Crystal growth of calcium apatites from dilute solutions

Abstract

The dependence of the kinetics of crystal growth of hydroxyapatite, HA, on concentration of seeds and degree of solution supersaturation is examined. Empirical relations are obtained between the initial rates of precipitation and the two aforementioned variables. The results clearly indicate that the precipitating phase, under the selected experimental conditions, is HA. The driving force for precipitation is the supersaturation with respect to this calcium phosphate. The data obtained from kinetic studies are consistent with the BCF theory of crystal growth which relates the mean linear rate of growth to the degree of supersaturation. This observation is also valid for systems in which the precipitating phases are fluoridated hydroxyapatites, Ca 5F x (OH) 1− x (PO 4) 3, FHA. In this case, supersaturation is calculated with respect to the specific FHA precipitated. The degree of fluoridation, x, appears to be determined by the activity of hydrofluoric acid in solution which remains fairly constant during precipitation but varies for each initial fluoride concentration used. Studies are reported on the kinetics of crystal growth of HA in the presence of biological inhibitors. Two proline-rich proteins and a peptide, statherin, of salivary origin are powerful inhibitors of HA crystal growth. It appears that their mode of action is related to their adsorption onto the surface of apatite seeds. The results suggest that the adsorption sites are the same as the sites where crystal growth takes place.