No abstract

We set new regularities between the effective compressibility factor at the critical point (Z*) and unit compressibility line (Zeno-line) parameters. For classical liquids, Z* is the ordinary compressibility factor Z(c), but for quantum liquids Z* depends on the de Boer parameters. As a result, we show that a wide group of real substances with the classical thermodynamical properties has Z(c) < 0.32. Classical mercury and quantum H(2), He(4), and He(3) have Z(c) or Z* > 0.37. Using the low temperature part of the liquid-gas coexistence curve obtained from experiment, we can find the critical parameters for metals (Al, Cu, W, U, and Zr) for which these parameters are unknown in advance and lie in the domain of parameters still inaccessible for experiment.

We use numerical simulation data for several model interatomic potentials to confirm the critical point-Zeno-line relations of similarity (CZS) for the liquid branch of the coexistence curve suggested earlier [E. M. Apfelbaum and V. S. Vorob'ev, J. Phys. Chem. B 112, 13064 (2008)]. These relations have been based on the analysis of experimental values for the critical point parameters and liquid-gas coexistence curves for a large number of real substances and two model systems. We show that the numerical modeling data as a whole confirm the CZS in the domain of the existence of liquid state. The deviations from CZS relations take place for two cases: (a) the numerically calculated coexistence curve gets into domain corresponding to solidification; (b) the liquid-vapor transition becomes metastable with respect to freezing.

We show that some of the thermodynamic regularities, following from the van der Waals (VDW) equation, are valid for the real substances and models described by completely different equations of state. These regularities relate to lines of ideal enthalpy, enthalpy minima, and isothermal compressibility maxima. The first one appears to be the straight line on the density-temperature plane, while for the two others there are universal relations, which are the same for various substances and models (argon and Lennard-Jones system as examples). The model systems were studied by Monte Carlo simulations (NVT MC), while experimental data were analyzed for the real substances. Our numerical calculations and the analysis of experimental data have shown that for the considered systems these curves are similar to the VDW ones.

The line of thermodynamic states with a unit value of the compressibility factor was calculated for a Lennard-Jones system using four different approaches. We show that all four approaches give rise to a straight line on the density-temperature plane. Thus, we theoretically confirm that the Lennard-Jones system satisfies Zeno line regularity.