Effect of Carbonate Content and Crystallinity on the Metastable Equilibrium Solubility Behavior of Carbonated Apatites

Abstract

The purpose of this investigation was to assess the applicability of the metastable equilibrium solubility (MES) concept, previously developed in our laboratory, over wide ranges of two independent variables, carbonate content and crystallinity, and also to examine the influences of these variables on the MES behavior of carbonated apatites (CAPs). The CAP samples were prepared by dicalcium phosphate dihydrate (DCPD) hydrolysis for 48 h in NaHCO3-containing media at 95, 70, and 50°C. This method of preparation gave CAP samples with varying carbonate contents and crystallinities. A previously developed technique with slight modification was used to determine the MES distribution for each of the CAP samples. The equilibration solutions were prepared both with and without extraction of fluoride with hydroxyapatite (HAP). From X-ray diffraction, the full width at half-maximum (FWHM) of the 002 reflection was used as a measure of crystallinity. The findings of this study showed that each of these preparations possessed an MES distribution and therefore provided further support that the MES distribution is a common phenomenon describing the dissolution behavior of CAPs, regardless of their carbonate content and crystallinities. The crystallinities of the CAPs decreased and the MES values increased with increasing carbonate content and decreasing temperature of synthesis. A plot of the mean MES value against 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 on the MES. When fluoride was not extracted from the equilibration solutions, the MES shifted to lower values. The concept of MES distribution and its dependence on the crystallinity of CAP may provide insight into the mechanism of dissolution of biological apatites, which may be considered imperfect crystalline substances.