Capillary condensation and capillary pressure of methane in carbon nanopores: Molecular Dynamics simulations of nanoconfinement effects

摘要

AbstractTwo main groups of thought have emerged over the past decade to improve thermodynamic modeling of capillary condensation of nanoconfined fluids. One approach has been developed on the premise of using shifted critical parameters for the condensed phase to account for the wall-fluid interactions, while the other one considers a pressure difference across the interface between the condensed and the bulk phases. This pressure difference is the capillary pressure that exists across a curved meniscus formed in a capillary pore. For nanoconfinement, a modified Laplace equation has been utilized to calculate the capillary pressure to take into account the confinement effects that become more prominent as the pore size reduces. For small pores of a few nanometers in size, however, the impact of structural forces known as nanoconfinement effects, on the pressure of the confined phase becomes more significant. In this work, we studied the capillary pressure of methane at the capillary condensation point in graphite pores smaller than 7?nm, to verify whether the confined phase can experience a negative pressure at capillary condensation point and whether we can accurately predict this pressure from thermodynamic equations. For this purpose, we used Molecular Dynamics (MD) simulations to investigate the pressure of methane molecules confined in graphite pores of various sizes. Furthermore, normal and tangential pressures of methane in selected pore sizes were obtained during capillary condensation at constant pressure. Our results indicated that for small pores there is a critical size below which capillary condensation did not occur. For larger pores, on the other