“Breaking” and “Making” of Water Structure at the Air/Water−Electrolyte (NaXO₃; X = Cl, Br, I) Interface

The prevalence of ions at the aqueous interface has been widely recognized, but their effect on the structure of interfacial water (e.g., hydrogen (H)-bonding) remains enigmatic. Using heterodyne-detected vibrational sum frequency generation (HD-VSFG) and Raman difference spectroscopy with simultaneous curve fitting (DS-SCF) analysis, we show that the ion-induced perturbations of H-bonding at the air/water interface and in the bulk water are strongly correlated. Specifically, the structure-breaking anions such as ClO₃^– decrease the average H-bonding of water at the air/water interface, as it does to the water in its hydration shell in the bulk. The structure-making anion of the same series (IO₃^–) does exactly the reverse. None of the electrolytes (NaXO₃; X = Cl, Br, I) form well-defined electric double layers that significantly increase or reverse the hydrogen-down (H-down) orientation of water at the air/water interface. These results provide a unified picture of specific anion effect at the air/water interface and in the bulk water.