Langmuir 1995,11, 2504-2507
2504
Formation of Mixed Aggregates at the Alumina-Aqueous Surfactant Solution Interface Ajay K. Vanjara and Sharad G. Dixit* Department of Chemical Technology, University of Bombay, Matunga, Bombay 400 019, India Received September 14, 1994. In Final Form: April 25, 1995@ The mixed critical hemimicelle concentrationsof dodecyl benzenesulfonate and dodecyl sulfate adsorbed on alumina from an aqueous solution have been determined using 5 potential measurements. The critical micelle concentrations of aqueous solutions of mixed surfactants have been determined using surface tension measurements. It is found that whereas mixed micelle formation shows ideal behavior, the mixed hemimicelle formation shows positive deviation from ideality. The results have been explained based on the equivalent contribution of 3.5 methylene groups by the aromatic benzene ring in dodecyl benzenesulfonate. Rubingh theory has been applied to the formation of the hemimicelle.
Introduction There are several applications in which mixtures of surfactants are invariably used. Among the applications are froth flotation of minerals, dispersiodflocculation, growth of particles in suspension, and enhanced oil recovery. The adsorption of surfactants at the solifliquid interface is therefore an important process. However, adsorption of surfactants from their mixture has not received adequate attention. Somasundaran and Fuerstenaul first established characteristic “S”-shaped adsorption isotherms revealing three regions. At very low concentration (region I), adsorption occurs mainly due to the electrostatic attraction between the surfactant ions and the charged sites on the solid surface. The beginning of region I1 is characterized by the onset of a lateral twodimensional aggregation process called hemimicelle formation by Gaudin and Fuerstenau.2 The minimum concentration required to form this aggregate is called .~ I11 the critical hemimicelle concentration ( c ~ c ) Region is located in the vicinity of the critical micelle concentration (cmc), above which adsorption remains more or less constant. Scamehorn et aL4had reported the formation of a bilayer in the case of adsorption of anionic surfactant on a mineral oxide surface. This bilayer formation is reported at much higher concentrations in the vicinity of the cmc of the surfactant. They called these bilayer aggregates the admicelle and the critical concentration required for their formation the critical admicelle concentration (cac). Yeskie et aL6 have also shown the possibility of bilayer formation under certain conditions. At low concentrations, it is difficult to form a bilayer as compared to a hemimicelle because the electrostatic contribution to the free energy of transferring a monolayer from a hemimicelle to an admicelle is potentially much larger in magnitude than the largest possible hydrophobic contribution to the free energy of transfer. ~
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Scamehorn et al.‘j investigated the adsorption of an anionic-anionic alkyl benzenesulfonate surfactant mixture from an aqueous solution on alumina and kaolinite. The phase transitions result in the formation of highdensity aggregates of a surfactant mixture on the solid surface (i.e., mixed hemimicelle) for the solution having a concentration much below the cmc. Adsorption of the surfactant having the least adsorbing tendency is enhanced because of the presence of the mixed hemimicelle. Ideal solution theory was proved to be a good approximation for adsorption eq~ilibrium.’-~Scamehorn et al.l0J1 were the first to investigate in detail the adsorption from the mixture of surfactants on the solid substrate. These authors determined adsorption isotherms for a binary mixture of anionic surfactants. They presented a semiempirical model based on the ideal solution In particular, adsorption of sodium decyl sulfate and sodium dodecyl sulfate on alumina was studied and the formation of a bilayer surface aggregate was found to show a positive deviation from ideal behavior, even though in solution the mixed micelle formation shows ideal behavior. The deviation was attributed to the methylene groups nearest the hydrophilic groups in dodecyl sulfate not being exposed to as hydrophobic an environment as in the case of the micelle formed in solution. The steric factors at the solid liquid interface and the unequal length of the hydrocarbon chain of the two surfactants resulted in such a behavior. Somasundaran et a1.I2have studied coadsorption of the anionic surfactant sodium p-octyl benzenesulfonate and the nonionic surfactant (dodecyloxy)(heptaethoxy)ethyl alcohol. Single anionic sulfonate was found to adsorb significantly, but the nonionic surfactant did not adsorb appreciably. However, in the mixed surfactant system, sulfonate induced significant coadsorption of the nonionic surfactant. This was attributed to the hydrophobic interaction between the surfactants and also to the reduction of electrostatic repulsion between sulfonate headgroups due to shielding by intervening nonionic surfactant molecules. However, the above authors did not investigate the formation of hemimicelle at the solid
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* Also Professor ofphysical Chemistry. To whom correspondence should be addressed. Abstract published in Advance ACS Abstracts, June 15,1995. (1) Somasundaran, P.;Fuerstenau, D. W. J . Phys. Chem. 1966,70, 90. (2)Gaudin, A. M.;Fuerstenau, D. W. Trans. A W E 1955,202,958. (3)Gaudin, A.M.; Soto, H.; Somasundaran, P. InReagents in Mineral Technology; Surfactant Science Series Vol. 27; Somasundaran, P., Moudgil, B. M., Eds.; Marcel Dekker: New York, 1987;p 79. (4)Scamehorn,J.F.;Schechter, R. S.;Wade, W. H. J . Colloid Interface Sci. 1982,85, 463. (5)Yeskie, A. M.;Hanvell, H. J. J . Phys. Chem. 1988,92,2345. @
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(6) Scamehorn,J. F.; Schechter,R. S.;Wade, W. H. J . Colloid Interface Sci. 1982,85, 479. (7)Tokiwa, F.; Dhok, K.; Kokubool. Bull Chem. SOC.Jpn. 1968,41,
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(8) Lange, H.; Beck, K. H. Kolloid Polym. 1973,251,425.
(9)Clint, J. H. J . Chem. Soc., Faraday Trans. 1975,71,1327. (10)Robert, B.L.; Scamehorn, J. F.; Harwell, J. F. InPhemmena in Mixed Surfactant Systems; ACS Symposium Series; Scamehorn, J. F., Ed.; American Chemical Society: Washington, DC, 1986;pp 200-215. (11)Scamehorn, J.F.; Schechter, R. S.;Wade, W. H. J . A m . Oil Chem. SOC.1983,60, 1345. (12) Somasundaran, P.;Fu, E.; Xu gun. Langmuir 1992,8, 1065.
0 1995 American Chemical Society
Formation of Mixed Aggregates
Langmuir, Vol. 11, No. 7, 1995 2505
liquid interface. In the present study, we have investigated the formation of hemimicelle,during the adsorption of sodium dodecyl sulfate and dodecyl benzenesulfonate, which have the same hydrocarbon chain but different headgroups,namely, benzenesulfonateand sulfate. It will be seen that this difference also results in deviation from ideality. The 5 potential measurements have been employed to determine the chc. Somasundaran and Fuerstenaul have earlier shown that adsorption as well as 5 potential measurements can be used to determine the critical hemimicelle concentration.
Materials and Methods Alumina (A12031 was procured from Aldrich Chemicals Co., Inc., and was 99.9% pure. An experimental sample having a particle size of 13 pm was prepared using CYCLOSIZER. The procedure was repeated 4 times to remove any coarse particles. The alumina was reported to be of the a variety. Surfactants. Sodium dodecyl sulfate (DDS), an anionic surfactant, was procured from Sigma Chemicals Co. and was 99% pure. The cmc of DDS was determined to be 6.019 x M in M NaCl solution. Sodium dodecyl benzenesulfonate (DDBS), another anionic surfactant, was obtained from Fluka Chemika, and its overall purity was found to be 99%) which contains more than 95% p-dodecyl benzenesulfonate and the remainder being other homologous isomers and traces of moisture. The cmc of DDBS was found to be low3M in M NaCl solution. The solubility of both surfactants was experimentally tested at a temperature of 30 "C, and it was found to be completely soluble over the entire concentration range. Both the surfactants were used as received. All the solutions were made in doubledistilled water. Adsorption Experiments. Fifty milliliters of surfactant solution in M NaCl solution having a pH of 5 0.1 was equilibrated with 0.5 g of alumina powder for 24 h in an Erlenmeyer flask maintained a t 30 "C. The solid particles were separated from the supernatant liquid by centrifugation for 15 min. The clear supernatant was removed for analysis of surface active agents. The concentration of DDBS was determined by measuring the absorption of U V radiation a t 223 nm by the phenyl ring. This method works in the concentration range below M.12 The concentration of DDS was determined by using the methylene blue method.l3 Measurement of the 5' Potential. The 5; potential Of A 1 2 0 3 particles (5 m) suspended in surfactant solution (50 mL) was measured with a 5; meter (Rank Brother Mark 11). The pH of the solution was maintained a t pH 5 f 0.1 by a Systronic pH meter using a combination electrode. The suspension of alumina in the surfactant solution in an Erlenmeyer flask was kept on a flask shaker at a speed of 200 rpm for 24 h a t constant temperature (30 "C)before potential measurements began. The 5; potential was calculated from the electrophoretic mobility using Smoluchowski's equation.14 The surface tension of the aqueous solution of single surfactants and mixed surfactants was measured by the Du Nuoy ring method. All'experiments were carried out a t 30 "C.
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