17 Relationship Between Surfactant Structure and Adsorption P. SOMASUNDARAN, R. MIDDLETON, and Κ. V. VISWANATHAN
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School of Engineering and Applied Science, Columbia University, New York,NY10027
Adsorption of a surfactant on solids is dependent, among other things, on the structure of both the hydro phobic and hydrophilic portions of it. There are a number of mechanisms proposed for surfactant adsorption and an understanding of the effects of the structure of the surfactant can help in elucidating the role of these mechanisms. In this study, the effect on adsorption on alumina of some structure variations of sulfonates (chain length and the branching and the presence of ethyoxyl, phenyl, disulfonate and dialkyl groups) is examined above and below CMC as a function of surfactant concen tration, pH and salinity. Co-operative action between an ionic alkylsulfonate and a nonionic ethoxylated alcohol is also studied. Surfactant adsorption on solids from aqueous solutions plays a major role in a number of interfacial processes such as enhanced oil recovery, flotation and detergency. The adsorption mechanism in these cases is dependent upon the properties of the solid, solvent as well as the surfactant. While considerable information is available on the effect of solid properties such as surface charge and solubility, solvent properties such as pH and ionic strength (1,2,3), the role of possible structural variations of the surfactant in determining adsorption is not yet fully understood. Adsorption is governed by a number of forces: covalent bond formation or electrostatic attraction or hydrogen bond formation between the adsorbate and the adsorbent, electrostatic repulsion among the adsorbate species, lateral associative interaction among adsorbed species, solvation of adsorbate or adsorbent surface species. Structural modifications can affect one or more of the above interactions that might be predominant in different concen tration regions, and it is the cumulative effect of a l l of these modifications on a l l interactions in various concentration regimes that will determine the overall adsorption behavior of a surfac tant (4,5). Thus while in practice chain length or branching will affect only the lateral interactions in the hemi-micellar region, 0097-6156/84/0253-0269506.50/0 © 1984 American Chemical Society
In Structure/Performance Relationships in Surfactants; Rosen, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1984.
STRUCTURE/PERFORMANCE RELATIONSHIPS IN SURFACTANTS
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presence of m u l t i f u n c t i o n a l groups (such as d i s u l f o n a t e s with an ether linkage) can a f f e c t both e l e c t r o s t a t i c and l a t e r a l a s s o c i a tion interactions. Coadsorption between d i f f e r e n t s u r f a c t a n t species also can be expected to be i n f l u e n c e d s i g n i f i c a n t l y by such structural variations. In t h i s paper the adsorption c h a r a c t e r i s t i c s of a s e r i e s of s t r u c t u r a l l y modified s u r f a c t a n t s w i l l be analyzed. Figure 1 summarizes t h i s s e r i e s showing the f o l l o w i n g s t r u c t u r a l v a r i a t i o n s : a r y l a d d i t i o n , chain length v a r i a t i o n , branching, xylene a l k y l a d d i t i o n , ether l i n k a g e and etheylene oxide a d d i t i o n , with alumina as the adsorbent. By understanding the e f f e c t of s t r u c t u r a l v a r i a t i o n s upon the adsorption mechanism a g u i d e l i n e may be e s t a b l i s h e d by which a s u r f a c t a n t may be t a i l o r e d with s p e c i f i c structural modifications for certain situations. Experimental Surfactants, η - s o d i u m dodecylsulfonate s p e c i f i e d to be 99.4% pure was purchased from A l d r i c h Chemicals. η - s o d i u m o c t y l , d e c y l , dodecyl and tetradecylbenzene s u l f o n a t e s were synthesized and p u r i f i e d i n our l a b o r a t o r y . Characterization of these chemicals using p-NMR, C-13 NMR, mass spectrometry and ALC showed these compounds to be i s o m e r i c a l l y pure. Branched hexadecyl benzene s u l f o n a t e was obtained from Conoco and used as r e c e i v e d a f t e r c h a r a c t e r i z a t i o n . A l k y l a r y l orthoxylene s u l f o n a t e s were a l s o i n v e s t i g a t e d . The f i r s t being a l i n e a r nonyl orthoxylene s u l f o n a t e and the second being a branched dodecyl one. Both were s u p p l i e d by the Exxon Corporation and c o n t a i n known amounts of unsulfonated hydrocarbons (14% and 25.2% r e s p e c t i v e l y ) . The d i s u l f o n a t e , Dowfax 3B2, used was a didecylphenoxyd i s u l f o n a t e c o n t a i n i n g 10% monosulfonated i m p u r i t i e s . HPLC a n a l y s i s showed t h i s s u r f a c t a n t to be a mixture of s e v e r a l com pounds . T r i t o n X-200 was used t o study the e f f e c t s of e t h o x y l a t i o n on the s u l f o n a t e s . Nonionic ethoxylated s u r f a c t a n t s were i n v e s t i g a t e d using Synfac 8216 obtained from the M i l l i k e n C o r p o r a t i o n . This was s t a t e d to be 100% a c t i v e with a molecular weight of 1100-1200. HPLC showed t h i s s u r f a c t a n t to be a mixture of s e v e r a l components. Mineral. Alumina used i n t h i s study was a high p u r i t y a - Linde sample purchased from the Union Carbide C o r p o r a t i o n . BET surface area was determined to be 15.0 m /g. 2
Procedure. A gram of the mineral was p r e c o n d i t i o n e d f o r 90 minutes with 5cc of a 0.2 kmol/m sodium c h l o r i d e s o l u t i o n a t 75°C on a w r i s t a c t i o n shaker. Then a 5cc s o l u t i o n of known s u r f a c t a n t concentration i s added and allowed to shake f o r four hours. Four hours mixing was found t o be s u f f i c i e n t to reach e q u i l i b r i u m from adsorption t e s t conducted as a f u n c t i o n of mixing time. The 3
In Structure/Performance Relationships in Surfactants; Rosen, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1984.
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SOMASUNDARAN ET AL.
Surfactant Structure and Adsorption
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