Crystals of Na+ ions at the surface of a silica hydrosol

TL;DR

This study uses x-ray diffraction to reveal that sodium ions at the surface of a silica hydrosol form a two-dimensional crystalline layer with specific symmetry and correlation length, providing insights into surface ion arrangements.

Contribution

It demonstrates that sodium ions at the silica hydrosol surface organize into a 2D crystal structure, a novel observation in colloidal surface chemistry.

Findings

Surface sodium ions form a 2D crystalline layer

The crystalline structure has p2 symmetry with four ions per unit cell

Correlation length between ions is approximately 30 Angstroms

Abstract

I used x-ray grazing incidence diffraction to measure the spatial correlations between sodium ions adsorbed with Bjerrum's density at the surface of a monodispersed 22-nm-particle colloidal silica solution stabilized by NaOH with a total bulk concentration mol/L. My findings show that the surface compact layer is in a two-dimensional crystalline state (symmetry p2), with four ions forming the unit cell and a ~30 Angstrom translational correlation length between sodium ions.