Second Hydration Shell of Mg<sup>2+</sup>: Competition between Ion–Water Interaction and Hydrogen Bonding Interaction
Weiyu Li, Zhaoru Sun
Abstract
High Resolution Image Download MS PowerPoint Slide Hydration in Mg 2+ 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 2+, 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 2+ in MgCl 2 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 2+ 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 2+ . Our results highlight the importance of accurately described ion–water interaction and HB interaction in the intermediate range.