EQUATION OF STATE FOR FLUID ALKALI-METALS - BINODAL

The three-parameter generalized van der Waals equation of state for liquids and gases is analyzed. This equation contains the generalized expression a/V-n for the molecular pressure; here the parameter n takes into account the specificity of intermolecular attractive forces for various substances, The equation is presented in the reduced form, from which follows the single-parameter law of corresponding states with the thermodynamic similarity parameter n. It is established that for alkali metals the value of the parameter n is the same and does not depend on temperature substantially. From the given generalized equation, the expressions for the binodal (equilibrium curve of the liquid and vapor phases) are obtained. For cesium, rubidium, and potassium, the temperature dependence of density is calculated over the temperature range from their melting point to the critical point; the results of the calculations agree with experimental data. It is established that for alkali metals, the law of rectilinear diameter breaks down in the vicinity of the critical point.