Molecular anatomy of the pressure anisotropy in the interface of one and two component fluids: local thermodynamic description of the interfacial tension

TL;DR

This paper investigates the molecular origins of pressure anisotropy at fluid interfaces, demonstrating that intermolecular forces primarily cause it and validating the density gradient theory through molecular dynamics simulations.

Contribution

It provides a local thermodynamic description of interfacial tension by decomposing pressure contributions and validating the density gradient theory with simulations.

Findings

Pressure anisotropy arises solely from intermolecular forces.

Pressure anisotropy is proportional to density gradient products.

Density gradient theory approximates free energy density in inhomogeneous systems.

Abstract

Through the decomposition of the pressure into the kinetic and the intermolecular contributions, we show that the pressure anisotropy in the fluid interface, which is the source of the interfacial tension, comes solely from the latter contribution. The pressure anisotropy due to the intermolecular force between the fluid particles in the same or the different fluid components is approximately proportional to the multiplication of the corresponding fluid density gradients, and from the molecular dynamics simulation of the liquid-vapor and liquid-liquid interfaces, we demonstrate that the density gradient theory (DGT) by van der Waals gives the leading order approximation of the free energy density in inhomogeneous systems, neglecting the Tolman length.