Computational study of interfaces between hydroxyapatite and water

Abstract

The hydration of (001) surfaces of hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂) was elucidated by means of molecular dynamics simulation. The investigated surface types were derived from differently cutting the ideal crystal structure perpendicular to the [001] direction. Three surface types with rather different electrostatic character were investigated. Both dynamical and static properties were calculated. During the hydration process, water was found to form strong electrostatic interactions with the calcium and the phosphate ions of the crystal surface. This leads to several hydration layers exhibiting a strongly ordered character. Within these layers water mobility is reduced drastically. For all surface types the embedding calcium triangles were found to prevent the interfacial water molecules from forming hydrogen bonds with the hydroxide ions. We therefore conclude that the apatite hydroxyl ions may not be directly protonated by aqueous protons. The decomposition of the embedding calcium triangles may be expected to be a prerequisite of such a process.