Insight into ion behavior at mixed organic/aqueous liquid surfaces is crucial for understanding the chemistry of atmospheric aerosols, which frequently contain mixtures of water, electrolytes, and organics. The addition of 1-butanol to an aqueous potassium iodide solution modifies the interfacial profile of ions at the liquid−vapor interface. Our experiments probe atomic composition at the liquid surface with ambient pressure X-ray photoelectron spectroscopy. Photoelectron kinetic energies are varied to produce a depth profile of the liquid−vapor interface. Molecular dynamics simulations of butanol in an aqueous electrolyte solution are used to develop a detailed understanding of the ion−solvent interactions in the interfacial region. Our previous work on pure aqueous salt solutions observed substantial ion concentrations at the liquid−vapor interface and an increased anion/cation ratio at the interface. A question has arisen as to whether covering the surface with an organic monolayer might change or suppress the interfacial ion concentrations. We observe that the direct interaction of both the cation and the anion with the butanol leads to changes in the ion concentrations in the region of the liquid interface. Substantial ion concentrations are still observed in the interfacial region in the presence of butanol. However, we do find that the presence of the butanol reduces the previously observed anion/cation separation in the interfacial region.

X-Ray photoemission spectroscopy operating under ambient pressure conditions is used to probe ion distributions throughout the interfacial region of a free-flowing aqueous liquid micro-jet of 6 M potassium fluoride. Varying the energy of the ejected photoelectrons by carrying out experiments as a function of X-ray wavelength measures the composition of the aqueous-vapor interfacial region at various depths. The F(-) to K(+) atomic ratio is equal to unity throughout the interfacial region to a depth of 2 nm. The experimental ion profiles are compared with the results of a classical molecular dynamics simulation of a 6 M aqueous KF solution employing polarizable potentials. The experimental results are in qualitative agreement with the simulations when integrated over an exponentially decaying probe depth characteristic of an APPES experiment. First principles molecular dynamics simulations have been used to calculate the potential of mean force for moving a fluoride anion across the air-water interface. The results show that the fluoride anion is repelled from the interface, consistent with the depletion of F(-) at the interface revealed by the APPES experiment and polarizable force field-based molecular dynamics simulation. Together, the APPES and MD simulation data provide a detailed description of the aqueous-vapor interface of alkali fluoride systems. This work offers the first direct observation of the ion distribution at an aqueous potassium fluoride solution interface. The current experimental results are compared to those previously obtained for saturated solutions of KBr and KI to underscore the strong difference in surface propensity between soft/large and hard/small halide ions in aqueous solution.

The chemistry of Br species associated with sea salt ice and aerosols has been implicated in the episodes of ozone depletion reported at Arctic sunrise. However, Br -is only a minor component in sea salt, which has a Br -/Cl -molar ratio of ~0.0015. Sea salt is a complex mixture of many different species, with NaCl as the primary component. In recent years experimental and theoretical studies have reported enhancement of the large, more polarizable halide ion at the liquid/vapor interface of corresponding aqueous alkali halide solutions. The proposed enhancement is likely to influence the availability of sea salt Br -for heterogeneous reactions such as those involved in the ozone depletion episodes. We report here ambient pressure x-ray photoelectron spectroscopy studies and molecular dynamics simulations showing direct evidence of Br -enhancement at the interface of an aqueous NaCl solution doped with bromide. The experiments were carried out on samples with Br -/Cl -ratios in the range 0.1% to 10%, the latter being also the ratio for which simulations were carried out. This is the first direct measurement of interfacial enhancement of Br -in a multi-component solution with particular relevance to sea salt chemistry.