Ultralow interfacial tensions and microemulsion formation in oil-water-surfactant systems

The thesis is concerned with aspects of the surface and colloid chemistry of various oil+water systems containing pure surfactants. In alkane plus aqueous NaCI systems containing the surfactant diethylhexyl sodium sulphosuccinate (AOT) , the alkane-aqueous solution interfacial tension becomes constant at the onset of surfactant aggregation, which can occur in either the aqueous or alkane phase. This constant tension,Yc , can attain ultralow values ( < 10-3 mN m-1 )and can pass through a minimum value with respect to salt concentration, temperature and alkane chain length. Surfactant transfer between phases, and phase inversion of macroemulsions are shown to occur around the condition which produces minimum Yc.The origin of the low tensions is thought to be due to monolayer adsorption. Aggregate in equilibrium with the plane monolayer are shown to be spherical microemulsion droplets, whose sizes increase as a minimum in Yc is approached.The results are discussed in terms of the effective molecular geometry of the surfactant and how this is affected by the variables of interest. Minimum Yc occurs for the condition such that the effective headgroup area is equal to the effective chain area at a plane interface.A thermodynamic treatment has been used to describe the tension variations; minimum Yc is shown to result when there is some kind of equivalence between the plane oil-water interface and the surface of the aggregates. For example, minimum tension with respect to salt concentration occurs (in the case of ionic surfactants) when the degree of dissociation of surfactant in the micelle and at the plane oil-water interface are equal.For the single-chain surfactant sodium dodecyl sulphate, salt alone cannot yield a minimum in Yc , nor is Yc very low. Addition of octanol as cosurfactant however can produce very low values of Yc and a minimum as cosurfactant concentration is varied. A geometrical description of these effects is given which is in accord with the findings relating to the composition of interfacial monolayers emerging from thermodynamics