14 Externe wissenschaftliche Einrichtungen
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Item Open Access Density profiles of ionic liquids at a hard wall(2008) Schramm, Sebastian M.In this work a high energy x-ray reflectivity study of deeply buried interfaces between room temperature ionic liquids (RTILs) and a sapphire hard wall is reported. For the first time the interfacial structure was obtained with molecular resolution. The experiments have been carried out at beamline ID15A (ESRF, Grenoble) using the HEMD (High Energy Micro Diffraction) instrument. The thorough analysis of the experimental reflectivities gives clear evidence of a pronounced molecular layering at the RTIL-solid interface. The periodicity of the molecular layering corresponds to correlation distances in the bulk liquid RTILs. The values of the surface tension seem to be unrelated to the interfacial structure. RTILs are molten salts consisting solely of ions with a melting point below 100 °C. Most RTILs are composed of relatively large (polyatomic) organic cations and inorganic anions. In the last few years the interest in them experienced an enormous growth. Their unique and useful properties like non-volatility, low melting point, and a wide electrochemical window render them suitable for a wide range of applications, i.e. as green solvents, or as electrolytes in a variety of electrochemical processes. In most of the applications of RTILs the RTIL-solid interface plays a crucial role. Four different RTILs were studied. Two of them share the same cation, 1-butyl-3-methylimidazolium, with the most widely used and extensively studied anions, tetrafluoroborate and hexafluorophosphate. The other two RTILs fall into a more recent class of RTILs with higher electrochemical stability. They share the same anion, bis(trifluoromethylsulfonyl)imide, and a pyrrolidinium-based and an imidazolium-based cation, respectively. The systematic exchange of the ion types within these four RTILs revealed a distinct impact of the ion pair on the interfacial behavior. Sapphire wafers with a (0001) surface served as a model system for a hard wall. Further information on interactions within the probed systems was gained by measurements of the interfacial tensions with air and n-hexane for all four RTILs at ambient condition. The surface tension of the RTILs is higher than the one for n-hexane but still smaller than the one for water. The measurements revealed that small changes in the nature of the ions have only a minor impact on the value of the surface and interfacial tension. In order to correlate the results for the interfacial structures with the bulk properties of the RTILs, bulk liquid x-ray scattering experiments were carried out at all four RTILs. These measurements revealed the presence of significant spatial correlations.