| Description |
The physical-chemical behavior of calcium hydroxyapatites in various mineralizing and demineralizing conditions has been studies for decades with the purpose of discovering its intrinsic influence on the mechanisms and extent of diseases of biologic hard tissue. There has been an assumption that thermodynamic equilibrium is reached in these experiments. All biologic and synthetic calcium apatites studies so far in our laboratory have, however, not shown the expected results of reaching thermodynamic equilibrium. Instead, they have shone a distribution of the metastable equilibrium solubility (MES) phenomenon. That is, on a time scale of months or less, a given biomineral does not reach thermodynamic equilibrium in partially saturated solutions but rather has a distribution of apparent solubilities-more properly, a distribution of metastable equilibrium solubilities. This thesis interprets the MES phenomenon in terms of mineral properties such as crystal morphology, specific surface area, microstrain, crystallite size, and carbonate content. In dissolution studies of constant solution composition, with a carbonated calcium hydroxyapatite (5.0 weight percent carbonate), it was found 1) pits are formed in the apatite crystals when exposed to severe undersaturation conditions, 2) vales of MES correlate with microstrain but not with crystallite size, 3z) the carbonate content ( and c-axis parameter) of the mass which remains after dissolution has occurred in less than tan of the original mass, 4) the specific surface area increases, and the crystallite size decreases, with dissolution. Is was concluded that the dissolution behavior of bulk calcium apatite is related to the linear an independent dissolution behavior of its component domains. This thesis secondarily evaluates the published value of the thermodynamic solubility product, K(SP,HAP), of the U.S. Government's standard for calcium hydroxyapatite, Standard Reference material 2910. The K(SP,HAP), claimed for this material in 10(-117.4), However, this value is consistent with the MES of only the more-soluble 10% of the material. The mean pKHAP of the material is less soluble be several pK units than the reported , K(SP,HAP). |
