(208g) Maxima and Minima of Thermodynamic Properties in Supercritical Fluids

The thermodynamic properties of supercritical fluids are of considerable interest for both practical applications such as supercritical fluid extraction and from a scientific perspective. It is of particular interset that some properties such as heat capacities exhibit a locus of maxima in the supercritical phase. In this work, molecular dynamics simulation results are reported for the energy, pressure, isothermal pressure coefficient, thermal expansion coefficient, isothermal and adiabatic compressibilities, isobaric and isochoric heat capacities, Joule-Thomson coefficient and speed of sound of fluids interacting via both the Lennard-Jones and Weeks-Chandler-Andersen potentials. These properties were obtained for a wide range of temperatures, pressures and densities. For each thermodynamic property an excess value is determined to distinguish between attraction and repulsion. It is found that the contributions of intermolecular interactions have varying effects depending on the thermodynamic property. The maxima exhibited by the isochoric and isobaric heat capacities, isothermal compressibilities and thermal expansion coefficient are attributed to interactions in the Lennard-Jones well. Repulsion is required to obtain physically realistic speeds of sound and both repulsion and attraction are necessary to observe a Joule-Thomson inversion curve.  Significantly, both maxima and minima are observed for the isobaric and isochoric heat capacities of the supercritical Lennard-Jones fluid. It is postulated that the loci of these maxima and minima converge to a common point via the same power law relationship as the phase coexistence curve with an exponent of β = 0.32. This provides an alternative explanation for the terminal isobaric heat capacity maximum in supercritical fluids.