Frequency-dependent fluidity and conductivity of an ionic liquid.

The frequency- and temperature-dependent shear fluidity, f(nu,T), of the ionic liquid [BMIm]BF(4) is presented and compared with its ionic conductivity, sigma(nu,T). [BMIm]BF(4) 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(multiply sign in circle)- 1/T) > 0. The observed superposition suggests that f(nu,T) should display a frequency dependence similar to sigma(nu,T(multiply sign in circle)). We have therefore measured f(nu,T) of [BMIm]BF(4) 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(multiply sign in circle)) are Fourier transforms of quite similar time correlation functions.