Spectroscopic Characterization of Salt Dissolution in Microhydrated Cluster Ions and at the Water / Vapor Interface

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Our understanding of salt dissolution in aqueous media and at their interfaces remains far from complete. Processes occurring at marine aerosol surfaces, for example, have a major impact on atmospheric chemistry, but their surface speciation is challenging to probe experimentally and to predict reliably using computational models. Spectroscopic studies on model systems provide crucial molecular level insight into specific properties of the water/vapor interface. Here, we propose to combine two powerful and highly complementary spectroscopic techniques to study how atmospherically-relevant ions, like Cl-, (NO3)-, (SO4)2-, Na+, Mg2+ and (NH4)+, are hydrated at aqueous surfaces in dependence of the salt concentration, the nature of the counter ion, the acidity and the presence of organic surfactants. Concentration depth profiles at the water/vapor interface of binary and ternary aqueous salt solutions will be determined using Neutral Impact Collision Ion Scattering Spectroscopy from which interfacial thermodynamic properties can be determined. Cryogenic Ion Trap Vibrational Spectroscopy will be used to study the structure and stability of smaller, size-selected microhydrated ions, ion pairs and larger ion complexes. The synergy of these approaches allows improving surface models, because gas phase clusters are amenable to higher level calculations, and critically evaluating the generality of the present cluster/interface approach. Our experiments are ultimately aimed at shedding new light on the molecular level structure of the water/vapor interface and its implications for atmospherically-relevant processes.