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Due to their unique properties, ionic liquids are promising candidates for various applications. The possibility to combine cations and anions almost arbitrarily leads to a virtually infinite number of different ionic liquids. The hope is that one could tailor an ionic liquid for a specific need in this way. However, to achieve a particular property, it would be essential to understand the ionic liquids on a microscopy level and not rely on trial and error. One crucial point is the dynamics of the ions, for example, concerning reaction media or electrolytes. However, the ion dynamics are quite complex due to the balanced interplay between Coulomb, hydrogen-bonding, and van der Waals interactions. Furthermore, in cases where cations are equipped with long, non-polar chains, it was found that nanosized aggregates form in neat ionic liquids. In this way, rotational and translational motions of cations and anions together with motions of aggregates possibly show up in the spectra of dynamical measurements. This has led to dynamic processes in the spectra being interpreted differently in the literature with respect to their microscopic origin. Therefore, this work aims to combine dielectric spectroscopy and depolarized light scattering to disentangle the rotational and translational contributions found in the relaxation spectra of ionic liquids. This is done for various neat ionic liquids, where the cations are equipped with non-polar chains of different lengths, thus varying the size ratio between cations and anions. Measurements are performed from far above room temperature down to the glass transition. In the case of dielectric spectroscopy, also pressures up to 550 MPa are applied. In this way, it could be shown that the dynamics of cations and anions separate in the case of a large size difference between the two ion species. Rotational motions of the cations are revealed to be the origin of a slow dielectric relaxation process, which was formerly often ascribed to motions of aggregates. It could be shown that such aggregates show up only in rare cases in the light scattering spectra at low frequencies, not accessible by dielectric spectroscopy. Furthermore, mixtures of an ionic liquid with water or 1-propanol are considered. The rotational contribution of the admixtures is discriminated from the ion dynamics and from signatures of hydrogen-bonding mediated orientational cross-correlations. Additionally, an ionic gel is prepared by mixing an ionic liquid with water and gelatin, and it is shown that the rotational and translational dynamics of the ions are hardly affected by the presence of the gelatin, although macroscopically, mechanical rigidity is introduced. More fundamental questions regarding the intensity of the scattered light and the shape of the rotational spectra, which have arisen during this work, are also addressed based on non-ionic systems.

An exact integral formula for the Kirkwood correlation factor of isotropic polar fluids $g_K$ is derived from the equilibrium averaged rotational Dean equation, which as compared to previous approaches easily lends itself to further approximations. The static linear permittivity of polar fluids $\epsilon$ is calculated as a function of temperature, density and molecular dipole moment in vacuo for arbitrary pair interaction potentials. Then, using the Kirkwood superposition approximation for the three-body orientational distribution function, we suggest a simple way to construct model potentials of mean torques considering permanent and induced dipole moments. We successfully compare the theory with the experimental temperature dependence of the static linear permittivity of various polar fluids such as a series of linear monohydroxy alcohols, water, tributyl phosphate, acetonitrile, acetone, nitrobenzene and dimethyl sulfoxide, by fitting only one single parameter, which describes the induction to dipole-dipole energy strength ratio. We demonstrate that comparing the value of $g_K$ with unity in order to deduce the alignment state of permanent dipole pairs, as is currently done is in many situations, is a misleading oversimplification, while the correct alignment state is revealed when considering the proper interaction potential. Moreover we show, that picturing H-bonding polar fluids as polar molecules with permanent and induced dipole moments without invoking any specific H-bonding mechanism is in many cases sufficient to explain experimental data of the static dielectric constant. In this light, the failure of the theory to describe the experimental temperature dependence of the static dielectric constant of glycerol, a non-rigid polyalcohol, is not due to the lack of specific H-bonding mechanisms, but rather to an oversimplified model potential for that particular molecule.

One of the hallmarks of molecular dynamics in deeply supercooled liquids is the non-exponential character of the relaxation functions. It has been a long standing issue if 'universal' features govern the lineshape of glassy dynamics independent of any particular molecular structure or interaction. In the paper, we elucidate this matter by giving a comprehensive comparison of the spectral shape of depolarized light scattering and dielectric data of deeply supercooled liquids. The light scattering spectra of very different systems, e.g. hydrogen bonding and van der Waals liquids but also ionic systems, almost perfectly superimpose and show a generic lineshape of the structural relaxation, approximately following a high frequency power law $\omega^{-1/2}$ . However, the dielectric loss peak shows a more individual shape. In systems with low dipole moment generic behavior is also observed in the dielectric spectra, while in strongly dipolar liquids additional crosscorrelation contributions mask the generic structural relaxation.

The dielectric Debye relaxation in monohydroxy alcohols has been subject of long-standing scientific interest and is presently believed to arise from the relaxation of transiently H-bonded supramolecular structures. Therefore, its manifestation might be expected to differ from a local dielectric probe as compared to the standard macroscopic dielectric experiment. In this work we present such local dielectric measurements obtained by triplet state solvation dynamics (TSD) and compare the results with macroscopic dielectric and light scattering data. In particular, with data from an improved TSD setup, a detailed quantitative comparison reveals that the Debye process does not significantly contribute to the local Stokes shift response function, while $��$- and $��$-relaxations are clearly resolved. Furthermore, this comparison reveals that the structural relaxation has almost identical time constants and shape parameters in all three measurement techniques. Altogether our findings support the notion that the transiently bound chain structures lead to a strong cross-correlation contribution in macroscopic dielectric experiments, to which both light scattering and TSD are insensitive, the latter due to its local character and the former due to the molecular optical anisotropy being largely independent of the OH bonded suprastructures., 8 pages, 9 figures