Chemistry Reference
In-Depth Information
1.4 Formation of Molecular Gels
Over the years the outcome of many studies on very different kinds of gelator
molecules and their gels has led to a fairly coherent view of gel characteristics
and gel formation from LMW gelators. Molecular gels from LMW gelators are
in most cases not the thermodynamically most stable state. Many gels suffer
from long-term instability by slowly developing into heterogeneous systems
that contain larger crystals in coexistence with the fibrous network.
29
Hence,
the gel state should be regarded as a metastable state, but unfortunately, there
is not yet a conclusive study towards the causes of this experimentally observed
instability. Nevertheless, two possible causes for this instability have been
proposed based on the structure and morphology of the gels. One possible
cause is the difference in surface free energy, due to the large interfacial area of
the fibrous network compared to crystals from the LMW gelators; another
possible cause are differences in lattice energy due to the many defects in the
fibrous network or even a different packing within the fibres, compared to
crystals.
Because most supramolecular gels are a metastable state, their formation and
properties are governed by kinetics and thermodynamics. Several studies have
indicated that the gel properties depend on the rate of cooling, and also recent
studies in which the rate of gel formation is controlled by catalytic action re-
vealed that the gel properties depend on the rate of formation. For gel for-
mation due to cooling, the gelation process proceeds through the following
steps
24
(Figure 1.4): (i) cooling of a solution of gelator to form a supersaturated
solution, (ii) formation of nuclei, (iii) growth of nuclei to fibres, (iv) fibre
bundling and branching, and (v) fibre entanglement to a 3D fibrous network.
The recent work by Meijer et al.
84
revealed that fibre formation by self-
assembly of small molecular building blocks indeed involves a nucleation step,
d
n
1
r
3
n
g
|
1
.
Figure 1.4
Schematic depiction of the process of supramolecular gel formation.
(Reproduced with permission of American Chemical Society from ref. 30).
