Interfacial tension minima in oil–water–surfactant systems. Effects of alkane chain length and presence of n-alkanols in systems containing aerosol OT

In alkane and aqueous NaCl systems containing Aerosol OT (AOT) the alkane–solution interfacial tension becomes constant at the onset of surfactant aggregation, which can occur in either the aqueous or alkane phase, depending on conditions. The constant tension, γc, can attain ultralow values (ca. 10–3 mN m–1), and has been shown previously to pass through a minimum value with respect to salt concentration and temperature. In the present paper we report an investigation into minima in γc brought about by variation of the chain length of the alkane, and by addition of n-alkanols (‘cosurfactants’) to the system. Results are discussed in terms of the effective molecular geometry of the surfactant and cosurfactant, and penetration of alkane into the surfactant monolayer. A thermodynamic treatment of the effect of cosurfactant demonstrates that minimum γc results when the ratio of surfactant to cosurfactant is equal at the plane alkane–water interface and in mixed aggregates, as expected from the simple geometrical picture.