Compact Layer of Alkali Ions at the Surface of Colloidal Silica

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 by either 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 the concentration of alkali base in the sol; it can be described by a two-layer model, that is, a ∼6−8-A-thick layer of directly adsorbed hydrated alkali ions with a surface concentration of ∼3 × 1018 m-2, and a ∼13-A-thick layer with a surface concentration of sodium ions of ∼8 × 1018 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 explain...