Atomic and micro-structure features of nanoporous aluminosilicate glasses from reactive molecular dynamics simulations
Not provided.
Materials Science↗
Engineering topics
Mahadevan, Thiruvilla S.
Publications and source records attributed to Mahadevan, Thiruvilla S..
Hydration and reaction mechanisms on sodium silicate glass surfaces from molecular dynamics simulations with reactive force fields
Recent development of reactive force fields have enabled molecular dynamics simulations of interactions between silicate glasses and water at the atomistic scale. While multicomponent silicate glasses encompass a wide variety of compositions and properties, one common structural feature in these glasses is the combination of the network structure that is made up of silica tetrahedra linked through corner sharing interspersed with network modifiers like alkali and alkaline-earth ions that break up the Si–O–Si linkages by forming nonbridging oxygen. In reactions with water, ion exchange between alkali ions in the glass and proton or hydronium in the solution, as well as hydrolysis reaction of the Si–O–Si linkages and subsequent silanol formation, is observed and well documented. Furthermore, we have used a set of recently developed reactive force field to investigate the reactions between water and the surfaces of silica and sodium silicate glasses of different compositions for reactions up to 8 nanoseconds. Our results indicate sodium leaching into water and diffusion of water molecules up to 25 Å into the glass surface. We examined the structural and compositional changes inside the glass and around the diffused ions and use these to explain the rates of silanol formation at the surface. We also observed proton transport in the glass which has an indirect influence on the silanol formation rates. While the surface of the glass was rough to start with, it undergoes further modification into a hydrated gel-like structure in the glass for up to 5 Å in the higher alkali containing glasses. It was found that the leached sodium ions remain close to the interface and that fragments of silicate network from the surface is capable of dislodging from the bulk glass and enter the aqueous solution. These simulations thus provide insights into the formation and structure of an alteration layers commonly observed in multicomponent silicate glasses corroded in aqueous solutions.
36 MATERIALS SCIENCE↗
Evaluating Water Reactivity at Silica Surfaces Using Reactive Potentials
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. Furthermore, 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.
36 MATERIALS SCIENCE↗