Comprehensive Molecular-level Understanding of MgO Hydration through Computational Chemistry

TL;DR

This study addresses the molecular mechanism of MgO hydration to Mg(OH)$_2$ by modeling MgO(100) with a two-layer MgO/water interface using potential-scaling MD (PS-MD) and first-principles refinements. It reveals that dissociative water adsorption precedes Mg$^{2+}$ dissolution, which can be exothermic if vacancy stabilization is achieved by proximal protons, and proceeds via a highly heterogeneous, HB-network–driven process with a typical barrier of $12$ kcal/mol. Mg(OH)$_2$ nucleation occurs through dissolution-precipitation of Mg$^{2+}$ ions into the aqueous layer, forming Mg-OH chains that serve as nuclei, with well-ordered crystalline nuclei emerging in bulk water environments. The findings support the dissolution-precipitation mechanism as the dominant pathway and provide a foundational molecular framework for this class of complex solid-surface reactions, with implications for material design and industrial processes.

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.

Figures (8)

• Figure 1: Representative MgO/water interfacial structures optimized at the DFT level (upper panels) and their corresponding $z$ coordinate (Å) distributions for Mg, O, and H atoms (lower panels) at various stages of the PS-MD cycle. The structures in the upper panels are optimized geometries from specific PS-MD cycles: (a) 0, (b) 45, (c) 92, (d) 162, (e) 205, and (f) 310. The distributions in the lower panels are averaged over five structures within the respective PS-MD cycle ranges: (a) 0–32, (b) 41–47, (c) 88–97, (d) 156–162, (e) 201–205, and (f) 306–310. In the upper panels, Mg, O, and H atoms are colored gray, red, and white, respectively, while, in the lower distribution plots, the lines for Mg, O, and H are red, green, and blue, respectively. Both panels display the interface above the second MgO layer from the bottom. Bonds between Mg and O atoms are shown for distances shorter than 2.3 Å. The $z$ coordinate is measured relative to the peak position of the outermost MgO layer.
• Figure 2: (a) Number of dissolved Mg$^{2+}$ ions (red) and adsorbed/penetrated protons (green) along the PS-MD cycle. The number of Mg$^{2+}$ ions in the second water layer (blue), whose $z$ coordinates are greater than 2.75 Å, is also shown. (b) Coordination number of dissolved Mg$^{2+}$ ion along the PS-MD cycle. The total coordination number is shown in red, and contributions to that from OH$^-$ (green), H$_2$O (blue), and MgO surface oxygen atoms (O$^*$; magenta) are also shown. The dotted lines are guides for the eye. Similar results from the other four runs are presented in Figures \ref{['figS:dMg_pH_others']} and \ref{['figS:ngroups_others']}. Note that data points are not shown for PS-MD cycles where the optimized structure is less stable than the initial one (see Section 2 in Supporting Information for details).
• Figure 3: (a) Fraction of the number of hydrogen bonds (HBs) per molecule in the first (red) and second (green) water layers. (b) Two-dimensional probability distribution of the orientation angles $\theta$ ($^{\circ}$) and $\varphi$ ($^{\circ}$), for water molecules in the first layer, where a darker color indicates higher probability. (c) Key hydrogen-bonded configuration involved in water dissociation. In panel (c), the blue bond represents the elongated O-H bond (1.05 Å), while the green (1.54 Å) and cyan (2.20 Å) dotted lines represent the HBs within the H$_2$O$\cdots$OH$^-$$\cdots$HO$^*$ complex. Note that the oxygen and hydrogen atoms connected by the cyan dotted line originally formed the downward O-H bond of a single water molecule before its dissociation.
• Figure 4: (a) Two-dimensional probability distribution of the octahedral order parameter Zimmerman2015 for the solvated Mg$^{2+}$ complex as a function of its $z$ coordinate calculated by the five independent runs. A darker color indicates higher probability. (b) Reaction energy (kcal/mol) plotted against the $z$ coordinate of the Mg$^{2+}$ ion obtained from the nudged elastic band calculations. Only dissolution processes with a locally stable product structure are used.
• Figure 5: (Upper) Boxplot of the reaction energy ($\Delta E$, kcal/mol) for the Mg$^{2+}$ dissolution process classified by (a) the number of ligands (OH$^{-}$ ions and H$_2$O molecules) coordinated to the dissolved Mg$^{2+}$ ion, $N_{\mathrm{ligand}}$, (b) the number of protons adsorbed on O$^*$ atoms adjacent to the vacancy generated by the dissolution, $N_{\mathrm{proton}}$, and (c) the sum of the number of the ligands and protons. The distance cutoffs for Mg$^{2+}$-O coordination and O$^*$-H$^+$ bonding are set to 2.5 Å and 1.2 Å, respectively. The red and black circles indicate mean reaction energy and outlier in the category, respectively, and the blue squares are used when there are less than four samples. (Lower) Change in the projected density of states of the dissolved Mg$^{2+}$ ion (d) and the O$^*$ atoms originally coordinated to the dissolved Mg$^{2+}$ ion (e) for the dissolution process. The density of states of the Mg and O$^*$ atoms are projected onto their (3s, 3p) and (2s, 2p) orbitals, respectively. The changes according to the process with negative and positive reaction energies are colored red and green, respectively. In both panels, the reference energy is set to the vacuum level in the slab model.