Composition Effect on Interfacial Reactions of Sodium Aluminosilicate Glasses in Aqueous Solution

Understanding the underlying reaction mechanisms responsible for aluminosilicate glass dissolution in aqueous environments is crucial for designing glasses for technological applications ranging from architecture windows and touch screens to nuclear waste disposal. This study investigated the glass composition effect on the interfacial reactions of sodium aluminosilicate (NAS) glasses using molecular dynamics (MD) simulations with recently developed reactive potentials. Glass-water interfacial models of six NAS glasses with varying Al₂O₃/Na₂O ratios were investigated for up to 4 nanoseconds (ns) to elucidate the interfacial reaction mechanisms at ambient temperature. The results showed that the coordination defects, such as undercoordinated Si and Al, as well as non-bridging oxygens (NBOs) accumulated at the glass surfaces, play a crucial role in the initial hydration reaction process of the glasses. They promote the formation of silanol (Si-OH) and aluminol (Al-OH) species together with the Na^+⇔ H⁺ ion-exchange reactions. The z-density profiles of H₂O and H⁺ ions affirmed the water/H⁺ propagation into the glass up to 2 nanometers after 4 ns reactions. The penetration depth depends on the composition and shows a nonlinear dependence, suggesting that the subsequent water penetration, particularly into the bulk glass, is supported by the availability of random channels. Aluminol formations, including Al-OH or Al-OH₂ near the surface, were found to form mainly through the hydrolysis of Al-O-Al bonds and hydration of Al⁺-NBO^- units. While water molecules are involved in initial interfacial reactions, water penetration into the bulk glass region is primarily achieved by proton transfer. Compared to highly mobile proton transfer involving silanol groups, proton transfer associated with [AlO₄]^- species is much more limited, particularly in the bulk glass region. These new insights into the role of aluminum in interfacial reactions of the NAS glasses can help to understand the initial dissolution mechanisms and in designing more durable glasses.