The Zeno (Z=1) Behavior of Water: A Molecular Simulation Study

Abstract

The regularity of fluid properties observed at the Zeno or Z=PV/RT=1 point has been proposed as a means of testing and improving volumetric equations of state. Previous research has shown that molecular interactions can be qualitatively and quantitatively related to the linear Z=1 contour of T_r vs ρ_r for pure fluids from the Boyle temperature to the triple point. In this study, we expand the molecular simulation analysis of previous work to gain a detailed microscopic understanding of the properties of Zeno-point systems. Our calculations show that popular semiempirical water models, such as SPC and SPC/E water, are able to replicate closely experimentally determined water properties in the Zeno-point region. Detailed molecular dynamics simulations of Z=1.00 and adjacent Z=0.75 and Z=1.25 states reveal common features over a wide range of temperatures and densities, from 77 to 1097°C and 1.01 to 0.029 g·cm⁻³. Radial distribution functions of high-temperature, high-density Zeno-point fluids display remarkable long-range structural correlation well above the critical temperature and pressure, and examination of hydrogen bonding within each system shows that large water–water hydrogen-bonded clusters persist at high temperatures and supercritical densities. These results are compared to the existing extended corresponding-states approaches for pure fluid properties.