The regularity exhibited by fluids along the contour of the unit compressibility factor in the temperaturedensity plane, where Z t 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 H 2 O, CH 4 , and CO 2 , 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 adequately capture the dynamic balance that exists between repulsive and attractive forces along the Zeno contour. From molecular dynamics simulations, the Lennard-Jones, the Simple Point Charge (SPC), and the extended SPC/E models of pure H 2 O also yielded Zeno lines close to experimentally measured values over a wide range of densities. Density-scaled radial distribution functions indicate that the degree of long-range ordering was nearly invariant along the Zeno line, but hydration numbers and cluster sizes varied markedly.

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