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The adsorption of glycine on the Ca-rich and P-rich HA(010) nonstoichiometric surfaces has been studied at B3LYP level using a polarized triple-ζ basis set within periodic boundary conditions. Although the Ca-rich and P-rich HA(010) nonstoichiometric surfaces exhibit different terminations, giving rise to different electrostatic features in the adsorption regions, glycine preferentially adsorbs as a zwitterion on both surfaces. When adsorbed in a canonical form, the proton of the COOH group is always transferred to the HA(010) surface except for one case, which, in turn, is also the least stable one. Glycine adsorbs by favorable electrostatic interactions between COO–/Ca2+ and NH3 +/PO4 2– species, while dispersion interactions play a minor role. The harmonic B3LYP vibrational spectrum is in very good agreement with the experimental one and, when merged with contributions due to glycine adsorbed on stoichiometric HA(010) surfaces, allows one to explain the origin of the “squat shape” of the COO– stretching band centered at about 1600 cm–1. The comparison also highlights that the Ca-rich rather than the P-rich termination, is the most suitable to explain subtle features of the experimental spectrum.

The affinity towards water of a selection of well-defined, nanostructured hydroxyapatite (HA) samples was investigated by H 2 O vapour adsorption microcalorimetry and infrared (IR) spectroscopy. A large hydrophilicity of all investigated materials was confirmed. The surface features of hydrated HA were investigated on the as-synthesized samples pre-treated in mild conditions at T = 303 K, whereas dehydrated HA features were characterized on samples activated at T = 573 K. The relatively large hydrophilicity of the hydrated surface (−D ads H ∼ 100-50 kJ mol −1 ) was due to the interaction of water with the highly polarized H 2 O molecules strongly coordinated to the surface Ca 2+ cations. At the dehydrated surface, exposing coordinatively unsaturated (cus) Ca 2+ cations, H 2 O was still molecularly adsorbed but more strongly (−D ads H ∼ 120-90 kJ mol −1 ). The use of CO adsorption to quantify the Lewis acidic strength of HA surface sites revealed only a moderate strength of cus Ca 2+ cations, as confirmed by both microcalorimetric and IR spectroscopic measurements and ab initio calculations. This result implies that the large HA/H 2 O interaction energy is due to the interplay between cus Ca 2+ sites and nearby hydrophilic PO 4 groups, not revealed by the CO probe. The lower density of cus Ca 2+ cations at the 573 K activated HA surface with respect to the *

The affinity of the (001) and of the water reacted (010)WR hydroxyapatite surfaces towards formic and alendronic acids is studied with density functional theory (PBE functional) using periodic boundary conditions based on Gaussian basis set. Structures, energetic of the adsorption and vibrational features of the adsorbates are computed in order to understand at the atomic level both the cariogenic processes (for the formic acid) and the features of anti-osteoporosis drugs (for the alendronic acid). For both molecules the interaction energy is very high on an absolute scale, and for all examined cases, it is higher on the (010)WR HA surface than on the (001) one. For the latter, a number of cases by which the acidic proton of the adsorbate is transferred to the HA surface are also characterized. For the formic acid case, experimental infrared spectra are also measured and the position and nature of the C=O stretching bands have been found to be in excellent agreement with the quantum mechanical simulations. For alendronic acid IR experiments are still not available and the present predicted infrared spectra will be useful as a guide to interpret future experimental studies.