Performance Relationships in Surfactants - ACS Publications

15 Binding of Alkylpyridinium Cations by Anionic Polysaccharides 1

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A. MALOVIKOVA and KATUMITU HAYAKAWA

Department of Chemistry, Dalhousie University, Halifax, Nova Scotia, Canada B3H 4J3 JAN C. T. KWAK

Downloaded by UNIV OF ARIZONA on January 5, 2013 | http://pubs.acs.org Publication Date: May 21, 1984 | doi: 10.1021/bk-1984-0253.ch015

Département de chimie, Université de Sherbrooke, Sherbrooke, Québec, Canada J1K 2R1

Solid state electrodes selective for alkylpyridinium cations are used to study the binding of these surfactants cations, with C , C and C alkyl chainlengths, to a number of anionic polyelectrolytes. The electrodes are shown to be effective from very low surfactant concentrations to the cmc, and can be used for accurate cmc determinations in solutions of high ionic strength. Binding isotherms of the alkylpyridinium cations with polyacrylate, alginate, pectate and pectinates are presented. A l l isotherms are highly cooperative. The surfactant chainlength dependence of the overall binding constant is identical to the case of micelle formation of the free surfactant, but for a given surfactant the overall binding constant depends strongly on the charge density of the polyion. 12

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The binding of ionic surfactants by polyions of opposite charge distinguishes itself from the much more widely studied case of binding by neutral water soluble polymers mainly because binding occurs at much lower free surfactant concentrations ( 1 2 ) · Thus, while the binding of ionic surfactants by neutral polymers is normally studied at concentrations close to or above the cmc of the surfactant, in the case of oppositely charged polyions and surfactant ions the first binding may take place at concentrations orders of magnitude below the cmc of the surfactant. It is therefore not surprising that a detailed study of this, binding process had to await the development of suitable analytical methods, notably surfactant selective electrodes (2-10). The pioneering paper by Satake and Yang (2) demonstrates the strongly cooperative 5

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Current address: Institute of Chemistry, Slovak Academy of Sciences, Bratislava, Czechoslovakia Current address: Department of Chemistry, Kagoshima University, Kagoshima, 890 Japan 2

0097-6156/ 84/ 0253-0225S06.00/ 0 © 1984 American Chemical Society

In Structure/Performance Relationships in Surfactants; Rosen, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1984.


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c h a r a c t e r of the b i n d i n g of d e c y l s u l f a t e anions by h i g h l y charged cationic polypeptides. These a u t h o r s i n t e r p r e t t h e i r b i n d i n g i s o therms i n terms of a n e a r e s t n e i g h b o u r i n t e r a c t i o n model, w i t h h y d r o p h o b i c i n t e r a c t i o n s b e t w e e n n e i g h b o u r i n g bound s u r f a c t a n t i o n s a c c o u n t i n g f o r the i n c r e a s e of the apparent b i n d i n g c o n s t a n t with increased binding. They a p p l y t h e t h e o r y o f Zimm and B r a g g , developed to d e s c r i b e the h e l i x c o i l t r a n s i t i o n i n p o l y p e p t i d e s (11), to the case of s u r f a c t a n t b i n d i n g by polymers, e f f e c t i v e l y f i t t i n g t h e o b s e r v e d b i n d i n g i s o t h e r m t o two p a r a m e t e r s , i . e . K, the " i n t r i n s i c b i n d i n g c o n s t a n t " between a n i s o l a t e d polymer b i n d i n g s i t e and a s i n g l e s u r f a c t a n t i o n , and u , a " c o o p e r a t i v e t y parameter" presumably determined by the hydrophobic i n t e r a c t i o n s b e t w e e n n e i g h b o u r i n g s u r f a c t a n t s . The S a t a k e - Y a n g t r e a t m e n t p a r a l l e l s the t h e o r y o f Schwarz developed t o d e s c r i b e t h e b i n d i n g of a n i o n i c d y e s t o l i n e a r b i o p o l y m e r s ( 1 2 ) , and i n f a c t t h e b i n d i n g o f l o n g c h a i n s u r f a c t a n t s may b e a much b e t t e r t e s t o f s u c h a t h e o r y t h a n t h e c a s e o f r e l a t i v e l y b u l k y and s t i f f dye m o l e c u l e s . S i n c e t h e w o r k o f S a t a k e and Y a n g , a number o f o t h e r s t u d i e s have appeared employing s u r f a c t a n t s e l e c t i v e e l e c t r o d e s to study t h e b i n d i n g o f a n i o n i c s u r f a c t a n t s b y c a t i o n i c p o l y m e r s (13-15) o r of c a t i o n i c s u r f a c t a n t s b y a n i o n i c polymers (7,10,16-19), w i t h m o s t o f t h e s e s t u d i e s r e l y i n g o n t h e t h e o r i e s o f Schwarz and S a t a k e and Yang t o d e s c r i b e t h e o b s e r v e d h i g h l y c o o p e r a t i v e b i n d i n g isotherms. A l t h o u g h i n a number o f c a s e s s p e c i a l a t t e n t i o n was g i v e n to c o n f o r m a t i o n a l changes o f the polymer induced b y s u r f a c t a n t a d s o r p t i o n (2,7,20,21) i t i s o f i n t e r e s t t o n o t e t h e d i f f e r e n c e b e t w e e n t h e m o d e l s u s e d t o d e s c r i b e t h e s e d a t a a t v e r y low s u r f a c t a n t and p o l y m e r c o n c e n t r a t i o n s , and t h e much more w i d e l y s t u d i e d c a s e o f b i n d i n g measurements c l o s e t o o r p a s t t h e s u r f a c t a n t cmc ( 1 , 2 2 - 2 8 ) . T h u s , w h e r e a s t h e g e n e r a l l y a c c e p t e d m o d e l f o r m i c e l l a r b i n d i n g e n v i s a g e s a complex where t h e polymer envel o p s many d i s t i n c t , m i c e l l a r - l i k e s u r f a c t a n t a g g r e g a t e s (24,26) i n s u r f a c t a n t i o n p o l y i o n b i n d i n g s t u d i e s a t low s u r f a c t a n t concent r a t i o n s i t i s g e n e r a l l y assumed t h a t t h e p o l y i o n m a i n t a i n s a w e l l d e f i n e d s o l u t i o n conformation, o r the p o l y i o n conformation i s not considered a t a l l , w i t h a d e s c r i p t i o n of the b i n d i n g isotherm i n t e r m s o f a n e a r e s t n e i g h b o u r m o d e l . Such a d e s c r i p t i o n l e a v e s open t h e q u e s t i o n w h e t h e r t h e h y d r o p h o b i c p a r t o f t h e bound s u r f a c t a n t r e m a i n s e x p o s e d t o t h e aqueous p h a s e , o r w h e t h e r a f t e r b i n d i n g the s u r f a c t a n t s aggregate i n t o m i c e l l e - l i k e groups, w i t h the polymer s u r r o u n d i n g the aggregates. I t i s important to note t h a t t h e b i n d i n g b e t w e e n o p p o s i t e l y c h a r g e d s u r f a c t a n t i o n s and p o l y i o n s i n i t i a l l y t a k e s p l a c e w i t h o u t p h a s e s e p a r a t i o n , and i s fully reversible. T h i s d i s t i n g u i s h e s t h e b i n d i n g measurements a t low s u r f a c t a n t c o n c e n t r a t i o n s f r o m t h e s t u d i e s o n p r e c i p i t a t i n g systems w i t h or w i t h o u t subsequent r e d i s s o l u t i o n (1,24). I n p r e v i o u s s t u d i e s we h a v e d e s c r i b e d t h e b i n d i n g o f a l k y l trimethylammonium i o n s to a v a r i e t y of p o l y a n i o n s (16-19). A s has been observed b y o t h e r a u t h o r s , b o t h the a l k y l c h a i n l e n g t h depend e n c e and t h e i o n i c - s t r e n g h t d e p e n d e n c e o f t h e b i n d i n g p r o c e s s


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Alkylpyridinium Cation Binding

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were found t o be r e m a r k a b l y s i m i l a r t o t h e case o f m i c e l l e forma t i o n , a p p a r e n t l y independent o f whether t h e p o l y i o n has a w e l l d e f i n e d b a c k b o n e c o n f i g u r a t i o n s u c h a s w i t h DNA o r p e c t i c a c i d ( 1 8 , 1 9 ) , o r may b e presumed t o b e e x t r e m e l y f l e x i b l e , e . g . d e x t r a n s u l f a t e o r p o l y ( s t y r e n e s u l f o n a t e ) (16,17). On t h e o t h e r h a n d , the b i n d i n g c o n s t a n t s f o r a g i v e n alkylammonium c a t i o n a r e found t o depend s t r o n g l y o n t h e p o l y m e r s t r u c t u r e a n d c h a r g e d e n s i t y (19). I n t h i s p a p e r we e x t e n d t h e s e measurements t o t h e c a s e o f a l k y l p y r i d i n i u m c a t i o n s w i t h d o d e c y l , t e t r a d e c y l , and h e x a d e c y l a l k y l groups ( t o be a b b r e v i a t e d as C i 2 y * C ^ P y ^ a n d C ^ P y * r e s p e c t i v e l y ) . A s p o l y a n i o n s we c h o o s e two p o l y s a c c h a r i d e s o f w e l l d e f i n e d s t r u c t u r e , i . e . a l g i n i c a c i d , a copolymer o f mannuronic and g u l u r o n i c a c i d , and p e c t i c a c i d , a l i n e a r polymer o f D - g a l a c t u r o n i c a c i d ( 2 9 ) . The i n f l u e n c e o f t h e c h a r g e d e n s i t y o f t h e p o l y i o n i s s t u d i e d by comparing p e c t a t e w i t h p e c t i n a t e s w i t h degrees o f e s t e r i f i c a t i o n o f t h e c o r r e s p o n d i n g p e c t a t e v a r y i n g f r o m 20 t o 70%. I n a d d i t i o n , r e s u l t s a r e p r e s e n t e d f o r b i n d i n g t o t h e sodium s a l t o f p o l y ( a c r y l i c a c i d ) ( P A A ) . P o l y m e r s t r u c t u r e s a r e shown i n F i g u r e 1.


p

Experimental Surfactants. C - ^ P y C l a n d C-^PyBr w e r e c o m m e r c i a l p r o d u c t s (Tokyo K a s e i K o g y i Co., L t d a n d Eastman Kodak Co., r e s p e c t i v e l y ) . T h e s e p r o d u c t s were p u r i f i e d b y repeated r e c r y s t a l l i z a t i o n from acetone and t r e a t m e n t w i t h a c t i v e c h a r c o a l . C-^PyBr was s y n t h e s i z e d b y r e a c t i n g t h e c o r r e s p o n d i n g a l k y l b r o m i d e (Eastman Kodak), p u r i f i e d by f r a c t i o n a l d i s t i l l a t i o n , w i t h a s l i g h t e x c e s s o f p y r i d i n e ( 3 0 ) . The c r u d e p r o d u c t was p u r i f i e d b y e x t r a c t i o n w i t h d i e t h y l e t h e r , f o l l o w e d b y up t o 6 r e c r y s t a l l i z a t i o n s f r o m a c e t o n e a n d t r e a t m e n t with active charcoal. Polysaccharides. N a - a l g i n a t e was i s o l a t e d f r o m L a m i n a r i a H y p e r b o rea. The g u l u r o n i c a c i d c o n t e n t was f o u n d t o b e 63.6%, c o r r e s p o n d i n g t o a n M/G ( m a n n u r o n i c a c i d / g u l u r o n i c a c i d ) r a t i o o f 0.57. The t o t a l c a r b o x y l a t e c o n c e n t r a t i o n was d e t e r m i n e d b y i o n e x c h a n g e t o t h e a c i d f o r m f o l l o w e d b y t i t r a t i o n w i t h NaOH. P e c t i n s o f v a r i o u s d e g r e e s o f e s t e r i f i c a t i o n , E, w e r e p r e p a r e d f r o m a p u r i f i e d c i t r u s p e c t i n (Genu P e c t i n , K o p e n h a g e n , Denmark) b y c o n t r o l l e d a l k a l i n e d e e s t e r i f i c a t i o n (31). Degree o f e s t e r i f i c a t i o n , i n t r i n s i c v i s c o s i t y [η] and v i s c o s i t y m o l e c u l a r w e i g h t w e r e d e t e r m i n e d u s i n g standard procedures (31,32). The c o n c e n t r a t i o n o f f r e e c a r b o x y l groups i n t h e i n i t i a l ( s t o c k ) s o l u t i o n s o f potassium p e c t a ^ t e a n d s o d i u m p e c t i n a t e w e r e d e t e r m i n e d b y p r e c i p i t a t i o n w i t h Cu (33,34). A n a l y t i c a l grade NaCl, p o l y v i n y l c h l o r i d e ( A l d r i c h , h i g h mol. w t . ) , b i s ( 2 - e t h y l h e x y l ) p h t a l a t e (GR, A l d r i c h ) , a n d t e t r a h y d r o f u r a n (AR, BDH C h e m i c a l s ) w e r e u s e d w i t h o u t f u r t h e r p u r i f i c a t i o n . Polya c r y l i c a c i d , m o l . w t . 250,000 ( A l d r i c h ) was t i t r a t e d w i t h NaOH t o o b t a i n a s t o c k s o l u t i o n o f NaPAA o f known c a r b o x y l a t e c o n c e n t r a tion. +




228

Potentiometry. F r e e s u r f a c t a n t c o n c e n t r a t i o n s were determined by means o f s o l i d s t a t e membrane e l e c t r o d e s r e s p o n d i n g t o t h e a l k y l p y r i d i n i u m c a t i o n s . The e l e c t r o d e s w e r e made as b e f o r e (16-19) e x c e p t t h a t t h e c a r r i e r c o m p l e x was p r e p a r e d b y r e a c t i n g t h e r e q u i r e d p u r i f i e d a l k y l p y r i d i n i u m bromide w i t h h i g h l y p u r i f i e d sod i u m d o d e c y l s u l f a t e and r e p e a t e d r e c r y s t a l l i z a t i o n f r o m a c e t o n e o f the r e s u l t i n g p r e c i p i t a t e . B i n d i n g curves were determined b y means o f a t i t r a t i o n t e c h n i q u e , w h e r e s u r f a c t a n t s o l u t i o n i s added t o t h e p o l y m e r s o l u t i o n b y means o f a m o t o r i z e d p i s t o n b u r e t . The polymer c o n c e n t r a t i o n i s kept c o n s t a n t by adding an equal volume of polymer s o l u t i o n of double the i n i t i a l c o n c e n t r a t i o n from a se cond p i s t o n b u r e t ( 1 8 ) . I n a r e c e n t improvement of our experimen t a l s e t - u p , t h e e l e c t r o m e t e r o u t p u t i s now d i g i t i z e d and s t o r e d i n j a m i c r o c o m p u t e r w h i c h a l s o c h e c k s f o r c o n s t a n c y o f t h e e.m.f. a n d a c t i v a t e s the p i s t o n b u r e t s . Thus c o m p l e t e b i n d i n g c u r v e s c a n b e determined unattended. I n a l l m e a s u r e m e n t s t h e t e m p e r a t u r e was c o n s t a n t a t 30.0 ± 0.1°C. R e s u l t s and

Discussion

The s u r f a c e t e n s i o n o f t h e p y r i d i n i u m s u r f a c t a n t s a t 30°C a s a f u n c t i o n o f c o n c e n t r a t i o n w e r e m e a s u r e d b y means o f a Du Nouy r i n g tensiometer (Figure 2). No m i n i m a a r e a p p a r e n t i n t h e C-^ and c u r v e s , b u t a s m a l l minimum i n t h e C-j^ c u r v e i n d i c a t e s t h e p r e s e n c e o f a m i n o r i m p u r i t y i n t h e c o m m e r c i a l C-joPyCl u s e d , e v e n a f t e r repeated r e c r y s t a l l i z a t i o n s . I n s p i t e of t h i s , our r e s u l t f o r the cmc, 1.40 (± 0.04) χ 10~2 m i s i n v e r y r e a s o n a b l e a g r e e m e n t w i t h l i t e r a t u r e d a t a r e p o r t e d a s 1.46 χ 10"" , 1.48 χ 10""* and 1.78 χ 10~2 f r o m c o n d u c t a n c e ( 3 5 - 3 7 ) and 1.62 χ 10"^ f r o m s u r f a c e t e n s i o n ( 3 7 ) , a l l a t 25°C. F o r C P y B r we f i n d a cmc o f 2.65 (± 0.05) χ 10""^ m, t y p i c a l l i t e r a t u r e v a l u e s a r e g i v e n a s 2.57 χ 10"" 3 m f r o m s u r f a c e t e n s i o n (35) and 2.63 χ 10"^ m f r o m c o n d u c t a n c e ( 3 6 ) . F i n a l l y , f o r C P y B r we o b t a i n 6.2 (± 0.1) χ 10"^ m, w h e r e l i t e r a t u r e v a l u e s v a r y r a t h e r w i d e l y , i . e . 5.8 χ 10~4 (35,38) a t 2 5 ° C and 7.05 χ 10""^ a t 3 0 ° C ( 3 6 ) , b o t h v a l u e s f r o m c o n d u c t a n c e , and 6.6 χ 10-4 25°C f r o m s u r f a c e t e n s i o n ( 3 9 ) . N o t e t h a t A n a c k e r (40) has p o i n t e d o u t some d i f f i c u l t i e s i n t h e d e t e r m i n a t i o n o f t h e cmc of C ^ P y C l by conductance. T y p i c a l e l e c t r o d e p e r f o r m a n c e s a r e shown i n F i g u r e 2 f o r C - j ^ and C;L£ p y r i d i n i u m c a t i o n s i n t h e p r e s e n c e o f a l a r g e e x c e s s o f N a C l . We n o t e t h a t t h e e l e c t r o d e s h a v e a n e x c e l l e n t s e l e c t i v i t y f o r t h e s u r f a c t a n t c a t i o n . R e s p o n s e i s N e r n s t i a n f r o m b e l o w 10~6 m t o t h e cmc f o r C ^ P y t and f r o m a b o u t 3 χ 10"^ m t o t h e cmc f o r C-j^Py and C - ^ P y ^ i n o t shown). In f a c t , the e l e c t r o d e s provide f o r a c o n v e n i e n t and a c c u r a t e m e t h o d t o d e t e r m i n e t h e cmc i n p a r t i c u l a r of the higher chainlength c a t i o n i c s u r f a c t a n t s i n s o l u t i o n s o f h i g h i o n i c s t r e n g t h , w h e r e o t h e r m e t h o d s become i n c r e a s i n g l y more d i f f i c u l t ( 4 0 ) . An example of t h e p o s s i b l e a p p l i c a t i o n of t h e s e e l e c t r o d e s i n t h e r m o d y n a m i c s t u d i e s i s shown i n F i g u r e 4, w h e r e t h e cmc a s o b t a i n e d f r o m d a t a s u c h a s i n F i g u r e 3 i s p l o t t e d v s the t o t a l c o u n t e r i o n c o n c e n t r a t i o n . When e x t r a p o l a t e d t o = 2

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MALOVIKOVA ET AL.

231


0, we o b t a i n f o r p u r e C - ^ P y C l l o g cmc = -2.37 ± 0.03, a n d f o r C-i^PyCl l o g cmc = -3.07 ± 0.02. O b v i o u s l y b e t t e r accuracy can be o b t a i n e d i n a more c o m p l e t e s t u d y w i t h d a t a a t l o w e r N a C l c o n c e n t r a t i o n s , b u t even t h e s e numbers a r e i n r e a o n a b l e a g r e e m e n t w i t h l i t e r a t u r e d a t a a t 25°C f o r l o g cmc q u o t e d a s -2.40 (36) a n d -3.05 (35) f o r C-j^PyCl a n d C ^ ^ P y C l r e s p e c t i v e l y . S i m i l a r l y , i f we c a l c u l a t e t h e f r e e e n e r g y o f m i c e l l i z a t i o n , AG f r o m t h e i n t e r c e p t a t l o g m c i = 0 we o b t a i n -29.9 k J / m o l e f o r C ^ P y C l a n d -23.5 k J / m o l e f o r C - ^ P y C l , i . e . a c o n t r i b u t i o n p e r CH2 g r o u p o f 3.2 k J / m o l e o r 1.27 RT, a n d f r o m t h e s l o p e o f t h e l o g cmc v s l o g m c ^ c u r v e s we c a l c u l a t e a n a p p a r e n t d e g r e e o f c o u n t e r i o n d i s s o c i a t i o n o f 0.30 ± 0.02 i n b o t h c a s e s . T h e s e numbers a r e i n good agreement w i t h e x p e c t e d v a l u e s ( 4 1 ) , perhaps even s u r p r i s i n g l y s o g i v e n t h e h i g h i o n i c s t r e n g t h o f t h e systems from w h i c h t h e s e v a l u e s have been c a l c u l a t e d . F o r t h e moment t h e y s e r v e t o u n d e r l i n e t h e r e l i a b l e performance o f the e l e c t r o d e s i n s o l u t i o n s o f w i d e l y v a r y i n g i o n i c s t r e n g t h , such as encountered i n the p r e s e n t b i n d i n g s t u d i e s , and we l e a v e t h e a p p l i c a t i o n o f t h e e.m.f. method t o cmc d e t e r m i n a t i o n s e s p e c i a l l y o f s y s t e m s w i t h l o w cmc a n d h i g h c o n c e n t r a t i o n o f v a r i o u s added e l e c t r o l y t e s , i n c l u d i n g m u l t i v a l e n t c o u n t e r i o n s , t o the future. I n p r e v i o u s p u b l i c a t i o n s (10,16) we h a v e d e s c r i b e d t h e p r o c e d u r e u s e d t o o b t a i n t h e d e g r e e o f b i n d i n g , 3, d e f i n e d a s N a


Na

β =

-

(

%

ν ξ ) /

%

(1)

f

where m i s t h e t o t a l s u r f a c t a n t c o n c e n t r a t i o n , m t h e f r e e s u r f a c t a n t c o n c e n t r a t i o n and m t h e monomolar p o l y i o n c o n c e n t r a t i o n ( i . e . m o l e s COO /kg H 0 ) , f r o m t h e e.m.f. d a t a . A l l o u r d a t a w i l l be p r e s e n t e d a s " b i n d i n g i o s t h e r m s " , w h e r e 3 i s p l o t t e d v s l o g m^. As h a s b e e n d e m o n s t r a t e d b e f o r e (19) t h e a v e r a g e l i n e a r c h a r g e s e p a r a t i o n on the polymer i s the predominant f a c t o r i n d e t e r m i n i n g the m|) r e g i o n w h e r e c o o p e r a t i v e b i n d i n g i s o b s e r v e d . T h i s c h a r g e s e p a r a t i o n on the p o l y i o n i s o f t e n expressed i n the form o f a charge d e n s i t y parameter ξ , D

D

p

2

ξ = e /cbkT 2

(2)

where e i s the p r o t o n i c c h a r g e , ε the d i e l e c t r i c c o n s t a n t , k t h e B o l z m a n n c o n s t a n t and Τ t h e t e m p e r a t u r e , a n d b i s t h e a v e r a g e l i near charge s e p a r a t i o n on the polymer, i . e . the average d i s t a n c e between charged groups on the f u l l y extended polymer. Thus t h e c h a r g e d e n s i t y p a r a m e t e r ξ o f p o l y a c r y l a t e has t h e v a l u e 2.83 t y p i c a l f o r v i n y l i c p o l y m e r c h a i n s . F o r p e c t a t e ξ = 1.61 i f we a s sume t h a t t h e 1 0 % n e u t r a l s u g a r s i n t h i s p o l y m e r a r e n o t r a n d o m l y distributed. A s i s d i s c u s s e d i n r e f . 19, i n t h e c a s e o f a l g i n a t e t h e r e a r e good r e a s o n s t o assume t h a t t h e c h a r g e d e n s i t y p a r a m e t e r ξ i s s l i g h t l y l a r g e r t h a n t h e v a l u e o f 1.43 e x p e c t e d f o r a polymannuronic acid chain. Both the presence o f g u l u r o n i c a c i d b l o c k s , and t h e l a r g e r f l e x i b i l i t y o f t h i s p o l y m e r (42) w o u l d i n d i c a t e a v a l u e i n b e t w e e n 1.43 a n d 1.61, t h e v a l u e f o r p e c t a t e ( 1 9 ) . A t y p i c a l e x a m p l e o f t h e i n f l u e n c e o f t h e c h a r g e d e n s i t y i s shown i n


232


F i g u r e 5 , w h e r e we compare t h e b i n d i n g o f C-j^Py*" i n t h e p r e s e n c e o f 0 . 0 1 m N a C l t o PAA, p e c t a t e and a l g i n a t e . We n o t e t h a t t h e o r d e r o f t h e o v e r a l l b i n d i n g c o n s t a n t Ku, d e f i n e d b y ( _ 2 , J ^ » 1 2 » 1 6 ) v a r i e s


Ku =

( J ^

(3)

i n t h e o r d e r PAA » a l g i n a t e > p e c t a t e . This order of Ku i s the same as was o b s e r v e d f o r a l k y l t r i m e t h y l a m m o n i u m i o n s ( 1 9 ) , b u t i n the p r e s e n t case of a l k y l p y r i d i n i u m i o n s the d i f f e r e n c e between a l g i n a t e and p e c t a t e i s s l i g h t l y more p r o n o u n c e d . A number o f o t h e r minor but t y p i c a l c h a r a c t e r i s t i c s can b e observed i n F i g u r e 5. B i n d i n g t o PAA r e a c h e s a s e c o n d c r i t i c a l p o i n t a r o u n d 3 = 0 . 7 , a g a i n s i m i l a r to the case of the corresponding trimethylammonium ions. A l l a l k y l p y r i d i n i u m b i n d i n g curves g i v e i n d i c a t i o n s of a t w o - s t e p b i n d i n g p r o c e s s , as may b e d e d u c e d f r o m t h e b e h a v i o u r o f the b i n d i n g isotherms below 3 0.5. The p e c t a t e b i n d i n g c u r v e seems t o l e v e l o f f above 3 = 1 , p o s s i b l y i n d i c a t i n g that the a l k y l p y r i d i n i u m i o n can b i n d t o the a p p r o x i m a t e l y 1 0 % n e u t r a l sugars present i n p e c t a t e but not i n a l g i n a t e . A l l the p e c t a t e b i n d i n g curves e x h i b i t a s i g n i f i c a n t l y lower c o o p e r a t i v i t y , i . e . the r i s e i n 3 w i t h i n c r e a s i n g m^ i s l e s s s t e e p , t h a n t h e c o r r e s p o n d i n g a l ginate binding curve. I n F i g u r e 6 we compare t h e b i n d i n g o f C i 2 P y a n d C-j^Py*" t o PAA to the case of t h e c o - r e s p o n d i n g d o d e c y l - and t e t r a d e c y l t r i m e t h y l ammonium i o n s (DTA and T T A ) b o t h i n t h e p r e s e n c e o f 0 . 0 1 m N a C l , and i n F i g u r e 7 a s i m i l a r c o m p a r i s o n i s made f o r C ^ 4 P y a n d C-j^Py"*" b i n d i n g t o a l g i n a t e and p e c t a t e . The r e m a r k a b l y c o n s i s t e n t b i n d i n g p a t t e r n s o f the v a r i o u s c a t i o n s and p o l y i o n s a t t e s t not o n l y t a the r e p r o d u c i b i l i t y of the r e s u l t s , but a l s o to the h i g h l y spe c i f i c c h a r a c t e r of the b i n d i n g p r o c e s s . In the case of p o s t - m i c e l l a r b i n d i n g the polymer c o n c e n t r a t i o n i s an important parameter (24). I n the p r e s e n t case the r e l a t i o n between t h e degree of b i n d i n g , 3 , and t h e f r e e s u r f a c t a n t c o n c e n t r a t i o n , m^, i s c o m p l e t e l y independent of the e q u i v a l e n t polymer c o n c e n t r a t i o n . For i n s t a n c e , «curves i n F i g u r e s 5-8 r e p r e s e n t p o l y m e r m o n o m o l a l c o n c e n t r a t i o n s o f 1 0 " 4 , 5 χ 1 0 4 , n d i f j - 3 , w i t h o u t any n o t i c e a b l e d i f f e r e n c e i n t h e t r e n d s o b s e r v e d . F i n a l l y , i n F i g u r e 8 we show t h e i n f l u e n c e of the polymer backbone, v a r y i n g the degree of e s t e r i f i c a t i o n o f t h e c a r b o x y l g r o u p i n p e c t i n a t e s d e r i v e d f r o m t h e same polypectate. R e l e v a n t p a r a m e t e r s c h a r a c t e r i z i n g t h e p e c t a t e and p e c t i n a t e s used are given i n Table I ( 3 1 , 4 3 ) . A l l binding iso therms i n F i g u r e 8 a r e f o r a n e q u i v a l e n t p o l y i o n c o n c e n t r a t i o n (COO c o n c e n t r a t i o n ) o f 1 χ 1 0 " 3 m, n o t e t h a t t h e r e f o r e t h e a c t u a l polymer c o n c e n t r a t i o n i n c r e a s e s as the degree of e s t e r i f i c a t i o n increases. The i n f l u e n c e o f t h e c h a r g e d e n s i t y i s e v i d e n t . I n a d d i t i o n , t h e i n f l u e n c e o f t h e added N a C l c o n c e n t r a t i o n shows a n i n t e r e s t i n g p a t t e r n : t h e d i f f e r e n c e b e t w e e n t h e m^ v a l u e s a t t h e h a l f - b o u n d p o i n t ( 3 = 0 . 5 ) between m i = 0 . 0 1 and 0 . 0 2 i s l a r g e s t f o r p e c t a t e (and a p p r o x i m a t e l y as e x p e c t e d compared e.g. t o t h e c a s e o f d e x t r a n s u l f a t e ( 1 6 ) ) and becomes p r o g r e s s i v e l y s m a l l e r as t h e p o l y i o n c h a r g e d e n s i t y d e c r e a s e s . =

+

+

+

+

_

a

m

N a C


MALOVIKOVA ET AL.



15.


233


234


m

m

F i g u r e 7. P o l y u r o n i d e b i n d i n g i s o t h e r m s . ^ " 0.01 « ο Na-alginate; à K-pectate. C u r v e s 1: C P y B r (1 χ 10_3 m u r o n i d e ) ; 2: C ^ P y B r , 3: C P y C l (1 χ 10 m u r o n i d e ) . N

a

C

1 6

1 2


15.

MALOVIKOVA ET AL.

TABLE I .

C h a r a c t e r i z a t i o n o f P e c t a t e and P e c t i n a t e s Polyuronides

K-pectate Na-pectinate (2) Na-pectinate (3) Na-pectinate (4)


3 M* η r

Ε -1

%

%

1 2 3

235


0 20.6 46.1 70.0

84.9 86.1 83.8 88.9

29,000 41,000 56,000 22,000

0.133 0.211 0.327 0.092

Degree o f e s t e r i f i c a t i o n Intrinsic viscosity M o l e c u l a r weight c a l c u l a t e d from v i s c o s i t y

1.61 1.28 0.87 0.48

( r e f . 43).

A l l b i n d i n g parameters d e r i v e d from f i t t i n g the b i n d i n g i s o therms t o t h e e q u a t i o n s o f S c h w a r z o r S a t a k e and Yang (2,16,19) a r e c o l l e c t e d i n T a b l e I I . A s has b e e n s t a t e d b e f o r e , t h e o v e r a l l b i n d i n g c o n s t a n t Ku can b e d e t e r m i n e d a c c u r a t e l y ( e s t i m a t e d a t ± 2 % ) , b u t t h e d e t e r m i n a t i o n o f Κ and u s e p a r a t e l y i s much more inaccurate. G e n e r a l l y we e s t i m a t e t h e p o s s i b l e e r r o r i n u a t ± 20%. Even i f the model c o n s i d e r a t i o n s w h i c h equate u t o a coop e r a t i v i t y p a r a m e t e r d e s c r i b i n g t h e a g g r e g a t i o n o f bound s u r f a c tant m o l e c u l e s , prove i n c o r r e c t o r i n a p p l i c a b l e , from a n e x p e r i m e n t a l p o i n t o f v i e w u may b e s e e n s i m p l y a s a p a r a m e t e r i n d i c a t i n g the s l o p e o f the b i n d i n g i s o t h e r m i n the c o o p e r a t i v e r e g i o n , i . e . h i g h e r u v a l u e s mean s t e e p e r b i n d i n g i s o t h e r m s . What i s m o s t obvious from Table I I i s the i d e n t i c a l s u r f a c t a n t c h a i n l e n g t h d e pendence o f the Ku v a l u e s f o r a l l p o l y m e r s , independent o f the py r i d i n i u m o f trimethylammonium head group o f the s u r f a c t a n t and o f t h e p r e s e n c e o f added s a l t . The d i f f e r e n c e p e r CIL? g r o u p i n I n Ku f o r a l l cases presented i n T a b l e I I averages 1.19 kT, v e r y c l o s e t o t h e v a l u e o f 1.23 kT f o u n d f o r t h e c a s e o f DTA a n d TTA b i n d i n g t o DNA w i t h o r w i t h o u t added N a C l ( 1 8 ) . I t i s hard t o f i n d any o t h e r e x p l a n a t i o n f o r t h i s r e m a r k a b l e c o n s t a n c y t h a n t o assume t h a t t h i s f a c t o r r e f l e c t s only the d i f f e r e n c e i n hydrophobic i n t e r a c t i o n s between the C ^ > 14> 1 6 l k y l groups, and t h a t t h e i n t r i n s i c b i n d i n g between s u r f a c t a n t and p o l y i o n i s u n a f f e c t e d b y the s u r f a c t a n t hydrophobic c h a i n l e n g t h . Of c o u r s e t h e s i m i l a r i t y between t h i s h y d r o p h o b i c e f f e c t i n s u r f a c t a n t b i n d i n g b y polymers and m i c e l l e f o r m a t i o n has b e e n p o i n t e d o u t many t i m e s , b u t i t i s n e v e r t h e l e s s s a t i s f y i n g t o see t h i s a l m o s t p e r f e c t c o r r e s p o n d e n c e between such w i d e l y v a r y i n g systems. +

c

o

r c

+

a

F i n a l l y , we w i l l c o n s i d e r t h e v a r i a t i o n i n K u w i t h s u r f a c t a n t h e a d g r o u p , p o l y m e r s t r u c t u r e a n d c h a r g e d e n s i t y , and added s a l t concentration. G e n e r a l l y , Ku d e c r e a s e s w i t h i n c r e a s i n g s a l t con c e n t r a t i o n , w h i c h i s e a s i l y e x p l a i n e d i n terms o f i n c r e a s e d s h i e l ding o f the polymer charges. I t remains t o be seen whether i n f a c t t h i s i s a c o r r e c t i n t e r p r e t a t i o n , i t might f o r i n s t a n c e be argued t h a t s i m i l a r t o the case o f m e t a l i o n b i n d i n g b y p o l y e l e c -



1 2

NaCl

1 2

+ c Py

45

3.42

1

200 100

u

3.69 3.58

l o g Ku

p r e c i s i o n ± 20% 19.

0 0.01 0 0.01 0 0.01 0.02 0.01 0.02 0.01 0.02 0.01 0.02

(mol/kg)

m

14

:

4.08 4.52 3.99 3.82 3.83 3.68 3.55 3.45 3.32 3.28

4.67

l o g Ku

C

+ Py

250

1

2300 770 250 320 26 50 12 26 12 26

u

16

5.00

5.08

l o g Ku

C

P y

1100

4800

u"'"

+

B i n d i n g C o n s t a n t s Ku and P a r a m e t e r s u f o r S u r f a c t a n t - P o l y i o n

Estimated reference

Pectinate ( 2 0 . 6 % Ε) Pectinate ( 4 6 . 1 % Ε) Pectinate ( 7 0 % Ε)

Pectate

Alginate

PAA

Polymer

TABLE I I .

3.35

5.39 4.48 4.48 3.88 4.46 3.86 70

15 500 70

4.43 3.43 3.30

Τ TA l o g Ku

+ 2

l o g Ku u"^

DTA

Binding.


1

20 600 150 2000 60 2000

u

2

15.

MALOVIKOVA ET AL.


t r o l y t e s i t i s the entropy gain of the released counterions ( i . e . Na ) which should be considered (44,45). Both approaches would p r e d i c t a d e c r e a s e i n Ku w i t h i n c r e a s i n g s a l t c o n c e n t r a t i o n f o r a g i v e n p o l y i o n - s u r f a c t a n t s y s t e m , and b o t h a p p r o a c h e s w o u l d a l s o p r e d i c t a s m a l l e r d e p e n d e n c e o f Ku o n t h e added 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 t h e p o l y i o n charge d e n s i t y . This l a s t e f f e c t i s sens i t i v e l y demonstrated not only i n our data f o r the p e c t i n a t e s , b u t a l s o i n t h e c o m p a r i s o n b e t w e e n t h e s a l t d e p e n d e n c e o f Ku f o r d e x t r a n s u l f a t e , p o l y s t y r e n e s u l f a t e (16) a n d p o l y a c r y l a t e ( T a b l e I I ) , a l l w i t h c h a r g e d e n s i t y p a r a m e t e r s o f 2.8, a n d a l g i n a t e a n d p e c t a te. The i n f l u e n c e o f t h e h e a d g r o u p , i . e . p y r i d i n i u m o r t r i m e thylammonium, o n t h e o v e r a l l b i n d i n g c o n s t a n t K u a n d t h e c o o p e r a t i v i t y parameter u i s r e l a t i v e l y s m a l l . I t i s perhaps s u r p r i s i n g t h a t i n a l l cases Ku f o r t h e p y r i d i n i u m s a l t i s l a r g e r t h a n f o r t h e t r i m e t h y l a m m o n i u m s a l t , a s c a n b e s e e n b e s t e.g. b y c o m p a r i n g C-j^Py and T T A , j u s t a s t h e cmc f o r p y r i d i n i u m s a l t s i s a l w a y s l o wer t h a n t h e cmc o f t h e c o r r e s p o n d i n g t r i m e t h y l a m m o n i u m s a l t . T h i s may b e c a u s e d b y two f a c t o r s . F i r s t o f a l l , i n t h e c a s e o f p y r i d i n i u m s a l t s t h e r e may b e a c o n t r i b u t i o n f r o m t h e h y d r o p h o b i c i n t e r a c t i o n s b e t w e e n n e i g h b o u r i n g bound h e a d g r o u p s ( a n e f f e c t which would n o t c o n t r i b u t e t o t h e f r e e energy o f m i c e l l e format i o n ) . S e c o n d l y , a s t e r i c h i n d r a n c e e f f e c t may p r e v e n t t h e p o s i t i v e c h r g e on t h e trimethylammonium head group from a p p r o a c h i n g c l o s e t o the p o l y i o n charge. In comparing b i n d i n g d a t a f o r t h e v a r i o u s polymers, i t i s c l e a r t h a t indeed t h e charge d e n s i t y i s t h e dominant f a c t o r govern i n g Ku. The i n v e r s i o n i n Ku v a l u e s o b s e r v e d b e t w e e n a l g i n a t e a n d p e c t a t e should then be a t t r i b u t e d t o t h e l a r g e r f l e x i b i l i t y o f t h e a l g i n a t e p o l y i o n (42) , a l l o w i n g i t t o b i n d a n d " e n v e l o p " t h e s u r f a c t a n t a g g r e g a t e s more e f f i c i e n t l y . I t i s noteworthy t h a t t h e Ku v a l u e s f o r D T A a t 0.01 m N a C l w i t h d e x t r a n s u l f a t e a n d p o l y a c r y l a t e a r e v i r t u a l l y i d e n t i c a l (16,190 b u t t h a t i n t h e c a s e o f p o l y s t y r e n e s u l f o n a t e K u f o r D T A i s much l a r g e r , and u v e r y much l o w e r . T h e s e t h r e e p o l y i o n s a l l h a v e a n i d e n t i c a l c h a r g e d e n s i t y paramet e r o f 2.8, and we c o n c l u d e t h a t o n l y i n t h e c a s e o f p o l y s t y r e n e s u l f o n a t e a t l e a s t p a r t o f t h e s u r f a c t a n t a l k y l group b i n d s t o t h e h y d r o p h o b i c p o l y m e r b a c k b o n e , a n d does n o t c o n t r i b u t e t o t h e c o o p e r a t i v e b i n d i n g b e t w e e n n e i g h b o u r i n g s u r f a c t a n t s . Thus i t seems l i k e l y t h a t , a l l o t h e r t h i n g s b e i n g e q u a l , more f l e x i b l e p o l y m e r s a r e more e f f i c i e n t i n b i n d i n g s u r f a c t a n t s , a s i s p a r t i c u l a r l y c l e a r f r o m t h e c a s e o f DNA ( 1 8 ) . We h a v e p o i n t e d o u t t h e s i m i l a r i t y between p o l y i o n s u r f a c t a n t i o n i n t e r a c t i o n and m i c e l l e formation of free surfactants. T h i s s i m i l a r i t y c a n b e s e e n f r o m e.g. the c h a i n l e n g t h dependence o f t h e o v e r a l l b i n d i n g c o n s t a n t a s d i s cussed above, o r from a comparison o f t h e thermodynamic parameters d e s c r i b i n g both processes (46). I n t h e p r e s e n t c o n t e x t i t now seems l i k e l y t h a t t h i s s i m i l a r i t y e x t e n d s t o t h e a c t u a l a g g r e g a t i o n p r o c e s s o f t h e bound s u r f a c t a n t s , w i t h t h e p o l y i o n e n v e l o p i n g the m i c e l l e - l i k e aggregates and n e u t r a l i z i n g t h e charge o f t h e +


237

+

+

+

+



238

s u r f a c t a n t head groups. The r e m a r k a b l e f a c t r e m a i n s t h a t t h i s charge n e u t r a l i z a t i o n allows aggregation t o take p l a c e a t f r e e s u r f a c t a n t c o n c e n t r a t i o n s o r d e r s o f m a g n i t u d e b e l o w t h e cmc, d e pendent on t h e p o l y i o n charge d e n s i t y .


Acknowledgments We a r e g r a t e f u l t o t h e N a t u r a l S c i e n c e s a n d E n g i n e e r i n g R e s e a r c h C o u n c i l o f Canada a n d t h e C z e c h o s l o v a k Academy o f S c i e n c e s f o r t h e award o f a s c h o l a r s h i p u n d e r t h e a u s p i c e s o f t h e s c i e n t i f i c e x c h a n g e s agreement b e t w e e n t h e C o u n c i l a n d t h e Academy, and t h e K i l l a m F o u n d a t i o n f o r t h e award o f a p o s t d o c t o r a l f e l l o w s h i p t o one o f t h e a u t h o r s (A.M.). The a u t h o r s a r e g r a t e f u l t o D r s . B. L a r s e n and 0. S m i d s r ^ d , I n s t i t u t e o f M a r i n e B i o c h e m i s t r y , U n i v e r s i t y o f T r o n d h e i m , Norway f o r t h e d o n a t i o n o f a f u l l y c h a r a c t e r i z e d s a m p l e o f N a - a l g i n a t e , a n d t o D r . R. Kohn, I n s t i t u t e o f Che m i s t r y , S l o v a k Academy o f S c i e n c e s , B r a t i s l a v a , C z e c h o s l o v a k i a , f o r p r e p a r i n g a n d c h a r a c t e r i z i n g p e c t i n s a m p l e s w i t h d i f f e r e n t de grees o f e s t e r i f i c a t i o n . This research i s supported by t h e Natu r a l S c i e n c e s a n d E n g i n e e r i n g R e s e a r c h C o u n c i l o f Canada t h r o u g h g r a n t s t o J.C.T.K.

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RECEIVED

March 6,

1984


Performance Relationships in Surfactants - ACS Publications

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