The Stability of Cerium Oxide Nanoparticles.

Interest has been shown in cerium oxide nanoparticles due to their potential use catalysts, gas sensors, and more. Unchecked use of these nano-materials could have profound environmental impacts, however, little is known about the fate and transport of these nanoparticles in aquatic systems. In this work, small (d ~ 15 nm) mostly spherical cerium oxide nanoparticles were synthesized in house and observed using transmission electron microscopy. The zeta potential of the particles was also measured across a range of pH values to determine the point of zero charge. Dynamic light scattering measurements demonstrate that these particles exist as supra-aggregates in solution and not as individual nanoparticles. The aggregation kinetics of cerium oxide supra-aggregates were measured across a range of sodium chloride concentrations using time resolved dynamic light scattering. Stability diagrams were constructed by comparing the measured aggregation rate constants to diffusion controlled rate constants. The stability of cerium oxide aggregates was modeled using conventional DLVO theory to determine a Hamaker constant for cerium oxide. The system showed significant deviations from DLVO theory at low ionic strengths where the supra-aggregates were less stable than predicted. The origins of these deviations will be discussed.