Adsorption and Substitution of Metal Ions on Hydroxyapatite as a Function of Crystal Facet and Electrolyte pH

Zinc and stannous ions are commonly used in oral care to reduce tooth enamel degradation. However, mechanistic understanding of the role of the ions in the protection of enamel against acid insults remains inadequate due to limitations of experimental techniques to validate interfacial interactions at the atomic scale. We overcome this problem by the examination of adsorption and subsurface exchange of the ions on common hydroxyapatite (001) and (010) surfaces in contact with electrolytes at pH values of 5 and 7 using molecular dynamics simulations in unprecedented accuracy. The surface chemistry under these conditions is characterized by the presence of dihydrogenphosphate ions and a 70/30 mixture of dihydrogenphosphate ions and monohydrogenphosphate ions. Zn(II) and Sn(II) ions favorably adsorb and coat the surfaces under all conditions, with stronger attraction at pH 5 than at pH 7 and a preference for the prismatic (010) surface over the basal (001) surface. Subsurface substitution is only significant for Zn(II) ions at pH 7 in small concentrations up to 6 mol % with free energies between 0 and 20 kcal/mol on both surfaces and largely unfavorable for Sn(II) ions. Zn(II) and Sn(II) ions can therefore coat the enamel surface and it is likely that Zn2+ ions incorporate below the surface and play a role to stabilize apatite surfaces from dissolution. Computed substitution free energies, lattice strains up to 1.5%, and changes in X-ray data agree very well with available experimental data for bulk apatites. The results provide first quantitative insights into enamel surface stabilization, and the methods can be applied to other mineral phases.