Insight into the roles of two typical ion clusters and their second hydration shells: Implication for the nucleation mechanism in MgSO4 aqueous solution

In this study, the ion association characteristics and the hydration dynamics of Mg2+ ions in MgSO4 aqueous solution at various concentrations were systematically investigated using density functional theory and classical molecular dynamics simulations. The results show that contact associated (CA) and solvent separated (SS) structures of Mg2+ and SO42− ions are the dominant species and play different roles in the crystallization and gelation processes of MgSO4 aqueous solution due to their distinctive association trend and the distinctive dynamics of their second hydration shells. The CA structures are more stable and the further aggregation of these clusters is sluggish in MgSO4 aqueous solution. The supersaturation or the formation of gel state can be partially attributed to the abundance of CA structures in supersaturated MgSO4 aqueous solution. In contrast, although the ion exchange in SS structures occurs frequently, the aggregation of SS structures is active and can be greatly promoted by increasing of the concentration. The dynamics of the second hydration shells around Mg2+ ions of SS structures also becomes more active as the concentration increases. The dynamic evolution characteristics of SS structures demonstrate their aggregation tendency for nucleation in the oversaturated MgSO4 aqueous solution, which helps to understand the nucleation mechanism in MgSO4 aqueous solution and the role of ion hydration dynamics in the nucleation process.