Reaction characteristics of hydroxyapatite with F– and PO3F2– ions. Chemical states of fluorine in hydroxyapatite

Abstract

The reactions of F^– and PO₃F^2– ions with hydroxyapatite (HAP) are of interest owing to their dental application. The mechanism of the anti-caries effect of the fluorides, however, is still not clearly understood. In the present study the reaction characteristics of HAP with sodium fluoride (NaF) and sodium monofluorophosphate (Na₂PO₃F) in aqueous solutions have been investigated using various techniques. F^– ions were more easily taken up by HAP than were PO₃F^2– ions, but no significant change in the X-ray diffraction pattern was observed, though CaF₂ was formed only in acidic solutions. X-Ray photoelectron spectroscopy studies have shown that F^– ions were ion-exchanged for structural OH^– ions of HAP to produce fluorapatite in neutral solution at an F^– concentration of 1000–10000 ppm, and that PO₃F^2– ions were chemisorbed onto the HAP surface or ion-exchanged for HPO^2–₄ ions. The PO₃F^2– ions were then partially hydrolysed to free F^– and orthophosphate ions, and this was followed by incorporation of F^– into HAP. Na⁺ ions were not incorporated into HAP in contrast to the divalent cations such as Sn²⁺ and Zn²⁺ ions reported previously.