Molecular structure and adsorption of dimethyl sulfoxide at the air/aqueous solution interface probed by non-resonant second harmonic generation

Abstract

In this study, non-resonant second harmonic generation (SHG) was used to investigate the molecular structure and adsorption of DMSO at the air/neat DMSO liquid and air/DMSO aqueous solution interfaces. The molecular orientation of interfacial DMSO as a function of the bulk DMSO concentration was investigated by quantitative polarization SHG analysis. For the air/neat DMSO liquid interface, the transition dipole moment of the SO group of DMSO is oriented 140° from the surface normal, where the SO group of DMSO is estimated to be 30° from the surface plane. The orientation of the SO group of interfacial DMSO is not dependent on the bulk DMSO concentration. Furthermore, the concentration-dependent SHG signal confirmed that the antiparallel double layer structure does not form at the air/DMSO water interface. The free energy of adsorption of DMSO at the air/DMSO aqueous solution interface was determined to be ΔG_ads = −5.6 ± 0.4 kJ mol⁻¹.