2.3 Microwave Spectrum: Dielectric Relaxation
67
only. In the condensed state, however, the model of orientational polarization leads to
a lower dielectric constant than observed experimentally (see Sect. 2.4). In addition,
it has been shown [20] that the relaxation time, calculated by the Stokes–Einstein
formula (τ D1 = 4πr
3
η/k B T , where r is the molecular radius and η is viscosity) is
too large if the macroscopic experimental viscosity is used.
6 The intermolecular
polarization mechanism, due to the dynamics of intrinsic ionic species, looks more
appropriate for the experimental data. Recently, a similar, nonrotational mechanism
of polarization has been discussed in monohydric alcohols, the dielectric properties
of which are very similar to those of water [21]. Both water and alcohols are two
typical representatives of polar liquids. Just like water, alcohols have a relatively
high dielectric constant and are good solvents. As alcohols have longer molecules
than those in water, rotational polarization is not possible. Thus, the nonrotational
mechanism of polarization suggested for alcohols is also possible for water, as their
spectra collapse to a single curve being scaled (see [21]).
2.3.3 Is Debye Relaxation a Unique Feature of Water?
Polarization ability governs various mechanical, chemical, and thermodynamic properties of dielectrics. Detectors, sensors, memory units, and electric-power storage systems all use this property. That is why different polar dielectrics have been studied in
detail. Although Debye relaxation is known as a hallmark of water, which is actually
missing in most dielectrics, a very similar relaxation band was also detected in a narrow class of materials such as organic solvents, molten salts, superionic conductors,
and ionic liquids. In the context of the microscopic dynamics, which accompany
dielectric relaxation, it is worth analyzing the spectroscopic data of these materials
and compare them with those for water.
Figure 2.7 compares dielectric spectra of water, a molecular liquid (glycerol), an
ionic liquid (ethylammonium nitrate), and amorphous quartz (SiO 2 ). The general
similarity of the spectra is observed for all substances except quartz. One can see
that the spectrum of SiO 2 , which served as an analog of water structure in X-ray
diffraction experiments (see Sect. 1.2), does not exhibit Debye relaxation, shown as
the yellow-shaded area in Fig. 2.7. Thus, Bernal–Fowler analogy between water and
quartz is not complete.
Nevertheless, the Debye relaxation is observed for some dielectrics, such as ferroelectrics, superionic conductors, alcohols, and ionic liquids. All these substances
are characterized by the ability of the atomic-molecular structure to rearrange. One
can see in Fig. 2.7 that the spectra of glycerol, ethylammonium nitrate, and water are
very similar in the microwave frequency range. The analogies between water and
6 The difference between experimental viscosity and that calculated by Stokes–Einstein formula is
observed for all polar liquids and, depending on the conditions, ranges from ten to several thousand
times.
67
only. In the condensed state, however, the model of orientational polarization leads to
a lower dielectric constant than observed experimentally (see Sect. 2.4). In addition,
it has been shown [20] that the relaxation time, calculated by the Stokes–Einstein
formula (τ D1 = 4πr
3
η/k B T , where r is the molecular radius and η is viscosity) is
too large if the macroscopic experimental viscosity is used.
6 The intermolecular
polarization mechanism, due to the dynamics of intrinsic ionic species, looks more
appropriate for the experimental data. Recently, a similar, nonrotational mechanism
of polarization has been discussed in monohydric alcohols, the dielectric properties
of which are very similar to those of water [21]. Both water and alcohols are two
typical representatives of polar liquids. Just like water, alcohols have a relatively
high dielectric constant and are good solvents. As alcohols have longer molecules
than those in water, rotational polarization is not possible. Thus, the nonrotational
mechanism of polarization suggested for alcohols is also possible for water, as their
spectra collapse to a single curve being scaled (see [21]).
2.3.3 Is Debye Relaxation a Unique Feature of Water?
Polarization ability governs various mechanical, chemical, and thermodynamic properties of dielectrics. Detectors, sensors, memory units, and electric-power storage systems all use this property. That is why different polar dielectrics have been studied in
detail. Although Debye relaxation is known as a hallmark of water, which is actually
missing in most dielectrics, a very similar relaxation band was also detected in a narrow class of materials such as organic solvents, molten salts, superionic conductors,
and ionic liquids. In the context of the microscopic dynamics, which accompany
dielectric relaxation, it is worth analyzing the spectroscopic data of these materials
and compare them with those for water.
Figure 2.7 compares dielectric spectra of water, a molecular liquid (glycerol), an
ionic liquid (ethylammonium nitrate), and amorphous quartz (SiO 2 ). The general
similarity of the spectra is observed for all substances except quartz. One can see
that the spectrum of SiO 2 , which served as an analog of water structure in X-ray
diffraction experiments (see Sect. 1.2), does not exhibit Debye relaxation, shown as
the yellow-shaded area in Fig. 2.7. Thus, Bernal–Fowler analogy between water and
quartz is not complete.
Nevertheless, the Debye relaxation is observed for some dielectrics, such as ferroelectrics, superionic conductors, alcohols, and ionic liquids. All these substances
are characterized by the ability of the atomic-molecular structure to rearrange. One
can see in Fig. 2.7 that the spectra of glycerol, ethylammonium nitrate, and water are
very similar in the microwave frequency range. The analogies between water and
6 The difference between experimental viscosity and that calculated by Stokes–Einstein formula is
observed for all polar liquids and, depending on the conditions, ranges from ten to several thousand
times.