Langmuir 1998, 14, 719-722
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Notes Inorganic Gelation in a Lyotropic Lamellar Phase L. Porcar,* P. Delord, and J. Marignan Groupe de Dynamique des Phases Condense´ es CC 26 (Unite´ Mixte de Recherche UM II/CNRS 5581), Universite´ Montpellier II, F-34095 Montpellier Cedex 5, France Received March 20, 1997. In Final Form: November 4, 1997
Introduction The sol-gel process is frequently used to produce oxide ceramic materials with controlled structure and composition for many applications.1 Particular advances have been made by performing sol-gel reactions in organized surfactant media.2-5 The first investigations of chemical reactions in organized media were published early by Friberg.6 Indeed, addition of a simple surfactant in the sol-gel process permits one to obtain homogeneous gel of a very reactive alkoxide. Thus, monodisperse spherical particles of different alkoxides, for example Ti and Zr, have been produced using reverse micellar systems.7 One could easily imagine that it should be possible to form as many inorganic structures as there exist lyotropic mesophases. Numerous research groups focus their attention on the synthesis of mesoporous (15-500 Å) materials with regular periodicities and high surface areas which have potential use in biotechnology applications, in largemolecule selective catalysis including heavy crude oil processing, and as highly ordered matrixes for optical data storage.8-10 A challenge and a potentially rewarding approach for the synthesis of mesodimensional porous media is to utilize an organized molecular array as a template for the condensation of inorganic species. An interesting way is to use lamellar phases as templates to realize the inorganic polymerization in the water layers and to obtain, after calcination, a pure inorganic mesoporous network with a pore size on the order of the lamellar spacing, i.e., about 50-100 Å. One attempt to produce “layered inorganic materials” through the sol-gel process has been developed by Dubois et al.11 using the lamellar phase as a template. The main difficulty of this process is to conserve the smectic order * Author for correspondence (e-mail:
[email protected]). (1) Brinker, C. J.; Scherer, C. W. Sol-Gel Science; Academic Press: New York, 1990. (2) Arriagada, F. J.; Osseo-Asare, K. Colloids Surf. 1992, 69, 105. (3) Friberg, S. E.; Yang, C. C.; Sjoblom, J. Langmuir 1992, 8, 372. (4) Osseo-Asare, K.; Arriagada, F. J. Colloids Surf. 1990, 50, 321. (5) Abadie, T.; Ayral, A.; Guizard, C.; Cot, L.; Robert, J. C.; Poncelet, O. In Better Ceramics Through Chemistry IV; Cheetham, K., Brinker, C. J., Mecartney, M. L., Sanchez, C., Eds.; Mater. Res. Symp. Proc., Vol. 346, Materials Research Society: Pittsburgh, PA, 1994; p 849. (6) Friberg, S. E.; Yang, C. C. In Innovations in Materials Processing Using Aqueous, Colloid, and Surface Chemistry; Doyle, F. M., Raghaven, S., Somasunaran, P., Warren, G. W., Eds.; The Minerals, Metals and Materials Society: Warrendale, PA, 1988; p 181. (7) Auvray, L.; Ayral, A.; Dabadie, T.; Cot, L.; Guizard, C.; Ramsay, J. D. F. Faraday Discuss. 1995, 101 (17). (8) Ozin, G. A. Adv. Mater. 1992, 4, 612. (9) Martin, C. R. Science 1994, 266, 1961. (10) Goltner, C. G.; Antonietti, M. Adv. Mater. 1997, 9, 431.
Figure 1. Small-angle X-ray scattering spectrum of a lamellar phase obtained for Triton X/water/decane solution (with 42.5 wt % Triton X and 15 wt % decane). The Bragg peak at q ) 0.0741 Å-1 (scattering vectors) corresponds to an interlamellar spacing of 85 Å. The sample consists of cylindrical Lindemann capillary tubes (diameter 1 mm) filled with the investigated mixture.
during the hydrolysis and condensation reactions of the precursor:
Si(OR)4 + 4H2O w Si(OH)4 + 4ROH Si(OH)4 + Si(OH)4 w (OH)3SiOSi(OH)3 + H2O Indeed, these reactions create mechanical constraints which can destroy the organized mesophase.3,12 In fact, the main effect that may induce the disruption of the smectic order is the release of alcohol during the first step of the sol-gel process.12 Another way to obtain various alkoxide structures is through biomimetic assembly. This method to make silicate mesoporous materials uses the specific interaction between silicate and surfactant.13-16 In this case, the silicate oligomers which act as multidendate ligands contribute to a lamellar organization of the surfactant.13 However, during the silica polymerization, the lamellar phase is transformed into an hexagonal network. This is one of the industrial processes to obtain mesoporous molecular sieves with regular and constant diameters in the range 15-100 Å.17 An interesting process to obtain silica stems after calcination of the surfactant18 is to use cylinders of a hexagonal mesophase: the polymerization occurs inside the hexagonal rods. Recently, organic polymers have been polymerized within the water layers of the lamellar phase.19 The (11) Dubois, M.; Gulik-Krzywicki, T.; Cabane, B. Langmuir 1993, 9, 673. (12) Friberg, S. E.; Ma, Z. J. Non-Cryst. Solids 1992, 147/148, 30. (13) Firouzi, A.; et al. Science 1995, 267, 1138. (14) Kresge, C. T.; et al. Nature 1992, 259, 710. (15) Monnier, A.; et al. Science 1993, 261, 1299. (16) Tanev, P. T.; Pinnavaia, T. J. Science 1996, 271, 1267. (17) Stucky, G. D.; et al. Mol. Cryst. Liq. Cryst. 1994, 240, 187. (18) Ne´, F. Private communication. (19) Laversanne, R. Macromolecules 1992, 25, 489.
S0743-7463(97)00304-1 CCC: $15.00 © 1998 American Chemical Society Published on Web 01/17/1998
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Figure 2. Texture of the lamellar phase corresponding to the sample of Figure 1. Focal conics and oily streaks, characteristic of lamellar phases, can be observed. Cell thickness: 75 µm. Crossed polarizers. Scale: 1 cm ) 50 µm.
smectic order is conserved until the end of the reaction. This polymerization is less constrained than the inorganic one, because no byproducts are formed. Nevertheless, even in this case, the lamellar structure is not always conserved.20 Because the organic polymerization in a lyotropic phase gives such good results, the inorganic one should give just as good results. So, Dubois et al.11 have recently polymerized a silica precursor in a swollen lamellar phase of DDAB (didodecyldimethylammonium bromide) surfactant. The polymerization occurs on the surface formed by the polar head groups of DDAB and not in the water layer; thus a substantial disorganization of the bilayer appears.11 Other works, using a lamellar phase made from a mixture of nonionic surfactant and water, reveal a silica cluster separation in the volume surrounding the microdomains of the lamellar phase.7,21 Before obtaining a mesoporous material after calcination of an inorganic template mixture, it is necessary to study the previous step, i.e., the gel. In this paper, we investigate a lamellar phase in which silica polymerization occurs with the smectic order unaffected. Our experimental process to obtain a “lamellar gel” requires two steps. First, pure lamellar phase must be obtained, and thereafter, the precursor is added. So, the polymerization occurs somewhere in the lamellar phase and never starts in isotropic solution. Nevertheless, as described by Dabadie,21 it is possible to obtain a lamellar phase at the end of the polymerization even if the reaction starts in an isotropic solution near a lamellar domain. The lyotropic phase we have studied is a mixture of nonionic surfactants, water, and decane. The lamellar gels, obtained after polymerization of the precursor, have been characterized by small-angle X-ray scattering (SAXS) and polarized light microscopy observations. (20) Holtzscherer, C.; Wittman, J. C.; Guillon, D; Candau, F. Polymer 1990, 31, 1978. (21) Dabadie, T. Thesis, University of Montpellier, 1994, unpublished.
Experimental Procedure The experimental starting system is a lamellar phase obtained from a ternary mixture of two nonionic commercial surfactants, Triton X100 (TX100) and Triton X35 (TX35), and water. The typical volume of the mixture used for the synthesis experiments is about 5 cm3. The weight ratios are TX100/TX35 ) 55/45 g/g and (TX100 + TX35)/H2O ) 1 g/g, and they are kept constant. Decane is incorporated in order to make the solution fluid and to obtain a much better contrast in small-angle X-ray scattering. It inserts between the tails of surfactants, increasing the bilayer thickness.22 The amount of decane is 26% in weight of surfactant. Pure lamellar phase has a periodic structure which is characterized by a series of diffraction peaks corresponding to q0, 2q0, 3q0, ... where q0 ) 2π/dp and dp is the smectic periodicity. Figure 1 shows a SAXS spectrum of a lamellar phase used as a template. Note the sharpness of the first Bragg peak (q0), which indicates a strong repulsive force between two adjacent neutral membranes. The stability of this lamellar phase is due to the steric repulsion described by Helfrich.23 These phases are transparent, viscous, and birefringent, and they show characteristic topological defects of an LR phase, i.e., oily streaks and focal conics in polarized light microscopy (Figure 2). As described below, the main difficulty in producing a lamellar gel appears in the second step: the addition and the mixing of the precursor into the lamellar phase to obtain a monophasic lyotropic mesophase. Indeed, the viscosity of the solution at room temperature and the fast reaction rate of silicon tetramethoxide (Si(OMe)4, or TMOS) with water make it difficult to mix the components and to obtain a homogeneous lamellar phase before the polymerization takes place. In our case, the presence of decane in the lamellar phase makes the solution fluid and the mixing easier. TMOS was added to the lamellar phase and vortexed vigorously. The samples were left at 20 °C, and gelation was estimated by optical observation of the fluidity of the sample. Thus, a homogeneous and transparent lamellar gel with 10% per weight of TMOS was obtained. At the onset of the polymerization, the samples remained transparent and birefringent, indicating that the lamellar structure is preserved and not affected by the (22) Porcar, L.; Ligoure, C.; Marignan, J. In preparation. (23) Helfrich, W. Z. Naturforsch. 1978, 33A, 305.
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Figure 3. (a, top) Texture of the lamellar gel corresponding to the sample of Figure 4. A homogeneous birefringence (marbled texture) confirms that there is no segregation at this scale. (b, bottom) A rotation of 90° lights dark domains and quenches the others, which is characteristic of an orientational order. Cell thickness: 75 µm. Crossed polarizers. Scale: 1 cm ) 50 µm. alkoxide. Keeping the lamellar structure requires a water/TMOS molar ratio above 20 (hereafter, this ratio is referred to as h). For h < 20, the organized mesophase is destroyed during the polymerization, and the silica gel obtained is opaque, not birefringent.
Results and Discussions At the end of the polymerization, the samples were studied by SAXS and polarized light microscopy. A first visual observation showed that the polymerization in the lamellar phase is fast: only a few hours is necessary to obtain the gelation.
A polarization microscopic picture of the lyotropic gel results in oily streaks (Figure 3). A veined texture shows a homogeneous lamellar solution at the microscale. A SAXS spectrum of silica gel (Figure 4) shows that a lamellar structure is maintained after the gelation process. Indeed, a Bragg peak is conserved, which is the signature of a lamellar phase. If we compare the position of the Bragg peak of the lamellar gel to that of the pure lamellar phase, a slight shift toward greater scattering vectors (q in Å-1), corresponding to a decrease in interlamellar spacing of about 3-4 Å, is observed. This is probably due to the water consumption during the sol-gel process or
722 Langmuir, Vol. 14, No. 3, 1998
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Figure 4. High-resolution small-angle X-ray scattering pattern in absolute units of a pure lamellar phase (b) and a lamellar gel (O). The volume fraction of membrane in these two samples is 0.59. Lamellar gel is obtained with h ) 40. Inset: Log I(q) vs Log q at small wave vectors in absolute units for the lamellar gel. Pure lamellar phase is not presented here because the diffusion in this range is not observed. In these experiments, the samples are held in a 1 mm cell.
to an increase of the surface per polar head due to the formation of methanol during the hydrolysis process which is presumably partly located in the surfactant monolayers. A combination of these two phenomena cannot be excluded. More details on this slight shift have been discussed in ref 24. Two experimental results tend to confirm that the polymerization occurs inside the lamellar phase: (1) Polarized light optical microscopy shows different textures between the lamellar phase and the lamellar gel (Figures 2 and 3). In the case of gelation out of the lyotropic phase, a mixture of birefringent microdomains (like the pure lamellar phase) and isotropic microphase corresponding to the silica network would be seen between crossed polarizers. As shown in Figure 3, an isotropic phase is never observed, and a rotation of the sample of 90° lights the dark domains and quenches the others. This suggests that, at the microscopic scale, there is no segregation. (2) The SAXS spectrum reveals an increase of the intensity in absolute units of the Bragg peak (Figure 4), characteristic of the lamellar phase. The height modulation of the Bragg peak is usually ascribed to the form factor of the bilayer which is directly related to the electron density contrast between the bilayers and the water layers.25 So, an increase of the intensity of the Bragg peak can be explained by an increase of contrast. Thus, the only way to obtain an increase of the Bragg peak intensity is to have silica located in the water layer or between bilayers since the electron density of silica is on the order of 2 times the electron density of water. At this stage, we can not specify where the polymerization occurs, on the polar heads of the surfactant bilayer or in the bulk of the water layer, but it seems to be clear that the polymerization occurs inside the lamellar phase. For an easier system (DDAB/water), Dubois et al.11 (24) Porcar, L.; Marignan, J. In preparation. (25) Nallet, F.; Laversanne, R.; Roux, D. J. Phys. II 1993, 3, 487.
observed during the polymerization of silicic acid monomers in the swollen cationic lamellar phase a shift of the Bragg peak corresponding to an increase in the interlamellar distance. By using contrast variation in neutron scattering, they claim that the polymerization occurs on the polar head of the cationic surfactant. Another feature of SAXS spectra of the lamellar gel is the diffuse scattering as Q f 0. For the pure lamellar phase this diffuse scattering is never observed. It is attributed to the scattering of a bushy silica structure which is the signature of a silica network. Freeze fracture experiments on this lamellar gel would allow the characterization of the texture, and it will be discussed in a forthcoming publication.24 Conclusion This paper indicates that the gelation of silicon tetramethoxide in a lyotropic lamellar phase made of nonionic surfactant is possible and that, at the end of the polymerization, the lamellar structure of the sample is conserved. SAXS experiments and polarized light microscopy are in favor of a gelation in the lamellar structure without segregation at the micrometer scale. As pointed out by the literature, all of the related surfactant-templated lamellar phases prepared to date collapse to an amorphous oxide upon template removal, but in each case, the inorganic walls are too thin to support the calcination process. The same problem appears using the hexagonal mesophase as a template to obtain a regular hexagonal array of uniform channels after calcination. In our experiments, the polymerization seems to perform in the bulk of the water layer, which supposes a silica thickness of about 30 Å. At this stage, we hope to preserve the lamellar array after the calcination and to obtain a particular mesoporous material. This approach could be transferred to template hexagonal phases, with the goal of creating large inorganic walls. LA9703048
