10 Polysaccharide Polyelectrolytes W. M. PASIKA Chemistry Department, Laurentian University, Sudbury, Ontario, Canada
Macromolecules which possess a large number of some functionality and ionize in aqueous media are called polyelectrolytes. Ionization of the attached function aids in the solubilization of the polyelectrolyte substance and is responsible for its unique properties. Although the ionogenic function may be regarded as a salt, dissolution of the polyelectrolyte substance is not comparable to the dissolution of a simple salt. A simple salt such as sodium chloride in solution produces a cation and an anion of comparable size. Each ion has independent mobility. A polyelectrolyte dissolves to yield a polyion and counter ions. The polyion holds a large number of charges in close proximity because they are attached to the macromolecular backbone. Although the polyion has mobility, the individual charges attached to the chain do not. They remain within the domain of the macromolecular c o i l . Not all the gegions or counterions are completely mobile. Anionic polyelectrolytes have positive counter ions whereas cationic polyelectrolytes have negative counter ions. Polyampholytes can acquire either positive or negative charge along the macromolecular backbone depending upon the composition of the solution. P i c t o r i a l l y , one has the following
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Because o f f r e e energy r e s t r i c t i o n s , n o t a l l the i o n o g e n i c groups " i o n i z e " . Many e x i s t as i o n p a i r s . A l a r g e number of p o l y s a c c h a r i d e p o l y e l e c t r o l y t e s can be i s o l a t e d from a v a r i e t y o f n a t u r a l s o u r c e s , h e p a r i n , h y a l u r o n i c a c i d , c h o n d r o i t i n and k e r a t i n , t o name a few, are i s o l a t e d from a n i m a l s o u r c e s . The more f a m i l i a r examples s u p p l i e d by the p l a n t w o r l d a r e p e c t i n i c a c i d s , a l g i n a t e s and carageenan. A number of p o l y s a c c h a r i d e p o l y e l e c t r o l y t e s , such as Xanthan, can be o b t a i n e d from nonpathogenic m i c r o organisms (1) . The common c h a r a c t e r i s t i c i s t h a t the macromolecular backbone i s composed o f s a c c h a r i d e r e s i d u e s c a r r y i n g i o n o g e n i c groups. The l a t t e r are more o f t e n than not c a r b o x y l or s u l f a t e f u n c t i o n s . " S y n t h e t i c " p o l y s a c c h a r i d e p o l y e l e c t r o l y t e s can be o b t a i n e d by s u i t a b l y d e r i v a t i z i n g p o l y s a c c h a r i d e s . The e n s u i n g d i s c u s s i o n w i l l focus on d e r i v a t i z e d d e x t r a n i n an attempt t o i l l u s t r a t e some o f the f a c t o r s which i n f l u e n c e the c h a r a c t e r i s t i c s o f p o l y saccharide p o l y e l e c t r o l y t e s . Viscosity. A l l macromolecular substances i n s o l u t i o n enhance the v i s c o s i t y o f the s o l v e n t c o n s i d e r a b l y . The l a r g e r the m o l e c u l a r weight o r macromolecular s i z e , the g r e a t e r the enhancement. In c h a r a c t e r i z i n g the macromolecular s i z e through the v i s c o s i t y enhancement, i t i s more c o n v e n i e n t l y done w i t h the v i s c o s i t y f u n c t i o n s l i s t e d i n F i g . 1. The dependence o f reduced v i s c o s i t y on c o n c e n t r a t i o n o f n e u t r a l macrom o l e c u l a r s u b s t a n c e s ( i . e . , dextran) i s l i n e a r as d e p i c t e d i n F i g . 1. E x t r a p o l a t i o n o f the v i s c o s i t y d a t a t o " z e r o " c o n c e n t r a t i o n y i e l d s the i n t r i n s i c v i s c o s i t y , which measures the hydrodynamic volume p e r a gram o f macromolecular s u b s t a n c e a t i n f i n i t e dilution. The reduced v i s c o s i t y which p e r t a i n s t o s o l u t i o n s o f f i n i t e c o n c e n t r a t i o n has the same u n i t s o f volume p e r gram o f s u b s t a n c e . P o l y e l e c t r o l y t e s ( i . e . , d e x t r a n s u l f a t e ) i n water do not e x h i b i t l i n e a r reduced v i s c o s i t y curves over the c o n c e n t r a t i o n range t h a t m a c r o m o l e c u l a r subs t a n c e s are u s u a l l y s t u d i e d ( 1%). The reduced v i s c o s i t y curve i s a c o n t i n u o u s l y i n c r e a s i n g f u n c t i o n with d i l u t i o n ( F i g . 2). The c o n t i n u a l i n c r e a s e w i t h d i l u t i o n does n o t o c c u r i n d e f i n i t e l y . At extremely low c o n c e n t r a t i o n s ( 10" ) the reduced v i s c o s i t y f u n c t i o n decreases very r p i d l y with f u r t h e r d i l u t i o n . S h o u l d the d i l u t i n g aqueous s o l v e n t c o n t a i n an e l e c t r o l y t e such as NaC3. e t c . , the reduced v i s c o s i t y a
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curves e x h i b i t maxima a t f i n i t e c o n c e n t r a t i o n s . The l a r g e r the e x t e r n a l s a l t c o n c e n t r a t i o n , the s m a l l e r the reduced v i s c o s i t y v a l u e s and t h e f u r t h e r t o t h e r i g h t t h e maximum reduced v i s c o s i t y v a l u e tends t o appear ( F i g . 2 ) . A l i n e a r dependence o f reduced v i s c o s i t y on p o l y e l e c t r o l y t e c o n c e n t r a t i o n i s o b t a i n e d i n the presence o f a s u f f i c i e n t l y high e x t e r n a l s a l t concentration. The v i s c o s i t y b e h a v i o u r o f p o l y e l e c t r o l y t e s i s governed by the f i r s t , second and t h i r d e l e c t r o v i s c o u s e f f e c t (2) ( F i g . 3 ) . The 1 s t e l e c t r o v i s c o u s e f f e c t a r i s e s because o f the d i f f e r e n c e i n s i z e o f t h e macro i o n and t h e c o u n t e r i o n s . I n an hydrodynamic g r a d i e n t , t h e s m a l l c o u n t e r i o n s a r e swept a l o n g more r a p i d l y than t h e much l a r g e r macro i o n . Charge s e p a r a t i o n o f t h e c o u n t e r i o n c l o u d from t h e macro i o n occurs. Because t h e two a r e c o u p l e d by a coulombic type i n t e r a c t i o n , t h e l a r g e r macro i o n a c t s as a b r a k e on t h e c o u n t e r i o n movement. This increases the v i s c o s i t y of the s o l u t i o n . I n s o l u t i o n , as t h e l i q u i d f l o w s , macro i o n s w i l l be d r i v e n p a s t each o t h e r because o f t h e hydrodynamic g r a d i e n t . Should the h i g h l y charged macro i o n s pass c l o s e l y , c o u l o m b i c r e p u l s i v e f o r c e s w i l l come i n t o p l a y . The f a s t e r moving macro i o n w i l l d e v i a t e from i t s i n i t i a l l i n e a r pathway. A g a i n , excess energy i s expended and t h e v i s c o s i t y o f t h e medium i s i n c r e a s e d . The l a r g e r the charge on t h e macro i o n , t h e s t r o n g e r w i l l be t h e 2nd e l e c t r o v i s c o u s e f f e c t . The 3 r d e l e c t r o v i s c o u s e f f e c t a r i s e s because o f t h e i n t e r a c t i o n o f t h e charges t h a t a r e a t t a c h e d t o t h e macromolecular backbone. I n t h e case o f a f l e x i b l e m a c r o m o l e c u l a r c o i l , t h i s i n t e r a c t i o n expands t h e c o i l t o an average c o n f o r m a t i o n which m i n i m i z e s t h e r e p u l s i v e i n t e r a c t i o n s . A t t h e new e q u i l i b r i u m c o n f o r m a t i o n ( l a r g e r than t h a t of the n e u t r a l macromolecule), the c o n t r a c t i l e f r e e energy o f t h e m a c r o m o l e c u l a r backbone i s e q u a l t o t h e e x p a n s i v e coulombic f r e e energy a r i s i n g from i o n i zation. The i n c r e a s e d m a c r o m o l e c u l a r c o i l s i z e enhances t h e v i s c o s i t y o f t h e s o l u t i o n . The v i s c o s i t y b e h a v i o u r t o t h e l e f t o f t h e maxima i n F i g . 2 i s p r i m a r i l y due t o t h e 2nd e l e c t r o v i s c o u s e f f e c t , w h i l e t h a t t o t h e r i g h t i s p r i m a r i l y due t o t h e 3 r d e l e c t r o viscous e f f e c t . Not a l l o f the c o u n t e r i o n s o f a p o l y e l e c t r o l y t e a r e f r e e t o move about. The f r e e i o n s form a counteion cloud about t h e p o l y i o n , whereas t h e i m m o b i l i z e d i o n s a r e bound t o a s p e c i f i c s i t e o r p o i n t o f t h e macromolecular backbone. T h i s model was p r e s e n t e d e a r l i e r i n the p o l y e l e c t r o l y t e d i s s o l u t i o n equation.
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As t h e p o l y e l e c t r o l y t e s o l u t i o n i s d i l u t e d more and more o f t h e s i t e bound c o u n t e r i o n s a r e r e l e a s e d . T h i s b u i l d s up t h e charge on t h e macro i o n which expands, which i n t u r n i n c r e a s e s t h e reduced v i s c o s i t y . E x p a n s i o n on d i l u t i o n , however, cannot o c c u r i n definitely. When t h e c o n c e n t r a t i o n o f t h e e x t e r n a l i o n s o f t h e s o l u t i o n become e q u a l t o o r g r e a t e r than t h a t of the counterions o f the p o l y e l e c t r o l y t e , i o n i z a t i o n of the p o l y e l e c t r o l y t e ceases. Further d i l u t i o n d e c r e a s e s t h e reduced v i s c o s i t y because e x p a n s i o n o f t h e c o i l has c e a s e d and t h e charged p a r t i c l e s a r e p l a c e d f u r t h e r and f u r t h e r a p a r t , c a u s i n g a r e d u c t i o n i n t h e 2nd e l e c t r o v i s c o u s e f f e c t . This i s t h e o r i g i n o f the maxima o f t h e r e d u c e d v i s c o s i t y curves. Dextran P o l y e l e c t r o l y t e
Behaviour.
A s u f f i c i e n t l y large external s a l t concentration w i l l y i e l d l i n e a r reduced v i s c o s i t y - c o n c e n t r a t i o n plots. L i n e a r i t y , however, does n o t i n s u r e t h a t t h e v i s c o s i t y b e h a v i o u r i s t h a t o f t h e n e u t r a l macromolecule. F i g . 4 shows t h e r e d u c e d v i s c o s i t y b e h a v i o u r o f a B-512 l i n e a r d e x t r a n ( Jjt^"] 0.164 d l / g ) and a b r a n c h e d d e x t r a n B - 7 4 2 ( f j \ J - 0.158 d l / g ) and the s u l f a t e d e r i v a t i v e s d e r i v e d from them. Despite l i n e a r i t y , the reduced v i s c o s i t i e s o f the s u l f a t e s are h i g h e r than t h o s e o f t h e n e u t r a l m o l e c u l e s by a f a c t o r o f about two. The d i f f i c u l t y i n c o l l a p s i n g t h e s u l f a t e macromolecular c o i l t o t h e s i z e o f t h e n e u t r a l macromolecule may stem from one o f two f a c t o r s or a combination o f both. Introduction of the s u l f a t e group may d e c r e a s e t h e f l e x i b i l i t y o f t h e macrom o l e c u l a r backbone. A r i g i d backbone tends t o produce a more extended m a c r o m o l e c u l a r c o n f o r m a t i o n which would e x h i b i t h i g h e r r e d u c e d v i s c o s i t i e s . Alternately, a l t h o u g h s t r o n g l o n g range coulombic i n t e r a c t i o n s have been e l i m i n a t e d by t h e e x t e r n a l s a l t , i t may be t h a t s h o r t range i n t e r a c t i o n s o f t h e i o n p a i r s e x i s t . E f f e c t o f Degree o f S u b s t i t u t i o n . The r e d u c e d v i s c o s i t i e s o f a number o f p o t a s s i u m d e x t r a n s u l f a t e s o f d i f f e r i n g degree o f s u b s t i t u t i o n d e r i v e d from B-742(CnJ]*0.158) a r e shown i n F i g . 5. Increasing the degree o f s u b s t i t u t i o n enhances t h e reduced v i s c o s i t y and s h i f t s t h e p o s i t i o n a t which t h e maximum r e d u c e d v i s c o s i t y appears t o t h e l e f t . I n c r e a s i n g t h e number o f i o n o g e n i c groups produces more charge on t h e macro i o n , c a u s i n g g r e a t e r expansion o f the c o i l . On d i l u t i o n , f u r t h e r i o n i z a t i o n
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and e x p a n s i o n o c c u r i n each c a s e . The h i g h e r the degree o f s u b s t i t u t i o n , the f u r t h e r must the p o l y e l e c t r o l y t e s o l u t i o n be d i l u t e d t o match the e x t e r n a l s a l t c o n c e n t r a t i o n w i t h the c o u n t e r i o n c o n c e n t r a t i o n of the p o l y e l e c t r o l y t e . S i m i l a r v i s c o s i t y behaviour i s o b s e r v e d f o r l i n e a r d e x t r a n s u l f a t e s and f o r branched and l i n e a r carboxymethyl d e x t r a n s . The t y p i c a l p o l y e l e c t r o l y t e v i s c o s i t y c u r v e s e x h i b i t e d by d e x t r a n s u g g e s t t h a t the macromolecular backbone i s f a i r l y f l e x i b l e and t h a t the c o i l can undergo e x p a n s i o n on a c q u i r i n g c h a r g e . E f f e c t o f M o l e c u l a r Weight. F i g . 6 i n d i c a t e s the e f f e c t o f m o l e c u l a r w e i g h t on p o t a s s i u m carboxymethyl d e x t r a n reduced v i s c o s i t y c u r v e s . The degree o f s u b s t i t u t i o n i s c o n s t a n t and the m o l e c u l a r w e i g h t v a r i e s from 73,000 t o 135,000. The r e d u c e d v i s c o s i t i e s i n c r e a s e w i t h m o l e c u l a r weight and the c o n c e n t r a t i o n at which the reduced v i s c o s i t y maximum appears i s i d e n t i c a l f o r a l l three molecular weights. I t would appear t h a t the m o l e c u l a r w e i g h t does n o t i n f l u e n c e the e x t e n t o r degree o f i o n i z a t i o n and t h a t the e x p a n s i o n i s d i r e c t l y p r o p o r t i o n a l t o the number o f s u b s t i t u t e d a n h y d r o g l u c o s e u n i t s i n the macromolecule £ ( \sf>/ )>τ*χχ 135,000 m o l e c u l a r weight sample a p p r o x i m a t e l y 2x ( T\* / c )VH*X o f 73,000 m o l e c u l a r weight samplej . T h i s s u g g e s t s t h a t the i n t e r a c t i o n of the i o n o g e n i c groups i s a l o c a l i z e d o r n e a r e s t neighbor i n t e r a c t i o n . S h o u l d i t be o t h e r w i s e , then each charge o f p o l y e l e c t r o l y t e would i n t e r a c t w i t h e v e r y o t h e r , compounding the i n t e r a c t i o n s . The h i g h e r m o l e c u l a r weight macromolecule c a r r y i n g more charge would r e g i s t e r a n o n - p r o p o r t i o n a t e reduced viscosity. The l i n e a r p r o p o r t i o n a l i t y between m o l e c u l a r w e i g h t and the maximum reduced v i s c o s i t y would n o t e x i s t . To show more q u a n t i t a t i v e l y t h a t the same i o n i z a t i o n and e x p a n s i o n p r o c e s s i s o c c u r r i n g w i t h the d i f f e r e n t m o l e c u l a r w e i g h t s , the d a t a o f F i g . 6 can be p l o t t e d i n terms o f a r e l a t i v e e x p a n s i o n f a c t o r R vs the c o n c e n t r a t i o n o f p o t a s s i u m carboxy methyl d e x t r a n as i n F i g . 7. The numerator o f R i s the maximum reduced v i s c o s i t y and the denominator i s the r e d u c e d v i s c o s i t y a t a p o l y e l e c t r o l y t e c o n c e n t r a t i o n g r e a t e r than t h a t a t which the maximum v i s c o s i t y appears. The c o i n c i d e n c e o f the l i n e a r p l o t s f o r the t h r e e m o l e c u l a r w e i g h t s i n d i c a t e s an i o n i z a t i o n e x p a n s i o n mechanism t h a t i s i d e n t i c a l f o r the t h r e e p o l y e l e c t r o l y t e samples. o
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