Dissolution kinetics of carbonate apatite and effects of solution ions

Recently, the concept that there exists a distribution of metastable equilibrium solubilities (MES) in biological dental minerals and synthetic apatites was proposed by Higuchi and Fox and coworkers at the University of Utah. The aim of this research was to employ the MES concept in a physical model to study quantitatively the dissolution kinetics of carbonate apatites (a synthetic model material for dental enamel) and to use this model in examining the effects of two important solution ions, pyrophosphate and fluoride. The dissolution kinetics of carbonate apatite (CAP) in powder suspensions was first studied. A novel quenching technique was developed that became a powerful experimental tool capable of following dissolution kinetics from 2 seconds to 2 hours. The results of the dissolution kinetics of CAP obtained from this study are consistent with a physical model for dissolution that assumed first-order kinetics and a dissolution rate controlling surface complex with the stoichiometry of hydroxyapatite, as well as a distribution of MES. This study demonstrated a quantitative relationship between the experimentally determined MES distribution and the dissolution kinetics of CAP. The dissolution kinetics of CAP was further explored in the presence of a dissolution inhibitor, pyrophosphate. The theoretical model was able to simulate the dissolution kinetics of CAP in both powder suspensions and a rotating disk system in the presence of pyrophosphate reasonably well. This demonstrated the potential for the broad application of the physical model and suggested that the model may be useful in studies of other dissolution inhibitors. The effect of fluoride in solution phase on the dissolution of CAP compressed disks and bovine teeth was also studied. A model independent data analysis (MIDA) technique was applied to calculate the intercrystalline solution composition inside the mineral matrix of a dissolving CAP compressed disk or an enamel block. The results of the data analysis supported the concept that a surface complex with fluorapatite stoichiometry on the surface of the crystals of CAP or enamel controlled the local microenvironmental threshold for dissolution. This, in turn, was responsible for the formation of lesions beneath the surface, leaving the region near the surface intact.