Effects of pH and Fluoride on Zeta Potential of Hydroxyapatite

Previous studies have reported zeta potentials of hydroxyapatite (HA) as a function of pH and solution compositions. Objective: To study zeta potential of HA under pHs and fluoride concentrations ([F]) relevant to oral F rinses. Materials and Methods: HA slurries were prepared by suspending 250 mg of HA in 100 mL of a solution that contained (0, 1, or 12) mmol/L of NaF. The solution also contained NaCl such that [NaCl]+[NaF] = 30 mmol/L in all cases. HA zeta potentials were measured using a Malvern Zetasizer instrument with the pH of the slurry adjusted from pH 9 to pH 4.5. F uptake by HA was determined by analyzing the F contents of the HA particles removed from the slurry at different pHs. Results: In the absence of F, HA zeta potential (mean ± standard deviation; n=3) increased linearly (R²=0.999) from (-16.2±4.8) mV at pH 9 to (6.7±0.8) mV at pH 4.5. Under [F] of 1 mmol/L, the zeta potential also increased approximately linearly (R²=0.987) from (-38.2±6.1) mV to (12.9±0.9) mV in the same pH interval. Under [F] of 12 mmol/L, the zeta potentials increased from (-43.0±7.1) mV to (14.4±1.2) mV, but it reached a maximum of (18.4±1.9) mV at pH 5.5. F uptake by HA, which ranged from 1190±20 to 9840±140 ppm, increased significantly (p<0.05) with decreasing pH or increasing [F]. Conclusions: F produced opposite effects on HA zeta potential depending on the pH. The decreases in the zeta potential at higher pHs suggested fluorapatite formation, while the increases under lower pH conditions can be attributed to CaF₂ formation. For a given [F], F uptake by HA increased with decreasing pH. Supported by NIDCR grants DE020824 and DE21148, Howard University, ADAF, and NIST.