Many-Body Interactions Govern Halide Distribution at the Air/Water Interface

Ion-specific surface propensities at the air/water interface remain debated. Here we use data-driven many-body energy (MB-nrg) potentials, which exhibit chemical accuracy across all phases, to compute Helmholtz free-energy profiles for the halides using a controlled model hierarchy. A polarizable baseline (TTM-nrg) reproduces the strong interfacial stabilization of I− (and smaller minima for Br−, Cl−) commonly found with empirical polarizable force fields. In contrast, the fully many-body MB-nrg treatment, which adds explicit 2-body and 3-body ion–water terms to many-body polarization, strongly suppresses interfacial adsorption across the series: only I− retains a shallow minimum on the order of kBT near the Gibbs dividing surface, where kB is Boltzmann's constant. Decomposition of Helmholtz free-energy shows that this residual preference arises from a narrow energetic minimum in the water–water contribution at the interface that compensates ion–water desolvation. These results reconcile classical image-charge exclusion with polarizable-anion continuum and interfacial-fluctuation models, and identify the collective water response, captured only with a quantitative many-body description, as the mechanism that governs halide surface propensity.