Atomic Insights into the Events Governing the Borosilicate Glass–Water Interface
Résumé
An atomistic understanding of the mechanisms that govern the borosilicate glass–water interface is highly needed to obtain initial assessments of the phenomena occurring during the degradation of nuclear waste forms. To this end, we have simulated a structural model of sodium borosilicate glass using classical molecular dynamics (CMD) simulations followed by a Car–Parrinello ab initio molecular dynamics (AIMD) simulation and periodic density functional theory (DFT) calculations to study both the reactivity and water adsorption events on a sodium borosilicate glass. The obtained results revealed that: (i) Na+ ions are directly involved in the first stages of the dissolution mechanism, namely, the penetration of water and the release of the most accessible sodium from the surface of the glass to the aqueous solution, (ii) due to the presence of a higher amount of sodium ions closer to BIV than BIII units, the boron atom with the BIII unit is more accessible to accommodate water molecules than the one with the BIV unit, and (iii) the attack of BIII by a water molecule occurs by a phenomenon of nondissociative adsorption followed by dissociative adsorption where the adsorbed water dissociates by the presence of another nearby water molecule. An analysis of the electronic structure, based on maximally localized Wannier functions, shows that the nondissociative adsorption presents a noncovalent bond between one of the lone pairs of the water molecule and BIII (B–OH2) by an interatomic distance of 1.57 Å, whereas the dissociative adsorption of this water molecule showed a B–OH covalent bond by an interatomic distance of 1.49 Å. Water molecules are collectively adsorbed on a B adsorption site, which ultimately leads to terminal B–OH and silanol H–NBO formations. As B–OH is formed, the boron site begins to attract more water molecules, which can be considered an early stage of solvation of the glass around this site. By combining DFT approaches and Bader charge analysis, our calculation reports that the adsorption of a single water molecule onto a sodium-rich site exhibits higher total interaction energy than onto a pure boron atomic site. On the other hand, DFT revealed that the water molecule adsorbed on the sodium-rich site exhibits a higher elongation of the H–OW bond length of about 0.05 Å, indicating the high probability of water dissociation and favoring the formation of H–NBO precipitates.