DOI: 10.1021/la9004714 8803Langmuir 2009, 25(15), 8802–8809
the gel fibers formed.
Already some attempts have been made to
further the understanding of gelators by X-ray crystal data
and elaborate studies have been done to design
nanoscaled materials with these self-assembled systems.
Here, we focus on the relative contribution of two distinct
supramolecular interactions involved in the gelation of solvents
by two classes of LMWGs (bisamide- and bisurea cyclohexyl-
based gelators (Figure 1)) and on how the solvent influences the
contribution of each of these interactions. The molecular scaffold
in both series of LMWGs is comparable, hence they provide an
excellent handle for understanding the relative contribution of the
supramolecular interactions involved in the gelation process.
Both systems consist of a cyclohexane framework on which two
hydrogen bonding moieties are connected at the 1,2 position in a
transoid fashion. The hydrogen bonding motifs (i.e., bisamide
and bisurea) are substituted with a linear alkyl tail. These motifs
are quite common in the design of LMWGs.
The syntheses of bisamide a11
and bisureas u4, u12,andu18
were first reported by Hanabusa et al. in 1996. The ability of
bisamides a7 and a11 and bisureas u4, u12,andu18 to gelate a
number of solvents has also been demonstrated.
classes, the gelation is presumed to be driven by two different
types of intermolecular interactions: van der Waals and hydrogen
bonding interactions. The overall sum of these interactions is
assumed to lead to an anisotropic aggregation orthogonal to the
cyclohexane ring. In the present contribution, we will describe the
results of gelation for a series of these systems with respect to their
structure, thermal stability, and melting enthalpy. The anisotropy
in the system is varied systematically through the length of the
n-alkyl unit and hence also the contributions of van der Waals
interactions and the number of hydrogen bonds (i.e., the bisamide
(2 H-bonds) and bisurea gelators (4 H-bonds). The strength of gels
in different solvents for each series of LMWGs allows for the role
of the solvent, in determining which intermolecular interaction is
primarily responsible for the gel fiber formation, to be identified.
Experimental Sectio n
All solvents and reagents for synthesis were reagent grade or
better and used as received. Solvents for gelation and spectro-
scopy were of HPLC or spectrophotometric grade. Bisamide
gelators a2-a17 were synthesized as described by Hanabusa et
al. for the preparation of bisamide a11.
One equivalent of
(1R,2R)-(-)-1,2-diaminocyclohexane is coupled with 2 equiv of
an alkyl acid chloride using excess of triethylamine in THF at
0 °C. Upon addition of the acid chloride, the reaction mixture
becomes viscous, and by heating the reaction mixture at reflux, the
aggregates break up. The product is purified by multiple washing
steps (the product is only sparely soluble in most organic solvents)
and after drying is isolated as a white solid (yields between 32%
and 93%). The lower isolated yields are due primarily to the
The synthesis of bisurea gelators u4-u18 was carried out
according to the procedure reported earlier for the synthesis of
One equivalent of (1R,2R)-(-)-1,2-diaminocyclohexane was
coupled to 2 equiv of an alkyl isocyanate in toluene. Immediately
after addition of the isocyanate, the reaction mixture became
viscous, and the mixture was heated at reflux to break up the
aggregates. Purification of the product was performed by multiple
washing steps yielding 24-93% of the product as a white powder
after drying. Full experimental details and analyses are provided
as Supporting Information.
Critical Gelation Concentration (cgc) Determinatio n.
compounds are insoluble at room temperature in most of the
solvents examined. Above the critical gelation concentration
(cgc), upon heating, they dissolve, and subsequent cooling to
room temperature results in the formation of gels. To determine
the cgc values for the different compounds, the gels were diluted,
heated, and then cooled to room temperature repeatedly until
either gels did not form upon cooling or the gels were too weak to
withstand gravity. The gels formed were examined after 1 day and
1 week to ensure that aging did not affect the results.
Gel Melting Temperature Determination.
Gels were pre-
pared at least 1 day prior to melting experiments. A stainless steel
ball with a diameter of 2.5 mm (6.238 mg) was placed on the gels
and held at 5 °Ch
, during which its position was monitored
via a video camera. The gel was considered to be melted when the
ball had reached the bottom of the vial. Dropping-ball experi-
ments were carried out at least in duplicate, and the melting
temperatures obtained were reproducible to within (1 °C.
Single-Crystal X-ray Analysis of a3.
Colorless thin platelet-
shaped crystals of a3 were obtained by recrystallization from
1-propanol with slow evaporation of the solvent. A crystal of
0.41 0.29 0.04 mm
, although providing only weak X-ray
scattering, was sufficient to give a final refinement on F
matrix least-squares techniques converged at wR(F
for 1574 reflections and R(F ) = 0.0425 for 1324 reflections with
g 4.0 σ(F
) and 267 parameters and 1 restraint. Because of the
lack of anomalous scatters, absolute structures could not be
determined reliably, although on the basis of the starting ma-
terials used to prepare a3 the configuration of C6 and C10 is
known to be R. For details, see Supporting Information, CCCD
reference number CCDC 717396.
Cyclohexane-based bisamide organogelators a2 to a17
and bisurea organogelators u4 to u18 employed in this study.
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Publication Date (Web): May 19, 2009 | doi: 10.1021/la9004714