Dielectric Relaxation Spectroscopy
apl. Prof. Dr. Richard
Buchner
Coworkers
Guests
Introduction
Dielectric Relaxation Spectroscopy (DRS)
probes the interaction of a macroscopic sample with a time-dependent electric
field [1]. The resulting polarization, either expressed by the
frequency-dependent complex permittivity and conductivity or as an impedance
spectrum, characterizes amplitude and timescale (via the relaxation time) of
the charge-density fluctuations within the sample. Such fluctuations generally
arise from the reorientation of the permanent dipole moments of individual
molecules or from the rotation of dipolar moieties in flexible molecules, like
polymers. Other possible mechanisms include the transport of ions or the
appearance of interfacial charges in heterogeneous systems. The timescale of
these fluctuations depends on the sample and on the relevant relaxation
mechanism. Relaxation times range from several picoseconds in low-viscosity
liquids to hours in glasses, probably marking DRS as the technique with the
most extensive coverage of dynamical processes. The corresponding measurement
frequencies range from 10-4 Hz to 1012 Hz, which requires
a series instruments for complete coverage. However, it is generally sufficient
to concentrate on a smaller frequency range adapted to the sample
properties.
In contrast to conventional spectroscopic methods, like NMR or vibrational
spectroscopy, DRS is especially sensitive to intermolecular interactions. DRS
is able to monitor cooperative processes and thus provides a link between
molecular spectroscopy, which monitors the properties of the individual
constituents, and techniques characterizing the bulk properties of the sample,
especially the viscoelastic and rheological behaviour. The decomposition of the
dielectric spectrum into its individual relaxation processes informs on the
relative amplitudes and characteristic times of the underlying molecular
motions.
DRS is widely applied in the characterization of ion-conducting solids,
polymers and mesophases [1-4]. But it is also of large
potential interest for the investigations of liquid [5-11] and
colloidal systems [12-14]. Additionally, the effects studied
by DRS are of increasing importance for technical applications like dielectric
heating or remote sensing [6,12,15-17]. Possible applications
include:
Static permittivity [4-7]
The static permittivity, e, is a central solvent
property which determines and reflects the magnitude of solute-solvent
interactions. e influences the solubility of
solutes, the formation of micelles or the osmotic coefficients of electrolytes.
Vice versa e strongly depends on the
composition of the solution (e.g. the static permittivity of pure
N-methylformamide at 25°C is 184, but for a 0.5 ,m NaCl solution
e=111 is observed). Note that DRS is the only
technique which allows the determination of the static permittivity of
electrically conducting liquids, like electrolyte solutions.
Dielectric heating [15,16]
DRS routinely determines the dielectric loss, e'' ,
or - for samples of conductivity k - the total loss,
h''=e'' +
k/(2pne0), as a function
of the frequency, n; e0 is the electric field constant (vacuum
permittivity). These quantities characterize the absorption of electromagnetic
energy by the sample and therefore determine the efficiency of dielectric
heating (e.g. in microwave ovens). Recently, dielectric heating has become a
popular tool in chemical synthesis because it is not only very efficient but
for several classes of reactions otherwise inaccessible product pathways are
followed.
Liquid-state Dynamics [7-9]
When extended into the far-infrared region DRS provides the full dynamics of
liquids from molecular librations to large-scale cooperative motions. Since DRS
probes the fluctuations of induced (in the far infrared) and permanent dipole
moments, i.e. of vectors, its information is complementary to
information from Raleigh, Raman and NMR spectroscopy which monitor the
fluctuation of tensor properties of molecules. Recently, it was shown [7] that the information provided by Raman and DRS on the
molecular dynamics is fully equivalent, whereas large-scale cooperative motions
are generally only detected by DRS.
Ion Solvation [9-11]
Ion-solvent interactions are of prime importance in a wide field of physical
chemistry, ranging from the physical (and thus implicit the physiological)
properties of seawater or body fluids via hydrometallurgy to protein stability.
DRS allows the determination of effective ion solvation numbers from the
analysis of the solvent relaxation processes. These solvation numbers generally
differ from coordination numbers obtained with scattering techniques or
computer simulation because they do not only reflect packing effects but
essentially monitor the relative strength of ion-solvent vs.
solvent-solvent interactions.
Speciation in Electrolyte Solutions [9,11]
In electrolyte solutions ion-ion interactions may lead to the formation of
aggregates, like ion-pairs or ion-triples. The identification and quantitative
determination of such species is of crucial importance for the understanding of
electrolyte properties and the modeling of geological and industrial processes.
DRS is specific for species with a permanent dipole moment, like ion-pairs. In
contrast to other spectroscopic techniques, which monitor only contact
ion-pairs, DRS is also able to detect solvent-separated ion-pairs.
Dynamics of complex liquid mixtures -
microhetrogeneities, hydrophobic hydration [6-11]
DRS is especially sensitive to cooperative processes. For instance in the case
of alcohol-water mixtures of increasing alcohol content DRS indicates the
transition of hydrated alcohol monomers to dimers to hydrated alcohol chains
with pockets of frustrated water molecules. Around hydrophobic ions a shell of
water molecules with strongly reduced mobility can be distinguished from bulk
water. DRS thus allows to monitor the influence of solute-solvent interactions
on structure and dynamics of the solvent as well as of the
solute. Such information is important for the understanding of
solubilization phenomena.
Emulsions and microemulsions [6,12,13]
DRS yields information on the type of emulsion (o/w or w/o), emulsion stability
and aggregation behaviour (including percolation phenomena), droplet size and
distribution. The dynamics of stabilizers and additives, as well as their
interaction with water can be investigated.
Colloids [6,12-14]
DRS monitors counterion diffusion on the surface of colloidal particles. From
the observed relaxation time information on particle size and shape can be
inferred.
Micelles and liposomes [6,12-14]
DRS yields information on the particle size and the surface diffusion
coefficient of bound counter ions. It allows the determination of the phase
diagram.From the individual spectral contributions specific information on the
headgroup dynamics of the surfactant molecules as well as on the structure of
the hydration shell can be obtained. Additionally, the interactions of polar
probe molecules with the different environments present in such systems can be
investigated.Due to its high sensitivity towards conducting contaminants, DRS
is very effective in the purity control of liposomes.
The timescales covered by these solution processes range from milli- to
femtoseconds, so that experiments must be conducted in the range of radio- and
microwaves, possibly extended into the far-infrared region. Due to now
available impedance analyzers dielectric relaxation studies up to 3 GHz are
becoming increasingly popular for the investigation of complex liquids, see
ref. [6]. However, a perusal of the literature reveals that
generally the interpretation of the data is not as unambiguous as one would
wish. This essentially arises from the large bandwidths of the spectral
components, so that precise spectra over a broad frequency range are necessary
for a meaningful decomposition. Since commercial instruments cannot cover the
relaxation range of the solvent appropriately (e.g. water peaks at ~20 GHz -
already in a frequency range difficult to access), such limited investigations
do not only forbid a characterization of the solvent response, but also
hamper a precise separation of the solute contribution or of specific
cooperative relaxation processes (e.g. of micelles, emulsions, colloids).
Coming from the investigation of ¨simple¨ liquids and solutions with
experience in the development of DRS instrumentation for highly conducting
electrolyte solutions, our laboratory is now able to perform DRS experiments in
the frequency range 0.01<=n/GHz <=89 with 2%
accuracy relative to e. We are unique in being able
to cover both solute and solvent response with high accuracy for
temperatures between -45°C and 65°C [5,11].
[1] F. Kremer and A. Schönhals
(eds.),Broadband Dielectric Spectroscopy, Springer, Berlin (2002).
[2] A.K.Jonscher, Dielectric Relaxation
in Solids, Chelsea Dielectric Press, London (1983).
[3] C.C.Ku and R.Liepins, Electrical Properties of
Polymers, Hanser, München (1987).
[4] L.M.Blinov, Electro-optical and Magneto-optical
Properties of Liquid Crystals, Wiley, Chichester (1983).
[5] Barthel and R. Buchner, Relative Permittivities of
Electrolytes, in: A.R.H. Goodwin, K.N. Marsh,
 and W.A. Wakeham (Eds.), Experimental
Thermodynamics, Vol. IV: Measurement of the Thermodynamic
 Properties of Single Phases, Ch.9.3,
Elsevier, Amsterdam (2003).
[6] D.Q.M.Craig, Dielectric Analysis of Pharmaceutical
Systems, Taylor & Francis, London (1995).
[7] T. Fukasawa, T. Sato, J. Watanabe, Y. Hama, W. Kunz and R.
Buchner, The relation between dielectric and
low-frequency Raman spectra of
hydrogen-bond liquids, Phys. Rev. Lett., 95 (2005) 197802.
[8] T. Sato und R. Buchner, Cooperative and molecular
dynamics of alcohol/water mixtures: the view of dielectric
spectroscopy,J. Mol. Liq. 117
(2005) 23-31 (Special Issue "Molecular Liquids --- New Routes from
Local
Order to Large-scale
Cooperativity").
[9] R. Buchner and J. Barthel ,Dielectric Relaxation in
Solutions , Annu. Rep. Prog. Chem., Sect. C, 91 (1994) 71-106;
97 (2001) 349-382
[10] U.Kaatze, The Dielectric Properties of Water in Its
Different States of Interaction, J. Solution Chem. 26 (1997)
1049-1112.
[11] R. Buchner, Dielectric Spectroscopy of Solutions, in:
J. Samios and V.A. Durov (eds.), Novel Approaches to the
Structure and Dynamics of Liquids:
Experiments, Theories and Simulations, NATO Science Ser. II: Mathematics,
Physics and Chemistry, Vol. 133, pp.
265-288, Kluwer, Dordrecht (2004).
[12] Yu. Feldman et al, Dielectric Spectroscopy on
Colloidal Systems - A Review, in: P. Becher (ed.),
Encyclopedia Handbook of Emulsion
Technology, Vol. 5, Marcel Dekker, New York (2001).
[13] K. Asami, Characterization of heterogeneous systems
by dielectric spectroscopy, Prog. Polym. Sci. 27 (2005)
1617-1659.
[14] R. Buchner et al, Dielectric Spectroscopy of Micelle
Hydration and Dynamics in Aqueous Ionic Surfactant
Solutions, J. Mol. Liq. 118
(2005) 179-187 (Special Issue "20th International Conference on Solution
Chemistry").
[15] C. Gabriel et al., Dielectric Parameters Relevant to
Microwave Dielectric Heating, Chem. Soc. Rev. 27 (1998)
213-223.
[16] A. de la Hoz et al, Microwaves in organic synthesis.
Thermal and non-thermal microwave effects, Chem. Soc. Rev.
34 (2005) 164-178.
[17] S. English et al, A comparison of the impact of TOVS
and ATOVS satellite sounding data on the accuracy of
numerical weather forecast, Q. J. R.
Meteorol. Soc. 126 (2000) 2911-2932
Facilities
- Auxiliary Data
Measurements of auxiliary data required for the analysis of the complex
permittivity spectra (conductivity, density, viscosity) are performed in our
laboratory.
- Data analysis
For the decomposition of the complex permittivity spectra into individual
relaxation processes and the visualisation of the results a software package
was developed which simultaneously fits the real and the imaginary parts of the
complex permittivity.Relaxation models with up to 10 relaxation processes may
be defined. Currently the equations of Debye, Cole-Cole, Davidson-Cole,
Havriliak-Negami, Froehlich, and Dissado-Hill, as well as the Damped
Harmonic Oscillator (for modelling intermolecular vibrations in the
far-infrared) are implemented as possible band-shape functions for the
individual processes. The quality of the fit can be judged by its reduced error
function (variance normalised to the number of parameters).
Research Projects
- Dynamics and phase behaviour of ionic and non-ionic microemulsions
W. Wachter, R. Buchner, W. Kunz
- The relevance of micelle hydration for the interpretation of SAXS
measurements
T. Sato (Shinshu Univ., Japan), T. Fukasawa (Ochanomizo Univ., Japan), K.
Aramaki (Yokohama State Univ., Japan), and R. Buchner
- Ion solvation and association in aqueous and non-aqueous solvents
H.A.U. Rahman, R. Buchner with A. Placzek, G.T. Hefter (Murdoch Univ.,
Australia)
- Cooperative dynamics of hydrogen-bonding liquids
T. Sato (Shinshu Univ., Japan), T. Fukasawa (Ochanomizo Univ., Japan), and
R. Buchner
- Dynamics of ionic liquids and their mixtures with polar compounds [DFG
SPP 1191]
J. Hunger, A. Stoppa and R. Buchner with G. Hefter (Murdoch Univ.,
Australia), A. Thoman,
M. Walther and H. Helm (Univ. Freiburg), D.A. Turton and K. Wynne (Strathclyde
Univ., U.K)
- Hydration and metal-ion binding of polyelectrolytes
R. Buchner with P. Sipos and I. Tóth (Univ. Szeged, Hungary), M.
Lukšic and B. Hribar-Lee (Univ. Ljubljana, Slovenia)
Recent Invited Lectures
- What can be learnt from Dielectric
Relaxation Spectroscopy about Ion Solvation and Association (11.2 MB)
30th International
Conference on Solution Chemistry, Perth (Australia), July 16-20, 2007.
- Dielectric Relaxation Spectroscopy of
Ion Association in Aqueous and Non-Aqueous Solvents (9.52 MB)
A New Step in Solution Chemistry from
Yokohama, Yokohama (Japan), November 20, 2007
- Cooperative Dynamics of Mixtures of
Ionic Liquids with Polar Solvents (11.2
MB)
Joint Conference of JMLG/EMLG Meeting
2007 and
30th Symposium on Solution
Chemistry of Japan, Fukuoka (Japan), November 21-25, 2007
Publications
see Publications R. Buchner and:
J. Barthel, H. Krienke, and W. Kunz, Physical Chemistry of Electrolyte
Solutions, Springer, Berlin, 1998.
J. Barthel, Ion Solvation and Ion Association Studied by Infrared and
Microwave Methods, J. Mol. Liq. 65/66 (1995) 177-185.
J. Barthel, Microwave Investigations on the Structure and Dynamics of Liquid
Systems, Atti della Accademia Peloritana dei Pericolanti, Classe I di
Scienze Fis. Mat. e Nat., Vol. LXX (1992), 1-61.
J.Barthel and M.Kleebauer, The Behaviour of Ion Pairs in High Frequency
Electric Fields Exemplified by Acetonitrile Solutions of Bu4NBr, J.
Solution Chem. 20 (1991) 977-993.
J. Barthel, Modern Aspects of Physical Chemistry of Ionic Solutions,
Portugaliae Electrochim. Acta, 9 (1991) 287-309.
J. Barthel, Der Einfluß von spezifischen intermolecularen Kräften
auf die dielektrischen Eigenschaften und die Leitfähigkeit von
Lösungen, in: J.P. Huyskens (ed.) Intensive Erasmus lessencyclus
"Intermoleculaire Krachten", Katholieke Universiteit Leuven, Vol. 1,
Chapter 11 (NL and German texts), 1989, 516-571.
J.Barthel and F.Feuerlein, Dielectric Properties of Propylene
Carbonate-1,2-Dimethoxyethane Mixtures and their Electrolyte Solutions of
NaClO4 and Bu4NClO4, Z. Phys. Chem. NF 148
(1986) 157-170.
J. Barthel: Transport Properties of Electrolytes from Infinite Dilution to
Saturation, Pure Appl. Chem. 57 (1985) 355-367.
J. Barthel: Transport Properties of Electrolytes from Infinite Dilution to
Saturation, Pure Appl. Chem. 57 (1985) 355-367.
J.Barthel and F.Feuerlein, Dielectric Properties of Propylene Carbonate and
Propylene Carbonate Solutions, J. Solution Chem. 13 (1984)
393-417.
J. Barthel, H.-J. Gores, G. Schmeer, and R. Wachter, Non-Aqueuos Electrolyte
Solutions in Chemistry and Modern Technology, Top. Curr. Chem. 111
(1983) 33-144.
J. Barthel, Electrolytes in Non-Aqueous Solvents, Pure Appl. Chem.
51 (1979) 2093-2124.
J.Barthel, J.Krüger, and E. Schollmeyer, Dielektrizitätskonstante
wäßriger und nicht-wäßriger Elektrolytlösungen. III.
Kritische Untersuchungen zur Meß- und Auswertemethode. Alkalifluoride,
-bromide, -jodide und -perchlorate in wäßriger Lösung, Z.
phys. Chem. NF 104 (1977) 59-72.
J.Krüger, E.Schollmeyer, and J.Barthel, The Influence of Higher Order
Modes upon the Accuracy of Dielectric Constant Determinations Using
Transmission Measurement Cells for Electrolyte Constants in the Giga Hertz
Range, Z. Naturforsch. 30a (1975) 1476-1480.
J.Barthel, F.Schmithals, and J.Barthel, Dielektrizitätskonstante
wäßriger und nicht-wäßriger Elektrolytlösungen. II.
Mikrowellenmessungen von Dielektrizitätskonstante und Relaxationszeit an
Lösungen der Alkalinitrate und -chloride in polaren
Lösungsmitteln, Z. Phys. Chem. NF 96 (1975) 73-88.
J. Barthel, H.Behret, and F.Schmithals, Dielectric Behaviour of Soltuions of
Alkali Chlorides and Nitrates in the Microwave Region, Ber. Bunsenges.
Phys. Chem. 75 (1971) 305-309.
J. Barthel, F.Schmithals, and H.Behret, Untersuchungen zur Dispersion der
komplexen Dielektrizitätskonstante wäßriger und
nicht-wäßriger Elektrolytlösungen. I. Auswahl der Methoden und
Messungen an wäßrigen Lösungen von 1-1-Eletrolyten bei
25°C im Bereich der cm-Wellen, Z. Phys. Chem. NF 71 (1970)
115-131.
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