Chemistry Reference
In-Depth Information
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Figure 2.19
Effect of temperature on CD intensity at four different concentrations
(left, data presented with a vertical offset for clarity - experimental data
shown as points, modelling to Oosawa-Kasai model shown as lines) and
the proposed mechanism of chiral self-assembly and disassembly (right).
Figure adapted from ref. 123 with kind permission of Science.
chiral organisation of IR-active units. This is a particularly powerful way to
study chiral hydrogen-bonding pathways that often drive organogelation and
experience a chiral environment within the self-assembled nanofibre. This
methodology can also been applied in combination with variable-temperature
methods in order to monitor the effect of temperature on self-assembly.
2.7 Characterising Solvent Effects in Gels
Gel-phase materials are usually constituted of ca. 99% of solvent, and in some
cases significantly more. Given that the solvent is the major component of a gel,
it can therefore play a vital role in controlling the material's performance and
properties. As such, the study of solvent effects on gelation is an important
topic, as understanding how solvent and self-assembled fibres interface can
provide significant insights into gelation. In many cases, researchers will try to
correlate a specific experimental parameter associated with the gel, with some
physical-organic characteristics of the solvent. Most commonly, authors will
simply attempt to correlate (i) whether gelation occurs, (ii) the minimum gel-
ation concentrations and/or (iii) the T
gel
value of the soft material - however, in
some cases other characteristics will be considered (such as the spectroscopic
shifts, etc.). Many authors attempt to understand gelation in terms of bulk
solvent parameters such as the dielectric constant.
82
However, such consider-
ations do not properly account for direct interactions between solvent and
gelator - which are important in mediating gelation. As such, other studies
have attempted to correlate solvent properties that allow for such interactions,
such as the Reichardt, Hildebrand and Hansen parameters,
43,129-133
and
Kamlet-Taft parameters.
134-136
Dissolution enthalpies and entropies have also
been used as a predictor for gelation in a given solvent - these are in some way,
experimentally derivable surrogates for the physical organic solvent parameters
but applied to a specific gelator.
137
The use of physical-organic solvent parameters can allow authors to dissect
the relative importance of solvent polarity and hydrogen-bonding effects in
terms of allowing gelation to take place. In general terms, solvent-gelator
3
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