Chemical Speciation in Mixtures of Ionic Liquids and Polar Compounds

Ionic liquids (ILs) are regarded as an exciting class of versatile materials, because they combine a number of outstanding properties, like high thermal and electrochemical stabilities, low volatility and flammability, as well as the option of tuning various physical and chemical properties by a straightforward change of cations or anions, which has led to the term ‘designer solvents’. In addition, the available property range of ILs can be enlarged even further by using binary IL + IL mixtures as this allows continuous and simultaneous adjustment of several key properties, such as viscosity, conductivity and polarity. Nevertheless, almost nothing is known about the structure and dynamics of these mixtures. Consequently, a model system consisting of two imidazolium-based ILs was studied in the thesis using dielectric spectroscopy and other techniques. The results were interpreted in terms of their excess quantities. For technological purposes ILs will rarely be employed in neat form: almost invariably they will be diluted either by reactants and products or by the presence of a co-solvent, deliberately added to optimize the physical and chemical characteristics of the IL. It is essential to be aware of the effects of added compounds, either impurities or co-solvents, on the physico-chemical properties of ILs. The thesis presents reliable values for the densities, conductivities, molar conductivities, and excess molar volumes of various imidazolium-based IL + polar solvent (acetonitrile or methanol) mixtures, which have established that much of the available literature data are of doubtful quality. Of major importance, the purity of ILs is a crucial topic. The main part of the thesis deals with the analysis of the dielectric spectra, yielding information on ion-ion and ion-solvent interactions and the transition from molten-salt like to electrolyte-solution like behavior in IL + polar solvent mixtures. The ILs retain their molten-salt like character, at least in terms of dielectric properties, up to high dilution with acetonitrile or methanol. At even higher dilution, the ILs behave as conventional electrolytes with moderate ion-association and solvated cations.