Field-driven Simulations to Probe the Impact of Ionic Correlations on Salt Transport Properties under Mixed-Salt Conditions

The transport of ionic solutes, especially in water-rich environments, has mostly been assumed to arise from the uncorrelated motion of cations and anions under applied fields. This forms the basis of the so-called Nernst-Einstein approximation, which assumes that cations and anions move independently of each other. However, ions exhibit significant electrostatic interactions leading to well-known phenomena such as ion aggregation in solution or condensation within polymeric systems, which are not accounted for within such an assumption. In recent years, the Onsager irreversible thermodynamic framework has been increasingly applied to measure ionic correlations (Onsager coefficients), which capture the degree of correlation between the motion of ions of the same or different species. However, such work has generally focused on statistically challenging equilibrium methods and systems used in electrically-driven processes, i.e., battery electrolytes. In this study, we apply efficient field-driven non-equilibrium atomistic molecular dynamics simulations to probe the impact of ionic correlations on mutual diffusion coefficients in aqueous mixed-salt systems. We observe that such methods are able to reproduce experimentally observed trends for ternary diffusion coefficients as a function of salt mole fraction. In future work, the methods will be applied to neutral and charged polymer membranes equilibrated with mixed-salt solutions.