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.

中文翻译：

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Mg2+ 的第二水合壳层：离子-水相互作用和氢键相互作用之间的竞争


Mg²⁺ 溶液中的水合在化学和生物工业中至关重要。研究人员已经确定了刚性的第一水合壳层，但离子-水和氢键 （HB） 相互作用在 Mg²⁺ 的第一层之外的影响，尤其是当离子对以高浓度形成时，仍然存在争议。在密度泛函理论的基础上，进行机器学习分子动力学，研究不同浓度的 MgCl₂ 溶液中 Mg²⁺ 的第二层。在低浓度下，虽然 SCAN 和 PBE+TS-vdW 都预测刚性第一层，但由于 HB 相互作用减弱，SCAN 提供了更柔软的结构。减弱的 HB 相互作用揭示了 Mg²⁺ 对第二壳层水重新定向的延迟作用。随着浓度的增加，SCAN 再现了水合数的下降趋势，该趋势源自 Cl^– 进入 Mg²⁺ 的第二个壳层。我们的结果强调了准确描述离子-水相互作用和中间范围内 HB 相互作用的重要性。