Reliability and limitations of the Hubbard-Onsager continuum dielectric friction theory for the limiting ionic mobility in sub- and supercritical water

Abstract

The validity of the Hubbard-Onsager (HO) continuum dielectric friction theory for the limiting ionic conductance is examined by its application to LiCl, NaCl, and KCl in sub- and supercritical water. At medium and higher densities (above 1.4 ϱ c = 0.45 g cm −3; ϱ c, the water critical density) and lower temperatures (below 0.9 T c = 300°C; T c, the critical temperature), the HO theory predicts an increase in the limiting conductance with increasing temperature and decreasing density. The HO theory explains the general trends of the experimental results along the liquid-vapor coexistence curve, the isochors, and the isotherms. The agreement between the theory and experiment becomes more quantitative at higher temperatures. This indicates that in these conditions the translational friction on a moving ion is dominated by the dielectric friction because the HO radius, the solvent scaling length, is larger than the ionic radius due to the small dielectric constant. At medium densities (slightly above 1.4 ϱ c) and higher temperatures (above 0.9 T c), the HO theory can reproduce the experimental observation that the limiting conductance increases with decreasing density with rather small temperature dependence. At lower densities (below 1.4 ϱ c) and supercritical temperatures, however, the HO theory fails to reproduce the density dependence of the limiting electrolyte conductance. With respect to the density dependence, the experimental limiting conductance shows a maximum or plateau at ∼ 1.4 ϱ c, whereas the theoretical limiting conductance increases monotonously with decreasing density as in the higher density regions. Thus the continuum model becomes invalid at densities around and below the critical density of water (< 1.4 ϱ c).