Influences of carbonate content and crystallinity on the solubility behavior of synthetic and biological apatites

The purpose of this research was to apply the metastable equilibrium solubility (MES) distribution concept previously developed, to CAPs synthesized by different methods and to bone mineral and also to examine the influences of the important physicochemical variables such as carbonate content, temperature of synthesis, and the crystallinity on the MES behavior of CAPs. In the first part of this research the x-ray powder diffraction full width at half maximum (FWHM) of the 002 reflection was taken as a measure of crystallinity. The MES distributions were determined by a previously developed technique. Each of the CAP preparations were found to possess an MES distribution. The crystallinities of the CAPs decreased and the MES values increased with increasing carbonate content and decreasing temperature of synthesis. Furthermore, a plot of the mean MES value against the CAP crystallinity revealed that the mean MES was a single valued function of crystallinity, i.e., when crystallinity was taken into account, there was no additional effect of carbonate upon the MES. The MES distribution concept appeared to be holding for bone apatites obtained from rats of different ages and the changes in the crystallinity of the bone apatite with age showed a good correlation with the corresponding MES values. By varying the pH of the acetate buffer solutions the MES controlling surface complex function was also deduced. A surface complex with HAP stoichiometry was found to be the function that best accounts for the MES behavior of bone apatite and CAP. Also, for the first time, an excellent synthetic prototype was obtained that closely mimics the physicochemical behavior of bone mineral. The effects of crystallite size and microstrain on the MES of the CAPs and human dental enamel were also investigated to determine the major solubility controlling factor. Two different approaches were used for the separation of crystallite size and microstrain components of the broadened intrinsic diffraction profiles of the CAPs and human dental enamel. The method of individual profile fitting within small angular ranges yielded values for the crystallite size and microstrain parameters that were consistent with those obtained by the Rietveld method of pattern-fitting structure-refinement. The values of the crystallite size parameters also showed a good correlation with the specific surface areas of the CAPs determined by the BET method. Finally, a strong correlation was obtained between the mean MES and the microstrain parameter whereas the crystallite size parameter exhibited a rather poor correlation with the MES values.