Specific behavior of excess enthalpy of binary fluid mixtures near critical loci

The excess enthalpies for model fluid mixture interacting through the Lennard-Jones potential were calculated using the Percus-Yevick integral equation over wide ranges of temperature and pressure and over the entire mole fraction range. The calculated results near the critical locus reproduced the several specific behaviors founded experimentally in many fluid mixtures, and all these specific behaviors were explained by the phenomena due to the change of the combination of the states of the mixture and its component fluids. Based on this result, the excess enthalpies of the model mixture and the mixture composed of ethane and ethene measured by Gruszkiewcz et al. (J. Chem. Thermodynamics, 27, 1995, 507-524) were divided into two contributions, that is, the contribution due to the difference of the states between the mixture and its component fluids and the contribution which reflects the details of the molecular interactions for each mixture. The following results were found from these considerations. The specific behaviors of excess enthalpies near the critical locus are determined by the first contribution, and the excess enthalpies at states far from the critical locus are determined by the second contribution. In intermediate region, the new type specific behavior may arise because of the compensation of these two contributions with similar magnitude.