Systematic THz Dielectric
Spectroscopy of Ionic Liquids
Zur Erlangung des Doktortitels der Naturwissenschaften
der Fakultät für Chemie und Biochemie
der Ruhr‐Universität Bochum
Matthias Patrick Krüger
This dissertation covers the results of my PhD studies at the Faculty of Chemistry and
Biochemistry of the Ruhr‐University Bochum in the group of Prof. Dr. M. Havenith‐Newen,
th thChair of Physical Chemistry II, in the time between July 11 , 2006 and April 15 , 2010.
stDissertation Defense: June 1 , 2010
First Examiner: Prof. Dr. M. Havenith‐Newen
Lehrstuhl für Physikalische Chemie II
Second Examiner: Prof. Dr. H. Weingärtner
AG "Physikalische Chemie der Flüssigkeiten"
Lehrstuhl für Physikalische Chemie II
Examination Chairman: to be announced
Faculty of Chemistry and Biochemistry
ITSP First Supervisor: Prof. Dr. M. Havenith‐Newen
Lehrstuhl für Physikalische Chemie II
ITSP Second Supervisor: Prof. Dr. M. Hofmann
Lehrstuhl für THz‐Technologie
“Don’t dream it, be it.”
Richard O’Brien ‐ The Rocky Horror Picture Show (1975)
“Another turning point, a fork stuck in the road.
Time grabs you by the wrist, directs you where to go.
So make the best of this test, and don't ask why.
It's not a question, but a lesson learned in time.
It's something unpredictable, but in the end it's right.
I hope you had the time of your life.”
Green Day ‐ Time of Your Life (1997)
This thesis would not have been possible without the help of many people. I would like to
thank all of them for their outstanding efforts, work, help, patience, care, and supervision.
You all have a big stake in my success.
Prof. Dr. Martina Havenith‐Newen supervised me during my Bachelor’s, Master’s and PhD
thesis. I would like to thank her for all the trust she put in me, for her patience and
competent supervision, and for giving me the opportunity to do high‐level research in such a
stimulating atmosphere. Also, I would like to thank her for letting me take part in other
projects besides my thesis.
Prof. Dr. Hermann Weingärtner is my second examiner and cooperation partner. Thank you
for providing the microwave dielectric spectra, for help with the data interpretation and for
kicking off the ionic liquids project.
Prof. Dr. Martin Hofmann is my second supervisor according to the Individual Training and
Supervision Plan (ITSP) of the Research School. Thank you for much advice and help and for
being the lecturer and examiner of my minor class exam.
I would like to thank Dr. Erik Bründermann for excellent guidance, outstanding competence
and supervision. Thank you for always being straightforward and supporting me. Moreover, I
would like to thank him for letting me take part in many of his interesting projects. I wish I
were able to give him back at least a little percentage of what he has given to me. Thank you
Erik also for proof‐reading this thesis.
Many thanks go to Asst. Prof. Dr. Özgür Birer, Dr. Rajib Mitra, Dr. Diedrich Schmidt and Dr.
Gerhard Schwaab for experimental help, proof‐reading, many fruitful scientific discussions
and support with the data analysis.
I would like to thank Prof. Dr. Martin Gruebele and Dr. Seung‐Joon Kim for letting me
participate in the development of the Kinetic THz Absorption (KITA) experiment.
Thank you to Prof. Dr. Wim van der Zande and Dr. Afric Meijer from Radboud University ‐
Nijmegen for cooperation and scientific exchange. viii Acknowledgements
I would like to thank Prof. Dr. Martin Thomas Zanni, Dr. Terry Ding and all other members
of the Zanni group at the University of Wisconsin ‐ Madison for the introduction to 2D‐IR
spectroscopy and three exciting months in the US. I also acknowledge the funding of this
research visit to the Foundation of German Economics (SDW).
Thank you to Prof. Dr. Uwe Weidner and Dipl.‐Ing. Sulamith Frerich for letting me take part
in a nice cooperation project on micro‐particles and for giving me an insight into state‐of‐
the‐art research on process engineering.
Thank you to Dr. Nils Gerhardt and Dr. Stefan Hoffmann for instant and useful help with
diodes, fibers and other optical and photonic devices.
I would like to thank Prof. Dr. Anja‐Verena Mudring and her group for carefully synthesizing
some of the ionic liquids and for fruitful discussion meetings.
Thank you to M.Sc. Mianmian Huang for help with the preparation and drying of the ionic
liquids and for providing the microwave dielectric spectra.
Thank you to Dr. Christoph Jahnke and Dr. Albrecht Bartels for their help with the ASOPS
system, their patience, and courtesy.
I would like to thank Dr. Benjmain Born, Dipl.‐Phys. Jessica Dielmann‐Gessner, Dr. Simon
Ebbinghaus, Dipl.‐Phys. Stefan Funkner, B.Sc. Matthias Heyden, M.Sc. Konrad Meister,
B.Sc. Anna Muhs, M.Sc. Gudrun Niehues, B.Sc. Luong Truong Quan and Dr. Konstanze
Schröck from the THz group for many years of fun, hard work, frustration and success in the
lab, for methodical introduction and for many helpful discussions.
Many thanks go to the group of Physical Chemistry 2, especially Christian Fester, Solveig
Förster, Ulla Knieper, Jochen Masuch, Hannelore Sommer, Reinhard Renzewitz and Sabine
Weiß for immediate help, care and support. I would like thank all current and former group
colleagues especially Dr. Anja Metzelthin and. Dr. Markus Ortlieb for a pleasant time,
excellent teamwork, friendship, and guidance.
Thank you to the machine shop and repositories of the Faculty of Chemistry & Biochemistry
for technical help and quick fabrication.
Many thanks go to Dr. Ursula Justus, Dr. Heide Schmidtmann, Maria Sprung and
Dr. Christiane Wüllner, the members of the Executive Board, and the fellows of the Ruhr‐
University Research School for nice experience and many interesting events in the Ruhr‐
University Research School.
I would like to thank Prof. Dr. Dieter Bender and Prof. Dr. Wilhelm Löwenstein for heading
the local chapter of the Foundation of German Economics (SDW) in Bochum, its scholars and
Ruth Lehnen as representative of the SDW for funding three years of my PhD studies and
many interesting events giving me a stimulating insight into economics and business.
Ich bedanke mich bei allen meinen Freunden, Kollegen, Stammtisch‐Brüdern vom
„Herrenabend Wanne‐Eickel“, Bekannten und Verwandten für eine immer wieder sehr tolle
Zeit außerhalb des Uni‐Lebens.
Mein größter Dank geht an meine Familie, meine Mutter Ulrike Krüger, meinen Vater Heinz‐
Günter Krüger, meine Großeltern, meinen Bruder Christopher Krüger und seine Freundin
Antje Sonntag für langjährige Unterstützung, Rat, Tat und Hilfe.
Ich danke euch allen sehr. x Abstract
Within the scope of this thesis a fast‐scanning Terahertz time‐domain spectrometer using
asynchronous optical sampling (ASOPS) has been prepared and extended to temperature‐
stabilized measurements of highly absorbing samples. The data analysis has been adapted to
a quartz cell for thin liquid samples and a metal pill holder for solid samples by advanced
pulse propagation and parameter extraction algorithms. In this context, the experimental
uncertainty and ambiguity have been studied in detail. This enables the measurement and
determination of the frequency‐dependent complex dielectric function of a chemical
component within a bandwidth of up to 3 THz. As test samples, homologous fatty acids have
been characterized in the THz region as well as other homologous organic samples such as
organic polymers and aromatic compounds. The main focus lies on a systematic combined
THz time‐domain and Fourier‐transform Infrared (FTIR) spectroscopic study of ionic liquids, a
new class of chemical compounds with a promising potential for scientific and industrial
applications. By combining THz, FTIR and microwave data, the absorption coefficient of
ethyl‐ammonium nitrate was measured over eight orders of magnitude in absolute size and
in frequency. Moreover, a series of homologous alkyl‐ammonium ionic liquids was
characterized. By simultaneous fitting of the dielectric spectra obtained with the THz
spectrometer and dielectric data in the microwave region, an insight into molecular
relaxation and interaction processes has been obtained. The taxonomy of the measured
samples allows correlations between dielectric behavior and chemical composition, which in
turn can help with prediction of physicochemical properties and customized design of ionic
Within the scope of this thesis, special measurement software packages for the TDS setup
and for the ASOPS spectrometer (cf. chapter 3.4) have been developed. Moreover, programs
for Gaussian beam propagation, parameter extraction from THz time‐domain measurements
and fitting of dielectric spectra have been written. Correspondence to: