An Overview of Phenomena Involving Surfactant ... - ACS Publications

1 An Overview of Phenomena Involving Surfactant Mixtures John F. Scamehorn

Downloaded by 132.174.255.116 on April 24, 2013 | http://pubs.acs.org Publication Date: June 5, 1986 | doi: 10.1021/bk-1986-0311.ch001

School of Chemical Engineering and Materials Science, University of Oklahoma, Norman, OK 73019

The effect of using mixtures of surfactants on micelle formation, monolayer formation, s o l u b i l i z a t i o n , adsorption, p r e c i p i t a t i o n , and cloud point phenomena i s discussed. Mechanisms of surfactant interaction and some models useful i n describing these phenomena are outlined. The use of surfactant mixtures to solve technological problems is also considered. S u r f a c t a n t s used i n p r a c t i c a l applications essentially a l w a y s c o n s i s t o f a m i x t u r e o f s u r f a c e - a c t i v e compounds. I s o m e r i c a l l y pure s u r f a c t a n t s a r e often expensive t o produce and g e n e r a l l y have o n l y a small potential advantage i n performance over the less expensive surfactant m i x t u r e s . I n many a p p l i c a t i o n s , m i x t u r e s o f d i s s i m i l a r s u r f a c t a n t s c a n have s u p e r i o r properties t o t h o s e o f t h e i n d i v i d u a l s u r f a c t a n t components i n v o l v e d . These s y n e r g i s t i c p r o p e r t i e s o f s u r f a c t a n t m i x t u r e s h a v e p r o v i d e d i m p e t u s f o r much o f t h e r e s e a r c h on i n t e r a c t i o n s between s u r f a c t a n t s . Individual surfactants vary ' i n t h e i r tendency t o form aggregated s t r u c t u r e s . Examples of such aggregates a r e m i c e l l e s , p r e c i p i t a t e , and m o n o l a y e r s . In s o l u t i o n s c o n t a i n i n g m i x t u r e s of s u r f a c t a n t s , t h e tendency t o form a g g r e g a t e d s t r u c t u r e s c a n be s u b s t a n t i a l l y d i f f e r e n t t h a n in solutions containing only t h e pure surfactants involved. F o r example, p r e c i p i t a t i o n may n o t o c c u r i n a surfactant mixture whose components individually precipitate when p r e s e n t a s s i n g l e components. The tendency f o r components t o d i s t r i b u t e t h e m s e l v e s between t h e u n a g g r e g a t e d s t a t e a n d an a g g r e g a t e may v a r y from component t o component f o r m i x t u r e s . T h e r e f o r e , f o r 0097-6156/ 86/ 0311 -0001 $08.00/ 0 © 1986 American Chemical Society

In Phenomena in Mixed Surfactant Systems; Scamehorn, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.


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P H E N O M E N A IN M I X E D S U R F A C T A N T S Y S T E M S

example, t h e s u r f a c t a n t c o m p o s i t i o n o f a m i c e l l e may differ g r e a t l y f r o m t h a t o f t h e s u r f a c t a n t monomer w i t h which i t i s i n e q u i l i b r i u m . T h i s i s important because the p r o c e s s e s o f i n t e r e s t may depend o n l y on e i t h e r monomer c o m p o s i t i o n o r on a g g r e g a t e c o m p o s i t i o n . For example, a d s o r p t i o n of s u r f a c t a n t on s o l i d s s u c h a s minerals depends o n l y on monomer composition and concentration, not m i c e l l a r properties. On t h e o t h e r hand, s o l u b i l i z a t i o n o f compounds i n t o m i c e l l e s depends o n l y on mi c e l l a r composition. The surfactant technologist n e e d s t o be a b l e t o predict and m a n i p u l a t e t h e t e n d e n c y f o r surfactant mixtures t o form aggregates, t h e p r o p e r t i e s of t h e s e a g g r e g a t e s , and t h e d i s t r i b u t i o n o f s u r f a c t a n t components between monomer and a g g r e g a t e . A c e n t r a l theme o f t h i s paper i s t h a t m i x t u r e s of s u r f a c t a n t s can a c h i e v e great s y n e r g i s m s i n v a r i o u s p r o c e s s e s by m a n i p u l a t i o n of t h e relative tendency t o form v a r i o u s aggregated s t r u c t u r e s . Often, t h e formation of a c e r t a i n aggregate w i l l i n h i b i t the formation of a l e s s d e s i r a b l e aggregate. For example, a d d i t i o n of n o n i o n i c surfactants t o anionic s u r f a c t a n t s enhances t h e f o r m a t i o n of m i c e l l e s , r e s u l t i n g in a reduced tendency f o r t h e a n i o n i c s u r f a c t a n t t o precipitate. This overview w i l l outline surfactant mixture properties and b e h a v i o r i n s e l e c t e d phenomena. Because of s p a c e l i m i t a t i o n s , n o t a l l o f t h e many p h y s i c a l p r o c e s s e s i n v o l v i n g s u r f a c t a n t m i x t u r e s c a n be c o n s i d e r e d h e r e , b u t some w h i c h a r e i m p o r t a n t and i l l u s t r a t i v e w i l l be d i s c u s s e d : these are m i c e l l e formation, monolayer formation, solubilization, surfactant precipitation, surfactant adsorption on s o l i d s , and cloud point phenomena. M e c h a n i s m s o f s u r f a c t a n t i n t e r a c t i o n w i l l be d i s c u s s e d , a s w e l l a s m a t h e m a t i c a l m o d e l s w h i c h h a v e been shown t o be u s e f u l in describing these systems. Practical applications will be c o v e r e d a s p a r t o f t h e c o n s i d e r a t i o n o f i n d i v i d u a l phenomena. T h i s overview w i l l attempt t o o u t l i n e t h e s t a t e of current k n o w l e d g e , w i t h o u t much comment on t h e a r e a s i n which f u r t h e r r e s e a r c h i s needed, t h e d i r e c t i o n t h e f i e l d is taking, and t h e i m p a c t o f t h e o t h e r c h a p t e r s i n t h i s book. These a r e r e s e r v e d f o r the Future Perspectives Chapter ( l a s t chapter of t h e book). M i c e l l e Formation The s t r u c t u r e and t h e r m o d y n a m i c s o f f o r m a t i o n o f mixed micelles i s of great t h e o r e t i c a l i n t e r e s t . M i c e l l e s a r e a l s o p r e s e n t and o f t e n i n t e g r a l l y i n v o l v e d i n practical processes. For example, in a small pore volume surfactant flooding process (sometimes c a l l e d m i c e l l a r flooding), t h e s o l u t i o n i n j e c t e d i n t o an o i l f i e l d g e n e r a l l y c o n t a i n s 5-12 w e i g h t V, s u r f a c t a n t 14), t h e l a t t e r e f f e c t d o m i n a t e s , l e a d i n g t o p o s i t i v e d e v i a t i o n from i d e a l i t y f o r s o l u b i l i z a t i o n . F o r an a n i o n i c / n o n i o n i c system with the anionic surfactant containing a benzene r i n g i n t h e h y d r o p h o b i c g r o u p , and 9 EO g r o u p s i n t h e n o n i o n i c surfactant, p o s i t i v e d e v i a t i o n f r o m i d e a l i t y i s s e e n when t h e b e n z e n e ring i s a t t a c h e d t o t h e a n i o n i c h y d r o p h i l i c group a t t h e end o f t h e l i n e a r a l k y l c h a i n ( 6 6 ) . However, when t h e benzene r i n g i s p o s i t i o n e d i n t h e h y d r o p h o b i c g r o u p away f r o m t h e h y d r o p h i l i c g r o u p , n e g a t i v e o r no d e v i a t i o n f r o m s o l u b i l i z a t i o n i d e a l i t y i s seen. This i s c o n s i s t e n t with t h e a t t r a c t i v e i n t e r a c t i o n between t h e b e n z e n e ring of the a n i o n i c s u r f a c t a n t and t h e EO g r o u p s o f t h e n o n i o n i c s u r f a c t a n t l e a d i n g t o a more compact m i c e l l e when t h e benzene ring i s i n t h e p a l i s a d e l a y e r of t h e m i c e l l e , leading t o increased s o l u b i l i z a t i o n . Negative deviation from ideality of micelle formation does not l e a d t o increased solubilization unless there i s a specific attraction between the hydrophilic groups of t h e d i s s i m i l a r s u r f a c t a n t s , not just electrostatic forces. The s p e c i f i c attraction can lead to a more compact micelle and higher solubilizations. As already discussed, this is consistent with aggregation numbers o f m i x e d m i c e l l e s . I f a s y s t e m w h i c h had l a r g e solubilization enhancement were d e s i r e d , an a n i o n i c / n o n i o n i c s y s t e m w o u l d be c h o s e n w i t h a benzene r i n g a t t a c h e d t o t h e a n i o n i c hydrophilic group and a l a r g e number o f EO g r o u p s i n t h e n o n i o n i c . This discussion only a p p l i e s t o systems in which solubilization i n the polyethoxylate portion of t h e n o n i o n i c s u r f a c t a n t i s s i g n i f i c a n t ( e . g . , Y e l l o w OB d y e ) . If t h e s o l u b i 1 i z a t e i s l o c a t e d only i n t h e hydrocarbon core of the micelle (e.g., hydrocarbons), large d e v i a t i o n s from i d e a l i t y would n o t be e x p e c t e d . This discussion i s consistent with other mixed s u r f a c t a n t s o l u b i l i z a t i o n d a t a (67,68).


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SCAMEHORN


Adsorption

Overview of Phenomena

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on S o l i d s

The adsorption of surfactants on solids affects f l o t a t i o n , a g r i c u l t u r a l s o i l c o n d i t i o n i n g , f o r m u l a t i o n of p h a r m a c e u t i c a l d i s p e r s i o n s , a n d d y e i n g ( 6 9 ) , j u s t t o name a few p r o c e s s e s . More s p e c i f i c a l l y , i n enhanced o i l recovery, a d s o r p t i o n o f s u r f a c t a n t on r e s e r v o i r m i n e r a l s can result i n a s u b s t a n t i a l l o s s o f s u r f a c t a n t (1). Selective adsorption of c e r t a i n s u r f a c t a n t components from the multicomponent mixture can lead to chromatographic separation of t h e s l u g (70,71), with subsequent disasterous results. In detergency, a d s o r p t i o n o f s u r f a c t a n t components on b o t h t e x t i l e s and contaminants (polar o r nonpolar d i r t s ) i s important ( 2 ) . When s u r f a c t a n t s a d s o r b on m e t a l oxide surfaces (e.g., m i n e r a l s ) , a t low c o n c e n t r a t i o n s , t h e adsorbate m o l e c u l e s a r e w i d e l y d i s p e r s e d enough t h a t no s i g n i f i c a n t i n t e r a c t i o n s between a d s o r b e d s u r f a c t a n t s o c c u r s . Above a certain critical concentration, dense surfactant a g g r e g a t e s f o r m on t h e s u r f a c e ( 7 2 ) . These a r e c a l l e d admicelles. For ionic surfactants, the admicelles are bilayered structures ( 7 2 ) . Above t h e CMC, t h e t o t a l a d s o r p t i o n of s u r f a c t a n t can i n c r e a s e o r decrease s l o w l y . When t w o s i m i l a r l y structured anionic surfactants a d s o r b on m i n e r a l s , t h e mixed a d m i c e l l e a p p r o x i m a t e l y obeys i d e a l s o l u t i o n theory (JUL)B e l o w t h e CMC, t h e t o t a l a d s o r p t i o n a t any t o t a l s u r f a c t a n t c o n c e n t r a t i o n i s intermediate between t h e p u r e component adsorption levels. A d s o r p t i o n o f e a c h s u r f a c t a n t component i n t h e s e s y s t e m s c a n be e a s i l y p r e d i c t e d f r o m p u r e component adsorption isotherms by c o m b i n i n g i d e a l s o l u t i o n t h e o r y w i t h an e m p i r i c a l c o r r e s p o n d i n g s t a t e s theory approach (75) . When an i o n i c / n o n i o n i c s u r f a c t a n t m i x t u r e a d s o r b s on a metal oxide surface, the admicelle e x h i b i t s negative deviation from ideality ( 7 4 * - T h i s means t h a t t h e adsorption level i s higher than i t w o u l d be i f t h e admicelle were ideal, at a specific surfactant concentration b e l o w t h e CMC. Above t h e CMC, the a d s o r p t i o n l e v e l i s d i c t a t e d by t h e r e l a t i v e enhancement of m i c e l l e formation vs. admicelle formation. In t h i s region, t h e l e v e l o f a d s o r p t i o n c a n be v i e w e d as the r e s u l t o f t h e c o m p e t i t i o n between m i c e l l e s a n d a d m i c e l l e s for s u r f a c t a n t . In analogy, t h e s u r f a c e t e n s i o n above the CMC c a n be v i e w e d a s c o m p e t i t i o n between the monolayer and m i c e l l e s f o r s u r f a c t a n t . The r e l a t i v e tendency f o r s u r f a c t a n t s o r s u r f a c t a n t m i x t u r e s t o f o r m a m i c e l l e compared t o a m o n o l a y e r i s approximately t h e same. However, t h e r e l a t i v e t e n d e n c y t o f o r m an a d m i c e l l e c a n be s u b s t a n t i a l l y d i f f e r e n t from that t o form m i c e l l e s o r monolayers. T h i s i s because t h e r e c a n be s p e c i f i c i n t e r a c t i o n s between t h e s o l i d s u r f a c e and t h e s u r f a c t a n t s a s w e l l a s i n t e r s u r f a c t a n t interactions in the aggregate. The surfactant technologist can take advantage of t h i s t o design



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m i n i m a l l y or maximally adsorbing systems. For example, as the number o-f e t h y l e n e o x i d e s i n a p o l y e t h o x y l a t e d s u r - f a c t a n t i n c r e a s e s , t h e a d s o r p t i o n on a m i n e r a l s u r f a c e decreases (74-76). A p o l y e t h o x y l a t e with a very large number of EO groups does not adsorb s i g n i f i c a n t l y (or even has n e g a t i v e a d s o r p t i o n ) . Y e t t h e t e n d e n c y of such nonionics to f o r m p u r e component m i c e l l e s o r n o n i d e a l mixed m i c e l l e s w i t h a n i o n i c s u r f a c t a n t s i s only weakly d e p e n d e n t on t h e number of EO g r o u p s . As a r e s u l t , a b o v e t h e CMC, a m i x t u r e of an a n i o n i c and n o n i o n i c s u r f a c t a n t shows h i g h a d s o r p t i o n f o r a s m a l l number of EO groups. However, f o r a l a r g e number of EO g r o u p s , a d s o r p t i o n of b o t h n o n i o n i c and a n i o n i c surfactant can be very low (74). This i s because the formation of t h e n o n i d e a l mixed m i c e l l e r e d u c e s t h e anionic surfactant monomer concentration, tending the reduce i t s adsorption. The n o n i o n i c has s o l i t t l e tendency t o adsorb because of negative i n t e r a c t i o n w i t h the s u r f a c e t h a t even w i t h the a n i o n i c s u r f a c t a n t i n the a d m i c e l l e , i t still exhibits negligible adsorption. In a s e n s e , t h e m i c e l l e has won t h e c o m p e t i t i o n f o r t h e monomer and the adsorption is reduced as the s u r f a c t a n t s p r e f e r t h e mixed m i c e l l e t o mixed a d m i c e l l e . Similarly, on silica gel, anionic surfactant has very low adsorption. When anionic s u r f a c t a n t i s added t o n o n i o n i c s u r f a c t a n t a b o v e t h e CMC, i t reduces the nonionic surfactant adsorption on the s i l i c a g e l , without adsorbing s i g n i f i c a n t l y i t s e l f (77). On hydrophobic surfaces, s i m i l a r e f f e c t s probably o c c u r and can e x p l a i n t h e d i f f e r e n c e i n a d s o r p t i o n of an anionic surfactant i n the a b s e n c e and p r e s e n c e of a n o n i o n i c s u r f a c t a n t a b o v e t h e CMC on c a r b o n ( 7 8 ) • In d e s i g n i n g s u r f a c t a n t s y s t e m s , i f a d s o r p t i o n of a given component i s t o be minimized, an additional s u r f a c t a n t s h o u l d be added t o t h e s y s t e m a b o v e t h e CMC. This surfactant should be selected so t h a t i t forms m i c e l l e s with high negative deviations from ideality, using the guidelines already discussed, and s o t h a t i t t e n d s n o t t o a d s o r b on t h e s o l i d of i n t e r e s t . This will be v e r y s p e c i f i c t o t h e p a r t i c u l a r s o l i d and may r e q u i r e e m p i r i c a l experiments to s p e c i f y the s u r f a c t a n t . Surfactant

Precipitation

P r e c i p i t a t i o n of s u r f a c t a n t s f r o m aqueous s o l u t i o n is generally undesirable in many surfactant-based technologies. Often, e i t h e r a d d i t i v e s must be included with the surfactant o r m i x t u r e s of s u r f a c t a n t s must be used t o p r e v e n t p r e c i p i t a t i o n . For example, i n enhanced oil recovery, alcohols are o f t e n added t o an i n j e c t e d s l u g (79) o r m i x t u r e s o f s u r f a c t a n t s (80) are used to permit f l o o d i n g of h i g h s a l i n i t y o r h a r d n e s s r e s e r v o i r s while avoiding surfactant p r e c i p i t a t i o n . In detergency, builder i s added t o a f o r m u l a t i o n (2) o r n o n - b u i l t h e a v y d u t y l i q u i d l a u n d r y d e t e r g e n t s may utilize mixtures of anionic and nonionic surfactants (55.81-83) t o p e r m i t washing i n hard water.



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SCAMEHORN


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P r e c i p i t a t i o n o f s u r f a c t a n t c a n be u s e f u l i n some applications, such a s i n s e l e c t i v e plugging of o i l r e s e r v o i r s t o i m p r o v e m o b i l i t y c o n t r o l (84) o r t o r e c o v e r surfactant from s u r f a c t a n t - b a s e d separation processes (85). T h e r e h a v e been two g e n e r a l a p p r o a c h e s t o s t u d y i n g surfactant precipitations measurement o f t h e K r a f f t temperature (temperature below which surfactant p r e c i p i t a t e s ) o r by m e a s u r i n g the precipitation phase boundary (added e l e c t r o l y t e concentration required t o cause s u r f a c t a n t p r e c i p i t a t i o n ) . An i n c r e a s e i n s a l i n i t y or hardness t o l e r a n c e i s d i r e c t l y measured f r o m phase b o u n d a r y s t u d i e s o r c a n be i n f e r r e d f r o m a d e c r e a s e i n the K r a f f t temperature. Since precipitation below t h e CMC i s governed by a simple solubility product relationship, this discussion will only consider surfactant s o l u t i o n a b o v e t h e CMC, where i n t e r e s t i n g e f f e c t s occur. Anionic/Anionic or C a t i on i c / C a t i on i c Surfactant Precipitation. When two s u r f a c t a n t s o f l i k e c h a r g e a r e mixed t o g e t h e r , and t h e s t r u c t u r e s a r e v e r y similar (e.g., c l o s e members o f an h o m o l o g o u s s e r i e s ) and c o n t a i n i n g t h e same c o u n t e r i o n , t h e K r a f f t t e m p e r a t u r e o f t h e m i x t u r e i s i n t e r m e d i a t e between t h e p u r e c o m p o n e n t s or shows o n l y a very shallow minimum ( 8 6 , 8 7 ) . The c r y s t a l s a r e mixed ( c o n t a i n both surfactants) in this case (86,87). If t h e s u r f a c t a n t s h a v e a more d i s s i m i l a r s t r u c t u r e or i f the counterion i s different with the same s u r f a c t a n t i o n (e.g., s o d i u m d o d e c y l s u l f a t e and c a l c i u m dodecyl s u l f a t e ) , t h e K r a f f t temperature of t h e mixture can be much l e s s t h a n e i t h e r p u r e component ( 8 7 - 8 9 ) . T h e s e s y s t e m s show t h e c l a s s i c a l e u t e c t i c t y p e behavior and t h e c r y s t a l s c o n t a i n o n l y one k i n d o f s u r f a c t a n t o r c o u n t e r i o n i n s u b s t a n t i a l amounts ( 8 7 - 8 9 ) . In t h e c a s e o f n o n - e u t e c t i c s y s t e m s , t h e s o l i d p h a s e shows n e a r l y ideal mixing, so that t h e surfactant components d i s t r i b u t e t h e m s e l v e s between t h e m i c e l l e and t h e s o l i d i n a b o u t t h e same r e l a t i v e proportions (i.e., b o t h t h e m i x e d m i c e l l e and m i x e d s o l i d a r e a p p r o x i m a t e l y ideal). However, i n t h e c a s e o f t h e e u t e c t i c type system, t h e c r y s t a l i s extremely n o n - i d e a l (almost a s i n g l e component), w h i l e t h e m i c e l l e has n e a r l y ideal mixing. As s e e n in earlier calculations f o r ideal systems, even though t h e t o t a l surfactant monomer concentration i s intermediate between t h a t o f t h e p u r e components, t h e monomer c o n c e n t r a t i o n o f an individual component d e c r e a s e s a s i t s t o t a l p r o p o r t i o n i n s o l u t i o n decreases. As t h e p r o p o r t i o n o f s u r f a c t a n t A d e c r e a s e s in s o l u t i o n ( p r o p o r t i o n of s u r f a c t a n t Β i n c r e a s e s ) from p u r e A, t h e r e i s a l o w e r monomer c o n c e n t r a t i o n o f A. Therefore, i t r e q u i r e s a lower temperature o r a higher added e l e c t r o l y t e level t o p r e c i p i t a t e i t . A t some



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composition, surfactant Β precipitates preferentially to A - t h i s i s t h e minimum i n t h e K r a f f t temperature. At s t i l l h i g h e r p r o p o r t i o n s o f Β i n t h e s y s t e m , t h e monomer concentration of Β i n c r e a s e s , increasing the Krafft temperature. I n t h e c a s e o f t h e s y s t e m s w i t h t h e same surfactant i o n , but d i f f e r e n t counterions, as t h e proportion of s u r f a c t a n t w i t h counterion A decreases, there i s a lower concentration of unbound A to precipitate t h e s u r f a c t a n t i o n . A t some c o m p o s i t i o n , counterion Β p r e f e r e n t i a l l y p r e c i p i t a t e s the surfactant and t h e minimum i n t h e e u t e c t i c o c c u r s . We may c o n s i d e r p r e c i p i t a t i o n i n these systems i n the context of c o m p e t i t i v e aggregate formation between m i c e l l e s and p r e c i p i t a t e . Even s y s t e m s f o r m i n g i d e a l mixed micelles can exhibit synergisms in salinity/hardness tolerance; i n such systems, t h e more components p r e s e n t , the higher the tolerance. This i s the reason that mixtures of i s o m e r i c surfactants g e n e r a l l y have K r a f f t temperatures considerably lower t h a n t h o s e o f t h e i n d i v i d u a l compounds ( 9 0 ) . Ionic/Nonionic Surfactant Mixtures. Since nonionic s u r f a c t a n t s a r e uncharged, they w i l l not form a salt precipitate with counterions l i k e the ionic surfactants. Also, a m i x t u r e o f i o n i c and n o n i o n i c s u r f a c t a n t s form nonideal micelles, resulting in a reduced ionic s u r f a c t a n t monomer c o n c e n t r a t i o n compared t o i d e a l mixed micelle formation. As a r e s u l t o f t h e s e e f f e c t s , t h e s a l i n i t y / h a r d n e s s t o l e r a n c e i n c r e a s e s (35,91,92) o r t h e Krafft t e m p e r a t u r e d e c r e a s e s (93) a s n o n i o n i c s u r f a c t a n t i s added t o a n i o n i c s u r f a c t a n t s o l u t i o n s i n i n c r e a s i n g proportions. I f t h e m i x e d m i c e l l e model a l r e a d y p r e s e n t e d i s u s e d t o p r e d i c t t h e i o n i c s u r f a c t a n t monomer c o n c e n t r a t i o n , and a s i m p l e c o n c e n t r a t i o n - b a s e d solubility product i s assumed t o h o l d between t h e unbound c o u n t e r i o n and monomer, t h e s a l i n i t y t o l e r a n c e o f an a n i o n i c / n o n i o n i c surfactant mixture c a n be a c c u r a t e l y p r e d i c t e d ( 9 1 ) , supporting this v i e w o f t h e mechanism o f t o l e r a n c e enhancement by n o n i o n i c s u r f a c t a n t . In o r d e r to define a ionic/nonionic surfactant solution with high salinity/hardness tolerance, the following criterion should be f o l l o w e d . The m i x e d m i c e l l e s h o u l d have as l a r g e of a n e g a t i v e d e v i a t i o n from i d e a l i t y as p o s s i b l e . Surfactant mixture c h a r a c t e r i s t i c s which r e s u l t i n t h i s have a l r e a d y been d i s c u s s e d . The nonionic surfactant should have a h i g h cloud point. O t h e r w i s e t h e amount o f n o n i o n i c s u r f a c t a n t w h i c h c a n be added t o t h e s y s t e m i s l i m i t e d t o l o w l e v e l s b e f o r e p h a s e separation occurs. I f p o s s i b l e , a mixed i o n i c s u r f a c t a n t should be u s e d f o r r e a s o n s j u s t d i s c u s s e d . T h e r e i s no such b e n e f i t t o u s i n g mixed nonionic surfactants, although t h i s i s not n e c e s s a r i l y detrimental e i t h e r . Anionic/Cationic

Surfactant

Mixtures.

Mixed

micelles


1.

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21

f o r m e d between a n i o n i c and c a t i o n i c s u r f a c t a n t s h a v e h i g h negative d e v i a t i o n from i d e a l i t y , as a l r e a d y discussed. Therefore, i t m i g h t seem t h a t s u r f a c t a n t w o u l d n o t t e n d t o p r e c i p i t a t e e a s i l y f r o m t h e s e s o l u t i o n s . However, t h e electrostatic a t t r a c t i o n s between the anionic and cationic surfactant can cause i o n p a i r i n g formation or c o a c e r v a t e phase f o r m a t i o n i n t h e s e systems (94)It i s beyond t h e s c o p e of t h i s a r t i c l e t o d i s c u s s t h e complex phase b e h a v i o r of the anionic/cationic surfactant systems, but use of t h e s e m i x t u r e s i s g e n e r a l l y n o t an effective way to solve precipitation problems. Conversely, i f p r e c i p i t a t i o n or phase s e p a r a t i o n is d e s i r a b l e , t h e s e m i x t u r e s can be b e n e f i c i a l ( 8 4 ) .


Cloud Point

Phenomena

As t h e t e m p e r a t u r e of d i l u t e a q u e o u s s o l u t i o n s c o n t a i n i n g ethoxylated nonionic surfactants is increased, the s o l u t i o n s may turn cloudy at a c e r t a i n temperature, c a l l e d the cloud p o i n t . At o r a b o v e t h e c l o u d p o i n t , t h e c l o u d y s o l u t i o n may s e p a r a t e i n t o two i s o t r o p i c phases, one c o n c e n t r a t e d i n s u r f a c t a n t ( c o a c e r v a t e p h a s e ) and t h e other containing a low concentration of surfactant ( d i l u t e phase). As an e x a m p l e o f t h e i m p o r t a n c e of this phenomena, d e t e r g e n c y i s s o m e t i m e s optimum j u s t b e l o w t h e cloud point, but a r e d u c t i o n i n t h e w a s h i n g e f f e c t can o c c u r above t h e c l o u d point (95). However, t h e phase separation can improve a c i d i z i n g o p e r a t i o n s in o i l r e s e r v o i r s (96). For s u r f a c t a n t m i x t u r e s , of p a r t i c u l a r interest i s the e f f e c t of m i x t u r e c o m p o s i t i o n on t h e c l o u d p o i n t and t h e d i s t r i b u t i o n of components between t h e two p h a s e s above t h e c l o u d p o i n t . The cloud point i s c l o s e t o , but not n e c e s s a r i l y equal t o the lower consolute s o l u t i o n temperature for polydisperse n o n i o n i c s u r f a c t a n t s (97). These a r e equal if the surfactant i s monodisperse. Since the lower consolute s o l u t i o n temperature i s l i k e a c r i t i c a l point for l i q u i d - l i q u i d mixtures, the dilute and coacervate p h a s e s have t h e same c o m p o s i t i o n , and t h e v o l u m e f r a c t i o n of s o l u t i o n w h i c h t h e c o a c e r v a t e c o m p r i s e s i s a maximum at t h i s temperature (98). If a coacervate phase c o n t a i n i n g a h i g h c o n c e n t r a t i o n of s u r f a c t a n t i s d e s i r e d , the s o l u t i o n should be a t a t e m p e r a t u r e w e l l a b o v e t h e cloud point. The c l o u d p o i n t of a m i x t u r e o f n o n i o n i c s u r f a c t a n t s i s i n t e r m e d i a t e between t h e pure nonionic surfactants involved ( 9 3 , 9 9 ) . The c l o u d p o i n t of a d i l u t e n o n i o n i c surfactant solution increases upon a d d i t i o n of ionic surfactant (93,98-104). The c o a c e r v a t e phase forms b e c a u s e of a t t r a c t i v e f o r c e s between t h e micelles in solution. The i n c o r p o r a t i o n o f i o n i c s u r f a c t a n t i n t o t h e nonionic micelles introduces electrostatic repulsion between m i c e l l e s , c a u s i n g c o a c e r v a t e p h a s e f o r m a t i o n to be h i n d e r e d , r a i s i n g t h e c l o u d p o i n t . The anionic/nonionic surfactant mixing in the



22


c o a c e r v a t e p h a s e can be approximated by equations resulting -from regular solution theory and the n o n i d e a l i t y of m i x i n g i s s i m i l a r t o t h a t o f m i x e d m i c e l l e f o r m a t i o n f o r t h e same s y s t e m ( 9 8 ) This i s reasonable because, at the cloud point, both phases are i d e n t i c a l and t h e c o a c e r v a t e i s just a micellar solution. The c o a c e r v a t e can be viewed as s i m p l y a very concentrated m i c e l l a r s o l u t i o n . The c o n c e n t r a t i o n of surfactant in the dilute phase i s always w e l l a b o v e t h e CMC, so micelles are present. If the total surfactant c o n c e n t r a t i o n i n the i n i t i a l s o l u t i o n i s below t h a t which w o u l d be present i n t h i s d i l u t e phase above the c l o u d point, the s o l u t i o n w i l l not e x h i b i t the cloud point phenomena. The e q u i l i b r i u m i n t h e s e systems above t h e c l o u d p o i n t then i n v o l v e s monomer—micelle e q u i l i b r i u m i n the dilute phase and monomer i n t h e dilute phase i n e q u i l i b r i u m w i t h t h e c o a c e r v a t e p h a s e . P r e d i c t i o n of t h e distribution of surfactant component between phases i n v o l v e s m o d e l i n g of b o t h of t h e s e e q u i l i b r i u m p r o c e s s e s (98). I t s h o u l d be k e p t i n mind t h a t the region under discussion here i n v o l v e s only a s m a l l f r a c t i o n of t h e t o t a l phase space i n t h e n o n i o n i c s u r f a c t a n t - w a t e r system (105). O t h e r c o m p o s i t i o n s may i n v o l v e more t h a n two e q u i l i b r i u m phases, l i q u i d c r y s t a l s , or other s t r u c t u r e s . As the temperature or surfactant composition or concentration i s varied, t h e s e r e g i o n s may be e n c r o a c h e d upon, s o m e t h i n g t h a t t h e s u r f a c t a n t t e c h n o l o g i s t must be wary of when w o r k i n g w i t h n o n i o n i c s u r f a c t a n t s y s t e m s . Conclusions T h i s b r i e f r e v i e w has a t t e m p t e d t o d i s c u s s some o f the i m p o r t a n t phenomena i n w h i c h s u r f a c t a n t m i x t u r e s can be involved. M e c h a n i s t i c a s p e c t s of s u r f a c t a n t i n t e r a c t i o n s and some m a t h e m a t i c a l m o d e l s t o describe the processes h a v e been o u t l i n e d . The a p p l i c a t i o n o f t h e s e p r i n c i p l e s t o p r a c t i c a l p r o b l e m s has been c o n s i d e r e d . For example, enhancement of solubilization or surface tension d e p r e s s i o n u s i n g m i x t u r e s has been d i s c u s s e d . However, i n many c a s e s , t h e v a r i o u s p r o c e s s e s i n w h i c h s u r f a c t a n t s interact generally c a n n o t be c o n s i d e r e d by t h e m s e l v e s , because they occur simultaneously. The surfactant technologist can use this to advantage t o accomplish certain o b j e c t i v e s . For example, the enhancement of mixed m i c e l l e f o r m a t i o n can l e a d t o a r e d u c e d t e n d e n c y f o r s u r f a c t a n t p r e c i p i t a t i o n , reduced adsorption, and a reduced tendency f o r coacervate formation. The s o l u t i o n t o a p a r t i c u l a r p r a c t i c a l problem involving surfactants is r a r e l y obvious because o f t e n the surfactants are i n v o l v e d i n m u l t i p l e s t e p s i n a p r o c e s s and optimization of a number of s i m u l t a n e o u s p r o p e r t i e s may be i n v o l v e d . An e x a m p l e of this i s d e t e r g e n c y , where adsorption, solubilization, foaming, emulsion formation, and o t h e r phenomena a r e a l l i m p o r t a n t . In e n h a n c e d o i l recovery,


1. S C A M E H O R N


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adsorption, microemulsion formation, precipitation, liquid crystal formation, and o t h e r processes a r e s i m u l t a n e o u s l y o c c u r r i n g . S o l u t i o n t o one problem i n t h e s e c o m p l i c a t e d s y s t e m s may i n t r o d u c e o t h e r problems. T h i s p a p e r h a s g i v e n some g u i d e l i n e s w h i c h s h o u l d a i d i n selection of surfactant mixtures t o achieve certain objectives, but t h e performance of proposed systems w i l l g e n e r a l l y need t o b e t e s t e d i n r e a l w o r l d a p p l i c a t i o n s .


Acknowledgments Financial support t o help obtain the original data reported h e r e was p r o v i d e d b y t h e Oklahoma M i n i n g and M i n e r a l s Resources Research I n s t i t u t e . B r u c e L. Roberts and James F. Rathman o b t a i n e d this data. K e v i n L. S t e l l n e r d r a f t e d t h e F i g u r e s i n t h i s paper.

Literature Cited 1. Gogarty, W.B. J. Pet. Technol. 1983, 35, 1581. 2. Cahn, Α.; Lynn, J.L. Jr. In "Kirk-Othmer Encyclopedia of Chemical Technology", Third Edition; Wiley: New York, 1983; Vol. 22, p. 332. 3. Dunn, R.Q.; Scamehorn, J.F.; Christian, S.D. Sep. Sci. Technol. 1985, 20, 257. 4. Shinoda, K. In "Colloidal Surfactants"; Shinoda, K.; Tamamushi, T.; Nakagawa, T.; Isemura, T., Eds.; Academic Press: New York, 1963; Chapter 1. 5. Kamrath, R.F.; Franses, E . I . J. Phys. Chem. 1984, 88, 1642. 6. Shinoda, K. J. Phys. Chem. 1954, 58, 541. 7. Mysels, K . J . ; Otter, R.J. J. Colloid Sci. 1961, 16, 474. 8. Barry, B.W.; Morrison, J.C.; Russell, G.F. J. Colloid Interface Sci. 1970, 33, 554. 9. Lange, H.; Beck, Κ.H., Kolloid Ζ. Ζ. Polym. 1973, 251, 424. 10. Nishikido, Ν.; Moroi, Y.; Matuura, R. Bull. Chem. Soc. Jpn. 1975, 48, 1387. 11. Scamehorn, J.F.; Schechter, R.S.; Wade, W.H. J. Colloid Interface Sci. 1982, 85, 479. 12. Holland, P.M.; Rubingh, D.N. J. Phys. Chem. 1983, 87, 1984. 13. Funasaki, N.; Hada, S. J. Phys. Chem. 1982, 86, 2504. 14. Clint, J.H. J. Chem. Soc. Faraday Trans. 1 1975, 71, 1327. 15. Shedlovsky, L . ; Jakob, C.W.; Epstein, M.B. J. Phys. Chem. 1963, 67, 2075. 16. Tokiwa, F . ; Ohki, K.; Kokubo, I. Bull. Chem. Soc. Jpn. 1968, 41, 2845. 17. Iwadare, Y. Bull. Chem. Soc. Jpn. 1970, 43, 3364. 18. Funasaki, N.; Hada, S. J. Phys. Chem. 1979, 83, 2471. 19. Schick, M.J. J. Am. Oil Chem. Soc. 1966, 43, 681. 20. Schick, M.J.; Manning, D.J. J. Am. Oil Chem. Soc. 1966, 43, 133.



24


21. Nishikido, N., J. Colloid Interface Sci. 1977, 60, 242. 22. Moroi, Y.; Akisada, H.; Saito, M.; Matuura, R. J . Colloid Interface Sci. 1977, 61, 233. 23. Kurzendorfer, C.P.; Schwuger, M.J.; Lange, H. Ber. Bunsen. Ges. Phys. Chem. 1978, 82, 962. 24. Moroi, Y.; Nishikido, N.; Saito, M.; Matuura, R. J . Colloid Interface Sci. 1975, 52, 356. 25. Rubingh, D.N. In "Solution Chemistry of Surfactants"; Mittal, K.L., Ed.; Plenum Press: New York, 1979; Vol. I, p. 337. 26. Hua, X.Y.; Rosen, M.J. J . Colloid Interface Sci. 1982, 90, 212. 27. Rosen, M.J.; Zhu, B.Y. J . Colloid Interface Sci. 1984, 99, 427. 28. Zhu, B.Y.; Rosen, M.J. J . Colloid Interface Sci. 1984, 99, 435. 29. Holland, P.M. In "Structure/Performance Relations in Surfactants"; Rosen, M.J., Ed.; ACS Symposium Series: Washington, D.C., 1984; p. 141. 30. Kamrath, R.F.; Franses, E.I. Ind. Eng. Chem. Fundam. 1983, 22, 230. 31. Scamehorn, J . F . ; Schechter, R.S.; Wade, W.H. J . Dispersion Sci. Technol. 1982, 3, 261. 32. Rosen, M.J.; Hua, X.Y. J . Am. Oil Chem. Soc. 1982, 59, 582. 33. Holland, P.M. Adv. Colloid Interface Sci. In Press. 34. Nguyen, C.M.; Rathman, J . F . ; Scamehorn, J . F . , J. Colloid Interface Sci., In Press. 35. Cox, M.F.; Borys, N.F.; Matson, T.P. J . Am. Oil Chem. Soc. 1985, 62, 1139. 36. Hey, M.J.; MacTaggart, J.W.; Rochester, C.H. J . Chem. Soc. Faraday Trans. 1 1985, 81, 207. 37. Osborne-Lee, I.W.; Schechter, R.S. J. Colloid Interface Sci. In Press. 38. Birdi, K.S. Proc. Int. Conf. Colloid Surf. Sci. 1975, 1, 473. 39. Hall, D.G.; Huddleston, R.W. Colloids Surf. 1985, 13, 209. 40. Rathman, J . F . ; Scamehorn, J . F . J . Phys. Chem. 1984, 88, 5807. 41. Tokiwa, F . ; Tsujii, K. J . Phys. Chem. 1971, 75, 3560. 42. Tokiwa, F . ; Tsujii, K. J . Colloid Interface Sci. 1972, 41, 343. 43. Tokiwa, F; Aigami, K. Kolloid Ζ. Z. Polym. 1970, 239, 687. 44. Kuriyama, K.; Inoue, H.; Nakagawa, T. Kolloid Ζ. Z. Polym. 1962, 183, 68. 45. Balzhiser, R.E.; Samuels, M.R.; Eliassen, J.D. "Chemical Engineering Thermodynamics"; Prentice-Hall: Englewood Cliffs, N . J . , 1972; p. 403-405. 46. Modell, M.; Reid, R.C. "Thermodynamics and Its Applications"; Prentice-Hall: Englewood Cliffs, N . J . , 1974; p. 279. 47. Hall, D.G.; Price, T . J . J . Chem. Soc. Faraday Trans.



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1 1984, 80, 1193. 48. Corkill, J.M.; Goodman, J . F . ; Tate, J.R. Trans. Faraday Soc. 1964, 60, 986. 49. Tokiwa, F; Moriyama, N. J . Colloid Interface Sci. 1969, 30, 338. 50. Nagarajan, R. Langmuir 1985, 1, 331. 51. Asakawa, T.; Johten, K.; Miyagishi, S.; Nishida, M. Langmuir 1985, 1, 347. 52. Mukerjee, P.; Yang, A.Y.S. J . Phys. Chem. 1976, 80, 1388. 53. Funasaki, N.; Hada, S. J . Phys. Chem. 1980, 84, 736. 54. Funasaki, N.; Hada, S.; Neya, S. Bull. Chem. Soc. Jpn. 1983, 56, 3839. 55. Funasaki, N.; Hada, S. J . Phys. Chem. 1983, 87, 342. 56. Shinoda, K.; Nomura, T. J . Phys. Chem. 1980, 84, 365. 57. Funasaki, N.; Hada, S. J . Colloid Interface Sci. 1980, 78, 376. 58. Meyer, M.; Sepulveda, L. J . Colloid Interface Sci. 1984, 99, 536. 59. Aratono, M.; Uryu, S.; Hayami, Y.; Motomura, K.; Matuura, R. J . Colloid Interface Sci. 1984, 98, 33. 60. Rosen, M.J. "Surfactants and Interfacial Phenomena"; Wiley: New York, 1978; Ch. 2. 61. Rosen, M.J.; Hua, X.Y. J . Colloid Interface Sci. 1982, 86, 164. 62. Rosen, M.J.; Zhao, F. J . Colloid Interface Sci. 1983, 95, 443. 63. Ingram, B.T. Colloid Polym. Sci. 1980, 258, 191. 64. Lucassen-Reynders, E.H. In "Progress in Surface and Membrane Science"; Cadenhead, D.A.; Danielli, J . F . , Eds.; Academic Press: New York, 1976; p. 253. 65. Dunn, A.S. In "Emulsion Polymerization"; Piirma, I., Ed.; Academic Press: New York, 1982; p. 221. 66. Tokiwa, F . ; Tsujii, K. Bull. Chem. Soc. Jpn. 1973, 46, 1338. 67. Tokiwa, F. J . Colloid Interface Sci. 1968, 28, 145. 68. Saito, H.; Shinoda, K. J . Colloid Interface Sci. 1967, 24, 10. 69. Hough, D.B.; Rendall, H.M. In "Adsorption from Solution at the Solid/Liquid Interface"; Parfitt, G.D.; Rochester, C.H., Eds.; Academic: New York, 1983; p. 247. 70. Harwell, J . H . ; Helfferich, F.G.; Schechter, R.S. AICHE J . 1982, 28, 448. 71. Harwell, J . H . ; Schechter, R.S.; Wade, W.H. AICHE J . 1985, 31, 415. 72. Scamehorn, J . F . ; Schechter, R.S.; Wade, W.H. J . Colloid Interface Sci. 1982, 85, 463. 73. Scamehorn, J . F . ; Schechter, R.S.; Wade, W.H. J . Am. Oil Chem. Soc. 1983, 60, 1345. 74. Scamehorn, J . F . ; Schechter, R.S.; Wade, W.H. J . Colloid Interface Sci. 1982, 85, 494. 75. Furlong, D.N.; Aston, J.R. Colloids Surf. 1982, 4, 121. 76. Rouquerol, J . ; Partyka, S.; Rouquerol, F. In "Adsorption at the Gas-Solid and Liquid-Solid Interface";



26


Rouqerol, J.; Sing, K.S.W., Eds.; Elsevier: Amsterdam, 1982; p. 69. 77. Gao, Y.; Yue, C.; Lu, S.; Gu, W.; Gu, T. J. Colloid Interface Sci. 1984, 100, 581. 78. Schwuger, M.J.; Smolka, H.G. Colloid Polym. Sci. 1977, 255, 589. 79. LaLanne-Cassou, C.; Carmona, I.; Fortney, L . ; Samii, Α.; Schechter, R.S.; Wade, W.H.; Weerasooriya, U.; Weerasooriya, V.; Yiv, S., SPE Paper No. 12035, Presented at the 58th Annual Technical Conference and Exhibition of the Society of Petroleum Engineers, San Francisco, October, 1983. 80. Novosad, J.; Maini, Β.; Batycky, J. J. Am. Oil Chem. Soc. 1982, 59, 833. 81. Cox, M.F.; Matson, T.P. J. Am. Oil Chem. Soc. 1983, 60, 1170. 82. Matson, T.P.; Berretz, M. Soap Cosmet. Chem. Spec. 1980, 56, 41. 83. Kravetz, L . ; Scharer, D.H.; Stupel, H. Tr. Mezhdunar. Konar. Poverkhn. Akt. Veshchestvam, 7th 1978, 3, 192. 84. Arshad, S.A.; Harwell, J.H. SPE Paper No. 14291, Presented at the 60th Annual Technical Conference of the Society of Petroleum Engineers, Las Vegas, September, 1985. 85. Scamehorn, J.F.; Christian, S.D. In "Surfactant-Based Separation Processes"; Scamehorn, J.F.; Harwell, J.H., Eds.; Marcel Dekker, In Press. 86. Murata, Y.; Motomura, K.; Matuura, R. Mem. Fac. Sci. Kyushu Univ. Ser. C 1978, 11, 225. 87. Tsujii, K.; Saito, N.; Takeuchi, T. J. Phys. Chem. 1980, 84, 2287. 88. Hato, M.; Shinoda, K. J. Phys. Chem. 1973, 77, 378. 89. Moroi, Y.; Oyama, T.; Matuura, R. J. Colloid Interface Sci. 1977, 60, 103. 90. Rosen, M.J. "Surfactants and Interfacial Phenomena"; Wiley: New York, 1978; p. 160. 91. Stellner, K.L.; Scamehorn, J.F. J. Am. Oil Chem. Soc. submitted for publication. 92. Gerbacia, W.E.F. J. Colloid Interface Sci. 1983, 93, 556. 93. Nishikido, N.; Akisada, H.; Matuura, R. Mem. Fac. Sci. Kyushu Univ. Ser. C 1977, 10, 91. 94. Tomlinson, E . ; Davis, S.S.; Mikhayer, G.I. In "Solution Chemistry of Surfactants, Vol. 1"; Mittal, K.L., Ed; Plenum Press: New York, 1979; p. 3. 95. Wirth, W. Tenside Deterg. 1975, 12, 245. 96. Iaks, C.G. U.S. Patent 4 163 727, 1979. 97. Smith, D.H.; Fleming, P.D. J. Colloid Interface Sci. 1985, 105, 80. 98. Yoesting, O.E.; Scamehorn, J.F. Colloid Polym. Sci., In Press. 99. Maclay, W.N. J. Colloid Sci. 1956, 11, 272. 100. Saito, H.; Shinoda, K. J. Colloid Interface Sci. 1967, 24, 10. 101. Nakagawa, T.; Shinoda, K. In "Colloidal


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SCAMEHORN


27

Surfactants"; Shinoda, K.; Tamamushi, B.; Nakagawa, T.; Isemura, T., Eds; Academic Press: New York, 1963; p. 130. 102. Kuriyama, K.; Inoue, H.; Nakagawa, T. Kolloid Ζ. Z. Polym. 1962, 183, 68. 103. Tadros, ThF. J. Colloid Interface Sci. 1974, 46, 528. 104. Corti, M.; Minero, C.; Degiorgio, V. J . Phys. Chem. 1984, 88, 309. 105. Mitchell, D.J.; Tiddy, G . J . T . ; Waring, L . ; Bostock, T.; McDonald, M.P. J. Chem. Soc. Faraday Trans. 1 1983, 79, 975.


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