Self-diffusion and structure of monovalent ions in two dimensions: A molecular dynamics study

The knowledge of the self-diffusion coefficient is indispensable for different processes in chemical engineering and in the design of new materials, which sometimes occur in restricted or confined geometries. In this work we studied the self-diffusion and structural properties of monovalent ions moving in two dimensions. The two-dimensional soft primitive model was used to describe the interaction between ions and molecular dynamics simulations of the electroneutral mixture were performed to calculate the properties. Radial distribution functions, density-density and charge-charge correlation functions as well as the mean square displacement, the velocity autocorrelation function and the diffusion coefficient were evaluated at supercritical conditions. For the lowest reduced temperature, the formation of clusters, chains and rings of ions influenced the cation-anion radial distribution function, particularly at low densities, where very high values of the first maximum were observed. Clusters and free ions were evaluated. These arrays also influence the density-density and charge-charge correlation functions. Concerning the self-diffusion, it increases as temperature is higher and decreases when density increases, as physically expected. The behavior of the self-diffusion coefficient was analyzed by trying different fitting models. We found that the density dependence is well described by a third-degree polynomial or a linear-logarithmic fit. When the simultaneous dependence with density and temperature is considered, we found that the diffusion is a unique function of the variable ργ/T, where the exponent γ is about 1.7, in contrast with three-dimensional m − 6 Lennard-Jones fluids, where it has been found to be in the range 3.4–13.5. We also investigated the type of diffusive motion. The features observed in both, the velocity autocorrelation function and the mean square displacement, indicated that the diffusion of the ions takes place as normal diffusion, in contrast with other two-dimensional fluids, where superdiffusive and subdiffusive motion have been observed.