  The spatial structure of the transition region between an insulator and an
electrolyte solution was studied with x-ray scattering.The electron density
profile across the n-hexane/silica sol interface (solutions with 5-nm, 7-nm,
and 12-nm colloidal particles) agrees with the theory of the electrical double
layer and shows separation of positive and negative charges. The interface
consists of three layers, i.e., a compact layer of Na+, a loose monolayer of
nanocolloidal particles as part of a thick diffuse layer, and a low-density
layer sandwiched between them. Its structure is described by a model in which
the potential gradient at the interface reflects the difference in the
potentials of "image forces" between the cationic Na+ and anionic nanoparticles
and the specific adsorption of surface charge. The density of water in the
large electric field (1-10 GV/m) of the transition region and the layering of
silica in the diffuse layer is discussed.
