(40e) Mechanisms of Nanoparticle Haloing for the Control of Colloidal Stability
Nanoparticle haloing is a mechanism by which binary colloidal suspensions can be stabilized as a result of particle size and charge asymmetry. As a model system, we have chosen negligibly charged silica microparticles mixed with low concentrations of highly charged zirconia nanoparticles. AFM measurements indicate an increase in the effective surface charge of silica particles with increasing zirconia nanoparticle concentration. This enhanced electrostatic repulsion between silica particles increases colloidal stability, which we attribute to nanoparticle haloing. We have characterized our model system over a range of conditions and discovered a narrow window of pH and nanoparticle concentration under which nanoparticle haloing is observed. Upon settling, colloidal packing was analyzed, and haloing was found to greatly affect equilibrium structures. DLVO model parameters were developed based on experimental measurements. Equilibrium properties of the binary system of silica and zirconia were evaluated by molecular dynamics simulations using HOOMD-blue. The free energy of microparticle association in various concentrations of nanoparticles was calculated from umbrella sampling simulations. Simulations predict the formation of loosely associated nanoparticle multilayers near the surface of each microparticle which we hypothesize leads to kinetic stabilization of the colloidal suspension in agreement with our experimental observations.