Hydrophobic Molecules Slow Down the Hydrogen-Bond Dynamics of Water

We study the spectral and orientational dynamics of HDO molecules in solutions of tertiary-butyl-alcohol (TBA), trimethyl-amine-oxide (TMAO), and tetramethylurea (TMU) in isotopically diluted water (HDO:D2O and HDO:H2O). The spectral dynamics are studied with femtosecond two-dimensional infrared spectroscopy and the orientational dynamics with femtosecond polarization-resolved vibrational pump-probe spectroscopy. We observe a strong slowing down of the spectral diffusion around the central part of the absorption line that increases with increasing solute concentration. At low concentrations, the fraction of water showing slow spectral dynamics is observed to scale with the number of methyl groups, indicating that this effect is due to slow hydrogen-bond dynamics in the hydration shell of the methyl groups of the solute molecules. The slowing down of the vibrational frequency dynamics is strongly correlated with the slowing down of the orientational mobility of the water molecules. This correlation indicates that these effects have a common origin in the effect of hydrophobic molecular groups on the hydrogen-bond dynamics of water.