The effect of an organic surfactant on the liquid-vapor interface of an electrolyte solution

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 I-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.