Interfacial tension minima in oil–water–surfactant systems. Effects of cosurfactant in systems containing sodium dodecyl sulphate

Abstract

In systems consisting of a non-polar oil in contact with aqueous NaCl, addition of an ionic surfactant reduces the oil–water interfacial tension until the aggregation point (c.m.c.) is reached and thereafter the tension remains constant (at γ_c) with increasing surfactant concentration. The value of γ_c depends on the salt concentration, m_s, and the concentration, m_co, of co-surfactant, e.g. an n-alkanol, if present. For the twin-tailed anionic surfactant AOT, addition of NaCl can produce ultra-low oil–water tensions and γ_c can be made to pass through a minimum. For the single-chain surfactant sodium dodecyl sulphate (SDS), however, salt alone cannot produce a minimum in γ_c, nor is γ_c very low. Addition of cosurfactant (octanol) to systems containing SDS above its c.m.c. can, however, result in very low values of γ_c and also a minimum as m_co is varied. These effects are discussed quantitatively in terms of the composition of interfacial monolayers.