7 Macromolecular Emulgents in Emulsion Stability S. N. SRIVASTAVA Downloaded via TUFTS UNIV on July 10, 2018 at 23:01:16 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
Department of Chemistry, Agra College, Agra, India
Introduction There has been always a l a c k of complete t h e o r e t i c a l under standing o f emulsion behaviour. E a r l i e r workers (1-6) attempted physicohemical i n t e r p r e t a t i o n of emulsion c h a r a c t e r i s t i c s i n a r a t h e r q u a l i t a t i v e manner. Later on some attempts were made i n a semiquantitative way to account f o r emulsion stability i n terms of the r a t e o f decrease o f s p e c i f i c i n t e r f a c e w i t h time (7). This was n o t , however, a very s a t i s f a c t o r y t h e o r e t i c a l approach and gave no c l e a r i n s i g h t i n t o the mechanism o f emulsion coagu lation except t h a t it did reflect a semiquantitative c o r r e l a t i o n between film strength and stabilizer c o n c e n t r a t i o n . Verwey (8) has considered the stability o f emulsions i n terms o f the e l e c trical double l a y e r which e x i s t s at the liquid-liquid interface. The total p o t e n t i a l drop was considered to occur i n both the phases, the greater drop being i n the oil phase. The D.L.V.O. theory of colloid stability should, t h e r e f o r e , be a p p l i c a b l e to s t a b l i s h e d emulsions. Recently t h i s theory has been used by van den Tempel (9) and A l b e r s and Overbeek (10) t o e x p l a i n the behaviour o f both O/W and W/O systems u s i n g a r b i t r a r y values o f the van der Waals constant. In the present work by the author the s t a b i l i z a t i o n of emul s i o n d r o p l e t s by e l e c t r o s t a t i c forces and by the v i s c o - e l a s t i c i t y of the adsorbed f i l m has been examined. Emulsion globules cov ered by adsorbed macromolecular emulgent f i l m s e x h i b i t a p p r e c i a b l e s t a b i l i t y to both f l o c c u l a t i o n and coalescence which t o g e t h er c o n s t i t u t e the o v e r a l l process o f s e p a r t i o n o f the d i s p e r s e phase. The q u a n t i t a t i v e theory o f these phenomena i s examined on one hand to e x p l a i n f l o c c u l a t i o n u s i n g the D . L . V . O . theory and c o n t r o l l i n g the double l a y e r p o t e n t i a l through the v a r i a t i o n o f pH and the bulk phase c o n c e n t r a t i o n . On the other hand, coalescence which follows f l o c c u l a t i o n i s found t o be governed by the rheology o f the f i l m . The f u n c t i o n a l van der Waals constants have a l s o been assessed. Present address:
Unesco Science C e n t r e , U n i v e r s i t y of A l e x a n d r i a (A. R. Egypt) 110
Mittal; Colloidal Dispersions and Micellar Behavior ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
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Theoretical The f l o c c u l a t i o n o f an emulsion i s s i m i l a r to that o f a lyophobie c o l l o i d . I t may be e i t h e r i r r e v e r s i b l e o c c u r i n g i n the primary minimum or r e v e r s i b l e o c c u r r i n g i n the secondary minimum ( i f i t s depth i s s m a l l ) . The q u a n t i t a t i v e aspects o f both the p o s s i b i l i t i e s have been d e a l t by the D . L . Y . O . theory. However, for the macromolecular s t a b i l i z e d emulsion under i n v e s t i g a t i o n , having an average p a r t i c l e r a d i u s greater than ο η β μ η ι , f l o c c u l a t i o n occurs at secondary minima. The p a r t i c l e s i n t h i s type o f r e v e r s i b l e f l o c c u l a t i o n are s t i l l separated by a d i s t a n c e of the order of 20 nm. T h i s i s diagrammatically depicted i n Figures 1-2. In the framework o f the D . L . V . O theory (11), the i n t e r a c t i o n energy o f the coated l i q u i d d r o p l e t s V = V R + V & where V R and V & are r e s p e c t i v e l y the r e p u l s i o n and a t t r a c t i o n e n e r g i e s . The r e p u l s i v e energy was c a l u l a t e d u s i n g an approximate e x p r e s s i o n :
XT
\
gaifto
2
.
= —f—
κ
- Η In (1 + e )
(1)
71
where V R i s the energy o f r e p u l s i o n , ε , d i e l e c t r i c c o n s t a n t , a , the p a r t i c l e r a d i u s , , κ , the Debye Huckel parameter and Η the i n t e r p a r t i c l e distance; Ψο i s the surface p o t e n t i a l assumed to be equal t o the z e t a p o t e n t i a l f o r the present system having κ a » 1. The a t t r a c t i o n energy V A symbolizing the p a r t i a l l y retarded van der Waals i n t e r a c t i o n was a s c e r t a i n e d from the f o l l o w i n g equations :
Α
Aa / 2 . 4 5 λ π ν 120 Η
2 λ 3 1045 HT
2
3 X _ _ 4 4 5.62x10 ~H
\
(2)
valid for H > 15 nm
valid for H < 15 nm The i n t e r a c t i o n V can be c a l c u l a t e d from the t o t a l number o f p a r t i c l e s a s s o c i a t e d w i t h a given p a r t i c l e i . e . the degree of a g gregation. D , can be computed from
3
2
D = 4 ft n a J S exp (-V/kT) ds
Mittal; Colloidal Dispersions and Micellar Behavior ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
(4)
C O L L O I D A L DISPERSIONS A N D M I C E L L A R
Figure 1.
Characteristic interaction energy profiles in the light of the D.L.V.O. theory
*v 25 kT
Hydrocarbon Droplet
Figure 2. Diagrammatic representation of the coalescence of the coated emulsion droplets
Mittal; Colloidal Dispersions and Micellar Behavior ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
BEHAVIOR
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where TIQ i s the number o f d r o p l e t s per u n i t volume o f the suspen sion and s = 2 + H / a . The i n t e r g r a n d i s evaluated g r a p h i c a l l y . In case o f r e v e r s i b l e f l o c c u l a t i o n under study, a s i n g l e t doublet e q u i l i b r i u m t e m p o r a r i l y e x i s t s i n the system and the extent o f f l o c c u l a t i o n can be determined from the formula JJ =
number of doublets number of singlets + number of doublets
For the above c a l c u l a t i o n s the value o f the van der Waals cons t a n t , A i s needed which was c a l c u l a t e d from the equation.
where of the to the lowing
^ i s the number o f atoms of k i n d i contained i n 1 c . c . matter of i t h kind and i s the London constant r e l a t i v e same substance, the l a t t e r being c a l c u l a t e d from the f o l three equations: 2
(a) The London e q u a t i o n ,
= 3/4 Q> hV^
(b) The Slater-Kirkwood B _
W
= 11.25 χ ΐ ο " "
2 4
2
3
^^ » ^
2
S—Κ (c ) The Slater-Kirkwood-Moelwyn-Hughes Β
l/2 2 . = 3/4 (s η) ηV α
S-K-M
'
κ
1 ' e where a i s the p o l a r i z a b i l i t y , V i s the c h a r a c t e r i s t i c London ^ frequency η i s the number o f valency e l e c t r o n s , V the e l e c t r o n i c v i b r a t i o n frequency and S i the e f f e c t i v e number o f d i s p e r s i o n e l e c t r o n s i s given by the e m p i r i c a l r e l a t i o n S i = 0.39n. Because the emulstion d r o p l e t s are immersed i n an aqueous medium, the net constant i s given by A = {A,. . + A„ -2A_. I (10) \ disperse phase H o disp.ph-B^o J Q
e
2
A i s thus i n v a r i a b l y p o s i t i v e and hence f o r two p a r t i c l e s o f any species embedded i n a second medium there w i l l always be a net a t t r a c t i v e f o r c e . Employing the above mathematical formulations the i n t e r a c t i o n o f emulsion d r o p l e t s s t a b i l i z e d by macromolecular sub stances such as bovine serum albumin (BSA), p e p s i n , sodium and gum a r a b i c has been examined q u a n t i t a t i v e l y i n the l i g h t of the s t a b i l i t y theory. The orthodox range o f c o l l o i d a l dimensions ( l - 100 nm) has been d e l i b e r a t e l y excluded and the range of the p a r t i c l e s i z e chosen i s such t h a t the f l o c c u l a t i o n i s r e v e r s i b l e and occurs i n the secondary minima of i n t e r a c t i o n energy p r o f i l e s . Since such i n t e r a c t i o n s may be a l t e r e d by adsorbed macromolecules an attempt has a l s o been made to study the i n f l u e n c e of a d s o r p t i o n
Mittal; Colloidal Dispersions and Micellar Behavior ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
114
C O L L O I D A L DISPERSIONS A N D M I C E L L A R
BEHAVIOR
on van der Waals interaction using the following equation from Void's mathematical treatment (12) described elswhere (13) A
MS
2
= "
H
A
H
H
2A
H
[ < V P S > P M ± {( P- PS) PM-< S A
