Surface Reactivity and Leaching of a Sodium Silicate Glass under an Aqueous Environment: A ReaxFF Molecular Dynamics Study

The presence of leachable ions in multicomponent silicate glasses complicates the understanding of their surface structure and chemistry under a humid or aqueous environment, in particular when compared to pure silica-water interactions. Here, we performed large-scale ReaxFF reactive molecular dynamics (MD) simulations and investigated the structural and chemical modifications that take place at the sodium silicate glass-water interface. From our simulation results, we observed that the water interaction gradually replaces sodium ions associated with the surface nonbridging oxygen (NBO) and found that molecular water dissociates into H⁺ and OH^- in the presence of these modifier cations. The protonation of the NBO site produces silanol (SiOH), and our results denote that protons can diffuse further into the bulk region by the discrete proton hopping between two adjacent NBO sites. Consequently, the number of silanol formation increases along the glass-water reaction, rather than reaching an equilibrium point within the nanosecond time scale of our MD simulations. In addition, sodium leaching and its mobility at the interface were studied to provide adequate information on how the ion-exchange process dictates their transport within the interface. Sodium ions are bound close to the surface as Na-OH ion pairs during the initial stages of the water interaction and exhibits a certain residence time at the surface before they are released to the aqueous media. The reactive MD simulation presented here can successfully provide physical insights needed to understand the surface chemistry with the time and length scales which would otherwise be difficult for first-principles or experimental studies to evaluate.