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

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 toward 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.

Salt Enrichment and Dynamics in the Interface of Supercooled Aqueous Droplets

“…We think that the increased exclusion of the ions to the subsurface as temperature decreases is due to an improved H-bonded coordinated network in the droplet interior. The differences in the H-bonded network in the interior and subsurface in supercooled droplets have been presented in our previous research . At supercooling, both the CIP and SSIP are mainly formed in the outer portion of the subsurface, where the dielectric constant is expected to be higher than that at 300 K. The higher dielectric constant at lower temperature may lead to a decrease in the free energy difference between the CIP and SSIP, which is consistent with the free energy profile at 200 K shown in Figure a.…”

“…The free-ion configurations account for ≈70% of the total number. In previous research we have found that the supercooled droplets with N > 200 exhibit heterogeneity that is manifested by three regions: a bulk-like interior, a subsurface, and a surface (Figure ). In N = 776 the distances from the droplet’s COM that delimit these regions are R B = 7.4 Å, R V = 14.8 Å, and R e = 17.7 Å (where R e denotes the equimolar radius), respectively.…”

“…TIP4P/2005 allows one to model the systems at supercooling, but it lacks the electronic polarization. In previous research it has been found that the SWM4-NDP model yields nonphysical water density at low temperature, ,, and for this reason for this model, we can only study the trends in dynamic and equilibrium properties in the temperature range T = 260–300 K.…”

“…Cold nano- and microdroplets have been studied through experiment and simulation in relation to crystallization and to their thermodynamic and structural properties. − ,, These cold nanoscopic systems have a surprisingly complex internal structure that varies with temperature and size, as revealed by computer simulations of the TIP4P/2005 model of water, − which we now summarize in the context of the present study.…”

“…Gray-colored layer: This is the surface region in the range R V < r < R e , where R e is the equimolar radius. The determination of R B and R V is presented in ref . A few H 2 O molecules that their O sites are depicted by small red-colored spheres are found at r > R e .…”