Metastable Equilibrium Solubility Behavior of Carbonated Apatites

Abstract

The purpose of this investigation was to examine the heretofore not considered hypothesis that carbonated apatite (CAP) preparations exhibit the phenomenon) of metastable equilibrium solubility (MES). The MES phenomenon may arise when crystal dissolution rates are very much greater than crystal-growth/precipitation rates when compared at equal but opposite (in sign) chemical potential differences between a crystal phase and solution. Preliminary studies had suggested that CAP preparations may be expected to exhibit the MES phenomenon and yield apparent solubilities very far removed from true thermodynamic equilibrium solubility. CAP samples prepared in our laboratories as well as samples prepared elsewhere were studied in the following manner. MES and MES distributions were determined by equilibrating under stirred conditions each of the CAP samples in a series or acetate buffers containing various levels of calcium and phosphate ions corresponding to various magnitudes of the solution ion activity productKHAP = (aCa)10(aPO46(aOH)2.A large solution-to-solid ratio was used to ensure that changes in the solution KHAP before and after equilibration would be negligible. In a typical set of experiments, the residues were recovered after 1, 2, 4, and 8 days, dried, and weighed to determine whether MES had been attained. Generally, dissolution proceeded rapidly and, after 2 to 4 days, further CAP dissolution could not be detected. The resulting apparent solubilities were plotted as fraction of CAP dissolved versus pKHAP and, typically, a normal distribution function satisfactorily fitted the experimental data. In order to demonstrate that the results corresponded to a true MES and not a steady-state situation involving equal dissolution and precipitation rates, (a) IR spectra of the residues were obtained and (b) residues from lower KHAP equilibrations were reequilibrated in buffers with higher KHAP values. Both these procedures showed that the apparent CAP solubilities and solubility distributions were closely consistent with the MES phenomenon and not a steady-state condition. Importantly, it has been shown in a separate study in this laboratory (J. Hsu, in press) that the MESs and MES distributions of the CAPs also govern the surface-controlled kinetics of 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 105 s. Also, this appears to be the first time that the driving force for dissolution of apatite crystallites has been independently and directly quantified via "solubility" measurements.