Role of the soft-core repulsion upon the prediction of characteristic curves with PC-SAFT

Abstract

In this work, the role of the soft-core repulsion upon the capability of the perturbed chain-statistical fluid association theory (PC-SAFT) of predicting coherent Brown's ideal curves (Amagat, Boyle, and Charles) was analyzed. The soft-core repulsion was introduced by using two different theoretical approaches to define the effective diameter into the Barker and Henderson thermodynamic perturbation theory (TPT). The Brown's characteristic curves were predicted for seven pure components, representing spherical, planar, and non-spherical molecules. For all modeled components (i.e., independent of the molecular size) the soft-core repulsion included into the original effective diameter of PC-SAFT fails to predict physically coherent Amagat curves, and this original diameter should be used with caution to perform calculations at extreme conditions of temperature and pressure. On the contrary, the soft-core model resulting of combining the Boltzmann factor criterion (BFC) approach and the generalized Lennard-Jones (GLJ) potential gives as a result a simple, analytical, and temperature-dependent effective diameter, which permitted to trace physically coherent Brown's characteristic curves with this equation of state (EoS). Finally, the BFC effective diameter was used to determine the effect of varying the intermolecular potential steepness (or softness) upon the Brown's characteristic curves, producing Amagat envelopes that increased at higher steepness (i.e., less softness), while the contrary behavior was found for both the Boyle and Charles curves.