Many-Body Effects Determine the Propensity of Halides for the Air-Water Interface.

Water's high polarity makes it a unique solvent for salts, where the nature and concentration of electrolytes at the surface are crucial. These factors significantly influence the hydration structure and reactivity of the solution. A detailed molecular-level understanding of the hydration properties of halide ions is essential to elucidate various environmental, industrial, and physiological processes. The hydration of halides is driven by the subtle interplay between ion-water and water-water interactions, which becomes increasingly significant near interfaces where ions undergo desolvation. These interactions must be captured accurately to predict the behavior of ions from gas phase to condensed phase. In this study, we employ our data-driven many-body energy (MB-nrg) potentials, trained on "gold-standard" coupled-cluster, CCSD(T), reference data to investigate the behavior of halides at the water surface. Here, we leverage the accuracy of the MB-nrg potentials in combination with enhanced-sampling methods to explore the surface propensity of halide ions at the air-water interface. Our simulations reveal that the subtle balance between ion-water and water-water many-body effects dictates the propensity of different halides for the water surface. Contrary to previous studies carried out with polarizable models, our MB-nrg simulations show that only iodide is slightly present at the air-water interface, with an adsorption free energy effectively comparable to thermal fluctuations.