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Nonionic Fluorinated Surfactant: Investigation of Phase Diagram and Preparation of Ordered Mesoporous Materials J. L. Blin, P. Lesieur, and M. J. Ste´be´* Equipe Physico-chimie des Colloı¨des, UMR 7565 Universite´ Nancy-1/CNRS, Faculte´ des Sciences, BP 239, F-54506 Vandoeuvre-les-Nancy cedex, France Received October 1, 2003. In Final Form: November 6, 2003 The behavior of fluorinated surfactant F(CF2)8C2H4(OC2H4)9OH in water solution was investigated, and the preparation of mesoporous molecular sieves was achieved. A direct micellar phase (L1) and a hexagonal (H1) liquid crystal were found. Small-angle X-ray scattering measurements proved that the hydrophobic chains are completely extended and that the cross sectional area remains constant in H1. At 80 °C, materials with a hexagonal array of their channel are prepared via a cooperative templating type mechanism in a wide range of surfactant concentrations (5-20 wt %). Decreasing the hydrothermal temperature leads to the formation of wormhole-like structure. In this case the channel arrangement is no longer governed by the surfactant behavior but by the silica condensation and polymerization. An increase of the mean pore diameter with heating temperature is noted. This result is associated with changes of aggregation number with temperature. A comparison of the characteristics of the materials obtained with both hydrogenated and fluorinated surfactants is also made.
1. Introduction Nonionic polyoxyethylene alkyl ether surfactants Cm(EO)n have been widely studied, and much physicochemical data, such as micelle formation, intermicellar interaction, clouding phenomenon, structural studies, and phase behavior in various solvents, have been reported in the literature.1-6 These surfactants have become commercially available, and they are used as emulsifiers, wetting agents, detergents, or solubilizers. More recently, polyoxyethylene alkyl ethers have been employed as templates for the preparation of mesoporous molecular sieves. Indeed, in 1995, Attard et al.7 with C12(EO)8 and C16(EO)8 reported the first syntheses of mesoporous molecular sieves through the N0I0 pathway. The assembly of the mesostructure is based on the hydrogen bonding interactions between the oxygen atoms of the oxyethylene group (N0) and the hydrogen atoms of the neutral inorganic precursor (I0). The obtained ordered materials with hexagonal channel arrangement exhibit pore sizes up to 3.0 nm. Then, using this type of templating agent, Stucky et al.8 reported new cubic (Pm3 h m) and hexagonal (P63/ mmc) mesoporous silica structure. Pinnavaia et al.9,10 also used the same family of surfactant and successfully * To whom correspondence may be addressed. Phone: +33-383-68-43-70. Fax: +33-3-83-68-43-22. E-mail: Jean-Luc.Blin@ lesoc.uhp-nancy.fr. (1) Nonionic Surfactant Physical Chemistry; Schick, M. J., Ed.; Surfactant Science Series 23; Marcel Dekker: New York, 1987. (2) Mitchell, D. J.; Tiddy, G. J. T.; Waring, L.; Bostock, T.; Mc Donald, M. P. J. Chem. Soc., Faraday Trans. 1 1983, 79, 975. (3) Meguro, K.; Takasawa, Y.; Kawahashi, N.; Tabata, Y. and Ueno, M. J. Colloid Interface Sci., 1982, 83, 50. (4) Ravey, J. C. In Microemulsions: Structure and dynamics; Friberg, S. E., Bothorel, P.; Eds.; CRC Press Inc.: Boca Ratan, FL, 1988; p 93. (5) Corti, M.; Degiorgio, V. Phys. Res. Lett. 1980, 45, 1045. (6) Kunieda, H.; Shigeta, K. and Ozawa, K. J. Phys. Chem. B 1997, 101, 7952. (7) Attard, G. S.; Glyde, J. C.; Go¨ltner, C. G. Nature 1995, 378, 366. (8) Zhao, D.; Huo, Q.; Feng, J.; Chmelka, B. F. and Stucky, G. D. J. Am. Chem. Soc. 1998, 120, 6024. (9) Bagshaw, S. A.; Prouzet, E.; Pinnavaia, T. J. Science 1995, 269, 1242 (10) Prouzet, E.; Pinnavaia, T. J. Angew. Chem., Int. Ed. Engl. 1997, 36, 516.
synthesized wormhole-like structured MSU (Michigan State University) mesoporous silica in near neutral conditions. Different mechanisms have been proposed to explain the formation of these materials. According to Mobil researchers, M41S type materials are formed through an LCT (liquid crystal templating) mechanism,11,12 in which aggregates of template molecules in combination with silicate species form a supramolecular structure. Later, based on the studies of different authors, the LCT mechanism was improved to lead to the CTM (cooperative templating mechanism).8,13,14 In the initial step, the interactions between silica and isolated spherical or cylindrical micelles drive to the formation of an organicinorganic mesophase. Then, the condensation of the inorganic precursor at the external surface of the micelles occurs. The ordered mesophase is obtained after intermicellar condensation. Finally, the hydrothermal treatment at higher temperature completes the assembly of micelles and the polymerization of the silica source. However, some disagreement concerning the first step of the CTM mechanism appears and numerous investigations were carried out in order to better understand how the surfactant and the inorganic precursor could affect the formation of the hybrid mesophase. Even if the debate concerning the initial step of the CTM mechanism is still opened, it is admitted that the interactions between surfactant and inorganic precursor are responsible for the mesoporous materials formation. It appears that this step is strongly affected by the behavior of surfactant in the synthesis solvent. For example, under the same synthesis conditions, i.e., pH, surfactant/silica molar ratio, hydrothermal temperature,and duration, hexagonal molecular sieves have been obtained with decaoxyethylene cetyl [C16(11) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1999, 359, 710. (12) 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. (13) 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. (14) Lee, Y. S.; Sujardi, D.; Rathman, J. F. Langmuir 1996, 12, 6202.
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(EO)10] and decaoxyethylene oleyl ether [C18(EO)10], the surfactant concentration in aqueous solution being in a wide range of around 10-25 wt %. However, if the surfactant concentration is raised higher than 30%, for example from 30 to 60 wt %, both mild acidic or basic media conduct to the formation of wormhole-like structures.15,16 The substitution of hydrogen atoms by fluorine ones enhances the chemical and thermal stability of the surfactant. For example the energy of the C-F bond is 552 kJ mol-1 instead of 338 kJ mol-1 for the C-H bond.17 The presence of fluorine atoms also strongly affects the properties of the surfactant and particularly its hydrophobicity and its critical micellar concentration (cmc).18 It was reported that for nonionic surfactants one CF2 group is equivalent to 1.7 CH2 groups,19 which triggers lower cmc’s. As an instance if the headgroup is not changed, a fluorinated surfactant with 7 carbon atoms has the same cmc as a hydrogenated one with 11 carbon atoms. As a consequence of the high hydrophobicity, fluorinated surfactants can decrease the surface tension of water from 30-40 to 15-20 mN‚m-1. The phase behavior of fluorinated surfactants is quite similar to the hydrogenated ones.20,21 However, some differences can be found, and a comparative study21 of fluorinated and nonfluorinated based systems has emphasized that lamellar liquid crystals and larger aggregates are preferentially formed with fluorinated surfactants. This difference in behavior has been ascribed to the fluorinated chain rigidity and to the higher value of the headgroup area of fluorinated surfactants. As their hydrogenated analogues, fluorinated surfactants are used in various domains. Indeed, due to their high stability, they can support strong temperature and severe pH conditions. Moreover, fluorinated surfactants allow cosolubilization of water and perfluoroalkanes,22,23 and the specific property of fluorocarbon to dissolve high quantities of oxygen and carbon dioxide make them very attractive for biomedical applications such as oxygen vectorization for instance.24,25 However, to the best of our knowledge, they have not yet been employed as templates for the preparation of mesoporous molecular sieves. In the present paper, we have investigated the ability of a fluorinated surfactant [F(CF2)8C2H4(OC2H4)9OH] to be used for the preparation of ordered mesoporous materials. As mentioned above, the properties of the molecular sieves depend on the phase behavior of the template in the synthesis solvent, so, as a first step, we have investigated the phase behavior of F(CF2)8C2H4(OC2H4)9OH in water. 2. Materials and Methods Phase Diagram Determination. The used fluorinated surfactant, which was provided by DuPont, had an average chemical structure of F(CF2)8C2H4(OC2H4)9OH, labeled as [RF8(15) Blin, J. L.; Le´onard, A.; Su, B. L. Chem. Mater. 2001, 13, 3542. (16) Blin, J. L.; Le´onard, A.; Herrier, G.; Philippin, G and Su, B. L. Stud. Surf. Sci. Catal. 2002, 141, 141. (17) Fluorinated Surfactants Synthesis properties Applications; Kissa, E., Ed.; Surfactant Science Series 50; Dekker: New York, 1994. (18) Shinoda, K.; Hato, M.; Hayashi, T. J. Phys. Chem. 1972, 76, 909. (19) Ravey, J. C.; Gherbi, A.; Ste´be´, M. J. Prog. Colloid Polym. Sci. 1998, 76, 234. (20) Ravey, J. C.; Ste´be´, M. J. Prog. Colloid Polym. Sci. 1997, 73, 127. (21) Ravey, J. C.; Ste´be´, M. J. Colloids Surf., A 1994, 84, 11. (22) Mathis, G.; Leempoel, P., Ravey, J. C.; Selve, C.; Delpuech, J. J. J. Am. Chem. Soc. 1984, 106, 6162. (23) Ravey, J. C.; Ste´be´, M. J.; Oberthu¨r R. In Surfactant in solution; Mittal, K. L., Bothorel, P., Eds.; Plenum: New York, 1987; Vol. 6, p 1421. (24) Riess, J. G. Chem. Rev. 2001, 101, 2797. (25) Hamza, M. A.; Serratrice, G.; Ste´be´, M. J.; Delpuech, J. J. J. Am. Chem. Soc. 1981, 103, 3733.
Blin et al. (EO)9]. The hydrogenated surfactant C16(EO)10 was purchased from Aldrich. In both cases, the hydrophilic chain moiety exhibited a Gaussian chain length distribution. The samples were prepared by weighing the required amounts of surfactant and water in well-closed glass vials to avoid evaporation. They were left at controlled temperature for some hours in order to reach equilibrium. The phase diagrams have been established between 10 and 60 °C in the whole water-surfactant composition. Liquid crystal phase domains have been identified by their texture observed with optical microscope equipped with cross polarizers. To find the exact limits of these domains, additional small-angle X-ray scattering (SAXS) measurements have also been performed at LURE (Orsay, France) on beam line D22 of the DCI synchrotron. The incident beam wavelength λ was 1.39 Å, and the sample to detector distance was 1800 mm. Mesoporous Preparation. Solutions with a weight percentage of RF8(EO)9 varying from 5 to 30 wt % were prepared by dissolving the surfactant in water at 40 °C. The pH value of the solutions was then adjusted to 2.0 using sulfuric acid. Tetramethoxysilane (TMOS), used as the silica source, was added dropwise. The surfactant/silica molar ratio was fixed to 0.5. The obtained gel was sealed in Teflon autoclaves and heated for 1, 2, and 3 days at 80, 60, and 40 °C, respectively. The final products were recovered after ethanol extraction with a Soxhlet apparatus over 30 h. Mesoporous Characterization. SAXS measurements were carried out using a home-built apparatus, equipped with a classical tube (λ ) 1.54 Å). The X-ray beam was focused using 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 further degassed 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.26
3. Results and Discussion F
3.1. R 8(EO)9/Water System. The temperaturecomposition phase diagram of the F(CF2)8C2H4(OC2H4)9OH-water system is reported in Figure 1A. In the investigated temperature range, and until 30 wt % of surfactant, a direct micellar L1 phase is detected. For 10 wt % of RF8(EO)9, only one rather broad line is observed on the SAXS pattern (Figure 2a), this line corresponds to a correlation distance of 13.7 nm (d ) 2π/q) and is related to the existence of micelles in solution. When the surfactant concentration is increased, this line is shifted toward higher angles (Figure 2b). For a 25 wt % of RF8(EO)9, the correlation distance is found to be equal to 9.0 nm, which is closed to the rod size of the hexagonal phase, which is given by the value of the cell parameter (as calculated below, a ) 7.7 nm for a 50 wt % of RF8(EO)9). When the weight percent of surfactant is increased up to 75 wt %, an optically anisotropic phase is detected. The fan-shaped texture (Figure 3) observed by optical microscopy with polarized light is characteristic of the structure defects of the direct hexagonal phase (H1). The hexagonal symmetry is confirmed by SAXS measurements. Indeed, lines located at 6.4 and 3.6 nm for a 60 wt % of RF8(EO)9 are detected on the pattern (Figure 2c). The relative positions of the Bragg reflections are 1, x3. Consequently, they can be attributed to the (100) and (110) reflections of the hexagonal structure. Contrary to the hydrogenated system, the (200) reflection is generally not observed. It should be noted that the H1 phase is separated from L1 by a twophase region (30 e wt % e 50). If the surfactant concentration is further increased (>75 wt %), the texture (26) Barret, E. P.; Joyner, L. G.; Halenda, P. P. J. Am. Chem. Soc. 1951, 73, 37.
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Taking into account the equivalence rule established for the phase behavior of hydrogenated and fluorinated surfactants,22 we can assume that C16(EO)10 is the hydrogenated analogue of RF8(EO)9. Indeed, except for the cubic phase between the L1 and H1 domains, the temperature-composition phase diagram established for the C16(EO)10-water system exhibits the same phase sequence (Figure 1B). The CF3-(CF2)7(OCH2CH2)16OHH2O system,27 which does not present a cubic domain at high surfactant concentration, presents similar characteristics as those of RF8(EO)9. However, for the hydrogenated equivalent system, the formation of the lamellar phase from H1 required the existence of a normal-cubicbicontinuous phase (V1). Thus, it seems that for the fluorinated system either the cubic domain is very small and we did not observed it or the presence of fluorine atoms does not favor the formation of cubic structure (V1) in water. The hexagonal phase is composed of infinite cylinders packed in a hexagonal array. In the case of direct systems, cylinders are filled by the hydrophobic chains and are covered by both headgroups and water. The hexagonal H1 phase is characterized by its typical SAXS profile with the relative peak positions, 1, x3, 2. The distance d associated to the first peak is related to the hydrophobic radius R by the relation28 Figure 1. Temperature-composition (wt%) phase diagram of RF8(EO)9 (A) and C16(EO)10 (B) in water.
VB x3πR2 ) VTA + RVE 2d2 where R stands for the number of water molecules per surfactant molecule and VB, VTA, and VE stand for the molar volumes of the hydrophobic part of the surfactant (VB ) 261 cm3/mol), the surfactant (VTA ) 626 cm3/mol), and water (VE ) 18 cm3/mol), respectively. The cell parameter a is given by the relation a ) 2d/x3. The crosssectional area S can then be deduced from the following relation:
S ) 2VB/0.6R
Figure 2. RF8(EO)9/water system: Evolution of the SAXS pattern with the concentration (wt %) of RF8(EO)9 in water at 25 °C: a, 10% (L1); b, 25% (L1); c, 60% (H1).
Results concerning the RF8(EO)9 surfactant are reported in Figure 4. As shown in Figure 4A, an increase in the d spacing and, thus, in the cell parameter is noted with increasing water content. This is due to the hydratation of the headgroup, and water could also form a film surrounding the surfactant. The hydrophobic radius (R), which provides direct information about the conformation of the hydrophobic chain, remains constantly equal to 1.8 nm. As the length of an extended chain with 10 carbon atoms is about 1.4 nm, we can deduce that the hydrophobic chains in the H1 liquid crystal phase are completely extended. The value of the cross sectional area is 4.9 nm. and it remains constant in the overall H1 domain (Figure 4B). 3.2. Preparation of Silicate Mesoporous Materials. 3.2.1 X-ray Analysis. Figure 5 depicts the variation of the SAXS pattern with the surfactant weight percentage in the aqueous solution of materials obtained after hydrothermal treatment at 80, 60, and 40°C. It has been reported29 that X-ray diffractograms of powdery hexagonal mesoporous materials exhibit a typical three-peak pattern
Figure 3. Characteristic texture of the hexagonal liquid crystal.
of the gel phase appears. If the temperature is increased, the overall phase limits are shifted toward higher surfactant concentration.
(27) Marczuk, P.; Lang, P.; Huang, H. N. J. Phys. Chem. 1996, 100, 13822. (28) Alibrahim, M.; Ste´be´, M. J., Dupont, G.; Ravey, J. C. J. Chim. Phys. 1997, 94, 1614. (29) Chen, C. Y.; Xiao, S. O.; Davis, M. E. Microporous Mater. 1995, 4, 20.
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Figure 4. Hexagonal liquid crystal RF8(EO)9/water: (A) repetition distance (d), cell parameter (a), hydrophobic radius (R); (B) cross-sectional area (S) as a function of R, the number of water molecules per surfactant molecule.
with a very strong feature at a low angle (100 reflection line) and two other weaker peaks at higher angles (110 and 200 reflection lines). These three reflection lines can be attributed to a hexagonal unit cell (a0 ) 2d100/x3) which corresponds to the sum of the pore diameter and the thickness of the pore wall. The absence of the last three peaks suggests a disordered structure of the mesoporous molecular sieves having wormhole-like channel systems, analogous to MSU-type materials. The broad peak that is observed on the SAXS pattern gives an indication of the average pore-to-pore distance in the disordered wormhole framework which presents a lack of long-range crystallographic order. As regards the sample recovered after hydrothermal treatment during 1 day at 80 °C, with a 5 wt % of RF8(EO)9, in addition to a sharp peak at 5.3 nm, two peaks at 3.2 and 2.6 nm are detected (Figure 5A, curve a). The presence of these last two peaks indicates a hexagonal organization of the channels in this material. According to Bragg’s law, the unit cell dimension (a0) can be calculated and found equal to 6.2 nm. With increasing the surfactant concentration in the solution until 20 wt %, the position of the sharp peak does not vary (Figure 5A, curves a-d), indicating that a0 remains constant. However, the results obtained by nitrogen adsorption-desorption analysis show that the pore diameter increases with the loading of surfactant (Table 1). It can then be inferred that the wall thickness decreases when the surfactant weight percentage of the surfactant is raised to 20 wt %.
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If surfactant concentration is progressively changed from 20 to 30 wt %, the (110) and (200) reflections, arising from the hexagonal structure, are not observed any longer (Figure 5A, curves e and f). As just discussed above, the presence of a single reflection indicates the formation of the wormhole-like structure. Above 30 wt % of surfactant, no reflection is detected any more, indicating that the channel array is completely disordered and that the pore diameter is not homogeneous. These results clearly demonstrate that, in the conditions reported here, with a concentration lower than 25 wt %, well-ordered silicas are obtained, whereas with a concentration higher than 25 wt %, disordered structures are formed. If the hydrothermal treatment is performed at a lower temperature, the formation of well-ordered mesoporous molecular sieves is not favored. Indeed, at 40 °C, whatever the surfactant concentration, only MSU-type materials are obtained (Figure 5C). At 60 °C (Figure 5B), the composition range of RF8(EO)9 leading to materials with a hexagonal array of their channels is shrunk from 5 to 10 wt % (Figure 5B, curves a and b). Moreover, the (110) and (200) reflections are less resolved and their intensity is lower than those obtained for sample prepared at 80 °C, which suggests that the channel array is less regular. 3.2.2. Particle Morphology. Figure 6 shows scanning electron microscopy pictures of the samples synthesized at different surfactant concentrations. After hydrothermal treatment for 1 day at 80 °C, the morphology of the materials prepared with a loading of RF8(EO)9 inferior to 25 wt % can be described as an agglomerate of gyroids, toroids (Figure 6a-c). Higher surfactant concentrations lead to the formation of particles with irregular shapes (Figure 6d). With decrease of the heating temperature to 60 °C, isolated gyroids, toroids, and ropes are present in the compound prepared with 5 wt % surfactant (Figure 6e,f). On the basis of many papers reporting such morphology, using different templates and synthesis conditions,30-33 in particular as described by Ozin et al.,34-37 it appears that no direct relationship between internal structure and external morphology can be deduced. No further investigation from this point of view was undergone in the present work. 3.2.3. Characterization by Nitrogen AdsorptionDesorption. All samples obtained after hydrothermal treatment at 80 °C during 1 day exhibit a type IV isotherm, characteristic of mesoporous materials according to the BDDT classification.38 A H1 type hysteresis loop in which adsorption and desorption branches are steep is observed for the samples obtained with surfactant concentration inferior to 25 wt % (Figure 7A, curves a-d), whereas a hysteresis loop similar to H2 type in which the desorption branch is steep but adsorption branch is more or less sloping corresponds to samples prepared with a surfactant concentration of 25 and 30 wt % (Figure 7A, curves e and (30) Lee, C. H.; Sung, S. P.; Choe, S. J.; Park, D. H. Microporous Mesoporous Mater. 2001, 46, 257. (31) Zhao, D.; Sun, J.; Li, Q.; Stucky, G. D. Chem. Mater. 2000, 12, 275. (32) Kim, J. M.; Sakamoto, Y.; Hwang, Y. K.; Kwon, Y. U.; Terasaki, O.; Park, S. A.; Stucky, G. D. J. Phys. Chem. B 2002, 106, 2552. (33) Sayari, A.; Hamoudi, S.; Yang, Y.; Moudrakovski, I. L.; Ripmeester, J. R. Chem. Mater. 2000, 12, 275. (34) Yang, H.; Coombs, N.; Ozin, G. A. Nature 1997, 386, 692. (35) Ozin, G. A.; Yang, H.; Sokolov, I.; Coombs, N. Adv. Mater. 1997, 9, 8, 662. (36) Yang, S. M.; Yang, H.; Coombs, N.; Sokolov, I.; Kresge, C. T.; Ozin, G. A. Adv. Mater. 1999, 11, 52. (37) Ozin, G. A.; Kresge, C. T.; Yang, H. Stud. Surf. Sci. Catal. 1998, 117, 119. (38) Brunauer, S.; Deming, L. S.; Deming, W. S.; Teller, E. J. Am. Chem. Soc. 1940, 62, 1723.
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Figure 5. Mesoporous materials: SAXS patterns of samples synthesized with a concentration (wt %) of RF8(EO)9: (a) 5%, (b) 10%, (c) 15%, (d) 20%, (e) 25%, and (f) 30%; hydrothermal treatment of 1 day at 80 °C (A), 2 days at 60 °C (B), and 3 days at 40 °C (C). Table 1. Mesoporous Material: Spacing Values (d), Cell Parameter (a0), Specific Surface Area (SBET), Pore Volume (Vp), and Pore Diameter Concerning the Samples Obtained at Different Surfactant Concentrations (wt %; temperature 80 °C; duration of hydrothermal treatment, 1 day) wt %
d (nm)
a0 (nm)
SBET (m2/g)
Vpa (cm3/(g STP))
diametera (nm)
5 10 15 20 25 30
5.3 5.3 5.3 5.4 5.8 6.0
6.2 6.2 6.2 6.3 b b
1044 844 950 831 744 660
1.28 1.28 1.55 1.44 1.58 1.63
4.3 4.6 5.0 6.0 7.8 9.3
a Values obtained from BJH method applied to the adsorption branch of the isotherm. b Wormhole-like structure.
f). The H2 type hysteresis loop can arise from the same type of open capillaries as are responsible for the H1 hysteresis loop which can be observed very commonly from MCM-41 type materials with a very regular arrangement of channels with very homogeneous openings. The H2 type of hysteresis loop is often encountered for disordered materials with a wormhole structure. For samples synthesized with a weight percentage of RF8(EO)9 inferior to 25 wt %, the value of the relative pressure at which the capillary condensation occurs slightly increases with the surfactant concentration. Since the p/p0 position of the inflection point is related to the pore diameter according to Kelvin’s equation, this observation suggests that the pore diameter of molecular sieves is raised when the concentration of RF8(EO)9 varies from 5 to 20 wt %. This is confirmed by the pore size distribution, whose maximum is shifted toward a higher value when the surfactant content is increased from 5 to 20 wt % (Figure 8a-d). However, if the surfactant content is further raised, the capillary condensation is spread out over a larger range of relative pressures, meaning that compounds become less homogeneous in pore sizes (Figure 7A, curves e and f). This is confirmed by the broader pore diameter distribution (Figure 8e,f), reflecting the transition from
a well-ordered mesoporous material with high specific surface area and a uniform pore diameter to a disordered structure with wormhole-like channels. The above analysis and the observations of two types of hysteresis loops are in agreement with the SAXS results. Thus it can be concluded that, for a hydrothermal treatment of 1 day at 80 °C, a weight percentage of templating agent equal to 20 wt % is the upper limit to obtain well-ordered mesoporous materials. If the hydrothermal is performed at a lower temperature, the situation is quite different. Indeed, at 40 °C, samples synthesized with a concentration of RF8(EO)9 ranging from 5 to 15 wt % exhibit isotherms (Figure 7B, curve a), which are intermediates between type I and IV. According to Dubinin,39 these kinds of isotherms are characteristic of supermicroporous materials, i.e., the pore size is located at the limit between the micro- and mesoporous domain. With increase of the surfactant content, isotherms become type IV (Figure 7B, curve b), meaning that the recovered materials are mesoporous. At 60 °C, all isotherms are characteristics of mesoporous materials (Figure 7B, curves c and d). In any case, an increase in pore diameter with heating temperature and with RF8(EO)9 concentration is noted. For example for 10% of RF8(EO)9, the pore diameter varies from 1.7 to 4.6 nm when the heating temperature is increased from 40 to 80 °C. 3.3. Discussion. 3.3.1. Mechanistic Consideration. Whatever hydrothermal treatment conditions are, if the concentration of RF8(EO)9 used for the preparation of the mesoporous material corresponds to the direct hexagonal H1 or to a biphasic (H1 + L1) domain, only materials with a disordered channel array are recovered. In this case, when the TMOS is added to the liquid crystal (H1), hydrogen bonding type interactions between the oxygen atoms of the oxyethylene group of the surfactant and hydrogen atoms (39) Dubinin, M. M. In Progress in Surface and Membrane Science; Cadenhead, D. A., Ed.; Academic Press: New York, 1975; Vol. 9, p 1.
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Figure 6. Variation of the particle morphology with concentration (wt %) of RF8(EO)9: a, 5%; b, 10%; c, 15%; d, 30% (hydrothermal treatment of 1 day at 80 °C); e, f, 5% (hydrothermal treatment of 2 days at 60 °C).
of the silica source occur. Then, when the polymerization of silica takes place, it disturbs the regular H1 array and the channels slightly move in order to minimize steric and electrostatic energies. Thus, the regular organization is lost and the obtained materials are disordered. Only one broad peak or no reflection at all can be detected on the SAXS pattern. When the preparation of the mesoporous material is carried out from the micellar solution L1 (concentrations of RF8(EO)9 between 5 and 20 wt %), the situation is very different. Indeed, at 80 °C, the obtained molecular sieves exhibit a hexagonal channel array. The formation of the mesostructure can be explained by a CTM-type mechanism. When the silica is added to the micellar solution, hydrogen bonding interactions between the oxygen atoms of the oxyethylene group of the surfactant and hydrogen atoms of TMOS are formed. To complete polymerization of tetramethoxysilane, these rodlike supramolecular assemblies (template-silica) have to pack together, leading to the formation of a hexagonal hybrid mesophase, whose features are analogous to the H1 liquid crystal. After surfactant removal, mesoporous molecular sieves with hexagonal channel array are synthesized. In these conditions, the behavior of the surfactant in aqueous solution
is the driving force in the formation of the regular array. Decreasing the hydrothermal treatment temperature, the channel arrangement is no longer governed by the surfactant behavior but by the silica condensation and polymerization. Indeed, even if some micelles remain, only a poor hexagonal arrangement is obtained at 60 °C with 5 and 10 wt % of RF8(EO)9, and if the synthesis is performed with a concentration of RF8(EO)9 between 15 and 25 wt %, disordered materials are recovered. At 40 °C, in the investigated range of concentrations, only wormhole-like structures are formed. These observations are in good agreement with those reported for the cetyltrimethylammonium-sodium silicate system.40 In a paper dealing with a kinetic study of the formation of MCM-41, we had reported that, at low heating temperature or short heating time, the (100) and (200) reflections were not detected by SAXS. This corresponds to the initial step of the synthesis, which refers to the nucleation step in zeolite synthesis. Following this step, the (100) and (200) reflections are present on the SAXS pattern of the mesoporous material. (40) Blin, J. L.; Otjacques, C.; Herrier G. and Su, B. L. Int. J. Inorg. Mater. 2001, 3, 959.
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Figure 7. (A) Nitrogen adsorption-desorption isotherms of compounds synthesized with a concentration (wt %) of RF8(EO)9: a, 5%; b, 10%; c, 15%; d, 20%; e, 25%; f, 30% (hydrothermal treatment of 1 day at 80 °C). (B) Nitrogen adsorption-desorption isotherms of compounds synthesized with a concentration (wt %) of RF8(EO)9: a, 10%; b, 25% (hydrothermal treatment of 2 days at 60 °C); c, 15%; d, 25% (hydrothermal treatment of 3 days at 40 °C).
Thus, the well-ordered hexagonal MCM-41 structure with high surface area is formed. 3.3.2. Variation of the Pore Diameter with the Concentration of Surfactant and the Hydrothermal Temperature. Two main tendencies can be drawn from the analysis of the pore size distribution. First, with increasing temperature, the pore diameter grows, for example, from 1.7 to 4.6 nm for 10 wt % of RF8(EO)9. Second, for a given temperature, in particular at 80 °C, the pore diameter increases sharply when the surfactant concentration, used for the preparation of the mesoporous molecular sieve, is raised from 5 to 30 wt % (from 4.3 to 9.4 nm). The first tendency is the result of a variation of the aggregation number of micelles (L1 phase). Indeed, it is well established that, for nonionic surfactants, an increase in temperature will involve an increase in the aggregation number.41,42 Thus, bigger micelles will be formed with increasing heating temperature, and consequently, materials with higher pore diameters will be recovered. The second tendency is that, for a given temperature, the pore diameter increases with increasing RF8(EO)9 content, employed for the synthesis of mesopo(41) Zana, R.; Weill, C. J Phys. Lett. 1985, 46, L953. (42) Zulauf, M.; Weckstro¨m, K.; Hayter, J. B.; Degiorgio, V.; Corti, M. J. Phys. Chem. 1985, 89, 3411.
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Figure 8. Pore size distribution of compounds synthesized with a concentration (wt %) of RF8(EO)9: (a) 5%; (b) 10%; (c) 15%; (d) 20%; (e) 25%; (f) 30% (hydrothermal treatment for 1 day at 80 °C).
rous materials, which can be explained by a change in micelle shapes. Indeed, when micelles are close together, due to the limitations on the conformation of oxyethylene groups arising from steric hindrance, repulsive forces occur. This leads to a micelle shape transition like sphereto-rod, which favors a higher degree of structure.2 As a consequence, the size of the micelle is increased and higher values of channel mean diameters are observed. When rod micelles are formed in solution, we can try to find a relationship between the pore diameter of the mesoporous materials and the conformation of the hydrophilic and hydrophobic chains of RF8(EO)9. If the synthesis is performed with 20 wt % of RF8(EO)9, the pore size distribution of the mesoporous material is centered at around 6.0 nm, which in the hexagonal hybrid mesophase (template-silica) corresponds to a contracted conformation of oxyethylene chains and to an extended conformation of the fluorocarbon chains. Indeed, in these conformations, the lengths of hydrophobic and hydrophilic part of RF8(EO)9 are 1.5 and 1.6 nm, respectively. Thus, the diameter of the rod in the hybrid mesophase can be estimated to 6.2 nm, which is very similar to the pore diameter of the mesoporous silica. The fluorocarbon chain adopts the same conformation in the hybrid mesophase (template-silica) and in H1 liquid crystal.
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3.3.3. Comparison with Mesoporous Prepared Using the C16(EO)10-TMOS System. Taking into account the equivalence rule between hydrogenated and fluorinated surfactants,22 we can consider that C16(EO)10 is the hydrogenated analogue of RF8(EO)9. Even if C16(EO)10 and RF8(EO)9 present similar characteristics in aqueous solution (Figure 1), materials prepared with both surfactants have quite different features, except morphology. In particular, materials synthesized with the fluorinated surfactant exhibit a high degree of organization, reflected by the better resolution of the (110) and (200) secondary reflections. For instance, for the C16(EO)10TMOS system, only one broad peak is detected on the SAXS pattern,16 whereas for compounds prepared with 20 wt % of RF8(EO)9 we can unambiguously distinguish the (110) reflection from that of (200) (Figure 5A, curve d). In this work, keeping the same synthesis conditions, the hexagonal-disordered hexagonal phase transition with increasing temperature from 40 to 80 °C which is reported for the C16(EO)10,43 is not observed. Those differences could be accounted for by the rigidity of the fluorocarbon chains, which prevent the flexibility of the surfactant-silica system. Moreover, the bigger size of the fluorine atom compared with that of the hydrogen might be responsible for the higher pore diameter noted in this study.
the phase sequence, phase behavior of RF8(EO)9 in aqueous solution was first investigated. We have delimited the different phase domains and determined the structural parameters of the H1 liquid crystal. We have shown that the hydrophobic chains are completely extended. No variation of the cross sectional area with the number of water molecules per surfactant molecule is noted. Mesoporous materials with a hexagonal channel array are prepared at 80 °C in a wide range of surfactant contents (5-20 wt %). If the hydrothermal treatment is performed at lower temperature, only poor mesostructures or wormhole-like structures are recovered. Thus, at high temperature, the channel arrangement is governed by the surfactant behavior in solution, whereas at lower temperature, the silica condensation and polymerization become the driving force. An increase in the pore diameter is noted with the rise in heating temperature. This phenomenon is the result of a change in aggregation number with temperature. A change in micelle shape is assumed to explain the variation of the pore diameter with surfactant concentration. Materials prepared with the fluorinated surfactant exhibit higher degrees of organization and higher pore diameters than those obtained with analogous hydrogenated surfactant [C16(EO)10].
4. Conclusions Mesoporous materials have been prepared using an aqueous solution of a fluorinated surfactant. To determine
Acknowledgment. The authors thank DuPont de Nemours Belgium for providing the RF8(EO)9, C. Borde for technical assistance during SAXS experiments, and A. Fisher for revising the English.
(43) Leonard, A.; Blin, J. L.; Robert, M.; Jacobs, P. A.; Cheetham, A. K.; Su, B. L. Langmuir 2003, 19, 5484.
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