Cation Depth-Distribution at Alkali Halide Aqueous Solution Surfaces

Using synchrotron radiation Near-Ambient Pressure X-ray photoemission spectroscopy (NAP-XPS) in surface and bulk sensitive conditions, we specifically explored the depth distribution of Na⁺ ions near the vapor/liquid interface of saturated solutions of sodium halides (NaCl, NaBr, and NaI) kept at 278 K under a pressure of 8 mbar. By varying the photoelectron kinetic energy, and hence the probing depth, we show that the Na⁺ depth distribution near the surface is highly sensitive to the size and polarizability of the anion. The large polarizable anions, Br^- and I^- that segregate at the surface, tend to drag the Na⁺ ions closer to the liquid-water surface. However, this situation is not observed for the NaCl solution, in line with recent theoretical results.