Ion-size effect at the surface of silica hydrosol

The forces of electrical imaging strongly polarize the surface of colloidal silica. I used X-ray scattering to study the ion-size effect in 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., a $\sim $ 6 - 8-Angstrom-thick layer of directly adsorbed hydrated alkali ions with a surface concentration $\sim 3\times 10^{18}$ m$^{-2}$, and a $\sim $ 13-Angstrom-thick layer with a surface concentration of sodium ions $\sim 8\times 10^{18}$ 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. Brookhaven National Laboratory is supported by U.S.D.O.E., Contract No. DEAC0298CH10886. X19C is partially supported through funding from the ChemMatCARS National Synchrotron Resource and the University of Chicago.