Relaxation of silica glass surfaces before and after stress modification in a wet and dry atmosphere: molecular dynamics simulations

Previous molecular dynamics simulations have shown that compression of silica glass surfaces occurs upon formation of an interface with a model crystal and that a structural change caused by this process is retained after glass and crystal are separated. The remnant structural modification caused by this stress was an increase in the concentration of siloxane bond angles less than 1508 in the near surface region of the glass. It was expected that the structural modification associated with interface formation and separation could represent an increase in the concentration of less stable siloxane bonds, particularly in the presence of water molecules. It was also recognized that a decreased stability could indicate greater reactivity with water molecules. Thus, water reaction on silica surfaces was simulated before and after stress modification and the subsequent structural relaxations in the glass surface were observed. Decreased stability, represented by a greater number of bond ruptures, existed after interface formation and removal. These bond ruptures were Si–O bonds breaking and reforming siloxane bonds with an angle nearer the average and also Si–O bonds breaking to react with water forming silanols. A greater number of silanols formed after interface formation and removal than before, demonstrating a greater reactivity with water after interface formation and separation. q 1998 Elsevier Science B.V. All rights reserved.