(216e) Synergestic Effects of Polymers and Nanoparticles On Colloidal Stability

The focus of this study was on understanding the synergistic effects of nanoparticles and polyelectrolytes on the depletion and structural forces between colloidal particles (order microns in size).  The experiments consisted of measuring the force-vs.-distance profile between a large silica particle and a silica plate in aqueous solutions using colloidal probe atomic force microscopy (CP-AFM).  The depletants used were silica nanoparticles (22 nm in diameter) and polyacrylic acid (Mw of 100k).  With low concentrations of either only nanoparticles or only polymer in solution, little, if any, depletion forces were detected between the microparticle and plate.  When both polymer and nanoparticles were mixed, however, significant depletion and structural forces were observed.  Calculation of the potential energy profile that would result from these forces indicated that the forces would be more than sufficient to induce flocculation of an otherwise stable dispersion of micron-sized particles.  Our hypothesis is that polymer chains adsorb to the surface of the nanoparticles, creating much larger, charged complexes that are more easily excluded from the gap region between the microparticle and plate, thus increasing the depletion force.