Strong Isotope Effect in the Vibrational Response of the Hydration Shells of Hydrophobic Ions

We have studied the properties of water molecules at the surface of salt solutions containing hydrophobic anions like triflate (CF₃SO₃-), ethanesulfonate (C₂H₅SO₃-), and butanesulfonate (C₄H₉SO₃-) using vibrational sum-frequency generation (VSFG) spectroscopy. The VSFG spectra reveal a surprisingly strong isotope effect in the intra- and intermolecular mixing of the water molecules contained in the hydration shells of the hydrophobic anions. The O–H stretch vibrations of H₂O molecules in the hydration shell are strongly mixed, whereas the O–D stretch vibrations of hydrating D₂O molecules are decoupled. This isotope effect is not observed for other ions like perchlorate (ClO₄-), and can be explained from the structure of the hydration shells of the hydrophobic ions.