Handbook of Solvents - George Wypych - ChemTech - Ventech!

550 Y. Y. Fialkov, V. L. Chumak
A small value σ cov in all considered systems can be explained by low energy of interaction
between the filled orbital of cation and anion. On the contrary, contribution of ion-ion
interaction to the process energy leads to high value σ el in comparison with the covalent
component even in media of high permittivity (not to mention media of low permittivities).
From [9.106], isodielectric solution formed by two mixed solvents I and II follows the
relationship:
( ) ( )
ln K = a / a ln K − a / a a + a
aI , 01, I 01, II aII , 01, I 01, II 00, II 00, I [9.108]
where:
00, 01 subscripts of coefficients of approximation for dependence on temperature and
permittivity
I, II subscripts for solvents I and II
The leveling (leveling solvent reduces differences between constants of ion association)
or differentiating (differentiating solvent enhances these differences) effect of the first
mixed solvent is defined by the ratio of coefficients of equation [9.106] or by the ratio of
el
vacuum electrostatic component of free energy of association process, δΔG ε=1.
One can calculate ratio a01, PC-DCB/a01, PC-Py =1.36 using the data from Tables 9.10 and
9.11 and assess the leveling effect of the solvent propylene carbonate-pyridine on electrolyte
strength. The ratio aPC-DCB/aPC-HAc = 3.1 indicates that propylene carbonate-acetic acid
has more pronounced leveling effect on electrolyte strength than propylene carbonate-pyridine.
Table 9.11. The components of free energy change (kJ mol -1 ) in ion association
process of different salts in conditionally-universal media at 298.15K
-σ el
-σ cov el
σε=1 Solvent
At max. ε
Lithium bromide
At min. ε
Propylene carbonate-o-dichlorobenzene 5.8 37.7 1.6 377
Propylene carbonate-pyridine 4.2 22.9 3.2 277
Propylene carbonate-acetic acid 1.9 20.0 4.6 122
Acetonitrile-chlorobenzene
Potassium thiocyanate
5.7 37.1 5.1 208
Propylene carbonate-chlorobenzene 3.2 37.9 2.7 212
DMSO-chlorobenzene 4.0 33.1 2.2 185
ε change of the solvent and universal media affect the ion concentration in solution.
The ion concentration in 0.1M solution of LiBr in the mixed solvent propylene carbonateo-dichlorobenzene
varies relative to its analytical concentration from 50% in propylene carbonate
to 0.1% in dichlorobenzene. The ion concentration in 0.1M solution of KSCN in the
solvent propylene carbonate-chlorobenzene varies from 60% in propylene carbonate to
0.06% in chlorobenzene.