Modeling the Water-Bioglass Interface by Ab Initio Molecular Dynamics Simulations

The hydration of the surface of a highly bioactive silicate glass was modeled using ab initio (Car-Parrinello) molecular dynamics (CPMD) simulations focusing on the structural and chemical modifications taking place at the glass-water interface immediately after contact and on the way in which they can affect the bioactivity of these materials. The adsorption of a water dimer and trimer on the dry surface was studied first, followed by the extended interface between the glass and liquid water. The CPMD trajectories provide atomistic insight into the initial stages relevant to the biological activity of these materials: following contact of the glass with an aqueous (physiological) Medium, the initial enrichment of the surface region in Na+ cations establishes dominant Na+-water interaction's at the surface, which allow water molecules. to penetrate into the open glass networks and start its partial dissolution. The model of a Na/H-exchanged interface shows. that ca(2+)-water interactions are mainly established after the-dominant fraction of Na is into the solution. Another critical role of modifier cations was highlighted: they provide the Lewis acidity necessary to neutralize OH- produced by water dissociation and protonation of nonbridging oxygen. (NBO) Surface sites. The CPMD simulations also highlighted an alternative, proton-hopping mechanism by which the same process can take place in the liquid Water film. The: main. features of the bioactive glass surface immediately after contact,with an aqueous medium, as emerged from the simulations, are (a) silanol groups formed by either water dissociation at undercoordinated Si sites or direct protonation of NBOs; (b) OH- groups generally stabilized by modifier cations and coupled with the protonated NBOs, and (c) small rings, relatively stable and unopened even after exposure to-liquid water. The possible role and effect of these sites in the bioactive process are discussed.