論文

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.