A simple theory for interfacial properties of dilute solutions

TL;DR

This paper introduces a simple dilute solution theory (DST) that accurately predicts interfacial properties of dilute multicomponent solutions by explicitly considering solute interactions, surpassing traditional models in accuracy.

Contribution

The paper develops a novel dilute solution theory that treats solute interactions exactly at lowest order, providing improved predictions of interfacial behavior in dilute solutions.

Findings

DST accurately describes dilute solutions compared to regular solution theory.

DST predicts three classes of solutes based on intrinsic interface preferences.

Interactions can invert surface preferences of weak solutes.

Abstract

Recent studies suggest that cosolute mixtures may exert significant non-additive effects upon protein stability. The corresponding liquid-vapor interfaces may provide useful insight into these non-additive effects. Accordingly, in this work we relate the interfacial properties of dilute multicomponent solutions to the interactions between solutes. We first derive a simple model for the surface excess of solutes in terms of thermodynamic observables. We then develop a lattice-based statistical mechanical perturbation theory to derive these observables from microscopic interactions. Rather than adopting a random mixing approximation, this dilute solution theory (DST) exactly treats solute-solute interactions to lowest order in perturbation theory. Although it cannot treat concentrated solutions, Monte Carlo (MC) simulations demonstrate that DST describes dilute solutions with much greater accuracy than regular solution theory. Importantly, DST emphasizes an important distinction between the `intrinsic' and `effective' preferences of solutes for interfaces. DST predicts that three classes of solutes can be distinguished by their intrinsic preference for interfaces. While the surface preference of strong depletants are relatively insensitive to interactions, the surface preference of strong surfactants can be modulated by interactions at the interface. Moreover, DST predicts that the surface preference of weak depletants and weak surfactants can be qualitatively inverted by interactions in the bulk. We also demonstrate that DST can be extended to treat surface polarization effects and to model experimental data. MC simulations validate the accuracy of DST predictions for lattice systems that correspond to molar concentrations.