Ca SPECIES AT THE SURFACE OF NANOSIZED HYDROXYAPATITE.A COMPUTATIONAL AB INITIO AND A MICROCALORIMETRIC/IRSPECTROSCOPIC STUDY.

Bioactive glasses, when implanted in the body or simply immersed in simulated body fluids (SBF), develop a biologically active hydroxyapatite (HA) layer which in turn does promote the bone-tissue formation. In fact hydroxyapatite, which exhibits strong similarities to the mineral phase of the mammalian bones and teeth, does play a key role during the bioactive glasses integration processes, in that it facilitates adhesion and subsequent proliferation of the osteocytes, so allowing the damaged bone tissues to be repaired. The first step of these processes is the adsorption of biomolecules at the active surface of HA. Therefore, studies aimed at quantitatively describing the structural and chemical properties of the HA surface are of greatest interest, in the attempt to elucidate at nano-level the interfacial processes involved in the biological fixation of inorganic materials to the living tissues. In the present study, ab initio methods and experimental techniques have been used to characterize the adsorption features of HA surfaces towards H2O, not only because water is ubiquitous in the biological environment but also because the nature and structure of surface hydrated layer does play a key role in the adsorption of biomolecules. Periodic ab initio B3LYP calculations using CRYSTAL06 code have run to characterize the (001) and (010) bare surfaces for both hexagonal and monoclinic HA phases. On the geometrically relaxed surfaces the adsorption of H2O has been simulated, from low to high coverage. Energies of adsorption and vibrational features of H2O adsorbed at the (001) surface have been calculated. In parallel, the RT-adsorption of H2O vapour on a nanosized HA specimen, previously dehydrated in vacuo (p . 10-5 Torr) following a controlled protocol, has been investigated by means of microcalorimetry and IR spectroscopy. The whole set of calculated and experimentally obtained results allowed to assess and quantitatively describe the dehydration-rehydration mechanism at the apatite surface, which was found to possess strongly hydrophilic properties. Indeed, on one hand the energy of the Ca-species/water interaction is quite high (. 100 kJ/mol, as indicated by both calculated and calorimetrically measured data) and on the other hand the adsorbed phase was only partially removed upon RT-evacuation. However, opposite to what initially thought, water does not react with the apatite surface giving rise (through a dissociative mechanism) to new hydrophilic and reactive Ca-OH sites, but is molecularly coordinated at the Ca-sites (at least the ones exposed at the 001 surface), as clearly indicated by computed and IR spectroscopic results. Indeed, the coordinatively unsaturated (cus) Ca2+ cations exposed at the surface are characterized by rather strong local electric fields (the effect of which does polarize the molecules over at least two shells of coordinated H2O), but their hydrophilicity is not sufficient to dissociate water molecules. This property has likely an implication in the adsorption processes at the hydrated layer interface and on the affinity of such materials towards proteins. Indeed, if reactive Ca-OH groups were present at the surface, denaturation of proteins would occur hampering cells adhesion.