Probing the surface activity of hydroxyapatite nanoparticles through their interaction with water molecules

Hydroxyapatite nanoparticles (nano-HAPs) are often used to promote the performance of various biomaterials, but their surface activity can be hardly evaluated directly. An indirect approach of using water molecules as a probe to measure the surface activity of nano-HAPs is proposed in this work. The interaction of nano-HAPs with their surrounding water molecules is studied by using a two-step simulation for the isothermal–isobaric and canonical ensembles successively. A part of water molecules forms a dense and structured layer around the particle, while the remaining molecules are in a disordered state. The particles annealed under high temperature show a lower activity toward water than the corresponding unannealed ones. Moreover, the small-sized particles possess greater interaction energy with water molecules than the large-sized particles, which indirectly reflects the higher activity of small-sized HAPs. Our computations not only suggest a new approach to study the surface activity of nanoparticles but also address the structure–activity relationship of nano-HAPs, which would be useful for regulating the surface activity of HAP-based biomaterials.