The Role of the Solvent in Polymer Adsorption: Displacement and

4 The Role of the Solvent in Polymer Adsorption: Displacement and Solvency Effects M. A. COHEN STUART, J. M. H. M. SCHEUTJENS, and G. J. FLEER

Downloaded by UNIV OF LIVERPOOL on October 13, 2015 | http://pubs.acs.org Publication Date: February 10, 1984 | doi: 10.1021/bk-1984-0240.ch004

Laboratory for Physical and Colloid Chemistry, Agricultural University, De Dreijen 6, 6703 BC Wageningen, The Netherlands In adsorption, the solvent always plays a double role, affecting both lateral interactions between the adsorbate molecules and determining the effec tive interaction between the surface and the adsor bate. For polymers, this means that they adsorb strongly from some solvents, whereas from others they do not at a l l . As a consequence, mixed sol vents can give rise to an adsorption/desorption transition: the polymer is desorbed by a so -called displacer. We investigate theoretically how the adsorp tion of the polymer varies with the displacer con centration. A simple analytical expression for the critical displacer concentration is derived, which is found to agree very well with numerical results from recent polymer adsorption theory. One of the applications of this expression is the determina tion of segmental adsorption energies from experi mental desorption conditions and the adsorption energy of the displacer. Illustrative experiments and other applications are briefly discussed. The r o l e of the solvent i n polymer adsorption has been the subject of much d i s c u s s i o n . For example, t h e o r i e s have made p r e d i c t i o n s about the e f f e c t of the polymer/solvent i n t e r a c t i o n ( i . e . F l o r y Huggins χ-parameter) on adsorption. For many systems, χ-parameters had already been tabulated so that a number of adsorption studies focused a t t e n t i o n on t h i s parameter. In s p i t e of much e f f o r t , a v a i l a b l e data are ambiguous, sometimes v e r i f y i n g and sometimes c o n t r a d i c t i n g the trends p r e d i c t e d by theory. Of course, many i n v e s t i g a t o r s have r e a l i s e d that changes i n solvent may not only a f f e c t the χ-parameter, but a l s o the e f f e c t i v e polymer/surface i n t e r a c t i o n , expressed by the adsorption

0097-6156/84/0240 0053$06.00/ 0 © 1984 American Chemical Society

In Polymer Adsorption and Dispersion Stability; Goddard, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

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energy parameter x . I n f a c t , i t i s almost impossible t o vary these parameters independently and t o o b t a i n a c l e a r p i c t u r e o f t h e i r separate e f f e c t s i n experimental s i t u a t i o n s . An i l l u s t r a t i v e e x a m p l e i s t h e w o r k o f C l a r k e t a l , on t h e c o n f o r m a t i o n o f p o l y ( v i n y l p y r r o l i d o n e ) (PVP) a d s o r b e d on s i l i c a (1) . These a u t h o r s d e t e r m i n e d bound f r a c t i o n s f r o m m a g n e t i c r e s o nance e x p e r i m e n t s . I n one i n s t a n c e t h e y added a c e t o n e t o a n aqueous s o l u t i o n o f PVP i n o r d e r t o a c h i e v e t h e t a c o n d i t i o n s f o r t h i s p o l y m e r . They e x p e c t e d t o o b s e r v e an i n c r e a s e i n t h e bound f r a c t i o n on t h e b a s i s o f s o l v e n c y e f f e c t s a s p r e d i c t e d by a l l modern p o l y m e r a d s o r p t i o n t h e o r y (2-6), b u t f o u n d e x a c t l y t h e o p p o s i t e e f f e c t . T h e i r e x p l a n a t i o n was p l a u s i b l e , namely t h a t acetone, with a b i l i t y t o a d s o r b s t r o n g l y on s i l i c a due t o i t s c a r b o n y l g r o u p , w o u l d be a b l e t o p a r t i a l l y d i s p l a c e t h e p o l y m e r by c o m p e t i n g f o r t h e a v a i l a b l e s u r f a c e s i t e s . Such d i s p l a c e m e n t e f f e c t s , a l t h o u g h o f t e n v e r y p r o n o u n c e d , have n o t y e t b e e n s t u d i e d s y s t e m a t i c a l l y . They w i l l be t h e s u b j e c t o f t h e p r e s e n t p a p e r . We w i l l d i s c u s s t h e a d s o r p t i o n o f p o l y m e r f r o m a m i x t u r e o f two s o l v e n t s and we w i l l see t h a t i n some c a s e s d r a s t i c e f f e c t s occur as a f u n c t i o n o f t h e m i x t u r e composition. A l s o , we e x p l o r e some c o n s e q u e n c e s a n d p r a c t i c a l a p p l i c a t i o n s o f displacement. I t turns out that displacement s t u d i e s not only i n c r e a s e o u r i n s i g h t on t h e r o l e o f t h e s o l v e n t i n p o l y m e r a d s o r p t i o n b u t c a n a l s o be u s e d t o d e t e r m i n e t h e s e g m e n t a l a d s o r p t i o n e n e r g y . So f a r , e x p e r i m e n t a l d a t a f o r t h i s q u a n t i t y were v e r y s c a r c e . Some i l l u s t r a t i v e e x p e r i m e n t s w i l l be d i s c u s s e d b r i e f l y .


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Theoretical For adsorption from s o l u t i o n t o occur, i t i s necessary that the exchange o f s o l v e n t a t t h e s u r f a c e w i t h a d s o r b a t e i n s o l u t i o n r e s u l t s i n a n e t d e c r e a s e o f t h e f r e e e n e r g y . The e x c h a n g e f r e e e n e r g y , i . e . t h e d i f f e r e n c e i n f r e e energy between s o l v e n t / s u r f a c e and a d s o r b a t e / s u r f a c e c o n t a c t s i s a measure o f t h e e f f e c t i v e i n t e r a c t i o n between s u r f a c e and a d s o r b a t e . S i l b e r b e r g (2) i n t r o duced t h e d i m e n s i o n l e s s segmental a d s o r p t i o n energy parameter d e n o t e d a s x w h i c h r e f e r s t o t h e a f o r e - m e n t i o n e d exchange p r o c e s s . Of c o u r s e , f o r a g i v e n p o l y m e r and s u r f a c e , x may have b o t h p o s i t i v e and n e g a t i v e v a l u e s , d e p e n d i n g on t h e s o l v e n t . I f x i s n e g a t i v e , no a d s o r p t i o n o f p o l y m e r o c c u r s . F o r p o s i t i v e x , adsorption i s p o s s i b l e , provided that x i s l a r g e r than a c r i t i c a l a d s o r p t i o n e n e r g y x . The r e a s o n f o r a n o n - z e r o x (usually est i m a t e d as a few t e n t h s o f kT) i s t h a t t h e l o s s i n c o n f o r m a t i o n a l e n t r o p y e x p e r i e n c e d by a n a d s o r b e d segment must be o u t w e i g h e d b y a n e t d e c r e a s e i n a d s o r p t i o n e n e r g y . Thus y s e p a r a t e s two r e g i m e s ; there are solvents f o r which x > x ( a d s o r p t i o n ) and s o l v e n t s for which x < x (no a d s o r p t i o n ) . An o b v i o u s c o n s e q u e n c e i s t h a t i n mixtures o f s o l v e n t s from e i t h e r regime a d s o r p t i o n - d e s o r p t i o n t r a n s i t i o n s o c c u r when t h e c o m p o s i t i o n o f t h e m i x t u r e i s v a r i e d . In d e a l i n g with binary solvent mixtures i t i s necessary t o d i s s

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t i n g u i s h b e t w e e n t h e two c o m p o n e n t s . We w i l l d e n o t e t h e more s t r o n g l y a d s o r b i n g component ( f o r w h i c h u s u a l l y x < X ) a s t h e displacer a n d t h e o t h e r one a s t h e solvent. An a d s o r p t i o n - d e s o r p t i o n t r a n s i t i o n i s i l l u s t r a t e d s c h e m a t i c a l l y i n F i g u r e 1, where we p l o t a d i s p l a c e m e n t i s o t h e r m , i . e . t h e a d s o r b e d amount o f a p o l y m e r a s a f u n c t i o n o f t h e c o m p o s i t i o n o f a m i x t u r e o f s o l v e n t a n d d i s p l a c e r . A t t h e l e f t i n F i g u r e 1, where t h e c o n c e n t r a t i o n o f d i s p l a c e r i s l o w , t h e p o l y m e r s u r f a c e e x c e s s i s p o s i t i v e . A s we i n c r e a s e t h e p r o p o r t i o n o f d i s p l a c e r i n t h e m i x t u r e , we o b s e r v e a d e c r e a s e i n t h e a d s o r b e d amount. A t a c e r t a i n c o m p o s i t i o n t h e a d s o r b e d amount o f p o l y m e r becomes z e r o . The c o n c e n t r a t i o n a t w h i c h t h e p o l y m e r s u r f a c e e x c e s s j u s t v a n i s h e s w i l l be d e n o t e d a s t h e critical displacer concentration 4> . Beyond , t h e s u r f a c e e x c e s s o f t h e p o l y m e r i s n e g a t i v e ( a n d very s m a l l i f t h e polymer c o n c e n t r a t i o n i s l o w ) . R e c e n t p o l y m e r a d s o r p t i o n t h e o r i e s , s u c h a s t h o s e o f Roe ( 3 ) and o f S c h e u t j e n s a n d F l e e r ( 4 ) a l l o w t h e c a l c u l a t i o n o f d i s p l a cement i s o t h e r m s , so t h a t we c o u l d s t u d y t h e d e p e n d e n c y o f t h e s e i s o t h e r m s o n v a r i o u s p a r a m e t e r s b y n u m e r i c a l m e t h o d s . However, a l l t h e e s s e n t i a l f e a t u r e s o f d i s p l a c e m e n t c a n a l s o be d e m o n s t r a t e d b y means o f a s i m p l e a n a l y t i c a l e x p r e s s i o n f o r t h e c r i t i c a l p o i n t , w h i c h c a n be d e r i v e d i n a s t r a i g h t f o r w a r d way. The model u n d e r l y i n g t h i s e q u a t i o n i s t h e l a t t i c e model f o r a r e g u l a r s o l u t i o n , a d j a c e n t t o an a d s o r b i n g w a l l . L a y e r s o f l a t t i c e s i t e s p a r a l l e l t o t h e w a l l a r e numbered 1, 2, .., i , ... and e a c h s i t e may b e o c c u p i e d b y e i t h e r a p o l y m e r segment ( p ) , a s o l v e n t molecule (o) o r a d i s p l a c e r molecule ( d ) . Each s i t e has ζ n e a r e s t neighbours, f r a c t i o n s λ and of which a r e i n the same,and i n e a c h o f t h e two n e i g h b o u r i n g l a y e r s , r e s p e c t i v e l y . A s we have t h r e e c o m p o n e n t s , we h a v e t h r e e a d s o r p t i o n e n e r g y parame t e r s , one f o r e a c h p a i r ; χ|° ( p o l y m e r f r o m s o l v e n t ) , (polymer f r o m d i s p l a c e r ) a n d χ|° ( d i s p l a c e r f r o m s o l v e n t ) . B e c a u s e o f t h e i : exchange c h a r a c t e r , these t h r e e parameters a r e n o t independent b u t a r e r e l a t e d i n a c y c l i c way: g

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56

t e l y a d j a c e n t t o the a d s o r b i n g s u r f a c e c o n t r i b u t e s t o the f r e e e n e r g y and we c a n d e s c r i b e t h e e q u i l i b r i u m e n t i r e l y by c o n s i d e r i n g o n l y the changes o c c u r i n g i n t h a t l a y e r . L e t us c a l c u l a t e t h e change i n f r e e e n e r g y àf a s s o c i a t e d w i t h t h e e x c h a n g e o f a d s o r b e d d i s p l a c e r m o l e c u l e s a g a i n s t segments i n t h e b u l k s o l u t i o n . Volume f r a c t i o n s φ w i l l be a s s i g n e d an u p p e r i n d e x r e f e r r i n g t o t h e c o r r e s p o n d i n g component (ρ, ο o r d ) , and a lower index r e f e r r i n g to e i t h e r the s u r f a c e l a y e r (s) or the b u l k s o l u t i o n ( * ) . We w i l l assume t h a t t h e s o l u t i o n i s d i l u t e i n p o l y mer: φ£ α ο

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p l a c e r a d s o r p t i o n i s o t h e r m s . S e g m e n t a l a d s o r p t i o n e n e r g i e s were c a l c u l a t e d , u s i n g data f o r a l l four d i s p l a c e r s . A f u l l account of the a n a l y s i s i s g i v e n i n r e f . 8. H e r e , we m e r e l y r e p o r t t h a t good agreement was o b t a i n e d and t h a t s e g m e n t a l a d s o r p t i o n e n e r g i e s o f 4 kT were f o u n d , b o t h f o r a d s o r p t i o n f r o m w a t e r and f r o m d i o x a n e . The a g r e e m e n t w i t h i n f e r e n c e s f r o m o t h e r e x p e r i m e n t s ( e . g . , c a l o r i m e t r y and s p e c t r o s c o p i c s t u d i e s ( 1 0 , 11)) is" q u i t e s a t i s f a c t o r y . I t i s hoped t h a t t h e method f i n d s w i d e r a p p l i c a t i o n . A d s o r p t i o n chromatography. A very i n t e r e s t i n g a p p l i c a t i o n of d i s placement i s i n a d s o r p t i o n chromatography. For e f f e c t i v e separa t i o n t o o c c u r , i t i s n e c e s s a r y t h a t t h e s o l u t e a d s o r b s w e a k l y on the s t a t i o n a r y phase. Since polymers i n a s i n g l e s o l v e n t are u s u a l l y e i t h e r not e l u t e d (because they adsorb too s t r o n g l y ) or move w i t h t h e e l u e n t f r o n t ( b e c a u s e t h e y do n o t a d s o r b a t a l l ) s o l v e n t m i x t u r e s must be f o u n d w h i c h b r i n g a b o u t t h e weak a d s o r p t i o n , i . e . c l o s e t o t h e c r i t i c a l p o i n t . E x p e r i m e n t s by G l o c k n e r e t a l . (12) and B e l e n k y (13) have i n d e e d shown t h a t e f f e c t i v e s e p a r a t i o n s occur o n l y i n a v e r y narrow range of c o m p o s i t i o n , thereby c l e a r l y demonstrating the ' c r i t i c a l c h a r a c t e r of d i s p l a cement. I t f o l l o w s t h a t φ may be q u i t e a c c u r a t e l y d e t e r m i n e d by c h r o m a t o g r a p h i c p r o c e d u r e s . An example i s shown i n F i g u r e 5, where t h e r e t e n t i o n f a c t o r Rp i s p l o t t e d as a f u n c t i o n o f m i x t u r e com p o s i t i o n . A sharp t r a n s i t i o n i s observed. Snyder (14) has g i v e n a d s o r p t i o n c h r o m a t o g r a p h y i t s t h e o r e t i c a l b a s i s . He c o l l e c t e d many a d s o r p t i o n e n e r g i e s f o r s o l v e n t s commonly u s e d as e l u e n t s , and d e v e l o p e d a method t o p r e d i c t t h e 'elution strength' ε ( e f f e c t i v e a d s o r p t i o n energy per u n i t of s u r f a c e a r e a ) o f a s o l v e n t m i x t u r e on t h e b a s i s o f an i d e a l s o l u t i o n ( L a n g m u i r a d s o r p t i o n ) m o d e l . A good c o r r e l a t i o n was f o u n d when s e g m e n t a l a d s o r p t i o n e n e r g i e s , c a l c u l a t e d by means o f ' i n c r e m e n t a d d i t i o n ' a c c o r d i n g t o S n y d e r , were compared w i t h v a l u e s a t a f i x e d r e t e n t i o n f a c t o r (12). Although Snyder's ex p r e s s i o n does n o t i n c l u d e non i d e a l i t y c o n t r i b u t i o n s , as does o u r E q u a t i o n 5, i t seems t o d e s c r i b e a l a r g e number o f e x p e r i m e n t a l d a t a r e a s o n a b l y w e l l . A l i t e r a t u r e s u r v e y (15) r e v e a l s many examples of s u c c e s f u l s e p a r a t i o n s of polymer homologues. Snyder's a d s o r p t i o n e n e r g i e s are a b s o l u t e ones, i n s t e a d of exchanged p a r a m e t e r s . T h u s , t h e r e s h o u l d be a r e l a t i o n b e t w e e n χ°Ρ and t h e d i f f e r e n c e i n a d s o r p t i o n e n e r g y b e t w e e n a segment and a s o l v e n t m o l e c u l e , as t a b u l a t e d by S n y d e r . A c o m p l i c a t i o n i s t h a t a c o r r e c t i o n i s necessary f o r d i f f e r e n c e s i n molecular/segmental area, and f o r s o l v e n c y e f f e c t s . H e n c e , a l t h o u g h a more t h o r o u g h a n a l y s i s i s w a n t e d , i t i s p r o b a b l e t h a t t h e t a b l e s by Snyder c o n s t i t u t e a v a l u a b l e s o u r c e o f i n f o r m a t i o n on x , a t l e a s t f o r s i l i c a and alumina substrates. 1

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O t h e r a p p l i c a t i o n s . D i s p l a c e m e n t must a l s o have i t s i m p a c t on c o l l o i d a l s t a b i l i t y . The r e l a t i o n b e t w e e n t h e a d s o r p t i o n o f p o l y mers on c o l l o i d a l p a r t i c l e s and t h e r e s u l t a n t s t e r i c s t a b i l i t y



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a g a i n s t f l o c c u l a t i o n was, a n d s t i l l i s , s t u d i e d e x t e n s i v e l y . R e c e n t l y , a n o n - a d s o r p t i v e f l o c c u l a t i o n mechanism h a s b e e n s t u d i e d by v a r i o u s i n v e s t i g a t o r s ( 1 6 - 2 0 ) . The c o n d i t i o n s f o r n o n - a d s o r b i n g p o l y m e r may be o b t a i n e d b y a d d i n g a s u i t a b l e d i s p l a c e r . I n o t h e r c a s e s , a s t e r i c a l l y s t a b i l i z e d s y s t e m may be d e s t a b i l i z e d b y merely removing t h e s t e r i c b a r r i e r by t h e a c t i o n o f a s u f f i c i e n t l y powerful d i s p l a c e r . Both p o s s i b i l i t i e s , being a l s o t e c h n o l o g i c a l l y i n t e r e s t i n g , d e s e r v e more a t t e n t i o n . So f a r , we d e a l t w i t h t h e d i s p l a c e m e n t o f s i m p l e h o m o p o l y m e r s . However, t h e phenomenon o f d i s p l a c e m e n t i s b y no means r e s t r i c t e d t o s u c h s i m p l e m a c r o m o l e c u l e s . C o p o l y m e r s have b e e n s u c c e s f u l l y e l u t e d b y means o f a d s o r p t i o n c h r o m a t o g r a p h y ( 1 3 ) . E v e n c o m p l i c a t e d , c h a r g e d m a c r o m o l e c u l e s l i k e p r o t e i n s c a n be s u c c e s f u l l y d i s p l a c e d . A s an example we g i v e i n F i g u r e 6 t h e d i s p l a c e m e n t i s o t h e r m f o r human p l a s m a a l b u m i n f r o m s i l i c a b y mor p h o l i n e ( 2 1 ) . Of c o u r s e , i n t h i s c a s e where c h a r g e e f f e c t s a n d a v a r i e t y o f segment/surface i n t e r a c t i o n s p l a y a r o l e , our simple E q u a t i o n 5 does n o t a p p l y . N e v e r t h e l e s s , f o r p r a c t i c a l w o r k i t i s i m p o r t a n t t o r e a l i z e t h a t most m a c r o m o l e c u l e s , o f t e n t h o u g h t t o be i r r e v e r s i b l y a d s o r b e d , c a n be removed c o m p l e t e l y f r o m t h e a d s o r b e n t s u r f a c e b y t h e c o n c e r t e d a c t i o n o f a l a r g e number o f small molecules. We c o n c l u d e t h a t b i n a r y m i x t u r e s o f s o l v e n t a n d d i s p l a c e r p r o v i d e u s w i t h a v a l u a b l e t o o l t o o b t a i n i n f o r m a t i o n on t h e i n t e r a c t i o n between a p o l y m e r i c a d s o r b a t e and t h e s u b s t r a t e . H o p e f u l l y d i s p l a c e m e n t s t u d i e s w i l l p r o v i d e u s w i t h many new i n s i g h t s i n the near f u t u r e . Legend o f symbols Some s y m b o l s c a r r y u p p e r i n d i c e s ο, ρ , d. These i n d i c e s r e f e r t o t h e r e s p e c t i v e components s o l v e n t , p o l y m e r , d i s p l a c e r . L o w e r i n d i c e s s and * r e f e r t o s u r f a c e phase and s o l u t i o n , r e s p e c t i v e l y . p d

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The Role of the Solvent in Polymer Adsorption: Displacement and

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