Cylindrical-to-Spherical Shape Transformation of Lecithin Reverse

Therefore, the free movement of the polar head of lecithin is limited and the ...... thanks to synchrotron and in-house small-angle X-rays scattering ...
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Cylindrical-to-Spherical Shape Transformation of Lecithin Reverse Micelles Induced by CO2 Yueju Zhao,† Jianling Zhang,*,† Qian Wang,† Wei Li,† Jianshen Li,† Buxing Han,*,† Zhonghua Wu,‡ Kunhao Zhang,‡ and Zhihong Li‡ †

Beijing National Laboratory for Molecular Sciences, Key Laboratory of Colloid and Interface and Thermodynamics, Institute of Chemistry, Chinese Academy of Sciences, and ‡Beijing Synchrotron Radiation Facility (BSRF), Institute of High Energy Physics, Chinese Academy of Sciences Received December 31, 2009. Revised Manuscript Received February 24, 2010

The effect of CO2 on the microstructure of L-R-phosphatidylcholine (lecithin) reverse micelles was studied. The smallangle X-ray scattering (SAXS) results show that CO2 could induce a cylindrical-to-spherical micellar shape transformation. Fourier transform infrared (FT-IR) and UV-vis techniques were also utilized to investigate intermolecular interactions and micropolarity in the reverse micelles at different CO2 pressures. The reduction of the degree of hydrogen bonding between surfactant headgroups and water with added CO2 was found to be the main reason for the micellar shape transformation. In the absence of CO2, the hydrogen bonding between water and PdO of lecithin forms a linking bridge in the interfacial layer. Therefore, the free movement of the polar head of lecithin is limited and the cylindrical reverse micelles are formed. Upon adding CO2 to the reverse micelles, the hydrogen bonds between lecithin and water in reverse micelles are destroyed, which is favorable to forming spherical micelles. Moreover, the CO2-combined reverse micelles were utilized in the synthesis of silica particles. Rodlike silica nanoparticles were obtained in the absence of CO2, and ellipsoidal and spherical mesoporous silica particles were formed in the presence of CO2. This method of tuning micellar shape has many advantages compared to traditional methods.

Introduction Reverse micelles have different structures in nonpolar solvents, such as spherical, rodlike, cylindrical, wormlike, disklike, ellipsoidal, and so forth. The functions and properties of surfactant systems depend strongly on their microstructures. Reverse micelles with different microstructures have attracted significant interest because of their tremendous applications in protein delivery,1,2 drug release,3 enzyme catalysis,4 and especially as relevant templates for the synthesis of nanomaterials with different morphologies.5-7 Therefore, controlling the shape of reverse micellar aggregation is important from both fundamental and application viewpoints. Usually, the micellar shape transformation can be accomplished by varying the solvent,8,9 *Corresponding authors. E-mail: [email protected]; [email protected]. Tel: 86-10-62562821. Fax: 86-10-62559373. (1) Lee, Y.; Ishii, T.; Cabral, H.; Kim, H. J.; Seo, J. H.; Nishiyama, N.; Oshima, H.; Osada, K.; Kataoka, K. Angew. Chem., Int. Ed. 2009, 48, 5309. (2) Nasongkla, X.; Shuai, H. A.; Weinberg, B. D.; Pink, J.; Boothman, D. A.; Gao, J. M. Angew. Chem., Int. Ed. 2004, 43, 6323. (3) Tan, J. P. K.; Kim, S. H.; Nederberg, F.; Appel, E. A.; Waymouth, R. M.; Zhang, Y.; Hedrick, J. L.; Yang, Y. Y. Small 2009, 5, 1504. (4) Mitra, R. N.; Dasgupta, A.; Das, D.; Roy, S.; Debnath, S.; Das, P. K. Langmuir 2005, 21, 12115. (5) Hillmyer, M. A. Science 2007, 317, 604. (6) Simmons, B. A.; Li, S. C.; John, V. T.; McPherson, G. L.; Bose, A.; Zhou, W. L.; He, J. B. Nano Lett. 2002, 2, 263. (7) Wang, M. F.; Kumar, S.; Lee, A.; Felorzabihi, N.; Shen, L.; Zhao, F.; Froimowicz, P.; Scholes, G. D.; Winnik, M. A. J. Am. Chem. Soc. 2008, 130, 9481. (8) Bang, J.; Jain, S. M.; Li, Z. B.; Lodge, T. P.; Pedersen, J. S.; Kesselman, E.; Talmon, Y. Macromolecules 2006, 39, 1199. (9) Huang, H. Y.; Hoogenboom, R.; Leenen, M. A. M.; Guillet, P.; Jonas, A. M.; Schubert, U. S.; Gohy, J. F. J. Am. Chem. Soc. 2006, 128, 3784. (10) Liu, M. C.; Sheu, H. S.; Cheng, S. J. Am. Chem. Soc. 2009, 131, 3998. (11) Liang, X. F.; Guo, C.; Ma, J. H.; Wang, J.; Chen, S.; Liu, H. Z. J. Phys. Chem. B 2007, 111, 13217. (12) Marsden, H. R.; Korobko, A. V.; van Leeuwen, E. N. M.; Pouget, E. M.; Veen, S. J.; Sommerdijk, N. A. J. M.; Kros, A. J. Am. Chem. Soc. 2008, 130, 9386. (13) Teixeira, C. V.; Itri, R.; do Amaral, L. Q. Langmuir 2000, 16, 6102. (14) Ryu, J. H.; Lee, E.; Lim, Y. B.; Lee, M. J. Am. Chem. Soc. 2007, 129, 4808.

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changing the temperature,10-12 adding organic additives13,14 or salts,15 and other methods.16 These processes are disadvantageous in that they suffer from economic and environmental costs or require a high energy input. Developing effective, controllable, and environmentally benign methods of tuning the micellar shape is challenging and of great importance. As an attractive green solvent, supercritical or compressed CO2 has been widely used in a variety of chemical and industrial processes because it is plentiful, inexpensive, and nontoxic and its physical properties can be changed continuously by pressure and/or temperature.17-19 In particular, supercritical or compressed CO2 can expand organic solvents, and the properties of organic solvents can be changed considerably by the dissolution of CO2.20,21 This unique feature of CO2 makes it possible to tune the formation of reverse micelles in organic solvents through an effective, controllable, economical, and environmentally benign route. It has been shown that compressed CO2 is efficient in tuning the microproperties of reverse micelles,22 increasing the solubilization capacity of reverse micelles to water, protein, and inorganic salt,23-25 (15) Tung, S. H.; Lee, H. Y.; Raghavan, S. R. J. Am. Chem. Soc. 2008, 130, 8813. (16) Chen, Q. J.; Zhao, H.; Ming, T.; Wang, J. F.; Wu, C. J. Am. Chem. Soc. 2009, 131, 16650. (17) Jacobson, G. B.; Lee, C. T.; Johnston, K. P.; Tumas, W. J. Am. Chem. Soc. 1999, 121, 11902. (18) Theyssen, N.; Hou, Z. S.; Leitner, W. Chem.;Eur. J. 2006, 12, 3401. (19) Anand, M.; Odom, L. A.; Roberts, C. B. Langmuir 2007, 23, 7338. (20) Jessop, P. G.; Subramaniam, B. Chem. Rev. 2007, 107, 2666. (21) Wei, M.; Musie, G. T.; Busch, D. H.; Subramaniam, B. J. Am. Chem. Soc. 2002, 124, 2513. (22) Zhang, J. L.; Han, B. X.; Liu, J. C.; Zhang, X. G.; Yang, G. Y.; He, J.; Liu, Z. M.; Jiang, T. J. Chem. Phys. 2003, 118, 3329. (23) Zhao, Y. J.; Zhang, J. L.; Han, B. X.; Zhang, C. X.; Li, W.; Feng, X. Y.; Hou, M. Q.; Yang, G. Y. Langmuir 2008, 24, 9328. (24) Shen, D.; Han, B. X.; Dong, Y.; Wu, W. Z.; Chen, J. W.; Zhang, J. L. Chem.;Eur. J. 2005, 11, 1228. (25) Liu, D. X.; Zhang, J. L.; Han, B. X.; Fan, J. F.; Mu, T. C.; Liu, Z. M.; Wu, W. Z.; Chen, J. J. Chem. Phys. 2003, 119, 4873.

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and synthesizing polymer microspheres through the micellization of polymer surfactants in supercritical carbon dioxide.26,27 Recently, the surfactant aggregation in CO2/heptane solvent mixtures was studied.28 L-R-Phosphatidylcholine (lecithin) is a major component of cell membranes and is a very useful biosurfactant. The reverse micelles of lecithin have attracted much attention not only because of their special properties but also for their potential in studying protein conformations and drug bindings.29-31 It is well known that lecithin is able to form cylindrical aggregates in organic solvents upon the addition of small quantities of water because of the hydrogen bonding between the phosphate of lecithin and water.32-35 In previous work,23 we found that CO2 could influence the critical micelle concentration of lecithin in cyclohexane and the solubilization of water, lysozyme, and PdCl2 in lecithin reverse micelles. In this work, we investigated the effect of CO2 on the shape and microstructure of water/lecithin/cyclohexane reverse micelles. It is demonstrated that the micellar shape can be tuned efficiently by compressed CO2 from cylindrical to spherical through an ellipsoidal micellar shape. The possible mechanism for the CO2-induced shape transformation of reverse micelles was proposed on the basis of the results of FT-IR and UV-vis spectra. Furthermore, silica particles with different morphologies were synthesized in CO2-combined lecithin reverse micellar solutions. This method of tuning micellar shape has many merits comparing to traditional methods. For example, the properties of reverse micelles can be regulated by the easy control of CO2 pressure; CO2 can be easily removed by depressurization, which makes postprocessing easier in comparison with the addition of conventional additives such as cosurfactants and salts, which usually cause contamination or a modification of the products; this method is environmentally benign.

Results and Discussion CO2 can dissolve in water/lecithin/cyclohexane solutions ([lecithin] = 40 mM) and expands them. (See the curve of the volume-expansion coefficient vs pressure shown in Figure S1.) When the pressure of CO2 was high enough, the reverse micellar solution became cloudy. The main reason is that the solvent strength is reduced markedly at high CO2 pressure, resulting in the precipitation of surfactant from the micellar solution. The pressure at which the micellar solution becomes cloudy is defined as the cloud-point pressure. For the water/lecithin/cyclohexane reverse micellar solution ([lecithin] = 40 mM) with W0 = 5 and 7, the cloud-point pressures determined are 5.42 and 5.37 MPa, respectively. To avoid phase separation, all of the following experiments were conducted below the cloud-point pressure. (26) Yoshida, E.; Mineyama, A. Colloid Polym. Sci. 2008, 286, 975. (27) Yoshida, E.; Mineyama, A. Colloid Polym. Sci. 2007, 285, 441. (28) Hollamby, M. J.; Trickett, K.; Mohamed, A.; Eastoe, J.; Rogers, S. E.; Heenan, R. K. Langmuir 2009, 25, 12909. (29) Mureseanu, M.; Galarneau, A.; Renard, G.; Fajula, F. Langmuir 2005, 21, 4648. (30) Avramiotis, S.; Papadimitriou, V.; Hatzara, E.; Bekiari, V.; Lianos, P.; Xenakis, A. Langmuir 2007, 23, 4438. (31) Van Horn, W. D.; Simorellis, A. K.; Flynn, P. F. J. Am. Chem. Soc. 2005, 127, 13553. (32) Angelico, R.; Ceglie, A.; Colafemmina, G.; Lopez, F.; Murgia, S.; Olsson, U.; Palazzo, G. Langmuir 2005, 21, 140. (33) Angelico, R.; Ceglie, A.; Colafemmina, G.; Delfine, F.; Olsson, U.; Palazzo, G. Langmuir 2004, 20, 619. (34) Arleth, L.; Bauer, R.; Ogendal, L. H.; Egelhaaf, S. U.; Schurtenberger, P.; Pedersen, J. S. Langmuir 2003, 19, 4096. (35) Cirkel, P. A.; Koper, G. J. M. Langmuir 1998, 14, 7095. (36) Shrestha, L. K.; Glatter, O.; Aramaki, K. J. Phys. Chem. B 2009, 113, 6290. (37) Shrestha, L. K.; Shrestha, R. G.; Varade, D.; Aramaki, K. Langmuir 2009, 25, 4435.

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Figure 1. Experimental SAXS profiles (A) and pair distance distribution function (p(r)) (B) of a water/lecithin/cyclohexane solution ([lecithin] = 40 mM, W0 = 7) at different CO2 pressures and 303.2 K. The arrow in plot B indicates the maximum core diameter (Dmax).

SAXS is a very useful tool for studying micellar size and shape.36-38 Figure 1A shows the typical SAXS curves of lecithin reverse micellar solutions ([lecithin] = 40 mM, W0 = 7) at different CO2 pressures. The forward-scattering intensity decreases with increasing CO2 pressure, indicating that the micellar size is decreased.36-38 The generalized indirect Fourier transformation (GIFT) gives the pair-distance distribution function, p(r), which is usually utilized to characterize the basic geometry of the aggregates (such as spherical, cylindrical, planar, etc.) in micellar systems.36,37 Figure 1B represents the pair-distance distribution function curves corresponding to the SAXS curves shown in Figure 1A. In the absence of CO2, the p(r) curve of the water/ lecithin/cyclohexane reverse micelle is asymmetrical, with a pronounced peak on the low-r side and an extended long tail on the high-r side, which is characteristic of rodlike or cylindrical particles.36-38 Its maximum is 4.6 nm, which is much smaller than the maximum core diameter (Dmax, marked in Figure 1B, 29 nm). This indicates that the lecithin reverse micelles with W0 = 7 present cylindrical microstructure, consistent with the results reported in the literature in which the lecithin reverse micelles are cylindrical when W0 > 3.39 Upon the addition of CO2 to the reverse micellar solution, the p(r) curves become more and more symmetric, as shown in Figure 1B. As the pressure is increased to 1.87 and 2.98 MPa, the Dmax values of the p(r) curve are decreased to 12 and 8.9 nm and the maxima are 4.4 and 3.7 nm, respectively. This demonstrates that the reverse micelles at these pressures are ellipsoidal. At 4.02 MPa, the p(r) curve is symmetric, bell-shaped, with a maximum of 3.1 nm and a Dmax of 6.8 nm, indicating that the reverse micelles are spherical. Moreover, the SAXS curves of (38) Sommer, C.; Deen, G. R.; Pedersen, J. S.; Strunz, P.; Garamus, V. M. Langmuir 2007, 23, 6544. (39) Schurtenberger, P.; Cavaco, C. Langmuir 1994, 10, 100.

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Figure 2. FT-IR spectra of the PdO stretching band in a water/ lecithin/cyclohexane solution ([lecithin] = 40 mM, W0 = 7) at different CO2 pressures and 303.2 K.

the reverse micellar solutions with W0 = 5 at different CO2 pressures were determined (shown in Figure S2). It was found that the cylindrical reverse micelles with W0 = 5 also could be changed to spherical reverse micelles by CO2. A schematic illustration of the cylindrical-to-spherical shape transformation of reverse micelles is shown in Scheme S1. FT-IR is a commonly used technique for studying the microstructure of reverse micelles,40 and lecithin reverse micelles in the absence of CO2 have been investigated by FT-IR spectroscopy.41 A typical feature is its potential to reveal specific interaction sites on phospholipids, which are responsible for phase transitions, structural and/or conformational changes, and other physical processes. Herein, FT-IR spectroscopy was used to obtain information on the structure change in lecithin reverse micelles induced by CO2. We focused on the PdO vibrational band because of the lack of interferences from solvent bands in this region, and the change in the PdO vibrational band provides evidence for the change in the lecithin reverse micellar structure.40-42 Figure 2 shows the IR spectra of the PdO stretching mode at different CO2 pressures in water/lecithin/cyclohexane reverse micellar solution ([lecithin] = 40 mM, W0 = 7). Evidently, the PdO stretching frequency is blue-shifted with increasing CO2 pressure. In the absence of CO2, the PdO vibrational band of the lecithin molecule is centered at 1251 cm-1, similar to the value reported by other authors.43 As the pressure is increased to 4.01 MPa, it gradually moves to 1265 cm-1. It is well known that the vibrational band of PdO of lecithin is dependent on the hydrogen bonding between the surfactant and water (i.e., the stretching band of PdO moves to a lower frequency as a result of hydrogen bonding).43 The addition of CO2 to the micellar solution may affect the vibrational band in two ways. First, according to previous study,44 CO2 is effective in removing water from silica. Herein, it is expected that the degree of hydrogen bonding between PdO and water would be reduced by adding CO2 to the lecithin reverse micellar solution, which may result in the shift of the PdO stretching band to higher frequency. Second, CO2 can be inserted into the surfactant interface of the reverse micelles and may decrease the micropolarity of the environment around the PdO group because CO2 is a nonpolar small molecule,22 which may also increase the stretching frequency. To clarify which factor plays a key role in the change in the PdO stretching frequency in water/lecithin/cyclohexane reverse micelles, the FT-IR spectra of lecithin/cyclohexane “dry” reverse (40) Zhang, J.; White, G. L.; Fulton, J. L. J. Phys. Chem. 1995, 99, 5540. (41) Hiibner, W.; Mantsch, H. H.; Paltauf, F.; Hauser, H. Biochemistry 1994, 33, 320. (42) Cavallaro, G.; La Manna, G.; Turco Liveri, V.; Aliotta, F.; Fontanella, M. E. J. Colloid Interface Sci. 1995, 176, 281. (43) Shervani, Z.; Jain, T. K.; Maitra, A. Colloid Polym. Sci. 1991, 269, 270. (44) Dickson, J. L.; Gupta, G.; Horozov, T. S.; Binks, B. P.; Johnston, K. P. Langmuir 2006, 22, 2161.

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Figure 3. UV-vis spectra of MO in a water/lecithin/cyclohexane solution ([lecithin] = 40 mM, W0 = 7) at different CO2 pressures and 303.2 K.

micellar solution at different CO2 pressures were investigated (Figure S3). In dry micellar solution, the hydrogen bonding between the surfactant and water is absent and thus its effect on the PdO stretching frequency can be ruled out and the insertion of CO2 can be solely taken into consideration. The results show that the PdO stretching frequency is slightly blue-shifted from 1261 cm-1 (without CO2) to 1263 cm-1 (4.01 MPa). In comparison with Figure 2, it can be seen that the blue-shift value in the dry lecithin reverse micelles is considerably smaller than that in the reverse micelles with W0 = 7. Therefore, it can be deduced that the change in hydrogen bonding between water and lecithin is a dominant factor in the movement of the PdO stretching frequency. In other words, the addition of CO2 induces the breakage of the hydrogen bonding between the hydrophilic headgroup of lecithin and water. The stretching band of PdO in reverse micellar solution at W0 = 7 and 4.01 MPa (1265 cm-1) is close to that in dry reverse micellar solution at the same pressure (1263 cm-1). This suggests that a considerable number of hydrogen bonds between lecithin and water in reverse micellar solution are broken at this pressure. The micropolarity of the lecithin reverse micelles at different CO2 pressures was determined by UV-vis spectra, using MO as a probe.45,46 Our experiments indicated that hydrophilic MO is not soluble in a cyclohexane/CO2 mixture, as evidenced by the fact that the absorbance of MO in the mixed solvent is negligible. Figure 3 shows the UV-vis spectra of MO in a water/lecithin/ cyclohexane reverse micellar solution ([lecithin] = 40 mM, W0 = 7) at different CO2 pressures. Evidently, the absorption of MO is blue-shifted by the increased CO2 pressure. In the absence of CO2, the maximum absorption wavelength (λmax) of MO is 416 nm. As the CO2 pressure is increased to 4.01 MPa, the λmax of MO is shifted to 408 nm. This suggests that the micropolarity of the surfactant interface decreases with increasing CO2 pressure. For comparison, the UV-vis spectra of MO in the lecithin/cyclohexane dry reverse micelles at different CO2 pressures were investigated (Figure S4). With increasing CO2 pressure, λmax is slightly blue-shifted from 408 nm (without CO2) to 407 nm (4.03 MPa). This indicates that the effect of CO2 insertion on the micropolarity of the surfactant interface is not remarkable. Therefore, it can be deduced that CO2 insertion is not the main reason for the remarkable micropolarity decrease at the surfactant interface of water/lecithin/cyclohexane reverse micelles ([lecithin] = 40 mM, W0 = 7) shown in Figure 3. The reduced (45) Johnston, K. P.; Harrison, K. L.; Clarke, M. J.; Howdle, S. M.; Heitz, M. P.; Bright, F. V.; Carlier, C.; Randolph, T. W. Science 1996, 271, 624. (46) Li, N.; Gao, Y.; Zheng, L. Q.; Zhang, J.; Yu, L.; Li., X. W. Langmuir 2007, 23, 1091.

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Figure 4. SEM and TEM images of silica particles obtained in a water/lecithin/cyclohexane solution ([lecithin] = 40 mM, W0 = 7) without CO2 (a-c) and at a CO2 pressure of 4.01 MPa (d-f). Scheme 1. Illustration of the Interaction of Lecithin with Water in the Interface of a CO2-Free Cylindrical Reverse Micelle (a) with a CO2-Combined Spherical Reverse Micelle (b)

number of hydrogen bonds of the polar headgroup of lecithin with water may be responsible for the decreased micropolarity of the surfactant interface, which is consistent with the above results of the IR study. The above results show that CO2 can induce the cylindricalto-spherical shape transformation of lecithin reverse micelles, during which some of the hydrogen bonds between the headgroup of surfactant and water are broken and the micropolarity in the surfactant interface is decreased. On the basis of these studies, a possible mechanism for the cylindrical-to-spherical shape transformation of the lecithin reverse micelles induced by CO2 is proposed, as shown in Scheme 1. In the absence of CO2, cylindrical reverse micelles are formed because of the hydrogen bonding of lecithin with water,31-34 as illustrated in Scheme 1a. In the interface of the lecithin reverse micelles, water is hydrogen bonded with PdO of lecithin, forming a linking bridge. Therefore, cylindrical reverse micelles are formed, and the free movement of 4584 DOI: 10.1021/la904917n

the polar head of lecithin is limited. Upon adding CO2 to the reverse micelles, some hydrogen bonds of water with PdO are destroyed, which is proven by FT-IR and UV-vis spectra. This results in the partial breakage of the linking bridge, and the ellipsoidal reverse micelles are formed. With further increasing CO2 pressure, more hydrogen bonds between lecithin and water in reverse micelles are destroyed, which is favorable for forming spherical micelles (Scheme 1b). Reverse micelles have been widely used in the synthesis of different materials.5-7 Herein, we prepared silica particles in a lecithin reverse micellar solution at different CO2 pressures. In the absence of CO2, lecithin/cyclohexane solutions containing an HCl aqueous solution (pH 3) were mixed with lecithin/cyclohexane solutions containing a sodium silicate aqueous solution ([Na2SiO3] = 0.2 M) in a 1:1 volume ratio and stirred for 24 h. Rodlike silica nanoparticles were obtained with a width of about 30 nm and a length in the range of 100-250 nm (Figure 4a-c). Langmuir 2010, 26(7), 4581–4585

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For the preparation of silica in the presence of CO2, no additional acid was added because CO2 can dissolve in water and form carbonic acid, which can be used as a special reactant for silica formation.47,48 At a pressure of 4.01 MPa, uniform spherical silica particles with a diameter of 75 ( 5 nm were obtained, as shown in Figure 4d,e. From the magnified TEM image shown in Figure 4f, it is evident that these silica particles are mesoporous. Similarly, mesoporous silica particles were also obtained at 2.01 MPa (Figure S5). All of the synthesized samples are amorphous silica, which are known from the powder X-ray diffraction pattern (Figure S6). The formation of mesoporous silica in the presence of CO2 is very interesting. During the addition of CO2 to the reverse micelles containing a sodium silicate aqueous solution, some CO2 molecules hydrolyze and react with sodium silicate to form silica gels. Simultaneously, other CO2 may be absorbed or trapped in the silica gels, which is released from the gels after the system is depressurized, thus creating the pores in the gels. These mesoporous silica particles may find potential applications in the fields of separation and catalysis.

Conclusions It was found that CO2 could induce the cylindrical-to-spherical shape transformation of lecithin reverse micelles. The maximum core diameter of reverse micelles decreased from 29 nm (without (47) Zhang, J. L.; Liu, Z. M.; Han, B. X.; Wang, Y.; Li, Z. H.; Y, G. Y. Microporous Mesoporous Mater. 2005, 87, 10. (48) Chattopadhyay, P.; Gupta, R. B. Ind. Eng. Chem. Res. 2003, 42, 465.

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CO2) to 6.8 nm (4.02 MPa). The results of FT-IR studies demonstrate that the breakage of hydrogen bonds between water and surfactant headgroups plays an important role in the micellar shape transformation. Furthermore, a UV-vis spectra investigation reveals that the micropolarity of the surfactant interface decreases with increasing CO2 pressure, which is consistent with the reduced number of hydrogen bonds between the polar headgroup of lecithin and water. The CO2-combined reverse micelles were used in the synthesis of porous silica particles, of which the size and shape can be easily tuned with CO2 pressure. In comparison to the conventional methods used to tune the micellar shape, this method has some advantages with respect to applications. For example, the transformation process can be easily controlled by the CO2 pressure, CO2 can be easily removed from the micellar system, and this method is environmentally benign. We believe that this method can be easily applied to the synthesis of other materials with controlled morphology. Acknowledgment. We acknowledge the National Natural Science Foundation of China (20633080 and 20873164), the Ministry of Science and Technology of China (2009CB930802), the Chinese Academy of Sciences (KJCX2.YW.H16), and the support of the K. C. Wong Education Foundation, Hong Kong. Supporting Information Available: Materials and experimental section details and supplementary figures. This material is available free of charge via the Internet at http://pubs.acs.org.

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