Computer simulation of the phase diagram for a fluid confined in a fractal and disordered porous material

TL;DR

This study uses grand canonical Monte Carlo simulations to explore how a Lennard-Jones fluid's phase diagram is affected by confinement in a fractal, disordered porous aerogel, revealing decreased critical temperature and density without liquid-liquid coexistence peaks.

Contribution

It introduces a simulation approach for analyzing fluid phase behavior in fractal porous materials, highlighting differences from previous studies with spherical matrices.

Findings

Critical temperature and density decrease under confinement.

No evidence of liquid-liquid phase coexistence peak.

High density fluctuations and disorder in coexisting phases.

Abstract

We present a grand canonical Monte Carlo simulation study of the phase diagram of a Lennard-Jones fluid adsorbed in a fractal and highly porous aerogel. The gel environment is generated from an off-lattice diffusion limited cluster-cluster aggregation process. Simulations have been performed with the multicanonical ensemble sampling technique. The biased sampling function has been obtained by histogram reweighting calculations. Comparing the confined and the bulk system liquid-vapor coexistence curves we observe a decrease of both the critical temperature and density in qualitative agreement with experiments and other Monte Carlo studies on Lennard-Jones fluids confined in random matrices of spheres. At variance with these numerical studies we do not observe upon confinement a peak on the liquid side of the coexistence curve associated with a liquid-liquid phase coexistence. In our case only a shouldering of the coexistence curve appears upon confinement. This shoulder can be associated with high density fluctuations in the liquid phase. The coexisting vapor and liquid phases in our system show a high degree of spatial disorder and inhomogeneity.