Interfacial tensions and microemulsion formation in heptane–aqueous NaCl systems containing aerosol OT and sodium dodecyl sulphate

Abstract

The tension, γ_c, between alkane and aqueous NaCl in systems containing an anionic surfactant (Aerosol OT or sodium dodecyl sulphate, SDS) at the aggregation point can be made ultralow (ca. 1 µN m^–1) by adjustment of the salt concentration or the concentration of an added ‘cosurfactant’(e.g. a long-chain alkanol). For the twin-tailed surfactant (AOT) an ultralow minimum in γ_c can be obtained in the absence of the cosurfactant, whereas the presence of the latter is required in systems containing SDS, which has only a single alkyl chain. Here we discuss how the tensions γ_c vary in the heptane–aqueous NaCl system containing mixtures of AOT and SDS (in the absence of cosurfactant), and consider the nature of the microemulsions which are formed in these systems. All the effects can be broadly understood in terms of well established ideas on the different molecular geometry of the two surfactants and the way in which monolayers tend to curve.