Micelle formation of polyoxyethylene-type nonionic surfactants in bmimBF4 studied by 1H NMR and dynamic light-scattering.
Critical micelle concentrations (cmc) of polyoxyethylene (POE)-type nonionic surfactants in a room temperature ionic liquid, bmimBF(4), were determined from the (1)H 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.
Department of Chemistry, Faculty of Science, Fukuoka University, Nanakuma, Jonan-ku, Fukuoka 814-01, Japan. inouetr@fukuoka-u.ac.jp
Tohru
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Inoue