Criticality points on the phase diagram of substances as scaling parameters in the law of corresponding states: alkali metals


 The law of corresponding states (LCS), based on a generalized van der Waals equation of state (gvdWEoS) has been formulated by rescaling its thermodynamic variables with liquid - vapour critical – point parameters, the spinodal, the thermodynamic limit of superheat (TLS) and the supercritical (SC) point. This gvdWEoS, which satisfactorily describes the properties of alkali metals, differs from the known van der Waals equation of state (vdWEoS) by the modified attractive term. The scaling properties have been derived on the basis of the gvdWEoS. The expressions for the isothermal compressibility, a derivate of Gibbs free energy have been derived. In all these cases of rescaling, the gvdWEoS has the similar functional form and the expression for isothermal compressibility has a similar functional form. Hence, it has been established that the parameters of at any spinodal-point on the phase diagram, including the TLS as a particular case, and the SC point, characteristic point on the quasispinodal, provide the alternate scales of parameterizing the fluid properties other than the liquid-vapour critical point.