Physical-Constant Equations-of-State for Argon Isotherms

Abstract

Using argon as the exemplary fluid, we report a representation of equations-of-state in fluid regions bounded by percolation loci with mainly physical constants that define state bounds and characterize the fluid phase diagram. Both gas- and liquid-state pressures can be represented by 3- or 4-term virial expansions. Gaseous states require only known virial coefficients and physical constants belonging to the fluid, i.e., Boyle temperature (TB), Tc, pc and coexisting densities of gas (ρcG) and liquid (ρcL) at Tc. A notable finding is that for isotherms below TB, the contribution of the fourth virial term is negligibly small within experimental uncertainty. In the supercritical mesophase, both gaseous and liquid states percolate the configurational phase volume, whereupon state functions of the density obey a linear combination equation-of-state in this region. We also find evidence for the percolation line that bounds the low-temperature existence of the pure liquid argon state giving rise to an equilibrium prefreezing mesophase. In this region, which probably exists for all liquids, there is a colloidal dispersion of the metastable crystalline structure with an energy density closest to that of the liquid, wherein coexisting amorphous and crystalline states both percolate the phase volume. We report equations-of-state for isotherms of argon, over the whole equilibrium fluid range; i.e., for four regions, gas, supercritical mesophase, liquid, and a liquid + bcc prefreezing and metastable fluid mesophase up to a glass transition pressure. We compare with experiment using the Tegeler–Span–Wagner equation via the NIST fluid thermophysical property database.