Equilibrium phase behaviour and emulsion stability in silicone oil+water+AOT mixtures

Abstract

We have investigated the equilibrium phase behaviour and the emulsion stability in mixtures of aqueous NaCl, anionic surfactant AOT and polydimethylsiloxane (PDMS) oils. For hexamethyldisiloxane, the transition of Winsor systems from I–III–II is effected by increasing the electrolyte concentration. Aggregated surfactant transfers from water to oil via a third phase, the composition of which resembles that of the L₃ phase. The PDMS/water interfacial tension passes through a low minimum around conditions of three phase formation. In single phase microemulsions, we have determined the uptake of silicone oil into aqueous surfactant solutions and the solubilisation of water into surfactant solutions in oil, both as a function of salt concentration and temperature. Maximum uptake of oil (water) occurs at a salt concentration equal to that at the Winsor I/III (III/II) boundary. The partitioning of salt between dispersed and excess water phases is in favour of the latter for small drops but becomes more nearly equal for larger drops. Macroemulsions prepared from the equilibrium coexisting phases invert from oil-in-water (o/w) to water-in-oil (w/o) at intermediate salt concentrations. For o/w emulsions, the stability to both creaming and coalescence falls markedly approaching the Winsor I/III transition. An argument based on the influence of the interfacial tension on the deformability of the drops is consistent with the findings. For w/o emulsions, the stability to both sedimentation and coalescence increases close to the boundary where three phases form. Three phase emulsions are extremely unstable. In systems containing PDMS oils of higher molecular weight, inversion of equilibrium systems from water to oil continuous occurs at higher salt concentrations. Both the width of the three phase region and the minimum interfacial tension increase with oil molecular weight. Emulsion inversion takes place over virtually the same range of salt concentration as that for which three phases are formed.