Surface Structure of Organic Carbonate Liquids Investigated by Molecular Dynamics Simulation and Sum Frequency Generation Spectroscopy

The vapor-liquid interface structures of two typical organic carbonates, propylene carbonate (PC) and dimethyl carbonate (DMC), are investigated in collaboration of sum frequency generation (SFG) spectroscopy and molecular dynamics (MD) simulation. The present general molecular model for organic carbonates well reproduces various liquid properties, including density, heat of vaporization, and infrared, Raman and SFG spectra. The MD simulation revealed contrasting interface structures between PC and DMC. The PC interface exhibits layered structure of oscillatory orientation, while the DMC interface is quite random. The structural feature of the PC interface is mainly attributed to dimer formation of PC molecules. We elucidated that the different surface structures are manifested in their Im[⁽²⁾] SFG spectra in the C - O stretching band, showing opposite signs of bipolar peaks between the two liquids.