Thermodynamic behavior of charged Lennard-Jones fluids

The Line of Ideal Isothermal Compressibility

We have considered the line along which the values of the isothermal compressibility of a system are the same as they would be for an ideal gas. It was called the κ_T line. Various substances and models have been studied with the use of the multi-parameter equations of states, implemented in REFPROP (Lemmon, E. W.; Bell, I. H.; Huber, M. L.; McLinden, M. O. NIST standard reference database 23: Reference fluid thermodynamic and transport properties-REFPROP, version 10.0 (National Institute of Standards and Technology, 2018)), and numerical simulations, respectively. It is shown that in the reduced variables, all κ_T lines can be described by the same relation following from the van der Waals equation. This similarity appears to be valid even for the systems with non-straight Zeno lines. Deviations within ∼5% are observed only near the binodals.

The Line of the Unit Compressibility Factor (Zeno-Line) for Crystal States

We study the behavior of the line of the unit compressibility factor (Zeno-line) in crystalline states. We used the Lennard-Jones system, experimental P-V-T data for a number of substances, and the Debye model. We found that, contrary to the case of the liquid states, the Zeno-line in a crystal is not a straight line at the density-temperature plane. However, the corresponding pressure-temperature dependence appears to be quasi-linear. As a result, this line in the solid state can be defined by the only one point where the Zeno-line crosses the melting curve.

Similarity Laws for the Lines of Ideal Free Energy and Chemical Potential in Supercritical Fluids

We have found the curves on the density-temperature plane, along which the values of free energy and chemical potential correspond to ideal gas quantities. At first, we have applied the van der Waals equation to construct them and to derive their equations. Then we have shown that the same lines for real substances (Ar, N₂, CH₄, SF₆, H₂, H₂O) and for the model Lennard-Jones system constructed on the basis of the measurements data and calculations are well matched with the derived equations. The validity and deviations from the obtained similarity laws are discussed.