Effects of neutral salts in a bench-scale caries model

In an earlier paper on bench-scale simulation of the caries process, it was shown that the passage of ions through ion-permselective barriers could have profound effects on the composition of the solution within the "lesion" at steady state. As indicated in earlier papers, these changes are produced by unequal rates of diffusion of Ca and PO4 ions prior to reaching steady state. Comparable effects are attributable to F ions when present. Here, we used the same two-compartment diffusion apparatus and membranes, as described in the earlier paper, to show that a neutral salt, such as NaCl, disproportionates under the influence of membrane potential. Thus, although the Na and Cl concentrations are nearly equal in the "plaque-saliva" compartment, they become very different in the "lesion" solution. An excess of Na over Cl is equivalent to the presence of the component NaOH, and an excess of Cl over Na is equivalent to the presence of HCl. With the highly permselective commercial membranes used in these experiments, the Ca/P ratio in the "lesion" solution changed from an initial value of 1.6 to a value as high as 53 or as low as 0.1 at steady state. These phenomena are relevant to the events taking place in a caries lesion and must be taken into account in devising physicochemical models of the caries process. A valid caries model, in turn, offers the possibility of identifying steps in the caries mechanism which might be blocked to prevent tooth decay.