The structure of fluids with impurities

TL;DR

This paper theoretically examines how dilute impurities affect the structure of a fluid solvent, extending the Ornstein-Zernike model to account for impurity effects, especially focusing on ionic size differences.

Contribution

It provides a model-independent extension of the solvent structure factor considering impurities, highlighting the role of ion size differences in structural influence.

Findings

Dilute impurities affect solvent structure only if ions differ significantly in size.

Non-ionic impurities or similar-sized ions do not qualitatively alter the structure.

The extended structure factor can help infer impurity-solvent interactions from scattering data.

Abstract

The influence of dilute impurities on the structure of a fluid solvent is investigated theoretically. General arguments, which do not rely on particular models, are used to derive an extension of the Ornstein-Zernike form for the solvent structure factor at small scattering vectors. It is shown that dilute impurities can influence the solvent structure only if they are composed of ions with significantly different sizes. Non-ionic impurities or ions of similar size are shown to not alter the solvent structure qualitatively. This picture is compatible with available experimental data. The derived form of the solvent structure factor is expected to be useful to infer information on the impurity-solvent interactions form measured scattering intensities.