Association of Kraft Lignin in Aqueous Solution - ACS Symposium

Chapter 11

Association of Kraft Lignin in Aqueous Solution Sri Rudatin, Yasar L . Sen, and Douglas L . Woerner Department of Chemical Engineering, University of Maine, Orono,

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ME 04469

A technique has been developed to study the self-association of kraft lignins in alkaline solution using ultrafiltration membranes. The effects of alkalinity, lignin concentration and molecular weight distribution, ionic strength and some additives have all been explored. The association process accompanies the neutralization of the phenoxide groups. Neutralization can be accomplished by protonation or chelation. Large molecules show a strong affinity to associate with small molecules. High lignin concentrations increase the extent of association. Association in a 5.0 M urea solution was greatly reduced at alkalinities less than pH 13.0, indicating that hydrogen bonding may play an important role. U t i l i z a t i o n of k r a f t l i g n i n s has been s t y m i e d b y several factors: the diffic u l t y i n p r o d u c i n g p u r i f i e d m a t e r i a l , i n a b i l i t y to separate the k r a f t l i g n i n b y m o l e c u l a r weight, a n d a general lack of u n d e r s t a n d i n g of k r a f t l i g n i n . I n general, the p h e n o m e n o n of association has g r e a t l y r e t a r d e d the u n d e r s t a n d i n g a n d u t i l i z a t i o n of k r a f t l i g n i n s . Disagreement o n the m o l e c u l a r weights a n d m o l e c u l a r weight d i s t r i b u t i o n s of l i g n i n p r e p a r a t i o n s is w i d e s p r e a d , u n d o u b t e d l y due to different degrees of association u n d e r m a n y different c o n d i t i o n s (1). W h e n the u n d e r l y i n g m e c h a n i s m a n d the c o n d i t i o n s affecting association are w e l l u n d e r s t o o d , m a n y o f the difficulties i n c h a r a c t e r i z a t i o n a n d u t i l i z a t i o n m a y be overcome. T h e v i s c o s i t y of k r a f t l i g n i n s o l u t i o n s can be modeled as c o l l o i d a l p h e n o m e n a . T h e o r i g i n of the c o l l o i d a l particles m a y be association. A s s o c i a t i o n of k r a f t l i g n i n is m a i n l y based o n i n t e r n a l a n d e x t e r n a l factors. T h e c o m p o s i t i o n a n d the f u n c t i o n a l groups w i t h i n the k r a f t l i g n i n s t r u c t u r e are i m p o r t a n t i n t e r n a l factors i n d e t e r m i n i n g the t h e r m o d y n a m i c b e h a v i o r of k r a f t l i g n i n . T h e m a j o r groups are the a r o m a t i c r i n g ( 1 / C 9 ) , 0097-6156/89/0397-0144$06.00/0 © 1989 American Chemical Society

In Lignin; Glasser, W., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

11.

RUDATINETAL.

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carboxyl groups ( 1 / C 9 ) , phenolic h y d r o x y l groups ( 0 . 6 / C 9 ) , aliphatic h y d r o x y l groups ( 0 . 4 8 / C 9 ) , a n d ether groups ( 1 / C 9 ) . T h e extent o f associ a t i o n i n k r a f t l i g n i n s o l u t i o n w i l l also change w i t h e x t e r n a l factors s u c h as solvent, a l k a l i n i t y , c o n c e n t r a t i o n , i o n i c c o m p o s i t i o n , o r g a n i c a d d i t i v e s , time, and temperature. T h e r e are four different m e c h a n i s m s o f m o l e c u l a r a s s o c i a t i o n : h y d r o g e n b o n d i n g ( Η - b o n d i n g ) , stereoregular a s s o c i a t i o n , l y o p h o b i c b o n d i n g , a n d charge transfer b o n d i n g . Η-bonding is a d i p o l e - d i p o l e i n t e r a c t i o n between t w o electronegative a t o m s o n w h i c h one has a h y d r o g e n a t o m covalently b o n d e d . T h e h i g h s t r e n g t h o f these b o n d s (20 k J ) arises because the dipoles are o n l y separated b y the very s m a l l h y d r o g e n a t o m . S t e r e o r e g u l a r associ a t i o n occurs between t w o h i g h l y o r d e r e d p o l y m e r s w h i c h share m a n y v a n der W a a l s i n t e r a c t i o n s a l o n g the p o l y m e r chains. M a n y b i o l o g i c a l p o l y m e r s d e m o n s t r a t e stereoregular b o n d i n g , the m o s t n o t a b l e b e i n g the D N A d o u ble h e l i x . L y o p h o b i c b o n d i n g is a m i s n o m e r for the e x c l u s i o n o f solutes f r o m s o l u t i o n because o f the s t r o n g i n t e r m o l e c u l a r a t t r a c t i o n between the solvent molecules. T h e m a j o r r e q u i r e m e n t for l y o p h o b i c b o n d i n g is the development o f a s t r u c t u r e o f the solvent molecules s u c h as w a t e r a n d b e n zene s h o w . C h a r g e transfer b o n d i n g is the s h a r i n g o f p o s i t i v e a n d n e g a t i v e charges as e x h i b i t e d between c a t i o n i c a n d a n i o n i c p o l y m e r s . A l o n g h i s t o r y o f a s s o c i a t i o n of k r a f t l i g n i n s i n s o l u t i o n exists i n the l i t e r a t u r e , almost a l l o f i t based o n the d e t e r m i n a t i o n o f m o l e c u l a r w e i g h t . I n 1958 G r o s s et al. (2) discussed Η-bonding w i t h respect t o i r r e p r o d u c i b i l i t y o f M W measurements b y cryoscopy. B e n k o i n 1964 (3) suggested t h a t Η-bonding or h y d r o p h o b i c b o n d i n g was responsible for the changes i n diff u s i v i t y a n d v i s c o s i t y o f k r a f t l i g n i n s . I n 1967 B r o w n (4) suggested t h a t Η-bonding was responsible for m o l e c u l a r a s s o c i a t i o n i n v a p o r pressure os m o m e t r y e x p e r i m e n t s for the d e t e r m i n a t i o n o f m o l e c u l a r weight. Lindstrôm i n 1979 a n d 1980 (5) a n d Y a r o p o l o v a n d T i s h c h e n k o i n 1970 (7,8) researched the changes i n v i s c o s i t y o f k r a f t l i g n i n s o l u t i o n s a n d a s c r i b e d the increasi n g v i s c o s i t y w i t h decreasing a l k a l i n i t y t o be due t o i n t e r m o l e c u l a r b o n d i n g . M i l l e d w o o d l i g n i n s have been s h o w n t o have i n t r a m o l e c u l a r h y d r o g e n b o n d i n g p r i m a r i l y between the p h e n o l i c , a l i p h a t i c a n d ether g r o u p s . S o m e i n t e r m o l e c u l a r h y d r o g e n b o n d i n g was observed w i t h the g a m m a a l i p h a t i c h y d r o x y groups (9). T h e m o s t recent w o r k i n t h i s field has been p e r f o r m e d b y S a r k a n e n a n d co-workers. A series o f papers (10-13) propose t h a t k r a f t l i g n i n u n d e r goes stereoregular association between h i g h l y ordered fragments o f the n a t i v e l i g n i n w h i c h r e m a i n after p u l p i n g . B o n d i n g is p r e d o m i n a n t l y H O M O L U M O i n t e r a c t i o n s o f the p i - o r b i t a l s o f the benzene r i n g s between large p o l y m e r s a n d s m a l l oligomers a n d is s t o i c h i o m e t r i c a l l y c o n s t r a i n e d . T h e m a j o r t o o l used i n t h i s w o r k is size e x c l u s i o n c h r o m a t o g r a p h y o f p r e c i p i t a t e d l i g n i n s , w i t h a v a r i e t y o f s u p p o r t i n g evidence. T h e m a j o r findings o f t h i s w o r k are: (1) complexes i n a v a r i e t y o f solvents c a n b e b r o k e n d o w n w i t h i o n i c a d d i t i v e s ; (2) the molecular weight d i s t r i b u t i o n o f p r e c i p i t a t e d k r a f t l i g n i n s is very s i m i l a r below 4000 d a l t o n s ; (3) d i s s o c i a t i o n is favored b y w o r k i n g at h i g h a l k a l i n i t y a n d low solute c o n c e n t r a t i o n .


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A s i m p l e m o d e l o f the degree o f a s s o c i a t i o n has been presented ( W o erner, D . L . ; M c C a r t h y , J . L . Macromolecules, i n press) i n w h i c h the degree of a s s o c i a t i o n is related to the s o l u b i l i t y of the p r o t o n a t e d l i g n i n i n n e u t r a l w a t e r a n d the degree of i o n i z a t i o n of the p h e n o l i c groups. T h e a s s u m p t i o n s are: (1) a l l i o n i z e d phenolic ions are i n s o l u t i o n ; (2) a c e r t a i n f r a c t i o n o f the p r o t o n a t e d p h e n o l i c groups is soluble; (3) a single K a n d s o l u b i l i t y l i m i t , S, are a p p l i c a b l e t o a l l l i g n i n species of a l l m o l e c u l a r weights. T h e f u n d a m e n t a l r e a c t i o n was i o n i z a t i o n ( E q u a t i o n 1) a n d the degree o f a s s o c i a t i o n is g i v e n i n E q u a t i o n 2. LOH = W + Η+ (1) a


1 - %ass

= S + (1 - S) x X where 1/X = 1 + — ^ - r

(2)

Κ [Un j a

Here S is the s o l u b i l i t y of the l i g n i n i n n e u t r a l water, X is the f r a c t i o n of p h e n o l i c groups t h a t have been i o n i z e d , a n d K is the d i s s o c i a t i o n constant of the p h e n o l i c groups. T h e p K w a s d e t e r m i n e d to be a p p r o x i m a t e l y 11.4 f r o m l i g h t s c a t t e r i n g results a n d the s o l u b i l i t y l i m i t was measured t o be a b o u t 0.16 mass f r a c t i o n . A reasonable fit of the p e r m e a t e c o n c e n t r a t i o n f r o m u l t r a f i l t e r e x p e r i m e n t s w i t h a 10,000 m o l e c u l a r weight cutoff ( M W C O ) was o b t a i n e d . a

a

Experimental Procedures T w o l i g n i n p r e p a r a t i o n s were used i n t h i s w o r k . T h e p r i m a r y k r a f t l i g n i n was i s o l a t e d f r o m a n o r t h e r n softwood m i l l l i q u o r b y p r e c i p i t a t i o n a n d w a s h i n g f o l l o w i n g the procedures developed b y K i m (14). T h e p r e c i p i t a t e d l i g n i n s were separated i n t o three groups: F A M ( f u l l y associated molecules) were the a s - p r e c i p i t a t e d molecules, S A M ( s m a l l a s s o c i a t i n g molecules) were molecules f r o m F A M w h i c h were able t o pass a 10,000 M W C O m e m b r a n e , a n d L A M (large a s s o c i a t i n g molecules) were the F A M molecules not able t o pass the 10,000 M W C O m e m b r a n e . T h e L A M s o l u t i o n c o n t a i n e d some low m o l e c u l a r weight m a t e r i a l . A f o u r t h l i g n i n s a m p l e was o b t a i n e d f r o m the n o n - p r e c i p i t a t e d l i g n i n s a n d called N A S M ( n o n - a s s o c i a t i n g s m a l l molecules). T h e second k r a f t l i g n i n was I n d u l i n A T o b t a i n e d f r o m W e s t v a c o C o r p . , C h a r l e s t o n , S . C . , a n d was used as o b t a i n e d . T h e l i g n i n was a n a l y z e d for c o n c e n t r a t i o n a n d m o l e c u l a r weight d i s t r i b u t i o n . T h e c o n c e n t r a t i o n was o b t a i n e d b y U V a b s o r p t i o n at 280 n m a s s u m i n g the v a l i d i t y of Beer's l a w w i t h a n a b s o r p t i o n coefficient of 20 c m m L / g . T h e m o l e c u l a r weight d i s t r i b u t i o n s were o b t a i n e d f o l l o w i n g S a r k a nen (10) u s i n g S e p h a d e x G - 1 0 0 i n a 25 m m b y 70 c m c o l u m n ( P h a r m a c i a ) . A l l d a t a were collected a n d a n a l y z e d b y F o r t r a n c o m p u t e r p r o g r a m s w r i t t e n at the U n i v e r s i t y o f M a i n e f o l l o w i n g Y a u (15). T h e m e m b r a n e s a m p l i n g technique was used extensively t o get samples w h i c h were representative of the s o l u t i o n i n e q u i l i b r i u m w i t h the associated complexes. B r i e f l y , the s a m p l e is e q u i l i b r i a t e d i n a sealed p o l y e t h y l e n e c o n t a i n e r u n d e r n i t r o g e n at least overnight. A 200 m L p o r t i o n of the s a m p l e is p l a c e d i n the u l t r a f i l t e r cell ( A m i c o n 8200) w i t h a n X M 3 0 0 m e m b r a n e ( A m i c o n 300,000 M W C O ) . E a r l i e r studies ( W o e r n e r , D . L . ; M c C a r t h y , J .


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147

L . , Macromolecules, i n press) h a d s h o w n t h a t t h i s m e m b r a n e d i d n o t a l l o w associated complexes t o pass t h r o u g h the m e m b r a n e . T h e cell is p r e s s u r i z e d w i t h n i t r o g e n . T h e first 5 m L o f permeate are d i s c a r d e d , a n d t h e n a 2 m L s a m p l e is t a k e n for a n a l y s i s . T h e u l t r a f i l t e r c o n d i t i o n s were a d j u s t e d t o m i n i m i z e c o n c e n t r a t i o n p o l a r i z a t i o n , C P (16), b y u s i n g a s t i r r e r r a t e o f 390 r p m a n d 60 k P a pressure d r o p across the m e m b r a n e . A t h i g h l i g n i n c o n c e n t r a t i o n s C P c o u l d n o t be avoided a n d m a n y different s t i r r e r rates a n d pressures were used t o p r o v i d e a c o r r e c t i o n for C P . T h e results are often presented i n t e r m s o f the rejection coefficient w h i c h is a dimensionless n u m b e r defined as Λ = 1 - C /C Downloaded by PURDUE UNIV on August 31, 2014 | http://pubs.acs.org Publication Date: July 31, 1989 | doi: 10.1021/bk-1989-0397.ch011

p

(3)

b

w h i c h allows direct c o m p a r i s o n o f results at different p a r e n t s o l u t i o n c o n c e n t r a t i o n s . T h e rejection coefficient is p r i m a r i l y a f u n c t i o n o f the m e m b r a n e pore size a n d the solute h y d r o d y n a m i c r a d i u s . A change i n the rejection coefficient w h e n the s o l u t i o n characteristics are changed is i n d i c a tive o f a change i n the degree o f a s s o c i a t i o n . A n increase i n the r e j e c t i o n coefficient i n d i c a t e s a decrease i n the n u m b e r o f molecules available t o p e n etrate the m e m b r a n e p r e s u m a b l y as a result o f a n increase i n the degree o f association.

Factors Affecting Association Alkalinity. T h e F A M s o l u t i o n was d i v i d e d i n t o several a l i q u o t s a n d the p H was adjusted w i t h H2SO4 t o a p H r a n g i n g f r o m 8.5 t o 13.8, each at a c o n c e n t r a t i o n o f a b o u t 11 g / L . E a c h s o l u t i o n was i n d i v i d u a l l y u l t r a f i l tered a n d a p e r m e a t e s a m p l e o b t a i n e d . T h e p H 8.5 s o l u t i o n was t i t r a t e d w i t h 0 . 1 M N a O H t o several alkalinities a n d u l t r a f i l t e r e d . T h e p e r m e a t e c o n c e n t r a t i o n s a n d rejection coefficients are presented i n T a b l e I, a n d the p e r m e a t e m o l e c u l a r weight d i s t r i b u t i o n s are s h o w n i n F i g u r e 1. T h e presence o f k r a f t l i g n i n a s s o c i a t i o n was i n d i c a t e d at a l l a l k a l i n i t i e s a n d a l l molecules p a r t i c i p a t e d . T h e a s s o c i a t i o n was s t r o n g l y p H dependent a n d c o m p l e t e l y reversible. H o w e v e r , the extent o f the a s s o c i a t i o n for the s m a l l molecules a n d the large molecules was different. T h e h i g h m o l e c u l a r weight molecules show considerable association i n the p H 13.5 t o p H 12.0 range, w i t h l i t t l e or n o f u r t h e r a s s o c i a t i o n at lower a l k a l i n i t i e s . T h e s m a l l molecules showed no association above p H 13.0 a n d the m a j o r a s s o c i a t i o n was i n the p H 13.0 t o 11.0 r e g i o n . B e l o w 10.0 there was n o f u r t h e r associ ation. T h e s e results i n d i c a t e t h a t p r o t o n a t i o n o f the phenoxide i o n is a nec essary step i n the a s s o c i a t i o n process, a n d the average K for i o n i z a t i o n o f the p h e n o l i c h y d r o x y l g r o u p increases w i t h m o l e c u l a r w e i g h t . a

Molecular Weight. T h e four K L s o l u t i o n s a n d 50:50 m i x t u r e s o f the S A M , N A S M a n d L A M w i t h F A M were u l t r a f i l t e r e d at p H 13.8 a n d 8.5 t o e x plore the effect o f the parent s o l u t i o n m o l e c u l a r weight d i s t r i b u t i o n o n the a s s o c i a t i o n process. T h e results at p H 13.8 are presented as rejection coef ficient d i s t r i b u t i o n s i n F i g u r e 2. T h e rejection coefficient d i s t r i b u t i o n was

American Chemical Society library

1155 16thW.,St., N.W, In Lignin; Glasser, et al.; Washington* DC. 20036 ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

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0.5

MOLECULAR WEIGHT

F i g u r e 1. M o l e c u l a r weight d i s t r i b u t i o n s o f the permeate o b t a i n e d f r o m the t i t r a t i o n o f F A M d e m o n s t r a t i n g the association o f large molecules i n t h e p H 13.8 t o 12.0 region a n d s m a l l molecules i n the p H 13.0 t o 10.0 range. Sephadex G-100/0.10 M N a O H .

MOLECULAR WEIGHT

F i g u r e 2. R e j e c t i o n coefficient d i s t r i b u t i o n o n the X M 3 0 0 m e m b r a n e s h o w i n g the association caused b y a n increase i n the c o n c e n t r a t i o n o f large molecules at constant t o t a l l i g n i n c o n c e n t r a t i o n .


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T a b l e I. T i t r a t i o n o f K r a f t L i g n i n U s i n g X M 3 0 0 M e m b r a n e


Run 1 2 3 4 5 6 7 8 9 10 11 12 13 14

PH

1

1

1

13.5 13.0 12.5 12.0 11.5 11.0 10.5 10.0 9.5 9.0 8.5 10.0 12.0 13.5

C feed, g / L

C permeate, g / L

12.10 10.80 10.75 10.65 10.70 10.80 11.10 11.10 11.00 11.00 11.00 11.00 10.50 10.10

7.00 5.35 4.50 4.10 4.00 3.85 3.85 3.50 3.00 2.75 2.50 3.25 4.35 7.00

Rej.

Coeff. 0.42 0.50 0.58 0.61 0.62 0.64 0.65 0.68 0.72 0.75 0.77 0.70 0.58 0.31

B a c k titration results.

o b t a i n e d b y d i v i d i n g the p e r m e a t e c o n c e n t r a t i o n b y the p a r e n t s o l u t i o n c o n c e n t r a t i o n at every m o l e c u l a r weight. T h e rejection coefficient s h o u l d be i d e n t i c a l except for changes i m p a r t e d b y a s s o c i a t i o n . T h e association of k r a f t l i g n i n was s t r o n g l y influenced b y i t s m o l e c u lar weight d i s t r i b u t i o n ( M W D ) at p H 13.8. T h e feed M W D a n d p e r m e a t e MWD profile o f these s o l u t i o n s s h o u l d coincide u p t o a c e r t a i n value of m o l e c u l a r weight regardless of the M W D of the p a r e n t s o l u t i o n i f associa t i o n is not present. T h e feed a n d p e r m e a t e M W D profile were i d e n t i c a l , i.e., rejection coefficient = 0, u p t o 3000 for N A S M , S A M , F A M - N A S M , and F A M - S A M . F o r the s o l u t i o n s h a v i n g m o r e large molecules ( F A M , L A M and F A M - L A M ) , the feed M W D a n d p e r m e a t e M W D c o i n c i d e d u p t o 600. A p p a r e n t l y w h e n a n excess of s m a l l molecules is present ( N A S M , S A M , N A S M + F A M a n d S A M + F A M ) the associated complexes a p p e a r e d t o b e c o m e s a t u r a t e d . O n the other h a n d m o r e large molecules i n the s o l u t i o n p r o v i d e d m o r e sites for a s s o c i a t i o n a n d s m a l l molecules were effectively rem o v e d f r o m the s o l u t i o n . T h i s result i n d i c a t e s t h a t there is considerable i n t e r a c t i o n between the s m a l l molecules a n d the large molecules at h i g h alkalinity. T h e a m o u n t of s m a l l molecules i n the p e r m e a t e at low a l k a l i n i t y i n creased w i t h the r e l a t i v e c o n c e n t r a t i o n o f s m a l l molecules ( F i g u r e 3). If h y d r o p h o b i c b o n d i n g or s i m p l e s o l u b i l i t y was the d o m i n a n t m e c h a n i s m for the a s s o c i a t i o n , the s o l u b i l i t y of l i g n i n molecules s h o u l d b e solely a f u n c t i o n o f p H regardless of the M W D o f the p a r e n t s o l u t i o n . C o n s e q u e n t l y , the p e r m e a t e M W D s h o u l d be a p p r o x i m a t e l y the same a n d not a f u n c t i o n o f the M W D o f the parent s o l u t i o n . Since the e x p e r i m e n t a l p e r m e a t e M W D v a r i e d w i t h the parent M W D , the s o l u b i l i t y or h y d r o p h o b i c i n t e r a c t i o n m e c h a n i s m was no longer accepted.


150


Lignin Concentration. L i g n i n concentrations have been e x a m i n e d i n the range o f 10 t o 100 g / L . These concentrations represent c o n c e n t r a t i o n s n o r m a l l y present i n b l a c k l i q u o r s a n d therefore are o f i n d u s t r i a l relevance. T y p i c a l l i g n i n concentrations i n b l a c k l i q u o r range f r o m 50 g / L for 1 5 % solids t o over 300 g / L at f i r i n g . T h e m e m b r a n e s a m p l i n g technique is m u c h m o r e c o m p l i c a t e d at these c o n c e n t r a t i o n s because of the effects o f C P at the m e m b r a n e surface. T h e t h e o r y o f rejection b y the C P layer (17) suggests t h a t a p l o t o f ln(C ) as a f u n c t i o n o f the reduced flux rate J w / J — 1 at several stirrer rates a n d t r a n s m e m b r a n e pressures s h o u l d y i e l d a s t r a i g h t l i n e . T h e intercept is ln(Cb a p x (1 — Rmem))- A t each c o n c e n t r a t i o n a p p r o x i m a t e l y 20 u l t r a f i l ter e x p e r i m e n t s are p e r f o r m e d at several s t i r b a r rates a n d pressures a n d p l o t t e d as s h o w n i n F i g u r e 4. T h e intercept values f r o m these p l o t s are d i v i d e d b y (1 — Rmem) a n d the parent l i g n i n s o l u t i o n c o n c e n t r a t i o n t o give the degree o f a s s o c i a t i o n . T h e results are s h o w n i n T a b l e I I . T h e effect o f l i g n i n c o n c e n t r a t i o n o n the m o l e c u l a r weight d i s t r i b u t i o n o f solutes i n e q u i l i b r i u m w i t h the associated complexes has n o t yet been ascertained because we need knowledge o f the diffusion coefficient o f k r a f t l i g n i n as a f u n c t i o n o f m o l e c u l a r weight a n d the rejection coefficient d i s t r i b u t i o n w i t h o u t assoc i a t i o n or c o n c e n t r a t i o n p o l a r i z a t i o n . p


t

P

T a b l e I I . A s s o c i a t i o n at H i g h L i g n i n C o n c e n t r a t i o n s

0

pH~~ Parent Solution Concentration

13.0

12.0

10.0

.940

1.0 .848

.479 .604

.77 .364

50 100

.846 .744

.748 .834

.740 .911

50 + 7*

.440

10 25

13.8

—

c

.696 c

--

T h e n u m b e r s are the p e r m e a t e concentrations d i v i d e d b y the b u l k c o n c e n t r a t i o n . A l l the corrections t o d e t e r m i n e the degree o f a s s o c i a t i o n are not yet k n o w n , b u t a decrease i n t h i s n u m b e r is evidence o f a n increase i n the degree o f a s s o c i a t i o n . T h e solute is I n d u l i n A T . T h e p e r m e a t e d a t a are o b t a i n e d f r o m the intercept o f figures s i m i l a r t o F i g u r e 4. 50 g / L s o l u t i o n , i o n i c s t r e n g t h 3.0 M w i t h the a d d i t i o n o f N a C l . T h i s a p p a r e n t decrease i n the degree o f association is under v e r i f i c a t i o n . T h e results based o n b u l k l i g n i n c o n c e n t r a t i o n d e m o n s t r a t e t h a t the degree o f association increases as the c o n c e n t r a t i o n o f the solute increases. T h i s result is observed at a l l a l k a l i n i t i e s . T h e degree o f a s s o c i a t i o n increases as the a l k a l i n i t y decreases at any solute c o n c e n t r a t i o n . T h e s i m p l e s o l u b i l i t y m o d e l w o u l d p r e d i c t t h a t the c o n c e n t r a t i o n i n the p e r m e a t e w o u l d r e m a i n constant as the t o t a l l i g n i n c o n c e n t r a t i o n increases.


11.

RUDATIN ET A L



pH 8.5

151

PERMEATES

70000 MOLECULAR WEIGHT

Figure 3. Molecular weight distributions of permeates obtained from parent solutions of differing molecular weight distributions at p H 8.5. Sephadex G 100/0.10 M N a O H .

Ln Cp v s . ( J w - J ) / J of 50 g / l pH=13.0 4.00

Ln (Cb*(1 - Rm))

3.75

3.50

£3.25

c

_j

3.00

2.75

2.50 0

10

20

30

40

50

CJw-JVJ

Figure 4. T h e correction for concentration polarization effects. Parent solution was 50 g / L at p H 13.0. Stirrer rates varied from 350 to 510 r p m , transmembrane pressure ranged from 34 to 207 k P . a



152


T h i s concept clearly overpredicts the degree of a s s o c i a t i o n at low a l k a l i n i t i e s a n d u n d e r p r e d i c t s t h a t at h i g h a l k a l i n i t i e s . Additives. S o d i u m d o d e c y l sulfate ( S D S ) , 5.0 M u r e a a n d 0.1 M b e t a i n e were a d d e d t o the F A M s o l u t i o n t o d i s r u p t the a s s o c i a t i o n process t o h e l p u n d e r s t a n d the m e c h a n i s m . B e t a i n e h a d p r e v i o u s l y s h o w n some d i s s o c i a tive p r o p e r t i e s , a n d u r e a is effective i n p r e v e n t i n g association caused b y Η-bonding. S o l u t i o n s at different alkalinities were p r o d u c e d as before a n d ultrafiltered. T h e rejection coefficients l i s t e d i n T a b l e I I I showed the r e d u c t i o n o f a s s o c i a t i o n f r o m F A M s o l u t i o n expressed b y the r e d u c t i o n of R at v a r i ous a d d i t i v e s usage. I n general the extent o f a s s o c i a t i o n o f F A M s o l u t i o n decreased m o d e r a t e l y w i t h the a d d i t i o n of b e t a i n e at a n y p H . U r e a i n creased the degree of d i s s o c i a t i o n at h i g h a l k a l i n i t y b u t m o r e i m p o r t a n t l y c o m p l e t e l y s t o p p e d association between p H 13.0 a n d 10.0, the r e g i o n i n w h i c h the m a j o r i t y of the p h e n o x i d e group p r o t o n a t i o n occurs. 0.14 M S D S showed a n i n i t i a l decrease i n the degree of a s s o c i a t i o n at h i g h a l k a l i n ity, b u t t h e n increased the degree o f a s s o c i a t i o n at a l l lower p H ' s . T a b l e I I I . R e j e c t i o n Coefficients of F A M S o l u t i o n a n d A d d i t i v e s PH Additive

13.7

13.0

10.0

8.5

None Urea Betaine SDS

0.47 0.41 0.36 0.33

0.53 0.45 0.44 0.58

0.64 0.46 0.51 0.79

0.68 0.55 0.56

-

T h e effectiveness of betaine c a n be e x p l a i n e d better b y i t s b l o c k i n g p r o t o n u p t a k e f r o m s o l u t i o n . B e t a i n e lessened the a m o u n t o f p r o t o n a t e d p h e n o l i c h y d r o x y l groups at any p H a n d consequently the i n t e r m o l e c u l a r a s s o c i a t i o n between k r a f t l i g n i n molecules. T h e c h e m i c a l reactions c a n be s h o w n as follows: LOH —+ LO- + # +

LCT+

+LO~

+B-

T h e e q u i l i b r i u m o f the i o n i z a t i o n r e a c t i o n was shifted t o a s m a l l e r m o l e f r a c t i o n b y b e t a i n e a d d i t i o n . However, as the p H was d r o p p e d , p r o t o n a t i o n of ionized groups t o o k place t o m a i n t a i n the e q u i l i b r i u m of b o t h reactions a n d association complexes were f o r m e d . T h e s e i n d i c a t i o n s suggest t h a t the p r e d o m i n a n t m e c h a n i s m of k r a f t l i g n i n association as the p H is decreased is h y d r o g e n b o n d i n g . H y d r o g e n b o n d i n g c o u l d be f o r m e d b y p h e n o l i c O H - p h e n o l i c O H linkages or p h e n o l i c O H - e t h e r linkages, w h i c h c o u l d be present a b u n d a n t l y i n k r a f t l i g n i n solute c o m p o n e n t s . C a r b o x y l groups have p K ' s less t h a n 5 a n d w o u l d be f u l l y i o n i z e d w h i l e the a l i p h a t i c h y d r o x y l groups o f the l i g n i n molecules w o u l d a


11.

RUDATIN ET A L


153


not ionize i n t h e a l k a l i n e region a n d therefore s h o u l d n o t c o n t r i b u t e t o the above results. A n o t h e r p o s s i b i l i t y is t h a t the hemicellulose a t t a c h e d t o l i g n i n forms h y d r o g e n bonds, a n d p r o t o n a t i o n o f the p h e n o x i d e is necessary o n l y t o reduce electrostatic repulsions. Ionic Strength. T h e role o f i o n i c s t r e n g t h m a y be p i v o t a l i n u n d e r s t a n d i n g the t h e r m o d y n a m i c s o f b l a c k l i q u o r solutions w h i c h have i o n i c strengths o f 2.0 M for 1 5 % solids liquor a n d about 7.0 M a t f i r i n g . P r e l i m i n a r y results show t h a t the i o n i c s t r e n g t h has a m a j o r effect o n the degree o f a s s o c i a t i o n at h i g h alkalinities as s h o w n i n T a b l e II. W h e n the i o n i c s t r e n g t h o f a 50 g / L l i g n i n s o l u t i o n at p H 13.8 was raised f r o m 1.0 M t o 3.0 M , the extent o f association increased s u b s t a n t i a l l y . A s o f t h i s t i m e t h e reason f o r t h i s is not k n o w n . A somewhat related effect i s d e m o n s t r a t e d i n the i m p o r t a n c e of i o n i c s t r e n g t h i n a d s o r p t i o n o f lignins o n Sephadex G 2 5 (13). Research i n t h i s area is c o n t i n u i n g t o verify the single result. Conclusions 1. T h e m e m b r a n e technique b e i n g developed a t the U n i v e r s i t y o f M a i n e is a powerful t o o l t o s t u d y i n t e r m o l e c u l a r i n t e r a c t i o n s w h e n one o f the solutes is a m a c r o m o l e c u l e or i n a separate phase. 2. P r o t o n a t i o n o f phenoxide is a key step i n the association process. 3. T h e Κ o f k r a f t l i g n i n s is a f u n c t i o n o f m o l e c u l a r weight. 4. T h e s i m p l e s o l u b i l i t y m o d e l is not adequate t o e x p l a i n the v a r i a t i o n of observed effects w i t h m o l e c u l a r weight, c o n c e n t r a t i o n a n d i o n i c s t r e n g t h . However, its s i m p l i c i t y a n d relative ease o f use w a r r a n t f u r ther m o d i f i c a t i o n . 5. T h e a d d i t i v e studies i n d i c a t e t h a t h y d r o g e n b o n d i n g plays a n i m p o r t a n t role i n the association process at low a l k a l i n i t y (less t h a n p H 13.0). It is not clear i f h y d r o g e n b o n d i n g occurs between the p h e n o l i c h y d r o x y l groups o r i f b o n d e d hemicellulose is responsible. 6. H i g h e r l i g n i n concentrations a n d higher i o n i c strengths increase t h e degree o f association. 7. T h e m e c h a n i s m o f association at h i g h a l k a l i n i t y m a y be different f r o m t h a t a t low alkalinity. a

Literature Cited 1. Chum, H . L.; Johnson, D . K . ; Tucker, M . P.; Himmel, M . E. Holz forschung 1980, 41, 97. 2. Gross, S. K.; Sarkanen, Κ. V.; Schuerch, C . Anal. Chem. 1958, 3 0 , 518. 3. Benko, J . Tappi 1964, 47, 508. 4. Brown, W . J. Appl. Polym. Sci. 1967, 1 1 , 2381. 5. Lindström, T . Colloid Polym. Sci. 1979, 2 5 7 , 277. 6. Lindström, T . Colloid Polym. Sci. 1980, 2 5 8 , 168. 7. Yaropolov, N . S.; Tishchenko, D . V . Zh. Prikl. Khim. 1970, 43, 1120. 8. Yaropolov, N . S.; Tishchenko, D . V . Zh. Prikl. Khim. 1970, 43, 1351. 9. Michell, A . J. Cell. Chem. Technol. 1982, 1 6 , 87.




154

10. Connors, W. J.; Sarkanen, S.; McCarthy, J. L. Holzforschung 1980, 34, 80. 11. Sarkanen, S.; Teller, D. C.; Hall, J.; McCarthy, J. L. Macromolecules 1981, 17, 426. 12. Sarkanen, S.; Teller, D. C.; Stevens, C . R.; McCarthy, J. L . Macro molecules 1984, 17, 2588. 13. Garver, T . M.; Sarkanen, S. Holzforschung 1986, 40, 93. 14. Kim, H. K. Ph.D. Thesis, University of Maine, Orono, ME, 1985. 15. Yau, W. W.; Kirkland, J. J.; Bly, D. D. Modern Size-Exclusion Chro matography; Wiley: New York, 1979. 16. Blatt, F.; Dravid, Α.; Michaels, A. S.; Nelson, L. In Membrane Science and Technology; Flinn, J. E . , Ed.; Plenum Press: New York, 1970. 17. Woerner, D. L.; McCarthy, J. L. AIChE Symp. Ser. 1986, 82, 77. RECEIVED February 27,1989


Association of Kraft Lignin in Aqueous Solution - ACS Symposium

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