The effect of resin particle size on the rate of ion release: interactions in mixed bed systems

The aim of the present study is to evaluate the influence of resin particle sizes on the rate of ions release from a mixture of ion-exchange resins (named NMTD) which supplies calcium, fluoride, and phosphate ions as the main mineral content, and to elucidate the different phenomena taking place through the related ion-exchange process. The final goal of the study, related to dental application (enamel restoration), is to limit the particle size range, since the rate of ion release is a key parameter in the successful achievement of such objective. Weak-type ion-exchange resins, loaded with the appropriate ions, were ground and sieved into granulometric fractions of bead diameters of 0.1–0.075, 0.075–0.063, and 0.063–0.05 mm. Particle size was controlled by a laser diffraction particle distribution analyzer. The experiments on the kinetics of ions release were carried out under batch conditions in artificial saliva desorption solution thermostatized at 37 °C. The release of Ca²⁺ and F^– was determined by corresponding ion-selective electrodes automatically controlled, whereas H₂PO₄^– was measured spectrophotometrically by the inductively coupled plasma–optical emission technique (ICP-OES). The results of this study show that the process of ion-exchange for the different particle size fractions of resins is critical for the study of the kinetics release of the ions immobilized in the corresponding mixed bed polymeric matrices. In fact, despite the apparent narrow range of particle sizes of the mixed bed systems studied, appreciable differences in the rate of ions release are obtained. Since the ion release rate is depending on the contact surface, an increase of factor of 2 in particle size represents an increase of an order of magnitude of the resin contact surface due to the resin porosity. In this concern, it has been observed that the rate of ions release increases when particle size decreases. The interactions occurring during the ion release from the mixed bed resins (containing calcium-, fluoride-, and phosphate-loaded resins) can be interpreted by the following phenomena: H₂PO₄^–, which hardly modifies its rate of release in the presence of Ca²⁺ and F^– in the mixture, promotes a considerable increase in the rate of Ca²⁺ release due to the formation of a calcium dihydrogen phosphate soluble complex. F^– also produces an acceleration in the rate of Ca²⁺ release due to the formation of solid CaF₂ on the surface of cationic resin particles, which in contrast leads to a decrease in the rate of F^– release.