Effects of ionic liquids on microstructure and thermal stability of microemulsions by broadband dielectric spectroscopy

Abstract

Phase behaviors of TX-100/[bmim][PF6]/water and TX-100/[bmim][BF4]/cyclohexane systems were investigated by dielectric relaxation spectroscopy. The dielectric analysis of the two systems showed that there were two relaxations in both systems at 1 ns and 50 ps, besides, all of them were originated from superimposed dipole polarization although induced by different groups. The relaxation parameters, which were obtained by fitting Cole-Cole equations to the dielectric data, were used to discuss the phase behaviors and percolation dynamics of microemulsions. It was found that the TX-100/[Bmim][PF6]/ water system presented a temperature-dependent phase transition, while the TX-100/[Bmim][BF4]/ cyclohexane system was not sensitive to temperature in the measured range, which proved that the hydrophilic/hydrophobic ionic liquid has a great influence on the thermal stability of microemulsion. For the TX-100/[bmim][PF6]/water system, the critical percolation temperature Tp was obtained by analysing the conductivity properties and verified by analyzing dielectric increment and relaxation time. Besides, according to percolation theory, there is the dynamic percolation process occured and caused the phase transion of TX-100/[bmim][PF6]/water microemulsion, however, that doesn't occurred in TX-100/[Bmim][BF4]/ cyclohexane system, which caused the different situation between two microemulsions in the rising the temperature. This work provides a new insight into the phase transition of sub-regions of IL-based microemulsion.