Thermophysical properties of [CN − 1C1im][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, [CN−1C1im][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 [C6C1im][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 [CN−1C1im][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, Aη, and the higher viscosity of the [CN−1C1im][PF6], when compared with the [CN−1C1im][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.