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.

Development of Water Reactive Potentials for Sodium Silicate Glasses.

Molecular dynamics (MD) simulations provide important insights into atomistic phenomena and are complement to experimental methods of studying glass-water interaction and glass corrosion. For simulations of glass-water systems using MD, there is a need to for a reactive potential that is capable not only to describe the bulk and surface glass structures but also reactions between glass and water. An important aspect of the glass water interaction is the dissociation of water and its interaction with glass components that can result in the dissolution and alteration in the structure of glass. These phenomena can be efficiently simulated using "Reactive" potentials that allow for the dissociation of water while properly describing the bulk physical properties of water. We demonstrate a method to develop parameters for simulations of sodium silicate glasses and their interactions with bulk water. The developed parameter set was used to simulate sodium silicate glasses of different compositions, and the local structure of the simulated glass is in good compliance with experimentally obtained structural information. We also demonstrate that the parameter set predicts an accurate value for the hydration number and dissociation reactions of NaOH in water. Based on these results, we posit that these simple and computationally efficient reactive potentials can be used for further studies of water-induced structural modifications in sodium silicate glasses.

Molecular mechanisms causing anomalously high thermal expansion of nanoconfined water.

Anomalously high thermal expansion is measured in water confined in nanoscale pores in amorphous silica and the molecular mechanisms are identified by molecular dynamics (MD) simulations using an accurate dissociative water potential. The experimentally measured coefficient of thermal expansion (CTE) of nanoconfined water increases as pore dimension decreases. The simulations match this behavior for water confined in 30 A and 70 A pores in silica. The cause of the high expansion is associated with the structure and increased CTE of a region of water approximately 6 A thick adjacent to the silica. The structure of water in the first 3 A of this interface is templated by the atomically rough silica surface, while the water in the second 3 A just beyond the atomically rough silica surface sits in an asymmetric potential well and displays a high density, with a structure comparable to bulk water at higher pressure.