The Joule-Thomson effect in confined fluids

The Joule-Thomson effect is discussed for a fluid composed of spherically symmetric Lennard-Jones(12,6) molecules (of "diameter" sigma) confined between two planar, rigid, structureless solid substrates separated by s(z)=10 and 20 sigma. The effect of "strong" and "weak" of the substrate is studied by employing fluid-substrate potentials with and without attractive interactions, respectively. The focal point of this study is the confinement-induced depression of the inversion temperature T-inv with respect to the bulk value. It is defined such that during a Joule-Thomson expansion the temperature of a (confined or bulk) gas remains constant. In the limit of vanishing gas density, T-inv is computed from the second virial coefficient defined through a density expansion of the transverse stress T-parallel to in the gas. For higher densities T-inv is computed from the (transverse) expansion coefficient alpha(parallel to) which is accessible through density and enthalpy fluctuations in mixed stress-strain ensemble Monte Carlo simulations. Results of these simulations are analyzed in terms of a mean-field theory which provides a qualitatively correct description of the Joule-Thomson effect in confined fluids. The smaller s(z) the more depressed (with respect to the bulk) is T-inv. The density dependence of T-inv is different for "strong" and "weak" substrates. Without attractive fluid-fluid interactions T-inv does not exist and the confined gas is always heated during a Joule-Thomson expansion. In this case alpha(parallel to) is independent of the substrate material. (C) 1999 Elsevier Science B.V. All rights reserved.