Broadband Conductivities and Fluidities of Fragile Ionic Liquids

Abstract

Conductivity and fluidity spectra of two room temperature ionic liquids, BMIm-BF4 and HMIm-BF4, taken at different temperatures, are presented and compared with each other. BMIm-BF4 and HMIm-BF4 are short for 1-butyl-3-methyl-imidazolium tetrafluoroborate and 1-hexyl-3-methyl-imidazolium tetrafluoroborate, respectively. In their temperature and frequency dependences, flow of charge and shear flow are found to display close similarities. The distinguishing features of the ion dynamics in fragile ionic liquids as compared to ionic solids are seen to arise from the absence of a network that provides pre-formed vacant sites for the ions to move in. In ionic liquids, the dispersive behaviour of the frequency-dependent conductivity is thus characterized by a temperature-independent end frequency. This key feature, along with Arrhenius activated elementary displacements, is shown to yield the well-known non-Arrhenius temperature dependence of the DC conductivity. In Arrhenius-type plots, DC conductivity and fluidity can be superimposed over the entire temperature range above the glass transition. As a consequence, ionic conduction and shear flow can be traced back to elementary displacements with identical activation energy. On the inverse temperature axis, a small positive shift, Δ=(1/T⊗−1/T), is required for the superposition. Remarkably, fluidities taken at temperature T correspond to conductivities taken at the slightly lower temperature T⊗ not only at DC, but in the entire frequency range studied. Indeed, according to the data as well as to our modelling, the entire conductivity and fluidity spectra superimpose when considered at T⊗ and T, respectively, with identical values of their end angular frequency. The still unknown time correlation function for shear flow is hence expected to have significant features in common with the current density autocorrelation function.