Chemodynamics of soft charged nanoparticles in aquatic media: fundamental concepts

A framework is presented for understanding the reactivity of nanoparticulate reactants with ions and small molecules. Without loss of generality, the formalism is developed for the case of nanoparticles in contact with environmentally relevant metal ions. In addition to reactive sites, nanoparticles generally carry indifferent electric charge distributed over either their surface (hard particles) or volume (soft particles). The ensuing structure and composition of the electric double layer formed within and/or outside the nanoparticulate reactants substantially govern the dynamics of their association and dissociation with ions in aquatic media. A defining feature of permeable nanoparticles is that their charges and reactive sites are spatially confined inside a particle body with an inner medium whose properties may be substantially different from those of the bulk solution. Consequently, the chemodynamic properties of nanoparticulate complexants may differ significantly from those of simple molecular ligands that are homogeneously dispersed in solution. The various physicochemical processes underlying the dynamic reactivity of nanoparticles toward metal ions are here identified, with a focus on the key role played by conductive-diffusion of both metal ions and nanoparticles, the partitioning of ions within the reactive nanoparticulate volume, and the dynamics of the local association/dissociation processes with the reactive sites. The nature of the rate-limiting step in the overall formation/dissociation of the nanoparticulate complexes is shown to depend on the size of the nanoparticle, its charge density, and the ionic strength of the bulk medium. The consequences of these features are further elaborated within the context of dynamics of metal partitioning at a macroscopic consuming biological interphase in the presence of metal complexing nanoparticles.