Where's the Salt? Sensing the Ionic Environment of the Air-Organic-Water Interface Using an Azide Probe

Understanding ionic behavior at aqueous interfaces is vital to fields spanning environmental chemistry, microdroplet reactivity, and electrochemical systems. Yet, whether ions accumulate or are depleted at aqueous boundaries remains debated. Using an uncharged, surface-anchored azide probe, we find that even at 5 M salt concentrations, the air-organic-water interface is depleted of small, inorganic ions, contrasting pronounced effects in bulk. Only under water-in-salt conditions (>10 M ZnCl₂) does evidence of direct cation coordination emerge at the interface. These findings extend the stratified salt model to the air-organic-water interface and provides molecular-level insights into interfacial reactivity, ion partitioning, and "on-water" transformations.