Molecular Behavior of Ethylene Glycol/1,2-Butanediol Mixtures at the Vapor–Liquid Interface

In this work, a binary azeotropic mixture of ethylene glycol (EG) and 1,2-butanediol (BUT) is used as a benchmark system to study how the molecular structure at the vapor–liquid interface depends on the concentration and temperature by using sum-frequency generation vibrational spectroscopy (SFG-VS), surface tension measurements, and molecular dynamics (MD) simulations. At 22 °C, the BUT molecules yield strong SFG-VS signals in the C–H stretching frequency range and override the signal of EG from the mole fraction of BUT at 0.3 to pure BUT solution. After the temperature is increased from 22 to 64 °C, the observed decrease in the SFG-VS intensity of the mixtures suggests that the dynamics of the adsorbed layer at the interface change with temperature. A detailed analysis of MD data shows that the methyl groups of BUT prefer an upright direction along the surface normal at 22 °C and that the bulk concentration change does not significantly affect the molecular orientation. The increase in the temperature reduces the "up-pointing" trend. The interfacial concentration analysis suggests that the interfacial concentration approaches the bulk mole fraction with increasing temperature. The interfacial behavior depends on the bulk properties of the liquid and contributes to vapor-phase behavior.