The Static Dielectric Constant of Ionic Liquids

Abstract

The static dielectric constant of an ionic liquid is not measurable by conventional methods because the samples are largely short-circuited by their intrinsic electrical conductance. It is, however, possible to determine this quantity by recording the frequency-dependent dielectric dispersion curve in the microwave regime, followed by extrapolation to quasi-static conditions. This article compiles the information on static dielectric constants available from such experiments and discusses trends in the cation- and anion-dependence for some widely used imidazolium, pyridinium, pyrrolidinium and alkylammonium salts. The dielectric constant is little sensitive to the nature of the cation, but highly sensitive to the anion. The results classify most ionic liquids as moderately polar solvents with dielectric constants of the order of ɛ = 10–12. Ionic liquids with higher dielectric constants can be designed by introducing highly polar anions such as ethylsulfate. The results are compared with polarity parameters derived from a variety of other experiments, including among others, solvatochromic shifts of probe dyes, polarity parameters deduced from gas chromatography with ionic liquids as stationary phases, and apparent dielectric constants derived from the OH vibrational frequency of dissolved water and from solvent effects on chemical reactions.