Low Density Interior in Supercooled Aqueous Nanodroplets Expels Ions to the Subsurface

The location of a single and multiple ions in aqueous droplets plays a key role in chemical reactivity of atmospheric and man-made aerosols. We report direct computational evidence that in supercooled aqueous nanodroplets a lower density core of tetrahedrally coordinated water molecules expels the sodium ions to a higher density and more disordered subsurface. In contrast, at ambient temperature the single Na+ density is higher in the core region and has a broad maximum at the droplet's center of mass. We analyse the expulsion of a single ion in terms of a general reference electrostatic model that we have developed. The energy of the system in the analytical model is expressed as the sum of electrostatic and surface energy of a fluctuating droplet. The model predicts that the energy associated with the distance of the ion from the droplet's center of mass is quadratic in this distance. We name thiseffect "electrostatic confinement". The predictions of the model are consistent with the simulations fndings for a single Na+ ion at ambient conditions. Our results assist in understanding the mechanisms of charging of macromolecules in spray-based ionization methods used in native mass spectrometry and the physical chemistry of atmospheric aerosols.