Insights into water-mediated ion clustering in aqueous CaSO solutions: pre-nucleation cluster characteristics studied by ab initio calculations and molecular dynamics simulations.

The molecular structure of growth units building crystals is a fundamental issue in the crystallization processes from aqueous solutions. In this work, a systematic investigation of pre-nucleation clusters and their hydration characteristics in aqueous CaSO solutions was performed using ab initio calculations and molecular dynamics (MD) simulations. The results of ab initio calculations and MD simulations indicate that the dominant species in aqueous CaSO solutions are monodentate ion-associated structures. Compared with charged ion clusters, neutral clusters are more likely to be present in an aqueous CaSO solution. Neutral (CaSO) clusters are probably the growth units involved in the pre-nucleation or crystallization processes. Meanwhile, hydration behavior around ion associated species in aqueous CaSO solutions plays an important role in related phase/polymorphism selections. Upon ion clustering, the residence of some water molecules around Ca in ion-associated species is weakened while that of some bridging waters is enhanced due to dual interaction by Ca and SO. Some phase/polymorphism selections can be achieved in aqueous CaSO solutions by controlling the hydration around pre-nucleation clusters. Moreover, the association trend between calcium and sulfate is found to be relatively strong, which hints at the low solubility of calcium sulfate in water.