Hydrogen-Bond Dynamics of Water in Ionic Solutions

We study the effects of ions on the structure and dynamics of the hydrogen bonds in liquid water. As a technique we use femtosecond two-color mid-infrared spectroscopy, since this technique allows a clear distinction of the dynamics of the first solvation (hydration) shell of water molecules from the dynamics of bulk water. We find that water molecules in the first hydration shell of the halogenic anions Cl − , Br − and I − show much slower hydrogen-bond dynamics than water molecules in the pure liquid. We also observe that the first hydration shell shows very slow collective orientational dynamics, and forms a rigid, long-living structure. Finally, we find that ions have surprisingly little effect on the hydrogen-bond dynamics of water molecules outside the first hydration shell, which implies that ions do not enhance or weaken the hydrogen-bond network of liquid water.