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

The interaction between water and ions within droplets plays a key role in the chemical reactivity of atmospheric and man-made aerosols. Here we report direct computational evidence that in supercooled aqueous nanodroplets
a lower density core of tetrahedrally coordinated water expels the Na+ ions to a denser and more disordered subsurface. In contrast, at room temperature, the radial distribution of a single Na+ ion in the droplet core
is nearly uniform. We analyze the spatial distribution of a single ion in terms of a new reference electrostatic model that we present here. The energy of the system in the analytical model is expressed as the sum of the electrostatic and surface energy of a deformable droplet. The model predicts that the ion is subject to a harmonic potential centered at the droplet's center of mass. We name this effect ``electrostatic confinement''.
The model's predictions are consistent with the simulation findings for a single Na+ ion at room temperature but not at supercooling. Because of the droplet's core organization at supercooling the distribution of multiple ions cannot be explained by the non-linear Poisson-Boltzmann equation.
Our study provides insight into the chemistry of atmospheric aerosols. We anticipate it to be the starting point for investigating the structure of supercooled electrosprayed droplets that are used to preserve the conformations of macromolecules originating from the bulk solution.