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The recrystallization and dissolution of acetylsalicylic acid Jamali, Fakhreddin 1973

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cof. 1 1 THE RECRYSTALLIZATION AND DISSOLUTION OF ACETYLSALICYLIC ACID by Fakhreddin Jamali Pharm. D., University of Tehran, Iran, 1969 A THESIS SUBMITTED' IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE IN PHARMACY i n the D i v i s i o n of Pharmaceutics of the Faculty of Pharmaceutical Sciences We accept t h i s thesis as conforming to the required standard. THE UNIVERSITY OF BRITISH COLUMBIA SPRING 1973 In presenting t h i s thesis i n p a r t i a l f u l f i l m e n t of the requirements for an advanced degree at the University of B r i t i s h Columbia, I agree that the Library s h a l l make i t f r e e l y a v a i l a b l e for reference and study. I further agree that permission for extensive copying of t h i s thesis for s c h o l a r l y purposes may be granted by the Head of my Department or by his representatives. 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 for f i n a n c i a l gain s h a l l not be allowed without my written permission. Faculty of Pharmaceutical Sciences The U n i v e r s i t y of B r i t i s h Columbia Vancouver 8, Canada Date: ABSTRACT Observations that some samples of commercial a c e t y l s a l i c y l i c acid had d i f f e r e n t d i s s o l u t i o n rates were followed by reports that a c e t y l s a l i c y l i c acid e x i s t s i n more than one polymorphic form. The evidence for polymorphism has been questioned by a number of authors and the e f f e c t s of such factors as c r y s t a l habit, p a r t i c l e s i z e , c r y s t a l imperfection, the presence of s a l i c y l i c acid and spherulites of a c e t y l s a l i c y l i c acid have been discussed i n an attempt to explain the anomalous behaviour. This work attempts to resolve the c o n f l i c t i n g points of view. A c e t y l s a l i c y l i c acid was r e c r y s t a l l i z e d from ethanol i n a c r y s t a l l i z e r which permitted control of the degree of supersaturation and, therefore, growth rate. In addition, a number of other r e c r y s t a l l i z a t i o n techniques and solvents were used to produce a c e t y l -s a l i c y l i c acid c r y s t a l s with a wide v a r i e t y of habits and varying amounts of s a l i c y l i c a cid. The c r y s t a l s were compressed in t o discs both with and without p r i o r s i z e reduction and s i e v i n g . I n t r i n s i c d i s s o l u t i o n rates, measured using a rotating d i s c technique showed that the rates were independent of c r y s t a l growth rate, c r y s t a l s i z e and habit, the content of s a l i c y l i c acid (up to 3.8% w/w) and the presence of spherulites of a c e t y l s a l i c y l i c acid. X-ray d i f f r a c t i o n patterns revealed no differences between the various c r y s t a l s and the o r i g i n a l commercial material. Melting points, however, were dependent on the method of measurement and the c r y s t a l s i z e and habit. Using the hot-stage method and heating at a rate of 0.2 per minute from a s t a r t i n g temperature of 2° below the approximate melting point I I (previously determined), a c e t y l s a l i c y l i c a c i d melted, with decomposition, i n the range of 128.3 to 132.7°, (excluding spherulites). When heating was started at 100° the melting range became very broad with small unaggregated p a r t i c l e s s t a r t i n g to melt at temperatures between 103° and 112°. Analysis of the melt showed that the proportion of s a l i c y l i c acid increased with decrease i n p a r t i c l e s i z e of the o r i g i n a l a c e t y l s a l i c y l i c acid c r y s t a l s . Hence, the depression of the melting point of i n d i v i d u a l c r y s t a l s i s r e l a t e d to the increased s u s c e p t i b i l i t y of small p a r t i c l e s to thermal decomposition with the formation of s a l i c y l i c acid. Evidence for the existence of metastable polymorphs of a c e t y l s a l i c y l i c acid rests on the reported properties of needle-like c r y s t a l s r e c r y s t a l l i z e d from n-hexane and spherulites grown i n t h i n films from saturated a l c o h o l i c s o l u t i o n . The needle-like c r y s t a l s melted over the range 123.9° to 130.1° using a heating rate of 0.2° per minute from a s t a r t i n g temperature 2° below the approximate melting point. The wide melting range i s probably due to decomposition of the f i n e needles and the formation of s a l i c y l i c a c i d as discussed above. Spherulites of a c e t y l s a l i c y l i c acid underwent a thermal trans-formation over the range 104° to 128° to form elongated prisms and a s o l u t i o n phase transformation into w e l l defined prisms when i n contact with a saturated s o l u t i o n of a c e t y l s a l i c y l i c a c i d i n various alcohols. The i n t r i n s i c d i s s o l u t i o n rates of compressed discs prepared from the needle-like c r y s t a l s andtspherulites were the same as the other a c e t y l -s a l i c y l i c acid c r y s t a l s . Moreover, X-ray d i f f r a c t i o n patterns of the needle-like c r y s t a l s , the spherulites and the c r y s t a l s formed from the spherulites a f t e r thermal and s o l u t i o n phase transformation were i d e n t i c a l with each other and the o r i g i n a l a s p i r i n . Hence, the needle-like c r y s t a l s and spherulites are not metastable polymorphic forms of a c e t y l s a l i c y l i c a c i d . It i s suggested that both the thermal and s o l u t i o n phase transformation are growth processes. SUPERVISOR IV LIST OF CONTENTS Page 1. INTRODUCTION; 1 2. LITERATURE SURVEY 2 2 - 1 . C r y s t a l and Amorphous States 2 2 - 2 . C r y s t a l Growth 4 I. S o l u b i l i t y and S u p e r s o l u b i l i t y 4 I I . Theories of C r y s t a l Growth 7 2 - 3 . Dendrite Growth 15 2 - 4 . Polymorphism 18 I. Microscopic O p t i c a l Crystallography ; 20 I I . X-Ray D i f f r a c t i o n 21 I I I . Infrared Spectroscopy 21 IV. D i f f e r e n t i a l Thermal Analysis 22 2 - 5 . D i s s o l u t i o n of C r y s t a l l i n e Compounds 22 I. D i s s o l u t i o n Phenomenon 22 I I . R eciprocity of Growth and D i s s o l u t i o n 23 I I I . D i s s o l u t i o n Models 25 IV. D i s s o l u t i o n Measurement Methods 28 V. Factors A f f e c t i n g D i s s o l u t i o n Rate. 30 2 - 6. The Polymorphism Question of A c e t y l s a l i c y l i c Acid 37 3. EXPERIMENTAL 42 3 - 1. Apparatus 42 3 - 2 . Materials 43 3 - 3 . Methods 44 I. S o l u b i l i t y of A c e t y l s a l i c y l i c Acid i n Absolute Ethanol 44 I I . R e c r y s t a l l i z a t i o n of A c e t y l s a l i c y l i c Acid 46 I I I . Determination of S a l i c y l i c Acid i n A c e t y l s a l i c y l i c Acid 52 IV. Measurement of the Melting Point of A c e t y l -s a l i c y l i c Acid 65 V. X-Ray D i f f r a c t i o n of A c e t y l s a l i c y l i c Acid 65 VI. D i s s o l u t i o n of A c e t y l s a l i c y l i c Acid 65 a. I n t r i n s i c D i s s o l u t i o n . . . . . . . 65 b. Bulk D i s s o l u t i o n 72 VII. E f f e c t of Various Factors on I n t r i n s i c D i s s o l u t i o n Rate a. E f f e c t of P a r t i c l e Size b. E f f e c t of S a l i c y l i c Acid c. E f f e c t of Supersaturation d. E f f e c t of Habit e. E f f e c t of R e c r y s t a l l i z a t i o n Solvent f. D i s s o l u t i o n Rate of Spherulites g. D i s s o l u t i o n Rate of Mixtures of Prismatic C r y s t a l s and Spherulites RESULT AND DISCUSSION.. 4 - 1 . Equilibrium S o l u b i l i t y of A c e t y l s a l i c y l i c Acid i n Absolute Ethanol 4 - 2 . Crystals of A c e t y l s a l i c y l i c Acid I. S a l i c y l i c Acid Content I I . Melting Point..... I I I . Solution-Phase Transformation IV. X-Ray D i f f r a c t i o n 4 - 3 . D i s s o l u t i o n of A c e t y l s a l i c y l i c Acid I. I n t r i n s i c D i s s o l u t i o n Rate I I . Bulk D i s s o l u t i o n I I I . E f f e c t of Various Factors on D i s s o l u t i o n Rate... a. P a r t i c l e Size b. S a l i c y l i c Acid c. Supersaturation d. Habit e. R e c r y s t a l l i z a t i o n Solvent f. D i s s o l u t i o n Rate of Spherulites and Mixture of Spherulites and Prismatic Crystals SUMMARY AND CONCLUSION REFERENCES VI LIST OF TABLES Page 2 - 1 . The seven c r y s t a l systems 3 2 - 2 . Ratio of v e l o c i t i e s of d i s s o l u t i o n over those of growth f o r some given substances 24 2 - 3 . V a r i a t i o n of growth of K^SO^ i n presence of some s a l t s 24 4 - 1 . S a l i c y l i c acid content of d i f f e r e n t samples of a c e t y l s a l i c y l i c acid c r y s t a l s during the melting process. 86 4 - 2 . R e c r y s t a l l i z a t i o n conditions and properties of a c e t y l s a l i c y l i c acid c r y s t a l s 93 4 - 3 . I n t r i n s i c d i s s o l u t i o n rate of a c e t y l s a l i c y l i c acid with d i f f e r e n t p a r t i c l e s i z e at 37* and 300 r.p.m 101 VII LIST OF FIGURES Page 2 - 1 . Theoretical s o l u b i l i t y - s u p e r s o l u b i l i t y diagram 6 2 - 2 . Structure of a low-index face (001) for a perfect c r y s t a l on Kossel-Stanski model 9 2 - 3 . C l a s s i f i c a t i o n of c r y s t a l imperfections 12 2 - 4 . The structure of an edge d i s l o c a t i o n 14 2 - 5 . The end of a screw d i s l o c a t i o n , 14 2 - 6. T y p i c a l dendrite i n two dimensions 16 3 - 1. S o l u b i l i t y Apparatus 45 3 - 2 . C r y s t a l l i z e r . . . 47 3 - 3 . Standard curve for the c o l o r i m e t r i c determination of s a l i c y l i c acid 53 3 - 4 . Gas-liquid chromatographic internals standard ca-Mbratibn curve" f o f . i s a l i c y l i e d a e i d 56 3 - 5 . Gas-liquid chromatogram of s a l i c y l i c acid i n commercial a c e t y l s a l i c y l i c acid 58 3 - 6 . Gas-liquid chromatogram of a c e t y l s a l i c y l i c acid i n mixture of s a l i c y l i c acid (1%) and a c e t y l s a l i c y l i c acid 59 3 - 7 . Standard curve for determination of s a l i c y l i c acid i n a c e t y l s a l i c y l i c acid using absorbance r a t i o method 60 3 - 8 . Standard curve for spectrophotofluorometry determination of s a l i c y l i c a c i d . . 62 3 - 9 . Metal mold to make a compressed dis k 67 3 - 10. I n t r i n s i c d i s s o l u t i o n aparatus 69 3 - 11. Standard curve for a c e t y l s a l i c y l i c acid i n 0.1 N hydrchloric acid 70 4 - 1. S o l u b i l i t y s u p e r s o l u b i l i t y diagram for the a c e t y l s a l i c y l i c acid - absolute ethanol system 76 4 - 2. Comparison of s o l u b i l i t y - s u p e r s o l u b i l i t y 77 4 - 3. A c e t y l s a l i c y l i c acid c r y s t a l habits 79 VIII Page 4 - 4 . Needle-like c r y s t a l s of a c e t y l s a l i c y l i c acid from n-hexane 80 4 - 5 . Hexagonal plates of a c e t y l s a l i c y l i c acid from ethanol 95% 80 4 - 6 . Spherulites of a c e t y l s a l i c y l i c acid 82 4 - 7 . Spherulites of v a n i l l i n 82 4 - 8 . Thermal transformed c r y s t a l s from spherulites 90 4 - 9 . Solution transformed c r y s t a l s from spherulites 90 4 - 10. X-ray d i f f r a c t i o n pattern of a c e t y l s a l i c y l i c acid 92 4 - 1 1 . Rotating d i s k d i s s o l u t i o n of a c e t y l s a l i c y l i c acid i n 0.1 N HC1 at 37* and 300 r.p.m 95 4 - 1 2 . F i n i t e d i f f e r e n c e s diagram for the d i s s o l u t i o n of a c e t y l s a l i c y l i c acid i n 0.1 N HC1 at 37* and 300 r.p.m... 96 4 - 1 3 . I n i t i a l d i s s o l u t i o n curve of a c e t y l s a l i c y l i c acid i n 0.1 N HC1 at 37* and 300 r.p.m 97 4 - 14. Bulk d i s s o l u t i o n of a c e t y l s a l i c y l i c acid i n 0.1 N HC1.... 99 IX ACKNOWLEDGEMENTS The author i s indebted to Dr.A/.G:Mitchel'l^fchesis supervisor for his guidance and general personal attention throughout the course of t h i s work and wishes to express h i s appreciation to the following people: Dr. F.S.Abbott f o r h i s s c i e n t i f i c advice and help with the a n a l y t i c a l aspects of t h i s i n v e s t i g a t i o n . Dr. M.Pernarowski for h i s academic advice and consideration. Mr. B.G.Butters, Department of Metallurgy, for h i s h e l p f u l assistance and guidance i n preparing X-ray d i f f r a c t i o n patterns. Mr. K.Kent, Department of Metallurgy, for h i s assistance with preparing comoressed disks. TO 1 1. INTRODUCTION. A c e t y l s a l i c y l i c acid was f i r s t synthesized i n 1853 by Gerhardt, but the f i r s t synthetic ester of s a l i c y l i c a c i d to be introduced into medicine was p h e n y l s a l i c y l a t e (Menchi, 1886). Meanwhile Hoffman, a chemist at Bayer's chemical works i n Germany, had found a simpler method of ac e t y l a t i n g s a l i c y l i c acid and had recognized the advantages of the acetylated compound over s a l i c y l i c a c i d i n the treatment of rheumatism. Dreser (1899) investigated the pharmacology of t h i s compound and eventually Bayer marketed a c e t y l s a l i c y l i c acid under the trade name of " A s p i r i n " . Hence a c e t y l s a l i c y l i c acid was one of the f i r s t chemothera-peutic agents and i t s properties have been studied s t e a d i l y over the years. A c e t y l s a l i c y l i c acid has a long h i s t o r y of giving trouble i n melting point determination. Reported values range from 100 to 144°. The anomalous physiochemical behavior of t h i s compound has been subject of many recent papers. Reported differences i n the d i s s o l u t i o n rate of commercial a c e t y l s a l i c y l i c a c i d were followed by reports that t h i s compound exists i n more than one polymorphic form. The evidence for polymorphism has been questioned by a number of authors, and the e f f e c t s of such factors as c r y s t a l habit, p a r t i c l e s i z e , c r y s t a l imperfections, the presence of s a l i c y l i c a c i d and spherulites of a c e t y l s a l i c y l i c acid have been discussed i n an attempt to explain the anomalous behavior.;' This work attempts to resolve the c o n f l i c t i n g points of view by r e c r y s t a l l i z i n g a c e t y l s a l i c y l i c acid using d i f f e r e n t methods and conditions of r e c r y s t a l l i z a t i o n followed by a study of the physicbchemical properties of the c r y s t a l s . 2 2 - LITERATURE SURVEY 2 - 1 . C r y s t a l and Amorphous States. Most s o l i d drugs are c r y s t a l l i n e i n nature, i . e . , the molecules of which they are composed are packed together i n a regular manner, forming a three-dimensional pattern. S o l i d drugs which appear as a powder or as i r r e g u l a r lumps consist of small c r y s t a l s as seen by microscope or examined by X-ray methods. A t y p i c a l c r y s t a l has a regular geometric form with sharp, s t r a i g h t edges and plane surfaces. When fractured, i t breaks i n t o pieces with plane faces meeting i n sharp edges. The regular external form of a c r y s t a l i s determined by a regular assembly of smaller units having a uniform geometrical form. The arrangement of these units i s the most important c h a r a c t e r i s t i c of a c r y s t a l , f o r , even i f the external form i s destroyed by powdering, the i n t e r n a l structure remains. Crystals may vary i n s i z e and i n the development of d i f f e r e n t faces owing to the conditions under which they are formed (habit m o d i f i c a t i o n ) . The constant geometrical form of the units composing the c r y s t a l i s exhibited i n the constancy of the angles between s i m i l a r faces (law of constancy of i n t e r f a c i a l angles). The symmetry possessed by c r y s t a l s i s expressed i n terms of t h e i r symmetry about c e r t a i n planes and axes of the c r y s t a l . An "axis of symmetry" i s an axis on which the c r y s t a l can be rotated through 360° and occupy the same p o s i t i o n more than once. I f a c r y s t a l possesses a su i t a b l e number of axes of symmetry, three of them may be chosen as " c r y s t a l l o g r a p h i c axes"; i n c r y s t a l s possessing less than Table 2 - 1 . The Seven C r y s t a l Systems. System Axes Angles Example Cubic Tetragonal Orthorhombic Monoclinic Rhombohedral Hexagonal T r i c l i n i c a = b = c a = b ; c a ; b ; c a ; b ; c a = b = c a = b ; c a ; b ; c = p = * = 9QC <* = ft = * = 90 90^ <*= (i= 90 ; * Sodium Chloride Calcium Oxalate Acetanilid;', V a n i l l i n - (I) Vitamin A (m) , A c e t y l s a l i c y l i c Acid. Cortisone /3 = 90 ; *=120 Styphnic acid ai; (i ; V Vitamin A * m, metastable ** 2, 4, 6. t r i n i t r o r e s o r c i n a l . 4 three axes of symmetry, other axes are chosen as the c r y s t a l l o g r a p h i c axes. For c l a s s i f i c a t i o n , a l l c r y s t a l s are referred to seven groups (Table 2 - 1), e.g. cubic (alum), t r i c l i n i c (copper sulphate), monoclinic ( a c e t y l s a l i c y l i c a c i d ) , which are defined by the p o s i t i o n of the c r y s t a l l o g r a p h i c axes. C r y s t a l shapes are often described i n general terms which are almost self-explanatory; needle-shape c r y s t a l s are often termed a c i c u l a r and p l a t e - l i k e c r y s t a l s , laminar. Such terms give no idea of the c r y s t a l l o g r a p h i c axes; for example, the prism occurs i n several groups. Certain s o l i d substances, e.g. glass and colophony, which do not e x h i b i t a l l the properties of c r y s t a l s , are regarded as "super-cooled l i q u i d s " or "amorphous substances". Supercooled l i q u i d s have a very high v i s c o s i t y , but are nevertheless capable of flowing very slowly. Under c e r t a i n conditions, c r y s t a l l i z a t i o n of the supercooled l i q u i d may occur; old glass w i l l c r y s t a l l i z e ( d e v i t r i f y ) when rendered less viscous by heating. The molecule arrangements of such substances are very complex and do not usually acquire the order necessary for a c r y s t a l l i n e form. However, with amorphous substances, i t i s by no means c e r t a i n that order i s e n t i r e l y lacking, and the word amorphous has therefore to be used with caution. 2-2. C r y s t a l Growth. I. S o l u b i l i t y and S u p e r s o l u b i l i t y . I t was known to early crystallographers that a c r y s t a l would not develop i n a just-saturated s o l u t i o n . "Supersaturation" was 5 recognised as the state i n which the s l i g h t e s t shock or disturbance w i l l bring about immediate c r y s t a l l i z a t i o n ; the region i n which t h i s state e x i s t s i s c a l l e d the "metastable region" and i s bounded by the s o l u b i l i t y and s u p e r s o l u b i l i t y curves. F i g . 2 - 1 shows a s o l u b i l i t y - s u p e r s o l u b i l i t y diagram. A represents a point i n the region of unsaturation which, i f undisturbed, would remain as a s i n g l e phase i n d e f i n i t e l y . A c r y s t a l placed i n such a s o l u t i o n would d i s s o l v e . There are two ways i n which we can bring s o l u t i o n A to the equilibrium "saturation" condition ( i . e . , the region i n which c r y s t a l s neither grow nor d i s s o l v e ) . F i r s t by lowering the temperature to the point B or, second, by evaporating o f f some of the solvent at constant temperature, when the conditions represented by point B ' w i l l be reached. With e i t h e r s o l u t i o n , at B or E*', a c r y s t a l could remain i n d e f i n i t e l y without growth or d i s s o l u t i o n taking place. T h e o r e t i c a l l y any further cooling below point B or withdrawal of solvent beyond point B ' should r e s u l t i n the separation of s o l i d ( i . e . , c r y s t a l -l i z a t i o n ) , but i t i s a matter of common experience that t h i s never occurs. Further cooling w i l l bring us to a point C, when the s l i g h t e s t shock or disturbance w i l l cause immediate c r y s t a l l i z a t i o n . The moment t h i s occurs, the heat of c r y s t a l l i z a t i o n w i l l prevent any further drop i n temperature u n t i l the c r y s t a l s have gathered to themselves most of the available excess represented by the distance from the equilibrium curve. P r a c t i c a l l y speaking i t i s inconvenient to bring a s o l u t i o n to the s u p e r s o l u b i l i t y point f o r c r y s t a l l i z a t i o n . A s l i g h t disturbance or seeding with a few c r y s t a l s i n the "metastable" region w i l l bring about immediate rapid c r y s t a l l i z a t i o n . The heat of c r y s t a l l i z a t i o n r e s u l t s i n a notable r i s e i n temperature. T h e o r e t i c a l l y the further t h i s region, Temperature, C" Th e o r e t i c a l s o l u b i l i t y - s u p e r s o l u b i l i t y diagram. 7 c a l l e d by Ostwald (1900) " l a b i l e " , i s penetrated the more and more unstable i t becomes and the greater i s the tendency to c r y s t a l l i z e . Opposing t h i s , however, i s the p o s s i b i l i t y of a big increase i n the v i s c o s i t y of the s o l u t i o n which i n time might overrule the tendency to c r y s t a l l i z e (Buckley 1951). Line AB'C' ;.(Fig. 2 - 1 ) represents withdrawal of solvent. As stated above, nothing w i l l happen to the s o l u t i o n at B'1 , but on evaporation to point C1' c r y s t a l l i z a t i o n w i l l occur spontaneously. I t i s l i k e l y , however, that the surface of the s o l u t i o n w i l l approach point C* , while the rest of the s o l u t i o n s t i l l remains i n the "metastable" region, so that the bulk never reaches the concentration, C .. This process occurs i n most cases of c r y s t a l l i z a t i o n by evaporation at constant temperature. II Theories of C r y s t a l Growth Verma & Krishna (1966) have reviewed and c l a s s i f i e d the theories of c r y s t a l growth in t o two parts: the theory of the growth of an i d e a l l y perfect c r y s t a l arid the theory of growth of an imperfect ( i . e . , real) c r y s t a l . Gibbs (1928) compared the growth of a l i q u i d drop from a mist with the growth of a c r y s t a l . The face energy that resides on the surface separating the two phases retards the formation of the second phase ins i d e the f i r s t . Therefore the s t a b i l i t y of an i s o l a t e d drop i s maximum when i t s surface free energy and hence i t s area are minimal. For a c r y s t a l i n equilibrium with i t s surroundings at constant temperature and pressure, t h i s condition implies that Gibbs free energy must be minimum for a given volume. It therefore follows that, for a given volume, those faces which lead to a minimum 8 t o t a l surface free energy w i l l develop. However, the atoms or molecules i n a l i q u i d are randomly arranged, whereas i n a c r y s t a l the s t r u c t u r a l units have a regular arrangement. This e s s e n t i a l difference between l i q u i d drops and c r y s t a l s was recognized by Gibbs. Curie (1885) calculated the shape and end forms of a c r y s t a l while Wulff (1901) deduced that the v e l o c i t i e s of growth of d i f f e r e n t faces are proportional to the appropriate s p e c i f i c surface free energies. Real c r y s t a l s are not i d e a l l y perfect. An i d e a l l y perfect c r y s t a l consists of a p e r i o d i c array of atoms or molecules whose arrangement would conform with the symmetry of one of the 230 space groups. The atomic theory of c r y s t a l growth can explain the formation of an i d e a l l y perfect c r y s t a l . Kossel (1927) and Stanski (1928) took a simple cubic model (Fig. 2 - 2 ) to i l l u s t r a t e the growth of an i d e a l l y perfect c r y s t a l . The atoms or molecules are represented as cubes. The cubes are placed face to face and each of these i s attached equally by a l l s i x neighbors. The energy required to separate two neighbors i s c a l l e d the "binding" energy. Unless the number of cubic b r i c k s happens to be the exact number of s i x that w i l l complete a l l the s i x faces of the c r y s t a l , at l e a s t one of the faces w i l l be incomplete. This s i t u a t i o n . produces a "monomolecular step" or simply a "step" on the c r y s t a l surface. This step i t s e l f i s l i k e l y to be incomplete, having a "kink". At absolute zero temperature, there i s no thermal v i b r a t i o n i n c r y s t a l molecules. I f the temperature i s r a i s e d , the molecules s t a r t to v i b r a t e r e l a t i v e to one another. On increasing the temperature, some molecules acquire enough energy to overcome the binding energy. They w i l l then break away from the body of the c r y s t a l , enter the space Figure 2 - 2. Structure of a low-index face (001) for a perfect c r y s t a l on Kossel Stanski model. surrounding i t and increase the concentration of the surrounding vapor or l i q u i d . Molecules at a kink p o s i t i o n are more l i k e l y to leave the c r y s t a l , though t h i s w i l l also happen for some molecules at other p o s i t i o n s . The molecules at the kink p o s i t i o n are bound by only three neighbors, while others have more neighbors and therefore more energy i s required to break the binding energy. This explanation shows that kinks act as "exchanging s i t e s " f or molecules on the c r y s t a l surface and those i n the surrounding phase. The departure of molecules from the c r y s t a l surface continues u n t i l an equilibrium i s reached between the c r y s t a l surface and the surrounding phase. At equilibrium, the rates at which molecules j o i n and leave the c r y s t a l surface are equal. When the concentration of the surrounding phase exceeds the equilibrium concentration c r y s t a l s w i l l s t a r t to grow. Volmer (1922) explained that between the time a molecule h i t s the surface of a c r y s t a l and the time i t evaporates again i n t o the vapor, the adsorbed molecule d i f f u s e s a considerable distance. He found that the growth rate could be explained only i f there was considerable surface d i f f u s i o n of adsorbed molecules during t h e i r l i f e t i m e on the surface of the c r y s t a l . Verma & Krishna (1966) explained that the process of growth on a perfect c r y s t a l whose surface has steps w i l l be the r e s u l t of three processes: ( i ) a transport of molecules from the surrounding phase to the surface to form the adsorbed layer (only one face i n contact); ( i i ) the d i f f u s i o n of the adsorbed molecules on the c r y s t a l surface, some of which i n t h e i r l i f e t i m e on the surface meet a step (two faces i n contact); and ( i i i ) the d i f f u s i o n of such adsorbed molecules along the edge of the step u n t i l they reach a kink (three faces i n contact). A 11 molecule which reaches a kink i s taken into the body of the c r y s t a l for the growth of the incomplete l a y e r . In continued c r y s t a l growth, new layers must be i n i t i a t e d a f t e r completing each layer. Burton and others (1951) calculated that new steps w i l l not be created by thermodynamic f l u c t u a t i o n s on a low-index face unless the temperature of the c r y s t a l i s raised to i t s melting point. Therefore, surface nucleation i s required to continue the growth. Gibbs (1928) explained that a small i s l a n d of molecules may c o l l e c t on the surface of the c r y s t a l . The i s l a n d spreads r a p i d l y across the whole face and the process of growth i s held up u n t i l a new nucleus i s formed. The s i z e of the nucleus plays an important r o l e i n growth. When the nucleus i s smaller than a c e r t a i n c r i t i c a l s i z e , the chances of new molecules being added are less than the chances of a molecule being evaporated from the nucleus. As indicated above a r e a l c r y s t a l has imperfections (Fig. 2 - 3 ) . Imperfections are the r e s u l t of c r y s t a l defects. I f the deviation from a p e r i o d i c arrangement i s l o c a l i z e d to the v i c i n i t y of only a few atoms, i t i s c a l l e d a point imperfection or point defect. Point imperfection may be e i t h e r a vacancy i n the regular arrangement or an extra atom i n an i n t e r s t i t i a l s i t e . Ion vacancies are c a l l e d "Schottky defects", and i t i s presumed that equal numbers of anion and cation vacancies are generated i n a c r y s t a l . In c e r t a i n c r y s t a l s an ion can leave i t s regular p o s i t i o n and take up an i n t e r s t i t i a l p o s i t i o n . Such a combination of i n t e r s t i t i a l ion and the consequent ion vacancy i s c a l l e d a "Frenkel defect". I t i s also possible for impurity atoms to substitute for the normal atoms. I t should be noted that a point defect produces s t r a i n only i n i t s neighborhood and does not a f f e c t the p e r f e c t i o n of more IMPERFECTIONS POINT IMPERFECTIONS VACANCIES EXTRA ATOMS SCHOTTKY FRANKEL SAME SUBSTANCE LATTICE IMPERFECTIONS LINE DEFECTS PLANE DEFECTS IMPURITIES Fig . 2-3. C l a s s i f i c a t i o n of c r y s t a l imperfections. d i s t a n t parts of the c r y s t a l (Verma & Krishna 1966). Sometimes the imperfection extends to the l a t t i c e of the c r y s t a l and r e s u l t s i n l i n e and plane defects. Line defects or d i s l o c a t i o n s occur when an i n t e r r u p t i o n occurs along a c e r t a i n d i r e c t i o n and d i s t o r t s the atomic l a t t i c e array. When l i n e defects c l u s t e r together i n one plane, they can form a plane unit or "plane defect". Lineage boundaries, grain boundaries and stacking f a u l t s are examples of plane defects. The concept of d i s l o c a t i o n s arises from a consideration of p l a s t i c flow i n a c r y s t a l . Consider a plane of atoms s l i d i n g i n a c e r t a i n c r y s t a l l o g r a p h i c d i r e c t i o n across a neighboring plane. I f both planes were r i g i d , the s l i p would be uniform over the en t i r e plane ( l i k e the s l i p p i n g of a pack of cards). However, the atoms i n a c r y s t a l are not r i g i d l y bound to each other. They are e l a s t i c a l l y coupled, so that thermal v i b r a t i o n and l o c a l i r r e g u l a r i t i e s make the forces acting over the g l i d e planes nonuniform. Therefore, d i f f e r e n t portions of a s l i p plane, i n general, s l i p over the neighboring plane by d i f f e r e n t amounts. A boundary may be pictured such that the two planes have slipped by d i f f e r e n t amounts on either side. The t r a n s i t i o n region separating the two areas that have slipped w i l l extend over a few atomic diameters only (Verma & Krishna, 1966) and the l i n e d i s c o n t i n u i t y may be regarded as a d i s l o c a t i o n . D islocations occur i n two standard o r i e n t a t i o n s : I, d i s l o c a t i o n s i n the edge orientations ( Fig. 2 - 4 ) ; and I I , d i s l o c a t i o n s i n the screw orien t a t i o n s ( F i g . 2 - 5 ) . Other intermediate cases can be regarded as combinations of edge and screw d i s l o c a t i o n s . Thomas (1970) explained that, i f a c r y s t a l surface has an emergent screw d i s l o c a t i o n , a permanent step i s a v a i l a b l e for nucleation i n supersaturated s o l u t i o n s , and a s p i r a l growth feature r e s u l t s . In general, the steps of the s p i r a l are monoatomic or monomolecular. Conversely, c r y s t a l d i s s o l u t i o n or vaporization at low undersaturations, i s enhanced at an emergent screw d i s l o c a t i o n , a s p i r a l depression being thereby formed. Dislocations create a great discrepancy between the t h e o r e t i c a l l y calculated rate of growth of a perfect c r y s t a l and that observed for r e a l c r y s t a l s (Verma & Krishna, 1966). Thomas (1970) explained that quantitive studies of the a c t i v a t i o n energy of d i s s o l u t i o n , at i d e a l and dis l o c a t e d s i t e s , i n d i c a t e that the energetics of d i s s o l u t i o n are s i g n i f i c a n t l y modified by the d i s l o c a t i o n . 2 - 3 . Dendritic Growth. Crystals grown from s o l u t i o n , melt, and vapor can e x i s t i n almost endless v a r i a t i o n s of t r e e - l i k e formations c a l l e d dendrites. These incude spherulites and s p h e r i c a l bodies (Buckley 1951). The f i r s t thing observed on the c r y s t a l l i z a t i o n of a s a l t from an aqueous so l u t i o n placed on a microscope s l i d e i s the formation of needle-like or fibrous crusts at the periphery of the drop. Once seeded, some of the c r y s t a l s develop inwards, and, as t h i s s o l u t i o n i s metastable, uniform c r y s t a l l i z a t i o n proceeds. At a l a t e r stage, the drop i s wearing th i n by evaporation and the increase i n concentration due to evaporation over-takes the slower depleting process of metastable growth. Hence the s o l u t i o n i n s i d e suddenly becomes l a b i l e (Fig. 2 - 1 ) . At c e r t a i n spots where t h i s occurs f i r s t , c r y s t a l apices j u t into the l a b i l e l i q u i d and rapid deposition takes place, so that extension occurs i n the d i r e c t i o n 6. T y p i c a l dendrite i n two dimensions. of the outward-pushing point. The s o l u t i o n on e i t h e r side of the point from which material has been extracted i s only metastable, and slow growth i s experienced at r i g h t angles to the r a p i d l y extending point. Some distance away, a s i m i l a r process i s occurring, and we get a serie s of long, p a r a l l e l needles advancing at rates c l e a r l y v i s i b l e to the eye, with gaps i n between where l i t t l e appears to be happening. Soon the l i q u i d between the limbs becomes i t s e l f l a b i l e again due to further evaporation, and the r e s u l t i s a succession of branchings from the main stems. Under optimum conditions, these primary branchings may be followed by secondary branchings, as shown i n F i g . 2 - 6 . This d e s c r i p t i o n refers to the almost two-dimensional s i t u a t i o n of a f l a t drop of s o l u t i o n . The f r o s t i n g of windows i s a f a m i l i a r i l l u s t r a t i o n of th i s phenomenon. Dendrites are c r y s t a l l i n e i n the sense that at c e r t a i n places, e s p e c i a l l y the extremities, they are usually bounded by plane faces. As far as t h e i r p h y s i c a l properties can be ascertained, each small portion i s s i m i l a r to a normal c r y s t a l , and they i n v a r i a b l y give i d e n t i c a l X-ray d i f f r a c t i o n patterns. Dendrite structures can be found with a degree of complexity ranging from those with p r a c t i c a l l y p a r a l l e l limbs, so that r e f l e c t i o n s of images of a s l i t from one of t h e i r f i n e l y subdivided faces are true to a minute of arc, down to hedgehog dendrites which are d i f f i c u l t to recon c i l e with any geometrical r e l a t i o n s h i p of the parts. There are many other aspects of growth where deviation from the normal mode takes up i n t e r e s t i n g forms. Some of these are of a r e p e t i t i v e character and are c a l l e d " s p h e r u l i t e s " . F i g . 4 - 6 i s a spherulite of a c e t y l s a l i c y l i c a c i d developed a f t e r a s o l u t i o n was poured on the surface of a microscope s l i d e as a t h i n f i l m . Popoff (1927) considered that the arrangement of the orientated portions of spherulites i s only a consequence of t h e i r common dependence upon an o r i g i n a l c e n t r a l seed. Buckley (1951) believed that, although spherulites may consist of a pure substance grown from s o l u t i o n and perhaps can be explained on purely p h y s i c a l grounds, such as a p e r i o d i c change i n the concentration of the surrounding s o l u t i o n as above, they also require other f a c t o r s , such as the presence of impurities, e s p e c i a l l y those which can form a s o l i d s o l u t i o n or orientated f i l m with the main substance. Shaftal (1968) explained that spherulites are formed on the c r y s t a l l i z a t i o n of many substances; they are s p e c i a l l y character-i s t i c of substances consisting of large molecules, p a r t i c u l a r l y polymers. Tawashi (1971) grew spherulites of a c e t y l s a l i c y l i c acid and suggested that substances of low molecular weight l i k e a c e t y l s a l i c y l i c acid do not normally give s p h e r u l i t e s . However, spherulites of a c e t y l s a l i c y l i c acid and other low molecular weight substances l i k e v a n i l l i n (Fig. 4 - 7 ) , a c e t a n i l i d , benzocaine and phenacetin have been grown i n t h i s work. 2 - 4 . Polymorphism. Just as any substance, i n general, can e x i s t i n the s o l i d , the l i q u i d , or the gaseous state, depending on the conditions of temperature and pressure, so also a s o l i d formed from s o l u t i o n , melt, or vapor phase can c r y s t a l l i z e i n more than one possible structure, depending on the conditions of temperature and pressure p r e v a i l i n g at the time of c r y s t a l l i z a t i o n . This phenomenon of the same chemical substance c r y s t a l l i z i n g i n more than one structure i s known as "polymorphism". The d i f f e r e n t structures are c a l l e d "polymorphs" or "polymorphic modifi-cations". There are also cases of polymorphic modifications that are not obtained d i r e c t l y from the s o l u t i o n , melt, or vapor phase by c r y s t a l l i z a t i o n , but are obtained by phase transformations i n the s o l i d phase. Polymorphism therefore includes every possible d i f f e r e n c e i n the c r y s t a l l i n e structure of a substance of constant chemical composition (Barth, 1934) except homogenous deformations. Since the d i f f e r e n t modifications have the same chemical composition they have s i m i l a r chemical properties. However, t h e i r p h y s i c a l properties, such as density, s p e c i f i c heat, conductivity, melting point, o p t i c a l behavior and X-ray d i f f r a c t i o n s , which depend on the arrangement of atoms i n the structure, may be widely d i f f e r e n t . Polymorphs of some compounds d i f f e r i n t h e i r degree of s t a b i l i t y . Under c e r t a i n conditions the le s s stable forms tend to convert to the stable form. T r a n s i t i o n from one form to the other takes place at a f i x e d temperature when observed under constant pressure. This temperature i s c a l l e d the " t r a n s i t i o n " temperature and i s one of the most important c h a r a c t e r i s t i c s of polymorphs. When the t r a n s i t i o n occurs i n one d i r e c t i o n (i.e.,the metastable to the stable form) i t i s said to be "monotropic"; when the t r a n s i t i o n i s r e v e r s i b l e , i t i s said to be "enantiotropic". T r a n s i t i o n can take place i n s o l u t i o n or i n the s o l i d s tate. T r a n s i t i o n phenomena i n both media can be used to i d e n t i f y polymorphism as w i l l be discussed l a t e r . The pharmaceutical applications of polymorphism have been reviewed by Haleblian and McCrofte (1969) and the subject i s covered i n several texts including those by Verma & Krishna (1966), O'Connor (1960) and McCrone (1957; 1965). The polymorphism of a c e t y l s a l i c y l i c acid has been the subject of many recent a r t i c l e s including those by Tawashi (1968, 1969, 1971) who reported the existence of two polymorphs of a c e t y l s a l i c y l i c a c i d (designated Form I and I I ) , and Summers & others (1970) who reported s i x polymorphisms of a c e t y l s a l i c y l i c acid (designated Form I to VI). However no d e f i n i t e evidence has been published to confirm the poly-morphism of a c e t y l s a l i c y l i c a c i d . The question of the polymorphism of a c e t y l s a l i c y l i c acid i s discussed i n d e t a i l i n part 2 - 6 of th i s work. Haleblian & McCrone (1969) reviewed the methods used to i d e n t i f y polymorphism. These methods are: o p t i c a l crystallography, X-ray d i f f r a c t i o n , i n f r a r e d spectroscopy, d i f f e r e n t i a l thermal a n a l y s i s , dilatometry, proton magnetic resonance spectroscopy, nuclear magnetic resonance spectroscopy, electron spectroscopy and magnetic anisotropy. Of these methods ,\ ;only microscopy, X-ray d i f f r a c t i o n , i n f r a r e d spectroscopy, and d i f f e r e n t i a l thermal analysis have been widely used. I. Microscopic O p t i c a l Crystallography - D i f f e r e n t polymorphs of a c r y s t a l may belong to one or two classes depending on the e f f e c t of the transmission of l i g h t i n d i f f e r e n t d i r e c t i o n s through the c r y s t a l s . These classes are i s o t r o p i c and a n i s o t r o p i c . In i s o t r o p i c c r y s t a l s the v e l o c i t y of l i g h t , or the r e f r a c t i v e index which depends on the v e l o c i t y of l i g h t , i s the same i n a l l d i r e c t i o n s . In anisotropic c r y s t a l s there may be two or three d i f f e r e n t v e l o c i t i e s or r e f r a c t i v e i n d i c e s . D i f f e r e n t polymorphs having d i f f e r e n t i n t e r n a l structures w i l l belong to d i f f e r e n t c r y s t a l systems and have d i f f e r e n t sets of r e f r a c t i v e i n d i c e s . Hot Stage Method - The p o l a r i z i n g microscope f i t t e d with a hot stage (or cold stage) i s a very useful t o o l for i n v e s t i g a t i n g poly-morphism. With t h i s combination an experienced microscopist can quickly t e l l whether polymorphism e x i s t s , the degree of s t a b i l i t y of the metastable forms, the t r a n s i t i o n temperature and melting points, the rate of t r a n s i t i o n under a l l temperatures and p h y s i c a l conditions and whether to pursue polymorphism as a route to an improved dosage form. These methods have been discussed by McCrone (1968) i n a t e c h n i c a l b u l l e t i n . I I . X-Ray D i f f r a c t i o n - C r y s t a l l i n e materials i n powder form give c h a r a c t e r i s t i c X-ray d i f f r a c t i o n patterns made up of peaks i n c e r t a i n positions and varying i n t e n s i t i e s . From the 2 l v a l u e of these peaks, the spacing values (d: distance) for the d i f f e r e n t planes of the c r y s t a l can be calculated using the Bragg equation, n X = 2d s i n $ where the wavelength of the X-ray source, X i s known, d i s the spacing of units i n a p e r i o d i c arrangement and 9 i s the angle between the X-ray beam and the plane of the c r y s t a l . I I I . Infrared Spectroscopy - In the i d e n t i f i c a t i o n of d i f f e r e n t polymorphs with IR spectroscopy, only s o l i d samples (as mineral o i l mulls, potassium bromide p e l l e t s or attenuated t o t a l reflectance) can be used, since i n s o l u t i o n the polymorphs of a compound have i d e n t i c a l spectra. Many authors have used IR spectroscopy to study polymorphism. Kendall (1953) claimed that, i n addition to being rapid, the technique i s both quantitative and q u a l i t a t i v e . Smakula & others (1957) reported that when d i f f e r e n t polymorphs of e s t r a d i a l -17/3 were t r i t u r a t e d as a mull for d i f f e r e n t time i n t e r v a l s the IR absorption spectra for these phases were changed to a common spectrum. 22 IV. D i f f e r e n t i a l Thermal Analysis - In d i f f e r e n t i a l thermal analysis (DTA), the heat loss or gain r e s u l t i n g from phy s i c a l or chemical changes occurring i n a sample i s recorded as a function of temperature as the substance i s heated at a uniform rate. Enthalpic changes, both exo- and endothermic, are caused by phase t r a n s i t i o n s . For example fus i o n , b o i l i n g , sublimation, vaporization, c r y s t a l l i n e structure immersion, s o l i d - s o l i d t r a n s i t i o n and water loss are generally endothermic reactions whereas c r y s t a l l i z a t i o n i s exothermic. One of the advantages of DTA i s the a b i l i t y to c a l c u l a t e the heats of t r a n s i t i o n from one polymorph to the other. 2 - 5 . D i s s o l u t i o n of C r y s t a l l i n e Compounds. I. D i s s o l u t i o n Phenomenon. Cr y s t a l s d i s s o l v e , when conditions are reversed from those of growth. C r y s t a l d i s s o l u t i o n takes places i n a manner s i m i l a r to c r y s t a l growth. The atoms or molecules are l o s t p r e f e r e n t i a l l y from the kink positions where the binding i s weakest. Continuous d i s s o l u t i o n requires successive nucleation of kinks. The rate of d i s s o l u t i o n i s affected by many factors which are discussed l a t e r . In addition, under constant experimental conditions, the d i s s o l u t i o n rate can vary due to the presence of c r y s t a l imperfections and the presence of i n h i b i t o r s . The free surface energy i n the imperfection s i t e s i s d i f f e r e n t from other spots (Thomas, 1970). The e f f e c t of impurities on the d i s s o l u t i o n and habit modification of c r y s t a l l i n e materials has been the subject of a number of papers including those by Buckley (1951) and Ives & Plewes (1965). P i c c o l o & Tawashi (1970) studied the i n h i b i t i n g e f f e c t of FD&C blue No. 1 on the d i s s o l u t i o n rates of s u l f a t h i a z o l e , s u l f a -guanidine and phenobarbital monohydrate. They suggested that the dye molecules 23 tend to deposit on the kinks and reduce the kink nucleation rate. I n h i b i t i o n caused by poisons that are c l o s e l y r e l a t e d chemically, e.g. cholesterol-sodium chelate reported by Saad & Higuchi (1965), and the sucrose-raffinose system reported by Albon & Dunning (1962) suggest that a s t e r i c f i t of the poison molecules on the c r y s t a l face i s necessary. I I . Reciprocity of Growth and D i s s o l u t i o n . Noyes & Whitney (1897) proposed an equation for c r y s t a l d i s s o l u t i o n | | = KS (Co-Ct) (1) Nerst (1904) modified the mentioned equation to include the process of growth • £ , J£ <C„-ct) (2) where Co i s the saturation concentration, Ct the concentration i n the bulk of the s o l u t i o n , dc the amount detached or attached i n a time dt, S the area of the c r y s t a l - l i q u i d i n t e r f a c e , h the thickness of quiescent layer through which d i f f u s i o n must take place, and D the d i f f u s i o n c o e f f i c i e n t . Using t h i s expression as a basis for t h e i r experiments, several workers have tested the v a l i d i t y of the assumption that S(Co-Ct) should be equal but of opposite sign, for growth and d i s s o l u t i o n , D/h should be the same i n both d i r e c t i o n s . Buckley (1951) reviewed many papers, most of which indi c a t e that these phenomena are not r e c i p r o c a l . The v e l o c i t i e s of growth are usually much less f or any given substance than those of d i s s o l u t i o n (Table 2 - 2). 24 Table 2 - 2 . Ratio of v e l o c i t i e s of d i s s o l u t i o n over those of growth for some given substances. V e l o c i t y of d i s s o l u t i o n Substance V e l o c i t y of growth Temperature NH4 - alum 4.3 0 K 2S04 7.7 0 K 2S04 10.0 9 K2Cr 207 5.7 0 The addition of another s a l t has a va r i a b l e e f f e c t on the v e l o c i t y of growth. The v e l o c i t i e s of growth of K 2S04 are varied i n the presence of added substance i n the manner shown i n Table 2 - 3 . Table 2 - 3 . Va r i a t i o n of growth of K 2S04 i n presence of some s a l t s . Substance Added V e l o c i t y Factor KC1 2.05 K 2C0 3 1.66 Na 2C0 3 1.14 K 2Cr04 0.75 KN0 3 0.65 A few substances showed an approximate r e c i p r o c i t y i n having equal growth and d i s s o l u t i o n v e l o c i t i e s but only at very high rates of s t i r r i n g (e.g., o x a l i c a c i d and KCIO3). The term r e c i p r o c i t y i t s e l f needs c l a r i f i c a t i o n . For complete r e c i p r o c i t y , the growth v e l o c i t i e s should be i d e n t i c a l (but i n the opposite sense)to the d i s s o l u t i o n v e l o c i t i e s . In addition, the same faces should be modified i n an i d e n t i c a l but opposing manner by the two processes. The f i r s t of these conditions necessitates the same c o e f f i c i e n t of d i f f u s i o n i n the two d i r e c t i o n s (an e a s i l y understandable condition), whereas the second requires that, over a given area of the c r y s t a l face, the atoms w i l l be l a i d down on the surface, or escape o f f again, with i d e n t i c a l readiness when the same degree of over, or under-saturation p r e v a i l s i n the adjacent s o l u t i o n . This i s an u n l i k e l y condition, however. During growth, many atoms or ions of impurities, even i n a highly p u r i f i e d s o l u t i o n , must be present and attached to the surface (e.g.,H or OH ions or H2O molecules i n a pure aqueous s o l u t i o n ) . Their presence, even i f they are ultimately rejected during growth, w i l l tend to slow down the l a t t e r process, whereas, during d i s s o l u t i o n they w i l l be the locations of the more unstable parts of the surface and, far from helping to slow down the d i s s o l u t i o n of the face, may a c t u a l l y provide f o c a l spots where the excess df solvent molecules can p r i s e open the l e s s - y i e l d i n g l a t t i c e surface. Moreover, as stated by Buckley (1951), the growth of c r y s t a l s i s s e r i o u s l y modified by proportions of impurity as low as one part i n 30,000 up to one part i n 70,000 of the c r y s t a l l i n e compound. Quantities of hundreds or thousands of times these amounts would be needed to produce an e f f e c t on the d i s s o l u t i o n process. I I I . D i s s o l u t i o n Models. In a d i s s o l u t i o n process, solute molecules must f i r s t be released from the surface of the s o l i d and then transferred into the bulk of the solvent.. Depending on the r e l a t i v e s i g n i f i c a n c e of these two processes and the means by which the transport i s effected, three p h y s i c a l models have been set up. Higuchi (1967) described these models as follows: a. d i f f u s i o n layer model, i n which i t i s assumed that there i s a s t a t i c l i q u i d f i l m adjacent to the s o l i d surface. The reaction at the i n t e r f a c e ( i . e . , s o l i d / l i q u i d film) i s assumed to be rapid, so that the rate of d i s s o l u t i o n i s governed e n t i r e l y by the d i f f u s i o n a l transport of the solute molecule through the l i q u i d f i l m . Once the solute molecules pass the l i q u i d f i l m i n t e r f a c e ( i . e . , film/bulk) rapid mixing occurs, and the concentration gradient i s destroyed; b. i n t e r f a c i a l b a r r i e r model, i n which the reaction at the s o l i d surface i s not instantaneous, due to the fact that a high free energy of a c t i v a t i o n i s required. This process at the s o l i d / l i q u i d i n t e r f a c e now becomes rate l i m i t i n g with respect to the transport process; c. Danckwerts' model. Danckwerts (1951) assumed that transport of solute away from the s o l i d surface i s achieved by means of macroscopic packets of solvent which attach themselves to the surface, absorb solute by normal d i f f u s i o n and are then replaced by fresh packets of solvent. Assuming the s o l i d surface reaction to be instantaneous, the rate at which the process occurs i s r e l a t e d to the solute transport rate. The rate laws predicted by the d i f f e r e n t mechanisms, both alone and i n combination, have been discussed by Higuchi (1967) i n an extension review of drug release rate processes. The simplest case i s that of a s i n g l e component, sin g l e phase, non-disintegrating s o l i d d i s s o l v i n g into a chemically non-reactive solvent under mild to high a g i t a t i o n condition. Under such circumstances, the d i f f u s i o n layer reaction obeys Nerst equation (Eq. 2). |f = ^ (Co-Ct) (2) When the d i s s o l u t i o n process i s controlled by the i n t e r f a c i a l r eaction, one can write: | | = kiS (Co-Ct) (3) when k i i s the e f f e c t i v e i n t e r f a c i a l transport rate constant. F i n a l l y , under s i m i l a r conditions to those s p e c i f i e d e a r l i e r , the Danckwerts model i s : | | = K g V s (Co-Ct) (4) where i s the mean rate at which fresh surface i s produced. The resemblance between the Noyes-Whitney equation (Eq. 1) and equations (2) through (4) i s obvious, i n that a l l four predict a f i r s t order dependence on (Co-Ct). In the Noyes-Whitney's expression, i t was assumed that a t h i n layer of a saturated s o l u t i o n of the solute was formed at the surface of the s o l i d and that the rate of d i s s o l u t i o n was governed by the rate of d i f f u s i o n from t h i s saturated layer into the bulk of the s o l u t i o n . This concept was very close to that for the d i f f u s i o n model (equation 2) . However, recently, S z i n a i & Hunt (1972) pointed out that as the rate of transport increases (e.g., by change of agitation) or as the rate of d i s s o l u t i o n decreases a stage i s reached when transport ( d i f f u s i o n ) to the bulk s o l u t i o n i s no longer the rate determining process. I t seems u n l i k e l y that compounds of very low s o l u b i l i t y could sustain a saturated layer even at low rates of a g i t a t i o n . In these cases the removal of solute molecules from the s o l i d must become the rate determining step. Thus one should consider a whole range of conditions governing d i s s o l u t i o n . At one end of the range are s o l i d s of such high s o l u b i l i t y i n a given solvent that they can i n fact sustain a saturated layer. In such cases the rate l i m i t i n g step i s governed by d i f f u s i o n as described by the c l a s s i c a l model. At the other end of the range are the much less soluble compounds (e.g., s t e r o i d s , many a l k a l o i d s and most synthetic drugs i n aqueous systems) where the rate of d i s s o l u t i o n i s governed not by d i f f u s i o n but by the energy changes of the processes at the i n t e r f a c e . T h e s e f l a t t e r are not covered by the assumptions inherent i n the Nbyes-Whitney type equations even i f the expressions f i t some of the experimental curves. IV. D i s s o l u t i o n Measurement Methods. Di s s o l u t i o n procedures can be c l a s s i f i e d i n a v a r i e t y of ways. The c l a s s i f i c a t i o n given below i s based on i n t r i n s i c and bulk d i s s o l u t i o n methods. An i n t r i n s i c d i s s o l u t i o n method i s a procedure whereby the surface area of the pure drug i s kept constant. This method uses non-d i s i n t e g r a t i n g tablets that have been compressed at very high pressure. In t h i s category a number of methods have been employed such as the beaker method (Levy & Hayes, 1960), the hanging disk method (Nelson, 1958), the s t a t i c disk method (Levy, 1963) and the rotating disk method (Levy & S a h l i , 1962; Nogami & others, 1966). The beaker method and the rotating disk method w i l l be discussed i n more d e t a i l . a. Beaker Method. Parrott & others (1955) reported on the use of a t w o - l i t r e three-necked, round bottomed f l a s k to follow the d i s s o l u t i o n of non-disintegrating s p h e r i c a l t a b l e t s . At the s t i r r i n g rate used (550 r.p.m.), the t a b l e t rotated f r e e l y i n the l i q u i d , rather than remaining on the bottom of the f l a s k . Nelson, i n 1957, described a d i s s o l u t i o n apparatus i n which a non-disintegrating drug p e l l e t , mounted on a glass s l i d e so that only the upper face was exposed, was placed at the bottom of a 600 ml. beaker i n such a manner that i t could not rotate when the d i s s o l u t i o n medium was s t i r r e d at 500 r.p.m. Using t h i s apparatus, Nelson was able to r e l a t e the blood l e v e l s of several o r a l l y administered theophylline s a l t s to t h e i r i n v i t r o d i s s o l u t i o n rates. 29 M i t c h e l l & S a v i l l e (1967) applied t h i s method to determine the i n t r i n s i c d i s s o l u t i o n of a c e t y l s a l i c y l i c a c i d . They used disks of 400 mg. and 1.3 cm. diameter of sample. Each disk was mounted on a microscope cover s l i p with a sui t a b l e water in s o l u b l e adhesive such as f l e x i b l e c o l l o d i o n or hard p a r a f f i n , so that only one face remained exposed. b. Rotating Disk Method. This method was developed by Nelson and described by Levy & S a h l i (1962). Non-disintegrating tablets are mounted i n a p l e x i g l a s holder, so that only one surface i s exposed to the d i s s o l u t i o n medium. The holder i s attached through a metal shaft free from v i b r a t i o n and any non-concentric movement to a v a r i a b l e speed p r e c i s i o n s t i r r i n g motor; the motor must be capable of maintaining a given rate of r o t a t i o n for extended periods of time. In the o r i g i n a l work of Levy & S a h l i , the tablet was immersed to a depth of one inch below the surface of 200 ml. of d i s s o l u t i o n f l u i d , maintained at 37° i n a 500 ml. three-necked, round bottomed f l a s k . The rate of ro t a t i o n was 555 r.p.m. Samples were removed at appropriate time i n t e r v a l s for analysis. Somewhat s i m i l a r conditions were employed by Nelson (1962) i n a d i s s o l u t i o n study of erythromycin and i t s esters. The s t i r r i n g mechanism was l a t e r modified by Levy & Tanski (1964) , so as to a f f o r d precisio.n c o n t r o l of r o t a t i o n , anywhere from approximately 3 to 400 r.p.m. A range of t h i s magnitude i s necessary i n order to determine the d i s s o l u t i o n rate at various rates of a g i t a t i o n and, thereby, characterize and elucidate the d i s s o l u t i o n mechanism. A further modification was proposed by Wood & others (1965), who used the compression die as the rot a t i n g holder. In order to prevent the disk or tablet from f a l l i n g out, the bottom of the die was threaded. In t h i s manner, as much as 75% of the disk can be dissolved without i t f a l l i n g out. M i t c h e l l & S a v i l l e (1967) studying the d i s s o l u t i o n rate of a c e t y l s a l i c y l i c acid using the r o t a t i n g disk method of Wood & others (1965) showed that i n t r i n s i c d i s s o l u t i o n rates were independent of pressure over 2 the compression range 2,000 to 13,000 Kg/cm , and were independent of the p a r t i c l e s i z e of a c e t y l s a l i c y l i c acid used i n preparing the compressed disks. Nogami & others (1966) used a disk of 3 cm. diameter to study the d i s s o l u t i o n rate of a number of drugs. Bulk d i s s o l u t i o n methods have been used by many authors. Excess compound i s added to the s t i r r e d d i s s o l u t i o n medium and the amount dissolved i s measured at s u i t a b l e time i n t e r v a l s . The change i n surface area precludes measurement of i n t r i n s i c d i s s o l u t i o n rates but the method i s useful f o r the study of phase transformations (Nogami & others, 1969; G r i f f i t h s & M i t c h e l l , 1971). V. Factors A f f e c t i n g D i s s o l u t i o n Rate. The physiochemical factors that control the rate of d i s s o l u t i o n include temperature, degree of a g i t a t i o n , pH, s o l u b i l i t y and concentration gradient, composition and v i s c o s i t y of the d i s s o l u t i o n medium and i t s p o t e n t i a l for m i c e l l a r s o l u b i l i z a t i o n , the presence of active or i n a c t i v e a d d i t i v e s , polymorphism, c r y s t a l mass, and e f f e c t i v e surface. These f a c t o r s , which must be taken into account when attempting to modify drug d i s s o l u t i o n so as to enhance, delay, or sustain b i o l o g i c a l a v a i l a b i l i t y , have been reviewed i n part by Wurster & Taylor (1965), Levy (1966), and Wood (1967). Of the many physiochemical factors involved, consideration w i l l be r e s t r i c t e d to the e f f e c t s of a g i t a t i o n i n t e n s i t y , drug s o l u b i l i t y , temperature, surface area, polymorphism and hydrate and solvate. a. A g i t a t i o n Intensity: It i s probably true to say that ( one of the most important variables to consider i n d i s s o l u t i o n i s the degree of a g i t a t i o n . From the e a r l i e r discussion on d i s s o l u t i o n mechanisms, i t i s apparent that a g i t a t i o n conditions w i l l most profoundly a f f e c t d i f f u s i o n c o n t r o l l e d d i s s o l u t i o n , since the thickness of the layer i s inversely proportional to the a g i t a t i o n or s t i r r i n g speed. Levich (1942) calculated that for a disk rotating i n a volume where wa l l e f f e c t s are minimal, the thickness of the d i f f u s i o n layer, h, should be r e l a t e d to the angular v e l o c i t y of the disk, w, by the expression h = 1.612D1/3 V X/6 W- -^ (5) where D i s the solute molecule d i f f u s i o n c o e f f i c i e n t and V the v i s c o s i t y . Therefore, for laminar flow K t 0 C W ^ (6) where Kt, the rate constant for the transport process, = V^^* For other types of s t i r r i n g K t o c W a (7) where the value of a, which l i e s between 0 and 1, depends on the type of a g i t a t i o n (laminar or turbulent), the geometry of the s t i r r e r and v e s s e l and the p o s i t i o n of the; s t i r r e r with respect to the d i s s o l v i n g substance (Bircumshaw & Riddiford 1952). Levich (through Cooper & Kingery, 1962) derived an equation, r e l a t i n g i n t r i n s i c d i s s o l u t i o n rate, DR, and r o t a t i o n rate RR. Levy & Procknal (1964) expressed t h i s i n the form: LogDR = a log RR + C (8) where a and c are constants. In d i f f u s i o n - c o n t r o l processes, a = 1. I f the rate of the i n t e r f a c i a l reaction i s the rate determining step, the 32 o v e r a l l d i s s o l u t i o n rate w i l l be independent of a g i t a t i o n and a = 0. A d i s s o l u t i o n system proceeding i n accord with Danckwerts' model may be expected to show some dependence on a g i t a t i o n i n t e n s i t y , since the d i s s o l u t i o n rate w i l l be function of the rate of transfer of the macroscopic packets of solvent and the generation of new surface. Presumably, a w i l l have a value approaching unity. The value of a may also vary with the type of a g i t a t i o n used (Wurster & Taylor, 1965). It should also be borne i n mind that a g i t a t i o n w i l l a f f e c t the degree of dispersion of a c o l l e c t i o n of p a r t i c l e s , and hence the t o t a l surface exposed to the d i s s o l u t i o n medium. b. Drug S o l u b i l i t y . The Noyes-Whitney equation (Eq. 1) shows a f i r s t - o r d e r dependence on the d i f f e r e n c e i n s o l u b i l i t y , (Co-Ct). Any means whereby Co can be increased and/orct reduced w i l l cause a corres-ponding increase i n dissolution„rate. When Co i s much greater than Ct, the i n i t i a l d i s s o l u t i o n rate, DR, w i l l be d i r e c t l y proportional to Co. This behavior was confirmed by Hamlin & others (1965), who showed that, for 55 sets of data obtained from suspended p e l l e t s i n b o t t l e s undergoing r o t a t i o n at 6 r.p.m. i n a Wruble (1930) apparatus: DR = (2.24+0.10) Co (9) -2 -1 -1 where DR i s mg. cm. hr , Co i s mg. ml and the constant (2.24) i s cm. hr ^. The value of +0.10 i s the 95% confidence i n t e r n a l for the constant. However, deviations from t h i s r u l e have been reported (Hamlin & Higuchi, 1966). The saturation s o l u b i l i t y of a weak acid or base i s not an in v a r i a n t property of that compound, but w i l l vary with the pH of the d i s s o l u t i o n medium. In both cases i t i s possible to increase Co and, consequently, the rate of d i s s o l u t i o n . 33 The pH-dependence of Co has been used to enhance the d i s s o l u t i o n rate of buffered a c e t y l s a l i c y l i c acid t a b l e t s . The addition of a small amount of buffer that w i l l r a i s e the pH of the environment immediately adjacent to the p a r t i c l e d i s s o l v i n g i n the media r e s u l t s i n an e f f e c t i v e increase i n Co i n the saturation layer surrounding the p a r t i c l e . Surface active agents, at strengths i n excess of the c r i t i c a l m i c e l l e concentration enhance the d i s s o l u t i o n rate due to increase i n Co (Bates & others, 1966; Wurster & P o l l i , 1961; Taylor & Wurster, 1965; Singh & others, 1968). According to the Noyes-Whitney equation, increase i n the concentration i n the d i s s o l u t i o n medium, Ct, r e s u l t s i n a decrease i n the d i s s o l u t i o n rate. The concentration Ct can be maintained at a very low value by: a. studying only the i n i t i a l d i s s o l u t i o n ; b. using very large volume of d i s s o l u t i o n f l u i d ; ' o r c. maintenance of sink conditions. Under these conditions j± = k Co = K (10) and the process i s pseudo zero order. According to G i b a l d i & Feldman (1967), sink condition can be assumed i f the t o t a l amount of drug i n s o l u t i o n does not exceed 10 to 20% of the saturation concentration. With drugs having only a very low s o l u b i l i t y i n the d i s s o l u t i o n medium, th i s can r e s u l t i n the use of very large volumes of f l u i d . G i b a l d i & Feldman (1967) superimposed an organic phase on the aqueous d i s s o l u t i o n medium to act as a r e s e r v o i r to maintain the sink conditions for the dissolved drug. Another approach, used by Wurster & P o l l i (1961), i s the addition of absorbent materials to the aqueous phase. Hersey & B a r z i l a y (1969) have advocated the use of d i a l y s i s as a s u i t a b l e technique to achieve sink conditions i n the d i s s o l u t i o n medium without r e s o r t i n g to the use of large volumes of f l u i d . c. Temperature: An increase i n temperature w i l l increase the rate of both the i n t e r f a c i a l reaction and the transport processes (Bircumshaw & Riddiford, 1952). For a transport c o n t r o l l e d d i s s o l u t i o n process the dependence of kfc on temperature can be expressed by the Arrhenius equation . „ -Et/RT k t = Z te (11). where k t i s given by the r a t i o of the d i s s o l u t i o n rate, DR, and s o l u b i l i t y , Co, provided the experimental conditions are such that the concentration of dissolved solute i s n e g l i g i b l e compared with Co, Zt i s a constant associated with the entropy of the reaction and/or c o l l i s i o n f a c t o r s , and i s c a l l e d the frequency f a c t o r , Et energy of a c t i v a t i o n , R gas constant and T, absolute temperature. For i n t e r f a c i a l reactions, the dependence of chemical v e l o c i t y constant per unit area at unit volume, kc on temperature may be also expressed by means of the Arrhenius equation k c = z ce - E C / R T (12) where Z c i s the c o l l i s i o n frequency and Ec the energy of a c t i v a t i o n . d. Surface Area: The Noyes-Whitney equation shows that the d i s s o l u t i o n rate i s d i r e c t l y proportional to the surface area, S. Reduction i n p a r t i c l e s i z e brings about an increase i n surface area and hence enhances the d i s s o l u t i o n rate. Enhanced d i s s o l u t i o n rates i n the presence of surfactants below the c r i t i c a l m i c e l l e concentration have been a t t r i b u t e d to an increase i n e f f e c t i v e surface area, r e s u l t i n g from a lowering of i n t e r f a c i a l tension and increased wetting (Wurster & S e i t z , 1960; Levy & Gumtow, 1963; Weintraub & G i b a l d i , 1969). e. Polymorphism: Many drugs e x i s t i n a number of polymorphic forms. The correct choice of polymorphic form may be important from the point of view of chemical and phy s i c a l s t a b i l i t y and also c l i n i c a l l y . Polymorphs are ei t h e r stable or metastable. The metastable forms are unstable and tend to change to the more stable c r y s t a l l i n e form. The metastable polymorphs usually have higher s o l u b i l i t i e s and f a s t e r d i s s o l u t i o n rates than the stable form. In the process of d i s s o l u t i o n the metastable form may be transformed to stable form and r e s u l t i n a decreased d i s s o l u t i o n rate. In t h i s case the l i n e a r curve of the i n i t i a l d i s s o l u t i o n process may be preceded by a steeper portion which i s due to simultaneous phase change. Nogami & others (1969) showed that the f i n a l slope of the d i s s o l u t i o n curve i s related to the s o l u b i l i t y of the stable form (A) by equation (13) |f = k t C o A (13) and the i n i t i a l slope i s related to the s o l u b i l i t y of the more soluble form (B) by |f = k t C o B (14) where Co A and COB are the respective s o l u b i l i t i e s and the other terms have been defined previously. f. Hydrate and Solvate: Many drugs associate with solvent molecules to produce c r y s t a l l i n e forms, termed "solvates". When water i s the solvate, the solvate formed i s c a l l e d a "hydrate". The s o l u b i l i t y and the rate of d i s s o l u t i o n of a solvate may be s i g n i f i c a n t l y d i f f e r e n t from the non-solvated form of the drug (Shafter & Higuchi, 1963). Whether the drug solvate demonstrates enhanced d i s s o l u t i o n c h a r a c t e r i s t i c over the non-solvated form of the drug depends on the nature of the solvate and the nature of the d i s s o l u t i o n medium. For example, Poole & others (1968) noted that the anhydrous form of 0 a m p i c i l l i n was more b i o l o g i c a l l y a v a i l a b l e when o r a l l y administered to human subjects, and produced higher peak blood l e v e l s of the a n t i b i o t i c than d i d the t r i h y d r a t e . However, recently H i l l & others (1972) found no s i g n i f i c a n t differences i n d i s s o l u t i o n rate of anhydrous and trihydrate forms of a m p i c i l l i n when measured i n s o l u t i o n of 0.53 N HC1. They postulated that the d i s s o l u t i o n rates of these two forms should not be r a t e - l i m i t i n g i n g a s t r o i n t e s t i n a l absorption and suggested that the reported differences i n b i o a v a i l a b i l i t y from o r a l products containing the two forms are rel a t e d to formulation f a c t o r s , and not to the hydration state of the raw mat e r i a l . Poole & Bahal (1968) also studied the d i s s o l u t i o n and s o l u b i l i t y c h a r a c t e r i s t i c s of the anhydrous and hydrate forms of an experimental a m i n o a l i c y c l i c p e n i c i l l i n and found r e s u l t s s i m i l a r to that reported f o r a m p i c i l l i n . In contrast, B a l l a r d & B i l e s (1964) determined the absorption rate of organic solvates of the t e r t i a r y b u t y l acetate esters of prednisolone and hydrocortisone following p e l l e t implantation i n r a t s . They found that the absorption rate of the monoethanol solvate of the prednisolone ester was 4.7 times that of the anhydrous form. 37 2 - 6 The Polymorphism Question of A c e t y l s a l i c y l i c Acid. M i t c h e l l & S a v i l l e (1967) reported differences i n the d i s s o l u t i o n rates of s i x d i f f e r e n t samples of commercial a c e t y l s a l i c y l i c acid (ASA). The lowest d i s s o l u t i o n rate was 0.995 and the highest was -2 -1 1.75 mg.cm. min. using the r o t a t i n g disk method of Wood & others (1965) at 37 and 430 r.p.m. The ASA with the lowest d i s s o l u t i o n rate was a free flow ASA granule and that with the highest d i s s o l u t i o n rate was a c r y s t a l l i n e ASA. I.R. and X-ray d i f f r a c t i o n studies showed no evidence of polymorphism. Tawashi (1968) reported the existence of two polymorphs of ASA. The stable form I (m.p. 143° to 144°) was obtained by slow r e c r y s t a l l i z a t i o n at room temperature, and the less stable form II (m.p. 123° to 125°) was obtained by slow r e c r y s t a l l i z a t i o n at room temperature from n-hexane. He claimed to have found differences i n the I.R. and X-ray spectra, but no evidence was produced to support t h i s claim. In addition to measuring the i n t r i n s i c d i s s o l u t i o n rates using a compressed disk technique, Tawashi used the s i n g l e c r y s t a l d i s s o l u t i o n method of Higuchi & others (unpublished work). He measured the d i s s o l u t i o n rates of three d i f f e r e n t faces (Tawashi uses the term axes) of form I (a hexagonal plate) and one face of form II (a t h i n needle-like c r y s t a l ) . The d i s s o l u t i o n rate of the needle c r y s t a l was greater than from any face of the hexagonal plate and Tawashi accepted t h i s as evidence of polymorphism. However, i t i s l i k e l y that Tawashi measured the d i s s o l u t i o n rates of faces (110), (100) and (010) for the plate whereas for the needle c r y s t a l the d i s s o l u t i o n rate of face (001) only was measured. Later, G r i f f i t h s & M i t c h e l l (unpublished work) and M i t c h e l l & others (1971) found no differences i n the I.R. spectra, ATR 38 spectra, X-ray d i f f r a c t i o n patterns, melting point, thermograms ( d i f f e r e n t i a l scanning calorimeter) or i n t r i n s i c d i s s o l u t i o n rates of ASA r e c r y s t a l l i z e d from ethanol 95% and n-hexane. These findings were confirmed by Schwartzman (1972) who, i n addition, found ethanol i n the ASA r e c r y s t a l l i z e d from t h i s solvent. It was not established whether the ethanol was present on the surface of the c r y s t a l , was trapped i n t e r s t i t i a l l y or was due to hydrogen bonding. Tawashi (1969) reported a difference i n the g a s t r o - i n t e s t i n a l absorption of forms I and II when given to human subjects as a dispersion i n water. The difference was at t r i b u t e d to polymorphism. However, i t i s possible that the differences i n absorption a r i s e from the marked differences i n habit which occur when ASA i s r e c r y s t a l l i z e d from ethanol 95% and n-hexane. Tawashi stated that both forms were of about the same p a r t i c l e s i z e , but even i f one c r y s t a l dimension., was-., of about the same s i z e , the differences i n the other dimensions would lead to s i g n i f i c a n t differences i n d i s s o l u t i o n rates and therefore absorption rates. The question of polymorphism was further complicated by the report by Summers, Carless & Enever (1970) that there are s i x polymorphs of ASA. These were distinguished on the basis of melting point (DSC and hot-stage microscopy) and densities but only minor differences were found i n the X-ray d i f f r a c t i o n patterns. They found that the least stable form melted at 100° and the most stable one melted at 133° to 135°. A sample which was sublimated under vacuum had a melting point of 108°. They also claimed to have noted solution-phase transformation of some of the forms at 20°C i n n-pentanol using microscopic method. G r i f f i t h s & M i t c h e l l (1971) reported a solution-phase trans-formation i n the process of bulk d i s s o l u t i o n of ASA. They showed that the concentration i n the bulk increased r a p i d l y and showed an i n i t i a l peak before the equilibrium s o l u b i l i t y was reached. They postulated that the peak i s due to an abrupt tran s f e r of solute from the large surface area of a more stable form. The bulk l i q u i d becomes super-saturated with respect to the more stable c r y s t a l l i n e form which c r y s t a l l i z e s out on the surface of the s o l i d . They also reported i n the i n t r i n s i c d i s s o l u t i o n process from a compressed disk of ASA, the l i n e a r curve was preceded by a steeper portion. They accepted that as a simultaneous phase change. They found that ATR spectra and X-ray pattern of the surface of the ASA disk, commercial ASA and c r y s t a l s before and a f t e r the bulk d i s s o l u t i o n experiment were i d e n t i c a l . K i l d s i g & others (1971) claimed to have found d i f f e r e n t pKa values for form I (pKa 8.99) and form II (pKa 9.19) i n dimethylformamide using tetrabutyl-ammonium hydroxide as the t i t r a n t . They ascribed the pKa values to differences i n i n t r a and intermolecular hydrogen bonding. P f e i f f e r (1971) questioned the evidence for the existence of polymorphs of ASA. He suggested that, differences i n s i z e and habit might: a. a f f e c t the d i s s o l u t i o n rate from a compressed disk through differences i n c a p i l l a r i t y or wetting; b. a f f e c t the determination of melting point and heat of fusion through differences i n rates of sublimation and decomposition; c. a f f e c t density measurement by variously i n t e r f e r i n g with the complete f i l l i n g of c a v i t i e s by the displacement f l u i d . He also proposed that, c r y s t a l s somehow d i f f e r with regard to imperfection, stresses or f i n e r s t r u c t u a l d e t a i l s , but these differences also would not j u s t i f y the use of polymorphic designations. Whatever the source of apparent extra thermodynamic a c t i v i t y exhibited by some of the ASA c r y s t a l s , exposure to heat, ultrasound, solvent, etc., could cause them to anneal, 40 grow or ripen; they would thus mimic polymorphic behavior by reverting to a "more stable" form but would not undergo changes i n t h e i r r o u t i n e l y determined X-ray d i f f r a c t i o n properties. Mulley & others (1971) showed that even ASA r e c r y s t a l l i z e d from nonaqueous solvents contained from 0.10% ( r e c r y s t a l l i z e d from ethanol 96% at 20 ) to 2.4% ( r e c r y s t a l l i z e d from a saturated s o l u t i o n i n n-hexane at 68°, a f t e r standing for fourteen hours at 0°) s a l i c y l i c acid (SA) as a r e s u l t of h y d r o l y s i s . They showed that samples of ASA which were sublimated at atmospheric pressures from 0.05 to 12 mm Hg contained from 1.3 to 60% SA and melted from 136.6 to 115° res p e c t i v e l y using d i f f e r e n t i a l thermal analysis method. He concluded that the phenomena at t r i b u t e d to polymorphism could be explained by the presence of s a l i c y l i c acid i n the samples p a r t i c u l a r l y as s i g n i f i c a n t differences i n X-ray d i f f r a c t i o n s were not found. Tawashi (1971) demonstrated the existence of ASA spherulites. On heating to 124° the spherulites were transformed to needle-like c r y s t a l s which melted at 125°. Borka (1972) considered that, spherulites are strong evidence for the existence of Tawashi's polymorphic form, form I I . In h i s work spherulites melted at 125° only when t h i s temperature was maintained. No information was given regarding the time required. He was unable to reproduce the thermal transformation of spherulites to needl e - l i k e prisms as described by Tawashi. Borka (1972) determined the melting point of ASA c r y s t a l s containing d i f f e r e n t amounts of s a l i c y l i c acid, using open and closed c a p i l l a r y tube methods. He showed that the decrease i n melting point from 131° (0% SA) to 114° (18% SA) i s almost l i n e a r when plotted against the percentage of SA i n ASA. He constructed a phase diagram for the 41 binary system of ASA arid SA which showed that approximately 17 or 72% of SA i s required to y i e l d a melting point of 123-125° which i s the melting point of the needle-like c r y s t a l s and s p h e r u l i t e s . Since the SA content of the spherulites i s i n s u f f i c i e n t to depress the melting point to 123-125°, Borka argued that, the depression of the melting point of ASA i s not due to the presence of SA impurities but i s evidence for the existence of polymorphic form I I . Borka also showed an almost l i n e a r r e l a t i o n s h i p between the heating rate and melting point of ASA. He reported a change i n melting point from 130° to 139° when using rates of 0.2° and 10° min r e s p e c t i v e l y and he showed that the choice of s t a r t i n g temperature also a f f e c t s the melting point. At constant conditions, a sample of ASA melted at 135.4° and 137.3° when s t a r t i n g temperatures were 130° and 135° r e s p e c t i v e l y . Hence i t i s c l e a r that p u r i t y , rate of heating and s t a r t i n g temperature play •- very important roles i n the melting point of ASA. Borka (1972) also demonstrated a solution-phase transformation from ASA spherulites to ASA prisms. He covered the ASA spherulites with a cover s l i p and allowed a drop of saturated s o l u t i o n of ASA i n isoamyl alcohol to flow under the cover s l i p . Spherulites changed to prismatic c r y s t a l s of ASA. He did not mention the time required for t h i s trans-formation to take place and accepted t h i s phenomenon as a phase-trans-formation and strong evidence of the existence of polymorphism. 42 3. EXPERIMENTAL. 3 - 1 . Apparatus. S o l u b i l i t y Measurement Apparatus (see 3 - 3, I ) : Waterbath (50 L., Ordinary f i s h tank). C i r c u l a t o r y C r y s t a l l i z e r (see 3 - 3 , I I ) : ISCO Model 310 Metering Pump. Mansostat V a r i s t a l t i c Advanced Model Pump. Haake Thermoregulator Type FE. Haake Thermoregulator Type R21. Fisher Dyna Mix S t i r r e r . Gas Dispersion Tubes, Kimble // 28630. Hi t a c h i Colman 124, Spectrophotometer. Micro-Tek 220 Gas-Liquid Chromatography equipped with Flame Ionization Detector and a 5% ov-210, Gas Chrom. Q. (6 f t . x 2 mm.) Column. Aminco-Bowman Spectrophotofluorometer. Buchi Rotavapor "R". Mettler FP2 and FP1 Melting Point Apparatus. P h i l l i p s High Angle X-ray Diffractometer with Tennelac TC-124 Counter Recorder. Metal Mold Punch and Die ^ See 3 - 3 VII, a(i)J U.S. Gauge Co. Hydrolic Press. Rotating Disk D i s s o l u t i o n Apparatus JSee 3 - 3, VII, a ( i i ) ) E l e c t r o n i c Applications Group Flask - Tac T r a n s i s t o r i s e d Stroboscope. Fisher Steadi-Speed Adjustable S t i r r e r . M a t e r i a l s . A c e t y l s a l i c y l i c Acid B.P. (ASA), B.D.H. Canada Lots 34755 and 27648, May and Baker Lot 27648, and Monsanto Granules S a l i c y l i c Acid (SA) Reagent Grade, B.D.H. Canada Lot 32533. Sodium Carbonate, Anhydrous Reagent. Absolute Ethanol, Reagent Grade. Ethanol 95%, Reagent Grade. Acetone, Reagent Grade. Chloroform, Reagent Grade. Chloroform, C e r t i f i e d Spectroanalyzed. Amyl Alcohol, Reagent Grade, n-hexane, Reagent Grade. F e r r i c Ammonium Sulfate B.P. Di l u t e N i t r i c Acid B.P. Diazomethane Solution. Tetrahydrofuran', Reagent Grade. Methyl 0-methoxybenzoate, Eastman, Highest P u r i t y . Hydrochloric Acid 0.1 Normal. Gl y c e r i n , Reagent Grade. Solvent Ether U.S.P. 3 - 3 . Methods. I. S o l u b i l i t y of A c e t y l s a l i c y l i c Acid i n Absolute Ethanol. The apparatus shown i n F i g . 3 - 1 consists of two 150 ml. f l a s k s containing glass wool. A narrow bore glass tubing bent at each end j o i n s the two f l a s k s . Excess ASA* was added to a given volume of absolute ethanol i n 'a'. The f l a s k s were connected and fastened on a rotator i n a temperature controlled water bath. In order to accelerate the approach to saturation, the apparatus was inverted at s u i t a b l e time i n t e r v a l s , so that s o l u t i o n passed through the glass wool i n t o the other f l a s k . The glass wool prevents the passage of ASA c r y s t a l s into f l a s k 'b', while the glass tubing permits displacement of a i r . F i n a l l y the saturated s o l u t i o n was transferred to the f l a s k ' l i * (previously weighed), the alcohol evaporated using a rotary evaporator and the f l a s k and contents dried to constant weight. At least three r e p l i c a t e gravimetric determinations were made and the r e s u l t s averaged. * B r i t i s h Drug Houses, Canada, Lot 34755. 45 Figure 3 - 1 . SOLUBILITY APPARATUS. a and b, 150 ml. f l a s k s ; c, connecting tube; d, narrow glass tube; e, glass wool; f, solvent; g, powder. 46 I I . R e c r y s t a l l i z a t i o n of A c e t y l s a l i c y l i c Acid. a. R e c r y s t a l l i z a t i o n from Absolute Ethanol: A c r y s t a l l i z e r was designed s i m i l a r to that described by Bujac (1970) on the basis of producing a saturated s o l u t i o n i n two separate but interconnected vessels, and then making a supersaturated s o l u t i o n by decreasing the temperature i n one of the vessels. A diagram of the apparatus i s shown i n F i g . 3 - 2 . Excess ASA was added to absolute ethanol at the desired temperature i n one of the jacketed beakers (reservoir) and s t i r r e d continuously. The s o l u t i o n was pumped through a f i l t e r s t i c k into the other jacketed beaker ( c r y s t a l l i z e r ) , and repumped into the r e s e r v o i r through another f i l t e r s t i c k . A f t e r reaching equilibrium, the temperature i n the c r y s t a l l i z e r was decreased slowly to produce a supersaturated s o l u t i o n . Some ASA c r y s t a l s were added as seeds. The r e s u l t i n g c r y s t a l s were removed from the c r y s t a l l i z e r , washed with solvent ether and dried i n a desiccator. The c i r c u l a t o r y c r y s t a l l i z e r proved s u i t a b l e f o r the production of large amounts of ASA c r y s t a l s . The equilibrium s o l u b i l i t y curve was determined together with the s u p e r s o l u b i l i t y curve and therefore the metastable region. Crystals were grown at d i f f e r e n t degrees of super-saturation a f t e r seeding (see 3 - 3 , c ) . The degree of s u p e r s a t u r a t i o n ,A c , was calculated from equation (15). A c = (Ct-Co) (15) where Ct i s the percent ASA i n s o l u t i o n and Co i s the equilibrium s o l u b i l i t y of ASA at the same temperature. The many factors which could a f f e c t nucleation and the rate of c r y s t a l growth are beyond the scope of the present work. However, i t i s w e l l known that nucleation varies with change i n a g i t a t i o n speed 47 Figure 3 - 2 . ^CRYSTALLIZES.. a, r e s e r v o i r ; j , c r y s t a l l i z e r vessel; b and i , thermometers; c and h, s t i r r e r s ( 3 cm. polyethylene, Nalgene); d and g, glass tubes; e and f, f i l t e r s t i c k s ; k and 1 , pumps; m, laboratory jack. 48 (Bujac, 1 9 7 0 ) . The main purpose of a g i t a t i o n i n a c r y s t a l l i z e r i s to suspend the c r y s t a l s . I f some of the c r y s t a l s remain on the vessel bottom, the o v e r a l l growth rate i s reduced as a l l the c r y s t a l surfaces are not exposed to the mother l i q u o r . Crystals l y i n g stationary on the bottom not only grow at a slow rate, but they also tend to form agglomerates or, worse, compact into a s o l i d cake. Hence the type of s t i r r e r and i t s p o s i t i o n were chosen to ensure that the c r y s t a l s were suspended at low speeds ( 6 0 - 7 0 r.p.m.). The nucleation points of A S A i n absolute ethanol at a given concentration l i e approximately 3.-5° below the equilibrium s o l u b i l i t y point (see F i g . 4 - 1 ) . Because of t h i s narrow metastable region, c r y s t a l growth under conditions i n which there was a precise difference i n the degree of supersaturation was d i f f i c u l t , since the apparatus required a degree of temperature co n t r o l beyond the c a p a b i l i t y of the apparatus used. b. R e c r y s t a l l i z a t i o n from Ethanol 9 5 % : The apparatus described i n II,a, was employed to r e c r y s t a l l i z e A S A from ethanol 9 5 % using the same method. c. R e c r y s t a l l i z a t i o n at D i f f e r e n t Degrees of Supersaturation: Excess A S A was added to a given volume of absolute ethanol i n the c r y s t a l l i z e r v e s s e l and a saturated s o l u t i o n of A S A was prepared at 2 5 . 8 ° . The temperature i n the c r y s t a l l i z e r v e s s e l was reduced to 2 4 . 8 ° (Ac = 1 . 1 % w/w) and some A S A c r y s t a l s were added as seeds. The c r y s t a l growth process was continued for t h i r t y minutes. The r e s u l t i n g c r y s t a l s were removed, washed with solvent ether and dried i n a desiccator. In another experiment the-.same process was followed but seeds were added to the supersaturated s o l u t i o n at 2 3 . 5 ° (Ac = 2 . 5 % w/w). d. Slow R e c r y s t a l l i z a t i o n from a S t i r r e d Solution: Excess ASA was added to a given amount of absolute ethanol at 35° and s t i r r e d . The temperature was increased slowly to 43° to obtain a clear s o l u t i o n , and then decreased gradually to 20° within three hours and the c r y s t a l s were removed from the s o l u t i o n , washed with solvent ether and dried i n a desiccator. e. Fast R e c r y s t a l l i z a t i o n : Excess ASA was added to a given amount of absolute ethanol i n a jacketed beaker at 35° and s t i r r e d . The temperature was increased to 45° to obtain a clear s o l u t i o n , and then reduced to 20° within twenty minutes. The r e s u l t i n g c r y s t a l s were c o l l e c t e d and washed with solvent ether and dried i n a desiccator. f. M i c r o c r y s t a l l i z a t i o n of A c e t y l s a l i c y l i c Acid: M i c r o c r y s t a l l i n e ASA was prepared following the method of Affonso & Naik (1971). 50 g. of ASA was dissolved i n 1120 ml. of g l y c e r i n at 80° to obtain a saturated s o l u t i o n of ASA i n g l y c e r i n . The cl e a r s o l u t i o n was transferred to a glass v e s s e l . S t i r r i n g and external cooling were started immediately and followed by the quick addition of i c e water. The temperature was dropped to 5° within f i f t e e n minutes. The microcrystals were f i l t e r e d under vacuum through Whatman No. 42 f i l t e r paper. F i l t r a t i o n was accelerated by the addition of i c e water. The product was washed with i c e water, and dried i n a desiccator. The s i z e of the c r y s t a l s was measured using a microscope f i t t e d with a c a l i b r a t e d eye-piece micrometer. g. R e c r y s t a l l i z a t i o n of Spherulites: 20 g. of ASA was dissolved i n 100 ml. of ethanol 95% at room temperature to make a concentrated so l u t i o n . One drop of s o l u t i o n was spread as a t h i n f i l m on a microscope s l i d e (previously washed with ethanol). The spherulites were observed to grow 50 a f t e r a few seconds, f i l l i n g the microscope f i e l d . Growth s t a r t s from the center of the sph e r u l i t e s . This experiment was also c a r r i e d out following the method of Tawashi (1971) using saturated solutions, One drop of a fresh, f i l t e r e d , saturated s o l u t i o n of ASA i n ethanol 95% was spread on a clean glass s l i d e , previously washed with ethanol. The spherulites grew from the center and r a p i d l y f i l l e d the microscope f i e l d . Small, w e l l defined prisms, located mostly on the edges of the s l i d e , were also observed. h. R e c r y s t a l l i z a t i o n of Crystals from Spherulites: Spherulites were grown on a microscope s l i d e and covered with a cover gla s s . One drop of a fre s h , f i l t e r e d , saturated s o l u t i o n of ASA i n „^namyl alcohol was allowed to flow under the cover glass. After a few minutes a l l the spherulites changed to small, well-defined prisms. i . R e c r y s t a l l i z a t i o n from a Solution of 0.1 N Hydrochloric Acid: A concentrated s o l u t i o n of ASA i n 0.1 N hydrochloric a c i d was prepared by continuing the rotating disk d i s s o l u t i o n experiment at 37° for three hours ( a s o l u t i o n of 380 mg./lOO ml. of ASA). The so l u t i o n was kept i n a r e f r i g e r a t o r for eighteen hours. Resulting c r y s t a l s were removed and washed with ether solvent and drie d i n a desiccator. In another experiment, excess ASA was added to a given volume of 0.1 N hydrochloric a c i d at 37°, s t i r r e d r a p i d l y for t h i r t y minutes. The s o l u t i o n was f i l t e r e d using a Whatman No. 1 f i l t e r paper and kept i n a r e f r i g e r a t o r f o r eighteen hours. The r e s u l t i n g c r y s t a l s were removed, washed with solvent ether and d r i e d i n a desiccator. j . Slow R e c r y s t a l l i z a t i o n at Room Temperature: Excess ASA was added to ethanol 95 % at room temperature, and s t i r r e d to obtain a saturated s o l u t i o n . The so l u t i o n was f i l t e r e d through a Whatman No. 1 f i l t e r paper, and kept at room temperature for fort y - e i g h t hours. The r e s u l t i n g c r y s t a l s were removed and washed with solvent ether and dried i n a desiccator. k. • R e c r y s t a l l i z a t i o n from n-hexane: Excess ASA was added to 1000 ml. n-hexane at 60°, s t i r r e d w e l l and f i l t e r e d . The clear s o l u t i o n was kept i n a r e f r i g e r a t o r for s i x hours. Resulting c r y s t a l s were c o l l e c t e d and washed with solvent ether and dried i n a desiccator. I I I . Determination of S a l i c y l i c Acid i n A c e t y l s a l i c y l i c Acid. The pharmacopeial l i m i t s f o r s a l i c y l i c acid (SA) i n commercial ASA are 0.1% USP 18 r e v i s i o n and 0.05% B.P. 1968. Hence the method used to measure the amount of SA i n ASA should be s e n s i t i v e enough to detect these l e v e l s or preferably l e s s , since i t i s possible that SA can be incorporated into the ASA c r y s t a l l a t t i c e as an i n t e r s t i t i a l defect. S a l i c y l i c acid present i n ASA was determined using colorimetry, g a s - l i q u i d chromatography, spectrophotometry, and absorbance r a t i o methods. a. Coloriimetry. In the presence of ac i d f e r r i c ammonium s u l f a t e , SA produces a v i o l e t color which absorbs at 530 nm. Preparation of Standard Curve (Fig. 3 - 3 ) - 100 mg. SA, accurately weighed, was dissolved i n 10 ml. of ethanol 95%, transferred to a 100 ml. volumetric f l a s k and water added to volume. 10 ml. of t h i s s o l u t i o n was transferred to another 100 ml. volumetric f l a s k and d i l u t e d to volume with water. 0.5, 1, 2 and 3 ml. of t h i s l a t t e r s o l u t i o n were transferred to f i v e 100 ml. volumetric f l a s k s and 2 ml. of acid f e r r i c ammonium s u l f a t e s o l u t i o n B.P. was added to each f l a s k , d i l u t e d to volume with water, and allowed to stand f o r one minute. The i n t e n s i t y of the color produced was measured at 530 nm and plo t t e d versus concentration on l i n e a r graph paper. The slope of the curve was 1.85 using regression a n a l y s i s . Preparation of Sample Solution - Approximately 1.0 g. of ASA accurately weighed was dissolved i n 10 ml. of ethanol 95% and transferred to a 100 ml. volumetric f l a s k . Two ml. of acid f e r r i c ammonium s u l f a t e 53 Figure 3 - 3. Standard curve for the co l o r i m e t r i c determination of s a l i c y l i c a c i d . 54 s o l u t i o n was added and water to make up to volume. The i n t e n s i t y of t h i s s o l u t i o n was measured at 530 nm a f t e r allowing to stand for one minute. In t h i s method the amount of ASA required i s more than the equilibrium s o l u b i l i t y of ASA i n the solvent. Hence t h i s method i s unsatisfactory for the determination of small amounts of SA i n ASA. b. Gas-Liquid Chromatography Determination of S a l i c y l i c Acid i n A c e t y l s a l i c y l i c Acid: The procedure involves the conversion of the two acids to t h e i r methyl esters with diazomethane i n tetrahydrofuran (THF) s o l u t i o n and subsequent isothermal e l u t i o n of the components from the column following the method of Watson and others (1971)• i Preparation of Diazomethane Solution - The method of De Baer & Backer (1956) was followed*. A 125 ml. d i s t i l l i n g f l a s k with a condenser set for d i s t i l l a t i o n was f i t t e d with a long-stem dropping funnel. The condenser was connected by means of an adaptor to a stoppered 250 ml. Erlenmeyer f l a s k . Through a second hole i n the stopper of the Erlenmeyer f l a s k an o u t l e t tube was placed which was bent so as to pass into and nearly to the bottom of a second unstoppered Erlenmeyer f l a s k . Both receivers were cooled i n an i c e - s a l t mixture; i n the f i r s t and second receivers r e s p e c t i v e l y , 2 ml. THF and 7 ml. of THF were placed. The i n l e t tube passed below the surface of the THF i n the second f l a s k . In the d i s t i l l i n g f l a s k , a s o l u t i o n of 1.29 g. of potassium hydroxide dissolved i n 2 ml. of water, 7.0 ml. of 2-ethoxy-ethanol, 2.0 ml. of THF, and a "Teflon"-coated magnetic s t i r r i n g bar were placed. * 2-ethoxy-ethanol was used instead of C a r b i t o l i n t h i s experiment. 55 The dropping funnel was attached and adjusted so that the stem was j u s t above the surface of the s o l u t i o n i n the d i s t i l l i n g f l a s k . A s o l u t i o n of 4.3 g. (0.02 M.) of p-tolylsulfonylmethylnitrosamide i n 2.5 ml. of THF was placed i n the dropping funnel. The d i s t i l l i n g f l a s k was heated i n a water bath at 70-75°, the s t i r r e r was started, and nitrosamide s o l u t i o n was added at a regular rate f o r f i f t e e n to twenty minutes. As soon as a l l the nitrosamide s o l u t i o n had been added, a d d i t i o n a l THF was trans-f e r r e d i n the dropping funnel and added u n t i l the d i s t i l l a t e was c o l o r l e s s . Preparation of Standard Curve of S a l i c y l i c Acid (Fig. 3 - 4) -A stock s o l u t i o n of 5 mg./ml. of SA i n THF was prepared; 0.2, 0.3, 0.4, 0.5 and 0.6 ml. of t h i s s o l u t i o n were transferred to f i v e 5 ml. volumetric f l a s k s r e s p e c t i v e l y . A stock s o l u t i o n of 5 mg./ml. of ortho-metoxybenzoate (OMBA) i n THF was also prepared for use as an i n t e r n a l standard. One ml. of th i s s o l u t i o n was transferred to each of the above 5 ml. volumetric f l a s k s . Diazomethane was added to the s o l u t i o n u n t i l a permanent yellow color appeared, and then THF was added to volume. O n e ^ l . of solutions of 0.2, 0.3 and 0.4 ml. of SA i n 5 ml. and 0.8^<1. of solutions of 0.5 and 0.6 ml. of SA i n 5 ml. THF were i n j e c t e d by means of a microsyringe i n t o a GC equipped with a flame-ionization detector unit and f i t t e d with a 5% 0V-210 GasChrom Q (100-120 mesh) U-shaped glass column (6 f t . x 2 mm.). The drugs were isothermally eluted as t h e i r methylated compounds under the following conditions: i n l e t temperature 180°, column temperature 90°, detector temperature 255°. Gas flows were: Ng 25 ml./min.; H£ 30 ml./min.; a i r 12 ml./min. The detector s i g n a l was recorded by a recorder with a 2 chart speed of 15 inches/hour, input attenuator 10 and output attenuator 32. The retention'time for SA.was 3.5 minutes. 56 Figure 3-4. Gas-liquid chromatographic internal standard calibration curve for salicylic acid. 57 The area r a t i o s of the methylated compound to i n t e r n a l standard were p l o t t e d against the weight r a t i o s of the drug to i n t e r n a l standard, Fig.(3 - 4) • The slope of the curve was obtained from regression analysis and was 1.070 which i s i n agreement with the value of 1.069 reported by Watson & others (1971). Sample Solution - 298 mg. of powdered c r y s t a l l i n e ASA was accurately weighed and transferred to a 5 ml. volumetric f l a s k . 1 ml. of OMBA stock s o l u t i o n and s u f f i c i e n t diazomethane were added to produce permanent yellow color, and then THF was added to volume. One jil. of t h i s s o l u t i o n was inj e c t e d into instrument by means of a microsyringe and the re s u l t was calculated (Figs. 3 - 5 and 3 - 6 ) . This method i s a time consuming procedure and the small amounts of SA normally found i n commercial ASA cannot be determined. A chromato-gram of the e l u t i o n of SA from ASA i s shown i n F i g . 3 - 5 . The sample was a commercial ASA containing 0.09% SA (spectrophotometric determination). SA has a very small peak l y i n g on the t a i l of the solvent peak. The isothermal e l u t i o n of a synthetic mixture of ASA containing 1% SA* shown i n F i g . 3 - 6 shows that although t h i s l e v e l of SA can be determined the method i s unsatisfactory for small amounts of SA. c. Determination of S a l i c y l i c Acid i n A c e t y l s a l i c y l i c Acid using Absorbance Ratio Technique. The absorbance of sample of ASA containing SA at 276 and 303 nm. 303 was measured and absorbance r a t i o ("275") calculated. Preparation of Standard Curve (Fig. 3 - 7 ) : Stock solutions of ASA (1 g./lOO ml. of ethanol 95%) and SA (0.1 g./lOO ml. of ethanol 95%) were prepared. 1, 5, 10, 15 and 20 ml. of SA stock s o l u t i o n were added to f i v e d i f f e r e n t 100 ml. volumetric flasks containing 10 ml. of ASA stock *The same experimental conditions except, output attenuator = 16. 58 F i g . 3 - 5 . Gas-liquid chromatogram of s a l i c y l i c acid i n commercial a c e t y l s a l i c y l i c acid.The peak due to isothermal e l u t i o n of a c e t y l s a l i c y l i c (retention time 37 minutes) i s not shown i n t h i s f i g u r e . 59 F i g . 3 - 6 . Gas-liquid chromatogram of a c e t y l s a l i c y l i c a c i d i n mixture of s a l i c y l i c a c i d (1%) and a c e t y l s a l i c y l i c acid. The peak due to isothermal e l u t i o n of a c e t y l -s a l i c y l i c a c i d ( retention time 37 minutes ) i s not shown i n t h i s figure. 60 l.o r-10 15 20 S a l i c y l i c Acid (per cent) Figure 3 - 7. Standard curve for determination of s a l i c y l i c acid in acetylsalicylic acid using absorbance ratio method. s o l u t i o n . Ethanol 95% was added to volume, and absorbance of each of the solutions was measured at 276 and 303 nm. re s p e c t i v e l y . The 303 absorbance r a t i o (^ ^^ ) was calculated, and plo t t e d as function of SA%. The slope of the curve calculated using regression analysis was 0.0415. Preparation of Sample Solution: A given sample of ASA was transferred into a f l a s k and dissolved i n ethanol 95%. The absorbance at 303 and 276 was measured and the absorbance r a t i o c a l c u l a t e d . No sample weighing or volume cor r e c t i o n i s required i n t h i s method. This method was used to measure the SA i n ASA melt a f t e r melting point determinations. This method cannot detect SA i n amounts less than 0.1%, because the amount of sample required exceeds the s o l u b i l i t y of ASA i n ethanol. I t i s very rapid and no volume correction or weighing are required. d. Spectrophotofluorometric Determination of S a l i c y l i c Acid i n A c e t y l s a l i c y l i c Acid : A cihlorof ormic s o l u t i o n of SA has a t y p i c a l fluorescence which i s detectable at an e x c i t a t i o n wavelength of 310 nm. and an emission wavelength of 450 nm. The method of Shane & Stillman (1971) was followed. Preparation of Standard Curve (Fig.3^8)- 100 mg. of SA was accurately weighed and transferred to a 100 ml. volumetric f l a s k and dissolved i n chloroform.* S u f f i c i e n t chloroform was added to make up to volume. One ml. of t h i s s o l u t i o n was d i l u t e d with chloroform to 100 ml. i n a 100 ml. volumetric f l a s k . Ten ml. of th i s l a t t e r s o l u t i o n was transferred to another 100 ml. volumetric f l a s k and made up to volume * Unless otherwise stated chloroform C e r t i f i e d Spectroanalyzed was used i n t h i s experiment. 62 Figure 3 - 8 . Standard curve for spectrophotofluorometry determination of s a l i c y l i c acid. , 63 with chloroform. One, 5, 10, 15 and 25 ml. of t h i s s o l u t i o n were trans-ferred to f i v e 100 ml. volumetric f l a s k s and d i l u t e d to volume with chloroform to make solutions of 0.001, 0.005, 0.010, 0.015 and 0.025 mg. SA per 100 ml. The r e l a t i v e fluorescence i n t e n s i t y of these solutions was obtained at e x c i t a t i o n and emission wavelengths of 310 and 450 nm. r e s p e c t i v e l y . The slope of the curve was 4.4 using regression analysis. The instrument was locked at a s e n s i t i v i t y of 30. The s l i t arrangement was 3, 2, 2, 3, 0.5 and the instrument was c a l i b r a t e d using a s o l u t i o n of 0.1^g./ml. quinine s u l f a t e i n 0.1 N sulphuric a c i d * ' . Preparation of Sample Solution - 100 mg. of ASA was accurately weighed and transferred to a 100 ml. volumetric f l a s k , dissolved i n and d i l u t e d to volume with chloroform. Ten ml. of t h i s s o l u t i o n was d i l u t e d to 100 ml. with chloroform i n a 100 ml. volumetric f l a s k and i t s r e l a t i v e fluorescence i n t e n s i t y was measured at e x c i t a t i o n and emission wavelengths of 310 and 450 nm. r e s p e c t i v e l y . The fluorescence i n t e n s i t y i s due to the presence of only SA, because ASA does not fluoresce."' I t i s a one-step operation; the complete an a l y s i s , exclusive of c a l i b r a t i o n , requires less than ten minutes. The fluorometer i s e s p e c i a l l y u seful i n the determination of very small amounts of SA i n the presence of a large amount of ASA, because the fluorescence per unit weight i s very strong and gives large peaks. Using t h i s method i t i s possible to detect as l i t t l e as 0.001% of SA i n a s o l u t i o n , and because * At the beginning and the end of each experiment the xenon lamp i n t e n s i t y was checked by remeasuring and comparing the r e l a t i v e fluorescence i n t e n s i t y of a s o l u t i o n of O.lyyg./ml. quinine s u l f a t e i n 0.1 N sulphuric acid at e x c i t a t i o n and emission wavelengths of 350 and 450 nm. r e s p e c t i v e l y . ** ASA does not exhibit fluorescence a c t i v i t y i n t h i s experimental -condition, of the high s o l u b i l i t y of ASA i n chloroform, there i s no s o l u b i l i t y problem. The only disadvantage i s the p o s s i b l i t y of a change i n the i n t e n s i t y of the xenon lamp. The introduction of errors due to t h i s cause was avoided by using a s o l u t i o n of quinine s u l f a t e as a standard s o l u t i o n to c a l i b r a t e the instrument before each analysis. In order to remove any background interference, highly pure chloroform* with almost zero fluorescence a c t i v i t y was used. This method was considered to be the most s u i t a b l e and convenient, and was used to determine small amounts of SA i n ASA. * Chloroform C e r t i f i e d Spectroanalyzed, Fisher S c i e n t i f i c Company. IV. Measurement of the Melting Point of the A c e t y l s a l i c y l i c Acid Samples. The melting point of d i f f e r e n t samples of ASA was determined using open c a p i l l a r y tube and hot stage methods. The sample was ground and f i l l e d into a c a p i l l a r y tube and/or was placed on a micro-scope s l i d e and covered with a cover s l i p . Spherulites were scraped from the s l i d e on which they were grown, f i l l e d i n t o a c a p i l l a r y tube and/or covered with a cover s l i p and placed on the hot stage and the melting point determined. Melting point of a l l samples determined under constant conditions, i . e . , a s t a r t i n g temperature of 2° below the estimated melting point (determined i n a preliminary experiment using a heating rate of 10°/min)i and heating rate of 0.2°/min. (Table 4 - 2 ) . V. X-Ray D i f f r a c t i o n of the A c e t y l s a l i c y l i c Acid Samples., Approximately 300 mg. of the ASA sample was f i l l e d i nto a sample s l i d e and exposed to CuKot r a d i a t i o n i n an X-ray diffractometer. The l o c a t i o n and i n t e n s i t y of the peaks at d i f f e r e n t values of 29 were studied. In the case of s p h e r u l i t e s , the samples were scraped from the microscope s l i d e and placed i n the sample s i t e i n the instrument. Compressed disks were used f o r X-ray analysis d i r e c t l y . VI. D i s s o l u t i o n of A c e t y l s a l i c y l i c Acid. a. I n t r i n s i c D i s s o l u t i o n Rate: The i n t r i n s i c d i s s o l u t i o n rate of ASA was measured using the r o t a t i n g disk method of Nogami & others (1966) as described previously by G r i f f i t h s & M i t c h e l l (1971). Xi) Preparation of Compressed Disk: A metal punch and die 66 (Nogami & others 1966) shown i n F i g . 3 - 9 was employed to compress the disk of ASA. I t consists of a die, 'a', upper punch, 'b', lower punch, 'c', base with three nuts, 'd', r i n g with a top f o r exhaustion, 'e', adaptor with two small p l a s t i c 0-rings i n the center hole, ' f ' , and two p l a s t i c 0-rings, 'g'. The 0-rings i n the center hole, ' f ' , make the assembly a i r t i g h t when 'b' i s inserted into the hole. The 0-rings 'g' make the assembly a i r t i g h t between 'd' and 'e'. and 'e' and ' f ' r e s p e c t i v e l y . A f t e r 'c' and 'a' were attached to 'd'. 3 g. of sample was put i n 'd' and then 'b' was inserted into ' d', as shown i n F i g . 3 - 9 , 2. F i n a l l y the punch and die was assembled as i n F i g . 3 - 9 , 3, and put on a hydraulic press*. The in s i d e of the assembly was exhausted under reduced pressure of about 2 mm. Hg for four minutes. Continuing the exhaustion, the powder sample was compressed by upper punch 'b' u n t i l the compression arrived at 2 approximately 3 .' 6 ' tons per cm **. Compression was continued for eight minutes i n that state. In order to remove the compressed disk, parts ' f , 'e', 'd' and 'c' were taken o f f from the assembly. Die 'a' and upper punch 'b' containing the compressed disk were placed above a 10 cm. metal tubing shown i n F i g . 3 r 9, 4, which was cut v e r t i c a l l y . Hydraulic pressure was applied manually and the disk was c a r e f u l l y forced out of the die. * U.S. Gauge Company. 2 ** The gauge pressure i s c a l i b r a t e d i n Kg./cm . 1 4-F i g . 3 -9. Metal mold to make a compressed disk, d i e ; b, upper punch ; c, lower punch . d, base p l a s t i c 0-rings. ( i i ) I n t r i n s i c D i s s o l u t i o n Apparatus and Procedure - The apparatus shown i n F i g . 3 - 1 0 was used for the measurement of d i s s o l u t i o n rate. A 3 cm. diameter compressed disk of sample 'q' was a f f i x e d to the s t a i n l e s s s t e e l holder of the same diameter, 'p', using n a i l polish*as an adhesive. So that only the bottom face of the disk was exposed to the l i q u i d , the edges of the disk were covered with the adhesive. The s t a i n l e s s s t e e l holder was connected to a motor, 'm'. The holder, 'p', with disk, 'q', was mounted above the jacketed beaker, 'n', which contained 250 ml. of 0.1 N hydrochloric acid s o l u t i o n . The jacketed beaker was placed on a laboratory jack. When the temperature i n the beaker, 'n', a r r i v e d at an equilibrium temperature of 37°, the disk was rotated at 300 r.p.m. (checked with an e l e c t r o n i c stroboscope) and the beaker was r a i s e d on the laboratory jack to immerse the disk to a f i x e d depth i n the s o l u t i o n . Care was taken to ensure that no a i r bubbles adhered to the disk surface. At appropriate i n t e r v a l s , 2 ml. samples we're taken from the beaker and d i l u t e d volumetrically to 10 or 25 ml. with 0.1 N hydrochloric a c i d . The analysis was c a r r i e d out spectrophotometrically at 276 nm. A c a l i b r a t i o n curve of concentration of ASA i n 0.1 N hydrochloric acid versus absorbance at 276 nm. ( s l i t width 0.5) was used to c a l c u l a t e the amount of ASA dissolved (Fig. 3 - 11). The a b s o r p t i v i t y value of ASA i n 0.1 N hydrochloric acid from the slope of the regression analysis was 6.45 when concentration was considered as grams per l i t e r . D i s s o l u t i o n r e s u l t s were p l o t t e d as mg. ASA dissolved (calculated using equation (16)1 * Cutex S p i l l p r u f P o l i s h . F i g u r e 3 - 10. INTRINSIC DISSOLUTION APPARATUS. 1, thermometer; m, s t i r r e r m o tor; n, j a c k e t e d b e a k e r ; o, l a b o r a t o r y j a c k ; p, sample h o l d e r ; q, compressed d i s k . Figure 3 - 11. Standard curve f o r determination of a c e t y l s a l i c y l i c acid i n 0.1 N HC1. 71 versus time A x Vd x 250 i - . , . , „ Ct = — = mg. ASA dissolved at time t (16) Vs x a where A i s absorbance, Vs the volume of sample, Vd the volume to which sample i s d i l u t e d and a i s the ab s o r p t i v i t y value. The d i s s o l u t i o n rate was calculated from equation (17) D i s s o l u t i o n rate = s l ° P e . , . . surface area of disk (17) = slope -2 . -1 -j mg. cm . mm Di s s o l u t i o n rate of six determinations for the ASA cont r o l were -2 -1 1.13, 1.15, 1.16, 1.20, 1.20 and 1.22 mg. cm . min ; mean value, = 1.18 -2 -1 mg. cm .min . The confidence l i m i t , CL, for the mean value was calculated using equation (18) CL = t n _ 1 ( S E ) (18) where t (2.571) i s the value obtained from tables of t - d i s t r i b u t i o n for a p r o b a b i l i t y of 0.05, n number of determinations and degrees of freedom n-1. _2 Standard error, SE (1.452 x 10 ), was calculated using equation (19) SE = y V a r i ; n C £ (19) _3 and variance (1.264 x 10 ) calculated from equation (20) Variance = 1 X * J (20) n—1 where x i s the mean value. The mean value f o r the i n t r i n s i c d i s s o l u t i o n rate of ASA from a 3 cm. diameter compressed disk, ro t a t i n g at 300 r.p.m. i n 0.1 N HC1 at 37° was 1.18 ±0.037 mg. cm - 2, m i n - 1 (confidence l i m i t = 0.95) b. Bulk D i s s o l u t i o n . Excess ASA* (5-10 times the equilibrium s o l u b i l i t y ) was added to 250 ml. 0.1 N HC1 i n a jacketed beaker, maintained at a constant temperature (30 or 37°) and s t i r r e d r a p i d l y (300 or 500 r.p.m.) with a 3 cm.Nalgene s t i r r e r . Samples were removed at suitable time i n t e r v a l s , using a pipet f i t t e d with a f i l t e r s t i c k , d i l u t e d and analyzed spectro-photometrically at 276 nm. The amount of ASA dissolved was calculated using the method described for i n t r i n s i c d i s s o l u t i o n and plotted versus time (Fig. 4 - 14). In another experiment a mixture of commercial ASA and spherulites (15% spherulites) was prepared and the bulk d i s s o l u t i o n studied at 37° and 300 r.p.m. VII. E f f e c t of Various Factors on the I n t r i n s i c D i s s o l u t i o n Rate, a. P a r t i c l e Size. ( i ) E f f e c t of P a r t i c l e Size Using Unground C r y s t a l s : 100 g. of commercial ASA was sieved through standard mesh sieves. Crystals which passed through mesh sizes 25, 35 and 45, and were retained by mesh sizes 35, 45 and 60 r e s p e c t i v e l y , were compressed and t h e i r d i s s o l u t i o n rates determined. Crystals r e s u l t i n g from d i f f e r e n t conditions of r e c r y s t a l l i z a t i o n ( i . e . , solvent, temperature, and etc.) had d i f f e r e n t s i z e s . The d i s s o l u t i o n rates of these c r y s t a l s were also determined and compared (Table 4 - 2). ( i i ) P a r t i c l e Size A f t e r Grinding: The method described i n 3 - 3 , VII, a ( i ) , was employed, but the c r y s t a l s were ground i n t o a f i n e powder by means of a pestle and mortar before compressing. Crystals which * B.D.H., Canada, Lots 27648, mesh s i z e 40, and 34755, mesh s i z e 40. 73 passed through mesh, sizes 25,45 and 60 and were retained by mesh sizes 35, 60 and 85 r e s p e c t i v e l y , were used i n this experiment. b. S a l i c y l i c Acid. The SA present i n d i f f e r e n t samples of ASA was determined spectrophotometrically, and the d i s s o l u t i o n rate was measured. The d i s s o l u t i o n rate of a p a r t i a l l y hydrolysed sample which had been kept at 37° i n an oven:, containing a beaker of water for f i f t y hours was also determined. c. Supersaturation. Crystals which were grown i n the c r y s t a l l i z e r using d i f f e r e n t degrees of supersaturation were compressed and t h e i r d i s s o l u t i o n rates determined and compared (see 3 - 3 , I I , c ) . d. Habit. ASA samples with d i f f e r e n t habits were prepared, i . e . prisms, plates and spherulites (Table 4 - 2). The samples were compressed without grinding and the d i s s o l u t i o n rate of the compressed disks was measured. e. R e c r y s t a l l i z a t i o n Solvent. Crystals grown from d i f f e r e n t solvents ( i . e . , absolute ethanol, ethanol 95% and 0.1 N HC1) were c o l l e c t e d , compressed and the d i s s o l u t i o n rates of the disks were measured. f. D i s s o l u t i o n Rate of Spherulites. Spherulites of ASA which were grown from ethanol 95% on several 20 x 20 cm. s l i d e s were scraped, compressed and the d i s s o l u t i o n rate measured. 74 g. D i s s o l u t i o n Rate of a Mixture of Prismatic Crystals and Spherulites. A mixture consisting of 80% c r y s t a l s grown from slow r e c r y s t a l l i z a t i o n from ethanol 95% at room temperature and 20% spherulites was prepared, compressed and d i s s o l u t i o n rate measured. 75 4. RESULTS AND DISCUSSION. 4 - 1 . Equilibrium S o l u b i l i t y of A c e t y l s a l i c y l i c Acid i n Absolute Ethanol. Absolute ethanol was used as the r e c r y s t a l l i z a t i o n solvent i n most of the r e c r y s t a l l i z a t i o n s , therefore the s o l u b i l i t y of ASA i n t h i s solvent was required. The r e s u l t s from the equilibrium s o l u b i l i t y gravimetric determination of ASA i n absolute ethanol are shown i n F i g . 4 - 1 . During the r e c r y s t a l l i z a t i o n experiments, conditions were attained, at constant temperature, where neither growth nor d i s s o l u t i o n occurred. The concentration of ASA i n s o l u t i o n under these conditions was also taken as the equilibrium s o l u b i l i t y ( F i g . 4 * 1 ) . From the nucleation observations and the determination of concentration and temperature at which no c r y s t a l growth or d i s s o l u t i o n took place, a s o l u b i l i t y - s u p e r s o l u b i l i t y diagram was constructed (F i g . 4 - 1 ) . The metastable region l i e s between the s o l u b i l i t y and the spontaneous nucleation l i n e s . Results obtained using the c r y s t a l l i z e r and the equilibrium s o l u b i l i t y apparatus are i n agreement. F i g . 4 - 2 compares the s o l u b i l i t y and s u p e r s o l u b i l i t y curves of ASA i n absolute ethanol found i n t h i s work with values determined by Glasby & Ridgway (1968) using a f l u i d i z e d bed c r y s t a l l i z e r as w e l l as a s o l u b i l i t y apparatus. Results from t h i s work are considerably greater than those reported by Glasby & Ridgway. 76 Figure 4 - 1 . Solubility-supersolubility diagram of the acetylsalicylic acid-ethanol system. O ,solubility determination in crystallizer; A t solubility determination in solubility apparatus; <3, nucleation. 77 Figure 4 - 2. Comparison of s o l u b i l i t y - s u p e r s o l u b i l i t y of a c e t y l s a l i c y l i c i n -absolute -ethanol., Sthis work; , Glasby & Ridgway(1968) 78 4 - 2 . Crystals of A c e t y l s a l i c y l i c Acid. In order to study the e f f e c t of habit, s i z e , s a l i c y l i c acid content and the rate of growth on the physico-chemical properties of ASA c r y s t a l s , a va r i e t y of methods of r e c r y s t a l l i z a t i o n were employed. These resulted i n a serie s of c r y s t a l s with d i f f e r e n t habits, sizes and percent SA. No attempt was made to i d e n t i f y or count the type and number of imperfections i n the r e s u l t i n g c r y s t a l s . A s i n g l e c r y s t a l of ASA grown by suspending a small c r y s t a l i n a saturated s o l u t i o n of ASA i n absolute ethanol was a tetragonal prism (Fig. 4 - 3, a) with two each of the following faces: (100), (010) and (001). Crystals from n-hexane were very t h i n needles ( F i g . 4 - 4 ) and those from ethanol 95% obtained by slow r e c r y s t a l l i z a t i o n at room temper-ature (evaporation) were sometimes hexagonal plates (Fig. 4 - 5 and 4 - 3, c) and sometimes tabular due to exaggerated growth of faces (110) (Fig. 4 - 3,f) . Crystals grown i n absolute ethanol and ethanol 95% using the c i r c u l a t o r y c r y s t a l l i z e r were hexagonal prisms (Fig. 4 - 3, b) which i n d i c a t e that plane (110) i n s t i r r e d suspended c r y s t a l s has a slower rate of growth than i n a sing l e suspended c r y s t a l . In some c r y s t a l s , plane (110) has a very slow rate of growth so that the c r y s t a l appears as a tetragonal prism with two (110), two (110), one (001) and one (OOl) faces (Fig. 4 - 3 , d). Crystals from an unsaturated s o l u t i o n of ASA i n 0.1 N HC1 (0.38 g./lOO ml.) were octahedral pyramids, which have 8 (111) faces (Fig. 4 - 3, e). When ASA was grown from a saturated s o l u t i o n of ASA i n 0.1 N HC1, i t appeared as hexagonal plates (Fig. 4 - 3, c ) . Spherulites of ASA (Fig. 4 - 6 ) r e c r y s t a l l i z e d i n a very t h i n layer. Microscopic observation of the spherulite growth process showed that growth occurs r a d i a l l y outward from the center. 79 c O p t i c a l axes of the monoclinic system (110) (100) or \001) (010) (110). (100) (010) (111) (111) (110) (111) (111) (001) Figure 4 - 3 . ACETYLSALICYLIC ACID CRYSTAL HABITS. a, tetragonal prism; b, hexagonal prism; c, hexagonal pla t e ; d, tetragonal prism; e, octahedral pyramid; f, p l a t e . A l l habits belong to'-the monoclinic system. F i g . 4-4. Needle-like c r y s t a l s of a c e t y l s a l i c y l i c acid from n-hexane (X100). F i g . 4 - 5 . Hexagonal plate of a c e t y l s a l i c y l i c acid from ethanol 9 5 % (X 10). I 81. Tawashi (1971) stated that the growth of ASA into two-dimensional spherulites i s an unusual phenomenon, and that substances of low molecular weight l i k e ASA do not normally c r y s t a l l i z e i n t h i s way. He argued that spherulite formation i s formed by conditions of high supersaturation and a high v i s c o s i t y of the c r y s t a l l i z a t i o n medium. These conditions are l i k e l y to be met when a saturated s o l u t i o n of ASA i n ethanol i s spread as a t h i n f i l m on a glass s l i d e . The a v a i l a b l e surface for evaporation i s large and the solvent mass transf e r rate becomes an important factor i n c r y s t a l l i z a t i o n . Tawashi (1971) postulated that, the reported v a r i a t i o n i n thermal and d i s s o l u t i o n c h a r a c t e r i s t i c s of commercial ASA and the surface phase transformation reported by G r i f f i t h s & M i t c h e l l (1971) could be a t t r i b u t e d to the presence of ASA spherulites on the surface of commercial ASA c r y s t a l s . However, Borka (1972) pointed out that spherulite formation on a c r y s t a l surface i s u n l i k e l y since preformed c r y s t a l s dominate and d i r e c t a d d i t i o n a l c r y s t a l growth; t h i s i s the basis of a l l c r y s t a l seedings. In our laboratory spherulites of phenacetin, v a n i l l i n (Fig. 4 - 7 ) , a c e t a n i l i d and benzocainehave been grown from absolute ethanol using the same method described for the growth of ASA s p h e r u l i t e s . Hence the existence of spherulites i s a normal phenomenon even for molecules with low molecular weight. I. S a l i c y l i c Acid Content. ." . - :.. *~ -M i t c h e l l & others (1971) suggested that the v a r i a t i o n i n the d i s s o l u t i o n rates of d i f f e r e n t samples of commercial ASA reported by M i t c h e l l & S a v i l l e (1967) may be due to differences i n the number and type of c r y s t a l imperfections. The most l i k e l y impurity to cause c r y s t a l defects F i g . 4 - 6 . Spherulites of a c e t y l s a l i c y l i c acid (X100) Fig . 4 - 7 . Spherulites of v a n i l l i n ( X 3 5 ). i n ASA c r y s t a l s i s the SA molecule, which by s u b s t i t u t i n g i n the l a t t i c e structure for an ASA molecule would give r i s e to a point imperfection ( s u b s t i t u t i o n a l d efect). Mulley & others (1971) demonstrated that i t i s almost impossible to r e c r y s t a l l i z e ASA from even nonaqueous solvent without producing a small amount of SA. Thomas (1971) pointed out that quantitative studies of a c t i v a t i o n energy of d i s s o l u t i o n at i d e a l and imperfect s i t e s , indicate,:; that the energetics of d i s s o l u t i o n are s i g n i f i c a n t l y modified by the imperfections. M i t c h e l l & others (1971) suggested that, further work to c l a r i f y the e f f e c t of c r y s t a l defects on the d i s s o l u t i o n rate of ASA i s necessary. In t h i s work, c r y s t a l s of ASA were d e l i b e r a t e l y prepared so as to contain SA i n d i f f e r e n t amounts (Table 4 - 2). The SA content of these c r y s t a l s a f t e r thorough washing with solvent ether ranged from 0.01 to 3.87%. I I . Melting Point of A c e t y l s a l i c y l i c Acid C r y s t a l s . The term melting point describes the temperature at which the s o l i d and l i q u i d forms of a substance are i n equilibrium. A pure substance melts sharply and completely over a narrow temperature range, while a mixture or a compound containing impurities melts gradually over a wide temperature range. A c e t y l s a l i c y l i c acid has a long h i s t o r y of giving trouble i n melting point determinations (Hayman & others, 1925). The reported values range from 100 (S ummers & others, 1970) to 144 (Tawashi, 1968).. This i s not a normal phenomenon for a pure monotropic substance. For more d e t a i l s on the melting point of ASA see 2 - 6 . In t h i s work, the melting point of ASA was determined using both hot-stage and c a p i l l a r y tube methods, under constant conditions, i . e . , a s t a r t i n g temperature of 2° below the estimated melting point (determined i n a preliminary experiment using a heating rate of 10°/minute) and heating rate of 0.2°/minute. As Table 4 - 2 shows, c r y s t a l s with d i f f e r e n t s i z e s , habits and SA content were examined. The melting points of a l l the samples were between 128.1° and 132.7° (excluding spherulites) where the melting point i s regarded as the temperature at which the c r y s t a l s melt completely. Tawashi's polymorphs (1968) (see 2 - 6 ) melted at 143-144° (form I, hexagonal plates) and 123-124° (form I I , n e e d l e - l i k e ) . No information was given regarding the s t a r t i n g temperature and the rate of heating. Schwartzman (1972) reported melting points of 126-137° for the plates (Tawashi's form I) and 127-133° for the needle-like c r y s t a l s (Tawashi's form II) using hot-stage apparatus. His report was also without any information about the s t a r t i n g temperature and the rate of heating. In our work hexagonal plates from 95% ethanol (Fig. 4 - 5 ) (Tawashi's form I) and needle-like c r y s t a l s from n-hexane (Fig. 4 - 4 ) (Tawashi's form II) melted at 130.3° and 130.1° r e s p e c t i v e l y under the experimental conditions described above. Even though the melting points of these two d i f f e r e n t c r y s t a l s were quite close, the needle-like c r y s t a l s melted over a very broad range (123.9 - 130.1°) when heating was started at 121°, while the plates were observed to melt over the range 128.8 - 130.9° under the same conditions. Borka (1972) showed that, the melting point of ASA depends on the rate of heating and the s t a r t i n g temperature. He constructed a phase diagram f o r the binary system of ASA and SA which showed that approximately 17 or 72% of SA i s required to y i e l d a melting point of 123-125° which i s the melting point of Tawashi's form I I . Since the SA content of the form II i s i n s u f f i c i e n t to depress the melting point to 124°, Borka argued that the depression of the melting point i s not due to the presence of SA impurities but i s evidence for the existence of polymorphic form I I . However, Borka was dealing with e u t e c t i c mixture. Mulley & others (1971), however, had suggested previously that, i n addition to the e f f e c t s of separate c r y s t a l s of SA, v a r i a t i o n i n the amount, l o c a t i o n and bonding of SA within the ASA c r y s t a l could influence the melting point. P f i e f f e r (1971) pointed out that differences i n s i z e and habit might a f f e c t the determination of melting point and heat of fusion through differences i n rates of sublimation and decomposition. In t h i s work, when heating was started at a low temperature, e.g., 100°, observation under the hot-stage apparatus showed that very small unaggregated p a r t i c l e s s t a r t to melt at a temperature between 103° and 112°. This process occurred at a very slow rate and was followed by a f a s t and sharp melting at a temperature between 128° and 132°, which corresponds to that obtained using the c a p i l l a r y tube method. I t i s suggested that small p a r t i c l e s of ASA c r y s t a l s are more susceptible to decomp-o s i t i o n during the heating process than the larger p a r t i c l e s and form a mixture of ASA and SA which melts over the temperature range of 103 - 128°. In order to confirm t h i s hypothesis, the true melting range of three d i f f e r e n t samples of ASA was measured ( s t a r t i n g temperature 100° and heating rate of 2°/min.) This range was considered to be the range i n which c r y s t a l s melt sharply and completely. The SA content was measured at the lower l i m i t of the melting range and i n the melt. As shown i n Table 4 - 1 , the SA content of needles and spherulites 86 Table 4 - 1 . S a l i c y l i c acid content of d i f f e r e n t samples of a c e t y l s a l i c y l i c acid c r y s t a l s during the melting process. Sample Approximate % I n i t i a l s i z e range s a l i c y l i c (/O acid W/W % s a l i c y l i c acid formed melting range* . I 1 / T T ° i n the heating period W/W ° c — lower l i m i t upper l i m i t Commercial 800-1000 0.10 130.1-131.2 2.50 8.80 Needle-like 100-500 0.92 125.2-130.1 4.80 11.40 Spherulites 0.10 127.7-130.7 5.12 12/90 * The range i n which c r y s t a l s melt sharply and completely. The true melting point i s the point at which a l l the c r y s t a l s melt completely (upper l i m i t of the melting range). of ASA was considerably higher than that of commercial ASA. These three samples of ASA were chosen i n t h i s experiment since they are d i f f e r e n t i n the habit and s i z e . Crystals of commercial ASA were prisms which were f a i r l y uniform i n a l l of t h e i r dimensions (800 - 1000^1) while the needle-l i k e c r y s t a l s were very t h i n i n one dimension (5 - 8 ) and very long i n the other (100 - 500^*); spherulites were i n a very t h i n c r y s t a l l i n e layer. This experiment revealed that the melting range pf ASA depends i n d i r e c t l y on the s i z e and habit of the sample through the d i f f e r e n c e i n s u s c e p t i b i l i t y to decomposition. Although the lower l i m i t s of the melting ranges are v a r i a b l e from one sample to another, the higher l i m i t s are i n good agreement; therefore i t i s recommended that, i n the melting point study of ASA, the temperature at which a l l c r y s t a l s melt completely be considered as the true melting point. The melting of small p a r t i c l e s of ASA at temperatures between 103 - 128° cannot be detected by means of the Metier FP1 Capillary-tube apparatus. The c a p i l l a r y tubes are scanned by a l i g h t source which i s s e n s i t i v e to changes i n the l i g h t transparency of the samples which occurs during the melting process. Apparently the amount of sample melted and/or becoming transparent at low temperatures does not permit the passage of s u f f i c i e n t l i g h t to operate the automatic i n d i c a t o r and ind i c a t e the melting point. However, the melting range of 128.1 - 132.7° was always f a i r l y constant. This range i s i n agreement with that of 130.9° reported by Borka (1972) using a Metier FP1 and s t a r t i n g temperature of 130° and a heating rate of 0.2°/minute. Tawashi (1971) showed that ASA spherulites underwent a trans-formation to needle-like c r y s t a l s at 124° and melted at 125° (see 2 - 6). Borka (1972) however was not able to reproduce t h i s . In t h i s work, the melting point of spherulites was not always constant and varied from batch to batch. They melted between 126.5 & 131.4° and underwent a thermal transformation to prismatic c r y s t a l s ( F i g . 4 - 8) at a temperature between 104° to 128° rather than 124° reported by Tawashi (.1970) As described before (see 4 - 4 ) the X-ray pattern of the s p h e r u l i t e s , transformed spherulites and the ASA c o n t r o l were i d e n t i c a l . Therefore the thermal transformation phenomenon i s not a polymorphic phase transformation and can be explained i n terms of c r y s t a l growth or c r y s t a l ripening. Spherulites are an unstable form of c r y s t a l which grow from a t h i n f i l m of s o l u t i o n . Tawashi (1971) considered them to be a mesomorphic phase. Buckley (1951) described spherulites as c r y s t a l l i n e i n the sense that, at c e r t a i n places, e s p e c i a l l y the extremities, they are usually bounded by plane faces. So f a r as t h e i r p h y s i c a l properties can be ascertained, each small portion i s s i m i l a r to a c r y s t a l , and they i n v a r i a b l y give X-ray d i f f r a c t i o n patterns. They are, i n f a c t , c r y s t a l s which have run r i o t under the p e c u l i a r condition of growth. Shaftal (1968) demonstrated that the si n g l e c r y s t a l d i f f r a c t i o n pattern given by the thinnest and smallest c r y s t a l s show that a spherulite arises from f o l d i n g i n a s i n g l e - c r y s t a l p l a t e l e t . P f e i f f e r (1971) pointed out that exposure to heat, ultrasound, solvent, etc., could cause c r y s t a l s to anneal, grow or ripen; they would thus mimic poly-morphic behaviour by r e v e r t i n g to a "more stable" form but would not undergo changes i n t h e i r r o u t i n e l y determined X-ray d i f f r a c t i o n properties. Therefore i t i s suggested that, thermal transformation of ASA spherulites to prismatic c r y s t a l s i s a growth process which occurs when some of the thinner portions of a s p h e r u l i t e melt, and the melt flows towards the neighboring portions of the s p h e r u l i t e where i t r e c r y s t a l l i z e s to b u i l d the prismatic c r y s t a l s of ASA. I I I . Solution-Phase Transformation of A c e t y l s a l i c y l i c Acid C r y s t a l s . An easy way to determine which of two forms of a compound i s stable at a given temperature i s to observe the r e l a t i o n s o l u b i l i t y of the two i n a solvent. This i s best and most ra p i d l y done by observing c r y s t a l s of both together i n a drop of saturated s o l u t i o n under the microscope. The less soluble form w i l l grow and the more soluble w i l l d i s s o l v e . This i s c a l l e d a s o l u t i o n phase transformation. Borka (1972) applied a method s i m i l a r to that mentioned to detect the s o l u t i o n phase transformation of the ASA spherulites (see 2 - 6). He allowed a drop of saturated s o l u t i o n of ASA i n isoamyl alcohol to flow under a cover s l i p placed over spherulites of ASA grown on a microscope s l i d e . Spherulites changed to prismatic c r y s t a l s of ASA. He did not mention the time required for t h i s change to take place. He suggested that t h i s was strong evidence for the existence of the form II polymorph of ASA described by Tawashi (1968). This experiment was repeated i n t h i s work and the same phenomenon was observed.(Fig. 4 - 9 ) . However, no differences were noted i n the X-ray d i f f r a c t i o n patterns of the spherulites and needle-like prisms. Hence t h i s phenomenon cannot be due to a polymorphic phase transformation. The spherulites changed into the prisms a f t e r two to f i v e minutes. In this period of time, evaporation takes place. Hence the concentration of the saturated s o l u t i o n increases and the s o l u t i o n becomes supersaturated. I t i s suggested that under these conditions the small c r y s t a l l i n e portions of a spherulite w i l l act as seeds and normal c r y s t a l s with w e l l defined faces are b u i l t up. Therefore, the s o l u t i o n phase trans-formation described by Borka (1972), l i k e thermal transformation, i s a growth process rather than a reversion from a metastable to a stable polymorphic form. 90 Fi g . 4-9. Solution transformed c r y s t a l s from spherulites (X100). IV. X-Ray D i f f r a c t i o n of A c e t y l s a l i c y l i c Acid C r y s t a l s . The most conclusive proof of polymorphism i s a diffe r e n c e i n the X-ray d i f f r a c t i o n pattern of the various forms. Each c r y s t a l l i n e form has i t s own c h a r a c t e r i s t i c X-ray d i f f r a c t i o n pattern. The X-ray d i f f r a c t i o n pattern of the ASA c o n t r o l shown i n F i g . 4 - 10, agrees with that reported by Wheatley (1964). It shows s i x sharp and many smaller peaks which are located between 7.8° and 44° 2$. Hence ASA control belongs to the monoclinic system with the same c r y s t a l structure as described by Wheatley. Diffractograms were prepared from c r y s t a l s and the compressed disk of a l l the samples shown i n Table 4 - 2 (in the case of octahedral pyramids and transformed spherulites only powder was examined) and also samples of ground ASA control with higher d i s s o l u t i o n rate £ see 4 - 3 I I I , a, (ii)j and compared with that of the ASA c o n t r o l . A l l the patterns were i d e n t i c a l . The absence of d i s t i n c t differences i n the X-ray diffractograms i s c l e a r evidence that ASA does not e x i s t i n polymorphic forms. In p a r t i c u l a r , the suggestion of Borka (1972) that spherulites correspond to the form II polymorph described by Tawashi (1968) cannot be supported. Figure 4 - 10. X-ray d i f f r a c t i o n pattern of a c e t y l s a l i c y l i c acid. Table 4 - 2 . RECRYSTALLIZATION CONDITIONS AND PROPERTIES OF ACETYLSALICYLIC ACID CRYSTALS. Crystals Approximate* S a l i c y l i c Melting Point° D i s s o l u t i o n Rate s i z e range0/0 acid % Hot-stage C a p i l l a r y mg.i cnr^min--'-. 800 - 1000 0.10 130.0 130.7 1.18 Hexagonal prisms from ethanol (Ac 1.1% w/w at 24.8 ); c i r c u l a t o r y c r y s t a l l i z e r 200 — 300 0.14 130.2 130.8 1.17 Hexagonal prisms from ethanol (Ac 2.5% w/w at 23.5 ); c i r c u l a t o r y c r y s t a l l i z e r 200 _ 300 0.14 130.4 129.9 1.19 Hexagonal prisms from ethanol (fast 150 _ 200 0.01 132.7 132.7 1.19 Hexagonal prisms from ethanol (slow 300 _ 400 0.10 132.0 131.8 1.18 Hexagonal prisms from 95% ethanol . . 300 - 400 0.12 131.8 131.8 1.18 Needle-like c r y s t a l s from n-hexane 100 - 500 0.92 130.1 - 11.21 Hexagonal plates from 95% ethanol (slow re-c r y s t a l l i z a t i o n at room temperature) 1000 _ 2000 1.40 130.3 131.2 1.19 M i c r o c r y s t a l l i n e hexagonal plates from g l y c e r i n 80 - 100 3.87 129.1 - 1.21 Hexagonal plates r e c r y s t a l l i z e d at 3° from saturated s o l u t i o n i n 0.1 N'HC1 1300 _ 1600 1.20 131.7 131.1 1.19 Octahedral pyramids r e c r y s t a l l i z e d at 3° from a s o l u t i o n of 0.38 g./lOO ml. of 0.1 N HC1 800 _ 2500 0.83 128.1 128.3 Spherulites from 95% ethanol 20 - 40 0.10 126.5-131.4 126.5-130.8 1.18 Prisms from spherulites (saturated s o l u t i o n added) . . . . 5 _ 50 0.12 128.1 130.8 _ Prisms from spherulites (after heating) .. 5 - 50 12.50 126.5-131.4 129.8 -* The longest c r y s t a l a x i s . ** A c = (Ct - Co) when&cis a measure of the degree of supersaturation, Ct i s the % a c e t y l s a l i c y l i c a c i d i n s o l u t i o n and Co i s the equilibrium s o l u b i l i t y of a c e t y l s a l i c y l i c acid at the same temperature. ,^ CO 4 - 3 . D i s s o l u t i o n of A c e t y l s a l i c y l i c Acid. I. I n t r i n s i c D i s s o l u t i o n Rate. A t y p i c a l concentration-time curve for the d i s s o l u t i o n of ASA up to 74% of the s o l u b i l i t y of ASA i n 0.1 N HC1 i s shown i n F i g . 4 - 1 1 . The d i s s o l u t i o n curve was used to construct a f i n i t e differences diagram (Fig. 4 - 12), from which the saturated concentration, Co, was estimated as described by Nog ami & others (1966). The equilibrium s o l u b i l i t y , Co, determined from the bulk d i s s o l u t i o n experiment was 5.7 mg./ml. while Co estimated k i n e t i c a l l y from the f i n i t e differences diagram was 5.5 mg./ml.' The values calculated by G r i f f i t h s & M i t c h e l l (1971) were 5.7 and 5.8 mg./ml. for the bulk d i s s o l u t i o n and f i n i t e differences methods r e s p e c t i v e l y . F i g . 4 - 1 3 shows a concentration-time curve for the d i s s o l u t i o n of ASA control over the i n i t i a l time period at 37° and 300 r.p.m. i n 0.1 N HC1 s o l u t i o n . The rate constants for the transport controlled process, kt and the o v e r a l l d i s s o l u t i o n rate constant, FCj., were calculated using equation (21) which i s developed from the Noyes-Nenst equation |f = k t (Co-Ct) = | K t (Co-Ct) = | | (Co-Ct) (21) where V i s the volume of the s o l u t i o n and the other terms have been defined i n equation (2) (see 2 - 5 , I I ) . Ct, the concentration at time t, can be neglected over the short time i n t e r v a l of the experiment. The rate constant for the transport c o n t r o l l e d process calculated from equation (10) f = k t C o <10> was 1.50 x 10 2 min "'" at 37° and 300 r.p.m. using the equilibrium s o l u b i l i t y obtained from the bulk d i s s o l u t i o n experiment (5.7 mg./ml.). The o v e r a l l d i s s o l u t i o n rate constant calculated from equation (22) = (22) was 2.12x10-3 c m m i n - l _ ' 1.25 Time (min.) Figure 4 - 11. Rotating disk d i s s o l u t i o n of a c e t y l s a l i c y l i c a c i d i n 0.1 N HC1 at 37 and 300 r.p.m. 96 tod Csl U C 1 (mg./ml.) Figure 4-12. F i n i t e differences diagram f o r the d i s s o l u t i o n of a c e t y l -s a l i c y l i c acid i n 0.1 N HC1 at 37° and 300 r.p.m. C-p concentration at time t-^; C^, concentration at time t£ = t^+ 10 mins.; -0-, C2 VS. C-^  and, , C-^  = C2. 2 5 0 h 0 5 10 15 20 2 5 Time (min.) Figure 4 - 13. I n i t i a l d i s s o l u t i o n curve of a c e t y l s a l i c y l i c acid i n 0.1 N.HC1. at 37° and 300 r.p.m. I I . Bulk D i s s o l u t i o n . The concentration of ASA i n the bulk increased r a p i d l y and reached the equilibrium s o l u b i l i t y as shown i n F i g . 4 - 1 4 . G r i f f i t h s & M i t c h e l l (1971) reported an i n i t i a l peak before the equilibrium s o l u b i l i t y was reached. I t was suggested that, t h i s was due to an abrupt transfer of solute from the large surface area of a more stable form. The bulk l i q u i d became supersaturated with respect to the more stable form which c r y s t a l l i z e d out on the surface of the s o l i d . In t h i s work no i n i t i a l peak was observed during the bulk d i s s o l u t i o n experiment. In order to study the possible e f f e c t of the presence of spherulites on-the d i s s o l u t i o n behaviour of ASA c r y s t a l s (see 4 - 2 ) a mixture of spherulites and ASA c r y s t a l s (the same ASA as i n the above experiment) was prepared and the bulk d i s s o l u t i o n was studied. Again no i n i t i a l peak was observed. I I I . E f f e c t of Various Factors on D i s s o l u t i o n Rate of A c e t y l s a l i c y l i c Acid, a. P a r t i c l e Size. ( i ) E f f e c t of P a r t i c l e Size Using Unground C r y s t a l s : No s i g n i f i c a n t differences i n d i s s o l u t i o n rate were found using commercial ASA* sieved i n t o d i f f e r e n t p a r t i c l e sizes (Table 4 - 3 ) nor when using c r y s t a l s which were grown i n d i f f e r e n t sizes (Table r;4 - 2). Hence the r e s u l t s obtained using the r o t a t i n g disk method confirm the previous findings of M i t c h e l l & other (1971) that, the d i s s o l u t i o n rate i s independent of p a r t i c l e s i z e . * B.D.H., Canada, Lot 27648 Figure 4 - 14. Bulk d i s s o l u t i o n of a c e t y l s a l i c y l i c acid i n 0.1 N. HC1. O.at 37° and 500 r.p.m.; A,at 37° and 300 r.p.m.; O, at 30 and 300 r.p.m. 100 ( i i ) P a r t i c l e Size A f t e r Grinding: Disks prepared a f t e r grinding and sieving ASA* into various s i z e ranges showed no differences i n d i s s o l u t i o n rate. The d i s s o l u t i o n rate was i n agreement with that of the ASA control (Table 4 - 3). Some powder of ASA which was ground by means of a pestle and mortar showed a remarkable ra i s e on d i s s o l u t i o n rate of ASA (DR = 1.55 mg. -2 -1 cm min ). Despite four reproducible determinations, further attempts to confirm t h i s unusual behaviour were unsuccessful. Therefore, p a r t i c l e s i z e and grinding do not a f f e c t the i n t r i n s i c d i s s o l u t i o n of ASA. This i s important since P f e i f f e r (1971) suggested differences i n s i z e might a f f e c t the d i s s o l u t i o n rate from a compressed disk through differences i n c a p i l l a r i t y or wetting, and Summers & others (1970) reported a transformation of form III to form II i n the process of grinding. b. S a l i c y l i c Acid. Since the SA content of ASA c r y s t a l s a f f e c t s the melting point of t h i s compound s i g n i f i c a n t l y (Mulley & others, 1971; Borka, 1972) i t seemed necessary to study the e f f e c t of the SA content on the i n t r i n s i c d i s s o l u t i o n rate of ASA. Impurities l i k e SA may be incorporated into the c r y s t a l l a t t i c e as a s u b s t i t u t i o n a l defect or as a s o l i d s o l u t i o n leading to d i s t o r t i o n of the host l a t t i c e (Khamskii, 1969). The most l i k e l y impurity i n the case of ASA i s SA which may be adsorbed onto the i n t e r n a l and external surface of the ASA c r y s t a l s or be present as separate c r y s t a l s . The SA content of the ASA c r y s t a l s produced i n t h i s work, af t e r thorough washing with solvent ether ranged from 0.01 to 3.87% but as shown i n Table 4 - 2 was without any e f f e c t on the i n t r i n s i c d i s s o l u t i o n rate of ASA. * B.D.H., Canada, Lot 27648. Table 4 - 3 . I n t r i n s i c d i s s o l u t i o n rate of a c e t y l s a l i c y l i c acid with d i f f e r e n t p a r t i c l e s i z e at 37° and 300 r.p.m. -2 -1 Mesh Size D i s s o l u t i o n Rate (mg.cm .min ) (see 3 - 3 , VII, a ( i ) ) ground unground 25 1.19 1.18 35 - 1.18 45 1.20 1.17 60 1.18 102 c. Supersaturation. M i t c h e l l & others (1971) speculated that the anomalous behavior of ASA may r e s u l t from differences i n the number and type of c r y s t a l imperfections. C r y s t a l imperfections are present i n a l l c r y s t a l s and are thermodynamically unstable. Thomas (1970) has emphasized the important, often cr u c i a l , r o l e played by d i s l o c a t i o n s i n the r e a c t i v i t y of c r y s t a l l i n e s o l i d s including such properties as c r y s t a l growth and d i s s o l u t i o n . V a r i a t i o n i n the conditions of c r y s t a l growth w i l l a f f e c t the type and number of defects i n a c r y s t a l . Glasby & Ridgway (1968) have shown, under the condition of t h e i r experiment, that incorporation of molecules into the c r y s t a l l a t t i c e i s the rate l i m i t i n g step i n the growth of ASA i n a fl a i d i z e d bed c r y s t a l l i z e r . When incorporation i s rapid i t i s reasonable to expect that a large number of imperfections w i l l be b u i l t into a c r y s t a l . By growing c r y s t a l s under d i f f e r e n t conditions of temperature and degree of saturation, i t i s possible to evaluate the e f f e c t s of growth rate, and hence i n d i r e c t l y v a r i a t i o n i n the number of imperfections, on the i n t r i n s i c d i s s o l u t i o n and other properties. As mentioned i n the experimental part of t h i s work, ASA was r e c r y s t a l l i z e d under a v a r i e t y of conditions. Two batches of c r y s t a l s were grown i n the c i r c u l a t o r y c r y s t a l l i z e r at d i f f e r e n t growth rates by varying the temperature and degree of supersaturation under co n t r o l l e d conditions (4C 1.1% w/w at 24.8° and&C 2.5% w/w at 23.5°). Also c r y s t a l s were grown from saturated solutions of ASA i n d i f f e r e n t solvents and d i f f e r e n t temperatures. Growth rates/were c o n t r o l l e d i n an approximate manner by changing the rate of cooling. The nucleation points of ASA i n absolute ethanol at a given 103 concentration l i e approximately 3.5° below the equilibrium s o l u b i l i t y ( F i g . 4 - 1 ) . The metastable region i s therefore very narrow and c r y s t a l growth under conditions i n which there was precise c o n t r o l of the degree of supersaturation was d i f f i c u l t to achieve. Nevertheless, no s i g n i f i c a n t differences were noted i n the d i s s o l u t i o n rates of c r y s t a l s grown i n absolute ethanol under co n t r o l l e d conditions of temperature and super-saturation arid ASA r e c r y s t a l l i z e d from other solutions under a va r i e t y of conditions. Although no attempt was made to measure the actual growth rates or count the number of imperfections, i t i s apparent that differences i n growth rate are without e f f e c t on subsequent d i s s o l u t i o n rates. d. Habit. The d i s s o l u t i o n rate of ASA with d i f f e r e n t habits, i . e . t e t r a -gonal prisms, hexagonal prisms and hexagonal plates (Fig. 4 - 3 ) were examined (Table 4 - 2 ) . Crystals were compressed without grinding and the i n t r i n s i c d i s s o l u t i o n rates were determined. No s i g n i f i c a n t differences were noted. This'.is important since Wood (personal communication through M i t c h e l l & others, 1971) pointed out that f a i l u r e to achieve a r e q u i s i t e degree of s i z e reduction may be responsible f o r the v a r i a t i o n i n d i s s o l u t i o n rates of compressed disks prepared from c r y s t a l s of d i f f e r e n t habit. e. R e c r y s t a l l i z a t i o n Solvent. A c e t y l s a l i c y l i c acid grown from d i f f e r e n t solvents, i . e . absolute ethanol, ethanol 95%, g l y c e r i n and 0.1 HC1, were compressed and the d i s s o l u t i o n rate determined (Table 4 - 2). Again no s i g n i f i c a n t differences were noted. 104 Schwartzman (1972) found ethanol i n the ASA r e c r y s t a l l i z e d from t h i s solvent. I t was not established whether the ethanol was present on the surface of the c r y s t a l s , was trapped i n t e r s t i t i a l l y or was due to hydrogen bonding. It i s concluded that the existence of ethanol i n c r y s t a l s of ASA does not a f f e c t the d i s s o l u t i o n rate. f. D i s s o l u t i o n Rate of Spherulites and a Mixture of Spherulites and Prismatic C r y s t a l s . Tawashi (1971) postulated that, the reported v a r i a t i o n i n thermal and d i s s o l u t i o n behavior of ASA could be due to the presence of s p h e r u l i t es on the surface of commercial ASA c r y s t a l s . Borka (1972) showed that, spherulites cannot grow on the surface of c r y s t a l s . He pointed out that, preformed c r y s t a l s dominate and d i r e c t a d d i t i o n a l c r y s t a l growth. This i s the basis of a l l c r y s t a l seeding. In t h i s work, spherulites were added d e l i b e r a t e l y to prismatic, c r y s t a l s (see 3 - 3 , VII, g) but no s i g n i f i c a n t differences were noted i n the subsequent d i s s o l u t i o n - 2 - 1 - 2 - 1 rates of the mixture (1.19 mg. cm min ), spherulites (1.18 mg. cm min ) and the ASA c o n t r o l . 105 5. Summary and Conclusions. I. A c e t y l s a l i c y l i c acid r e c r y s t a l l i z e s i n d i f f e r e n t habits a l l of which belong to the monoclinic system. I I . Spherulites of ASA exhibit the same X-ray d i f f r a c t i o n patterns as other forms of ASA. I I I . The existence of spherulites i s not an unusual phenomenon. Spherulites of other low molecular weight organic compounds, i . e . , phen-acet i n , acetanilidv. , v a n i l l i n and benzocaine, were grown i n t h i s work. IV. D i f f e r e n t methods of r e c r y s t a l l i z a t i o n give ASA c r y s t a l s with d i f f e r e n t amounts of SA. V. The melting point of ASA depends on the experimental conditions. A c e t y l s a l i c y l i c acid melts between 128.3 and 132.7° (excluding spherulites) using a s t a r t i n g temperature 2° below the estimated melting point and a heating rate of 0.2°/min. VI. Very small unaggregated p a r t i c l e s of ASA s t a r t to melt at a temperature between 103 and 112° at a very slow rate. Melting continues with increased temperature up to 128° when fast and sharp melting of a l l the remaining c r y s t a l s occurs between 128 and 132°. This range i s always f a i r l y constant. The s u s c e p t i b i l i t y of ASA towards thermal decomposition becomes greater with decrease i n p a r t i c l e s i z e and the depression of the melting point of i n d i v i d u a l c r y s t a l s i s re l a t e d to the amount of SA formed. VII.Spherulites of ASA melt between 126.5 and 131.4° and produce more SA i n the process of melting than the o r i g i n a l commercial ASA. VIII.The thermal and s o l u t i o n transformation observed with 106 spherulites are processes of c r y s t a l growth. IX. No surface transformation was noted during the bulk d i s s o l u t i o n process. X. The i n t r i n s i c d i s s o l u t i o n rate of r e c r y s t a l l i z e d ASA i s independent of the conditions of r e c r y s t a l l i z a t i o n , the SA content, at least up to 3.87%, habit and s i z e of the c r y s t a l s . XI. No s i g n i f i c a n t differences were found i n the i n t r i n s i c d i s s o l u t i o n rate of any commercial or r e c r y s t a l l i z e d ASA. XII. The low i n t r i n s i c d i s s o l u t i o n rate of a free flow-granular ASA reported by M i t c h e l l & S a v i l l e (1967) was confirmed using the same sample. Other properties of t h i s sample were i d e n t i c a l . I t i s suggested that the low d i s s o l u t i o n rate may be due to the e f f e c t of granulating agents used i n the manufacturing process. XIII. I t i s suggested that, the observed difference between the two forms of ASA reported by Tawashi (1968) i s simply the r e s u l t of an attempt to compare the d i s s o l u t i o n rates from d i f f e r e n t faces. XIV. The differences i n g a s t r o - i n t e s t i n a l absorption of d i f f e r e n t c r y s t a l l i n e forms when given to human subjects as a dispersion i n water (Tawashi, 1969) i s most l i k e l y due to the 'differences i n c r y s t a l habit which occurs when ASA i s r e c r y s t a l l i z e d from ethanol and n-hexane. The c r y s t a l grown from ethanol i s a tabular c r y s t a l while the one from n-hexane i s needle-like. Hence, even though t h e i r longest dimensions may be the same s i z e , the differences i n other dimensions of the c r y s t a l w i l l lead to s i g n i f i c a n t differences i n d i s s o l u t i o n rates and therefore absorption rates. 107 6. REFERENCES. 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