Molecular Gels and their Fibrillar Networks - Soft Fibrillar Materials: Fabrication and Applications - page 28

O

O

H
N

N

O N

H

O

33

O

Figure 1.25 Structure of 33 LMOG . Reproduced from Ref. [87] with permission of The
Royal Society of Chemistry.

In addition, CD and FL have been used to investigate the effect of changing the
relative volume fractions of toluene/CCl 4 mixtures on the packing of naphthalimide
moieties of peptide

s [87]. One of the
naphthalimide gelators is shown in Figure 1.25. The chirality of the packing of
the

LMOG molecules ( 33 ) within their SAFIN

as well.
Clearly, the bulk properties of the liquid mixtures (as modulated by the volume
fractions of toluene and CCl 4 ) and the manner in which each liquid type interacts
with the LMOG molecules as they aggregate in the sol phase upon cooling influence
the eventual packing within the SAFIN s.
In another approach, NMR measurements of solubilities in toluene of 4

33 molecules can be altered by small structural changes to the LMOG

LMOG s

consisting of alkanes with α -and ε -amino acid groups of l-lysinelysine (Figure 1.26,

34-37 ) have been used in a van't Hoff analysis to calculate temperatures at

which the s are completely solubilized at various concentrations [88]. These

temperatures can be related to the CGCs of the

LMOG

s in the liquid components,
and therefore constitute a predictor of several gel properties, such as the so-called
''plateau region'' where the values of T g vary little with

LMOG

concentration.
Others have employed the van't Hoff equation (Equation 1.2 in which

LMOG

H diss
and S diss are the molar enthalpy and entropy for dissolving a solid in a liquid
and Sol is the solubility at T eq ) and related equations such as the Schroder-van
Laar equation [55] to describe the thermodynamic properties of sol-gel transitions
in other systems, but without the same level of detailed analysis. A problem
endemic to the use of such treatments is that they suppose that the sol phases are
''ideal'' solutions; because aggregation frequently persists above T g , they are not.
Nevertheless, in some systems, such as the one mentioned here, interesting and
useful information can be obtained.

R (1.2)
Several attempts to correlate the bulk properties of solvents with the ability
to be gelated by specific gelators have appeared recently. These physical-organic
approaches will be very useful if they can be shown to apply to several structurally
diverse classes of gelators. In addition to permitting researchers to make a priori
assessments of which liquids probably will and will not be gelated by a specific
molecule, they will broaden the scope of the anti-solvent approach to making gels
[44a, 89]. In it, gelation is induced at room temperature by adding a miscible solvent
to a solution in which the gelator is insoluble.

ln ( S ) =− H diss / RT eq + S diss /