A theoretical study on the anomalous pressure dependence of the transport properties of ionic liquids: Comparison among lithium bromide, silica, and water

The transport coefficients of three ionic liquids, lithium bromide ~LiBr!, rubidium bromide ~RbBr!, and molten silica (SiO2) are calculated by the mixture mode-coupling theory. The static partial structure factors required are obtained from the interionic interaction potential by the Ornstein– Zernike/hypernetted-chain integral equation. The anomalous pressure dependence of the transport properties, the increase in the molar ionic conductivity of LiBr and the fluidity of SiO2 in the low-pressure region, is reproduced qualitatively well by our theoretical calculation. The calculated results are analyzed in the similar way as that for water performed by Yamaguchi et al. @T. Yamaguchi, S.-H. Chong, and F. Hirata, J. Chem. Phys. 119, 1021 ~2003!#, and we found that the friction on the electric current caused by the coupling between the chargeand number-density modes is effective to the increase of the transport coefficient with pressure, as is the case of water. Comparing the results for LiBr and RbBr, the contribution of the electrostatic friction is smaller for RbBr, which leads to the normal pressure dependence of its molar ionic conductivity. The negative values of the Nernst–Einstein deviation parameter for the ionic conductivity of LiBr and SiO2 reported by previous MD simulations are also explained consistently. Furthermore, it is shown that the mechanism for the anomalous pressure dependence of the fluidity of molten SiO2 demonstrated in this work is consistent with a conventional picture that the five-coordinated silicon atom is important to enhance the ionic mobility. © 2003 American Institute of Physics. @DOI: 10.1063/1.1622652#