Stabilization of Weakly Charged Microparticles Using Highly Charged Nanoparticles

An experimental study was performed to understand the ability of highly-charged nanoparticles to stabilize weakly-charged microsphere dispersions.  The experiments involved adding either anionic (sulfate) or cationic (amidine) latex nanoparticles to dispersions of micron-sized silica spheres near the silica IEP.  Although both types of nanoparticle increased the zeta potential of the silica microspheres above the value at which dispersions containing only silica spheres remained stable, only with the amidine nanoparticles was stability obtained.  Adsorption tests on flat silica slides indicated that the amidine nanoparticles deposited more densely than sulfate nanoparticles, producing multi-layer surface coverage, roughly three times that of the sulfate nanoparticles.  Calculated DLVO energy profiles between nanoparticle-coated microspheres predicted stability for both systems.  It is hypothesized that the relatively low coverage of the sulfate nanoparticles (≤25%) led to bare silica patches on the microspheres that could align during interaction due to Brownian motion.  These results indicate that highly-charged nanoparticles can be effective stabilizers, provided sufficient levels of adsorption, and that zeta potential alone is insufficient for predicting coated microsphere stability.