Colloidal Stability of Microcrystalline Cellulose in Aqueous Salt Solutions

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Downloaded by UNIV OF MASSACHUSETTS AMHERST on October 13, 2015 | http://pubs.acs.org Publication Date: February 10, 1984 | doi: 10.1021/bk-1984-0240.ch023

Colloidal Stability of Microcrystalline Cellulose in Aqueous Salt Solutions P H I L I P L U N E R and T.

1

CHOU

Empire State Paper Research Institute, College of Environmental Science & Forestry, Syracuse, NY 13210 The coagulation of microcrystalline cellulose was re -investigated over a wider range of solids than previously reported. At solids less than 230 ppm only one CCC was found. As the solids concentration was increased to 410 ppm coagulation was followed by an apparent redispersion region. Further addition of salt led to coagulation. At higher solids ~600 ppm, the CCC at the lower salt concentration was eliminated. The redispersion region coagulated at temperatures higher than 23°C, the extent varying with solids concentration. The flocs i n this region differed considerably from that found at higher salt concentrations. While the system conforms to coagulation i n a secondary minimum, the redispersion region is best accounted for i n terms of gel formation originating from the rod-like shape of the particles and hydrated surface. During the f i n a l coagulation process additional attractive forces such as dipolar and hydrogen bonding form flocs which are i r r e v e r s i b l e and denser than those formed at a lower salt concentration. I n a number o f r e c e n t p u b l i c a t i o n s ( 1 , 2) microcrystaîline c e l l u l o s e d i s p e r s i o n s (MCC) h a v e b e e n u s e d a s m o d e l s t o s t u d y d i f f e r e n t aspects o f t h e papermaking p r o c e s s , e s p e c i a l l y w i t h regard t o i t s stability. One o f t h e c e n t r a l p o i n t s i n t h e w e l l e s t a b l i s h e d DLVO theory of c o l l o i d a l s t a b i l i t y i s the c r i t i c a l coagulation concentrat i o n (CGC). I n p r a c t i c e , i t r e p r e s e n t s t h e minimum s a l t c o n c e n t r a t i o n t h a t causes r a p i d c o a g u l a t i o n o f a d i s p e r s i o n and i s an i n t i m a t e p a r t o f t h e t h e o r e t i c a l f r a m e w o r k o f t h e DLVO t h e o r y ( 3 ) . K r a t o h v i l e t a l ( 4 ) h a v e s t u d i e d t h i s a s p e c t o f t h e DLVO t h e o r y w i t h MCC a n d g i v e n v a l u e s f o r t h e CCC f o r many s a l t s , c a t i o n i c 1

Current address: The General Tire and Rubber Company, Akron, OH 44305. 0097-6156/ 84/ 0240 0377506.00/ 0 © 1984 American Chemical Society

In Polymer Adsorption and Dispersion Stability; Goddard, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

POLYMER ADSORPTION AND DISPERSION STABILITY


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s u r f a c t a n t s and m e t a l c h e l a t e s . I t was f o u n d t h a t t h e MCC s o l behaved q u i t e d i f f e r e n t l y towards these d e s t a b i l i z i n g agents t h a n o t h e r more c o n v e n t i o n a l s o l s , s u c h a s l a t e x and A g i . I n t h e context of the present study s u f f i c e i t t o say that i n the s o l i d s r a n g e o f 100 t o 400 ppm t h e CCC d i d n o t v a r y a n d was 1.3 χ 1 0 " % NaCl. I n o u r p r e v i o u s s t u d i e s , s l i g h t l y h i g h e r v a l u e s were found w h i c h was a t t r i b u t e d t o e i t h e r s a m p l e o r i g i n , p r e p a r a t i o n t e c h niques, p a r t i c l e s i z e or s o l i d s content. I n t h e c o u r s e o f f u r t h e r w o r k o n c h a r a c t e r i z i n g t h e MCC s o l s i t was f o u n d t h a t t h e CCC o f a v a r i e t y o f s a l t s v a r i e d b o t h w i t h t h e s o l i d s c o n t e n t a n d t e m p e r a t u r e . I n v e s t i g a t i o n o f t h e s e param e t e r s forms t h e b a s i s o f t h e study. I t w i l l be shown t h a t a s a r e s u l t o f p a r t i c l e shape, c o n c e n t r a t i o n and s u r f a c e c h a r a c t e r i s t i c s , c o a g u l a t i o n l e a d s t o a g e l - l i k e s t r u c t u r e . On f u r t h e r a d d i t i o n o f s a l t the coagulated g e l - l i k e s t r u c t u r e aggregates i n t o floes that are i r r e v e r s i b l e . I n t h i s p a p e r , we o u t l i n e t h e e x p e r i m e n t a l p a r a m e t e r s w h i c h l e a d t o t h e s e phenomena and p r e s e n t some possible explanations. Experimental S o l P r e p a r a t i o n . Microcrystaîline c e l l u l o s e ( A v i c e l PH 105) was s u p p l i e d b y t h e FMC C o r p o r a t i o n , P h i l a d e l p h i a , PA., a s a w h i t e powder. Twenty grams o f A v i c e l PH 105 w e r e m i x e d w i t h 180 m l o f w a t e r i n a W a r i n g b l e n d e r f o r 5 m i n u t e s . The t h i c k s l u r r y was t h e n t r a n s f e r r e d t o a l£. v o l u m e t r i c f l a s k and g r a d u a l l y d i l u t e d t o l£. under constant s t i r r i n g . The d i l u t e d s u s p e n s i o n was l e f t u n d i s t u r b e d f o r 3 d a y s . The s u p e r n a t a n t w i t h a v o l u m e o f a b o u t 650 m l was removed. T h i s c o n s t i t u t e d t h e microcrystaîline c e l l u l o s e (MCC) s t o c k s o l . The s o l i d s c o n c e n t r a t i o n o f t h e s o l s were d e t e r m i n e d b y o v e n d r y w e i g h i n g and was a b o u t ( 1 . 2 g / l ) . The u n a d j u s t e d pH was 5.6. A l l t h e s a l t s , h y d r o c h l o r i c a c i d and s o d i u m h y d r o x i d e u s e d w e r e c e r t i f i e d A.C.S. g r a d e f r o m F i s h e r S c i e n t i f i c . S t a b i l i t y Measurements. S t a b i l i t y e x p e r i m e n t s were performed by t h e a d d i t i o n o f e l e c t r o l y t e s o r HC1 t o t h e MCC s o l s . Two s e r i e s o f graduated c y l i n d e r s were prepared. One s e r i e s c o n t a i n e d 15 m l o f s o l w i t h t h e same s o l i d s c o n c e n t r a t i o n i n e a c h c y l i n d e r . The o t h e r s e r i e s o f c y l i n d e r s c o n t a i n e d 15 m l o f t h e e l e c t r o l y t e o r HC1 o f varying concentrations. P a i r s o f g r a d u a t e d c y l i n d e r s (one f r o m e a c h s e r i e s ) w e r e c o m b i n e d b y p o u r i n g t h e c o n t e n t s b a c k and f o r t h b e t w e e n t h e c y l i n d e r s s e v e r a l t i m e s t o g i v e a t o t a l combined v o l u m e o f 30 m l . HC1 o r NaOH was added t o t h e s e m i x t u r e s when n e c e s s a r y t o a d j u s t t o t h e f i n a l pH v a l u e . These s a m p l e s w e r e l e f t u n d i s t u r b e d f o r 2 h o u r s . A t t h e end o f 2 h o u r s , t h e u p p e r 20 m l o f t h e s u p e r n a t a n t was w i t h d r a w n w i t h a l o n g n e e d l e s y r i n g e f o r t u r b i d i t y measurement. The s o l i n w a t e r s e r v e d a s a b l a n k . The t u r b i d i t y r e a d i n g s o f a l l t h e s a m p l e s w e r e compared w i t h t h e r e a d i n g o f t h e b l a n k , and t h i s r a t i o , τ was u s e d a s t h e c r i t e r i o n f o r s t a b i l i t y . The t u r b i d i t i e s w e r e d e t e r m i n e d w i t h a t u r b i d i m e t e r , M o d e l DRT100,



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o b t a i n e d f r o m HF I n s t r u m e n t s , F r e d o n i a , N.Y. The t u r b i d i t i e s w e r e e x p r e s s e d i n N e p h e l o m e t e r T u r b i d i t y U n i t (NTU). I n most e x p e r i m e n t s t h e s m a l l e s t amount o f e l e c t r o l y t e needed t o c o a g u l a t e t h e s o l s m e a s u r e d a f t e r 2 h o u r s s t a n d i n g was c h o s e n a s t h e CCC. When u s i n g HC1, t h i s p o i n t i s t h e c r i t i c a l c o a g u l a t i o n pH. A c o n s t a n t t e m p e r a t u r e w a t e r b a t h was u s e d f o r t e m p e r a t u r e d i f f e r e n t t h a n 23°C. The pH v a l u e s w e r e m e a s u r e d w i t h a Beckman M o d e l 96A pH m e t e r and a F i s h e r c o m b i n a t i o n e l e c t r o d e . The e l e c t r o p h o r e t i c m o b i l i t y measurements w e r e made w i t h a L a s e r D o p p l e r Electrophoresis apparatus. These e x p e r i m e n t s w e r e p e r f o r m e d by Mr. J . K l e i n of the Chemistry Department, Syracuse U n i v e r s i t y . F i l t r a t i o n T e s t . The f i l t r a t i o n t e s t e q u i p m e n t c o n s i s t e d o f a g l a s s c o l u m n , w i t h a f r i t t e d d i s c 2 cm. d i a m e t e r , f i t t e d w i t h a s t o p c o c k and a 25 m l g r a d u a t e d c y l i n d e r . A f t e r m i x i n g t h e MCC s o l s w i t h e l e c t r o l y t e s o l u t i o n , t h e m i x t u r e was i m m e d i a t e l y p o u r e d i n t o t h e g l a s s c o l u m n w i t h t h e s t o p c o c k c l o s e d . The t i m e r e q u i r e d t o c o l l e c t w a t e r ( b l a n k ) and f i l t e r e d s o l b e t w e e n 5 t o 25 m i s i n t h e g r a d u a t e d c y l i n d e r a f t e r 2 h r s . was d e t e r m i n e d . The f r i t t e d g l a s s was b a c k w a s h e d w i t h w a t e r t h o r o u g h l y a f t e r e a c h measurement and a new b l a n k was e s t a b l i s h e d . The r e l a t i v e f i l t r a t i o n t i m e b e t w e e n s a m p l e and w a t e r was u s e d t o compare d i f f e r e n c e s b e t w e e n s a m p l e s . Results E f f e c t o f P a r t i c l e C o n c e n t r a t i o n s on t h e CCC. F i g u r e 1 gives the p l o t s of the r e l a t i v e t u r b i d i t y , versus l o g NaCl c o n c e n t r a t i o n a t d i f f e r e n t s o l i d c o n c e n t r a t i o n a t 23°C. A t a s o l i d s c o n c e n t r a t i o n b e l o w 230 ppm ( F i g u r e 1A) t h e CCC i n c r e a s e s somewhat w i t h a d e c r e a s e i n s o l i d c o n c e n t r a t i o n . However, when t h e s o l i d c o n c e n t r a t i o n i s b e t w e e n 300 ppm and 620 ppm, two CCC v a l u e s a r e o b s e r v e d ( F i g u r e I B ) . The minimum o f t h e f i r s t u n s t a b l e r e g i o n g r a d u a l l y d e c r e a s e s as t h e s o l i d s c o n c e n t r a t i o n i s i n c r e a s e d u n t i l f i n a l l y a t a b o u t 670 ppm, o n l y one CCC i s o b s e r v e d a t a h i g h e r e l e c t r o l y t e c o n c e n t r a t i o n (Figure 1C). As t h e r e d i s p e r s i o n r e g i o n may be t h e r e s u l t o f a c h a r g e r e v e r s a l , t h e e l e c t r o p h o r e t i c m o b i l i t i e s o f t h e MCC s o l s as a f u n c t i o n o f N a C l c o n c e n t r a t i o n w e r e d e t e r m i n e d . No c h a r g e r e v e r s a l was d e t e c t e d and t h e m o b i l i t y o f t h e p a r t i c l e s d e c r e a s e d f r o m 3.5 t o 2.6 m o b i l i t y u n i t s i n a l i n e a r manner 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 i n d i c a t i n g t h a t t h e r e d i s p e r s i o n r e g i o n was n o t c a u s e d by c h a r g e r e v e r s a l . T h r e e a d d i t i o n a l e l e c t r o l y t e s , L i C l , C a C l 2 and B a C l w e r e u s e d as d e s t a b i l i z i n g e l e c t r o l y t e s and t h e s e c o a g u l a t i o n c u r v e s a r e shown i n F i g u r e 2. A g a i n , two CCC's a r e o b s e r v e d w i t h a l l t h e e l e c t r o l y t e s s t r o n g l y i n d i c a t i n g t h a t t h i s phenomenon d o e s depend on e l e c t r o l y t e t y p e . The c o a g u l a t i o n c o n c e n t r a t i o n s o f B a C l 2 and C a C l 2 a r e l o w e r t h a n t h o s e o f L i C l and N a C l , w h i c h a g r e e s w i t h r e s u l t s o f p r e v i o u s s t u d i e s (4) and p o i n t s t o t h e f a c t t h a t we a r e d e a l i n g w i t h an e l e c t r o s t a t i c t y p e o f d e s t a b i l i z a t i o n . Figure 2 2




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F i g u r e 1 . R e l a t i v e T u r b i d i t y (τ%) v e r s u s N a C l c o n c e n t r a t i o n f o r MCC s o l s a t d i f f e r e n t s o l i d s c o n c e n t r a t i o n .


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a l s o shows t h a t t h e CCC v a l u e o f N a C l i s l o w e r t h a n t h a t o f L i C l . Bohm a n d L u n e r (5) have o b s e r v e d b y p o t e n t i o m e t r i c t i t r a t i o n t h a t t h e o r d e r i n t h e a d s o r p t i o n o f e o u n t e r i o n s w i t h o x i d i z e d MCC increases as follows: L i + < N a < K < Mg + < C a < Ba A s s u m i n g t h a t t h e same a d s o r p t i o n o r d e r w o u l d o c c u r when t h e s e e o u n t e r i o n s a r e a d s o r b e d o n t o t h e MCC s o l s u s e d h e r e , t h i s c o a g u l a t i n g sequence f o l l o w s t h e a d s o r p t i o n o r d e r n i c e l y . Downloaded by UNIV OF MASSACHUSETTS AMHERST on October 13, 2015 | http://pubs.acs.org Publication Date: February 10, 1984 | doi: 10.1021/bk-1984-0240.ch023

+

+

2

2 +

2 +

E f f e c t o f Temperature on the S t a b i l i t y / I n s t a b i l i t y Regions. Figure 3 shows t h e e f f e c t o f i n c r e a s i n g t e m p e r a t u r e o n t h e s t a b i l i t y o f t h e MCC s o l s . Two m a i n f e a t u r e s may b e n o t e d . F i r s t , w h i l e o n l y one CCC i s o b s e r v e d w i t h t h e 670 ppm s o l s a t 23°C, b y i n c r e a s i n g t h e t e m p e r a t u r e t o 29°C c r e a t e s a n i n s t a b i l i t y r e g i o n a t a s a l t c o n c e n t r a t i o n o f 1 χ 1 0 " " % w h i c h i n c r e a s e s w i t h t e m p e r a t u r e . The s e c o n d f e a t u r e made a p p a r e n t w i t h i n c r e a s i n g t e m p e r a t u r e i s t h e s y s t e m a t i c d i s a p p e a r a n c e o f the " r e s t a b i l i z e d zone". Indeed, t h e 670 ppm s o l shows o n l y o n e CCC a t 44°C, a n d t h e 410 ppm s o l shows o n l y o n e CCC a t 34°C. S i m i l a r l y , t h e 410 ppm s o l shows o n l y one CCC a t 17°C. A n i n s t a b i l i t y r e g i o n a p p e a r s a t 23°C a n d 1 χ 1 0 " % s a l t w h i c h i n c r e a s e s p r o g r e s s i v e l y a n d t h e " r e s t a b i l i z e d " zone i s e l i m i n a t e d a t 45°, i . e . , o n l y one CCC i s o b s e r v e d a t a l o w e r s a l t concentration (Figure 4). The d i f f e r e n t s t a b i l i t y r e g i o n s a r e shown s c h e m a t i c a l l y i n F i g u r e 5 a n d i n s u b s e q u e n t d i s c u s s i o n r e f e r e n c e w i l l be made t o t h e r e g i o n s i n d i c a t e d i n t h i s F i g u r e . T h e s e r e s u l t s show t h a t t h e two CCC v a l u e s a r e s e n s i t i v e b o t h t o t e m p e r a t u r e a n d t o p a r t i c l e c o n c e n t r a t i o n and there e x i s t c r i t i c a l p o i n t s o f p a r t i c l e concentra t i o n and temperature where e i t h e r r e g i o n I I c o u l d be s t a b i l i z e d o r r e g i o n I I I d e s t a b i l i z e d w h i c h f i n a l l y r e s u l t s i n o n l y o n e CCC value. I n order t o determine the c r i t i c a l temperature and s o l i d s con c e n t r a t i o n p o i n t s , s t a b i l i t y domain d i a g r a m s w e r e c o n s t r u c t e d a s a f u n c t i o n o f s o l i d s c o n c e n t r a t i o n a n d s a l t , ( F i g u r e 6) a s w e l l a s a f u n c t i o n o f temperature and s a l t a t f i x e d v a l u e s o f s o l i d s (Figure 7). From t h e d a t a g i v e n s o f a r a s w e l l a s d a t a n o t r e p o r t e d , a two CCC d o m a i n d i a g r a m w i t h t e m p e r a t u r e a n d s o l i d c o n c e n t r a t i o n a s c o o r d i n a t e s was c o n s t r u c t e d a n d shown i n F i g u r e 8. The s h a d e d a r e a r e p r e s e n t s t h e two CCC d o m a i n . E f f e c t o f pH o n t h e CCC. I t h a s b e e n shown t h a t t h e n e g a t i v e c h a r g e o f MCC o r i g i n a t e s f r o m t h e d i s s o c i a t i o n o f c a r b o x y l g r o u p s ( 6 ) . A change i n pH v a l u e o f t h e MCC s o l w i l l l e a d t o a change i n t h e degree o f d i s s o c i a t i o n o f t h e c a r b o x y l g r o u p s , r e s u l t i n g i n a change o f t h e s u r f a c e p o t e n t i a l o f t h e MCC p a r t i c l e s . Since surface p o t e n t i a l i s an important f a c t o r f o r the s t a b i l i t y o f t h e c o l l o i d s y s t e m , t h e e f f e c t o f pH o n t h e s t a b i l i t y o f MCC s o l s was e x a m i n e d .



382


F i g u r e 2. R e l a t i v e T u r b i d i t y (τ%) v e r s u s s a l t f o r MCC s o l s a t 23°C and 410 ppm s o l i d s .

concentration

F i g u r e 3. R e l a t i v e T u r b i d i t y (τ%) v e r s u s N a C l c o n c e n t r a t i o n f o r MCC s o l s a t 670 ppm s o l i d s a t v a r i o u s t e m p e r a t u r e s .


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F i g u r e 4. R e l a t i v e T u r b i d i t y (τ%) v e r s u s N a C l c o n c e n t r a t i o n a t 410 ppm and a t v a r i o u s t e m p e r a t u r e s .

ccc

ELECTROLYTE CONCENTRATION

CCC

F i g u r e 5. S c h e m a t i c r e p r e s e n t a t i o n o f s t a b i l i t y and i n s t a b i l i t y regions.


384

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Cb)

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NoCI CONCENTRATION (mM)

F i g u r e 6. S t a b i l i t y domain d i a g r a m s o f MCC s o l s , a s a f u n c t i o n of s o l i d c o n c e n t r a t i o n , a ) 23°C b ) 29°C c ) 34°C. The s h a d e d a r e a s r e p r e s e n t c o a g u l a t e d and s e t t l e d MCC s o l s .

F i g u r e 7. S t a b i l i t y domain d i a g r a m s o f MCC s o l s a s f u n c t i o n o f t e m p e r a t u r e and N a C l c o n c e n t r a t i o n a t a s o l i d s c o n c e n t r a t i o n o f a) 410 ppm, and b ) 670 ppm. The shaded a r e a s r e p r e s e n t c o a g u l a t e d and s e t t l e d MCC s o l s .


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F i g u r e 9A a n d 9B show t h e c o a g u l a t i o n c u r v e s a t pH 4.7 and 6.4 a s w e l l a s t h e u n a d j u s t e d o n e , pH 5.6, a t 23°C w i t h a s o l i d c o n t e n t o f 410 ppm. When c o m p a r i n g t h e c o a g u l a t i o n c u r v e f o r pH 5.6, i t i s s e e n t h a t a t pH 4.7, t h e w h o l e c u r v e was s h i f t e d i n t h e d i r e c t i o n o f l o w e r e l e c t r o l y t e c o n c e n t r a t i o n . A t pH 6.4, o n l y one CCC i s o b s e r v e d a t 23°C, b u t , o n i n c r e a s i n g t h e t e m p e r a t u r e t o 29°C, two C G C s a r e o b s e r v e d . Downloaded by UNIV OF MASSACHUSETTS AMHERST on October 13, 2015 | http://pubs.acs.org Publication Date: February 10, 1984 | doi: 10.1021/bk-1984-0240.ch023

1

Floe R e v e r s i b i l i t y by Changing NaCl Concentration. To evaluate more f u l l y the nature of the c o a g u l a t i o n process 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 a number of observations were made on the f l o e s . The s o l i d c o n c e n t r a t i o n of MCC s o l f o r t h i s study was 410 ppm a t 23°C. In region I I , the NaCl c o n c e n t r a t i o n could be reduced by r e p l a c i n g the supernatant w i t h water to make the f i n a l NaCl concent r a t i o n the same as that i n r e g i o n I . A f t e r a d j u s t i n g the concent r a t i o n to r e g i o n I the o r i g i n a l l y formed aggregates were r e d i s persed by gently r e v e r s i n g the c y l i n d e r s s e v e r a l times. The readj u s t e d d i s p e r s i o n was s t a b l e and no v i s i b l e f l o e s formed during two hours which i n d i c a t e d r e v e r s i b i l i t y between region I I and I . However, the f l o e s of r e g i o n IV always reformed no matter which r e g i o n they were d i l u t e d t o . These r e s u l t s i n d i c a t e that the type and extent of i n t e r m o l e c u l a r f o r c e s vary with s a l t c o n c e n t r a t i o n . Floe R e v e r s i b i l i t y by Changing Temperature. At 23°C with a s o l i d c o n c e n t r a t i o n of 670 ppm, the MCC s o l i n r e g i o n I I was s t a b l e . On r a i s i n g the temperature from 23°C to 34°C, the s o l was d e s t a b i l i z e d and f l o e s formed. When the temperature was lowered again to 23°C (the c y l i n d e r reversed s e v e r a l times to r e d i s p e r s e the aggregates) , the f l o e s were redispersed and the d i s p e r s i o n was s t a b l e . In another t e s t , the MCC s o l at 23°C and 410 ppm was unstable (region I I ) . A f t e r decreasing the temperature to 20°C, the MCC s o l was s t a b i l i z e d . I f the temperature was r a i s e d back to 23°C, the MCC s o l d e s t a b i l i z e d and f l o e s formed again. The f l o e s i n r e g i o n II seem to be temperature r e v e r s i b l e . The r e v e r s i b i l i t y of the MCC s o l s i n r e g i o n I I I was a l s o studied a t 410 ppm a t 23°C. Three NaCl concentrations were chosen f o r t h i s t e s t , i . e . , 1.33 χ 10" M, which was c l o s e to the NaCl c o n c e n t r a t i o n of r e g i o n I I , 1.67 χ 10" M, l o c a t e d i n the middle of r e g i o n I I I , and 2.25 χ 1 0 " % , was near the NaCl c o n c e n t r a t i o n of region IV. A f t e r two hours, a l l the s o l s were s t a b l e . Then when the temperature was r a i s e d from 23°C to 34°C, f l o e s formed i n a l l three samples. A f t e r two hours, the temperature was lowered to 23°C. The s o l s with NaCl concentrations of 1.33 χ 10" M and 1.67 χ 10" 3M were r e s t a b i l i z e d , whereas the f l o e s formed i n 2.25 χ 10 M NaCl were i r r e v e r s i b l e . 3

3

3

3

Floe Sedimentation. Although f l o e s formed i n both regions I I and IV, t h e i r formation r a t e and sediment volume were d i f f e r e n t . The


386

POLYMER ADSORPTION AND DISPERSION STABILITY 50h

p 4ομ UJ

QC ?

30

Ui

i

20|


UJ

F i g u r e 8, Two CCC domain d i a grams o f MCC a s a f u n c t i o n o f t e m p e r a t u r e and s o l i d s concen tration. The shaded a r e a r e p r e s e n t s t h e two CCC d o m a i n s .

Τ

1 I I I I I 11

10

200 400 600 800 1000 SOLID CONCENTRATION(ppm)

1

NaCl

! IIIMI!

1

1 I Mill

M/t

F i g u r e 9. The e f f e c t o f pH and t e m p e r a t u r e on t h e c o a g u l a t i o n o f MCC s o l s a t a s o l i d c o n c e n t r a t i o n o f 410 ppm.



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p a r t i c l e s o f r e g i o n IV coagulated immediately a f t e r mixing the s o l w i t h NaCl w h i l e t h e p a r t i c l e s o f region I I d i d not coagulate u n t i l an h o u r l a t e r o r e v e n l o n g e r a f t e r m i x i n g . F i g u r e 10 shows t h e s e d i m e n t a t i o n r e s u l t s a t 23°C a n d 410 ppm i n a t o t a l v o l u m e o f 30 m l . F o r b o t h r e g i o n s , t h e i n i t i a l v o l u m e o f s u b s i d e n c e c o u l d n o t b e r e c o r d e d b e c a u s e t h e r e was no c l e a r boundary between sediment and s u p e r n a t a n t . From t h i s f i g u r e , we s e e t h a t t h e s e d i m e n t a t i o n r a t e i n r e g i o n I V was h i g h e r t h a n t h a t i n r e g i o n I I , w h i l e t h e s e d i m e n t a t i o n v o l u m e i n r e g i o n I I was t w i c e that of region IV. F i l t r a t i o n Test. The s o l s i n r e g i o n s I a n d I I I a p p e a r e d i d e n t i c a l by t u r b i d i t y measurements a t 23°C a t a s o l i d s c o n c e n t r a t i o n o f 410 ppm. However, t h e p r e v i o u s d a t a showed t h a t t h e s o l s i n r e g i o n I I I coagulate a t h i g h temperatures o r a t lower s o l i d c o n c e n t r a t i o n s , w h i l e t h e s o l s i n r e g i o n I a r e s t a b l e u n d e r t h e s e c o n d i t i o n s . One p o s s i b l e e x p l a n a t i o n i s t h a t t h e p a r t i c l e s i n r e g i o n I I I formed a t y p e o f n e t w o r k ( g e l ) s t r u c t u r e . Such a s t r u c t u r e i m m o b i l i z e s t h e p a r t i c l e s and g i v e s t h e appearance o f s t a b l e s o l s . One way t o t e s t t h i s p o s s i b i l i t y i s t o r e f i l t e r samples from t h e d i f f e r e n t r e g i o n s . I f a network s t r u c t u r e i s formed i n r e g i o n I I I , t h e r e ought t o be an e x t r a r e s i s t a n c e t o f i l t r a t i o n a n d l e a d i n g t o l o n g e r f i l t r a t i o n times than those i n r e g i o n I . Table I g i v e s t h e r e s u l t s

T a b l e 1.

R e l a t i v e F i l t r a t i o n Time o f MCC S o l s a s a F u n c t i o n o f N a C l C o n c e n t r a t i o n a t 23°C

Solids 0 ppm B l a n k 410 760

1.17 1.19

5x10-4 Reg. I 1.22 1.26

N a C l C o n c e n t r a t i o n (M) 9xl0~4 1.33x10- 1.67x10" 2.25x10" 4 x 1 0 " Reg. I I Reg. I l l Reg. I l l Reg. I l l Reg. I V 3

1.43 1.39

1.50 1.75

3

1.65 1.93

3

1.63 1.97

3

6.40 9.52

for t h e r e l a t i v e f i l t r a t i o n time as a f u n c t i o n o f NaCl concentration. The r e s u l t s i n d i c a t e t h a t t h e d i f f e r e n t r e g i o n s have d i f f e r ent f i l t e r a b i l i t y i n d i c a t i n g t h a t t h e f l o e s have d i f f e r e n t s t r u c tures. Discussion F i g u r e 1A shows t h a t t h e CCC d e c r e a s e s w i t h i n c r e a s i n g s o l c o n c e n t r a t i o n i n t h e r a n g e o f 50 t o 200 ppm. The v a r i a t i o n o f t h e CCC w i t h s o l c o n c e n t r a t i o n i s b y no means a n u n e x p e c t e d o b s e r v a t i o n i n v i e w o f p r e v i o u s w o r k w i t h o t h e r s y s t e m s ( 7 ) . R e c e n t l y Βensley a n d H u n t e r (8) have i n v e s t i g a t e d t h i s a s p e c t o f c o l l o i d a l s t a b i l i t y w i t h l a t i c e s a n d f o u n d t h a t up t o a v o l u m e f r a c t i o n o f 0.10 t h e CCC was i n d e p e n d e n t o f t h e v o l u m e f r a c t i o n . These a u t h o r s a l s o p o i n t e d o u t t h e d i f f i c u l t i e s i n v o l v e d i n m e a s u r i n g t h e s t a b i l i t y o f more



388


F i g u r e 10· S e d i m e n t v o l u m e ( m l ) v e r s u s s e t t l i n g t i m e ( h r ) f o r MCC s o l s i n r e g i o n I I and IV a t 23°C and a s o l i d s c o n c e n t r a t i o n o f 410 ppm.



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concentrated d i s p e r s i o n s and the importance of mixing during the c o a g u l a t i o n process. In view of the r o d - l i k e shape of the MCC p a r t i c l e s , t h e i r d e n s i t y (1.52), and wide s i z e d i s t r i b u t i o n , using the more r i g o r o u s o p t i c a l techniques to determine the CCC would be a formidable task. Two more convenient but l e s s accurate approaches were adopted; measuring the r e s i d u a l t u r b i d i t y and sediment volume a f t e r s e t t l i n g f o r a f i n i t e time. While n e i t h e r of these methods i s absolute, they have proved convenient and i n f o r m a t i v e . The most s t r a i g h t f o r w a r d explanation f o r the data i n F i g u r e 1A l i e s i n the f l o e s i z e formed on c o a g u l a t i o n . As the s o l conc e n t r a t i o n i n c r e a s e s , l a r g e r f l o e s are formed which s e t t l e more q u i c k l y and g i v e an apparent decrease i n the CCC. Experiments made a t longer standing times than two hours showed somewhat lower CCC v a l u e s . As the s o l c o n c e n t r a t i o n reaches 300 ppm, an apparent r e d i s p e r s i o n occurs which i s followed by another CCC a t a higher s a l t c o n c e n t r a t i o n . In a l l i n s t a n c e s , the r e d i s p e r s i o n occurs over a very narrow range of s a l t c o n c e n t r a t i o n . Indeed, much of our p r e l i m i n a r y work pointed to a very i r r e p r o d u c i b l e system u n t i l t h i s phenomenon was recognized as r e a l . The e l e c t r o p h o r e t i c r e s u l t s and the two CCC behavior with other mono and d i v a l e n t s a l t s seem to e l i m i n a t e the s p e c i f i c a d s o r p t i o n of c a t i o n s as an e x p l a n a t i o n f o r t h i s phenomenon. When s t i l l higher s o l concentrations are used (Figure 1C) only one CCC, at a higher s a l t c o n c e n t r a t i o n was observed. The t u r b i d i t y values i n d i c a t e that r e g i o n I and I I I are s i m i l a r , i . e . , both regions appear as a d i s p e r s e d s o l . To e s t a b l i s h whether t h i s i s indeed the case, s o l s from the d i f f e r e n t s t a b i l i t y zones were c e n t r i f u g e d a t low speed. I t was found that at a s a l t c o n c e n t r a t i o n g r e a t e r than 1 χ 10" M, i . e . , between the two CCC v a l u e s , the s o l s sedimented much more r a p i d l y than a t s a l t con c e n t r a t i o n s below 1 χ 10~3M i n d i c a t i n g that r e g i o n I I I i s composed of l a r g e r aggregates which d i d not s e t t l e on standing; thus, g e l a t i o n may be o c c u r r i n g a t a s a l t c o n c e n t r a t i o n > 1 χ 10~3M < 5 χ 10~3 and at a s o l p a r t i c l e c o n c e n t r a t i o n g r e a t e r than 300 ppm. The r e f i l t r a t i o n data i n Table 1 and the d i f f e r e n t e q u i l i b r i u m sediment values i n regions I I and IV seems to confirm that a d i f f e r e n t s t r u c t u r e e x i s t s i n these i d e n t i f i a b l e r e g i o n s . Lastly, the l a c k of r e d i s p e r s i b i l i t y of the s o l s coagulated a t h i g h con c e n t r a t i o n , i n c o n t r a s t to the r e d i s p e r s i b i l i t y of the s o l s coag u l a t e d at 1 χ 10 3M, f u r t h e r shows that s t r u c t u r a l d i f f e r e n c e s e x i s t between f l o e s i n regions I I and IV. 3

-

DLVO Theory A p p l i e d to MCC S o l s . In order to i n t e r p r e t the s t a b i l i t y of MCC s o l s , s e v e r a l s t a b i l i t y c a l c u l a t i o n s were made. F i r s t , by using a s p h e r i c a l p a r t i c l e r a d i u s of 706A, A = 2.6 kT, ψο = 14 mV and Τ = 296°K at a s a l t c o n c e n t r a t i o n of 0,9 χ 10"3M (region I I ) , a secondary minimum was obtained at 500 A, = 4 kT a t V = 0 a t 14 A. At higher s a l t c o n c e n t r a t i o n s , the DLVO p l o t s showed n e a r l y complete i n s t a b i l i t y . Similar calculations were made using f l a t p l a t e geometry with A = 2.94 χ 10~20j T


P O L Y M E R A D S O R P T I O N A N D D I S P E R S I O N STABILITY

390

(9, 1 0 ) . I t was found that = 0.5 χ l O - i O j / m with a broad shallow secondary minimum of 600 A. K r a t o h v i l et a l (4) have accounted f o r the high s e n s i t i v i t y of MCC s o l towards e l e c t r o l y t e based on a secondary minimum and indeed the c a l c u l a t i o n s presented above make t h i s a reasonable explanation. With t h i s i n mind, the f i r s t CCC < 2 0 0 ppm i s a r e s u l t o f c o a g u l a t i o n i n a secondary minimum. However, g i v e n s u f f i c i e n t p a r t i c l e s , 410 ppm, and a s a l t c o n c e n t r a t i o n greater than 1 χ 1 0 - 3 M the p a r t i c l e s a r e brought s t i l l c l o s e r together and a three dimensional s t r u c t u r e (gel) i s formed. This g e l imbibes s u f f i c i e n t water so as to prevent i t s sedimentation completely. Further a d d i t i o n of s a l t , (second CCC) breaks the g e l s t r u c t u r e and the f l o e s s e t t l e out. The temperature r e v e r s i b i l i t y o f f l o e s i n regions I I and I I I i n c o n t r a s t to those of r e g i o n IV, as w e l l as the d i f f e r e n c e i n sediment volumes and r e f i l t r a t i o n time between the two regions, i n d i c a t e that a t the higher s a l t concen t r a t i o n , i . e . , a t the second CCC, the p a r t i c l e s may indeed be a t a primary minimum. Observations such as those presented here have been reported p r e v i o u s l y f o r C e 0 2 by Kruyt ( 1 1 ) . T h e i r r e s u l t s i n d i c a t e d that at 1.64 χ 1 0 " M NaCl, the C e U 2 s o l coagulates slowly w h i l e a t 1.82 χ 1 0 - 1 M NaCl, the C e 0 2 s o l forms a g e l and the p a r t i c l e s do not s e t t l e . S i m i l a r g e l - s t r u c t u r e phenomenon have been observed for A I 2 O 3 ( 1 2 ) and S 1 O 2 s o l s (13). Overbeek (14) has proposed that t h i s type o f g e l was as a l o o s e network s t r u c t u r e formed by p a r t i c l e - p a r t i c l e i n t e r a c t i o n s . T h i s network s t r u c t u r e immobi l i z e s the p a r t i c l e s as w e l l as occluded l i q u i d and prevents the p a r t i c l e s from c o a g u l a t i n g . Since the p a r t i c l e s cannot coagulate, the system which forms a g e l appears l i k e a s t a b l e s o l . Two recent reviews (15, 16) have r e c e n t l y focused a t t e n t i o n on t h i s type of c o a g u l a t i o n and g e l a t i o n phenomena, e s p e c i a l l y w i t h r o d - l i k e par t i c l e s . While c o n s i d e r a b l e progress has been made, a complete explanation of these phenomena i s s t i l l l a c k i n g . Much of the data are c o n s i s t e n t w i t h c o a g u l a t i o n a t secondary minimum, but two ex perimental r e s u l t s need f u r t h e r e l a b o r a t i o n ; the lower f l o e volume of r e g i o n IV compared to r e g i o n I I and the temperature s e n s i t i v i t y of r e g i o n I I I . On the b a s i s of v i s c o s i t y and sedimentation a n a l y s i s , Marchessault et a l (17) have proposed a t a c t o i d shape u n i t comprising about 2 0 m i c r o c r y s t a l s i n d i l u t e phosphate b u f f e r . Other evidence (18) a l s o suggests that c e l l u l o s e m i c r o c r y s t a l s have a tendency to aggregate e a s i l y i n a s i d e - b y - s i d e f a s h i o n . I t i s our c o n t e n t i o n that r e g i o n IV c o n s i s t s l a r g e l y of t a c t o i d type aggregates. Cal c u l a t i o n s show that the occluded volume f r a c t i o n o f the s o l i n the sediment i n r e g i o n IV i s 0.16 while the occluded volume f r a c t i o n o f the o r i g i n a l s o l i s 0.90. I t would appear that the par t i c l e s i n t h i s s a l t r e g i o n a r e subjected to a d d i t i o n a l a t t r a c t i v e f o r c e s . These may o r i g i n a t e from d i p o l e and/or hydrogen bonding f o r c e s (15) r e s u l t i n g i n a more t i g h t l y packed f l o e .


2

2


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As the g e l i s very d i l u t e , the temperature s e n s i t i v i t y i n t h i s r e g i o n i s l i k e l y r e l a t e d to the increased Brownian motion which breaks the weakly bonded s t r u c t u r e i n t o d i s c r e t e f l o e s . However, whether t h i s occurs as a r e s u l t o f changes o f p a r t i c l e charge and p o t e n t i a l and/or dehydration o f the dispersed phase i s unknown a t t h i s time (19).


Concluding Remarks This study has shown that MCC forms a g e l - l i k e s t r u c t u r e a t low s o l i d s c o n c e n t r a t i o n i n the presence o f s a l t . I t has a l s o been known f o r a c o n s i d e r a b l e time (20) that MCC forms aqueous g e l s and pastes but a t much higher s o l i d s c o n c e n t r a t i o n . I t would appear that f u r t h e r s t u d i e s with the MCC g e l i n d i l u t e form may e l u c i d a t e the mechanism o f g e l a t i o n more d e f i n i t i v e l y . Acknowledgment s We would l i k e to acknowledge the h e l p f u l d i s c u s s i o n s with Dr. Robert Evans and Mr. A. S h r i n a t h . Mr. Shrinath a l s o c o n t r i b u t e d the experimental work d e a l i n g with the MCC s o l s . C o n t r i b u t i o n No. 167 Empire State Paper Research I n s t i t u t e .

Literature Cited 1. Sandell, L., and Luner, P., J. Appl Polymer Sci 18: 2075 (1974). 2. Goossens, J.W.S., and Luner, P., TAPPI 59 No. 2 89 (1976). 3. Verwey, E.J.W., and Overbeek, J.Th.G., "Theory of Stability of Lyopholic Colloids" Elsevier, Amsterdam, 1948. 4. Kratohvil, S., Janauer, G.E., and Matijevic, E., J. Colloid Interface Sci 29 187 (1969). 5. Bohm, J., and Luner, P. unpublished results. 6. Edelson, M.R., and Hermans, J. Jr., J. Polymer Sci Part C No. 2 145 (1963). 7. "Colloid Science", H.R. Kruyt, ed. Vol. 1, p. 321, Elsevier, Amsterdam, 1952. 8. Bensley, C.N., and Hunter, R., J. Colloid Interface Sci 88, No. 2 546 (1983). 9. Visser, J . , Adv. Colloid Interface Sci. 3 331 (1972). 10. Visser, J . , J. Colloid Interface Sci 34 26 (1970). 11. Kruyt, H.R., and van der Made, J.E.M., Rec. Trav. Chem. 42, 277 (1923). 12. Gann, J.Α., Kolloid Chem. Beihefte 8 64 (1916). 13. Kruyt, H.R., and Postma, J . , Rec. Trav. Chem. 44 765 (1925). 14. See Ref. 7, p. 335. 15. I.F. Efremov in "Surface and Colloid Science"; Matijevic, E., Ed., Wiley Interscience: 1976 Vol. 8 p. 85. 16. W. Heller in "Polymer Colloids II"; Fitch, R.M., Ed., Plenum Press, 1980; p. 153.


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17. Marchessault, J. Jr., Koch, M.J., and Yang, J.T., J. Colloid Sci 16 345 (1961). 18. Hermans, J. Jr., J. Polymer Sci Part C, No. 2, 129 (1963). 19. Baran, Α.Α., Solomentseva, I.M., and Kurilenko, O.D., Kolloidnyi Zhurnal 37 No. 2 p. 219 (1975). 20. Battista, O.A., Cruz, M.M., and Ferraro, C.F., in "Surface and Colloid Science; Matijevic, E., Ed., Wiley Interscience 1971 Vol. 3 p. 241. RECEIVED

October 7, 1983


Colloidal Stability of Microcrystalline Cellulose in Aqueous Salt Solutions

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