Leaching and Reactivity at the Sodium Aluminosilicate Glass-Water Interface: Insights from a ReaxFF Molecular Dynamics

H Jabraoui and S Gin and T Charpentier and R Pollet and JM Delaye, JOURNAL OF PHYSICAL CHEMISTRY C, 125, 27170-27184 (2021).

DOI: 10.1021/acs.jpcc.1c07266

In this study, we used ReaxFF reactive force field molecular dynamics (MD) simulations to investigate the dynamics associated with reactivity at the sodium aluminosilicate (NAS) glass-water interface. By combining van Hove correlation functions and visual analysis of individual trajectories, we found that when a Na+ ion leaves its initial position at the glass surface under the effect of water, it can be replaced either by a H+ ion or by another diffused Na+, resulting in a (Na+ reversible arrow H+) ion exchange or (Na+ reversible arrow Na+) ion exchange, respectively. These rearrangements at the NAS glass-water interface take place through many events such as the formation of oxygen-bearing functional groups (silanol Si(OH), germinal silanol Si(OH)(2), and Al(OH)) and the agglomeration of Na+ ions on the glass surface. The high affinity between water and Na+ ions leads to two events, namely, the penetration of the water molecule into the first glass layer and the release of Na+ ions into the bulk water. The release of sodium ions into bulk water takes place via several successive steps: (1) leaving their original position to an area more exposed to bulk water, (2) diffusion to the surface of the glass, (3) leaving the surface toward the solution, and (4) coming back to the glass surface. Statistical analysis shows that oxygen-bearing functional group formation occurs both by protonation of the nonbridging oxygen (NBO) site and by filling defects such as Si-III, Al-III, and Al-IV by OH- resulting from the dissociation of water molecules, while in the bulk, they could be produced by the proton jumps between two adjacent NBO sites and dissociation of the water diffused in deep layers of glass. Germinal silanol groups (Si(OH)(2)) are present in small amounts in the first layer of the glass after the protonation of two NBOs from a single Si-IV. This process is accompanied by the conversion of Al-III and Al-IV units into Al-V units and Si-III units into Si-IV units. By comparing the networks of Si and Al, we found that Al is more impacted by the occurrence of water molecules than Si, which is explained by an increase in the diffusion coefficient of Al compared with Si when these units become protonated (Si(OH) and Al(OH)).

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