Surface concentrations and molecular interactions in binary mixtures of surfactants

The regular solution treatment of Rubingh for mixed surfactant micelles has been extended to adsorption at the aqueous solution/air interface. Three systems were investigated: C12H25(OC2H4)3-OH-C12H25(OC2H4)8OH, C12H25SO3Na-C12H25(OC2H4)8OH, and C12H25C5H5NBr-C12H25(OC2H4)8-OH, all in aqueous solution. Mole fractions at the aqueous solution/air interface of the surfactants in these mixtures, calculated by this treatment, are in good agreement with those calculated by use of the Gibbs adsorption equation. Values of the molecular interaction parameter, β, at the aqueous solution/air interface, were also calculated by the regular solution treatment. For the systems investigated, the order of decreasing interaction was anionic-nonionic > cationic-nonionic > nonionic-nonionic.