Ion-Size Effect at the Surface of a Silica Hydrosol

TL;DR

This study uses synchrotron x-ray reflectivity to investigate how alkali ion size influences their distribution at silica hydrosol surfaces, revealing larger ions tend to accumulate more than smaller ones, aligning with electrostatic free energy predictions.

Contribution

It provides experimental evidence of ion-size effects at silica hydrosol surfaces, supporting theoretical models of ion distribution based on electrostatic free energy.

Findings

Larger alkali ions preferentially accumulate at the silica sol surface.

Ion distribution correlates with ion size and electrostatic free energy.

Results align with the Kharkats-Ulstrup model predictions.

Abstract

The author used synchrotron x-ray reflectivity to study the ion-size effect for alkali ions (Na$^+$, K$^+$, Rb$^+$, and Cs$^+$), with densities as high as $4 \times 10^{18}- 7 \times 10^{18}$ m$^{-2}$, suspended above the surface of a colloidal solution of silica nanoparticles in the field generated by the surface electric-double layer. According to the data, large alkali ions preferentially accumulate at the sol's surface replacing smaller ions, a finding that qualitatively agrees with the dependence of the Kharkats-Ulstrup single-ion electrostatic free energy on the ion's radius.