Aggregation kinetics of cerium oxide nanoparticles in monovalent and divalent electrolytes

As a result of the commercial availability and use of nanoparticulate cerium oxide, CeO2, it is extremely likely that this material will be introduced into the environment requiring knowledge of its fate, transport and bioavailability in natural aquatic systems. To this end, this work probes the physicochemical interactions that govern the aggregation kinetics of cerium oxide nanoparticles using time-resolved dynamic light scattering (TR-DLS) over a range of monovalent, Na+, and divalent, Ca2+, electrolyte concentrations. Sets of nanoparticles were synthesized by precipitation in aqueous solutions containing varying concentrations of methanol. The point of zero charge (pzc) of these nanoparticles changes as a result of synthesis method. Those produced in the absence of methanol had pzc = 6.5. As predicted by the theories of Derjaguin–Landau–Verwey–Overbeek (DLVO), both reaction-limited and diffusion-limited aggregation were observed in each solution type. The experimental critical coagulation concentrations (CCC) at pH 11.0 was ca. 80 mM and ca. 16 mM for the monovalent (NaCl) and divalent (CaCl2) salts, respectively. DLVO theory proved to be an adequate predictor for the interactions between cerium oxide nanoparticles albeit the derived Hamaker constant of 1.0 × 10−20 J was somewhat smaller than experimental Hamaker constants determined for other metal oxide nanoparticles. Deviations between experimental data and DLVO theory are discussed.