Free energy calculations along entropic pathways III. Nucleation of capillary bridges and bubbles

TL;DR

This study uses molecular simulations to analyze the free energy and nucleation mechanisms of capillary condensation and evaporation of Argon in nanopores, focusing on entropy as a reaction coordinate.

Contribution

It introduces a novel approach by defining entropy as a reaction coordinate to study free energy profiles of capillary phase transitions in nanopores.

Findings

Capillary condensation proceeds via nucleation of a liquid bridge.

Capillary evaporation involves destabilization of layered structures and bubble formation.

Free energy profiles reveal multi-stage nucleation processes.

Abstract

Using molecular simulation, we analyze the capillary condensation and evaporation processes for Argon confined in a cylindrical nanopore. For this purpose, we define the entropy of the adsorbed fluid as a reaction coordinate and determine the free energy associated with both processes along entropic pathways. For capillary condensation, we identify a complex free energy profile resulting from the multi-stage nature of this phenomenon. We find capillary condensation to proceed through the nucleation of a liquid bridge across the nanopore, followed by its expansion throughout the pore to give rise to the stable phase of high density. In the case of capillary evaporation, the free energy profile along the entropy pathway also exhibits different regimes, corresponding to the initial destabilization of the layered structure of the fluid followed by the formation, and subsequent expansion, of a bubble across the nanopore.