Monolithic organic aerogels derived from single amino-acid based supramolecular gels: physical and thermal properties

Abstract

Highly porous materials have numerous applications in research fields such as filtration and collection devices, catalysis and electric devices or in acoustic and thermal insulation. In the latter, inorganic and/or macromolecular chemical gels have been extensively studied. Herein, a new type of monolithic aerogel, made of self-assembled small organic molecules, is described. The low-molecular weight aerogels (LMWA) are prepared from amino acid-based (phenylalanine or leucine) supramolecular gels using a CO₂ supercritical drying process. The organogels and aerogels exhibit very interesting properties. Indeed, the gelator molecules are able to immobilize aromatic solvents such as toluene or tetralin at low concentrations. The melting enthalpies (ΔH) were calculated for both gelator molecules and for both solvents. In the case of the aerogels, an important hydrophobic character, a very low density and remarkable thermal properties were observed. For the former, the measured contact angles were found to be between 110 and 114°. As for the second, the organic aerogels belong to the family of the lightest porous materials in the world with densities as low as 4.3 kg m⁻³. Finally, the thermal measurements show that these LMWAs present a low thermal conductivity under atmospheric pressure (λ = 26.5 mW m⁻¹ K⁻¹) and very low thermal conductivity under vacuum (λ = 4 mW m⁻¹ K⁻¹ at 10⁻² mbar). Moreover, measurement of the radiative conductivity demonstrated that the LMWAs are a good scattering materials with 80 to 90% of infrared radiation stopped. Such characteristics make these materials viable candidates for use in thermal insulation.

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Introduction

Supramolecular gels are usually the result of a self-assembly process, in a suitable solvent, of small molecules into a three dimensional fibrillar network, which prevents solvent flow.^1,2 The molecular weight limit of the gelator molecules, which allows one to classify the as-obtained materials in the wide family of low molecular weight gels (LMWGs),³ is usually lower than 3000 g mol⁻¹. The interactions involved between the gelator molecules (such as hydrogen bonding, π–π stacking interactions, van der Waals interactions, etc.) are of the non-covalent type^4,5 and can therefore be more or less easily broken, which gives these materials a dynamic and reversible nature. The formation and disruption of these supramolecular aggregates can be driven by external stimuli such as temperature, solvent properties, pH, chemical species, light, electrical or magnetic fields and redox reactions.^6–8 Considering this, applications of LMWGs in materials science are obvious and numerous.⁶ Indeed, supramolecular gels can be used, among others, in the fields of organic electronics,^9,10 medicine (as biosensors or drug delivery systems),^11,12 reaction vessels and catalytic systems^13,14 or as templates for the elaboration of nanostructured materials.^15,16

Classification of gelator molecules, leading to self-assembled fibrillar networks (SAFINs),¹⁷ can be made according to the nature of the trapped liquid within their structure. The two major families, which imply water and organic solvents as immobilized fluid phase, are the low molecular weight hydrogelators (LMWHs)^18,19 and low molecular weight organogelators (LMWOs),²⁰ respectively. Another specific case is the aerogels family,²¹ where the fluid found within the matter is of the non-condensed type and is simply air.

Because of the simultaneous presence of donor and acceptor hydrogen bonding sites, amide units are particularly suitable for the building of unidirectional self-assembled aggregates, which are the primary steps towards the formation of gel frameworks.²² In that way, thanks to two major structural and molecular properties, peptides and peptidomimetics are well known to form gels.^23–29 Another advantage of using this family of organic compounds is the easy and well-known chemistry that can be used to obtain new gelator molecules. Finally, the bio-inspired molecules are also interesting because of their biocompatibility and relatively low cost.

Aerogels are highly porous solid materials^30,31 made of an interconnected nanometer scale network of fibers or walls. The specific structure of aerogels is closely linked to the structure of the previously used wet gels. Indeed, aerogels are usually obtained from a supercritical CO₂ drying process. This specific solvent extraction from the cavities of wet gels occurs with very little disruption of the 3D solid network developed during the gelation process. In contrast, by drying the same wet gels either under vacuum or upon increasing the temperature, xerogels are obtained. In this case, shrinkage of the cavities due to capillary forces with a simultaneous and drastic overall volume contraction of the initial gel is observed. These materials are usually in a monolithic form, i.e. they are made of a continuous phase leading to a single object. However, in some cases the drying step can give fluffy powders or cotton wool-like structures. Concerning the chemical structure, the aerogels described in the literature are mainly inorganic with for instance the presence of silica³² or metal oxides ones.^33,34 Organic aerogels are less frequent and are either made of carbon^35–38 (graphene, graphene oxide, carbon nanotubes) or are of the polymeric type such as polystyrene³⁹ or polysaccharide.^40,41 Because of their highly divided solid nature, the main structural properties of an aerogel are: a low apparent density and a high surface area. According to this, potential applications^30,31,42,43 of these materials are, for instance, in the area of filtration and collection devices, catalysis, electric devices or in acoustic and thermal insulation.^44–49 In the latter field, inorganic (nanoporous silica),⁵⁰ polymeric⁵¹ or composite⁵² aerogels exhibit very interesting thermal properties, with conductivity smaller than 30 and 8 mW m⁻¹ K⁻¹ at atmospheric pressure and at 1 mbar, respectively.

In the present paper, we report the elaboration and properties of the first monolithic organic aerogels obtained by a CO₂ supercritical drying process of organogels made through the self-assembly of single amino acids-based low molecular weight gelators. To the best of our knowledge, only one example of an aerogel derived from supramolecular gels has been described. This is the case of aerogels obtained from pure 2,3-didecyloxyanthracene (DDOA)⁵³ or DDOA blended with organic⁵⁴ or inorganic⁵⁵ compounds. In all cases, the elaborated materials resemble cotton wool. In contrast, the final aerogels we have obtained resemble a marshmallow with more defined outlines. Concerning their properties, the materials are very hydrophobic and exhibit a rather low thermal conductivity, which indicates that these materials are good candidates for the development of thermal insulators.

Experimental

Preparation of the supramolecular organogels

The desired amount of gelator and solvent were placed in a round-bottomed flask equipped with a reflux condenser and submitted, under stirring, to microwave irradiation in order to obtain a complete isotropic solution. The apparatus used was a SEM Discover Microwave with the following parameters; temperature: 80 °C, power: 150 W, speed of stirrer: medium, run time: 10 min and hold temperature time: 2 min. Once the gelator was completely solubilized, the hot solution was poured into a glass cylindrical mould and allowed to cool down to room temperature before the drying step, or stored at 4 °C in case of subsequent drying. The sol–gel transition temperatures T_g were measured with the “falling ball” method.⁵⁶ A steal ball (4 mm diameter, 0.26 g) was placed at the top of the gel previously formed in a test tube (11 mm diameter). Upon heating, the gel melts slowly into solution and the ball drops in the tube. The temperature when the ball reaches the bottom of the tube is the T_g. This temperature was measured at least three times.

Preparation of the supramolecular xerogel and aerogels

The xerogels were simply prepared, either in the open air or under vacuum, via the evaporation of the solvent from organogels. In the case of the aerogels, drying was performed using supercritical CO₂ using the following procedure: a sample of the organogel (around 5 to 7 g) was introduced into the autoclave. Supercritical drying was performed following two steps according to the thermodynamic data: a static step followed by a dynamic or continuous supercritical drying. The conditions used for the first step for a toluene organogel was those of liquid CO₂ (T = 15 °C, P = 90 bar with a contact duration between organogel and CO₂ of 15 min). Then, the temperature was increased to 45 °C (P = 90 bar, solvent: toluene, duration: 20 min). In the second step, the system was in the supercritical state and the solvent extraction was performed. At the end of the procedure, the pressure was released to atmospheric pressure and the LMWA was extracted out of the autoclave. Using this process, the solvent extraction was quantitative and could be controlled by the dissolving aerogel in CHCl₃ followed by GPC analysis.

Characterization methods

The bulk densities (ρ_b) of the aerogels were estimated on the basis of their mass-to-volume ratio. The skeletal density (ρ_s = 1346 kg m⁻³), on the other hand, was determined by helium pycnometry measurements. The porosities were calculated with the help of the two values of the density using the following equation:

Porosity = (1 − ρ_b/ρ_s) × 100% (1)

The surface area was measured by N₂ absorption and analyzed using the Brunauer–Emmett–Teller (BET) method, whereas the pore size distribution was determined from the thermal conductivity measurements.⁵⁷

Scanning electron microscopy experiments were performed on a Philips XL30-ESM instrument or a high resolution Hitachi FEG-SEM S4800 coupled with a dispersion spectrometer. The accelerating voltage used for SEM was 30 kV. The dry samples, either aerogels or xerogels, were coated with gold (15 Å) over 4 min of physical vapor deposition (PVD).

The contact angle (θ) measurements were performed with a contact angle meter Dataphysics OCA 15 EC in order to evaluate the degree of hydrophobicity of the aerogels. The experiments were achieved as follows: on the plane and sanded surface of the aerogel (2 or 3 wt%), a 20 or 30 μL droplet of water was placed and the contact angle measured.

The thermal conductivities (λ) were measured using the three layer method,⁵⁸ which is a new measuring device developed for lightweight insulators and superinsulators. This apparatus is comprised of a stack of three layers: a brass plate – the sample – a brass plate. A thermal disturbance is imposed on one brass plate and the transient temperature variations of the two plates are recorded. This enables one to identify the transfer function between the two temperatures and to estimate the thermal conductivity of the sample using an inverse method. The transfer model is purely analytical and takes into account the bidirectional transfer in the sample. Validation of the method was carried out in two stages: the analytical model was first validated by comparison with a numerical 3D simulation (the mathematical models were discretized by the Finite Element Method (FEM) using COMSOL Multiphysics software) and then, the experimental device and the estimation methods were validated on insulators and superinsulators with a thermal conductivity between 18 and 180 mW m⁻¹ K⁻¹ measured using other methods. The measurement accuracy was estimated to be 5%. The complete device comprised of a sealed chamber and a vacuum pump that allowed one to perform measurements at room temperature at pressures ranging from 10⁻³ mbar to 10³ mbar.

The volumetric heat capacities (ρ_c) were measured on a Setaram DSC 131 Evo (C_p = 1210 J kg⁻¹ K⁻¹) and the radiative thermal conductivities were given by transmission measurements in the infrared region.

Results and discussion

The formation and properties of the organogels

It is a decade now since our research laboratory started to work on the synthesis and gelation studies of a family of low molecular weight gelator molecules bearing a single (L)-amino acid unit.⁵⁹ These molecules are very interesting building blocks because: (1) in the case of natural amino acids there are 20 different side-chain residues available, (2) there is a chiral center and (3) various functionalities can be introduced on N and C-terminal positions. Amino acids are therefore highly versatile and allow the synthesis of a large number of potential gelator molecules. We have shown, in our previous work⁶⁰ that gelators 1 and 2 (Fig. 1) are able to immobilize a large variety of aromatic and chlorinated solvents.

Fig. 1 The respective molecular structures of the phenylalanine and leucine-based gelators 1 and 2.

These molecules are L-phenylalanine and L-leucine based gelators bearing carboxybenzyl (Z) as an amine protecting group and a naphthalimide moiety at the C-terminus position as a fluorescent chromophore. It has been demonstrated that in the case of these gelators, the immobilization of solvent was not due to the formation of a gel but to a jammed solution. This jammed solution can be described as a system combining the elasticity of a solid network formed by the clusters of organogelator molecules and by the viscosity of the suspending fluid.⁶¹ The supramolecular gels are first macroscopically characterized by their critical gelation concentration (CGC), which is the minimum amount of molecules necessary to immobilize solvent. In the literature, the CGC values can be expressed as a weight percentage (wt%), as a molar concentration or even as a gelation number (GN), which is the number of solvent molecules trapped per molecule of gelator. This value reflects the gelation efficiency of gelator molecules. Indeed, for a given solvent, the lower the amount of gelator needed to form a gel, the more efficient the gelator is. The second macroscopic characterization of thermoreversible supramolecular gels is the sol–gel transition temperature (T_g); the higher this temperature is, the stronger the gel is.

Efficiency of gelation. If we compare the gelation ability of compounds 1 and 2 towards toluene or tetralin, we notice several things. First, whatever the solvent, the phenylalanine-based gelator 1 was always a better gelator than 2. Indeed, the lowest CGC (2.99 and 3.76 mM versus 5.68 and 6.73 mM) was observed for gelator 1 (see Table 1). In other words, by looking at the GN for each solvent, 1 is almost able to trap the double the amount of solvent molecules when compared to gelator 2. Second, according to the CGC and GN values, toluene seems to be easier to immobilize than tetralin. Indeed, if we compare the GN found for toluene (3120) and tetralin (1950) corresponding to gelator 1, we notice that there were around 35% more molecules of toluene trapped when compared to tetralin. This ratio was found to be almost similar in the case of gelator 2 where the GNs are equal to 1650 and 1090 for toluene and tetralin, respectively. All these differences are probably due to the fact that: (1) gelator 1 has a more pronounced aromatic character than gelator 2 and is, therefore, more efficient to immobilize aromatic solvents, and (2) toluene is smaller than tetralin and the number of trapped molecules, for a fixed volume of solvent, is therefore higher in the former case than in the latter.

Table 1 The critical gelation concentration (CGC) and gelation number (GN) of gelator 1 and 2 in toluene and tetralin

	Toluene		Tetralin
	CGC (wt%/mM)	GN	CGC (wt%/mM)	GN
1	0.17/2.99	3120	0.18/3.76	1950
2	0.30/5.68	1650	0.30/6.73	1090

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Strength of the gels. Regarding the nature, the amount of gelator molecule involved and the nature of the solvent, the thermal stabilities of the gels are different. The phase diagram presented in Fig. 2a shows the evolution of T_g as a function of the concentration of gelator 1 and 2 in toluene and tetralin, respectively. First, for both gelators and in both solvents, an increasing concentration induces an increasing of T_g value. The amount of used matter has a direct influence on the thermal stability of the 3D SAFIN. Indeed, the higher the concentration is, the denser the fibrillar network is and the more stable towards temperature the gel is. Second, the T_g for 1 was always higher, in a given solvent, than that found for 2. This indicates that 1, in addition to being the most efficient gelator, also gives the strongest gels. Finally, for both gelators, there is an exponential increase in the T_g value as a function of concentration. However, in the case of tetralin, this increase was more pronounced at lower concentrations than for toluene. From the exponential curves, an Arrhenius plot of ln[c] versus reciprocal transition temperature (1/T_g) gave a linear relationship (Fig. 2b). This behaviour is often observed for a wide range of gelators.² If we consider that the gel-to-sol phase transition can be compared to a dissolution process of microcrystals, the gelator concentration is given by Schrader's equation:^62–64where [c] is the gelator concentration expressed in mol L⁻¹, ΔH is the melting enthalpy of the gel (kJ mol⁻¹), R is the gas constant and T_g is expressed in Kelvin. Then, from the derived eqn (2), the ΔH values can be calculated from the slopes of the straight line graphs shown in Fig. 2b. In this way, the melting enthalpies ΔH in toluene were found to be around 45 and 35 kJ mol⁻¹ for 1 and 2, respectively, and 65 and 45 kJ mol⁻¹ in the case of tetralin. These values are in good agreement with data found in the literature for supramolecular gels.^62–64 As the gel–sol transition enthalpies are correlated to the strength of the intermolecular interactions in the fibrillar networks, the highest values of ΔH correspond to the strongest interactions. So, the presence of an extra aromatic ring in the case of the phenylalanine gelator gives stronger intermolecular interactions probably due to supplementary π–π stacking interactions. In addition, when compared to toluene, the use of tetralin as the solvent during the gelation process seems to induce a higher crystalline character in the fibers leading then to higher values of ΔH.

Fig. 2 T_g as a function of gelator 1 (star) and gelator 2 (disc) concentration in toluene (black) and tetralin (red).

Gelation at the molecular level. Different spectroscopic techniques such as ¹H NMR, FT-IR, fluorescence and circular dichroism (CD) have been used to study the self-assembly process at a molecular level.⁶⁵ We have shown that a sequential pathway was followed during gel formation. The first step consists of a head-to-tail hydrogen bonded driven stacking-up of gelator molecules (Fig. 3 left). The as-obtained primary columns can be compared to the anti-parallel beta-sheet secondary structure observed for peptides. In the second step, after folding of the Z protecting group, the primary columns self-assemble in turn via intercolumnar π–π stacking interactions between the naphthalimide units (Fig. 3 right). The further organization of the as-formed objects leading to the final fibers will be discussed elsewhere.

Fig. 3 A schematic representation of the sequential self-assembly process to form primary columns followed by secondary intercolumnar association.

Formation and properties of the xerogels and aerogels

The xerogels and aerogels were formed by drying the corresponding organogel either under vacuum or via a supercritical CO₂ process, respectively.

Macroscopic aspects and characterization. In the case of the xerogels, a dramatic shrinkage of the material was observed. In contrast, the obtained aerogel, which we call LMWAs for aerogels obtained from low molecular weight organogels, has almost the same volume as the initial organogel. More interestingly and newly, they are obtained in a monolithic form, they can be handled with care without cracking and have a marshmallow appearance (Fig. 4).^66–68 The dried materials are extremely porous and therefore, extremely light (Table 2). Indeed, on the one hand, the porosities vary from 98 to 99.7% for the aerogels made from the 3 to 0.5 wt% phenylalanine-based organogels. On the other hand, the bulk densities vary from 26.8 to 4.3 kg m⁻³ for the same organogel concentration. These values are very low when compared to silica aerogels (from 4 to 500 kg m⁻³ with a typical value of 100 kg m⁻³),³⁰ polystyrene aerogels (100 kg m⁻³)³⁹ or even extruded polystyrene (about 35 kg m⁻³). In addition, the skeletal density was evaluated to be 1346 kg m⁻³. The aerogel obtained from the 2 wt% organogel of gelator 1 in toluene (17.7 kg m⁻³) has a specific surface area of 90.5 m² g⁻¹, which was smaller than silica aerogels (100 to 1600 m² g⁻¹ with a typical value of 600)³⁰ but larger than the 10 m² g⁻¹ of the other organic low molecular weight aerogel (DDOA based) found in the literature.⁵³

Fig. 4 Pictures of the organogel (left), xerogel (middle) and aerogel (right).

Table 2 The bulk and skeletal densities, porosity and specific surface area of gelator 1 at different concentrations in toluene

Gelator conc. (wt%)	Bulk density (kg m^−3)	Skeletal density (kg m^−3)	Porosity	Specific surface area (m^2 g^−1)
0.5	4.3	1346 ± 2	99.7	—
1	8.7		99.4	—
2	17.7		98.7	90.5
3	26.8		98	—

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Scanning electron microscopy. Scanning electron microscopy (SEM) shows a highly entangled fibrillar network for all the dried gels. In our previous work on xerogels,⁶⁰ we have shown that the nature of the solvent had an influence on the fibers size. In the case of toluene, the diameter of the fibers for gelator 1 was in the range of 150 to 200 nm. In addition, as we can see on SEM pictures (Fig. 5), xerogels obtained from the 0.23 and 1 wt% organogels in toluene show similar structures and the diameter of the fibers seems to be invariant with concentration. As the xerogels are in fact 2D structures obtained from the collapsed 3D structures, the real fiber densities can't be evaluated. The observed densities using SEM are therefore not relevant and seem to be identical for both concentrations.

Fig. 5 Scanning electron microscopy pictures of the xerogels of compound 1 from toluene gels (a) 0.23 wt% (×20 000) and (b) 1 wt% (×20 000).

A comparison of the xerogel and aerogel made from the same organogel (i.e. gelator 1 in 1 wt% toluene, Fig. 5b and 6a) shows that the fiber diameter was larger in the case of the xerogel (150 to 200 nm) than that found for the aerogel (40 to 130 nm). This difference can be explained by the fiber aggregation phenomena observed during classical evaporative drying, which does not occur during the supercritical drying process.⁵³

Fig. 6 SEM pictures of the aerogels of compound 1 obtained from the (a) 1 wt% toluene (×20 000), (b) 1 wt% tetralin (×20 000) and (c) 0.5 wt% tetralin (×25 000) organogels.

The fiber diameters were in the nanometer range size for both aerogels prepared from the 1 wt% toluene and 1 wt% tetralin organogels. Nevertheless the distribution was different: 40 to 130 nm for the first one and 80 to 200 nm.

Lowering the density of the aerogels (obtained from the organogels of tetralin) from 13 to 4.3 kg m⁻³ (Fig. 6b and c) leads, as expected, to a less dense fibrillar structure. However, the maximum fiber diameters sizes are higher for the less dense aerogel (250 nm) than for the aerogel prepared from the 1 wt% organogel (200 nm). As shown in Fig. 6c, large fibers seem to be in fact a bundle of several smaller fibers as observed in the Fig. 6b.

Hydrophobic properties. Phenylalanine and leucine, regarding the nature of the side chain, are classified in the neutral and non-polar family of amino acids. In addition, functionalization of the C and N-terminal positions with aromatic groups tilts the balance in favour of strong hydrophobicity. Indeed, when a piece of aerogel was placed in water, it floats on the surface and tries to minimise its contact with water. Additionally, when water is laid down on a piece of aerogel, it forms droplets on the surface (Fig. 7a) with contact angles of 110° and 114° in the case of the aerogels made from the 2 wt% and 3 wt% organogels in toluene of gelator 1, respectively (Fig. 7b). Knowing that Teflon, for instance, has an average contact angle value of 103° and that materials considered as superhydrophobic have contact angles higher than 150°,⁶⁹ the obtained values indicate clearly the high hydrophobic character of our materials.

Fig. 7 (a) A photograph of the water droplets formed on the organic aerogel and (b) the water contact angle on the surface of aerogel obtained from the 3 wt% of 1 in toluene organogels.

Thermal properties. Considering the intrinsic characteristics of LMWAs, a potential application field is thermal insulation. However, because of their very low density, making imprecise the surface temperature measurements, none of the existing methods for the measurement of thermal conductivity can be applied to LMWAs. Indeed, the thermal characterization (thermal conductivity and diffusivity measurements) of high insulating materials (i.e. with thermal conductivity values lower than air) is particularly difficult to perform either in steady-state or transient regimes. The classical methods (flash method, thermal probes, hot-wire technique, hot plates, etc.) have to be modified to take into account the semi-transparent character of the material and the thermal coupling with the apparatus and the surrounding that are often more diffusive than the material itself. So, in order to analyse lightweight insulators and superinsulators, a new measuring device, based on the “three layer” method (Fig. 8), was developed by Jannot et al.,⁵⁸ which leads to satisfying precision for the estimation of conductivity (<5%).

Fig. 8 A schematic representation of the “three layer” measurement method of lightweight material thermal conductivity.

Three LMWAs, 4.3, 17.7 and 26.8 kg m⁻³ prepared from the toluene organogels of gelator 1 at concentrations of 0.5, 2 and 3 wt%, respectively were thermally characterized at atmospheric pressure using the three layer method. Concerning the volumetric heat capacities (ρ_c), the difficulties in obtaining reliable values (those obtained by the three layer method are too high) let us to perform measurements by calorimetry. The estimated parameters are gathered in Table 3. The thermal conductivity (λ) is a data, which reflects the thermal insulation ability of a material. When the λ values are lower than 25 mW m⁻¹ K⁻¹, then the materials are considered as superinsulators. In the case of our aerogels (17.7 and 26.8 kg m⁻³ obtained from toluene), the λ values are found to be between 26 and 27 mW m⁻¹ K⁻¹, which is very close to the value found for air (26 mW m⁻¹ K⁻¹). In addition, it should be noted that the aerogels become superinsulators at low pressure (Fig. 9a). Indeed, reducing the pressure to 10⁻² mbar induces, in the case of the 3 wt% aerogel, a reduction in this λ value to 4 mW m⁻¹ K⁻¹. The establishment of an equivalent thermal conductivity model suitable for lightweight insulators, associated with measurements of the thermal conductivity for air pressures ranging from 10⁻³ to 10³ mbar, allowed one to estimate the pore size distribution in a 26.8 kg m⁻³ aerogel obtained from toluene (Fig. 9b).⁵⁷ It was found that this aerogel was a macroporous type with around 60% of the pores having a size of 1 μm, 30% being 10 times larger and the rest even larger. These values are rather high if we compare them with the mean pore diameter of a silica aerogel (20 to 150 nm).³⁰ Such differences can be used to explain the gap in the thermal conductivity observed between our organic and inorganic aerogels. Finally, transmission measurements in the infrared region (between 5 and 25 μm) allowed us to reach values of 2.8 and 2.9 mW m⁻¹ K⁻¹ for the radiative conductivity at room temperature of the 4.3 and 26.8 kg m⁻³ aerogels obtained from toluene, respectively. These values indicate that the aerogels are good scattering materials towards infrared radiation. Therefore, around 20% of the IR light was transmitted in the case of the 4.3 kg m⁻³ aerogel and this rate dropped to around 10% in the case of the highest concentration (Table 4).

Table 3 The estimated parameters (conductivity λ, volumic heat capacity ρ_c, exchange coefficient h and radiative resistance R) of the 2 and 3 wt% aerogels prepared from gelator 1 in toluene

Gelator concentration (wt%)	Bulk density (kg m^−3)	λ (mW m^−1 K^−1)	ρc (J m^−3 K^−1)	h (W m^−2 K^−1)	R (K W^−1)
2	17.7	26.9	48 600	5.8	370
3	26.8	26.5	50 400	5.8	310

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Fig. 9 (a) The thermal conductivity as a function of pressure and (b) the pore size distribution as a function of volumic fraction of the 3 wt% aerogel.

Table 4 The radiative properties of the 0.5 and 3 wt% aerogels prepared from 1

Gelator concentration (wt%)	Transmission rate (%)	Radiative conductivity (mW m^−1 K^−1)
0.5	18.9	2.8
3	9.4	2.9

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Conclusions

In summary, we have designed and synthesized new monolithic aerogels (LMWA) obtained from low molecular weight organogels (LMWO) using a supercritical CO₂ drying process. Hydrogen bonding and π–π stacking interactions are responsible for the gelation phenomenon, which is due to a primary head-to-tail self-assembly process of the gelator molecules into columns, which further self-assemble into fibers. The phenylalanine and leucine-based compounds showed a high gelation efficiency and gel strength, mainly in the case of aromatic solvents such as toluene and tetralin. Replacement of solvent by air during supercritical drying doesn't affect the morphology of the 3D fibrillar network observed in the organogel. In other words, no collapse of the network was observed during the drying step. These new materials exhibit very interesting properties. First, the hydrophobic character, comparable to Teflon, was rather high. Second, thanks to a new measuring device using a tri-layer stack, we could measure a thermal conductivity of about 0.027 W m⁻¹ K⁻¹ for our LMWAs. Given their remarkable properties, low thermal conductivity, scattering in the infrared region and hydrophobic character, these new materials can be used in the field of thermal insulation.

Acknowledgements

The authors thank the National Research Agency (ANR) (MULOWA Blan08-1_325450) for financial support.

Notes and references

1. P. Terech and R. G. Weiss, Chem. Rev., 1997, 97, 3133.
2. R. G. Weiss and P. Terech, in Molecular Gels. Materials with Self-Assembled Fibrillar Networks, Springer, Dordrecht, 2006.
3. D. J. Abdallah and R. G. Weiss, Adv. Mater., 2000, 12, 1237.
4. J. W. Steed, D. R. Turner and K. J. Wallace, in Core Concepts in Supramolecular Chemistry and Nanochemistry, Wiley, Chichester, 2007.
5. R. E. Melendez, A. J. Carr, B. R. Linton and A. D. Hamilton, Struct. Bonding, 2000, 96, 32.
6. N. M. Sangeetha and U. Maitra, Chem. Soc. Rev., 2005, 34, 821.
7. T. Ishi-I and S. Shinkai, Top. Curr. Chem., 2005, 258, 119.
8. X. Yang, G. Zhang and D. Zhang, J. Mater. Chem., 2012, 22, 38.
9. S. S. Babu, S. Prasanthkumar and A. Ajayaghosh, Angew. Chem., Int. Ed., 2012, 51, 1766.
10. A. Das and S. Ghosh, Angew. Chem., Int. Ed., 2014, 53, 2038.
11. A. Vintiloiu and J.-C. Leroux, J. Controlled Release, 2008, 125, 179.
12. V. F. M. Segers and R. T. Lee, Drug Discovery Today, 2007, 12, 561.
13. D. Diaz Diaz, D. Kühbeck and R. J. Koopmans, Chem. Soc. Rev., 2011, 40, 427.
14. B. Escuder, F. Rodriguez-Llansola and J. F. Miravet, New J. Chem., 2010, 34, 1044.
15. M. Llusar and C. Sanchez, Chem. Matter., 2008, 20, 782.
16. F. Delbecq, Adv. Colloid Interface Sci., 2014, 209, 98.
17. K. L. Caran, D.-C. Lee and R. G. Weiss, in Soft Fibrillar Materials: Fabrication and Applications, Wiley, Chichester, 2013, pp. 3–75.
18. L. A. Estroff and A. D. Hamilton, Chem. Rev., 2004, 104, 1201.
19. M. de Loos, B. L. Feringa and J. H. van Esch, Eur. J. Org. Chem., 2005, 3615.
20. M. George and R. G. Weiss, Acc. Chem. Res., 2006, 39, 489.
21. A. Du, B. Zhou, Z. Zhang and J. Shen, Materials, 2013, 6, 941.
22. F. Fages, F. Vögtle and M. Zinic, Top. Curr. Chem., 2005, 256, 77.
23. C. Tomasini and N. Castelluci, Chem. Soc. Rev., 2013, 42, 156.
24. J. Raeburn, A. Zamith Cardoso and D. J. Adams, Chem. Soc. Rev., 2013, 42, 5143.
25. D. J. Adams, Macromol. Biosci., 2011, 11, 160.
26. D. J. Adams and P. D. Topham, Soft Matter, 2010, 6, 3707.
27. H.-W. Jun, S. E. Paramonov and J. D. Hartgerink, Soft Matter, 2006, 2, 177.
28. A. Aggeli, M. Bell, L. M. Carrick, C. W. G. Fishwick, R. Harding, P. J. Mawer, S. E. Radford, A. E. Strong and N. Boden, J. Am. Chem. Soc., 2003, 125, 9619.
29. J. D. Hartgerink, E. Beniash and S. L. Stupp, Proc. Natl. Acad. Sci. U. S. A., 2002, 99, 5133.
30. N. Hüsing and U. Schubert, Angew. Chem., Int. Ed., 1998, 37, 22.
31. A. C. Pierre and G. M. Pajonk, Chem. Rev., 2002, 102, 4243.
32. H. Maleki, L. Duraes and A. Portugal, J. Non-Cryst. Solids, 2014, 385, 55.
33. N. Gaponik, A.-K. Herrmann and A. Eychmüller, J. Phys. Chem. Lett., 2012, 3, 8.
34. D. R. Robinson and B. Dunn, J. Mater. Chem., 2001, 11, 963.
35. M. Antonietti, N. Fechler and T.-P. Fellinger, Chem. Mater., 2014, 26, 196.
36. W. Ren and H.-M. Cheng, Nature, 2013, 497, 448.
37. S. Nardecchia, D. Carriazo, M. L. Ferrer, M. C. Gutierrez and F. del Monte, Chem. Soc. Rev., 2013, 42, 794.
38. M. Zeng, W.-L. Wang and X.-D. Bai, Chin. Phys. B, 2013, 22, 098105–098111.
39. C. Daniel, S. Longo, R. Ricciardi, E. Reverchon and G. Guerra, Macromol. Rapid Commun., 2013, 34, 1194.
40. K. S. Mikkonen, K. Parikka, A. Ghafar and M. Tenkanen, Trends Food Sci. Technol., 2013, 34, 124.
41. P. Tingaut, T. Zimmermann and G. Sèbe, J. Mater. Chem., 2012, 22, 20105.
42. G. M. Pajonk, Colloid Polym. Sci., 2003, 281, 637.
43. S. M. Jones, J. Sol-Gel Sci. Technol., 2006, 40, 351.
44. E. Cuce, P. M. Cuce, C. J. Wood and S. B. Riffat, Renewable Sustainable Energy Rev., 2014, 34, 273.
45. M. M. Koebel, A. Rigacci and P. Achard, Aerogels Handbook, 2011, p. 607.
46. P. M. Norris and S. Shrinivasan, Annu. Rev. Heat Transfer, 2005, 14, 385.
47. K. Richter, P. M. Norris and C.-L. Tien, Annu. Rev. Heat Transfer, 1995, 6, 61.
48. R. Stangl, W. Platzer and W. Wittwer, J. Non-Cryst. Solids, 1995, 186, 256.
49. V. Wittwer, J. Non-Cryst. Solids, 1992, 145, 233.
50. K. Duer and S. Svendsen, Sol. Energy, 1998, 63, 259.
51. A. Rigacci, J.-C. Marechal, M. Repoux, M. Moreno and P. Achard, J. Non-Cryst. Solids, 2004, 350, 372.
52. D. Ge, L. Yang, Y. Li and J. Zhao, J. Non-Cryst. Solids, 2009, 355, 2610.
53. F. Placin, J.-P. Desvergne and F. Cansel, J. Mater. Chem., 2000, 10, 2147.
54. S. Banerjee, R. Das, P. Terech, A. de Geyer, C. Aymonier, A. Loppinet-Serani, G. Raffy, U. Maitra, A. Del Guerzo and J.-P. Desvergne, J. Mater. Chem. C, 2013, 1, 3305.
55. N. M. Sangeetha, S. Bhat, G. Raffy, C. Belin, A. Loppinet-Serani, C. Aymonier, P. Terech, I. Maitra, J.-P. Desvergne and A. Del Guerzo, Chem. Mater., 2009, 21, 3424.
56. P. Terech, C. Rossat and F. Volino, J. Colloid Interface Sci., 2000, 227, 363.
57. V. Felix, Y. Jannot and A. Degiovanni, Rev. Sci. Instrum., 2012, 83, 054903.
58. Y. Jannot, A. Degiovanni and G. Payet, Int. J. Heat Mass Transfer, 2009, 52, 1105.
59. N. Brosse, D. Barth and B. Jamart-Grégoire, Tetrahedron Lett., 2004, 45, 9521.
60. P. Curcio, F. Allix, G. Pickaert and B. Jamart-Grégoire, Chem.–Eur. J., 2011, 17, 13603.
61. D. Collin, R. Covis, F. Allix, B. Jamart-Grégoire and P. Martinoty, Soft Matter, 2013, 9, 2947–2958.
62. N. Amanokura, K. Yoza, H. Shinmori, S. Shinkai and D. N. Reinhoudt, J. Chem. Soc., Perkin Trans. 2, 1998, 2585.
63. M. Bielejewski, A. Lapinski, J. Kaszynnska, R. luboradzki and J. Tritt-Goc, Tetrahedron Lett., 2008, 49, 6685.
64. S. H. Seo and J. Y. Chang, Chem. Mater., 2005, 17, 3249.
65. F. Allix, P. Curcio, Q. N. Pham, G. Pickaert and B. Jamart-Grégoire, Langmuir, 2010, 26, 16818.
66. B. Jamart-Grégoire, N. Brosse, Q. N. Pham, D. Barth, A. Scondo and A. Degiovanni, Patent INPL Br. Fr. 09/53363, 2009.
67. B. Jamart-Grégoire, N. Brosse, Q. N. Pham, D. Barth, A. Scondo and A. Degiovanni, PCT 133798 A2, 2010.
68. B. Jamart-Grégoire, N. Brosse, Q. N. Pham, D. Barth, A. Scondo and A. Degiovanni, Patent US 2012/0097884 A1, 2012.
69. M. A. Nilsson, R. J. Daniello and J. P. Rothstein, J. Phys. D: Appl. Phys., 2010, 43, 045301.

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