Interfacial tension minima in oil–water–surfactant systems. Behaviour of alkane–aqueous NaCl systems containing aerosol OT

In heptane and aqueous NaCl systems containing diethylhexyl sodium sulphosuccinate (AOT) very low interfacial tensions, γ, can be attained. For fixed temperature (T) and salt concentration, γ falls as the AOT concentration increases and levels off at a value of γC at a concentration (the critical micelle concentration, c.m.c.) corresponding to the onset of surfactant aggregation in either the aqueous or oil phase, depending on T and salt concentration. For a given T, at low salt concentration AOT resides in the aqueous phase both below and above the c.m.c. At higher salt concentrations surfactant transfers to the oil phase and leaves the aqueous phase close to the c.m.c. expected if no excess of oil were present, but devoid of micelles. In this case the oil phase is shown to be a dilute water-in-oil microemulsion. The transition between behaviour at high and low salt concentrations corresponds to the attainment of a minimum γC. The variation in γC with salt concentration is in part a consequence of the way in which the c.m.c. and the surfactant activity coefficients change with salt concentration. The minimum γC occurs when the apparent degree of dissociation of surfactant in the micelle and at the oil–water interface are equal and close to zero.