The interaction between water and ions within droplets plays a key role in the chemical reactivity of atmospheric and man-made aerosols. The structure of the droplets changes as a function of size, temperature and charge. We present bridging of electrostatic properties of droplets charged with (macro)ions between the nanoscopic and microscopic scale, as revealed by atomistic, continuum modelling, and analytical theory.

It is found that in supercooled aqueous nanodroplets kosmotropic ions (Na+, Li+, F-) are expelled to the droplet's subsurface. In contrast, at room temperature, depending on the nature of the ion, the number density in the droplet core is nearly uniform or elevated towards the center. The number density of chaotropic ions (Cl-, Br-) exhibit two maxima, one in the surface and the other in the subsurface, while only the surface one has been reported in the literature for over a decade. We analyze the spatial distribution of a single ion in terms of a reference electrostatic model that we have developed. 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 also model the NaCl ion-pairing, one of the fundamental reactions in chemistry, at a temperature range of 200~K-300~K. We identify distinct trends in the equilibrium constant between the contact-ion pair and the solvent-separated ion pair and the dynamics as a function of temperature that are different from those reported for planar interfaces. We provide suggestions of how to optimize mass spectrometry ionization methods, for aerosolizing directly a biological sample from a physiological solution in experiments. The use of a physiological solution is critical problem in mass spectrometry that has not been resolved yet.