Dielectric properties and cooperative dynamics of protic and aprotic room-temperature ionic liquids

by Thomas Sonnleitner

Institution: Universität Regensburg
Department: Chemie und Pharmazie
Degree: PhD
Year: 2014
Record ID: 1113270
Full text PDF: http://epub.uni-regensburg.de/29499


For more than two decades ionic liquids (ILs) - defined as a sub-class of molten salts with a melting point below 100°C - have been attracting increasing attention in scientific research and industrial applications due to their unique combinations of properties. In the present thesis dielectric properties and intermolecular dynamics of representative neat protic ILs (PILs) and non-imidazolium aprotic ILs (AILs) are investigated by means of dielectric relaxation spectroscopy (DRS) and, for most of the samples, by optical Kerr-effect spectroscopy (OKE) over the extraordinary wide frequency range 200MHz ≤ v ≤ 10THz between 5°C and 65°C. Particular focus is on the low-frequency part (v ≤ 100GHz), where relaxation processes dominate both DR and OKE spectra. All investigated neat PILs and AILs have in common that the relaxation of anions and cations occurs through large-angle jumps rather than rotational diffusion as a result of the strong inter-ionic interactions. In contrast to PILs, however, the major contribution to the OKE intensity at MHz frequencies of all studied AILs arises from fluctuations of mesoscale aggregates. In the far-infrared region the intensity of DR and OKE spectra is associated with intermolecular vibrations and librations. Although their contributions typically overlap and are strongly coupled, assignment of individual modes to specific molecular processes was possible for two of the studied PILs. Furthermore, a binary mixture of ethylammonium nitrate (EAN) and acetonitrile (AN) is investigated by means of DRS in a limited frequency range (0.2 ≤ v/GHz ≤ 89). This study reveals that in IL-rich mixtures the dynamics are dominated by those of the IL, whereas at low IL concentrations typical behavior of conventional electrolyte solutions is observed including pronounced formation of contact ion-pairs. The extensive ion association of EAN in AN is explained by strong hydrogen-bond interactions in addition to coulomb forces. Das Interesse an ionischen Flüssigkeiten (ionic liquids, ILs), welche als Salze mit einen Schmelzpunkt unter 100°C definiert sind, ist innerhalb der letzten zwei Jahrzehnte sowohl in der Forschung als auch hinsichtlich industrieller Anwendungen stetig gewachsen. In der vorliegenden Dissertation werden die dielektrischen Eigenschaften und die intermolekulare Dynamik von typischen, reinen protischen (PILs) und aprotischen ILs (AILs) mittels dielektrischer Relaxationsspektroskopie (DRS) über einen sehr weiten Frequenzbereich (200MHz ≤ v ≤ 10THz) zwischen 5°C und 65°C untersucht. Als komplementäre Technik zur DRS wird für die Mehrzahl der untersuchten Proben zusätzlich die optische Kerr-Effekt Spektroskopie (OKE) verwendet. In beiden Fällen liegt der besondere Fokus auf dem Niederfrequenzbereich (v ≤ 100GHz) der entsprechenden Spektren, welcher in erster Linie durch Relaxationsprozesse charakterisiert ist. Dabei haben alle untersuchten PILs und AILs gemein, dass aufgrund der starken interionischen Wechselwirkungen die Anionen- und Kationen-Relaxation nicht durch…