Thermophysical properties of [C(N-1)C(1)im][PF6] ionic liquids

Abstract

Densities, viscosities and refractive index, as a function of temperature, and isobaric thermal expansion coefficient, were determined for 1-alkyl-3-methylimidazolium hexafluorophosphate, [C(N - 1)C(1)im]PF6] (where N = 5 to 10) series of ionic liquids. The density presents a regular decrease along the ionic liquid series, with no detectable alkyl chain length dependence of the thermal expansion coefficient. Both the refractive index and viscosity show a trend shift along the series around the [C(6)C(1)im][PF6] that could be associated to the impact of the change in the nanostructuration of the ionic liquids. The experimental results are compared with the analogous [C(N - 1)C(1)im][NTf2] series and the effect of the anion is analyzed. The sphericity of the [PF6](-) anion is reflected in the lower pre-exponential VTF parameter, A(eta), and the higher viscosity of the [C(N - 1)C(1)im][PF6]. when compared with the [C(N - 1)C(1)im][NTf2] series, is ruled by the higher energy barriers, which are related with stronger electrostatic interaction due to the low charge dispersion of the [PF6](-),anion. (C) 2013 Elsevier B.V. All rights reserved.