Effect of Halide Anions on the Structure and Dynamics of Water next to an Alumina (0001) Surface

While ions are known to perturb hydrogen bonding networks in bulk water, our understanding of such effects is less developed for interfaces. Alumina/water interfaces are highly ordered due to strong hydrogen bonding interactions between interfacial water molecules and adjacent aluminol groups. However, how ions alter this interaction is not yet known. Herein, to address the effect of sodium halide salts on the hydrogen bonding environment of interfacial water, we investigated charged alumina (0001) surfaces using steady-state and time-resolved vibrational sum frequency generation (vSFG) spectroscopy. Our results indicate that the effect of halide anions on the attenuation of the vSFG signal next to positively and negatively charged alumina surfaces followed the sequence F^- ≫ Br^- > Cl^- > I^- (slightly varied direct Hofmeister series) and Br^- > I^- ≈ Cl^- > F^- (slightly varied indirect Hofmeister series), respectively. Additionally, time-resolved vSFG reveals that only F^- perturbs the vibrational lifetime of water next to a positively charged alumina surface by presumably breaking the strong hydrogen bonding interaction between the surface aluminol groups and the nearby water molecules.