Chapter 10
Adsorption of Binary Anionic Surfactant Mixtures on α-Alumina 1
Jeffrey J. Lopata, Jeffrey H. Harwell , and John F. Scamehorn School of Chemical Engineering and Materials Science, University of Oklahoma, Norman, OK 73019 The adsorption of b i n a r y mixtures of a n i o n i c s u r f a c t a n t s of a homologous series (sodium o c t y l sulfate and sodium dodecyl sulfate) on alpha aluminum oxide was measured. A thermodynamic model was developed to describe i d e a l mixed admicelle (adsorbed surfactant bilayer) formation, for concentrations between the critical admicelle concentration and the critical micelle concentration. Specific homogeneous surface patches were examined by considering constant levels of adsorption. T h i s model was shown to accurately describe the experimental r e s u l t s obtained, as w e l l as previously reported results of another binary anionic/anionic surfactant system. Theoretical predictions of i d e a l mixture adsorption can be made on an a priori b a s i s if the pure component adsorption isotherms are known. The a d s o r p t i o n o f m i x t u r e s of surfactants on mineral o x i d e s u r f a c e s i s i m p o r t a n t i n d e t e r g e n c y , f l o t a t i o n , and enhanced o i l r e c o v e r y (EOR), among o t h e r t e c h n o l o g i e s . I f t h e thermodynamics o f mixed s u r f a c t a n t adsorption on mineral surfaces were known, i t m i g h t be p o s s i b l e t o formulate m i x t u r e s t o e i t h e r enhance o r reduce t h e t o t a l surfactant adsorption, thereby saving reagent costs i n f l o t a t i o n (1-3) and e n h a n c e d o i l r e c o v e r y (A). Technical problems, such as the s e l e c t i v i t y o f m i n e r a l separation (3,5) and t h e c h r o m a t o g r a p h i c s e p a r a t i o n o f s u r f a c t a n t s l u g s (6,7), could also be systematically addressed. Surfactants a r e almost always used as mixtures i n 1
Correspondence should be addressed to this author. 0097-6156/88/0373-0205$06.00/0 © 1988 American Chemical Society
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practical applications, since surfactants are usually manufactured as mixtures. These mixtures are often c o m p r i s e d o f homologs o f a s u r f a c t a n t s e r i e s t h a t differ only by t h e a l k y l chain length, and t h e y a r e u s u a l l y t r e a t e d t h e o r e t i c a l l y as a s i n g l e component surfactant, w i t h t h e mean p r o p e r t i e s o f t h e m i x t u r e . A thermodynamic knowledge of the adsorption o f homologous m i x t u r e s o f s u r f a c t a n t s w o u l d a l l o w b e t t e r p r e d i c t i o n s t o be made o f the s u r f a c t a n t ' s performance. Most o i l - f i e l d e x p e r i e n c e w i t h t h e u s e o f s u r f a c t a n t derives from so-called micellar/polymer or chemical f l o o d i n g EOR, i n which s u r f a c t a n t s a r e used t o produce u l t r a - l o w i n t e r f a c i a l tensions through the formation o f a third phase, which c o e x i s t s w i t h t h e aqueous a n d o l e i c phase and c o n t a i n s most o f t h e s u r f a c t a n t . By t h e p h a s e rule, in a three-phase s y s t e m o f t h r e e components ( o r pseudocomponents) t h e c o m p o s i t i o n s o f t h e phases a r e fixed. So long as t h e system c o m p o s i t i o n s t a y s w i t h i n the t i e t r i a n g l e , r e m o v a l o f one o f t h e components from the system d o e s n o t change t h e c o m p o s i t i o n s o r p h y s i c a l p r o p e r t i e s o f t h e phases. This s t a b i l i z e s the properties o f t h e system a g a i n s t l o s s o f s u r f a c t a n t from the fluid phases due t o a d s o r p t i o n on t h e v e r y l a r g e s u r f a c e a r e a o f t h e p o r o u s r o c k {S). N e v e r t h e l e s s , loss of surfactant due t o a d s o r p t i o n has long been a major concern i n micellar/polymer EOR ( 9 - 1 1 ) . The s u r f a c t a n t s y s t e m s u s e d f o r m o b i l i t y c o n t r o l i n miscible flooding do n o t f o r m a s u r f a c t a n t r i c h t h i r d phase, and l a c k i t s " b u f f e r i n g " a c t i o n a g a i n s t s u r f a c t a n t adsorption. Furthermore, f o r o b v i o u s economic reasons, it i s d e s i r a b l e t o keep t h e s u r f a c t a n t c o n c e n t r a t i o n as low a s p o s s i b l e , which i n c r e a s e s t h e s e n s i t i v i t y o f t h e dispersion stability to surfactant loss. Hence, surfactant adsorption i s necessarily an e v e n greater c o n c e r n i n t h e u s e o f foams, emulsions, and d i s p e r s i o n s for mobility control in miscible-flood EOR. The importance of surfactant adsorption i n surfactant-based m o b i l i t y c o n t r o l i s w i d e l y r e c o g n i z e d by r e s e a r c h e r s . A decision t r e e has even been p u b l i s h e d f o r s e l e c t i o n o f a mobility-control surfactant based on adsorption c h a r a c t e r i s t i c s (12). P u r e component s u r f a c t a n t a d s o r p t i o n h a s b e e n w i d e l y studied (9,13-24). Figure 1 illustrates the four d i s t i n c t adsorption regions that e x i s t when an anionic surfactant adsorbs on a p o s i t i v e l y charged surface. Region I i s c a l l e d t h e Henry's Law region and t h e surfactant molecules adsorb because o f t h e e l e c t r o s t a t i c a t t r a c t i o n between t h e s u r f a c t a n t head groups and t h e oppositely charged surface, and t h e i n t e r a c t i o n o f t h e s u r f a c t a n t t a i l groups w i t h the s u r f a c e . At a critical concentration, t h e a d s o r p t i o n i s g r e a t l y enhanced by t h e association of the surfactant tail groups. A twod i m e n s i o n a l p h a s e t r a n s i t i o n i s b e l i e v e d t o t a k e p l a c e on the highest energy patches on t h e s o l i d s u r f a c e . The c o n c e n t r a t i o n a t which the f i r s t s u r f a c t a n t aggregate i s
10. LOPATA ET AL.
Adsorption ofBinary Anionic Surfactant Mixtures
τ
1
Γ
EQUILIBRIUM CONCENTRATION (MICROMOLE/L)
F i g u r e 1. T y p i c a l A n i o n i c S u r f a c t a n t A d s o r p t i o n I s o t h e r m on a P o s i t i v e l y C h a r g e d M i n e r a l O x i d e Surface.
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formed i s c a l l e d the C r i t i c a l Admicelle C o n c e n t r a t i o n or CAC (1_8) , or the Hemimicelle Concentration or HMC (20,23) . At the CAC, the surfactant aggregate (or a d m i c e l l e ) forms on the most energetic patch on the surface. As the surfactant concentration increases, s u c c e s s i v e l y l e s s e n e r g e t i c p a t c h e s have a d m i c e l l e s form on them i n R e g i o n s I I and I I I . T h e r e i s no f u n d a m e n t a l s i g n i f i c a n c e t o the Region I I to Region III transition; Region III e x i s t s due to h i s t o r i c a l p r e c e d e n t as t h e r e g i o n where t h e a d s o r p t i o n i n c r e a s e s l e s s rapidly with concentration on a log-log plot. With each a d s o r p t i o n l e v e l i n Region I I , ^there i s a corresponding e q u i l i b r i u m concentration (CAC ) that is required to form an a d m i c e l l e on a p a t c h o f a s p e c i f i c e n e r g y level. Region IV begins w i t h the formation of m i c e l l e s . M i c e l l e s a c t as a chemical p o t e n t i a l sink for any additional surfactant added to the s o l u t i o n , thereby keeping the monomer c o n c e n t r a t i o n n e a r l y c o n s t a n t , and t h e a d s o r p t i o n l e v e l nearly constant. Hence, Region IV is sometimes c a l l e d the p l a t e a u a d s o r p t i o n r e g i o n {9). The adsorption of mixtures of surfactants has r e c e i v e d c o m p a r a t i v e l y l i t t l e a t t e n t i o n . The adsorption of m i x t u r e s o f n o n i o n i c and a n i o n i c s u r f a c t a n t s has b e e n s t u d i e d (10,25-27) and s t r o n g negative deviations from ideality were observed (10,27). A t t e m p t s t o model t h e degree of non-ideality using regular solution theory failed (2J7) . The a d s o r p t i o n o f m i x t u r e s o f 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 w o u l d be expected to exibit even l a r g e r d e v i a t i o n s f r o m i d e a l i t y (2_8) . Wilson and co-workers developed a statistical mechanical model for single component surfactant adsorption (29-31) and expanded i t to a b i n a r y system (2,3). D i f f e r e n t adsorption curves were generated by varying the Van d e r Waals i n t e r a c t i o n p a r a m e t e r s . The m i x e d a d s o r p t i o n e q u a t i o n s t h a t were d e v e l o p e d were very complex and were n o t a p p l i e d t o e x p e r i m e n t a l data. Scamehorn et. a l . expanded a single component a d s o r p t i o n e q u a t i o n (9) to describe the adsorption of binary mixtures of a n i o n i c s u r f a c t a n t s o f a homologous s e r i e s (1_1) . I d e a l s o l u t i o n t h e o r y was f o u n d t o d e s c r i b e the system f a i r l y w e l l . The m i x e d adsorption equations worked very well i n p r e d i c t i n g the mixture adsorption, b u t t h e e q u a t i o n s were complex and w o u l d be d i f f i c u l t to extend beyond a b i n a r y system. Scamehorn et. al. (32) also developed a reduced adsorption equation to describe the adsorption of mixtures of anionic s u r f a c t a n t s , w h i c h a r e members o f homologous s e r i e s . The e q u a t i o n s were s e m i - e m p i r i c a l and were b a s e d on i d e a l s o l u t i o n t h e o r y and the theory of corresponding states. To apply these equations, a c r i t i c a l c o n c e n t r a t i o n f o r each pure component in the mixture is chosen, so that when the equilibrium c o n c e n t r a t i o n s o f t h e p u r e component a d s o r p t i o n isotherms are divided by their critical concentrations, the adsorption isotherms would coincide. The a d v a n t a g e o f
10. LOPATA ET AL.
Adsorption of Binary Anionic Surfactant Mixtures
these equations i s that little adsorption data i s r e q u i r e d t o make a p p r o x i m a t e m i x t u r e p r e d i c t i o n s . The adsorption of binary mixtures of anionic s u r f a c t a n t s i n t h e b i l a y e r r e g i o n has a l s o been modeled by u s i n g j u s t t h e p u r e component a d s o r p t i o n i s o t h e r m s a n d ideal s o l u t i o n theory t o d e s c r i b e t h e f o r m a t i o n o f mixed a d m i c e l l e s (3_3) . 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 i n t h e m i x e d a d m i c e l l e p h a s e was r e p o r t e d , and t h e n o n - i d e a l i t y was attributed to the planar shape o f t h e a d m i c e l l e . However, a c o m p u t a t i o n a l e r r o r was made i n c o m p a r i s o n o f the ideal s o l u t i o n theory equations t o the experimental data, even though t h e t h e o r e t i c a l equations presented were correct. Thus, t h e p o s i t i v e d e v i a t i o n from i d e a l m i x e d a d m i c e l l e f o r m a t i o n was i n e r r o r . Experimental Materials. Sodium d o d e c y l s u l f a t e ( C ^ S O ^ was p u r c h a s e d from Fisher Scientific with a manufacturer reported p u r i t y o f a t l e a s t 95.01%. The C ^ S O ^ was r e c r y s t a l l i z e d one time from r e a g e n t grade a l c o h o l and water. Sodium decyl sulfate (C*QSO^), f r o m Eastman Kodak Company, was recrystallized t w i c e u s i n g t h e same p r o c e d u r e a s f o r t h e 12 4* Sodium o c t y l s u l f a t e (CgSO.), f r o m Eastman Kodak Company, was r e c r y s t a l l i z e d two t i m e s f r o m boiling ACS grade 2-propanol and water, and then r i n s e d t h r e e t i m e s w i t h ACS g r a d e d i e t h y l ether. The c r y s t a l s were then d r i e d i n a h o o d f o r two d a y s , a n d t h e n u n d e r a vacuum f o r t h r e e days. The alpha aluminum o x i d e was p u r c h a s e d f r o m A l p h a Products, Thiokol/Ventron Division. The aluminum oxide had a p a r t i c l e s i z e o f 40 m i c r o n s , a s u r f a c e a r e a o f 160 m / g , and c o n s i s t e d o f 90% A l 0 a n d 9% H 0 , a c c o r d i n g t o the manufacturer. The aluminum o x i d e was d r i e d i n 50 g b a t c h e s u n d e r a vacuum f o r two d a y s b e f o r e u s e . Other m a t e r i a l s u s e d were ACS g r a d e s o d i u m c h l o r i d e and s o d i u m c a r b o n a t e , HPLC g r a d e m e t h a n o l , a n d 0.1 Ν a n d 0.01 Ν h y d r o c h l o r i c a c i d and sodium h y d r o x i d e s o l u t i o n s . All o f these chemicals were purchased from Fisher Scientific. The w a t e r was d i s t i l l e d a n d d e i o n i z e d . C
S 0
2
3
2
Methods. The b r i n e s o l u t i o n s t h a t were u s e d t o make t h e sample solutions were 0.15 M sodium chloride and contained 0.0015 g/1 s o d i u m c a r b o n a t e . Sodium c a r b o n a t e was added t o b u f f e r t h e s o l u t i o n s against the carbonic a c i d t h a t f o r m e d upon t h e a b s o r p t i o n o f c a r b o n d i o x i d e i n the solutions. The pH of the brine solutions were a d j u s t e d t o a n i n i t i a l v a l u e o f 7.8, so t h a t when t h e solution was equilibrated with the alumina, the e q u i l i b r i u m pH was 8.4. T e n m i l l i l i t e r s o f t h e sample solution were t h e n p i p e t e d t o t e s t t u b e s w h i c h c o n t a i n e d 0.5 grams o f aluminum o x i d e . The t e s t tubes were then centrifugea f o r 45 m i n u t e s a t 1000 rpm a n d p l a c e d i n a water bath that was kept a t a constant temperature o f 30 C. A f t e r four to five days, t h e pH v a l u e s o f t h e
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SURFACTANT-BASED MOBILITY CONTROL
e q u i l i b r a t e d s a m p l e s were m e a s u r e d a n d t h e supernatants o f t h e s a m p l e s were removed f r o m t h e s o l i d s . The concentrations of the anionic surfactants (CgSO^, C S0 , and C- S0 ) i n the i n i t i a l and equilibrated samples were measured using a high performance liquid chromatograph with a conductivity detector. D e t a i l s of the a n a l y t i c a l procedure may be found elsewhere {34). 1 Q
4
2
4
Theory The pure component adsorption isotherms of the surfactants used i n this study (C S0 , C^^SO^, and C S O - ) a r e shown i n F i g u r e 2. A l l o f the adsorption isotherms were c o n t i n u o u s i n Regions I I and I I I , which s u g g e s t s t h a t t h e d i s t r i b u t i o n o f e n e r g y l e v e l p a t c h e s on the s u r f a c e o f the alpha aluminum oxide was nearly continuous. The admicelle standard states a r e d e f i n e d as the e q u i l i b r i u m monomer c o n c e n t r a t i o n s t h a t a r e r e q u i r e d t o form the pure admicelles on a s p e c i f i c energy l e v e l patch. T h i s p a r t i c u l a r p a t c h i s assumed t o c o r r e s p o n d t o t h e same a d s o r p t i o n l e v e l f o r e i t h e r p u r e s u r f a c t a n t s o r mixtures i n R e g i o n s I I a n d I I I . The e q u i l i b r i u m monomer concentration that i s required t o form the mixed admicelles on t h e same energy level patch, i s then compared t o t h e monomer concentration predicted from ideal s o l u t i o n theory. By d e f i n i n g t h e s t a n d a r d s t a t e s at constant l e v e l s o f adsorption, we can look a t one homogeneous e n e r g y l e v e l p a t c h on t h e s u r f a c e a t a t i m e . As we l o o k a t i n c r e a s i n g l y h i g h e r a d s o r p t i o n l e v e l s , the effects o f lower energy level p a t c h e s and i n c r e a s i n g t o t a l surface coverage on a d m i c e l l e formation c a n be examined. The a d m i c e l l e s t a n d a r d s t a t e s were d e t e r m i n e d f r o m t h e p u r e component a d s o r p t i o n i s o t h e r m s by reading the e q u i l i b r i u m monomer c o n c e n t r a t i o n s t h a t c o r r e s p o n d e d t o a n a r b i t r a r y a d s o r p t i o n l e v e l o f i n t e r e s t {33) . The a p p r o a c h u s e d t o develop the i d e a l solution theory equations t o describe binary mixed admicelle formation, was s i m i l a r t o t h a t u s e d by R o b e r t s et.a l . (33). The t o t a l monomer e q u i l i b r i u m c o n c e n t r a t i o n t h a t i t takes t o reach a s p e c i f i e d level of adsorption was used a s t h e v a r i a b l e w h i c h was p r e d i c t e d f r o m t h e m o d e l . The p a r t i a l fugacities c a n be w r i t t e n f o r both the monomer and a d m i c e l l e p h a s e s : g
4
1 2
mon
f
=
Y
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ι f . m
o
a
d
m
mon i l adm
=
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A
C
* m (2)
where f and j partial fugacities of component i i n t h e monomer and admicelle phases, respectively; Y. i s the surfactant-only based mole f r a c t i o n o f component i i n t h e e q u i l i b r i u m solution; Z. i s t h e s u r f a c t a n t - o n l y b a s e d m o l e f r a c t i o n o f component i a
r
e
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i
e
10. LOPATA ET AL.
Adsorption ofBinary Anionic Surfactant Mixtures
1000.0 ρΗ 8.4 TEMP 30°C 0.15 M NaCl SOLUTION/SOLID RATIO 0.02 (L/G)
CMC
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