New insights on the molecular organization of binary azeotrope at the vapor/liquid interface affected by temperature

The vapor/liquid interface plays an important role in the distillation of azeotropic mixture. The understanding of interfacial structure will allow us to improve the separation of azeotropic mixture in a rational way. Herein, we use sum-frequency generation vibrational spectroscopy (SFG-VS) and molecular dynamics (MD) simulation to study how the interfacial molecular structure of the mixture of 1,2-butanediol and ethylene glycol depends on concentration and temperature. At 64 ℃, an obvious decline is observed in the SFG-VS intensity of this system contrast to the strong signal yield by 1,2-butanediol at 22 ℃. MD simulation results show that bulk concentration would not significantly affect the molecular orientation. But a temperature rise would make 1,2-butanediol gradually twist around its C2-O2 bond and turn to a lying orientation. This work explores the structure of azeotrope from molecular level and carves the path for the further study on the vapor/liquid interface affected by temperature.