Water at hydroxyapatite surfaces: the effect of coverage and surface termination as investigated by all-electron B3LYP-D* simulations

Hydroxyapatite [HA, Ca-10(PO4)(6)(OH)(2)], the main constituent of bones and teeth enamels, is a widely studied and employed biomaterial. Its applications span from dental to orthopedic implants, including bone tissue engineering scaffolds, coating, filler and many others. Previous theoretical and experimental studies have already characterized the physical-chemical foundations of water adsorption on a number of HA surfaces, an essential step in the mechanism of biomaterial integration. Here, we extend such knowledge by simulating, at a hybrid DFT level of theory, different HA surface terminations, both stoichiometric and non-stoichiometric, as free and in interaction with water. Such a goal is achieved at an unprecedented accuracy, with a large all-electron basis set and including dispersion forces contributions. The calculated results are then compared with experimental micro-calorimetric data, showing a good agreement in the loading trend of the (010) surfaces. More generally, this theoretical approach is confirmed to be an efficient tool to analyze these biomaterials, giving the possibility to investigate the HA behavior toward more complex molecules, from amino acids to collagen, at the here-presented level of theory, to shed some light on the complex biomineralization process of human bones and teeth.