THERMODYNAMIC TESTING OF EQUATIONS OF STATE OF DENSE SIMPLE LIQUIDS

The isobaric thermal expansivities (alpha(p)) Of n-hexane and of methane measured over large pressure and temperature ranges have been proposed as a thermodynamic property lest for theoretical and semi-empirical equations of state for simple liquids at high densities. The correlation equations for a(p) (p, T) and the coordinates of the crossing point of isotherms of a(p) are given for both substances. A number of selected equations of state with various modifications of the attractive and repulsive contribution to the pressure have been tested with respect to their ability to reproduce the proposed test property. The analysis has revealed that at high densities the equations of state with modified repulsive term reproduce the lest property better than the equations with modified attractive term. A reproduction of the most characteristic feature of the lest property, the crossing of isotherms of a(p) can be only obtained with equations in which the repulsion term is made softer than the simple rigid-sphere term RT/(V-m-b). The best reproduction of the test property was obtained with the soft-sphere equation constructed on 8-4 Lennard-Jones intermolecular pair potential. The anisotropy in molecular shape of n-hexane and methane has only a little influence on the results of testing.