The Dielectric Response of Room-Temperature Ionic Liquids: Effect of Cation Variation

In continuation of recent work on the dielec. response of imidazolium-based ionic liqs. (ILs) (J. Phys. Chem. B, 2006, 110, 12682), the authors report on the effect of cation variation on the frequency-dependent dielec. permittivity up to 20 GHz of ionic liqs. The salts are comprised of pyrrolidinium, pyridinium, tetraalkylammonium, and triethylsulfonium cations combined with the bis((trifluoromethyl)sulfonyl)imide anion. The dielec. spectra resemble those obsd. for imidazolium salts with the same anion. In all cases, the major contribution results from a diffusive low-frequency response on the time scale of several 100 ps, which shows a broadly distributed kinetics similar to that of spatially heterogeneous states in supercooled and glassy systems rather than that obsd. in fluid systems. There is evidence for a weak secondary process near 10-20 ps. Perhaps the most interesting difference to imidazolium salts is founded in the missing portions of the spectra due to processes beyond the upper cutoff frequency of 20 GHz. These are lower than that obsd. for imidazolium-based salts and seem to vanish for tetraalkylammonium and triethylsulfonium salts. As for imidazolium salts, the extrapolated static dielec. consts. are on the order of eS equiv. 10-13, Classifying these ILs as solvents of moderate polarity.