Wigner crystals of Na+ ions at the surface of a silica hydrosol

The symmetry of the surface of an electrolyte solution can be anisotropic, regardless of the bulk's isotropic symmetry, because of spatial correlations between adsorbed ions. The author used x-ray grazing-incidence diffraction to measure the spatial correlations between sodium ions in "classical one-component plasma" adsorbed with Bjerrum's density at the surface of a monodispersed 22 nm particle colloidal silica solution stabilized by NaOH with a total bulk concentration approximately 0.05 mol/L. The authors findings show that the surface compact layer is in a two-dimensional crystalline state (symmetry p2), with four sodium ions forming the unit cell and a approximately 30 A translational correlation length between the ions.