The ionic liquid BmimBr: a dielectric and thermal characterization

Abstract

The thermal and dielectric properties of ionic liquid 1-butyl-3-methylimidazolium bromide (BmimBr) were measured in the glassy, supercooled liquid and equilibrium liquid states, covering the temperature range from −120 to 100 °C. From thermally stimulated depolarization current (TSDC) data we determined the glass transition temperature, that was in very good agreement with the calorimetric one (T_g = −48 °C). The kinetic parameters associated to the main relaxation, i.e. activation energy and the pre-exponential factor, were also estimated. From dielectric relaxation spectroscopy (DRS), two sub-T_g relaxations were clearly detected. One of them presents a good correlation with that also detected using TSDC, and exhibiting characteristics compatible with those of a genuine Johari–Goldstein secondary process. Additionally, the temperature dependence of d.c. conductivity obeys the Vogel–Fulcher–Tammann–Hesse (VFTH) equation. The degree of decoupling between charge transport and the structural α-relaxation was also estimated for BmimBr.