Comprehensive Molecular-Level Understanding of MgO Hydration through Computational Chemistry

The hydration of magnesium oxide (MgO) to magnesium hydroxide (Mg(OH)₂) is a fundamental solid-surface chemical reaction with significant implications for materials science. Yet its molecular-level mechanism from water adsorption to Mg(OH)₂ nucleation and growth remains elusive due to its complex and multistep nature. Here, we elucidate the molecular process of MgO hydration based on structures of the MgO/water interface obtained by a combined computational chemistry approach of potential-scaling molecular dynamics simulations and first-principles calculations without any a priori assumptions about reaction pathways. The result shows that the Mg²⁺ dissolution follows the dissociative water adsorption. We find that this initial dissolution can proceed exothermically even from the defect-free surface with an average activation barrier of ∼12 kcal/mol. This exothermicity depends crucially on the stabilization of the resulting surface vacancy, achieved by proton adsorption onto neighboring surface oxygen atoms. Further Mg²⁺ dissolution then occurs in correlation with proton penetration into the solid. Moreover, we find that the Mg(OH)₂ nucleation and growth proceeds according to the dissolution–precipitation mechanism, rather than a solid-state reaction mechanism involving a direct topotactic transformation. In this process, Mg²⁺ ions migrate away from the surface and form amorphous Mg–OH chains as precursors for Mg(OH)₂ nucleation. We also demonstrate that sufficient water facilitates the formation of more ordered crystalline nuclei. This computational study provides a comprehensive molecular-level understanding of MgO hydration, representing a foundational step toward elucidating the mechanisms of this class of complex and multistep solid-surface chemical reactions.