Thermoreversible gelation of organic liquids by arylcyclohexanol derivatives: Synthesis and characterisation of the gels

Article abstract of DOI:10.1039/a801922c

A variety of organic liquids can be immobilised using certain 4-tert-butyl-1-arylcyclohexanol derivatives (BACOl). Only the isomers with axial aryl groups are active as gelling agents. The BACOl-hydrocarbon systems have been characterised with respect to their rheological properties under shear. The gels are viscoelastic solids with high elastic shear moduli (G′ ≈ 70350 Pa at C ≈ 2.2 wt.% in dodecane) and high yield stresses (σ ≈ 620 Pa). A 3D network with high cohesive energy and long lifetimes of the related structures is in thermal equilibrium with the surrounding solution. Diffraction experiments have characterised the crystallinity of the gel networks made up of assemblies of bimolecular BACOl associations (d ≈ 14.3 A). Xerogels exhibit a mesomorphic organisation with a 76 A repeating unit. Phase diagrams have been determined as a function of the solvent and gelator types and the related thermodynamic parameters were deduced. IR spectroscopy has demonstrated that H-bonding is responsible for the aggregation of the BACOl molecules.

Full text of DOI:10.1039/a801922c

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Thermoreversible gelation of organic liquids by arylcyclohexanol
derivatives
Synthesis and characterisation of the gels
Charles M. Garner,a Pierre Terech,b* Jean-Jacques Allegraud,b Brian Mistrot,a Phuc Nguyen,a
Arnaud de Geyerb and Daniel Riveraa
a Department of Chemistry, Baylor University, W aco, T exas 76798, USA
b L aboratoire Physico-Chimie Mole
Departement de Recherche Fondamentale sur la Matie
Martyrs, 38054 Grenoble Cedex 09, France

culaire, UMR 5819 CEA-CNRS-Universite

J. Fourier,

`
re Condensee, C.E.A.-Grenoble, 17, rue des

A variety of organic liquids can be immobilised using certain 4-tert-butyl-1-arylcyclohexanol derivatives (BACOl). Only the
isomers with axial aryl groups are active as gelling agents. The BACOlÈhydrocarbon systems have been characterised with
respect to their rheological properties under shear. The gels are viscoelastic solids with high elastic shear moduli (G@ B 70350 Pa
at C B 2.2 wt.% in dodecane) and high yield stresses (p B 620 Pa). A 3D network with high cohesive energy and long lifetimes of
the related structures is in thermal equilibrium with the surrounding solution. Di†raction experiments have characterised the
crystallinity of the gel networks made up of assemblies of bimolecular BACOl associations (d B 14.3 Ó). Xerogels exhibit a
mesomorphic organisation with a 76 Ó repeating unit. Phase diagrams have been determined as a function of the solvent and
gelator types and the related thermodynamic parameters were deduced. IR spectroscopy has demonstrated that H-bonding is
responsible for the aggregation of the BACOl molecules.
azobenzene-appended11 cholesterol based gelators), long-
1
Introduction
chain alkylamide derivatives,12,13 organometallic deriv-
atives,14 sorbitols,15 bisurea derivatives16 and acylated
cellobioses,17 etc. All these are non-ionic molecules which gel
apolar hydrocarbons, unsaturated liquids, chlorinated sol-
vents, nitriles and long-chain alcohols more or less selectively.
A special class of ionic gelators (quaternary ammonium salts
containing a cholestanyl group) is also known to form gels in
alkanes, silicone oil, and alcohols, and one of them is even a
gelator of propanolÈwater mixtures!18 A large number of
experimental observations4 show both that gel formation and
thermal stability of the related gels (as measured by their
Gels are heterogeneous materials known for their spectacular
ability to immobilise certain liquids. This property results
from interfacial interactions between the liquid and a three-
dimensional (3D) solid-like network mainly or totally consti-
tuted with the gelator itself. The thermal stability of the
networks distinguishes the so-called “chemicalÏ gels, which are
irreversible, from the “physicalÏ gels which exhibit a reversible
liquid-to-gel phase transition. Physical gels exhibit a reversible
melting transition from an elastic to a viscous behaviour at
temperatures which can be currently below ca. 150 ¡C. A gel
network can be pictured as a mesh of threads connected by
nodes whose cohesive energy accounts for its thermal stability.
The nodes or junction zones of physical gel networks involve
weak forces, such as van der Waals interactions, which can be
reinforced by electrostatic interactions, such as hydrogen
bonding, electron transfer, ionic interaction, or weak
organometallic coordination bonding.
Besides the well known class of synthetic or natural
macromolecules1 which form reversible gels2 under appropri-
ate concentration and solvent conditions, another important
class of physical gels is constituted by special small molecules.
Examples of low molecular weight compounds which can gel
water3 or organic liquids4 are known. Such small gelator mol-
ecules have speciÐc chemical functionalities responsible for the
autoassociative behaviour operative in certain concentrationÈ
temperatureÈliquid conditions.
melting temperature, abbreviated as T )12 are dependent
GS
upon the type of gelled organic liquids. These phenomenologi-
cal features are far from being understood. Dramatic varia-
tions are frequently19 observed when minor changes are
introduced in the chemical constitution (of either the gelator
or the solvent) of the binary systems. At this stage, the com-
bination of long aliphatic tails with potential site(s) for direc-
tional molecular connections are among the essential criteria
for a molecule to be a gelator. These conditions illustrate the
balance between attractive (i.e. speciÐc bonding mechanisms)
and repulsive conditions (ÐbreÈÐbre interactions) which make
the occurrence of gelation a relatively rare phenomenon in
which unidirectional (1D) crystallisation must occur rather
than the ordinary tridimensional (3D) one.
The present paper reports the study of a family of 4-tert-
butyl-1-arylcyclohexanols, (BACOl, Schemes 1 and 2), which
can form reversible organogels under surprising molecular
conÐguration conditions. The BACOl structure consists of a
phenyl group grafted to a 4-tert-butyl-1-cyclohexanol mol-
ecule: it appears that only the diastereoisomer with the aryl
group in an axial conÐguration is an organogelator while the
equatorial substitution does not lead to gels! BACOl gels also
show a thermal stability clearly dependent upon the liquid
type. The route for the synthesis of BACOl derivatives and the
related solÈgel phase diagrams are determined and analysed.
Gelator concentrations of at least ca. 0.5% are usually
necessary to observe the hardening e†ect of organic liquids.
Apolar organic liquids provide conditions for attenuated
dipolar interactions between non-ionised species in the
absence of the so-called hydrophobic interactions found with
aqueous systems. Di†erent types of small polarised molecules
are available to form physical organogels. These include, for
instance: fatty acids,5h7 steroids,8 aromatic gelators,9
aromaticÈsteroid based gelators (for instance anthryl-10 or
J. Chem. Soc., Faraday T rans., 1998, 94(15), 2173È2179
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chromatography (Harrison Research, Palo Alto, CA) using
100% dichloromethane as eluant, with the trans isomer
eluting after the cis isomer. 1H NMR (CDCl ): 0.77 (s, 9H),
3
0
2
.9È1.1 (m, 2H), 1.1È1.12 (m, 1H), 1.7È1.8 (m, 5H), 2.5È2.6 (m,
H), 7.2È7.3 (m, 1H), 7.3È7.4 (m, 2H), 7.5È7.6 (m, 2H). 13C
NMR (CDCl ): 24.9, 27.6, 32.2, 38.8, 47.8, 73.4, 126.4, 127.4,
3
128.5, 144.3.
trans-4-tert-Butyl-1-(4-Ñuorophenyl)cyclohexanol (3) was
prepared as follows. Magnesium (1.48 g, 61 mmol) was sus-
pended in anhydrous ether (20 ml) and 4-Ñuorobromobenzene
(5.0 ml, 45.5 mmol) was added gradually over a 1 h period,
during which time the initiation of the reaction was evident.
After stirring for an additional 60 min, the Grignard reagent
was cooled to [78 ¡C in a dry-iceÈacetone bath, and a solu-
tion of 4-tert-butylcyclohexanone (5.6 g, 36.3 mmol) in 6 ml of
ether was added dropwise. The reaction mixture was allowed
to warm slowly to room temperature, then treated cautiously
Scheme 1 BACOl derivatives: gelator 1 (trans isomer: phenyl group
is axial); non-gelator 2 (cis isomer: aryl group is equatorial)
The mechanical characterisation of the gels is shown by rheol-
ogy. IR and wide-angle X-ray scattering (WAXS) experiments
provide some further clariÐcation of the molecular aggre-
gation mechanism.
with 10% NH Cl solution (50 ml). An additional 50 ml of
4
ether was added, and the organic phase was separated and
dried over MgSO . Concentration by rotary evaporation
4
yielded Ðrst a gel then, after prolonged exposure to a vacuum,
an o†-white solid, which was powdered with a mortar and
pestle and dried under vacuum to constant weight. This gave
2
Experimental
8
.1 g (yield B 90%) of a solid which by capillary GC analysis
Except as noted, all reagents were used as received. 4-
Fluorobromobenzene, pentaÑuorobromobenzene, magnesium,
phenylmagnesium bromide and 4-tert-butylcyclohexanone
were obtained from Aldrich Chemical Co. Anhydrous diethyl
ether was obtained from Malinkrodt. Preparation and use of
Grignard reagents were entirely under argon. Capillary gas
chromatography was on an SE-54 column (30 m ] 0.25 mm);
the cleanliness of the injection port was important, because
otherwise dehydration of these tertiary benzylic alcohols
would occur to a signiÐcant extent (30% or more). With atten-
tion to this detail, dehydration was not important (\1%).
Proton NMR was at 360 MHz, and carbon NMR spectra
were obtained at either 90 or 75 MHz and Ñuorine spectra
were at 282 MHz.
contained 64% trans (3) and 36% cis isomers. The isomers
were easily separable by chromatography using dichloro-
methane as eluant (the trans isomer eluting after the cis
isomer). 1H NMR (CDCl ): 0.8 (s, 9H), 0.9È1.1 (m, 2H),
3
1.1È1.2 (m, 1H), 1.6È1.8 (m, 4H), 2.0 (s, 1H), 2.4È2.5 (m, 2H),
7.0 (t, 2H, J \ 8.8), 7.49 (dd, 2H, J \ 8.8, 5.4). 13C NMR
(CDCl ): 24.9, 27.5, 32.2, 39.0, 47.7, 73.0, 115.1 (d, J \ 21),
3
128.2 (d, J \ 7), 140.1 (d, J \ 4), 161.4 (d, J \ 244). 19F NMR
(CDCl ): [115.8 (tt, J \ 8.8, 5.4).
3
trans-4-tert-Butyl-1-(pentaÑuorophenyl)cyclohexanol (4) was
prepared as follows. Magnesium (1.31 g, 54 mmol) was sus-
pended in anhydrous ether (10 ml) and pentaÑuorobenzene
(5.0 ml, 40.1 mmol) was added in small amounts until reaction
had commenced, with the remainder added gradually over a 2
h period. After stirring for an additional 2 h, the Grignard
reagent was diluted with additional ether (20 ml). A solution
of 4-tert-butylcyclohexanone (4.9 g, 32 mmol) in 20 ml of ether
was added dropwise to the Grignard reagent with the Ñask
immersed in a room temperature water bath. The reaction
mixture was stirred at room temperature for 1 h, then treated
trans-4-tert-Butyl-1-phenylcyclohexanol (1) was prepared as
previously reported;20 a summary of a typical procedure
follows. A solution of phenylmagnesium bromide (300 mmol)
in 700 ml of anhydrous ether was cooled to [78 ¡C and
treated slowly with a solution of 4-tert-butylcyclohexanone
(
41.6 g, 270 mmol) in 150 ml of ether. After warming slowly to
room temperature, water (40 ml) was cautiously added with
good stirring. Within a few minutes, a precipitate formed,
leaving a clear organic phase. The organic phase was passed
through a 2 cm pad of anhydrous sodium sulfate, and the
precipitated magnesium salts were rinsed with additional
ether. The Ðltrate was concentrated by rotary evaporation to
yield Ðrst a gel, then, after prolonged exposure to a vacuum to
constant weight, 61.5 g, of an o†-white solid (crude
yield B 98%), which contained 62% trans (1) and 38% cis (2)
isomers by capillary GC analysis. The isomers were easily sep-
arable by either column silica chromatography21 or radial
cautiously with 10% NH Cl solution (100 ml). The aqueousÈ
4
organic mixture was Ðltered through a 2 cm pad of silica gel,
and rinsed with an additional 50 ml of ether. The brown
organic phase was separated, washed once with saturated
sodium chloride solution and dried over MgSO . Concentra-
4
tion by rotary evaporation yielded Ðrst a gel then, after pro-
longed exposure to a vacuum, a tan solid. This material could
not reproducibly be analysed by capillary GC because partial
decomposition to 4-tert-butylcyclohexanone was observed to
occur in the GC injection port and was sensitive to the injec-
tion port temperature. However, the cis and trans isomers
were easily separable by either radial or column chromatog-
raphy using 100% as eluant, with the trans isomer 4 eluting
after the cis isomer. 1H NMR (CDCl ): 0.82 (s, 9H), 0.9È1.2
3
(
(
4
m, 3H), 1.55È1.7 (m, 2H), 1.75È1.9 (m, 2H), 2.45 (s, 1H), 2.78
br d, 2H, J \ 13.6). 13C NMR (CDCl ): 25.1, 27.5, 32.2, 40.0,
3
7.2, 76.0, 118.1 (br s), 137.9 (br d, J \ 248), 140.2 (br d,
J \ 254), 146.1 (br d, J \ 250). 19F NMR (CDCl ): [138.2
3
(
m, 2F), [155.9 (tt, 1F, J \ 21, 4), [162.3 (m, 2F).
BACOl is a solid of low solubility in non-polar solvents
cyclohexane, benzene, carbon tetrachloride, etc.). However,
(
with warming, the solid dissolves, and upon subsequent
cooling, a gel results. The turbid appearance of concentrated
gels was sensitive to the solvent type and concentration.
Toluene or benzene gels appeared transparent while concen-
trated gels in heptane or dodecane were turbid. The gel-like
Scheme 2 Substituted aryl groups incorporated in 1-aryl-4-tert-
butylcyclohexanols; only the ring-Ñuorinated compounds 3 and 4 are
organogelators
2174
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samples were colloidal in nature as demonstrated by the
visible trace of an incident laser beam in the gel. Gels were
often stable for months before a solid liquid phase separation
process was observed for some of them (i.e. in heptane).
Rheology experiments, conducted in the linear regime of
deformations, used a Haake RS100 stress rheometer in a fre-
quency range 0.001È61 Hz. A plateÈplate geometry was used
(
20 mm diameter, 0.25 mm gap) with serrated surfaces so as to
prevent sliding due to the liquid Ðlm expelled by certain dilute
samples. In addition to the choice of an organic liquid with a
high boiling point (dodecane: T \ 216 ¡C), a special device
b
was used to limit any solvent evaporation. The temperature
was regulated with an accuracy better than 0.1 ¡C. The gel was
allowed to form between the two plates of the rheometer after
preliminary introduction of the heated solution. After an
equilibration time of ca. 1 h of the gelling system, the measure-
ments of the rheological behaviour were started.
To determine the gel melting temperatures, T , the gels
GS
were prepared in 5 mm tubes closed at one end and capped at
the other with a septum. A small TeÑon-coated magnetic stir
bar was included. The dropping of the stir bar during the very
slow temperature ramp (to which the gel is submitted) indi-
cates the gel melting temperature. For each solvent, the mea-
surement was repeated with three separate samples at each
concentration, and was typically reproducible to ^0.5 ¡C.
WAXS experiments were performed with an Elliot GX 20
rotary anode generator, using the Cu Ka radiation from a
copper anode. X-Rays from a Ðne focus source (100 lm ] 100
lm) were Ka/Kb Ðltered (j \ 1.54 Ó) and 2D focused by
using the total reÑection from two orthogonally curved
mirrors (Ni-coated glass Ñats). A linear gas-Ðlled detector
Fig. 1 ConcentrationÈtemperature phase diagrams ln(/ ) vs. T of
ag
BACOl (compound 1) and Ñuorinated derivative organogels. Each
continuous line separates the Ñuid solution domain (high temperature)
from the solid-like gel domain (low temperature). A, Solvent type
dependence;
… \ heptane,
L \ toluene,
) \ ether,
> \
dichloromethane, ] \ ethyl acetate. B, Comparison of BACOl
gelling agents in heptane (numbers refer to structures as in Scheme 1
and 2); L \ (3) p-Ñuorophenyl, … \ phenyl (1), ) \ (4) pentaÑuo-
rophenyl. The bold cross ( ] ) indicates the melting point (T \ 308.8
K) of a gel made up of a mixture (C \ 1.375 wt.%) of the gelling
isomer (1, 51%) and non-gelling isomer (2, 49%).
(
4000 channels) was used at a sample-detector distance l \ 193
mm and the angular range was 0.9¡ \ 2h \ 10.4¡
corresponding to a transfer momentum range Q \ 4p/j(sin h)
[
0
tallisation does not intervene. The hardening of organic
liquids is not a†ected by the presence of the inactive isomer in
the system (Fig. 1B). The gelator can “crystalliseÏ to a Ðbrous
solid if a germination process is induced by either seeding,
evaporation of the solvent or possibly by mechanical agita-
tion. However, crystallisation is easily avoided, particularly in
low volatility solvents (e.g. mineral oil). Even when crys-
tallisation is induced, it is exceedingly slow in the gelled state
and generally requires gentle warming. Among the liquids
studied, only methanol and ethanol could not be gelled with
BACOl since crystallisation occurred immediately.
In order to foster the understanding of the mechanisms that
underlie the molecular aggregation/gelation phenomena, we
investigated the structural variations which could be accom-
plished without losing the gelation properties associated with
compound 1. To this purpose, a variety of derivatives with
substituted aromatic rings were prepared (Scheme 2). The
BACOl system proved to be quite sensitive to such structural
variations: of all the derivatives mentioned in Scheme 2, only
the ring-Ñuorinated compounds 3 and 4 (diastereoisomers
having an axial aryl group) showed a gelation ability. It was
impossible to obtain any of the “gel activeÏ diastereoisomers as
single crystals suitable for X-ray analysis; “crystallisationÏ
results only in very Ðbrous solids. However, the diastereo-
isomers with an equatorial aryl group are crystalline, and the
stereochemistry was conÐrmed by X-ray analysis of the cis-p-
Ñuoro derivative. Fig. 1B shows the phase diagrams for the
BACOl derivatives 1, 3 and 4 in heptane.
.0634 \ Q \ 0.74 Ó~1]. Experimental intensities (in arbitrary
units) were normalised to the detector response by dividing
spectra by a Ñat Ñuorescence signal provided by a 25 lm thick
cobalt foil.
IR studies used a solution (C \ 4 wt.%) of gelling agent 1 in
CCl transferred warm to a variable-temperature IR cell
4
(
Wilmad Glass Co.) with ca. 1 mm pathlength in a Perkin
Elmer 1600 FTIR spectrometer. UV absorption spectra were
obtained (not shown) from a Beckman spectrometer, but
failed to show signiÐcant di†erences between the gelled and
liquid states.
3
Results
In the course of synthesising a series of substituted cyclo-
hexanones, new organic gelling agents were discovered. The
arylcyclohexanol gelators are 4-tert-butyl-1-arylcyclohexanol
derivatives (1 and 2, abbreviated to BACOl), which are pre-
pared as shown in Scheme 1. The gelation phenomenon was
mentioned20 many years ago but not studied. The trans
isomer 1 predominates in an approximately 65 : 35 ratio,
depending on the reaction temperature. Interestingly, only the
trans diastereoisomer with the axial aryl group (1), which is
also the more polar, exhibits a gelation ability. The gel is
stable to the presence of additional hydrocarbon solvents or
water and liqueÐes with a well deÐned “melting pointÏ T
GS
upon heating. T depends both on the polarity and type of
GS
A rheological characterisation of the BACOlÈdodecane
system is shown in Fig. 2 and 3. Fig. 2A displays the viscous
(GA) and elastic (G@) responses of a gel (C \ 2.2 wt.%) sub-
mitted to an oscillatory stress with constant amplitude (p \ 10
Pa) in the linear regime of deformations. The rheological
parameters are calculated22 respectively from the phase lag
between the applied shear stress and the related Ñow and from
the ratio of the amplitudes of the imposed oscillation to the
response of the gel. G@, GA, g* (complex viscosity) are linked
organic liquid and on the concentration of the gelling agent:
less polar solvents are immobilised at lower concentrations,
and higher concentrations of 1 always result in higher melting
points. For example, heptane is immobilised at room tem-
perature with only 0.7 wt.% of the gelling agent. Fig. 1A
shows the relationship between the melting points of several
gelled liquids and the concentrations of the gelator. The
process is completely reversible with warming, provided crys-
J. Chem. Soc., Faraday T rans., 1998, V ol. 94
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Fig. 4 WAXS patterns of BACOl crystalline powder (curve 1), a gel
in dodecane (curve 2) at C \ 10.0 wt.%, a xerogel (curve 3) obtained
from a cyclohexane gel. The bold arrow points at Q B 0.44 Ó~1 corre-
sponding to the bimolecular distance ca. 14.3 Ó. For the xerogel
pattern (curve 3), dotted arrows indicate three additional di†raction
features at Q B 0.0825 Ó~1 (B ), Q B 0.165 Ó~1 (B ) and Q B 0.44
1
2
Ó~1 (B ).
3
The structural investigation was pursued towards smaller
length-scales using the IR and UV spectroscopies. Since
BACOl is an hydroxylated compound (Scheme 1), it was rea-
sonable to expect the participation of H-bonds in the mecha-
nism of molecular aggregation leading to the gels. In the
Fig. 2 Rheology of the BACOl (1)Èdodecane system (C \ 2.2 wt.%).
A, Dynamic experiment: a constant oscillatory stress (p \ 10 Pa,
OwH stretching region (Fig. 5), a gel at C \ 4 wt.% in CCl
1
0~3 \ l \ 61 Hz) is applied to the organogel in the linear regime of
4
deformations at T \ 20 ¡C. The elastic modulus G@ (…) and the loss
modulus GA (L) are shown. B, Flow experiment: the gel sandwich is
submitted to a constantly increasing stress p (dp/dt B 0.15 Pa s~1),
while its deformation c (proportional to the angular displacement) is
measured. The gel starts to Ñow at p* B 620 Pa.
exhibited three broad absorptions (3189, 3280, 3450 cm~1;
intensity I estimated by the area of the absorption bands is ca.
8
0% of total OH bands) and a narrow peak (3603 cm~1,
I B 20%). Upon warming above the melting point, only the
3
increasing signiÐcantly (I B 50% at 80 ¡C) as the temperature
increased.
450 and 3603 cm~1 absorptions remained, with the latter
together as follows o g*(u) o \ [[G@2(u) ] GA2(u)]1@2/u]1@2
where u is the angular frequency (u \ 2pl). At this concentra-
tion, the elastic shear modulus G@ (70350 Pa), at l \ 1 Hz, is
much greater than GA (3202 Pa) and about constant over at
least 4 frequency decades in the 10~3È10 Hz range. Fig. 2B
shows a Ñow experiment with a BACOlÈdodecane system at
C \ 2.2 wt.%: no signiÐcant Ñow occurs for stresses below a
limiting stress p* B 620 Pa. Fig. 3 illustrates the thermal
reversibility of the sol to gel and gel to sol phase transitions as
probed by rheology. Related values of the transition tem-
peratures, T \ 28.7 ¡C and T \ 45.7 ¡C, can be estimated
4 Analysis and Discussion
The solid-like consistency of the BACOlÈorganic liquid
systems is typical of a self-supported gel. The recognition of
SG
GS
and a large hysteresis e†ect is clearly observed.
Wide-angle X-ray di†raction experiments (WAXS) were
employed to characterise the crystallinity of the systems. Fig.
4
presents the di†raction patterns of a BACOl crystalline
powder (curve 1) used to prepare the gels, a xerogel (curve 2)
obtained from a cyclohexane gel and a gel in dodecane at
C \ 10 wt.% (curve 3).
Fig. 3 Thermal reversibility of the sol to gel phase transition of a
BACOlÈdodecane system (C \ 2.2 wt.%). The elastic modulus G@ is
recorded in an oscillatory experiment (p \ 10 Pa and l \ 1 Hz) while
the temperature T is varied (dT /dt B 0.83 ¡C min~1) in the range 20È
Fig. 5 IR spectra of a BACOl organogel in CCl (C \ 4 wt.%) at
4
7
0 ¡C. Bold vertical arrows are for the transitions: melting (…), (I,
increasing temperatures. 1: T \ 29.0 ¡C (gel phase); 2: T \ 40.0 ¡C; 3:
T
B 45.7 ¡C) and subsequent re-gelation on cooling (L), (II, T
B
T \ 80.0 ¡C (liquid phase). The absorption in the 3000 cm~1 region is
GS
8.7 ¡C). The rheograms delimit the gel and sol domain.
SG
2
due to CH stretching vibrations.
2176
J. Chem. Soc., Faraday T rans., 1998, V ol. 94

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the gel state can be formalised by taking advantage of its
rheological properties:23 the storage modulus G@(l) exhibits a
pronounced plateau extending to times of at least 1000 s and
which is signiÐcantly greater than the loss modulus GA(l). The
large G@ value (70350 Pa at l \ 1 Hz and C \ 2.2 wt.%) can
be compared to common values observed for molecular gels
with crystalline 3D networks (i.e. made of rigid Ðbres fused
into crystalline microdomains). For instance, the 12-hydroxy-
stearic acidÈdodecane gel is a solid-like crystalline gel with a
high elasticity G@ B 8220 Pa (while GA B 560 Pa) at C \ 1.27
wt.%. In this case, the high value of the storage modulus was
interpreted as resulting from the existence of crystalline
microdomains7 whose monoclinic symmetry was sensitive to
the solvent type. It has been shown24 that with idealised net-
works in which frozen crosslinks are permanent and rigid (i.e.
not freely hinged) and chains are straight, the elastic modulus
scales as the second power of the volume fraction (i.e.
G@ B /2). Taking into account the concentration di†erence,
the value G@ \ 70350 Pa for the BACOl system must be com-
pared to the value G@ B 24666 Pa of the fatty acid “reference
systemÏ. A signiÐcant di†erence exists which may reveal the
speciÐcity of the BACOl molecular aggregation mechanism in
relation to the bending energy of the constitutive rigid rods.
The large values of the storage elastic modulus and yield
stress indicate that interactions between the BACOl colloidal
aggregates are energetic associations able to form long-lived
rigid networks. The present rheology experiments qualify the
BACOlÈhydrocarbon systems as viscoelastic solid-like gels.
Gelation of organic solutions by BACOl is a thermorevers-
ible transition and exhibits a large hysteresis e†ect (ca. 20 ¡C)
as shown in Fig. 3. Hysteresis is a common feature with the
class of low-mass organogels. Close analogies exist between
gelation and the crystallisation behaviour of supersaturated
solutions. Crystallisation results from two simultaneous pro-
cesses, the formation of nuclei of the new phase and the
growth of crystals which is frequently a Ðrst-order process,
specially when involving hydrogen bond connections. The ger-
mination reactions of the sol to gel phase transition can be
kinetically the limiting steps compared to the speed of the
imposed temperature sweep experiment as revealed by the
large temperature di†erence observed with Fig. 3 (ca. 17 ¡C).
The shearing stress can also a†ect the aggregation process (i.e.
through orientational e†ects) as suggested by the incomplete
elastic recovering observed at T \ 20 ¡C during the sol to gel
transition of Fig. 3 (Cycle II, vertical axis).
The phase diagrams of the BACOlÈhydrocarbon systems
are shown in Fig. 1 in a speciÐc representation ln (/ ) vs. T
ag
(/ being the unit mole fraction of gelator at temperature T ).
ag
The speciÐc representation is convenient when thermodyna-
mic parameters are to be determined since they are pro-
portional to the slope L ln / /LT [see eqn. (1)]. It also
ag
facilitates the distinction of di†erent groups of auto-
associative systems according to their enthalpic values. Thus,
three groups of liquids can be distinguished: heptane, toluene
and a group with ether, ethyl acetate and dichloromethane.
Heptane gels have a higher melting point while gels with
larger alkanes exhibit increasingly higher melting points at a
Ðxed concentration (Fig. 6). Thermodynamic parameters of
the aggregate formation can be evaluated using eqn.
(1).12,22,25 T is the absolute temperature, *G¡ is the standard
ag
free energy, *H¡ is the standard free enthalpy and *S¡ is the
ag
ag
entropy of formation of the aggregates through a phase
separation process. Interestingly, the growing aggregates of
this class of low-mass gelators have been shown to be Ðbres
with polydisperse lengths as they result from thermal equi-
libria while their cross-sections can be frequently mono-
disperse (with Ðnite cross-sectional aggregation numbers).7
*
G¡ \ RT ln(/ )
(1a)
ag
ag
A
L ln(/ )
LT
B
*H¡ \ [RT 2
ag
(1b)
ag
A
L ln(/ )
B
*
S¡ \ [R ln(/ ) [ RT
ag
(1c)
ag ag
LT
The thermoreversible gelation transition occurs during the
course of the molecular aggregation reactions forming Ðbrillar
structures. The thermodynamic parameters which describe the
aggregation also describe the physical gelation from small
molecules. Tables 1 and 2 show that the enthalpic contribu-
tion compensates for the entropic change in all investigated
situations (liquid or gelator changes). The standard enthalpy
Table 1 Gel/sol phase diagrams [ln(/ ) vs. T ] of BACOl in hydrocarbons
ag
L ln(/ )a,b
LT
solvent
ag
*H¡ c/kJ mol~1
ln(/ )a
*S¡ d/J K~1 mol~1
m
ag
m
heptane
toluene
ether
dichloromethane
ethyl acetate
0.048057
0.053197
0.031005
0.038277
0.029773
[35.6
[39.7
[23.0
[28.5
[22.2
[5.29
[3.88
[3.64
[3.46
[3.67
[75.5
[100.2
[46.8
[66.5
[43.5
a / is the unit mol fraction; T is in K. b L ln(/ )/LT is the slope of the phase diagram
ag ag
(
K~1). c *H¡ is the standard enthalpy at 20 ¡C for the melting transition of the gels. d *S¡
m
m
is the standard entropy at 20 ¡C for the melting transition of the gels.
Table 2 Gel/sol phase diagrams [ln(/ ) vs. T ] of 1, 3 and 4 derivatives in heptane
ag
L ln(/ )b,c
LT
derivativea
ag
*H¡ d/kJ mol~1
ln(/ )b
*S¡ e/J K~1 mol~1
m
ag
m
1
3
4
0.048221
0.047386
0.0546
[36.0
[35.2
[40.6
[5.75
[5.73
[5.01
[72.0
[70.3
[94.2
a Numbers refer to the formulae in Schemes 1 and 2. b / is the volume fraction; T
ag
is in K. c L ln(/ )/LT is the slope of the phase diagram (K~1). d *H¡ is the stan-
ag
m
dard enthalpy at 20 ¡C for the melting transition of the gels. e *S¡ is the standard
m
entropy at 20 ¡C for the melting transition of the gels.
J. Chem. Soc., Faraday T rans., 1998, V ol. 94
2177

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of melting is dependent upon the liquid type which might
reveal a sensitivity of the BACOl structures with the liquid
type in a similar way to the polytypism phenomenon encoun-
tered with long-chain derivatives26 such as the mentioned
“
referenceÏ fatty acid (vide supra). Due to the chemical consti-
tution of BACOl, it is reasonable to expect the participation
of hydroxy groups in the aggregation mechanism (vide infra).
A typical value for hydrogen bonding is of the order of
*
H B 16È21 kJ mol~1 27 which suggests that the interaction
which makes *H negative might arise not only from hydrogen
bonding between the hydroxy groups (a contribution from
dipolar interactions between the aromatic polarised rings and
van der Waals interactions may also be required).
Fig. 6 Melting temperatures T
of gels (C \ 1 wt.%) of straight-
GS
chain hydrocarbons as a function of carbon number n in n-alkanes.
A comparison of the phase diagrams for the three gelators
Alkanes (C H
) are indicated by C labels. The value for a mineral
n
2n`2
n
oil (]) is forced to lie on the best straight line (0.926n ] 28.6) by
1, 3 and 4 (Fig. 1B and Table 2) shows that the standard free
assuming 25 carbons.
energy is comparable and suggests that the mechanisms of
aggregation and the related degrees of molecular ordering
within the aggregates are similar. WAXS experiments (Fig. 4)
show that the molecular structure of the BACOl crystal and
xerogel present both similarities and di†erences. An intense
and narrow Bragg peak at Q B 0.44 Ó~1 is common to the
two solids (curves 1 and 3) and can also be distinguished with
the organogel in dodecane (curve 2). The corresponding reti-
cular distance is ca. 14.3 Ó which is approximately the BACOl
bimolecular length (2 ] 6.6 Ó). It can be concluded that the
BACOl networks include crystalline-like heterogeneities made
up of organised associations of bimolecules. These micro-
domains can be assimilated with the junction zones of the
BACOl elastic networks. It is remarkable to observe that most
of the typical di†raction features observable on the pattern of
the crystalline solid (curve 1) can be distinguished in the
pattern of the dodecane gel (curve 2). For instance, two addi-
tional intense peaks of the crystalline powder at Q B 0.521
Ó~1 and Q B 0.546 Ó~1 are also emerging in the gel pattern
and appear broadened. BACOl colloids and the crystalline
powder are constituted of bimolecular associations. The
xerogel is further organised over much larger distances as
3450 cm~1. Simultaneously, the amplitudes of the bands
typical of the large aggregates are decreased (3280, 3189 cm~1
bands). The band at 3450 cm~1 keeps a signiÐcant amplitude
even at high temperatures in the sol domain which is consis-
tent with the existence of strong bimolecular associations
which are the building units of the BACOl colloids in the gel
phase. The present phenomenology has already been observed
with di†erent hydroxylated auto-associative molecules.5,29
The enthalpy values of Tables 1 and 2 indicate that H-
bonding is not a unique driving force for the BACOl aggre-
gation reactions. Attractive interactions, such as van der
Waals and dipolar interactions involving polarised BACOl
molecules with r and p electronic densities, are probably addi-
tional components for the free energy. A 3D network of
crystalline-like colloids is formed during the sol to gel phase
transition and exhibits both a lifetime and an elastic shear
modulus revealing the existence of strong interactions. The
solid-like network is in thermal equilibrium (Fig. 1) with the
liquid surrounding medium which contains “isolatedÏ BACOl
species or small non-interconnected aggregates (sol part of the
phase diagram). The thermal hysteresis and the phase separa-
tion behaviours are phenomena which suggest that nucleation
and crystal growth reactions are important parameters for the
growth and subsequent topology of the interconnected
network of colloidal crystalline-like aggregates. It is inter-
esting to note that hydrogen bonding is the most common
aggregation mechanism encountered with hydroxylated or
aminato organogelators. Some non-hydrogen bonding
organogelators are also known to develop Ðbres through
stacking mechanisms between polarised aromatic molecules9
or organometallic coordinated species.14 Di†erent systems
have already shown an aggregation ability and/or structures
of the resulting aggregates a†ected by the spatial conÐgu-
ration at the major site for molecular aggregation of the
gelator. As an example, azobenzene organogelators develop a
marked dependency of their gelation behaviour upon the con-
Ðguration at the C3 of the steroid moiety.11 A second example
is that of organogels made from substituted octadecanoic
acids which show relationships between the optical activity of
the gelator and the helicity of the Ðbre-like aggregates of the
gels.30 In organic liquids, the inÑuence of the molecular con-
Ðguration of the gelator upon the aggregation/gelation capac-
ity is usually not as important7 as it can be with aqueous
analogues, where racemic gelators are known to produce gels
with restricted lifetimes as a consequence of the so-called
“chiral bilayer e†ectÏ.31 Here, with BACOl derivatives, the
conÐguration at the carbon bearing the hydroxy group
remarkably determines the aggregation/gelation behaviour.
Incidentally, a close derivative, 4-tert-butyl-1-phenylcyclohex-
ane,20 has shown that the phenyl group prefers the equatorial
position of the cyclohexane ring (i.e. the trans conformation is
more stable than the cis one by *H \ 15.0 kJ mol~1). If such
a situation is preserved in BACOl diastereoisomers (1 and 2),
characterised by the existence of peaks B (Q B 0.0825 Ó~1),
1
B (broad, Q B 0.165 Ó~1) and B (very broad, 0.3 \ Q \ 0.4
2
3
Ó~1). A repeating distance d \ 2p/0.0825 B 76 Ó is typical of
the mesomorphic organisation of the xerogel. The xerogel
results from the collapse and merging process of the native
solvated colloids (by solvent evaporation) which can form
locally oriented bundles of colloids (most frequently Ðbres).
The distance 76 Ó characterises such basic colloids further
ordered in a symmetry revealed by B and B peaks over rela-
2
3
tively small correlation distances (as suggested by the width of
peaks). Due to the broadness of the B and B peaks, a clear
2
3
identiÐcation of the symmetry of the xerogel mesomorphic
organisation cannot be obtained. Assuming that the typical
distance d \ 76 Ó is the diameter of Ðbrillar colloids, the
cross-sectional molecular organisation appears to be complex
since no simple relation appears to link the bimolecular length
to d. Further experiments are required to characterise the
morphology of the BACOl colloids and will be presented in a
forthcoming paper.28 The increase of T
with the chain
GS
length of the gelled alkanic liquid (Fig. 6) suggests that crys-
tallinity and melting temperatures of BACOl gels can be cor-
related.
IR spectroscopy shows (Fig. 5) that H-bonding is a major
mechanism for the aggregation of BACOl molecules to the
colloids of the gels. The OwH stretching vibration at 3603
cm~1 (narrow) is due to the unaggregated BACOl species
while the two bands at 3189 and 3280 cm~1 (broad) corre-
spond to the stretching of OwH bonded links. An increase of
temperature (in the sol domain) disorganises the BACOl col-
loids made up with associated bimolecules. As a consequence,
the fraction of isolated gelators is increased (enhancement of
the band at 3603 cm~1) as well as the fraction of small n-mers
(i.e. dimers) to which corresponds the increase of the band at
2178 J. Chem. Soc., Faraday T rans., 1998, V ol. 94

View Article Online
it may account for the metastability of 1 organogels despite
the fact that the conversion of 1 to 2 remains impossible.
In conclusion, the thermal and mechanical characterisations
of BACOl organogels have shown that a solid-like network
was formed by bimolecular associations. Crystalline micro-
domains are acting as elastically active junction zones of the
gel networks. The related xerogels exhibit a long-range meso-
morphic organisation with a characteristic repeating unit of
13 K. Hanabusa, M. Yamada, M. Kimura and H. Shirai, Angew.
Chem., Int. Ed. Engl., 1996, 35, 1949.
1
4
P. Terech, C. Chachaty, J. Gaillard and A. M. Giroud-Godquin,
J. Phys. France, 1987, 48, 663.
1
5
S. Yamasaki and H. Tsutsumi, Bull. Chem. Soc. Jp n ., 1994, 67,
906.
16 J. van Esch, S. De Feyter, R. M. Kellog, F. De Schryver and B. L.
Feringa, Chem. Eur. J., 1997, 3, 1238.
1
7
8
N. Ide, T. Fukuda and T. Miyamoto, Bull. Chem. Soc. Jp n ., 1995,
6
8, 3423.
L. Lu and R. G. Weiss, J. Chem. Soc., Chem. Commun., 1996,
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19 R. Mukkamala and R. G. Weiss, J. Chem. Soc., Chem. Commun.,
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20 E. W. Garbisch and D. B. Patterson, J. Am. Chem. Soc., 1963, 85,
228.
76 Ó size. H-bonding is a major mechanism in the BACOl
1
gelation phenomenon which form colloidal structures through
solvent-dependent dipolar interactions.
2
1
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Paper 8/01922C; Received 9th March, 1998
J. Chem. Soc., Faraday T rans., 1998, V ol. 94
2179