Hydration effects on a magnesium silicate glass by using a combination of classical approach and ab-initio Metadynamics
Résumé
The hydration of silica glass leads to the so-called hydrolysis phenomenon, which reflects the breaking of Si-O-Si bonds and the formation of Si-OH groups. As a result, physical and/or chemical properties like glass chemical durability can be altered. Changing the glass composition by introducing alkali ions or intermediate species like Al or Mg can result in complex phenomena between the glass network and H2O molecules. The arising difficulties can be addressed by classical molecular dynamics, but also by means of Density Functional Theory (DFT). In this context, hydrated enstatite glass (MgSiO3-H2O) is a useful system to investigate, as its dried structure involves a large Mg coordination distribution (around 68% of [4]Mg, 28% [5]Mg and 4% [6]Mg) and a large fraction of non-bonded oxygen ions (around 60%). Furthermore, as chemical reactions between H2O molecules and Si-O-X (X=Si or Mg) bonds are unlikely to occur in normal simulation conditions, the use of Metadynamics (which can be seen as a field potential biasing technique) becomes an efficient way to increase the probability of occurrence of these phenomena. We will show that a hydrated glass structure obtained by the Empirical Potential Structure Refinement technique (EPSR) can be geometrically optimised by DFT, and used as a starting point for Metadynamics calculations. After H2O dissociation, several metastable states involving H+ and OH- appear, but with different local environments. Moreover, another state can occur where both H+ ions separate from their initial water-related oxygen ion, leading to the complete dissociation of the water molecule. Finally, the derived effective atomic charges reveal that a water molecule interacts with the network structural units via dipole-dipole or dipole-charge interactions. All these processes will be discussed in relation with energetic barriers and the choice of collective variables. O. Bouty, Reaction mechanisms in hydrated magnesium silicate glass investigated by Ab-Initio methods and Metadynamics, Computational Materials Science 232 112632 (2024).