Ions Salting Out Ions at the Water-Air Interface

In common aqueous systems, including naturally occurring salt water as well as tap water, multiple ion species are co-solvated. At the water-air interface, these ions are known to affect chemical reactivity, aerosol formation, climate, and water odor. Yet, the composition of ions at the water interface has remained enigmatic. Here, using surface-specific vibrational spectroscopy, i.e., sum-frequency generation spectroscopy, we quantify the relative surface activity of two co-solvated ions in aqueous electrolyte solution. Through the interfacial water’s O-H stretch mode signal, we find that more hydrophobic ions are salted out to the interface in the presence of the hydrophilic ions. This tendency of the salting-out follows the Hofmeister series. Simulations show that the solvation free energy difference between the ions and the intrinsic surface propensity of ions determine the extent of salting-out of ions by other ions. This mechanism provides a unified view of the salting-out of monatomic and polyatomic ions at interfaces of electrolyte solutions with air.