Where is liquid-vapor interface located in solutions?

TL;DR

This paper develops a theoretical model to precisely locate the liquid-vapor interface in solutions, distinguishing between insoluble and soluble surfactants, and discusses applications to surface electrostatics.

Contribution

It introduces a model that accurately determines the interface location based on surface tension and mass density excess, extending to electrostatic considerations.

Findings

Surface phase coincides with the equimolecular interface for insoluble surfactants.

Surface phase is on the zero total mass density excess surface for soluble surfactants.

Model applicability to surface electrostatics via a two-dimensional Maxwell equation.

Abstract

Two-component liquid-vapor systems are modeled as two bulk phases divided by a two-dimensional surface phase and the mass and momentum balances are theoretically studied. Comparing the derived equations with some typical models of surface rheology, useful information about the interface location is obtained. It is demonstrated that the surface phase, set on the surface of tension, coincides with the equimolecular interface for insoluble surfactants, whereas it is placed on the surface of zero total mass density excess for soluble ones. The applicability of the model to surface electrostatics is also discussed by introduction of a two-dimensional Maxwell equation for the surface phase.