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Behavior o f Water o n the Surface o f K a o l i n M i n e r a l s R. F. Giese, Jr., and P. M. Costanzo Department of Geological Sciences, State University of New York at Buffalo, 4240 Ridge Lea Road, Amherst, NY 14226 Study of hydrated kaolinites shows that water molecules adsorbed on a phyllosilicate surface occupy two different structural sites. One type of water, "hole" water, is keyed into the ditrigonal holes of the silicate layer, while the other type of water, "associated" water, is situated between and is hydrogen bonded to the hole water molecules. In contrast, hole water is hydrogen bonded to the silicate layer and is less mobile than associated water. At low temperatures, all water molecules form an ordered structure reminiscent of ice; as the temperature increases, the associated water disorders progressively, culminating in a rapid change in heat capacity near 270 K. To the extent that the kaolinite surfaces resemble other silicate surfaces, hydrated kaolinites are useful models for water adsorbed on silicate minerals. To a large extent, the study of terrestrial geology is the study of the interaction of water and rock materials. Much of the modification of the earth s surface, involving chemical weathering, transport, and deposition of sediment, results from the contact of water, often containing reactive chemical species, with the surfaces of mineral grains. The majority of the chemical activity, as far as we presently know, takes place on a microscopic scale; at the interface between a mineral and the first, or perhaps the first few, layers of adsorbed water molecules. Such interfacial regions have complex physical chemical properties, often very different from the phases which they separate. This is compounded, in the case of the watersilicate interface, by the fact that the structure of bulk water is very complex itself, and, while we know in general terms the crystal structures of the major silicate minerals, we often do not have a clear picture of the structure of the mineral surface, nor do we know in detail the structure of disordered minerals, at least not on an atomic scale. The amount of water in the interfacial region is very small compared to the bulk water in the system. For many experimental 1
0097-6156/ 86/ 0323-0037S06.00/ 0 © 1986 American Chemical Society
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t e c h n i q u e s , t h i s means t h a t the s i g n a l from the s u r f a c e water i s too weak t o be e a s i l y s e p a r a t e d from t h a t o f the b u l k w a t e r . One s o l u t i o n t o t h i s problem i s t o s t u d y m a t e r i a l s w i t h l a r g e s p e c i f i c s u r face areas. T r a d i t i o n a l l y , t h e s e have been c l a y m i n e r a l s , z e o l i t e s , and g e l s . Of t h e s e , c l a y m i n e r a l s a r e c o n c e p t u a l l y the s i m p l e s t because they p r e s e n t two d i m e n s i o n a l p l a n a r s u r f a c e s t o the e x t e r n a l environment. T h i s paper i s d i v i d e d i n t o two p a r t s ; an i n t r o d u c t i o n t o the p r o p e r t i e s o f water a t the m i n e r a l - w a t e r i n t e r f a c e , followed by a d e s c r i p t i o n o f the work done i n our l a b o r a t o r y on the s t r u c t u r e o f water i n the i n t e r l a y e r r e g i o n o f k a o l i n i t e w i t h p a r t i c u l a r r e f e r e n c e t o the s t r u c t u r e and p r o p e r t i e s o f water a t the i n t e r f a c e . Clay Mineral
Structures
The b a s i c s t r u c t u r e s o f the c l a y m i n e r a l s were d e s c r i b e d by P a u l i n g (J_, 2) and i l l u s t r a t i o n s o f each type c a n be found i n the t e x t o f Grim Q ) . These s t r u c t u r e models are based on r e g u l a r t e t r a h e d r a and o c t a h e d r a formed by oxygen o r h y d r o x y l g r o u p s , s y m m e t r i c a l l y d i s p o s e d i n p l a n a r l a y e r s . The models a r e e s t h e t i c a l l y p l e a s i n g , but a c t u a l m i n e r a l s a r e f a r more complex (see (4.) f o r a r e c e n t summary). The two t y p e s o f c l a y m i n e r a l s t r u c t u r e s which a r e o f i n t e r e s t i n the p r e s e n t d i s c u s s i o n a r e the expanding 2:1 s t r u c t u r e s ( t h e s m e c t i t e s and v e r m i c u l i t e s ) and the 1:1 s t r u c t u r e s ( t h e k a o l i n s ) . The s m e c t i t e s and v e r m i c u l i t e s have a fundamental l a y e r made up o f two s h e e t s o f t e t r a h e d r a which i n c o r p o r a t e s m a l l , h i g h l y charged c a t i o n s and one s h e e t o f o c t a h e d r a c o o r d i n a t i n g l a r g e r c a t i o n s . The o c t a h e d r a share e d g e s , the t e t r a h e d r a share c o r n e r s , and the t h r e e s h e e t s s h a r e oxygens i n common p l a n e s t o form the 2:1 l a y e r ( F i g u r e 1A). A s i m i l a r scheme, but i n v o l v i n g o n l y one s h e e t o f t e t r a h e d r a and one o f o c t a h e d r a , produces the 1:1 l a y e r s i l i c a t e s ( F i g u r e 1B) o f which k a o l i n i t e i s perhaps the most i m p o r t a n t m i n e r a l . I n the s m e c t i t e s and v e r m i c u l i t e s , s u b s t i t u t i o n o f d i f f e r e n t l y charged i o n s i s common. These may i n v o l v e aluminum f o r s i l i c o n i n the t e t r a h e d r a l s i t e s , and, i n the o c t a h e d r a l s i t e s , f e r r o u s o r f e r r i c i r o n f o r aluminum, magnesium f o r aluminum, o r l i t h i u m f o r magnesium; i n a d d i t i o n , v a c a n t s i t e s a r e commonly f o u n d . The s u b s t i t u t i o n s c r e a t e a charge imbalance which i s n e u t r a l i z e d by the a d s o r p t i o n o f c a t i o n s on the e x t e r n a l and i n t e r n a l s u r f a c e s o f the crystals. These compensating c a t i o n s a r e n o t f i r m l y a t t a c h e d t o the c l a y s u r f a c e s and they c a n be exchanged by t r e a t m e n t w i t h d i l u t e s o l u t i o n s of appropriate s a l t s . When exposed t o water and many o r g a n i c m o l e c u l e s , the l a y e r s o f t h e s e m i n e r a l s s e p a r a t e a l l o w i n g the g u e s t m o l e c u l e s t o e n t e r between the l a y e r s . Thus, b o t h s u r f a c e s o f e v e r y l a y e r o f the c l a y c r y s t a l become e q u i v a l e n t t o e x t e r n a l s u r f a c e s and the t o t a l s u r f a c e a r e a i n c r e a s e s t o as much as 800 m /g i n the c a s e o f the s m e c t i t e s . The 1:1 k a o l i n s t r u c t u r e s a r e c h e m i c a l l y s i m p l e r ; the t e t r a h e d r a l s i t e s a r e o c c u p i e d by s i l i c o n and the o c t a h e d r a l s i t e s by aluminum. There i s a minor amount o f s u b s t i t u t i o n , l a r g e l y o f f e r r i c i r o n f o r aluminum, but the amounts a r e g e n e r a l l y o n l y a few t e n t h s o f a p e r c e n t by weight o f o x i d e . The k a o l i n m i n e r a l s do n o t expand i n the p r e s e n c e o f water and t h e i r s u r f a c e a r e a , a p p r o x i m a t e l y 10 t o 15 m / g , r e p r e s e n t s the e x t e r n a l a r e a o f the c r y s t a l s . Because o f the d i f f e r e n c e between the 2:1 and 1:1 s t r u c t u r e s , t h e i r e x t e r n a l and i n t e r n a l s u r f a c e s a r e f u n d a m e n t a l l y d i f f e r e n t .
3.
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A
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Β
F i g u r e 1. P o l y h e d r a l r e p r e s e n t a t i o n s o f the l a y e r s i n 2:1 smec t i t e s (A) and 1:1 k a o l i n i t e s ( B ) . Only a s i n g l e l a y e r i s shown i n each. The view i s down t h e [041] a x i s o f the u n i t c e l l .
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G E O C H E M I C A L P R O C E S S E S AT M I N E R A L S U R F A C E S
A l l s u r f a c e s o f t h e 2:1 c l a y s ( e x c l u d i n g adsorbed s p e c i e s ) c o n s i s t o f oxygen, w h i l e the 1:1 m i n e r a l s have one s u r f a c e o f each l a y e r formed by oxygen and the o t h e r s u r f a c e formed by h y d r o x y l g r o u p s . Hydration of Clavs. There i s g e n e r a l agreement, and ample e x p e r i m e n t a l and t h e o r e t i c a l e v i d e n c e t o s u p p o r t the p o i n t o f v i e w , t h a t w a t e r m o l e c u l e s i n c o n t a c t w i t h o r i n the c l o s e v i c i n i t y o f c l a y surfaces are perturbed. The d i s t a n c e o v e r which t h e s e p e r t u r b a t i o n s produce measurable changes i n the p r o p e r t i e s o f t h e adsorbed w a t e r m o l e c u l e s i s much more c o n t r o v e r s i a l . Two c o n f l i c t i n g models f o r w a t e r adsorbed on c l a y m i n e r a l s u r f a c e s have been p r o p o s e d ; one s t a t e s t h a t the p e r t u r b a t i o n by the s u r f a c e i s l i m i t e d t o between 3 and 4 water l a y e r s ( r o u g h l y 10A) (5.), w h i l e t h e o t h e r h o l d s t h a t the i n f l u e n c e o f the c l a y s u r f a c e s e x t e n d s much f u r t h e r , up t o 30 o r more water l a y e r s ( r o u g h l y 100A) (6,). E v a l u a t i n g the e v i d e n c e f o r e a c h model i s c o m p l i c a t e d because t h e y a r e based on d i f f e r e n t e x p e r i m e n t a l a p p r o a c h e s which probe t h e c l a y - w a t e r i n t e r a c t i o n s a t d i f f e r e n t time s c a l e s ( £ ) . In a d d i t i o n , i t i s d i f f i c u l t to separate the i n f l u e n c e o f t h e c l a y l a y e r i t s e l f from t h e s t r u c t u r i n g e f f e c t o f t h e exchangeable c a t i o n s adsorbed on o r near t h e s u r f a c e . The f o l l o w i n g d i s c u s s i o n i s n o t meant t o be an e x h a u s t i v e e v a l u a t i o n o f t h e two m o d e l s , but r a t h e r a s e l e c t i v e d i s c u s s i o n o f what i s known about the s t r u c t u r e o f water i n c l o s e c o n t a c t w i t h a c l a y m i n e r a l s u r f a c e and what t h e g e n e r a l arguments a r e i n f a v o r o f o r a g a i n s t t h e s h o r t - r a n g e and l o n g - r a n g e i n t e r a c t i o n m o d e l s . More d e t a i l e d d i s c u s s i o n o f t h e problem c a n be found i n (2.) and (8.). Water on H a l l o v s i t e . C e n t r a l t o t h e c o n t r o v e r s y i s the o b s e r v a t i o n t h a t c l a y c r y s t a l s p r e s e n t a p l a n a r a r r a y o f oxygens (and h y d r o x y l s i n t h e c a s e o f k a o l i n i t e ) w h i c h have hexagonal ( o r n e a r l y ) symmetry w i t h a p e r i o d i c i t y s i m i l a r t o t h a t found i n the c r y s t a l s t r u c t u r e o f i c e . Because o f t h i s g e o m e t r i c s i m i l a r i t y , i t has f r e q u e n t l y been assumed t h a t water adsorbed on a c l a y s u r f a c e w i l l p r e f e r e n t i a l l y adopt an i c e - l i k e c o n f i g u r a t i o n . When l o o k e d a t i n d e t a i l , i t i s d i f f i c u l t to f i n d unequivocal evidence to support t h i s . H a l l o y s i t e - 1 0 A i s f r e q u e n t l y r e f e r e n c e d as an example o f w a t e r m o l e c u l e s w i t h an i c e - l i k e s t r u c t u r e , e p i t a x i a l l y adsorbed o n a c l a y s u r f a c e (2_). I n f a c t , t h i s model has been so a p p e a l i n g t h a t t h e o r i g i n a l f i g u r e i l l u s t r a t i n g t h e s t r u c t u r a l model o f H e n d r i c k s and J e f f e r s o n (5.) has been r e p r o d u c e d i n n u m e r a b l e t i m e s by o t h e r s ( s e e ( £ ) and (10.) f o r e x a m p l e ) . I t i s o f t e n s t a t e d i n the l i t e r a t u r e t h a t the H e n d r i c k s and J e f f e r s o n model i s based on X - r a y d i f f r a c t i o n d a t a , i m p l y i n g t h a t a s t r u c t u r e was proposed and t e s t e d by c o m p a r i son o f o b s e r v e d and c a l c u l a t e d i n t e n s i t i e s . What seems t o have been o v e r l o o k e d i s the f a c t t h a t the o r i g i n a l paper o f H e n d r i c k s and J e f f e r s o n p r e s e n t e d no s u b s t a n t i a l e x p e r i m e n t a l e v i d e n c e t o s u p p o r t t h e i r model o t h e r t h a n the o b s e r v e d i n c r e a s e i n the t h i c k n e s s o f t h e c l a y s t r u c t u r e r e s u l t i n g from h y d r a t i o n . That d i s t a n c e , r o u g h l y 2 . 9 A, does impose some g e o m e t r i c r e s t r i c t i o n on the p o s s i b l e a r r a n g e ment o f the i n t e r l a y e r water m o l e c u l e s , b u t h a r d l y a d e f i n i t i v e o n e . Much subsequent work has shown t h a t i t i s v e r y d i f f i c u l t t o o b t a i n good agreement between o b s e r v e d and c a l c u l a t e d i n t e n s i t i e s even f o r the s i m p l e c a s e o f the o n e - d i m e n s i o n a l s t r u c t u r e a l o n g the £ - a x i s which i n v o l v e s o n l y the & c o o r d i n a t e o f t h e atoms (see (11) for discussion).
3.
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H a l l o y s i t e - 1 0 A r e p r e s e n t s a s t r u c t u r e w i t h few i f any i n t e r l a y e r c a t i o n s , a l l o w i n g one t o i n v e s t i g a t e t h e r e l a t i v e l y s i m p l e c a s e o f water i n t e r a c t i n g w i t h a c l a y s u r f a c e . Similarly, ice-like models have been proposed f o r water adsorbed on s m e c t i t e and v e r m i c u l i t e s u r f a c e s ( £ , J_2, 12.). These r e p r e s e n t c a s e s o f charged c l a y l a y e r s w i t h adsorbed exchangeable c a t i o n s . Water on V e r m i c u l i t e . F o r low water c o n t e n t s ( t h a t i s , one o r two water l a y e r s ) , the e v i d e n c e f o r h i g h l y s t r u c t u r e d water i n t h e i n t e r l a y e r s p a c e s o f s m e c t i t e s and v e r m i c u l i t e s i s most e a s i l y s e e n i n X-ray d i f f r a c t i o n s t r u c t u r e determinations of ordered hydrate s t r u c t u r e s s u c h as the two-water l a y e r h y d r a t e o f C a - v e r m i c u l i t e (11. 15.) and N a - v e r m i c u l i t e (15., 16). I n the C a - v e r m i c u l i t e , the i n t e r l a y e r c a l c i u m i o n s a r e o f two t y p e s ; one w i t h s i x water m o l e c u l e s i n an o c t a h e d r a l arrangement, the o t h e r w i t h e i g h t water m o l e c u l e s a r r a n g e d i n a d i s t o r t e d c u b e . The v e r m i c u l i t e l a y e r s a r e s t a c k e d so t h a t the d i t r i g o n a l h o l e s i n the t e t r a h e d r a l s u r f a c e s a r e o p p o s i t e each o t h e r , as a r e the t e t r a hedral s i t e s . The s i x - c o o r d i n a t e d c a l c i u m i o n s l i e between a d j a c e n t t e t r a h e d r a where t h e y c a n most e f f e c t i v e l y compensate t h e c h a r g e d e f i c i e n c y r e s u l t i n g from t h e aluminum f o r s i l i c o n s u b s t i t u t i o n . In c o n t r a s t , t h e e i g h t c o o r d i n a t e d c a l c i u m i o n s a r e p o s i t i o n e d between the d i t r i g o n a l h o l e s , a p o s i t i o n where t h e y a r e a l s o v e r y c l o s e t o the t e t r a h e d r a l c h a r g e d e f i c i t s , the c a t i o n s i n t h e N a - v e r m i c u l i t e a r e a l l o c t a h e d r a l l y c o o r d i n a t e d by w a t e r m o l e c u l e s and l i e e x c l u s i v e l y between t e t r a h e d r a l oxygens o f t h e a d j a c e n t l a y e r s . Water on S m e c t i t e s . Compared t o v e r m i c u l i t e s , s m e c t i t e s p r e s e n t a more d i f f i c u l t e x p e r i m e n t a l system because o f t h e l a c k o f s t a c k i n g o r d e r o f the l a y e r s . F o r t h e s e m a t e r i a l s , the t r a d i t i o n a l t e c h n i q u e o f X - r a y d i f f r a c t i o n , e i t h e r u s i n g the Bragg o r n o n - B r a g g i n t e n s i t i e s , i s of l i t t l e use. Spectroscopic techniques, e s p e c i a l l y nuclea r magnetic r e s o n a n c e and i n f r a r e d , as w e l l as n e u t r o n and X - r a y s c a t t e r i n g have p r o v i d e d d e t a i l e d i n f o r m a t i o n about the p o s i t i o n o f the water m o l e c u l e s , the dynamics o f the water m o l e c u l e m o t i o n s , and the c o o r d i n a t i o n about the i n t e r l a y e r c a t i o n s . As an example, i n f r a r e d s p e c t r o s c o p y has shown t h a t the l o w e s t s t a b l e h y d r a t i o n s t a t e f o r a L i - h e c t o r i t e has a s t r u c t u r e i n which the l i t h i u m c a t i o n i s p a r t i a l l y keyed i n t o t h e d i t r i g o n a l h o l e o f t h e h e c t o r i t e and has 3 water m o l e c u l e s c o o r d i n a t i n g the exposed p a r t o f t h e c a t i o n i n a t r i a n g u l a r arrangement (17 ), as proposed i n t h e model o f Mamy ( 1 3 ) . The water m o l e c u l e s e x h i b i t two k i n d s o f m o t i o n ; a slow r o t a t i o n o f the whole h y d r a t i o n s p h e r e about an a x i s t h r o u g h the t r i a n g l e o f t h e water m o l e c u l e s , and a f a s t e r r o t a t i o n o f each water m o l e c u l e about i t s own Cp a x i s (18). A similar s t r u c t u r e f o r adsorbed water a t low water c o n t e n t s has been o b s e r v e d f o r C u - h e c t o r i t e , C a - b e n t o n i t e , and C a - v e r m i c u l i t e ( 1 7 ) . M u l t i l a y e r A d s o r p t i o n o f Water. As the amount o f w a t e r i n the c l a y i n c r e a s e s o v e r t h a t needed f o r a o n e - o r t w o - l a y e r h y d r a t e , t h e s t u d y o f the p r o p e r t i e s o f the water becomes e x p e r i m e n t a l l y more difficult. T h i s i s i m p o r t a n t because i t i s o n l y a t water c o n t e n t s i n e x c e s s o f the t w o - l a y e r h y d r a t e t h a t a c o n f l i c t a r i s e s between t h e s h o r t - r a n g e and l o n g - r a n g e i n t e r a c t i o n models. In support o f t h e s h o r t - r a n g e model, two s t u d i e s a r e n o t e w o r t h y . A small angle
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X - r a y d i f f r a c t i o n s t u d y o f c l a y - w a t e r g e l s (19) has shown t h a t t h e r e a r e s t r o n g i n t e r a c t i o n s between the c l a y s u r f a c e s and t h e i r adsorbed i o n s and water m o l e c u l e s , but t h e s e i n t e r a c t i o n s e x i s t o n l y o v e r short distances. An e n t i r e l y d i f f e r e n t approach has been t a k e n by F r i p i a t e t a l . (5.). They examined c l a y - w a t e r m i x t u r e s o v e r a wide range o f c l a y c o n c e n t r a t i o n s u s i n g k a o l i n i t e and s m e c t i t e s . For m i c r o s c o p i c water p r o p e r t i e s , the n u c l e a r magnetic r e s o n a n c e r e l a x a t i o n time f o r hydrogen and d e u t e r i u m were measured. The m a c r o s c o p i c p r o p e r t i e s were d e t e r m i n e d by m e a s u r i n g the h e a t s o f w e t t i n g o f previously equilibrated clay-water mixtures. Both e x p e r i m e n t a l t e c h n i q u e s , o p e r a t i n g on v e r y d i f f e r e n t time s c a l e s , showed t h a t the number o f water l a y e r s i n f l u e n c e d by t h e c l a y was l e s s t h a n 5, w i t h an average f o r a l l samples o f 3.4 l a y e r s . T h i s g i v e s an average t h i c k n e s s o f about 10 A f o r t h e p e r t u r b e d w a t e r . The view t h a t the c l a y s u r f a c e p e r t u r b s water m o l e c u l e s a t d i s t a n c e s w e l l i n e x c e s s o f 10 A has been l a r g e l y based on measurements o f thermodynamic p r o p e r t i e s o f t h e adsorbed w a t e r as a f u n c t i o n o f the water c o n t e n t o f t h e c l a y - w a t e r m i x t u r e . There i s an e x t e n s i v e l i t e r a t u r e on t h i s s u b j e c t which has been summarized by Low (6_). The p r o p e r t i e s examined a r e , among o t h e r s , the a p p a r e n t s p e c i f i c heat c a p a c i t y , the p a r t i a l s p e c i f i c volume, and the a p p a r e n t s p e c i f i c e x p a n s i b i l i t y (6_). These measurements were made on samples p r e p a r e d by m i x i n g p r e d e t e r m i n e d amounts o f water and smect i t e t o a c h i e v e t h e d e s i r e d number o f adsorbed w a t e r l a y e r s . The number o f water l a y e r s adsorbed on t h e c l a y i s d e r i v e d from the amount o f water added t o the c l a y and the s u r f a c e a r e a o f the c l a y . The v a l u e o f the thermodynamic p r o p e r t y i n q u e s t i o n i s the d i f f e r e n c e i n v a l u e s f o r the c l a y - w a t e r sample and the same measurement on an e q u i v a l e n t amount o f p u r e , anhydrous c l a y (6.). This procedure i n v o l v e s two a s s u m p t i o n s : 1) t h e added water i s u n i f o r m l y adsorbed on a l l c l a y l a y e r s , and 2) the thermodynamic p r o p e r t i e s o f the c l a y i t s e l f do n o t change when the c l a y expands and i s i n t e r c a l a t e d by water m o l e c u l e s . The a s s u m p t i o n t h a t t h e water i s adsorbed i n u n i f o r m l a y e r s on a l l t h e c l a y s u r f a c e s f o r a wide r a n g e o f m i x t u r e s has been c r i t i c i z e d (5., 20 ). The argument i s t h a t the i n d i v i d u a l c l a y p a r t i c l e s i n t h e c l a y - w a t e r m i x t u r e do n o t expand beyond a c e r t a i n d i s t a n c e r e g a r d l e s s o f the q u a n t i t y o f water which i s added. The c l a y l a y e r s group t h e m s e l v e s i n t o t a c t o i d s r e s u l t i n g i n two p o p u l a t i o n s o f w a t e r ; t h o s e m o l e c u l e s which a r e found between the t a c t o i d s and t h o s e d i r e c t l y p e r t u r b e d by t h e c l a y l a y e r s . I f t r u e , t h i s would i n v a l i d a t e the p r o c e d u r e used t o c a l c u l a t e the thermodynamic p r o p e r t i e s o f the adsorbed w a t e r . However, o t h e r workers have r e p o r t e d c o m p l e t e d e l a m i n a t i o n o f c e r t a i n s m e c t i t e s (21, 2 2 ) . I t i s not c l e a r under what c o n d i t i o n s t a c t o i d s w i l l f o r m , o r n o t , and t h i s u n c e r t a i n t y i s u n d e r l i n e d i n (21) (see remarks by Nadeau and F r i p i a t , pages 1 4 6 - 1 4 7 ) . The v a l i d i t y o f the a s s u m p t i o n t h a t the v a r i o u s thermodynamic p r o p e r t i e s o f the s m e c t i t e remain i n v a r i a n t , r e g a r d l e s s o f t h e s t a t e o f h y d r a t i o n , has been a d d r e s s e d i n d e t a i l by S p o s i t o and P r o s t (JJ · They p o i n t o u t t h a t one would, f o r example, e x p e c t h y d r o l y s i s o f t h e c l a y t o o c c u r a t h i g h w a t e r c o n t e n t s , and a l s o , i t i s l i k e l y t h a t t h e exchangeable c a t i o n s w i l l change t h e i r s p a t i a l r e l a t i o n s h i p w i t h the c l a y l a y e r s . T h u s , the d e r i v e d thermodynamic p r o p e r t i e s o f the adsorbed water would not r e p r e s e n t c o r r e c t v a l u e s .
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F i n a l l y , the whole c o n c e p t o f u s i n g m a c r o s c o p i c ( i . e . thermodynamic) p r o p e r t i e s t o d e r i v e a m i c r o s c o p i c p i c t u r e o f the adsorbed water i s open t o q u e s t i o n ( £ , 8.). I t i s d i f f i c u l t t o r e c o n c i l e t h e s e v e r y d i f f e r e n t views o f the i n t e r a c t i o n o f water and c l a y s u r f a c e s . S p o s i t o (8.) has attempted this. He p o i n t s o u t t h a t the thermodynamic p r o p e r t i e s have an e s s e n t i a l l y i n f i n i t e time s c a l e , whereas the s p e c t r o s c o p i c measurements l o o k a t some v a r i a n t o f the v i b r a t i o n a l o r a p r e d e c e s s o r o f the d i f f u s i o n a l s t r u c t u r e o f w a t e r . I t i s p o s s i b l e t h a t the thermodynamic p r o p e r t i e s r e f l e c t a number o f c o o p e r a t i v e i n t e r a c t i o n s which c a n be seen o n l y on a v e r y l o n g time s c a l e . S t i l l , the X - r a y d i f f r a c t i o n s t u d i e s s e e m i n g l y a l s o o p e r a t e on as l o n g a time s c a l e as the thermodynamic p r o p e r t i e s . There i s s t i l l n o t a c l e a r c h o i c e between the s h o r t - r a n g e and l o n g - r a n g e i n t e r a c t i o n m o d e l s . E x p e r i m e n t a l S t u d i e s o f Water on K a o l i n M i n e r a l s I n g e n e r a l , the 2:1 c l a y s a r e n o t v e r y s i m p l e systems i n which t o s t u d y the i n t e r a c t i o n o f water and s u r f a c e s . They have complex and v a r i a b l e c o m p o s i t i o n s and t h e i r s t r u c t u r e s a r e p o o r l y u n d e r s t o o d . Water o c c u r s i n s e v e r a l d i f f e r e n t e n v i r o n m e n t s : z e o l i t i c water i n the i n t e r l a y e r r e g i o n s , water adsorbed on t h e e x t e r n a l s u r f a c e s o f the c r y s t a l l i t e s , water c o o r d i n a t i n g the exchangeable c a t i o n s , a n d , o f t e n , as pore water f i l l i n g v o i d s between t h e c r y s t a l l i t e s . Thus, t h e r e a r e many v a r i a b l e s and the e f f e c t s o f each on t h e p r o p e r t i e s o f water a r e d i f f i c u l t t o s e p a r a t e . I n view o f the problems a s s o c i a t e d w i t h the expanding 2:1 c l a y s , the s m e c t i t e s and v e r m i c u l i t e s , i t seemed d e s i r a b l e t o use a d i f f e r e n t c l a y m i n e r a l s y s t e m , one i n which the i n t e r a c t i o n s o f s u r f a c e adsorbed water a r e more e a s i l y s t u d i e d . An o b v i o u s c a n d i d a t e i s t h e h y d r a t e d form o f h a l l o y s i t e , but s t u d i e s o f t h i s m i n e r a l have shown t h a t h a l l o y s i t e s a l s o s u f f e r from an e q u a l l y i n t r a c t a b l e s e t o f d i f f i c u l t i e s (J_0). These a r e p r i n c i p a l l y the poor c r y s t a l l i n i t y , the n e c e s s i t y t o m a i n t a i n the c l a y i n l i q u i d water i n o r d e r t o p r e v e n t l o s s o f the s u r f a c e adsorbed ( i n t e r c a l a t e d ) w a t e r , and the h i g h l y v a r i a b l e morphology o f the c r y s t a l l i t e s . I t seemed t o u s p r e f e r a b l e t o s t a r t w i t h a c h e m i c a l l y p u r e , w e l l - c r y s t a l l i z e d , and w e l l - k n o w n c l a y m i n e r a l ( k a o l i n i t e ) and t o i n c r e a s e t h e n o r m a l l y s m a l l s u r f a c e a r e a by i n s e r t i n g water m o l e c u l e s between the l a y e r s through chemical treatment. Thus, the water would be i n c o n t a c t w i t h b o t h s u r f a c e s o f e v e r y c l a y l a y e r i n the c r y s t a l l i t e s r e s u l t i n g i n an e f f e c t i v e s u r f a c e a r e a f o r water a d s o r p t i o n o f a p p r o x i m a t e l y 1000 m g . The s y n t h e t i c k a o l i n i t e h y d r a t e s t h a t r e s u l t e d from t h i s work a r e n e a r l y i d e a l m a t e r i a l s f o r s t u d i e s o f water adsorbed on s i l i c a t e s u r f a c e s . Kaolin Minerals. The 1:1 s t r u c t u r e s i n c l u d e a group o f a l u m i n o s i l i c a t e m i n e r a l s which a r e termed c o l l e c t i v e l y the k a o l i n m i n e r a l s ; s p e c i f i c a l l y t h e s e a r e k a o l i n i t e , d i c k i t e , n a c r i t e , and h a l l o y s i t e . The b a s i c 1:1 l a y e r f o r a l l o f t h e s e m i n e r a l s has t h e c o m p o s i t i o n A l S i 0 ( 0 H ) . ; t h e r e i s a s m a l l amount o f s u b s t i t u t i o n o f i r o n f o r aluminum, ana f l u o r i d e f o r h y d r o x y l i o n . A l l , except h a l l o y s i t e , a r e n o r m a l l y anhydrous and do not expand (as do the s m e c t i t e s ) upon exposure t o water and most o r g a n i c m o l e c u l e s . As a r e s u l t , t h e y ^ g e n e r a l l y have a r a t h e r s m a l l s u r f a c e a r e a , on the o r d e r o f 10 m 2
2
5
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G E O C H E M I C A L PROCESSES AT M I N E R A L S U R F A C E S
g . I n s p i t e o f t h e i r r e l a t i v e l y s i m p l e c o m p o s i t i o n and the abundance o f w e l l c h a r a c t e r i z e d s a m p l e s , the s m a l l s u r f a c e a r e a o f k a o l i n i t e and d i c k i t e h a s , u n t i l r e c e n t l y , e f f e c t i v e l y e l i m i n a t e d them as c a n d i d a t e s f o r s u r f a c e c h e m i s t r y s t u d i e s . Assuming l i t t l e o r no i s o m o r p h i c s u b s t i t u t i o n , the b o n d i n g between l a y e r s i s l a r g e l y due t o hydrogen bonds from the h y d r o x y l s o f one s u r f a c e t o a d j a c e n t oxygens o f the next s u r f a c e (22, 2 4 ) . Hydrogen bonds a r e n o t n o r m a l l y thought o f as b e i n g v e r y s t r o n g r e l a t i v e t o i o n i c and c o v a l e n t bonds, and one m i g h t , t h e r e f o r e , e x p e c t t h a t t h e l a y e r s o f t h e k a o l i n m i n e r a l s would be e a s i l y s e p a r a t e d , a t l e a s t by s m a l l m o l e c u l e s and w a t e r . Such i s not the c a s e f o r w a t e r , and i n t e r c a l a t i o n i s found t o be p o s s i b l e o n l y f o r a r e l a t i v e l y s m a l l number o f o r g a n i c m o l e c u l e s and s a l t s . The t y p e s o f o r g a n i c m o l e c u l e s t h a t a r e a b l e t o i n t e r c a l a t e the k a o l i n m i n e r a l s a r e g e n e r a l l y s m a l l w i t h l a r g e d i p o l e moments. These i n c l u d e h y d r a z i n e , d i m e t h y l s u l f o x i d e (DMSO), formamide and some d e r i v a t i v e s (N-methylformamide and d i m e t h y l f o r m a m i d e ) , acetamide and some d e r i v a t i v e s , and p y r i d i n e N - o x i d e . Some s a l t s s u c h as p o t a s s i u m a c e t a t e a l s o i n t e r c a l a t e k a o l i n i t e s . Once i n t e r c a l a t e d by one o f t h e s e s m a l l m o l e c u l e s o r s a l t s , o t h e r m o l e c u l e s w h i c h n o r m a l l y do n o t d i r e c t l y i n t e r c a l a t e k a o l i n s c a n be i n t r o d u c e d by replacement. F u r t h e r , the e x p o s u r e o f t h e i n n e r s u r f a c e s by i n t e r c a l a t i o n g i v e s one the o p p o r t u n i t y t o a l t e r the i n t e r l a y e r b o n d i n g o f t h e k a o l i n l a y e r s by c h e m i c a l m o d i f i c a t i o n o f the i n n e r s u r f a c e s . Synthesis of K a o l i n i t e Hydrates. Our work i n s y n t h e s i z i n g a w a t e r s i l i c a t e system t h a t has a s i m p l e r c h e m i s t r y t h a n the s m e c t i t e s o r v e r m i c u l i t e s i s based on the c o n c e p t o f r e d u c i n g the t o t a l number o f i n t e r l a y e r hydrogen bonds by a c h e m i c a l r e p l a c e m e n t o f some i n n e r s u r f a c e h y d r o x y l s by f l u o r i n e (25.). I n p r i n c i p l e , one need o n l y expand t h e k a o l i n i t e by i n t e r c a l a t i o n w i t h an a p p r o p r i a t e o r g a n i c m o l e c u l e and t h e n expose the i n t e r c a l a t e d c l a y t o an environment containing fluoride ions. In p r a c t i c e , t h e o r g a n i c m o l e c u l e , t h e t y p e o f f l u o r i d e s a l t added, as w e l l as the time and t e m p e r a t u r e a t e a c h s t a g e i n the development o f a h y d r a t e d k a o l i n i t e , p l a y an i m p o r t a n t r o l e i n d e t e r m i n i n g the s u c c e s s o f the s y n t h e s i s and the y i e l d of hydrated c l a y . E a r l y work showed t h a t a 10A h y d r a t e , s i m i l a r t o n a t u r a l l y h y d r a t e d h a l l o y s i t e , c o u l d be s y n t h e s i z e d from a w e l l - c r y s t a l l i z e d k a o l i n i t e from C o r n w a l l , E n g l a n d (26.). The p r o c e d u r e was t o i n t e r c a l a t e the c l a y w i t h DMSO which c o n t a i n e d about 8% by weight o f water. The c l a y expanded from 7 . 2 A t o 11 A upon i n t e r c a l a t i o n by DMSO. Ammonium f l u o r i d e was d i s s o l v e d i n the DMSO s o l u t i o n a n d , p r e s u m a b l y , the f l u o r i d e d i f f u s e d i n t o t h e i n t e r l a y e r r e g i o n where i t r e p l a c e d some h y d r o x y l g r o u p s . T h i s s u s p e n s i o n was c o n t i n u o u s l y s t i r r e d a t 6 0 C f o r p e r i o d s v a r y i n g from a few h o u r s t o as much as 12 h o u r s o r more. The c l a y was t h e n s e p a r a t e d from the water-DMSO by c e n t r i f u g a t i o n and r e d i s p e r s e d i n d i s t i l l e d w a t e r . This cycle of water washing was r e p e a t e d s e v e r a l t i m e s t o remove i n t e r c a l a t e d DMSO molecules. I n s t e a d o f c o l l a p s i n g back t o 7 A, t h e k a o l i n i t e e x h i b i t e d a 10 A s p a c i n g s i m i l a r t o t h a t o f a f u l l y h y d r a t e d h a l l o y s i t e - 1 0 A . I n f r a r e d s p e c t r a showed a broad a b s o r p t i o n i n the 3 4 0 0 3500 cm" r e g i o n and a s i n g l e band near 1650 cm i n d i c a t i n g that t h e k a o l i n i t e was i n t e r c a l a t e d p r i m a r i l y by w a t e r . As i s t h e c a s e w i t h h y d r a t e d h a l l o y s i t e s , t h i s 10A p r o d u c t was not s t a b l e under e
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ambient h u m i d i t y and t h u s was o f l i m i t e d use i n e x a m i n i n g the p r o p e r t i e s o f s u r f a c e adsorbed w a t e r . Subsequent work showed t h a t a m o d i f i c a t i o n o f t h e s y n t h e s i s p r o c e d u r e produced a 10A h y d r a t e w h i c h , i f d r i e d c a r e f u l l y , would m a i n t a i n the i n t e r l a y e r water i n the absence o f e x c e s s w a t e r ( 2 1 ) . T h i s m a t e r i a l i s o p t i m a l f o r adsorbed water s t u d i e s f o r a number o f reasons: the parent c l a y i s a w e l l - c r y s t a l l i z e d k a o l i n i t e w i t h a n e g l i g i b l e l a y e r c h a r g e , t h e r e a r e few i f any i n t e r l a y e r c a t i o n s , t h e r e i s no i n t e r f e r e n c e from pore water s i n c e t h e amount i s m i n i m a l , and the i n t e r l a y e r water m o l e c u l e s l i e between u n i f o r m l a y e r s o f known s t r u c t u r e . T h u s , t h e h y d r a t e p r o v i d e s a u s e f u l model f o r s t u d y i n g the e f f e c t s o f a s i l i c a t e s u r f a c e on i n t e r l a y e r w a t e r . C h a r a c t e r i z a t i o n o f I n t e r l a v e r Water. X-ray d i f f r a c t i o n studies of the 10A h y d r a t e show no h k l r e f l e c t i o n s i n d i c a t i n g a l a c k o f r e g u l a r i t y i n the s t a c k i n g o f the k a o l i n l a y e r s . In a d d i t i o n to the 10A h y d r a t e , two o t h e r l e s s h y d r a t e d k a o l i n i t e s were s y n t h e s i z e d . B o t h have one m o l e c u l e o f water f o r each f o r m u l a u n i t i n c o n t r a s t t o the 10A h y d r a t e which has two. These l e s s h y d r a t e d c l a y s c o n s e q u e n t l y have s m a l l e r d(001) s p a c i n g s o f 8.4 and 8.6 A. The s y n t h e s i s c o n d i t i o n s f o r t h e s e two h y d r a t e s a r e d e s c r i b e d i n ( 2 £ ) . By s t u d y i n g the i n t e r l a y e r water i n the 8.4 and 8.6A h y d r a t e s , i t was p o s s i b l e t o f o r m u l a t e a model o f the w a t e r i n the more c o m p l i c a t e d 10A h y d r a t e . An i s o l a t e d h y d r o x y l g r o u p , s u c h as i s found i n the 2:1 m i c a s , a b s o r b s i n f r a r e d r a d i a t i o n a t a p p r o x i m a t e l y 3700 cm (28). Kaol i n i t e i s more complex s i n c e t h e r e a r e two d i s t i n c t t y p e s o f h y d r o x y l g r o u p s , the s i n g l e h y d r o x y l a t the i n t e r f a c e between the t e t r a h e d r a l and o c t a h e d r a l s h e e t s ( t h e i n n e r h y d r o x y l ) , and t h r e e h y d r o x y l s ( t h e i n n e r s u r f a c e h y d r o x y l s ) which form one o f t h e e x t e r n a l s u r f a c e s o f the k a o l i n i t e l a y e r . Because the i n n e r h y d r o x y l i s i s o l a t e d i t p r o d u c e s a s i n g l e v i b r a t i o n a t a p p r o x i m a t e l y 3620 cm" . The t h r e e e x t e r n a l h y d r o x y l s do n o t v i b r a t e i n d e p e n d e n t l y ; t h e i r s t r e t c h i n g v i b r a t i o n s a r e c o u p l e d t o produce s e v e r a l bands, none o f which c a n be r e l a t e d t o a s p e c i f i c h y d r o x y l group (22., 1 0 ) . T h e r e a r e t h r e e o f t h e s e bands f o r k a o l i n i t e ; a s t r o n g a b s o r p t i o n a t 3695 cm" , and two much weaker a b s o r p t i o n s a t 3665 and 3650 c m . The f o u r bands may v a r y i n f r e q u e n c y and i n t e n s i t y o f a b s o r p t i o n from sample t o sample. I n c o n t r a s t , the v i b r a t i o n a l bands o f water m o l e c u l e s i n t h e s o l i d phase o c c u r a t 3350 and 3250 cm" w i t h a b e n d i n g mode a t 1640 cm" ( 2 1 , 2 £ ) . Hence the i n f r a r e d bands from t h e s t r u c t u r a l h y d r o x y l s and i n t e r c a l a t e d water do n o t o v e r l a p . Of i m p o r t a n c e t o t h e p r e s e n t s u b j e c t i s the f a c t t h a t t h e t h r e e bands c o r r e s p o n d i n g t o the i n n e r s u r f a c e h y d r o x y l s c a n be used t o i n f e r the b o n d i n g s t a t e o f m o l e c u l e s i n the i n t e r l a y e r r e g i o n o f t h e kaolinite. I n s e r t i o n o f g u e s t m o l e c u l e s between t h e k a o l i n i t e l a y e r s d i s r u p t s the hydrogen bonds o f the o r i g i n a l k a o l i n i t e and r e s u l t s i n the f o r m a t i o n o f new bonds w i t h the g u e s t m o l e c u l e s . To r e c o r d the i n f r a r e d s p e c t r a , samples o f the p a r e n t k a o l i n i t e and the t h r e e h y d r a t e s were d i s p e r s e d i n a f l u o r i n a t e d h y d r o c a r b o n . The m u l l s were squeezed between c a l c i u m f l u o r i d e p l a t e s and t h e sample was p l a c e d d i r e c t l y i n the beam o f a P e r k i n - E l m e r 683 s p e c trometer. T h i s mounting t e c h n i q u e r e s u l t s i n a tendency f o r t h e c l a y l a y e r s t o a l i g n t h e m s e l v e s p e r p e n d i c u l a r t o t h e beam o f t h e spectrometer. I n f r a r e d s p e c t r a o f t h e s e m a t e r i a l s have been p u b -
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l i s h e d e l s e w h e r e (22.) and t h e r e a r e s l i g h t d i f f e r e n c e s i n f r e q u e n c i e s ( 2 - 3 cm"" o r l e s s ) and band i n t e n s i t i e s compared t o the r e s u l t s reported here. There a r e s e v e r a l r e a s o n s f o r t h e s e d i f f e r e n c e s : for t h i s work, we used f r e s h l y p r e p a r e d h y d r a t e s which seem t o g i v e a b e t t e r s i g n a l , t h e sample p r e p a r a t i o n f o r the i n f r a r e d s p e c t r o m e t e r has improved o v e r our o r i g i n a l work, and the p r e s e n t s p e c t r a were run i n the absorbance r a t h e r t h a n i n the t r a n s m i s s i o n mode. The s p e c t r a o f the h y d r a t e d k a o l i n i t e s ( F i g u r e 2A, 2B, 2C) d i f f e r s u b s t a n t i a l l y from e a c h o t h e r and a l s o from t h e p a r e n t k a o l i n i t e ( F i g u r e 2D). The 8.4A h y d r a t e ( F i g u r e 2A) has an o r d e r e d l a y e r s t a c k i n g , as shown by X - r a y d i f f r a c t i o n ( H ) . The c r y s t a l s t r u c t u r e shows t h a t the water m o l e c u l e s a r e a s s o c i a t e d w i t h and a r e keyed i n t o the d i t r i g o n a l h o l e s of the t e t r a h e d r a . The c o n d i t i o n s o f the 8.4A s y n t h e s i s r e s u l t i n up t o 20% o f t h e i n n e r s u r f a c e h y d r o x y l s b e i n g r e p l a c e d by f l u o r i d e i o n s . Because i t seems l i k e l y t h a t some o f the i n t e r l a y e r water m o l e c u l e s a r e hydrogen bonded t o f l u o r i d e i o n s o f t h e a d j a c e n t l a y e r , a t l e a s t two hydrogen bond schemes c a n be e n v i s i o n e d : one where water bonds t o t h e oxygens o f the a d j a c e n t t e t r a h e d r a l s h e e t ; the o t h e r where the w a t e r bonds t o an oxygen o f one l a y e r and a f l u o r i d e o f t h e o t h e r l a y e r ( o r , p o s s i b l y , t o two f l u o r i d e s o f t h e same l a y e r ) . Hydrogen bonds t o f l u o r i d e a r e n o r m a l l y s t r o n g e r t h a n s i m i l a r bonds t o oxygen so one would e x p e c t t o see s e p a r a t e a b s o r p t i o n bands i n the i n f r a r e d spectrum o f t h e 8.4A h y d r a t e i f t h e r e a r e s u b s t a n t i a l numbers o f water m o l e c u l e s b o n d i n g to f l u o r i d e (27). I n c o n t r a s t t o the 8.4A h y d r a t e , t h e 8.6A h y d r a t e ( F i g u r e 2B) has o n l y a minor amount o f r e p l a c e m e n t o f f l u o r i d e f o r h y d r o x y l i o n s and a n o n - c r y s t a l l i n e c h a r a c t e r , l i k e the 10A h y d r a t e . Comparison o f the i n f r a r e d s p e c t r a o f the 8.4 and 8.6A h y d r a t e s ( F i g u r e s 2A, 2B) does i n d e e d show more bands f o r t h e former i n t h e r e g i o n between 3600 and 3200 c m , i n agreement w i t h t h e argument s t a t e d above. In t h e b e n d i n g mode r e g i o n , the 8.4A h y d r a t e has two c l e a r l y s e p a r a t e d bands, as one would e x p e c t f o r water hydrogen bonded t o oxygen (1645 c m " ) and a l s o hydrogen bonded t o f l u o r i d e (1590 c m ) . In c o n t r a s t , t h e 8.6A h y d r a t e has a s i n g l e band a t 1653 cm" . 1
Because the i n n e r h y d r o x y l i s b u r i e d i n the k a o l i n i t e l a y e r and i s n o t p e r t u r b e d s u b s t a n t i a l l y by i n t e r c a l a t i o n , i t s v i b r a t i o n , a t 3620 cm" , i s common t o a l l f o u r c l a y s i n F i g u r e 2. The 8.4A h y d r a t e has two h i g h f r e q u e n c y bands above 3620 cm" ; 3690 and 3650 c m . S i m i l a r bands a r e e v i d e n t i n t h e 8.6A h y d r a t e a l t h o u g h t h e i r i n t e n s i t i e s a r e somewhat d i f f e r e n t . Below 3620 c m , the 8.4A h y d r a t e has s m a l l bands a t 3584 and 3538 cm" and l a r g e r bands a t 3443 and 3340 cm" . The l a t t e r two do n o t appear i n the s p e c t r u m o f t h e 8.6A h y d r a t e and have been a s s i g n e d t o hydrogen b o n d i n g from water m o l e c u l e s t o f l u o r i d e . Bands s i m i l a r t o the 3584 and 3538 cm bands o f the 8.4A h y d r a t e appear i n t h e 8.6A h y d r a t e s p e c t r u m , b u t s h i f t e d s l i g h t l y t o 3595 and 3547 c m . The c l o s e match between t h e s e comparable bands a t f r e q u e n c i e s above 3500 cm" i n the two h y d r a t e s i s good e v i d e n c e t h a t t h e s e a r e due t o h y d r o x y l s and w a t e r molecules i n s i m i l a r environments. The water m o l e c u l e s a t t a c h e d t o t h e d i t r i g o n a l h o l e s have been termed " h o l e w a t e r " (22.). These r e p r e s e n t a d i s c o n t i n u o u s monolayer o f water adsorbed o n t o a s i l i c a t e s u r f a c e (33.). Weight l o s s measurements (22.) show t h a t t h e r e a r e as many water m o l e c u l e s as t h e r e a r e d i t r i g i o n a l h o l e s . Comparison o f
the bands a t f r e q u e n c i e s
above 3500 cm
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F i g u r e 2 . I n f r a r e d a b s o r p t i o n s p e c t r a o f the 8.4A h y d r a t e ( A ) , t h e 8.6A h y d r a t e ( B ) , t h e 10A h y d r a t e ( C ) , and the o r i g i n a l k a o l i n i t e used t o s y n t h e s i z e the t h r e e h y d r a t e s ( D ) .
American Chemical Society Library 1155 16th St., N.W. Washington, D.C. 20036
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10A h y d r a t e spectrum w i t h t h e o t h e r two h y d r a t e s shows t h a t t h e g e n e r a l f e a t u r e s a r e s i m i l a r ; the 10A h y d r a t e has two bands a t 3585 and 3548 cm" from h o l e w a t e r v i b r a t i o n modes, and the h i g h f r e q u e n cy bands above 3620 cm" have f e a t u r e s which a r e g r o s s l y s i m i l a r t o t h o s e i n the 8.6A h y d r a t e . The s i m i l a r i t y i n bands between 3600 and 3500 cm f o r the 10A and 8.6A h y d r a t e s i n d i c a t e s the e x i s t e n c e o f h o l e water i n the f o r m e r . Below 3500 cm" , the 10A h y d r a t e , i n common w i t h h y d r a t e d h a l l o y s i t e s , shows a v e r y broad a b s o r p t i o n c e n t e r e d a p p r o x i m a t e l y a t 3250 c m . T h i s f e a t u r e i s a b s e n t from t h e two o t h e r h y d r a t e s and from t h e k a o l i n i t e . This absorption f a l l s i n t h e range o f l i q u i d water and has been a s s i g n e d t o t h e e x t r a water i n t r o d u c e d t o expand the h y d r a t e d c l a y from 8 . 4 / 8 . 6 A t o 10A. Thus, the i n f r a r e d s p e c t r a a l o n g w i t h the o b s e r v e d i n c r e a s e i n l a y e r t h i c k n e s s from r o u g h l y 8.4 t o 10 A, and t h e d e h y d r a t i o n d a t a (5., 27) i n d i c a t e t h a t when a d i h y d r a t e i s formed, h a l f t h e water ( h o l e water) i s a t t a c h e d t o t h e d i t r i g o n a l h o l e s o f t h e s i l i c a t e s u r f a c e w h i l e the o t h e r h a l f o c c u p i e s o t h e r s i t e s a t a g r e a t e r d i s t a n c e from the s i l i c a t e s u r f a c e . L a c k i n g a t h r e e d i m e n s i o n a l l y o r d e r e d 10A h y d r a t e , we c a n n o t d i r e c t l y d e t e r m i n e the l o c a t i o n o f the second l a y e r o f water m o l e c u l e s . We do know t h a t they must occupy s i t e s o t h e r t h a n the d i t r i g o n a l h o l e s because t h e s e a r e c o m p l e t e l y f i l l e d by h o l e water ( £ ) . To d i s t i n g u i s h t h e two t y p e s o f w a t e r , the n o n - h o l e water i s termed " a s s o c i a t e d w a t e r " . Heat C a p a c i t y Measurements and I n t e r l a v e r Water S t r u c t u r e . The h e a t c a p a c i t y o f the i n t e r l a y e r water has been measured f o r the 10A, 8 . 6 A , and 8.4A h y d r a t e s between 110 and 275 Κ (24) and p r o v i d e s an i m p o r t a n t c l u e i n d e t e r m i n i n g t h e s t r u c t u r e o f t h e a s s o c i a t e d water molecules. A t a l l t e m p e r a t u r e s , the Cp f o r the i n t e r l a y e r w a t e r i n the 8.4A and 8.6A h y d r a t e s was n o t s i g n i f i c a n t l y d i f f e r e n t from published values for i c e . The water i n t h e 10A h y d r a t e d e v i a t e d from the i c e v a l u e s a t r o u g h l y 160 Κ and t h e d e v i a t i o n i n c r e a s e d as the temperature r o s e . A s h a r p peak i n t h e Cp began a t r o u g h l y 240 Κ and ended near the m e l t i n g p o i n t o f i c e ( F i g u r e 3 ) . The i n i t i a l d e p a r t u r e o f t h e Cp from i c e v a l u e s c o i n c i d e d w i t h an i n c r e a s e i n t h e p r o t o n NMR s i g n a l (25.). T h i s b e h a v i o r s u g g e s t s t h a t a t v e r y low t e m p e r a t u r e s , the water m o l e c u l e s occupy r e l a t i v e l y s t a t i c p o s i t i o n s i n the i n t e r l a y e r r e g i o n . The i c e - l i k e Cp o f t h e h o l e water t h r o u g h o u t the t e m p e r a t u r e range i n v e s t i g a t e d i s i n agreement w i t h the s u p p o s i t i o n d e s c r i b e d e a r l i e r t h a t the hole water i s r e l a t i v e l y s t r o n g l y bonded t o t h e d i t r i g o n a l h o l e s and r e m a i n s f i x e d . The peak i n t h e Cp t h e n must i n v o l v e p r i m a r i l y the a s s o c i a t e d water m o l e cules. Examination o f a p r o j e c t i o n o f the t e t r a h e d r a l s u r f a c e o f the k a o l i n l a y e r shows a p s e u d o - h e x a g o n a l arrangement o f oxygen atoms (Figure 4). When a l l the d i t r i g o n a l s i t e s a r e o c c u p i e d by h o l e water (open c i r c l e s i n t h e f i g u r e ) , i t c a n be s e e n t h a t t h e y a r e approximately 5 A apart (Figure 4A). Hence, t h e r e i s no p o s s i b i l i t y o f hydrogen b o n d i n g between h o l e water m o l e c u l e s i n e i t h e r the 8.4A and 8.6A h y d r a t e s . The a s s o c i a t e d w a t e r m o l e c u l e s ( f i l l e d c i r c l e s i n the f i g u r e ) c a n be added i n two d i f f e r e n t o r d e r e d a r r a n g e m e n t s , one o f which i s shown i n F i g u r e 4. I n e i t h e r arrangement, t h e d i s t a n c e between a s s o c i a t e d and h o l e water m o l e c u l e s i s on t h e o r d e r o f 3 A, a r e a s o n a b l e d i s t a n c e f o r hydrogen b o n d i n g t o o c c u r between h o l e water and a s s o c i a t e d w a t e r . These hydrogen bonds from a s s o c i -
3.
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F i g u r e 4. A schematic r e p r e s e n t a t i o n o f the t e t r a h e d r a l surface o f k a o l i n i t e ( t r i a n g l e s ) showing t h e p o s i t i o n o f t h e h o l e water m o l e c u l e s (open c i r c l e s ) k e y i n g i n t o t h e d i t r i g o n a l h o l e s . The a s s o c i a t e d water ( f i l l e d c i r c l e s i n A) m o l e c u l e s a r e a r r a n g e d i n an o r d e r e d p a t t e r n which e x i s t s a t low t e m p e r a t u r e s . Disorder i n the a s s o c i a t e d water ( f i l l e d c i r c l e s i n B) i s c r e a t e d by i n c r e a s i n g the temperature.
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a t e d water t o h o l e water a r e s t r o n g e r t h a n the bonds from the h o l e water t o the d i t r i g o n a l oxygens as shown by the lower i n f r a r e d a b s o r p t i o n f r e q u e n c i e s o f the former. As the t e m p e r a t u r e r i s e s , t h e model s u g g e s t s t h a t a few a s s o c i a t e d water m o l e c u l e s become m o b i l e . Those which have s u f f i c i e n t k i n e t i c energy c a n jump t o the v a c a n t s i t e s o f the a l t e r n a t e c o n f i g u r a t i o n (as i n F i g u r e 4 B ) . This d i s r u p t s the o r d e r l y hydrogen bond scheme w h i c h e x i s t s a t l o w e r t e m p e r a t u r e s and makes i t e a s i e r f o r o t h e r a s s o c i a t e d m o l e c u l e s t o jump. As the t e m p e r a t u r e c o n t i n u e s t o r i s e , t h i s jumping between the two c o n f i g u r a t i o n s , s i m i l a r t o m e l t i n g , l e a d s t o the peak i n Cp w i t h a maximum a t about 270 Κ ( F i g u r e 3 ) . I n summary, the model s u g g e s t s t h a t the water i n d i r e c t c o n t a c t w i t h the m i n e r a l s u r f a c e ( h o l e water) i s s t r o n g l y bonded t o t h e s i l i c a t e l a y e r . The second l a y e r o f water ( a s s o c i a t e d water) behaves v e r y d i f f e r e n t l y because i t has few i f any hydrogen bonds d i r e c t l y t o the s i l i c a t e l a y e r . A p p l i c a t i o n o f R e s u l t s to o t h e r S i l i c a t e M i n e r a l s Our model f o r the a d s o r p t i o n o f water on s i l i c a t e s was d e v e l o p e d f o r a system w i t h few i f any i n t e r l a y e r c a t i o n s . However, i t s t r o n g l y r e s e m b l e s the model proposed by Mamy (13) f o r s m e c t i t e s w i t h monovalent i n t e r l a y e r c a t i o n s . The p r e s e n c e o f d i v a l e n t i n t e r l a y e r c a t i o n s , as shown by s t u d i e s o f s m e c t i t e s and v e r m i c u l i t e s , should r e s u l t i n a strong s t r u c t u r i n g of t h e i r primary h y d r a t i o n s p h e r e and p r o b a b l y the next n e a r e s t n e i g h b o r water m o l e c u l e s as well. I f t h e c o n c e n t r a t i o n o f the d i v a l e n t c a t i o n s i s low, t h e n t h e water i n i n t e r l a y e r space between t h e d i v a l e n t c a t i o n s w i l l c o r r e spond t o the p r e s e n t model. On the o t h e r h a n d , i f t h e c o n c e n t r a t i o n o f d i v a l e n t c a t i o n s approaches the number o f d i t r i g o n a l s i t e s , t h i s model w i l l n o t be a p p l i c a b l e . Such a s i t u a t i o n would o n l y be found i n concentrated e l e c t r o l y t e s o l u t i o n s . I n d i s c u s s i n g the a p p l i c a b i l i t y o f t h e p r e s e n t model t o s i l i c a t e m i n e r a l s i n g e n e r a l , t h e r e a r e two c o n s i d e r a t i o n s : t o what e x t e n t do the exposed s u r f a c e s o f a g i v e n s i l i c a t e m i n e r a l mimic t h e d i t r i g o n a l h o l e s o f the c l a y m i n e r a l s , a n d , d u r i n g c h e m i c a l w e a t h e r i n g , i s the s i l i c a t e m i n e r a l d i r e c t l y exposed t o the aqueous phase o r i s t h e r e an i n t e r v e n i n g phase o f d i f f e r e n t s t r u c t u r e and c o m p o s i tion? The f i r s t p o i n t i s f a i r l y e a s i l y d e t e r m i n e d by i n s p e c t i o n o f t h e c r y s t a l s t r u c t u r e s o f the major s i l i c a t e g r o u p s . Nesosilicates ( i s o l a t e d t e t r a h e d r a ) and t h e s o r o s i l i c a t e s ( d o u b l e t e t r a h e d r a ) have l i t t l e resemblance t o the s t r u c t u r e o f t h e c l a y m i n e r a l s , t h e i n o s i l i c a t e s ( s i n g l e and d o u b l e c h a i n s ) which have c o r n e r shared t e t r a h e d r a a r e s i m i l a r , p a r t i c u l a r l y the d o u b l e c h a i n s , p h y l l o s i l i c a t e s ( s h e e t s t r u c t u r e s ) c l e a r l y a r e s i m i l a r , and the t e k t o s i l i c a t e s may o r may n o t be s i m i l a r , depending on the e x t e r n a l s u r f a c e s exposed t o the aqueous phase. To the e x t e n t t h a t t h e m i n e r a l s a r e s i m i l a r , one would e x p e c t the model t o a p p l y . I t i s w e l l known t h a t d i s s o l u t i o n o f s i l i c a t e s , p a r t i c u l a r l y the c o m p o s i t i o n a l l y complex ones s u c h as t h e f e l d s p a r s , i s o f t e n i n c o n g r u e n t and t h a t p r e c i p i t a t e d p r o d u c t may c o a t the m i n e r a l s u r f a c e s . These c o a t i n g s may be ( c o m p o s i t i o n a l l y or s t r u c t u r a l l y ) s i m i l a r to c l a y minerals or r e l a t e d l a y e r s t r u c t u r e s ( k a o l i n i t e , s m e c t i t e , b o e h m i t e ) , and under t h e s e c o n d i t i o n s , t h e model f o r adsorbed water may a l s o be v e r y g o o d . Little i s known i n d e t a i l about t h e amorphous phases which form a t t h e i n t e r f a c e between s i l i c a t e m i n e r a l s and the aqueous phase, and the
3.
GIESE A N D COSTANZO
Water on the Surface of Kaolin
model must be used w i t h c a u t i o n f o r cates.
Minerals
m i n e r a l s other than
51
phyllosili-
Summary An u n d e r s t a n d i n g o f much o f aqueous g e o c h e m i s t r y r e q u i r e s an a c c u r a t e d e s c r i p t i o n o f the w a t e r - m i n e r a l i n t e r f a c e . Water m o l e c u l e s i n c o n t a c t w i t h , o r c l o s e t o , the s i l i c a t e s u r f a c e a r e i n a d i f f e r e n t environment t h a n m o l e c u l e s i n b u l k w a t e r , and i t i s g e n e r a l l y agreed t h a t t h e s e adsorbed water m o l e c u l e s have d i f f e r e n t p r o p e r t i e s than bulk water. Because t h i s i n t e r f a c i a l c o n t a c t i s so i m p o r t a n t , t h e adsorbed water has been e x t e n s i v e l y s t u d i e d . S p e c i f i c a l l y , two major q u e s t i o n s have been examined: 1) how do the p r o p e r t i e s o f s u r f a c e adsorbed water d i f f e r from b u l k w a t e r , and 2) t o what d i s t a n c e i s water p e r t u r b e d by the s i l i c a t e s u r f a c e ? These a r e d i f f i c u l t q u e s t i o n s t o answer because the i n t e r f a c i a l r e g i o n n o r m a l l y i s a v e r y s m a l l p o r t i o n o f the w a t e r - m i n e r a l s y s t e m . To i n c r e a s e the p r o p o r t i o n o f s u r f a c e t o b u l k , the expanding c l a y m i n e r a l s , w i t h t h e i r l a r g e s p e c i f i c s u r f a c e a r e a s , have proved t o be u s e f u l e x p e r i mental m a t e r i a l s . Based on t h e s t u d y o f e x p a n d i n g c l a y m i n e r a l s , two models o f water adsorbed on s i l i c a t e s u r f a c e s have been p r o p o s e d . One s t a t e s t h a t o n l y a few l a y e r s (
