Physics – Condensed Matter – Soft Condensed Matter
Scientific paper
2006-09-18
J. Phys. Chem. C, 111, 930 (2007)
Physics
Condensed Matter
Soft Condensed Matter
32 pages 10 figures
Scientific paper
10.1021/jp065538r
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.
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