Reaction characteristics of dental and synthetic apatites with Al3+ and La3+ ions in acidic solutions

Abstract

Previous studies have shown that divalent cations such as Sn²⁺, Zn²⁺ and Fe²⁺ are taken up by dental and synthetic hydroxyapatities by a cation-exchange reaction with molar ratios of Ca²⁺/Sn²⁺, Zn²⁺, Fe²⁺≈ 1.0. In the present study the reaction characteristics of dental apatite and synthetic hydroxyapatites, the latter being model substances of the dental apatites, have been investigated in detail in trivalent cation solutions. In acidic solutions with pH values below 4.0, Al³⁺ ions were easily taken up by apatite samples to yield stable amorphous AlPO₄·nH₂O. In alkaline solutions at pH 11.0, however, the Al(OH)^–₄ formed in solution had a low reactivity with the hydroxyapatite. La³⁺ ions reacted with the apatite samples to form stable crystalline LaPO₄ in acidic solution. The ionic radii of Ca²⁺(1.05 Å) and La³⁺(1.22 Å) are close enough together that solid solutions could be formed. XPS studies, however, suggested that an ion-exchange reaction forming a solid solution of La-H apatite would be unlikely. Both Al³⁺ and La³⁺ ions were taken up into the apatites by an attack reaction of these ions for Ca²⁺ ions of the apatite samples, assisted by loosening the crystal structures of the apatite samples with H⁺ ions in solution. These results should provide important physicochemical information for clinical investigations of the roles played by the trivalent cations in inhibiting dental caries and dentine hypersensitivity.