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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, T c (x), P c (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 e ij and surface fractions are used and (ii) the Flory-Huggins excess combinatorial A EC is added to [Math]. When A EC is neglected, wrong values of [Math] are obtained. This is a first proof that A EC must be considered on the gas-liquid critical locus curve.

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.

Critical-locus curves Tc(x) and Pc(x) have been determined for the carbon dioxide + benzene and sulfur dioxide + benzene systems. The data, together with existing liquid-vapor equilibrium data, are used to construct the complete phase diagrams at high pressures.

An accurate equation of state has been utilized to model miscibility gaps for binary liquid mixtures at high pressures. The hypothetical system studied here is an argon + argon system with imposed specific interactions. In addition to the known effects of electrostatic interactions, which may produce an upper or both an upper and a lower critical solution temperature, a strong effect on the shape of the miscibility gap is found to arise from the value of the mixture collision diameter σ ij , determined by the constant k ij . When k ij is assumed to be a function of pressure (or of reduced density), it is possible to model the rare case of a miscibility gap that vanishes with increasing pressure and then reappears at very high pressures.

The relation for the compressibility of hard convex bodies (Boublik, 1975) is combined with the power series in (u/k T) and (V°/V) (Alder et al., 1972) to obtain the most general and accurate equation of state of fluids. 24 universal constants of the series were fitted to the internal energy and PVT data for argon. The equation also contains 5 characteristic constants that were fitted to the properties of 11 fluids to show its validity at all densities ranging from zero (second virial coefficients) to the liquid density at T/Tc = 0.55 that corresponds to the triple point of argon. The interaction energy u/k appears to be equal to the critical temperature Tc for small molecules but it becomes a function of the temperature for non-spherical molecules, in agreement with the theory of non-central forces.

The liquid-vapor equilibria and critical loci of the systems N2 + CO2, CH4 + CO2 and CH4 + H2S have been correlated on the basis of experimental data using an accurate equation of state (EOS) including a semiempirical correction for vanishing ordering effects in the critical regions. The EOS predicts gas-gas equilibria for the N2 + CO2 and CH4 + H2S systems and confirms the data of Kohn and Kurata (1958) indicating no critical locus for CH4 + H2S in the composition region 0.54 ⩽ xCH4 ⩽ 0.89. The predicted excess enthalpies and compressibilities of the gaseous mixtures agree well with the data. Deviations from the random-mixing approximation as they affect calculated liquid densities are discussed.

Semi-empirical three-parameter relations for the average intermolecular energy ūm and the excess functions of nonrandom mixtures have been derived from a hypothesis that the asymmetry of the functions result from the formation of molecular clusters. It is postulated that changes in cluster concentration upon dilution in an inert solvent can be interpreted as changes in the apparent three-body interaction parameters. Comparison with data for binary systems where there is unusual composition dependence of the excess chemical potentials demonstrates the accuracy of the relations. Conclusions regarding the sources of deviations from random mixing come from computer simulations and other data.

We have measured the density of an equimolar gas mixture of C2H6–CH3Cl between 323.15 and 473.15 K at pressures to 14.0 MPa. Interaction second virial coefficients and dew-point pressures are extracted from the measurements. Intermolecular force constants for the Lennard-Jones and Stockmayer models are determined optimally for like and unlike pair interactions. Recent methods of prediction for second virial coefficients, both pure and interaction, are tested against experiment.

The critical temperature T c and critical pressure p c were measured at five compositions for each of five binary mixtures formed by propane with perfluoropropane, perfluoro- n -hexane, hexafluoroacetone, hexafluorobenzene, and trifluoroacetonitrile, and the two binary mixtures of n -hexane with perfluoro- n -hexane and hexafluorobenzene, and of perfluorocyclobutane with pentene-1. The composition dependences of T c and p c calculated by means of the modified theory of conformal solutions are in satisfactory agreement with the experimental results. In particular, the unusual S-shaped curve of p c against mole fraction for C 3 H 8 + CF 3 CN, predicted by the theory, is confirmed by the experimental results. The peculiar properties of mixtures of fluoro-compounds with inert solvents and the deviations from the harmonic mean rule for e 11 are discussed. Additionally, the densities of liquid (CF 3 ) 2 CO and the densities and vapor pressures of CF 3 CN were determined at reduced temperatures from about 0.6 to 1.