Second Hydration Shell of Mg²⁺: Competition between Ion-Water Interaction and Hydrogen Bonding Interaction

Hydration in Mg²⁺ 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 Mg²⁺, 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 Mg²⁺ in MgCl₂ 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 Mg²⁺ 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 Mg²⁺. Our results highlight the importance of accurately described ion-water interaction and HB interaction in the intermediate range.