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 less than 1.4 nm, that correspond to less than 255 H2O molecules, have been extensively studied by computations. However, the surface abundance of Cl-, Br- and 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. 3.6 nm that corresponds to approx. 6000 H2O molecules, the relative abundance of the ions in the vapor-liquid interface is near to that observed in the planar interface at room temperature. At elevated temperatures the surface propensity of Cl- decreases gradually, while that of 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 mechanisms and salt crystallization via solvent evaporation. We also argue that the commonly used approach of nullifying parameters in a force field in order to determine the factors that determine the ion location does not provide transferable insight.