Chemistry Reference
In-Depth Information
between their polar cores. The most direct way to evidence this is to estimate the
structure factor at zero angles in a scattering experiment. The osmotic compress-
ibility increase is a direct translation of the attractive interparticle potential (
109
).
Intermicellar interactions lead to third-phase formation under certain conditions as
described above. In the presence of reverse micelles, the molecular speciation of spe-
cies remains complex and relatively unknown. Generally, the authors are interested in
the structure of the organic phases as a function of their compositions rather than as a
function of the complex stoichiometry. The sticky hard-sphere potential approach can
be used if the global composition of the polar core of the micelle is considered with a
spherical shape for the reverse micelle. The limit of the model is obtained if the shape
of the reverse micelle is strongly modified by the extracted salt, as for example a transi-
tion from sphere to rod.
Chiarizia et al. (7, 22, 23) systematically studied the TBP,
n
-dodecane/HNO
3
,
UO
2
(NO
3
)
2
solvent-extraction system using SANS. They observed an increase in the
diameter of the aggregates by increasing the amount of UO
2
(NO
3
)
2
or HNO
3
. This
reflects the swelling of the reverse micelles when solutes are solubilized in the polar
core of the reverse micelles. At the same time, an increase in the short-range attrac-
tion forces between polar cores of the micelles due to dipole-dipole interactions is
also observed. As shown in Figure 7.14 (a) (redrawn from Ref. (7)), the attraction
potential becomes more attractive as the amount of solute increases in the reverse
micelles, and reaches about -1.8
k
B
T
(which corresponds to an effective Hamaker
constant of about 4.3
k
B
T
) at the limit of third-phase formation.
Similar results have been obtained for the diamide/dodecane (
37, 70
) system con-
tacted with an HNO
3
, or UO
2
(NO
3
)
2
, or Nd(NO
3
)
3
aqueous phase (Figure 7.14 (b)).
For all the systems, assuming the spherical reverse micelles are interacting through
(a)
(b)
-1.5
-1.5
-1.55
-1.6
-1.65
-2
-1.7
-1.75
-1.8
0
-2.5
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
Solute
org,eq
Solute
org,eq
FIGURe 7.14
Interaction potential
U
/
k
B
T
versus solute concentration in the organic phase
at equilibrium. (a) The data are obtained from Baxter modeling of SANS data for the TBP
(0.73 M),
n
-dodecane system equilibrated with uranyl nitrate (
n
), or nitric acid (×), or a mix-
ture of both salts ( ) (Redrawn from R. Chiarizia, K. L. Nash, M. P. Jensen, P. Thiyagarajan,
and K. C. Littrell,
Langmuir
, 19: 9592-9599, 2003). (b) The data are obtained from Baxter
modeling of SANS data for the DMDBTDMA, dodecane system equilibrated with (×) nitric
acid or (
n
) uranyl nitrate, LiNO
3
(1 M), HNO
3
(0.01 M), or (∆) neodymium nitrate, LiNO
3
(0.1
M), HNO
3
(0.01 M). (From F. Testard, P. Bauduin, L. Martinet, B. Abécassis, L. Berthon, and
Madic C,
Radiochim. Acta
, 96: 1-8, 2008. With permission.)
