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Design of Ordered Bimodal Mesoporous Silica Materials by Using a Mixed Fluorinated-Hydrogenated Surfactant-Based System F. Michaux, J. L. Blin, and M. J. Ste´be´* Equipe Physico-chimie des Colloı¨des, Faculte´ des Sciences, UMR SRSMC No. 7565, UniVersite´ Henri Poincare´ sNancy 1/CNRS, BP 239, F-54506 VandoeuVre-les-Nancy cedex, France ReceiVed October 23, 2006. In Final Form: NoVember 21, 2006 Mesoporous silica materials have been prepared using aqueous solutions of hydrogenated-fluorinated surfactant mixtures. The phase behavior of the C18H35(OC2H4)10-C6F15C2H4(OC2H4)11OH [RH18(EO)10-RF6(EO)11] mixture in aqueous solution was first established at the temperature at which the silica source is added, i.e., 20 or 40 °C. We have delimited the different phase domains. Concerning the mesostructured silica, whatever the temperature at which the silica source is added, mesoporous material with a hexagonal array of their channel is formed via a cooperative templating mechanism (CTM), if the content of RF6(EO)11 in the surfactant mixture is lower than 50%. Moreover, when the silica source is added at 40 °C, the recovered materials exhibit a bimodal pore size distribution. The appearance of this bimodality has been related to the coexistence of hydrogenated micelles with fluorinated wormlike micelles. By contrast, the bimodality is not observed when the silica source is added at 20 °C.
1. Introduction Since the discovery of MCMs (Mobil crystalline materials) by Mobil’s scientists in 1992,1,2 the surfactant-templating strategy has been widely used to synthesize a variety of mesoporous compounds, and different kinds of materials have been obtained through an electrostatic route or a neutral pathway.3-10 The synthesis of pure silica mesoporous molecular sieves through the CTM (cooperative templating mechanism)11-13 consists in the condensation and polymerization of an inorganic source of silica around the micelles of surfactants. The final mesostructure is obtained after the surfactant removal by solvent extraction or by calcination. These silica supports are of particular interest in various domains, such as adsorbents,14 catalysts,15-17 host matrixes for electronic and photonic devices,18,19 drug delivery systems,20 and sensors.21 However, before development of such (1) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1999, 359, 710. (2) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T. W.; Olson, D. H.; Sheppard, E. W.; McCulle, S. B.; Higgins, J. B.; Schlender, J. L. J. Am. Chem. Soc. 1992, 114, 10834. (3) Attard, G. S.; Glyde, J. C.; Go¨ltner, C. G. Nature 1995, 378, 366. (4) Bagshaw, S. A.; Pinnavaia, T. J. Angew. Chem., Int. Ed. Engl. 1996, 10, 1102. (5) Prouzet, E.; Pinnavaia, T. J. Angew. Chem., Int. Ed. Engl. 1997, 36, 516. (6) Blin, J. L.; Le´onard, A.; Su, B. L. Chem. Mater. 2001, 13, 3542. (7) Blin, J. L.; Lesieur, P.; Ste´be´, M. J. Langmuir 2004, 20, 491. (8) Han, Y.; Zhao, D.; Song, J.; Yang, X.; Li, N.; Di, Y.; Li, C.; Wu, S.; Xu, X.; Meng, X.; Lin, K.; Xiao, F. S. Angew. Chem., Int. Ed. 2003, 42, 3633. (9) Tan, B.; Dozier, A.; Lehmler, H. J.; Knutson, B.; Rankin, S. E. Langmuir 2004, 20, 6981. (10) Groenewolt, M.; Antonietti, M.; Polarz, S. Langmuir 2004, 20, 7811. (11) Zhao, D.; Huo, Q.; Feng, J.; Chmelka, B. F.; Stucky, G. D. J. Am. Chem. Soc. 1998, 120, 6024. (12) Firouzi, A.; Kumar, D.; Bull, L. M.; Besier, T.; Sieger, P.; Huo, Q.; Walker, S. A.; Zasadzinski, J. A.; Glinka, C.; Stucky, G. D. Science 1995, 267, 1138. (13) Lee, Y. S.; Sujardi, D.; Rathman, J. F. Langmuir 1996, 12, 6202. (14) Llewellyn, P. L.; Ciesla, U.; Decher, H.; Stadler, R.; Schu¨th, F.; Unger, K. K. Stud. Surf. Sci. Catal. 1994, 84, 2013. (15) Brunel, D.; Blanc, A. C.; Galarneau, A.; Fajula, F. Catal. Today 2002, 73 139. (16) Ayala, V.; Corma, A.; Iglesias, M.; Sanchez, F. J. Mol. Catal. A 2004, 221, 201. (17) Corma, A. Chem. ReV. 1997, 97, 2373. (18) Agger, J. R.; Anderson, M. W.; Pemble, M. E.; Terasaki, O.; Nozue, Y. J. Phys. Chem. B 1998, 102, 3345. (19) Wu, C.; Bein, T. Chem. Mater. 1994, 6, 1109. (20) Vallet-Regi, M.; Ramila, A.; Del, Real, R. P.; Pe´rez-Pariente, J. Chem. Mater. 2001, 13, 308.
applications, it is important to control their properties, in particular the morphology, the channel arrangement, and the pore diameter. For example, the occurrence of pores with a bimodal pore size distribution in mesoporous materials is important and useful for catalysis and for the engineering of pore systems.22 Indeed, it was reported that a hierarchical combination of mesopores reduces transport limitations in catalysis, resulting in higher activities and better controlled selectivity.23 The pore arrangement of the recovered materials, i.e., hexagonal, cubic, lamellar, or wormholelike structure, is affected by several physicochemical factors such as the molecular structure of the surfactant, the silica/ surfactant molar ratio,2,24 the pH value of the solution,25 the heating duration, and the temperature.26 For example, using a nonionic fluorinated surfactant, we have reported7 the preparation of mesoporous materials with a hexagonal channel array at 80 °C via a CTM-type mechanism in a wide range of surfactant concentrations (5-20 wt %). However, decreasing the hydrothermal temperature or increasing the surfactant concentration leads to the formation of wormhole-like mesostructure. Another way to control the mesopore arrangement consists in varying the ratio between the volumes of the hydrophilic head (VA) and the hydrophobic part (VB) of the surfactant. Indeed, in a paper dealing with the structural design of mesoporous silica by micelle-packing control using blends of amphiphiles [CnH2n+1(OCH2CH2)xOH), n ) 12-18, x ) 2-23], Stucky and co-workers27 have established a correlation between the final structure and the ratio between the volumes of the hydrophilic head (VA) and the hydrophobic part (VB) of the surfactant. In this way it was evidenced that, with a fixed surfactant concentration of 4 wt %, VA/VB ratios between 0.5 and 1.0 result in the formation of a lamellar mesostructure, (21) Blin, J. L.; Ge´rardin, C.; Carteret, C.; Rodehu¨ser, L.; Selve, C.; Ste´be´, M. J. Chem. Mater. 2005, 17, 1479. (22) Keshavaraja, A.; Ramaswamy, V.; Soni, H. S.; Ramaswamy, A. V.; Ratnasamy, P. J. Catal. 1995, 157, 501. (23) Coppens, M. O.; Froment, G. F. Fractals 1997, 5, 493. (24) Leonard, A.; Blin, J. L.; Robert, M.; Jacobs, P. A.; Cheetham, A. K.; Su, B. L. Langmuir 2003, 19, 5484. (25) Blin, J. L.; Otjacques, C.; Herrier, G.; Su, B. L. Int. J. Inorg. Mater. 2001, 3, 959. (26) Leonard, A.; Blin, J. L.; Jacobs, P. A.; Grange, P.; Su, B. L. Microporous Mesoporous Mater. 2003, 63, 59. (27) Kim, J. M.; Sakamoto, Y.; Hwang, Y. K.; Kwon, Y. U.; Terasaki, O.; Park, S. E.; Stucky, G. D. J. Phys. Chem. B 2002, 106, 2552.
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values of 1.0-1.7 yield 2-D hexagonal mesostructures, and 3-D hexagonal and cubic mesostructures can be obtained in the range of 1.2-2.0.27 Both hydrogenated [RHm(EO)n] and fluorinated [RFm(EO)n] nonionic surfactants can be used as templates for the preparation of mesoporous materials. However, due to the presence of the fluorine atom, fluorinated surfactants have unique properties; in particular, they exhibit a high thermal stability.28 As a consequence, since the hydrothermal treatment can be performed at higher temperature, we could expect that the recovered materials would exhibit a higher hydrothermal stability than the ones prepared using a hydrogenated surfactant-based system. Moreover, fluorinated surfactants can be mixed with other surfactants, and mixtures of hydrogenated and fluorinated surfactants are useful in many practical applications. In regard to the synthesis of mesoporous molecular sieves, the surfactant mixtures can be used to control the characteristics of the final material, such as the channel arrangement and the pore size distribution. In the present paper, we have investigated the ability of a mixed fluorinated-hydrogenated surfactant-based system to be used for the design of well-ordered bimodal mesostructured silica. Indeed, in this kind of system we can assume that a mixture of surfactants form one mixed entity having a mixed hydrophilic and a mixed hydrophobic volume. This consideration allows the continuous variation of the volumes of both the hydrophilic (VA) and the hydrophobic (VB) parts of the surfactant. 2. Materials and Methods The used fluorinated surfactant, which was provided by DuPont, had an average chemical structure of C6F13C2H4(OC2H4)11OH, labeled as RF6(EO)11. The hydrogenated surfactant C18H35(OC2H4)10, labeled as RH18(EO)10, was purchased from Aldrich (Brij 97). In both cases, the hydrophilic chain moiety exhibited a Gaussian chain length distribution. 2.1. Phase Diagram Determination. The samples were prepared by weighing the required amounts of fluorinated, hydrogenated surfactant, and water in well-closed glass vials to avoid evaporation. They were left at controlled temperature for several hours to reach equilibrium. Micellar and liquid crystal domains were identified by visual observations. Dynamic light scattering experiments were performed with a Malvern 300HSA Zetasizer instrument. 2.2. Mesoporous Preparation. A micellar solution containing 10 wt % surfactant in water was prepared. The content of RF6(EO)11 in the surfactant mixture was varied from 0% to 100%. The pH value of the solution was kept at 7.0. Tetramethoxysilane (TMOS), used as the silica source, was added dropwise into the micellar solution at 40 or 20 °C. The surfactant/silica molar ratio was adjusted to 0.5. The obtained samples were sealed in Teflon autoclaves and heated for 1 day at 80 °C. The final products were recovered after ethanol extraction with a Soxhlet apparatus for 48 h. 2.3. Mesoporous Characterization. X-ray measurements were carried out using a home-built apparatus, equipped with a classical tube (λ ) 1.54 Å). The X-ray beam was focused by means of a curved gold/silica mirror on the detector placed 527 mm from the sample holder. Nitrogen adsorption-desorption isotherms were obtained at -196 °C over a wide relative pressure range from 0.01 to 0.995 with a volumetric adsorption analyzer, TRISTAR 3000, manufactured by Micromeritics. The samples were degassed further under vacuum for several hours at 320 °C before nitrogen adsorption measurements. The pore diameter and the pore size distribution were determined by the BJH (Barret, Joyner, Halenda) method.29 Although it is well-known that this method gives an underestimated pore size and that some new methods have been developed,30 we (28) Ravey, J. C.; Ste´be´, M. J. Colloids Surf., A 1994, 84, 11. (29) Barret, E. P.; Joyner, L. G.; Halenda, P. P. J. Am. Chem. Soc. 1951, 73, 37. (30) Jarionec, C. P.; Kruk, M.; Jarionec, M. Phys. Chem. B 1998, 102, 5503.
Figure 1. Temperature-composition phase diagram (wt %) of the RH18(EO)10/RF6(EO)11/water system at 20 °C (A) and 40 °C (B). The dotted line corresponds to the water concentration of the solution from which the silica materials have been prepared. use it here for the sake of simplicity, and the use of this mathematical algorithm does not significantly affect our results as it is a systematic comparison.
3. Results and Discussion 3.1. RF6(EO)11/RH18(EO)10/Water Ternary Phase Diagram. When mesoporous materials are prepared through the CTM mechanism, the surfactant has to form micelles in water. To check whether the RF6(EO)11-RH18(EO)10 mixtures fit with this condition, we have investigated the behavior of the RF6(EO)11RH18(EO)10 mixtures in water at the temperature at which the surfactant and the silica source are mixed, i.e., 40 or 20 °C in our case. The phase diagram of the RF6(EO)11/RH18(EO)10/water system is reported in Figure 1. This kind of system can also be considered as the solubilization of a surfactant into another in the presence of water. At 20 °C (Figure 1A), above 85 wt % water, i.e., at lower surfactant content, the system is micellar (L1) and both hydrogenated and fluorinated surfactants are completely miscible. The compatibility between the head groups, which are almost the same, i.e., respectively 11 and 10 oxyethylene units for RF6(EO)11 and RH18(EO)10, allows the formation of mixed micelles in all proportions. This observation is in good agreement with results reported in the literature. Indeed, the mutual oleophoby
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Figure 2. SAXS patterns of samples synthesized from the surfactant mixture with (a) 0%, (b) 25%, (c) 50%, (d) 70%, and (e) 100% RF6(EO)11. The silica source was added at 20 °C (A) or 40 °C (B).
of fluorinated surfactants is attenuated when the hydrophilic chains are symmetrical, and in this case an ideal mixing behavior is noted.31 For high loading of fluorinated surfactants, the micellar domain is extended up to 55 wt % water for a concentration of RH18(EO)10 lower than 5 wt %. By contrast for a high loading of hydrogenated surfactant and for a water content lower than 84 wt %, only a small fraction of RF6(EO)11 can be accommodated in the hydrogenated micelles. Nevertheless, mixed micelles are formed for water loadings higher than 70 wt % with an RF6(EO)11 concentration comprised between 5 and 9 wt %. Moreover, for high concentrations of surfactant, whatever the ratio between the hydrogenated and the fluorinated surfactants, only one hexagonal liquid crystal phase composed of mixed infinite cylinders packed in a hexagonal array is formed. This observation is in good agreement with the existence of real mixed liquid crystals.31,32 At 40 °C, the situation is quite different (Figure 1B). Indeed, the micellar zone is smaller than that detected at 20 °C, and the dehydration of the oxyethylene chains with the increase of temperature exhibits the oleophobic feature of the fluorinated surfactant. As a consequence, two domains of the hexagonal phase, one fluorocarbon-rich, HF1, which can incorporate up to 15 wt % RH18(EO)10, and one hydrocarbon-rich, HH1, which can accommodate up to 13 wt % RF6(EO)11, are detected. However, as our main goal is to prepare mesoporous silica through the CTM mechanism, the liquid crystal phases were not investigated in detail. 3.2. Mesoporous Materials. All the silica materials were prepared from a solution which contains 90 wt % water and 10 wt % surfactant (dotted line in Figure 1). 3.2.1. Structural InVestigations. First the TMOS used as the silica source was added to the surfactant solution at 20 °C. The SAXS pattern of the mesoporous silica prepared only with RH18(EO)10 exhibits three peaks located at 5.8, 3.2, and 2.9 nm (Figure 2Aa). The presence of the last two peaks is characteristic of a hexagonal organization of the channels. The unit cell a0, which is the sum of the pore diameter and the thickness of the pore wall, (31) Ravey, J. C.; Gherbi, A.; Ste´be´, M. J. Prog. Colloid Polym. Sci. 1989, 79, 272. (32) Tamori, K.; Esumi, K.; Meguro, K. J. Colloid Interface Sci. 1991, 142, 236.
Figure 3. TEM micrographs of sample prepared with 25% RF6(EO)11 in the surfactant mixture: (a) transversal view, (b) longitudinal view.
can be deduced from the relation a0 ) 2d100/31/2, and its value is equal to 6.7 nm. By contrast, only one broad peak situated at 5.1 nm is evidenced on the SAXS pattern of the materials prepared only with RF6(EO)11 (Figure 2Ae). In this case, the mesoporous molecular sieve exhibits a wormhole-like channel system, analogous to MSU (Michigan State University)-type materials. The broad peak observed in the XRD pattern is an indication of the average pore-to-pore separation in the disordered wormhole framework, which presents a lack of long-range crystallographic order. When the mixed RF6(EO)11-RH18(EO)10 surfactant-based system is employed for the synthesis of the porous materials, if the content of RF6(EO)11 in the surfactant mixture is lower than 50%, in addition to a sharp peak at 5.7 nm, two peaks at 3.1 and 2.8 nm are detected in the SAXS pattern (Figure 2Ab,c), meaning that the recovered materials adopt a hexagonal channel array. The hexagonal arrangement of the channels is further confirmed by TEM micrographs. Indeed, the characteristic honeycomblike arrangement and the parallelism of well-oriented channels are clearly evidenced in parts a and b, respectively, of Figure 3. If the content of RF6(EO)11 in the surfactant mixture is higher than 50%, the secondary reflections are not detected anymore (Figure 2Ad), indicating the formation of a disordered structure. Similar results are obtained when the silica source is added to the micellar solution at 40 °C (Figure 2B). Results obtained by SAXS measurement clearly evidenced that the addition of the hydrogenated surfactant involves a pore ordering of the mesoporous materials. 3.2.2. Particle Morphology. From Figure 4, which shows the scanning electron micrographs as a function of the content of RF6(EO)11 in the surfactant mixture, we can conclude that the particle morphology is not significantly influenced by the loading of RF6(EO)11. Large agglomerates are formed, and irregular edge-
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Figure 4. Variation of the particle morphology with the content of RF6(EO)11: (a) 0%, (b) 50%, (c) 75%, (d) 100% (silica source added at 40 °C). Table 1. Mesoporous Material Specific Surface Area (SBET), Pore Volume (Vp), and Pore Diameter (L) as a Function of the Fluorinated Surfactant Content and of the Temperature at Which the Silica Source Is Added (T) [RF6(EO)11] (%) 0 25 50 75 100
T (°C)
SBET (m2 g)
Vp (cm3/g)
L (nm)
40 20 40 20 40 20 40 20 40 20
1047 1069 1048 949 877 1107 1124 1101 1124 1125
1.20 1.16 0.92 1.14 0.78 0.83 0.75 0.45 1.09 0.80
4.1 4.1 2.5-3.8 4.1 2.5-4.2 3.3 2.7-3.4 3.2 3.2 3.2
Table 2. RH18(EO)10/RF6(EO)11/Water System: Variation of the Hydrophilic Volume (VA), of the Hydrophobic Volume (VB), and of the VA/VB Ratio as a Function of the Percentage of Fluorinated Surfactant in the Surfactant Mixture [RF6(EO)11] (%)
VA (cm3/mol)
VB (cm3/mol)
VA/VB
0 25 50 75 100
415 428 442 459 475
361 339 314 285 252
1.15 1.26 1.41 1.61 1.88
shaped particles can be observed. However, it should be noted that when the materials are prepared only with the fluorinated surfactant, particles tend to form bigger spherical particles (Figure 4d). 3.2.3. Characterization by Nitrogen Adsorption-Desorption. The mesoporous silica materials, prepared only with RH18(EO)10, exhibit a type IV isotherm (Figure 5Aa), according to the BDDT
classification.33 The adsorption branch of the isotherm can be decomposed into three parts: the monolayer, the multiple adsorption of N2 on the wall of the mesopores, i.e., the capillary condensation of nitrogen within the mesopores, and then the saturation. An H1-type hysteresis loop in which adsorption and desorption branches are steep is observed for this sample. The isotherm of the materials obtained only with RF6(EO)11 presents similar features (Figure 5Ae). The pore size distribution determined by using the BJH method is quite narrow and centered at 4.1 nm (Figure 5Ba) and at 3.2 nm (Figure 5Be), respectively, for the samples synthesized from the hydrogenated and fluorinated surfactants. Whatever the content of RF6(EO)11 in the surfactant mixture and the temperature at which the silica source is added to the surfactant mixture, all compounds obtained from the mixed RF6(EO)11-RH18(EO)10 surfactant-based system exhibit a type IV isotherm (Figure 5Ab-d). In any case, it should be noted that the adsorbed volume of nitrogen increases significantly at high relative pressures instead of remaining constant due to saturation. These compounds show very large interparticle porosity. Nevertheless, this supplementary textural porosity is observed for both the materials prepared with only the hydrogenated (Figure 5Aa) and the fluorinated (Figure 5Ae) surfactants. This phenomenon cannot be due to use of the mixture of surfactants and should rather be related to the conditions of synthesis and in particular to the pH value. Indeed, for the RH16(EO)10 hydrogenated system in a paper dealing with the effect of the pH of the synthesis gel on the preparation of mesoporous silica, we have reported26 that a considerable amount of secondary porosity appears upon pH increase. The appearance of secondary or interparticular porosity has been correlated with the SEM (33) Brunauer, S.; Deming, L. S.; Deming, W. S.; Teller, E. J. Am. Chem. Soc. 1940, 62, 1723.
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Figure 5. Evolution of the nitrogen adsorption-desorption isotherm (A) and the corresponding BJH pore size distribution curve (B) with the content of RF6(EO)11 in the surfactant mixture: (a) 0%, (b) 25%, (c) 50%, (d) 75%, (e) 100% (silica source added at 40 °C).
observations that show the formation of very small particles with increasing pH values of the synthesis gel. Whatever the synthesis conditions, the value of the specific surface area of the recovered materials is high (>1000 m2/g, Table 1). The maximum of the pore size distribution is progressively shifted toward lower values when the loading of
RF6(EO)11 is progressively increased from 0% to 100% (Table 1). Moreover, it is interesting to note that, when the silica source is added at 40 °C, with increasing fluorinated surfactant content the pore size distribution reveals that there are two mesopore sizes in the material (Figure 5B). However, since the values of the two pore diameters are close to each other, only one inflection
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Figure 6. Scheme illustrating the type of micelles that can be present in the solution before the addition of the silica source: (A) 20 °C, (B) 40 °C.
point, related to the capillary condensation step, is detected on the isotherm (Figure 5Ab-d). Results obtained by nitrogen adsorption-desorption analysis reveal that a bimodal pore size distribution appears when the silica source is added at 40 °C. 3.3. Discussion. Two main tendencies can be drawn from the analysis of the results obtained in this study. First, increasing the proportion of the hydrogenated amphiphile in the mixture leads to the formation of mesoporous silica with a hexagonal array. Second, if the silica source is added at 40 °C, the recovered material, prepared from the mixed RF6(EO)11-RH18(EO)10 surfactant-based system, exhibits a bimodal pore size distribution. In regard to the mesoporous materials, it should be remembered that the overall concentration of surfactant is kept equal to 10 wt % in water (dotted line in Figure 1) and that the materials have been prepared by varying the proportion between the hydrogenated and fluorinated surfactants in the aqueous solution, which was kept at 20 or 40 °C during the addition of TMOS. So from the mesoporous point of view all the conditions are optimal to get well-ordered mesoporous silica through the CTM. However, results obtained by SAXS and TEM analysis evidenced that ordered molecular sieves are recovered only when the content of RF6(EO)11 in the surfactant mixture is lower than 50%. Materials prepared with a higher content of RF6(EO)11 exhibit the MSU structure. As indicated in the introduction, according to Stucky et al.27 the pore ordering can be related to the ratio between the volumes of the hydrophilic head group (VA) and the hydrophobic part (VB) of the surfactant. For the RH18(EO)10-RF6(EO)11 mixture, when the content of the fluorocarbon surfactant is changed from 0% to 100%, the VA/VB ratio varies from 1.15 to 1.88 (Table 2). According to the scale established by Stucky and co-workers, whatever the content of RF6(EO)11, a hexagonal mesostructure should be recovered. However, SAXS patterns show that in our case the hexagonal structure is obtained only for VA/VB ratios between 1.15 and 1.41. To take into account the
high hydrophobicity of the fluorinated surfactant,34 we can suppose that this scale will be shifted. Moreover, looking at the study reported by Esquena et al.,35 it appears that the VA/VB ratio is not the only parameter that should be taken into account to design the structure of mesoporous materials. Indeed, these authors have investigated the formation of mesostructured silica using CF3(CF2)7SO2(C3H7)N(C2H4O)nH as the surfactant; the value of n was varied from 6 to 20. Referring to the VA/VB concept, lamellar, hexagonal, and cubic structure should be obtained when the number of oxyethylene units is increased. However, mesoporous silica with a hexagonal channel array is formed only with n ) 10; other values of n lead to disordered structure. To explain the appearance of the bimodality when TMOS is added at 40 °C, we should have a look at the phase diagram reported in Figure 1. Indeed, for a concentration of surfactant equal to 10 wt %, at 20 °C hydrogenated and fluorinated surfactants form mixed micelles in all proportions. Thus, whatever the proportion between the two surfactants, only one kind of micelle is formed. This result is confirmed by the quasielastic light scattering (QELS) experiments. At 20 °C for a surfactant concentration located at around 1 wt %, the hydrodynamic radius of the RH18(EO)10 micelles is found to be equal to 9 nm. By contrast the size of the fluorinated micelles cannot be obtained from this technique. Indeed, birefringence experiments under weak shear flow36 have been performed with fluorinated surfactants, analogous to that used in the present study, and the results evidenced that the micelles exhibited an elongated shape (>300 nm). This observation is in good agreement with the results reported by Acharya et al.37 Indeed, in a paper dealing (34) Ravey, J. C.; Ste´be´, M. J. Prog. Colloid Polym. Sci. 1988, 76, 234. (35) Esquena, J.; Rodriguez, C.; Solans, C.; Kunieda, H. Microporous Mesoporous Mater. 2006, 92, 212. (36) Decruppe, J. C.; Pontin, A. Eur. Phys. J. 2003, 10, 201. (37) Acharya, D. P.; Sharma, S. C.; Rodriguez-Abreu, C.; Aramaki, K. J. Phys. Chem. B 2006, 110, 20224.
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with a vicoelastic micellar solution in nonionic fluorinated surfactant systems, these authors reported that the aggregate structures in the fluorinated surfactant can be explained, like hydrogenated surfactants, in terms of the critical parameter. However, since fluorocarbon chains are bulkier than the hydrocarbon chain, cylindrical micelles are often observed at solution conditions where spherical micelles are expected in hydrogenated surfactant systems. These cylindrical micelles undergo enormous one-dimensional growth and form flexible treadlike aggregates called wormlike micelles. Concerning the mixture of surfactants, for a solution containing 50% RF6(EO)11 and 50% RH18(EO)10, the results indicate that the micelles have a diameter of about 8 nm, a value close to that observed for the RH18(EO)10 micelles. If the hydrogenated content is decreased, the value of the radius determined by QELS is shifted toward a higher value, suggesting a progressive evolution of the micelle shape from a sphere to a rod (Figure 6A). When the silica source is added to the surfactant solution, hydrogen-bonding interactions between the oxygen atoms of the oxyethylene groups of the surfactant and hydrogen atoms of TMOS are formed, and after hydrothermal treatment and surfactant removal by ethanol extraction, mesoporous material with a single pore diameter is recovered. By contrast, for a concentration of surfactant equal to 10 wt %, at 40 °C, from the phase diagram it is evidenced that a demixion occurs when the content of RF6(EO)11 in the surfactant mixture is decreased. We can assume that, in the presence of TMOS, the limits of the different domains of the phase diagram will be shifted and that the phenomenon of demixion will also take place for higher RF6(EO)11 contents. Therefore, hydrogenated micelles, which accommodate a small fraction of R F6(EO)11, and wormlike fluorinated micelles, which incorporate a small fraction of RH18(EO)10, will form a biphasic system containing two types of micelles (Figure 6B). The proportion between the two kinds of micelles depends on the proportion between the two surfactants. This idea is strengthened by the phase behavior of the surfactant
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mixture. Indeed, it should be remembered that, at this temperature and for high concentrations of surfactants, two domains of the hexagonal phase, one fluorocarbon-rich, HF1 and one hydrocarbonrich, HH1, are evidenced. During the material preparation, the silica source polymerizes both around the two kinds of micelles. As a consequence the final molecular sieve exhibits a bimodal pore size distribution.
4. Conclusions Bimodal ordered mesoporous materials have been prepared using aqueous solutions of hydrogenated-fluorinated surfactant mixtures. The phase behavior of the RH18(EO)10-RF6(EO)11 mixture in aqueous solution was first established at the temperature at which the silica source is added, i.e., 20 or 40 °C. At 40 °C, above 92 wt % water, i.e., at lower surfactant content, the system is micellar (L1) and both hydrogenated and fluorinated surfactants are completely miscible. When the water concentration is decreased, a gap of miscibility appears. In this case spherical hydrogenated micelles, which accommodate a small fraction of RF6(EO)11, and wormlike fluorinated micelles, which can incorporate a small fraction of RH18(EO)10, coexist. At 20 °C both surfactants are completely miscible in all proportions. Whatever the synthesis conditions, starting from this hydrogenated-fluorinated surfactant-based system, mesostructured silicas with a hexagonal array of their channels are prepared via a CTM, if the content of RF6(EO)11 in the surfactant mixture is lower than 50%. Moreover, when the silica source is added at 40 °C, the recovered materials exhibit a bimodal pore size distribution. The appearance of this bimodality has been related to the coexistence of hydrogenated micelles with fluorinated cylindrical ones. The latter undergo enormous one-dimensional growth. Acknowledgment. We thank DuPont de Nemours Belgium for providing the fluorinated surfactant. LA063103P
