Separation processes are expensive, non-spontaneous processes that constitute 40 to 70% of both the capital and operating cost of a chemical plant. Distillation constitutes approximately 90-95% of the separation processes used in practice, and in the United States accounts for approximately 11% of all in-plant energy consumed [1,2]. Of contemporary interest is the separation of binary azeotropic mixtures. The presence of an azeotrope places a limit on the achievable separation using conventional distillation. We present simple criteria for azeotrope formation in terms of solvation free energies. The expressions compare the solvation free energy of each component in the other at infinite dilution relative to in itself, requiring four total solvation free energy calculations per binary system. The solvation free energies can be related to intermolecular interactions, and therefore shed insight into why an azeotrope occurs. The developed criteria may additionally be used as an additional constraint/check in the development of molecular models. We demonstrate use with solvation free energies computed using electronic structure calculations with continuum solvent models, molecular simulation, and linear solvation energy relationships.
