On the structure of a confined ideal gas: A statistical mechanical description with an external field

All fluids are somehow confined; even the ideal gas is by definition a confined fluid model. For all practical purposes, if the distance between the confining walls is reasonably large, the confinement effect is negligible. Nevertheless, whenever this distance is about a few nanometers, the confinement effect is unavoidable, and is manifested as an inhomogeneity in the fluid particles distribution. Here, a statistical mechanical framework from which the structure of a confined ideal gas can be directly calculated is derived. As a case study, the structure of an ideal gas confined by planar graphite walls is determined with the proposed theory and compared to Monte Carlo simulations. The gratifyingly perfect agreement between these two approaches shows the correctness of the proposed theory. The influences of pore size and temperature are analyzed. The theoretical results obtained here serve as a limit that is obeyed by any low density confined fluid. (C) 2019 Elsevier B.V. All rights reserved.