Quantifying Hydrogen-Bond Populations in Dimethyl Sulfoxide/Water Mixtures.

Dimethyl sulfoxide (DMSO) disrupts the hydrogen-bond networks in water. The widespread use of DMSO as a cosolvent, along with its unusual attributes, have inspired numerous studies. Herein, infrared absorption spectroscopy of the S=O stretching mode combined with molecular dynamics and quantum chemistry models were used to directly quantify DMSO/water hydrogen-bond populations in binary mixtures. Singly H-bonded species are dominant at 10 mol %, due to strong DMSO-water interactions. We found an unexpected increase in non-hydrogen-bonded DMSO near the eutectic point (ca. 35 mol %) which also correlates with several abnormalities in the bulk solution properties. We find evidence for three distinct regimes: 1) strong DMSO-water interactions (<30 mol %), 2) ideal-solution-like (30-90 mol %), and 3) self-interaction, or aggregation, regime (>90 mol %). We propose a "step in" mechanism, which involves hydrogen bonding between water and the DMSO aggregate species.