Chemistry Reference
In-Depth Information
C
A
D
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B
Figure 5.2
Microscopic aspect of the gel network and schematic representation of a
network's compartment and guest molecules. (A: gelator molecules in the
fibres; B: gelator in solution; C: interacting guest molecules; D: noninter-
acting guest molecules).
On the other hand, the presence of the fibrillar network introduces a barrier
for diffusion of molecules through the gel that will be slow compared to dif-
fusion in pure solvent. Quite often, gels are loaded with other compounds by
adding a supernatant solution and allowing free diffusion. Even in the absence
of strong intermolecular interactions between the added compounds and the gel
phase, diffusion of these solutions may take several hours. For some appli-
cations this may not be the ideal case. Alternatively, additives may be en-
trapped during the gelation process by codissolution with the gelators (in situ
loading). In such a case they must not interfere with the gelator self-assembly,
otherwise the gel may even be destroyed.
What happens once a guest molecule has reached a solvent compartment
within the gel network? As is schematised in Figure 5.2, guest molecules will be
distributed between the solution and the gel phases depending on their relative
anities.
It has been shown that noninteracting guests within the solvent pools held by
the gel network behave as if they were in true independent solutions. For in-
stance, photochemical quenching rates, which measure diffusion rates in the
range of nanometres, showed that molecules trapped into solvent pools of a
hyaluronan gel diffuse as fast as in solution.
13
A similar behaviour has been
described by Galindo and coworkers for a molecular gel.
14
On the other hand,
Hanabusa and coworkers have studied organogel electrolytes and have found
that the ionic conductivity is only slightly affected by the presence of the gel
network.
15
Furthermore, Duncan and Whitten have studied molecular gels by
NMR and reported that the line width of NMR solvent signals is not affected
by gel formation, revealing that the macroscopic viscosity increase does not
affect the tumbling rates of the solvent molecules.
16
In our group we have used NMR in order to study interaction and molecular
recognition of small molecules in molecular gels and we have also observed that
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