Molecular Orbital Study of an Environmentally Enhanced Crack Growth Process in Silica

Abstract

We have used molecular orbital calculations to investigate effects of environmental substances and strain on fracture of the Si─O bond in an H6Si2O7 molecule. We believe that the tendencies observed simulate crack growth in silica in the vicinity of a crack tip. This study is focused on the initial Stage of environmental enhancement, in which a molecule of a substance from the environment approaches a reaction site in silica. Five environmental substances—ammonia, water, formamide, nitrogen, and argon—were considered. The total energy of each H6Si2O7–environmental molecule system was calculated with H6Si2O7 in each of two strain conditions and with the environmental molecule at two different distances from the bridging O in H6Si2O7. This provides estimates of the average force required to move an environmental molecule toward the reaction site in silica. The energy difference due to environmental molecule position is relatively small for ammonia and water, and is relatively large for nitrogen and argon. Experimentally, ammonia and water have shown the greatest tendency to enhance crack growth in silica, whereas nitrogen has shown virtually no such tendency. The tendency for a substance to enhance crack growth therefore appears to be at least partly an inverse function of the energy required to move a molecule of that substance toward the reaction site on the silica surface. Electron population analyses indicate that oxygen atoms adjacent to the reaction site engage in antibonding interactions with certain environmental‐molecule atoms. These interactions are much smaller in magnitude when H6Si2O7 is strained than when it is unstrained. It therefore appears possible that in a crack in silica, where an environmental molecule would experience restricted mobility, strain reduces steric hindrances to the approach of environmental molecules to the reaction site. Straining H6Si2O7 also increases the magnitudes of the charges on Si and O at the reaction site, probably leading to increased electrostatic attractions of environmental molecules and H6Si2O7.