  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 cosmotropic ions to
the denser and more disordered subsurface. In contrast, at room temperature,
depending on the nature of the ion the radial distribution in the droplet core
is nearly uniform or elevated towards the center. We analyze the spatial
distribution of a single ion in terms of a reference electrostatic model. 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 ion at
room temperature but not at supercooling. We anticipate this study to be the
starting point for investigating the structure of supercooled (electro)sprayed
droplets that are used to preserve the conformations of macromolecules
originating from the bulk solution.
