Simulations of the Surfaces of Soda Lime Aluminoborosilicate Glasses Exposed to Water

Molecular dynamics simulations of 7 compositionally different sodium calcium alumino-borosilicate glasses showed formation of B-4 and Al-5 more consistent with experimental data without compromising the other structural features that match experimental results observed in recent simulations of these glasses. Analysis of the dry surfaces of these glasses show a lack of B-4 in the top 5-6 angstrom of the surface in comparison to the bulk concentration for all glasses and no Al-5. Upon exposure to water, the simulations show that the B-3 in the top 5-6 angstrom of the glasses are preferentially attacked, decreasing the number of B bonds to O originally from the glass, indicating a change in the glass network. Inclusion of all B-O bonds in the top 5-6 angstrom (i.e., including O from water) shows a decrease in B-3 but an increase in B-4 that is consistent with NEXAFS analysis, which the simulations show are hydroxylated. There is an increase in the concentration of Al-3 in the dry surface in comparison to the bulk, but exposure to water converts almost all of these Al-3 to Al-4. Hydroxyl concentrations vary from 2.6/nm(2) to 4.1/nm(2), with SiOH and BOH dominating these surface hydroxyls. Upon exposure to water, network linkages to B are preferentially ruptured. This, and the preferential loss of the nonbridging oxygen sites attached to Na, provide atomistic evidence of the initial stages of removal of B and Na from glass surfaces exposed to water.