Frequency-dependent fluidity and conductivity of an ionic liquid

The frequency- and temperature-dependent shear fluidity, f(nu, T), of the ionic liquid [BMIm]BF4 is presented and compared with its ionic conductivity, sigma(nu,T). [BMIm]BF4 is short for 1-butyl-3-methyl-imidazolium tetrafluoroborate. Its DC fluidity, f(DC)(T), and DC conductivity, sigma(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, Delta = (1/T-circle times - 1/T) > 0. The observed superposition suggests that f(nu, T) should display a frequency dependence similar to sigma(nu, T-circle times). We have therefore measured f(nu, 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(nu, T) and sigma(nu, T-circle times) are Fourier transforms of quite similar time correlation functions.