Theoretical Calculations of Segregation Behavior of Zinc and Magnesium at Hydroxyapatite Surface in Contact with Water

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Density functional theory (DFT) calculations were performed to investigate atomic structures and segregation behavior of Zn²⁺ and Mg²⁺ ions at a (10\bar{1}0) surface of hydroxyapatite (HAp). In order to take account of aqueous solution environment surrounding the HAp surface, the conductor-like screening model (COSMO) was used. Ionic exchange energies of Zn²⁺ and Mg²⁺ at Ca sites around the surface were evaluated, under an assumption of chemical equilibrium between HAp and aqueous solution. It was found that Zn²⁺ can segregate energetically more favorably at the HAp surface, as compared to Mg²⁺. This may correspond to Zn²⁺ effects on inhibition of HAp crystal growth, as suggested by experiment.