Interfacial Molecular Organization at Aqueous Solution Surfaces of Atmospherically Relevant Dimethyl Sulfoxide and Methanesulfonic Acid Using Sum Frequency Spectroscopy and Molecular Dynamics Simulation

The molecular organization at the aqueous dimethyl sulfoxide (DMSO) and methanesulfonic acid (MSA) surfaces was investigated using vibrational sum frequency generation (VSFG) spectroscopy and molecular dynamics (MD) simulation. The molecular orientation of surface DMSO and MSA is deduced based on the VSFG spectra of both C-H stretch and S-O stretch regions. The S-O stretch region was studied for the first time and is shown to be critical in molecular orientation determination. On average, the CH(3) groups of DMSO and MSA are preferentially pointing outward into the air. However, the DMSO S═O group points slightly inward away from the surface, while the SO(3) vector of dissociated MSA points nearly straight down. In addition, MD simulations reveal that the orientation distribution of surface DMSO is relatively broad in contrast with a narrow distribution of surface MSA, which agrees with the experiment findings.