Handbook of Solvents - George Wypych - ChemTech - Ventech!

9.4 Mixed solvent influence on the chemical equilibrium 541
Padova 36 has developed this approach to non-ideal solutions. He has proposed an
equation based on electrostatic interaction which relates molar fractions of the components
(xB - in the mixed solvent and x′ B - in the solvate shell) to the activity coefficient of components
of the binary solvent:
[ ( 1 x ) ( 1 x ) ]
α = ln − ′ / − = lnγ
2
B B B [9.95]
Strengthening or weakening interaction (ion-dipole interaction or dipole-dipole interaction)
of universal solvation leads to re-distribution of molecules in the mixed solvate and
to the change of the composition of solvate shell in contrast to the composition of mixed solvent.
The method for determination of average filling of molecules` coordination sphere of
dissolved substance by molecules of the mixed solvent (with one solvate-inert component)
has been proposed. 37 The local permittivity is related to average filling of molecules’ coordination
sphere expressed by the equation:
where:
ε = ε x′ + ε x′
[9.96]
p A A B B
( ) ( )
x′ = z / z + z ; x′ = z / z + z
[9.97]
A A a B B B A B
where:
zA,zB average numbers of A and B molecules in the first solvate shell
The last equations can be used for development of the next expression permitting to
calculate the relative content of B molecules in the solvate shell
( ε ε ) ( ε ε )
′ = − / −
[9.98]
x B p A B A
Figure 9.11. Selective solvation of 3-aminophthalimid
(A) by n-butanol (B) from the mixed solvent hexanen-butanol.
where:
εp permittivity of binary solvent
εA, εB permittivities of components
Value x′ B can be found from the equation
linking the location of maximum of absorption
band of IR spectrum with refraction
index and ε of the solution.
The data on selective solvation of
3-aminophthalimid by butanol from
butanol-hexane mixture are presented in
Figure 9.11. The data have been calculated
from the equations presented above. Alcohol
content in solvate shell has higher concentration
than in solvent composition even at low
concentration of alcohol in the solvent. For
example, when molar fraction of n-butanol
in the mixture was 7%, the relative molar
fraction of n-butanol in the solvate shell of