Effect of ions on the structure and dynamics of liquid water

We use femtosecond two-colour mid-infrared spectroscopy to study the dynamics of aqueous solutions of salts in HDO:D₂O. We find that the lifetime of the O-H stretch vibration of HDO molecules in the solvation shell of the halogenic anions Cl^-, Br^-, and I^- is much longer than the lifetime of the O-H stretch vibration of the HDO molecule in bulk D₂O solution. This difference in lifetime allows for a clear separation of the response of the solvation shell from that of the bulk liquid. We observe that the solvating HDO molecules show much slower hydrogen-bond and reorientation dynamics than bulk water molecules. In contrast, the dynamics of the water molecules outside the first ionic hydration shell are observed to be negligibly affected by the ions.