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

摘要

The transport coefficients of three ionic liquids,lithium bromide (LiBr),rubidium bromide (RbBr0,and molten silica (SiO_2) 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 SiO_2 in the low-pressure region,is reproduced qualitatively well by our theoretical calculation.The claculated 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 w found that the friction on the electric current caused by the coup0ling between the charge- nd 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 SiO_2 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 SiO_2 dmonstrated in this work is consistent with a conventional picture that the five-coordinated silicon atom is important to enhance the ionic mobility.