Mixture Equation of State for Water with an Associating Reference Fluid

Development of equations of state for water is challenging due to the structural transition to tetrahedral symmetry of liquid water at ambient conditions. Recently, the concept of associated reference perturbation theory (APT) was introduced. APT includes association in the reference fluid to the isotropic attractions. This allows for the structural transition of water to tetrahedral symmetry to be incorporated in the perturbation theory. APT was shown to give substantial improvements over standard SAFT approaches in the description of pure water. In this paper, we further refine this approach by including an associating reference fluid in the polar PC-SAFT equation of state. This allows for a more accurate representation of pure water thermodynamics. It is demonstrated that the new approach gives a substantial improvement in the correlation of liquid densities, hydrogen-bonded structure, and the second virial coefficient as compared to a standard polar PC-SAFT equation of state. The approach qualitatively reproduces the minima in isothermal compressibility and isobaric heat capacity; however it does not reproduce the density maximum. The new approach is also shown to give a good representation of water/hydrocarbon mixture phase equilibria.