Self-Diffusion Coefficient of Water in the Presence of Alkali Halide Ions

The self-diffusion coefficient of water in aqueous solutions of alkali halides at different concentrations was studied using the polarizable BK3 model of water [1]. As ion potentials, initially the alkali halide force field fitted to this water potential by the solvation free energy, ion-water clusters as well as, for the purpose of crosschecking, to the ambient crystal energy and density was investigated. It was found that the strength of attraction between the central ion and the water molecules in the hydration shell is stronger than in reality, which manifests itself in the reduced mobility of water. For small ions, this discrepancy is small. By omitting the dipolar polarization of larger ions, the estimates could be improved. Similarly, increasing the size of the anions within reasonable limits also enhances the results but the structural breaking effect is insufficient. If the relative permittivity of water is smaller than that measured experimentally, typically ionic charges in non-polarizable water models can be decreased to recover the correct semi-macroscopic Coulomb energy. Since our polarizable water model reasonably estimates the relative permittivity, smaller ionic charges were used to quantify the differences in the strengths of ion-water attraction. Our estimates of self-diffusion in aqueous solutions by varying the concentration of ions are presented and discussed for LiCl, NaCl, NaBr, NaI, RbF and RbI.