Coordination chemistry of Ca sites at the surface of nanosized hydroxyapatite: interaction with H2O and CO

The affinity towards water of a selection of well-defined nanostructured hydroxyapatite HA samples was investigated by room temperature H2O vap adsorption microcalorimetry and IR spectroscopy. A large hydrophilicity of all investigated materials was confirmed. The surface features of hydrated HA were investigated on the as-synthesized samples treated in mild conditions at T = 303K, whereas dehydrated HA features were characterized on samples activated at T = 573K. The relatively large hydrophilicity of the hydrated surface (−D ads H ~100 – 50 kJ/mol) was due to the interaction of water with the highly polarized H 2 O molecules strongly coordinated to the surface Ca2+ cations. At the dehydrated surface, exposing coordinatively unsaturated (cus) Ca2+ cations, H 2 O was still molecularly adsorbed but more strongly (- ads H ~120-90 kJ/mol). However, microcalorimetric and IR measurements as well as ab initio calculations of CO adsorption revealed only a moderate Lewis strength of HA cus Ca2+ cations. This implies that the large HA/H 2 O interaction energy is due to the interplay between cus Ca2+ sites and nearby hydrophilic PO 4 groups, not revealed by CO probe. The lower density of cus Ca2+ cations at the 573K-activated HA surface with respect to the pristine one, did not affect the whole hydrophilicity of the surface, as the polarizing effect of Ca sites is so strong to extend up to the fourth hydrated layer, as confirmed by both high-coverage microcalorimetric and IR spectroscopic data. No specific effects due to the investigated specimens preparation method and/or different morphology were observed.