The Zeno (Z = 1) Behavior of Equations of State: An Interpretation across Scales from Macroscopic to Molecular

Abstract

The regularity exhibited by fluids along the contour of the unit compressibility factor in the temperature−density plane, where Z ⋮ PV/RT = 1, is explored as a means for testing and improving the volumetric equations of state. For a wide range of pure fluids, this contour, known as the Zeno line, has been empirically observed to be nearly linear from the Boyle temperature of the low-density vapor to around the triple point in the liquid region. The Z = 1 contour thus offers a quantitative criterion for interpreting intermolecular interactions. For H2O, CH4, and CO2, experimental results are compared with predictions from macroscopic PVT equations of state and with predictions from NVT molecular simulations. Reasonably straight Zeno lines result from commonly used macroscopic EOS models, such as the Redlich−Kwong−Soave (RKS) and the Peng−Robinson (PR). Although quantitative agreement between predicted and experimental Zeno behavior was not exact, the general trends suggest that these macroscopic models ade...