Diffusion Controlled Dissolution and Precipitation*

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, P04, and OH, need to be transported from the lesion below the tooth surf'ace 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 (Figures 1. and 2.) 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. Effect of membrane permselectivity on the composition of »lesion« solution: In this bench-scale simulation of the caries process one compartment (the »lesion«) contained an excess of hydroxyapatite, and the solution was kept saturated by stirring. An undersaturated acidic calcium phosphate solution flowed continuously through the other compartment (the »plaque-saliva«), thus providing the driving force for dissolution of the crystals as modified by the permselectivity of the membrane. It was found that the passage of ions through the permselective barrier has profound effects on