(627a) Molecular Dynamics Simulations Uncover Mechanisms of Increased Stability in Binary Colloidal Suspensions of Microparticles and Nanoparticles

This study focuses on the stability of neutral colloidal silica microparticles in aqueous suspension with low concentrations of highly charged zirconia nanoparticles. DLVO theory modified with additional depletion interactions was applied to describe particle interaction potentials and forces. Atomic force microscopy measurements identified and quantified an effective repulsion between microparticles that increased as nanoparticle concentration increased, suggesting that charged nanoparticles can be used to modulate the stability of microparticle suspensions. To better understand the mechanisms by which the effective repulsion arose, molecular dynamics (MD) simulations were performed. Systems containing 10^-6 to 10^-3 volume fraction of zirconia nanoparticles were simulated over timescales up to ten seconds. In the absence of nanoparticles, microparticles quickly aggregated during simulations. However, even in the smallest concentration of nanoparticles studied, an apparent kinetic barrier arose that slowed microparticle aggregation significantly. To quantify the apparent kinetic barrier and the effective pair interactions between microparticles in the presence of nanoparticle halos, enhanced sampling was employed. Umbrella sampling was used to ensure sampling of rare states and to construct a continuous potential of mean force as a function of microparticle-microparticle distances. Simulation results indicate a kinetic barrier ranging from about 2 to 6 k_BT exists for microparticle-microparticle interactions, in agreement with previous theories. This barrier grows as nanoparticle concentrations increase, suggesting that nanoparticle concentration is a driving factor in stabilizing microparticle suspensions as was observed in atomic force microscopy experiments. Ultimately, stabilization and aggregation were studied for systems of many microparticles using MD by applying potential of mean force curves from the umbrella sampling. Thus, the stabilizing effects of nanoparticles were preserved while their physical presence in the simulation box was no longer required. Simulations indicate that the kinetic barriers found during umbrella sampling simulations affect the rate of microparticle aggregation, matching experimental observations.