Comparison of Predictions of the PC-SAFT Equation of State and Molecular Simulations for the Metastable Region of Binary Mixtures

Metastable states are omnipresent. For example boiling requires superheating, condensing requires supercooling. Effects like retardation of boiling or cavitation are due to metastablility. However, experimental studies of... [ view full abstract ]

Metastable states are omnipresent. For example boiling requires superheating, condensing requires supercooling. Effects like retardation of boiling or cavitation are due to metastablility. However, experimental studies of metastable states are difficult. Using molecular simulations, it is possible to obtain reliable information on the metastable region, as the intermolecular potentials on which the simulations rely are state-independent. Also equations of state (EOS), can be used. In contrast to molecular simulation the theories on which EOS are based generally contain semi-empirical approximations. Hence, it is necessary to check their validity in the metastable region.

In the present work, the metastable region of five binary mixtures is examined with the PC-SAFT equation of state and compared to molecular simulations performed with ms2. The studied mixtures are: Methane + Ethane, Methanol + Ethanol, Carbon Dioxide + Toluene, Carbon Dioxide + Hydrochloric Acid, and Hydrochloric Acid + Toluene. In order to calculate the spinodal, the second partial derivatives with respect to the components molarities are analytically determined for the PC-SAFT EOS. Pure component PC-SAFT and molecular simulation models, as well as binary interaction parameters are mostly taken from the literature.

Thermodynamic properties in the vapor-liquid equilibrium (VLE), metastable states as well as the spinodal points determined using PC-SAFT and molecular simulation are compared for the different mixtures. The role of polarity and size of the mixture molecules is discussed. The investigations show that the PC-SAFT EOS is well suited for predicting metastable states and spinodals of mixtures. The deviations between the results from the molecular simulations and those from PC-SAFT for the metastable states are typically of the same order as those obtained for stable states. This is especially important for the application of PC-SAFT in connection with the density gradient theory.