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15 Adsorption of a Mixture of Anionic Surfactants on Alumina 1

Downloaded by PENNSYLVANIA STATE UNIV on February 16, 2013 | http://pubs.acs.org Publication Date: June 5, 1986 | doi: 10.1021/bk-1986-0311.ch015

Bruce L. Roberts, John F. Scamehorn, and Jeffrey H . Harwell

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

The a d s o r p t i o n of sodium d e c y l s u l f a t e and sodium d o d e c y l s u l f a t e and w e l l defined mixtures thereof was measured on alumina. The thermodynamics of mixing upon formation of the bilayered surface aggregates (admicelles) was s t u d i e d as w e l l as t h a t a s s o c i a t e d with mixed m i c e l l e formation f o r the system. Ideal s o l u t i o n theory was obeyed upon formation of mixed m i c e l l e s , but p o s i t i v e d e v i a t i o n from i d e a l s o l u t i o n theory was found at all mixture compositions upon mixed admicelle formation. The mixed m i c e l l e s are i d e a l because the hydrophobic groups of d i f f e r e n t length are i n approximately the same environment in the s p h e r i c a l hydrophobic core of the mixed m i c e l l e as they a r e i n the pure component m i c e l l e s . In the p l a n a r core of the admicelle, however, the methylene groups nearest the h y d r o p h i l i c group on the longer hydrocarbon chain surfactant are not exposed to as hydrophobic an environment as in the pure component a d m i c e l l e , resulting in a reduced tendency f o r the mixed a d m i c e l l e t o form, compared t o an i d e a l solution. The a d s o r p t i o n of s u r f a c t a n t mixtures on metal o x i d e surfaces (e.g., minerals) from aqueous s o l u t i o n s i s an important process i n such a p p l i c a t i o n s as enhanced o i l recovery and detergency. S i n c e s u r f a c t a n t s used i n r e a l world applications are almost always mixtures, i n t e r a c t i o n s between different adsorbed surfactant 1

To whom correspondence should be addressed. 0097-6156/ 86/ 0311 -0200$06.00/ 0 © 1986 American Chemical Society

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


15.

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s p e c i e s , a s w e l l a s s u r f a c t a n t - s u r - f a c e i n t e r a c t i o n s , must be studied t o p r o v i d e a comprehensive u n d e r s t a n d i n g of these systems. The adsorption of single-component anionic s u r f a c t a n t s on m e t a l oxide surfaces has been studied extensively (1-1Q). The s u r f a c e o f t h e a d s o r b e n t c a n be v i e w e d a s b e i n g composed o f p a t c h e s , each patch having a characteristic e n e r g y of adsorption (or f o r m a l l y , a c h a r a c t e r i s t i c standard s t a t e chemical potential change upon a d s o r p t i o n of s u r f a c t a n t on t h a t p a t c h ) . A t low surfactant concentrations, the s u r f a c t a n t s are adsorbing as individual, unassociated m o l e c u l e s on e a c h p a t c h on the surface. At a c e r t a i n specific concentration, the surfactants form a surface a g g r e g a t e on the most e n e r g e t i c p a t c h on t h e s u r f a c e . The a d s o r p t i o n on this patch increases sharply when t h i s occurs. As the surfactant concentration is increased further, successively less energetic p a t c h e s have a s u r f a c t a n t a g g r e g a t e f o r m e d on them. On a s u b s t r a t e like alumina, the d i s t r i b u t i o n of p a t c h e n e r g i e s i s n e a r l y c o n t i n u o u s , so that the adsorption isotherm is essentially continuous. Surface a g g r e g a t e s formed by ionic surfactant adsorption on o p p o s i t e l y c h a r g e s u r f a c e s h a v e been shown to be bi layered structures and are called admicelles(2) i n t h i s paper, though t h e y a r e sometimes r e f e r r e d t o as hemimicel1 es. The c o n c e n t r a t i o n a t which a d m i c e l l e s f i r s t f o r m on t h e most e n e r g e t i c s u r f a c e p a t c h is c a l l e d t h e C r i t i c a l A d m i c e l l a r C o n c e n t r a t i o n (CAO in analogy to the Critical Micelle Concentration (CMC), where m i c e l l e s a r e f i r s t f o r m e d . Again, i n much o f t h e literature, t h e CAC i s referred t o as the Hemimicellar Concentration (HMO. At concentrations below the CAC, there is no significant interaction between adsorbed surfactant molecules. Therefore, the adsorption in this region o b e y s H e n r y ' s law and i s p r o p o r t i o n a l t o c o n c e n t r a t i o n . T h i s r e g i o n of t h e a d s o r p t i o n i s o t h e r m i s known a s the Henry's law region. At the CAC, the adsorption vs. c o n c e n t r a t i o n isotherm e x h i b i t s a sharp i n c r e a s e i n s l o p e a s s u c c e s s i v e p a t c h e s on t h e s u r f a c e become f i l l e d with admicelles. Adsorption may continue to increase with increasing surfactant concentration until complete bilayer coverage occurs over the entire surface. However, o f t e n t h e CMC i s r e a c h e d before this occurs. The activity or chemical potential of t h e s u r f a c t a n t varies little at concentrations above the CMC. Therefore, for single component s u r f a c t a n t s , the a d s o r p t i o n changes s l o w l y w i t h surfactant concentration above t h e CMC. The resulting c h a r a c t e r i s t i c isotherm shape f o r a single-component surfactant adsorption is illustrated in Figure 1 f o r t h e two p u r e s u r f a c t a n t s shown. The thermodynamics of formation of admicelles composed of two o r more s u r f a c t a n t s i s t h e f o c u s o f t h i s


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study. O t h e r m i x e d a g g r e g a t e s -formed by s u r f a c t a n t s have been s t u d i e d , s u c h a s mixed micelles (11-13)« mixed m o n o l a y e r s (13,14)„ m i x e d m i c r o e m u l s i o n s ( 1 5 - 1 7 ) , mixed l i q u i d c r y s t a l s (.17) , and mixed c o a c e r v a t e above the cloud point (18)? t o name a few. However, mixed a d m i c e l l e s have r e c e i v e d l i t t l e a t t e n t i o n , despite their c o m m e r c i a l and t h e o r e t i c a l i n t e r e s t . Scamehorn e t . a l . (19) r e p o r t e d the adsorption isotherms f o r a b i n a r y m i x t u r e of a n i o n i c s u r f a c t a n t s . A f o r m a l a d s o r p t i o n model d e v e l o p e d f o r s i n g l e surfactant systems (1_) was e x t e n d e d t o t h i s b i n a r y s y s t e m and shown t o a c c u r a t e l y d e s c r i b e t h e mixed adsorption isotherms (19.). That t h e o r e t i c a l l y b a s e d model was v e r y complex and i s p r o b a b l y not f e a s i b l e t o e x t e n d beyond two s u r f a c t a n t components. Scamehorn e t . a l . (20) a l s o p r e s e n t e d a s i m p l e , s e m i - e m p i r i c a l method 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 concept of reduced a d s o r p t i o n i s o t h e r m s t o p r e d i c t t h e m i x e d a d s o r p t i o n i s o t h e r m and a d m i c e l l a r c o m p o s i t i o n f r o m t h e p u r e component i s o t h e r m s . I n t h i s work, we p r e s e n t a more g e n e r a l theory, b a s e d o n l y on ideal solution theory, and p r e s e n t d e t a i l e d m i x e d s y s t e m d a t a f o r a b i n a r y m i x e d s u r f a c t a n t s y s t e m (two members o f a homologous series) and use i t t o t e s t t h i s model. The t h e r m o d y n a m i c s o f a d m i c e l l e f o r m a t i o n i s a l s o compared t o t h a t o f m i c e l l e f o r m a t i o n f o r t h i s same s y s t e m . Experimental Materials. The s o d i u m n - d e c y l s u l f a t e (CioSO*) f r o m Kodak and t h e s o d i u m n - d o d e c y l s u l f a t e ( C i S 0 * ) f r o m F i s h e r were purified by r e c r y s t a l 1 i z a t i o n from water and from m e t h a n o l , f o l l o w e d by d r y i n g u n d e r a vacuum. The a l u m i n a u s e d was Aluminum O x i d e C (Degussa I n c . ) , a primarily gamma a l u m i n a , w i t h a s u r f a c e a r e a o f 100 m /g. The NaCl was F i s h e r r e a g e n t g r a d e and t h e w a t e r was d i s t i l l e d and deionized. 2

2

Methods. Adsorption i s o t h e r m s were r u n a t c o n s t a n t f e e d m o l a r r a t i o o f C S 0 ^ / C S 0 ^ . The f e e d s o l u t i o n s had a pH o f 4.25 and a NaCl c o n c e n t r a t i o n o f 0.15 M. Ten ml o f feed solution was added t o 0.5 g a l u m i n a i n a s c r e w t o p c e n t r i f u g e t u b e and c e n t r i f u g e d a t 700 RPM f o r 45 m i n u t e s a t room t e m p e r a t u r e . The t u b e was t h e n p l a c e d i n a w a t e r b a t h a t 30°C f o r f o u r d a y s , t h e l i q u i d d e c a n t e d f r o m t h e m i n e r a l and a n a l y z e d . The s u r f a c t a n t c o n c e n t r a t i o n s were analyzed using high performance l i q u i d chromatography with a conductivity detector. The solution pH after equilibration was d e t e r m i n e d u s i n g pH e l e c t r o d e s . The e q u i l i b r i u m pH i n c r e a s e d t o 6.8 at equilibrium because t h e PZC of a l u m i n a i s a p p r o x i m a t e l y 9. l o

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c o n c e n t r a t i o n s s i g n i f i c a n t l y h i g h e r t h a n t h e CAC, a l m o s t a l l a d s o r b e d s u r f a c t a n t on t h e s u r f a c e i s i n t h e f o r m o f a d m i c e l l e s ; i . e . , t h e p a t c h e s w i t h H e n r y ' s Law a d s o r p t i o n c o n t r i b u t e an i n s i g n i f i c a n t amount o f a d s o r b e d s u r f a c t a n t to the total. Consider a pure s u r f a c t a n t adsorption i s o t h e r m a t a s u r f a c t a n t c o n c e n t r a t i o n a b o v e t h e CAC. A t a specific total surfactant adsorption level, certain s p e c i f i c p a t c h e s on t h e s u r f a c e c o n t a i n a d m i c e l l e s , with the r e s t of t h e s u r f a c e patches c o n t a i n i n g o n l y sparse coverage of s u r f a c t a n t . F o r t h e other pure s u r f a c t a n t component and f o r a n y s u r f a c t a n t m i x t u r e at that same adsorption level, i t i s assumed t h a t t h e same p a t c h e s (and o n l y t h e s e p a t c h e s ) c o n t a i n a d m i c e l l e s as f o r the first pure s u r f a c t a n t . Taking t h i s p h y s i c a l view of t h e surface, i t i s c o n v e n i e n t t o model t h e t o t a l surfactant concentration required t o c a u s e a d m i c e l l e s t o f o r m on those surface patches corresponding t o a s p e c i f i c total adsorption level. S i n c e t o t a l a d s o r p t i o n i s a monotonie f u n c t i o n of t o t a l s u r f a c t a n t concentration f o r a s p e c i f i c surfactant composition, predicting concentration as a f u n c t i o n o f a d s o r p t i o n y i e l d s t h e same r e s u l t a s t h e more traditional p r e d i c t i o n of adsorption as a f u n c t i o n of concentration. I f t h e composition of t h e adsorbate were known, t h e a d s o r p t i o n of each individual surfactant component c o u l d a l s o be p r e d i c t e d i n s u r f a c t a n t m i x t u r e s . Equations t o p r e d i c t these quantities for mixture adsorption, b a s e d on i d e a l m i x i n g i n t h e a d m i c e l l e , w i l l be p r e s e n t e d h e r e . Consider t h e pure s u r f a c t a n t adsorption isotherms shown i n F i g u r e 1. A t c o n c e n t r a t i o n s between t h e CAC and the CMC, there i s a unique concentration level corresponding t o each a d s o r p t i o n level f o r each pure component. Since this concentration corresponds to formation of admicelles on s p e c i f i c p a t c h s on t h e surface, f o r component i , we c a l l the concentration CAC**; t h e v a r i a b l e CAC (no s u p e r s c r i p t ) w i l l be r e s e r v e d to refer t o the concentration which corresponds t o a d m i c e l l e f o r m a t i o n on t h e most e n e r g e t i c patch on t h e surface. We w i l l only consider binary mixtures of surfactants, so t h e s u b s c r i p t i can r e f e r t o either component A o r B. F o r a s u r f a c t a n t m i x t u r e , the total s u r f a c t a n t c o n c e n t r a t i o n r e q u i r e d t o reach a specified a d s o r p t i o n l e v e l i s d e f i n e d a s CAC *. The mixed a d m i c e l l e i s very a n a l o g o u s t o mixed micelles, t h e thermodynamics of f o r m a t i o n of which has been w i d e l y studied. I f t h e s u r f a c t a n t mixing i n t h e m i c e l l e c a n be d e s c r i b e d by i d e a l solution theory, t h e Critical Micelle Concentration (CMC) or minimum concentration a t which m i c e l l e s f i r s t form can be d e s c r i b e d by (21.): M

CMC

M

= CMC«CMC /(y«CMC e

e

+ y CMC«) e

(1)

where CMC i s t h e m i x t u r e CMC, and CMC« o r CMC» a r e t h e CMC v a l u e s f o r p u r e component Â o r B, and y * o r y» a r e M


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the 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 s o-f component A o r Β i n t h e monomer i n s o l u t i o n . The c o m p o s i t i o n o-f t h e micelles i n e q u i l i b r i u m w i t h t h e monomer, a c c o r d i n g t o i d e a l s o l u t i o n t h e o r y , can be d e s c r i b e d by ( 1 1 ) :


κ

= y«CMC /CMC«

Λ

(2)

M

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VA

+ y» = 1

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Χ α

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1

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X

B

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where κ or x a r e t h e sur-f a c t a n t - o n l y b a s e d mole fractions o f component A o r Β i n t h e m i x e d m i c e l l e . E q u a t i o n s 1-5 p r o v i d e a p r i o r i p r e d i c t i o n o f m i x e d CMC v a l u e s and c o m p o s i t i o n o f m i c e l l e s . At a specific adsorption level, we c a n v i e w t h e surfactant monomers a s b e i n g i n equilibrium with admicelles on s p e c i f i c surface patches, just as the monomer i s i n equilibrium with the micelles at a monomeric c o n c e n t r a t i o n o f t h e CMC. T h e r e f o r e , CAC* i s analogous f o r admicelles t o t h e CMC f o r micelle formation. I f mixing between s u r f a c t a n t components i s ideal i n the admicelle, at a s p e c i f i c adsorption level, i n analogy t o Equations l-3s Λ

B

CAC » = CAC«*CAC */(y*CAC»* + y CAC«*)

Z A = y«CAC */CAC«*

(7)

z© = y»CAC */CAC»*

(8)

ζ

(9)

M

e

M

M

Λ

+ ζ

β

= 1

e

where ζ o r ζ» a r e t h e s u r f a c t a n t - o n l y b a s e d mole f r a c t i o n s o f component A o r Β i n t h e m i x e d admicelle. Once CAC** a n d CAC»* a r e d e f i n e d f o r a n y a d s o r p t i o n l e v e l from t h e pure s u r f a c t a n t a d s o r p t i o n isotherms, Equations 6-9 c a n b e u s e d t o predict the total a d s o r p t i o n and admicelle composition f o r any m i x t u r e c o m p o s i t i o n and c o n c e n t r a t i o n i n s o l u t i o n ( a t c o n c e n t r a t i o n s between t h e CAC and t h e CMC f o r t h a t m i x t u r e ) . From t h i s , t h e i n d i v i d u a l s u r f a c t a n t component adsorption l e v e l s can a l s o be c a l c u l a t e d , a l l on an a p r i o r i b a s i s . A p r e v i o u s l y proposed theory t o describe mixed a d s o r p t i o n i n t h e s e s y s t e m s (20) depended n o t o n l y on ideal s o l u t i o n theory, b u t a l s o on t h e c o r r e s p o n d i n g s t a t e s theory t oapply t osurfactant mixtures. In that model, i t was assumed t h a t t h e a d s o r p t i o n i s o t h e r m s f o r t h e pure components c o i n c i d e d when p l o t t e d a g a i n s t a reduced concentration. This occurs when t h e r a t i o CAC */CAC«* i s t h e same a t a n y a d s o r p t i o n level. When true, t h i s s i m p l i f i e s t h e p r e d i c t i o n o f mixed a d s o r p t i o n i s o t h e r m s somewhat, b u t t h a t model i s r e a l l y a special c a s e o f t h e model p r e s e n t e d h e r e . Λ

B


206



R e s u l t s and D i s c u s s i o n Mixed Micelles, The CMC v a l u e s -for t h e t w o p u r e s u r f a c t a n t s and w e l l d e f i n e d m i x t u r e s t h e r e o f a r e shown in Figure 2. The e x p e r i m e n t s were r u n a t a h i g h added salt level (swamping e l e c t r o l y t e ) s o t h e c o u n t e r i o n contributed by t h e d i s s o l v e d s u r f a c t a n t i s n e g l i g i b l e . Predicted mixture CMC values f o r ideal mixing from Equation 1 a r e a l s o shown. Ideal solution theory describes mixed m i c e l l e f o r m a t i o n very well, as i s usually t h e case f o r s i m i l a r l y structured surfactant m i x t u r e s (12,19,21-24). Mixed A d m i c e l l e s . The t o t a l s u r f a c t a n t a d s o r p t i o n o f t h e two p u r e s u r f a c t a n t s and m i x t u r e s t h e r e o f on a l u m i n a a r e shown i n Figure 3. The m i x t u r e s are at constant s u r f a c t a n t r a t i o i n t h e feed o r i n i t i a l s o l u t i o n , but not necessarily i n the final equilibrium solution. The concentration on the abscissa i s the equilibrium concentration. The i n d i v i d u a l surfactant adsorption isotherms f o r t h e p u r e s u r f a c t a n t s and i n t h e m i x t u r e s a r e shown i n F i g u r e s 4 and 5. The e x p e r i m e n t s were r u n at t h e same swamping e l e c t r o l y t e c o n c e n t r a t i o n a s were t h e CMC d a t a . The predicted adsorption isotherms from ideal s o l u t i o n t h e o r y ( E q u a t i o n s 6-9) a r e a l s o shown i n F i g u r e s 3-5. Since i t i s difficult t o s e e d e g r e e o f f i t on a log-log plot, the a b i l i t y t o describe the data i s better illustrated i n F i g u r e s 6-9, where t h e C A C * i s p l o t t e d f o r s e v e r a l a d s o r p t i o n l e v e l s a s a f u n c t i o n o f monomer c o m p o s i t i o n a l o n g w i t h p r e d i c t i o n s f r o m E q u a t i o n 6. From Figures 3 and 6-9, the predicted total adsorptions f o r surfactant mixtures are higher than observed values. Therefore, t h e m i x e d a d m i c e l l e s showed p o s i t i v e d e v i a t i o n from ideality a t a l l compositions. This remarkable behavior h a s n o t been o b s e r v e d b e f o r e b e c a u s e d a t a o f t h e a c c u r a c y and r a n g e r e p o r t e d h e r e h a s not (to our k n o w l e d g e ) p r e v i o u s l y been reported. Observation of t h e expected i d e a l behavior f o r t h e CMC data indicate that this i s probably n o t due t o a p e c u l i a r i t y of t h e s u r f a c t a n t s used. The e x p l a n a t i o n f o r t h i s e f f e c t l i e s i n t h e s t e r i c c o n s t r a i n t s of t h e two-dimensional admicelle. In t h e admicelle, t h e t h i c k n e s s of t h e hydrophobic p o r t i o n of the admicelle i s p r o b a b l y d i c t a t e d by t h e l e n g t h o f t h e longest a l k y l chain present ( i . e . , t h e dodecyl chain). In t h e m i x e d a d m i c e l l e , the decyl chain i s almost c o m p l e t e l y s u r r o u n d e d by o t h e r a l k y l chains and s o i s exposed t o a p p r o x i m a t e l y t h e same e n v i r o n m e n t a s i n a pure CioSO^ a d m i c e l l e . However, t h e two a d d i t i o n a l m e t h y l e n e g r o u p s on t h e d o d e c y l chain a r e not i n as hydrophobic an e n v i r o n m e n t a s i n t h e pure C* S0* a d m i c e l l e , s i n c e neighboring decyl chains cannot i n t e r a c t M

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