Steric and Electrostatic Contributions to the Colloidal Properties of

21 Steric and Electrostatic Contributions to the Colloidal Properties of Nonaqueous Dispersions 1

Downloaded by UNIV OF GUELPH LIBRARY on September 8, 2012 | http://pubs.acs.org Publication Date: February 10, 1984 | doi: 10.1021/bk-1984-0240.ch021

F. M. FOWKES and R. J. PUGH

Department of Chemistry, Lehigh University, Bethlehem, PA 18015

Dispersions of carbon black in dodecane and kerosene have been prepared with a basic polyisobutene succinamide, the most extensively used commercial non-aqueous dispersant. The basic anchoring groups adsorb strongly onto the acidic surface sites of the carbon black, and with 1 w% or more of dispersant (based on the carbon black) the 50 Å- long polyisobutene chains extend out into the solution to provide a steric barrier 100 Å thick between particles, as evidenced by a million-fold decrease in electrical conductivity and a twenty-fold decrease in viscosity, but with no degree whatsoever of deflocculation. Only at much higher dispersant levels is deflocculation observed. When the concentration of unadsorbed dispersant in the oil phase exceeds 0.1%, the conductivity increases as counterions form, large negative zeta-potentials develop (-140 to -300 mV), the stability ratios climb towards infinity and the dispersions become completely deflocculated. D i s p e r s i o n s o f f i n e l y d i v i d e d s o l i d s i n non-aqueous m e d i a h a v e been i m p o r t a n t f o r p a i n t s , i n k s , r e i n f o r c e d polymers and l u b r i c a t i n g o i l s , b u t w i t h the development o f l i q u i d t o n e r systems and " u l t r a - s t r u c t u r e " p r o c e s s i n g o f c e r a m i c s a s f i n e powders d i s p e r s e d i n o r g a n i c m e d i a , t h e u n d e r s t a n d i n g and o p t i m i z a t i o n o f s u c h s y s t e m s i s more i m p o r t a n t t h a n e v e r .

1

Current address: Ytkemiska Institutet Box 5607, Institute for Surface Chemistry, Stockholm, Sweden S-114 86. 0097-6156/84/0240-O331$07.00/0

© 1984 American Chemical Society

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

332


Theories

POLYMER ADSORFFION AND DISPERSION STABILITY o f C o l l o i d a l S t a b i l i t y i n Non-Aqueous

Media

E l e c t r o s t a t i c s i n Non-Aqueous M e d i a . A p o p u l a r m i s c o n c e p t i o n i n s t u d i e s o f non-aqueous d i s p e r s i o n s c o n c e r n s e l e c t r o s t a t i c e f f e c t s . B e c a u s e t h e s e a r e more d i f f i c u l t t o measure t h a n i n aqueous m e d i a , t h e r e h a s b e e n a g e n e r a l t e n d e n c y t o i g n o r e them c o m p l e t e l y . However, t h e few i n v e s t i g a t o r s who h a v e measured z e t a - p o t e n t i a l s o r e l e c t r o d e p o s i t i o n w i t h t h e s e s y s t e m s h a v e become c o n v i n c e d o f t h e i r i m p o r t a n c e . W i t h t h e a d v e n t o f modern c o m m e r c i a l i n s t r u m e n t a t i o n f o r z e t a - p o t e n t i a l s i n non-aqueous m e d i a i t i s t o hoped t h a t t h e s e e f f e c t s w i l l be m e a s u r e d r a t h e r t h a n i g n o r e d . T h i r t y y e a r s ago v a n d e r M i n n e and H e r m a n i e showed t h a t c a r b o n b l a c k i n b e n z e n e d e v e l o p e d a p p r e c i a b l e z e t a p o t e n t i a l s and c o l l o i d a l s t a b i l i t y i n t h e p r e s e n c e o f c a l c i u m s o a p s ( w h i c h gave p o s i t i v e z e t a - p o t e n t i a l s ) o r q u a t e r n a r y ammonium p i c r a t e s ( w h i c h gave n e g a t i v e z e t a - p o t e n t i a l s ) ; t h e i r m i x t u r e s gave l o w z e t a p o t e n t i a l s and c o l l o i d a l i n s t a b i l i t y . ( 1 , 2 ) Twenty y e a r s ago t h e s e n i o r a u t h o r and c o - w o r k e r s showed t h a t t h e mechanism o f e l e c t r o s t a t i c c h a r g i n g i n non-aqueous i s t h e r e v e r s e o f t h a t i n aqueous m e d i a . (3) I n aqueous s o l u t i o n s s u r f a c t a n t s i o n i z e and t h e n t h e s u r f a c e - a c t i v e ions adsorb onto surfaces to give e l e c t r o s t a t i c a l l y c h a r g e d s u r f a c e s . However i n o r g a n i c m e d i a b a s i c d i s p e r s a n t s a d s o r b a s n e u t r a l m o l e c u l e s o n t o a c i d i c s u r f a c e s where p r o t o n - t r a n s f e r from a c i d i c s i t e s charges the adsorbed bases positively. I f the concentration of dispersant d i s s o l v e d i n the o i l p h a s e i s s u f f i c i e n t l y h i g h t h a t d y n a m i c a d s o r p t i o n and d e s o r p t i o n o c c u r s , some p r o t o n - c a r r y i n g d i s p e r s a n t s w i l l d e s o r b i n t o s o l u t i o n , and p r o v i d e t h e c o u n t e r i o n s f o r t h e n e g a t i v e c h a r g e s l e f t on t h e s u r f a c e . F i g u r e 1 i l l u s t r a t e s t h i s mechanism, w h i c h h a s b e e n d e m o n s t r a t e d i n some d e t a i l by u s i n g c a r b o n - 1 4 t a g g e d b a s i c p o l y m e r i c d i s p e r s a n t s ( 4 , 5 ) and t r i t i u m - t a g g e d a c i d i c s i t e s on s u r f a c e s . I n F i g u r e 1 a p o l y m e r i c d i s p e r s a n t i s i n d i c a t e d , f o r t h e r a d i o t r a c e r s t u d i e s w e r e done w i t h h i g h m o l e c u l a r w e i g h t polyalkylmethacrylates having v i n y l p y r i d i n e basic s i t e s . Howe v e r t h e mechanism h a s a l s o b e e n d e m o n s t r a t e d w i t h l o w e r m o l e c u l a r w e i g h t m a t e r i a l s and e v e n w i t h m i c e l l a r m e t a l s o a p s and s u l f o nates. T h i s mechanism r e s u l t s i n n e g a t i v e l y - c h a r g e d a c i d i c p a r t i c l e s w i t h b a s i c d i s p e r s a n t s and p o s i t i v e l y - c h a r g e d b a s i c p a r t i c l e s w i t h a c i d i c d i s p e r s a n t s , a p r i n c i p l e t h a t h a s b e e n demons t r a t e d e x p e r i m e n t a l l y a g r e a t many t i m e s . ( 4 , 5 ) Steric Stabilization. S t e r i c s t a b i l i z a t i o n was a t e r m f i r s t i n t r o d u c e d by H e l l e r t o e x p l a i n how a d s o r b e d p o l y e t h y l e n e o x i d e polymers increased the s a l t c o n c e n t r a t i o n r e q u i r e d f o r f l o c c u l a t i o n o f n e g a t i v e l y c h a r g e d aqueous s u s p e n s i o n s . ( 6 ) H e l l e r ' s s y s t e m s w e r e s t a b i l i z e d by b o t h m e c h a n i s m s , a s a r e most c o m m e r c i a l d i s p e r s i o n s t o d a y , aqueous and n o n - a q u e o u s . Much o f t h e more r e c e n t l i t e r a t u r e on s t e r i c s t a b i l i z e r s h a s b e e n p r e o c c u p i e d w i t h s o l u b i l i t y requirements, f o r the s o l u b i l i t y of polymers i s a d e l i c a t e m a t t e r and v e r y s e n s i t i v e t o t e m p e r a t u r e and s o l v e n t



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F O W K E S AND PUGH

Colloid Properties of Nonaqueous

Dispersions

333

composition. T h i s preoccupation w i t h s o l u b i l i t y requirements has l e d some i n v e s t i g a t o r s t o b e l i e v e t h a t p r e c i p i t a t i o n o f s t a b i l i z e r s i s t h e m a i n mechanism o f c o l l o i d a l i n s t a b i l i t y , r a t h e r t h a n the t r a d i t i o n a l d i s p e r s i o n f o r c e a t t r a c t i o n s between p a r t i c l e s . I t s h o u l d b e o b v i o u s t h a t a d i s p e r s a n t and s o l v e n t s y s t e m must b e chosen so t h a t t h e d i s p e r s a n t does n o t p r e c i p i t a t e , b u t t h i s l i m i t a t i o n h a s n o t h i n g t o do w i t h t h e mechanisms o f c o l l o d i a l stability. In studies of s t e r i c s t a b i l i z e r s too l i t t l e attentioni s g e n e r a l l y p a i d t o t h e d i s p e r s i o n f o r c e a t t r a c t i o n s between p a r t i c l e s and t h e c r i t i c a l s e p a r a t i o n d i s t a n c e (H ) n e e d e d t o k e e p p a r t i c l e s from f l o c c u l a t i n g . Adsorbed s t e r i c s t a b i l i z e r s can p r o v i d e a c e r t a i n f i l m t h i c k n e s s on each p a r t i c l e b u t i f t h e sep a r a t i o n d i s t a n c e between c o l l i d i n g p a r t i c l e s i s l e s s than H the particles w i l l flocculate. The c a l c u l a t i o n o f H i s not d i f f i c u l t a n d measurements t o p r o v e o r d i s p r o v e s u c h c a l c u l a t i o n s are n o t d i f f i c u l t e i t h e r . For equal-sized spheres o f substance 1 w i t h r a d i u s o r i n medium 2 ^ t h e Hamaker e q u a t i o n f o r t h e d i s p e r s i o n f o r c e a t t r a c t i v e e n e r g y (ϋχ2ΐ) a t c l o s e a p p r o a c h i s ( 7 ) : Ul21

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m

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(D

w h e r e A121> t h e Hamaker c o n s t a n t , c a n b e r e l a t e d t o ^ t h e f o r c e c o n t r i b u t i o n t o t h e s u r f ace e n e r g i e s γ χ a n d γ :

dispersion

2

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Αχ2i = 1.5 χ 10

Hamaker c o n s t a n t s c a n s o m e t i m e s b e c a l c u l a t e d f r o m r e f r a c t i v e i n d e x d a t a b y t h e L i f s h i t z e q u a t i o n s ( 8 ) , b u t i t now a p p e a r s t h a t γ v a l u e s a r e c l o s e l y r e l a t e d t o r e f r a c t i v e i n d i c e s and a r e a d i r e c t measure o f t h e L i f s h i t z a t t r a c t i o n s . I n E q u a t i o n 1 a c o r r e c t i o n f a c t o r f f o r " r e t a r d a t i o n " o f d i s p e r s i o n f o r c e s i s shown w h i c h c a n b e d e t e r m i n e d f r o m F i g u r e 2, a g r a p h o f 1 / f a t v a r i o u s v a l u e s o f Η and a a s a f u n c t i o n o f λχ, t h e c h a r a c t e r i s t i c wave l e n g t h o f t h e most e n e r g e t i c d i s p e r s i o n f o r c e s , c a l c u l a b l e a n d t a b u l a t e d i n t h e l i t e r a t u r e (9). T a b l e I l i s t s some c h a r a c t e r i s t i c wave l e n g t h s f r o m t h e w o r k o f G r e g o r y ( 9 ) . The c a l c u l a t i o n s o f f shown i n F i g u r e 2 a r e t a k e n f r o m t h e w o r k o f C l a y f i e l d and Lumb.(lO) By u s i n g t h e s e c a l c u l a t i o n s one c a n d e t e r m i n e t h e a t t r a c t i v e e n e r g y p e r p a i r o f p a r t i c l e s a t v a r i o u s s e p a r a t i o n d i s t a n c e s , a n d d e t e r m i n e f o r any p a r t i c u l a r v a l u e g f A121, ΐ > a n d r a d i u s ( a ) t h e c r i t i c a l v a l u e o f Η t h a t makes U i = - k T , w h e r e k i s t h e B o l t z m a n n c o n s t a n t a n d kT i s t h e a v e r a g e v i b r a t i o n a l e n e r g y o f ^ a p a i r o f p a r t i c l e s f l o c c u l a t e d a t s e p a r a t i o n d i s t a n c e H. I f U ^ i i s g r e a t e r t h a n -kT the p a r t i c l e s w i l l n e a r l y always bounce a p a r t on c o l l i s i o n , b u t i f i t i s l e s s t h a n -kT t h e p a r t i c l e s t e n d t o f l o c c u l a t e . F i g u r e 3 i s a graph o f Η v s . p a r t i c l e radius (a) f o r carbon p a r t i c l e s i n _ o i l w h e r e t f i e Hamaker c o n s t a n t i s r e l a t i v e l y h i g h (A 2i 2.8xl0 e r g s ) , f o r polystyrene p a r t i c l e s i n water λ

1 2

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POLYMER ADSORPTION AND DISPERSION STABILITY

F i g u r e 1. M e c h a n i s m o f e l e c t r o s t a t i c c h a r g i n g i n o i l o f p a r t i c l e s w i t h a c i d i c s i t e s (AH) b y a p o l y m e r i c d i s p e r s a n t w i t h b a s i c s i t e s ( B ) . Reproduced w i t h p e r m i s s i o n from Ref. ( 5 ) . C o p y r i g h t 1982, American Chemical S o c i e t y .

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F i g u r e 2. R e t a r d a t i o n c o r r e c t i o n f a c t o r ( f ) f o r d i s p e r s i o n f o r c e a t t r a c t i o n s between s p h e r i c a l p a r t i c l e s o f r a d i u s (a) a t s e p a r a t i o n d i s t a n c e ( Η ) , w i t h d i s p e r s i o n f o r c e wave l e n g t h X. (10) t


21.

FOWKES AND PUGH =

Colloid Properties of Nonaqueous Dispersions

335

llf

(Αΐ2ΐ 5·3χ10 ergs)» and f o r o i l - i n - w a t e r e m u l s i o n s w h e r e t h e Hamaker c o n s t a n t i s q u i t e l o w ( A i 21-9.3x10 e r g s ) . For the system under s t u d y i n t h i s paper ( c a r b o n i n o i l , w i t h average r a d i u s a= 0.2 \im) i t i s s e e n t h a t Η i s 260 X and t h a t t h e a d s o r b e d s t e r i c b a r r i e r m o l e c u l e s mus£ provide f i l m s 130 X i n thickness a t c o l l i s i o n to prevent f l o c c u l a t i o n . Since the d i s p e r s a n t u s e d i n t h i s p a p e r p r o v i d e s 50 X f i l m s and a n H a t c o l l i s i o n o f 100 X i t s h o u l d be no s u r p r i s e t h a t when z e t a p o t e n t i a l s are small the p a r t i c l e s f l o c c u l a t e i n every c o l l i s i o n . 1 5


r

Table I .

C h a r a c t e r i s t i c Wave L e n g t h s o f D i s p e r s i o n F o r c e A t t r a c t i o n s (9)

Substance water n-octane benzene quartz polystryene

in X 896 882 1181 793 1145

The c a l c u l a t i o n s i n F i g u r e 3 w e r e made o n t h e a s s u m p t i o n t h a t t h e Hamaker c o n s t a n t s o f t h e a d s o r b e d d i s p e r s a n t f i l m s a r e t h e same a s t h e l i q u i d m e d i a . This i s an e x c e l l e n t assumption ^ f o r p o l y m e r s , s i n c e t h i s means t h a t t h e s o l u b i l i t y p a r a m e t e r (6 ) o f t h e d i s p e r s a n t and o f t h e medium must b e a b o u t t h e same, w h i c h i s the b a s i c requirement f o r s o l u b i l i t y of polymers. I n t h e e a r l i e r l i t e r a t u r e o n s t e r i c s t a b i l i z a t i o n t h e r e was a tendency t o i g n o r e the importance o f a n c h o r i n g s i t e s f o r s t e r i c stabilizers. Without s t r o n g anchors t h e adsorbed molecules a r e e a s i l y swept a s i d e i n c o l l i s i o n s , a s h a s b e e n s o w e l l i l l u s t r a t e d w i t h p o l y s t y r e n e adsorbed onto carbon b l a c k s i n hydrocar bon s o l u t i o n s ; i n t h e s e s y s t e m s p a r t i c l e s f l o c c u l a t e i n e v e r y collision.(11) Many i n v e s t i g a t o r s o f s t e r i c s t a b i l i z a t i o n h a v e m e a s u r e d c o l l o i d a l s t a b i l i t y w i t h o u t t a k i n g t h e e f f o r t t o f i n d out whether the s t a b i l i t y a c t u a l l y r e s u l t e d from e l e c t r o s t a t i c s t a b i l i z a t i o n . I n many p u b l i s h e d a r t i c l e s i t h a s b e e n c o n c l u d e d t h a t s t e r i c s t a b i l i z a t i o n h a d b e e n a t t a i n e d and f u r t h e r s t u d y showed t h i s was n o t t h e c a s e . One s u c h e x a m p l e i s a r e c e n t p a p e r o n " s t e r i c s t a b i l i z a t i o n b y a n a d d i t i v e o f t h e same t y p e u s e d i n t h i s w o r k . ( 1 2 ) The p u b l i s h e d p h o t o g r a p h shows t h e s i l i c a p a r t i c l e s i n o i l s t a b i l i z e d a t i n t e r p a r t i c l e separations s e v e r a l times the d i s t a n c e s p r o v i d e d b y t h e a d s o r b e d f i l m s ; no e l e c t r i c a l m e a s u r e ments had b e e n made, b u t i t t h e y h a d , t h i s p a r t i c u l a r d i s p e r s a n t w o u l d h a v e p r o v i d e d a b o u t -200 mV o f z e t a - p o t e n t i a l and g i v e n excellent electrostatic repulsion. The r e a d e r s h o u l d b e w a r y o f any c l a i m s o f s t e r i c s t a b i l i z a t i o n u n l e s s t h e e l e c t r o s t a t i c c o n t r i b u t i o n has been measured. 1 1



336


Combined E l e c t r o s t a t i c a n d S t e r i c S t a b i l i z a t i o n . The c o m b i n a t i o n o f t h e two mechanisms i s i l l u s t r a t e d i n F i g u r e 4, t a k e n f r o m Shaw's t e x t b o o k , (13) w h e r e t h e r e p u l s i o n o f t h e s t e r i c b a r r i e r d u r i n g a c o l l i s i o n f a l l s o f f so r a p i d l y as t h e c o l l i d i n g p a r t i c l e s bounce a p a r t t h a t t h e d i s p e r s i o n f o r c e a t t r a c t i o n s h o l d t h e p a r t i c l e s t o g e t h e r i n t h e " s e c o n d a r y minimum". T h i s i s e x a c t l y what happens i n t h e s y s t e m i n v e s t i g a t e d i n t h i s p a p e r . I n such systems t h e requirement o f t h e e l e c t r o s t a t i c c o n t r i b u t i o n t o c o l l o i d a l s t a b i l i t y i s q u i t e d i f f e r e n t t h a n when no s t e r i c b a r r i e r i s present. I n t h e l a t t e r case an energy b a r r i e r o f a b o u t 30 kT i s d e s i r a b l e , w i t h a Debye l e n g t h l/κ o f n o t more t h a n 1000 X. T h i s i s a t t a i n a b l e i n non-aqueous s y s t e m s ( 5 ) , b u t n o t b y most d i s p e r s a n t s . However when t h e s t e r i c b a r r i e r i s present, the only requirement f o r the e l e c t r o s t a t i c r e p u l s i o n i s t o e l i m i n a t e t h e s e c o n d a r y minimum and t h i s i s e a s i l y a c h i e v e d w i t h z e t a - p o t e n t i a l s f a r below those r e q u i r e d t o operate e n t i r e l y by t h e e l e c t r o s t a t i c mechanism. Dispersant The d i s p e r s a n t u s e d i n t h e s e s t u d i e s i s C h e v r o n C h e m i c a l s OLOA 1200, a p o l y b u t e n e o f a b o u t 70 c a r b o n atoms a t t a c h e d t o a s u c c i n i c a c i d group w h i c h i s r e a c t e d w i t h d i e t h y l e n e t r i a m i n e t o p r o v i d e t h e b a s i c a n c h o r i n g g r o u p . F i l m b a l a n c e s t u d i e s showed t h a t t h e a d s o r b e d f i l m s h a v e a f i l m t h i c k n e s s o f 50 X. This d i s p e r s a n t i s s u p p l i e d a s a 50 w% s o l u t i o n i n a m i n e r a l o i l . I t c a n be d e o i l e d b y a d s o r p t i o n f r o m t o l u e n e o n t o s i l i c a w i t h e l u t i o n b y acetone. I n t h i s p a p e r t h e w% o f d i s p e r s a n t r e f e r s t o t h e d e o i l e d material. The b a s i c i t y o f OLOA 1200 h a s b e e n e v i d e n c e d b y i t s i n t e r a c t i o n w i t h t h e o i l - s o l u b l e a c i d i c i n d i c a t o r d y e , Brom P h e n o l M a g e n t a Ε (ΕΚ 6810) w h i c h i s n o r m a l l y y e l l o w b u t t u r n s b l u e a n d t h e n magenta w i t h i n c r e a s i n g b a c i c i t y . The a c i d i c f o r m h a s a n a d s o r p t i o n p e a k a t 390 nm, t h e b a s i c a t 610 nm, a n d t h e i s o b e s t i c p o i n t i s a t 460 nm. T h e s e s p e c t r a h a v e b e u s e d t o d e t e r m i n e t h e c o n c e n t r a t i o n o f OLOA 1200 i n s o l u t i o n f o r a d s o r p t i o n i s o t h e r m s . Adsorption

Isotherms on Carbon B l a c k (14)

The c a r b o n b l a c k u s e d i n a l l o f t h e s e s t u d i e s was C a b o t ' s S t e r l i n g NS ( 2 5 m / g ) , a f u r n a c e b l a c k w h i c h was e x t r a c t e d o f s o l u b l e s w i t h a c e t o n e and h e x a n e and t h e n d r i e d a t 60°C. u n d e r vacuum. C a r b o n b l a c k a d s o r b e d OLOA 1200 v e r y s t r o n g l y , e v e n t u a l l y p i c k i n g up 5% o f i t s w e i g h t i n d i s p e r s a n t . The f i r s t 2% a d s o r b e d a l m o s t i n s t a n t l y , b u t a d d i t i o n a l i n c r e m e n t s a d s o r b e d more and more s l o w l y ( s e e F i g u r e 5 ) . Such t i m e - d e p e n d e n c e i n a d s o r p t i o n o f l a r g e m o l e c u l e s i s q u i t e common ( 1 5 ) , b u t i s s e l d o m s t u d i e d . The a d s o r p t i o n i s o t h e r m s d e t e r m i n e d a f t e r 48 h o u r s o f t u m b l i n g a t 25° 2



FOWKES AND PUGH


337

a, pm

F i g u r e 3. C r i t i c a l s e p a r a t i o n d i s t a n c e (H ) o f s p h e r i c a l p a r t i c l e s o f r a d i u s ( a ) t o p r e v e n t floccuîation a t 25°C. C a r b o n - i n - o i l d i s p e r s i o n s (C/0), p o l y s t y r e n e i n w a t e r l a t e x e s (PS/W), and o i l - i n - w a t e r e m u l s i o n s (0/W).

(Μ L

Μ,+ν +Λ* λ

\

\ 1 — \

0

\

v

.

+

^

interpartielc separation

/ ./

_

Η

/

Λ

ν,

,Ί

^

ν,-ν,

t / Ι / I / ι F i g u r e 4. P o t e n t i a l e n e r g y d i aιg r a m s f o r a p a i r o f p a r t i c l e s w i t h : o n t h e l e f t , a s t e r i c b a r r i e r (V ) a n d d i s p e r s i o n f o r c e a t t r a c t i o n (V ) ; and o n t h e r i g h f , w i t h e l e c t r o s t a t i c r e p u l s i o n (V ) added. R e p r o d u c e d w i t h p e r m i s s i o n f r o m Ref. ( 1 3 ) . C o p y r i g h t 1980, B u t t e r w o r t h s .





21.

FOWKES AND PUGH

Colloid

339

Properties of Nonaqueous Dispersions

and a t 50° ( s e e F i g u r e 6 ) a r e p r o b a b l y n o t e q u i l i b r i u m v a l u e s , a s s u g g e s t e d b y t h e g r e a t e r a d s o r p t i o n a t 50°C. The a r e a p e r m o l e c u l e w i t h 5 w% o f d i s p e r s a n t a d s o r b e d o n t o t h e c a r b o n b l a c k i s ioo£ . 2


C a l o r i m e t r i c Heats o f A d s o r p t i o n on Carbon

Black(14)

Heats o f a d s o r p t i o n were determined w i t h a M i c r o s c a l Flow M i c r o c a l o r i m e t e r , u s i n g f l o w r a t e s o f one m l . p e r h o u r u n d e r a g r a v i t i o n a l h e a d , a n d 70 t o 80 mg o f S t e r l i n g NS i n t h e b e d . The r e s u l t s o f a t y p i c a l r u n a r e shown i n F i g u r e 7 » w h i c h i l l u s t r a t e s t h e r a p i d i t y o f a d s o r b i n g 2.25% OLOA 1 2 0 0 . The a r e a u n d e r t h i s p e a k c o r r e s p o n d s t o t h e g e n e r a t i o n o f 6.8 m i l l i c a l o r i e s o f h e a t , a AH o f -4.7 k c a l / m o l e . The r e s u l t s o f s e v e r a l s u c h e x p e r i ments a r e summarized i n T a b l e I I . a d g

T a b l e I I . H e a t s o f A d s o r p t i o n o f OLOA 1200 o n C a r b o n B l a c k By F l o w M i c r o c a l o r i m e t r y (40°C.)

a)

A l i q u o t s o f d i s p e r s a n t i n j e c t e d a t 2-3 h o u r

P a r t s OLOA 1 2 0 0 / 100 p a r t s o f i n j e c t e d (no bed change)

Approximate* Surface area per molecule (X)

1.5 1.5 1.5

0.75 1.5 2.25

Amount o f heat absorbed (millicalories)

Heat o f adsorption* (Kcal/mole)

2

330 167 110

b) F r e s h l y p r e p a r e d aliquot injected.

intervals.

-6.7 -5.5 -4.7

beds o f c a r b o n b l a c k prepared

660 330 220

-3.6 -6.7 -6.8

-7.0 -5.7 -4.9 before

each

-7.5 -7.0 -4.7

* A s s u m i n g a n a v e r a g e m o l e c u l a r w e i g h t o f 1200 f o r t h e d i s p e r s a n t and a s u r f a c e a r e a o f 25 m /gm f o r t h e c a r b o n b l a c k . I n p a r t (a) o f Table I I i s i s seen that t h e f i r s t t h i r d o f a m o n o l a y e r t o a d s o r b t e n d s t o o c c u p y t h e more s t r o n g l y a c i d s i t e s , f o r each s u c c e s s i v e increment i s l e s s s t r o n g l y adsorbed. I n p a r t (b) t h e decrease i n average heats o f a d s o r p t i o n w i t h i n c r e a s i n g coverage i s q u i t e c o n s i s t e n t w i t h t h e above. 2

E l e c t r i c a l C o n d u c t i v i t y o f Carbon B l a c k D i s p e r s i o n s i n Dodecane(16) S t e r l i n g NS i s a f a i r l y c o n d u c t i v e

c a r b o n b l a c k and d i s p e r s i o n s o f



340


EQUILIBRIUM CONCENTRATION, | / l F i g u r e 6. Amount o f OLOA-1200 a d s o r b e d o n c a r b o n b l a c k f r o m o d o r l e s s k e r o s e n e i n 48 h o u r s . R e p r o d u c e d w i t h p e r m i s s i o n from R e f . (14) C o p y r i g h t 1983, E l s e v i e r S c i e n c e P u b l i s h e r s .

1

i

i

I

I

I

S

10

15

20

26

1

tint* (minutes)

F i g u r e 7. E x o t h e r m f o r a d s o r p t i o n o f OLOA-1200 f r o m o d o r l e s s kerosene onto carbon b l a c k by f l o w m i c r o c a l o r i m e t r y . Reproduced w i t h p e r m i s s i o n from Ref ( 1 4 ) . C o p y r i g h t 1983, E l s e v i e r Science P u b l i s h e r s .


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341


1

10 w% i n d o d e c a n e h a v e a c o n d u c t i v i t y o f a b o u t 5x10 **ohm ^m , measured w i t h p l a t i n u m e l e c t r o d e s i n a s t i r r e d conductance c e l l , using a Keithley electrometer. This c o n d u c t i v i t y r e s u l t s from c h a i n s o f p a r t i c l e - t o - p a r t i c l e c o n t a c t s w h i c h a r e b r o k e n and r e made d u r i n g s t i r r i n g , w i t h e l e c t r o n - t u n n e l l i n g c u r r e n t s b e t w e e n c l o s e l y adjacent conductive carbon p a r t i c l e s (17). With i n c r e m e n t a l a d d i t i o n s o f OLOA-1200 t o t h e s e d i s p e r s i o n s t h e a d s o r b e d l a y e r s i n c r e a s e t h e t h i c k n e s s o f t h e i n s u l a t i n g gap be tween c a r b o n p a r t i c l e s , and s i n c e t u n n e l l i n g c u r r e n t s d e c r e a s e e x p o n e n t i a l l y w i t h s e p a r a t i o n d i s t a n c e (17), the conductance i s s e e n i n F i g u r e 8 t o d e c r e a s e e x p o n e n t i a l l y w i t h OLOA-1200 c o n t e n t up t o 1 w% and t h e n t o d e c r e a s e more s l o w l y up t o 2 w% w h e r e t h e c o n d u c t a n c e i s a b o u t one m i l l i o n t h o f what i s was w i t h no OLOA1200. The minimum c o n d u c t i v i t y o b s e r v e d i n t h i s s y s t e m i s i n d i c a t i v e o f t h e 100 X s e p a r a t i o n b e t w e e n p a r t i c l e s e x p e c t e d w i t h OLOA-1200, f o r e l e c t r o n - t u n n e l l i n g c u r r e n t s become n e g l i g i b l y s m a l l a c r o s s i n s u l a t i n g gaps o f 100 A t h i c k n e s s . In F i g u r e 8 the conductance i s seen to i n c r e a s e w i t h d i s p e r s a n t c o n t e n t a f t e r r e a c h i n g a minimum. T h i s i s b e c a u s e t h e c o n t e n t o f d i s p e r s a n t l e f t i n s o l u t i o n becomes s i g n i f i c a n t i n t h e r e g i o n a b o v e t h e minimum. F o r e x a m p l e , t h e c o n d u c t a n c e m e a s u r e d a f t e r 45 m i n u t e s r i s e s a f t e r a minimum a t 2.5% OLOA-1200; t h i s c a n compared w i t h F i g u r e 5 w h i c h shows t h a t 2% i s a d s o r b e d i n a b o u t 10 m i n u t e s and 2.5% i s a d s o r b e d i n a b o u t an h o u r . On t h e o t h e r h a n d , t h e 1 2 - h o u r c o n d u c t a n c e r e a c h e d a minimum a t a b o u t 4% i n F i g u r e 8 and F i g u r e 5 shows t h a t a b o u t 4% i s a d s o r b e d i n 12 h o u r s . Thus t h e r i s e o f c o n d u c t a n c e a f t e r t h e minimum i s a good m e a s u r e o f t h e p r e s e n c e o f s i g n i f i c a n t c o n c e n t r a t i o n s o f d i s p e r s a n t i n s o l u t i o n , which i s a requirement f o r the develop ment o f e l e c t r o s t a t i c c h a r g i n g , i l l u s t r a t e d i n F i g u r e 1. The i n c r e a s e i n o v e r a l l d i s p e r s a n t l e v e l s r e q u i r e d t o p r o v i d e an excess i n s o l u t i o n i n c r e a s e s w i t h time because of the slow k i n e t i c s of a d s o r p t i o n i n the approach to s a t u r a t e d s u r f a c e con c e n t r a t i o n s , as d i s c u s s e d e a r l i e r . The i n c r e a s i n g c o n d u c t i v i t y o f the d i s p e r s i o n , once e x c e s s d i s p e r s a n t i s l e f t i n s o l u t i o n , i s p r o b a b l y due m o s t l y t o t h e i n c r e a s i n g c o n c e n t r a t i o n o f p r o t o n c a r r y i n g OLOA-1200 m o l e c u l e s , t h e p o s i t i v e c o u n t e r - i o n s i l l u s t r a t e d i n F i g u r e 1. S o l u t i o n s o f OLOA-1200 i n d o d e c a n e h a v e some c o n d u c t a n c e , h o w e v e r , and i t i s f a i r l y l i n e a r w i t h c o n c e n t r a t i o n , so n o t a l l o f t h e i n c r e a s i n g c o n d u c t a n c e c a n be a t t r i b u t e d t o counterions. Z e t a - P o t e n t i a l s of Carbon B l a c k D i s p e r s i o n s i n Hydrocarbons w i t h OLOA-1200 D i s p e r s a n t (14-16)" I n i t i a l s t u d i e s w e r e made w i t h t h e Rank B r o s , e l e c t r o p h o r e s i s u n i t , u s i n g the d i l u t e supernatant suspension over a d i s p e r s i o n o f 3.33g o f c a r b o n b l a c k p e r l i t e r o f d o d e c a n e e q u i l i b r a t e d f o r 24 h o u r s w i t h t h e added OLOA-1200. The e l e c t r o p h o r e t i c m o b i l i t y (μ) o f 1-3 ym c l u m p s o f p a r t i c l e s was o b s e r v e d a t a f i e l d o f 100 v o l t s p e r c e n t i m e t e r . The z e t a - p o t e n t i a l s (ζ) w e r e c a l c u l a t e d



342


M I S QUA 1200 DISPOSAMT/100 PARTS C t t M l

MOI

F i g u r e 8. C o n d u c t i v i t y o f s t i r r e d 1 0 % s u s p e n s i o n s o f c a r b o n b l a c k i n d o d e c a n e w i t h OLOA-1200 d i s p e r s a n t . Triangles- 5 m i n u t e s a f t e r d i s p e r s a n t a d d e d ; c i r c l e s - 45 m i n u t e s a f t e r a d d i t i o n ; a n d s q u a r e s - 12 h o u r s a f t e r a d d i t i o n . R e p r o d u c e d w i t h p e r m i s s i o n from Ref. ( 1 6 ) . C o p y r i g h t 1983, E l s e v i e r Science P u b l i s h e r s .


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Dispersions

343


f r o m t h e H u c k e l e q u a t i o n (18) f o r s y s t e m s i n w h i c h t h e Debye l e n g t h exceeds t h e p a r t i c l e r a d i u s :

in is of in the

w h i c h η i s t h e v i s c o s i t y , ε i s t h e d i e l e c t r i c c o n s t a n t and ε t h e p e r m i t t i v i t y o f f r e e s p a c e . F i g u r e 9 shows t h e m a g n i t u d e the n e g a t i v e z e t a - p o t e n t i a l s i n t h i s system, w i t h p o t e n t i a l s e x c e s s o f 100 mV when, t h e d i s p e r s a n t l e v e l e x c e e d e d 2 w% o f carbon black. L a t e r s t u d i e s , u s i n g t h e P e n Kern 3000, employed s o n i c a t e d a n d w e l l - d i s p e r s e d carbon b l a c k i n kerosene systems i n which a l l o f the p a r t i c l e s w e r e s u b - m i c r o n i n s i z e . T h i s i n s t r u m e n t u s e s a D o p p l e r - s h i f t t y p e o f measurement a n d a n a l y s e s t h e m o t i o n o f many p a r t i c l e s s i m u l t a n e o u s l y by a F o u r i e r t r a n s f o r m o f the m u l t i p l e s e n s i n g s . F i g u r e 10 shows a t y p i c a l h i s t o g r a m o f t h e d i s t r i b u t i o n o f 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 c a r b o n b l a c k s i n 0.2% s o l u t i o n s o f OLOA-1200 i n k e r o s e n e ; i n t h i s c a s e t h e a v e r a g e mobility was -1.0x10 a n d t h e a v e r a g e z e t a p o t e n t i a l was -153mV. Table I I I l i s t s t h e average z e t a - p o t e n t i a l s o b t a i n e d w i t h d i s p e r s i o n s o f 10 mg o f c a r b o n b l a c k s o n i c a t e d i n 100 m l . o f k e r o s e n e w i t h 0,0.2,0.5, a n d 0.8% OLOA-1200 i n s o l u t i o n . 9

T a b l e I I I . Average Z e t a - P o t e n t i a l s o f Carbon B l a c k i n Kerosene D i s p e r s i o n s D e t e r m i n e d b y P e n Kern 3000 %OLOA-1200 0 0 0.2 0.2 0.2 0.5 0.5 0.5 0.5 0.8 0.8 0.8 0.8

Time C o n s t a n t , s e c . 60 60 10 10 60 10 62 255 600 10 10 60 60

ζ-Potenital, mV +0.1 +0.5 -156-, -124 ( -144 -153) -181K -144/ -137/ -177 -247/ -319. -136 j -218 -201 1 -213 J

The t i m e c o n s t a n t i n t h e a b o v e t a b l e i s t h e t i m e a f t e r t h e f i e l d i s a p p l i e d and b e f o r e the m o b i l i t i e s a r e measured. T h i s time i s n e e d e d t o e s t a b l i s h t h e f i e l d a n d i s a n RC t i m e c o n s t a n t o f t e n s e c o n d s when t h e c o n d u c t i v i t y i s 10 ohm m . The a b o v e s o l u t i o n s had c o n d u c t i v i t i e s s l i g h t l y g r e a t e r than t h i s v a l u e . 9

1

1



344


0

1

2

3

4

5

6

7

PARTS OLOA 1200 DISPERSANT/100 PARTS CARBON BLACK

F i g u r e 9. Z e t a - p o t e n t i a l o f c a r b o n b l a c k d i s p e r s e d i n d o d e c a n e w i t h OLOA-1200 d i s p e r s a n t (23°C.). Sampled f r o m d i s p e r s i o n s o f 3.33 g o f c a r b o n b l a c k p e r l i t e r o f dodecane. R e p r o d u c e d w i t h p e r m i s s i o n f r o m R e f . ( 1 4 ) . Copyr i g h t 1983, E l s e v i e r S c i e n c e P u b l i s h e r s .

F i g u r e 10. D i s t r i b u t i o n o f z e t a - p o t e n t i a l s o f carbon b l a c k i n k e r o s e n e w i t h 0.1% OLOA-1200. A v e r a g e z e t a - p o t e n t i a l was -HOmV.


21.

F O W K E S AND


H

PUG

Dispersions

345


The z e t a - p o t e n t i a l s of Table I I I are greater than those i n Figure 9 because the concentration of OLOA-1200 i n s o l u t i o n was appreciably higher. The r e s u l t s are q u i t e c o n s i s t e n t at the same concentration i n s o l u t i o n . These f i n d i n g s show that the z e t a - p o t e n t i a l i n organic media i s not a f u n c t i o n of how much dispersant i s adsorbed, but how much d i s p e r s a n t i s l e f t i n s o l u t i o n . This i s i n c o n t r a s t to the s t e r i c b a r r i e r s , which depend on how much i s adsorbed and not on how much i s l e f t i n solution. The E l e c t r o s t a t i c Energy B a r r i e r s The energy of e l e c t r o s t a t i c r e p u l s i o n (per p a i r of p a r t i c l e s ) which r e s u l t s from z e t a - p a r t i c l e s i s estimated from the Deryagin equation (19): U?21 = 2πε ε

3ψ

1ΔΙ

2

In ( l + e " "

KH)

(4)

T O O

where l/κ i s the Debye length and ψ i s the surface p o t e n t i a l , approximated here by the z e t a - p o t e n ? i a l . This equation a p p l i e s e q u a l l y w e l l to aqueous or non-aqueous media. The Debye length l/κ tends to be much longer than i n aqueous media, but not always ( 5 ) . For t h i s system we used the equation of Klinkenberg and van der Minne:(20) l/κ = /ε ε D/2o r ο

(5)

where D i s the d i f f u s i o n constant f o r the charge-carrying species and σ i s the c o n d u c t i v i t y . The value of D can be c l o s e l y es timated from the S t o k e s - E i n s t e i n equation f o r spheres D

ο

= kT/o-rrrn

(6)

corrected with an appropriate value of f / f D = D χ ο

f/f

f o r rods : (7)

ο

For OLOA-1200 we assume a molecular weight of 1200, a d e n s i t y of 0.9, and_one e l e c t r o n i c charge per molecule (5), and f / f =1.2; D=2.7xl0 . The c o n d u c t i v i t y of a 0.2% s o l u t i o n of OLoX-1200 i n kerosene i s 1x10 ohm ^m , so the Debye length i s 0.5 um(500oX). This Debye length i s about the same as i n the i r i d e s c e n t aqueous polystyrene l a t e x e s which have had serum replacement with d i s t i l l e d water. In these i r i d e s c e n t latexes the extensive Debye lengths give e l e c t r o s t a t i c r e p u l s i o n s that provide a d i s t a n c e between the centers of hexagonally packed p a r t i c l e s matching v i s i b l e l i g h t wavelengths so that Bragg r e f l e c t i o n s occur. These i r i d e s c e n t latexes are s t a b i l i z e d by the e l e c t r o s t a t i c r e p u l s i o n at Debye lengths of the same order as occur i n the OLOA s o l u t i o n s i n kerosene. 1 0

8

1


346

POLYMER ADSORPTION AND DISPERSION STABILITY I

n

d Figure 11 the sum of the d i s p e r s i o n f o r c e a t t r a c t i o n ( U l ) °f equation (1) and of the e l e c t r o s t a t i c r e p u l s i o n ( U ^ l ) of equation (4) are p l o t t e d as a f u n c t i o n of the s e p a r a t i o n d i s t a n c e H between carbon b l a c k p a r t i c l e s with r a d i u s 200 nm, the average diameter of our carbon b l a c k as determined by e l e c t r o n microscopy of a thoroughly d e f l o c c u l a t e d sample,and using a λ of 100 nm. Two curves are shown, one f o r s o l u t i o n s with 0.2 % OLOA-1200 (1/K=500 nm, ζ=-144 mV) and another f o r s o l u t i o n s w i t h 0.8% OLOA-1200 (1/κ=250 nm, ζ=-218 mV). The maximum s l o p e , a measure of the f o r c e between p a r t i c l e s , i s 0.23 kT/nm and 0.9 kT/nm r e s p e c t i v e l y . During the approach of p a r t i c l e s i n a c o l l i s i o n , the energy l o s s per 100 nm i n climbing the above slopes would be 23 kT and 90 kT, s u f f i c i e n t to slow and stop the approach i n almost a l l c o l l i s i o n s . The dashed l i n e j u s t above the peak of e l e c t r o s t a t i c r e p u l s i o n represents the 5 nm s t e r i c b a r r i e r of adsorbed OLOA-1200 which tends to extend the b a r r i e r much h i g h e r . 1 2


χ

F l o c c u l a t i o n Rates and S t a b i l i t y Ratios

(16)

D i s p e r s i o n s of 3.33 g of carbon b l a c k per l i t e r of dodecane (matching the compositions used i n determing z e t a - p o t e n t i a l s i n Figure 9) and d i s p e r s i o n s of 0.33 g. per l i t e r were i n v e s t i g a t e d f o r f l o c c u l a t i o n r a t e s by t u r b i d i t y measurements immediately a f t e r s o n i c a t i o n . From the slope of a b s o r p t i v i t y v s . time the f l o c c u l a t i o n h a l f - t i m e was determined. The h a l f - t i m e s determined with no d i s p e r s a n t were used as the r a p i d f l o c c u l a t i o n h a l f - t i m e (t ) used as the b a s i s f o r determining the s t a b i l i t y r a t i o (W). In Table IV t i s 6 minutes f o r the d i s p e r s i o n s c o n t a i n i n g 3.33 g / l i t e r and 40 minutes with 0.33 g / l i t e r . Upon adding d i s persant the f l o c c u l a t i o n time increased and the r a t i o of the longer f l o c c u l a t i o n time to t i s the s t a b i l i t y r a t i o . As the OLOA-1200 c o n t e n t o f these d i s p e r s i o n s increased i t should be remembered that the s t e r i c b a r r i e r i s well-developed a t 1% and optimum at about 2% OLOA-1200, as evidenced by the con d u c t i v i t y and v i s c o s i t y measurements of Figures 8 and 13. However i n Table IV we see no i n c r e a s e i n W at 1%, and only a small i n c r e a s e at 2% of d i s p e r s a n t . The value of W increases r a p i d l y at about the same c o n c e n t r a t i o n that the c o n d u c t i v i t y i n c r e a s e s , the counterion c o n c e n t r a t i o n increases and the z e t a p o t e n t i a l i n c r e a s e s . At OLOA-1200 l e v e l s of 3.5% and higher the s t a b i l i t y r a t i o exceeds 5x1ο *, with h a l f - t i m e s i n excess of seven months ; these s t a b i l i t y r a t i o s developed when z e t a - p o t e n t i a l s were -120 mV or more. These f i n d i n g s show that the s t e r i c b a r r i e r provided by the 50 X adsorbed f i l m s was inadequate to d e f l o c c u l a t e the d i s p e r s i o n at a l l , even though i t provided a m i l l i o n times more e l e c t r i c a l r e s i s t a n c e and reduced the v i s c o s i t y very a p p r e c i a b l y . On the other hand, the e l e c t r o s t a t i c b a r r i e r was very e f f e c t i v e i n d e f l o c c u l a t i n g the system, but i t took more d i s p e r s a n t than r

1



F O W K E S A N D PUGH


Dispersions

F i g u r e 1 1 . P o t e n t i a l e n e r g y d i a g r a m f o r two s p h e r i c a l c a r bon b l a c k p a r t i c l e s o f r a d i u s 0.2 ym w i t h Debye l e n g t h s a n d z e t a p o t e n t i a l s d e t e r m i n e d f o r 0.2% and 0.8% s o l u t i o n s o f OLOA-1200 i n o d o r l e s s k e r o s e n e .


347


348

Table IV.

S t a b i l i t y o f C a r b o n B l a c k i n Dodecane


(A) C o n c e n t r a t i o n 3.33 g l " P a r t s OLOA 1200 p e r 100 p a r t s carbon b l a c k 0

Flocculation sedimentation time 6 min 6 min 7 min 9 min 1 .5 h 5 h 24 h >7 months >7 months >7 months >7 months >7 months

k 1 1% 2 2% 3 3k 4 hh 5 6

(B) C o n c e n t r a t i o n 0.33 g l P a r t s OLOA 1200 p e r 100 p a r t s carbon b l a c k 0 k 1 Ik 2 2k 3 3k 4 4% 5

1

W (stability

1 1 1.16 1.5 15 50 2.4xl0 5 xlO >5 xlO*

ratio )

2

>5 >5 >5

4

4

h

xl0 xl0 xl0

k k

1

Flocculation sedimentation time 40 m i n 40 50 m i n 90 m i n 3 h 24 h 2 .5 d a y s 10 d a y s >6 months >6 months >6 months

W (stability

1 1 1.25 2.25 4.5 36 90 3.6xl0 >6 x l O >6 xlO >6 x l O

ratio )

2

3

3

3



21.

FOWKES AND PUGH


Dispersions

349

needed f o r the s t e r i c b a r r i e r to provide the excess i n s o l u t i o n needed to promote counterion formation. The e l e c t r o s t a t i c b a r r i e r i s seen i n F i g u r e 11 to be very s e n s i t i v e to the c o n c e n t r a t i o n of OLOA-1200 l e f t i n s o l u t i o n , with a slope p r o p o r t i o n a l to that c o n c e n t r a t i o n . Concentrations of OLOA-1200 i n s o l u t i o n probably need to exceed 0.1% f o r r e a l l y e f f e c t i v e e l e c t r o s t a t i c d e f l o c c u l a t i o n . Figure 12 compares the s t a b i l i t y r a t i o to the conduct i v i t y (at 12 hours) to i l l u s t r a t e that the s t e r i c b a r r i e r forms i n the 1-2% range of d i s p e r s a n t , but that i n t h i s range so much i s adsorbed that none i s l e f t over to provide counterions i n s o l u t i o n s . At about 4% d i s p e r s a n t the c o n d u c t i v i t y r i s e s as counterions develop,the Debye length decreases and the z e t a p o t e n t i a l climbs to provide a r a p i d i n c r e a s e i n the s t a b i l i t y ratio W. V i s c o s i t y of D i s p e r s i o n s of Carbon Black i n Hydrocarbons

(16)

V i s c o s i t i e s of concentrated suspensions of carbon b l a c k i n a white mineral o i l (Fisher " p a r a f f i n " o i l of 125/135 Saybolt v i s c o s i t y ) were measured with a B r o o k f i e l d viscometer as a f u n c t i o n of OLOA-1200 content. F i g u r e 13 shows the v i s c o s i t i e s of d i s persions with 30 w%, 35 w% and 70 w% carbon b l a c k . In a l l cases the v i s c o s i t y f e l l r a p i d l y as the OLOA-1200 content increased from 0 to 1%, then f e l l more g r a d u a l l y and l e v e l l e d o f f as the OLOA-1200 content approached 2%. In many respects the r e d u c t i o n i n v i s c o s i t y with i n c r e a s i n g OLOA-1200 content p a r a l l e l s the c o n d u c t i v i t y measurements; both phenomena are sensing the b u i l d up of the s t e r i c b a r r i e r , and t h i s s t e r i c b a r r i e r weakens, s o f t e n s , and l u b r i c a t e s the i n t e r p a r t i c l e c o n t a c t s . As evidenced i n foregoing s e c t i o n s , the p a r t i c l e s are s t i l l f l o c c u l a t e d but can be e a s i l y s t i r r e d and separated mechanically. The onset of e l e c t r o s t a t i c r e p u l s i o n at OLOA-1200 contents i n excess of 2.5% did not a f f e c t v i s c o s i t i e s . Sedimentation Volumes of Carbon Black D i s p e r s i o n s i n Kerosene

(14)

In the foregoing s t a b i l i t y measurements the 1-20 ym agglomerates of carbon b l a c k (as received) were broken down to 0.1-1 ym part i c l e s f o r f l o c c u l a t i o n s t u d i e s , but f o r sedimentation s t u d i e s the agglomerates were l e f t i n t a c t and these sedimented r a p i d l y under g r a v i t a t i o n a l f o r c e s . The sediment volume tends to be a measure of 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 e n e r g i e s . I f these f o r c e s are strong, c o l l i d i n g p a r t i c l e s may s t i c k together t i g h t l y on f i r s t contact and form a very loose voluminous sediment. I f added d i s p e r s a n t reduces the i n t e r a c t i o n energy s u f f i c i e n t l y , p a r t i c l e s may s l i d e past one another under g r a v i t a t i o n a l f o r c e s and form a t i g h t l y - p a c k e d sediment. In the sedimentation s t u d i e s the d i s p e r s i o n s were tumbled i n a r o l l i n g m i l l f o r 25, 50, 75, or 150 hours, allowed to sediment over night and the volumes read. As can be seen i n Figure14 the



350


PARTS OLOA 1200 DISPERSANT/100 PARTS CARBON BLACK

F i g u r e 12. Comparison o f t h e d i s p e r s a n t c o n c e n t r a t i o n and dependence o f t h e s t a b i l i t y r a t i o W t h e c o n d u c t i v i t y o f c a r b o n b l a c k d i s p e r s i o n s i n dodecane. Reproduced w i t h p e r m i s s i o n from Ref. ( 1 6 ) . Copyright 1983, E l s e v i e r Science Publishers.


21.


FOWKES AND PUGH

Dispersions

351


3O0Or

Ql

, I S OLOA 1200

0

, 2

ί3

F i g u r e 1 3 . E f f e c t o f OLOA-1200 c o n c e n t r a t i o n ( p a r t s o f OLOA-1200 p e r 100 p a r t s c a r b o n b l a c k ) o n t h e B r o o k f i e l d v i s c o s i t y ( a t 30 r.p.m.) o f d i s p e r s i o n o f c a r b o n b l a c k i n p a r a f f i n o i l . S q u a r e s - 30 W% c a r b o n b l a c k ; t r i a n g l e s 35 W%; and c i r c l e s 70 W% c a r b o n b l a c k .

AGITATION TtME(fcMfS)

• ι •

, IS

, 1.1

, tl

, M

, 2.S

, 3.1

, 3J

, 4.0

PARTS O L O A 1211/ 111 PARTS CARBON BLACK

F i g u r e 14» S e d i m e n t a t i o n v o l u m e o f 10 W% d i s p e r s i o n s o f c a r b o n b l a c k i n o d o r l e s s k e r o s e n e a s a f u n c t i o n o f OLOA1200 c o n t e n t a n d a g i t a t i o n t i m e . S e d i m e n t a t i o n t i m e was 24 h o u r s . R e p r o d u c e d w i t h p e r m i s s i o n f r o m R e f . ( 1 4 ) . C o p y r i g h t 1983, E l s e v i e r S c i e n c e P u b l i s h e r s .



352

POLYMER ADSORPTION AND

DISPERSION STABILITY

sediment volumes i n c r e a s e d q u i t e a p p r e c i a b l y w i t h i n c r e a s e d time o f a g i t a t i o n , p o s s i b l y b e c a u s e t h e a g g l o m e r a t e s became l e s s d e n s e . The most i m p o r t a n t f i n d i n g s a r e t h a t t h e s e d i m e n t v o l u m e s dec r e a s e d w i t h i n c r e a s e d d i s p e r s a n t l e v e l s , as i s a l s o e v i d e n c e d i n t h e p h o t o g r a p h s o f F i g u r e 15 H e r e i t i s s u r p r i s i n g t o see t h a t t h e d e n s e s t s e d i m e n t was f o u n d f o r t h e d i s p e r s i o n w i t h no d i s p e r s a n t , and t h e l e a s t d e n s e w i t h 0.2% o f OLOA-1200, w h i c h a l l o w s o n l y a v e r y s m a l l f r a c t i o n o f an a d s o r b e d m o n o l a y e r . The v e r y v o l u m i n o u s s e d i m e n t s o b t a i n e d a t l o w d i s p e r s a n t l e v e l s w e r e most e a s i l y s t i r r e d , w h i l e the dense sediments o b t a i n e d a t h i g h d i s p e r s a n t l e v e l s w e r e h a r d and d i f f i c u l t t o s t i r . Some f u r t h e r l i g h t may be s h e d on c a r b o n f l o e s t r u c t u r e by t h e e l e c t r o n m i c r o g r a p h s o f F i g u r e 16 s h o w i n g an a p p r e c i a b l e decrease i n f l o e s i z e w i t h increase i n dispersant content ( f o r t h e s u s p e n s i o n s t u m b l e d f o r 150 h o u r s ) . One m i g h t c o n c l u d e t h a t t h e OLOA-1200 was an e f f i c i e n t " g r i n d i n g a i d " Conclusions 1. OLOA-1200, a p o l y b u t e n e s u c c i n a m i d e w i t h a b a s i c a n c h o r i n g g r o u p and a 50 X e x t e n d e d p o l y b u t e n e c h a i n , p r o v i d e d b o t h s t e r i c and e l e c t r o s t a t i c s t a b i l i z a t i o n t o d i s p e r s i o n s ôf c a r b o n b l a c k i n hydrocarbon media. 2. The s t e r i c b a r r i e r d e v e l o p e d upon a d s o r p t i o n o f 1-2% o f t h e d i s p e r s a n t was e v i d e n c e d by a m i l l i o n - f o l d d e c r e a s e i n c o n d u c t i v i t y , a twenty-fold decrease i n v i s c o s i t y , a two-fold i n c r e a s e i n s e d i m e n t v o l u m e , b u t no d e f l o c c u l a t i o n o f any d e g r e e . 3. The e l e c t r o s t a t i c b a r r i e r d e v e l o p e d o n l y a f t e r enough d i s persant adsorbed t h a t a c o n c e n t r a t i o n of d i s s o l v e d d i s p e r s a n t of a b o u t 0.1% o r more r e m a i n e d i n t h e o i l p h a s e , w h e r e c o u n t e r i o n s d e v e l o p e d as e v i d e n c e d by i n c r e a s e d c o n d u c t i v i t y , t h e d e v e l o p m e n t of l a r g e negative z e t a p o t e n t i a l s , s t e e p l y r i s i n g s t a b i l i t y r a t i o s , and c o m p l e t e d e f l o c c u l a t i o n . Acknowledgments The a u t h o r s w i s h t o e x p r e s s t h e i r a p p r e c i a t i o n and t h a n k s t o Dr. V i t t o r i o de N o r a and t h e Diamond Shamrock C o r p o r a t i o n f o r t h e e s t a b l i s h m e n t o f t h e V i t t o r i o de Nora-Diamond Shamrock P o s t D o c t o r a l F e l l o w s h i p s a t Lehigh U n i v e r s i t y , funds from which supported t h i s research p r o j e c t . The Pen Kem 3000 measurements w e r e made by C h r i s t i n a B l o m , a summer s t u d e n t f r o m S t o c k h o l m . The 70% c a r b o n b l a c k v i s c o s i t i e s w e r e m e a s u r e d by D o u g l a s S e i f e r t , a L e h i g h g r a d u a t e s t u d e n t , and s e v e r a l c o n t r i b u t i o n s t o t h e e l e c t r o s t a t i c s t u d i e s w e r e made by Dr. T r i s n o M a k g a w i n a t a , t h e c u r r e n t de Nora-Diamond Shamrock Fellow.



F O W K E S AND P U G H

Colloid

Properties of Nonaqueous

Dispersions

353

F i g u r e 15. Photograph o f s e d i m e n t a t i o n o f carbon b l a c k (10 W%) i n o d o r l e s s k e r o s e n e as a f u n c t i o n o f OLOA-1200 content. A g i t a t e d 150 h o u r s , s e d i m e n t e d 24 h o u r s . OLOA1200 c o n t e n t s : ( a ) - 0 , ( b ) - 0 . 2 , ( c ) - 0 . 4 , ( d ) - 0 . 6 , ( e ) - 0 . 8 , ( f ) - 1 . 0 , ( g ) - 1 . 2 , ( h ) - 1 . 4 , ( i ) - 2 . 0 , ( j ) - 2 . 8 , and (k)-4.0 p a r t s OLOA-1200 p e r 100 p a r t s c a r b o n b l a c k . Reproduced w i t h p e r m i s s i o n from R e f . (14) E l s e v i e r S c i e n c e P u b l i s h e r s .

F i g u r e 16. E l e c t r o n m i c r o g r a p h s s h o w i n g f l o e s t r u c t u r e o f c a r b o n b l a c k d i s p e r s e d i n o d o r l e s s k e r o s e n e a f t e r 150 h o u r s of a g i t a t i o n . P a r t s OLOA-1200 p e r 100 p a r t s o f c a r b o n b l a c k : (a)-0, (b)-0.4, (c)-2.0, (d)-4.0. These a r e from samples ( a ) , ( c ) , ( i ) and ( k ) o f F i g u r e 15. Reproduced w i t h p e r m i s s i o n from R e f . ( 1 4 ) . C o p y r i g h t 1983, E l s e v i e r S c i e n c e Publishers.




354

Literature Cited 1. van der Minne, J. L.; Hermanie, P. H. J. J. Colloid Sci. 1952, 7, 600. 2. van der Minne, J. L.; Hermanie, P. H. J. J. Colloid Sci. 1953, 8, 38. 3. Fowkes, F. M.; Disc. Faraday Soc. 1966, 42, 246. 4. Tamaribuchi, K.; Smith, M. L., J. Colloid Interface Sci. 1966, 22, 404. 5. Fowkes, F. M.; Jinnai, H.; Mostafa, Μ Α.; Anderson, F. W.; Moore, R. J . , in "Colloids and Surfaces in Reprographic Technology"; Hair, M.; Croucher, M. D., Eds.; ACS SYMPOSIUM SERIES No. 200, American Chemical Society: Washington, D. C., 1982, p 307. 6. Heller, W.; Pugh, T. J. Poly. Sci. 1960, 47, 203-219. 7. Hamaker, H. C., Physica 1937, 4, 1068. 8. Parsegian, V. Α., in "Physical Chemistry: Enriching Topics from Colloid and Surface Science", van Olphen & Mysels, Eds.; Theorex, La Jolla, CA., 1975, p 27. 9. Gregory, J., Adv. Colloid Interface Sci. 1970, 2, 396. 10. Clayfield, E. J . ; Lumb, E. C.; Mackey, P. Η., J. Colloid Interface Sci. 1971, 37, 382. 11. Achorn, P. Ph.D. Thesis, Lehigh University, Bethlehem, PA., 1970. 12. de Hek, H.; Vrij, Α., J. Colloid Interface Sci. 1981, 79, 289. 13. Shaw, D. J. "Introduction to Colloid and Surface Chemistry", Butterworths, London, 1966, p 211. 14. Pugh, R. J.; Fowkes, F. M. submitted to Colloids and Surfaces 1983. 15. Fowkes, F. M. J. Phys. Chem. 1962, 66, 385. 16. Pugh, R. J . ; Matsunaga, T.; Fowkes, F. M. "Colloids and Surfaces", in press, 1983. 17. Sichel, E. K.; Gittleman, G. I.; Sheng, P. in "Carbon-BlackPolymer Composites. The Physics of Electrically Conducting Composites", Sichel, Ε. Κ., Ed., Dekker, 1982, p 51-77. 18. Huckel, E. Phys. Z. 1924, 25, p 204. 19. Deryagin, Β. V.; Landau, L. D. Acta Physicochim URSS, 1941 14, 633. 20. Klinkenburg, Α.; van der Minne, J. L. "Electrostatics in the Petroleum Industry", Elsevier, 1958, p 40. RECEIVED October 19, 1983


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