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Thermoreversible Gelation of Organic Liquids by Arylcyclohexanol Derivatives: a Structural Study P. Terech,*,† J. J. Allegraud,† and C. M. Garner‡ Laboratoire Physico-Chimie Mole´ culaire, UMR 5819, CEA-CNRS-University J. Fourier, De´ partement de Recherche Fondamentale sur la Matie` re Condense´ e, CEA-Grenoble, 17, rue des Martyrs, 38054 Grenoble Ce´ dex 09, France, and Department of Chemistry, Baylor University, Waco, Texas 76798 Received February 10, 1998 Physical organogels of a 4-tert-butyl-1-arylcyclohexanol derivative (BACOl) are investigated. The smallangle scattering (SAS) technique (neutrons and X-rays) demonstrates that gel networks result from the entanglement of long, solidlike, and rigid fibers. The related cross-sections are shown to be circular (r ≈ 38 ( 2 Å) with a density comparable to that of the crystalline state and are rather monodisperse. Upon a concentration increase, the fibers merge into crystalline-like heterogeneities (junction zones) which are randomly dispersed in the mesh. The kinetics of BACOl molecular aggregation, during which physical gelation occurs, is studied by SAS, and the typical kinetical times of the supersaturated system are evaluated. Anisotropic scatterings reveal orientational effects in the gel which are sensitive to the solvent type. Electron microscopy (SEM) confirms the fibrillar random texture of the xerogels while it demonstrates that the phase-separated solids from metastable gels present oriented microdomains of bundles of fibers. The calorimetry and scattering techniques indicate that colloids in BACOl gels exhibit complex solvent-dependent mesomorphic relationships involving the basic BACOl bimolecular units (d ≈14.2 Å).
1. Introduction Low-mass organogels constitute a special class of physical gels. Numerous studies1 have been dedicated to the structural investigation of the network morphology and to the determination of the molecular aggregation mechanisms involved in the network growth process from the bulk solution. The number of known organogelators has increased dramatically in recent years, lending increased urgency to our understanding of the relationships both between the gelator structure and gelation ability and between the nanoscopic/microscopic aggregate structures and colligative properties of the gels. To date, these objectives are still challenging goals and gelators are mainly discovered by chance. A first necessary approach consists of an as exhaustive as possible investigation of a representative organogel system. To this purpose, a demonstrative methodology for studying the nanostructures of such heterogeneous and fragile materials can use both electron microscopy2,3 and radiation scattering,4 which are complementary techniques. Both techniques are essential as they can provide structural information on a nanoscopic scale which rely upon realand reciprocal-space observations, respectively. Recently, a low-mass gelator, 4-tert-butyl-1-phenylcyclohexanol (abbreviated as BACOl in the following), has been shown to be an excellent gelator of different organic liquids.5 The system exhibits a remarkable specificity since only the diastereoisomer with the aryl * To whom correspondence should be addressed. † CEA Grenoble. ‡ Baylor University. (1) Terech, P.; Weiss, R. G. Chem. Rev. 1997, 97, 3133. (2) Lin, Y.-c.; Kachar, B.; Weiss, R. G. J. Am. Chem. Soc. 1989, 111, 5542. (3) Tachibana, T.; Mori, T.; Hori, K. Bull. Chem. Soc. Jpn 1980, 53, 1714. (4) Terech, P.; Rodriguez, V.; Barnes, J. D.; McKenna, G. B. Langmuir 1994, 10, 3406. (5) Garner, C. M.; Terech, P.; Allegraud, J. J.; deGeyer, A.; Mistrot, B.; Nguyen, P.; Rivera, D. J. Chem. Soc., Faraday Trans. 1998, 94.
Chart 1. BACOl Derivatives: Gelator (Trans Isomer, Aryl Group Axial); Nongelator (Cis Isomer, Aryl Group Equatorial)
group in an axial configuration is an organogelator (Chart 1). The equatorially substituted isomer does not lead to gels, but rather immediately crystallites from solution. The thermal stabilities of BACOl gels appear to be dependent upon the organic liquid type, as illustrated with their related phase diagrams.5 Gels in heptane melt at higher temperatures than gels in toluene. In the present work, the small-angle X-ray and neutron scattering techniques (SAXS and SANS, respectively) are used to characterize the nanostructures of the BACOl gels. The scattering techniques provide the morphological features of the interacting colloidal aggregates in the gels in the presence of their liquids. A scanning electron microscopy (SEM) study is used to reveal the texture of the networks in the solids derived from the native gels: xerogel and phase-separated solid (on a long time scale, certain organogels can evolve toward a solid-liquid-phase separation). The different structural organizations of the solids (xerogel, phase-separated solid, and crystalline powder) induce different thermal behaviors which are distinguished by a differential scanning calorimetry (DSC) study. 2. Experimental Section The synthesis of BACOl and its derivatives has been described in a previous work.5 BACOl is only sparingly soluble at room temperature in nonpolar solvents (cyclohexane, benzene, carbon
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3992 Langmuir, Vol. 14, No. 15, 1998 tetrachloride, etc.) but is moderately soluble in these solvents when hot. On subsequent cooling, a transparent (benzene, toluene) or more or less turbid (dodecane, cyclohexane) gel forms. Xerogels were obtained by slow evaporation of the organic liquid from a gel: the gelatinous mass shrunk progressively to a collapsed and solid compact residue. Conversely, the phaseseparated solid obtained from a heptane gel over a period of a few days exhibited a characteristic feltlike texture and consistency. SANS experiments used the PAXE spectrometer (Laboratoire Le´on Brillouin, Orphe´e reactor, Saclay, France). The momentum transfer Q (Å-1) for elastic scattering was defined as Q ) (4π/ λ)(sin θ), where θ is half the scattering angle and λ is the wavelength of the incident radiation. A two-dimensional BF3 multidetector 64 × 64 cm2 with 1 × 1 cm2 cells was used to improve the statistics of the scattering signals and to check their isotropic character. The experimental Q range was 0.006 Å-1 < Q < 0.35 Å-1 which corresponded to two sample-detector distances, D ) 5.05 m (at λ ) 12 Å) and D ) 2.55 m (at λ ) 6 Å). These conditions gave access to a 20-1000 Å distance range typical of colloidal systems. Radial averaging procedures and the usual corrections for background subtraction, transmission and normalization were applied.6 SAXS data were obtained at the DCI synchrotron source “Laboratoire pour l’Utilisation du Rayonnement Electromagne´tique” (LURE, Orsay, France), using the D22 instrument at λ ) 1.458 Å (E ) 8500 eV). A gas-filled (Xe-CO2) detector with a 0.217 mm per channel spatial resolution placed at appropriate distances (1.75 and 0.714 m) was used to cover a 0.005-0.3 Å-1 Q range. Data were transmission corrected for the empty beam signal and solvent scattering. Complementary experiments were using the high-brilliance beamline (ID2, BL4) of the European synchrotron source (ESRF, Grenoble, France). Gels in deuterated liquids were prepared directly within 1 mm thickness quartz cells for SANS experiments while, for SAXS, gels in protonated liquids were introduced into 1 mm cells with Capton windows. SEM investigations of the BACOl xerogel and phase-separated solid used a JEOL JSM 840A microscope. The acceleration voltage was 10 keV, and the working distances were 8 or 39 mm. The solids were fractured at room temperature and pasted on a brass substrate with a carbon-conducting glue. Before observation, the fractured face of the solid was sputtered with a 200 Å thickness gold layer. Differential scanning calorimetry (DSC) experiments used a Mettler FP85 apparatus.
3. Results A previous rheological investigation of the BACOl/ hydrocarbon systems has characterized their solidlike viscoelastic character typical of gels. Wide-angle diffraction experiments (WAXS) have further characterized the gels as made of crystalline-like 3-D networks (see Discussion).5 SANS is known to be a very appropriate technique7 to probe colloidal aggregates in the 20-2000 Å length range. Figure 1A shows the SANS curves of BACOl/cyclohexane-d (C6D12) gels at three concentrations. The neutron intensity, isotropically scattered at low angles around the primary beam, is shown as a QI vs Q representation specific for fiberlike particles (vide infra). From low to large angles, the scattering curve appears as a plateau followed by a sharp decay (around Q ≈ 0.05 Å-1) and eventually a bump at Q ≈ 0.127 Å-1. These scattering features are expected to characterize the nanoscopic structure of the aggregates in BACOl gels and will be analyzed below. To elucidate the relationships between the structure and the thermal stability of the gels as a function of the solvent type, the neutron scattering of (6) Lindner, P.; Zemb, T. Neutron, X-ray and light scattering: introduction to an investigative tool for colloidal and polymeric systems; Bombannes: France, 1990. (7) Kratky, O. Prog. Colloid Polym. Sci. 1988, 77, 1.
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Figure 1. Logarithmic plots of the SANS cross-sectional intensity decay (QI vs Q) of BACOl organogels. (A) BACOl/ cyclohexane-d systems: (1, b) C ) 1.21 wt %; (2, O) C ) 3.11 wt %; (3, ×) C ) 6.11 wt %. The arrow points at an intensity bump at Q ≈ 0.127 Å-1 and the dotted horizontal line is a reference for a Q-1 intensity decay in the representation used. (B) Influence of the solvent type: (1, b) 1/octane-d, C ) 1.38 wt %; (2, ×) 1/decane-d, C ) 13.24 wt % (for the sake of clarity the intensity has been divided by 3000); (3, O), 1/decane-d, C ) 0.671 wt %. The dotted lines are references for a Q-1 (horizontal) and a Q-4 (sloped) decay of the intensity I.
BACOl in other deuterated solvents was considered. Figure 1B summarizes some of the observed scattering trends. The scattered neutron intensity integrated within a fixed low-angle region is used to trace the kinetics of formation of the BACOl colloids during the sol to gel phase transition. Figure 2 shows the sigmoid variation which depends strongly upon the concentration and temperature of the gelling solution: the asymptotic plateau of the neutron intensity vs time is reached at shorter times for more concentrated gelling solutions. SEM was also used to characterize the morphologies of the colloids in the BACOl gel networks. The observations were first made with dried gels (xerogels). In the Discussion, the SEM of phase-separated solids obtained from metastable gels in heptane will be also considered (vide infra). Due to collapses of the brittle structures in the 3-D network during the shrinking step (evaporation of the liquid), SEM focuses on the general shapes and morphologies rather than on absolute quantities such as diameters, lengths, or topologies. Figure 3A clearly shows very long and rigid fibers which are entangled in a porous matrix. The thickness is variable (0.1-2 µm) and Figure 3B reveals that thinner fibrils (0.05 µm) are also present which exhibit a moderate flexibility. Figure 3C displays a special morphology where fibers are emanating from a central point. Thermograms of the crystalline solid, the xerogel and the phase-separated solid are shown in Figure 4. During
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Figure 2. Kinetics of formation of colloidal particles in a BACOl/toluene-d system (C ) 4.43 wt %) at T ) 21 °C. Ordinates are the number of neutrons scattered within a 0.05 Å-1 < Q < 0.35 Å-1 range. The continuous line is an indicative theoretical variation obtained by solving the differential equation attached to a two-step model of successive reactions (autocatalytic followed by a first-order step):14 d2R/dt2 + dR/dt[[1/k0 - k0k1(1 - R)2 + k2(1 - R)]/(1 - R)] + k0k2 (1 - R) ) 0. R is the reaction rate and k0, k1, and k2 are the related kinetic constants (0.004, 0.031, and 0.022, respectively).
the heating procedure, a single endothermal peak (M) at T ) 157 °C (∆H ≈ 8.0 kcal mol-1) is observed with the crystalline powder while an additional exothermal peak (C) at 83 °C (∆H ≈ 0.6 kcal mol-1) is also observed with the xerogel. With the phase-separated solid (curve 3, Figure 4), an endothermal peak at ca. 140 °C precedes the peak M. 4. Analysis (i) Theory. The importance of the small-angle scattering of radiations (light, X-ray, neutron) as a technique for studying the structures of colloids has been thoroughly demonstrated.7,8 The nondestructive character of the method contributes to its success in colloid science and especially for molecular organogels9 whose colloids are most often fibers. Interferences10 between scattered waves (neutrons or X-rays) by a single fiber in dilute systems, generate the so-called form-factor intensity IF. Interferences between correlated fibers in semidilute or concentrated systems provide a structure intensity IS. From IF, information can be extracted concerning the shape, sizes, monodispersity, homogeneity, and mass per unit length of fibers, while IS reveals their ordering degree. With molecular gels, the concentrations involved are ca. 1 wt %, so that interferences between colloids can be easily recognized as manifested in the scattering profiles: the related long distance interaction forces can generate a very-low angle contribution. These interparticle forces are also responsible for the typical viscoelastic behavior of the gels which comes from the 3-D connection of fibers and microdomains of fused fibers (junction zones). As a first step of the structural study, the analysis will focus on the morphological information extracted from IF. IF is the average over all orientations of the rods (length L, cross-section A) with respect to the momentum transfer ks (k Bi and B ks are the wavenumbers of the incident Q B )B ki - B and scattered beams, respectively). Expression 1 describes the situation for a collection of isolated cylindrical fibers.11,12 (8) Cabane, C. Surfactant Solutions. In Surfactant Science Series; Zana, R. Ed.; M. Dekker: New York, 1987; Vol. 22, pp 57-145. (9) Terech, P. Croat. Chem. Acta 1992, 65, 425. (10) Guinier, A.; Fournet, G. Small Angle Scattering of X-rays; Wiley: New York, 1955. (11) Hayter, J. B.; Penfold, J. Colloid Polym. Sci. 1983, 261, 1022.
Figure 3. Scanning electron microscopy of fractured xerogels obtained from BACOl/cyclohexane gels: (A) random repartition of very long and rigid fibers forming a porous solid; (B) detail showing that thinner and more flexible fibrils are present; (C) detail of the collapsed gel network where fibers emanating from a central point are clearly seen.
F(Q B ) ) ∆FL
sin[(QL cos γ)/2] (QL cos γ)/2
F(Q B ) ) F(Q,γ) ) 2∆FL
∫∫e-iQB ‚rb dA
sin(QL/2 cos γ) J1(Qr sin γ) QL/2 cos γ Qr sin γ (1)
where γ is the angle between Q B and the rod axis and J1 is the Bessel function of the first kind. If the length of the rodlike scatterers is much larger than the diameter,12 due to the properties of the sin(x)/x (12) Glatter, O.; Kratky, O. Small-Angle X-ray Scattering; Academic Press: London, 1982.
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Figure 4. DSC thermograms of BACOl solids (heating rate: 10 K min-1): (1) crystalline solid (endothermal peak M at T ) 157 °C); (2) xerogel from a cyclohexane gel (exothermal peak C at T ≈ 83 °C); (3) phase-separated solid from a heptane gel (endothermal peak at T ≈ 140 °C).
vs x variation, the rods make a contribution to the diffraction only when they lie nearly perpendicular to Q B. The form-factor for scattering by rods can then be written as the product of two nearly independent terms: axial (proportional to 1/Q) and cross-sectional (proportional to J1 for a circular shape). In a random gel network, the long fibers exhibit all possible orientations with respect to any given Q B orientation, and there is no dependence of the resulting scattered intensity IF with the angular position of the gel with respect to Q B . The intensity reduces to expression 2 and the detector reveals an isotropic scattering whose isointensity contour plots are circles.
IF )
[
]
J1(Qr) πC 2 ∆b ML 2 Q Qr
2
(2)
where ∆b is the neutron specific contrast of the rod with respect to the solution,9 ML is the mass unit per length of the rod. Conversely, with oriented systems, the fibers are position and/or orientation correlated and the scattering is dependent upon the angular position of the gel with respect to Q B . Two extreme situations can be examined corresponding to a perfect orientation of the fibers (no defects). First, if the director (direction of orientation) is vertical and orthogonal to the incident beam, then, if L/2r > ca. 10, expression 1 gives a nonzero contribution only for γ ) π/2: a horizontal band is observed on the bidimensional detector. Actually, a gel in absence of any external orientation forces (shearing stress, electric field, etc.) is usually not perfectly oriented and is composed of domains with a distribution of orientation around a main director b n. Each fiber in a domain is referred in polar coordinates by the polar angles θ, φ. When orientation is observed in a sheared material, Q| is conveniently taken to be parallel to the direction of shear. A probability function p(θ,φ) describes the orientational distribution of the rods in the sample. The anisotropic scattering evolves to a scattering with elliptical isointensity contours which reveal, for each ψ angular value in the detector plane, a population of fibers which satisfies the condition of orthogonality of Q B with the related fiber axes. With the second extreme situation, the director is parallel to the b is orthogonal to the (x b,z b) detector plane), incident beam B ki (n expression 1 reduces to expression 2 and an isotropic scattering (circular isointensity contours) independent of the Q B position in the (x b,z b) plane is observed.
Figure 5. SANS of BACOl/cyclohexane-d gel (C ) 1.21 wt %) with a Guinier plot ln(QI) vs Q2 appropriate for fibrillar structures. The low-angle extra-scattering (O) has not been taken into account for the determination of the Gaussian decay of the cross-sectional intensity according to expression 3. The full straight line has a slope which corresponds to r ≈ 39 Å.
When the long (L . 2r) fibers are interacting, as is the case in the semidilute regime (C > 1/L3) and especially for concentrated systems (C > 1/(2rL2), the two-dimensional structure factor can add specific diffraction features depending upon the symmetry of the cross-sectional ordering (hexagonal, rectangular, etc.) of the fibers.13 (ii) BACOl Organogels. The isotropic neutron scatterings of BACOl/cyclohexane gels of Figure 1A exhibit a flat plateau characteristic of the low-angle asymptotic scattering of linear particles (axial term Q-1 in expression 2). In this context, the sharp decay which follows can be attributed to the finite size of the cross-sections of fibers. Expression 3 is an expansion in the low-angle limit (1/L < Q < 1/r) of expression 2, which shows that the crosssectional intensity decay is Gaussian and depends on the radius of the cylindrical fibers.
IF )
φ(πr∆F)2 exp(-Q2r2/4) Q
(3)
At this stage of the analysis, the intensity bump at Q ) 0.127 Å-1 can be generated by the Bessel function of expression 2, and it constitutes a characteristic scattering feature of the cross-sectional shape (possibly circular, rectangular, elliptic). This issue will be detailed below using an adjustment of the theoretical expression 2 to a normalized experimental intensity (Figure 6). The relative invariance of the scattering profiles in cyclohexane with the BACOl concentration up to C ≈ 6 wt % (Figure 1A) validates the analysis of the bump as a form-factor oscillation. An additional argument is provided with a previous WAXS study5 which has shown that the diffraction features of crystalline-like aggregates in BACOl gels show up at high Q values, beyond 0.4 Å-1. BACOl gels are made up of fiberlike aggregates, and appropriate use of expression 3 gives access to the related radii values, assuming a circular cross-section. Figure 5 shows such an example of a ln(QI) vs Q2 plot from which r values are estimated as a function of the concentration and solvent type. The best linear straight lines are determined for Q e 1/r (validity condition of expression 3) and exclude the innermost extra-scattering whose origin is discussed below. Taking into account a set of SANS experiments, BACOl fibers have a mean diameter of ca. 76 ( 2 Å in the range 1-7 wt % in cyclohexane. Dilute gels in octane, in decane (respectively, curves 1 and 3 in (13) Kekicheff, P.; Cabane, C. Acta Crystallogr. 1988, 44, 395.
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Figure 6. SANS of BACOl/cyclohexane-d gel (C ) 0.671 wt %). The cross-sectional intensity is normalized by the scattering invariant INV. A best adjustment according to expression 5 is shown (full line) for r ) 39 Å, ) 0.1. assumes that the radial polydispersity ( ) ∆r/r) can be described by a Gaussian distribution function.
Figure 1B), or in toluene contain similar aggregates (diameter ≈ 80 Å). If the concentration is strongly increased (Figure 1B, curve 2), the scattering evolves from the typical Q-1 form-factor profile for isolated fibers toward a monotonic Q-4 intensity decrease consistent with expression 4, which is an expansion of expression (2) in the large-angle limit. ∆F is the volume contrast of the system.
S I f 2π∆F2 Q-4 V Qf∞
(4)
Such a power-law intensity decrease is typical of the existence of a significant fraction of large aggregates,12 which exhibit sharp interfaces with the surrounding liquid. A large increase of the gelator concentration in decane leads to a further aggregation of the fibers into microdomains acting as junction zones in the 3-D network and in such a proportion that their specific low-angle scattering overwhelms the signal. The corresponding wide-angle diffraction contribution from these domains is found at Q > 0.4 Å-1.5 The related low-angle Q-4 “signature”, analyzed as a form-factor of large interfaces, might also be interpreted as resulting from a random distribution of a large fraction of heterogeneities (vide infra). The heterogeneities of the gel result from the merging of fibers in the junction zones of the energetic network, which affects the elastic modulus of the gels, as shown previously.5 In an appropriate range of concentrations (C < 6 wt % in cyclohexane), the fiber diameter remains about constant as indicated by the SANS curves of Figure 1A, while a further increase of concentration (i.e., ca. 10% in decane) induces an increase of the cross-sectional dimensions and consequently raises both the bend constant of the fibers and the elastic modulus of the gel network. A limiting situation for extremely concentrated networks can be that of xerogels in which structural collapses concern the largest spread possible of distances. SEM views (Figure 3) of BACOl xerogels exhibit networks with an extremely large polydispersity of the cross-sectional dimensions of fibers and junction zones, which is also a consistent feature observed with the scattering of concentrated gels (Figure 7). Since the scattering profile is not very sensitive to the BACOl concentration in cyclohexane (for C e 6 wt %), the time evolution of the scattered intensity within a 0.05 Å-1
Figure 7. SAXS of BACOl/hydrocarbon gels: (1) C ) 1 wt %, dodecane; (2) C ) 3.0 wt %, dodecane; (3) C ) 5.04 wt %, dodecane; (4) C ) 9.63 wt %, toluene. The dotted straight line is a guide for a Q-4 intensity I decay. The full line is a best agreement according to expression 2 for rodlike scatterers, r ) 36 Å, ) 0.15 (intensities have been vertically shifted for sake of clarity).
< Q < 0.35 Å-1 Q range can be assumed to be proportional to the amount of rodlike colloids in the system. The sigmoid curve (Figure 2) exhibits all of the characteristics of a collective kinetical process: induction delay (time during which no scattering can be detected, td ≈ 160 s), a second-order (autocatalytic) startup reaction, and a firstorder reaction rate of the terminal process. The agreement of such a model with the data of Figure 2 is satisfactory, and the kinetic constants can be extracted (see caption). The kinetic behavior of BACOl organogels is similar to that of supersaturation systems.14 The warmed and homogeneous BACOl/hydrocarbon solutions are unstable at temperatures in the gel domain of the phase diagram5 and evolve toward gelation. Since the BACOl solubility CTsol at temperature T is low in most alkanic liquids, the supersaturation degree (ST ) C/CTsol) is usually high. Most of the BACOl molecules are involved in the aggregation/ gelation process during which unidirectional crystallization and fiber entanglement processes develop. The variation of the induction time (as short as ST is high)15 as well as the kinetics of crystallization (as fast as ST is high)16 are the usual behaviors resulting from nucleation phenomena in supersaturated systems. A definitive demonstration of the unidirectional character of the BACOl colloids is given using the whole Q range of a neutron scattering curve normalized by the scattering invariant INV. INV is the integral of the intensity over the reciprocal space and is related to the mean square fluctuation of the contrast according to expression 5. INV is independent of any other feature of the statistically isotropic colloidal system and can be conveniently used to calibrate the data.
INV )
∫0∞ Q2I(Q) dQ ) (∆F)2φ(1 - φ)2π2
(
)
2J1(Qr) QI(Q) r2 ) INV Qr 2(1 - φ)
2
(5)
The cross-sectional intensity QI normalized by the scattering invariant INV is dependent upon only a single parameter r, since the importance of the volume fraction φ is minimized with dilute systems. Figure 6 demonstrates (14) Terech, P. J. Colloid Interface Sci. 1985, 107, 244. (15) Keller, D. M.; Massey, R. E.; Hileman, O. E. Can. J. Chem. 1978, 56, 3096. (16) Heughebaert, J. C.; Nancollas, G. H. J. Phys. Chem. 1984, 88, 2478.
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Figure 8. Growth of large aggregates in BACOl/d-decane gels induced by a concentration increase: (1, b), C ) 0.67 wt %, dilute gel, r ) 39 Å, ) 0.10; (2, O), C ) 13.2 wt %, concentrated gel, r ≈ 300 Å, ) 0.2. Full lines are IQ4 vs Q fits according to expression 2 for SANS data.
that the fibrillar scatterer hypothesis, used with BACOl gels, is appropriate since the intensity level of the Q-1 plateau as well as the Q location of the oscillation (Q ≈ 0.13 Å-1) of the theoretical scattering curve (expression 5) calculated for r ) 39 Å agrees correctly with the experimental data. SAXS experiments (Figure 7) confirm that if the concentration is increased in dodecane (from 1 to 5 wt %), a low-angle extra-scattering (typical of large aggregates) progressively blurs the Q-1 form-factor signal of isolated fibers. At a fixed concentration, the amplitude of this component is in the sequence dodecane > decane > cyclohexane > toluene, which is also the order of turbidity of the gels. Curve 3 (Figure 7) shows that, at high concentration, the scattering curve reduces to a Q-4 intensity decay. Such a power-law decay can be typical either of the high-Q interfacial scattering of large aggregates or of the scattering of a random distribution of microcrystalline junction zones, modeled according to the so-called Debye-Bueche random-two-phase model.17 In the latter hypothesis, the scattering depends on the size and volume fraction of the heterogeneities distributed according to an exponential correlation function. Such a description has already been successfully used to describe comparable scattering evolutions as a function of the concentration with other organogels (such as those of a benzohydroxamic acid derivative).18 Considering that the junction zones in BACOl gel networks can be assimilated with heterogeneities which grow (number and/or size) with concentration, it is reasonable to assume that BACOl gels can also be modeled as a random two-phase model. Such organogels can be described as “crystalline organogels” if there junction zones are crystalline-like microdomains. As mentioned with BACOl gels, the increase of amplitude of the Q-4 component is accompanied with an increase of amplitude of Bragg peaks accounting for the internal ordering of the clusters of fibers (at Q > 0.4 Å-1).5 The amplitude of the Q-4 low-angle signal is found to be typical of the increase of the overall crystallinity of the gels (estimated from the low to large-angle parts of their scattering patterns) in the sequence dodecane > decane > cyclohexane > toluene. To illustrate the structural evolution with the concentration of the aggregates in decane, Figure 8 shows that the radius of cylindrical aggregates varies from 39 Å (at (17) Debye, P.; Bueche, A. M. J. Appl. Phys. 1949, 20, 518. (18) Terech, P.; Coutin, A.; Giroud, A. M. J. Phys. Chem. B 1997, 101, 6810.
Figure 9. SAXS of BACOl/decane gels: (1) C ) 1.42 wt %; (2) C ) 2.36 wt % obtained at the brilliant ESRF synchrotron source. Agreement is seen with expression 2 of the form-factor for cylindrical fibers (r ) 36 Å, ) 0.10) in two complementary representations. A: QI vs Q plots. The low-angle extrascattering for the more concentrated system 2 is clearly seen (see text). B: Q4I vs Q plot which demonstrates the monodispersity of the cross sections of the fibers in the organogels.
C ) 0.67 wt %, curve 1) to a range of thicker fibers with r ≈ 300 Å (at C ) 13.2 wt %, curve 2). The Porod representation IQ4 vs Q emphasizes the oscillations of the cross-sectional form-factor of the fibers (expression 2). This trend is also evident with Figure 9, showing highresolution data obtained with the brilliant ESRF synchrotron source: the bump at ca. 0.13 Å-1 is well-defined, and the enhancement of the amplitude of the low-angle component upon a concentration increase (from 1.4 to 2.4 wt %) is clearly seen (Figure 9A, curve 2). The agreement of a fit using expression 2 is quite satisfactory and confirms that the mean radius is ca. 36 ( 1 Å in decane. The quality of these spectra is such that the monodispersity of the cross-sections of the fibers is clearly revealed: two formfactor oscillations are seen and modeled in a Q4I vs Q plot (Figure 9B). 5. Discussion SANS isointensity contour plots (Figure 10) of BACOlcyclohexane gels are most often circular and characterize isotropic scatterings (Figure 10, image A) from a network of randomly distributed fibers (Theory section). At a similar concentration, gels in octane can exhibit elliptical contours characteristic of anisotropic scatterings (Figure 10, image B). Referring to the Theory section, it can be concluded that domains of imperfectly oriented BACOl fibers are present in octane gels which exhibit a vertical orientation direction. The fiber orientation is accidental and can be either due to the confinement of the gelling system within the thin gap of the measuring cell whose vertical axis is a preferential axis for the rod growth or
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Figure 10. SANS isointensity contour plots of BACOl/ hydrocarbon gels in the range 0.04 e Q e 0.2 Å-1. The central part is a beam stop used to protect the detector from the direct beam. Key: top left, cyclohexane-d gel at C ) 1.21 wt %, isotropic scattering, top right, octane-d gel at C ) 1.38 wt %, anisotropic scattering; bottom, anisotropic scatterings; bottom left, octane-d gel, C ) 1.38 wt %, horizontal position of the measuring cell; bottom right, vertical position of the cell.
due to a vertical shear of the gelling solution developed during the filling procedure of the cell. To confirm the existence of a vertical orientation direction (long axis of the cell), the scattering has been examined at two orthogonal orientations of the cell. As shown with images C and D (Figures 10), a vertical position of the cell corresponds a horizontal “elliptical” anisotropic scattering, while if the cell is rotated by 90° (horizontal position), the anisotropic scattering becomes vertically located. The long axis of the cell is the orientation director of the BACOl fibers. No signs for a higher degree of ordering, such as localized spots, can be seen. Despite the low concentration of the gels, it cannot be excluded that locally (i.e., within the junction zones) such an ordering could be present, but the amplitude of its diffraction would not allow a clear identification. The dependence of the anisotropy of organogels upon the solvent type is a phenomenon already observed with some other gelators19 and is probably related to the rigidity of the constitutive fibers and their aggregates in a network whose topology is sensitive to any shear stress and excluded volume effects during the growth step. The turbidity of BACOl organogels is in the sequence decane > cyclohexane > toluene and is a consequence of the light scattering by polydisperse structures whose characteristic dimensions lie within the visible wavelength range. The heterogeneous distribution of microheterogeneities, such as crystalline junction zones, can play such a role, and the greater the crystallinity is the more wave interferences in the long distance range there will be. This situation has been already pointed out with the existence of the extra Q-4 low-angle scattering with concentrated systems and additional WAXS diffraction features at Q > 0.4 Å-1. As mentioned in the Introduction and the Results, the long-term kinetical evolution of the BACOl/hydrocarbon gels can exhibit a solid-liquid phase separation. The typical time scales are on the order of months and depend (19) Terech, P. J. Phys. Fr. 1989, 50, 1967.
Figure 11. SEM of the phase-separated solid obtained from a BACOl/heptane gel: (A) thick bundles of fibers; (B, C) high orientation degree of the fibers.
on the solvent type. Microphase separations cannot be excluded in relation with the small amounts of liquids expelled by some dilute gels. Figure 3C shows a spherulitic nodule with a central point from which are emanating the BACOl fibers which might constitute such a nucleation center for the demixing reaction. The solid-liquid phaseseparation process develops through collapses of fibers into oriented bundles. SEM micrographs of Figure 11 show that the phase-separated solid is made of fibers associated in locally aligned bunches. This confirms that the true equilibrium state of such a collection of very long and rigid fibers is the biphasic system. The major structural difference (over nanoscopic distances) between the xerogel and the demixed solid lies in the degree of orientation and the distribution of the patchwork of microdomains made of collapsed bundles of fibers. The ordering degree of the fibers in xerogels, as observed by
3998 Langmuir, Vol. 14, No. 15, 1998
SEM, is confirmed by a WAXS study5 and is consistent with the present SAS investigation. The WAXS patterns of BACOl xerogels show, in addition to a narrow and intense Bragg peak at Q ≈ 0.44 Å-1, a first peak at Q ≈ 0.0825 Å-1 and a second broad bump at 0.14 < Q < 0.19 Å-1. To the first peak corresponds a periodic distance d ≈ 2π/0.0825 ≈ 76 Å which compares exactly to the BACOl fiber diameter. Due to the width of the second diffraction feature, the cross-sectional symmetry of the bundles of fibers in the xerogels cannot be clearly identified (between hexagonal and lamellar) but nevertheless characterize the mesomorphic organization of the xerogel. A differential scanning calorimetry (DSC) analysis of the first-order transitions in the different solids (xerogel, phase-separated solid, crystalline solid) is shown with Figure 4. The crystalline solid exhibits a single endothermal peak (peak M) which corresponds to the melting transition to an isotropic liquid, while with the xerogel, an additional exothermal peak (peak C) precedes the melting transition. Interestingly, the mesomorphic organization of BACOl xerogels appears less stable than that of the phase-separated solid. The temperature range ∆T ) TM - TC is 157 - 83 ) 74 °C for the xerogel while it is only 157 - 140 ) 17 °C for the demixed solid. These observations suggest that the textures in the solids are not equivalent and the mesomorphic organization within the fibers of the BACOl gels is transformed to the crystalline structure through the exothermic reaction (peak C). The transition from unidirectional aggregates to tridimensional ones is irreversible in the solid state (not shown) and the mesomorphic organization in the fibers can only be obtained through the sol to gel phase transition. Comparable observations were made with different representatives of the class of low-mass organogelators.1 With some of these examples it was shown that, depending on the heating rate, an endothermal peak can precede the exothermal cold crystallization transition (peak C). This is not clearly observed with the BACOl xerogel, unlike the phase-separated solid for which the endothermal process is seen, while the subsequent exothermal cold crystallization is probably masked by the intense melting transition M. The degree of molecular organization within the fibers of the BACOl organogels is high, as confirmed by the diffraction features of the WAXS patterns.5 The sensitivity of the mesomorphic organization to the solvent type is illustrated by the displacement of the cold crystallization peak C as a function of the xerogel history. It is interesting to observe that the enthalpy of the melting DSC transition M (ca. 8 kcal mol-1) is comparable to the values ∆Hag of the gel to sol transition determined by the falling-ball method.1,5 The absolute SANS intensities can be used to support the DSC conclusion and to validate the hypothesis of the existence of crystalline-like structures in the gels. Using expression 2, the number (nL ) ML/M) of gelator molecules aggregated per linear angstrom of BACOl fiber is extracted from the ordinate to origin (QI)o of Guinier plots ln(QI) vs Q2 (expression 3). For a gel in cyclohexane-d at C ) 9.08 × 10-3 g cm-3, the value (QI)o ) 0.5514 cm-1 Å-1 has been determined which gives nL ≈ 13.2 mol Å-1 assuming ∆b2 ) 38.02 × 1020 cm2 g-2 for BACOl aggregates in C6D12. The related Guinier plot (not shown) has also provided a radius value r ) 37.5 Å. The experimental nL value can be compared to the theoretical estimation for a crystalline BACOl unidirectional aggregate (density ≈ 1 g cm-3) with a comparable geometry.
Terech et al.
Assuming a circular cross-section, the theoretical nL value is 11.5 mol Å-1 while it is 15 mol Å-1 for a square crosssection (75 Å side); the mean value between these two similar configurations is 13.2 mol Å-1 and compares exactly to the experimental value (13.2 mol Å-1). It can be claimed that BACOl gel networks are constituted of dense crystalline fibers and bundles of fibers. Radii values deduced from the analysis of SAXS data can be compared to those deduced from SANS data at similar concentrations in identical solvents. The SAXS curve 4 in Figure 7 of a BACOl gel in toluene is instructive as it shows a profile in which the nodal (Q-4 low-angle component) and the fiber (Q-1 behavior in the intermediate Q-range) components can be easily identified. A careful examination of the most dilute gels is recommended so that the determination of r values cannot be affected by the increase of crystallinity observed with concentrated gels. The mean radii in toluene (r ≈ 35 Å) or in cyclohexane (r ≈ 36 Å) measured by SAXS appears to be slightly lower than those measured by SANS (rSANS ≈ 40 Å and r ≈ 38 Å, respectively). A similar remark can be made with fibers in decane (rSAXS ≈ 36 ( 2 Å, rSANS ≈ 39 ( 2 Å). Surfactant aggregates have usually regions of different chemical constitution (i.e., oxygenated and alkanic parts) which are responsible for the existence of a heterogeneous electronic (or neutron) contrast. A previous WAXS study5 has indicated that a diffraction feature common to gels, xerogels, and solids was found at Q ≈ 0.44 Å-1 corresponding to reticular distances of ca. 14.2 Å and which is attributed to head to head BACOl bimolecular associations. In such a context, the external shell in BACOl fibers is expected to generate a contrast effect (between neutron and X-ray radiations) of ca. 2-3 Å (between oxygenated and alkanic parts) with the measured radii values. The experimental observation agrees correctly with the above estimation and supports the description of BACOl bimolecules aggregated in fibers with outer alkanic shells (reverse organization). At this stage, it is not possible with the present data to detail the internal structure of the cross-sections. Further X-ray diffraction and Infrared absorption experiments are under progress to determine the crystallographic relationships between the diameter (76 Å) and the bimolecular repeating distance (14.2 Å) in the different states of the system. BACOl organogels have been investigated at various length scales from microscopic to nanoscopic to macroscopic ranges. The thermoreversible networks are constituted of dense fibers which are very long and rigid. The cross sections (r ≈ 38 Å) are rather monodisperse, as demonstrated by the occurrence of the 0.13 Å-1 crosssectional form-factor bump. The diameter of the fibers is large with respect to the BACOl bimolecular length (ca. 2 × 6.6 Å between the hydroxyl and tert-butyl groups), which indicates that several bimolecules are involved per repeating unit along the fiber axis. A subtle sensitivity of the structures and of the stabilities of the systems is observed as a function of the solvent type. Xerogels are mesomorphic solids resulting from ordered fiber associations. Acknowledgment. Drs. J. P. Lesieur (LURE), O. Diat (ESRF), and J. Teixera (LLB) are thanked for their support during the scattering experiments. LA980160C
