Frequency-dependent fluidity and conductivity of an ionic liquid

Abstract

The frequency- and temperature-dependent shear fluidity, f(ν,T), of the ionic liquid [BMIm]BF₄ is presented and compared with its ionic conductivity, σ(ν,T). [BMIm]BF₄ is short for 1-butyl-3-methyl-imidazolium tetrafluoroborate. Its DC fluidity, f_DC(T), and DC conductivity, σ_DC(T), are non-Arrhenius and superimpose in an Arrhenius-type representation if the respective inverse temperature axes are made to differ by a small amount, Δ = (1/T^⊗− 1/T) > 0. The observed superposition suggests that f(ν,T) should display a frequency dependence similar to σ(ν,T^⊗). We have therefore measured f(ν,T) of [BMIm]BF₄ over five decades of frequency at different temperatures. The spectra thus obtained corroborate our expectations. We model our experimental results in terms of the MIGRATION concept and arrive at the conclusion that f(ν,T) and σ(ν,T^⊗) are Fourier transforms of quite similar time correlation functions.