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The kinetics of seeded crystal growth of calcium apatites were studied in dilute supersaturated solutions at various levels of fluoride concentrations. Initial precipitation rates were enhanced by fluoride concentrations higher than 0.05 ppm. The analytical results are consistent with the precipitation of fluoridated hydroxyapatites, CaF-(OH)(PO), FHA. The degree of fluoridation, X, appears to be determined by the activity of HF in solution, which varies for the various initial fluoride levels but remains fairly constant during precipitation. Thus the composition of the precipitating phase was the same for a given solution whether 25 or 10 mg of hydroxyapatite was added as seeds. All the experimental results are consistent with the BCF theory, which relates the mean linear rate of growth, R, to the supersaturation, DS, by the expression R=CT(DS-1)1n(DS)tanh(C/T 1n DS), in which DS is the supersaturation defined by mean molar activities with respect to the precipitating FHA, T the absolute temperature, and C and C are constants calculated from the experimental results. Consequently, the crystal growth appears to take place in surface kinks and to be controlled by surface diffusion. Since crystal growth in most biological systems takes place at fluoride concentrations within the experimental range used, it seems probable that it occurs along the model advanced in the present investigation. [ABSTRACT FROM AUTHOR]

The effect of fluoride derived from fluorhydroxyapatite (FHAp) minerals on bacterial glycolysis under aerobic and strictly anaerobic conditions was studied to validate the claims that this mineral could be used as a reservoir of fluoride in plaque. To isolate the direct effect of fluoride on bacterial glycolysis from that of an indirect pH-buffering effect of hydroxyl or phosphate ions which are also dissolved from the mineral, we equalized the pH-fall time course of reactions by manually adding KOH or HCI. This ensured that pH effects on glycolysis were minimized. Under controlled pH-fall and strictly anaerobic conditions, fluoride derived from the dissolution of FHAp containing more than 30,100 ppm fluoride (i.e., when the substitution of OH by F in the mineral was greater than 80%) had a direct inhibitory effect on lactic acid production in Streptococcus mutans. Under free pH-fall and strictly anaerobic conditions, increasing amounts of fluoride in FHAp (starting as low as 2000 ppm fluoride), appeared to have a pronounced indirect inhibitory effect on lactic acid production. This was probably mediated through a reducing pH buffer effect of the mineral. Even in the presence of high-fluoride FHAp, only 0.01 to 0.025 mmol/L fluoride was found in the reaction mixtures, a probable result of non-stoichiometric dissolution of FHAp. In spite of such low levels of fluoride, marked inhibitory effects on bacterial glycolysis were demonstrated. The results of this study suggest that high- fluoride FHAp may serve as a reservoir of fluoride for the inhibition of anaerobic acid production by S. mutans. [ABSTRACT FROM AUTHOR]

This work shows that mechanochemical synthesis allows the obtaining of hydroxyapatite in which fluorine ions replace hydroxyl ions either partially or completely after 30 ​min of synthesis. The crystal structure of the materials obtained and their thermal stability when heated in the air were studied. It has been shown that the fluorine ions in the hexagonal channel of hydroxyapatite hinders diffusion of hydroxide ions at high temperatures, which leads to increased thermal stability of the apatite. Image 1 • A new approach to the synthesis of F-substituted apatites is proposed. • Dry mechanochemical method allows reducing the synthesis time down to 30 ​min. • The product of synthesis is single-phase with an average crystallite size of 25 ​nm. • Partially and fully substituted F-apatites are stable up to 1400C. [ABSTRACT FROM AUTHOR]