Thermodynamics of micellization of ionic liquids C6mimBr and orientation dynamics of water for C6mimBr-water mixtures: a dielectric spectroscopy study.

Abstract

The aggregation behavior of C6mimBr and its interaction with water at different concentrations and temperatures have been investigated by dielectric spectroscopy over frequency ranges from 500 MHz to 40 GHz. Dielectric determinations show that micellar aggregations form when the C6mimBr concentration is higher than 0.85 M, and the size of micellar aggregations varies inversely to temperature. The thermodynamic quantities of the micellization at different temperatures, ΔGm, ΔHm, and ΔSm were calculated, and it was found that the main driving forces of C6mimBr aggregation were controlled by different thermodynamic quantities at different temperatures: the micelle formation process is controlled by an enthalpy effect at low temperature while it is entropically driven at high temperature. Two remarkable relaxations that originate from the orientation polarization of "bulk water" and "bound water" interacting with C6mimBr were observed at about 8.2 and 15 GHz. The relaxation parameters obtained by fitting the dielectric spectra data were used to estimate the number of bound water per C6mim(+) in the micellar core. The enthalpy and entropy closely linked to the relaxation processes of bound water and bulk water were calculated using the relaxation time according to Eyring equations. The average number of hydrogen bonds of a C6mimBr-water system in different microenvironments was calculated, and the essence of cooperative orientation dynamics of water was described.