Specific ion effects at the air/water interface

Theoretical and computational results on single ions' clusters and extended interfaces are reviewed including experimental studies and comparison with simulation results of surface tension, surface potentials, heterogeneous chemical processes, photoelectron spectroscopy and surface selective nonlinear spectrocopies. Traditionally, surfaces of aqueous electrolytes are described as inactive and practically devoid of ions. This turns out to hold for hard ions while multiply charged ions are strongly repelled from the surface, but due to strong electrostatic interaction, they tend to structure and broaden the interfacial water layer. However, due to specific ion effects and polarization interactions in particular, soft anions exhibit a propensity for the air/water interface. Similarly, the hydronium cation shows anaffinity for the aqueous surface, albeit primarily due to its specific hydrogen bonding features. It is likely that many more hitherto unexplored effects will be discovered now that the surface propensity of hydronium cations and soft inorganic anions has been established.