Abstract
A model describing the kinetics of dissolution of calcium hydroxyapatite, Ca 10(PO 4) 6(OH) 2, microcrystals in aqueous solution is presented. In this model the rate-controlling step is formation of dissolution nuclei, the lateral growth rate of which depends on the concentration of hydrogen ions. This dependence on hydrogen ion concentration is explained by hydrogen ion catalysis of the exchange of phosphate between the crystal surface and the solution. According to this model, the rate of dissolution should be practically zero when the size of a critical dissolution nucleus is comparable to the ratio of the surface area to the number of inhibitor units adsorbed to the crystal surface. A simple model for Langmuir adsorption of inhibitors onto crystals, taking into account the protolytic forms of the inhibitor in the solution and on the crystal surface is presented. The effects of pyrophosphoric acid, 1-hydroxy-ethylidene-1, 1-diphosphonic acid, methylene diphosphonic acid, 3-phosphoglyceric acid, 3-phosphoserine, 3-amino-1,1,3-propanetricarboxylic acid (γ-carboxyglutamic acid) and phosphoethanolamine on the rate of dissolution of calcium hydroxyapatite crystals in aqueous syspension at pH = 6.8 and pH = 7.2 have been studied.
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