EFFECTS OF ALUMINUM(III) AND FLUORIDE ON THE DEMINERALIZATION OF BOVINE ENAMEL POWDER AND HYDROXYAPATITE - IN-VITRO KINETIC-STUDIES SHOWING SYNERGISTIC INHIBITION

The effect of Al(III) and fluoride on the rate of dissolution of powdered enamel in 0.1 M lactate solution has been studied at pH 4.5 and 5.0. The dissolution rate of synthetic hydroxyapatite (HAP) microcrystals in similar solutions is reported for pH 5.0. A combination of Al(III) and fluoride has a stronger inhibitory effect on dissolution of enamel and HAP at pH 5.0 than that of these substances separately, even though the concentration of free fluoride ions decreases in the presence of Al(III). The effects on dissolution of enamel at pH 4.5 are similar, but less pronounced. It is suggested that Al(III) and its major complexes is slightly acidic solution, being positively charged, are adsorbed at different sites on the mineral surface from fluoride, thereby producing a synergistic effect. When the dissolution process takes place far from equilibrium, inhibitor concentrations greater than about 100muM (approximately 3 ppm for Al(III) and approximately 2 ppm for F-) show significant inhibition under the conditions studied. In such solutions the real concentration of Al3+ is of the order 1muM. Close to equilibrium, the total inhibitor concentration need only be of the order 1muM (i.e. 0.03 ppm for Al(III), 0.02 ppm for F-) for a strong effect. In such solutions, the real concentration of Al3+ is in the nanomolar range.