reference prism was a Wilks MIR 1 i n t e r n a l r e f l e c t i o n p l a t e h o l d e r . The sample prism was supported by an a l l t e f l o n l i q u i d c e l l . The c e l l body has a shape s i m i l a r to that of a Wilks MIR 3 l i q u i d sample holder except that the r e a c t i o n v e s s e l was enlarged to c o n t a i n 10 ml of the l i q u i d . This volume of the l a u r i c a c i d s o l u t i o n was introduced i n t o the c e l l w i t h the a i d of a hypodermic s y r i n g e . The c e l l was cl o s e d to the atmosphere du r i n g r e c o r d i n g of " i n s i t u " s p e c t r a . For measurements made a f t e r exposure to the l i q u i d , the samples were d r i e d i n an argon j e t . The presence of the m u l t i p l e r e f l e c t i o n attachment and the i n t e r n a l r e f l e c t o r p l a t e (not coated) i n the path of the sample beam r a d i a t i o n P r e l i m i n a r y t e s t s , conducted using p a r a l l e l and p e r p e n d i c u l a r p o l a r i z a -t i o n , f a i l e d to i n d i c a t e any improvement i n the q u a l i t y of the s p e c t r a by the use of the p o l a r i z e r (band h e i g h t s were independent of p o l a r i z a t i o n ) . On the c o n t r a r y , the i n t r o d u c t i o n of the p o l a r i z e r decreased the t o t a l energy reaching the det e c t o r i m p a i r i n g instrument performance. - 68 -lowered the t r a n s m i t t e d power l e v e l of the instrument from 100% t r a n s m i s s i o n to approximately 45% (measured at 2100 cm "*") . Using an i n t e r n a l r e f l e c t o r p l a t e coated w i t h cuprous oxide, the l e v e l was lowered to 25-30% and the presence of the aqueous medium i n the ATR c e l l ("in s i t u " experiments) brought t h i s l e v e l to values as low as 15% t r a n s m i s s i o n . - 69 -CHAPTER IV RESULTS AND DISCUSSION 4.1 F l o t a t i o n Experiments (a) The Role of M i n e r a l Surface Charge i n T e n o r i t e F l o t a t i o n F l o t a t i o n t e s t s c a r r i e d out on t e n o r i t e using a n i o n i c (2 x 10 M l a u r i c a c i d s o l u t i o n ) and c a t i o n i c (2 x 10 M l a u r y l amine s o l u t i o n ) c o l l e c t o r s , i n the pH range of 3 to 11.5, i n d i c a t e d a c l o s e r e l a t i o n -s h i p between f l o t a t i o n recovery and m i n e r a l surface charge. In F i g . 15 f l o t a t i o n r e c o v e r i e s are p l o t t e d a g a i n s t pH_^ , i n i t i a l pH measured i n step 4 of Procedure I ( S e c t i o n 3.1.c). The curves i n t e r c e p t i n the neighborhood of the IEP of t e n o r i t e (approximately pH 9.4). L a u r i c a c i d i s seen to be an e f f e c t i v e c o l l e c t o r at pH's below the IEP, where the surface of t e n o r i t e i s p o s i t i v e l y charged; l a u r y l amine i s e f f e c t i v e at pH's above the IEP (negative surface charge). I d e n t i c a l behavior has been observed f o r other oxide minerals i n s i m i l a r f l o t a t i o n systems ( 4 ) . I n v e s t i g a t o r s have i n t e r p r e t e d the symmetry of a n i o n i c and c a t i o n i c f l o t a t i o n curves about the IEP as a support f o r the i o n a d s o r p t i o n theory (mechanisms B . i i , Table 1, S e c t i o n 2.1). In F i g . 16, the r e s u l t s of t e n o r i t e f l o t a t i o n t e s t s conducted u s i n g l a u r i c a c i d as c o l l e c t o r , i n the absence (Procedure I I ) and i n the presence of atmospheric carbon d i o x i d e (Procedure I) are compared. 100 90 80 70 60 S 50 o a Q) pi 40 30 20 U 10 <5 2 x 10 M l a u r i c a c i d A 2 x 10 ^ M l a u r y l amine 1 2 3 4 5 6 7 pHL ( i n i t i a l pH) F i g ure 15. F l o t a t i o n of t e n o r i t e w i t h a n i o n i c and c a t i o n i c c o l l e c t o r s (Procedure I) Figure 16. F l o t a t i o n of t e n o r i t e i n the presence (Procedure I) and i n the absence (Procedure I I ) of carbon d i o x i d e contaminations. 2 x 10"^ M l a u r i c a c i d s o l u t i o n s . - 72 -Recoveries are p l o t t e d against i n i t i a l pH (pH_^) and average f l o t a t i o n pH (pH ) . In the pH range 7 to 9.4, the recovery versus pH curve r r occurs at more a c i d pH's: i . when carbon d i o x i d e i s present as opposed to absent; and, i i , when compared w i t h the corresponding plK curve (carbon d i o x i d e p r e s e n t ) . This can be assumed to be caused by f a s t pH d r i f t s a s s o c i a t e d w i t h carbonate r e a c t i o n s i n the 7 to 9.4 pH range (as discussed i n p a r t b below). The r e c o v e r i e s obtained i n the absence of carbon d i o x i d e i n d i c a t e that m i neral c o l l e c t i o n occurs under a l l pH's below the IEP of t e n o r i t e , which supports the view t h a t s urface charge pla y s the most important r o l e i n the process. Experimental data r e l a t e d to the f l o t a t i o n t e s t s and i l l u s t r a t e d i n F i g s . 15 and 16 are given i n more d e t a i l i n Tables 2 and 3, r e s p e c t i v e l y . These t a b l e s a l s o i n c l u d e r e s u l t s of "blank t e s t s " ; i . e . , t e s t s c a r r i e d out w i t h no c o l l e c t o r present i n the s o l u t i o n . The purpose of these t e s t s was to evaluate the r o l e of p u r e l y mechanical f a c t o r s i n the t e s t procedures used. Very low " r e c o v e r i e s " (< 0.6%) were obtained which i n d i c a t e d that p a r t i c l e s c a r r i e d over by mechanical f o r c e s made a n e g l i g i b l e c o n t r i b u t i o n to the t o t a l weight of the "concentrates". (b) Changes i n Pulp pH The pH d r i f t s shown i n Table 2 (Procedure I , carbon d i o x i d e present i n the f l o t a t i o n system) suggest that chemical e q u i l i b r i u m was not reached i n the i n t e r v a l of time used to c o n d i t i o n and f l o a t the samples. Independently of the presence or absence of c o l l e c t o r , pulp pH's presented a tendency to i n c r e a s e under a c i d i c c o n d i t i o n s and to decrease - 73 -Table 2 Results of Flotation Tests on Tenorite (Procedure I) Collector ^ pR\ pH c pH f pH Drift pRp % Recovery ( p H i - p H f } (pH c + PH f/2) "Blank" 4.05 5.45 5.75 +1.70 5.60 0.3 5.50 6.30 6.45 +0.95 6.375 0.5 7.50 6.80 6.70 -0.80 6.75 0.3 9.00 6.85 6.80 -2.20 6.825 0.2 9.90 9.40 9.30 -0.60 9.35 0.6 2 x 10~5 M Lauric Acid 2.90 3.30 3.30 +0.40 3.30 99.0 3.60 4.00 4.00 +0.40 4.00 99.6 5.30 5.70 5.80 +0.50 5.75 99.8 5.70 6.00 6.10 +0.40 6.05 95.2 6.90 6.60 6.60 -0.30 6.60 62.2 6.95 6.60 6.70 -0.25 6.65 65.8 7.35 6.70 6.90 -0.45 6.80 38.4 8.50 7.05 6.80 -1.70 6.925 15.9 8.85 7.50 7.45 -1.40 7.475 2.2 9.25 8.55 8.50 -0.75 8.525 3.0 9.30 8.60 8.45 -0.85 8.525 1.8 9.70 9.20 9.10 -0.60 9.15 1.5 9.90 9.50 9.45 -0.45 9.475 2.5 10.40 10.30 10.25 -0.15 10.275 0.7 10.60 10.50 10.45 -0.15 10.475 0.5 11.10 11.00 11.00 -0.10 11.00 1.5 2 x 10~5 M Lauryl Amine 4.00 5.35 5.75 +1.75 5.55 0.4 6.05 6.30 6.30 +0.25 6.30 0.9 8.90 7.30 7.20 -1.70 7.25 0.7 9.25 8.30 7.90 -1.35 8.10 2.3 9.55 9.35 9.20 -0.35 9.275 7.6 9.60 9.35 9.20 -0.40 9.275 9.7 9.70 9.50 9.40 -0.30 9.45 8.3 9.80 9.50 9.30 -0.50 9.40 10.4 9.90 9.60 9.55 -0.35 9.575 35.0 10.55 10.30 10.20 -0.35 10.25 59.2 10.90 10.70 10.70 -0.20 10.70 66.2 11.30 11.20 11.20 -0.10 11.20 82.5 Recovery = 100 x weight "concentrate"/(weight of "concentrate" + weight of non-floated) Material losses were of the order of 1% of the weight of the original samples. - 74 -Table 3 Results of F l o t a t i o n Tests on Ten o r i t e (Carbon Dioxide Contamination Minimized; Procedure I I ) C o l l e c t o r PH. pH r c pH f pH D r i f t P RF % Recove "Blank" 3.90 5.20 5.75 +1.85 5.475 0.3 5.50 6.50 6.60 +1.10 6.75 0.4 7.55 7.55 7.60 +0.05 7.575 0.2 8.90 8.90 8.90 0 8.90 0.6 9.70 9.70 9.65 -0.05 9.675 0.6 10.10 10.00 9.95 -0.15 9.975 0.4 2 x 10~ 5 M L a u r i c A c i d 4.10 5.65 .5.. 9 5 +1.85 5.80 99.3 5.25 6.05 6.60 +1.35 6.325 99.5 6.95 7.10 7.10 +0.15 7.10 99.2 7.70 7.65 7.55 -0.15 7.60 36.8 8.40 8.50 8.45 +0.05 8.45 15.8 8.85 8.80 8.80 -0.05 8.80 22.7 9.35 9.35 9.30 -0.05 9.325 3.5 9.95 . 9.85 9.75 -0.20 9.80 0.7 10.60 10.50 10.50 -0.10 10.50 1.9 - 75 -under b a s i c c o n d i t i o n s . A q u a l i t a t i v e e x p l a n a t i o n f o r t h i s behavior can be put forward on the b a s i s of the f o l l o w i n g arguments: ( i ) D i s s o l u t i o n of c u p r i c oxide i s p r i m a r i l y r e s p o n s i b l e f o r the pH in c r e a s e s i n the a c i d i c media. At pH's below the IEP, the h y d r o l y s i s of c u p r i c oxide proceeds according to r e a c t i o n s which tend to in c r e a s e the pH of the s o l u t i o n , as f o r example: CuO + H 20 ^2 Cu + 2 (OH) 2CuO + 2H 20 C u 2 ( O H ) 2 + + + 2(OH)' CuO + H„0 *• Cu(OH) + + (OH)' z •* The higher s o l u b i l i t y of c u p r i c oxide i n a c i d i c water i s i n d i c a t e d by i t s s o l u b i l i t y diagram ( F i g . 2 ). ( i i ) The pH d r i f t s i n b a s i c media are due mainly to r e a c t i o n s of carbonate ions (formed by adso r p t i o n and h y d r o l y s i s of carbon d i o x i d e from the a i r ) w i t h the c u p r i c oxide surface and i t s h y d r o l y s i s products. These r e a c t i o n s l e a d to the formation of b a s i c c u p r i c carbonate (malachite) as suggested by thermodynamic data (see F i g . B3, Appendix B). The e q u i l i b r i a of the carbon d i o x i d e h y d r o l y s i s r e a c t i o n s 2 (gas) * 2(aq) C 02(aq) + V ^ H 2 C ° 3 - 76 -H 2 C 0 3 7^ H C 0 3 + H HCO ~ H CO„ = + H + 3 •* 3 ( e q u i l i b r i u m diagram presented i n Appendix B, F i g . B4) are changed i n a manner to favour the l e f t to r i g h t r e a c t i o n s during formation of malachite because of the consumption of carbonate ions (HCO^ of CO^ )• The o v e r a l l pH dependence of these r e a c t i o n s can e x p l a i n the decrease i n pH of a b a s i c pulp. In the absence of carbon d i o x i d e (see Table 3) the pH d r i f t s i n a c i d i c media were d i c t a t e d by the h y d r o l y s i s of c u p r i c oxide. In b a s i c media, the pH d r i f t s were i n s i g n i f i c a n t and probably r e l a t e d to other phenomena such as a d s o r p t i o n of p o t e n t i a l determining ions and c o l l e c t o r d i s s o c i a t i o n r e a c t i o n s . (c) E f f e c t of Sulphide Ions A d d i t i o n of sulphide ions to the pulp expanded the pH range of f l o a t a b i l i t y of t e n o r i t e w i t h l a u r y l amine as shown i n F i g . 17. The reverse e f f e c t was obtained i n the case of l a u r i c a c i d f l o t a t i o n of t h i s m i n e r a l ( F i g . 18). Although i t has not been proven e x p e r i m e n t a l l y , s u l p h i d e ions (HS or.S ) are l i k e l y to be s p e c i f i c a l l y adsorbed on o x i d i z e d copper minerals and change t h e i r e l e c t r i c a l surface charge towards more negative values. The f a c t that c a t i o n i c and a n i o n i c f l o t a t i o n of t e n o r i t e were improved and depressed, r e s p e c t i v e l y , by the presence of negative sulphide ions i n the pulp i s i n agreement w i t h the preceding r e s u l t s which pointed out the importance of the m i n e r a l surface charge i n i o n i c f l o t a t i o n of copper oxide. ioo H Q i ® 90 80 70 60 50 o <j 40 30 20 10 0 ® No Na2S added & 1 x 10~4 M Na2S J I L 7 / 8 10 11 . 12 Figure 18. Anionic flotation of tenorite with and without sulphide ions in solution. (2 x 10-5- M lauric acid, Procedure I) . co - 79 -(d) E f f e c t of Added Copper Ions > F i g . 19 shows that by i n c r e a s i n g the c o n c e n t r a t i o n of copper ions i n s o l u t i o n , an i n h i b i t i n g e f f e c t i s observed i n both c a t i o n i c and a n i o n i c f l o t a t i o n of t e n o r i t e . S p e c i f i c a d s o r p t i o n of hydroxo-complexes, e s p e c i a l l y l a r g e r complexes, has pronounced e f f e c t s on the surface charge of some oxides (71). P e r r i n (112) has shown that near I | n e u t r a l pH's C^COH^ i s the predominant copper species i n s o l u t i o n . Copper ions were added to the f l o t a t i o n s o l u t i o n s i n the e x p e c t a t i o n that l a r g e r concentrations of copper hydroxo-complexes would favour more p o s i t i v e values f o r the surface charge of t e n o r i t e and improve i t s f l o a t -a b i l i t y w i t h a n i o n i c c o l l e c t o r . The r e s u l t s obtained ( F i g . 19) were opposite to those expected. However, they are c o n s i s t e n t w i t h the model proposed i n S e c t i o n 1.2.c. According to t h i s model, H + and OH should be the main p o t e n t i a l - d e t e r m i n i n g ions of t e n o r i t e and the c o n c e n t r a t i o n of copper ions i n s o l u t i o n should not a f f e c t to any l a r g e extent the nature and magnitude of the zeta p o t e n t i a l of the system. The depressing a c t i o n of added copper ions was observed i n a l l the systems s t u d i e d . I t seems to be r e l a t e d to a decrease i n c o l l e c t o r c o n c e n t r a t i o n due to bulk i n t e r a c t i o n s of the copper species ( d i s s o l v e d complexes or suspended c o l l o i d s ) w i t h the c o l l e c t o r . Experimental support f o r t h i s p r o p o s i t i o n was obtained during the s t u d i e s of a d s o r p t i o n of l a u r i c a c i d on copper oxide s u b s t r a t e s (to be presented i n S e c t i o n 4.2). (e) F l o t a t i o n of T e n o r i t e w i t h O l e i c A c i d and Amyl Xanthate The r e s u l t s of the t e s t s c a r r i e d out using o l e i c a c i d and xanthate as c o l l e c t o r are shown i n F i g s . 20 and 21, r e s p e c t i v e l y . The recovery Figure 19. F l o t a t i o n of t e n o r i t e w i t h a n i o n i c and c a t i o n i c c o l l e c t o r and i n the presence of added copper ions (1 x 10~ 5M CuSO ) (Procedure I ) . 1 2 3 4 5 6 7 8 9 10 11 12, F i g u r e 20. F l o t a t i o n of t e n o r i t e w i t h o l e i c a c i d (Procedure I ) . - 83 -curves are not symmetrical about the IEP of t e n o r i t e i n d i c a t i n g that the b a s i c phenomena of c o l l e c t i o n cannot be accounted f o r by the i o n adsorption theory when these c o l l e c t o r s are used. The optimum f l o t a t i o n c o n d i t i o n as obtained from the recovery curves w i t h o l e i c a c i d as c o l l e c t o r , i s a pH of the pulp of about 9 . The recovery curves f o r xanthate as c o l l e c t o r i n d i c a t e t h a t pH i s not a c r i t i c a l v a r i a b l e . However, the presence of s u l p h i d e ions can be seen to i n c r e a s e recovery s i g n i f i c a n t l y . The above c o n d i t i o n s are i n agreement w i t h p l a n t p r a c t i c e f o r f l o t a t i o n of o x i d i z e d copper ores. - 84 -4.2 I n f r a r e d Spectroscopic Studies > 4.2.1 Q u a l i t a t i v e and Q u a n t i t a t i v e A n a l y s i s bf the Spectra (a) The Assignment of Bands To f a c i l i t a t e the q u a l i t a t i v e i n t e r p r e t a t i o n of the IR s p e c t r o -s c o p i c data obtained during the s t u d i e s of adsorption of aqueous l a u r i c a c i d onto copper oxide substrates (to be discussed i n su b - s e c t i o n 4.2.2), an i n t r o d u c t o r y d i s c u s s i o n on band assignment i s presented. The f o l l o w i n g s p e c t r a are analyzed: ( i ) t r a n s m i s s i o n reference s p e c t r a of o x i d i z e d copper m i n e r a l s ; Cii) specular r e f l e c t i o n s p e c t r a of gold and copper oxide s u b s t r a t e s ; ( i i i ) ATR s p e c t r a of cuprous oxide f i l m s (alone, i n the presence of H 20 and D 20); Civ.) t r a n s m i s s i o n .reference s p e c t r a of c a r b o x y l i c acids and soaps; and Cv) specular r e f l e c t i o n s p e c t r a of c a r b o x y l i c compounds. These i n c l u d e : (v.1) t h i n f i l m s of l a u r i c and s t e a r i c a c i d condensed onto gold substrates from the vapour phase; (v.2) l a u r i c a c i d adsorbed onto gold surfaces from H 20 and D 20 s o l u t i o n s , and (v.3) t h i n f i l m s of l a u r i c and s t e a r i c a c i d condensed onto c u p r i c oxide s u b s t r a t e s . ( i ) Transmission Reference Spectra of Oxidized Copper Mi n e r a l s The absorption s p e c t r a of c u p r i e oxide, cuprous oxide, c u p r i c hydroxide and m a l a c h i t e , which are the o x i d i z e d copper minerals most - 85 -p e r t i n e n t to t h i s work, are shown i n F i g s . 22 to 25. These s p e c t r a are presented i n terms of absorbance versus wavenumber and they have been r e p l o t t e d from s p e c t r a recorded i n a l i n e a r percent t r a n s m i s s i o n mode. Band assignment (shown i n the f i g u r e s ) was made on the b a s i s of the data presented i n Appendix C ( t a b l e of c h a r a c t e r i s t i c f r e q u e n c i e s ) . ( i i ) Specular R e f l e c t i o n Spectra of Gold and Copper Oxide Substrates In F i g s . 26 (wavenumber region of 3700 to 2600 cm"1) and 27 (1800 to 700 cm ' region) the s p e c t r a ( a c t u a l t r a c e i n 20X expanded l i n e a r t r a n s m i s s i o n mode) of f r e s h l y prepared g o l d (curve A) and c u p r i c oxide (curve B) substrates are shown i n order to i l l u s t r a t e the t y p i c a l behaviour of the b a s e l i n e s obtained during the s p e c t r o s c o p i c s t u d i e s which were conducted using the eleven r e f l e c t i o n , 70° i n c i d e n c e angle, specular r e f l e c t i o n technique. The s p e c t r a l regions represented were the most u s e f u l regions as f a r as the i d e n t i f i c a t i o n of surface c a r b o x y l i c compounds were concerned. A l l the s u b s t r a t e s (gold or copper oxide) used i n these s t u d i e s showed a tendency t o adsorb organic contaminants from the surroundings. This was i n d i c a t e d by the presence of a weak band at approximately 2900 cm 1 (C-H. s t r e t c h i n g v i b r a t i o n s of organic compounds) which i n c r e a s e d w i t h time of exposure of the m i r r o r s to the atmosphere. The i n t e n s i t y of the CH band i n the spectrum shown i n F i g . 26A represents the maximum l e v e l of contamination accepted. M i r r o r s f o r which the spectrum contained _ Spectra of cuprous oxide substrates (not shown) presented e s s e n t i a l l y the same behavior as that of c u p r i c oxide s u b s t r a t e s ( F i g s . 26 and 27 B). ** This l e v e l corresponds to l e s s than a tenth of "complete coverage of the m i r r o r surface w i t h organic species as estimated f o r a monolayer of s t e a r i c a c i d (to be shown i n P a r t (c) of t h i s s e c t i o n ) . 4000 4000 Figure 22 - 4000 4000 Figure 23. 3500 3500 3000 2500 2000 1500 Wavenumber (cm ) Absorption spectrum of c u p r i c oxide (KBr d i s c ) . 3500 3000 2500' 2000 - 1 N 1500 Wavenumber (cm ) Absorption spectrum of cuprous oxide (KBr d i s c ) . 1000 1000 1000 500 250 500 250 500 250 Absorbance Absorbance O o o r o o o u i U l o t o U l > LO cr U l CD o o o i-i xs r t H * o 3 CO V. LO B O O i-i o o r h & fu M CU t o O U l ^ o o r t CD fa < ro W 3 c 3 o- cr t o ro o CD i-i o o o N—' •-^ • n B U l o o o o o U l o o t o U l o t o U l o (TO c H CD to .e-c ? CD O H >0 o 3 co ro o o o o LO U i O O LO O O o of o c to T3 to U l i-f U l o H- o o O o s: to G . <! H ro o 3 c H- 3 &. cr ro CD ro to o i-i o o o o o w o g i a. H' CO o *-—* M • • M U l U l o O o o 3575 3425 ( f r e e ) -fN o o o LO U l O O -LO O O O t o U i o o t o o o o carbonate (?) U l o o i - 1 o o o Cu-OH U l o o t o U l o t o U l o o U l U l o to U l LO - L2 -2700 2600 100 JL 3700 3600 3500 3400 3300 3200 3100 3000 2900 Wavenumber (cm -!) Fig u r e 26. (A) Spectrum of gold m i r r o r s ( a c t u a l t r a c e , 3700-2600 cm" IR r e g i o n ) . 2800 2700 -1 97 2600 (B) Spectrum of c u p r i c oxide f r o n t surface gold m i r r o r s ( a c t u a l t r a c e , 3700-2600 cm . IR r e g i o n ) . 1800 1700 1600 1500 1400 1300 1200 1100 . 1000 900 800 700 3 » t ! s i _ i i ; — i 1 J — ; 1 J 1800 1700 1600 1500 1400 1300 1200 . 1100 1000 900 800 700 Wavenumber (cm ) _^ Figure 27. (A) Spectrum of gold m i r r o r s ( a c t u a l t r a c e , 1800-700 cm region) _^ (B) Spectrum of c u p r i c oxide covered f r o n t surface gold m i r r o r s ( a c t u a l t r a c e , 1800-700 .cm ^ r e g i o n ) . - 90 -a CH band more intense than that were discarded. ' Copper oxide substrates y i e l d e d s p e c t r a showing a p a i r of very -1 -1 weak bands around 1500 cm and 1400 cm " which was probably due to the presence of minor carbonate contaminants formed during exposure of the m i r r o r s to atmospheric a i r c o n t a i n i n g carbon d i o x i d e . The bands shown i n F i g . 27B are t y p i c a l examples. S l i g h t d i f f e r e n c e s i n i n t e n s i t y and p o s i t i o n were observed i n carbonate bands of s p e c t r a of 113 the v a r i o u s copper oxide substrates used i n t h i s research. Mrmaj' a l s o noted the presence of s i m i l a r bands i n r e f l e c t i o n s p e c t r a of copper metal m i r r o r s . Representative r e f l e c t i o n s p e c t r a of cuprous oxide and c u p r i c oxide covered f r o n t surface gold m i r r o r s are presented i n F i g s . 28 and 29, ..respectively. The assignment -of the predominant band at 645 cm 1 i n F i g . 28 to Cu^O (or CuO ^ ) l a t t i c e v i b r a t i o n s , i s i n agreement w i t h p u b l i s h e d d a t a . ^ ' ^ A shoulder i n t h i s main band (shown at approximately 500 cm "*") was observed i n almost a l l s p e c t r a of cuprous oxide s u b s t r a t e s . I t may be a s s o c i a t e d w i t h the presence of s m a l l concentrations of cupric oxide i n those f i l m s , as suggested by t h e i r 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 (see S e c t i o n 3.2). The assignment of the predominant band at 570 cm to CuO (see F i g . 29) i s a l s o 85 c o n s i s t e n t w i t h previous observations. The shoulder at about 450 cm 1 appears to be r e l a t e d to the presence of c u p r i c oxide but i t s r e a l nature was not determined. ( i i i ) ATR Spectra of Cuprous Oxide Fi l m s ATR s p e c t r a of t h i n f i l m s of cuprous oxide deposited onto KRS-5 prisms are shown i n F i g . 30. When the spectrum was recorded * . 0 5 < , 1 0 4 0 0 0 l O O i 75 *—* QJ.50 a c cd S 25 <4-l Q) Pi 0 4 0 0 0 F i g u r e 2 8 . 3500 3000 —I 2500 2000 — I — 1 5 0 0 X X 3500 3000 2 5 0 0 JL o o o o i r i cd C o & u cd a (cm 1 ) 1 5 0 0 1 0 0 0 ! 5 0 0 1 0 0 0 2000 Wavenumber R e f l e c t i o n spectrum of cuprous oxide covered f r o n t s u r f a c e , gold m i r r o r s , o CM 3 CJ 5 0 0 250 0 100 75 0 250 0 . 0 5 50 i 0 . 1 0 25 i Pi < . 0 5 , 1 0 4 0 0 0 loop 75 CJ o 50 o CD 25 01 Pi 0 3500 3000 2500 —I 2000 i 1 5 0 0 JL 1 0 0 0 ~ ~ 5 — JL 4 0 0 0 3500 3000 2500 2000 _ ± 1 5 0 0 1 0 0 0 Wavenumber (cm ) Figu r e 2 9 . R e f l e c t i o n spectrum of c u p r i c oxide covered f r o n t s u r f a c e , gold m i r r o r s , 5 0 0 250 T 5 0 0 250 4000 3500 300 . 2500 . 2000 1500 1000 500 250 u • ' • « . i i i 4000 3500 3000 2500 ••2000 . 1500 1000 500 250 Wavenumber (crrT ) F i g u r e 30. ATR s p e c t r a of cuprous oxide f i l m s : (A) alone, (B) i n the presence of H 90, (C) i n the presence of D„0. - 9 3 -without any l i q u i d i n the r e a c t i o n v e s s e l ( F i g . 30A), the Cu^O band occurred at 615 cm \ i n c l o s e agreement w i t h values obtained by t r a n s m i s s i o n spectroscopy (see F i g . 23). The shoulder to the main band, l o c a t e d at approximately 520 cm X may be due to the presence of c u p r i c oxide i n the f i l m as aforementioned. The presence of weak carbonate bands can a l s o be n o t i c e d . The bands at 1200 and 1145 cm X are a t t r i b u t e d to the C-F groups of t e f l o n . They p o s s i b l y arose as a r e s u l t of contact of the ATR prism w i t h the t e f l o n gasket used to s e a l the r e a c t i o n v e s s e l . In F i g . 30, s p e c t r a B and C were taken w i t h pure H^ O and D^ O i n the r e a c t i o n v e s s e l , r e s p e c t i v e l y . They i l l u s t r a t e the range of a v a i l a b i l i t y of the IR s p e c t r a l r e g i o n f o r r e c o r d i n g of " i n s i t u " SDectra o'f adsorbed surface species« ( i v ) Transmission Reference Spectra of C a r b o x y l i c Acids and Soaps The IR s p e c t r a of c a r b o x y l i c acids have been e x t e n s i v e l y s t u d i e d and the assignment of t h e i r c h a r a c t e r i s t i c frequencies i s w e l l e s t a b l i s h e d (see Appendix C). C a r b o x y l i c a c i d s occur as dimers i n the s o l i d or l i q u i d s t a t e due to strong hydrogen bonding of the form y0 HO „ r N (R = hydrocarbon r a d i c a l ) X 0 H 0 The t r a n s m i s s i o n s p e c t r a of l a u r i c and s t e a r i c a c i d are shown i n F i g s . 31 and 32, r e s p e c t i v e l y . These two s p e c t r a are s i m i l a r i n appearance w i t h minor d i f f e r e n c e s i n the r e l a t i v e i n t e n s i t y of the CH^ s t r e t c h i n g - y6 -- 95 -and bending v i b r a t i o n bands, and i n the number and p o s i t i o n of the bands i n the region 1320 to 1170 cm 1 (band p r o g r e s s i o n due to CH^ wagging modes). The carbonyl s t r e t c h i n g frequency (strong band at approximately 1700 cm X ) i s c h a r a c t e r i s t i c of the dim e r i c s t r u c t u r e of the a c i d molecules and can be used to d i f f e r e n t i a t e the dimeric a c i d s from the monomers and from the c a r b o x y l a t e ions ( s a l t s ) . Monomers of some sat u r a t e d f a t t y a c i d s have, been observed i n d i l u t e s o l u t i o n s of non-polar organic s o l v e n t s or i n the vapour phase. T h e i r carbonyl s t r e t c h i n g v i b r a t i o n s occur at approximately 1760 cm \ A l s o , the monomeric species give r i s e to a f r e e OH s t r e t c h i n g band (sharp) at 3560-3500 cm X i n s t e a d of the broad band of the dimers at 3000-2500 cm 1 (band shown at approximately 2700 cm 1 i n F i g s . 31 and ,32) • ,In .addition, a pure (uncoupled) C-OH s t r e t c h i n g band i s observed at around 1375 cm 1 i n the s p e c t r a of the monomers. The s p e c t r a of some c a r b o x y l i c a c i d s contain carbonyl bands at 1715 cm 1 and the s i n g l e - b r i d g e s t r u c t u r e . . .0 OH...0 OH...0 OH... / C R R R has been proposed. 115 In the case of the s a l t s , the carbox y l a t e i o n 0 R - C 0 - 96 -i s formed. Consequently, bands a s s o c i a t e d w i t h the OH groups are > not present ± 1 t h e i r s p e c t r a . In the s p e c t r a of sodium and c u p r i c l a u r a t e shown i n F i g s . 33 and 34, the c h a r a c t e r i s t i c carbonyl -1 a b s o r p t i o n i s replaced by two bands at 1610-1550 cm and 1450-1300 cm corresponding to the asymmetric and symmetric s t r e t c h i n g v i b r a t i o n s of the COO group, r e s p e c t i v e l y . The carboxylate s t r e t c h i n g bands are observed at 1555 and 1420 cm X , f o r sodium l a u r a t e , and at 1585 and -1 9' 1433 cm , f o r c u p r i c l a u r a t e , i n agreement w i t h reported f r e q u e n c i e s . The OH bands shown i n the s p e c t r a (at about 3300 cm X) are a s s o c i a t e d w i t h water absorbed i n t o the potassium bromide d i s c and not w i t h the c a r b o x y l i c s a l t s . (v) -Specular - R e f l e c t i o n Spectra of C a r b o x y l i c Compounds ( v . l ) Thin f i l m s of l a u r i c and s t e a r i c a c i d condensed onto gold substrates from the vapour phase. Except f o r some d i f f e r e n c e s i n the r e l a t i v e i n t e n s i t i e s of bands, the r e f l e c t i o n s p e c t r a of t h i n f i l m s of l a u r i c and s t e a r i c a c i d evaporated onto gold s u b s t r a t e s (shown i n F i g s . 35 and 36) are very s i m i l a r to the corresponding t r a n s m i s s i o n s p e c t r a of these a c i d s ( F i g s . 31 and 32). A c i d molecules i n the dimeric form seem to be the only component of the condensed f i l m s . However, treatment of the f i l m s w i t h organic s o l v e n t s removed most of the dimeric species exposing a l a y e r of a d i f f e r e n t c a r b o x y l i c s t r u c t u r e i d e n t i f i e d from s p e c t r a These d i f f e r e n c e s are r e l a t e d to the phenomenon of molecular o r i e n t a t i o n i n the f i l m s and i t s e f f e c t on specular r e f l e c t i o n band i n t e n s i t y . This matter w i l l be discussed i n p a r t (b) of t h i s s u b - s e c t i o n . 4000 3500 3000 2500 2000 1500 1000 500 250 Figure 33. Absorption spectrum of sodium laurate (KBr disc). 4000 3500 3000 2500 2000 1500 1000 500 250 4000 3500 3000 2500 ^2000 1500 1000 500 250 Wavenumber (cm ) Figure 34. Absorption spectrum of cupric laurate (KBr disc). 4000 .01 4 .02 1000 4000 3500 3000 2500 2000 1500 1000 Wavenumber (cm ) Figure 35. Specular reflection.spectrum of l a u r i c a c i d evaporated onto gold s u b s t r a t e s . 500 250 oo 55 - 0 2 <3 .04 4000 3500 3000 2500 2000 -I. 1500 1000 Wavenumber (cm ) Figure 36. Specular r e f l e c t i o n spectrum of s t e a r i c a c i d evaporated onto gold s u b s t r a t e s . 500 250 0.02 0.04 - 99 -as the monomeric form of the acid. A representative spectrum of such residual films is presented i n F i g . 37A. This spectrum was recorded after immersing a pair of gold mirros bearing evaporated stearic acid films (same sample used for recording F i g . 36 spectrum) i n 200 ml of carbon tetrachloride for 30 minutes and rinsing with 100 ml of the solvent. The carbonyl band characteristic of the dimer (1700 cm \ in F i g . 36) was not detected i n F i g . 37A spectrum but another carbonyl stretching band was exposed at around 1730 cm This band together with a sharp band at 1380 cm 1 and the CH stretching vibrations i n the 2900 cm L region were the only bands observed. Identification of this spectrum with that of monomeric carboxylic acid was based upon the studies of adsorption of lauric acid onto gold substrates from H^O and D20 solutions, which w i l l be the subject of a later discussion (Part v .2 ) . No attempt was made to establish the real nature of the bonds between the monomeric acid species and the gold surface. Strength of Adhesion of Molecules i n the Condensed Films Several tests showed that the dimeric acid molecules ( lauric or stearic acid) , vapour deposited onto the gold substrates, were weakly bound to the sol id surface. Spectra of films recorded after storing the mirrors in argon for a few hours showed a noticeable decrease in the intensity of the IR bands associated with the dimeric acid as compared with the spectra of the films recorded immediately after the deposition. This indicates that part of the f i lm probably evaporated into the environment. Also, as above mentioned, short time immersion i n organic solvents (hexane, carbon tetrachloride or acetone) removed most < ,0011 .002 on . o o i i < ,002^  es .001' .002 Figure 37, 4000 lOOf-3500 3000 4000 3500 3000 2000_1 (cm ) 1500 1000 2500 Wavenumber Spectra of monomeric carboxylic acid formed on gold by(A) washing with carbon-tetrachl stearic acid films), (B) adsorption of lauric acid from H2O solutions, (C) adsorption D2O solutions (spectra recorded 15 minutes after deposition). 500 250 1 .001 .002 1 .002 o o .001 .002 oride (evapora of lauric acid ted from - 101 -of the dimeric species present i n the condensed f i l m s . On the other hand the r e s i d u a l l a y e r seemed somewhat more adherent. A s i g n i f i c a n t f r a c t i o n of t h i s l a y e r was s t i l l detected i n the s p e c t r a of m i r r o r s t r e a t e d w i t h the organic s o l v e n t s f o r s e v e r a l hours ( f o r example, the sample used f o r r e c o r d i n g F i g . 37 spectrum was subjected to a f u r t h e r immersion i n carbon t e t r a c h l o r i d e f o r 48 hours and i t s spectrum recorded; a AR value of 0.0006 f o r the CH^ asymmetric s t r e t c h i n g band at 2920 cm 1 was measured and the other bands a s s o c i a t e d w i t h the monomeric a c i d were s t i l l detected). The r e s i d u a l f i l m s were h i g h l y i n s o l u b l e i n water. Spectra of m i r r o r s c o n t a i n i n g these f i l m s , recorded before and a f t e r immersion i n double d i s t i l l e d water (500 ml) f o r more than 50 hours, showed no d i f f e r e n c e s i n band i n t e n s i t i e s or - p o s i t i o n s . I t was observed that the r e s i d u a l l a y e r on the gold surface i n c r e a s e d the hydrophobic nature of the s u b s t r a t e . Covered gold m i r r o r s were completely hydrophobic i n c o n t r a s t w i t h the p a r t i a l l y , h y d r o p h i l i c uncovered gold m i r r o r s . (v.2) L a u r i c a c i d adsorbed onto gold surfaces from H^O and D^ O s o l u t i o n s . The spectrum of gold m i r r o r s exposed to a l a u r i c a c i d s o l u t i o n -5 * (2 x 10 M l a u r i c a c i d i n H^O, pH = 5.7, 15 minutes exposure time, aqueous system kept under argon environment) i s presented i n F i g . 37B. Band i n t e n s i t i e s were not changed by exposing the m i r r o r s f o r longer periods of time (up to 12 hours). Adhesion of the l a u r i c a c i d f i l m s to the a i b s t r a t e was comparable to that of the c a r b o x y l i c a c i d r e s i d u a l l a y e r s as estimated by s o l u b i l i t y t e s t s ( i n water and i n organic s o l v e n t s ) analogous to those r e f e r r e d to i n ( v . l ) . - 102 -The presence of the carbonyl band at approximately 1730 cm X and of the^ 1380 cm X band suggested that the l a u r i c a c i d f i l m adsorbed from aqueous s o l u t i o n i s of the same nature as the r e s i d u a l l a y e r of c a r b o x y l i c a c i d observed during the s t u d i e s conducted on f i l m s formed by evaporation (compare F i g . 37A w i t h F i g . 37B). E f f e c t of Deuteration on the Spectrum of the Adsorbed F i l m To c l a r i f y the assignment of the bands at 1730 and 1380 cm X , gold m i r r o r s were exposed to a B^O s o l u t i o n of l a u r i c a c i d (2 x 10 ~* M, argon environment, 15 minutes exposure time) and the spectrum of the adsorbed deuterated a c i d species (C^B^COOD) was recorded ( F i g . 37C) . The p o s i t i o n of the 1730 cm X band was u n a l t e r e d . However, the i n t e n s i t y of the 1380 cm X band seems much reduced i n comparison w i t h the same band i n F i g . 37B (hydrogenated s p e c i e s ) , i n d i c a t i n g that the 1380 cm 1 band i s a s s o c i a t e d w i t h a group c o n t a i n i n g OH. In t h i s case a band a s s o c i a t e d w i t h the corresponding deuterated group (C-OD) should have been recorded but decreased r e s o l u t i o n i n s p e c t r a recorded at lower frequencies may have obscured i t s i d e n t i f i c a t i o n . The presence of a weak band i n the spectrum of the deuterated f i l m at 1380 cm might be r e l a t e d w i t h the f a s t exchange of OD by OH groups e x i s t e n t i n the form of water vapour i n the atmosphere. In f a c t , i t was observed that the i n t e n s i t y of t h i s band increased w i t h time of exposure of the f i l m s to a i r ; immersion of the m i r r o r s i n H^ O brought the i n t e n s i t y of t h i s band to the same order of magnitude of the one shown i n F i g . 37B f o r the completely hydrogenated s p e c i e s . - 103 -_ i , Assignment of the 1730 cm and .1380 cm bands -1 -1 The 1730 cm and 1380 cm bands were i d e n t i f i e d w i t h the C=0 and C-OH s t r e t c h i n g modes,respectively, of monomeric a c i d molecules adsorbed on the gold s u r f a c e . In Chapter I I i t was pointed out that s e v e r a l i n v e s t i g a t o r s have reported the presence of a weak band around 1735 cm L during s t u d i e s of IR s p e c t r a of adsorbed c a r b o x y l i c a c i d species onto v a r i o u s s u b s t r a t e s . They have assigned t h i s band to the carbonyl s t r e t c h i n g of an " e s t e r " surface compound. Formation of a s i m i l a r compound on the gold surface seems u n l i k e l y . In a d d i t i o n , the " e s t e r " hypothesis would not e x p l a i n the presence of the r e l a t i v e l y strong band at 1380 -1 93 cm . Eischens, working w i t h a c e t i c a c i d a t t r i b u t e d a band at 1389 cm 1 to CH^ bending v i b r a t i o n s of the surface " e s t e r " , A s i m i l a r assignment to the 1380 cm 1 band of the s p e c t r a of f i l m s adsorbed on gold ( F i g s . 37A and 37B) can be r u l e d out i n view of the r e s u l t s of the experiments conducted using D^ O s o l u t i o n s of l a u r i c a c i d which i n d i c a t e d that t h i s band i s a s s o c i a t e d w i t h a hydroxylated group. The -1 114 f a c t that C=0 and C-OH bands have been reported at 1735 cm and -1 117 1375 cm (see Appendix C), r e s p e c t i v e l y , f u r t h e r supports the proposed assignments. (v.3) Thin f i l m s of l a u r i c and s t e a r i c a c i d condensed onto c u p r i c oxide s u b s t r a t e s . L a u r i c and s t e a r i c a c i d f i l m s deposited from the vapour phase onto c u p r i c oxide s u b s t r a t e s y i e l d e d s p e c t r a s i m i l a r to those recorded f o r f i l m s condensed on gold surfaces ( F i g s . 35 and 36) i n d i c a t i n g that the dimeric form of the acids i s the major component of these, f i l m s . - 104 -However, a band at 1583 cm very weak i n s p e c t r a recorded immediately a f t e r d e p o s i t i o n but i n c r e a s i n g w i t h time, was observed i n the case of c u p r i c oxide s u b s t r a t e s . This i s i l l u s t r a t e d i n F i g s . 38 and 39 which show the s p e c t r a of s t e a r i c a c i d f i l m s evaporated onto c u p r i c oxide s u b s t r a t e s and recorded 30 minutes and 90 hours a f t e r the d e p o s i t i o n , r e s p e c t i v e l y . The 1585 cm 1 band was assigned to the asymmetric s t r e t c h i n g of c u p r i c c a r b o x y l a t e (see t r a n s m i s s i o n spectrum of c u p r i c l a u r a t e given i n F i g . 34) formed by r e a c t i o n of the a c i d molecules w i t h c u p r i c oxide. This band incr e a s e s i n i n t e n s i t y as the carbonyl s t r e t c h i n g of the dimeric a c i d (1700 cra^) decreases. The symmetric s t r e t c h i n g of the carbo x y l a t e ions (expected at 1433 cm "*") was not observed probably because of s u p e r p o s i t i o n of the 1430 and 1410 cm 1 bands of the a c i d (C-OH s t r e t c h i n g coupled w i t h i n - p l a n e OH bendings). A n o t i c e a b l e decrease i n i n t e n s i t y of the CH^ s t r e t c h i n g bands (2900 cm 1 region) w i t h longer r e a c t i o n times i n d i c a t e d that the c a r b o x y l i c species were l o o s e l y bound i n the f i l m . Washing the m i r r o r s w i t h o r g a n i c s o l v e n t s exposed a r e s i d u a l l a y e r of c a r b o x y l i c surface compounds. A r e p r e s e n t a t i v e spectrum of the r e s i d u a l l a y e r i s shown i n F i g . 40. This spectrum was recorded a f t e r immersion of the sample, used f o r r e c o r d i n g the spectrum of F i g . 39 i n carbon t e t r a c h l o r i d e (200 ml) f o r 30 minutes and r i n s i n g w i t h 100 ml of the s o l v e n t . I t i s apparent from the spectrum that v i r t u a l l y a l l c u p r i c s t e a r a t e and dimeric a c i d molecules have been removed by the s o l v e n t . The low c o n c e n t r a t i o n of the r e s i d u a l adsorbate and the f a c t that carbonate bands are p o s s i b l y u n d e r l y i n g the bands a s s o c i a t e d w i t h the c a r b o x y l i c species i n the r e g i o n 1500 to 1400 cm X (see spectrum of the s u b s t r a t e alone; F i g . 27) 4000 3500 3000 2500 2000 1500 1000 500 250 4000 3500 3000 2500 2000 , 1500 1000 500 250 Wavenumber (cm ) Figure 38. Specular reflection spectrum of stearic acid deposited on cupric oxide substrates from the vapour phase (recorded 30 minutes after deposition). 4000 3500 3000 2500 2000 1500 1000 500 250 Wavenumber (cm ) Figure 39. Specular reflection spectrum of stearic acid evaporated onto cupric oxide substrates (90 hours after deposition). 4000 100 250 100 Pi < .001 .002 .001 .002 4000 3500 3000 2500 2000 1500 1000 500 Wavenumber (cm ) Figure 40. Spectrum of s t e a r i c a c i d f i l m d eposited by ev a p o r a t i o n onto c u p r i c oxide s u b s t r a t e ( l a y e r observed a f t e r treatment of m i r r o r s w i t h carbon t e t r a c h l o r i d e ) . 250 r e s i d u a l 2000 lOOff 1750 100 0) o 0 4-1 o 0) Pi o ON 2000 1750 1500 1250 1000 Wavenumber (cm ) Fig u r e 41. D i f f e r e n c e spectrum of the r e s i d u a l l a y e r (spectrum of F i g . 40 minus spectrum o f unreacted c u p r i c oxide s u b s t r a t e ) . - 107 -make the assignment of bands of the spectrum of the remaining species 1 r a t h e r d i f f i c u l t . However, the d i f f e r e n c e spectrum* of the f i l m ( F i g . 41) i n d i c a t e d that the r e s i d u a l l a y e r i s composed of two d i f f e r e n t c a r b o x y l i c compounds. One compound y i e l d i n g a carbonyl band at approximately 1730 cm 1 i s probably the monomeric a c i d hydrogen bonded to the o x i d i z e d copper surface. The assignment of the 1730 cm 1 band to the carbonyl s t r e t c h i n g of the monomer was discussed i n the preceding p a r t of t h i s s e c t i o n (v.2) and the p r o p o s i t i o n that the molecules were hydrogen bonded to the s u r f a c e seems reasonable i n view of the composition of the c u p r i c oxide surface (mainly p o l a r groups c o n t a i n i n g oxygen and hydrogen atoms). The C-OH s t r e t c h i n g band which appeared at 1380 cm 1 i n the spectrum of monomeric a c i d attached to the ..gold..surface i s not c l e a r l y .dis.cer.nable i n F i g . 41 spectrum. This i s probably due to the presence of stronger bands around 1400 cm 1 to -1 1450 cm . Another p o s s i b i l i t y i s that hydrogen bonding between C-OH groups of the monomeric a c i d molecules and the o x i d i z e d copper surface might have s h i f t e d t h i s s t r e t c h i n g band from 1380 cm 1 ( p o s i t i o n f o r the f r e e group) to a higher wavenumber (1400-1500 cm 1 ) . The carbonyl s t r e t c h i n g of a second surface compound i s shown at approximately 1530 cm 1 i n F i g . 41. This frequency i s more c h a r a c t e r i s t i c of an i o n i c carboxylate compound (see Appendix C) suggesting that e l e c t r o s t a t i c f o r c e s are i n v o l v e d i n the process of ad s o r p t i o n of t h i s second compound. B e t t e r q u a l i t y s p e c t r a of analogous carboxylate The d i f f e r e n c e spectrum i s obtained by s u b t r a c t i n g the spectrum of the unreacted s u b s t r a t e from the spectrum of the m i r r o r s covered w i t h the adsorbate. D i f f e r e n c e s p e c t r a are u s e f u l f o r q u a l i t a t i v e a n a l y s i s only. - 108 -species were observed during the s t u d i e s of d e p o s i t i o n of monolayers onto c u p r i c oxide substrates from the a i r - w a t e r i n t e r f a c e . They w i l l be discussed i n P a r t (c) of t h i s s e c t i o n together w i t h q u a n t i t a t i v e aspects of the specular r e f l e c t i o n s p e c t r a . (b) E f f e c t s of Molecular O r i e n t a t i o n on IR Specular R e f l e c t i o n  Spectra Monolayer Tra n s f e r r e d from the Air-Water I n t e r f a c e onto Smooth Gold Substrates The most s t r i k i n g e f f e c t s of molecular o r i e n t a t i o n were observed during the specular r e f l e c t i o n s p e c t r o s c o p i c s t u d i e s conducted on s t e a r i c a c i d monolayers which had been deposited as s i n g l e l a y e r s on smooth gold surfaces according to the Langmuir-Blodgett method. The spectrum of one of these monolayers, recorded 10 minutes a f t e r the d e p o s i t i o n , i s shown i n F i g . 42. The spectrum of the same monolayer recorded 24 hours l a t e r ( m i r r o r s s t o r e d i n argon) i s presented i n F i g . 43 showing a s u b s t a n t i a l i n c r e a s e i n the i n t e n s i t y of bands. The i n t e n s i t y (AR) of the CH^ asymmetric s t r e t c h i n g band (2920 cm x) i s 15 times greater i n F i g . 43 than i n F i g . 42. The experiment has been repeated s e v e r a l times; as the lapse of time between the d e p o s i t i o n and the recording of the s p e c t r a increased the r e f l e c t i o n bands a l s o increased. The r a t e of increase of i n t e n s i t y of the bands v a r i e d q u i t e c o n s i d e r a b l y between experiments, b u t , i n a l l of them, band i n t e n s i t i e s showed a maximum value (AR = 0.017 +_ 0.002) f o r the CH^ asymmetric v i b r a t i o n band. This behavior might be e x p l a i n e d by the F r a n c i s and E l l i s o n theory (Chapter I I ) i f the f o l l o w i n g assumptions are made: Pi < 4000 3500 3000 2500 2000 1500 1000 500 250 § Wavenumber (cm ) Figure 42. Spectrum of a monolayer of s t e a r i c a c i d deposited onto smooth s u r f a c e , g o l d m i r r o r s (spectrum recorded 10 minutes a f t e r d e p o s i t i o n ) . F i g u r e 43. Spectrum of a monolayer of s t e a r i c a c i d deposited onto smooth s u r f a c e , gold m i r r o r s (recorded 24 hours a f t e r d e p o s i t i o n ) . - 110 -( i ) The monlayers were t r a n s f e r r e d from the trough to the gold as "hydrous l a y e r s " , and w i t h the a c i d molecules o r i e n t e d w i t h the hydrocarbon chains i n a p o s i t i o n n e a r l y normal to the surface as i l l u s t r a t e d s c h e m a t i c a l l y below. The d i p o l e -moment changes of the CIL ^^Hy d r o c a r b o n Chain 6cVx)6ax) • • • I n t e r v e n i n g Water Layer //'/'/// S S o l i d v i b r a t i o n modes were o r i e n t 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 s u r f a c e , i n the most unfavourable p o s i t i o n f o r i n t e r a c t i o n w i t h the component of the r a d i a t i o n p a r a l l e l to the plane of i n c i d e n c e (only component capable of i n t e r a c t i o n ) . As a consequence, the CH 2 s t r e t c h i n g bands appeared extremely weak i n F i g . 42 (compare r e l a t i v e i n t e n s i t y of bands i n t h i s spectrum w i t h that i n spectrum of F i g . 32). The presence of an i n t e r v e n i n g water l a y e r i s i n d i c a t e d by the (weak) bands at approximately 3300 and 1600 cm 1 i n the s p e c t r a of the monolayers. Another support f o r the assumption mentioned above i s the presence of the carbonyl band at 1710 cm 1 i n F i g . 42. This frequency i s c l o s e r to th a t reported -1 115 f o r the carbonyl s t r e t c h i n g of s i n g l e - b r i d g e a c i d molecules (1715 cm ) than those of the dimer (1700 cm 1 ) or the monomer attached to the gold surface (1730 cm ^) observed i n t h i s work. This then suggests t h a t * 118 Hydrous l a y e r i s the denomination given by Langmuir f o r monolayers deposited w i t h a water l a y e r between them and the s u b s t r a t e ( u s u a l l y a non-reactive s u b s t r a t e ) . - I l l -the molecules are arranged i n the t r a n s f e r r e d f i l m s i n a way analogous' to t h e i r d i s p o s i t i o n at the a i r - w a t e r i n t e r f a c e . I n other words, the s i n g l e hydrogen bonds between the carbonyl groups of the f a t t y a c i d molecules formed i n s o l i d i f i e d monolayers of the a c i d , as proposed by 119 previous i n v e s t i g a t o r s , are preserved i n the t r a n s f e r r e d l a y e r s . ( i i ) . As the i n t e r v e n i n g water l a y e r evaporates, a subsequent rearrangement of the molecules occurs. Dimeric a c i d molecules are formed and grouped together i n c r y s t a l l i t e s w i t h t h e i r hydrocarbon chains l y i n g p r e f e r e n t i a l l y p a r a l l e l to the s u r f a c e . This 90° r o t a t i o n of the molecular a x i s b r i n g s the predominant d i r e c t i o n of the d i p o l e moment change of the CH^ v i b r a t i o n modes from the most unfavourable • p o s i t i o n f o r i n t e r a c t i o n w i t h the p a r a l l e l component of the r a d i a t i o n , to the most favourable ( d i p o l e moment changes n e a r l y normal to the s u r f a c e ) . Consequently, an enormous i n c r e a s e i n the i n t e n s i t y of the CH2 s t r e t c h i n g bands of s p e c t r a recorded a f t e r a l l o w i n g time f o r evaporation of the i n t e r v e n i n g water l a y e r ( F i g . 43) was observed. The f a c t that water bands present i n the s p e c t r a recorded immediately a f t e r d e p o s i t i o n ( F i g . 42) were not observed i n s p e c t r a recorded at a l a t e r stage ( F i g . 43) i s i n agreement w i t h the hypothesis that the i n t e r v e n i n g water l a y e r evaporates. A l s o , the carbonyl s t r e t c h i n g band present at 1710 cm X i n F i g . 42 was s h i f t e d to a s l i g h t l y lower frequency (1700 cm x) i n F i g . 43 g i v i n g support f o r the replacement of the s i n g l e - b r i d g e s t r u c t u r e by dimeric a c i d s p e c i e s . "Overturning" of molecules and formation of c r y s t a l l i t e s (spots or s t r i a t i o n s ) i n monolayers deposited onto n o n - r e a c t i v e substrates have been reported i n , , 1 . . 118,120 the l i t e r a t u r e . - 112 -E s t i m a t i o n of AR Values on the Basis of the F r a n c i s and E l l i s o n Theory A AR value of 0.0079 was c a l c u l a t e d using F r a n c i s and E l l i s o n ' s equation [8] (Chapter I I ) f o r the CH^ asymmetric s t r e t c h i n g band (2920 cm X ) i n a h y p o t h e t i c a l r e f l e c t i o n spectrum of a monolayer composed of randomly o r i e n t e d s t e a r i c a c i d molecules (see c a l c u l a t i o n i n Appendix G). The AR maximum 0.017 + 0.002) measured f o r the CH 2 asymmetric s t r e t c h i n g band i n the spectrum of s t e a r i c a c i d monolayers deposited onto smooth gold substrates (hydrocarbon chain of the a c i d molecules o r i e n t 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) i s approximately 2.15 times g r e a t e r than that c a l c u l a t e d f o r the case of an i s o t r o p i c f i l m (r = AR „TT I /AR ~ T . = 2.15). v vas,CH 2±' vas,CH 2 random I f e f f e c t s of a n i s o t r o p y on the p r o b a b i l i t y that a molecule (or group) w i l l undergo a t r a n s i t i o n are taken i n t o account, a r a t i o r = AR v Appendix H). This value of r i s i n c l o s e agreement w i t h t h a t obtained „„ i ( c a l c u l a t e d ) / A R „7T , =2.46 might be expected (see vas,CK_X vas,CH_ random 6 r by u s i n g the value of AR | measured from the spectrum of the s t e a r i c a c i d monolayer. This supports the c o r r e l a t i o n drawn between the p r e d i c t i o n s of the F r a n c i s and E l l i s o n theory and the i n t e r p r e t a t i o n s of these s p e c t r o s c o p i c s t u d i e s . Monolayer T r a n s f e r r e d to Rough Gold Substrates F u r t h e r support f o r the F r a n c i s and E l l i s o n theory was obtained from the a n a l y s i s of s p e c t r a of s t e a r i c a c i d monolayers ( s i n g l e l a y e r ) * This i s so i n view of the s p e c t r o p h o t o m e t r y e r r o r i n measuring AR (+10%) and the approximations i n v o l v e d i n the c a l c u l a t i o n of ^ v a s CH random t* 1 £ ' i y P o t n e t : ' - c a l monolayer (Appendix G) . - 113 -deposited onto rough gold m i r r o r s (the degree of roughness of these \ m i r r o r s i s i l l u s t r a t e d i n F i g . 11; compare the topography of the surface of these m i r r o r s w i t h that of smooth gold s u b s t r a t e shown i n F i g . 10). The i n t e n s i t y of the CH^ s t r e t c h i n g bands of the sp e c t r a approached that c a l c u l a t e d f o r a h y p o t h e t i c a l randomly o r i e n t e d monolayer (Appendix G). Spectra of monolayers deposited on rough gold are shown i n F i g s . 44 (recorded 10 minutes a f t e r d e p osition) and 45 (24 hours l a t e r ) . S i m i l a r to the case of s t e a r i c a c i d monolayers deposited on smooth gold s u r f a c e s , discussed above, band i n t e n s i t i e s i n c r e a s e d w i t h the i n t e r v a l of time between the recording of the s p e c t r a and the d e p o s i t i o n of the monolayer. This i n d i c a t e s that s i n g l e - b r i d g e molecules l y i n g w i t h t h e i r hydrocarbon chains approximately normal to the sur f a c e were converted to d i m e r i c s p e c i e s , as the i n t e r v e n i n g water l a y e r was evaporated. However, i n the case of rough s u b s t r a t e s , the maximum AR value f o r the CH^ asymmetric s t r e t c h i n g band was s u b s t a n t i a l l y lower ( F i g . 45) than that observed f o r the case of smooth su b s t r a t e s ( F i g . 43). The measured maximum AR „„ value of 0.0065 + 0.0010 i s vaSjCH^ _ c l o s e to that c a l c u l a t e d f o r the h y p o t h e t i c a l i s o t r o p i c monolayer (0.0079), suggesting that the dimeric a c i d molecules are disposed w i t h t h e i r hydrocarbon chains f o l l o w i n g the rough contour of the s u b s t r a t e . Films deposited from the Vapour Phase Comparison between specular r e f l e c t i o n s p e c t r a of c a r b o x y l i c a c i d molecules condensed onto gold or copper oxide substrates (Figs. 35, 36, and 38) and the corresponding t r a n s m i s s i o n s p e c t r a (Figs. 31 and 32) 4000 ^100 500 250 r ^ 1 0 0 % °- 0 0 5|-l7.5 - - 97.5 1 .005 .010 92.5 4000 Wavenumber (cm "*") Figure 44. Spectrum of a monolayer of s t e a r i c a c i d deposited onto rough s u r f a c e , gold m i r r o r s (recorded 1 10 minutes a f t e r d e p o s i t i o n ) . 3000 1 Pi < .97.5 100 T O .005 .010 cu Pi 4000 3500 3000 2000 (cm ) 1500 1000 500 Figure 45. 2500 Wavenumber Spectrum of a monolayer of s t e a r i c a c i d deposited onto rough s u r f a c e , gold m i r r o r s ( 24 hours a f t e r d e p o s i t i o n ) . 92.5 250 recorded - 115 -showed remarkable d i f f e r e n c e s i n r e l a t i v e band i n t e n s i t i e s which can > be e x p l a i n e d by assuming that the evaporated f i l m s have a n i s o t r o p i c o p t i c a l p r o p e r t i e s . The most obvious d i f f e r e n c e (see f o r example F i g . 36 spectrum) i s tie much higher i n t e n s i t y of the CH^ asymmetric s t r e t c h i n g band i n r e l a t i o n to any other band of the spectrum. The CH^ v i b r a t i o n s (e.g., 2960 cm 1 band), the carbonyl s t r e t c h i n g (1700 cm X band) and the bands a s s o c i a t e d w i t h c o u p l i n g of C-OH s t r e t c h i n g and OH i n - p l a n e deformation modes (1430, 1410, and 1300 cm X ) are weaker i n comparison to the CH^ asymmetric s t r e t c h i n g band (2920 cm x ) but not i n comparison w i t h the other bands of the spectrum. The OH s t r e t c h i n g of the dimer (3000- 2500 cm x r e g i o n , i n the a b s o r p t i o n spectra) and the out-of-plane bending of -pure .OH groups (approximately 935 cm x) are p r a c t i c a l l y n o n existent i n the r e f l e c t i o n s p e c t r a . A l s o , the bands r e l a t e d w i t h CH^ symmetric s t r e t c h i n g (2850 cm x ) and bending v i b r a t i o n s ( s c i s s o r i n g at 2470 cm X) appear abnormally weak i n the r e f l e c t i o n s p e c t r a . The above d i f f e r e n c e s i n r e l a t i v e band i n t e n s i t i e s can be explained on the b a s i s of the F r a n c i s and E l l i s o n theory i f i t i s assumed that molecules i n the evaporated f i l m s were condensed w i t h t h e i r hydrocarbon chains o r i e n t 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 s u r f a c e . In a d d i t i o n , i t i s necessary to assume f u r t h e r that the hydrogen atoms of the CH^ groups are disposed n e a r l y normal to the s u r f a c e and the r i n g - l i k e s t r u c t u r e formed by the hydrogen bonded dimeric carbonyl groups l i e s i n planes approximately normal to the surface (see model i l l u s t r a t i n g the proposed o r i e n t a t i o n of the molecules i n F i g . 46). For a c i d molecules o r i e n t e d i n t h i s way, the d i r e c t i o n of the d i p o l e moment change produced - 116 -Figure 46. O r i e n t a t i o n of s t e a r i c a c i d molecules (dimeric) i n f i l m s deposited from the vapour phase. - 117 -by the CIL^  asymmetric s t r e t c h i n g mode, H H i s n e a r l y normal to the sur f a c e (most favourable p o s i t i o n f o r i n t e r a c t i o n w i t h the " a c t i v e " p a r a l l e l component of the r a d i a t i o n ) , e x p l a i n i n g the abnormally strong 2920 cm 1 band i n the r e f l e c t i o n s p e c t r a . On the other hand, the CR^ , symmetric s t r e t c h i n g , H the CrL^  s c i s s o r i n g , H the OH s t r e t c h i n g and the OH out-of-plane bending. 0*-*H 0 / © * R-C C-R * e / 0....H«-»0 produce d i p o l e moment changes n e a r l y p a r a l l e l to the r e f l e c t i n g s u r f a c e (most unfavourable p o s i t i o n ) , y i e l d i n g extremely low i n t e n s i t y bands at 2850, 1470, 3000-2500, and 935 cm - 1, r e s p e c t i v e l y . A l s o , according to the model shown i n F i g . 46, d i p o l e moment changes a s s o c i a t e d w i t h i n - p l a n e OH deformation modes, - 118 -0-H 0 ' 1 ^ R-C • C-R 0 H-0 are o r i e n t e d p r e f e r e n t i a l l y normal to the sur f a c e and the i n t e n s i t y of the r e f l e c t i o n bands a r i s i n g from t h e i r c o u p l i n g w i t h the C-OH s t r e t c h i n g v i b r a t i o n s (1430, 1410, and 1300 cm L ) might be expected to be s l i g h t l y enhanced. The d i r e c t i o n of the d i p o l e moment changes of other v i b r a t i o n modes such as CH^ v i b r a t i o n s , C=0 and C-OH s t r e t c h i n g v i b r a t i o n s should not be s t r o n g l y a f f e c t e d by o r i e n t a t i o n of the molecules. (c) Langmuir-Blodgett Monolayers of S t e a r i c A c i d Deposited onto  Cupric Oxide Substrates  Composition of the F i r s t Layer A r e p r e s e n t a t i v e spectrum of a monolayer of s t e a r i c a c i d t r a n s f e r r e d from the Langmuir trough to c u p r i c oxide coated f r o n t surface gold m i r r o r s i s shown i n F i g . 47 ( a c t u a l t r a c e , 3700-2600 cm L region) and F i g . 48 (1800-700 cm \ d i f f e r e n c e spectrum a l s o shoxm). The s e n s i t i v i t y of the m u l t i p l e specular r e f l e c t i o n technique to record s p e c t r a of sur f a c e f i l m s at sur f a c e c o n c e n t r a t i o n l e v e l s of monolayer coverage i s i l l u s t r a t e d . These s p e c t r o s c o p i c r e s u l t s i n d i c a t e d that the major component of the f i r s t l a y e r t r a n s f e r r e d from the ai r - w a t e r i n t e r f a c e to c u p r i c oxide s u b s t r a t e s i s an i o n i c c a r b o x y l a t e surface compound. Some monomeric a c i d molecules are a l s o present i n the f i l m , but no evidence f o r the formation of c u p r i c s t e a r a t e or dimeric a c i d molecules was observed. 3700 3600 3500 3400 3300 3200 3100 3000 2900 2800 2700 2600 Wavenumber (cm ) Figure 47. Spectrum of a monolayer of s t e a r i c a c i d deposited on c u p r i c oxide s u b s t r a t e ( a c t u a l t r a c e , 3700-2600 cm - 1 r e g i o n ) . 1800 1700 1600 1500 1400 1300 1200 1100 1000 900 800 700 L J i ; i i i i i 1 1 1 s J 1800 1700 1600 1500 1400 1300 1200 1100 1000 900 800 700 Wavenumber (cm - 1) Fig u r e 48' Spectrum of a monolayer of s t e a r i c a c i d deposited on c u p r i c oxide s u b s t r a t e : (A) A c t u a l t r a c e (1800-700 cm-*- region) , (B) D i f f e r e n c e spectrum. - 121 -The bands at approximately 1530 and 1400 cm 1 i n F i g . 48 were assigned to the asymmetric and symmetric s t r e t c h i n g v i b r a t i o n s of the s t e a r a t e i o n adsorbed at the s u r f a c e . A weak band at around 1730 cm ' suggested the presence of the monomeric a c i d . The other bands shown i n the spectrum of the monolayer ( F i g s . 47 and 48) are a s s o c i a t e d w i t h s t r e t c h i n g or bending v i b r a t i o n s of CH^ and CH^ groups. The c h a r a c t e r i s t i c bands of the dimeric a c i d (1700 cm ') or the c u p r i c s t e a r a t e (1585 cm ') were not observed. Composition of the Subsequent Layers Fourteen a d d i t i o n a l monolayers were t r a n s f e r r e d from the a i r - w a t e r i n t e r f a c e to c u p r i c oxide s u b s t r a t e covered w i t h the f i r s t l a y e r ( t o t a l of 15 l a y e r s ) . Spectra of the subsequent l a y e r s (odd number l a y e r s only) were recorded. Dimeric s t e a r i c a c i d and c u p r i c s t e a r a t e were analyzed as the components of the subsequent l a y e r s . The s p e c t r a of 3 and 9 l a y e r s are shown, as examples, i n F i g s . 49 and 50, r e s p e c t i v e l y . Spectrum B i n F i g . 49 represents the d i f f e r e n t spectrum of the 2nd and 3rd monolayers (spectrum of 3 l a y e r s minus spectrum of the f i r s t monolayer). The i d e n t i f i c a t i o n of s t e a r i c a c i d (dimer) and c u p r i c s t e a r a t e as the only components of the subsequent l a y e r s i s apparent from the comparison of F i g s . 49 and 50 w i t h the a b s o r p t i o n s p e c t r a of c a r b o x y l i c compounds given i n Part (a) of t h i s s e c t i o n . Some P r o p e r t i e s of the Langmuir-Blodgett Monolayers Films formed by d e p o s i t i n g successive monolayers of s t e a r i c a c i d onto c u p r i c oxide substrates from the Langmuir trough were more s t a b l e than those condensed onto the m i r r o r s from the vapour phase. As ' 4000 100 500 250 0.005 0.010 97.5 4 0.00 0.01 4000 3500 3000 2500 2000 1500 1000 Wavenumber (cm ) Fi g u r e 49. (A) Spectrum of 3 l a y e r s of s t e a r i c a c i d deposited on c u p r i c oxide s u b s t r a t e s . (B) D i f f e r e n c e spectrum of 2nd and 3rd l a y e r s . 500 250 .01 .02 4000 Fig u r e 50. Spect 3500 3000 2500 2000 1500 1000 Wavenumber (cm ) rum of 9 l a y e r s of s t e a r i c a c i d deposited on c u p r i c oxide s u b s t r a t e . 500 250 - 123 -discussed i n Pa r t ( a ) , evaporated f i l m s of s t e a r i c a c i d showed a 1 tendency to escape i n t o the atmosphere and a l s o to re a c t w i t h the o x i d i z e d copper s u b s t r a t e to form c u p r i c s t e a r a t e . Spectrum of 15 monolayers (not shown) recorded a f t e r storage of the m i r r o r s i n argon f o r s e v e r a l days presented no d e t e c t a b l e d i f f e r e n c e i n band i n t e n s i t y or p o s i t i o n as compared w i t h the spectrum of the same f i l m of 15 l a y e r s recorded immediately (10 minutes) a f t e r the d e p o s i t i o n . The f a c t that no r e a c t i o n between the a c i d molecules and the s u b s t r a t e was observed i n t h i s experiment suggested that the c u p r i c s t e a r a t e present i n the subsequent l a y e r s was formed i n s i d e the trough, probably by the i n t e r a c t i o n of d i s s o l v e d copper species w i t h the s o l i d i f i e d A monolayers. In f a c t , a decrease i n th i c k n e s s of the c u p r i c oxide o s u b s t r a t e of the order of 50 A was n o t i c e d a f t e r immersing the m i r r o r s i n the trough s o l u t i o n (30 minutes) at pH 4.5 f o r d e p o s i t i o n of the f i r s t l a y e r . Immersion of the s u b s t r a t e i n the trough f o r d e p o s i t i o n of the subsequent l a y e r s produced f u r t h e r d i s s o l u t i o n but, i n t h i s case, at a much slower r a t e ; only 30 A decrease i n thickness a f t e r 7 immersions r e q u i r e d f o r d e p o s i t i o n of the 14 subsequent l a y e r s . Treatment of the Langmuir-Blodgett f i l m s w i t h organic s o l v e n t s promoted f a s t removal of the subsequent l a y e r s but f a i l e d to a f f e c t the f i r s t monolayer, supporting the view that the i o n i c c a r b o x y l a t e species (and the monomeric a c i d molecules) were chemisorbed onto the c u p r i c oxide s u b s t r a t e . Spectra of m i r r o r s covered w i t h a monolayer of s t e a r i c A Changes i n c u p r i c oxide f i l m t hickness were c a l c u l a t e d from measured AR values at band maximum (570 cm - 1) and by means of equation I 2 given i n Appendix I. - 124 -acid measured after immersion in carbon tetrachloride, hexane or acetone (up to 48 hours) yielded identical bands to those shown in Figs. 47 and 48 (untreated monolayer). Other mirrors were immersed in pure water for several days and again no change in the composition of the adsorbed monolayer was observed. The presence of the monolayer on the mirrors changed the hydrophilic nature of the cupric oxide surface to hydrophobic. The high s t a b i l i t y of the Langmuir-Blodgett films can be understood by considering that molecules in the s o l i d i f i e d monolayers were in a close-packed arrangement and consequently strong van der Waals attractions between hydrocarbon chains were developed. Band Intensity Spectra of stearic acid monolayers transferred from the air-water interface to cupric oxide substrates were recorded with the objective of establishing standard references for quantitative analysis of similar carboxylic films. However, the following points must be observed: (i) More than one carboxylic compound was present in the deposited monolayers which made impossible the determination of their actual concentrations from the trough data. Therefore, bands associated with vibration modes of carbonyl groups are not useful for quantitative purposes. ( i i ) The transferred films presented anisotropic optical properties due to preferential orientation of the molecules. The concentration of CH„ groups in the monolayers can be estimated from the trough data but - 125 -e f f e c t s of molecular o r i e n t a t i o n on the s pecular r e f l e c t i o n s p e c t r a (discussed i n P a r t (b)) should be taken i n t o account before using the i n t e n s i t i e s of the CH^ bands as q u a n t i t a t i v e r e f e r e n c e s . AR values f o r the CH^ asymmetric s t r e t c h i n g band (2920 cm X ) were measured from the s p e c t r a of the t r a n s f e r r e d monolayers and p l o t t e d against the number of l a y e r s i n F i g . 51. P o i n t P l (one monolayer) represents the average of a s e r i e s of measurements made using f i v e d i f f e r e n t samples prepared under s i m i l a r c o n d i t i o n s . The AR values f o r the CR^ asymmetric s t r e t c h i n g band of the s p e c t r a of these f i r s t monolayers were measured as 0.0025 + 0.0001. According~to the F r a n c i s and E l l i s o n theory, a l i n e a r p l o t of AR of the CH^ asymmetric s t r e t c h i n g band versus f i l m thickness (or number of l a y e r s ) .should be expected i f the hydrocarbon chain of the molecules were i d e n t i c a l l y o r i e n t e d i n each l a y e r . The c o n c e n t r a t i o n of CH^ groups was the same f o r any l a y e r (same t r a n s f e r r a t i o , 1 + 0.02), t h e r e f o r e suggesting that the n o n - l i n e a r i t y observed i n F i g . 51 p l o t might be r e l a t e d to d i f f e r e n c e s i n o r i e n t a t i o n of molecules i n the v a r i o u s l a y e r s . E f f e c t s of Molecular O r i e n t a t i o n on the Spectra of Monolayers Deposited  on Cupric Oxide Substrates The behavior of the curve obtained by p l o t t i n g AR (CH^ asymmetric s t r e t c h i n g ) v e r s u s number of l a y e r s of s t e a r i c a c i d t r a n s f e r r e d from the trough to c u p r i c oxide su b s t r a t e s ( F i g . 51) might be e x p l a i n e d by d i f f e r e n c e s i n molecular o r i e n t a t i o n i n the v a r i o u s l a y e r s . The observed d e v i a t i o n from l i n e a r i t y i n the curve shown i n F i g . 51 i s manifested i n two ways: 0.030 0.020 -CN < CO CO 0.010 .030 020 - .010 Fig u r e 51. 5 7 Number of Layers ^ Measured AR values f o r the CH.2 asymmetric s t r e t c h i n g band (2920 cm ) of s p e c t r a of s t e a r i c a c i d monolayers t r a n s f e r r e d to c u p r i c oxide s u b s t r a t e s . - 127 -1. The i n t e n s i t y of the CH^ s t r e t c h i n g band of the s p e c t r a of the f i r s t l a y e r i s somewhat higher than the average i n t e n s i t y per l a y e r of the corresponding band i n 3pectra of subsequent l a y e r s . This might be due to the f a c t that CH^ groups of subsequent l a y e r s are o r i e n t e d i n planes n e a r l y p a r a l l e l to the r e f l e c t i n g s urface and the molecules i n the f i r s t l a y e r are more randomly o r i e n t e d . In t h i s case, the chemisorbed molecules of the f i r s t l a y e r are disposed w i t h t h e i r hydrocarbon chains s t i c k i n g out of the c u p r i c oxide substrate but f o l l o w i n g the r e l a t i v e l y rough su r f a c e contour. D i s s o l u t i o n of the f i l m i n s i d e the trough i s r e s p o n s i b l e f o r an i n c r e a s e i n roughness of the s u b s t r a t e . This was v e r i f i e d by i n t e r f e r o m e t r i c and e l e c t r o n - m i c r o s c o p i c s t u d i e s conducted on c u p r i c oxide surfaces which have been exposed to chemical environments s i m i l a r to those which e x i s t i n . t h e trough. The in c r e a s e i n the degree of roughness of c u p r i c oxide surfaces r e s u l t i n g from immersion i n K^SO^ s o l u t i o n (pH 4.5) f o r 30 minutes (same c o n d i t i o n s used during a c t u a l d e p o s i t i o n of a monolayer) i s i l l u s t r a t e d i n F i g . 52. 2. The c o n t r i b u t i o n of i n d i v i d u a l l a y e r s to the i n t e n s i t y of the CH^ asymmetric s t r e t c h i n g band of s p e c t r a of m u l t i l a y e r s i n c r e a s e s w i t h the number of l a y e r s ( i n c r e a s i n g slope i n F i g . 51 curve). This phenomenon can be a t t r i b u t e d to a p a r t i a l degradation of molecular o r i e n t a t i o n i n the t h i c k e r f i l m s or (and) an in c r e a s e i n the angle formed by the planes c o n t a i n i n g o r i e n t e d CH^ groups w i t h the r e f l e c t i n g s u r f a c e . 121 Takenaka and co-workers used a p o l a r i z e d i n f r a r e d ATR technique f o r c a l c u l a t i n g the angle formed by the plane of CH^ groups w i t h the - 128 -52(a) Fizeau interference fringes (fringe-to-fringe spacing = o 2945 A, 15 x horizontal magnification). 52(b) Transmission electron micrograph (acetate replica) Figure 52. Surface topopgraphy of cupric oxide substrates exposed to -4 5 10 " M H_S0. solution for 30 minutes. - 129 -s o l i d s u rface i n b u i l t - u p f i l m s of s t e a r i c a c i d (up to 201 l a y e r s ) > deposited onto a germanium prism. T h e i r r e s u l t s showed angles (average) v a r y i n g from 27-30° f o r t h i n n e r f i l m s (e.g., 9 and 11 l a y e r s ) to 35° f o r t h i c k e r f i l m s . They considered the angle formed by the molecular a x i s w i t h the plane normal to the surface i n the case of t h i n f i l m s (29-30°) to be clo s e to the c r y s t a l l o g r a p h i c angle between the C- and C'- axes of the s t e a r i c a c i d c r y s t a l (26.4°). They e x p l a i n e d the i n c r e a s e i n t h i s angle w i t h i n c r e a s e i n the number of l a y e r s by degradation of molecular o r i e n t a t i o n i n the t h i c k e r f i l m s . Table 4 shows values f o r the average angles formed by planes c o n t a i n i n g CH^ groups w i t h the s o l i d surface ( f i r s t l a y e r not included) as estimated u s i n g AR values from F i g . 51 and applying the F r a n c i s and E l l i s o n theory .(see...de.tail of c a l c u l a t i o n .in Appendix J) . The c a l c u l a t e d angles v a r i e d from 15.4 to 17.4. These angles were s i g n i f i c a n t l y lower than the corresponding angles reported by Takenaka and a s s o c i a t e s mentioned above. The discrepancy might be due to the lower surface pressure (16.5 dynes.cm 1 ) used by them during t r a n s f e r of the monolayers, as compared to the pressure of 2 7 + 1 dynes.cm X employed i n t h i s work. These angles are c l o s e r to the angle (minus 90°) made by the carbon t e t r a h e d r a l bonds w i t h the a x i s of the hydrocarbon chain (109° 28' -90° = 19° 28*) ( r e f . 122) than to the c r y s t a l l o g r a p h i c angle of the s t e a r i c a c i d c r y s t a l (26.4°). - 130 -Table 4. Average angle made by planes c o n t a i n i n g CH^ groups of s t e a r i c a c i d molecules i n the subsequent l a y e r s w i t h the s o l i d s u r f a c e . Number of l a y e r s (m) AR AR -AR, m 1 Average AR AR -AR, m 1 m-l Average angle 0 1 .0025 - (.0025) (21°) 3 .0053 .0028 .00140 15.6° 5 .0081 .0056 .00140 15.6° 7 .0107 .0082 .00137 15.4° 9 .0141 .0116 .00145 15.8° 11 .0174 .0149 .00149 16.0° 13 .0223 .0198 .00165 16.9° 15 .0267 .0242 .00174 17.4° D e t a i l of c a l c u l a t i o n given i n Appendix J . - 131 -4.2.2 Adsorption of Aqueous L a u r i c A c i d onto Copper Oxide Substrates (I) Specular R e f l e c t i o n Spectroscopic Studies In agreement w i t h the r e s u l t s of the f l o t a t i o n t e s t s discussed i n S e c t i o n 4.1", a d s o r p t i o n experiments conducted using the s p e c u l a r r e f l e c t i o n s p e c t r o s c o p i c technique showed that m i n e r a l surface charge was the major v a r i a b l e a f f e c t i n g a d s o r p t i o n of aqueous l a u r i c a c i d onto c u p r i c oxide s u b s t r a t e . The r e l a t i o n s h i p between a d s o r p t i o n of c o l l e c t o r , s u r f a c e charge and f l o a t a b i l i t y was i n t h i s way e s t a b l i s h e d . Chemisorption of l a u r a t e ions as counter ions i n the i n t e r n a l part of the double l a y e r (mechanism B . i i . c , Table 1) was p o s t u l a t e d as the predominant adsorption mechanism l e a d i n g to c o l l e c t i o n . Co-adsorption of monomeric l a u r i c a c i d molecules was observed as a secondary phenomenon. I t was suggested that the n e u t r a l a c i d molecules were attached to the s o l i d s u r f a c e by.means of strong hydrogen bonding (mechanism A . i i i , Table 1). Experiments c a r r i e d out using cuprous oxide s u b s t r a t e y i e l d e d r e s u l t s analogous to those obtained w i t h c u p r i c oxide. V a r i a b l e s such as added copper i o n s , s u r f a c t a n t c o n c e n t r a t i o n , time of exposure, p a r t i a l pressure of carbon d i o x i d e , the presence of oxygen i n the system and a g i t a t i o n of the aqueous medium were i n v e s t i g a t e d and t h e i r r o l e s i n the adsorption process were found to be i n s i g n i f i c a n t i n comparison w i t h the e f f e c t s of pH. (a) E f f e c t s of pH on Adsorption of L a u r i c A c i d onto Cupric Oxide  Substrate The e f f e c t of pH on adsorption of aqueous l a u r i c a c i d onto c u p r i c - 132 -oxide covered f r o n t surface gold m i r r o r s was i n v e s t i g a t e d by r e c o r d i n g IR s p e c t r a of m i r r o r s before and a f t e r t h e i r exposure to l a u r i c a c i d s o l u t i o n s which contained no added copper i o n s and were kept under a c o n t r o l l e d argon atmosphere. A'R values f o r the CH^ asymmetric s t r e t c h i n g band of s p e c t r a of adsorbates measured during t e s t s conducted using 2 x 10 M l a u r i c a c i d s o l u t i o n s (same c o l l e c t o r c o n c e n t r a t i o n employed during f l o t a t i o n t e s t s ) , are p l o t t e d a g a i n s t pH ranging from 4.7 to 10.5 i n F i g . 53. The m i r r o r s were immersed i n t o the s o l u t i o n f o r 5 minutes (time e q u i v a l e n t to that used f o r c o n d i t i o n i n g the f l o t a t i o n samples) and the s o l u t i o n was not s t i r r e d during the exposure. T y p i c a l behavior of the adsorbate s p e c t r a are shown i n F i g . 54 (spectrum A, s o l u t i o n at pR 4.7; spectrum B, pH 8.4; and spectrum C, pH 10.5). C o r r e l a t i o n between Adsorption and F l o t a t i o n The c l o s e c o r r e l a t i o n between m i n e r a l f l o a t a b i l i t y and a d s o r p t i o n of l a u r i c a c i d onto c u p r i c oxide s u b s t r a t e i s apparent from the comparison of the adsorption curve ( F i g . 53) w i t h the f l o t a t i o n curve ( F i g . 16). This i n d i c a t e s that m i n e r a l surface charge i s the major f a c t o r governing a d s o r p t i o n of the a n i o n i c s u r f a c t a n t . F i g . 53 shows that c u p r i c oxide does not adsorb l a u r i c a c i d from s o l u t i o n s at pH's above the IEP (approximately pH 9.4). A l s o , v i s u a l i n s p e c t i o n of the m i r r o r s i n d i c a t e d t h a t t h e i r s urface remained h y d r o p h i l i c a f t e r the exposure to these s o l u t i o n s . As the pH of the s o l u t i o n was lowered to values below the IEP, and consequently as the p o s i t i v e surface charge i n c r e a s e d , a d s o r p t i o n a l s o - 133 -0.0020 CM Ed u CO ca Pi < 0.0015 0.0010 0.0005 © 9 PH 9 IEP 10 Fig u r e 53. Adsor p t i o n of aqueous l a u r i c . a c i d (2 x 10 "* M) onto c u p r i c oxide s u b s t r a t e (system under argon atmosphere, no copper ions added, no a g i t a t i o n of the s o l u t i o n , 15 minutes r e a c t i o n time). 4000 100 a o u 01 P-. o c cd o rH 4-1 P i Figure 54. R e f l e c t i o n s p e c t r a of c u p r i c oxide s u b s t ? ^ ™ ^ ^ ? a q u e o u s l a u r i c a c i d (2 x l O ^ M system under argon atmosphere, no added copper ions,no a g i t a t i o n , 15 minutes exposure time) (A) pH 4.7; (B) pH 8.4; (C) pH 10.5 - 135 -incr e a s e d up to a maximum at approximately pH 7.5. Below t h i s pH no . f u r t h e r change i n the adsorbate c o n c e n t r a t i o n was observed. The surface of the m i r r o r s t r e a t e d w i t h s o l u t i o n s at pH's between 9.5 and 7.5 had p a r t i a l l y hydrophobic p r o p e r t i e s . Complete hyd r o p h o b i c i t y of the s u r f a c e was only observed f o r m i r r o r s exposed to s o l u t i o n s at pH's below 7.5. Monolayer Coverage The expected AR value f o r the CH^ asymmetric s t r e t c h i n g band of the spectrum of a monolayer formed by adsorption of l a u r i c a c i d onto c u p r i c oxide can be estimated, from s p e c t r o s c o p i c data obtained i n sub-section 4.2.1, as 0.00156 + 0.0001, Since i n the r e g i o n of maximum adsorption ( F i g . 53) the observed AR has values 0.0015 + 0.0001, i t can be assumed that "complete" surface coverage by a monolayer occurred when the sub s t r a t e was t r e a t e d i n s o l u t i o n s at pH's corresponding to those of best f l o a t a b i l i t y of t e n o r i t e (pH's below 7.5). Composition of the Surface Films Representative s p e c t r a of samples exposed to s o l u t i o n s at pH's c h a r a c t e r i s t i c of the three d i f f e r e n t adsorption r e g i o n s , i . e . r e g i o n of "complete" surface coverage, r e g i o n of p a r t i a l coverage, and, no adsorp t i o n r e g i o n , are shown i n F i g . 54 (spectrum A, B, and C, r e s p e c t i v e l y ) . Laurate ions and monomeric l a u r i c a c i d were i d e n t i f i e d as the components of the adsorbed f i l m s . _ In sub-section 4.2.1 i t was shown that AR f o r the s p e c t r a of vas,CH 2 v s t e a r i c a c i d monolayers (16 CH2 groups) deposited as s i n g l e l a y e r s on c u p r i c oxide was 0.0025 + 0.0001. For a monolayer of l a u r i c a c i d (10 CH2 groups), assuming that the CH2 groups are o r i e n t e d s i m i l a r l y to those of the s t e a r i c a c i d molecules: A R = (0.0025 + 0.0001) 10/16 = 0.00156 + 0.0001. - 136 -Spectrum A resembles that of a s i n g l e monolayer of s t e a r i c a c i d deposited onto c u p r i c oxide s u b s t r a t e by the Langmuir-Blodgett method (compare d i f f e r e n c e s p e c t r a of F i g . 54A w i t h that of F i g . 48). Bands at approximately 1530 and 1400 cm 1 were assigned to the asymmetric and symmetric s t r e t c h i n g of the carbonyl group (COO ) of the adsorbed l a u r a t e i o n s . The presence of a weaker carbonyl band at 1730 cm 1 suggests that monomeric a c i d molecules were a l s o adsorbed. Spectra of f i l m s formed at pH's between 7.5 and 9.5 (spectrum B, F i g . 54) show carbonyl bands f o r the i o n i c species and the monomer a c i d of the same order of magnitude. F i g . 54, spectrum C, i l l u s t r a t e s the f a c t t h a t organic species were not adsorbed from s o l u t i o n s at pH's g r e a t e r than 9.5. Adhesion O f the Adsorbed Species Tests c a r r i e d out w i t h the purpose of i n v e s t i g a t i n g the s t r e n g t h -o f adhesion of the adsorbed c a r b o x y l i c f i l m i n d i c a t e d that strong • chemical f o r c e s were developed between the adsorbate and the c u p r i c oxide s u b s t r a t e . Spectra of adsorbed f i l m s ( t o t a l or p a r t i a l s u rface coverage) recorded before and a f t e r s t o r i n g the m i r r o r s i n argon, or immersing them i n pure double ' d i s t i l l e d water (pH 6.7) , f o r periods of time up to 100 hours showed no n o t i c e a b l e d i f f e r e n c e i n i n t e n s i t y or p o s i t i o n of bands. Furthermore, the f i l m s were a l s o r e s i s t e n t to organic s o l v e n t s : immersion (up to 12 hours) i n the s o l v e n t s employed i n t h i s work f a i l e d to produce any s i g n i f i c a n t d i f f e r e n c e i n the s p e c t r a _ Immersion of the m i r r o r s i n water at pH s gr e a t e r than approximately 9.5 removed the adsorbed f i l m . - 137 -of the adsorbed f i l m s (except f o r a s m a l l decrease i n the i n t e n s i t y 1 of the 1730 cm X band during prolonged exposure to a strong p o l a r s o l v e n t such as acetone). Support f o r the Mechanisms of Adsorption The hypothesis that i o n i c l a u r a t e species were chemisorbed as counter ions i n the i n t e r n a l p a r t of the double l a y e r i s supported by: ( i ) the e s t a b l i s h e d c o r r e l a t i o n between adsorption and s u b s t r a t e s u r f a c e charge; ( i i ) the presence of COO carbonyl bands i n the s p e c t r a of the adsorbed f i l m s ; ( i i i ) the f a c t that a d s o r p t i o n d i d not exceed "complete" s u r f a c e cover-age; and, ( i v ) the strong adhesion of the adsorbate to the s u b s t r a t e . Chemisorption of n e u t r a l a c i d species i s supported by: ( i ) i d e n t i f i c a t i o n of the 1730 cm 1 band of the s p e c t r a of the adsorbed f i l m s w i t h the carbonyl band a s s o c i a t e d w i t h monomeric a c i d molecules probably hydrogen bonded to the s u b s t r a t e (see S e c t i o n 4.2.1(a)), ( i i ) the strong nature of the adhesion, and ( i i i ) the "sub-monolayer" coverage of the s u r f a c e . In g e n e r a l , adsorption of n e u t r a l species i s not a f f e c t e d by the s u b s t r a t e surface charge. Lack of a d s o r p t i o n of monomeric a c i d species onto c u p r i c oxide from s o l u t i o n s at pH's above 9.5 might be due to d e p l e t i o n of n e u t r a l a c i d molecules i n the bulk by d i s s o c i a t i o n at more b a s i c pH's (see i o n i z a t i o n constant, Appendix A). - 138 -(b) E f f e c t of Time of Exposure and A g i t a t i o n i Further support f o r the proposed adsorption mechanisms was obtained from the. r e s u l t s of a d s o r p t i o n t e s t s c a r r i e d out u s i n g longer exposure times and/or s t i r r i n g the s o l u t i o n (other c o n d i t i o n s were i d e n t i c a l to those of .the experiments reported i n P a r t ( a ) ) . M i r r o r s exposed to s o l u t i o n s at pH's lower than 7.5 f o r longer periods of time (up to 10 hours) showed no d e t e c t a b l e i n c r e a s e i n the amount of c a r b o x y l i c species adsorbed (measured from A R of the v as, 2 spectra) beyond that observed i n experiments discussed i n P a r t ( a ) ) . This o b s e r v a t i o n i s c o n s i s t e n t w i t h the hypothesis of formation of a "complete" monolayer by chemisorption. Films adsorbed from s o l u t i o n s at pH's i n the 7.5 to 9.5 range, showed an increase i n A R of .spectra, .recorded a f t e r longer exposure t i m e , but a d s o r p t i o n never exceeded the c a l c u l a t e d value f o r a "complete" monolayer i n agreement w i t h the proposed chemical nature (strong hydrogen bonding) of the a d s o r p t i o n of the monomeric a c i d molecules. A corresponding i n c r e a s e i n the 1730 cm 1 monomeric a c i d band was a l s o n o t i c e d i n t h i s case. Such i n c r e a s e supports the e x p l a n a t i o n given i n P a r t (a) f o r the decrease i n adsorption of the n e u t r a l species at more b a s i c pH's ( d i s s o c i a t i o n of the a c i d , d e p l e t i o n of n e u t r a l species i n the b u l k ) . At no time d i d adsorption occur i n samples t r e a t e d w i t h s o l u t i o n s at pH's above 9.5. Tests c a r r i e d out w i t h the s o l u t i o n being s t i r r e d y i e l d e d s p e c t r a of adsorbed f i l m s s i m i l a r to those obtained w i t h stagnant s o l u t i o n s i n d i c a t i n g that a g i t a t i o n was i n e f f e c t i v e i n promoting a d s o r p t i o n or d e s o r p t i o n of the chemisorbed organic f i l m s . However, s t i r r i n g - 139 -a f f e c t e d the r a t e of d i s s o l u t i o n of the s u b s t r a t e s l i g h t l y . For example, i n the absence of a g i t a t i o n , no d i s s o l u t i o n of the c u p r i c oxide f i l m was detected i n t e s t s conducted under b a s i c pH c o n d i t i o n s ; m i r r o r s immersed i n s o l u t i o n s at pH 5.5 f o r 10 hours r e s u l t e d i n a decrease i n f i l m t hickness of approximately 30 8(measured from the decrease i n AR values of the CuO band at 570 cm ^ ) . . When m i r r o r s were exposed to o a g i t a t e d s o l u t i o n s , d i s s o l u t i o n was observed i n b a s i c media (a 20 A decrease i n thickness of f i l m s t r e a t e d at pH 10.5 f o r 10 h o u r s ) ; m i r r o r s immersed i n pH 5.5 s o l u t i o n s (10 hours) showed a decrease i n f i l m t hickness of approximately 60 A. (c) E f f e c t of S u r f a c t a n t Concentration Experiments s i m i l a r to those described i n P a r t (a) were conducted using s o l u t i o n s at v a r i o u s l a u r i c a c i d c o n c e n t r a t i o n l e v e l s . R e s ults i n d i c a t e d that i n the c o n c e n t r a t i o n range i n v e s t i g a t e d (1 x 10 ^ M to 4 x 10 M), a d s o r p t i o n of the i o n i c species ( l a u r a t e ions) was independent of the c o n c e n t r a t i o n of the a c i d i n the bulk. This i s i n agreement w i t h the proposed hypothesis t h a t the i o n i c adsorbate was chemisorbed (coverage up to a "complete" monolayer). S l i g h t i n c r e a s e i n the amount cf n e u t r a l a c i d adsorbed at pH's above 7.5 w i t h i n c r e a s i n g bulk c o n c e n t r a t i o n was observed. (d) E f f e c t of Added Copper Ions Cupric sulphate (1 x 10 M) was added to l a u r i c a c i d s o l u t i o n s (2 x 10 ~* M) and a d s o r p t i o n onto c u p r i c oxide s u b s t r a t e s was s t u d i e d at v a r i o u s pH's. Evidence f o r the i n t e r a c t i o n between l a u r i c a c i d and copper species i n the bulk of the s o l u t i o n were obtained supporting - 140 -the hypothesis ( S e c t i o n 4.1) put forward to e x p l a i n depression of 1 t e n o r i t e f l o t a t i o n by added copper i o n s . I t was assumed that copper ions depress t e n o r i t e f l o t a t i o n by r e a c t i n g w i t h c o l l e c t o r o u t side the i n t e r f a c e ( d e p l e t i o n of e f f e c t i v e c o l l e c t o r s p e c i e s ) . A c i d S o l u t i o n s During t e s t s c a r r i e d out i n a c i d i c s o l u t i o n s , formation of p h y s i c a l l y adsorbed l a y e r s composed e s s e n t i a l l y of c u p r i c l a u r a t e (some monomeric a c i d as secondary component) was observed. Concentration of the adsorbed species increased s u b s t a n t i a l l y w i t h time of exposure supporting the proposed p h y s i c a l nature of the phenomena. F i g . 55, spectrum A, shows the spectrum of m i r r o r s exposed to a pH 5.5 s o l u t i o n •f-o-r-one four. The i n t e n s i t y of the CH^ asymmetric s t r e t c h i n g band at 2920 cm - 1 (AR = 0.0020; grea t e r than AR = 0.0015 f o r a monolayer) suggested that m u l t i l a y e r adsorption occurred. The COO asymmetric (1585 cm "*") and symmetric (1430 cm ') s t r e t c h i n g v i b r a t i o n s assigned to c u p r i c l a u r a t e were the predominant carbonyl bands. The presence of ' n e u t r a l a c i d molecules i n c o n c e n t r a t i o n somewhat higher than those observed i n s i m i l a r s p e c t r a of f i l m s formed i n the absence of added copper ions (see F i g . 54, spectrum A) was i n d i c a t e d by the comparatively stronger 1730 cm X band. P h y s i c a l a d s o r p t i o n of copper hydroxo complexes c o n t a i n i n g hydrogen bonded monomeric a c i d molecules can be p o s t u l a t e d to e x p l a i n the above phenomenon. Bands a s s o c i a t e d w i t h chemisorbed species ( l a u r a t e ions and monomeric acid) were not evident i n spectrum A of F i g . 55. However, washing the m i r r o r s w i t h organic s o l v e n t s (30 minutes immersion i n 4000 100 2000 — J — 1500 ! 1000 — I 500 250 I o 4000 3500 3000 2500 2000 1500 1000 500 „. c_ _ Wavenumber (cm 0 Figure 55. R e f l e c t i o n s p e c t r a of l a u r i c a c i d f i l m s adsorbed onto c u p r i c oxide s u b s t r a t e s f c o n t a i n i n g aided copper ions (1 x I O - 5 M CuS04; 1 hour exposure): (A) pH 5.5; (B but s p e c t r a recorded a f t e r 30 minutes immersion i n hexane; (C) pH 8.5 97 96 95 100 99 98 97 96 95 100 99 98 250 rom s o l u t i o n s ) same as A i r-» I - 142 -200 ml of the s o l v e n t ) removed the p h y s i c a l l y adsorbed species > exposing a chemisorbed l a y e r (spectrum B, F i g . 55) i d e n t i c a l to that observed i n experiments of p a r t (a) (adsorption i n the absence of added copper i o n s , spectrum A, F i g . 5 4 ) . The low adhesion of the copper l a u r a t e and l a u r i c a c i d - copper complex molecules to the s u b s t r a t e i n d i c a t e s that these species have been p h y s i c a l l y adsorbed. B a s i c Solutions Formation of m u l t i l a y e r s a l s o occurred during a d s o r p t i o n t e s t s conducted at pH's s l i g h t l y b a s i c (approximately pH 7 to 9). In t h i s case, the presence of c u p r i c l a u r a t e was not observed. P h y s i c a l l y adsorbed l a y e r s contained only monomeric a c i d molecules, probably hydrogen bonded .to .cupric hydroxide p r e c i p i t a t e s . P h y s i c a l a d s o r p t i o n on top of a chemisorbed l a y e r of monomeric a c i d ( i d e n t i c a l , to that i d e n t i f i e d during t e s t s c a r r i e d out without a d d i t i o n of copper i o n s ; spectrum B, F i g . 54) was i n d i c a t e d by organic s o l v e n t t e s t s s i m i l a r to those mentioned above. Spectrum C, F i g . 55, was recorded a f t e r exposing a p a i r of c u p r i c oxide covered f r o n t s u r f a c e gold m i r r o r s to a pH 8.5 s o l u t i o n f o r one hour (mi r r o r s became p a r t i a l l y hydrophobic). The i n t e n s i t y of the band at 1730 cm \ a s s o c i a t e d w i t h the monomeric a c i d molecules, appears s u b s t a n t i a l l y higher than that observed i n s p e c t r a of f i l m s formed i n absence of copper ions and at s i m i l a r pH's ( F i g . 54B). The presence of bands at approximately 3300 and 930 cm \ i n F i g . 55C, frequencies c l o s e to the OH s t r e t c h i n g and bending v i b r a t i o n s of - 143 -A c u p r i c hydroxide (see F i g . 24), seems to support the hypothesis that ' monomeric a c i d molecules were present i n the adsorbed f i l m , hydrogen A A bonded to copper hydroxide p r e c i p i t a t e s . I n a d d i t i o n , the bands assigned to c u p r i c hydroxide were not observed i n s p e c t r a of m i r r o r s recorded a f t e r treatment w i t h organic s o l v e n t s (removal of p h y s i c a l l y adsorbed species) suggesting that t h e i r appearance was a s s o c i a t e d w i t h the presence of the p h y s i c a l l y adsorbed l a y e r s . (e) E f f e c t of D i s s o l v e d Carbon Dioxide The t e s t s discussed i n the preceding p a r t s of t h i s s e c t i o n were conducted using s o l u t i o n s s a t u r a t e d w i t h an i n e r t gas. The e f f e c t of the presence of carbon d i o x i d e i n the system,and consequently of i t s h y d r o l y s i s products,was i n v e s t i g a t e d by c a r r y i n g out experiments s i m i l a r to those aforementioned but nox^ using s o l u t i o n s s a t u r a t e d w i t h a 300 ppm CC>2 i n gas mixture. Results of these t e s t s showed that the presence of carbon d i o x i d e d i d not change to any' great extent the q u a l i t a t i v e and q u a n t i t a t i v e character of the corresponding adsorbed f i l m s formed i n s o l u t i o n s s a t u r a t e d w i t h the i n e r t gas. The only n o t i c e a b l e d i f f e r e n c e , caused by the presence of carbon d i o x i d e , i n the s p e c t r a of adsorbates was A Other c h a r a c t e r i s t i c bands of the c u p r i c hydroxide spectrum were not observed; the r e l a t i v e l y strong Cu-OH bending v i b r a t i o n at 680 cm~l might have been obscured by the presence of the very strong and broad CuO band at 570 cm - 1 i n F i g . 55C. A A According to the s o l u b i l i t y diagram f o r c u p r i c hydroxide (shown i n F i g . 2 ) , p r e c i p i t a t i o n should occur at pH's grea t e r than approximately pH 7, i n solutions c o n t a i n i n g 1 x 10 -^ t o t a l d i s s o l v e d copper. - 144 -the appearance of stronger carbonate bands at approximately 1500 cm and 1400 cm \ when mirrors were treated in basic solutions. The carbonate bands were more pronounced in the spectra of substrates exposed to carbonated solutions containing added copper ions. The above observations were i n agreement with the equilibrium diagram for the system (Fig. B3, Appendix B) and supported the explanation presented i n Section 4.1 for pH d r i f t s during f lotat ion tests. pH d r i f t s i n f lotat ion tests conducted at basic pH's and i n the presence of carbon dioxide were assumed to be caused by reactions of carbonate ions with the cupric oxide surface and with i t s hydrolysis product. (f) Adsorption of Aqueous Lauric Acid onto Cuprous Oxide Substrate Adsorption tests conducted using cuprous oxide covered front surface goldi irrors yielded results similar to those obtained during tests on cupric oxide substrates (discussed i n parts (a) to (e) above). Spectra of films adsorbed onto cuprous oxide substrates under conditions identical to those used in the aforementioned tests were recorded and compared with the corresponding spectra of films formed on cupric oxide substrate. No major differences were found. To i l l u s t r a t e this , spectra of films formed on cuprous oxide surfaces during a sequence of tests similar to that reported i n F ig . 54 for the case of cupric oxide substrate are presented in F ig . 56. Spectra of films formed i n the presence of oxygen (air saturated solutions) were also obtained i n the present case of cuprous oxide. 1 0 0 9 9 9 8 9 7 9 6 9 5 1 0 0 9 9 9 8 9 7 9 6 9 5 1 0 0 9 9 9 8 9 7 9 6 9 5 4( 5 6 2 0 0 0 1 5 0 0 1 0 0 0 I 5 0 0 2 5 C J d i f f e r e n c e spectrum 1 3 U 1 d i f f e r e n c e spectrum Jm 3 0 0 0 2 5 0 0 2 0 0 0 " l x 1 5 0 0 1 0 0 0 5 0 0 2 5 0 Wavenumber ( c m ) t r a of cuprous oxide substrates exposed to l a u r i c a c i d s o l u t i o n s ( 2 x 1 0 ~ ^ M (A) pH 6 . 0 ; (B) pH 8 . 6 , ( C ) pH 1 0 . 0 . - 1 4 6 -Re s u l t s of t e s t i n the presence of oxygen were analogous to those us i n g i n e r t gas s a t u r a t e d s o l u t i o n s . The above r e s u l t s seems to i n d i c a t e that the cuprous oxide f i l m s were overlayed by a t h i n c u p r i c oxide l a y e r . This i s c o n s i s t e n t w i t h the e q u i l i b r i u m diagram f o r the system ( F i g . B2, Appendix B) which suggests that under c o n d i t i o n s analogous to those used i n the present work, the c u p r i c form i s more s t a b l e than the cuprous form of the oxide. ( I I ) " In s i t u " ATR Studies I n f r a r e d ATR s p e c t r a of f i l m s adsorbed onto cuprous oxide s u b s t r a t e from aqueous (H^O and D 20) s o l u t i o n s of l a u r i c a c i d (2 x 10 M, pH 5.5) were recorded " i n . s i t u " . Q u a l i t a t i v e a n a l y s i s of the s p e c t r a i n -d i c a t e d that the composition of the adsorbed f i l m was not a l t e r e d to any s i g n i f i c a n t extent by the removal of the s u b s t r a t e from the aqueous phase. Laurate ions and monomeric l a u r i c a c i d molecules were i d e n t i f i e d as components of the adsorbed f i l m s i n agreement w i t h the r e s u l t s of the t e s t s conducted using the specular r e f l e c t i o n technique (S e c t i o n 4.2.2 ( I ) ) . H 20 S o l u t i o n F i g . 57 shows an " i n s i t u " spectrum obtained using l a u r i c a c i d s o l u t i o n i n H 20 (30 minutes exposure). Besides the bands as s o c i a t e d w i t h the a i b s t r a t e , the l i q u i d phase and the t e f l o n s e a l (see F i g . 30), the f o l l o w i n g bands were observed and assigned to species present at the i n t e r f a c e : 2800-3000 cm 1 r e g i o n , CH and CH„ s t r e t c h i n g v i b r a t i o n s 4000 3500 3000 2500 2000 1500 1000 500 . 250 Wavenumber (cm ) . • Figure 57. "In s i t u " ATR spectrum: cuprous oxide s u b s t r a t e i n 2 x 10 M l a u r i c a c i d s o l u t i o n i n H-0 (30 minutes exposure time). F i g u r e 58. ATR spectrum of the cuprous oxide, s u b s t r a t e a f t e r being exposed to H^ O s o l u t i o n of l a u r i a c i d (dry sample; H O removed from l i q u i d c e l l a f t e r r e c o r d i n g of the spectrum shown i n F i g . 57). - 148 -_1 1 of the organic adsorbate; 1730 cm , carbonyl v i b r a t i o n of monomeric l a u r i c a c i d ; 1530 cm \ asymmetric s t r e t c h i n g of adsorbed l a u r a t e i o n s ; 1400 cm \ symmetric s t r e t c h i n g of the l a u r a t e i o n s ; 1460-1400 cm \ and CH^ bending v i b r a t i o n s and C-OH coupled w i t h OH bending v i b r a t i o n s ; and, 1500 cm 1 and 1360 cm \ probably a s s o c i a t e d w i t h carbonates. P o s i t i o n and i n t e n s i t y of these bands were not a f f e c t e d by v a r y i n g the exposure times (up to 2 hours). Spectra recorded a f t e r formation of the c a r b o x y l i c f i l m and subsequent to removal of the aqueous medium (see F i g . 58) showed t h a t , except f o r the almost complete absence of the water bands, a l l the bands present i n the " i n s i t u " spectrum ( F i g . 57) were preserved. This suggests that the composition of the i n t e r f a c i a l organic f i l m was not a l t e r e d by dr y i n g the m i r r o r s . D 20 S o l u t i o n R e s u l t s of adso r p t i o n t e s t s c a r r i e d out u s i n g ' D^ O ( F i g s . 59 and 60) were s i m i l a r to those presented i n the preceding d i s c u s s i o n f o r the case of H 20 s o l u t i o n s . However, the absence of a strong H 20 band at 1600 cm L i n " i n s i t u " s p e c t r a recorded using D 20 permitted the obse r v a t i o n of a weak band at 1580 cm 1 ( c h a r a c t e r i s t i c frequency of the COO asymmetric s t r e t c h i n g of the c u p r i c l a u r a t e molecules). The presence of t h i s tand i s probably due to the formation of c u p r i c l a u r a t e i n the outer p a r t of the double l a y e r i n t e r f a c e ( r e a c t i o n of p h y s i c a l l y adsorbed l a u r a t e ions w i t h the h y d r o l y s i s product of the copper oxide s u r f a c e ) . The 1580 cm 1 band disappeared w i t h the removal of the s o l u t i o n ( F i g . 60). 4000 3500 3000 2500 2000 1500 1000 500 250 4000 3500 3000 . 2500 2000 1500 1000 500 250 M ^ Wavenumber (cm ) Figure 59. "In s i t u " ATR spectrum: cuprous oxide s u b s t r a t e i n 2 x 10~"' M l a u r i c a c i d s o l u t i o n s i n ' D2O (30 minutes exposure time). V 4000 3500 3000 2500 2000 1500 1000 500 250 4000 3500 3000 2500 2000 _ ± 1500 1000 5000 250 Wavenumber (cm ) Fig u r e 60. ATR spectrum of the cuprous oxide s u b s t r a t e a f t e r being exposed to D 20 s o l u t i o n of l a u r i c a c i d (dry sample, D 20 removed from l i q u i d c e l l a f t e r r e c o r d i n g of the spectrum shown i n F i g . 59). - 150 -SUMMARY AND CONCLUSIONS The f l o t a t i o n behaviour of copper oxide minerals and the mechanisms of a d s o r p t i o n of i o n i c c o l l e c t o r s on copper oxide substrates were i n v e s t i -gated, using micro f l o t a t i o n t e s t s and i n f r a r e d spectroscopy. A h i g h l y s e n s i t i v e specular r e f l e c t i o n s p e c t r o s c o p i c technique was employed f o r q u a l i t a t i v e and q u a n t i t a t i v e analyses of adsorbates at surface concentra-t i o n l e v e l s never before attempted .in t h i s system. " In s i t u " i n f r a r e d s p e c t r a of the cuprous oxide/aqueous l a u r i c a c i d were recorded. The r e s u l t s l e d to the f o l l o w i n g major c o n c l u s i o n s , l i s t e d below. (1) A d s o r p t i o n of i o n i c c o l l e c t o r s and f l o a t a b i l i t y of copper oxides are g r e a t l y dependent on m i n e r a l surface charge. (2) The " i o n a d s o r p t i o n theory" can e x p l a i n the mechanism of c o l l e c t i o n of copper oxide minerals by l a u r i c a c i d : l a u r a t e ions are chemisorbed as counter ions i n the i n t e r n a l part of the e l e c t r i c a l double l a y e r . Monomeric l a u r i c a c i d molecules are a l s o present i n the adsorbed f i l m s but t h e i r r o l e i n the c o l l e c t i o n process i s assumed of secondary importance. (3) In the presence of excess copper i o n s , l a u r a t e species i n t e r a c t w i t h copper hydroxo complexes i n the bulk of the s o l u t i o n d e p l e t i n g the co n c e n t r a t i o n of e f f e c t i v e c o l l e c t o r species and consequently depressing f l o t a t i o n . (4) The behaviour of specular r e f l e c t i o n s p e c t r a of organic f i l m s can be explained by the F r a n c i s and E l l i s o n theory and gi v e u s e f u l inform-a t i o n on molecular o r i e n t a t i o n . Band i n t e n s i t i e s i n sp e c t r a of i s o t r o p i c - 151 -f i l m s are c l o s e l y described by equation [8] using o p t i c a l constants a v a i l -able i n the l i t e r a t u r e f o r s i m i l a r bulk compounds. For the case of a n i s o -t r o p i c f i l m s , band i n t e n s i t i e s are h i g h l y dependent on the o r i e n t a t i o n of the molecules. However, equation [8]is s t i l l a p p l i c a b l e provided that o r i e n t a t i o n e f f e c t s . o n the p r o b a b i l i t y that a molecule (or group) w i l l undergo a t r a n s i t i o n are taken i n t o account. ' (5) For the p a r t i c u l a r case of f i l m s adsorbed onto copper oxide s u b s t r a t e s from aqueous l a u r i c a c i d s o l u t i o n s , the composition of the chemisorbed l a y e r i s not a l t e r e d by the removal of the s u b s t r a t e from the aqueous phase. - 152 -SUGGESTIONS FOR FURTHER WORK (1) In the present work, i n t e r a c t i o n s of copper oxides w i t h simple i o n i c c o l l e c t o r s have been i n v e s t i g a t e d . S i m i l a r s t u d i e s c a r r i e d out on other o x i d i z e d minerals of copper and i n the presence of d i f f e r e n t reagents might c o n t r i b u t e to a b e t t e r understanding of the problem of f l o t a t i o n of o x i d i z e d copper m i n e r a l s . (2) Up to 15 monolayers of s t e a r i c acid.have been deposited on c u p r i c oxide s u b s t r a t e according to the Langmuir-Blodgett method and t h e i r specular r e f l e c t i o n s p e c t r a recorded. Analogous s p e c t r o s c o p i c s t u d i e s on f i l m s c o n t a i n i n g a much higher number of l a y e r s of s t e a r i c a c i d (or other s u r f a c t a n t s ) deposited on v a r i o u s s u b s t r a t e s might y i e l d r e s u l t s u s e f u l to the development of the t h e o r i e s of r e f l e c t i o n spectroscopy. A l s o , a b e t t e r understanding of the c h a r a c t e r i s t i c s of monolayers t r a n s f e r r e d to the s o l i d surfaces might be achieved. (3) "In s i t u " i n f r a r e d s p e c t r a of the cuprous oxide/aqueous l a u r i c a c i d i n t e r f a c e have been recorded. A p p l i c a t i o n of the technique f o r r e c o r d i n g IR s p e c t r a of other i n t e r f a c e s might be p o s s i b l e . Optimum c o n d i t i o n s must be e x p e r i m e n t a l l y determined f o r each system. - 153 -APPENDIX A S o l u b i l i t y Data f o r Selected Organic Compounds I. S o l u b i l i t y products at 20°C (values r e f e r to i n f i n i t e d i l u t i o n and are negative logarithms of s o l u b i l i t y p r o d u c t s ) : Compounds S o l u b i l i t y Products References L a u r i c a c i d 5.3 (123) L a u r y l amine 3.4(at 25°C) (124) S t e a r i c a c i d 13.8 (65) Sodium s t e a r a t e 6.2 (65) Cupric s t e a r a t e 23.0 (65) O l e i c a c i d 12.3 (65) Sodium o l e a t e 5.7 (65) Cupric o l e a t e 19.. 4 (65) S o l u b i l i t y i n water (mg/1 of water): Compounds Temperature References 20°C 25°C 30°C 60°C L a u r i c a c i d 55 63 87 (124) L a u r i c a c i d 4.6 (123) L a u r y l amine 42 (1) S t e a r i c a c i d 2.9 3.4 5.0 (124) S o l u b i l i t y i n organic s o l v e n t s at 20°C (g/100 g of s o l v e n t ) . Compounds Solvents References Methanol Acetone Carbon t e t r a -c h l o r i d e L a u r i c a c i d 120 60.5 87 (124) S t e a r i c a c i d 0.1 1.5 6.0 (124) - 154 --10 -20 -25 -30 CM -35 P4 60 o -40 -45 -50 -55 -60 APPENDIX B E q u i l i b r i u m Diagrams Mala c h i t e Cu 2(OH) 2C0 3 + • H2° A z u r i t e C u 3 ( 0 H ) 2 ( C 0 3 ) 2 ' + H 0 Cup r i t e Cu 20 + H 20 Native Copper Cu + H 20 -20 -15 -10 l o g P :o +5 +10 CO, F i g u r e B l . S t a b i l i t y of copper compounds as f u n c t i o n s of p a r t i a l pressures of oxygen (P ) and carbon d i o x i d e (P„ n ) , at 25°C and 1 atmosphe t o t a l pressure. Pure l i q u i d water i s assumed present ( r e f . 125). - 155 -+1.0 +0.8 +0.6 Y „ ++ +o.4 r +0.2 EH 0.0 -0.2 -0.4 |--0.6 -0.8 -1.0 -i r X -4 -6 logCCu"^] Cu aq H 20 -v. log[Cu0 2] -6 -4J, (Cu +aq) • V H 2 0 Native Copper 12 14 2 4 6 8 10 pH Figure B2. P o t e n t i a l (Eh)-pH e q u i l i b r i u m diagram f o r the system Cu-H 20-0 2 at 25°C and 1 atmosphere t o t a l pressure. (Ref. 125). - 156 -+1.0 +0.8 +0.6 • +0.4. Cu aq v. . Cu S ^. 2 N >. c h a l c o c i t e -0.6--0.8--1.0 "T 2 - r 4 ~I 1 6 8 pH' 10 12 14 Figure B3. P o t e n t i a l (Eh)-pH e q u i l i b r i u m diagram f o r the system Cu-R^O-0„=C09~S at 25°C and i atmosphere t o t a l pressure. -3.5 P a r t i a l pressure of carbon d i o x i d e ( P r n ) = 10 " ( t y p i c a l content i n a i r ) . T o t a l d i s s o l v e d s u l f u r species = 0.1 M. (Ref. 125). - 157 -+2L Figure B 4 . Logarithmic c o n c e n t r a t i o n diagrams f o r carbonate species i n aqueous s o l u t i o n i n e q u i l i b r i u m w i t h 1 0 ~ 3 . 5 atmosphere p a r t i a l pressure of carbon d i o x i d e (P Q Q )» at 2 5 ° C ( E q u i l i b r i u m constants from Ref. 1 2 6 ) . 2 - 158 -APPENDIX C i C h a r a c t e r i s t i c Group Frequencies Conditions governing data i n t a b l e of s e l e c t e d c h a r a c t e r i s t i c group freq u e n c i e s . i ) ' C h a r a c t e r i s t i c a b s o r p t i o n frequencies of groups most p e r t i n e n t to t h i s work are presented i n the form of a t a b l e (pages 159 to 162 ). i i ) Unless otherwise s t a t e d , the bands u s u a l l y appear w i t h i n + 10 cm 1 of the p o s i t i o n shown i n the t a b l e . i i i ) N o t a t i o n : v = s t r e t c h i n g v i b r a t i o n ; 6 = bending v i b r a t i o n ; as = asymmetric; s - symmetric. i v ) General r e f e r e n c e s : 127, 128, and 114. Other re f e r e n c e s : as s t a t e d i n brackets ( ). 2. Table C Selected C h a r a c t e r i s t i c Group Frequencies Group Band (cm ^ ) Assignment Remarks -CH 3 2960 2870 1460-1450 1380 v as v s 6 as 6 s -CH 2- . 2925 2850 1465 1350-1180 720 v as v s s c i s s o r i n g wagging and t w i s t i n g ( C H 2 ) n rocking i Regular s e r i e s of bands i n f a t t y a c i d s and soaps (band p r o g r e s s i o n ) . Jg i 0-H f r e e 3700-3500 ^1250 * 225 V 6 (in-plane) 6 (out-of-plane) Sharp band i n c u p r i c hydroxide (85) : 3580 cm In monomeric c a r b o x y l i c a c i d s : 3560-3500 cm -^(127). 0-H i n t r a -molecular hydrogen bond 3570-2500 3200-2500 V V S i n g l e - b r i d g e compounds. Sharp. Chelate compounds. Broad. 0-H i n t e r -molecular hydrogen bond 3550-3450 V S i n g l e - b r i d g e compounds. Sharp. I n d i l u t e s o l u t i o n s of c a r b o x y l i c a c i d s i n dioxane: ^3100 cm--'-; due probably t o the complex -C CH.-CH-N0H-.-.0 / 2 2 ^ 0 (114) N C H 2 - C H 2 ^ Table C (Cont.) Group Band (cm L ) Assignment Remarks -In dimeric c a r b o x y l i c a c i d s : 3000-2500 cm L ; (broad) ,0--C" OH-•HO C- (127) 3400-3200 0-H i n t e r -molecule hydrogen bond (cont.) 1500-1300 ^650 6 (in-plane) 6 (out-of-plane) -Polymeric a s s o c i a t i o n . Broad. Water of c r y s t a l -l i z a t i o n : a l s o at 3600-3100 cm - 1. I n ma l a c h i t e : 3430 cm--1 (129). In copper hydroxide: 3310 c m - i (130). I n deuterium oxide: 0-D s t r e t c h i n g at 2530 cm" 1 (131). -Broad. Frequently two. S h i f t e d to hi g h e r frequencies w i t h stronger hydrogen bond. In dimeri c c a r b o x y l i c a c i d s : 1430, 1410 and 1300 cm - 1 (coupled w i t h C-OH s t r e t c h i n g modes) (132 ) . -Broad. Higher frequencies w i t h stronger hydrogen bonds. In d i m e r i c c a r b o x y l i c a c i d s : 935 + 15 cm--'-. Monomeric a c i d s do not show OH 6 (out-of-plane) (133). -0-H bending i n l i q u i d water: 1650-1600 cm - 1 (87). In malachite: 1050, 880 and 780 cm' -1 (86) In a z u r i t e : 957, 841 and 772 cm--'- (86). In copper hydroxide: 940 cm - 1 (85). -0-D bending i n deuterium oxide: 1210 cm" (87). C-OH 1430-1030 v - I n monomeric c a r b o x y l i c a c i d s : 1375 cm - 1 (117). In d i m e r i c a c i d s : 1430, 1410 and 1300 cm 1 (coupled w i t h OH<5(out-of-plane) modes). Table C (Cont.) Group Band (cm ) Assignment Remarks C=0 1900-1580 v -Saturated ketones: 1715 cm L Saturated aldehydes: 1730' cm - x Saturated e s t e r s : 1735 cm"-'-Saturated carbonates: 1750 cm - 1 Saturated c a r b o x y l i c a c i d s : monomer ( i n carbon t e t r a c h l o r i d e ) : 1760 cm - 1 (114); monomer ( i n dioxane): 1735 cm--'- (114). dimer: 1710-1700 cm"! . . .0 OH...0 OH.. . s i n g l e - b r i d g e a c i d s , ^ ^ >^ / -1 1 1 1715 cm (115). -In d i m e r i c c a r b o x y l i c a c i d s : 680 (134). Ref. (116). Metal l a u r a t e s ( i n the s o l i d s t a t e ) (94): v as (cm - 1) v s (cm - 1) Sodium 1563,1551 1426 Potassium 1560 1420 Tha l l i u m 1544 1500 Cupric 1585 1437 C-Cl 800-600 - I n carbon t e t r a c h l o r i d e : 784 and 764 cm \ C-F 1400-1000 -I n t e f l o n : 1219 and 1149 -1 cm CO^ 1450-1410 - I n b a s i c carbonates a b s o r p t i o n i n the r e g i o n 880-840 1430 cm - 1 i s almost always doubled. A s e r i e s of bands appear between 1110 and 700 cm - 1. M a l a c h i t e shows four bands i n the re g i o n 1550-1350 cm - 1. 1610-1550 1400-1300 v as v s C ' Table C (Cont.) Group Band (cm "*") Assignment Remarks co 3= (cont.) These are: 1520, 1500, 1426, and 1393 cm - 1. Other m a l a c h i t e bands due to the carbonate i o n are: 1095, 823 and 713 cm - 1. A z u r i t e shows the f o l l o w i n g carbonate bands: 1505, 1470, 1424, 1094, 823, and 747 cm - 1 (130). -Bicarbonates do not absorb i n the 1430 cm X r e g i o n . Instead two w i d e l y separated bands appear on e i t h e r s i d e (127). S 0 . = 4 1130-1080 680-610 S i l i c a t e s 1100-900 -Quartz Cu-OH 680 fi -Strong (129). Cuprous oxide 623-609 l a t t i c e v i b r a t i o n s -(84,85,90,135,136). Cupric oxide 510 - ( 8 5 ) . - 163 -APPENDIX D Determination of the IEP of T e n o r i t e The IEP of t e n o r i t e (reagent grade c u p r i c oxide, F i s h e r C 474, used i n the f l o t a t i o n t e s t s ) was estimated at pH 9.4 using Mular and Roberts method (103). The t e n o r i t e was stage ground i n an agate mortar and 10 samples weighing 1 gm were placed i n 100-ml beakers each c o n t a i n i n g 50.ml of 0.01 N Na 2S0^ s o l u t i o n s . The pH (pIL) was adjusted w i t h NaOH. The i o n i c s t r e n g t h was r a i s e d to 10 X by adding the r e q u i r e d amount of the supporting e l e c t r o l y t e (Na 2S0^). The f i n a l pH (pH^) was measured a f t e r s t i r r i n g the s o l u t i o n w i t h a g l a s s rod f o r three minutes. Values of ApH = pH. - pH are p l o t t e d against pH i n F i g . D. • - 164 -Figure D. ApH versus pH^ f o r t e n o r i t e (IEP determination based on Mular and Roberts method). - 165 -APPENDIX E C a l i b r a t i o n of the Torsion-Wire Langmuir Balance The " c a l i b r a t i n g weights method", d i s c r i b e d i n Ref. 77 was used f o r the c a l i b r a t i o n of the balance. The t o r s i o n - w i r e was a 10" monel w i r e , 15 cm long. w grams of the c a l i b r a t i n g weight r e q u i r e the t o r s i o n head to be r o t a t e d 8 degrees to r e t u r n the f l o a t to the zero p o s i t i o n . The s e n s i t i v i t y S (dynes.cm "'".degree x ) of the balance i s given by: S = w g l c / e l f L , E l where 1 = 7.0 cm, i s the l e v e r arm f o r the c a l i b r a t i n g weight ( h o r i z o n t a l d i s t a n c e from the weights to the w i r e ) ; 1^ = 12.5 cm, i s the l e v e r arm f o r the f l o a t ( v e r t i c a l d i s t a n c e from the f l o a t to the w i r e ) ; L = 14.0 cm, i s the e f f e c t i v e length of the f l o a t (taken to be the l e n g t h of _2 the f l o a t plus h a l f the width of the gaps at i t s end); g = 980.6 cm.sec , i s the a c c e l e r a t i o n of g r a v i t y . A p l o t of w versus 6 ( F i g . E) g i v e s : w -3 - -1 - — = 5.3 x 10 gm.degree Using formula E l and the values of the parameters given above, the s e n s i t i v i t y of the balance i s c a l c u l a t e d as S = 0.21 dynes, cm ''".degree X : a pressure of 1 dyne.cm 1 corresponds to 4.76° r o t a t i o n angle of the t o r s i o n head. - 166 --1 6 (degrees) Figure E. Torsion-wire Langmuir balance c a l i b r a t i o n curve. - 167 -APPENDIX F T y p i c a l Behavior of S t e a r i c A c i d Films Spread at the Air-Water I n t e r f a c e An example of the experimental data obtained during compression of s t e a r i c a c i d monolayers i n the Langmuir trough i s presented i n F i g . F (pressure-area curve) and Table F (numerical values used f o r p l o t t i n g F i g . F c u r v e ) . The area occupied by a molecule of s t e a r i c a c i d i n the s o l i d i f i e d °2 f i l m was measured at 20.9 A . This i s i n good agreement w i t h l i t e r a t u r e °2 values of 20.4 A obtained f o r s t e a r i c a c i d monolayers spread under s i m i l a r c o n d i t i o n s (77). - 168 -A molecule Figure F. Pressure-area curve for a typical stearic acid monolayer. (24°C, 10"4 N H 2S0 4; pH 4.4). - 169 -TABLE F _2 Langmuir Trough Data (Spreading S o l u t i o n : 0.045 ml of 10 Molar S t e a r i c A c i d i n Normal Hexane. Number of A c i d Molecules i n the F i l m : 2.7 x 10 Measured area (a) of the f i l m (cm 2) ( w i t h i n + 5 cm 2) 853 31.6 0.5 0.1 826 30.6 0.5 0.1 783 29.0 1 0.2 751 27.8 1 0.2 724 26.8 2 0.4 697 25.8 3 0.6 683 25.3 10 2.1 675 25.0 13 2.7 661 24.5 20 4.2 653 24.2 24 5.0 645 23.9 30 6.3 637 23.6 34 7.1 629 23.3 40 8.4 624 23.1 45 9.5 613 22.7 50 10.5 607 22.5 56 11.8 599 22.2 63 13.2 591 21.9 70 14.7 586 21.7 78 16.4 578 21.4 85 17.8 570 21.1 . 90 18.9 564 20.9 94 19. 7 559 20.7 100 21.0 556 20.6 105 22.0 555 20.55 110 23.1 553 20.5 114 23.9 552 20.45 117 24.6 551 20.4 119 25.0 549 20.35 125 26.3 549 20.35 130 27.3 548 20.3 135 28.4 548 20.3 140 29.4 547 20.25 142 29.8 545 20.2 150 31.5 545 20.2 152 31.9 Area occupied by 0 (measured w i t h i n Surface Pressure one molecule + .1 degree) (0 x .21) i n (=a x lOj-6/2.7 x dynes . cm--'-10 1? A 2) - 170 -APPENDIX G C a l c u l a t i o n of AR „TT , aSjCH^ random Let i t be assumed that a "monolayer" of s t e a r i c a c i d p r e s e n t i n g the p r o p e r t i e s given below can be formed on the surface of a s o l i d . ( i ) The CH^ groups of the molecules l i e i n planes randomly o r i e n t e d w i t h respect to the s o l i d s u r f a c e . Such a h y p o t h e t i c a l f i l m would have i s o t r o p i c o p t i c a l p r o p e r t i e s and equation [ 8 ] , Chapter I I ( F r a n c i s and E l l i s o n theory) describes the r e l a t i o n between band i n t e n s i t y and f i l m t h i c k n e s s . ( i i ) The thickness (d) of the "monolayer" ( c o n c e n t r a t i o n term i n the e x p r e s s i o n of AR) i s " e q u i v a l e n t " to that of a s o l i d i f i e d monolayer at the a i r - w a t e r i n t e r f a c e (d = 24.4 A 0 as reported by B l o d g e t t i n Ref. 137). ( i i i ) The r e f r a c t i v e index (N^) of the f i l m at the band maximum (2920 cm x ) i s approximately the same as the r e f r a c t i v e index (n ) of bulk s t e a r i c a c i d 0$ = n Q = 1.4299; sodium l i g h t , 25°C, Ref. 138). ( i v ) The molecular e x t i n c t i o n c o e f f i c i e n t (.£ , n n n n -1\) of the (.zy/u cm ) s t e a r i c a c i d molecules i n the f i l m i s approximately equal to the molecular e x t i n c t i o n c o e f f i c i e n t of CH^ groups (times 16) i n bulk —18 2 organic compounds (0.125 x 10 cm per group, Ref. 139). The a b s o r p t i o n constant (K^) f o r the s t e a r i c a c i d molecules i n the f i l m can be c a l c u l a t e d from the value of e / o n ~ ~ - 1 N i n the f o l l o w i n g way. (2920 cm ) ° — —18 —18 2 e ,„ n„ n -1.. = 16 x 0.125 x 10 = 2 x 10 cm per molecule (2920 cm ) r a(2920 cm"1) = 2 , 3 0 3 x p x e(2920 c m " 1 ) ( e x P r e s s i o n t ^ ] , Chapter I I ) - 171 -p = density x Avogrado's number/molecular weight p = .9408 x 6.022 x 1023/284.5 (values from Ref. 138) hence, 9.17 x 10 -3 -1 a (2920 cm ) cm and, K. 1 a(2920) X 4TT = 0.25 (X = 1/2920 ="3.42 x 10 4 cm). By substituting the values of d = 24.4 A°, N 1 = 1.4299, = 0.25, 6 •= 70° and X = 3.42 x 10 4 A° i n equation [8], AR for the CH2 asymmetric stretching band in a hypothetical specular reflection spectrum of a monolayer composed of randomly oriented stearic acid molecules can be calcualted as AR as,CR9 random = 0.0079. - 172 -APPENDIX R C a l c u l a t i o n of a T h e o r e t i c a l Value f o r AR „„ i /AR „tt , = r as,CH 2X' as,CH_2 random Let P^ be the p r o b a b i l i t y that a s t r e t c h i n g v i b r a t i o n mode of CH^ groups l y i n g i n planes normal to the surface w i l l i n t e r a c t w i t h the component of the e l e c t r i c f i e l d (E,, ) p a r a l l e l t o the plane of in c i d e n c e of the r a d i a t i o n . C a l l ?^ the p r o b a b i l i t y i n the case of randomly o r i e n t e d CH^ groups. As discussed i n Chapter I I . AR n v s , ^  P, , and AR , ^ P„ as,CH 2_L 1' as,CH 2 random 2 I t f o l l o w s that r = P l / P 2 From expression [ 9 ] , Chapter I I : 1 h 2 \ z > _ An m 2 f 2 2 0 *«2z h//,z t h e r e f o r e , W l z , 2 r = <rf=-r [El] V2z where, y^^ i s the component (normal to the surface) of the matrix element - 173 -of the d i p o l e moment change f o r the s t r e t c h i n g v i b r a t i o n mode of CR^ groups l y i n g i n planes normal to the s u r f a c e , and i s the average value f o r the normal components of the matrix element of the d i p o l e moment change i n the case of randomly o r i e n t e d groups. Considering that f o r randomly o r i e n t e d molecules the component of the ma t r i x element can form angles w i t h the s o l i d s u rface v a r y i n g from zero to ir r a d i a n s , t h i s average value can be expressed as p. sine r A n i r ., Iz [-u, cose] /o.. H l z o 2z ir TT TT ^ l z Hence, y l z 2 r = , ( 2 l z r = 2 . 4 6 7 y i z - 174 -APPENDIX I Measurement of Copper Oxide F i l m Thickness from AR Values The thi c k n e s s (d) of an i s o t r o p i c f i l m i s expected to be d i r e c t l y p r o p o r t i o n a l to the measured AR values of the specular r e f l e c t i o n s p e c t r a of the f i l m . Equation [8] (F r a n c i s and E l l i s o n ) can be w r i t t e n : d.„ = x • AR where 3 . XN^.cose x = — 2 16 ITS i n 6K^ Es t i m a t i n g x ( i ) Cuprous oxide f i l m 90 x can be c a l c u l a t e d using N^ and values given by O'Keefe (N x = 1.5; K x = 0.75, at 645 cm" 1). 0 = 70°, X = 15.5u = 1.55 x 10 5 A°, t h e r e f o r e , x = 5,600 A 0" 1, and d A O = 5,600 AR,,C -1 [ I I ] A 645 cm ( i i ) Cupric oxide f i l m Since values of N^ and (at 570 cm X ) f o r c u p r i c oxide f i l m s were not a v a i l a b l e , x was determined from a p l o t ( F i g . I) of d c obtained by independent thickness measurements ( i n t e r f e r o m e t r y ) versus observed ARr.,rt -1 val u e s . From F i g . I , x = slope = 6,400 A° X , 570 cm ° r hence, d = 6,400 AR -1 [12] A 570 cm - 175 -- 176 -APPENDIX J Calculation of Angles Formed by Planes Containing CH^ Groups of Stearic Acid Monolayers with the Solid Surface An expression (similar to equation H^, Appendix H) can be obtained, on the basis of the Francis and Ellison theory, for the ratio AR „TT /AR „TT , = r: aSjCIL, aSjCH^ random ,p3z,2 r = ( ) where u-jz i s the component (normal to the surface) of the matrix element of ftie dipole moment change for the stretching vibration mode of the CH^ groups lying in planes forming 0 (unknown) degrees with the 2 surface, and u2z - ~ V^z a s given in Appendix H. ^3z = Hz S i n 9 thus, r = t]Z 1 A2, and 0 = s i n " 1 ^ r 1 / 2 ) [Jl] 7 y i z Expression J l was used to calculate the average angle made by planes containing CB^ groups of stearic acid molecules with the solid surface, in films deposited oh cupric oxide substrates according to the Langmuir-Blodgett method; r values were obtained by dividing average AR values given in Table 4 by 0.0079 (AR _„ , for a hypothetical • J aSjCH^ random J r monolayer as calculated in Appendix G). - 177 -REFERENCES 1. 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