Dissolution is an important process that alters nanoparticle abundance and properties. Here we used EDTA (ethylenediaminetetraacetic acid) to dissolve ZnS nanoparticles and develop a predictive model for the effects of particle size and pH on nanoparticle reactivity. Synchrotron in situ small-angle X-ray scattering (SAXS) was used to monitor changes in scattering intensity during the dissolution of ZnS nanoparticles over the pH range 9-10. The nanoparticle mass, size, and size distribution were derived from fitting of the SAXS data. Thermodynamic analysis showed that both particle size and pH play important roles in dissolution. Kinetic modeling of the extent of dissolution as a function of time revealed that the reaction is reversible and first order with respect to the concentration of reagents and products and the total surface area of ZnS nanoparticles. The dissolution equilibrium constant and the interfacial free energy of ZnS nanoparticles were derived from parameters obtained from the kinetic modeling. Analysis of the variation of the equilibrium constant with and particle size showed that the lower the pH and the smaller the particle size, the higher the solubility of ZnS nanoparticles in the experimental pH range. The method developed in this study revealed the effect of particle size on dissolution-precipitation equilibria and provides a basis for description of nanoparticle reactivity.
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