Second Hydration Shell of Mg: Competition between Ion-Water Interaction and Hydrogen Bonding Interaction.

Hydration in Mg solutions is critical in the chemical and biological industries. Researchers have identified a rigid first hydration shell, but the effects of ion-water and hydrogen bonding (HB) interactions beyond the first shell of Mg, especially when ion pairs form at high concentrations, remain controversial. On the basis of density functional theory, machine learning molecular dynamics was performed to study the second shell of Mg in MgCl solutions at various concentrations. At low concentrations, while both SCAN and PBE+TS-vdW predict a rigid first shell, SCAN affords a more softened structure due to its weakened HB interaction. The weakened HB interaction reveals the retarding effect of Mg on the water reorientation of the second shell. With an increasing concentration, SCAN reproduces the decreasing trend of the hydration number, which originates from Cl entering the second shell of Mg. Our results highlight the importance of accurately described ion-water interaction and HB interaction in the intermediate range.