Temperature dependence of the dielectric properties and dynamics of ionic liquids

Abstract

Dielec. spectra were measured for eight, mostly imidazolium-based, room temp. ionic liqs. (RTILs) over a wide range of frequencies (0.2 ≤ ν/GHz ≤ 89) and temps. (5 ≤ θ/°C ≤ 65). Detailed anal. of the spectra shows that the dominant low frequency process centered at ca. 0.06 to 10 GHz (depending on the salt and the temp.) is better described using a sym. broadened Cole-Cole model rather than the asym. Cole-Davidson models used previously. Evaluation of the temp. dependence of the static permittivities, effective dipole moments, vols. of rotation, activation energies, and relaxation times derived from the dielec. data indicates that the low frequency process cannot be solely due to rotational diffusion of the dipolar imidazolium cations, as has been thought, but must also include other contributions, probably from cooperative motions. Anal. of the Debye process obsd. at higher frequencies for these RTILs is not undertaken because it overlaps with even faster processes that lie outside the range of the present instrumentation.