Surface properties of silica-based biomaterials: Ca species at the surface of amorphous silica as model sites

The adsorption of CH₃OH vapor at the surface of Ca-modified silica was studied by means of the combined use of an experimental and a theoretical approach. Parallel IR spectroscopic and microcalorimetric experiments were performed to describe quantitatively and energetically the surface features of nanosized Ca-modified specimens (A200/Cax, activated in mild conditions) as a function of Ca loading and in comparison with the unmodified parent silica (Aerosil 200). The presence of Ca species at the surface enhances the adsorption capacity with respect to the unmodified parent silica and creates a rather complex reactivity. Ab initio simulation provided microscopic information on the energetic of coordinated CH₃OH adducts formed at the Ca sites (BE = 104 kJ/mol vs q^diff ∼ 100 kJ/mol) and on the possible reaction path toward products. The methoxylation of the surface, yielding Si-OCH₃ and Ca-OH species (and not Ca-OCH₃ and Si-OH) occurs only to a limited extent (30-40% of the total methanol uptake) and depends on both CH₃OH pressure and time contact, according to the activated reaction pathway, as provided by ab initio simulation. Data from both volumetric-calorimetric and IR spectroscopic experiments indicated, in good agreement with ab initio simulation results, that the overall interaction involves both chemical and physical adsorption processes which, to a large extent, occur simultaneously, the relevant energy transfers being very similar.