Synergism in binary mixtures of surfactants: V. Two-phase liquid—liquid systems at low surfactant concentrations

Abstract

Nonideal solution theory has been used to derive equations that predict interfacial concentrations and conditions for synergism in binary mixtures of surfactants in liquid-liquid systems at low surfactant concentrations. Equations were derived that allow prediction of synergism in three phenomena: (1) interfacial tension reduction efficiency; (2) mixed micelle formation; and (3) interfacial tension reduction effectiveness. These equations involve the experimentally determined parameters, β LL σ and β LL M, related to the molecular interactions between the two surfactants in the mixed monolayer at the liquid-liquid interface and in the mixed micelles, respectively. The experimental data needed to determine whether a liquid-liquid binary surfactant system is capable of synergism in the three areas mentioned above are: (1) the interfacial tension vs log total concentration curves of the individual surfactants in the vicinity of their critical micelle concentrations (CMC); and (2) the interfacial tension vs log total concentration curve of at least one mixture in the vicinity of its CMC. φ, the ratio of the volume of the oil phase to the volume of the aqueous phase in each system investigated, and K, the partition coefficient of each surfactant, drop out of the pertinent equations provided that their values are the same in the mixed system and in the pure surfactant systems. This is generally the case in systems containing low concentrations of surfactants. Two systems were investigated: C 12H 25(OC 2H 4) 7OH-C 12H 25SO 3 ⊝Na ⊕ and C 12H 25N ⊕(CH 2C 6H 5)(CH 3)CH 2COO ⊝-C 12H 25 SO 3 ⊝Na ⊕ in heptane-water mixtures. In the systems studied, theoretical predictions are in good agreement with experimental results.