Comprehensive Molecular-level Understanding of MgO Hydration through Computational Chemistry

TL;DR

This study uses advanced computational methods to reveal the detailed molecular mechanism of MgO hydration, including water adsorption, Mg2+ dissolution, and Mg(OH)2 nucleation, providing fundamental insights into this complex solid-surface reaction.

Contribution

It offers the first detailed molecular-level elucidation of MgO hydration mechanisms using combined molecular dynamics and first-principles calculations, without prior assumptions.

Findings

Mg2+ dissolution follows dissociative water adsorption

Dissolution can be exothermic even on defect-free surfaces

Mg(OH)2 nucleation proceeds via dissolution-precipitation, not direct solid-state transformation

Abstract

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 multi-step 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 Mg$^{2+}$ 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 $\sim$12 kcal/mol. This exothermicity depends crucially on the stabilization of the resulting surface vacancy, achieved by proton adsorption onto neighboring surface oxygen atoms. Further Mg$^{2+}$ 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, Mg$^{2+}$ 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 multi-step solid-surface chemical reactions.