Spatial modulation of the composition of a binary liquid near a repulsive wall

TL;DR

This study investigates how the composition of a binary liquid near a strongly repulsive wall exhibits spatial modulation, comparing molecular dynamics simulations and density functional theory to the Guggenheim relation, revealing conditions of validity and violation.

Contribution

The paper demonstrates the qualitative validity of the Guggenheim relation at wall-induced interfaces for negative mixing energy and identifies density-dependent violations for positive mixing energy through simulations and theory.

Findings

Guggenheim relation holds for negative mixing energy systems.

Violation of the relation occurs at high densities with positive mixing energy.

Density functional theory supports simulation results.

Abstract

When a binary liquid is confined by a strongly repulsive wall, the local density is depleted near the wall and an interface similar to that between the liquid and its vapor is formed. This analogy suggests that the composition of the binary liquid near this interface should exhibit spatial modulation similar to that near a liquid-vapor interface even if the interactions of the wall with the two components of the liquid are the same. The Guggenheim adsorption relation quantifies the concentrations of two components of a binary mixture near a liquid-vapor interface and qualitatively states that the majority (minority) component enriches the interface for negative (positive) mixing energy if the surface tensions of the two components are not very different. From molecular dynamics simulations of binary mixtures with different compositions and interactions, we find that the Guggenheim relation is qualitatively satisfied at wall-induced interfaces for systems with negative mixing energy at all state points considered. For systems with positive mixing energy, this relation is found to be qualitatively valid at low densities, while it is violated at state points with high density where correlations in the liquid are strong. This observation is validated by a calculation of the density profiles of the two components of the mixture using density functional theory with the Ramakrishnan-Yussouff free-energy functional. Possible reasons for the violation of the Guggenheim relation are discussed.