Molecular Gels and their Fibrillar Networks - Soft Fibrillar Materials: Fabrication and Applications - page 7

OR

HO OR

OR

RO

RO OH

RO

R = C 16 H 33
4 2 μ m

1 μ m

(a) (b)

(c)

Figure 1.3 Structure of gelator 4 (a), SEM (b) and AFM (c) images from xerogels of 4 .
Reprinted with permission from Ref. [50]. Copyright 2005 Wiley.

a freeze-etching replication, electron-microscopy method (Figure 1.2a) [48]. Albeit
less well resolved, POM can also provide the

structure of an organogel.
Figure 1.2b demonstrates the POM image of a native gel of n -hexatriacontane (

SAFIN

3 )

in silicone oil prepared in a flattened, sealed glass capillary [49].
Fibrillar structures can be clearly visualized from xerogels by SEM and AFM
techniques, as shown in Figure 1.3 with gelator [50]. However, as mentioned

above, a correlation between such images and those of the gel itself should be made
only if supported by additional characterization techniques, such as small angle
scattering (SAS) [51], which relate the SAFIN structure and xerogels.
SAS, including X-rays (SAXS) and neutrons (SANS), is a powerful technique to
provide structure information about native gels. It has been used to provide insights
into many gel structures [52]. As a result of their high intensity, synchrotron sources
can enable characterization of native gels better than conventional X-ray sources.
To perform SANS experiments, either deuterated gelators or deuterated solvents
(or other contrasting liquids) are required. The difficulty to deuterate significant
portions of most gelator structures has resulted, as expected, in the vast majority
of studies being conducted with deuterated liquid components.
SAS is a model-based approach involving extensive mathematical operations;
fortunately, many fitting programs are available. When SAS profiles of a native
gel are obtained, an appropriate model needs to be chosen (e.g., rigid-rod, tubule,
ribbon, or cylinder). Then, comparison is made between the simulated and exper-
imental SAS profiles to validate the chosen model after the fitting parameters for
size, persistence length, and so on, have been optimized. Terech and co-workers
have reported many SAS investigations on gels, revealing the morphology of fibers
as well as their junction zones [53]. For example, gelator

4

[53c] in decane formed
hexagonally packed bundles (from structure factor analysis at large-angle scattering)
of cylinders (from form factor analysis at low-angle scattering) (Figure 1.4). In addi-
tion, a solvent-dependent morphology change to more rectangular ribbon-shaped
objects was observed in 1-alkanols.
Sakurai et al . have employed synchrotron SAXS to support a previously
proposed model [54] for molecular arrangement in a helical fiber of an
azobenzene-cholesterol-based gelator ( 6 ) [52a]. A hollow cylinder model exhibited
better agreement with the experimental SAXS profile than a solid cylinder model,

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