SDS Vesicle Formation and Use As Template

Jun 8, 2010 - Silica hollow spheres, with diameters 30−60 nm and a wall thickness of 8−10 nm, were prepared by using the vesicles as the templates...
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C12mimBr Ionic Liquid/SDS Vesicle Formation and Use As Template for the Synthesis of Hollow Silica Spheres Jie Yuan, Xiangtao Bai, Mingwei Zhao, and Liqiang Zheng* Key Laboratory of Colloid and Interface Chemistry, Shandong University, Ministry of Education, Jinan, Shandong, 250100, People’s Republic of China Received March 28, 2010. Revised Manuscript Received May 23, 2010 The phase behavior of an aqueous catanionic surfactant system, composed of a long-chain imidazolium ionic liquid 1-dodecyl-3-methylimidazolium bromide (C12mimBr) and sodium dodecyl sulfate (SDS), is described. The phase diagram of the catanionic system was determined by electrical conductivity measurements and the formation of vesicles in a birefringent LR phase characterized by transmission electron microscopy (TEM) and freeze-fracture transmission electron microscopy (FF-TEM). Rheological measurements were used to characterize the macroscopic properties of the birefringent LR phase. Both electrostatic and hydrophobic interactions contribute to the vesicle formation in the catanionic system. Compared to the DTAB/SDS aqueous solution, differences between the imidazolium and trimethylammonium headgroups geometric packing and charge density induce the different phase behavior in each system. Silica hollow spheres, with diameters 30-60 nm and a wall thickness of 8-10 nm, were prepared by using the vesicles as the templates. The hollow silica spheres were characterized by TEM, scanning electron microscopy (SEM), and nitrogen adsorption-desorption. The results suggest additional application for ionic liquid based vesicles to be used as templates for the synthesis of hollow inorganic materials.

Introduction Vesicles, as a kind of supramolecular aggregate formed by phospholipids and synthetic surfactants, have elicited much interest because of their broad applications as models of biological membranes,1 use in drug delivery,2 and application as microreactors for the production of inorganic materials.3 Since Kaler et al. reported spontaneous vesicle formation in aqueous solution,4 many routes have been explored to study the microstructures and properties of vesicles from single surfactants or from mixtures of cationic and anionic surfactants (catanionic vesicles).5-7 In catanionic surfactant systems, strong electrostatic interactions between the oppositely charged headgroups result in a reduced area per headgroup in the bilayers, and thermodynamically stable vesicles may be constructed at the appropriate ratios of cation and anion.8 Most of the investigations related to vesicle formation in catanionic surfactant system have focused on mixtures that include excess salt, such as aqueous sodium dodecyl sulfate (SDS) and alkyltrimethylammonium halides solutions.9-11 In these systems, a birefringent LR phase is usually observed, and precipitates form when there is an equal molar ratio of the cationic and anionic surfactants.

Vesicles may be used as soft templates in the preparation of various organized inorganic materials. Different reaction environments in and around vesicles may be utilized for the assembly of different materials by using vesicle templates. Materials may be formed in the “bulk” solution outside the vesicles, in the inner chamber of the vesicles, on the outside surfaces of the vesicles, or at the hydrophobic palisade layer of the vesicles.12 A number of reports have detailed the syntheses of silica hollow spheres using vesicles as templates.13-16 Wang et al. synthesized various hollow silica spheres with and without mesostructures by using unilamellar DDAB vesicles and studied the effect of template phase on the resulting silica structures.17 Hollow silica spheres were also synthesized inside the membrane of equilibrated surfactant vesicles by Nowakowska et al.18 These hollow silica spheres possess many unique characteristics which include low density, adiabatic capacity, and encapsulating ability.19,20 The spheres may also have applications as catalysts, in pigments and pharmaceuticals, in film substrates, as components of light sensitive protection materials, and in humidity sensors.21,22 Recently, investigations regarding vesicles formed in/with ionic liquids (ILs) have attracted attention. Ionic liquids, a class of

*Corresponding author. Tel.: þ86 531 88366062; fax: þ86 531 88564750. E-mail address: [email protected].

(12) Dong, R. H.; Weng, R.; Dou, Y. Y.; Zhang, L.; Hao, J. C. J. Phys. Chem. B 2010, 114, 2131–2139. (13) Hubert, D. H. W.; Jung, M.; Frederik, P. M.; Bomans, P. H. H.; Meuldijk, J.; German, A. L. Adv. Mater. 2000, 12, 1286–1290. (14) Tan, B.; Vyas, S. M.; Lehmler, H. J.; Knutson, B. L.; Rankin, S. E. Adv. Funct. Mater. 2007, 17, 2500–2508. (15) Wang, H.; Wang, Y.; Zhou, X.; Zhou, L.; Tang, J.; Lei, J.; Yu, C. Adv. Funct. Mater. 2007, 17, 613–617. (16) Kepczynski, M.; Ganachaud, F.; Hemery, P. Adv. Mater. 2004, 16, 1861– 1863. (17) Zhang, L.; Li, P.; Liu, X.; Du, L.; Wang, E. Adv. Mater. 2007, 19, 4279– 4283. (18) Kepczynski, M.; Lewandowska, J.; Romek, M.; Zapotoczny, S.; Ganachaud, F.; Nowakowska, M. Langmuir 2007, 23, 7314–7320. (19) Sugama, T.; Carciello, N. Adv. Cement Based Mater. 1996, 3, 45–53. (20) Sugama, T.; Lipford, B. J. Mater. Sci. 1997, 32, 3523–3534. (21) Vasquez, Y.; Sra, A. K.; Schaak, R. E. J. Am. Chem. Soc. 2005, 127, 12504– 12505. (22) Herbert, G. J. Eur. Ceram. Soc. 1994, 14, 205–214.

(1) Chevalier, Y.; Zemb, T. Rep. Prog. Phys. 1990, 53, 279–371. (2) Guo, X.; Szoka, F. C. Acc. Chem. Res. 2003, 36, 335–341. (3) Antonietti, M.; Forster, S. Adv. Mater. 2003, 15, 1323–1333. (4) Kaler, E. W.; Murthy, A. K.; Rodriguez, B. E.; Zasadzinski, J. A. N. Science 1989, 245, 1371–1374. (5) Laughlin, R. G. The Aqueous Behavior of surfactants, Academic Press, London, 1994. (6) Suzana, S.; Durdica, T. Adv. Colloid Interface Sci. 2006, 121, 51–75. (7) Yin, H.; Lin, Y.; Huang, J.; Ye, J. Langmuir 2007, 23, 4225–4230. (8) Bergstrom, M.; Pedersen, J. S.; Schurtenberger, P.; Egelhaf, S. U. J. Phys. Chem. B 1999, 103, 9888–9897. (9) Marques, E. F.; Regev, O.; Khan, A.; Miguel, M. G.; Lindman, B. J. J. Phys. Chem. B 1998, 102, 6746–6758. (10) Bergstr€om, M.; Pedersen, J. S.; Schurtenberger, P.; Egelhaaf, S. U. J. Phys. Chem. B 1999, 103, 9888–9897. (11) Sohrabi, B.; Gharibi, H.; Tajik, B.; Javadian, S.; Hashemianzadeh, M. J. J. Phys. Chem. B 2008, 112, 14869–14876.

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organic molten electrolytes at or near ambient temperature, are of interest to researchers because of their exceptional properties.23 Desirable IL characteristics include insignificant vapor pressure, potential catalytic properties, high ion conductivity, and nonflammability. As such, ionic liquids have many potential applications as electrolytes or solvent media for reactions or extraction.24-27 In some reports, ionic liquids have been used as the reaction medium for vesicle formation. For example, Hao et al. studied vesicles formed by Zn2þ-fluorous surfactant in the ionic liquids, 1-butyl-3-methylimidazolium tetrafluoroborate (C4mimBF4) and 1-butyl-3-methylimidazolium hexafluorophosphate (C4mimPF6).28 Nobuo’s group has studied vesicle formation in ether-containing ionic liquids.29 Ionic liquids have also been used as host molecules to construct vesicles, as a part of a catanionic system. When ionic liquids are mixed with oppositely charged surfactants, it will lead to the formation of advanced aggregates and surfactant phases, since under these conditions, ILs behave in a manner similar to ionic surfactants. For example, Marangoni’s group studied the vesicle formation and nanoscale aggregates formed in mixtures of amphiphilic ionic liquids, hexylpyridinium bromide (HexPyBr) or hexylpyridinium tetrafluoroborate (HexPyBF4), and sodium dodecyl sulfate (SDS).30 Ionic liquids composed of 1-alkyl-3-methylimidazolium cation (Cnmimþ) have been extensively investigated in colloid and interface science. Our group has found that this kind of IL can self-assemble in both aqueous solutions31,32 and in other ILs.33 Currently, there are no reports describing vesicle formation with long-chain imidazolium ionic liquids. Herein, we report the phase behavior and vesicle formation in cationic-rich aqueous mixtures of a long-chain imidazolium ionic liquid, 1-dodecyl-3-methylimidazolium bromide (C12mimBr), and an anionic surfactant, sodium dodecyl sulfate (SDS). The template effect of the IL-SDS vesicles for the synthesis of hollow silica spheres is also described.

Experimental Section Materials. C12mimBr was prepared according to the litera-

ture,31 and its purity was confirmed by 1H NMR in CDCl3. Sodium dodecyl sulfate (SDS, g 99%) and L-ascorbic acid (g99.7%) were purchased from Shanghai Chemical Reagents Company (China). Tetraethylorthosilicate (TEOS, g 98%) was obtained from Beijing Yili Chemical Reagents Co. Ltd. (China). Ammonia solution was purchased from Laiyang Chemical Reagents Co. Ltd. (China). All reagents were analytical grade and used as received. Triply distilled water was used for all experiments.

Methods. Phase Behavior of the C12mimBr/SDS Aqueous Solution. A stock solution with a concentration of 100 mM C12mimBr was prepared. Various amounts of SDS were added into 10 mL of the C12mimBr stock solution. The mole ratio r (r = n(SDS):n(C12mimBr)) was varied from 0 to 1. The mixtures were (23) Li, R. X. Green Solvent: Synthesis and Application of Ionic Liquids; Chemistry Technology Press: Beijing, 2004. (24) Mcewen, A. B.; Ngo, H. L.; Lecompte, K.; Goldman, J. L. J. Electrochem. Soc. 1999, 146, 1687–1695. (25) Welton, T. Chem. Rev. 1999, 99, 2071–2084. (26) Blanchard, L. A.; Hancu, A.; Bechman, E. J.; Brennecke, J. F. Nature 1999, 399, 28–29. (27) Anthony, J. L.; Maginn, E. J.; Brennecke, J. F. J. Phys. Chem. B 2002, 106, 7315–7320. (28) Hao, J. C.; Song, A.; Wang, J.; Chen, X.; Zhuang, W.; Shi, F.; Zhou, F.; Liu, W. Chem.;Eur. J. 2005, 11, 3936–3940. (29) Takuya, N.; Nobuo, K. Chem. Lett. 2002, 10, 1018–1019. (30) Singh, K.; Marangoni, D. G.; Quinn, J. G.; Singer, R. D. J. Colloid Interface Sci. 2009, 335, 105–111. (31) Dong, B.; Li, N.; Zheng, L.; Yu, L.; Inoue, T. Langmuir 2007, 23, 4178– 4182. (32) Dong, B.; Zhang, J.; Zheng, L.; Wang, S.; Li, X.; Inoue, T. J. Colloid Interface Sci. 2008, 319, 338–343. (33) Li, N.; Zhang, S.; Zheng, L.; Dong, B.; Li, X.; Yu, L. Phys. Chem. Chem. Phys. 2008, 10, 4375–4377.

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stirred until the SDS had dissolved and then stirred for an additional 10 min. Bubbles in the solution were removed by ultrasonication. The sample solutions were thermostatted at 25.0 ((0.1) °C for at least 4 weeks in order to reach an equilibrium state. The phase behavior of the solutions was examined by visual inspection with and without crossed polarizers. The phase diagram was obtained by observing the samples at the same temperature of 25.0 ((0.1) °C, and the boundaries of the phase regions were determined by conductivity measurements. Vesicle-Templated Preparation of Hollow Silica Spheres. In a typical experiment, 0.4 mL of 50 mM L-ascorbic acid solution and 0.75 mL of 0.15 mM ammonia solution were added to 5 mL of a vesicle solution (100 mM C12mimBr, r = 0.63) with stirring at 25 °C. Next, 16 μL TEOS was added to the stirred mixture, and after an additional 3 h, another 20 μL aliquot of TEOS was added, and the mixture stirred for two additional hours. A white precipitate gradually appeared and was collected by centrifugation, washed several times with distilled water and absolute ethanol, and then dried. Samples were calcined at 550 °C in air for 5 h resulting in a fine white powder. Equipment. Electrical Conductivity Measurements. A low-frequency conductivity analyzer (model DDS-307, Shanghai Precision & Scientific Instrument Co., Ltd., accuracy of 1%) was used to measure the electrical conductivities of the solutions at 25 °C. The electrical conductivities of the two-phase solutions were measured with stirring. Polarized Microscopy Observations (POM). The birefringence of the samples was checked by visual inspection with and without crossed polarizers using a Carl Zeiss Axiosk 40 light microscope.

Transmission Electron Microscopy (TEM) and FreezeFracture Transmission Electron Microscopy (FF-TEM). Negative staining TEM was used for the observation of the vesicle samples. A drop of the sample solution was placed on a Formvarcovered TEM grid (copper grid, 3.02 mm, 200 mesh) and stained with a drop of 2 wt % phosphotungstic acid aqueous solution. The excess solution was removed by blotting with a filter paper. For TEM observation of the silica hollow spheres, the samples were ultrasonically dispersed in ethanol, and a drop of the suspension placed onto Formvar-coated Cu grid. A JEOL 100CX-II TEM, operating at 100 kV, was used to examine the samples after the grids were dried at room temperature. Fracturing and replication were carried out on Balzers BAF-400D (Germany) freeze-fracture device at a temperature and pressure of -110 °C and 10-4 Pa, respectively. The samples were examined with a Philips Tecnai 20 and Jeal JEM-100CX electron microscope. Rheological Measurements. The oscillatory shear of the C12mimBr/SDS LR phase was investigated using a Haake Rheostree RS75 rheometer with a concentric cylinder system (Z41 Ti). The sample was placed in the temperature-controlled vessel at 25.0 ((0.1) °C for 5 min to reach equilibrium before measurements. The oscillatory shear frequency was varied from 0.01 to 10 Hz. Scanning Electron Microscopy (SEM). SEM images were taken to measure the sizes and morphologies of the silica hollow spheres by using a field emission scanning electron microscope (JEOL JSM-7600F) operating at 3.0 kV. Nitrogen Absorption and Desorption Measurements. Nitrogen absorption and desorption measurements were performed on a QuadraSob SI apparatus. The surface areas were calculated by the Brunauer-Emmett-Teller (BET) method, and the pore size distribution was calculated from the adsorption isotherm curves using the Brunauer-Joyner-Halenda (BJH) method.

Results and Discussion Phase Behavior of the C12mimBr/SDS Aqueous Solution. C12mimBr aqueous solution with a concentration of 100 mM is a low-viscosity L1 phase (a micellar solution). When SDS is added DOI: 10.1021/la101221z

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Figure 1. Optical photographs of aqueous C12mimBr (100 mM) with increasing amounts of SDS observed without (top) and with (bottom) polarizers at 25.0 ((0.1) °C.

into 100 mM C12mimBr aqueous solution, a series of phase changes were observed as the mole ratio r (r = n(SDS):n(C12mimBr)) was varied from 0 to 1. Photographs of typical samples with and without crossed polarizers are shown in Figure 1. For solutions with r between 0 and 0.4, a transparent solution with a low-viscosity L1 phase (a micellar solution) is observed. Increasing r from 0.4 to 0.52 causes the solution to become viscous, and a slightly bluish L1 phase is observed. When r is 0.52-0.58, a two-phase region appears with an isotropic L1 phase at the bottom and a birefringent LR phase that has the appearance of creamy white floating precipitates at the top. The volume of the top LR phase grows with an increase in the amount of added SDS. As r increases from 0.58 to 0.64, a birefringent, turbid, and bluish LR phase is observed, this phase contains the vesicles described below. Solutions with r between 0.64 and 0.75 show a flow birefringence LR phase, which is transparent and bluish, this may indicate that the flow birefringence LR phase contains a small amount of vesicles.34 For samples with r in the range 0.75-1, a two-phase region with an L1 phase at the top and thick precipitates at the bottom is observed (not illustrated here). This precipitate formation, which is common in catanionic surfactant systems, especially those close to equimolar ratios of cationic and anionic surfactant, is usually attributed to the electrostatic interactions between the oppositely charged headgroups.9,35 The sequence of the phase regions on the cationic-rich side with increasing SDS amounts can be summarized as follows: L1, L1/ LR, LR (turbid), LR(flow birefringence), L1/thick precipitate. Figure 2 illustrates the phase diagram of 100 mM C12mimBr solution with various amounts of SDS (r = 0-1); the different phase boundaries were determined by electrical conductivity measurements. In the L1 phase (micellar solution), the amount of counterions increased with the addition of SDS, which results in an increase of electrical conductivity. Further, when vesicles formed in the next phase, part of the solution is enwrapped, so the conductive ions as charge carriers in the solution are also being enwrapped; as a consequence, the conductivity of the mixture is (34) Shen, Y. W.; Hoffmann, H.; Hao, J. C. Langmuir 2009, 25, 10540–10547. (35) Hao, J. C.; Yang, J. L. Self-assembled structures of cationic-anionic surfactants and templating applications as nano-reactors, In Recent Research Developments in Physical Chemistry: Surfaces and Interfaces of Nanostructured Systems, Wang, C., Ed.; Kerala, India, 2007; Chapter VI, pp 141-163. (36) Li, X.; Dong, S.; Jia, X.; Song, A.; Hao, J. C. Chem.;Eur. J. 2007, 13, 9495–9502.

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Figure 2. Phase diagram of the C12mimBr (100 mM)/SDS system with varied SDS amounts at 25.0 ((0.1) °C. The conductivity (b) and bottom phase volumes (O) of the two phases are included in the diagram.

reduced,36 as illustrated in Figure 2. The reduction in conductivity, in addition to the TEM and FF-TEM observations below, confirms the formation of vesicles in the catanionic system. The phase behaviors of the C12mimBr/SDS and DTAB/SDS aqueous solutions are significantly different. In the DTAB/SDS system, the LR phase is only a small region on the anionic-rich side; no LR phase exists on the cationic-rich side according to the ternary phase diagram reported by Kaler et al.37 In contrast, vesicle formation occurs in a relatively large LR phase on the cationic-rich side of the C12mimBr/SDS system. Comparing the two systems, C12mimBr and DTAB possess equal alkyl chain lengths, but differ in the imidazolium and trimethylammonium headgroups leading to a difference in geometric packing. In addition, the charge density of the headgroups will affect the solubility mismatch of the ion pairs and the electrical interactions between the cationic and anionic surfactants.38 The results indicate that the packing parameter of the C12mim-DS ion pairs is shifted into a region on the cationic-rich side more favorable for vesicle formation than for the DTA-DS ion pairs. In summary, (37) Herrington, K. L.; Kalar, E. W. J. Phys. Chem. 1993, 97, 13792–13802. (38) Siliva, B. F. B.; Marques, E. F.; Olsson, U.; Pons, R. Langmuir 2010, 26, 3058–3066.

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Figure 3. Negatively stained (phosphotungstic acid) TEM images of birefringent samples from the LR phase: (a) 100 mM C12mimBr and r = 0.6; (b) 100 mM C12mimBr and r = 0.63.

the differences of the geometric packing effects and charge density between the imidazolium and trimethylammonium headgroups induce the different phase behavior of the two systems. Vesicle Formation in the Lr Phase. Figure 3 shows typical TEM images of the vesicles formed in the C12mimBr/SDS LR phase. The presence of aggregates (vesicles) can be readily inferred from the images. The average diameter of the vesicles formed with r = 0.6 is about 100 nm (Figure 3a). When r is increased to 0.63, as shown in Figure 3b, the vesicle diameters become larger, approximately 100-250 nm. The textures of the birefringent LR phase were further characterized by polarized optical microscopy. Supporting Information Figure S1 shows two typical cross-polarized optical images of the LR phase that exhibit the “Maltese Cross” pattern, which indicates the presence of lamellar structures. These lamellar structures include vesicles that were further confirmed by FFTEM. Supporting Information Figure S2 illustrates the vesicles formed with r = 0.63. The sizes of C12mimBr/SDS vesicles are polydisperse with diameters of 100 to 200 nm, consistent with the TEM results. Unfortunately, it is difficult to distinguish whether these vesicles are multi- or unilamellar structures. Vesicle formation in the C12mimBr/SDS aqueous solution is driven by both electrostatic and hydrophobic interactions.12 First, the oppositely charged headgroups, C12mimþ and DS-, can form ion pairs because of the strong electrostatic interaction. Then the C12mimþ-DS- ion pairs form aggregates that grow into vesicles via hydrophobic interactions. On the cationic-rich side of C12mimBr/SDS aqueous solution, the electrostatic repulsion of the excess C12mimþ and steric exclusion make further aggregation of ion pairs difficult. So, vesicles can be formed by the synergistic combination of electrostatic and hydrophobic interactions. The birefringent LR phase is highly viscoelastic and exhibits yield stress. Air bubbles can be trapped in the vesicle solution. The macroscopic properties of the birefringent LR phase were characterized by a rheological study. A rheogram of a typical LR phase sample with r = 0.63 is shown in Figure 4, which indicates that the solution behaves like a Bingham fluid. Across the shear frequency range, the storage modulus G0 and the loss modulus G00 remain more or less constant at about 3 and 0.4 Pa, respectively. G00 is about 1 order of magnitude lower than G0 , and the complex viscosities, |η*|, decrease with a slope of -1 over the whole frequency range. The rheogram of the C12mimBr/SDS LR phase Langmuir 2010, 26(14), 11726–11731

Figure 4. Rheogram of the oscillatory shear parameters of a birefringent LR phase sample (100 mM C12mimBr and r = 0.63; 25.0 ( 0.1 °C).

exhibits essentially the same characteristics as other catanionic surfactant vesicle solutions systems.12 Vesicle-Assisted Preparation of Hollow Silica Spheres. The LR phase at a concentration of 100 mM C12mimBr and r = 0.63 was employed as a template for the synthesis of inorganic materials. Using the vesicles and TEOS in a templated silication reaction, described in the Experimental Section, resulted in hollow silica spheres. A modified St€ober approach was adopted for the synthesis.13,39 Ethanol, produced during the hydrolysis of the TEOS in silication reactions, has two potential effects on the stability of the vesicle templates in the catanionic system. On one hand, ethanol will reduce the hydrophobic interactions between the surfactants, which will damage the vesicles. On the other hand, it will reduce the dielectric constant of the solvent and induce an increase in the electrostatic interactions between the polar head groups, the packing parameter will also increase, which will make the vesicles more stable.40 We have made an approximate calculation about (39) Zhang, L.; Li, P.; Liu, X.; Du, L.; Wang, E. Adv. Mater. 2007, 19, 4279– 4283. (40) Huang, J. B.; Han, F.; Wu, T. Acta Phys. Chim. Sin. 2003, 19, 779–784.

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Figure 5. Typical TEM images of hollow silica spheres prepared at 25.0 ((0.1) °C before (a and b) and after (c and d) calcination.

the dielectric constant change during the reaction. According to the following reaction equation SiðOC2 H5 Þ4 þ 4H2 O ¼ SiðOHÞ4 þ 4C2 H5 OH

ð1Þ

The added 0.036 mL TEOS produces about 0.037 mL ethanol during the silication reaction in 6.15 mL aqueous solution. The ratio of V(water)/V(ethanol) is about 166. Following is the most well-know equation for the calculation of the dielectric constant of a compound41 εRr ¼

X

Vi εRri

ð2Þ

i

where Vi and εri are the volume fraction and the relative dielectric constant of the ith material, and R is a constant. According to the equation, the content of the ethanol in the solution during the reaction is so little that the dielectric change is very small (nearly maintain the value of the water). Generally speaking, the destructive effect of the ethanol is the main factor. Ascorbic acid can accelerate the hydrolysis and condensation of TEOS, but the most appropriate pH range for the silication reaction is 8.0-10.0 as reported in the literature.13 Here, the pH, measured over the whole reaction, is maintained around 9.0 by using a mixture of the ascorbic acid and ammonia, which serves to buffer the solution and may avoid excessive damage to the vesicle templates.39 Typical TEM images of hollow silica spheres prepared using vesicle templates are shown in Figure 5. Figure 5a,b shows the hollow silica spheres before calcination and spheres with a diameter between 30 and 60 nm and wall thickness of about 8 nm are observed. The hollow silica spheres after calcination are shown in Figure 5c,d and possess nearly the same diameters and wall thickness. Supporting Information Figure S3 shows SEM (41) Hippel, A. R. Dielectrics and Waves; Wiley: New York, 1954.

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images of the as-synthesized hollow silica spheres (diameter 30-60 nm). Some broken spheres (indicated by the black arrows) show that the wall thickness is about 10 nm, consistent with the TEM observation. In some reports, the structure of hollow silica spheres prepared with vesicle templates exhibit changes in diameter and shell thickness before and after calcination.39 The small changes observed before and after calcination in the C12mimBr/ SDS vesicles suggest that the hollow silica spheres prepared using ionic liquid vesicle templates may be more stable. We also note that the hollow silica spheres prepared here are obviously smaller than the vesicles themselves (Figure 3 and Supporting Information Figure S2), and some of them are irregular spheres. This will be illustrated in detail below. Nitrogen adsorption-desorption isotherms are used to determine the specific surface area and corresponding BarrettJoyner-Halenda (BJH) pore-size distributions of silicate materials. The isotherm for the calcined hollow silica spheres prepared here are of typical type IV and exhibits a distinct H3 hysteresis in the range 0.5-1.0 P/P0 (Figure 6a). This behavior demonstrates the presence of both mesoporous and macroporous material in the sample according to IUPAC classification.42 Using the BJH method, the corresponding pore size distribution is plotted in Figure 6b. The sharp peak indicates a narrow pore diameter distribution at about 4.0 nm. The pore volume and the BET surface area of the hollow silica spheres prepared here are 1.158 cm3/g and 805.9 m2/g, respectively. This surface area is relatively large compared to previously reported surface areas (280.0-520.0 m2/g) for similarly prepared materials.43 Proposed Mechanism for Vesicle-Assisted Synthesis of Hollow Silica Spheres. A possible mechanism for the formation of hollow silica spheres is illustrated in Figure 7. In the catanionic (42) Wang, W. S.; Zhen, L.; Xu, C. Y.; Shao, W. Z. J. Phys. Chem. C 2008, 112, 14360–14366. (43) Rana, R. K.; Mastai, Y.; Gedanken, A. Adv. Mater. 2002, 14, 1414–1418.

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Figure 7. Proposed mechanism of formation for the hollow silica spheres prepared using vesicle templates.

termination of hydrolysis, hollow silica spheres are formed in the palisade layer of the smaller vesicles. Washing and calcination result in the final hollow silica spheres. We propose that the deformation and size decrease of the vesicles results in the formation of both the most stable hollow silica spheres and some instances of irregular spheres.

Conclusions

Figure 6. Dinitrogen (N2) adsorption-desorption isotherms for calcined hollow silica spheres (a) and their corresponding pore-size distribution curve (b).

C12mimBr/SDS aqueous solution, ion pairs can be formed because of the strong electrostatic interaction between the oppositely charged headgroups, and vesicles are formed with an appropriate cation/anion proportion. The vesicles are fragile and may be deformed easily, and even collapse when the solution conditions are changed.44 Initially, when TEOS is added to the vesicle solution, it will pool in the hydrophobic palisade layer of the vesicles because of its lipophilic nature.45,46 The TEOS then hydrolyzes and concentrates to form a discrete silica layer in the palisade layer. Both the discrete silica layer and ethanol expelled from the hydrolysis of TEOS act to destroy the vesicles. Sustaining hydrolysis and silication in the vesicle phase also may cause the vesicles to divide into smaller more stable structure.39 At the (44) Fan, W.; Gao, L. J. Colloid Interface Sci. 2006, 297, 157–160. (45) Zhao, M. W.; Zheng, L. Q.; Li, N.; Yu, L. Mater. Lett. 2008, 62, 4591–4593. (46) Hubert, D. H. W.; Jung, M.; Frederik, P. M.; Bomans, P. H. H.; Meuldijk, J.; German, A. L. Adv. Mater. 2000, 12, 1291–1294.

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Vesicles are formed in the aqueous catanionic surfactant solution of a long-chain imidazolium ionic liquid, C12mimBr, and SDS on the cationic-rich side of the system. Electrostatic and hydrophobic interactions between the cationic and anionic surfactants are the driving forces for the vesicle formation. Differences in geometric packing and charge density between surfactant headgroups induce different phase behaviors for C12mimBr/SDS and DTAB/SDS aqueous solutions. Hollow silica spheres, with a diameter of 30-60 nm and a wall thickness of 8-10 nm, were prepared using the ILconstructed vesicle templates. We expected that IL vesicles may have wide potential for the synthesis of other nanomaterials. Acknowledgment. The authors thank the National Natural Science Foundation of China (no. 50972080, 20773081) and the National Basic Research Program (2007CB808004, 2009CB930101) for financial support. This work was also partially supported by the Laboratory of Organic Optoelectronic Functional Materials and Molecular Engineering, TIPC, CAS. The authors thank Dr. J. David Van Horn (Visiting Professor, Shandong University) for editorial assistance. Supporting Information Available: SEM images of calcined hollow silica spheres, the polarized images and FFTEM images of the samples from the LR phase are given. This material is available free of charge via the Internet at http://pubs.acs.org.

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