Halide Affinity for the Water−Air Interface in Aqueous Solutions of Mixtures of Sodium Salts

Spontaneous Appearance of Triiodide Covering the Topmost Layer of the Iodide Solution Interface Without Photo-Oxidation

Ions containing iodine atoms at the vapor-aqueous solution interfaces critically affect aerosol growth and atmospheric chemistry due to their complex chemical nature and multivalency. While the surface propensity of iodide ions has been intensely discussed in the context of the Hofmeister series, the stability of various ions containing iodine atoms at the vapor-water interface has been debated. Here, we combine surface-specific sum-frequency generation (SFG) vibrational spectroscopy with ab initio molecular dynamics simulations to examine the extent to which iodide ions cover the aqueous surface. The SFG probe of the free O-D stretch mode of heavy water indicates that the free O-D group density decreases drastically at the interface when the bulk NaI concentration exceeds ∼2 M. The decrease in the free O-D group density is attributed to the spontaneous appearance of triiodide that covers the topmost interface rather than to the surface adsorption of iodide. This finding demonstrates that iodide is not surface-active, yet the highly surface-active triiodide is generated spontaneously at the water-air interface, even under dark and oxygen-free conditions. Our study provides an important first step toward clarifying iodine chemistry and pathways for aerosol formation.

Surface tension between liquids containing antagonistic and regular salts

We investigate theoretically the surface tension between two immiscible liquids containing antagonistic and regular salts. The mean-field model includes the electrostatic energy and the Gibbs transfer energy of the ions. We find the Donnan potential difference between the liquids and solve the Poisson-Boltzmann equation to calculate the potential and ion density profiles. When the liquids contain only a regular salt, the surface tension increases when compared to the no-salt case; in contrast, the surface tension is reduced when they contain only an antagonistic salt. When both salts are present, the surface tension can either further decrease or increase depending on the preferential solvation of the ions, in agreement with published experimental observations.

Proton Traffic Jam: Effect of Nanoconfinement and Acid Concentration on Proton Hopping Mechanism

The properties of the water network in concentrated HCl acid pools in nanometer-sized reverse non-ionic micelles were probed with TeraHertz absorption, dielectric relaxation spectroscopy, and reactive force field simulations capable of describing proton hopping mechanisms. We identify that only at a critical micelle size of W0=9 do solvated proton complexes form in the water pool, accompanied by a change in mechanism from Grotthuss forward shuttling to one that favors local oscillatory hopping. This is due to a preference for H+ and Cl- ions to adsorb to the micelle interface, together with an acid concentration effect that causes a "traffic jam" in which the short-circuiting of the hydrogen-bonding motif of the hydronium ion decreases the forward hopping rate throughout the water interior even as the micelle size increases. These findings have implications for atmospheric chemistry, biochemical and biophysical environments, and energy materials, as transport of protons vital to these processes can be suppressed due to confinement, aggregation, and/or concentration.

Insight into the Ionizing Surface Potential Method and Aqueous Sodium Halide Surfaces

Complementing the microscopic picture of the surface structure of electrolyte solutions set out by previous theoretical and experimental studies, the ionizing surface potential technique offers a unique approach to quantifying the impact of aqueous inorganic ions upon the interfacial electric field of the air-aqueous interface. In this Feature Article, we review the vulnerability of theoretical and empirically derived χ_water values as a normative reference for aqueous ion surface potentials. Instead, we recognize and evaluate aqueous ion surface potentials relative to well-known ionic surfactants cetyltrimethylammonium bromide (CTAB) and sodium dodecyl sulfate (SDS). Additionally, we also explore factors that impact the magnitude of the measured surface potentials using the ionizing method, particularly in the type of reference electrode and ionizing gas environment. With potential measurements of sodium halide solutions, we show that iodide has a dominant effect on the air-aqueous electric field. Compared to chloride and bromide, iodide is directly observed with a net negatively charged surface electric field at all salt concentrations measured (0.2 to 3.0 mol/kg water). Also, above the 2 M region, bromide is observed with a net negatively charged surface. Although several scenarios contribute to this effect, it is most likely due to the surface enrichment of bromide and iodide. While the results of this study are pertinent to determining the specific interfacial reactivity of aqueous halides, these anions seldom transpire as single-halide systems in the natural environment. Therefore, we also provide an outlook on future research concerning surface potential methods and more complex aqueous electrolyte systems.

Reactive Uptake of Ozone to Simulated Seawater: Evidence for Iodide Depletion

The reaction of ozone with iodide in the ocean is a major ozone dry deposition pathway, as well as an important source of reactive iodine to the marine troposphere. Few prior laboratory experiments have been conducted with environmentally relevant ozone mixing ratios and iodide concentrations, leading to uncertainties in the rate of the reaction under marine boundary layer conditions. As well, there remains disagreement in the literature assessment of the relative contributions of an interfacial reaction via ozone adsorbed to the ocean surface versus a bulk reaction with dissolved ozone. In this study, we measure the uptake coefficient of ozone over a buffered, pH 8 salt solution replicating the concentrations of iodide, bromide, and chloride in the ocean over an ozone mixing ratio of 60-500 ppb. Due to iodide depletion in the solution, the measured ozone uptake coefficient is dependent on the exposure time of the solution to ozone and its mixing ratio. A kinetic multilayer model confirms that iodide depletion is occurring not only within ozone's reactodiffusive depth, which is on the order of microns for environmental conditions, but also deeper into the solution as well. Best model-measurement agreement arises when some degree of nondiffusive mixing is occurring in the solution, transporting iodide from deeper in the solution to a thin, diffusively mixed upper layer. If such mixing occurs rapidly in the environment, iodide depletion is unlikely to reduce ozone dry deposition rates. Unrealistically high bulk-to-interface partitioning of iodide is required for the model to predict a substantial interfacial component to the reaction, indicating that the Langmuir-Hinshelwood mechanism is not dominant under environmental conditions.

Influence of the Sea Surface Microlayer on Oceanic Iodine Emissions

The influence of organic compounds on iodine (I₂) emissions from the O₃ + I^- reaction at the sea surface was investigated in laboratory and modeling studies using artificial solutions, natural subsurface seawater (SSW), and, for the first time, samples of the surface microlayer (SML). Gas-phase I₂ was measured directly above the surface of liquid samples using broadband cavity enhanced absorption spectroscopy. I₂ emissions were consistently lower for artificial seawater (AS) than buffered potassium iodide (KI) solutions. Natural seawater samples showed the strongest reduction of I₂ emissions compared to artificial solutions with equivalent [I^-], and the reduction was more pronounced over SML than SSW. Emissions of volatile organic iodine (VOI) were highest from SML samples but remained a negligible fraction (<1%) of the total iodine flux. Therefore, reduced iodine emissions from natural seawater cannot be explained by chemical losses of I₂ or hypoiodous acid (HOI), leading to VOI. An interfacial model explains this reduction by increased solubility of the I₂ product in the organic-rich interfacial layer of seawater. Our results highlight the importance of using environmentally representative concentrations in studies of the O₃ + I^- reaction and demonstrate the influence the SML exerts on emissions of iodine and potentially other volatile species.

Surface Propensity of Aqueous Atmospheric Bromine at the Liquid-Gas Interface

Multiphase reactions of halide ions in aqueous solutions exposed to the atmosphere initiate the formation of molecular halogen compounds in the gas phase. Their photolysis leads to halogen atoms, which are catalytic sinks for ozone, making these processes relevant for the regional and global tropospheric ozone budget. The affinity of halide ions in aqueous solution for the liquid-gas interface, which may influence their reactivity with gaseous species, has been debated. Our study focuses on the surface properties of the bromide ion and its oxidation products. In situ X-ray photoelectron spectroscopy carried out on a liquid jet combined with classical and first-principles molecular dynamics calculations was used to investigate the interfacial depth profile of bromide, hypobromite, hypobromous acid, and bromate. The simulated core electron binding energies support the experimentally observed values, which follow a correlation with bromine oxidation state for the anion series. Bromide ions are homogeneously distributed in the solution. Hypobromous acid, a key species in the multiphase cycling of bromine, is the only species showing surface propensity, which suggests a more important role of the interface in multiphase bromine chemistry than thought so far.

A kinetic model for ozone uptake by solutions and aqueous particles containing I- and Br-, including seawater and sea-salt aerosol

The heterogeneous interactions of gaseous ozone (O3) with seawater and with sea-salt aerosols are known to generate volatile halogen species, which, in turn, lead to further destruction of O3. Here, a kinetic model for the interaction of ozone (O3) with Br- and I- solutions and aqueous particles has been proposed that satisfactorily explains previous literature studies about this process. Apart from the aqueous-phase reactions X- + O3 (X = I, Br), the interaction also involves the surface reactions X- + O3 that occur via O3 adsorption on the aqueous surface. In single salt solutions and aerosols, the partial order in ozone and the total order of the surface reactions are one, but the apparent total order is second order because the number of ozone sites where reaction can occur is equal to the surficial concentration of X- ([X-]surf). In the presence of Cl-, the surface reactions are enhanced by a factor equal to , where and . Therefore, we have inferred that Cl- acts as a catalyst in the surface reactions X- + O3. The model has been applied to estimate ozone uptake by the reaction with these halides in/on seawater and in/on sea-salt aerosol, where it has been concluded that the Cl--catalyzed surface reaction is important relative to total ozone uptake and should therefore be considered to model Y/YO (Y = I, Br, Cl) levels in the troposphere.

The opposing effect of butanol and butyric acid on the abundance of bromide and iodide at the aqueous solution-air interface

The efficient oxidation of iodide and bromide at the aqueous solution-air interface of the ocean or of sea spray aerosol particles had been suggested to be related to their surface propensity. The ubiquitous presence of organic material at the ocean surface calls for an assessment of the impact of often surface-active organic compounds on the interfacial density of halide ions. We used in situ X-ray photoelectron spectroscopy with a liquid micro-jet to obtain chemical composition information at aqueous solution-vapor interfaces from mixed aqueous solutions containing bromide or iodide and 1-butanol or butyric acid as organic surfactants. Core level spectra of Br 3d, Na 2s, C 1s and O 1s at ca. 160 eV kinetic energy and core level spectra of I 4d and O 1s at ca. 400 eV kinetic energy are compared for solutions with 1-butanol and butyric acid as a function of organic concentration. A simple model was developed to account for the attenuation of photoelectrons by the aliphatic carbon layer of the surfactants and for changing local density of bromide and iodide in response to the presence of the surfactants. We observed that 1-butanol increases the interfacial density of bromide by 25%, while butyric acid reduces it by 40%, both in comparison to the pure aqueous halide solution. Qualitatively similar behavior was observed for the case of iodide. Classical molecular dynamics simulations failed to reproduce the details of the response of the halide ions to the presence of the two organics. This is attributed to the lack of correct monovalent ion parameters at low concentration possibly leading to an overestimation of the halide ion concentration at the interface in absence of organics. The results clearly demonstrate that organic surfactants change the electrostatic interactions near the interface with headgroup specific effects. This has implications for halogen activation processes specifically when oxidants interact with halide ions at the aqueous solution-air interfaces of the ocean surface or sea spray aerosol particles.

Water at surfaces with tunable surface chemistries

Aqueous interfaces are ubiquitous in natural environments, spanning atmospheric, geological, oceanographic, and biological systems, as well as in technical applications, such as fuel cells and membrane filtration. Where liquid water terminates at a surface, an interfacial region is formed, which exhibits distinct properties from the bulk aqueous phase. The unique properties of water are governed by the hydrogen-bonded network. The chemical and physical properties of the surface dictate the boundary conditions of the bulk hydrogen-bonded network and thus the interfacial properties of the water and any molecules in that region. Understanding the properties of interfacial water requires systematically characterizing the structure and dynamics of interfacial water as a function of the surface chemistry. In this review, we focus on the use of experimental surface-specific spectroscopic methods to understand the properties of interfacial water as a function of surface chemistry. Investigations of the air-water interface, as well as efforts in tuning the properties of the air-water interface by adding solutes or surfactants, are briefly discussed. Buried aqueous interfaces can be accessed with careful selection of spectroscopic technique and sample configuration, further expanding the range of chemical environments that can be probed, including solid inorganic materials, polymers, and water immiscible liquids. Solid substrates can be finely tuned by functionalization with self-assembled monolayers, polymers, or biomolecules. These variables provide a platform for systematically tuning the chemical nature of the interface and examining the resulting water structure. Finally, time-resolved methods to probe the dynamics of interfacial water are briefly summarized before discussing the current status and future directions in studying the structure and dynamics of interfacial water.

The distribution of iodide at the sea surface

Recent studies have highlighted the impact of sea surface iodide concentrations on the deposition of ozone to the sea surface and the sea to air flux of reactive iodine. The use of models to predict this flux demands accurate, spatially distributed sea surface iodide concentrations, but to date, the observational data required to support this is sparse and mostly arises from independent studies conducted on small geographical and temporal scales. We have compiled the available measurements of sea surface iodide to produce a data set spanning latitudes from 69°S to 66°N, which reveals a coherent, large scale distribution pattern, with highest concentrations observed in tropical waters. Relationships between iodide concentration and more readily available parameters (chlorophyll, nitrate, sea surface temperature, salinity, mixed layer depth) are evaluated as tools to predict iodide concentration. Of the variables tested, sea surface temperature is the strongest predictor of iodide concentration. Nitrate was also strongly inversely associated with iodide concentration, but chlorophyll-a was not.