Water Adsorption on the Stoichiometric (001) and (010) Surfaces of Hydroxyapatite: A Periodic B3LYP Study

H2O adsorption on hexagonal hydroxyapatite (001) and (010) stoichiometric surfaces has been studied at B3LYP level with a localized Gaussian basis set of polarized double-zeta quality using the periodic CRYSTAL06 code. Because four Ca2+ cations are available at both surfaces, the considered H2O coverages span the 1/4 <= theta <= 5/4 range. The affinity of both HA surfaces for H2O is large: on the (001) surface, H2O adsorbs molecularly (binding energies BE approximate to 80 U mol(-1) per adsorbed molecule), whereas it dissociates on the (010) surface, giving rise to new surface terminations (CaOwHw and POHw). The highly negative reaction energy for H2O dissociation (between -250 and -320 kJ mol(-1) per adsorbed H2O molecule) strongly suggests that the pristine (010) surface "as cut" from the hydroxyapatite bulk cannot survive in aqueous environment. Conversely, on the reacted surface, H2O adsorbs molecularly with BE similar to those computed for the (001) surface. The B3LYP BEs have been contrasted to the experimental water adsorption enthalpies measured by microcalorimetry on polycrystalline hydroxyapatite samples, showing a fairly good agreement and supporting the suggestion that H2O vapor adsorbs on the already reacted (010) crystalline faces. Harmonic B3LYP vibrational features of adsorbed H2O show, when compared to modes of the gas-phase H2O, a hypsochromic shift of the HOH bending mode (<Delta delta(HOH)> = 49 cm(-1)) and a bathochromic shift of the OH stretching modes larger than 1700 cm(-1) (<Delta nu(OH)> = 427 cm(-1)), which are both in good agreement with literature experimental data.