Mechanism of Action of Adsorbed Fluoride Ions on the Dissolution Kinetics of Apatite Powder

Abstract The kinetics of dissolution of calcium hydroxyapatite powder (HAP) is studied at 37°C at a constant pH 5 or 6 in the presence of 1 or 10 ppm fluoride ions. Experiments were carried out with an automatic setup which recorded continuously the proton uptake and the concentrations of calcium and fluoride in solution, using specific electrodes. We demonstrate that fluoride ions have a dual action. Their presence at the interface reduces the apatite solubility, and, as a consequence, the equilibrium is attained earlier. However, the adsorption of fluoride ions onto the apatite surface accelerates the initial dissolution process. Both processes are fully interpreted using the quantitative model recently proposed for HAP dissolution. According to this model, apatite dissolution is autoinhibited by the adsorption of calcium ions at the solid interface, thus forming a cationic semipermeable layer. Fluoride ions interact with this layer and decrease its autoinhibiting ability, especially in the case of higher fluoride concentrations where interfacial CaF 2 is formed.