The variation of surface propensity of halides with droplet size and temperature

Abstract

Relative to a planar interface, nanodroplets are characterized by substantial mass density gradients between the interior and the surface regions and different forces when a sole ion is embedded within them. The radial number density of halide and alkali ions in aqueous clusters with equimolar radius up to approximately 1.4 nm, that corresponds to approximately 250 water molecules, has been extensively studied. However, the abundance of Cl-, Br- and I- on the surface relative to the bulk interior in these smaller clusters may not be representative of the larger systems. Indeed, here we show that the small droplet sizes with equimolar radius up to approximately 1.4 nm are significantly different in their structure and mass density and thus, in the relative surface abundance of halides from their larger counterparts composed of > 800 water molecules (equimolar radius > 1.75 nm). Starting from equimolar radius approximately 1.75 nm converging values in the ion location are observed. The chloride number density profile is the most sensitive to the droplet size and temperature and of the iodide least. The observed trend is that the larger the droplet is, the lower the relative surface abundance of chaotropic halides is. At elevated temperatures Cl- loses gradually its surface propensity, while I- still preserves it. The relative interfacial free energy of solvation of the ions is in the range of a few kJ/mol.