Equilibrium interfacial tension measurements of a recommended system for liquid–liquid extraction investigations, toluene + water, at 25.0 °C are carried out to examine interfacial properties of individual and mixture of two widely used surfactants, octylphenol decaethylene glycol ether (Triton X–100) and cetyl trimethyl ammonium bromide (CTAB) with concentrations much lower than their CMC. Based on Szyszkowski equation, Triton has higher adsorption tendency than that of CTAB. The results obtained for mixtures are analyzed by the theory of non–ideal binary mixtures (NIBM) and interfacial composition and interaction parameter in the mixed adsorbed monolayer are determined. Accordingly, the attractive interaction finds a maximum value at nearly equal bulk mole fraction. The calculated parameters show that synergism is achieved for Triton bulk mole fractions of 0.30 and more and the highest degree of synergism (40.6%) belongs to bulk mole fraction near 0.5 and the lowest investigated constant interfacial tension of 28.0 mN•m1. A correlation was developed for the variation of interaction parameter. The results help to find choices to enhance mass transfer across interface, while interfacial tension is maintained constant.
