Solvent isotope effect on ion mobility in water at high pressure. Conductance and transference number of potassium chloride in compressed heavy water

Limiting molar conductances of the K+ and Cl− ions in heavy and light water have been determined at 25 °C as a function of pressure up to 2 kbar from the measured conductances and transference numbers of KCl. The residual friction coefficients (Δζ) are obtained for the cation and the anion in D2O and H2O by using the determined limiting conductance and the bulk viscosity of solvent and compared with the corresponding values predicted by applying the Hubbard–Onsager (HO) dielectric friction theory at various pressures below 1 kbar. At atmospheric and high pressures Δζ of the K+ ion in D2O is larger than that in H2O just as predicted by the HO theory, but Δζ of the Cl− ion in D2O is smaller than that in H2O on the contrary to the theoretical prediction. The reverse solvent isotope effect on Δζ(Cl−) suggests that a microscopic viscosity in the vicinity of the relatively large ion is smaller than the bulk viscosity used in the continuum theory.