METASTABLE EQUILIBRIUM SOLUBILITY BEHAVIOR OF CARBONATED APATITES

The purpose of this investigation was to examine the heretofore not co nsidered hypothesis that carbonated apatite (CAP) preparations exhibit the phenomenon of metastable equilibrium solubility (MES). The MES ph enomenon may arise when crystal dissolution rates are very much greate r than crystal-growth/precipitation rates when compared at equal but o pposite (in sign) chemical potential differences between a crystal pha se and solution. Preliminary studies had suggested that CAP preparatio ns may be expected to exhibit the MES phenomenon and yield apparent so lubilities very far removed from true thermodynamic equilibrium solubi lity. CAP samples prepared in our laboratories as well as samples prep ared elsewhere were studied in the following manner. MES and MES distr ibutions were determined by equilibrating under stirred conditions eac h of the CAP samples in a series of acetate buffers containing various levels of calcium and phosphate ions corresponding to various magnitu des of the solution ion activity product K-HAP = (a(Ca))(10)(a(OH))(6) (a(OH))(2). A large solution-to-solid ratio was used to ensure that ch anges in the solution K-HAP before and after equilibration would be ne gligible. In a typical set of experiments, the residues were recovered after 1, 2, 4, and 8 days, dried, and weighed to determine whether ME S had been attained. Generally, dissolution proceeded rapidly and, aft er 2 to 4 days, further CAP dissolution could not be detected. The res ulting apparent solubilities were plotted as fraction of CAP dissolved versus pK(HAP) and, typically, a normal distribution function satisfa ctorily fitted the experimental data. In order to demonstrate that the results corresponded to a true MES and not a steady-state situation i nvolving equal dissolution and precipitation rates, (a) IR spectra of the residues were obtained and (b) residues from lower K-HAP equilibra tions were reequilibrated in buffers with higher K-HAP values. Both th ese procedures showed that the apparent CAP solubilities and solubilit y distributions were closely consistent with the MES phenomenon and no t a steady-state condition. Importantly, it has been shown in a separa te study in this laboratory (J. Hsu, in press) that the MESs and MES d istributions of the CAPs also govern the surface-controlled kinetics o f CAP crystallite dissolution in acetate buffers. One infers from this that the CAP MESs determined in the present study govern dissolution of CAPs over, at least, a time-scale range from seconds to 10(5) s. Al so, this appears to be the first time that the driving force for disso lution of apatite crystallites has been independently and directly qua ntified via ''solubility'' measurements. (C) 1994 Academic Press, Inc.