  The forces of electrical imaging strongly polarize the surface of colloidal
silica. I used X-ray scattering to study the adsorbed 2-nm-thick compact layer
of alkali ions at the surface of concentrated solutions of 5-nm, 7-nm, and
22-nm particles, stabilized either by NaOH or a mixture of NaOH and CsOH, with
the total bulk concentration of alkali ions ranging from 0.1- to 0.7-mol/L. The
observed structure of the compact layer is almost independent of the size of
the particles and concentration of alkali base in the sol; it can be described
by a two-layer model, i.e., an ~ 8 Angstrom thick layer of directly adsorbed
hydrated alkali ions with a surface concentration 3x10(18) m(-2), and a ~ 13
Angstrom thick layer with a surface concentration of sodium ions 8x10(18)
m(-2). In cesium-enriched sols, Cs+ ions preferentially adsorb in the first
layer replacing Na+; their density in the second layer does not depend on the
presence of cesium in the sol. The difference in the adsorption of Cs+ and Na+
ions can be explained by the ion-size-dependent term in the electrostatic Gibbs
energy equation derived earlier by others. I also discuss the surface charge
density and the value of surface tension at the sol's surface.
