Chemistry Reference
In-Depth Information
Three main effects are universal and do not depend on the system studied. The
favorable effect of a cation on third-phase formation is measured by the slope of the
energy of attraction between the reverse micelles plotted versus the cation concentra-
tion in the organic phase or the total nitrate concentration for different salt. Whatever
the nature of the extracted cations, third-phase formation is observed when the energy
of attraction is near 2
k
B
T
. Finally, the tendency toward phase splitting correlates well
with the hydration enthalpy of the cations.
7.3.2.2 extraction of Inorganic Acids and Polarizability
Inorganic acids are not equal in promoting third-phase formation, as the phenom-
enon is strongly dependent on the nature of the anions. Condamines et al. (
111
) have
shown that inorganic acids such as HClO
4
, H
2
SO
4
, H
3
PO
4
, and HCl are less extracted
than HNO
3
by dialkylamide diluted in alkane. This is surprising in comparison with
the classical Hofmeister series (
112, 113
), HNO
3
should not be the best extracted in
the series given according to Hofmeister. They explained this by the hydrophobicity
of the amide, which prevents the coextraction of water. Inversions in the Hofmeister
series are also observed in protein separations and are related to the polarizability of
the “active” site (
114
). Nigond et al. (
66
) have observed that HClO
4
is more effective
than HNO
3
in promoting a third phase in the amide-TPH system. For TBP systems,
Chiarizia et al. concluded that HCl is more effective than HNO
3
in forming a third
phase (
26, 27
). In similar systems, it has been shown that HClO
4
is also more effec-
tive than either HNO
3
or HCl in promoting TBP phase splitting (
115, 116
). Thus,
contrary to the case of cations, the tendency toward phase splitting seems not to be
correlated with the hydration enthalpy of the anions (
88
). Chiarizia et al. (
26, 88
)
recently investigated the liquid/liquid extraction of several mineral acids (HNO
3
,
HClO
4
, H
2
SO
4
, HCl, and H
3
PO
4
) by TBP under identical conditions, to compare
the efficiency of the acids in promoting third-phase formation with their specific
properties. They evidenced the important role of coextracted water on the efficiency
of anions in promoting third-phase formation. In an earlier study, Nave et al. (
49
)
described the relation between third-phase formation and polar-core polarizability
for the extraction of two acids, perchloric acid and nitric acid. In the case of nitric
or perchloric acid extraction by TBP in
n
-dodecane, the third phase is obtained at
lower concentration with the most polarizable anion (perchloric acid). Phase splitting
is observed near an acidity of 2 M for HClO
4
and 15 M for HNO
3
for TBP (1.1 M)
in dodecane (Figure 7.16 (b)). SAXS attributes this difference to the higher effec-
tive attraction interaction between polar cores when perchloric acid is extracted.
Figure 7.16 (a) compares the SAXS patterns for TBP (1.1 M) solution contacted with
H
2
O, 2 M HNO
3
, and 0.5 M HClO
4
, respectively. The aggregates formed in the three
cases have the same shape, as all the three plots merge into the same curve at high
q
values, while the large increase in intensity in the low
q
range indicates high osmotic
compressibility and hence strong attractive interparticle interactions for the HClO
4
solution. Reverse micelles containing ClO
4
-
ions thus have a more polarizable polar
core, inducing predominant dispersion forces (van der Waals interactions) between
polar cores, as suggested by Ninham et al. (
117
) (polarizability of ClO
4
-
and NO
3
-
are, respectively, 7.47 and 4.13 Å
3
(
110
)).
