Abstract
In this study we investigated the adsorption kinetics of fluoride ions on the surface of hydroxyapatite (HAP). Time dependence of adsorption was measured in both an aqueous solution and in a foam solution containing fluoride ions. The amount of fluorine ions adsorbed was determined using a F- ion electrode in the buffer solution at pH 5.3, after HAP particles adsorbed with fluoride ions were dissolved with perchloric acid. Our results were as follows: (1) The formations of ion complexes or ion pairs could not be detected in any direct interactions between the foam making surfactants and NaF. (2) Sodium dodecylsulfate ions, which were one of foam making surfactants, were not adsorbed on the HAP surfaces. (3) The adsorption kinetics of fluoride ions on the HAP surfaces from the solution in foam was about 10 times faster than that from the bulk aqueous solution. (4) The decay rate of a slice specimen of human molar tooth, which had been treated for 3 min with the solution or the foam at the same concentration of fluoride ions, was investigated by dipping the specimen in an acetic acid solution at ambient temperature. Experimental observations by the CMR technique agreed with the results obtained in the HAP study, in that the foam treatment appeared to be more effective in preventing decay because the fluoridation rate with the foam was faster than the rate with the solution. (5) Physicochemical analysis was conducted to investigate why adsorption of the fluoride ions was faster than in the foam solution, and from the results we speculate that the electrostatic effect of hetero-electric double layers generated between the HAP surface and the adsorbed surface of SDS in foam focilitated the effect. In conclusion we suggest that this effect makes foam very useful in suppling fluoride ions onto the surface of teeth.
