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-ζ quality using the periodic CRYSTAL06 code. Because four Ca2+ cations are available at both surfaces, the considered H2O coverages span the 1/4 ≤ θ ≤ 5/4 range. The affinity of both HA surfaces for H2O is large: on the (001) surface, H2O adsorbs molecularly (binding energies BE ≈ 80 kJ 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 adsorp...