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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.

We present results from theoretical studies of aqueous ionic solvation of alkali halides aimed at developing a microscopic description of structure and dynamics at the interface between air and salt solutions. The traditional view has depicted the air/solution interface of simple electrolytes as being devoid of ions. However, it is now firmly established that polarizable anions, such as the heavier halides, occupy the surface of small to medium sized water clusters. Using a combination of theoretical calculations, including ab initio quantum chemistry, Car-Parrinello molecular dynamics simulations, and primarily molecular dynamics simulations based on polarizable force fields, we present a unified view of the interfacial structure of aqueous ionic clusters and bulk solutions. Indeed, we demonstrate that the heavier halogen anions have a propensity for the interface that is proportional to their polarizabil- * Electronic mail: jungwirt@jh-inst.cas.cz † Electronic mail: dtobias@uci.edu 2

A combination of experimental, molecular dynamics, and kinetics modeling studies is applied to a system of concentrated aqueous sodium chloride particles suspended in air at room temperature with ozone, irradiated at 254 nanometers to generate hydroxyl radicals. Measurements of the observed gaseous molecular chlorine product are explainable only if reactions at the air-water interface are dominant. Molecular dynamics simulations show the availability of substantial amounts of chloride ions for reaction at the interface, and quantum chemical calculations predict that in the gas phase chloride ions will strongly attract hydroxl radicals. Model extrapolation to the marine boundary layer yields daytime chlorine atom concentrations that are in good agreement with estimates based on field measurements of the decay of selected organics over the Southern Ocean and the North Atlantic. Thus, ion-enhanced interactions with gases at aqueous interfaces may play a more generalized and important role in the chemistry of concentrated inorganic salt solutions than was previously recognized.

The molecular structure of the interfacial regions of aqueous electrolytes is poorly understood, despite its crucial importance in many biological, technological, and atmospheric processes. A long-term controversy pertains between the standard picture of an ion-free surface layer and the strongly ion specific behavior indicating in many cases significant propensities of simple inorganic ions for the interface. Here, we present a unified and consistent view of the structure of the air/solution interface of aqueous electrolytes containing monovalent inorganic ions. Molecular dynamics calculations show that in salt solutions and bases the positively charged ions, such as alkali cations, are repelled from the interface, whereas the anions, such as halides or hydroxide, exhibit a varying surface propensity, correlated primarily with the ion polarizability and size. The behavior of acids is different due to a significant propensity of hydronium cations for the air/solution interface. Therefore, both cations and anions exhibit enhanced concentrations at the surface and, consequently, these acids (unlike bases and salts) reduce the surface tension of water. The results of the simulations are supported by surface selective nonlinear vibrational spectroscopy, which reveals among other things that the hydronium cations are present at the air/solution interface. The ion specific propensities for the air/solution interface have important implications for a whole range of heterogeneous physical and chemical processes, including atmospheric chemistry of aerosols, corrosion processes, and bubble coalescence.

Ions differ in their ability to salt out proteins from solution as expressed in the lyotropic or Hofmeister series of cations and anions. Since its first formulation in 1888, this series has been invoked in a plethora of effects, going beyond the original salting out/salting in idea to include enzyme activities and the crystallization of proteins, as well as to processes not involving proteins like ion exchange, the surface tension of electrolytes, or bubble coalescence. Although it has been clear that the Hofmeister series is intimately connected to ion hydration in homogeneous and heterogeneous environments and to ion pairing, its molecular origin has not been fully understood. This situation could have been summarized as follows: Many chemists used the Hofmeister series as a mantra to put a label on ion-specific behavior in various environments, rather than to reach a molecular level understanding and, consequently, an ability to predict a particular effect of a given salt ion on proteins in solutions. In this Feature Article we show that the cationic and anionic Hofmeister series can now be rationalized primarily in terms of specific interactions of salt ions with the backbone and charged side chain groups at the protein surface in solution. At the same time, we demonstrate the limitations of separating Hofmeister effects into independent cationic and anionic contributions due to the electroneutrality condition, as well as specific ion pairing, leading to interactions of ions of opposite polarity. Finally, we outline the route beyond Hofmeister chemistry in the direction of understanding specific roles of ions in various biological functionalities, where generic Hofmeister-type interactions can be complemented or even overruled by particular steric arrangements in various ion binding sites.

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Water autoionization reaction 2H2O 3 H3O ؊ ؉ OH ؊ is a textbook process of basic importance, resulting in pH ‫؍‬ 7 for pure water. However, pH of pure water surface is shown to be significantly lower, the reduction being caused by proton stabilization at the surface. The evidence presented here includes ab initio and classical molecular dynamics simulations of water slabs with solvated H3O ؉ and OH ؊ ions, density functional studies of (H2O)48H ؉ clusters, and spectroscopic isotopic-exchange data for D2O substitutional impurities at the surface and in the interior of ice nanocrystals. Because H3O ؉ does, but OH ؊ does not, display preference for surface sites, the H2O surface is predicted to be acidic with pH < 4.8. For similar reasons, the strength of some weak acids, such as carbonic acid, is expected to increase at the surface. Enhanced surface acidity can have a significant impact on aqueous surface chemistry, e.g., in the atmosphere.density functional theory ͉ IR spectroscopy ͉ molecular dynamics ͉ water autoionization ͉ ice nanocrystals I n room-temperature liquid, one in 6 ϫ 10 8 water molecules is autoionized, yielding the standard value of pH ϭ 7. Autoionization in crystal ice should be less favorable, because, in contrast to water, ice is a very poor solvent of ionic and polar substances (1). As recently realized (2-5), the chemistry and composition of aqueous surfaces are quite distinct from that of the bulk; therefore, autoionization behavior should be reexamined at the surface.A number of recent computations (6-8) indicated the preference of hydronium cations for surface positions. Surface propensity of H 3 O ϩ was also deduced from vibrational spectroscopy of large protonated water clusters (6), as well as vibrational sum frequency generation (8, 9) and second harmonic generation (10) spectroscopic experiments probing extended aqueous interfaces. Interestingly, zeta potential measurements and titration experiments on oil droplets dispersed in water indicated the presence of negative charges at the interface, interpreted as adsorbed OH Ϫ ions (11). Similar conclusions have also been drawn from zeta potentials of air bubbles in water (12). More work is clearly needed to reconcile this apparent discord between predictions of surface-selective spectroscopies and molecular simulations on one side and electrochemical measurements on the other side.H 3 O ϩ forms three strong proton-donor bonds to H 2 O, but acts as a poor proton acceptor. A surface position with only H atoms hydrogen-bonded is preferred to interior positions, because the latter are associated with disruption of the approximately tetrahedral hydrogen-bond network in water (10). The present work focuses on the effect of surface stabilization of hydronium on water autoionization and surface pH. CalculationsOverview. Modeling of proton-tranfer systems is a nontrivial problem, because standard (empirical) potential energy surfaces do not include a possibility of proton hopping between different water molecules or transitions between the two li...