| A complete understanding of the demineralization processes involving mineralized tissues, viz. bone and dental enamel, has eluded researchers because of the poor knowledge of apatite solubility. Such insights will be valuable in our assessment of pathological disorders like osteoporosis and dental caries, and would allow for the development of better therapy for its prevention and cure. The present study used synthetic carbonated apatites (CAPs) as model compounds for characterizing the solubility behavior of biological apatites.; Research in our laboratory on CAPs has demonstrated that they exhibit the metastable equilibrium solubility (MES) distribution phenomenon. The purpose of this research was to investigate the applicability of the MES concept to a wide range of CAPs that differed in carbonate content, temperature of synthesis, method of preparation, and crystallinity, and to test our hypothesis that the MES of a given CAP is governed by a surface complex with a stoichiometry different from that of the dissolving apatite.; The determination of the correct stoichiometry for the surface complex that governs the dissolution of CAPs required the accurate knowledge of all the equilibria parameters applicable in the dissolution system. Hence the first part of this research was aimed at the determination of the essential equilibrium constants relevant in calcium-hosphate systems in acetate buffers using dicalcium phosphate dihydrate (DCPD) as a probe.; MES experiments were conducted using fifteen CAP samples over a wide range of CAP crystallinity, in partially saturated dissolution media prepared in 0.1M acetate buffer, to which calculated amounts of calcium and phosphate were added for achieving the desired ion activity products. A wide range of solution compositions was used for the study by choosing different calcium-to-phosphate ratios and pHs. Also, the influence of the equilibration time on the determined surface complex stoichiometry was investigated.; The following were the important findings. All CAP samples of high crystallinity yielded surface complex stoichiometries close to that of hydroxyapatite {lcub}Ca 10(PO4)6(OH)2; Ca/P ratio = 1.67{rcub} The results of experiments with CAPs of low crystallinity were consistent with surface complex stoichiometries possessing a somewhat lower Ca/P ratio of around 1.55.; A late development in this research was the discovery that some of the fifteen CAP preparations of the study exhibited a "volume effect" (or slurry density effect), i.e., for some of the samples, the determined MES was found to be a function of the solution solution-to-solid ratio. Further work is warranted to clarify the uncertainties associated with this matter. |
