Atomistic simulation of ion solvation in water explains surface preference of halides

Abstract

Water is a demanding partner. It strongly attracts ions, yet some halide anions-chloride, bromide, and iodide-are expelled to the air/water interface. This has important implications for chemistry in the atmosphere, including the ozone cycle. We present a quantitative analysis of the energetics of ion solvation based on molecular simulations of all stable alkali and halide ions in water droplets. The potentials of mean force for Cl − , Br − , and I − have shallow minima near the surface. We demonstrate that these minima derive from more favorable water-water interaction energy when the ions are partially desolvated. Alkali cations are on the inside because of the favorable ion-water energy, whereas F − is driven inside by entropy. Models attempting to explain the surface preference based on one or more ion properties such as polarizability or size are shown to lead to qualitative and quantitative errors, prompting a paradigm shift in chemistry away from such simplifications.ozone layer | GROMACS | aerosol | thermodynamics S ea water becomes airborne in the form of droplets formed at the sea surface by the action of waves (1). These droplets can be carried by the wind, and they interact with other constituents of the atmosphere (Fig. 1A). On the surface of the small water droplets, halide anions can be converted into halogen atoms by absorption of light (2, 3), and the air-water interface serves to increase certain reaction probabilities (4, 5). One example is the oxidation of Cl − and Br − by OH radicals or O 3 that can occur at the air-water interface, with mechanisms different from those in the bulk phase (6), leading to natural ozone depletion in the stratosphere.Both theoretical and experimental studies have shown that Cl − , Br − and I − prefer to be at the surface of water droplets (Fig. 1A), whereas F − and alkali cations strive to become fully solvated in the bulk of water droplets (7-12). The Br − concentration is enhanced more at the surface than the Cl − concentration (13), an effect that may be enhanced in mixtures containing both Br − and Cl − , such as seawater (14). Because Br − is more reactive than Cl − , the surface preference has an impact on the chemistry in the atmosphere, despite the fact that the content of Cl − in bulk sea water is about three orders of magnitude higher than that of Br − (15). The energetic and structural mechanisms underlying the surface preference of large halide anions have been studied in quite some detail (16-18), but the phenomenon is still not understood quantitatively (10). Only very recently has the surface absorption free energy of one halide anion, Br − , been determined experimentally (19). Recent progress in development of force fields for water (20) and ions (21) finally allows establishment of the surface preferences for all halide and alkali ions quantitatively.Here we present the energetics of ion solvation in a water droplet with a radius of approximately 1.1 nm for all stable halide anions (F − , Cl − , Br − , I − ) and alkali cations (Li þ , Na þ , K þ...

Organic molecules on the surface of water droplets – an energetic perspective

“…38 using constrained molecular dynamics simulations. Along the reaction coordinate r, that is the distance of the center of mass (COM) of the solute and the COM of the water droplet, 59 positions were selected in the range 0.15 nm ≤ r ≤ 2 nm, with a distance of 0.025 nm between adjacent positions in the range of 0.6 nm ≤ r ≤ 1.65 nm, and a distance of 0.05 nm between all other adjacent positions.…”

“…In previous studies of the evaporation from pure water droplets 36 and water droplets containing ions 37,38 we have studied evaporation properties as well as ion PMFs. The results of these simulations were corraborated by Otten et al recently 39 .…”

Communication: Electronic structure of the solvated chloride anion from first principles molecular dynamics

“…The structural and electronic properties of the latter differ from those of the bulk anions, as Cl − anions are known to preferably reside at the surface. [52][53][54][55] Hence, the signal detected experimentally may have an overall significant component originating from surface anions. Finally, we note that the concentration of the solution simulated here (∼0.87 M) is lower than those studied experimentally (2 M 51, 56 and 3 M 15 ) and the concentration difference may also be partially responsible for the discrepancy with experiments.…”

Ion Transport through a Water–Organic Solvent Liquid–Liquid Interface: A Simulation Study

“…32,34 The approaches taken are dominantly classical, point-like partial charge models although the need for polarizable, or quantum mechanical models, has been argued. 17,35 Polarizable models have been used for the study of ions at the water -vapor interface 17,[36][37][38] but also for ion interactions at liquid-liquid interfaces in, e.g., refs 39-43, and ion transport in, e.g., refs 40,44-46. These studies show that accounting for polarization in the model affects the ion-interface behavior, especially when large, polarizable anions are considered; whereas for sodium, a small and hard ion, the effect is reduced.…”