Interfacial tension minima in oil–water–surfactant systems. Systems containing pure non-ionic surfactants, alkanes and inorganic salts

Abstract

Data are presented for the effects of salts and temperature on the interfacial tensions between alkane and aqueous phases in alkane–water systems containing non-ionic surfactants of the type C₁₂H₂₅(OCH₂CH₂)_N OH for n= 3–6. The tensions of interest are those attained for surfactant concentrations above the aggregation point (‘c.m.c.’) in either the aqueous or alkane phase. Such tensions are frequently very low (10^–3–10^–2 m Nm^–1) and can pass through a minimum with respect to both temperature and salt concentration. The condition for minimum tension corresponds to the phase inversion of coarse emulsions formed by agitation of the systems. The temperatures corresponding to minimum tensions and to phase inversion are shown to be 2 or 3 K above the cloud points of the aqueous solutions saturated with the appropriate alkane (but in the absence of excess alkane phase). The effects of salt concentration and of temperature on the tensions are discussed using a recent simple thermodynamic treatment. It is shown that the salt effect on interfacial tension can be understood in terms of the difference in the salting out of surfactant monomers in a plane monolayer and in an aggregate.