Frequency-dependent fluidity and conductivity of an ionic liquid

Abstract

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.

First and Second Universalities: Expeditions Towards and Beyond

“…Indeed, the first-universality-type scaled conductivity, see Fig. 21, is very well reproduced with this value [24,25].…”

“…In this section, we show that a more coherent view is provided by considering the information contained in the conductivity spectra, which display the features of the first universality [82,83]. Even more remarkably, the same line of reasoning has been found to be valid for shear fluidities (inverse shear viscosities), which have many properties in common with ionic conductivities, not only regarding their temperature dependence at low frequencies, but also regarding their first-universality-type frequency dependence at fixed temperatures [24,25].…”

“…(22) and Eq. (23) are considered valid, and the same value, E * ≈ 0.20 eV, results for the activation energy for elementary displacements when derived from DC conductivities and DC fluidities [24,25].…”

“…The surprising statement made by the first universality is that very much the same function or "master curve" is obtained for a large number of ion conductors which largely differ in their structures. These include level-three crystals [10,19], glassy [10,18,20] and polymer electrolytes [21,22], and even ionic melts [23] and liquids [24,25]. Figure 2 exemplifies the meaning of "second universality".…”

Supramolecular Ionic‐Liquid Gels with High Ionic Conductivity

“…[22,23] The latter concept of strong interactions and correlated ion dynamics was also highlighted in astudy on the relationship between charge transport and shear flow in [BMIm] [BF 4 ]. [24] On the other hand, the introduction of gelator molecules into an IL and the gelationp rocess involve ion-gelator interactions that depend on the nature of the IL ions and gelator molecule. Therefore, the question arises how the oxalamide gelator influences the ionic interactions in gels.…”