Synergism in mixed monolayers of alkylpolyglucoside and alkylsorbitan surfactants at liquid/liquid interface

Abstract

A study of nonideal behavior in the formation of mixed monolayers at the oil-water interface was performed for a nonionic-nonionic surfactant system. Mixtures containing alkylpolyglucoside and alkylsorbitan derivatives were investigated. As the oily phase, colza-rapeseed and olive oils were used. To evidence a synergetic effect in the interfacial tension reduction in the oil-water-surfactant-cosurfactant system, the model based on the regular solution theory was modified for the case of both surfactants being soluble in the water as well as in the oily phase. For determination of the condition for the synergism and the point of the maximum synergetic effect, the molar fraction in the mixed monolayer X S and the interaction parameter β S were calculated, using experimental data for the interfacial tension and for the partition coefficient. A set of general equations was developed, to allow the analysis of a mixture containing a water-soluble and an oil-soluble surfactant. The equations are applied according the characteristics of studied quaternary systems. The mathematical model was tested with literature data, and the results were compared with those obtained from the phase diagram of oil-water-mixed surfactant system. The systems water-vegetable oil-alkylpolyglucosidealkylsorbitan show a maximum synergetic effect at molar fractions between 0.85 and 0.90. The liquid-liquid interfacial tension and partition coefficient data were used to calculate the point of the maximum synergetic effect, i.e., the surfactant-cosurfactant ratio, which ensures the interfacial tension minimum. The dramatic reduction in interfacial tension due to the presence of the surfactant mixture at the interface at the point of the synergism maximum is related to the formation of threephase and single-phase microemulsions. The results were applied to obtain single-phase microemulsion in water-vegetable oil-alkylpolyglucoside-alkylsorbitan systems.Paper no. S1348 in JSD 6, 259-264 (July 2003) KEY WORDS: Liquid-liquid interface, microemulsion, mixed surfactant system, synergism.The nonideal behavior of surfactant mixtures is of interest from both a theoretical and a practical point of view. Most industrial applications of surfactants involve the use of mixed surfactants or technical surfactants, which can be considered to be a mixture of surfactants. The existence of synergetic effects in mixed surfactant systems has been studied for several decades, but the published results have been based on a qualitative approach to this behavior, and quantitative analysis has been reported only for some particular cases. Some mathematical models were elaborated to study the nonideal behavior of the mixed surfactant systems. Rubingh (1,2) devised a model based on regular solution theory, as applied to the mixed micellization process in the aqueous solution of mixed surfactants. One can calculate the molar fraction of the surfactant in the mixed micelle and the interaction parameter in the mixed aggregate from the variation of critical micelle concentrations...