LAUSR

Molecular dynamics simulations of seven compositionally different sodium calcium alumino-borosilicate glasses showed formation of 4B and 5Al 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 4B in the top 5-6A of the surface in comparison to the bulk concentration for all glasses and no 5Al. Upon exposure to water, the simulations show that the 3B in the top 5-6A 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 5-6A (i.e. including O from water) shows a decrease in 3B but an increase in 4B that is consistent with NEXAFS analysis, which the simulations show are hydroxylated. There is an increase in the concentration of 3Al in the dry surface in comparison to the bulk, but exposure to water converts almost all of these 3Al to 4Al. Hydroxyl concentrations vary from 2.6/nm2 to 4.1/nm2, 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 non-bridging oxygen sites attached to Na, provide atomistic evidence of the initial stages of removal of B and Na from glass surfaces exposed to water. This article is protected by copyright. All rights reserved.