Low density interior in supercooled aqueous nanodroplets expels ions to the subsurface

TL;DR

This study uses computational methods to show that supercooled aqueous nanodroplets have a low-density core that pushes ions to the denser subsurface, contrasting with behavior at room temperature, and introduces a model for ion confinement.

Contribution

It provides the first direct computational evidence of ion expulsion to the subsurface in supercooled droplets and develops an electrostatic confinement model for ion distribution.

Findings

Ions are expelled to the subsurface in supercooled droplets.

The electrostatic model predicts harmonic confinement at room temperature.

Simulation results differ at supercooling, indicating complex behavior.

Abstract

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.