Solution Properties of Polysaccharides - American Chemical Society

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Interaction Between Metal Cations and Anionic Polysaccharides G . PASS and P. W . H A L E S Department of Chemistry, University of Salford, Salford, M5 4WT, U . K .

A wide variety of techniques has been used to investigate the interaction between inorganic cations and polyelectrolytes in aqueous solution. The results are usually given in the form of a sequence of metal cations listed in order of increasing interaction with the polyelectrolyte. For the a l k a l i metal cations the sequence of increasing interaction with carboxylated (1, 2) and phosphated polyelectrolytes (3) parallels the decrease in ionic radius. The sequence is reversed for the interaction with sulphated polyelectrolytes. (4, 5, 6) A theoretical model (7) has been developed to explain the behaviour of polyelectrolytes in aqueous solution in terms of a linear charge parameter, defined for monovalent charged groups and counterions as

where e D b k Τ

= = = = =

the the the the the

charge on the proton bulk dielectric constant of water average distance between charged groups Boltzmann constant thermodynamic temperature

When ξ > 1 a proportion of the counterions w i l l condense on to the polyanion to give an effective value of ξ = 1, the remaining uncondensed counterions interact with the polyelectrolyte through Debye-Hückel forces. The development from this theory of mathematical expressions for certain physical properties of polyelectrolyte solutions has allowed more quantitative investigations to be undertaken. Measurements have usually been made on the sodium salt of the polyelectrolyte and good agreement is obtained between theory and experiment. (8, 9, 10) When different salts of the polyelectrolyte are used the results reflect the sequential behaviour already described. (11, 12, 13) 0097-6156/81/0150-0349$05.00/0 © 1981 A m e r i c a n C h e m i c a l Society

Brant; Solution Properties of Polysaccharides ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

350

SOLUTION

PROPERTIES

OF

POLYSACCHARIDES

An e x p r e s s i o n h a s b e e n d e r i v e d f r o m t h e l i n e a r c h a r g e m o d e l f o r the enthalpy o f d i l u t i o n o f p o l y e l e c t r o l y t e s o l u t i o n s . (14) T h i s h a s b e e n a p p l i e d t o t h e e n t h a l p y o f d i l u t i o n o f samples o f sodium c a r b o x y m e t h y I c e l l u l o s e and sodium p o l y a c r y l a t e . (15) B e t t e r agreement b e t w e e n t h e o r e t i c a l a n d e x p e r i m e n t a l v a l u e s was o b t a i n e d w i t h sodium c a r b o x y m e t h y I c e l l u l o s e . T h i s was a t t r i b u t e d t o t h e NaCMC c h a i n more c l o s e l y a p p r o a c h i n g t h e r o d - l i k e g e o m e t r y r e q u i r e d by t h e l i n e a r charge model o f a p o l y e l e c t r o l y t e . S a t i s f a c t o r y agreement b e t w e e n t h e o r y a n d e x p e r i m e n t a l v a l u e s was a l s o r e p o r t e d f o r s o d i u m p o l y v i n y l s u l p h o n a t e s . ( 1 4 ) The e f f e c t o f c a t i o n s i z e on t h e e n t h a l p y o f d i l u t i o n o f p o l y s t y r e n e s u l p h o n a t e s has b e e n r e p o r t e d a t l o w c o n c e n t r a t i o n s ( 1 6 ) a n d a l s o a t r e l a t i v e l y high concentrations (17). I n t h e p r e s e n t w o r k we h a v e i n v e s t i g a t e d t h e e f f e c t o f t h e c a t i o n on t h e e n t h a l p y o f d i l u t i o n o f a l k a l i m e t a l s a l t s o f c a r b o x y m e t h y l c e l l u l o s e , a l g i n a t e , and d e x t r a n s u l p h a t e . An e x p r e s s i o n h a s a l s o b e e n d e r i v e d f r o m t h e l i n e a r c h a r g e m o d e l f o r t h e e n t h a l p y change when a p o l y e l e c t r o l y t e i s m i x e d w i t h a s a l t s o l u t i o n c o n t a i n i n g t h e same c o u n t e r i o n . ( 1 8 ) We have now i n v e s t i g a t e d the enthalpy of mixing of a l k a l i metal s a l t s of a l g i n a t e and d e x t r a n s u l p h a t e w i t h s o l u t i o n s o f a l k a l i m e t a l c h l o r i d e s t o e s t a b l i s h w h e t h e r t h e c a t i o n h a s a n y i n f l u e n c e on t h e e n t h a l p y change. Experimental Sodium c a r b o x y m e t h y l c e l l u l o s e (7L2P) was s u p p l i e d b y t h e H e r c u l e s Company, U.K. The s t r u c t u r e o f t h e p o l y a n i o n c o n s i s t s o f r e p e a t i n g u n i t s o f £-1,4 l i n k e d a n h y d r o g l u c o s e r e s i d u e s , s u b s t i t u t e d by s o d i u m c a r b o x y m e t h y 1 g r o u p s , w i t h a r e p e a t i n g l e n g t h o f 0.515 nm. The s a m p l e ( 1 9 ) h a d a d e g r e e o f s u b s t i t u t i o n (DS) o f 0.83. Sodium a l g i n a t e ( M a n u c o l L B ) was s u p p l i e d b y A l g i n a t e I n d u s t r i e s , U.K. The p o l y a n i o n c o n t a i n s a n h y d r o m a n n u r o n a t e a n d a n h y d r o g u l u r o n a t e u n i t s , w i t h a r e p e a t i n g l e n g t h o f 0.50 nm. The sample ( 2 0 ) h a d a DS = 0.92. S o d i u m d e x t r a n s u l p h a t e was s u p p l i e d b y t h e Sigma C h e m i c a l C o . , USA. The m a i n s t r u c t u r a l f e a t u r e c o n s i s t s o f l i n e a r c h a i n s o f a-1,6 l i n k e d g l u c o p y r a n o s e r e s i d u e s , c o n t a i n i n g t h e s u l p h a t e g r o u p s , w i t h a r e p e a t i n g l e n g t h o f 0.51 nm. The s a m p l e had a DS = 2.13. A l l t h e s a m p l e s w e r e d i a l y s e d f o r 48 h o u r s a g a i n s t d i s t i l l e d w a t e r b e f o r e u s e . D i a l y s i s t u b i n g , V i s k i n g s i z e 8, was o b t a i n e d f r o m M e d i c e l l I n t e r n a t i o n a l L t d . The d i a l y s e d s o l u t i o n s were c o n c e n t r a t e d on a r o t a r y e v a p o r a t o r a t 40 C, a n d t h e c o n c e n t r a t e d s o l u t i o n was e i t h e r f r e e z e d r i e d o r u s e d i m m e d i a t e l y to prepare s o l u t i o n s o f the r e q u i r e d c o n c e n t r a t i o n s . The d i a l y s e d s o l u t i o n s o f t h e s o d i u m s a l t s w e r e c o n v e r t e d t o the l i t h i u m o r p o t a s s i u m s a l t f o r m s b y p a s s i n g t h e s o l u t i o n s t h r o u g h an i o n e x c h a n g e r e s i n , Z e r o l i t 3 2 5 , s u p p l i e d b y B.D.H., UK. The a c i d f o r m o f t h e r e s i n was c o n v e r t e d t o t h e a p p r o p r i a t e c a t i o n f o r m b y p a s s i n g t h e a l k a l i m e t a l c h l o r i d e s o l u t i o n (1M) t h r o u g h t h e c o l u m n , a n d t h e column washed u n t i l t h e w a s h i n g s were f r e e o f


24.

PASS A N D H A L E S

Metal

Cations

and Anionic

Polysaccharides

351

c h l o r i d e i o n . A s o l u t i o n o f the sodium p o l y e l e c t r o l y t e , o f s u i t a b l y low v i s c o s i t y , was p a s s e d t h r o u g h t h e column a n d c o l l e c t e d w i t h t w i c e t h e volume o f w a s h i n g s . The s o l u t i o n s w e r e c o n c e n t r a t e d u s i n g t h e same p r o c e d u r e a d o p t e d f o r t h e s o d i u m s a l t s , and t h e p u r i t y o f t h e p r o d u c t s d e t e r m i n e d b y f l a m e p h o t o m e t r y . The column was r e g e n e r a t e d b y f i r s t c o n v e r t i n g t o t h e a c i d f o r m . L i t h i u m c h l o r i d e , s o d i u m c h l o r i d e and p o t a s s i u m c h l o r i d e were d r i e d a t 105°C f o r 48 h o u r s b e f o r e u s e . A stock s o l u t i o n o f each c h l o r i d e was p r e p a r e d c o n t a i n i n g 16 χ 10"^ m o l 1~*. Aliquots of these s o l u t i o n s were d i l u t e d as r e q u i r e d . C a l o r i m e t r i c measurements were made u s i n g a f l o w c a l o r i m e t e r (LKB P r o d u k t e r , Bromma, Sweden. M o d e l 2 1 0 7 ) . The two s o l u t i o n s a r e pumped t h r o u g h the s p i r a l t u b i n g (1mm diam. χ 60 cm) o f t h e f l o w m i x i n g c e l l a t a r a t e o f 14 cm^ h ~ l , g i v i n g a r e s i d e n c e t i m e o f a p p r o x i m a t e l y 1 m i n u t e . The h e a t change o c c u r r i n g i n t h e c e l l f l o w s t h r o u g h two t h e r m o p i l e s t o o r f r o m a l a r g e a l u m i n i u m b l o c k . The v o l t a g e g e n e r a t e d i n t h e t h e r m o p i l e s i s a m p l i f i e d ( K e i t h l e y I n s t r u m e n t s M o d e l 150B m i c r o v o l t ammeter) and d i s p l a y e d on a c h a r t r e c o r d e r (LKB M o d e l 2066) r e l a t i v e t o a b a s e l i n e e s t a b l i s h e d w i t h a f l o w of d e i o n i s e d water r e p l a c i n g the p o l y e l e c t r o l y t e . Peak h e i g h t s a r e c a l i b r a t e d a g a i n s t a known i n p u t o f h e a t a t c o n t r o l l e d c u r r e n t and h e a t e r r e s i s t a n c e . The c a l o r i m e t e r i s k e p t i n an i n s u l a t e d box w h i c h i n t u r n i s i n a t h e r m o s t a t t e d a i r b a t h w i t h a t e m p e r a t u r e s t a b i l i t y o f ± 0.02°C. The two s o l u t i o n s t o b e m i x e d a r e b r o u g h t t o t h e same t e m p e r a t u r e b y two h e a t e x c h a n g e c o i l s b e f o r e e n t e r i n g the f l o w m i x i n g c e l l . Using the c a l i b r a t e d h e a t e r , h e a t changes i n t h e m i x i n g c e l l down t o 50 uW c o u l d be d e t e r m i n e d t o ±2%. The p e r f o r m a n c e o f the c a l o r i m e t e r was c h e c k e d b y m e a s u r i n g t h e h e a t o f d i l u t i o n o f s o d i u m c h l o r i d e w h i c h gave r e s u l t s ±2% o f the l i t e r a t u r e v a l u e s . R e s u l t s and

Discussion

The e n t h a l p y changes o c c u r r i n g i n t h e i n t e r a c t i o n b e t w e e n p o l y e l e c t r o l y t e s and c o u n t e r i o n s may be c o n s i d e r e d i n t e r m s o f a t least three p o s s i b l e e f f e c t s . The f i r s t i n v o l v e s t h e e n t h a l p y changes o c c u r i n g on d i l u t i o n , t h e s e c o n d i n v o l v e s e n t h a l p y changes a r i s i n g f r o m i n t e r a c t i o n b e t w e e n t h e i o n atmosphere o f t h e p o l y e l e c t r o l y t e a n d t h e c o u n t e r i o n , and t h e t h i r d c o n c e r n s t h e e n t h a l p y changes i n v o l v i n g the c o n d e n s e d c o u n t e r i o n s . Enthalpy o f d i l u t i o n . The e n t h a l p y o f d i l u t i o n o f aqueous s o l u t i o n s o f s i n g l e s a l t s o f c a r b o x y m e t h y l c e l l u l o s e (CMC) was d e t e r m i n e d o v e r t h e c o n c e n t r a t i o n r a n g e 8 χ 10 - 0.5 χ 10"" e q u i v . I " . The CMC s o l u t i o n was d i l u t e d w i t h a n e q u a l volume o f w a t e r and t h e sum o f t h e h e a t changes was c a l c u l a t e d t o a l l o w d e t e r m i n a t i o n o f the e n t h a l p y o f d i l u t i o n from a g i v e n i n i t i a l c o n c e n t r a t i o n t o a f i n a l c o n c e n t r a t i o n o f 0.5 χ 10 e q u i v . 1"" . The e n t h a l p y o f d i l u t i o n o f a p o l y e l e c t r o l y t e may b e c a l c u l a t e d f r o m t h e e x p r e s s i o n (14) 1


SOLUTION

352

AU Δ

Η

_ -ΚΓξ f "

i m

„ ( 1

+

. Τ dD. D dT>

OF

^ i m

l n

-> m Ρ

where m

η

PROPERTIES

Ρ

POLYSACCHARIDES

. 1

Ρ

= the f i n a l p o l y e l e c t r o l y t e

concentration

Ρ the i n i t i a l

η

polyelectrolyte

concentration

= 1 when ξ < 1 and n = - l when ξ > 1

The e x p r e s s i o n i s a l i m i t i n g l a w i n t h e D e b y e - H l i c k e l s e n s e and i s s t r i c t l y v a l i d o n l y i n d i l u t e s o l u t i o n . For a reaction at 25 C t h e e n t h a l p y o f d i l u t i o n f « m ΔΗ . = 1.067 χ H T ξ l o g J mol" 2 m -> m m Ρ Ρ Ρ η

where (1 + ~

.

i s t a k e n as e q u a l t o -0.374.

1

(2J.)

The e n t h a l p y o f d i l u t i o n i s t h e r e f o r e d e p e n d e n t on t h e e x t e n t o f t h e c h a r g e on t h e p o l y e l e c t r o l y t e , w h i c h i s g i v e n i n t e r m s o f the l i n e a r charge parameter, ξ. F o r t h e sample o f CMC t h e d e g r e e o f s u b s t i t u t i o n (DS) was 0.83 a n d t a k i n g t h e l e n g t h (22) o f a r e p e a t i n g u n i t i n t h e p o l y m e r as 0.515 nm t h i s g i v e s a v a l u e o f ξ = 1.15. The e x p e r i m e n t a l r e s u l t s f o r t h e l i t h i u m , s o d i u m , and p o t a s s i u m s a l t s o f CMC a r e g i v e n i n F i g u r e 1 w i t h t h e t h e o r e t i c a l l i n e o b t a i n e d f o r ξ = 1.15. The r e s u l t s f o r t h e s o d i u m s a l t a r e i n s a t i s f a c t o r y agreement w i t h p r e v i o u s l y r e p o r t e d r e s u l t s . (15) The r e s u l t s i n F i g u r e 1 show t h a t t h e e n t h a l p y o f d i l u t i o n becomes i n c r e a s i n g l y e x o t h e r m i c i n t h e sequence KK, b u t t h e r e i s no a p p a r e n t d i f f e r e n c e i n t h e b e h a v i o u r o f a c a r b o x y l and a c a r b o x y m e t h y l s u b s t i t u e n t i n t h e p o l y a n i o n . (18)


24.

PASS A N D H A L E S

Figure 1.

Metal

Polysaccharides

Enthalpy of dilution of salts of CMC: (a) KCMC; LiCMC;(t) theoretical line from Equation 2.

I.O

Figure 2.

Cations and Anionic

- l o g mj,

1.5

(b) NaCMC;

353

(c)

2.0

Enthalpy of dilution of alginates: (a) Κ alginate; (b) Ν a alginate; (c) Li alginate; (t) theoretical line.


354

SOLUTION PROPERTIES OF POLYSACCHARIDES

Ion atmosphere e f f e c t s . The e n t h a l p y change o c c u r r i n g when a 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 mixed w i t h a simple s a l t s o l u t i o n c o n t a i n i n g t h e same c o u n t e r i o n may b e c a l c u l a t e d ( 1 8 ) f r o m ~ ΔΗ .

mix

= 1.067 χ 10

ξ

Π

(m^ + 2am^) l o g -2— 5. ι m Ρ

where i n a d d i t i o n t o t h e t e r m s d e f i n e d i n e q u a t i o n s = the f i n a l s a l t

3

:

1 and 2,

concentration

s a Υ

= 1 when ζ < 1 and α = ξ when ξ > 1. m"^ + 2am^ = -B-^ 51

m Ρ S o l u t i o n s o f a g i v e n s a l t f o r m o f a l g i n a t e were p r e p a r e d c o n t a i n i n g 10 equiv. 1 and m i x e d w i t h e q u a l v o l u m e s o f t h e a l k a l i metal chloride s o l u t i o n . Thus s a m p l e s o f s o d i u m a l g i n a t e ( 1 0 ~ ^ e q u i v . 1" ) , were m i x e d w i t h a s e q u e n c e o f s o d i u m c h l o r i d e s o l u t i o n s of d i f f e r e n t concentrations. The e n t h a l p y o f m i x i n g as c a l c u l a t e d from the equation r e f e r s only t o the c o n t r i b u t i o n from the p o l y e l e c t r o l y t e . The e x p e r i m e n t a l e n t h a l p y changes were c o r r e c t e d f o r the e n t h a l p y o f d i l u t i o n o f the sodium c h l o r i d e s o l u t i o n , w h i c h was d e t e r m i n e d u n d e r i d e n t i c a l r e a c t i o n c o n d i t i o n s , e x c e p t t h a t t h e s o d i u m a l g i n a t e s o l u t i o n was r e p l a c e d b y d e i o n i s e d w a t e r . The c o r r e c t e d e n t h a l p i e s o f m i x i n g l i t h i u m a l g i n a t e w i t h l i t h i u m c h l o r i d e and p o t a s s i u m a l g i n a t e w i t h p o t a s s i u m c h l o r i d e were d e t e r m i n e d i n t h e same way. The r e s u l t s i n F i g u r e 3 show t h a t t h e c o r r e c t e d e n t h a l p y o f m i x i n g becomes p r o g r e s s i v e l y more e n d o t h e r m i c a s t h e c o n c e n t r a t i o n o f t h e added s a l t i s i n c r e a s e d , a l t h o u g h i n t h e c a s e o f t h e potassium s a l t t h i s trend i s reversed a t the highest salt concentrations. D i l u t i o n o f t h e i o n atmosphere o f t h e p o l y e l e c t r o l y t e p r o d u c e s a s e q u e n c e K

Solution Properties of Polysaccharides - American Chemical Society

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