Exploring the surface of bioactive glasses: Water adsorption and reactivity

Car-Parrinello Molecular Dynamics simulations and structural optimizations have been carried out to investigate the surface of a highly bioactive phosphosilicate glass, containing Na and Ca modifier cations. The identification and characterization of the active surface sites was performed on the basis of their interaction with a water molecule. Spontaneous water dissociation was observed on the strongest surface sites, represented by three-coordinated Si atoms combined with a proton acceptor such as a nonbridging oxygen. Additional adsorption sites are represented by Na/Ca cation modifiers, which appear to be directly involved in the glass dissolution mechanism, by providing favorable pathways which allow water to penetrate under the surface. Small silica rings, 2- or 3-membered, are stable features of the surface, despite a favorable energetic balance for their opening upon water dissociation. The minimum energy paths and barriers for alternative 2 M ring-opening mechanisms, obtained by means of String Method Car-Parrinello calculations, show that the opening of small rings is hindered by an energy barrier. Together with the availability of alternative, more favorable molecular adsorption sites where water can migrate before dissociation, this can result in some of the small rings not being open and hydroxylated upon immersion in a physiological environment, thus leaving them available to assist nucleation of dissolved Ca and P ions on the glass surface, as it had been previously proposed to interpret the initial stages of the bioactive fixation mechanism.