Controlling nanoparticles dispersion in ionic liquids by tuning the pH

Hypothesis: Getting colloidally stable dispersions of nanoparticles in ionic liquids is a challenging task. Indeed, long-range electrostatic repulsions often involved in molecular solvents are screened in ionic liquids and cannot counterbalance the interparticle attractions. Using a polyelectrolyte coating should provide a good stabilisation of the nanoparticles. Investigating the role of the polyelectrolyte charge on the dispersion state should yield to a better comprehension of the stabilisation mechanisms. Experiments: Polyacrylate coated maghemite nanoparticles were transferred from water to ethylammonium nitrate, a protic ionic liquid, for various polymer chain length and nanoparticles size. Titrations of coated nanoparticles and of free polymer chains were performed in water and in ethylammonium nitrate. The dispersion state of the nanoparticles was monitored at different pH by small-angle X-ray scattering. Findings: Polyacrylate coating stabilised the nanoparticles in ethylammonium nitrate. However, reversible aggregation with the pH was observed. Surprisingly, this control was not directly related to the surface charge like in water but to the solvent quality for the polyelectrolyte. This study is the first report on the use of the pH to tune the dispersion state of nanoparticles in an ionic liquid. It provides a better understanding of the mechanisms responsible for colloidal stability in ionic liquids. (C) 2015 Elsevier Inc. All rights reserved.