Langmuir 2006, 22, 5491-5496
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Tuning the Mesostructures of Vinyl Silica by Adjusting the Micellar Curvature Yanqin Wang,*,† Yaojun Wang,† Chia-Min Yang,‡ Guanzhong Lu,† and Ferdi Schu¨th§ Lab for AdVanced Materials, Research Institute of Industrial Catalysis, East China UniVersity of Science and Technology, Shanghai 200237, P. R. China; Department of Chemistry, National Tsing Hua UniVersity, Hsinchu, 300, Taiwan; Max-Planck-Institut fu¨r Kohlenforschung, Kaiser-Wilhelm-Platz 1, 45470, Mu¨lheim an der Ruhr, Germany ReceiVed January 20, 2006. In Final Form: April 5, 2006 In our previous study (Wang, Y. Q.; Yang, C.-M.; Zibrowius, B.; Spliethoff, B.; Linde´n, M.; Schu¨th, F. Chem. Mater. 2003, 15, 5029), mesoporous vinyl-functionalized silica (vinyl silica) with hexagonal P6mm and cubic Ia3d structures has been synthesized at different loadings of vinyl groups and at different concentrations of sodium chloride when triblock copolymer P123 was used as a template. Our further investigations presented in this article reveal that at a loading of 10% vinyl groups, well-ordered cubic Ia3d structure was obtained at a low concentration of Na2SO4 (0.5 M) and the hexagonal structure was produced at 1.0 M NaCl. When NaNO3 was used as the inorganic salt, the hexagonal structure was still maintained even at a salt concentration of 2.0 M. The result is in accordance with the Hofmeister series order (salting-out effect): SO42- > Cl- > NO3-. The lowering of the acidity also induced the formation of the cubic Ia3d structure. At 20% loading, hexagonal structure can be obtained by adding the more hydrophilic Pluronic F127 (EO106PO70EO106) to the acidic solutions of P123, but the hexagonal structure cannot be produced with pure P123 under the synthesis conditions investigated. All of these results can be rationalized through hydrophilic-hydrophobic balance and the change in micellar curvature. Furthermore, 10% mercaptopropyl-functionalized mesoporous silica with cubic Ia3d structure was designed and synthesized successfully with the assistance of an inorganic salt (NaCl) in an acidic solution of P123, which is the first example of mercaptopropyl-functionalized large-pore mesoporous silica with high loadings.
Introduction Mesoporous materials with different mesostructures (hexagonal, cubic, and lamellar), morphologies (spheric, rodlike, fiber, etc.), and pore sizes (2-30 nm) have been synthesized with cationic, anionic, and neutral surfactants and block copolymers as templates. The pore structure, such as channel connectivity and pore size, is one of the most important physical parameters of these mesoporous materials for practical applications and must be designed with respect to their intended use. One route to designing the silica/surfactant mesostructure is to adjust the micellar packing parameter (g ) V/a0l, where V is the total volume of the hydrophobic surfactant chain, a0 is the effective hydrophilic headgroup size at the aqueous-micellar surface, and l is the kinetic surfactant tail length). The value of g can be influenced by the addition of a cosurfactant, the polarity of the solvent, and the addition of electrolytes (especially the type of anions). The expected mesophase sequence as a function of the packing parameter is g
mesophase
1/3 1/2 1/2-2/3 1
cubic (Pm3n) hexagonal (p6m) cubic (Ia3d) lamellar
These transitions reflect a decrease in surface curvature from cubic (Pm3n) through hexagonal (p6m) and cubic (Ia3d) to lamellar.2,3 * Corresponding author. E-mail:
[email protected]. Phone: +0086-21-64253824. Fax: +0086-21-64253703. † East China University of Science and Technology. ‡ National Tsing Hua University. § Max-Planck-Institut fu ¨ r Kohlenforschung.
Because the concept of the micellar packing parameter was initially developed for dilute surfactant solutions, it is not immediately transferable to block copolymers. However, it still can be used qualitatively to tailor the mesostructures of silica. Another concept of the hydrophilic/hydrophobic volume ratio VH/VL is suggested especially in block copolymer templating systems to account for the formation of different mesophases.4 By adjusting the hydrophilic/hydrophobic balance (VH/VL or micellar curvature), large-pore cubic Ia3d pure silica has been synthesized by the addition of cosurfactants (e.g., mercaptopropyltrimethoxysilane5), less-polar solvents (e.g., butanol6), or electrolytes (e.g., NaI7), but it seems to be especially difficult to synthesize cubic materials (Ia3d) using triblock copolymer alone. The results obtained with mercaptopropyl trimethoxysilane and butanol suggest that the hydrophilic/hydrophobic balance under synthesis conditions determines the phase that is eventually formed. Recently, much attention has been paid to the applications of functionalized cubic Ia3d large-pore mesoporous silicas in the fields of catalysis, adsorption, medicine, and nanotechnology. The synthesis system that we targeted is mesoporous vinyl silica. It serves as a model system for the investigation of mesophase transformations because the vinyl group is fairly stable under the reaction conditions for the synthesis of the mesostructured material (1) Wang, Y. Q.; Yang, C.-M.; Zibrowius, B.; Spliethoff, B.; Linde´n, M.; Schu¨th, F. Chem. Mater. 2003, 15, 5029. (2) Huo, Q.; Leon, R.; Petroff, P. M.; Stucky, G. D. Science 1995, 268, 1324. (3) Huo, Q.; Margolese, D.; Stucky, G. D. Chem. Mater. 1996, 8, 1147. (4) Kim, J.; Sakamoto, Y.; Hwang, Y.; Kwon, Y.; Terasaki, O.; Park, S.; Stucky, G. J. Phys. Chem. B 2002, 106, 2552. (5) Liu, X.; Tian, B.; Yu, C.; Gao, F.; Xie, S.; Tu, B.; Che, R.; Peng, L. M.; Zhao, D. Y. Angew. Chem., Int. Ed. 2002, 41, 3876. (6) Kleitz, F.; Choi, S. H.; Ryoo, R. Chem. Commun. 2003, 2136. (7) Flodstro¨m, K.; Alfredsson, V.; Ka¨llrot, N. J. Am. Chem. Soc. 2003, 125, 4402.
10.1021/la0601915 CCC: $33.50 © 2006 American Chemical Society Published on Web 05/11/2006
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and is sufficiently reactive to allow further postsynthesis functionalization.8 Very recently, mesoporous vinyl silica was used for the immobilization of penicillin acylase (PA), which shows good initial enzymatic activity for the hydrolysis of penicillin G (Pen-G).9 The loading of PA on mesoporous vinyl silica is not high compared to that of other organic-groupfunctionalized silica, but the initial enzymatic activity of PA immobilized on the 1:20 TEVS/TEOS support (here, TEVS is triethoxyvinylsilane and TEOS is tetraethoxysilane) was 2 times higher than that of the free enzyme, showing that the vinyl group is a promising functionality for the immobilization of PA. This may be due to the strong affinity of the substrate (Pen-G) for the vinyl groups on the support material, thus increasing the affinity of the immobilized enzyme for the substrate as well as causing changes in the microenvironment of the enzyme to favor the reaction. In our previous study,1 mesoporous vinyl silica with hexagonal p6mm and cubic Ia3d structures were synthesized at different loadings of vinyl groups and at different concentrations of sodium chloride when P123 was used as the template. At a loading of 10% vinyl groups, the hexagonal structure has been maintained even with 1.5 M NaCl, whereas the ordered cubic Ia3d structure was obtained at an only slightly high NaCl concentration of 2 M. At a loading level of 20% vinyl groups, the mesostructure changed from a disordered structure to the cubic Ia3d structure with the addition of 1.5 M NaCl. In this article, we report further investigations of the influence of the molar ratio of TEVS/ TEOS: the type, concentration of anionic ions, addition of cosurfactant, and concentration of HCl that can be rationalized by the change in micellar curvature. This result has also been extended to the synthesis of mercaptopropyl-functionalized mesoporous silica. Experimental Section Chemicals. Pluronic P123 and F127 were obtained from Aldrich and BASF, respectively. Tetraethoxysilane (TEOS) was purchased from Gelest, and triethoxyvinylsilane (TEVS) and mercaptopropyltrimethoxysilane (MPTMS) were purchased from Aldrich. Synthesis of Mesoporous Vinyl Silica. The synthesis process of mesoporous vinyl silica has been published elsewhere.1 Typically, 5.68 g of Pluronic P123 and calculated amounts of inorganic salts were dissolved in 160 mL of a 1.0 M HCl aqueous solution. After a homogeneous solution was obtained at 35 °C, a mixture of TEVS and TEOS (53.2 mmol in total) was added slowly under stirring. The mixture was stirred at 35 °C for 4 h more and was then aged at 80 °C for 24 h. The product was filtered, washed with deionized water, ethanol, and acetone, and dried at 80 °C overnight. The polymer was removed by 60 wt % H2SO4 treatment. As indicated in the Results and Discussion section, one experiment was performed with 0.5 M HCl, and in another, a mixture of F127 and P123 was used. Synthesis of Mesoporous 10%Mercaptopropyl Silica. The synthesis procedure is the same as that for mesoporous vinyl silica except that MPTMS was used in place of TEVS as the functional organosilane. In all experiments and discussions, the mole percent of TEVS or MPTMS indicates the mole percent (based on silicon) in the starting mixture rather than in the final product. Characterization. Small-angle X-ray diffraction patterns (XRD) were recorded with a Stoe STADI P or Rigaku D/MAX 2550 PC diffractometer with Cu KR radiation (scanning step: 0.01°/s). Transmission electron microscope (TEM) images were obtained with an HF2000 electron microscope from Hitachi with a cold field emission cathode. N2 sorption isotherms were obtained at 77 K on (8) Asefa, T.; Kruk, M.; MacLachlan, M. J.; Coombs, N.; Grondey, H.; Jaroniec, M.; Ozin, G. A. AdV. Funct. Mater. 2001, 11, 447. (9) (a) Chong, A. S. M.; Zhao, X. S. Appl. Surf. Sci. 2004, 237, 398. (b) Chong, A. S. M.; Zhao, X. S. Catal. Today 2004, 93-95, 293.
Wang et al. a Micromeritics ASAP 2020 sorption analyzer. The solvent-extracted samples were evacuated at 80 °C for ca. 10 h before measurements. The BET surface areas were obtained from the adsorption branches in the relative pressure range of 0.05-0.20. The pore size distributions were calculated from the desorption branches using the BarrettJoyner-Halenda (BJH) method. The total pore volume was calculated at a relative pressure of p/p0 ) 0.97.
Results and Discussion In our previous paper, co-condensation of TEVS and TEOS in the presence of a triblock copolymer surfactant was shown to allow the synthesis of hexagonal and Ia3d cubic silica with vinyl groups bearing silicon atoms incorporated into the material. The TEVS molar percentage governs the nature of the phase formed, and the addition of inorganic salts improves the quality of the materials. Effect of the Loading Amount. In the presence of 1.5 M NaCl, the mesostructure developed from hexagonal to cubic Ia3d structure with increasing fraction of TEVS (Figure 1, XRD and TEM), indicating the influence of the hydrophobicity of TEVS on the curvature of the micelles (i.e., TEVS has a cosurfactant effect). Such cubic Ia3d structure has also been obtained with the addition of small amounts of mercaptopropyl-trimethoxysilane4,10 to the silicon source TEOS, whereas it seems to be especially difficult to synthesize cubic materials (Ia3d) via triblock copolymer alone. The influence of the TEVS/(TEVS + TEOS) ratio on the phase behavior of the studied systems is caused by the hydrophobicity of TEVS, which exhibits cosurfactant properties; that is, it will be preferentially adsorbed close to the hydrophobic PPO portion of the micelles and therefore cause a decrease in the interfacial curvature, eventually favoring the formation of the Ia3d phase instead of the 2D hexagonal material. Effects of the Salt Concentration and Anion Type. The Hofmeister series order (salting-out effect) is SO42-, Cl-, NO3-. The influence of the concentration of NaCl on the mesostructure is fully investigated for 10% vinyl silica. XRD patterns in Figure 2 (left) show that a pure hexagonal structure was obtained in the absence of salt and at 0.5 M NaCl. With the addition of more NaCl (1.0 and 1.5 M), additional intensity on the right-hand side of the (100) reflection of the hexagonal structure is observed, which could indicate the coexistence of the cubic phase. At 2.0 M NaCl, the cubic Ia3d structure is fully developed. When Na2SO4 or NaNO3 was used as the inorganic salt instead of NaCl, different phenomena occurred. XRD patterns in Figure 2 (right) show that a well-ordered cubic Ia3d structure has been synthesized with 0.5 M Na2SO4, but the hexagonal structure is still maintained even at 2.0 M NaNO3. As mentioned above, at 1.0 M NaCl, the hexagonal structure is obtained, probably with small amounts of a coexisting cubic structure. At a high concentration of NaCl (2.0 M), the pure cubic structure can be obtained. This phenomenon can be explained by Hofmeister anion effects on surfactant self-assembly (salting-in and saltingout effects). The Hofmeister series of ions was initially established for protein solubility, and it was soon be confirmed that anions have a much stronger impact on protein solubility than cations. This concept of the Hofmeister series of ions has been extended to surfactant solution, and anions were separated into salting-in (increasing micellar curvature) and salting-out (decreasing micellar curvature) groups. The Hofmeister series orders ions with increasing salting-in potency from left to right, as follows:11 (10) Hodgkins, R. P.; Garcia-Bennett, A. E.; Wright, P. A. Microporous Mesoporous Mater. 2005, 79, 241.
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Figure 1. XRD patterns and TEM images (insets are Fourier diffractograms) of vinyl silicas with different ratios of TEVS/TEOS synthesized in 1.5 M NaCl.
Figure 2. XRD patterns of 10% vinyl silica with different mesostructures. (Left) at different concentrations of NaCl; (right) for different types of inorganic salts.
SO42-, HPO42-, OH-, F-, HCOO-, CH3COO-, Cl-, Br-, NO3-, I-, SCN-, ClO4Ions on the left of Cl-, which represents a borderline case, reduce the solubility of proteins or decrease the curvature of surfactant micelles (salting-out ions). The opposite holds for ions on the right of Cl-, known as salting-in ions, which enhance the solubility of proteins or increase the curvature of surfactant micelles. Under our experimental conditions, the anion sequence that affects the phase behavior of block-copolymer-templated mesostructured vinyl silica is exactly in accordance with the Hofmeister series (i.e., SO42- leads to a salting-out effect, causing the formation of the bicontious cubic structure with low interfacial curvature, whereas NO3- shows a salting-in effect, leading to the formation of the hexagonal structure with high interfacial curvature even at high concentration (2.0 M)). This result differs from the one observed during investigations of the phase transformation of mesostructured silica templated by nonionic block copolymers.12 In that work, an unusual anion sequence with respect to the phase behavior of block-copolymer-templated mesostructured solids was described (i.e., SO42-(HSO4-) > NO3- > Br- > Cl-), and the authors explained this in terms of (11) Leontidis, E. Curr. Opin. Colloid Interface Sci. 2002, 7, 81. (12) Tang, J. W.; Yu, C. Z.; Zhou, X. F.; Zhao, D. Y. Chem. Commun. 2004, 2240.
radii and dehydration effects. Besides the fact that anions were found to influence the mesostructures in this work and in other studies,7,12,13 recent work on mesoporous silica formation has also disclosed that anions change the hydrolysis rates of the silicate precursors,1 affect the morphologies of the final products,14 and improve the ordering of the mesostructures.1,7,15-18 Effect of Acid Concentrations. The cubic Ia3d structure for 10% vinyl silica was formed at low HCl concentration. As discussed above, for samples with 10% TEVS (10% vinyl silica) synthesized at 1.0 M HCl, hexagonal structures were observed for concentrations of 0-1.5 M NaCl, whereas cubic Ia3d structures were obtained with 0.5 M Na2SO4 or a high concentration of NaCl (2.0 M), depending on the salting-out effect. In addition, cubic Ia3d structures can also be produced when the concentration of acid is decreased, as shown in the small-angle XRD pattern in Figure 3 (left). This earlier onset of the phase change from hexagonal to cubic Ia3d structure has (13) Che, S. N.; Lim, S. Y.; Kaneda, M.; Yoshitake, H.; Terasaki, O.; Tatsumi, T. J. Am. Chem. Soc. 2002, 124, 13962. (14) Yu, C. Z.; Fan, J.; Tian, B. Z.; Zhao, D. Y.; Stucky, G. D. AdV. Mater. 2002, 14, 1742. (15) Wang, Y. Q.; Zibrowius, B.; Yang, C. M.; Schuth, F. Chem. Commun. 2004, 46. (16) Guo, W. P.; Park, J. Y.; Oh, M. O.; Jeong, H. W.; Cho, W. J.; Kim, I.; Ha, C. S. Chem. Mater. 2003, 15, 2295. (17) Yu, C. Z.; Tian, B. Z.; Fan, J.; Stucky, G. D.; Zhao, D. Y. J. Am. Chem. Soc. 2002, 124, 4556. (18) Lu, D. L.; Kondo, J. N.; Domen, K. J. Am. Chem. Soc. 2002, 124, 11256.
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Figure 3. XRD pattern (left) and N2 sorption isotherm (right) of mesostructured 10% vinyl silica synthesized from a 0.5 M HCl solution in the presence of 1.5 M NaCl.
Figure 4. XRD pattern (left), TEM image (middle), and N2 sorption isotherm (right) of 20% vinyl silica with hexagonal structure. Table 1. Structural Properties of 10% Vinyl-Functionalized Mesoporous Silica Aged at 80 °C phase
[HCl]/ M
salt concentration
a0/ nm
SBET/ m2 g-1
Dp/ nm
Vt/ cm3 g-1
P6mm P6mm P6mm Ia3d Ia3d Ia3d
1.0 1.0 1.0 1.0 0.5 1.0
0.5 M NaCl 1.0 M NaCl 1.5 M NaCl 2.0 M NaCl 1.5 M NaCl 0.5 M Na2SO4
11.0 10.8 10.8 22.5 24.2 24.0
573 576 580 574 535 547
7.3 7.3 7.4 6.9 7.1 7.5
1.13 1.09 1.04 1.04 1.08 1.10
P6mm
1.0
2.0 M NaNO3
10.2
574
7.4
1.05
also been observed in the synthesis of pure mesoporous silica after decreasing the acidity (from 1.3 to 1.2 M HCl) under the influence of butanol.12 It is understandable that with the decreasing acidity the protonation of triblock copolymers decreases, leading to the phase transformation from the more hydrophilic hexaganol structure to the relatively hydrophobic Ia3d structure. The N2 sorption isotherm in Figure 3 (right) shows a typical IV isotherm with clear H1-type hysteresis loops at high relative pressure, indicating the presence of large pores with a narrow pore size distribution. The calculated surface area, pore size, and pore volume are 535 m2 g-1, 7.1 nm, and 1.08 cm3 g-1, respectively. All of the above results indicate that the type of salt, the salt concentration, and the acid concentration affect the mesostructures of 10% vinyl silica. Table 1 summarizes the structural properties. High surface areas, large pore sizes, and large pore volumes were obtained for all samples. Effect of the Addition of More Hydrophilic Polymers. The addition of more hydrophilic F127 to the acidic solution of P123 leads to the formation of hexagonal structure for 20% vinyl silica. In our previous work,1 for samples with 20% TEVS, a welldefined cubic Ia3d structure was obtained in the presence of NaCl, whereas the structures could not clearly be assigned from the XRD patterns for samples prepared without salt. In addition, the pore volume for such samples was substantially lower. This
suggests that the ratio between the hydrophobic and hydrophilic silica source plays an important role but that the salt has an additional effect on the ordering of the material, possibly in the sense of a salting-out. If anions such as NaI (or NaSCN) displaying the salting-in effect are added, as reported in ref 7, then the hexagonal structure could not be obtained under our experimental conditions. However, if the more hydrophilic F127 surfactant was added as a cosurfactant, then mesoporous 20% vinyl silica with hexagonal structure can be synthesized. This result indicates that a more hydrophilic surfactant is needed to counteract the hydrophobicity of TEVS if the hexagonal structure is to be formed. The hexagonal structure can be clearly seen from the smallangle XRD pattern and the TEM image shown in Figure 4, and the mesoporosity is demonstrated by the N2 sorption isotherm (Figure 4, right). A high surface area of 540 m2 g-1 and a large pore size of 7.3 nm were obtained after H2SO4 treatment. Cubic Im3m Structure. A structure analogous to SBA-16 was obtained for 10% vinyl silica when F127 was used as the template. When the more hydrophilic F127 surfactant was used instead of P123, the cubic Im3m structure with high curvature was obtained for 10% vinyl silica, which is analogous to SBA16 19 synthesized in an acidic solution of F127. The XRD pattern and TEM image are shown in Figure 5. All of the above results show that mesostructures can be tuned by adjusting the micellar curvature, and the latter can be influenced by the hydrophobic-hydrophilic properties of the cosurfactant, anion types, solution acidity, and solvent polarity. All of these can be summarized in Scheme 1: TEVS plays the role of a cosurfactant, which is more hydrophobic than TEOS and decreases the interfacial curvature by being preferentially solubilized close to the interface between the dehydrated and hydrated PEOs. This leads to the formation of cubic Ia3d structure (20% TEVS). The addition of NaCl or Na2SO4 induces a salting-out effect, which leads to the formation of the cubic Ia3d structure, (19) Zhao, D. Y.; Huo, Q. S.; Feng, J. L.; Chmelka, B. F.; Stucky, G. D. J. Am. Chem. Soc. 1998, 120, 6024.
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Figure 5. XRD pattern (left) and TEM image (right) of 10% vinyl silica with cubic Im3m structure. Scheme 1. Interfacial Curvatures and Mesostructures of Surfactant Solutions Affected by Different Factors
and the influence of Na2SO4 is more obvious than Cl -, which is in accordance with the Hofmeister series. Lowering the acidity also induces the formation of the cubic Ia3d structure. For 20% vinyl silica, the hexagonal structure cannot be obtained with pure P123 as a template because of the hydrophobicity of TEVS, but the addition of the more hydrophilic F123 can counteract the hydrophobicity of TEVS and leads to the formation of hexagonal structure. When pure F127 was used as a template, the more highly curved cubic Im3m structure was attained. Actually, in all of the above synthesis processes, 1.0 M HCl (and occasionally 0.5 M HCl) was present, and the influence of the acid type (such as HCl, HNO3, and H2SO4) on the mesostructures has been investigated before.13 In our experiments, additional inorganic salt plays an important role in tuning the mesostructues. This additional effect is probably due to the higher counterion concentration, which would be needed to exert a sufficiently strong salting-out effect. This was further confirmed by the synthesis of 10% mercaptopropyl-functionalized mesoporous silica. Synthesis of Mesoporous 10% Mercaptopropyl Silica. The addition of NaCl induces the formation of well-ordered cubic Ia3d structure. According to the above discussion, mercaptopropyl-funtionalized mesoporous silica (mercaptopropyl silica) with cubic Ia3d structure can be designed by the addition of
salting-out anions. From an acidic solution of P123, 10% mercaptopropyl silica with cubic Ia3d structure has been obtained in the presence of 1.5 M NaCl (XRD pattern in Figure 6). This is the first example of mesoporous mercaptopropyl silica with high loading (10%, based on silicon) and well-ordered structures synthesized from an acidic solution of P123. In the former studies of Zhao9 and Wright,10 only mesoporous mercaptopropyl silica at low concentrations (2-5%) was obtained.
Figure 6. XRD pattern of mesostructured 10% mercaptopropyl/ silica synthesized in 1.0 M HCl solution in the presence of 1.5 M NaCl.
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Conclusions
hexagonal structure. When pure F127 is used as a template, the more highly curved cubic Im3m structure was obtained. Furthermore, cubic Ia3d structures of 10% mercaptopropylfunctionalized mesoporous silica have been successfully prepared, which is the first example of mercaptopropyl-functionalized mesoporous silica with high loading (10%).
In this study, mesoporous vinyl silica with different mesostructures has been designed and synthesized by adjusting the micellar curvature, which can be influenced by adding different kinds of inorganic salts, lowering the acidity of the aqueous solution of P123, or adding the cosurfactant F127. The addition of NaCl or Na2SO4 induces a salting-out effect, leading to the formation of the cubic Ia3d structure, and the influence of Na2SO4 is more obvious than that of Cl -, which is in accordance with the Hofmeister series. The lowering of the acidity also induces the formation of the cubic Ia3d structure. For 20% vinyl silica, the hexagonal structure cannot be obtained with pure P123 as a template because of the hydrophobility of TEVS, but the addition of the more hydrophilic F123 can counteract the hydrophobicity of TEVS and leads to the formation of the
Acknowledgment. Y.Q.W. is grateful for support from the National Basic Research Program of China (no. 2004CB719500) and the Shanghai Government (04ZR14036, 05PJ14032, 0552nm030), China. This work is partially supported by the Leibniz program of the DFG and by basic support from the MPI fu¨r Kohlenforschung, Germany. We thank Dr. Freddy Kleitz and Dr. Bodo Zibrowius for fruitful discussions. LA0601915
