Partitioning of atmospherically relevant ions between bulk water and the water/vapor interface

Abstract

Recently, surface-sensitive spectroscopy data and molecular dynamics simulations have generated intense interest in the distribution of electrolyte ions between bulk water and the air͞water interface. A partitioning model for cations and anions developed for biopolymer surface is extended here to interpret the effects of selected acids, bases, and salts on the surface tension of water. Data for electrolytes were analyzed by using a lower-bound value for the number of water molecules in the surface region [0.2 H 2O Å ؊2 (approximately two layers of water)], obtained by assuming that both Na ؉ and SO 4 2؊ (i.e., Na2SO4) are fully excluded from this region. Surface-bulk partition coefficients of atmospherically relevant anions and the proton are determined. Notably, we find that H ؉ is most strongly surface-accumulated, I ؊ is modestly accumulated, NO 3 ؊ is evenly distributed, and OH ؊ is weakly excluded.surface tension ͉ surface-bulk partition coefficients ͉ inorganic salt ions T he distribution of ions between bulk water and water located at a surface has major implications for processes in biochemistry, atmospheric chemistry, and technology. As early as 1957, the observation of negative surface potentials for aqueous solutions of alkali metal salts of I Ϫ , ClO 4 Ϫ , SCN Ϫ , and PF 6 Ϫ led to the proposal that these anions (but not the cations) were present in the interfacial region of aqueous salt solutions (1, 2). However, the traditional proposal of an ion-free water layer on the surface of salt solutions, based on their positive surface tension increments (d␥͞dm 2 Ͼ 0), went relatively unchallenged until a decade ago, when Hu and coworkers observed that the kinetics of halogen uptake by NaBr and NaI solutions were too rapid to be explained by a simple bulk-phase reaction mechanism (3). The authors concluded that significant concentrations of bromide and iodide must be present in the surface layer, even at low bulk salt concentrations. Since then, considerable attention has been focused on this topic because of its implications for atmospheric chemistry (4, 5). Surface-sensitive spectroscopic experiments (6-9), molecular dynamics simulations (10-12), and various theoretical͞experimental collaborations (13-15) provide evidence, recently reviewed (16), that, although ''hard'' and multiply charged ions are excluded from the surface, large polarizable anions (e.g., SCN Ϫ and I Ϫ ) and the proton accumulate there, in some cases at concentrations greater than in the bulk solution. Only when both ions are hard and͞or multiply charged (e.g., NaF and Na 2 SO 4 ) do the simulations show both ions excluded from the surface, in agreement with the classical picture of a salt-free layer. In other cases, almost complete cationic exclusion and various degrees of anionic accumulation at the surface result in a range of net ion depletion at the surface (13); for these cases, a correspondingly wide range of positive surface tension increments have been reported (17). We apply a recently developed two-state, single-ion partitio...

Thermodynamic Origin of Hofmeister Ion Effects

“…As seen in Table 2, the trends observed for molecular hydrocarbon surface parallel those previously reported for ion partitioning at the air-water surface; 13,14 comparisons of ion distributions near these surfaces, obtained from molecular dynamics simulations, yield a similar conclusion. 36 Alkali metal cations are strongly excluded, GuH + is much less excluded than the other cations, and the rank order of partitioning of anions matches their placement in the traditional biopolymer Hofmeister series.…”

“…1A. Also tabulated in Table 1 are the corresponding surface tension increments 13,14 and the intrinsic (composite) thermodynamic quantities b 1 (K p,3 -1) characterizing the net accumulation or exclusion of these salts for both molecular hydrocarbon and air-water surfaces (Eq. 1).…”

“…complete exclusion of Na + and from the air-water surface) yielded a similar b 1 (0.19 H 2 O Å −2 ) for the number of water molecules per unit area at the air-water surface. 13,14 To obtain composite partition coefficients K p,3 characterizing electrolyte-model compound interactions, we assume that b 1 is a molecular property of the model compound hydration layer, independent of the concentration and nature of the electrolyte. Values of K p , 3 are then analyzed to obtain individual ion K p,i using Eq.…”

“…13,14 Partition coefficients K p of ions, defined as ratios of the local concentration of the ion in surface water to the bulk concentration, are found to be independent of salt concentration and of the nature of the companion ion. Alkali metal cations and sulfate and carbonate anions are found to be highly excluded from the air-water interface (local concentration much lower than bulk; K p << 1).…”

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

“…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. 98 Oxidants such as OH and O 3 are also predicted to be stable at the interface, [99][100][101] increasing the interface reactivity.…”