Theoretical examination of the global fluid phase behavior and critical phenomena in carbon dioxide plus n-alkane binary mixtures

The phase behavior of carbon dioxide + n-alkane binary mixtures is studied using the statistical associating fluid theory for potentials of variable attractive range. Extensive experimental data is available in the literature concerning the homologous series of carbon dioxide + n-alkane systems; these provide an excellent example of the continuity of fluid phase behavior in binary mixtures. Continuous transitions in the types of phase behavior are observed as the molecular weight of the n-alkane is increased. In our work, the carbon dioxide and n-alkane molecules are treated as attractive spherical segments tangentially bonded together. The attractive interactions are included as square-well potentials of depth epsilon and range lambda. The pure component parameters of carbon dioxide are obtained by fitting to experimental vapor pressure and saturated liquid density data from the triple to the critical point, and the optimized molecular parameters of n-alkanes are taken from a set of transferable parameters previously presented [McCabe and Jackson, Phys. Chem. Chem. Phys. 1999, 1, 2057]. The optimized conformal molecular parameters (segment size and dispersive energy) are rescaled in order to give the best description of the experimental critical points. The critical lines and critical end-points, which determine the types of phase behavior, are predicted by the SAFT-VR approach in very good agreement with experimental data. This is particularly gratifying as the unlike dispersion interaction and range of the potential between carbon dioxide and alkane segments are obtained from a single mixture (CO2 + n-tridecane) and then transferred to the other systems.