Liquid/Vapor Interface of Dimethyl Carbonate-Methanol Binary Mixtures Investigated by Sum Frequency Generation Vibrational Spectroscopy and Molecular Dynamics Simulation.

In the present work, the dimethyl carbonate (DMC)-methanol binary mixture was used as a benchmark system to study the molecular structures of the liquid/vapor interface of organic-organic mixtures by sum frequency generation vibrational spectroscopy (SFG-VS) and molecular dynamics (MD) simulations. It was discovered that both the methanol and DMC molecules are anisotropically oriented at the surface, yielding strong SFG-VS signals in the C-H stretching frequency range for both molecules. The detailed analyses of the spectroscopic and MD data reveal that the increase of the methanol bulk concentrations reduces the orientational order of the methyl groups for both the interfacial DMC and methanol molecules but does not significantly affect the orientations of the carbonyl group in DMC. Moreover, no obvious correlations were found between the room-temperature orientations of the surface molecules and the azeotropic mole fraction. The present work paves the road for future investigations on the molecular structures of the liquid/vapor interfaces of other organic-organic mixtures, especially those that are important in industrial separations.