Surface properties and mineralization kinetics of calcium phosphates and calcium sulfates

The mechanism of crystallization of fluorapatite (FAP) has been investigated using the constant solution composition method. The rate data, as a function of supersaturation, were interpreted in terms of the Burton-Cabrera-Frank (BCF) model. Experiments with non-stoichiometric solutions indicated that the rate was not a single valued function of supersaturation but depended on the solution molar ratio of lattice ions. This dependence was explained in terms of the zeta-potentials of the microcrystals which governed FAP dispersion stability against flocculation. Thus flocculation was shown to be an important factor in determining the reaction rates.; The influence of clinically important dentifrice additives, pyrophosphate, zinc and magnesium ions on the mineralization of FAP was studied at a series of constant additive concentrations. All three ions inhibited the crystallization of FAP in order of decreasing effectiveness: pyrophosphate {dollar}>{dollar} zinc {dollar}>{dollar} magnesium. It was speculated that the additives exerted their inhibition action through adsorption on the growing crystals, thus increasing the interfacial energy.; The interfacial energy of calcium phosphate powders was determined through contact angle measurements using the thin layer wicking technique. The much smaller interfacial energy of octacalcium phosphate (OCP) in contact with water compared with that of hydroxyapatite (HAP) supported the observation that the crystallization of the latter was usually preceded by OCP and that OCP could serve as a template for apatite growth.; The dispersion stability of calcium phosphates of colloidal dimensions was investigated. It was found that the relative stability of OCP, dicalcium phosphate dihydrate (DCPD), and HAP dispersions could be explained by invoking the concepts of Lewis acid-base interactions in addition to the two other more traditional types of interactions that are involved in the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory: electrical double-layer repulsion and van der Waals attraction. On the other hand, the dispersion of FAP seemed to follow DLVO theory quite well, presumably due to fluoride substitution for the hydroxide in the HAP lattice resulting in a dramatic reduction in Lewis base parameter.; This thesis also includes a study of gypsum (CSD), a very important building material. Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) have revealed its epitaxial crystallization from structurally similar DCPD.