Applications of the augmented Van der Waals theory of fluids

The residual molar Helmholtz energy [Math] at the gas-liquid critical point, calculated from the accurate equation of state (Part I) for 19 substances, appears to be constant, — 0.99 ± 0.02. This result leads to an accurate correlation of critical locus curves, Tc (x), Pc (x) and [Math] of mixtures. It is shown that the combining rule for pair interactions [Math], derived for systems of small molecules (Part II), is valid also for large molecules if (i) segment interactions εij and surface fractions are used and (ii) the Flory-Huggins excess combinatorial AEC is added to [Math]. When AEC is neglected, wrong values of [Math] are obtained. This is a first proof that AEC must be considered on the gas-liquid critical locus curve

Applications of the augmented van der waals theory of fluids

Basic relations for the thermodynamic functions of multicomponent systems are derived from the equation of state, developed in part I. These functions are calculated from the total (temperature dependent) interaction energies uii, ujj, and uij. A simple combining rule for uij. is derived on the basis of the square-well potential. Comparisons with experimental data (limited to binary systems) show that the predicted values agree satisfactorily with the experimental data only for systems with small or zero values of η/k whereas for those involving large molecules (or large η/k) the ratio of polarizabilities in the combining rule for uij. requires an empirical correction. The calculations include thermodynamic excess functions at P = 0 and high-pressure liquid-vapor equilibria. Weighing of the interaction energies by means of mole fractions appears to be satisfactory only for systems of small molecules