Molecular structure and adsorption of dimethyl sulfoxide at the surface ofaqueous solutions

Surface vibrational sum frequency generation (VSFG) spectroscopy complement ed with surface tension measurements has been utilized to probe the air/ di methyl sulfoxide (DMSO) interface as a function of DMSO concentration in wa ter. For the neat DMSO surface, the DMSO methyl groups extend away from the liquid phase and VSFG polarization studies show that the methyl transition dipole moments of pure DMSO are on average oriented a maximum of 55 degree s from the surface normal. A blue shift of the methyl symmetric stretch is observed with decreasing DMSO concentration and attributed to an electronic interaction between the sulfur and the methyl groups of DMSO. From surface tension data of the aqueous DMSO system, it is shown that DMSO number dens ities are higher at the surface of DMSO-water solutions relative to bulk DM SO concentrations revealing surface partitioning effects. Structural change s of surface DMSO an discussed in terms of monomers, dimers, and clusters w hich could account for the large differences in VSFG intensities and surfac e number densities. From surface tension measurements and utilizing DMSO ac tivities, Delta G(ads)(0), is calculated to be -19.8 (+/-0.4) kJ/mol.