(683c) Tuning the Stabilization Mechanism in Nanoparticle Regulated Complex Fluids

Over the past decade, charged nanoparticles have been found to enhance the stability of colloidal suspensions. This stabilization mechanism is known as “nanoparticle haloing” and refers to the nanoparticle layer that presumably forms around a larger colloidal surface with a non-zero separation distance between the two species. The charge on the nanoparticles leads to an induced electrostatic repulsion between the larger colloidal spheres. However, recent studies indicate that the stabilization mechanism may rely on nanoparticle adsorption, meaning instead of segregating to the region near the larger colloids, nanoparticles directly deposit onto their surfaces. These previous works assumed the mechanisms of nanoparticle haloing and adsorption were mutually exclusive and focused on specific, but largely distinct, nanoparticle concentrations. In this study, we find that these two mechanisms work across a continuum to regulate the stability of colloidal suspensions over increasing nanoparticle concentrations. Firstly, AFM force measurements show that highly charged zirconia nanoparticles built up an electrostatic repulsion between negligible charged silica surfaces, thereby preventing them from aggregating at zirconia nanoparticle volume fractions from 10^-5 to 10^-2. The follow-up adsorption studies and force modeling indicate that minor adsorption of nanoparticles is expected at lower volume fractions of 10^-5 – 10^-3, but adsorption dramatically increases with the increasing nanoparticle volume fraction beyond 10^-3. Based on these results, we propose that the fundamental mechanism of nanoparticle-regulated stabilization is “nanoparticle haloing” at lower nanoparticle concentrations and transitons to “adsorption” at higher concentrations. This does imply that there is a transition region within which the stabilization can be influenced by both nanoparticle haloing and adsorption. This transition was observed around a nanoparticle volume fraction of 10^-3 in our experimental studies. Our study suggests that when using highly charged nanoparticles to stabilize a weakly charged colloidal suspension, the mechanism of stabilization, and hence the accessibility of the colloidal surfaces, can be largely controlled by simply tuning the nanoparticle concentration.