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 (CaO_wH_w and POH_w). The highly negative reaction energy forH₂O dissociation (between -250 and -320 kJ mol^-1 per adsorbed H ₂O 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 (<Δ>δ_hoh) ) 49 cm^-1) and a bathochromic shift of the OH stretching modes larger than 1700 cm^-1 (<Δv_oh>) = 427 cm^-1), which are both in good agreement with literature experimental data.