Femtosecond Mid-Infrared Study of the Dynamics of Water Molecules in Water–Acetone and Water–Dimethyl Sulfoxide Mixtures

We study the vibrational relaxation dynamics and the reorientation dynamics of HDO molecules in binary water–dimethyl sulfoxide (DMSO) and water–acetone mixtures with polarization-resolved femtosecond mid-infrared spectroscopy. For low solute concentrations we observe a slowing down of the reorientation of part of the water molecules that hydrate the hydrophobic methyl groups of DMSO and acetone. For water–DMSO mixtures the fraction of slowed-down water molecules rises much steeper with solute concentration than for water–acetone mixtures, showing that acetone molecules show significant aggregation already at low concentrations. At high solute concentrations, the vibrational and reorientation dynamics of both water–DMSO and water–acetone mixtures show a clear distinction between the dynamics of water molecules donating hydrogen bonds to other water molecules and the dynamics of water donating a hydrogen bond to the S═O/C═O group of the solute. For water–DMSO mixtures both types of water molecules show a v...