Micelle formation of polyoxyethylene-type nonionic surfactants in bmimBF 4 studied by 1H NMR and dynamic light-scattering

Abstract

Critical micelle concentrations (cmc) of polyoxyethylene (POE)-type nonionic surfactants in a room temperature ionic liquid, bmimBF 4, were determined from the 1H NMR chemical shift of the surfactant as a function of concentration at various temperatures. Thermodynamic parameters derived from the temperature dependence of the cmc revealed that the micelle formation process of the surfactants in bmimBF 4 is entropy-driven at low temperature, while it changes to enthalpy-driven at high temperature. Similarity of this behavior to the case of aqueous system suggests that the micellization of surfactant molecules in bmimBF 4 is caused by a solvophobic interaction between hydrocarbon tails of the surfactants, which is analogous to a hydrophobic interaction in aqueous system. Compared with aqueous system, the solvophobic effect in bmimBF 4 is much weaker. It was demonstrated that the enthalpy–entropy compensation relation holds for the micelle formation in bmimBF 4 as well as in aqueous system.