Compact Layer of Alkali Ions at the Surface of Colloidal Silica

TL;DR

This study uses X-ray scattering to characterize a consistent two-layer alkali ion surface structure on colloidal silica, revealing ion-specific adsorption behaviors and implications for surface charge and tension.

Contribution

It provides detailed structural insights into the alkali ion layers on colloidal silica surfaces, highlighting ion-specific adsorption patterns and independence from particle size and concentration.

Findings

Two-layer model describes alkali ion surface structure

Cs+ ions replace Na+ in the first layer in cesium-enriched sols

Surface structure is independent of particle size and alkali concentration

Abstract

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.