Femtosecond study of the effects of ions on the reorientation dynamics of water

TL;DR

This study uses femtosecond mid-infrared spectroscopy to investigate how different ions influence the reorientation dynamics of water molecules, revealing distinct time scales and effects based on ion type and hydration environment.

Contribution

It provides new insights into ion-specific effects on water reorientation dynamics using polarization-resolved femtosecond spectroscopy.

Findings

Reorientation of water around halide ions occurs on two distinct time scales.

Hydrophobic tetra-alkylammonium ions significantly slow water reorientation.

Fast wobbling motion preserves hydrogen bonds during reorientation.

Abstract

We study the effects of ions on the reorientation dynamics of liquid water with polarization-resolved femtosecond mid-infared spectroscopy. We probe the anisotropy of the excitation of the O-D stretch vibration of HDO molecules in solutions of NaCl, NaI and tetra-alkylammonium bromide salts in 8 percent HDO:H2O. We find that the reorientation O-D groups of HDO molecules hydrating the Cl- and I- anions occurs on two different time scales with time constants of 2pm0.3 ps and 9pm2 ps. The fast component is due to a wobbling motion of the O-D group that keeps the hydrogen bond with the halogenic anion intact. For solutions of tetra-alkylammonium bromide salts we observe a very strong slowing down of the reorientation of water that is associated with the hydration of the hydrophobic alkyl groups of the tetra-alkylammonium ions.