Many-body interactions resolve halide distribution at the air/water interface

Whether dissolved ions approach or avoid the air/water interface remains unsettled, despite its importance for atmospheric and interfacial chemistry. Here we show, using chemically accurate data-driven many-body models and a controlled hierarchy of interaction descriptions, that the intrinsic surface propensity of halide ions depends critically on short-range many-body interactions beyond classical polarization. A conventional polarizable model predicts strong iodide adsorption and weaker stabilization of bromide and chloride. When short-range two- and three-body quantum-mechanical interactions are included, adsorption is strongly suppressed: fluoride, chloride, and bromide remain bulk-favored, while iodide retains only a shallow free-energy minimum comparable to thermal energy. Energy decomposition reveals that iodide's residual preference reflects localized reorganization of interfacial water that offsets the cost of partial dehydration, but is opposed by entropy. These results unify classical electrostatic exclusion and molecular many-body descriptions, showing that halide distributions are controlled not by ion size or polarizability alone, but by a finely balanced collective response of the ion-water system.