X-ray study of the electric double layer at the n-hexane/nanocolloidal silica interface

TL;DR

This study uses x-ray scattering to analyze the structure of the electric double layer at the n-hexane/silica interface, revealing a three-layer arrangement consistent with double layer theory and detailing charge separation and layering phenomena.

Contribution

It provides detailed experimental evidence of the interfacial structure and charge distribution at the n-hexane/silica interface, supporting and extending the electrical double layer model.

Findings

The interface has three distinct layers: a Na+ layer, a nanocolloid monolayer, and a low-density layer.

The electron density profile aligns with the electrical double layer theory.

Layering of silica and water density variations are observed in the transition region.

Abstract

The spatial structure of the transition region between an insulator and an electrolyte solution was studied with x-ray scattering.The electron density profile across the n-hexane/silica sol interface (solutions with 5-nm, 7-nm, and 12-nm colloidal particles) agrees with the theory of the electrical double layer and shows separation of positive and negative charges. The interface consists of three layers, i.e., a compact layer of Na+, a loose monolayer of nanocolloidal particles as part of a thick diffuse layer, and a low-density layer sandwiched between them. Its structure is described by a model in which the potential gradient at the interface reflects the difference in the potentials of "image forces" between the cationic Na+ and anionic nanoparticles and the specific adsorption of surface charge. The density of water in the large electric field (1-10 GV/m) of the transition region and the layering of silica in the diffuse layer is discussed.