Characterization of sol-gel bioglasses with the use of simple model systems: a surface-chemistry approach

The present work deals with the physico-chemical characterization of the surface features of two powdery sol–gel-synthesized bioglass specimens (58S and 77S) employed in repair of human bones and soft tissues. In-situ transmission FTIR spectroscopy and adsorption microcalorimetry (with the use of water as a probe molecule) were adopted in parallel to study on one hand bioglasses (amorphous Si, Ca and P oxides) and on the other hand pure and doped silica specimens, as reference materials. It was observed that the presence of Ca and P moieties induces the formation of surface sites that possess higher strength and higher capability towards water coordination than do the hydroxy groups present on pure silica. In particular, the presence of some irreversibly coordinated water was observed both on these bioglasses and on doped silica. The Ca/P ratio seems to play a major role: (i) an excess of Ca induces the presence of coordinatively unsaturated surface (cus) Ca2+ ions, that generate Lewis acid sites that can strongly interact with water and can yield surface carbonates; (ii) the presence of P species increases the surface hydrophilic character and leads to the formation of Si–O–P labile bridges, which exhibit an enhanced propensity to dissociate water.