Modes of Association Between Kraft Lignin Components - American

Chapter 12

Modes of Association Between Kraft Lignin Components Sunil Dutta, Theodore M . Garver, Jr., and Simo Sarkanen

1

Department of Forest Products, University of Minnesota, St. Paul, MN 55108

Ultraviolet-visible spectral concomitants have been identified for the associative/dissociative processes which affect the apparent molecular weight distributions of kraft lignin preparations in aqueous alkaline solution. Their kinetic time course has a form which appears to be the same as that of the weight-average molecular weight itself. The results indicate that the velocities for both dissociation and association of the individual molecular species are independent of molecular weight. These findings suggest that the kraft lignin components interact productively with the associated complexes in a particular order. Protonation of the kraft lignin phenoxide moieties facilitates, in nonaqueous polar solvents such as D M F , much more extensive association; here the underlying intermolecular forces are different from, yet in their selectivity complementary to, those operative in aqueous alkaline solution. The severe restrictions prevailing upon associative/dissociative behavior imply that the macromolecular kraft lignin complexes embody a well-defined regular structure derived, presumably, from the original configuration of the native biopolymer. It is now quite well established that the behavior of the molecular species comprising various lignin derivatives is strongly affected by attractive intermolecular interactions between them (1-9). There is nothing inherently Address correspondence to this author.

1

0097-6156/89A)397-0155$06.25A) © 1989 American Chemical Society

156

LIGNIN: PROPERTIES AND MATERIALS

u n i q u e about t h i s , except perhaps i n the current divergence of o p i n i o n a m o n g workers i n the field r e g a r d i n g the extent, i m p o r t a n c e a n d i m p l i c a t i o n s of such p h e n o m e n a . Indeed a c o m p a r i s o n of r e p o r t e d findings suggests t h a t at least t w o k i n d s of i n t e r m o l e c u l a r forces exert t h e i r respective i n f l u ences under rather different circumstances (6,9). However, a declared i n t e n t i o n to e m b a r k u p o n d e t a i l e d studies of n o n covalent i n t e r a c t i o n s between kraft l i g n i n components is a l m o s t c e r t a i n t o cause some s u r p r i s e . T h e m a j o r i t y of chemists w o u l d find it difficult to e n t e r t a i n a more u n a t t r a c t i v e prospect t h a n the i n v e s t i g a t i o n of c o m p l i c a t e d macromolecular structures constituted from (p-hydroxyphenyl)propane u n i t s by random p r o p o r t i o n a t e d i s t r i b u t i o n s o f t e n different linkages. T h e i r concerns w o u l d h a r d l y be m i t i g a t e d b y the belief (7) t h a t such s t r u c t u r e s m i g h t adequately describe n a t i v e l i g n i n s , the second most a b u n d a n t g r o u p of b i o p o l y m e r s . B u t to b e d e v i l the s i t u a t i o n further b y e x p o s i n g these macromolecules to the i n d u s t r i a l k r a f t process (developed, after a l l , for c o n v e r t i n g w o o d into p u l p ) w o u l d surely confound even the most dedicated a n a l y s t . T w o - h o u r contact at 170°C w i t h aqueous s o l u t i o n s c o n t a i n i n g 45 g L " N a O H a n d 12 g L " N a S can engender quite d r a s t i c m o d i f i c a t i o n s t h r o u g h i n t e r u n i t covalent b o n d cleavage a n d f o r m a t i o n , redox reactions, a n d numerous other t r a n s f o r m a t i o n s , b o t h i n t e l l i g i b l e a n d obscure (10,11). 1

1

2

Y e t sometimes p r o d u c t i v e i n s i g h t s can appear f r o m quite u n e x p e c t e d quarters, a n d i t is the contention of t h i s chapter t h a t the associative i n teractions p r e v a i l i n g between k r a f t l i g n i n components i n s o l u t i o n afford s u c h a n occasion. T h e e n s u i n g account embodies selected excerpts f r o m recent work c a r r i e d out at the U n i v e r s i t y of M i n n e s o t a (12,13) t h a t w i l l be p u b l i s h e d i n complete d e t a i l elsewhere. T h e k r a f t l i g n i n a d o p t e d for i n v e s t i g a t i o n was derived f r o m D o u g l a s fir (Pseudotsuga menziesii ( M i r b . ) F r a n c o ) a n d recovered f r o m a n i n d u s t r i a l black l i q u o r s a m p l e d o n a t e d b y the Weyerhaeuser C o . f r o m their m i l l i n E v e r e t t , W a s h i n g t o n , U . S . A . (8).

Molecular Weight Distributions of Kraft Lignins T h e experience of p r i o r work has repeatedly confirmed t h a t the m o l e c u l a r weight d i s t r i b u t i o n s of k r a f t l i g n i n p r e p a r a t i o n s c a n be r e l i a b l y deduced f r o m Sephadex G 1 0 0 / a q u e o u s 0.10 M N a O H e l u t i o n profiles b y u l t r a c e n trifuge s e d i m e n t a t i o n e q u i l i b r i u m techniques (5, 6, 8). A l t h o u g h t h e i r reso l u t i o n does not a p p r o a c h t h a t o b t a i n e d under H P S E C c o n d i t i o n s , such profiles have a p a r t i c u l a r advantage i n d e p i c t i n g the p o p u l a t i o n s of m a c r o m o l e c u l a r complexes a n d i n d i v i d u a l components p r e v a i l i n g at p H ' s greater t h a n 11.5: below this p o i n t the effects of i n t e r m o l e c u l a r hydrogen b o n d i n g are no longer counteracted b y the negative charge densities a r i s i n g f r o m u n p r o t o n a t e d phenoxide groups o n the species i n v o l v e d (9). Indeed w e l l defined associative processes m a y be r e a d i l y detected i n aqueous alkaline s o l u t i o n s c o n t a i n i n g k r a f t l i g n i n concentrations greater t h a n 100 g L " at p H ' s i n a f a i r l y n a r r o w range i m m e d i a t e l y above 11.5; conversely dissoc i a t i o n occurs at k r a f t l i g n i n concentrations less t h a n 1 g L i n aqueous 0.10 M N a O H (6). 1

- 1

12.

DUTTAETAL.

Association Between Kraft Lignin Components

157

Ultraviolet-Visible Spectral Concomitants of Associative Interac tions between Kraft Lignin Species F i g u r e 1 i l l u s t r a t e s the u l t r a v i o l e t - v i s i b l e s p e c t r a l changes a c c o m p a n y i n g the d i s s o c i a t i o n o f a Douglas fir kraft l i g n i n (8) d u r i n g i n c u b a t i o n at 0.047 g L " i n aqueous 0.10 M N a O H . I n the regions a r o u n d 230 a n d 290 n m , a n d above 450 n m , the m a r k e d r e d u c t i o n i n a b s o r p t i v i t y is c h a r a c t e r i z e d b y a t i m e course w h i c h is reasonably w e l l described b y s u p e r i m p o s e d first a n d pseudo-zero order k i n e t i c rate processes. O w i n g t o t h e fact t h a t t h e respective velocities are c o m p a r a b l e i n m a g n i t u d e , t h r o u g h t h e L e v e n b e r g M a r q u a r d t n o n l i n e a r regression procedure (14) i t is difficult t o determine the c o r r e s p o n d i n g first order rate coefficients a c c u r a t e l y f r o m d a t a i n these s p e c t r a l regions. A r o u n d 350 n m , however, the absorbance increases w i t h t i m e i n a first order process s u p e r i m p o s e d u p o n a m u c h smaller zero order v e l o c i t y ( F i g u r e 2). Indeed, a p p r o p r i a t e s u b t r a c t i o n o f the l i n e a r a s y m p t o t e t o t h e change reveals t h a t t h e p r e d o m i n a n t rate process obeys first order k i n e t i c s quite accurately for at least t w o half-lives ( F i g u r e 3). Interestingly, t h e net extent o f t h i s first order process increases a p p r e c i a b l y w i t h decreasing k r a f t l i g n i n c o n c e n t r a t i o n i n s o l u t i o n (Table I ) . D e s p i t e the accentuated errors i n herent i n their n u m e r i c a l analyses, the s p e c t r a l changes a r o u n d 230 n m a n d above 450 n m are n o t inconsistent w i t h their first order c o m p o n e n t s ' b e i n g characterized by rate coefficients o f t h e same m a g n i t u d e as t h a t d e d u c e d at 350 n m . 1

T a b l e I. V a r i a t i o n i n E x t e n t o f K r a f t L i g n i n D i s s o c i a t i o n w i t h C o n c e n t r a t i o n i n A q u e o u s 0.10 M N a O H at 2 5 . 0 ° C a

Concentration gL" 1

4.62 χ 1 0 " 4.68 χ Ι Ο " 4.72 χ Ι Ο " a

b

c

1

2

3

k

x

h" » 1

3.9 χ Ι Ο " 4.2 χ 1 0 " 4.1 χ Ι Ο "

Spectrophotometric Pathlength, cm

Aoo- A at 350 n m

0.1 1.0 10.0

0.059 0.064 0.118

3

3

3

0

c

D e t e r m i n e d s p e c t r o p h o t o m e t r i c a l l y at 350 n m . R a t e coefficient for first order p h y s i c o c h e m i c a l process. O v e r a l l change i n absorbance a r i s i n g f r o m first order process.

Kinetic Changes in Weight-Average

Molecular Weight

T h e foregoing u l t r a v i o l e t - v i s i b l e s p e c t r a l changes are d i r e c t l y correlated w i t h t h e v a r i a t i o n i n m o l e c u l a r weight d i s t r i b u t i o n of the k r a f t l i g n i n s a m ple under c o m p a r a b l e c o n d i t i o n s i n aqueous 0.10 M N a O H ( F i g u r e 4 ) . P h e n o m e n o l o g i c a l l y , the decrease i n weight-average m o l e c u l a r weight, M , w i t h t i m e also k i n e t i c a l l y conforms t o a s u p e r p o s i t i o n of first a n d pseudozero order rate processes ( F i g u r e 5). E m p l o y i n g the L e v e n b e r g - M a r q u a r d t procedure (14) t o enable s u b t r a c t i o n o f the pseudo-zero order c o n t r i b u tion, M , f r o m the o v e r a l l change i n M t> the r e m a i n i n g p l o t o f w

W}00}t

Wi


158 2.5 ι

1

250 3 0 0 350 400 450 500 550 wavelength, nm Figure 1. Ultraviolet-visible spectral concomitants of dissociation and ox idative cleavage of kraft lignin preparation during incubation at 0.047 g L in aqueous 0.10 M N a O H between (1) 0 h and (5) 1340 h. 1

0

200

400

600

800

1000

1200

time, h

Figure 2. First and pseudo-zero order kinetic components of change in ab sorbance at 350 n m with time for kraft lignin preparation during incuba tion at 0.46 g L in aqueous 0.10 M N a O H at 25.0 °C; k = 0.0039 h , v = 0.0000026 A ^ - (0.1 cm pathlength). 1

0

x

1

1

DUTTAETAL.

12.


159

2.0

0

100

200

300

400

500

time, h Figure 3. Rate plot for first order kinetic component of change in absorbance at 350 nm with time for kraft lignin preparation during incubation at 0.46 g l / in aqueous 0.10 M N a O H at 25.0° C; = 0.0039 h . 1

1

0

0.5

1.0

1.5 V

20

10

5

2 M

w

χ

2.0

R

10"

1

0.5

3

Figure 4. Dissociation and oxidative cleavage of kraft lignin preparation during incubation at 0.50 gL* in aqueous 0.10 Af N a O H for (1) 48 h, (2) 168 h, (3) 336 h, (4) 840 h and (5) 1780 h. (Sephadex G100/aqueous 0.10 M N a O H elution profiles monitored at 320 nm.) 1


160

ln(M t — M t) against t i m e , t, reveals t h a t the e s t i m a t e d first order rate coefficient, 5.0 ± 0.3 χ 1 0 ~ h " , is quite close i n m a g n i t u d e to t h a t (4.05 ± 0.15 χ 1 0 ~ h " ) c h a r a c t e r i z i n g the v a r i a t i o n i n A350. I n a s i m i l a r m a n n e r , the p h e n o m e n o l o g i c a l rate coefficient d e s c r i b i n g the first order c o n t r i b u t i o n to the change i n M d u r i n g the d i s s o c i a t i o n at 0.50 g L " of a preassociated k r a f t l i g n i n s a m p l e i n aqueous 0.10 M N a O H ( F i g u r e 6) was f o u n d to be 3.9 ± 0 . 4 χ 1 0 " h " . Wt

W}00t

3

3

1

1

1

w

3

1

Implications at the Molecular Level A n u m b e r of s t r u c t u r e s t h o u g h t to be present i n k r a f t l i g n i n s , i n c l u d i n g stilbenes (15), s t y r y l a r y l ethers (16, 17) a n d a r o m a t i c rings of p h e n o l i c moieties (18), are susceptible t o cleavage b y oxygen i n aqueous alkaline s o l u t i o n . Indeed the oxygen content of k r a f t l i g n i n fractions increases s i g n i f i c a n t l y d u r i n g i n c u b a t i o n for extended periods at 0.5 g L " i n aqueous 0.10 M N a O H s o l u t i o n (12). If the pseudo-zero order components of the changes i n a b s o r p t i v i t y a n d M are identified w i t h the results of s u c h o x i d a tive cleavage reactions, the first order processes m u s t reflect the d i s s o c i a t i o n of the m a c r o m o l e c u l a r k r a f t l i g n i n complexes. Indeed the characteristics of the u l t r a v i o l e t - v i s i b l e s p e c t r a l changes for the k r a f t l i g n i n s o l u t i o n i n aqueous 0.10 M N a O H d u r i n g the course of d i s s o c i a t i o n a n d covalent cleavage differ a c c o r d i n g to w h i c h process c o n t r i b u t e s to the greater extent (Iwen, M . L . , S a r k a n e n , S., U n i v e r s i t y of M i n n e s o t a , u n p u b l i s h e d results). I n i t i a l l y , w h e n the decrease i n m o l e c u l a r weight o r i g inates p r i m a r i l y f r o m d i s s o c i a t i o n , quasi-isosbestic p o i n t s reside near 305 a n d 455 n m ; subsequently, w h e n covalent cleavage p r e d o m i n a t e s , q u a s i isosbestic p o i n t s appear a r o u n d 330 a n d 385 n m . T h e o v e r a l l effect is one where the change i n a b s o r p t i v i t y is least i n the regions about 315 a n d 400 n m ( F i g u r e 1). T h e e q u i l i b r i u m constant c h a r a c t e r i z i n g the association of i n d i v i d u a l k r a f t l i g n i n components w o u l d be expected t o increase r a p i d l y w i t h m o l e c u l a r weight above a value c o r r e s p o n d i n g to the c r i t i c a l c h a i n l e n g t h for m a c r o m o l e c u l a r c o m p l e x f o r m a t i o n (19). A l t h o u g h t h i s change i n e q u i l i b r i u m constant w o u l d not be d i c t a t e d e x c l u s i v e l y b y differences i n the phe n o m e n o l o g i c a l rate coefficients for d i s s o c i a t i o n , i t is reasonable t o a n t i c i p a t e t h a t k r a f t l i g n i n components of higher m o l e c u l a r weight w o u l d dissociate m o r e slowly t h a n those of lower m o l e c u l a r weight. S u c h a n effect m a y be exemplified by the r e p o r t e d i n t e r a c t i o n between p o l y ( A T - v i n y l p y r r o l i d o n e ) a n d d a n s y l labeled p o l y ( a c r y l i c acid) w i t h Μ = 5.9 χ 1 0 , the d a t a for w h i c h s u p p o r t the existence of h y d r o g e n b o n d donor a n d acceptor groups i n equivalent concentrations (20): i n t r o d u c t i o n of a n over five-fold m o l a r excess of u n l a b e l e d p o l y ( a c r y l i c acid) w i t h Μ = 6.9 χ 1 0 f a c i l i t a t e d a rate of interchange t h a t is ten t i m e s slower for 5.7 χ 1 0 t h a n for 1.0 χ 1 0 m o l e c u l a r weight p o l y ( T V - v i n y l p y r r o l i d o n e ) (21). 1

w

5

η

5

η

4

4

C o n s e q u e n t l y a n y m o d e l proposed t o account for the d i s s o c i a t i o n of i n d i v i d u a l components f r o m associated k r a f t l i g n i n complexes s h o u l d p a y p a r t i c u l a r a t t e n t i o n to r e s t r i c t i o n s t h a t w o u l d r a t i o n a l i z e the c o n c o m i t a n t first

12.

D U T T A E T AL.

Association Between Kraft Lignin Components 6.0 ι

3.0

1

1

0

161

1

1

500

1

1000

1500

1

2000

time, h Figure 5. First and pseudo-zero order kinetic components of change in M with time for kraft lignin preparation during incubation at 0.50 gL* in aqueous 0.10 M N a O H (calculated from Sephadex G100/aqueous 0.10 M N a O H elution profiles monitored at 320 nm.) w

1

I

I

I

I

I

20

10

5

2

1

M

W

χ

I

0.5

1er

3

Figure 6. Dissociation of preassociated kraft lignin sample during incuba tion at 0.50 gL* in aqueous 0.10 M N a O H for (1) 0 h, (2) 25 h, (3) 118 h, (4)171 h, (5) 401 h, (6) 575 h and (7) 1340 h. (Sephadex G100/aqueous 0.10 M N a O H elution profiles monitored at 280 nm.) 1


162

order b e h a v i o r o f b o t h a b s o r p t i v i t y a n d weight-average m o l e c u l a r weight. T h e f o l l o w i n g scheme i n v o l v i n g the consecutive release o f c o m p o n e n t s L\ f r o m complexes C +i i n a specific order c o u l d successfully meet these re quirements: c

Cc+1

C'c+2

+ £1+1

c

Cc+1

+ Li

c

C

+

c

Cc-2

^l-l

+ £/-2

S t o i c h i o m e t r y i m p l i e s [ C + i ] - [C +i]o = - [£f+i]o - ( [ £ / ] - [£/]o), where [ C + i ] o , [£/+i]o a n d [L/]o are the m o l a r concentrations [ C + i ] , a n d [ L / ] , respectively, o f the c o m p l e x C + i a n d c o m p o n e n t s L / + i a n d L / at a r b i t r a r y t i m e t = 0. A d o p t i n g m t o represent m o l e c u l a r w e i g h t , since m i = m + mj, c

c

c

c

c

c +

c

Y^m\[C ] C,/

+ mJ[L,])

e

= £>i2 C,/

+ 1

[Ce

-

+ 1

]o + m"

+ 1

([L,

+ 1

] -

[I/+i]o)

- [Li]o) - 2 m m / ( [ L / ] -

[L,] )

c

0

-m?([L,]-[L|]o) + m? [L,+i]) + 1

= S(^+i[Cc+i]o+m?[L,] c,/ -

2ro m,([L,] -

0

[L,] )).

e

0

A c c o r d i n g l y , t h e weight-average molecular weight, M for t h e s y s t e m of i n t e r a c t i n g species at a n y t i m e d u r i n g t h e a s s o c i a t i v e / d i s s o c i a t i v e process c a n be w r i t t e n as W1

—

_ Z c , / ( ™ c + i [ C W i ] o + rnj[Li]

0

'

- 2 < m m , > {[L{\ -

[L,] ))

c

0

E c i K I C J + rmlL,])

where < m m / > = 5 3 / c / ( [ ^ / ] - [ ^ / ] ο ) / ( Σ / ( [ ^ ' ] [^j]o)) ~ d e r s t o o d t h a t a u n i q u e r e l a t i o n s h i p exists between c a n d /. T h e number-average ( w i t h respect t o the i n d i v i d u a l c o m p o n e n t s re leased) p r o d u c t o f the m o l e c u l a r weights, < m m\ > , o f i n t e r a c t i n g species, { C , L / } , w i l l r e m a i n constant i f the e v o l u t i o n w i t h t i m e o f {[L{\ — [L/]o) follows e x a c t l y t h e same k i n e t i c course for each L\. T h i s is r e m i n i s c e n t o f earlier findings t h a t t h e relative ratios o f k r a f t l i g n i n c o m p o n e n t s w i t h m o l e c u l a r weights below 3500 d o not v a r y w i t h the degree o f a s s o c i a t i o n for the s a m p l e as a whole (6). A n a p p r o p r i a t e l y p o s i t i o n e d r a t e - d e t e r m i n i n g step i n the sequence o f dissociative events w o u l d impose s u c h a r e s t r i c t i o n u p o n the s y s t e m . I f the step i n question were t o e x h i b i t first order b e h a v ior, so also w o u l d the changes i n M a n d a b s o r p t i v i t y a c c o m p a n y i n g the o v e r a l l dissociative process. c

m

m

a

c

c

w

n

d

l t

i s

u n

12.

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Associative/Dissociative Homology among Kraft Lignin Samples T h e a s s o c i a t i v e / d i s s o c i a t i v e processes i n w h i c h k r a f t l i g n i n species p a r t i c i pate are reversible: association a n d d i s s o c i a t i o n take place respectively at h i g h a n d low k r a f t l i g n i n concentrations i n aqueous a l k a l i n e s o l u t i o n . F o r e x a m p l e , a k r a f t l i g n i n s a m p l e t h a t has been preassociated ( d u r i n g i n c u b a t i o n at 190 g L * " i n 1.0 M i o n i c s t r e n g t h aqueous 0.6 M N a O H ) s p o n taneously dissociates w h e n d i l u t e d i n aqueous 0.10 M N a O H to 0.5 g L " ( F i g u r e 6). Conversely, a k r a f t l i g n i n s a m p l e t h a t has been predissociated ( t h r o u g h i n c u b a t i o n at 0.5 g L " i n aqueous 0.10 M N a O H a n d subsequent recovery b y u l t r a f i l t r a t i o n w i t h a n o m i n a l l y 500 m o l e c u l a r weight cutoff m e m b r a n e ) associates spontaneously w h e n dissolved to a level of 160 g L " i n 1.0 M i o n i c s t r e n g t h aqueous 0.40 M N a O H ( F i g u r e 7). O n t o a l l such a s s o c i a t i v e / d i s s o c i a t i v e processes, the effects of o x i d a t i v e covalent cleavage reactions w i l l be a p p e n d e d t o v a r y i n g extents d e p e n d i n g u p o n the p r e v a i l i n g circumstances. C o n s e q u e n t l y the m o l e c u l a r weight d i s t r i b u t i o n s depicted by the c o r r e s p o n d i n g e l u t i o n profiles e x h i b i t p o i n t s of intersection t h a t m a y v a r y quite w i d e l y ( F i g u r e s 4 a n d 6). H o w e v e r , the k r a f t l i g n i n species capable of i n t e r a c t i n g w i t h one another can be r e a d i l y separated f r o m those components t h a t , o w i n g to covalent m o d i f i c a t i o n , c a n n o t . T h i s m a y be a c c o m p l i s h e d t h r o u g h the s t r a i g h t f o r w a r d e x p e d i ent o f p a r t l y r e l a x i n g the i m p e d i m e n t s to a s s o c i a t i o n i m p o s e d b y s o l u t i o n c o n d i t i o n s ; a p p r o p r i a t e l y e l a b o r a t e d c h r o m a t o g r a p h i c f r a c t i o n a t i o n of the m a c r o m o l e c u l a r complexes w i l l t h e n complete the t a s k . T o t h i s end k r a f t l i g n i n samples possessing different degrees of associat i o n i n aqueous s o d i u m h y d r o x i d e solutions c a n be secured b y a c i d i f i c a t i o n to p H 7.5 a n d freeze-drying. T h e r e u p o n d i s s o l u t i o n i n a p o r t i o n of eluent c o n t a i n i n g 0.325 M N a O H a n d subsequent f r a c t i o n a t i o n t h r o u g h S e p h a d e x L H 2 0 w i t h aqueous 3 5 % dioxane separates the s a m p l e i n t o two groups of species: a subset o f associated complexes a p p e a r i n g at the v o i d v o l u m e is segregated f r o m the r e m a i n i n g components e l u t i n g j u s t after the salt b a n d ( F i g u r e 8; cf. réf. 8). A f t e r freeze-drying, r e f r a c t i o n a t i o n o f the l e a d i n g p e a k t h r o u g h Sephadex L H 2 0 w i t h aqueous 3 5 % dioxane allows a net 606 5 % of the o r i g i n a l k r a f t l i g n i n to be recovered as a n associated salt-free specimen. 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 depicted b y the Sephadex G 1 0 0 / aqueous 0.10 M N a O H e l u t i o n profiles ( F i g u r e 9) of these recovered k r a f t l i g n i n p r e p a r a t i o n s reflect the degrees of association for the c o n s t i t u e n t subsets o f species i n the o r i g i n a l samples. T h e y e x h i b i t a c o m m o n i n t e r section p o i n t , a n d as such represent the m o l e c u l a r weight d i s t r i b u t i o n s of p r e p a r a t i o n s t h a t are homologous f r o m a n a s s o c i a t i v e / d i s s o c i a t i v e p o i n t of v i e w : the r e l a t i o n s h i p between successive members of the series is u n i f o r m l y confined to s y s t e m a t i c differences i n t h e i r degrees o f association w i t h o u t p e r t u r b a t i o n s a r i s i n g f r o m the effects of covalent cleavage reactions. T h i s h a r m o n i o u s o u t c o m e f r o m a s i m p l e procedure has f a r - r e a c h i n g i m p l i c a t i o n s . T h a t the species a p p e a r i n g at the v o i d v o l u m e i n aqueous 3 5 % dioxane f r o m Sephadex L H 2 0 are associated is r e a d i l y c o n f i r m e d b y the corr e s p o n d i n g size-exclusion c h r o m a t o g r a p h i c profiles f r o m a 1 0 À pore-size 1

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Figure 7. Association of predissociated kraft lignin sample (92% retained by nominally 500 molecular weight cutoff ultrafiltration membrane) during incubation at 160 gL" in 1.0 M ionic strength aqueous 0.40 M N a O H for ( l ) 0 h , (2) 50 h and (3) 480 h. 1

salt band

J

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elution volume Figure 8. Kraft lignin fractionation into two subsets of species during desalting by elution with aqueous 3 5 % dioxane through Sephadex L H 2 0 of an initially 4.5 g L ' sample solution at 2.3 M ionic strength containing 0.32 M aqueous N a O H . 1

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p o l y ( s t y r e n e d i v i n y l b e n z e n e ) c o l u m n . W h i l e the i n t e r m o l e c u l a r forces caus i n g association i n aqueous dioxane necessarily differ f r o m those i n aqueous a l k a l i n e s o l u t i o n , t h e i r respective s t o i c h i o m e t r i c selectivities towards the i n d i v i d u a l k r a f t l i g n i n components must be d i r e c t l y c o m p l e m e n t a r y . T h e r e l a t i v e l y r a p i d association f a c i l i t a t e d b y l o w e r i n g the p H is p r e s u m a b l y m e d i a t e d b y h y d r o g e n b o n d i n g (9) a n d / o r d i p o l a r i n t e r a c t i o n s ; the slower associative processes encountered i n aqueous alkaline s o l u t i o n s are p r o b a b l y caused b y n o n b o n d e d o r b i t a l i n t e r a c t i o n s , the c o n c o m i t a n t s of w h i c h c o n t r i b u t e t o the long-wavelength absorbance changes i n the u l t r a v i o l e t - v i s i b l e s p e c t r u m ( F i g u r e 1).

T h e Behavior of < m m\ > during Association and Dissociation c

It has not been possible e x p e r i m e n t a l l y t o detect s y s t e m a t i c differences i n the u l t r a c e n t r i f u g e s e d i m e n t a t i o n e q u i l i b r i u m weight-average m o l e c u l a r weight c a l i b r a t i o n curves for the Sephadex G 1 0 0 / aqueous 0.10 M N a O H e l u t i o n profiles of k r a f t l i g n i n p r e p a r a t i o n s w i t h different degrees of associa t i o n ( F i g u r e 10). C o n s e q u e n t l y the same r e l a t i o n s h i p has been e m p l o y e d t o c a l c u l a t e the o v e r a l l weight- a n d n u m b e r -average m o l e c u l a r weights, M a n d M , of the p r e p a r a t i o n s as associative or dissociative processes are underway. w

n

A c c o r d i n g l y , the average p r o d u c t of the m o l e c u l a r weights, < m m\ >, of i n t e r a c t i n g k r a f t l i g n i n species (vide supra: Implications at the Molecular Level) c a n be e v a l u a t e d n u m e r i c a l l y b y p l o t t i n g M versus l/M : the slope at a n y p o i n t o n the r e s u l t i n g curve is given b y —2 < m m / > (4). It is hereby evident t h a t the value, 1.03 χ 1 0 , o f < m m / > c h a r a c t e r i z i n g the r e l a t i o n s h i p between the members of the homologous series of k r a f t l i g n i n p r e p a r a t i o n s is, w i t h i n e x p e r i m e n t a l error, i d e n t i c a l i n m a g n i t u d e t o those encountered d u r i n g association of the o r i g i n a l k r a f t l i g n i n s a m p l e at 195 g L " " i n 1.0 M i o n i c s t r e n g t h aqueous 0.4 M N a O H ( F i g u r e 11), d i s s o c i a t i o n of the preassociated kraft l i g n i n p r e p a r a t i o n ( F i g u r e s 6 a n d 11), a n d association o f the predissociated k r a f t l i g n i n p r e p a r a t i o n ( F i g u r e s 7 a n d 11). c

w

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D u r i n g d i s s o c i a t i o n of the o r i g i n a l k r a f t l i g n i n s a m p l e at 0.46 g L " i n aqueous 0.10 M N a O H ( F i g u r e 4), however, the apparent value, 5.1 χ 1 0 , o f < m m\ > is considerably lower ( F i g u r e 11). Indeed the m a n n e r i n w h i c h M varies w i t h 1 / M d u r i n g the d i s s o c i a t i o n of the preassociated k r a f t l i g n i n p r e p a r a t i o n ( F i g u r e s 6 a n d 11) suggests t h a t < m m\ > s i m i l a r l y becomes s m a l l e r w h e n a c o m p a r a b l e range of degree of association has been reached. P r e s u m a b l y the m a g n i t u d e of < m m\ > under these c o n d i t i o n s is influenced b y a d d i t i o n a l c o n t r i b u t i o n s a r i s i n g f r o m covalent cleavage of i n d i v i d u a l k r a f t l i g n i n c o m p o n e n t s , the effects of w h i c h w o u l d be m o r e extensive w h e n the degree of association is s m a l l e r , the p H higher, a n d the s o l u t i o n more d i l u t e ( w h e r e u p o n the s t o i c h i o m e t r i c r a t i o of dissolved oxygen to i n d i v i d u a l k r a f t l i g n i n c o m p o n e n t s is larger). I n t h i s c o n n e c t i o n i t s h o u l d be p o i n t e d out t h a t , i n aqueous a l k a l i n e s o l u t i o n s c o n t a i n i n g the highest k r a f t l i g n i n concentrations where a s s o c i a t i o n is favored, the p H tends t o approach a value a r o u n d 11.6, j u s t above the region where the 1

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Figure 9. Apparent molecular weight distributions representing a homolo gous series of kraft lignin samples secured by desalting after association and dissociation in aqueous alkaline solutions for: (1) 300 h, (2) 144 h and (3) 48 h at 170 g L in 1.0 M ionic strength aqueous 0.40 M N a O H ; (4) 0 h; (5) 144 h and (6) 644 h at 0.50 g L ' in aqueous 0.10 M N a O H . (Sephadex G100/aqueous 0.10 M N a O H elution profiles monitored at 320 nm.) 1

1

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Figure 10. Molecular weight calibration curves for kraft lignin samples dif fering in degree of association eluted from Sephadex G100 with aqueous 0 . 1 0 M N a O H : « > ) associated sample after 385 h at 180 g L in 1.0 M ionic strength aqueous 0.40 M N a O H ; (O) original preparation; ( • ) dis sociated sample precipitated upon acidification to p H 3.0 after 2000 h at 0.50 gL" in aqueous 0.10 M N a O H . 1

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m o l e c u l a r weights of the m a c r o m o l e c u l a r complexes f a l l most r a p i d l y d u r i n g t i t r a t i o n of the component phenolic groups w i t h h y d r o x i d e (9). R e m a r k a b l y , the m a g n i t u d e of < m m / > d e l i n e a t i n g the associative or dissociative influences u p o n m o l e c u l a r weight d i s t r i b u t i o n does not appear to be affected b y the s m a l l v a r i a t i o n s i n c o m p o n e n t c o m p o s i t i o n r e s u l t i n g f r o m differences i n the p r e p a r a t i v e histories of the samples. Indeed the same constant value of < ra ra/ > prevails d u r i n g a s s o c i a t i o n a n d d i s s o c i a t i o n despite m a r k e d changes i n the relative p o p u l a t i o n s of the c o n s t i t u e n t species. B o t h processes t h u s e m b o d y r e s t r i c t i o n s whereby the i n d i v i d u a l components are c a p t u r e d b y or released f r o m the m a c r o m o l e c u l a r complexes at respective rates of w h i c h the k i n e t i c forms are i n d e p e n d e n t of m o l e c u l a r weight. I n the context i m p o s e d b y the dissolved k r a f t l i g n i n p r e p a r a t i o n , therefore, the e q u i l i b r i u m constants for a l l p a i r s of i n t e r a c t i n g species are o p e r a t i o n a l l y the same as one another. c

c

C o n t r a r y t o a priori expectations (19), t h e n , the e q u i l i b r i u m constant for the association of k r a f t l i g n i n components does n o t increase w i t h the degree of p o l y m e r i z a t i o n . R a t h e r the b e h a v i o r encountered indicates t h a t the rates of association a n d dissociation are governed b y a p a r t i c u l a r rate d e t e r m i n i n g step. S u c h a r e s t r i c t i o n s t r o n g l y suggests t h a t the i n d i v i d u a l m o l e c u l a r species interact w i t h c o m p l e m e n t a r y l o c i o n the c o r r e s p o n d i n g m a c r o m o l e c u l a r complexes i n a specific order. It is difficult to conceive h o w s e l e c t i v i t y o n t h i s scale c o u l d be sustained i n a s y s t e m w i t h o u t a h i g h level of s t r u c t u r a l r e g u l a r i t y i n , a n d molecular o r g a n i z a t i o n a m o n g , the species involved.

Further Evidence for Nonrandom Interactions Lignin Species

between Kraft

T h e r e are other quite independent i n d i c a t i o n s t h a t the associative processes t a k i n g place i n l i g n i n samples are not r a n d o m . T h i s is exemplified by the c o m p o s i t i o n of a p a r t i a l l y dissociated k r a f t l i g n i n p r e p a r a t i o n secured b y p r e c i p i t a t i o n ( 5 9 % of o r i g i n a l sample) u p o n a c i d i f i c a t i o n t o p H 3.0 of a 0.50 g L s o l u t i o n i n aqueous 0.10 M N a O H t h a t h a d been i n c u b a t e d for 2000 h . P a u c i d i s p e r s e fractions selected f r o m the Sephadex G l O O / a q u e o u s 0.10 M N a O H e l u t i o n profile of the p r e p a r a t i o n can be separated i n t o t w o subsets of components Β a n d C b y e l u t i n g w i t h water t h r o u g h S e p h a d e x G 2 5 i n the presence of a h i g h ( i n i t i a l l y 2.0 M ) i o n i c s t r e n g t h salt b a n d (8). T h e components i n subsets Β a n d C respectively c o n t r i b u t e p r i m a r i l y t o the higher a n d lower m o l e c u l a r weight regions of the Sephadex G 1 0 0 / a q u e o u s 0.10 M N a O H e l u t i o n profile ( F i g u r e 12). T h e b e h a v i o r of the k r a f t l i g n i n species d u r i n g e l u t i o n t h r o u g h Sephadex G 2 5 is d e t e r m i n e d b y three coupled p h y s i c o c h e m i c a l processes, viz. a d s o r p t i o n at h i g h s o l u t i o n ionic strengths of the lower m o l e c u l a r weight components o n t o the gel, i n t e r m o l e c u l a r association, a n d diffusion of the higher m o l e c u l a r weight entities t h r o u g h the salt b a n d . T h e o v e r a l l effect facilitates more complete s e p a r a t i o n between the s m a l l e r a n d larger k r a f t l i g n i n species w h e n the differences i n t h e i r m o l e c u l a r weights are greater. T h e weight-average m o l e c u l a r weights of the c o m p o n e n t subsets - 1

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Figure 11. Relationship between M and M during associative/dissoci ative processes between molecular kraft lignin species in aqueous alkaline solutions: ( Q ) desalted homologous series of samples with different de grees of association; association of (O) preparation at 180 gL" and ( Λ ) predissociated sample at 160 g L ' , both in 1.0 Af ionic strength aqueous 0.40Af N a O H ; (O) dissociation of preassociated sample and ( φ ) dissoci ation and covalent cleavage of preparation, both at 0.50 g L ' in aqueous 0.10 Af N a O H . w

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Figure 12. Contributions of component subsets Β ( φ ) and C ( • ) to Sephadex G100/0.10 Af aqueous N a O H elution profile for dissociated kraft lignin sample precipitated upon acidification to p H 3.0 after 2000 h at 0.50 gL in 0.10 M aqueous N a O H . -1

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B a n d C are j u x t a p o s e d i n F i g u r e 13 to those of the p a r e n t paucidisperse fractions f r o m the p a r t i a l l y dissociated k r a f t l i g n i n p r e p a r a t i o n . T h i s c o m p a r i s o n establishes t h a t a p a r t i c u l a r subset of components c h a r a c t e r i z e d b y a weight-average m o l e c u l a r weight o f 3500 c a n be s p o n t a n e o u s l y l i b e r a t e d f r o m the entire range of k r a f t l i g n i n species w i t h m o l e c u l a r weights i n i t i a l l y f a l l i n g between 10,000 a n d 23,000. A n i m p o r t a n t p h y s i c o c h e m i c a l c o n c o m i t a n t of the a s s o c i a t i v e / d i s s o c i a tive processes i n w h i c h kraft l i g n i n components p a r t i c i p a t e can be detected i n the u l t r a v i o l e t - v i s i b l e s p e c t r a of paucidisperse fractions i n aqueous a l kaline s o l u t i o n derived f r o m k r a f t l i g n i n p r e p a r a t i o n s characterized b y different degrees o f a s s o c i a t i o n . T h e r a t i o of the a b s o r p t i v i t y at 230 n m to t h a t at 300 n m generally decreases w i t h i n c r e a s i n g frequency o f p h e n o x ide moieties a m o n g the species present (22). E x c e p t i n the very lowest reaches of the m o l e c u l a r weight range, t h i s r a t i o for those c o m p o n e n t s w i t h m o l e c u l a r weights less t h a n 3500 is independent of degree o f a s s o c i a t i o n for the entire k r a f t l i g n i n p r e p a r a t i o n ( F i g u r e 14). O n the other h a n d , the frequencies of u n p r o t o n a t e d phenoxide groups a m o n g the higher m o l e c u l a r weight species clearly increases w i t h degree of association for the s a m p l e as a w h o l e . These observations are consistent w i t h the s u p p o s i t i o n (6) t h a t the associative processes involve preferential interactions between the subset of components w i t h m o l e c u l a r weights below 3500 a n d the c o m p l e m e n t a r y higher m o l e c u l a r weight entities; a c c o m p a n y i n g p r o t o n a t i o n of the p h e n o x ide moieties, w h i c h tends to reduce the charge d e n s i t y o n the associated complexes, e v i d e n t l y need not be complete.

Association in Nonaqueous Polar Solvents W h e n the p h e n o x i d e moieties o f the k r a f t l i g n i n components p a r t i c i p a t i n g i n associated c o m p l e x f o r m a t i o n are f u l l y p r o t o n a t e d , far more extensive a s s o c i a t i o n can occur i n a c c o m m o d a t i n g solvents. S u c h a n o u t come m a y be r e a d i l y d e m o n s t r a t e d for the series of k r a f t l i g n i n p r e p a r a t i o n s differing homologously w i t h respect to their degrees of association i n aqueous 0.10 M N a O H ( F i g u r e 9). W h e n c a l i b r a t e d w i t h paucidisperse p o l y s t y r e n e s t a n d a r d s , the c o r r e s p o n d i n g e l u t i o n profiles ( F i g u r e 15) i n D M F f r o m 1 0 À pore-size p o l y ( s t y r e n e - d i v i n y l b e n z e n e ) e x h i b i t a p p a r e n t m o l e c u l a r weight d i s t r i b u t i o n s characterized b y weight-average m o l e c u l a r weights 1.5 to 1.9 χ 1 0 times greater t h a n those i n aqueous 0.10 M N a O H (Table II). 6

4

W h e n measured t h r o u g h a 26 n m b a n d w i d t h interference filter (reject i n g most fluorescence), R a y l e i g h scattered light intensities ( m u l t i p l i e d b y r e c i p r o c a l s o l u t i o n t r a n s m i t t a n c e to correct for absorbance) f u r n i s h weightaverage m o l e c u l a r weights for the same series of k r a f t l i g n i n p r e p a r a t i o n s i n D M F t h a t are between 190 a n d 1100 times larger t h a n those i n aqueous 0.10 M N a O H ( T a b l e I I ) . T h e trends i n , r a t h e r t h a n the absolute values of, the r e p o r t e d n u m b e r s have the greater significance. T h e forms of the respective Z i m m plots ( F i g u r e 16) are t y p i c a l of a s s o c i a t i n g m a c r o m o l e c u l a r species (23): the apparent second v i r i a l coefficients, c a l c u l a t e d f r o m the

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Figure 13. Comparison of weight-average molecular weights for component subsets Β (φ) and C ( • ) with those for complete paucidisperse fractions ( • ) isolated from Sephadex G100/0.10 Af aqueous N a O H elution profile of the dissociated kraft lignin sample precipitated upon acidification to pH 3.0 after 2000 h at 0.50 g L in 0.10 Af aqueous NaOH. 1

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Figure 14. Variation with molecular weight exhibited by ratios of absorbance values at 230 and 300 nm for paucidisperse kraft lignin fractions in aqueous 0.10 Af N a O H from ( ^ ) associated sample after 385 h at 180 gL" in 1.0 M ionic strength aqueous 0.40 M N a O H ; (O) original preparation; ( • ) dissociated sample precipitated upon acidification to p H 3.0 after 2000 h at 0.50 gL* in aqueous 0.10 M N a O H . 1

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2x10

7

extrapolated molecular weight calibration from polystyrenes Figure 15. Apparent molecular weight distributions in D M F of kraft lignin samples secured by desalting after association and dissociation in aqueous alkaline solutions for: (1) 300 h and (2) 144 h at 170 g L in 1.0 A f ionic strength aqueous 0.40 A f N a O H ; (3) 0 h; (4) 144 h and (5) 644 h at 0.50 gL" in aqueous 0.10 A f N a O H . (Profiles from 10 À pore-size 20Λ» particle poly(styrene-divinylbenzene) column monitored at 320 nm.) 1

1

6

12.

DUTTA ET AL.


173

Figure 16. Z i m m plot of 514.5 nm light-scattering data characterizing associated kraft lignin sample in D M F following desalting after incubation at 170 g L for 300 h in 1.0 M ionic strength aqueous 0.40 M N a O H . 1


174 Table II. C o m p a r i s o n of Homologous A q u e o u s 0.10 M N a O H

K r a f t Lignin Samples i n D M F and

a

Parameter Mjv,NaOH

e

M DMF .DMF

3

4.67 χ 1 0

5.89 χ 1 0

3

1.3 x 1 0

5

190

W

cm mol

Original Kraft Lignin

8.7 χ 1 0

5

W

Modes of Association Between Kraft Lignin Components - American

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