A semiempirical equation of state for non-polar and polar substances on the basis of perturbation theory

A new semiempirical equation of state applicable both to non-polar and to polar substances was developed by a modification of the perturbation theory. This equation is composed of four terms, each characterized by one of the four kinds of interactions: repulsion, symmetric, non-polar asymmetric and polar asymmetric interactions. For the first one, the hard convex fluid equation of Boublik was used. The functional forms of the second and third ones were determined by analogy to the Pade function of Stell and the second virial coefficient equation of Tsonopoulos. The last one was approximated by the polar deviation function of Stell. The universal constants of the second and third terms were obtained by use of the P-V-T data of the reference substances having the corresponding interaction. Four characteristic parameters, with well-defined physical meaning, were required to characterize the pure substances. The values of these parameters are presented for non-polar and polar substances. The proposed equation produced good correlation results for the P-V-T relations of the non-polar and polar substances, except in the critical region of polar substances.