Adsorption of solutes at liquid-vapor interfaces: Insights from lattice gas models

TL;DR

This study uses simplified lattice gas models to explain the unexpected adsorption behavior of ions at liquid-vapor interfaces, emphasizing the role of fluctuations and identifying conditions for maximum surface affinity.

Contribution

It demonstrates that coarse lattice gas models can replicate complex interfacial thermodynamic trends observed in detailed studies, highlighting the importance of fluctuations.

Findings

Energy and entropy are minimized near the surface for various ions.

Capillary wave-like fluctuations influence ion adsorption.

Surface propensity varies with temperature and interaction strength.

Abstract

The adsorption behavior of ions at liquid-vapor interfaces exhibits several unexpected yet generic features. In particular, energy and entropy are both minimum when the solute resides near the surface, for a variety of ions in a range of polar solvents, contrary to predictions of classical theories. Motivated by this generality, and by the simple physical ingredients implicated by computational studies, we have examined interfacial solvation in highly schematic models, which resolve only coarse fluctuations in solvent density and cohesive energy. Here we show that even such lattice gas models recapitulate surprising thermodynamic trends observed in detailed simulations and experiments. Attention is focused on the case of two dimensions, for which approximate energy and entropy profiles can be calculated analytically. Simulations and theoretical analysis of the lattice gas highlight the role of capillary wave-like fluctuations in mediating adsorption. They further point to ranges of temperature and solute-solvent interaction strength where surface propensity is expected to be strongest.