Solubilization of single-component and binary mixtures of hydrocarbons in aqueous micellar solutions

Abstract

The solubilization of hydrocarbon molecules in micellar solutions has been investigated by injecting directly in a gas chromatograph the micellar solutions containing solubilizates. This method was used after an evaluation of the techniques already reported in literature showed that none was as satisfactory for multicomponent hydrocarbon solubilizates. In the single component solubilization experiments, the molar solubilization ratio of seven organic molecules in three different surfactant solutions (cetyl pyridinium chloride, dodecyl ammonium chloride, and sodium dodecyl sulfate surfactants, at 0.1 M concentration and at 25°C) have been measured. The data show that the molecular volume of the solubilizate and its polarity play an important role in determining the extent of solubilization. The interfacial tension of the solubilizate against water was chosen as a measure of its polarity. Correlations between the measured molar solubilization ratios and either the molecular volume or the combined molecular volume-polarity parameter have been developed. In general, a solubilizate with smaller molecular volume and some polarity is preferentially solubilized. Solubilization experiments involving binary hydrocarbon mixtures show that selective solubilization of benzene over hexane occurs in all the five surfactant systems examined. The selectivity ratio for benzene increases with decreasing concentrations of benzene in the bulk solubilizate phase which is in equilibrium with the aqueous micellar solution. A selectivity ratio as large as 7 occurs in dodecyl trimethyl ammonium chloride solution. However, the systems exhibiting high selectivity ratios do so only at the cost of a decrease in the total solubilization capacities for the different solubilizates. Binary solubilization experiments involving the solubilizate pairs benzene + hexane, benzene + cyclohexane and cyclohexane + hexane suggest that the order of selectivity follows that which would be expected from the corresponding pure component molar solubilization ratios. Pairs of solubilizates that have widely different pure component solubilization ratios have the largest selectivity ratio when solubilized as binary mixtures. Finally, the measured molar solubilization ratio for hexane in all the mixed solubilization experiments exhibits an interesting synergistic behavior in the presence of small amounts of benzene. This suggests that surface solubilization of benzene occurs at low amounts of benzene in the micelles.