Structural Relationships in 2,3-Bis-n-decyloxyanthracene and 12

Aug 16, 2010 - Sukumaran Santhosh Babu , Vakayil K. Praveen , and Ayyappanpillai ... Sounak Dutta , Tanmoy Kar , Deep Mandal , and Prasanta Kumar Das...
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J. Phys. Chem. B 2010, 114, 11409–11419

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Structural Relationships in 2,3-Bis-n-decyloxyanthracene and 12-Hydroxystearic Acid Molecular Gels and Aerogels Processed in Supercritical CO2 Pierre Terech,*,† Cyril Aymonier,‡ Anne Loppinet-Serani,‡ Shreedhar Bhat,§ Supratim Banerjee,§ Rajat Das,§ Uday Maitra,§ Andre´ Del Guerzo,⊥ and Jean-Pierre Desvergne⊥ INAC/SPrAM, UMR 5819 (CEA-CNRS-UJF) CEA-Grenoble, 38054 Grenoble cedex 9, France, CNRS, UniVersite´ de Bordeaux, ICMCB, 87 AVenue du Dr. Albert Schweitzer, 33608 Pessac Cedex, France, Department of Organic Chemistry, Indian Institute of Science, Bangalore 560012, India, and Groupe Nanostructures Organiques, Institut des Sciences Mole´culaires UMR 5525, UniVersite´ Bordeaux 1, CNRS, 351 Cours de la Libe´ration, 33405 Talence Cedex, France ReceiVed: May 26, 2010; ReVised Manuscript ReceiVed: July 27, 2010

Supercritical carbon dioxide is used to prepare aerogels of two reference molecular organogelators, 2,3-bisn-decyloxyanthracene (DDOA) (luminescent molecule) and 12-hydroxystearic acid (HSA). Electron microscopy reveals the fibrillar morphology of the aggregates generated by the protocol. SAXS and SANS measurements show that DDOA aerogels are crystalline materials exhibiting three morphs: (1) arrangements of the crystalline solid (2D p6m), (2) a second hexagonal morph slightly more compact, and (3) a packing specific of the fibers in the gel. Aggregates specific of the aerogel (volume fraction being typically φ ≈ 0.60) are developed over larger distances (∼1000 Å) and bear fewer defaults and residual strains than aggregates in the crystalline and gel phases. Porod, Scherrer and Debye-Bueche analyses of the scattering data have been performed. The first five diffraction peaks show small variations in position and intensity assigned to the variation of the number of fibers and their degree of vicinity within hexagonal bundles of the related SAFIN according to the Oster model. Conclusions are supported by the guidelines offered by the analysis of the situation in HSA aerogels for which the diffraction pattern can be described by two coexisting lamellar-like arrangements. The porosity of the aerogel, as measured by its specific surface extracted from the scattering invariant analysis, is only 1.8 times less than that of the swollen gel and is characteristic of a very porous material. Introduction A gel is a heterogeneous system made up of a 3D interconnected solidlike network embedded in a liquid. With molecular gels,1 the structure of the connected units is hierarchically developed from molecules to self-assembled fibers, their bundles, and the resulting self-assembled fibrillar network (SAFIN). The volume fraction of the SAFIN in a gel is classically less than 1%, and the relationship between the chemical structure of the low-mass molecular gelator and its ability to develop spontaneous 1D self-assembly is still largely unknown, except for a few particular systems.2 The panel of gelators and liquids available for developing appropriate intermolecular interactions and subsequent supramolecular connectivity and gels is extremely large. The complexity of a multiscale structural investigation of such materials is due to the heterogeneous nature of the SAFINs, the random distribution of mesoscaled domains, and the intrinsic metastability of the gels. There are a variety of molecular packing modes that can use either arrangements specific of a type of liquid in which gels are formed or simply the molecular ordering of crystals grown in nongelating liquids.2,3 The main characteristic of a SAFIN is the extreme dispersion degree of the molecular gelator distributed in the fibers. The * To whom correspondence should be addressed. E-mail: pierre.terech@ cea.fr. † CEA-CNRS-UJF. ‡ CNRS, Universite´ de Bordeaux. § Indian Institute of Science. ⊥ Institut des Sciences Mole´culaires UMR 5525, Universite´ Bordeaux.

residual interfiber potential is short-range-attractive and is responsible for the intrinsic metastability of the gels. The dispersion property can be interesting, in particular, for catalytic4 purposes. The tunable porosity of the self-assembled mesh can also be used for the entrapment and slow release of active drugs at biological targets under physiological pH.5 The precise molecular aggregation mechanism, the shape and dimensions of the cross-section, the flexibility of the fiber, the supramolecular chirality, the aggregation number per unit length, the radial monodispersity, the scission energy of the fiber, the interaction and relaxation modes, and the spatial distribution of fibers and junction zones in the SAFIN are all parameters that are system-dependent and sensitive to the thermal and mechanical histories of the specimen. Such features make SAFINs complex molecular materials. The swollen SAFINs found in molecular gels have been extensively studied6-9 because of their fascinating rheological properties. From the gels, two types of solids can be generated: xerogels and aerogels. Xerogels are made up by evaporation of the liquid component from a swollen gel and following various experimental conditions (fast versus slow evaporation, freeze-drying, etc.). Because the migration of the liquid-air interface at the meniscus induces significant changes of the contact angle together with a modification of the flow near the contact line,10 SAFINs undergo severe damage at various length scales. The morphological consequences were demonstrated by transmission electron microscopy (TEM) experiments on an androstanolbased cyclohexane gel delivering helical fibers.11 In such a system, overtwisted structures were observed in the filamentary

10.1021/jp104818x  2010 American Chemical Society Published on Web 08/16/2010

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SCHEME 1a

a

Left, DDOA; right, HSA.

structures after shrinkage of the SAFIN in the manner of overtwists in DNA supercoils.12 By contrast, in an aerogel obtained by nondestructive elimination of the solvent, the hierarchical architecture of the SAFIN should be preserved. Replacing the solvent in swollen gels with gas without shrinkage of the SAFIN can deliver aerogels. The challenge was first attempted by S. S. Kistler in 193113 who used appropriate liquids (in terms of solubility and chemical activity) and finally replaced them with air via the supercritical domain. Silica and alumina aerogels but also cellulose, agarose, and egg albumin aerogels were prepared using such a method. Aerogels can now be classically prepared using liquid CO2 exchange followed by supercritical CO2 extraction. Silica aerogels can be obtained through the sol-gel route followed by a supercritical drying step. These materials exhibit very low densities (only twice as dense as air), low thermal conductivities, and specific refractive indices and sound velocities. Since then, the chemistry of aerogels has been explored, and sophisticated potentialities for a range of applications have appeared in the fields of thermal or acoustic insulation, Cherenkov counters, luminescent aerogels, nuclear waste storage, catalysis, life science, etc.14 Swollen SAFINs in molecular gels are very fragile structures that cannot bear a liquid substitution operation. In addition, the SAFIN-solvent interaction parameter, responsible for the aggregation-dispersion balance between the fibers, is very sensitive to the solvent type. A first example, combining realand reciprocal-space techniques (namely, TEM and small-angle neutron scattering (SANS)) to investigate a low-pressure sublimation-etching protocol was performed with a molecular organogel network made up of the previously mentioned androstanol derivative in cyclohexane. TEM studies were used to support SANS data and proved that the structures were not altered in the nanoscopic scale by the solvent removal protocol.15 The small-angle X-ray scattering (SAXS) technique is also suitable for the investigation of aerogels, as demonstrated, for instance, by a study of the structural evolution, during the gelation kinetics, of the sol-gel polycondensation of resorcinolformaldehyde and supercritical drying process with CO2.16 This paper reports for the first time on the role of supercritical carbon dioxide (scCO2) in the molecular organization and fibrillar morphology of aerogels of low mass organogelators. The aerogel formation takes advantage of the specific and tunable properties of scCO2 from the one of a liquid to the one of a gas without crossing the liquid-gas equilibrium curve.17 The structural data at the nanoscale extracted from the scattering features of the aerogels will be compared with swollen gels and the polycrystalline solid. Under these conditions, it is expected that the gelator develops hierarchical arrangements related to packing modes found in the crystalline powder, the gel, or xerogel phases. A fatty acid (12-hydroxstearic acid, HSA) organogel is also used as a reference system and, in particular, for scattering measurements.18 2,3-Didecyloxyanthracene (DDOA) is so far the only molecular gelator to form an aerogel directly in supercritical CO2 (two other examples are known but with compounds including H-bonding urea groups).19,20 DDOA is known as a versatile gelator, with unique luminescent properties

and showing a propensity to embed inorganic nanoparticles.17,21-24 The small-angle scattering technique is a noninvasive technique suitable for the present investigation of the hierarchical structures in molecular gels, aerogels, xerogels, and solid networks.25 Experimental Section Aerogels prepared from supercritical (sc) CO2 are highly porous fibrillar materials. The absence of interface for CO2 pressures and temperatures above pc (7.38 MPa) or Tc (31 °C) preserves the system from destructive effects of the liquid-air meniscus migrating through the fragile SAFIN during evaporation. The direct formation of the aerogel in scCO2 can require higher pressures and temperatures than for a classical supercritical drying, since the first step is the solubilization of the gelator.17,26 The homogeneity of the solution was verified by means of a high-pressure/-temperature cell equipped with sapphire windows. Taking advantage of previous studies of the nucleation and growth of crystalline particles in supercritical fluids27 and the physical chemistry of molecular gels,1 the context of the present study is thereby open to the potential observation of various crystallographic morphs, nanoscale morphologies, and hierarchical interconnectivities. Synthesis of DDOA Aerogels. In a typical experiment, DDOA in the form of a powder is charged into a 25 mL stainless steel reactor, and the CO2 pressure is adjusted to 5 MPa. The temperature of the reactor is increased to 80 °C. Under these conditions, DDOA melts and remains in the form of a liquid. The pressure is adjusted to 25 MPa, and the temperature, to 90 °C. DDOA is dissolved in scCO2, and the system is slowly (30 min) cooled to 40 °C with a pressure decrease to 10 MPa. The pressure is then released at a temperature between 35 and 40 °C, not to cross the liquid-gas equilibrium curve. The material isolated (density ≈ 2 mg cm-3) from the top portion of the reactor is a white, light fibrous aerogel. Typically, 20 mg of DDOA was charged in the reactor. The aerogel was conserved in a glass pill box under Ar coated with an aluminum film for a light protection. Synthesis of HSA Aerogels. In a typical experiment, HSA in the form of a powder (as received from Aldrich, purity 99%) is charged into a 25 mL stainless steel reactor, and the CO2 pressure is adjusted to 5 MPa. The temperature is then increased to 95 °C, corresponding to a pressure of 11 MPa (batch reactor). The CO2 pressure of the reactor is then adjusted to 25 MPa. Under these conditions, HSA melts and is solubilized in scCO2. The system is then cooled at 40 °C with an external water bath, and the pressure drops to 10 MPa. At this stage, the pressure is released at a temperature between 35 and 40 °C, not to cross the liquid-gas equilibrium curve. The material isolated from the top portion of the reactor is a white, light fibrous aerogel. Typically, 20 mg of HSA was charged in the reactor. The aerogel was conserved in a glass pill box under Ar coated with an aluminum film for a light protection. SANS and SAXS Measurements. Small angle neutron scattering experiments were performed at the European neutron source of the Institut Laue Langevin (ILL, Grenoble, France) using the large dynamic small-angle diffractometer D22.28 Fully deuterated solvents (toluene, octanol) were used to restrict the incoherent scattering to the protons of the DDOA or HSA

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Figure 1. SAXS of the DDOA crystalline solid. The first five peaks are considered: the sequence is 0.0961, 0.168, 0.193, 0.256, 0.288 Å-1 for a spacing of 1, 1.75, 2.01, 2.67, 3.00. For a 2D p6m hexagonal crystallographic space group, reflections [10], [11], [20], [21], [30] would deliver a theoretical spacing sequence of 1, 3, 4, 7, 9 (1, 1.732, 2.0, 2.645, 3.0).

molecules themselves. The isotropic 2D scattering patterns were radially averaged using standard correction procedures. Three distances (1.1, 5.0, and 20 m) at a wavelength λ ) 6 Å were used to cover the range of scattering vector Q from 0.0022 to 0.34 Å-1 where |Q| ) Q ) (4π sin θ)/λ, λ being the neutron wavelength, and θ, half the scattering angle. In particular, the subtraction of the incoherent scattering was made assuming that the aggregates exhibit sharp interfaces with the surrounding liquid phase. A flat incoherent background arising from the protons of the DDOA or HSA molecules was subtracted, and its level was adjusted so that the intensity decay in the large-Q range is the closest to a Q-4 profile. The samples were contained in Suprasil Quartz (Hellma) cells (path lengths: 1 mm for gels and 1 mm or 1 cm for aerogels). Small angle X-ray scattering measurements were achieved at the European synchrotron source (European Synchrotron Radiation Facility, ESRF, Grenoble) using the ID01 and BM32 beamlines.29 Protonated solvents were used, and samples were contained in homemade cells with Capton or mica windows. The experimental Q-range submitted to analysis was 0.0035-0.35 Å-1 (sample-detector distance ) 75.4 cm, E ) 8.048 06 keV, λ ) 12.398/E ) 1.540 55 Å). Scanning electron microscopy (SEM) experiments used a field emission gun microscope Hitachi 4100-Zeiss at an acceleration voltage in the range 1-5 kV. Results and Analysis 1. DDOA. The molecular organization in the crystalline DDOA can be considered as the “reference” state for the structural investigation of aerogels and gels. DDOA molecules in the solid state are known to pack according to a hexagonal arrangement.30 Figure 1 shows the diffraction pattern of the DDOA solid as observed in the present experimental SAXS conditions. The first five peaks, 0.0961-0.168-0.193-0.2560.288 Å-1 (relative spacing sequence 1:1.75:2.01:2.67:3.00), correspond to a 1:3:4:7:9 theoretical sequence for [10], [11], [20], [21], [30] diffracting planes in a 2D p6m hexagonal ordering satisfying relation 1,

Qhk )

2π√h2 + hk + k2 d

(1)

h, k being the Miller indices, and d, the Bragg lattice spacing.

Figure 2 presents the SAXS pattern for a DDOA aerogel (Figure 2A) and focuses on diffraction differences between the solid and aerogel states (Figure 2B). First, the aerogel presents a sharp low-Q intensity decay (feature I, absent in the solid). Second, a broad peak at ∼0.11 Å-1 appears in the pattern (feature II) that is absent in the solid. Third, the sequence (feature III) of peaks (0.0946-0.166-0.190-0.249-0.282 Å-1) corresponding to a sequence 1:1.754:2.00:2.63:2.98) confirms that the aerogel is also a crystalline organization with a p6m hexagonal packing having a slightly larger cell parameter than for the solid (66.4 Å-1 versus 65.4 Å-1). Furthermore, the diffraction pattern is enriched with new peaks more visible at large Q-values. The relative amplitudes of the peaks in the hexagonal sequence remain roughly similar for the aerogel and solid states. The difference in the value of the cell parameter is minute (∼1 Å) and is only a small fraction of the molecular length 〈lmol〉 ≈ 24 Å. Figure 2B clearly shows that the large-Q diffraction peaks corresponding to [21], [30] planes are split into two components. One of the components corresponds to the genuine crystalline arrangement (Figure 1). It is also observed that the resolution of the diffraction peaks is greater than that for the solid phase (see Figure 2B). To illustrate, reflection [11] shows a width at midheight of 0.0099 Å-1 for a position Q ) 0.166 Å-1. The average size Sc of the crystalline objects can be extracted from expression 2a.

SC )

β1/22 )

[

0.94λ β1/2 cos θ

0.94λ SC cos θ

]

2

+ [4ε tan θ]2

(2a)

(2b)

where λ is the source wavelength (1.540 55 Å), β1/2 is the full width at half-maximum (FWHM) at the peak position 2θ, and ε is the strain.31 Although the extraction of absolute values of the average size of the crystallites should take into account the instrumental broadening, an indicative value of ∼900 Å can already be mentioned for the aerogel. For the solid, β1/2 ) 0.0099 Å-1 at Q ) 0.168 Å-1 provides an identical Sc value for the coherently diffracting domains. This result is surprising, since the highest dispersion degree of DDOA should be expected in the aerogel. The similarity of the average size Sc for the crystallized domains is only apparent because, as mentioned, the diffraction pattern is split. For instance, the apparent width of the diffraction peaks corresponding to the [11] and [20] planes in the aerogel is comparable to that in the solid, since two split peaks contribute to their widths (Figure 2B). To extract the width of each diffracting component, the Scherrer determination is attempted with [21] and [30 diffraction planes, for which the split is clearly apparent. Figure 3 shows such an analysis for the [30] diffraction, in comparison with the corresponding single peak in the solid powder. The peaks are modeled by Gaussian profiles: a single one for the solid DDOA and two for the aerogel. The width at midheight is related to the variance of the Gaussian following FWHM ) 2(2 ln(2))σ ) 2.3548σ. The variance of the Gaussian for the solid or the aerogel is significantly larger than the one for each component (0.005, as compared with 0.0033 and 0.0025). Interestingly, the component at Q ) 0.2571 Å-1 is located at a position (0.2564 Å-1) very comparable to that of the solid. It is concluded that the molecular ordering in the aerogel is developed over larger distances than in the solid.

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Figure 2. A: SAXS diffraction patterns of DDOA aerogel and DDOA crystalline solid states. Features I, II, III are pointed out by arrows (see text). B: Enlargement of the large-Q region to reveal the split of the diffraction peaks ([11], [20], [21], [30] diffracting planes).

Figure 3. Diffraction by [21] reticular planes in DDOA aerogel and crystalline powder (dots are experimental data, and full lines are best agreements with Gaussian profiles). Solid: I ) 2.1(1/σ(2π)) exp -(((Q - 0.2564)/σ)2/2) + 89; σ ) 0.005. Aerogel: first component at Q ) 0.25 Å-1, I ) 0.6(1/σ(2π)) exp -(((Q - 0.25)/σ)2/2) + 86; σ ) 0.0025. Second component at Q ) 0.2571 Å-1, I ) 0.525(1/σ(2π)) exp -(((Q-0.2571)/σ)2/2) + 86; σ ) 0.0033.

Similar conclusions can be drawn if Lorentzian shapes are used for the modeling (not shown). At this stage, it is speculated that DDOA aggregates in the aerogel bear significantly fewer defaults and residual strains than in a polycrystalline aggregate grown from a solution. The proportions of the two diffracting components can be estimated from areas A of the related peaks (A ∝ kσ2, k being the prefactor of the Gaussian components). Thereby, the first component at Q ) 0.250 Å-1 and σ ) 0.0025 (twice as narrow as in the solid) has a volume fraction φ ≈ 0.60. Nevertheless, the Debye-Scherrer expression 2a should be replaced by the more general Williamson-Hall expression 2b, including the strain effects.32 The diffraction patterns are all issued from the same SAXS setup with identical collimation conditions. Consequently, the instrumental broadening effect is identical for aerogels, gels, or solids. The existence of another very similar hexagonal morph with a slightly larger cell parameter (compared to the reference solid state) is indicative of the expansion caused by an uniform strain. There is no broadening associated with this type of strain. In addition, nonuniform strain may exist and could lead to systematic shifts of atoms and therefore to peak broadening. Point defects, plastic deformation and poor crystallinity can usually generate this type of strain. Because the size and strain broadening show a different θ dependence, the two effects could be distinguished by plotting β1/22 cos2 θ versus sin2 θ. Williamson and Hall proposed a method32 for size- and strain-

broadening deconvolution: the crystallite size SC could be extracted from the y-intercept of the fit, whereas the strain is related to its slope. Unfortunately, the present experimental X-ray diffraction patterns of DDOA aerogels exhibit an insufficient Q-separation between the peaks of the two components to extract their individual β1/2 parameters (see Figure 2a). To be exhaustive, the stacking fault density and the potentiality for nonuniform deformation should also be taken into account. It is reasonable to expect a density of ordering defects much lower in aerogels due to the annealing effect of high temperatures used, and the supercritical conditions should also significantly lower the occurrence of stresses (generated by migration of the liquid-air meniscus). It is noteworthy that, due to the polycrystalline nature of the aggregates, the size of a coherently diffracting domain is not the size of the embedding aggregate. As far as low-molecularweight gelators are concerned, there is always a conflict between the crystallization and gelation phenomena: good gelators are difficult to crystallize and single crystals may not be easily obtainable. DDOA is an excellent organogelator, and the growth of single crystals has not yet been possible.24 It is a classical method in scattering experiments to vary the contrast between the scatterer and its embedding medium so as to reveal specific zones in heterogeneous aggregates.33 Changing the nature of the incident source is also a method for contrast variation.25 Thereby, SANS experiments were performed in an extended Q-range with DDOA gels and aerogels to complete the structural information. Figure 4A shows the scattering differences between the two states of the DDOA gelator (gel versus aerogel). The gel shows a sharp low-Q intensity decay that can be accounted with a Debye-Bueche model (DB)34 (expression 3) describing a biphasic system made up of a random distribution of heterogeneities (bundles in a SAFIN). Figure 4B illustrates such a DB analysis from which the average correlation length, ξ, of the heterogeneities is deduced: ξ is ∼350 Å.

I(Q) )

I0

(1 + ξ2Q2)2

(3)

where ξ is the correlation length of the heterogeneities and I0 is the zero angle intensity. The aerogel shows a more pronounced low-Q intensity decay corresponding to larger aggregates with sharp scattering interfaces. The Q-profile of the decay is IRQ-3.7, and the exponent 3.7 is close to the expected one (4) for interfacial scattering of large aggregates. The DB analysis (Figure 4B) confirms that the correlation length is significantly increased (up to ∼1200 Å).

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Figure 4. SANS of DDOA gel and aerogel obtained in scCO2. A: Comparison between DDOA/d-butanol gel (C ) 0.024 g cm-3) and the aerogel. Dotted vertical bars point at two Bragg peaks (Q ) 0.100 and 0.175 Å-1). B: Debye-Bu¨eche analysis following expression 3 for a gel (DDOA/ d-decane, C ) 0.033 g cm-3 ξgel ) 345 Å) and an aerogel (ξaerogel ) 1200 Å).

Figure 5. SAXS of DDOA aerogel and swollen organogels. EtOH gel, C ) 0.02 g cm-3; DMSO gel, C ) 0.02 g cm-3. A: global view. B: close-up of the first diffraction peak.

In the gel, at Q ≈ 0.100 Å-1 (〈d〉 ≈ 63 Å), an intense and broad peak is known to correspond to the periodic [10] diffracting planes of the hexagonal molecular ordering in the fibers and their bundles of the related SAFIN.30 Interestingly, the peak is also found in the SAXS pattern of the aerogel (Figure 2A) at Q ≈ 0.11 Å-1 (feature II, 〈d〉 ≈ 57 Å). The neutron data demonstrate that the material obtained by the scCO2 route has similarities with the SAFIN in a gel (Figure 4A). It exhibits larger crystalline-like aggregates with a reduced level of defects. Indeed, two diffraction peaks can be seen in Figure 4A (0.100 and ∼0.175 Å-1), but only one is clearly seen in the gel (Q ≈ 0.100 Å-1). The corresponding diffracting sequence is consistent with a p6m hexagonal symmetry (1:3 # 1.75). Stresses generated by the sol-gel transition are transported through the interconnected SAFIN and finally are revealed by a broadened reflection corresponding to [10] planes. Nevertheless, the scCO2 protocol is not stress-free. DDOA cannot be obtained as single crystals, and it can reasonably be speculated that SC should be small in the solid (obtained from a saturated solution) and might be increased in the aerogel (assuming that stacking faults, dislocations and antiphase boundaries are less promoted). To summarize the present analysis, a DDOA aerogel can have up to three types of coexisting molecular arrangements: (i) a p6m morph, (ii) a p6m morph uniformly strained (dilation), and (iii) a morph for the packing in the fibers of the swollen gel. Peaks observed in the most diffracting gels (Figures 5) are drastically broadened in comparison with the aerogel. In a DMSO gel, FWHM is ∼0.0069 Å-1, but it is 0.0029 Å-1 in the aerogel and 0.0088 Å-1 in the solid. The global interplay

of the spatial extension of coherently diffracting domains and the density of their defects and strains suggests that the aerogel is the most ordered and stress-minimized arrangement. After focusing on the width of the peaks, the variation of their Q-positions (with respect to the reference solid) is instructive. Table 1 lists the position of the diffraction features of the different gels and solid and aerogel materials. Table 1 shows that the Q-sequence of the diffraction peaks is almost unchanged among the solid, aerogel, and crystalline gels (ethanol and DMSO). The latter are less stable than the ones in butanol or decane. It is accepted that structural features at the nanoscale can affect in a complex manner the macroscopic behaviors. In particular, the metastability of the gels depends on the fiber-fiber interaction potential, and the propensity to form bundles of fibers (junction zones in the SAFIN), revealed by diffraction peaks in SAXS curves, is frequently concomitant with the startup of a solid-liquid phase separation process. With DDOA, the Q-sequence of diffraction peaks is very close to the expected one for a p6m crystallographic space group. Nevertheless, there are variations of the absolute Q-positions that deserve a special focus. The [10] diffraction is shifted to larger Q-values (Table 1) in the following sequence: aerogel < DMSO gel < butanol gel < decane gel. The [10] diffraction can also be shifted to lower Q-values in the unique case of the ethanol gel. The aerogel presents [10] diffracting planes at a periodic distance smaller than that for the solid (64.5 versus 65.4 Å). Smaller characteristic distances are observed in the gels (decane, 〈d〉 ) 64.6 Å; butanol, 〈d〉 ) 61.0 Å) except for ethanol (〈d〉 ) 65.6 Å). These minute variations of the packing reflect the influence of the solvent in

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TABLE 1: X-ray Diffraction of DDOA Aerogel, Gel and Solid Statesa system

status

DDOA/decane (C ) 0.0334 g cm-3) DDOA/butanol (C ) 0.0235 g cm-3) DDOA/EtOH (C ) 0.02 g cm-3)

gel gel gel

DDOA/DMSO (C ) 0.02 g cm-3)

gel

DDOA

solid

DDOA

aerogel scCO2

first peak (Å-1), [11]

second peak (Å-1), [11]

third peak (Å-1), [20]

fourth peak (Å-1), [21]

∼0.0972 ∼0.103 0.0957 1 1 0.09788 1 1 0.0961 1 1 0.09735 1 1

0.167 1.74 8.89 0.1691 1.73 0.08 0.1677 1.74 0.58 0.1689 1.73 0.75

0.193 2.02 2.67 0.1953 2.00 0.05 0.1927 2.00 0.26 0.1949 2.00 0.35

0.255 2.66 1.19 0.2567 2.62 0.02 0.2573 2.68 0.21 0.2452 2.52 0.007

a First row, position of Bragg peaks (Å-1); second row, sequence of relative position with respect to the first peak; third row, relative intensity sequence of the first four peaks (when available) normalized to the first Bragg peak ([10] planes).

the molecular ordering and the corresponding cell parameter of the resulting SAFIN. For gels, the diffraction features related to their SAFINs are assumed to depend on concentration and aging conditions of the gels and reveal mainly the existence of nodal zones elaborated by merging the fibers. The sequence of the relative intensities of the peaks for DDOA in the solid and aerogel states is comparable (except the very attenuated [21] diffraction in the aerogel). By contrast, the DDOA/ ethanol gel exhibits a strikingly different intensity sequence strongly amplifying the [11] diffraction pattern. It is interesting to observe that several features (Q-positions, sequence of intensities of the peaks) distinguish the ethanol gel from the others while its main macroscopic originality is its high metastability. Within the bundles of a SAFIN, cylindrical fibers are frequently arranged along a hexagonal lattice and thereby generate a specific diffraction signature.35 This can be the situation in molecular organogels but also in aerogels. The present scCO2 protocol used with DDOA (or HSA) delivers a dispersed network of aggregated gelators without passing through the swollen organogel state. A very subtle balance of interactions (intra and inter) governs the molecular structure of a crystal. There is no physical reason stating that the molecular packing in the aerogel should be identical to that either in the crystalline solid or in the swollen molecular organogel. The solid aerogel is built from a completely different route from which usual thermally equilibrated aggregation-micellization reactions are probably not the principal driving force. Several length scales of organizations are involved in a SAFIN of DDOA molecules: (i) the internal structure of the fibers already involves a hexagonal ordering of the molecules, (ii) the form factor of the aggregates is 1D (cylindrical fibers), (iii) the fibers are arranged along a hexagonal symmetry within the bundles and finally, and (iv) the bundles (nodal zones) are randomly distributed on a macroscopic scale (as confirmed by the isotropic character of the scattering and the SAXS analysis along the Debye-Bueche model). Similarly, the intensity of the X-ray diffraction pattern of a crystallite can be described as the square of the crystal structure factor being the Fourier transform of the convolution of a lattice function multiplied by a size function with a basis function.35-37 As an attempt to account for the variations of Q-locations and sequence of intensities of the diffraction peaks (Table 1), it is instructive to consider a simple theoretical modeling involving cylindrical scatterers distributed along a p6m 2D symmetry. Oster35 and, finally, Glatter36 have described the expected theoretical dif-

fraction patterns in a hexagonal arrangement of n infinitely long cylindrical rods (expression 5),

I)

()

1 2 F (QR) n2

n

n

p

q

∑ ∑ J0(Qrpq)

(5)

where J0 is the zero-order cylindrical Bessel function, rpq is the distance between the centers of the pth and qth cylinder of radius R, and Q is the scattering vector already defined. F(QR) is the scattering form factor of the 1D scatterers. For DDOA, cylindrical fibers are grown in the gels, and expression 6 can be used.

F2(Q) )

[

2J1(QR) πL A∆F Q QR

]

2

(6)

where R is the radius of cylindrical fibers of cross section A and volume contrast ∆F. Expressions 7a and 7b describe the scattering by systems including seven cylinders (first circle of scatterers) or 61 cylinders (fourth circle of scatterers), respectively,38

I)

( 491 )F ( 2γx )[7 + 24J (x) + 6J (2x) + 12J (√3x)] 2

0

0

0

(7a) I ) (1/62)F2(x/(2γ))[61 + 312J0(x) + 264J0(√3x) + 258J0(2x) + 432J0(√7x) + 174J0(2√3x) + 204J0(3x) + 336J0(√13x) + 264J0(√19x) + 96J0(3√3x) + 150J0(4x) + 240J0(√21x) + 180J0(2√7x) + 120J0(√37x) + 30J0(4√3x) + 96J0(5x) + 144J0(√31x) + 96J0(√39x) + 72J0(7x) + 54J0(6x) + 72J0(√43x) + 36J0(2√13x) + 24J0(√57x) + 6J0(8x)] (7b) with x ) 2γQR In the case that plain cylinders are in contact (γ ) 1), the use of expression 5 for n ) 61 shows that the peaks are narrower and large-Q shifted, and the second peak (corresponding to [20] diffracting planes) is enhanced (Figure 6A) as compared with

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Figure 6. Theoretical modeling (expressions 5 and 7) describing the diffraction pattern of plain cylindrical fibers arranged along a hexagonal symmetry. A: Fibers in contact (γ ) 1), n ) 61 (full line), peaks are at 6.93,12.42,14.78,19.03; n ) 7 (dashed line), peaks are at 8.66,11.62,15.54,18.32. B: Separated fibers (γ ) 0.5), n ) 61, peaks are at 7.55,12.39,14.50,19.93; n ) 7, peaks are at 8.06,11.85,14.66,18.32. Abscissa is in reduced units (2QR).

the case n ) 7. The sequence of peaks (see the caption for Figure 6A) evolves from 1 to 1.34:1.79:2.1 at n ) 7 to 1:1.79:2.13:2.7 at n ) 61, whereas for the infinite and perfect crystal for which the diffraction nodes are punctual, the sequence is 1:1.73:2.0: 2.65. The effects of the convolution of the punctual p6m diffraction with the scattering form factor function of cylinders can be summarized as follows: The sequence of peaks comes closer to the asymptotic “punctual limit” when the number of fibers in contact is increased. Nevertheless, the coincidental occurrence at comparable Q-values of a structure peak, and a form factor oscillation may strongly affect the resulting amplitude of the peak.39 The absolute position of the first peak is decreased when the size of the bundle is increased. Figure 6B shows a parallel situation in which the cylindrical fibers are separated (γ ) 0.5). The sequence of spacing varies from 1:1.47:1.82:2.27 for n ) 7 to 1:1.64:1.93.2.51 for n ) 61. Similarly, the sequence of relative spacing of the peaks is closer to the theoretical limit of punctual scatterers for n increasing. In contrast to the situation of close contact (Figure 6A), the low-Q shift of the first peak appears less sensitive to the variation of the number n of fibers per bundle. Such theoretical qualitative trends could be one option to interpret the DDOA scattering data listed in Table 1. First, the experimental sequences of relative spacing are close to the expected one for a p6m symmetry: 〈n〉 is large. Previous rheological measurements confirm that DDOA organogels are physical gels with large values of the yield stress at which SAFINs are broken and a liquidlike flow occurs.30 As mentioned before, the absolute variations of the Q-position of the first peak can be considered with reference to the solid state (Q[10] ) 0.0961 Å-1). EtOH gels exhibit a SAXS with a lower Q[10] value (0.0957 Å-1) and are very metastable, whereas gels in DMSO or butanol exhibit larger Q[10] values (0.0978 and 0.103 Å-1, respectively) and are more stable. The absolute value itself is related to the internal packing of the fibers and is not discussed here. Considering only the relative trend of the Q[10] position with the packing parameter γ of the fibers within the bundle, it could be speculated that separated fibers lead to increased metastability of the network. EtOH gels also exhibit a remarkable enhancement of the intensity of [11] diffraction (see Table 1). These features might be considered as indicators of the metastability of the DDOA gels. Most of the gels exhibit only the first Bragg peak that limits the analysis along the above guidelines and, in particular, the variation of the macroscopic stability with 〈n〉.

At this stage, it is interesting to refine the knowledge of the structural effects of the protocol using scCO2 with a different organogelator. The racemic acid HSA is known to form organogels in a large variety of organic liquids. This choice is justified, since the SANS signal presents low and intermediate Q-domains rich in scattering features characterizing the nanoscale structures of the fibers. HSA is an excellent organogelator but, like many fatty acids, it is very difficult to crystallize, and only very small crystals can be obtained (0.7 × 0.4 × 0.02 mm).40 2. HSA. The aforementioned SAXS-SANS approach is used with HSA in the organogel, aerogel, and solid phases. The structural specificities of the HSA system are believed to be favorable for a deeper investigation of the structural relationships between these different states. Indeed, the residual free energy potential is less attractive, such that thinner and more isolated fibers are present in the SAFIN that is less prone to phase separation through bundle formation processes. Consequently, the scattering signal is enriched by (i) a clear low-Q asymptotic decay (Q-1), (ii) several form factor oscillations revealing the shape and composition of the cross-section, and (iii) Bragg peaks characterizing the internal structure of the aggregates. These features can be analyzed using the form factor scattering function for fibers having rectangular cross sections (see expression 8),

I(Q) )

(

2Aφ∆F2 Q

)∫ [ π/2

0

sin(Qa sin φ) sin(Qb cos φ) 2 dφ Qa sin φ Qb cos φ (8)

]

where a, and b are half the sides of the section of area A, ∆F is the volume contrast of the 1D species and φ is the trigonometrical integration variable describing the rectangular (or square cross section). The effect of the cross-sectional polydispersity can be calculated by a convolution of expression 8 with a normalized Gaussian with ∆a/a ) ε. The structure of the HSA molecular gels has been extensively studied by SANS and SAXS.41 In toluene gels at C > 3 wt %, the scattering curves are typical of a loose fibrillar network. Typical scattering features of the fibers can be observed as well as the evidence for their interconnections in the network. Thus, a typical scattering curve in the gel phase consists of a low-Q Q-1 decay characterizing the long length of the fibers preceding a sharp Q-4 decay due to the finite size of the cross sections. Then, at larger Q-values, several more or less damped form

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Terech et al.

Figure 7. SAXS curves of HSA aerogel and the reference solid phase (A) and swollen gel in toluene (B, C ) 0.03 g.cm-3).

factor oscillations characterize square cross sections and are followed by Bragg peak(s) typical of periodic HSA reticular planes in monoclinic arrangements. For concentrated gels, a low-Q component of the Debye-Bueche type can complete the pattern and is assigned to the scattering by the nodal domains of the SAFIN. Comparatively, DDOA does not offer such a panel of scattering features that are as many indicators for the SAXS analysis of the effects of the scCO2 protocol in the 10-3000 Å length scale. Figure 7 compares the scattering profiles of the HSA aerogel with the two reference states, swollen gel and crystalline solid. Figure 7A clearly shows that the scattering profiles of the aerogel and swollen gel exhibit differences and similarities in both the low-Q and large-Q regions. Consequently, it can be stated that the scCO2 protocol does not deliver a solid network that has all structural characteristics of the SAFIN in the gel. Figure 7B compares the scattering behaviors of the HSA aerogel with those of the solid. In particular, the low-Q part of the aerogel can be modeled by a power law I ≈ KQ-3.97, and the solid can be described by I ≈ KQ-2.7. This is a first indication that HSA aggregates in aerogels are large crystallites with sharp interfaces. The solid also exhibits diffraction peaks at 0.135, 0.270 (very weak), and 0.408 Å-1 (not shown). The lack of suitable single crystals explains that the polymorphic structure of HSA is still not well understood, although triclinic42 or monoclinic40 organizations have been proposed. To bypass this drawback, the structure of a close homologue, the stearic acid (SA) (CH3(CH2)16COOH), can be considered for tentative structural extrapolations to HSA. SA can exist in the form of monoclinic or triclinic crystals but, due to the lack of the OH group on the alkyl backbone, SA is not a gelator.41 In the triclinic form, the three first peaks (from [010], [020], [030] diffracting planes) are at Q ) 0.133, 0.266, 0.400 Å-1; in the monoclinic form, the sequence is Q ) 0.143, 0.286 (weak), 0.428 Å-1. Figure 7B shows that both HSA aerogel and solid exhibit a first broadened peak (0.135 Å-1) that may correspond to the first peak of the triclinic solid ([010] diffracting planes). The second experimental peak of the HSA solid at 0.270 Å-1 also appears in the aerogel: it is broadened, composite, and with a weak intensity. It may correspond to the [020] diffracting planes of the “extrapolated” triclinic HSA solid deduced from SA. In the experimental HSA aerogel (Figure 7B), the peak at 0.160 Å-1 cannot be indexed in this context (see also Figure 8, showing an extended diffraction pattern of the aerogel). Due to the propensity of SA and HSA to exhibit polymorphism, it can be envisaged that HSA molecules are distributed along two closely related morphs when submitted to the particular scCO2 protocol. The diffraction pattern of Figure 8 can be qualitatively modeled by two coexisting lamellar-like arrangements, the first one with a

Figure 8. X-ray diffraction pattern of HSA aerogel. Indexation according to two lamellar (L1, L2) packing modes. L1 mode, full vertical bars; L2 mode, dotted vertical bars. 0.135, 0.270, 0.405 Å-1; relative spacing sequence 1:2.0:3.0. L2 mode, dashed vertical bars; 0.160, 0.320, 0.480 Å-1; relative spacing sequence 1:2.0:3.0.

periodic distance of 46.5 Å (sequence of peaks at 0.135, 0.270, 0.405 Å-1) and the second one with a periodic distance of 36.9 Å (sequence of peaks at 0.160, 0.320, 0.480 Å-1). The first system corresponds to a packing of HSA bimolecules (2 × 25 Å) with a moderate tilt angle with respect to the fiber axis (∼21°); the second system would involve a 2-fold tilt angle (42°) or interdigitation of the molecules in the manner of Lβ lamellar organizations. In such a scenario, the supercritical CO2 protocol would generate a second and more compact crystalline morph. All diffraction features of the solid HSA (Figure 8) are indexed with such a simplified description. The observation of the generation of a second crystallographic morph was also made with DDOA. Here with HSA, the naive modeling along lamellar (L) organizations emphasizes the identification of the second morph attached to the supercritical protocol. At this stage, it is interesting to evaluate the porosity of the aerogel to compare with that of a swollen gel. A Porod analysis in the large Q-range can be attempted with SAXS data. The slope at Q f ∞ of the scattering function is given by expression 9, S being the total interface of the aggregates.

lim I(Q) ) (∆F)2

2π S Q4

Qf∞

(9)

The asymptotic limit at large Q is identifiable as a plateau in a Q4I-versus-Q plot. Figure 9 shows such a plateau (horizontal dashed line) preceding Bragg peaks in the case of the aerogel and the swollen gel. For the gel, a first well-resolved oscillation

DDOA, HSA Processed in Supercritical CO2

Figure 9. SAXS of HSA gel in toluene (C ) 0.02 g cm-3) and aerogel. Full lines are best theoretically modeled according to expression 8 for plain fibers with square cross sections. Gel: a ) 90 Å, ε ) 0.20. Aerogel: a ) 190 Å, ε ) 0.25.

can be seen that characterizes the monodispersity of the cross sections of the fibers.18 Figure 9 shows that the experimental data are shifted from large to low-Q values from the swollen gel to the aerogel, indicating the presence of larger scatterers when the selfassembly is conducted in liquid CO2 rather than in liquid toluene. Indicative theoretical profiles, following expression 8, confirm the doubling of the characteristic cross-sectional size (from a ) 90 to 190 Å). The first form factor oscillation is less resolved in the aerogel, suggesting that the fiber-fiber interaction potential in toluene is less residually attractive than in carbon dioxide. At larger Q-values, the intensity sharply increases due to the presence of the first Bragg peak around Q ≈ 0.1 Å-1 for both gel and aerogel. Confirmation that HSA aggregates in the aerogel material are fibrillar is also given by electron microscopy experiments. Figure 10A is a view of the aerogel appearing as a piece of cotton

J. Phys. Chem. B, Vol. 114, No. 35, 2010 11417 formed by the overlap of fiberlike species. Figure 10B shows a view of the corresponding SAFIN that is quite similar to that classically observed in molecular organogels.1 The present SEM images are not dedicated to an accurate evaluation of the diameters of fibers in the SAFINs. This latter is better realized from SANS/SAXS measurements as they operate on macroscopic illuminated volumes from which a statistical average can be deduced. As expected, SEM also confirms the fibrillar morphology of the DDOA aerogel network (Figure 10C). In this latter case, a mean minimal diameter of ∼700 Å can be measured that corresponds well to estimations from previous SANS work demonstrating the presence of bundles of at least 500 Å diameter.30 Scattering data can be used to extract the surface area per volume ∑ of the aerogel material. The comparison with the value for the swollen gel is instructive, since this later can be considered as the asymptotic value for HSA in protocols operating in liquid phases. It is related to the porosity of the SAFINs grown in these two distinct conditions (scCO2 versus liquid toluene). Such a Porod analysis assumes aggregates with sharp interfaces (thickness , 2π/Q) and data at large Q with good statistics. To avoid any initial assumption on the structure and contrast of the scatterers (volume fraction, φ), it is advantageous to use the scattering invariant INV, as written in expression 10a. INV captures the total amount of scattering as written in expression 10b. The analytical procedure does not require the use of calibrated data.

Σ ) S/V ) π lim I(Q)Q4 /INV INV )

∫

∞

0

(10a)

Q2 I(Q) dQ ) 2π2V γ(0) ) (∆F)2 φ(1 - φ) 2π2

(10b)

For long 1D scatterers, the contribution of the ends of the rods to the calculated interface is negligible, and it results in Σ

Figure 10. SEM of aerogels obtained by a scCO2 protocol (see the Experimental section). A: individual HSA fibers emerge from the mesh. B: HSA SAFIN. The scale bar (20 µm) is shown. C: DDOA SAFIN. The scale bar (2 µm) is shown.

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Figure 11. Closeup of the first Bragg reflection in HSA aerogel and HSA/cyclohexane gel (C ) 0.03 g cm-3). Vertical bars points at the [010] diffraction: 0.135 Å-1 (aerogel), 0.129 Å-1 (gel), and the dotted thin vertical bar localizes the peak at Q ) 0.160 Å-1 of the second morph L2.

# 2/R. For HSA aerogel, lim Q4I is ∼1.41 × 10-6 cm-1 Å-4 and INV ) 0.000 306 6 cm-1 Å-3, whereas in the toluene organogel, lim Q4I is ∼1.17 × 10-7 cm-1 Å-4 and INV ) 0.000 013 838 cm-1 Å-3. It becomes Σaerogel ) 0.460π10-2 Å-1 and for the gel Σgel ) 0.845π10-2 Å-1. The ratio Σgel/Σaerogel ≈ 1.8 demonstrates that the aerogel is a very porous structure (in particular, as compared with a bulk solid phase or a xerogel). The porosity is of the order of magnitude of the one in the very opened structure of the corresponding HSA SAFIN in a toluene gel. Nevertheless, it is about half as porous as the genuine organogel. This latter feature is also consistent with the observation of two times thicker fibers in the aerogel than in the swollen gel. The close-up view in Figure 11 shows that the peak at Q ) 0.160 Å-1 in the aerogel has completely vanished in the gel. This peak was assigned to the more compact of the two lamellarlike packing modes (L2; see the caption for Figure 8) used in the speculative model to describe the aerogel diffraction pattern. The first peak at Q ) 0.135 Å-1 in the aerogel (identical to the one observed in the solid state) is shifted to low-Q values (Q ) 0.129 Å-1) in the gel. The internal structure of the aggregates in the swollen gel is less compact, implying either a decrease in the tilt angle of the HSA dimer with respect to the fiber axis or the insertion of solvent molecules. The first Bragg peak in the gel is much broader (FWHM ) 0.0325 Å-1) than in the aerogel (FWHM ) 0.0108 Å-1), suggesting that the number of defects and residual strains is dramatically larger in the solidlike aggregates of the gel than in those of the aerogel. Such a broadening of the peak(s) in the gel phases was also observed with DDOA (vide supra). Conclusion The analysis of the present SAXS and SANS experiments provided first-hand information on the structural organization of SAFINs formed with the small-sized DDOA and HSA gelators. The in-depth investigation of their aerogels, organogels, and microcrystalline phases shows the structural implications of the supercritical CO2 protocol. The process does not provide a perfect replica of the SAFIN formed in the swollen gel. The aerogel materials are fibrillar networks with a very high porosity (twice that of the gel) formed with thicker fibers (diameter is doubled) and bundles exhibiting highly crystalline arrangements (reduced level of imperfections). Remarkably, it has been discovered that the process generates two packing modes

Terech et al. coexisting in the aerogel: one is the genuine solid arrangement, and the other is a more compact version of the same crystallographic packing. The supercritical procedure exploited herein appears, thus, as a method of choice to produce highly crystalline SAFINs. The conclusions emerging from these SAXS and SANS experiments also contribute to more confidently extrapolate a precise supramolecular organization of DDOA in gels. We thus propose that the DDOA molecules assemble in the gel SAFIN the same way as analogues with shorter alkoxy chains pack in large crystals.42] This structure corresponds to coaxial anthracenes organized at 60° into triads that pack head-to-tail into sheets forming the larger structures. This hexagonal structure was previously shown to be compatible with spectroscopic analyses and molecular modeling.24,43 This unusual packing seems inherently related to the 2,3-disubstitution of the anthracene and is found in self-assembled organo- and aerogels as well as in crystals. The same driving forces, mostly consisting of weak intermolecular interactions and the packing of the chains, appear to be at the origin of the self-assembly as well as the crystallization processes. Acknowledgment. This work has been funded by the IndoFrench Center for the Promotion of Advanced Scientific Research (IFCPAR, Project 3605-2). The authors thank the CNRS, the French Ministry of Education and Research, and the Re´gion Aquitaine for financial support. P.T. is grateful to O. Glatter for useful discussion. Institut Laue Langevin (ILL, Grenoble, France) and the European Synchrotron Radiation Facility (ESRF), Grenoble, France are acknowledged for providing access and technical help at the beamlines. References and Notes (1) Molecular Gels: Materials with Self-Assembled Fibrillar Networks; Weiss, R. G., Terech, P., Eds.; Springer: Dordrecht, The Netherlands, 2006, p 976. (2) Dastidar, P. Chem. Soc. ReV. 2008, 37, 2699–2715. (3) Ostuni, E.; Kamaras, P.; Weiss, R. G. Angew. Chem., Int. Ed. Engl. 1996, 35, 1324–1326. (4) Rodriguez-Lansola, F.; Escuder, B.; Miravet, J. F. J. Am. Chem. Soc. 2009, 131, 11478–11484. (5) Naskar, J.; Palui, G.; Banerjee, A. J. Phys. Chem. B 2009, 113, 11787–11792. (6) Terech, P.; Weiss, R. G. Chem. ReV. 1997, 97, 3133–3159. (7) Abdallah, D. J.; Weiss, R. G. AdV. Mater. 2000, 12, 1237–1247. (8) Estroff, L. A.; Hamilton, A. D. Chem. ReV. 2004, 104, 1201–1217. (9) Sangeetha, N. M.; Maitra, U. Chem. Soc. ReV. 2005, 34, 821–836. (10) Berteloot, G.; Pham, C. T.; Daerr, A.; Lequeux, F.; Limat, L. EPL 2008, 83, 14003. (11) Terech, P.; Wade, R. H. J. Colloid Interface Sci. 1988, 1252, 542– 551. (12) Charvin, G.; Strick, T. R.; Bensimon, D.; Croquette, V. Biophys. J. 2005, 89, 384–392. (13) Kistler, S. S. J. Phys. Chem. 1932, 36, 52–64. (14) Pierre, A. C.; Pajonk, G. M. Chem. ReV. 2002, 102, 4243–4265. (15) Wade, R. H.; Terech, P.; Hewat, E. A.; Ramasseul, R.; Volino, F. J. Colloid Interface Sci. 1986, 114, 442–451. (16) Tamon, H.; Ishizaka, H. J. Colloid Interface Sci. 1998, 206, 577– 582. (17) Placin, F.; Desvergne, J. P.; Cansell, F. J. Mater. Chem. 2000, 10, 2147–2149. (18) Terech, P. J. Phys. II France 1992, 2181–2195. (19) Shi, C.; Huang, Z.; Kilic, S.; Xu, J.; Enick, R. M.; Beckman, E. J.; Carr, A. J.; Melendez, R. E.; Hamilton, A. D. Science 1999, 286, 1540– 1543. (20) Paik, I.-H.; Tapriyal, D.; Enick, R. M.; Hamilton, A. D. Angew. Chem., Int. Ed. 2007, 46, 3284–3287. (21) Brotin, T.; Utermo¨hlen, R.; Fages, F.; Bouas-Laurent, H.; Desvergne, J. P. Chem. Soc., Chem. Comm. 1991, 416–418. (22) Placin, F.; Colomes, M.; Desvergne, J.-P. Tetrahedron Lett. 1997, 38, 2665–2668.

DDOA, HSA Processed in Supercritical CO2 (23) Shklyarevskiy, I. O.; Jonkheijm, P.; Christianen, P. C. M.; Schenning, A. P. H. J.; Del Guerzo, A.; Desvergne, J.-P.; Meijer, E. W.; Maan, J. C. Langmuir 2005, 21, 2108–2112. (24) Olive, A. G. L.; Raffy, G.; Allouchi, H.; Le´ger, J.-M.; Del Guerzo, A.; Desvergne, J.-P. Langmuir 2009, 25, 8606–8614. (25) Terech, P. Small-angle Scattering and Molecular Gels. In Molecular Gels: Materials with Self-Assembled Fibrillar Networks; Weiss, R. G., Terech, P., Eds.; Springer: Dordrecht, the Netherlands, 2006; p 275-324. (26) Cansell, F.; Aymonier, C.; Loppinet-Serani, A. Curr. Opin. Solid State Mater. Sci. 2003, 7, 331–340. (27) Cansell, F.; Aymonier, C. J. Supercrit. Fluids 2009, 47, 508–516. (28) Www.Ill.Fr. (29) Www.Esrf.Fr. (30) Terech, P.; Desvergne, J. P.; Bouas-Laurent, H. J. Colloid Interface Sci. 1995, 174, 258–263. (31) Scherrer, P. Nachr. Ges. Wiss. Gottingen, Math-Phys. Kl. 1918, 26, 98–100. (32) Williamson, G. K.; Hall, W. H. Acta Metallurg. 1953, 1, 22–31. (33) Jacrot, B. Rep. Prog. Phys. 1976, 39, 911–953.

J. Phys. Chem. B, Vol. 114, No. 35, 2010 11419 (34) Debye, P.; Bueche, A. M. J. Appl. Phys. 1949, 20, 518–526. (35) Oster, G.; Riley, D. P. Acta Crystallogr. 1952, 5, 272–276. (36) Freiberger, N.; Glatter, O. J. Phys. Chem. B 2006, 110, 14719– 14727. (37) Inagaki, S.; Sakamoto, Y.; Fukushima, Y.; Terasaki, O. Chem. Mater. 1996, 8, 2089–2095. (38) Glatter, O. Personal communication. (39) Terech, P.; Jean, B.; Ne, F. AdV. Mater. 2006, 18, 1571–1574. (40) Kuwahara, T.; Nagase, H.; Endo, T.; Ueda, H.; Nakagaki, M. Chem. Lett. 1996, 435–436. (41) Terech, P.; Rodriguez, V.; Barnes, J. D.; McKenna, G. B. Langmuir 1994, 10, 3406–3418. (42) Kamijo, M.; Nagase, H.; Endo, T.; Ueda, H.; Nakagaki, M. Anal. Sci. 1999, 15, 1291–1292. (43) Placin, F.; Desvergne, J.-P.; Belin, C.; Buffeteau, T.; Desbat, B.; Ducasse, J.-C.; Lassegues, J.-C. Langmuir 2003, 19, 4563–4572.

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