Combined analysis of self-diffusion, conductivity, and viscosity data on room temperature ionic liquids

Abstract

The self-diffusion coefficient of anions and cations, the ionic conductivity and the viscosity of four room-temperature ionic liquids, 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF 4), 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMITFSI), 1-butylpyridinium tetrafluoroborate (BPBF 4), and 1-butylpyridinium bis(trifluoromethylsulfonyl)imide (BPTFSI) were analysed within a comprehensive ionic transport model. The experimental data, covering wide temperature ranges, were taken from the work of Noda et al. [J. Phys. Chem. B, 105, 2001]. To compare the dc conductivity with the self-diffusion coefficients, the former quantity was converted into a charge diffusivity D σ using the Nernst–Einstein equation. In a similar way, we calculated a viscosity-related diffusivity D η with the aid of the Stokes–Einstein equation. Taking three mobile species into account, i.e., single cations, single anions and neutral ion pairs, our model yields their individual diffusivities that obey a uniform Vogel–Tammann–Fulcher-like temperature dependence described by common values of the parameters B and T 0 . Evaluating simultaneously all experimental data within this model yields the contribution of ion pairs to the self-diffusion coefficient of cations and anions separately. We further obtain the temperature-independent cation transference number and the effective hydrodynamic radius, which are important characteristics of charge and mass transport in these ionic liquids.