Interfaces of Ionic Liquid Systems

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Ionic liquids (ILs) are molten salts with a melting point below 100°C. Their structural diversity results into nearly unlimited possibilities to combine cations and anions with different properties. This allows tailoring their physico-chemical properties over a wide range and adapting them to specific applications. After an initial focus on the bulk properties of ILs, the importance of the interface of an IL with its environment has also been recognized for a large number of application areas, in particular also for new concepts in catalysis. Due to their low vapour pressure, ILs can be investigated by ultrahigh vacuum-based methods and the area of Ionic Liquid Surface Science has started to establish itself within the last years. The applicants group has significantly contributed to this development, in particular using angle-resolved X-ray photoelectron spectroscopy. However, while the presently available studies contain information on the specific properties of particular systems, a general picture of the underlying mechanisms and driving forces is still missing. This lack of fundamental understanding of processes and mechanisms governing the properties of IL/gas (vacuum) and IL/solid interfaces, and of processes in IL multiphase systems define the stage for this project: Four specific topics will be investigated: 1) Temperature-dependence of surface composition, orientation and enrichment effects: addresses molecular orientation at IL surfaces, surface composition of binary IL mixtures, and the influence of bulk liquid-liquid phase transitions on surface orientation. The temperature dependences of these effects are of pivotal importance for many applications, but up to now have been mostly ignored in surface-related studies. 2) Dissolution of metal complexes: aims at developing general concepts to overcome the low solubility of many metal complexes in ILs and to control surface activity of these complexes. We will apply complex ligands with IL-type moieties and study metal-complex-containing ILs. 3) Ionic liquid/support interactions: concern the interfaces of ILs with solid supports or with the surface of a second IL, making use of ultra-thin IL films deposited by established and new in situ techniques from the gas phase. 4) Monitoring of chemical reactions in ionic liquid-based systems: exploits the potential of X-ray photoelectron spectroscopy for in situ monitoring. We will address reactions between ILs, the metalation of free base porphyrins in ILs, and adsorption and reaction of small gaseous molecules in ILs and at dissolved metal complexes.