Particle Restabilization in Silica/PEG/Ethanol Suspensions: How Strongly do Polymers Need To Adsorb To Stabilize Against Aggregation?

Abstract

We study the effects of increasing the concentration of a low molecular weight polyethylene glycol on the stability of 44 nm diameter silica nanoparticles suspended in ethanol. Polymer concentration, c(p), is increased from zero to that characterizing the polymer melt. Particle stability is accessed through measurement of the particle second-virial coefficient, B(2), performed by light scattering and ultrasmall angle X-ray scattering (USAXS). The results show that at low polymer concentration, c(p) < 3 wt %, B(2) values are positive, indicating repulsive interactions between particles. B(2) decreases at intermediate concentrations (3 wt % < c(p) < 50 wt %), and particles aggregates are formed. At high concentrations (50 wt % < c(p)) B(2) increases and stabilizes at a value expected for hard spheres with a diameter near 44 nm, indicating the particles are thermodynamically stable. At intermediate polymer concentrations, rates of aggregation are determined by measuring time-dependent changes in the suspension turbidity, revealing that aggregation is slowed by the necessity of the particles diffusing over a repulsive barrier in the pair potential. The magnitude of the barrier passes through a minimum at c(p) ≈ 12 wt % where it has a value of ∼12 kT. These results are understood in terms of a reduction of electrostatic repulsion and van der Waals attractions with increasing c(p). Depletion attractions are found to play a minor role in particle stability. A model is presented suggesting displacement of weakly adsorbed polymer leads to slow aggregation at intermediate concentration, and we conclude that a general model of depletion restabilization may involve increased strength of polymer adsorption with increasing polymer concentration.