MD simulation and evaluation of the self-diffusion coefficients in aqueous NaCl solutions at different temperatures and concentrations

Abstract

Aqueous NaCl solutions are the most common electrolyte with a wide range of applications. One of the most important transport properties of aqueous electrolytes are the self-diffusion coefficients. In this study, molecular dynamics (MD) simulations of aqueous NaCl solution are carried out to study the effect of temperature, which is not often reported in the literature, and also of salt concentration on the self-diffusion coefficients. The self-diffusion coefficients of water, Na+ and Cl− are calculated from the long-time limit of the mean square displacements (MSD) and the results are validated with experimental results from the literature. The simulation results show that the self-diffusion coefficients of water, Na+ and Cl− increase with increasing temperature, whereas they decrease with increasing salt concentration. The temperature effect on the self-diffusion coefficient is more obvious at low concentrations. The radial distribution functions for all atom pairs in the NaCl electrolyte are plotted and the effect of temperature on these functions is investigated. The effects of temperature and concentration on the ion hydration numbers and the Na+–Cl− coordination numbers are also studied. Furthermore the self-diffusion coefficient of the NaCl salt is calculated in two ways, directly from the simulation, and also from the Cussler equation, which has been proposed to evaluate the average value of electrolyte diffusion coefficient from the diffusion coefficients of the constituent ions. A good consistency was observed between the obtained results with the two methods.