How Polar Are Ionic Liquids? Determination of the Static Dielectric Constant of an Imidazolium-based Ionic Liquid by Microwave Dielectric Spectroscopy

Abstract

In a pilot study of the dielectric constant of room-temperature ionic liquids, we use dielectric spectroscopy in the megahertz/gigahertz regime to determine the complex dielectric function of five 1-alkyl-3-methylimidazolium salts, from which the static dielectric constant epsilon is obtained by zero-frequency extrapolation. The results classify the salts as moderately polar solvents. The observed epsilon-values at 298.15 K fall between 15.2 and 8.8, and epsilon decreases with increasing chain length of the alkyl residue of the cation. The anion sequence is trifluoromethylsulfonate > tetrafluoroborate approximately tetrafluorophosphate. The results indicate markedly lower polarities than found by spectroscopy with polarity-sensitive solvatochromic dyes.

Ionic Liquids—Progress on the Fundamental Issues

“…The pattern in the measurements indicates 'static' dielectric constants between 10 and 15 in most of these liquids, [13] somewhat lower than estimates on the basis of effective polarity would seem to indicate. [14] However, these measurements are carried out at GHz frequencies and 'static' in this context is any timescale longer than about 1 ns.…”

Ionic Liquids in Separation Techniques

“…The anion sequence is trifluoromethylsulfonate > tetrafluoroborate ≈ tetrafluorophosphate. The results indicate markedly lower polarities than the ones found by spectroscopy with polarity-sensitive solvatochromic dyes ( Wakai et al, 2005).…”

“…To compare the collected parameters with solvents commonly used in analytical chemistry, methanol, water and acetonitrile have been added at the end of the above-mentioned Table 1. Wakai and coworkers ( Wakai et al, 2005) determined the static dielectric constants of 1-alkyl-3-methylimidazolium ionic liquids by microwave dielectric spectroscopy in the megahertz/gigahertz regime. The obtained results classify the ILs as moderately polar solvents.…”

Communication: Are hydrodynamic models suited for describing the reorientational dynamics of ions in ionic liquids? A case study of methylimidazolium tetra(hexafluoroisopropoxy)aluminates

“…spectral range is limited at low frequencies by interfacial polarizations at the walls of the sample cell. 12 The dispersion curve Ј͑ ͒ covers, however, a sufficiently large segment of the putative decay from the static limit ͑"dielectric constant"͒ to the infinite frequency limit ϱ due to electronic polarization to enable accurate parametrization. Due to the high electrical conductivities of the solutions ͑Table I͒ the corresponding dielectric loss Љ͑ ͒ is swamped by the 1 / -dependent Ohmic loss.…”