176
5 Electrodynamics of Aqueous Media
for the mechanisms of solvation,
5 which, as shown by IR spectroscopy [11], is
very important for the dynamic structure of electrolytes. The fourth problem is that
the theory gives large mobilities for large ions [12]. For instance, the ions of Li
+ ,
Na
+ , K
+ , Rb
+ , Cs
+ , etc., are of the same charge, and ordered by the increasing
crystallographic radius, and, consequently, their mobilities should gradually decrease
from left to right. However, their mobilities in aqueous solutions appear to show
the opposite behavior, demonstrating an increase of mobility as the size increases.
Although this effect is usually explained by smaller ions getting larger solvation
shells, such an assumption contradicts the general physical principles of electrostatic
interactions. Finally, the theory is only valid for the static DC current and says nothing
about the frequency dependence of the conductivity. In particular, it does not explain
the similar microwave absorption of pure water and strong electrolytes. All these
points can be accounted for, but the initially simple model was getting so complicated
that its practical application is complicated. As a result, in practice, the transfer
phenomena in aqueous solutions (including electrical properties) are described by
the empirical coefficients [12], while the real atomic-molecular structure of aqueous
electrolytes is missing.
Summarizing, at the beginning of the twentieth century, the mechanism of the
conductivity of electrolytes was better understood than the mechanism of electrical
conduction in metals. The simplicity of Arrhenius’ ideas found many supporters,
and his ideas were transformed into the modern theory of solvation, which, however,
still has some open questions, as described above. Nowadays everything is upside
down, while the theory of metals has been significantly elaborated for decades [13],
the structure and dynamics of aqueous electrolytes have not changed much in comparison with the knowledge that we had at the beginning of the twentieth century.
Interestingly, earlier views on the structure of aqueous electrolytes are still very popular in textbooks, and the basic understanding of the kind of phenomenology discussed
above is still used for the description of many electrochemical processes. However,
recent progress in quantum chemistry, computational methods, and especially femtochemistry require a significant reconsideration of the quasi-static approach developed
more than hundred years ago.
5.1.2 The Frequency-Dependent Conductivity of Aqueous
Electrolytes
Hasted et al. conducted [14] the first systematic study of the frequency-dependent
dielectric function of different aqueous electrolytes. The results are assembled in [15].
In the past decade, the accuracy of the data and the frequency coverage have been
significantly increased and this has allowed us to systematize the electrodynamic data
of the basic aqueous electrolytes from the static parameters, such as the dielectric
constant and DC current, up to the terahertz region. Figure 5.3 shows the frequency
5 Solvation describes the interaction of the solvent with the dissolved molecules.
5 Electrodynamics of Aqueous Media
for the mechanisms of solvation,
5 which, as shown by IR spectroscopy [11], is
very important for the dynamic structure of electrolytes. The fourth problem is that
the theory gives large mobilities for large ions [12]. For instance, the ions of Li
+ ,
Na
+ , K
+ , Rb
+ , Cs
+ , etc., are of the same charge, and ordered by the increasing
crystallographic radius, and, consequently, their mobilities should gradually decrease
from left to right. However, their mobilities in aqueous solutions appear to show
the opposite behavior, demonstrating an increase of mobility as the size increases.
Although this effect is usually explained by smaller ions getting larger solvation
shells, such an assumption contradicts the general physical principles of electrostatic
interactions. Finally, the theory is only valid for the static DC current and says nothing
about the frequency dependence of the conductivity. In particular, it does not explain
the similar microwave absorption of pure water and strong electrolytes. All these
points can be accounted for, but the initially simple model was getting so complicated
that its practical application is complicated. As a result, in practice, the transfer
phenomena in aqueous solutions (including electrical properties) are described by
the empirical coefficients [12], while the real atomic-molecular structure of aqueous
electrolytes is missing.
Summarizing, at the beginning of the twentieth century, the mechanism of the
conductivity of electrolytes was better understood than the mechanism of electrical
conduction in metals. The simplicity of Arrhenius’ ideas found many supporters,
and his ideas were transformed into the modern theory of solvation, which, however,
still has some open questions, as described above. Nowadays everything is upside
down, while the theory of metals has been significantly elaborated for decades [13],
the structure and dynamics of aqueous electrolytes have not changed much in comparison with the knowledge that we had at the beginning of the twentieth century.
Interestingly, earlier views on the structure of aqueous electrolytes are still very popular in textbooks, and the basic understanding of the kind of phenomenology discussed
above is still used for the description of many electrochemical processes. However,
recent progress in quantum chemistry, computational methods, and especially femtochemistry require a significant reconsideration of the quasi-static approach developed
more than hundred years ago.
5.1.2 The Frequency-Dependent Conductivity of Aqueous
Electrolytes
Hasted et al. conducted [14] the first systematic study of the frequency-dependent
dielectric function of different aqueous electrolytes. The results are assembled in [15].
In the past decade, the accuracy of the data and the frequency coverage have been
significantly increased and this has allowed us to systematize the electrodynamic data
of the basic aqueous electrolytes from the static parameters, such as the dielectric
constant and DC current, up to the terahertz region. Figure 5.3 shows the frequency
5 Solvation describes the interaction of the solvent with the dissolved molecules.