The partitioning of solutes between water-in-oil microemulsions and conjugate aqueous phases

Abstract

The partitioning of solutes between an aqueous phase containing sodium chloride and an equilibrium, conjugate, water-in-oil microemulsion phase has been measured. The two-phase system consists of a water phase containing a very low concentration of monomeric surfactant in equilibrium with an upper, oil phase consisting of a water-in-oil microemulsion containing virtually all the surfactant. The surfactant used was sodium bis(2-ethylhexyl) sulphosuccinate (AOT) and the oil was n-heptane. The size of the water-in-oil microemulsion aggregates present in the upper phase was varied by altering the sodium chloride concentration present in the aqueous phase. The solutes investigated were sodium chloride, the cationic dimidium ion, the neutral p-nitroaniline species and the anionic species murexide. The sodium chloride partitions only weakly in favour of the bulk water phase. Dimidium ion partitions strongly to the AOT interface, particularly so at low salt concentrations. p-Nitroaniline partitions mainly to the AOT interface but less strongly than the DMD. The negative murexide ion partitions between the bulk and dispersed water of the microemulsion phase with a partition coefficient of near unity. The involvement of electrostatic and other forces in determining the partitioning is discussed. The study of partition coefficients in these two-phase systems can yield information about the interaction of the solute with the microemulsion which is relevant to the kinetics of chemical reactions in microemulsions.