THERMODYNAMIC TESTING OF EQUATIONS OF STATE OF DENSE SIMPLE LIQUIDS

The isobaric thermal expansivities (alpha(p)) Of n-hexane and of metha ne measured over large pressure and temperature ranges have been propo sed as a thermodynamic property lest for theoretical and semi-empirica l equations of state for simple liquids at high densities. The correla tion equations for a(p) (p, T) and the coordinates of the crossing poi nt of isotherms of a(p) are given for both substances. A number of sel ected equations of state with various modifications of the attractive and repulsive contribution to the pressure have been tested with respe ct to their ability to reproduce the proposed test property. The analy sis has revealed that at high densities the equations of state with mo dified repulsive term reproduce the lest property better than the equa tions with modified attractive term. A reproduction of the most charac teristic 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 rep roduction of the test property was obtained with the soft-sphere equat ion constructed on 8-4 Lennard-Jones intermolecular pair potential. Th e anisotropy in molecular shape of n-hexane and methane has only a lit tle influence on the results of testing.