7 A Primer on Gels (with an Emphasis on Molecular Gels)
303
7.3 Molecular Gels and Approaches to Their Analyses
In this chapter, we focus on the structures and properties of molecular gels; the techniques for obtaining the structural information, usually indirect methods such, as
neutron and X-ray scattering, or direct methods such as atomic force and electron
microscopies will not be discussed here. For an excellent coverage of methods to
determine gel structures at different distance scales, see Ref. [29]. Only in a very
limited number of examples have the molecular packing arrangements of gelator
molecules within their gel networks been determined. The difficulties to obtain this
information arise from several factors: (1) many molecular gelators are polymorphous, so that the phase within a gel may be different from that obtained from single
crystal X-ray analyses, even if an appropriate crystal can be grown; (2) the gels
networks are dynamic, so that the molecules within them are not fixed ‘permanently’
in space; (3) the habit within the crystalline network may differ depending on the
liquid from which it is grown; (4) objects within the network may include disordered
liquid molecules; (5) removal of the liquid from a gel risks changing the morph of
the xerogel left behind; (6) intrinsically, the individual gelator objects within the gel
network are very small, despite the gelator molecular sizes, shapes and conformational labilities varying over a wide range. An example of the latter is the family of
acyclic to pentacyclic triterpenes whose structures and gelation properties have been
compared [30]. Despite these difficulties, methods for determining the crystalline
organization of gelator molecules within the gel networks are being developed and
there is promise of others [31]. They rely on solid state (magic-angle) NMR techniques [32], synchrotron radiation analyses [33], and correlations between powder
diffraction patterns from intrinsic gels and derived from single crystal data [34]. Also,
the orientation of gelator molecules within a single gel fiber can be determined, in
principle, from linearly polarized radiation and knowledge of the direction of the
transition dipoles of the chromophores or lumophores [35]. Although this method
holds great promise, it is still in a state of development and will not be applicable to
all molecular gelator assemblies.
The structural analyses have been and continue to be aided by calculations at
different levels of sophistication on single gelator molecules and ensembles of them.
In that regard, density functional theory (DFT), molecular dynamics, statistical
mechanical, and other types of calculations are being used to discern details of association between molecular gelators at the early and latter stages of aggregation, and
to correlate the results of those calculations with experimental observations [36–39].
Even with these data, additional structural challenges will remain. By necessity,
gelator networks must be 3-dimensional in order to immobilize the liquid component
(usually by attractive short-range interfacial and capillary forces and longer-range
repulsive interactions [23, 40]). Those networks rely on a fusion of the objects at
‘junction zones’ (i.e., at the intersection points between the constituent objects).
Even though more information is forthcoming about the shapes and properties of the
objects within the gelator networks, and they constitute the vast majority of the mass
of the networks, very little is known about the nature of the molecular organizations
303
7.3 Molecular Gels and Approaches to Their Analyses
In this chapter, we focus on the structures and properties of molecular gels; the techniques for obtaining the structural information, usually indirect methods such, as
neutron and X-ray scattering, or direct methods such as atomic force and electron
microscopies will not be discussed here. For an excellent coverage of methods to
determine gel structures at different distance scales, see Ref. [29]. Only in a very
limited number of examples have the molecular packing arrangements of gelator
molecules within their gel networks been determined. The difficulties to obtain this
information arise from several factors: (1) many molecular gelators are polymorphous, so that the phase within a gel may be different from that obtained from single
crystal X-ray analyses, even if an appropriate crystal can be grown; (2) the gels
networks are dynamic, so that the molecules within them are not fixed ‘permanently’
in space; (3) the habit within the crystalline network may differ depending on the
liquid from which it is grown; (4) objects within the network may include disordered
liquid molecules; (5) removal of the liquid from a gel risks changing the morph of
the xerogel left behind; (6) intrinsically, the individual gelator objects within the gel
network are very small, despite the gelator molecular sizes, shapes and conformational labilities varying over a wide range. An example of the latter is the family of
acyclic to pentacyclic triterpenes whose structures and gelation properties have been
compared [30]. Despite these difficulties, methods for determining the crystalline
organization of gelator molecules within the gel networks are being developed and
there is promise of others [31]. They rely on solid state (magic-angle) NMR techniques [32], synchrotron radiation analyses [33], and correlations between powder
diffraction patterns from intrinsic gels and derived from single crystal data [34]. Also,
the orientation of gelator molecules within a single gel fiber can be determined, in
principle, from linearly polarized radiation and knowledge of the direction of the
transition dipoles of the chromophores or lumophores [35]. Although this method
holds great promise, it is still in a state of development and will not be applicable to
all molecular gelator assemblies.
The structural analyses have been and continue to be aided by calculations at
different levels of sophistication on single gelator molecules and ensembles of them.
In that regard, density functional theory (DFT), molecular dynamics, statistical
mechanical, and other types of calculations are being used to discern details of association between molecular gelators at the early and latter stages of aggregation, and
to correlate the results of those calculations with experimental observations [36–39].
Even with these data, additional structural challenges will remain. By necessity,
gelator networks must be 3-dimensional in order to immobilize the liquid component
(usually by attractive short-range interfacial and capillary forces and longer-range
repulsive interactions [23, 40]). Those networks rely on a fusion of the objects at
‘junction zones’ (i.e., at the intersection points between the constituent objects).
Even though more information is forthcoming about the shapes and properties of the
objects within the gelator networks, and they constitute the vast majority of the mass
of the networks, very little is known about the nature of the molecular organizations