(687c) Effects of Highly-Charged Nanoparticles On the Interaction Force Between Weakly-Charged Nanoparticles and Surfaces

Colloidal probe atomic force microscopy was used to measure the force-vs.-distance profile between a micron-size particle and solid plate in aqueous solutions containing spherical nanoparticles.  These particular experiments focused on systems where the interaction between the nanoparticle and microparticle/plate was weakly repulsive, which was achieved using nanoparticles that were highly charged and microparticles/plates that were weakly charged but of the same charge sign as the nanoparticles.  One goal of this work was to experimentally investigate the proposed nanoparticle halo effect.  Experiments with a variety of nanoparticle-microparticle/plate systems, including polystyrene-glass, polystyrene-silica, zirconia-silica, all showed qualitatively similar behavior: the nanoparticles adsorb to the microparticle and plate at lower concentrations, increasing the magnitude of the electrostatic repulsion between the microparticle and plate.  Confirmation of this adsorption was obtained using tests in which the substrate was immersed in a nanoparticle solution, gently rinsed to remove excess fluid, dried and them imaged using scanning electron microscopy.  At increasing nanoparticle concentrations, depletion forces, followed by longer-ranged oscillatory structural forces, became evident.