engleski

Ion dynamics in ionic liquids: Correlation between charge transport and shear flow

The ion dynamics in three imidazolium-based ionic liquids (ILs), BMIm-BF4, HMIm-BF4 and BMIm-PF6 have been investigated by measuring the frequency-dependent conductivity and shear fluidity (inverse viscosity). For all ILs, DC conductivity and DC fluidity show non-Arrhenius temperature dependence and superimpose exactly, if their respective inverse temperature axes are slightly shifted. Strikingly, fluidity taken at a particular temperature corresponds to conductivity taken at a slightly lower temperature, not only for the DC values, but in the entire frequency range studied. For BMIm-BF4, these aspects have been analyzed within the framework of the MIGRATION concept. We find that the same activation energy for the elementary displacement of an individual ion is involved in both flow of charge and shear flow. However, in order to contribute to charge transport or shear flow, this elementary step of an ion has to be followed by suitable movements of neighbouring ions. For shear flow, these latter movements need to fulfil more specific requirements than for flow of charge, which results in slower ion dynamics. In particular, we are able to show that replacement of the anion (BMIm-PF6) rather than the cation (HMIm-BF4) has a larger effect in further slowing down the ion dynamics of shear flow. These findings and their implications will be presented in detail.

Ionic liquid; Ionic conductivity; Shear fluidity

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