Equation of State for Molten Alkali Metals from Surface Tension. Part II

Abstract

This work presents a new method for predicting the equation of state for molten alkali metals, based on statistical–mechanical perturbation theory from two scaling constants that are available from measurements at ordinary pressures and temperatures. The scaling constants are the surface tension and the liquid density at the boiling temperature (γb, ρb). Also, a reference temperature, T Ref, is presented at which the product (T Ref T b 1/2 ) is an advantageous corresponding temperature for the second virial coefficient, B 2(T). The virial coefficient of alkali metals cannot be expected to obey a law of corresponding states for normal fluids, because two singlet and triplet potentials are involved. The free parameter of the Ihm–Song–Mason equation of state compensates for the uncertainties in B 2(T). The vapor pressure of molten alkali metals at low temperatures is very low and the experimental data for B 2(T) of these metals are scarce. Therefore, an equation of state for alkali metals from the surface tension and liquid density at boiling temperature (γb, ρb) is a suitable choice. The results, the density of Li through Cs from the melting point up to several hundred degrees above the boiling temperature, are within 5%.