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of measurable cation coextraction indicates that this anionic complex is exchanged
for the anionic component of the IL (i.e., Tf
2
N
-
).
In contrast, in a subsequent study of the extraction of uranium by
bis
-(2-ethyl-
hexyl)phosphoric acid (HDEHP) and of Ans (e.g., Am
3 +
) a nd la nt ha n ides by Cya nex
272, Cocalia et al.
92
found through a combination of radiotracer partitioning mea-
surements, UV-visible spectroscopy, and EXAFS investigations that both the gen-
eral extraction behavior and metal ion coordination environment of the investigated
ions are the same in either [C
10
mim
+
][Tf
2
N
-
] or dodecane. In either solvent, for
example, uranyl ion is extracted by a pair of H-bonded extractant dimers, while
the extraction of Am requires three such monodeprotonated dimers. Thus, despite
obvious differences in structure between dodecane and the IL, in this instance, the
two diluents behave nearly identically in many respects as extraction solvents. It is
important to note, however, that higher metal ion distribution ratios were observed
in dodecane than in the IL. (In the case of Am extraction from dilute nitric acid,
the difference was nearly a factor of 10.) Thus, the unique solvation environment of
the IL does not necessarily lead to enhanced extraction efficiency. Equally impor-
tant to note is the fact that this study employed a relatively hydrophobic IL, one for
which prior work with strontium extraction by DCH18C6 (described above) would
have suggested that similarities to the behavior of a conventional solvent are to be
expected. Taken together with the results of Jensen
91
, these results suggest that the
mechanistic complications (i.e., ion exchange and the concomitant IL solubiliza-
tion losses) arising from the use of ILs bearing relatively short alkyl side chains as
diluents (e.g., C
n
mim
+
, with
n
≤ 8) are not confined to systems employing neutral
extractants.
11.3.5 t
a S k
-S
P e C i f i C
i
of n i C
l
i Q u i d S
(tSil
S
)
In contrast to conventional SX systems, in which the solvent (diluent) and extractant
constitute distinct entities, ILs offer the possibility of incorporating an extracting
moiety into either the cationic or anionic component of the solvent. Since the first
description of such “task-specific” ILs (TSILs) by Davis et al.
93
in 2001, a number of
examples of this unique class of solvents have been prepared and characterized,
94,95
among them several TSILs capable of the extraction of Ans.
96-98
Davis, for exam-
ple, has described the synthesis of a phosphonamide IL in which a phosphine oxide
group (such as that seen in CMPO) is appended to the imidazolium cation. Although
the extraction of tri-, tetra-, and hexavalent Ans using the hexafluorophosphate form
of the TSIL is very efficient, it has also been found to be both nonselective and pH
independent,
96
clearly problematic from the perspective of the design of a workable
extraction process. Subsequent work by Ouadi et al
.
97
also employed an imidazolium
substructure onto which was grafted an extracting moiety, in this case, a 2-hydroxy-
benzylamine functionality. Evaluation of the resultant TSIL as an extractant for
americium, either neat or as a solution in C
4
mim
+
Tf
2
N
-
, showed that maximum
extraction is achieved at a pH of ca. 10. At lower pH values, the TSIL is mainly in
its LH
3
+
(acidic) form, while at higher pH, the solubility of the TSIL in the aqueous
phase becomes significant. In both cases, the result is reduced extraction efficiency.
Thus, the TSIL is not suitable for the extraction of Am (and by analogy other Ans)
