Charcterization of the nature of interactions in ionic liquid co-solvent mixtures by dynamic light scattering (DLS)
Final Report Abstract
The aim of the research project was to contribute to a more fundamental understanding of the nature of molecular interactions present in ionic liquid (IL) co-solvent mixtures. For this, conventional methods for the determination of thermophysical properties as well as different light scattering techniques were used. Both dynamic light scattering (DLS) from the bulk of fluids and the application of DLS to fluid surfaces, also called surface light scattering (SLS), were applied not only to access transport properties, i.e., mutual diffusion and viscosity as well as interfacial tension with reasonable accuracy, but also to gain a close insight into the bulk structure and the fluid structure at the phase boundary of IL co-solvent mixtures. It could be shown that the nature of the co-solvent distinctly affects the mixture viscosity. Here, internal hydrogen bonding between co-solvent molecules such as water as well as varying strength of van-der-Waals interactions and entanglement effects due to different alkyl chain lengths of the ions seem to be dominating. For the behavior of mutual diffusivities, similar influences of the ion properties could be found. Comparing different co-solvents, smaller mutual diffusivities were measured for mixtures with ethanol than for those with acetone, for example. This observation could be related to the formation of hydrogen bonds between ethanol and the ions, retarding the diffusion process. The viscosity of the pure ILs proved to be no reliable indicator for the mutual diffusivity in the mixtures. For mixtures with water, very weak light scattering intensities in the DLS experiments hinted at the presence of water clusters in these systems. This seems to be confirmed by the correlation length calculated from viscosity and binary diffusion coefficients. Here, the smallest values were found for mixtures with water. Furthermore, the mixtures with water showed less negative excess molar volume values than those with, e.g., acetone or acetonitrile although water is the smallest of all considered co-solvent molecules. In this case, water cluster formation obviously prevents efficient occupation of interstitials in the IL structure. In contrast, the data for the excess molar volume for mixtures with acetone show that this rather small and angulated aprotic molecule can easily occupy interstitials. It could also be shown with the help of interfacial tension data that virtually all co-solvents are more or less surface-active agents in combination with ILs and seem not to be present at the liquid-gas interface up to a critical concentration. The diffusion of various co-solvents over the phase boundary into the gas phase at higher concentrations is indicated by SLS results, but the presence of co-solvent in the gas phase close to the phase boundary could not be investigated as planned by confocal Raman scattering or fluorescence spectroscopy due to financial restrictions. The application of conventional Raman scattering to the liquid phase was severely distorted by fluorescence effects, while no SLS signals could be found for liquid-liquid phase boundaries in systems including ILs. In contrast, the accessibility of sound speed data for ILs by DLS from the bulk of fluids was successfully demonstrated.
Publications
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Characterization of ionic liquids and their mixtures with co-solvents by dynamic light scattering (DLS), Proc. 9th Asian Thermophysical Properties Conference (Beijing, China 2010)
Andreas P. Fröba, Alfred Leipertz
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Densities and excess molar volumes for binary mixtures of ionic liquid 1-ethyl-3-methylimidazolium ethylsulfate with cosolvents, Journal of Chemical & Engineering Data 55, 4068-4074 (2010)
Julia Lehmann, Michael H. Rausch, Alfred Leipertz, Andreas P. Fröba
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Density and surface tension of ionic liquids, Journal of Physical Chemistry B 114, 17025-17036 (2010)
Claudia Kolbeck, Julia Lehmann, Kevin R. J. Lovelock, Till Cremer, Nathalia Paape, Peter Wasserscheid, Andreas P. Fröba, Florian Maier, Hans-Peter Steinrück
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Thermal conductivity of ionic liquids – measurement and prediction, International Journal of Thermophysics 31, 2059-2077 (2010)
Andreas P. Fröba, Michael H. Rausch, Kamil Krzeminski, Daniel Assenbaum, Peter Wasserscheid, Alfred Leipertz
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Mutual diffusion in ionic liquid solvent mixtures by dynamic light scattering (DLS), Physical Chemistry Chemical Physics 13, 9525-9533 (2011)
Michael H. Rausch, Julia Lehmann, Alfred Leipertz, Andreas P. Fröba
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Prediction of the thermal conductivity of ionic liquids based on measurements with a guarded parallel plate instrument, Proc. 31st Thermal Conductivity Conference (Saguenay, Quebec, Canada 2011)
Michael H. Rausch, Kamil Krzeminski, Alfred Leipertz, Daniel Assenbaum, Peter Wasserscheid, Andreas P. Fröba
