Salt partitioning in Winsor Type II systems

Abstract

In systems in which an excess aqueous phase is in equilibrium with a micellar solution, so-called microemulsion, i.e., Type II in Winsor's nomenclature, a nonuniform partitioning of sodium chloride has been observed. This partitioning has been investigated under a variety of surfactant and salt concentrations with both nonionic and anionic surfactants. In both cases, the salt concentration in the excess brine phase has been found higher than in the brine contained in the micellar phase. The magnitude of the effect is, however, lower in the case of nonionics. With nonionic surfactants, the results are interpreted by considering that some salt-free water molecules in the micellar phase are devoted to hydrate the surfactant heads; in the remaining water of the micellar phase, the salt concentration is assumed identical to that in the excess aqueous phase. With anionic surfactant, beside the hydration of the polar heads by some salt-free water molecules, the partial surfactant ionization has also to be taken into account through a dissociation constant. For both systems, despite the large variety of experimental conditions, the calculated excess aqueous phase salinities are found to agree well with the experimental results.