5.1 The Dielectric Response of Electrolyte Solutions
175
the charge carriers in electrolytes many years earlier, firmly entered the history of
the physical chemistry of solutions. Apart from Faraday’s assumption that ions are
produced by an external electric field, Arrhenius proposed that, even in the absence
of an electric field, solutions contain ions. He thus introduced the idea that chemical
reactions in solution were reactions between ions only. No water molecules were
involved in his consideration of the electric current at all. According to his theory,
aqueous electrolytes conduct due to the presence of long-lived free ions. The idea
that more free ions provide more current was in line with the ideas of Kohlrausch.
Interestingly, neither Kohlraush nor Arrhenius ever saw the Debye relaxation of
water, which appears at frequencies higher than those available at that time and was
discovered years later. As the optical transparency window (see Fig. 2.2) is almost
on the same level as DC conductivity, they probably even did not expect that a huge
absorption band exists in the gigahertz-to-IR region. Nor did they know that the
microwave part of the spectrum is almost unaffected by the electrolyte concentration
(see Sect. 5.1.2), and thus were unable to include high-frequency mechanisms in
their considerations. Maybe if they had, the theory of electrolytes would be more
general and completely different.
Mendeleev did not agree with the postulates by Arrhenius [6, 7]. His opinion was
based on the experience of chemists, who dealt with chemical kinetics, and the idea
of the physical concept of the electrolytes, where ionic species exist as spherical
particles and do not interact with the solvent (water) was controversial. He noted [8]
that rather than accepting that for the salt MX the dissociation of its particles into ionic
species M+X, we should first, following our knowledge on the nature of electrolytes,
look for aqueous electrolytes of the salt MX in their interaction with H 2 O, which
gives MOH + HX. In other words, Mendeleev’s idea was to account for local chemical
reactions and the chemical exchange between the species of solute and solvent. His
ideas and arguments were actively supported by Armstrong, Crompton, Pickering,
Traube, Wiedemann, and others. Nevertheless, Arrhenius [9], enlisting the support
of Ostwald and Van ’t Hoff, defended himself by the argument that the isotonic
coefficient i
4 can be explained by his theory of electrolytic dissociation. Providing
his arguments, he only rejected that part of Mendeleev’s idea related to clathrates but
was not able to appreciate the pivotal idea of the ion–molecular exchange interaction
proposed by Mendeleev, which would significantly move forward the understanding
of the dynamic structure of aqueous electrolytes.
There several points that show the imperfection of the theory of Arrhenius (and all
subsequent theories on this basis). First, the theory does not explain the mechanism of
dissolution. It operates with the already dissolved ionic species without explanation
of where water gets the huge amount of energy (hundreds of kJ/mol) required to
separate the ions of the initially neutral solvent. The second problem is that the
theory works for very diluted electrolytes (usually below 0.001 mol/l) but fails to
reproduce results for higher concentrations (even accounting for the Debye–Hückel
amendments [10]). The third problem is that the model of Arrhenius does not account
4 The isotonic coefficient (or van ’t Hoff’s coefficient) is the proportionality coefficient between the
total concentration of a solute and the concentration of solute particles.
175
the charge carriers in electrolytes many years earlier, firmly entered the history of
the physical chemistry of solutions. Apart from Faraday’s assumption that ions are
produced by an external electric field, Arrhenius proposed that, even in the absence
of an electric field, solutions contain ions. He thus introduced the idea that chemical
reactions in solution were reactions between ions only. No water molecules were
involved in his consideration of the electric current at all. According to his theory,
aqueous electrolytes conduct due to the presence of long-lived free ions. The idea
that more free ions provide more current was in line with the ideas of Kohlrausch.
Interestingly, neither Kohlraush nor Arrhenius ever saw the Debye relaxation of
water, which appears at frequencies higher than those available at that time and was
discovered years later. As the optical transparency window (see Fig. 2.2) is almost
on the same level as DC conductivity, they probably even did not expect that a huge
absorption band exists in the gigahertz-to-IR region. Nor did they know that the
microwave part of the spectrum is almost unaffected by the electrolyte concentration
(see Sect. 5.1.2), and thus were unable to include high-frequency mechanisms in
their considerations. Maybe if they had, the theory of electrolytes would be more
general and completely different.
Mendeleev did not agree with the postulates by Arrhenius [6, 7]. His opinion was
based on the experience of chemists, who dealt with chemical kinetics, and the idea
of the physical concept of the electrolytes, where ionic species exist as spherical
particles and do not interact with the solvent (water) was controversial. He noted [8]
that rather than accepting that for the salt MX the dissociation of its particles into ionic
species M+X, we should first, following our knowledge on the nature of electrolytes,
look for aqueous electrolytes of the salt MX in their interaction with H 2 O, which
gives MOH + HX. In other words, Mendeleev’s idea was to account for local chemical
reactions and the chemical exchange between the species of solute and solvent. His
ideas and arguments were actively supported by Armstrong, Crompton, Pickering,
Traube, Wiedemann, and others. Nevertheless, Arrhenius [9], enlisting the support
of Ostwald and Van ’t Hoff, defended himself by the argument that the isotonic
coefficient i
4 can be explained by his theory of electrolytic dissociation. Providing
his arguments, he only rejected that part of Mendeleev’s idea related to clathrates but
was not able to appreciate the pivotal idea of the ion–molecular exchange interaction
proposed by Mendeleev, which would significantly move forward the understanding
of the dynamic structure of aqueous electrolytes.
There several points that show the imperfection of the theory of Arrhenius (and all
subsequent theories on this basis). First, the theory does not explain the mechanism of
dissolution. It operates with the already dissolved ionic species without explanation
of where water gets the huge amount of energy (hundreds of kJ/mol) required to
separate the ions of the initially neutral solvent. The second problem is that the
theory works for very diluted electrolytes (usually below 0.001 mol/l) but fails to
reproduce results for higher concentrations (even accounting for the Debye–Hückel
amendments [10]). The third problem is that the model of Arrhenius does not account
4 The isotonic coefficient (or van ’t Hoff’s coefficient) is the proportionality coefficient between the
total concentration of a solute and the concentration of solute particles.