We study the effects of ions on the reorientation dynamics of liquid water with polarization-resolved femtosecond mid-infrared spectroscopy. We probe the anisotropy of the excitation of the O–D stretch vibration of HDO molecules in solutions of NaCl, NaI and N(alkyl)4Br (tetra-alkylammoniumbromide) salts in 8% HDO:H2O. We find that the reorientation of the O–D groups of HDO molecules hydrating the Cl and I anions occurs on two different time scales with time constants of 2 ± 0.3 ps and 9 ± 2 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 N(alkyl)4Br 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 N(alkyl)4+ ions.

Amsterdam: Elsevier
J. Mol. Liq.
Ultrafast Spectroscopy

van der Post, S.T, Scheidelaar, S, & Bakker, H.J. (2012). Femtosecond study of the effects of ions on the reorientation dynamics of water. J. Mol. Liq., 176, 22–28. doi:10.1016/j.molliq.2012.05.014