Organic Molecule-Modulated Phase Evolution of Inorganic

Feb 8, 2008 - The involvement of alkane in the P123-TEOS-NH4F-H3O+ synthesis system alters the phase behavior of the complex emulsion system ...
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Langmuir 2008, 24, 2372-2380

Organic Molecule-Modulated Phase Evolution of Inorganic Mesostructures Junming Sun,† Ding Ma,† He Zhang,† Feng Jiang,† Yi Cui,† Rong Guo,‡ and Xinhe Bao*,† State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, PR China, and School of Chemistry and Chemical Engineering, Yangzhou UniVersity, Yangzhou, Jiangsu 250002, PR China ReceiVed September 28, 2007. In Final Form: NoVember 8, 2007 The involvement of alkane in the P123-TEOS-NH4F-H3O+ synthesis system alters the phase behavior of the complex emulsion system dramatically. Changing one of the reaction parameters (such as the initial reaction temperature, IRT) will result in diverse solution mesostructures. With subsequent condensation of silicate species, interesting inorganic materials with various mesostructures are obtained. The present work is aimed at understanding the phase evolution behavior of this complex alkane (C6-C12)-P123-TEOS-NH4F-H3O+ emulsion system, with emphasis on the influence of alkane chain number (ACN) and IRT. HREM (high-resolution electron microscopy), XRD (X-ray diffraction), nitrogen sorption, FFEM (freeze-fracture electron microscope), and interfacial tension techniques have been used to investigate the phase behavior of the emulsion system and the structure of the inorganic products. A linear relationship between the phase-transformation temperature (PTT) and ACN has been established, which could be attributed to the modification of alkane with respect to the hydrophobic-hydrophilic properties of the complex emulsion system. Moreover, the right combination of reaction temperature, ACNs, and thus-induced swelling of hydrophobic PPO blocks as well as the modification of hydrophilicity of PEO brushes by silicate oligmers is the driving force in altering the packing parameter/geometry of the copolymers surfactant (P123) aggregates. This leads to the diverse structures of the obtained mesoporous silicas. A temperature-induced phase-transformation mechanism has also been proposed and discussed.

1. Introduction Amphiphilic block copolymer surfactants exhibit rich phase behavior in aqueous solutions,1-3 which have been extensively usedtoconstructinorganicmaterialswithvariousmesostructures.4-10 Basically, the morphologies of surfactant assemblies determine the final mesostructures of the inorganic materials. Parameters such as composition of surfactants,5 concentration, ionic strength, and synthesis temperature11 were demonstrated to have significant effects on the self-assembly of surfactants and the shaped mesostructured materials. Specifically, because of the hydration/ dehydration of PPO/PEO blocks, poloxamer-type block copolymers (e.g., P123) often possess temperature-dependent micellization properties.2,3 This greatly affects the structural parameters * To whom correspondence should be addressed. E-mail: xhbao@ dicp.ac.cn. Fax: 86-411-84694447. † Chinese Academy of Sciences. ‡ Yangzhou University. (1) Fo¨rster, S.; Antonietti, M. AdV. Mater. 1998, 10, 195. (2) Malmsten, M.; Lindman, Macromolecules 1992, 25, 5440. Alexandridis, P.; Holzwarth, J. F.; Hatton, T. A. Macromolecules 1994, 27, 2414. (3) Santore, M. M.; Discher, D. E.; Won, Y. Y.; Bates, F. S.; Hammer, D. A. Langmuir 2002, 18, 7299. Wanka, G.; Hoffmann, H.; Ulbricht, W. Macromolecules 1994, 27, 4145. (4) Yang, P. D.; Zhao, D. Y.; Margolese, D. I.; Chmelka, B. F.; Stucky, G. D. Chem. Mater. 1999, 11, 2813. (5) Zhao, D.; Huo, Q.; Feng, J.; Chmelka, B. F.; Stucky, G. D. J. Am. Chem. Soc. 1998, 120, 6024. (6) Schu¨th, F. Chem. Mater. 2001, 13, 3184 and references therein. (7) Lu, Y.; Fan, H.; Stump, A.; Ward, T. L.; Rieker, T.; Brinker, C. J. Nature 1999, 398, 223. (8) Kim, S. S.; Karkamkar, A.; Pinnavaia, T. J.; Kruk, M.; Jaroniec, M. J. Phys. Chem. B 2001, 105, 7663. Meng, X. J.; Li, D. F.; Yang, X. Y.; Yu, Y.; Wu, S.; Han, Y.; Yang, Q.; Jiang, D. Z.; Xiao, F. S. J. Phys. Chem. B 2003, 107, 8972. (9) Liang, C. D.; Dai, S. J. Am. Chem. Soc. 2006, 128, 5316. Zhang, F. Q.; Meng, Y.; Gu, D.; Yan, Y.; Yu, C. Z.; Tu, B.; Zhao, D. Y. J. Am. Chem. Soc. 2005, 127, 13508. (10) Andre´, P.; Ninham, B. W.; Pileni, M. P. AdV. Colloid Interface Sci. 2001, 89-90, 155. Pileni, M. P. Langmuir 2001, 17, 7476. (11) Antonietti, M.; Go¨ltner, C. Angew. Chem., Int. Ed. 1997, 36, 910.

(e.g., pore geometry and wall thickness) of the obtained mesoporous inorganic materials. For instance, by controlling the ratio of the hydrophobic and hydrophilic moieties of the block copolymers, mesoporous materials with diverse pore structures (e.g., 2-D hexagonal, 3-D cubic, and lamellar) have been synthesized by Zhao et al.5 In addition, the chain length of the polymer and the synthesis temperature also have significant effects on the structure of mesoporous inorganic materials.12,13 The addition of organic counterions/cosolvents is able to alter the phase behavior of the synthesis mixture. It was reported that some kinds of organic molecules such as 1,3,5-trimethylbenzene (TMB) can be used as the micelle swelling agent in the synthesis of SBA-15 and other mesoporous silica, which led to enlarged spherical cages (as large as 45 nm).14,15 Very recently, we have reported that by using alkane (decane) in the synthesis mixture, unusual mesoporous SBA-15 with parallel channels running along the short axis can be obtained, which is of great importance for applications where fast mass transfer is required. It was proposed that a large amount of decane has created the discrete spaces during the synthesis process. This confined the formation of very small mesoporous silica particles at the same time that the channel orientation is reversed by swelling the rod micelles.16 Indeed, organic counterions (cosolvents) play a key role in controlling the structural and morphologic polymorphism of amphiphilic copolymers in ternary surfactant-water-organic (12) Kipkemboi, P.; Fogden, A.; Alfredsson, V.; Flodstro¨m, K. Langmuir 2001, 17, 5398. (13) Martines, M. A. U.; Yeong, E.; Larbot, A.; Prouzet, E. Microporous Mesoporous Mater. 2004, 74, 213. (14) Lettow, J. S.; Han, Y. J.; Schmidt-Winkel, P.; Yang, P. D.; Zhao, D. Y.; Stucky, G. D.; Ying, J. Y. Langmuir 2000, 16, 8291. (15) Amatani, T.; Nakanishi, K.; Hirao, K.; Kodaira, T. Chem. Mater. 2005, 17, 2114. (16) Zhang, H.; Sun, J. M.; Ma, D.; Bao, X. H.; Klein-Hoffmann, A.; Weinberg, G.; Su, D. S.; Schlo¨gl, R. J. Am. Chem. Soc. 2004, 126, 7440.

10.1021/la702991r CCC: $40.75 © 2008 American Chemical Society Published on Web 02/08/2008

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solvent emulsion systems.17,18 Different organic cosolvents have different solubilization sites in amphiphilic copolymers micelles, and the way that they influence the structural polymorphism of amphiphilic copolymers in the synthesis mixture is also different.18 For the amphiphilic block copolymers surfactants, the polar organic and aromatic counterions would preferentially stay at the polar/apolar interface of the micelles, which could tune the interfacial area of each surfactant and the curvature of the surfactant aggregates significantly. Therefore, the morphology of the surfactant aggregates was changed17,18 Instead, aliphatic alkanes (e.g., n-alkanes) prefer to reside at the hydrophobic cores of the copolymer surfactant aggregates,19 which will increase the effective volume of the hydrophobic part of the aggregates and thus change their packing parameters.18 As a result, various phases (eg., L1, I1, H1, LR, and H2) have been observed in ternary block copolymer-water-organic solvent systems.17,18 For example, by using the proper amount of butanol, Ryoo et al. reported the transformation of highly ordered large-pore silica mesophases (from Fm3m, Im3m to p6mm) in a triblock copolymer (i.e., F127)-butanol-water system.20-22 Our recent studies showed that besides the pore-size expansion effect, the use of aliphatic alkanes in the P123-TEOS-water system can alter the phase behavior of the synthesis mixture and thereby change the structure and the morphology of the obtained mesostructured silica. With the suitable interactions of alkane with the P123 compolymer surfactants, highly ordered mesoporous silicas with tunable pore size and pore length and chemically significant morphologies have been obtained successfully by strictly controlling IRTs.23-26 Despite that, a temperature-induced phase transformation from amorphous to hexagonal (H1) to lamellar (LR) and then to mesocellular foam structures (swollen L1) was observed in the alkane-added P123TEOS-NH4F-H3O+ mixture.23 No insightful understanding of this important alkane-P123-TEOS-NH4F-H3O+ system, especially its phase evolution toward temperature variation, has been reached. In this article, a detailed investigation of the phase behavior of the alkane (C6-C12)-P123-TEOS-NH4F-H3O+ complex emulsion systems has been made. As a result, a synthesis phase diagram (IRT vs ACN) was drawn from (HR)EM observations, XRD, and nitrogen sorption. A linear relationship between the PTT and ACN has been established and discussed in detail. Meanwhile, what has happened in the solution phase before the precipitation of the inorganic material has been tracked by FFEM, which was used to confirm the formation mechanism proposed on the basis of the phase evolution diagram. An understanding of the phase evolution behavior of the complex emulsion systems would shed light on the temperature-dependent solubilization behavior of the oil phase within the copolymer surfactant micelles and, most importantly, the construction of novel functional materials with designated structure and morphology. (17) Alexandridis, P.; Olsson, U.; Lindman, A. Langmuir 1998, 14, 2627. (18) Holmqvist, P.; Alexandridis, P.; Lindman, B. Macromolecules 1997, 30, 6788. (19) Nagarajan, R.; Barry, M.; Ruckenstein, E. Langmuir 1986, 2, 210. (20) Kleitz, F.; Choi, S. H.; Ryoo, R. Chem. Commun. 2003, 2136. (21) Kim, T. W.; Kleitz, F.; Paul, B.; Ryoo, R. J. Am. Chem. Soc. 2005, 127, 7601. (22) Kleitz, F.; Kim, T. W.; Ryoo, R. Langmuir 2006, 22, 440. (23) Sun, J. M.; Ma, D.; Zhang, H.; Wang, C.; Bao, X. H.; Su, D. S.; KleinHoffmann, A.; Weinberg, G.; Mann, S. J. Mater. Chem. 2006, 16, 1507. (24) Sun, J. M.; Zhang, H.; Ma, D.; Chen, Y.; Bao, X. H.; Klein-Hoffmann, A.; Pfa¨nder, N.; Su, D. S. Chem. Commun. 2005, 5343. (25) Zhang, H.; Sun, J. M.; Ma, D.; Weinberg, G.; Su, D. S.; Bao, X. H. J. Phys. Chem. B 2006, 110, 25908. (26) Sun, J. M.; Zhang, H.; Tian, R. J.; Ma, D.; Bao, X. H.; Su, D. S.; Zou, H. F. Chem. Commun. 2006, 1322.

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2. Experimental Section 2.1. Synthesis. According to the typical synthesis procedure,23 2.4 g of EO20PO70EO20 (P123) was dissolved in 84 mL of HCl solution (1.30 M), followed by the addition of 0.027 g of NH4F. The mixture was stirred at room temperature until the solution became clear and was then heated to a given temperature (from 278 to 353 K). Different alkanes (from hexane to dodecane) and TEOS were premixed and then added to the solution under mechanical stirring (300-360 rpm) (P123/HCl/NH4F/H2O/TEOS/alkane molar ratios of 1:261:1.8:11 278:x:y; x ) 36-60, y ) 134). The above mixture was stirred at the given temperature for another 20 h and then transferred to an autoclave for further reaction at 373 K for 48 h. The products were collected by filtration, dried in air, and calcined at 813 K for 5 h to remove the templates. 2.2. Characterization. SEM was carried out on the Hitachi S4800 field-emisson scanning electron microscope. TEM images were obtained with a JEOL JEM-2000EX and a Philips CM 200 transmission electron microscope equipped with a CCD camera. XRD patterns were collected on a Rigaku D/MAX 2400 diffractometer equipped with a Cu KR X-ray source operating at 40 kV and 50 mA. The N2 adsorption-desorption isotherms were recorded on the ASAP 2000 and the Quantachrome autosorb-1 instrument. Before sorption analysis, samples were put under vacuum at 623 K for at least 6 h. 2.3. Freeze-Fracture Replication. Freezing (fracture), etching, and replication of the complex emulsions were carried out in a Balzer BAF 400D apparatus. The preparation of the synthesis mixture was exactly the same as that described in section 2.1. Specifically, after adding the mixtures of alkane (i.e., decane) and TEOS (P123/HCl/ NH4F/H2O/TEOS/alkane molar ratios ) 1:261:1.8:11 278:48:134), a small part of the sample (tens of microliters) would be taken out at the given time interval under stirring and quenched rapidly using liquid nitrogen. The frozen sample was then transferred to the Balzer BAF 400D apparatus for fracture (123 K), etching (ca. 10 min at 163 K), and replication with Pt-C (at a pressure of 2.0 × 10-6 mbar). EM images of the replicas were observed with a JEOL JEM2000EX transmission electron microscope equipped with a CCD camera 2.4. Interfacial Tension Measurement. The measurement of the interfacial tension between alkane (decane) and P123-ethanolNH4F-H3O+ was carried on a Texas-500 spinning drop interfacial tensiometer. To avoid the interference of silicate species, ethanol (an equal amount of that produced by the hydrolysis of TEOS during the synthesis) instead of TEOS was used in the interfacial tension measurement to simulate the emulsion system. Specifically, a mixture of P123-ethanol-NH4F-H3O+ (P123/HCl/NH4F/H2O/ethanol/ alkane molar ratios ) 1:261:1.8:11 278:240:134) is loaded into a glass holder, and then a small drop of alkane was injected into the mixtures by a micrometer syringe. After sealing, the holder was fixed on the tension meter to test the interfacial tension as a function of temperature. The spin rate was set at 5000 rpm

3. Results and Discussion 3.1. Synthesis Phase Diagram. Alkanes from hexane to dodecane have been used in the synthesis. When IRT is lower than 283 K, only amorphous silica gel is obtained in the emulsion systems studied (Figures S1 and S2). This can be attributed to the complete hydration of copolymer surfactants and thus the failure to template the formation of inorganic mesostructures at lower temperatures. When IRT is higher than 283 K, mesoporous silica materials with diverse mesostructures are obtained, depending on IRTs and ACNs. In the case of undecane, when IRT is 308 K, ultrafine mesoporous nanofibers bundles with almost 100% yield are observed in the products (Figure 1a). With increasing IRT, multilamellar vesicles would appear and gradually become predominant in the products (Figure 1b-d, SEM and HRSEM images are shown in Figure S3). When IRT is 323 K, the yield of multilamellar vesicles is higher than 95%

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Figure 2. Synthesis phase diagram (ACN vs reaction temperature) obtained in the alkane-P123-TEOS-NH4F-H3O+ emulsion system. (P123/HCl/NH4F/H2O/TEOS/alkane molar ratio ) 1:261: 1.8:11 278:48:134; A - amorphous; H - hexagonal; H′ - wormlike; H + H′ - mixture of H and H′; L - lamellar; V - vesicles; F - foams. Note that the single phase marked in the synthesis phase diagram means that the yield of the phase is more than 95% with respect to the products.)

Figure 1. TEM images of mesoporous silica materials with diverse mesostructures prepared by using undecane as cosolvents at (a) 308, (b) 313, (c) 318, (d) 323, (e) 333 K (the arrows refer to the MLVs), and (f) 343 K. (P123/HCl/NH4F/H2O/TEOS/undecane ) 1:261: 1.8:11 278:48:134.)

(Figure 1d). With further increasing IRT, mesoporous cellular foams (MCFs) with a very large cage size (>30 nm) appear and mix with a small number of multilamellar vesicles, which indicates a phase transformation from lamellar to swollen spherical micelles (Figure 1e). When IRT is higher than 343 K, pure mesoporous cellular foams are obtained (Figure 1f). According to the EM, XRD, and nitrogen sorption analysis of the obtained inorganic materials, a synthesis phase diagram (phase vs IRT and ACN) was plotted in Figure 2. It can be seen that the PTTs are different, depending on the alkane used. (For example, for hexane (ACN ) 6), the first PTT is around 290 K; for dodecane (ACN ) 12), it is around 325 K.) The temperature increases with the ACN. When alkanes with higher ACNs (i.e., dodecane) are employed, the PTT is close to that without using alkanes (Figure 2), although the space for the existence of some single-phase products, such as vesicles, is much narrower. Significantly, there exists a linear relationship between PTT and ACN, which indicates that the micellization behavior is tunable by ACNs. For all of the alkanes investigated here, the PTT is lower than that of the traditionally synthesized mesoporous materials (without alkanes). This decrease in the PTT can be attributed to the solubilization of alkanes in the hydrophobic PPO blocks (swelling) and the modification of hydrophobichydrophilic properties as well as the micellization behavior of the P123 copolymer surfactant. Details will be discussed in subsections 3.6 and 3.7.

It should be mentioned that when IRT is close to 283 K the structure of the obtained mesoporous silicas becomes less ordered; that is, wormlike mesopores (in the phase diagram, H’ was used to mark this part) have been obtained (Figure S4), whereas only amorphous silica can be observed at such a low temperature without alkanes. 3.2. Characterizations of the Obtained Materials. It is observed that in a specific region, although the morphoarchitectures show some difference (Figure 2), the structures are same. In the H region (hexagonal mesoporous silica), when dodecane is used, submicrometer-sized rodlike particles that are similar to those of traditional SBA-15 would be predominant in the products. When decane is employed, however, mesoporous nanofiber (ca. 40 to 85 nm) bundles would be obtained. Upon decreasing the carbon chain length of the alkane to C9 (i.e., nonane), ultrathin mesoporous nanofibers (most less than 50 nm with a pore size of around 13 nm), which are randomly aligned to form the bundles, are produced. To our knowledge, they are the thinnest mesoporous nanofibers ever reported. These hierarchical mesostructures could be further confirmed by their nitrogen sorption results (Figure S5; also note that a huge adsorption at a relative pressure of more than 0.9 in the sample prepared with nonane indicates their ultrathin nanofiber characteristics23). Upon further decreasing the carbon chain length to less than C8, mesoporous silica nanoparticles are observed in the products, which can be related to both the swelling ability of alkanes and IRTs.25 HRSEM images (inset in Figure 2) and TEM images (Figure S6) of the obtained samples indicate that they are highly ordered mesoporous silicas. SAXD (small-angle X-ray diffraction) patterns of all of the obtained materials give at least three diffraction peaks (Figure 4), which can be assigned to the (100), (110), and (200) diffractions. They are characteristics of 2-D hexagonal mesostructures, which is consistent with the TEM results. Noticeably, those mesoporous nanofibers with a diameter of less than 100 nm can give only well-resolved (100) and (110) reflections, as well as a shoulder peak in the (200) diffraction. On the contrary, when the diameter of the primary mesoporous silica particles is larger than 100 nm (i.e., prepared in the presence of dodecane), the diffraction peaks are better resolved (Figure 4). The arrangement of silica nanotubes in the ultrafine particles is close to the minimum requirement for the hexagonal structures, which

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Figure 3. SEM images of 2-D hexagonal mesoporous silicas with various morphologies prepared by using (a) dodecane at 313 K, (b) decane at 303 K, (c) nonane at 298 K, and (d) octane at 298 K. (P123/HCl/NH4F/H2O/TEOS/alkane molar ratio ) 1:261:1.8:11 278:48:134.)

leads to the broadening of the diffraction peaks.23 In addition, the diffraction peaks shifted to lower angle with the decrease in ACN, suggesting the expansion of the pore size of the obtained mesoporous silicas. This can be further confirmed by their nitrogen sorption results (Figure S5). In the V (lamellar) region, it is observed that the mesostructures are very sensitive to the reaction temperature, especially for those with high ACNs (i.e., dodecane). When dodecane is used, spherical particles (100-150 nm in diameter) would be obtained at ca. 328 K. TEM images of the corresponding sample indicate that they are multilamellar vesicles with one to three homocentric uneven layered structures (Figure 5). With a slight deviation from this temperature (e.g., 323 or 333 K), a mixture of multilamellar vesicles and 2-D hexagonal or multilamellar vesicles and mesoporous cellular foams would result (Figures S7 and S8). When decane is used, multilamellar vesicles (ca. 150 nm) are obtained at ca. 313 K, but TEM images show that the obtained multilamellar vesicles possess three to five homocentric layered structures (Figure 5b). When the ACN is lower than 10 (e.g., nonane), large spherical particles (∼200 nm) are observed in the products, and TEM images show that they are also multilamellar vesicles but with more homocentric layers (five to seven layers, Figure 5c). SAXD patterns of the obtained multilamellar vesicles show no diffraction peaks (not shown), which can be attributed to the uneven lamellar structures (inset in Figure 5). Nitrogen sorption isotherms of the typical multilamellar vesicles (Figure S9) give an H3 hysteresis loop that could be assigned to N2 filling in the slit-shaped pores,23 which is consistent with the EM observations. In the F (MCFs) region, HRSEM and TEM images show that all of the obtained mesoporous cellular foams are aggregated spherical particles with a diameter of ca. 1 to 2 µm (Figure 6). At the same time, there are no significant differences in the cage sizes of the obtained MCFs prepared in the presence of different alkanes, which is further confirmed by their nitrogen sorption results (Figure S10 and Table S1). 3.3. Effects of the Amount of TEOS on the Mesostructures: Slight Phase Shift from Lamellar to Hexagonal. Besides the reaction temperature, it seems that the lamellar region is also sensitive to the change in the amount of TEOS added. For instance, in the decane-P123-NH4F-H3O+ emulsion system, keeping

other synthesis parameters the same, the amount of TEOS has a significant effect on the mesostructures of the obtained materials. Nitrogen sorption results (Figure 7) are able to show the pore structure difference of the materials with different TEOS/P123 molar ratios. From Figure 7, when the TEOS/P123 molar ratio is smaller than 54, a huge H3 hysteresis loop27 is obtained, which is an indication of multilamellar vesicular structures.23 When the TEOS/P123 molar ratio is larger than 60, the H3 hysteresis loop transforms into a large H1 hysteresis loop, which indicates a significant change in the mesostructures of the obtained materials. TEM images of the cross-sectioned samples indicate that they have macroporous cavities with circularly ordered mesoporous structures (Figure S11). It suggests that increasing the amount of TEOS would result in the recovery of the hexagonal phase from the lamellar phase, which could be attributed to the change in the microbalance of packing parameters by the silicate species.1,11 Therefore, these special hierarchically macromesoporous structures are the result of the coexistence of lamellar (i.e., vesicles) and hexagonal surfactant phases. It should be mentioned that when the TEOS/P123 molar ratio is less than 42 (i.e., 36) amorphous silica gels result. Therefore, the TEOS/ P123 ratio influences not only the morphologies of ordered mesoprous silicas25 but also the hydrophobic-hydrophilic properties and thus the micellization behavior of the emulsion system. 3.4. Tracking the Micellization Process of the DecaneP123-TEOS-NH4F-H3O+ Complex Emulsion Systems by FFEM. To know what has happened in the solution phase during the synthesis, especially when the precipitation of inorganic materials has not yet happened, FFEM was used to monitor the micellization process. FFEM presents the direct observation of solution structures. In FFEM, samples are rapidly frozen, fractured, and replicated with metal (e.g., Pt)-C thin films. Therefore, the microstructure of the solution, which is well maintained by this technique, can be observed with transition electron microscope (TEM).28 In the present study, the target is to explore the temperature-induced phase evolution of the solution (27) Rojas, F.; Kornhauser, I.; Felipe, C.; Esparza, J. M.; Cordero, S.; Dominguez, A.; Riccardo, J. L. Phys. Chem. Chem. Phys. 2002, 4, 2346. (28) Jahn, W.; Strey, R. J. Phys. Chem. 1988, 92, 2294. Burauer, S.; Belkoura, L.; Stubenrauch, C.; Stey, R. Colloids Surf., A 2003, 228, 159.

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Figure 4. SAXD patterns of the obtained 2-D hexagonal mesoporous silicas in the presence of alkanes with different ACNs. (P123/HCl/ NH4F/H2O/TEOS/alkane ) 1:261:1.8:11 278:48:134.)

microstructure in the complex emulsion system and find out whether they have the kind of relationship with final structures of the obtained inorganic silicas. The solution microstructures of the decane-P123-TEOS-NH4F-H3O+ complex emulsion systems captured at different reaction temperatures and times (before precipitation) are shown in Figure 8. At 303 K, a small amount of the lamellar phases appeared in the synthesis solutions after 3 min of reaction (Figure 8a). With time going to 6 min, interestingly, the modulated lamellar phase in a large areas was observed (Figure 8b). At the same time, rodlike micelles within the lamellar phases could be observed

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(Figure 8c, white arrow marked), indicating that the confined formation of rodlike micelles occurred At 313 K, lamellar phases (with the interplanar distance smaller than that prepared at 303 K) are observed after 2 min (Figure 8d). After 4 min of reaction, although the lamellar phase could still be found (Figure S12), a large number of spherical particles appeared (Figure 8e), indicating a significant structural transformation of the emulsion systems. With reaction time going to ca. 5 min, separated spheres with lamellar mesostructures are observed (Figure 8f, white arrow marked). After that, the condensation of silica species finishes, and the inorganic precipitates are obtained. Upon further increasing the reaction temperature to 333 K, another phase transformation is observed (Figure 8g), where lamellar mesostructures are split into swollen micelles (Figure 8g, white arrow marked). This indicates that swelling of the lamellar phase results in the reorganization of the surfactant bilayers and thus the formation of the swollen L1 phase. When the reaction time increased to 4 min, aggregated spherical micelledirected foam structures were dominant in the emulsion system (Figure 8h). Comparing the FFEM results of the solution phase with EM pictures of the obtained inorganic materials (section 3.2), it can be concluded that the temperature indeed could induce significant changes in the solution microstructures and geometry of the surfactant composite assemblies. Directed by those solution microstructures, diverse inorganic mesostructures are obtained in the complex emulsion systems. It should be mentioned that we cannot differentiate whether there exist silicate species in the solution microstructures (those just before inorganic materials precipitate) because the images taken with EM are of just a Pt-C replication. However, the hydrolysis of TEOS and thus the produced silicate species must take part in the self-assembly, which alters the emulsion phase and surfactant micellization behavior during the synthesis. This supports the point of coorganization of organic-inorganic composites during the synthesis of mesoporous silica materials.29 At the same time, the phase evolution in the solution versus time (at a fixed reaction temperature) indicates that the final micelle form could be a thermodynamically favored phase and the phase evolution is a thermodynamically driven process. The obtained inorganic product should be a transient phase that balances the thermodynamically favored phase and the silicate condensation quenched phase. 3.5. Interfacial Tension Evolution with Temperature in the Decane-P123-NH4F-Ethanol-H3O+ Complex Emulsion Systems. To further uncover the phase behavior of the complex emulsion system and its influence on the morphoarchitectures of the shaped mesoporous silicas, the interfacial tension of the decane-P123-TEOS-NH4F-H3O+ emulsion system is monitored as a function of temperature. To avoid the interference of silicate species, TEOS is not involved in the interfacial tension measurement. However, ethanol (with an equal number of those molecules produced by the hydrolysis of TEOS during the synthesis) is introduced into the system to simulate the emulsion system. The interfacial tension as a function of temperature is shown in Figure 9. When the temperature is lower than 308 K, there exists a constant ultralow interfacial tension between the decane and the P123-ethanol-H3O+ mixture, which is only ca. 0.021 mN m-1. However, when the temperature is higher than 308 K, a sudden (29) Firouzi, A.; Kumar, D.; Bull, L. M.; Besier, T.; Sieger, P.; Huo, Q.; Walker, S. A.; Zasadzinski, J. A.; Glinka, C.; Nicol, J.; Margolese, D.; Stucky, G. D.; Chmelka, B. F. Science 1995, 267, 1138.

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Figure 5. SEM and TEM images of the obtained lamellar mesoporous silica materials prepared by using (a) dodecane at 328 K, (b) decane at 313 K, (c) nonane at 313 K, and (d) heptane at 303 K. (P123/HCl/NH4F/H2O/TEOS/alkane molar ratio ) 1:261:1.8:11 278:48:134.)

Figure 6. HRSEM images of the mesoporous cellular foams prepared by using (a) dodecane, (b) decane, (c) nonane, and (d) octane. (P123/HCl/NH4F/H2O/TEOS/alkane molar ratio ) 1:261:1.8:11 278:48:134.)

increase in interfacial tension is observed,30 indicating a significant change in the hydrophobic-hydrophilic properties and the phase behavior of the emulsion system, which agrees well with the results of FFEM observations. After 320 K, the interfacial tension would exceed the range of the spinning drop interfacial tensiometer since an oil drop ellipsoid is observed. Herein, it is interesting that without ethanol, although the interfacial tension evolution with temperature shows the same trend as for ethanol, (30) The temperature-induced significant change in the interfacial tension could be related to the configurations of the P123 copolymer surfactant at the oil (alkanes)/ water interface. At lower temperature, the PEO brushes and even the PPO blocks of the P123 copolymers surfactant should be stretched and spread separately at the oil/water interface, which results in a significant decrease in interfacial tension. As the temperature increases, as a result of the dehydration of PPO blocks and thus increasing hydrophobicity, more and more P123 copolymer surfactant would aggregate into large micelles with alkanes solubilized with their hydrophobic cores, which will lead to a sudden decrease in the number of P123 surfactant molecules at the oil/water interface and thus a sudden increase in interfacial tension.

the value of the former is much lower than that of the latter (Figure S13). 3.6. Temperature-Induced Mesostructure Evolution. Basically, the driving force for the phase evolution of the surfactant assemblies (e.g., the shape and size of the micelle) is closely related to the hydrophobic-hydrophilic balance or the packing parameters of surfactant in the emulsion.18,31 Block copolymers consisting of polyethylene oxide (PEO) and polypropylene oxide (PPO) exhibit amphiphilic character and can self-assemble in the aqueous solution.2,3 The hydrophilic-lipophilic character of these polymers and thus the structure of their assemblies can be readily altered by varying their molecular weight and chemical composition. Specifically, the change in temperature could lead to the hydration and dehydration of PEO and PPO blocks and can be used to tune the packing parameter of those copolymer (31) Antonietti, M.; Fo¨rster, S. AdV. Mater. 2003, 15, 1323.

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Figure 7. Nitrogen sorption results of the obtained lamellar structures prepared in the decane-P123-NH4F-H3O+ emulsion system (P123/ HCl/NH4F/H2O/TEOS/decane molar ratio ) 1:261:1.8:11 278:x: 134, x ) 42-60) with TEOS/P123 molar ratios of (a) 42, (b) 48, (c) 54, and (d) 60; IRT ) 313 K.

surfactants (e.g., P123).3,18 This special phase behavior in the binary block copolymer-water system has been widely used for the synthesis and modification of mesostructures of inorganic materials.5,12,13 In the ternary copolymers surfactant-water-oil emulsion systems, the phase behavior is much more complicated, largely as a result of the participation of the organic counterions. The solubilization sites of the organic molecules in the surfactant micelles are critical for phase behavior modulation. Normally, polar organic counterions,20-22 which would predominantly stay at the polar/apolar interface, can tune the interfacial energy and effective area per headgroup and thus the hydrophilic-lipophilic balance as well as packing parameters (v/(al)).31 For the apolar organic solvents, there are two possibilities depending on the molecular structures of the solvent. For the aromatics, the strong interactions between the π electrons of aromatics and positively charged headgroups of the surfactants make the aromatic molecules dissolve preferentially at or near the interfaces of the aggregates.32,33 For the aliphatic molecules (i.e., n-alkanes), however, they would predominantly be dissolved in and swell the hydrophobic microdomains of the micelles,16,19 which increases the effective volume of the hydrophobic blocks (e.g., PPO) and thus the packing parameters of the copolymer surfactant (e.g., P105).18,31 In the current n-alkane-P123-TEOS-NH4F-H3O+ emulsion system, n-alkanes would predominantly stay at the hydrophobic cores of P123 micelles.16,25 Furthermore, increasing the temperature results in a significant increase in the solubilization ability of alkanes within the hydrophobic microdomains.34 It is of great importance because the swelling of n-alkanes would not only increase the hydrophobicity of the copolymer surfactant and thus decrease their cmc23-25 but also increase the effective volume of the hydrophobic part18 and the packing parameters of the P123 block copolymer surfactants. Therefore, it is definitely different as to whether alkane exists in the P123-water system. Without alkanes, increasing the reaction temperature would just lead to the dehydration of PPO and PEO blocks, thus modifying the hydrophobic-hydrophilic properties slightly (the last column in Figure 2). A relatively high temperature is required to induce the phase transformation. With the addition of alkanes, however, increasing reaction temperature not only results in the dehydration of PPO blocks but also swells the PPO block because of the (32) Hoffmann, H.; Ulbricht, W. J. Colloid Interface Sci. 1989, 129, 388. Wanka, G.; Hoffmann, H.; Ulbricht, W. Colloid Polym. Sci. 1990, 268, 101. (33) Tornblom, M.; Henriksson, U. J. Phys. Chem. B 1997, 101, 6028. (34) Lebens, P. J. M.; Keurentjes, J. T. F. Ind. Eng. Chem. Res. 1996, 35, 3415.

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increased solubilization ability. In this way, a slight increment of the reaction temperature would dramatically shift the hydrophobic-hydrophilic equilibrium of the alkane-P123TEOS-NH4F-H3O+ emulsion system from the H1 to LR region. (In the H1 phase, PPO blocks aggregates together to reach a minimum energy requirement. With more alkanes residing in the PPO moiety, the LR phase is much more comfortable for the emulsion system because only with this conformation can the PPO moiety become more relaxed in terms of its spatial arrangement.) Indeed, it is further confirmed by the present TEM results of the obtained inorganic materials, where a phase transformation from highly ordered 2-D hexagonal mesoporous silicas to lamellar structures is observed (Figure 1). It is interesting that multilamellar vesicles instead of planar lamellar structures (even though we still find some planar lamellar mesostructures in the octane-P123-TEOS-NH4F-H3O+ emulsion system, Figure S14) are obtained in the products. This is reasonable because the shearing force (in the present case, mechanical stirring) was involved during the synthesis, which is reported to be able to drive those lamellar structures into multilamellar vesicles.35 This holds in this case. The phase evolution has been further verified by the FFEM observations (Figures 8d-f and S12), where the structures in the emulsion phase were well preserved and replicated into inorganic materials. The attachment of silicate species onto the charged PEO blocks during the synthesis could also tune the effective areas per headgroups31 and subsequently the packing parameters and the morphology of P123 surfactant aggregates. Increasing the amount of TEOS in the synthesis mixtures would result in the formation of larger silicate oligomers.36 It would lead to a slight increase in the effective area per headgroup of the P123 surfactant and subsequent decrease in packing parameters. At this stage, if the packing parameters just stay at the boundary of two mesophases (i.e., H1 and LR), then a slight increase in packing parameters might result in the transition of mesophases of the emulsion system and therefore the mesostructures of the obtained inorganic materials. Indeed, the present results (section 3.3, Figure S11) showed that increasing the amount of TEOS leads to the reappearance of 2-D hexagonal phases in the products, which further confirmed the aforementioned conclusion. Moreover, on the basis of this proposal, we could even fine tune the hydrophobic-hydrophilic balance of the emulsion system to match the emulsification of the oil phase and the self-assembly of the copolymer surfactant (i.e., P123) so as to build hierarchically ordered macromesoporous silica materials (Supporting Information).37 Upon further increasing the temperature, large mesoporous cellular foams are obtained, which should be directed by a highly swollen normal micelle (L1). Normally, increasing the synthesis temperature would favor the structures with curvature toward water,18 and thus the inverse micellar solution (L2) phase is usually preferred. As a result, inorganic nanoparticles would be produced within the confinement of so-called nanoreactors.11 That is obviously not the case in this study. One possible interpretation is that the temperature is not sufficient high to reverse the curvature of the copolymer surfactant (P123); however, increasing the reaction temperature would result in the further dehydration and hydrophobicity of PPO blocks and thus would increase the solubilization ability of alkanes within the hydrophobic domains. (35) Roux, D.; Nallet, F. Phys. ReV. E 1995, 51, 3296. Zipfel, J.; Lindner, P.; Tsianou, M.; Alexandridis, P.; Richtering, W. Langmuir 1999, 15, 2599. (36) Chao, M.; Lin, H. P.; Sheu, H.; Mou, C. Y. Stud. Surf. Sci. Catal. 2002, 141, 387. (37) Sun, J. M.; Ma, D.; Zhang, H.; Bao, X. H.; Su, D. S.; Weinberg, G. Microporous Mesoporous Mater. 2007, 100, 356.

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Figure 8. TEM images of the freeze-fracture-replicated samples taken at a given reaction temperature and reaction times: (a) 3 min at 303 K; (b, c) 6 min at 303 K (the precipitation happened at 7 to 8 min at 303 K); (d) 2 min at 313 K; (e) 4 min at 313 K; (f) 5 min at 313 K (the inset of Figure 5b is the enlarged image corresponding to part of Figure 5b, and the precipitation happened at 6 to 7 min at 313 K); (g) 3 min at 333 K; and (h) 4 min at 333 K (the precipitation happened at 4 to 5 min at 333 K). (P123/HCl/NH4F/H2O/TEOS/decane molar ratio ) 1:261:1.8:11 278:48:134.)

In this manner, further swelling of the lamellar phase of the surfactant happened, which eventually leads to the formation of a swollen L1 phase. Indeed, our FFEM experiments confirm that

multilamellar vesicles are split into smaller micelles as a result of temperature-induced further solubilization of the surfactant bilayers.

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For the given alkane-P123-TEOS-NH4F-H3O+ emulsion systems, eq 3-1 can be simplified to eq 3-2. Furthermore, when the LR phase is reached, HLD would be zero. As the result, the onset temperature for the phase transformation shows a linear relationship with the ACN, which is consistent with our experimental results. Therefore, it can be concluded that alkanes shows different solubilization (or swelling) abilities within the hydrophobic microdomains of the P123 surfactant in the current studies. It can tune the hydrophobic-hydrophilic balance and subsequently the microstructures of the surfactant assemblies and the final structures of the mesoporous silica materials.

bT ) HLD + aACN (a and b are constants) Figure 9. Interfacial tension evolution as a function of temperature in the decane-P123-NH4F-ethanol-H3O+ complex emulsion systems. (The shadow-area-marked points indicates the interfacial tension that exceeds the range that the spin-drop tension meter can detect.)

Therefore, it is clear that IRT, alkanes, and silicate doping play key roles in successfully tuning the hydrophobic-hydrophilic properties/packing parameters of P123 surfactants and thus the microstructure of mesoporous silica materials during the interfacial assembly in the current n-alkane-P123-TEOS-NH4FH3O+ emulsion system. 3.7. Relationship between the PTT and ACN. In the alkaneP123-TEOS-NH4F-H3O+ emulsion systems, the relationship between the PTT and ACN can be correlated to their solubilization (or swelling) abilities;25 that is, the swelling of the PPO blocks increased the effective volume of the P123 surfactant and thus the packing parameters. The smaller the ACN, the larger the effective volume of the P123 aggregates and the larger the packing parameters that would result. In this way, at a constant temperature (i.e., 313 K), with a decrease in the ACN, a phase transformation from H1 to LR is possible in the alkane-P123-TEOS-NH4FH3O+ emulsion systems. (See also section 3.6.) Indeed, our results (Figure 2) show that there exists such a phase transformation, further confirming the validity of our proposals. Interestingly, in the cationic surfactant systems, Huang et al. have also observed a similar trend in the alkane-induced micelle-vesicle transition (MVT).38 Another significant aspect that needs to be noted is that a linear relationship is shown between PTT and ACN (Figure 2). Equation 3-1 shows the well-known empirical expressions found in the alkane/ethoxylated nonionic surfactant/water emulsion systems39

HLD ) R-EOH - kACN + bS + φ(A) + cT(T - Tref) (3-1) where HLD is the hydrophile-lipophile deviation, R is a characteristic parameter of the hydrophobic part of the surfactant, EOH is the number of ethylene oxide groups per surfactant molecule, ACN is the number of carbon atoms in the alkane molecule (or equivalent), S is the salinity of the aqueous phase in wt % NaCl (or equivalent), ψ(A) is function of the alcohol type and concentration, T is the temperature (°C), Tref is generally taken at 25 °C; and k, b, and CT are constants that are characteristic of the surfactant type and electrolyte. (38) Yin, H.; Lei, S.; Zhu, S.; Huang, J.; Ye, J. Chem.sEur. J. 2006, 12, 2825. (39) Allouche, J.; Tyrode, E.; Sadtler, V.; Choplin, L.; Salager, J.-L. Langmuir 2004, 20, 2134.

(3-2)

Finally, it must be kept in mind that although each point with same symbol in Figure 2 (there are 17 H points in the H region) is the same in structure, the morphologies are different in most cases (Figure 3). The morphology of the products is determined by the combination of various complex factors, such as the free energy of the system, temperature, and competition between dynamics and thermodynamics40-43 (Supporting Information), which is beyond the scope of the current study and will not be discussed here.

4. Conclusions The temperature as well as the ACN-dependent solubility of alkanes within the PPO hydrophobic microdomains and the attachment of silicate oligmers to the PEO brushes could fine tune the hydrophobic-hydrophilic properties and thus the microstructures of organic-inorganic composites in the alkaneP123-TEOS-NH4F-H3O+ complex emulsion system. The rationalization of the self-assembly process and the relationship between synthesis parameters such as ACN, PTT, and IRT in the alkane-P123-TEOS-NH4F-H3O+ system have been explored. These will be helpful in understanding the phase behavior of the oil-water-surfactant emulsion systems and will therefore promote the rational construction of various functional materials with designated structure and morphology. Acknowledgment. We are grateful for the support of the National Natural Science Foundation of China (no. 90206036) and the Ministry of Science and Technology of China (2005CB221405). We thank Professor Dangsheng Su (Department of Inorganic Chemistry, Fritz-Haber Institute of the Max Planck Society), Professor Yuanhua Ding, and Professor Weihong Qiao (State Key Laboratory of Fine Chemicals, Dalian University of Technology) for their help with EM characterization and testing the interfacial tension. J.S. thanks Professor Fengshou Xiao (Jilin University) and Dr. Gang Hu (Oxford University) for helpful discussions. Supporting Information Available: SEM/HRSEM and TEM images of amorphous silica gels, 2-D hexagonal mesoporous silicas, multilamellar vesicles and the mixture of multilamellar vesicles and 2-D hexagonal mesoporous silicas; FFEM images of the replicas obtained in the nonane-P123-TEOS-NH4F-H3O+ emulsion system; nitrogen sorption isotherms of all of the obtained silica materials; and interfacial tension of the decane-P123-NH4F-H3O+ emulsion system. This material is available free of charge via the Internet at http://pubs.acs.org. LA702991R (40) Sen, T.; Tiddy, G. J. T.; Casci, J. L.; Anderson, M. W. Microporous Mesoporous Mater. 2005, 78, 255. (41) Zhao, D. Y.; Sun, J. Y.; Li, Q. Z.; Stucky, G. D. Chem. Mater. 2000, 12, 275. Sujandi; Park, S. E.; Han, D. S.; Han, S. C.; Jin, M. J.; Ohsuna, T. Chem. Commun. 2006, 4131. (42) Yu, C. Z.; Fan, J.; Tian, B. Z.; Zhao, D. Y. Chem. Mater. 2004, 16, 889. (43) Kosuge, K.; Sato, T.; Kikukawa, N.; Takemori, M. Chem. Mater. 2004, 16, 899. Bao, X. Y.; Zhao, X. S. J. Phys. Chem. B 2005, 109, 10727.