Molecular Origin of Interfacial Anomalies in Azeotropic Refrigerant Mixtures

Interfacial anomalies such as the formation of an aneotrope have been well-established for binary refrigerant blends with polar and nonpolar compounds. This remains largely unexplored for mixtures of polar refrigerants. This study investigates the phase equilibria and interfacial properties of pure refrigerants and their binary mixtures, specifically focusing on the molecular origin of interfacial anomalies. The polar soft-SAFT equation coupled with density gradient theory (DGT) was used to model the interfacial properties of 16 pure refrigerants, as well as phase equilibria and surface tension of selected binary mixtures with available data. Once the model was validated, the phase equilibria and interfacial tension for mixtures with R134a were systematically predicted with emphasis on azeotrope and aneotrope formation. It was determined that, unlike polar + nonpolar refrigerant mixtures, the occurrence of azeotropy is not a strict prerequisite for manifesting aneotropic-like behavior. The aneotrope composition was always consistent with the composition at which zero relative adsorption was observed rather than the composition of the azeotrope. Distinct interfacial enrichment in these mixtures was absent, indicating a subtle preferential adsorption rather than significant accumulation at the interface. This research provides a deeper understanding of interfacial anomalies and molecular-level insights for the rational design of sustainable next-generation refrigerants.