Unusual Effect of Iodine Ions on the Self-Assembly of Poly(ethylene glycol)-Capped Gold Nanoparticles

We use synchrotron X-ray reflectivity and grazing incidence small-angle X-ray scattering to investigate the surface assembly of the polyethylene glycol (PEG) grafted gold nanoparticles (PEG-AuNPs) induced by different salts. We find that NaCl and CsCl behave as many other electrolytes, namely drive the PEG-AuNPs to the vapor/suspension interface to form a layer of single particle depth and organize them into very high quality two-dimensional (2D) hexagonal crystals. By contrast, NaI induces the migration of PEG-AuNPs to the aqueous surface at much higher surface densities than the other salts (at similar concentrations). The resulting 2D ordering at moderate NaI concentrations is very short-ranged and at a higher NaI concentration the high density monolayer is amorphous. Considering NaCl, CsCl and the majority of salts behave similarly, this implicates the anomaly of iodine ion (I$^-$) in this unusual surface population. We argue that the influence of most electrolytes on the PEG corona preserves the polymer in the $\theta$-point with sufficient flexibility to settle into a highly ordered state, whereas I$^-$ has a much more severe effect on the corona by collapsing it. The collapsed PEG renders the grafted AuNP a non-spherical shaped complex that, although packs at high density, cannot organize into a 2D ordered arrangement