Cation Depth-Distribution at Alkali Halide Aqueous Solution Surfaces
Résumé
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.