Chemistry Reference
In-Depth Information
the solution separates into a dilute and a concentrated surfactant phase. Cloud-point
phenomena have been modeled (
92
) with the Flory-Huggins theory of polymer solu-
tions. The same approach has been used by Lefrançois et al. (
93
) to interpret and
parameterize the third-phase formation in the extraction of HNO
3
by a malonamide
in dodecane. Reverse micelles, instead of polymers in the Flory-Huggins theory, are
assumed to arrange on a lattice. The interaction parameter
χ
12
can be estimated from
the Hildebrand solubility parameters and is equal to
χ
12
= (
V
1
/RT) (d
1
− d
2
)
2
, where
V
1
is the molar volume of solvent, and di
i
is the solubility parameter of the solvent
(1) or the solute (2). The interaction parameter
χ
12
between aggregates and diluent has
been shown to be correlated with the nitric acid content in the organic phase.
χ
12
is
another way of expressing the penetrating power (
94
) of the diluent in the apolar chain
of the extractants. Lefrançois et al. (
93
) obtained good agreement between the theo-
retical phase-separation curves and the experimental data. By entering the relevant
molecular parameters (molar volume and solubility) in the model, the authors derived
the third-phase formation for different diluents: the more penetrating the diluent, the
higher the ionic strength required to obtain a third phase. As for the model based on
the Baxter approach, this parameter model can be used if the supramolecular struc-
ture with the interaction potential of the solution is known. To our knowledge, the use
of phase-separation theory of polymers for reverse aggregates has not been extended
to other systems, particularly when metal salt or modifiers are added to the system.
7.2.6 e
f f e C t
o n
C
of n d u C t i v i t y
Conductivity measurement is an effective way of following the transitions of the
supramolecular structures in the organic extractant phase. The conductivity of apo-
lar solvent is typically between 10
-10
and 10
-16
µS m
-1
, rising to 1−10 µS m
-1
when
reverse micelles are present in the solvent. Moreover, the conductivity increases with
the clustering or connection of the reverse micelles. The structure of the organic phase
can thus be followed by conductivity measurements (
68, 95, 96
). An increase or a
decrease in the normalized conductivity along dilution lines in a phase diagram indi-
cates a change in the structure of the solution or a change in the interaction between
aggregates. Experimentally, for a given extractant concentration, an increase in the
conductivity is observed when approaching a third phase with increasing amounts of
extracted salt in the organic phase at a given extractant concentration (
83
). Using the
Baxter approximation to describe the structure of the solution allows the observed
behavior of conductivity to be rationalized. The simplest approach is to consider that
the conductivity is proportional to the number of first neighbors, λ, of a given reverse
micelle. This number can be obtained analytically for sticky hard spheres using the
analytical description of the Baxter model (
90
). The comparison between the cal-
culated and observed conductivity is shown in Figure 7.11(a) for the DMDBTDMA,
n
-dodecane system contacted with nitric acid phase. As a rough approximation, it
can be concluded that the conductivity of extractant aggregate solutions is due to ions
exchanging between polar cores of micelles.
Another example with TBP is shown in Figure 7.11(b), where it is shown that
the conductivity of TBP-dodecane solution equilibrated with nitric acid aqueous phase
decreases as the temperature rises. The cmc is known to increase with temperature
