Theoretical Basis for Coupling Colloid Dispersion and Dissolution Potentials with Respect to Engineered Nanoparticles.

Efforts in developing new regulatory framework considers nanomaterials as a dual toxicity risk, arising from the potential of the nanometer -sized particles to cross biological membranes and the potential for the particles to dissolve and release toxic constituents.  Yet, theoretical treatments for simultaneously considering these processes are inherently incompatible.  For example, generalized equations model particle aggregation in terms of second-order kinetics while particle dissolution is modeled as a first-order reaction based on the difference between the concentration of dissolved constituent and its aqueous solubility.  The incompatibility in these theories lies in their inherent lack of reversibility.  Aggregation emphasizes mechanisms of particle size growth while dissolution focuses on decrease in particle size.  Thus, it is unclear which theory should be considered for a given nanoparticle suspension.  Here, we present the theoretical framework for thermodynamic descriptions of nanoparticle behavior based on classical concept of mineral solubility.  For this theory, we presume that dissolution is the reverse process of aggregation, representing a behavior exhibited at particle saturation.  Evidence collected from ongoing laboratory studies demonstrates the potential for the solubility approach to explain nanoparticle behavior and dynamics in aqueous solution.  Yet the real theoretical challenge lies in defining the actual solubility constant for nanoparticles that almost always contain organic coatings.  We hypothesize that it is the presence of these coatings that allow nanomaterials to violate classical solubility limits related to the pure material's composition.  We demonstrate these differences using classical surface complexation modeling of actual nanoparticle formulations.