A new interatomic potential for dissociative water was developed for use in molecular dynamics simulations. The simulations use a multibody potential, with both pair and three-body terms, and the Wolf summation method for the long-range Coulomb interactions. A major feature in the potential is the change in the shortrange O-H repulsive interaction as a function of temperature and/or pressure in order to reproduce the densitytemperature curve between 273 K and 373 at 1 atm, as well as high-pressure data at various temperatures. Using only the change in this one parameter, the simulations also reproduce room-temperature properties of water, such as the structure, cohesive energy, diffusion constant, and vibrational spectrum, as well as the liquid-vapor coexistence curve. Although the water molecules could dissociate, no dissociation is observed at room temperature. However, behavior of the hydronium ion was studied by introduction of an extra H + into a cluster of water molecules. Both Eigen and Zundel configurations, as well as more complex configurations, are observed in the migration of the hydronium.

Molecular dynamics (MD) computer simulation of the adsorption of water molecules onto the vitreous silica surface was performed using a new dissociative water potential. 58 The simulations showed dissociative chemisorption of water molecules onto the silica surface, forming silanol (SiOH) groups at a concentration consistent with experimental data. Water penetration and silanol formation ∼7-8 Å below the outermost oxygen are observed. Because of the dissociative nature of the water potential, formation of hydronium ions is allowed, and, whereas seldom observed in the simulations of bulk water, hydronium ions are formed during the reactions causing the formation of the silanols. The formation of hydronium ions has also been observed in ab initio calculations of water adsorption onto silica surfaces. The time evolution of the reactions involving hydronium ions in our MD simulations is similar to that observed in first-principles MD calculations. Hydronium ions offer a mechanism by which initially singly coordinated terminal oxygen (Si-O -) receives a H + ion from a relatively distant chemisorbed H 2 O molecule via multiple H + ion transfer, creating two SiOH sites.

Understanding the dissolution of silicate glasses and minerals from atomic to macroscopic levels is a challenge with major implications in geoscience and industry. One of the main uncertainties limiting the development of predictive models lies in the formation of an amorphous surface layer––called gel––that can in some circumstances control the reactivity of the buried interface. Here, we report experimental and simulation results deciphering the mechanisms by which the gel becomes passivating. The study conducted on a six-oxide borosilicate glass shows that gel reorganization involving high exchange rate of oxygen and low exchange rate of silicon is the key mechanism accounting for extremely low apparent water diffusivity (∼10−21 m2 s−1), which could be rate-limiting for the overall reaction. These findings could be used to improve kinetic models, and inspire the development of new molecular sieve materials with tailored properties as well as highly durable glass for application in extreme environments.

Table 1c.

Dilatometric measurement of the thermal expansion of water in porous silica shows that the expansion coefficient increases systematically as the pore size decreases below about 15 nm. This behavior is quantitatively reproduced by molecular dynamics (MD) simulations based on a new dissociative potential. According to MD, the structure of the water is modified within approximately 6 A of the pore wall, so that it resembles bulk water at a higher pressure. On the basis of this observation, it is possible to account for the measured expansion, as the thermal expansion coefficient of bulk water increases with temperature over the range considered in this study.

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.

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.

Understanding the interactions between amorphous silica surfaces and water provides insight into material degradation of silicate glasses and minerals in aqueous environment. Molecular dynamics (MD) simulations of water and nanometer sized silica structures were used in this work to evaluate the reactivity of flat silica surface and surfaces with curvature. We compared two dissociable water/silica potentials, namely the Reactive Force Field (ReaxFF) and the Mahadevan–Garofalini water/silica force field (MGFF) that have been in development over the past decade, to study their performance in simulating bulk water as well as silica–water interactions. Significant differences in the properties of bulk water as well as surface interactions were observed between the two types of potentials, as well in the same potential type with two parametrizations for ReaxFF, suggesting a need for improvement of the existing water/silica ReaxFF potentials. Our simulation results show that a majority of the silanols were formed by reactions between water and strained siloxane bonds that mainly exist on the surface of amorphous silica, within a few nanoseconds of the simulation time scale, in agreement with previous studies. Effect of surface curvature on the reactivity with water was investigated. Our results indicate that defect concentration at the surface bears a strong correlation to the concentration of silanols (Si–OH) that eventually form. We observe undercoordinated Si’s at the surface that are attacked by water before the hydrolysis reaction of the siloxane bonds and demonstrate possible mechanisms of water reacting with these undercoordinated Si’s. We also find that the method of generating surfaces in simulation determines the defect concentration and hence influences the reactivity of the amorphous silica surface.