Liquid-Vapour Coexistence Curves from a Model Equation of State

Abstract

Using a model equation of state proposed by Chapman and March, which is sufficiently general to contain the models of van der Wails and Dietetic, the thermodynamics of the coexistence curve is set up. The requirement that the critical exponent is less than 1/2 is shown to require a specific non-analytic behavior of a function appearing in the model equation of state. In turn this provides a constraint having the nature of a relation between average density of liquid and vapour and temperature, transcending rectilinear diameters. Comparison between theory and experiment is made to the fluid alkali Cs and for some molecular fluids.

Key Words:

• Critical exponent
• non-analyticity
• rectilinear diameter.