Phase diagrams and capillarity condensation of methane confined in single- and multi-layer nanotubes

Abstract

Using a newly developed algorithm in the Gibbs ensemble, the liquid–vapour phase diagram of methane adsorbed in single and multilayer nanotubes was considered. The method was based on improving statistical sampling by combining the cavity-bias and the parallel tempering formalisms in Gibbs ensemble Monte Carlo simulations (PTCBGMC). Two models were constructed in order to describe the liquid–vapour equilibrium of methane in nanotubes. The first model consisted of two simulated nanotube-shaped boxes, one containing the vapour and the other containing the liquid. The vapour and liquid were kept in thermodynamic equilibrium using the PTCBGMC algorithm. Cailletet–Mathias phase diagrams showed that upon adsorption of methane in the nanotubes, the critical temperature and density of methane decreased upon confinement. However, the behaviour of the liquid and vapour phases differed depending on whether the nanotube was single- or multi-layered. A second computational model was used to consider in detail the capillarity condensation of methane in the nanotubes. The results obtained were explained in terms of the difference in the strength of the intermolecular forces.