  I used x-ray grazing incidence diffraction to measure the spatial
correlations between sodium ions 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 mol/L. My findings show that the surface
compact layer is in a two-dimensional crystalline state (symmetry p2), with
four ions forming the unit cell and a ~30 Angstrom translational correlation
length between sodium ions.
