PHASE-BEHAVIOR OF AN IONIC MICROEMULSION SYSTEM AS A FUNCTION OF THE COSURFACTANT CHAIN-LENGTH

The phase behaviour of a microemulsion system consisting of equal volumes of brine and oil, sodium dodecyl sulphate (SDS) as surfactant and alcohols of different chain lengths (pentanol, hexanol and heptanol) as cosurfactant was studied. In the case of pentanol, at low surfactant concentrations and increasing alcohol concentration, the well-known progression of phase equilibria, Winsor I-Winsor III-Winsor II, was observed. The use of hexanol or heptanol as cosurfactant leads to rather different phase behaviour. At low surfactant concentrations and increasing alcohol concentration the following sequence of phase equilibria is observed: Winsor I-Winsor III-four-phase equilibrium (water-lamellar phase-isotropic microemulsion phase-oil)-Winsor III-Winsor II. Increasing the surfactant concentration starting from the four-phase equilibrium first leads to a three-phase equilibrium (lamellar phase-isotropic microemulsion-oil), then a two-phase equilibrium (lamellar-bicontinuous microemulsion) followed by a monophasic lamellar system. The observed change in phase behaviour as a function of the cosurfactant chain length is discussed in terms of phenomenological interfacial models of microemulsions.