The variation of surface propensity of halides with droplet size and temperature; The planar interface limit

The radial number density profiles of halide and alkali ions in aqueous clusters with equimolar radius $\lessapprox 1.4$~nm, that correspond to $\lessapprox$255~\ce{H2O} molecules, have been extensively studied by computations. However, the surface abundance of \ce{Cl-}, \ce{Br-} and \ce{I-} relative to the bulk interior in these smaller clusters may not be representative of the larger systems. Indeed, here we show that the larger the cluster is, the lower the relative surface abundance of chaotropic halides is. In droplets with equimolar radius of $\approx 2.45$~nm that corresponds to $\approx 2000$~H2O molecules, the polarizable halides show a number density maximum in the droplet's bulk-like interior. A similar pattern is observed in simulations of the aqueous planar interface with halide salts at room temperature. At elevated temperature the surface propensity of \ce{Cl-} decreases gradually, while that of \ce{I-} is partially preserved. The change of the chaotropic halide location at higher temperatures relative to room temperature may considerably affect photochemical reactivity in atmospheric aerosols, vapor-liquid nucleation and growth mechanisms, and salt crystallization via solvent evaporation. We argue that the commonly used approach of nullifying parameters in a force field in order to find the factors that determine the ion location does not provide transferable insight to other force fields.