Surface energetics of the hydroxyapatite nanocrystal-water interface: a molecular dynamics study

Face-specific interfacial energies and structures of water at ionic crystal surfaces play a dominant role in a wide range of biological, environmental, technological, and industrial processes. Nanosized, plate-shaped crystals of calcium phosphate (CaP) with nonideal stoichiometry of hydroxyapatite (HAP, ideal stoichiometry Ca10(PO4)6(OH)2) comprise the inorganic component of bone and dentin. The crystal shape and size contribute significantly to these tissues' biomechanical properties. Plate-shaped HAP can be grown in the presence of biomolecules, whereas inorganically grown HAP crystals have a needlelike shape. Crystal morphology reflects the relative surface areas of the faces and, for an ideal inorganically grown crystal, should be governed by the surface energies of the faces with water. Interfacial energies and dynamics also affect biomolecule adsorption. Obtaining face-specific surface energies remains experimentally challenging because of the difficulty in growing large HAP single crystals. Here we employed molecular dynamics (MD) simulations to determine nanocrystalline HAP-water interfacial energies. The (100) face was found to be the most favorable energetically, and (110) and (004) were less hydrophilic. The trend in increasing interfacial energy was accompanied by a decrease in the average coordination number of water oxygen to surface calcium ions. The atomic-level geometry of the faces influenced interfacial energy by limiting lateral diffusion of water and by interrupting the hydrogen bond network. Such unfavorable interactions were limited on (100) compared to the other faces. These results provide a thermodynamic basis for the empirically observed trends in relative surface areas of HAP faces. The penetration of charged biomolecules through the interfacial water to form direct interactions with HAP faces, thus potentially influencing morphology, can also be rationalized.