Direct observation of segregation of sodium and chloride ions at an ice surface.

Distribution of electrolyte ions near the surface of water or ice is a subject of interest in both fundamental chemistry and atmospheric chemistry of sea-salt aerosols and ice particles. The ion distribution at the surface of these particles may affect their reaction with ozone and organic species in the troposphere and the generation of reactive halogen species. 2] In early studies, the surface of aqueous solutions containing simple electrolytes such as alkali halides was thought to be deficient of ions, as inferred from the surface tension of solutions. Molecular dynamics (MD) simulations have been widely employed since the 1990s to study the molecular details of solvation and segregation of atomic ions in water clusters. MD calculations predict that the large and polarizable anions are more readily available at the surface than the small nonpolarizable cations, in both water slab models and clusters of finite sizes. 7,9] Photoelectron spectroscopic studies of anionic water clusters in the gas phase support the surface residence of the larger halide anions by comparing the ionization energies with calculations. Further studies of the anionic clusters [X (H2O)1–5, X=F, Cl, Br and I] using vibrational spectroscopy indicate that the larger halides (Cl , Br and I ) are solvated at the surface of water clusters. The spectra from clusters with the larger halides reveal a hydrogen-bonding network of water molecules, which supports surface solvation of anions, whereas the spectra from the F -containing clusters lack such hydrogen-bonding features. A limited number of experimental studies were performed to investigate the ion distribution at the surface of real aqueous solutions, and basically none at ice surfaces. Morgner and co-workers measured He(I) photoelectron spectra from the surface of concentrated aqueous solutions of CsF and observed that the salt concentration is strongly depleted in the surface region. MD simulations from the same group explain the phenomena by the circumstance that the ions near the surface mostly keep their first solvation shell intact. Using Xray photoelectron spectroscopy and scanning electron microscopy, Ghosal et al. observed selective segregation of Br ions to the surface of a NaCl crystal, which was slightly doped with Br ions, under conditions of relative humidity, where the condensed water films caused partial dissolution of the crystal surface. Their results provide the first experimental evidence for preferential segregation of large halide anions to the surface of mixed alkali halides. More recently, vibrational sum frequency generation spectroscopy for sodium halide solutions