Formation of reactive aerogels and their reactivity in aqueous media. Wettability induces hydrophobic vs. hydrophilic selectivity

Article abstract of DOI:10.1039/c2jm30184a

Aerogels were formed from organogels of diamides by a supercritical drying process. The used organogels are composed of self-assembled nanotubes of 29 nm in diameter. SEM studies reveal that the resulting aerogels are made of fibers with diameters comprised between 40 and 200 nm, corresponding to bundles of the starting nanotubes, while WAXS indicated that most of the crystalline structure detected in the self-assemblies of the starting gel is preserved in aerogels. Two different reactive diamides bearing respectively an alkynyl and an azido function were investigated. We have tested the reactivity of the resulting aerogels under copper-catalyzed azide-alkyne cycloaddition in aqueous solution. These aerogels react easily with hydrophobic compounds although reactants are in separate phases. In contrast, they do not react with hydrosoluble compounds, because of their superhydrophobicity.

Full text of DOI:10.1039/c2jm30184a

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Formation of reactive aerogels and their reactivity in aqueous media.
Wettability induces hydrophobic vs. hydrophilic selectivity†
*
ꢀ
Thi-Thanh-Tam Nguyen, Franc¸ois-Xavier Simon, Niaz Ali Khan, Marc Schmutz and Philippe J. Mesini
Received 10th January 2012, Accepted 22nd February 2012
DOI: 10.1039/c2jm30184a
we have used low molecular weight organogelators.5 It has been
shown that organogels can be transformed easily into aerogels.6 The
purpose of this study is to form a reactive organogel, transform it into
a reactive aerogel and then study the reactivity of the resulting aerogel
in water. It can be anticipated that such aerogels will not return to
their initial gel state when brought in contact with water. They are
expected to behave like a macroporous solid with high specific area
and better mechanical stability than the starting gels. As such, they
are good candidates to support catalysis or reactions in aqueous
media. However aerogels prepared from organogels and able to
undergo reaction have not been reported and their reactivity in water
has not been probed.
Aerogels were formed from organogels of diamides by a supercrit-
ical drying process. The used organogels are composed of self-
assembled nanotubes of 29 nm in diameter. SEM studies reveal that
the resulting aerogels are made of fibers with diameters comprised
between 40 and 200 nm, corresponding to bundles of the starting
nanotubes, while WAXS indicated that most of the crystalline
structure detected in the self-assemblies of the starting gel is
preserved in aerogels. Two different reactive diamides bearing
respectively an alkynyl and an azido function were investigated. We
have tested the reactivity of the resulting aerogels under copper-
catalyzed azide–alkyne cycloaddition in aqueous solution. These
aerogels react easily with hydrophobic compounds although reac-
tants are in separate phases. In contrast, they do not react with
hydrosoluble compounds, because of their superhydrophobicity.
The present study explores the possibility of functionalizing aero-
gels with alkynyl or azido groups, then letting them react with
external reagents under copper-catalyzed azide–alkyne cycloaddi-
tion.7 This model reaction was chosen not only because of its effi-
ciency and selectivity, but also because of its effectiveness even in
heterogeneous conditions,8 including with fibrillar materials, such as
in organogels.9–11 Recent studies on organogels have shown that
organogels are able to support other organic reactions9,12 and organic
catalysis.13 The studied reactions take place at the interface of the gel
fibrils and the same solvent in which the gel had been formed. We
wondered whether the reactions still occur after replacing the solvent
by water. The formation of aerogels allows exploration of this point.
We have previously shown that compound 1 (Scheme 1) forms
organogels in alkanes.14 These gels are obtained simply by dissolving
a small amount of the compound in hot solvent and letting it cool
down to 25 ꢀC. Freeze fracture electron microscopy and small angle
neutron scattering show that the gel is composed of nanotubes with
an external diameter of 29 nm. The compound self-associates
through non-covalent bonds: H-bonds between the amides and p–p
stacking between the aromatic units. Insertion of an alkynyl or an
azido group at the end of the ester chain of compound 1 preserves the
properties of forming nanotubes.10 Freeze fracture TEM (Fig. 1A) or
SAXS11 shows that 2 and 3 self-assemble into tubes with diameters
Aerogels are light materials with an alveolar and macroporous
structure.1 They are formed from gels by replacing the solvent of the
gel by air while preserving the structure of the gel network.1 The
principle is very simple, but the removal of the solvent is technically
complex. Simple drying does not afford an aerogel since capillary
forces collapse the gel fibers. It causes a dramatic shrinkage of the gel
network and thus yields a material without a macroporous structure.
In order to preserve that structure, the technique of supercritical
drying was developed.2 This technique consists in immersing the gels
in a low critical pressure liquid such as freon, nitrous oxide or carbon
dioxide in a sealed vessel. The solvent of the gel is exchanged by this
liquid below the critical point, and the temperature is raised above the
critical point where the liquid transforms into a supercritical fluid.
The release of the fluid lowers the pressure and transforms the
supercritical fluid into gas. The overall transformation from liquid to
gas is achieved without passing any liquid–gas interface.
Owing to their low density and high porosity, aerogels are
attractive materials for applications including acoustic or thermal
insulation3 and catalysis.4 The range of aerogel applications may be
extended by functionalizing them chemically. To address this issue,
Institut Charles Sadron, CNRS UPR 022, Universiteꢀde Strasbourg, 23 rue
du Loess BP 84047, 67034 Strasbourg Cedex 2, France. E-mail: mesini@
ics-cnrs.unistra.fr; Fax: +33 388 414 099; Tel: +33 388 414 070
† Electronic supplementary information (ESI) available: Experimental
details for the formation of the aerogels, reaction with the aerogels,
and analysis of the products. See DOI: 10.1039/c2jm30184a
Scheme 1 Structure of the gel forming compounds.
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scattering), indicating that the supercritical drying of the gel does not
cause a drastic reorganization of self-assembly within the fibrils. The
comparison of those spectra with that of the solid powder confirms
that removal of the solvent does not yield the crystalline structure of
ꢂ1
ꢁ
the initial powder. The increase in the intensities of peaks at 0.18 A
ꢁ
and 0.54 Aꢂ1 in the aerogel may indicate a slight recrystallization.
1, 2 and 3 are water insoluble and the corresponding aerogels are
hydrophobic. Especially, they do not reform any gel when being in
contact with water. After its removal from water, the aerogel dewets
instantaneously and there is no need to proceed with a new super-
critical drying process. This advantage makes them useful as a cata-
lytic support for reaction in aqueous solutions. In contrast, when the
aerogel is brought in contact with the organic solvent, the organogel
reforms.
The reactivity of aerogels has been tested first with reagents soluble
in water. The aerogels of 2 and 3 were reacted with an aqueous
solution of CH3(OCH2CH2)3N3 or propargyl alcohol (15 equiv.)
respectively in water in the presence of CuSO4 and ascorbic acid
(0.2 and 0.8 equiv. resp.) (Scheme 2). Because of their low density and
hydrophobicity, aerogels float above the solution and thus have
limited contact with the reaction medium. In order to increase this
surface, they have been trapped in a heavy wire mesh holder
immersed in the reaction medium and the air trapped in the aerogel
was removed under vacuum. The resulting aerogels were analyzed
(after dissolution and extraction in dissociating solvents) with stan-
dard chromatographic and spectroscopic methods. Under the
described conditions, no reaction occurs with aerogels of 2 and 3
(Table 1). Surprisingly, as a comparison, solid powders of 2 and 3 can
react with those reagents under the same conditions to afford the
desired products (resp. 4 and 6) in good yields.
Fig. 1 (A) Freeze fracture-TEM of a gel of 2 in C6H12 (2 wt%); (B and C)
SEM micrographs of the aerogel formed from 2 and (D) aerogel from 3.
comparable with those of 1 (25.3 ꢁ 3 nm for 2 and 26.5 ꢁ 0.3 nm
for 3).
Gels of 1, 2 or 3 were formed in cyclohexane and the solvent was
removed by supercritical drying with CO2. A white and cotton-like
material was obtained; its volume is about 80% of that of the starting
organogels, indicating a partial collapse of the structure. SEM
microscopy shows that the fibrillar network is preserved (Fig. 1C
and D).
Large field micrographs show some fluctuation in the fiber density
and some spherical areas where fibers are more compact (Fig. 1B).
The volume loss cannot be attributed to those dense areas since such
areas are also observed in the gels before drying. The thicknesses of
the observed fibrils are polydisperse, between 40 nm and 180 nm, with
an average value of 60 nm. It is clear that fibers in aerogels arise from
the association of several starting nanotubes. The aerogels of three
compounds have the same morphology especially the same alveolar
structure and fiber size. This structural homogeneity reflects that of
the organogels as observed by freeze fracture-TEM.
The reactivity of aerogels vs. hydrophilic reagents was compared
on the same self-assemblies but in the organogels i.e. still in the
organic solvent. We have reported11 that self-assembled gels of 2 and
3 react with C10H21N3 and propargylic alcohol to afford 5 and 6 with
a yield of 61% and 70% respectively. Since the reactions were con-
ducted in the organic gelled solvent, a different catalyst, the orga-
nosoluble Cu(PPh3)3Br, was used. Although quantitative comparison
cannot be done, the formation of the final compound observed only
in the case of organogel shows the difference in reactivity of gels and
aerogels.
WAXS measurements have been performed on the studied
compounds in different states. Fig. 2 compares the diffraction pattern
from 1 in aerogels, organogels and solid state. The latter corresponds
to the powder obtained from synthesis, without any self-assembly or
heating/cooling cycle in organic solvent. The spectrum of the aerogel
is similar to that of the organogel (after subtraction from the solvent
The comparison between gels and aerogels proves that the lack of
reactivity of aerogels is not the result of a reduced mobility of the
reactants inside a tight network structure. As a matter of fact, it has
been shown by conductimetry,15 fluorescence quenching16 and
NMR17 that transport within gels is the same as in pure solvents.
Organogels have been used as a reagent9–12 or as a selective binding
Fig. 2 WAXS of 1; top: powder; middle: gel (6.5 wt% in C6H12) after
subtraction of solvent scattering and bottom: aerogel.
Scheme 2 Reactions tested with aerogels.
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Table 1 Yields of reactions of 2 and 3 in different states
conducted to explore the reactivity with binary mixtures of solvents
and with higher pressures.
Aerogela
Gelsb
Powder
2 + N3(CH2CH2O)3CH3
3 + Propargylic alcohol
2 + C10H21N3
0
0
63
—
70
61
80 (60c)
Acknowledgements
Quant. (50c)
54 (30c)
This project was funded by the International Center for Frontier
ꢀ
Research in Chemistry (icFRC), Strasbourg and by the Region
a
Yields in purified compounds. b In C6H12, catalyst Cu(Ph3)3Br. c Yields
without stirring.
Alsace. We thank J. Faerber for the SEM experiments, B. Heinrich
and F. Schnell for the WAXS measurements. We acknowledge Dr T.
Ondarc¸uhu for the fruitful discussions.
medium18 and diffusion never prevents reactions. However since
the reactions with the organic gels were carried out without stirring,
the complete matter transport through the whole volume is slow
(typically 1 week for a 2 cm3 gel). Therefore, we have conducted blank
experiments with powders without stirring in order to mimic the
unfavorable conditions of the reactions in organogels and in aerogels.
In this case, the yield is lower but is still significant (Table 1).
The low reactivity of aerogels toward hydrosoluble reagents is
clearly not due to an intrinsic lack of reactivity. In order to study the
role of reagent hydrophilicity, we have conducted a reaction between
aerogels of 2 and hydrophobic reagents such as C10H21N3, in water
under the same conditions as described for hydrosoluble compounds.
The azido compound is a water-insoluble oil and the three compo-
nents of the reaction (alkyne, azide and catalyst) were thus in separate
phases. Despite these unfavorable conditions, the reaction proceeds
in good yield (63%). The morphology of the aerogels, checked by
SEM, before and after reaction shows no change in the macroporous
architecture as well as in the diameters of the fibers.
Notes and references
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The reactivity of the aerogel with a hydrosoluble reactant is null,
whereas it is significant with hydrophobic species. Hydrophobicity of
the aerogels can explain this phenomenon: the hydrophobic reagents
tend to adsorb on the fibers of aerogels. The porous structure of
aerogels favors the adsorption via capillary force. On the other hand,
hydrophobicity of aerogels cannot explain their low reactivity with
hydrosoluble species, since the powders react with good yields. The
most relevant property to explain the difference of reactivity between
aerogels and powder is the superhydrophobicity of the aerogels. This
property is the magnification of the hydrophobicity of a surface by its
roughness.19 This property is observed in nanopatterned surfaces,19
fibrillar materials such as electrospun mats20 and porous materials
like foams.21 Laplace’s law can be used to derive the pressure
difference between the air and the liquid necessary for the liquid to
penetrate into pores: DP ¼ G/2r, where G is the superficial tension of
water (72 mN mꢂ1) and r the radii of the pores, assuming that they are
spherical. After removal of the air under vacuum, DP cannot exceed 1
bar under the experimental conditions, which allows the penetration
of pores of radii larger than 1.5 mm. The pores of the studied aerogels
have radii of this order or below and cannot be wetted by water at
room pressure.
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In summary, we have been able to prepare reactive aerogels from
organogels. The morphology of the aerogel reflects that of the
organogel. The hydrophobicity of the aerogel severely screens the
reactants: it is inert toward hydrosoluble reactants, but highly reactive
toward liposoluble reactants. This selectivity is explained by the
superhydrophobic nature of the material. Further studies will be
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