Cation and anion sizes influence in the temperature dependence of the electrical conductivity in nine imidazolium based ionic liquids

Abstract

In this paper we present experimental data on the temperature dependence of the electrical conductivity, σ, in nine different imidazolium based ionic liquids. We have measured four 1-(alkyl chain)-3-methyl imidazolium tetrafluoroborate (CnMIM-BF4) ionic liquids, with Cn representing ethyl, butyl, hexyl and octyl chains, to study the dependence of σ with the cation length. Moreover, to study the influence of the anion size in the electrical conductivity, we measured six different EMIM-X, with X being, from smaller to bigger sizes, Cl−, Br−, BF4−, PF6−, ethyl sulfate and tosylate. The measurements were performed at atmospheric pressure, and the studied temperature range covers the liquid phase of the analyzed compounds. We have fitted the electrical conductivity data of the nine ionic liquids using a Vogel–Tamman–Fulcher (VTF) equation with high precision. We observe from the measured data that the electrical conductivity decreases its value as the alkyl chain of the cation increases. In contrast, we do not observe that dependence with the anion size, where there seems to be an optimal size (that of BF4−) for which σ reaches its maximum value, being lower for smaller or bigger anion sizes. Finally, if we plot the natural logarithm of σ versus the distance in temperature to the glass transition one for each IL, we observe that the resulting straight lines are ordered with the anion (or cation) sizes for all nine compounds measured, i.e., lower σ values for bigger sizes.