EMULSION-TYPE BELOW AND ABOVE THE CMC IN AOT MICROEMULSION SYSTEMS

In systems containing the pure anionic surfactant AOT (sodium bis-2-ethylhexyl sulphosuccinate), heptane and aqueous sodium chloride, the effect of surfactant concentration on emulsion type with respect to salt concentration has been studied by conductivity measurement. At high enough AOT concentrations, inversion of emulsions, prepared from equilibrium microemulsions and their excess phases, from oil-in-water (o/w) to water-in-oil (w/o) emulsions occurs close to salt concentrations around which three phases appear in equilibrium systems. At low salt concentrations, where aggregates (once formed) are o/w microemulsions, emulsions are o/w both below and above the CMC of the aqueous phase. At high salt concentrations, where aggregates (once formed) are w/o microemulsions, inversion of emulsions (from o/w to w/o) occurs with increasing surfactant concentration, at concentrations approximately five times the CMC. The results are discussed in relation to Bancroft's rule and to the distribution of both monomeric and aggregated surfactant.