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 mo del crystal and that a structural change caused by this process is ret ained after glass and crystal are separated. The remnant structural mo dification caused by this stress was an increase in the concentration of siloxane bond angles less than 150 degrees in the near surface regi on of the glass, It was expected that the structural modification asso ciated with interface formation and separation could represent an incr ease in the concentration of less stable siloxane bonds, particularly in the presence of water molecules. It was also recognized that a decr eased stability could indicate greater reactivity with water molecules . Thus, water reaction on silica surfaces was simulated before and aft er stress modification and the subsequent structural relaxations in th e glass surface were observed. Decreased stability, represented by a g reater number of bond ruptures, existed after interface formation and removal. These bond ruptures were Si-O bonds breaking and reforming si loxane bonds with an angle nearer the average and also Si-O bonds brea king to react with water forming silanols, A greater number of silanol s formed after interface formation and removal than before, demonstrat ing a greater reactivity with water after interface formation and sepa ration. (C) 1998 Elsevier Science B.V. All rights reserved.