J. Vila,^a C. Franjo,^a J.M. Pico,^a L.M. Varela,^b O. Cabeza*,^a

^a Dpto. de Física, Fac. de Ciencias, Universidade da Coruña. Campus da Zapateira s/n,

15072 A Coruña, SPAIN

^b Grupo de Nanomateriales y Materia Blanda. Dpto. de Física de la Materia Condensada. Fac. de Física, Universidad de Santiago de Compostela. E-15782. Santiago de Compostela. SPAIN

Abstract

In this paper we present experimental measurements of the temperature dependence of the electrical conductivity, s, in four ionic liquid compounds (ILs) in both the liquid and solid states and at atmospheric pressure. The chemicals measured are composed by the 1-ethyl-3-methyl-imidazolium (EMIM⁺) cation, which has been combined with four different anions: Cl^-, Br^-, BF₄^- and ethyl sulfate (ES^-). In the liquid state, the temperature dependence of s, for the four ILs follows the Vogel-Tamman-Fulcher (VTF) equation with high precision. Around the transition between the solid and liquid states, the electrical conductivity of the three ILs with lighter anions presents an hysteresis loop, that can be explained as the apparition of supercooled liquid. In contrast, the EMIM-ES presents a smooth transition, without any jump in the s, value or any hysteresis loop (probably because its melting point is reported to be below the minimum temperature measured by us). Finally, the jump in s, is not related with the glass transition because its temperature value is well below the minimum temperature measured, and at that glass transition temperature the s, value is below the resolution of our conductivity meter (2 nS/cm).

Keywords: ionic liquids, electrical conductivity, temperature, phase transition, hysteresis

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* Corresponding author. E-mail address: oscabe@udc.es