Mechanical and thermodynamic routes to the liquid-liquid interfacial tension and mixing free energy by molecular dynamics

TL;DR

This paper compares mechanical and thermodynamic methods using molecular dynamics simulations to measure liquid-liquid interfacial tension and mixing free energy, revealing differences based on component miscibility.

Contribution

It introduces a combined mechanical and thermodynamic approach to accurately determine interfacial tension and free energy in liquid-liquid systems via molecular dynamics.

Findings

Good agreement between methods for immiscible liquids when all contributions are included.

Significant differences in measurements for miscible liquids due to mixing free energy.

Identification of free energy of mixing as a key factor in miscible systems.

Abstract

In this study, we carried out equilibrium molecular dynamics (EMD) simulations of the liquid-liquid interface between two different Lennard-Jones components with varying miscibility, where we examined the relation between the interfacial tension and isolation free energy using both a mechanical and thermodynamic approach. Using the mechanical approach, we obtained a stress distribution around a quasi-one-dimensional (1D) EMD systems with a flat LL interface. From the stress distribution, we calculated the liquid-liquid interfacial tension based on Bakker's equation, which uses the stress anisotropy around the interface, and measures how it varies with miscibility. The second approach uses thermodynamic integration by enforcing quasi-static isolation of the two liquids to calculate the free energy. This uses the same EMD systems as the mechanical approach, with both extended dry-surface and phantom-wall (PW) schemes applied. When the two components were immiscible, the interfacial tension and isolation free energy were in good agreement, provided all kinetic and interaction contributions were included in the stress. When the components were miscible, the values were significantly different. From the result of PW for the case of completely mixed liquids, the difference was attributed to the additional free energy required to separate the binary mixture into single components against the osmotic pressure prior to the complete detachment of the two components, i.e., the free energy of mixing.