Diffusion Controlled Dissolution and Pre

Dissolution and precipitation reactions that occur in biological systems and in non-stirred in vitro systems generally are diffusion-controlled. Subsurface demineralization of tooth enamel in dental caries, which has been the subject of considerable interest, is an example of such processes. In the caries process ions from solubilized tooth mineral, i.e., Ca, PO4 and OH, need to be transported from the lesion below the tooth surface to the plaque and eventually into the saliva. Concomitantly, plaque acid ions must diffuse into the caries lesion to affect demineralization. Several mathematical models have been proposed to describe the diffusion process during caries lesion progression. An important factor which until recently had not been taken into consideration in caries models is the effect of the difference in rates of diffusion of the various ions that participate in the demineralization process. We report here the results of a study in which a diffusion cell comprising two compartments separated by a membrane of known ion permselectivity was used as an experimental model to study how the dynamics of diffusion affect dissolution and precipitation of hydroxyapatite and other sparingly soluble calcium phosphates.