Molecular Structure of Salt Solutions:  A New View of the Interface with Implications for Heterogeneous Atmospheric Chemistry

Abstract

Most salts raise the surface tension of water. Interpretation of this phenomenon via the Gibbs adsorption equation has led to the commonly held belief that the ions are repelled from the air/solution interface. Here, we report results from molecular dynamics simulations of a series of sodium halide solution/air interfaces. The simulations reproduce the experimentally measured increases in surface tension relative to pure water. Analysis of the structure reveals that the small, nonpolarizable fluoride anion is excluded from the interface, in accord with the traditional picture. However, all of the larger, polarizable halide anions are present at the interface, and bromide and iodide actually have higher concentrations in the interfacial region than in the bulk. On the basis of the simulations we develop a molecular picture of hydrogen bonding in the interfacial region that might be tested by surface sensitive spectroscopic experiments. The novel, microscopic view of the interfacial structure of aqueous salt solutions presented in this paper has implications for the reactivity of sea salt aerosols in the marine boundary layer, and bromine chemistry in the remote Arctic at polar sunrise.

Reactions at surfaces in the atmosphere: integration of experiments and theory as necessary (but not necessarily sufficient) for predicting the physical chemistry of aerosols

“…For example, Hu et al 89 had proposed that the unusually fast uptake measured for Cl 2 and Br 2 on solutions containing I À and Cl À was caused by a contribution of a reaction at the interface. However, molecular dynamics (MD) simulations [90][91][92][93][94][95][96][97] of solutions of NaX where X = F À , Cl À , Br À and I À (Fig. 3) showed that while F À prefers to be fully solvated in the bulk, the other ions are increasingly present at the interface as one proceeds down the series, which has also been predicted in theoretical studies using a partitioning model.…”

The effect of cations on NO2 production from the photolysis of aqueous thin water films of nitrate salts

“…1 However, a combination of computational [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] and experimental studies [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33] has shown that ions can reside and even be enhanced at the interface.…”

“…Nitrate ions are a common constituent of sea salt aerosols (100-400 mM) 52 due to oxidation of surface halide ions by oxides of nitrogen (N 2 O 5 , NO 2 , NO 3 and ClONO 2 ), as well as uptake of nitric acid from the gas phase. 81 Nitrate ions photolyze with actinic radiation, producing NO 2 and OH via (1a) 82 and (2), 83 and NO 2 À and O( 3 P) through (1b).…”

“…At room temperature the bulk-phase quantum yields are B0.01 for OH production and an order of magnitude lower (B0.001) for O( 3 P) formation (1b) at 305 nm. 82,[90][91][92][93] In smaller water clusters (32-300 water molecules), 86 NO 3 À ions are predicted to be present at the air-water interface. In the interface region, nitrate ions are less solvated than those in the bulk, 86 resulting in increased NO 2 production yields.…”

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

“…In particular, the binding strength of the ion hydration shell has direct consequences for their interfacial behavior; small hard ions that form a strong hydration shell, such as alkali cations or fluoride, are depleted from the interface, whereas large polarizable ions, for example heavier halides and hydronium, may show propensity for the interface. 2,5,[18][19][20][21] The possibility to characterize the liquid-liquid interface on a microstructural level and the detailed dynamics information readily available have raised significant interest on molecular dynamics simulations of ions and their interactions at immiscible water -organic solvent interfaces since 1990s, 4,[22][23][24][25][26][27][28][29][30][31][32][33] but only recently molecular simulations have reached the system sizes and timescales sufficient to describe interfacial coarsening and transport through the interface in atomistic detail. 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.…”