Probing the surface structure of hydroxyapatite through its interaction with hydroxyl: a first-principles study

Understanding the interaction of the hydroxyapatite (HAp) surface with hydroxyl originating from either the alkalescent physiological environment or HAp itself is crucial for the development of HAp-based biomaterials. Periodical density functional theory calculations were carried out in this study to explore the interaction of the HAp (100), (010) and (001) facets with hydroxyl. Based on a comparison study of Ca-rich, PO₄-rich and Ca-PO₄-OH mixed surfaces, the interaction pattern, interaction energy and effect of an additional water molecule on the Ca-OH interaction were comprehensively studied. The formation of CaOH on the Ca-rich surface was energetically favored on (100) and (001), while Ca(OH)₂ was energetically favored on (010). The Ca-water interaction was competitive, but had lower interaction energy than Ca-OH. Furthermore, Ca-O bonding and its influence on the OH stretching vibration were analyzed. Our calculations suggest that the hydroxyl-coated surface structure is more appropriate than the commonly used Ca-terminated surface model for studying HAp surface activity in its service environments.