Tuning the Stabilization Mechanism of Nanoparticle-Regulated Complex Fluids

TL;DR

This paper investigates how charged nanoparticles stabilize colloidal suspensions, revealing a transition from nanoparticle haloing to adsorption as nanoparticle concentration increases, which influences the stabilization mechanism.

Contribution

It clarifies the dual stabilization mechanisms in nanoparticle-regulated fluids and identifies the concentration-dependent transition between haloing and adsorption.

Findings

Nanoparticle haloing dominates at low concentrations.

Adsorption increases significantly beyond a critical concentration.

Stabilization involves a transition from haloing to adsorption around 10^-3 volume fraction.

Abstract

In nanoparticle haloing, charged nanoparticles have been found to enhance the stability of colloidal suspensions by forming a non-adsorbing layer surrounding neutral colloids which induces an electrostatic repulsion between them. However, there has been some debate that nanoparticles may directly deposit onto the colloidal surfaces and that the stabilization mechanism relies on nanoparticle adsorption. In this study, we have found that these two mechanisms control the stability of colloidal suspensions across a continuum over increasing nanoparticle concentrations. AFM force measurements showed that highly charged zirconia nanoparticles built up an electrostatic repulsion between negligibly charged silica surfaces, preventing them from aggregating. The follow-up adsorption measurements and force modeling indicated that minor adsorption of nanoparticles is expected at volume fractions of 10-5 to 10-3, but the amount of nanoparticle adsorption dramatically increases with increasing the nanoparticle volume fraction beyond 10-3. Based on these results, we propose that the fundamental mechanism of nanoparticle-regulated stabilization is nanoparticle haloing at low nanoparticle concentrations which transitions to adsorption at higher concentrations. Accordingly, at a nanoparticle volume fraction of around 10-3 where the transition happens, the stabilization can be influenced by both nanoparticle haloing and adsorption.