Excess Functions for Moderately Dense Gas Mixtures

Although the virial equation of state can be applied to moderately dense gases, the difficulty in determining third and fourth interaction virial coefficients makes its application to mixtures impractical. It has become increasingly common in dealing with moderately dense gas mixtures to use the experimental approaches and theories usually associated with liquid mixtures. For example, one can describe the volumetric properties of gas mixtures by the excess volume rather than in terms of virial coefficients. The focus is on excess functions rather than on the properties of the mixture. 

Methods for Measuring and — Beenakker and co-workers were among 

Low-pressure enthalpies of mixing calculated from data have been reported for hydrocarbon and fluorocarbon mixtures, and for mixtures of benzene with hexane, heptane, and octane. Direct measurements of enthalpies of mixing of moderately dense gases have been reported by a number of groups.  

The pressure is held constant by the controller G and is measured by the Bourdon gauge, H. Flow rates are determined at atmospheric pressure by a gasometer I and the mixture is analysed by a gas chromatograph J. The calorimeter must be carefully designed to ensure complete mixing without producing a significant pressure drop. Trial measurements in which the same substance is used in both 

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