Chemistry Reference
In-Depth Information
The adjunction of a modifier thus changes the physicochemical properties of the
extractant and directly influences the maximum solute concentration in the organic
phase (LOC). The role of the modifier is generally attributed to a specific experimen-
tal system and is generally not linked to a particular supramolecular organization.
Abecassis et al. (
89
) studied the impact of octan-1-ol used as a modifier on the self-
assembling properties of the DMDBTDMA in dodecane. Octanol (an H-bonded liq-
uid) is known to form oligomers through intermolecular hydrogen bonds constituting
regions of high electronic density (
130, 131
) compared to the aliphatic parts. Octanol
is not a molecular solution, and its addition to a diamide/alkane solution organized into
reverse micelles must have some consequences on the supramolecular organization.
When small amounts of octanol are added, the organization into reverse micelles is
maintained and a cosurfactant effect is observed. A “cosurfactant” is a molecule that
cannot form micelles by itself in a given solvent, but once a micelle is formed by a sur-
factant or an extractant molecule, a cosurfactant molecule participates in the micellar
structure. The scattering spectra are typical of reverse micelles, but the external specific
surface area of the polar aggregates is modified by the presence of the cosurfactant. The
dynamic properties of the extractant film can be modified with possible consequences
on extraction capacities, but, to date, no systematic studies have been done to relate the
molecular exchange, extraction kinetics, and structural properties of the aggregates.
When octanol is added in large amount and used as a cosolvent, a structural tran-
sition occurs in the molecular organization of the solution. A new structure appears
organized in a hydrogen-bond network that contains the diamide. When present, this
hydrogen-bonded network has a specific SAXS signature (
132-134
). Indeed, upon the
addition of octanol, the scattering spectrum is modified from a classical reverse-mi-
celle spectrum to a spectrum containing a broad peak, characteristic of a mean distance
in the sample between regions of high electronic density, as shown in Figure 7.19.
0.2
0.15
0.1
0.05
0
0.1
0.2
0.3
0.4
q
(Å
-1
)
FIGURe 7.19
Small-angle X-ray scattering spectra of solutions of [DMDOHEMA] = 0.7 M in
various solvents: (×): extractant reverse micelles in dodecane; or extractant dispersed in structured
solvent: (
▲
) for φ
dodecane
/φ
octanol
= 24/76 and (
¢
) octanol. (From F. Testard, P. Bauduin, L. Martinet,
B. Abécassis, L. Berthon, and C. Madic,
Radiochim. Acta
, 96: 1-8, 2008. With permission.)
