Thermodynamics of Liquid (Xenon + Methane) Mixtures

Abstract

Twenty years after the publication of the seminal book by Rowlinson and Swinton, Liquids and Liquid Mixtures , 1 there is still a need for good experimental data on a certain number of model systems. Given the fact that a whole array of molecular featuresssize, shape, flexibility, etc scontribute to the thermodynamic behavior of a liquid mixture, it is useful to look at families or series of systems so that trends can be detected and correlations explored. Among the first to be studied were mixtures of spherical and quasi-spherical molecules. Typical examples of the former are the noble gases, and an obvious choice for the latter is methane. Detailed thermodynamic studies of liquid mixtures of methane with both argon 1 and krypton 2 have been published. (Krypton and methane have, in fact, very similar sizes and interaction energies, leading to the almost ideal behavior of (krypton + methane) mixtures. 2 ) A whole series of mixtures of xenon with light alkanes, viz., ethane, propane, n-butane, and i-butane, have also been investigated. (Only methane is missing from the list.) It thus seemed worthwhile to study the (xenon + methane) system, the missing link between those series (of methane with the noble gases and of xenon with the lower alkanes). As far as we are aware, no liquid-vapor equilibrium study has been performed on such system. The reason is obvious: unlike the other mixtures in the two families of systems, which can be studied at low pressures, the liquid overlap of xenon and methane happens at moderately high pressures. (The vapor pressure of methane at the xenon triple point is 16 bars.) From a theoretical point of view, a major development in our understanding of liquid mixtures has been the emergence in the late 1980s of molecularly based equations of state (EOS) such as the statistical associating fluid theory (SAFT). 3,4