Molecular dynamics study of ion clustering in concentrated electrolyte solutions for the estimation of salt solubilities

The structure and formation of ion clusters for different 1:1 electrolytes in aqueous solution were evaluated over a wide concentration range using molecular dynamics (MD) simulations. Six molecular models for alkali halides from the literature were investigated – four different models for NaCl, one model for KCl, and one model for NaI – all in combination with the SPC/E or TIP4P-Ew water models. For all model combinations, solubility data from thorough solubility calculations are available in the literature. Since such calculations are challenging and computationally very demanding, in this work we propose a simple approach for solubility estimation, which can be used on the fly or as a simple post-processing step in standard MD simulations. This approach is based on analyzing the ion clusters in solution using a clustering algorithm from the literature. In essence, we suggest that the solubility is reached at the concentration for which clusters containing six or more ions are found in solution. We compare our approach to an empirical rule for solubility estimation from the literature, which is based on the number of contact ion pairs only. For the investigated molecular models, our new approach yields results closer to the solubility data reported in the literature compared to the empirical rule from the literature. Altogether, considering its simplicity and low computational cost, it provides reasonable solubility estimates that can, e.g., also be used during the development of new models, which would be infeasible with thorough solubility calculations. Furthermore, the cluster analysis provided interesting insights into the possible formation of a hydrate in the case of the investigated NaI model.