Ionic depletion at the crystalline Gibbs layer of PEG-capped gold nanoparticles brushes at aqueous surfaces.*

In-situ surface sensitive X-ray diffraction and grazing incidence X-ray fluorescence spectroscopy (GIXFS) methods are combined to determine the ionic distributions across the liquid interfaces of thiolated-polyethylene-glycol capped gold nanoparticles (PEG-AuNPs) solutions. Induced by the addition of salts (i.e., Cs₂SO₄) to PEG-AuNPs solutions, 2D hexagonal lattices of PEG-AuNPs form spontaneously at the aqueous surfaces, as has been demonstrated by X-ray reflectivity (XR) and grazing incidence small-angle X-ray scattering (GISAXS). By taking advantage of element specificity with GIXFS method, we find that the cation Cs⁺ concentration at the crystalline film is significantly reduced in parts of the PEG-AuNPs film compared to that in the bulk.