Langmuir 2008, 24, 1031-1036
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Surfactant Solubilization and the Direct Encapsulation of Interfacially Active Phenols in Mesoporous Silicas Grace Tan,†,⊥ Christy Ford,†,⊥ Vijay T. John,*,† Jibao He,‡ Gary L. McPherson,§ and Arijit Bose| Department of Chemical and Biomolecular Engineering, Coordinated Instrumentation Facility, and Department of Chemistry, Tulane UniVersity, New Orleans, Louisiana 70118, and Department of Chemical Engineering, UniVersity of Rhode Island, Kingston, Rhode Island 02881 ReceiVed September 18, 2007. In Final Form: NoVember 20, 2007 The solubilization of phenols in micelles of cetyltrimethyl ammonium bromide leads to microstructural changes from spherical micelles to wormlike micelles and then to vesicles. These microstructures are then used to template silicas. There is a transition from highly ordered hexagonal mesoporous silicas of the M41S family to lamellar structures, as the phenolic dopant concentration is increased. The results have implication to the removal of phenols from aqueous waste streams through the micellar enhanced ultrafiltration process. The entrapment of phenols in mesoporous silicas provides a way to sequester such contaminants in concentrated form.
Introduction Above the critical micelle concentration, surfactant molecules self-assemble in water to form a variety of microstructures such as spheres, discs, biaxial ribbons, rods, lamellar bilayers, or vesicles in dilute solutions. The microstructure is highly dependent on the nature of the surfactant molecules and can be predicted approximately by the value of the packing parameter g ) V/aol, where V is the effective volume of the surfactant tail region, ao, the effective head group area at the micelle surface, and l the surfactant tail length.1 Small values of the packing parameter (g < 1/3) typically lead to the spherical micelles, while at the other extreme (g > 1) inverse micelles are formed. At intermediate values, cylindrical wormlike micelles, lamellar structures, and vesicles are predicted.2 The range of self-assembled microstructures is considerably expanded by the interactions of multiple surfactants with different packing characteristics.3-5 Modifications of microstructure can also be carried out by the introduction of dopants into a given self-assembled system. For example, ionic salts can modulate surfactant microstructure either by counterion binding of the salts to the charged surfactant head groups (in the case of inorganic salts) or by shielding of the electrostatic repulsion between individual charged surfactant head groups (organic salts). The latter is achieved by penetration of the hydrophobic part of the organic salt into the surfactant palisade region. The transition of CTAB micelles from spherical to rodlike or wormlike structures in particular has been widely investigated for ionic salts such as * Corresponding author. Phone: 504-865-5883. E-mail:
[email protected]. † Department of Chemical and Biomolecular Engineering, Tulane University. ‡ Coordinated Instrumentation Facility, Tulane University. § Department of Chemistry, Tulane University. | University of Rhode Island. ⊥ These authors contributed equally to the paper. (1) Israelachvili, J. N.; Mitchell, D. J. Biochim. Biophys. Acta 1975, 389 (1), 13. (2) Israelachvili, J. Intermolecular & Surface Forces, 2nd ed.; Academic Press: San Diego, 1991. (3) Simmons, B.; Li, S.; John, V. T.; McPherson, G. L.; Bose, A.; Zhou, W.; He, J. Nano Lett. 2002, 2 (4), 263. (4) Nieh, M. P.; Raghunathan, V. A.; Glinka, C. J.; Harroun, T. A.; Pabst, G.; Katsaras, J. Langmuir 2004, 20, 7893. (5) Li, X.; Kunieda, H. Curr. Opin. Colloid Interface Sci. 2003, 8, 327.
potassium bromide,6 sodium chloride,7 sodium salicylate,8-10 sodium p-toluene sulfonate,10,11 and sodium butyl benzene sulfonate.12 Nonionic dopants have also been found to alter microstructure, and the literature cites evidence of the use of benzene,13 pentanol,14 and benzyl alcohol.15 We have previously reported the role of 4-ethylphenol, a polar but nonionic dopant on altering the structure of cationic micelles of cetyltrimethylammonium bromide (CTAB).16,17 Due to the low dissociation constant of 4-ethylphenol (pKa ∼ 10), the counterion effect is not expected to play an important role in modulating micelle shape and size. However, the polarity of the molecule influences its incorporation into the surfactant palisade region and affects surfactant self-assembly. We have observed a systematic structural transition from spherical micelles to elongated micelles to globular vesicles as evidenced by cryoTEM imaging of the microstructures.16,18 The phenomenon is common to a range of para-subsituted phenols but the transitions take place at different doping levels of the phenols in CTAB.18 CTAB micelles have two major application areas. First, they are extremely effective in the synthesis of ordered mesoporous silicas of the M41S family19,20 and have been widely studied in (6) Michels, B.; Waton, G. J. Phys. Chem. A 2003, 107 (8), 1133. (7) Ionescu, L. G.; Do Aido, T. H. M.; Kid, B. J. Bol. Soc., Chil. Quim. 1990, 35 (1), 105. (8) Manohar, C.; Rao, U. R. K.; Valaulikar, B. S.; Iyer, R. M. Chem. Commun. 1986, 379. (9) Hassan, P. A.; Bhattacharya, K.; Kulshreshtha, S. K.; Raghavan, S. R. J. Phys. Chem. B 2005, 109 (18), 8744. (10) Hassan, P. A.; Yakhmi, J. V. Langmuir 2000, 16, 7187. (11) Imai, S.; Shikata, T. Langmuir 1999, 15 (23), 7993. (12) Pal, O. R.; Gaikar, V. G.; Joshi, J. V.; Goyal, P S.; Sawal, V. K. Langmuir 2002, 18 (18), 6764. (13) Hedin, N.; Sitnikov, R.; Furo, I.; Henriksson, U.; Regev, O. J. Phys. Chem. B 1999, 103, 9631. (14) Han, S.; Hou, W.; Yan, X.; Li, Z.; Zhang, P.; Li, D. Langmuir 2003, 19, 4269. (15) Zhang, W.-C.; Li, G.-Z.; Mu, J.-H.; Zheng, L.-Q.; Liang, H.-J.; Wu, C. J. Dispersion Sci. Technol. 2000, 21 (5), 605. (16) Singh, M.; Ford, C.; Agarwal, V.; Fritz, G.; Bose, A.; John, V. T.; McPherson, G. L. Langmuir 2004, 20, 9931. (17) Ford, C.; Singh, M.; Lawson, L.; He, J.; John, V.; Lu, Y.; Papadopoulos, K.; McPherson, G.; Bose, A. Colloids Surf., B 2004, 39, 143. (18) Agarwal, V.; Singh, M.; McPherson, G.; John, V.; Bose, A. Colloids Surf., A 2006, 281, 246. (19) Kresge, C.; Leonowicz, M.; Roth, W.; Vartuli, J.; Beck, J. Nature 1992, 359, 710.
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this regard.21-23 Hydrolysis of silica precursors occurs at the micelle-water interface and surfactant charge stabilization of the hydroxylated intermediates causes condensation to occur in the aqueous side of the micelle-water interface. Such interactions lead to ordered silicas of the M41S family that exhibit a range of hexagonal, cubic, and lamellar mesopore morphologies.19,20 Second, CTAB micelles are very useful in trapping environmental contaminants and have applications in environmental remediation. The micellar enhanced ultrafiltration (MEUF) process first described by Scamehorn and co-worker24 is a remarkably successful concept where organic contaminants are allowed to partition into surfactant micelles. The micelles are too large to cross the ultrafiltration membranes, allowing only the surfactant monomer and small amounts of the contaminant to pass through. The retentate is a highly concentrated stream containing surfactant micelles that solubilize the contaminant. The retentate is usually processed in a further step to separate the surfactant from the contaminants. This can be achieved by precipitation of the surfactant or by air/vacuum stripping, depending on the type of contaminants involved. The surfactant-free contaminant stream is then subjected to treatment methods, e.g., biodegradation to clear organics such as phenol.25 The use of the MEUF process for remediation of waste streams containing phenols has been well-documented in the literature.26,27 CTAB micelles are considered viable in MEUF because they are very effective in solubilizing phenols, which are ubiquitous contaminants.28 In this paper, we couple the two concepts. We conduct a systematic study of the use of such phenol-doped CTAB micelles on the synthesis of mesoporous silicas. In addition to using microstructural changes in surfactant self-assembly to modify the characteristics of mesoporous silicas, the concept of sequestering phenols in silicas has application potential in environmental technologies by significantly decreasing the volume production of wastes. Although there are reports on the use of additives to alter the shape of CTAB micelles, there are only a few that have extended this concept to inorganic materials synthesis. Zink and co-workers have demonstrated the ability to control placement of luminescent molecules in specific regions of sol-gel thin films.23,29 White and co-workers investigated surfactant-templated silicate films in the presence of inorganic salts,30 while Han and co-workers14 examined in particular the effect of pentanol in moderating morphology and pore structure of inorganic silica. We envision the route for silica encapsulation of phenolic contaminants as a step in a post-MEUF process to have phenolic contaminants solubilized in CTAB micelles sequestered in silicas and thus removed from solution in concentrated form. Our (20) Beck, J.; Vartuli, J.; Roth, W.; Leonowicz, M.; Kersge, C.; Schmitt, K.; Chu, C.; Olson, D.; Sheppard, E.; McCullen, S.; Higgins, J.; Schlenker, J. J. Am. Chem. Soc. 1992, 114, 10834. (21) Lin, V.; Radu, D.; Han, M.; Deng, W.; Kuroki, S.; Shanks, B.; Prusk, M. J. Am. Chem. Soc. 2002, 124, 9040. (22) Honma, L.; Zhou, H. Chem. Mater. 1998, 10, 103. Lim, M.; Blanford, C.; Stein, A. Chem. Mater. 1998, 10, 467. (23) Hernandez, R.; Franville, A.; Minoofar, P.; Dunn, B.; Zink, J. J. Am. Chem. Soc. 2001, 123, 1248. (24) Christian, S. D.; Scamehorn, J. F. Surfactant-Based Separation Processes; Scamehorn, J. F., Harwell, J. H., Eds.; Marcel Dekker: New York, 1989; Chapters 1-2. (25) Olguı´n-Lora, P.; Razo-Flores, E. J. Chem. Technol. Biotechnol. 2004, 79, 554. (26) Kim, J. H.; Kim, C. K.; Kim, D. W.; Kim, S. S.; Park, S. K.; Lee, M. C.; Lim, J. C. Sep. Sci. Technol. 2003, 38, 1791. (27) Kim, B.-K.; Baek, K.; Yang, J.-W. Water Sci. Technol. 2004, 50, 227. (28) Witek, A.; Koltunieqicz, A.; Kurczewski, B.; Radziejowska, M.; Hatalski, M. Desalination 2006, 191, 111. (29) Minoofar, P. N.; Hernandez, R.; Chia, S.; Dunn, B.; Zink, J. I.; Franville, A.-C. J. Am. Chem. Soc. 2002, 124, 14388. (30) Ruggles, J. L.; Holt, S. A.; Reynolds, P. A.; White, J. W. Langmuir 2000, 16, 4613.
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objectives in this work are to understand how shape changes in micelle microstructure upon doping with phenol affects the morphology of templated silica and to attempt to sequester phenols that are entrapped in CTAB micelles into such mesoporous silicas. Experimental Section Materials. Hexadecyltrimethyl ammonium bromide (CTAB) (99%) was purchased from Sigma-Aldrich Co. Phenol (99.99%), 4-cresol (99%), tetramethoxysilane (TMOS) (98%), sodium hydroxide (98.6%), 4-n-octylphenol (99%), and 4-ethylphenol (99%) were purchased from Aldrich Co. 4-n-Butylphenol (98%) was purchased from Alfa Aesar. All chemicals were used as received. Methods. A 50 mM CTAB (2% w/w) stock solution was prepared using distilled water. Required amounts of the phenolic compounds were dissolved into the micellar solution by stirring with a magnetic stir bar in 20 mL of scintillation vials until isotropic solutions formed at 25 °C. For sol-gel synthesis, 0.625 mL of tetramethoxysilane (TMOS) was added per 10 mL volume of CTAB-phenol sample and stirred to ensure proper dispersion. Later, 120 µL of 50 wt % NaOH was added and the vial contents were agitated as precipitates started appearing in a matter of seconds. The samples were left to stand for 2 h before being filtered with a 0.22 µm filter membrane and the precipitates were washed with hot water. The samples were then dried in air and crushed into fine powder prior to XRD characterization. Samples for surface area analysis through the Brunauer-Emmet-Teller isotherm (BET) were further heated at a ramp rate of 1 °C/min and calcined at 500 °C for 3 h before analysis. Characterization. Powder small-angle X-ray diffraction was performed on a Siemens-500D diffractometer at 40 kV and 30 mA. The diffraction data were recorded at a 0.02° step size for a 2θ range of 1°-10° at a scan speed of 1 deg/min. TEM images were obtained on a JEOL 2011 transmission electron microscope operated at an acceleration voltage of 120 kV. The powdered specimen was dispersed in ethanol and a drop of the sample was placed on a carbon-coated Formvar grid. Mesoporous silica porosity after calcination was determined by nitrogen sorption at 77 K (Micromeritics, ASAP 2010). Samples were degassed at 200 °C for several hours prior to measurements. The Brunauer-Emmet-Teller (BET) equation was used for the determination of specific surface areas. Pore diameters were analyzed using the Barret-JoynerHalenda (BJH) method for the adsorption isotherms.
Results and Discussion A series of para-substituted phenols were used in this study. For convenience, we choose 4-ethylphenol as the model phenol to be elaborated on, although the results for all phenols are presented. 4-Ethylphenol is readily solubilized in CTAB micelles when stirred vigorously at 25 °C, resulting in fully stable solutions at room temperature. At a 4-ethylphenol content of m < 0.67, colorless transparent solutions are obtained, which turn increasingly translucent and turbid with increased 4-ethylphenol content, indicating the presence of larger assemblies in the system. Figure 1 is a schematic of the transformation from spherical micelles to long wormlike micelles to vesicles as the dopant level is increased.16 The cryo-transmission electron micrographs are simply added to clarify the concept, and details of the procedure with full cryo-TEMs of all phenols with CTAB can be found in our recent work.16,18 We note from our previous work that the transitional m values (molar ratio of phenol to CTAB) needed to change from spherical to wormlike micelles, and then to vesicles decreases as the alkyl chain length of the phenol increases. With unsubstituted phenol, even at m ) 1, there is no evidence of vesicle formation and the microstructure is that of wormlike micelles and globular micelles. With 4-ethylphenol, vesicles are observed at m ) 1, and with the butylphenols, vesicles are observed at m values as low as 0.33. These transitions in microstructure can be explained by a combination of hydrogen
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Figure 1. Transitions from spherical micelles to wormlike micelles and then to vesicles upon doping CTAB with increasing levels of 4-ethylphenol. The cryo-TEMS are fully explained in our earlier publications.16,18
Figure 3. p-Ethylphenol silica nanocomposites transition from hexagonal (h) to lamellar (l) structure with increasing levels of p-ethylphenol incorporation. m ) [p-ethylphenol]/[CTAB].
Figure 2. Schematic of the process of phenol solubilization in CTAB micelles followed by encapsulation in mesoporous silica.
bonding and packing parameter arguments. Phenols are strong hydrogen-bonding compounds and the incorporation of phenols into spherical micelles of CTAB tends to rigidify the interface and reduce curvature, leading to the transition from spherical micelles to the surfaces of lower curvature such as wormlike micelles and vesicles. The transition is aided by the effective increase in the tail volume (V) in the packing parameter as the alkyl tail length is increased. The effective tail length (l) is still defined by the tail length of CTAB, while the incorporation of the polar phenolic dopant shields the head group repulsions of the CTAB molecules, which results in a diminished effective head group area, also aiding the increase in the packing parameter. Figure 2 is a schematic of the process of encapsulation, drawn to represent the system at low m values where spherical micelles persist. After doping the CTAB micelles with the phenolic component, the silica precursor is added together with a base to precipitate silicas. At the basic conditions used in silica synthesis, reaction occurs rapidly upon the addition of TMOS, and silica precipitates in a matter of seconds. As shown schematically in Figure 2, the objective is to trap the surfactant and organic
contaminant in mesoporous silicas, leading to sequestration of phenol in a solid. X-ray diffraction structure analysis of the washed 4-ethylphenol silica composites in Figure 3 reveal highly textured hexagonal pore structures at low dopant content (m < 1), which is in agreement with the literature data for undoped CTAB.19 Interestingly, the XRD data show a transition to lamellar silicas at higher 4-ethylphenol dopant levels (m g 1). The TEM image in Figure 4a shows the corresponding images of the 4-ethylphenol silica composite at low ethylphenol content (m ) 0.17) that has been washed and calcined prior to imaging. A highly ordered hexagonal structure is observed clearly resembling MCM-41 mesoporous silica.19,20 Several studies on the morphological transitions during the formation of templated mesoporous materials suggest that adsorbed silicate at the micellar interface may reduce the spontaneous curvature of spherical micelles and induce an attractive interaction between the micelles. These lead to the formation of cylindrical structures and subsequent formation of hexagonal silicas.31-33 As the ethylphenol to CTAB molar ratio is raised to unity, a lamellar structure is prevalent (Figure 4b), which appears to be (31) Gov, N.; Borukhov, I.; Goldfarb, D. Langmuir 2006, 22, 605. (32) Flodstro¨m, K.; Wennerstro¨m, H.; Alfredsson, V. Langmuir 2004, 20, 680. (33) Impe´ror-Clerc, M.; Grillo, I.; Khodakov, A. Y.; Durand, D.; Zholobenko, V. L. Chem. Commun. 2007, 8, 834.
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Figure 4. Transmission electron micrographs of (a) hexagonal p-ethylphenol silica composites at m ) [p-ethylphenol]/[CTAB] ) 0.17 (calcined, scale bar ) 40 nm) and (b) lamellar p-ethylphenol silica composites at m ) 1 (washed with hot water and calcined (inset), scale bar ) 20 nm).
the result of templating by vesicles. Upon calcination at high temperatures, the structure collapses, a characteristic of lamellar mesophases, as shown in the inset to Figure 4b. Thus, the hexagonal to lamellar structural change in the mesoporous silica materials is a consequence of modification of template structure. It is therefore evident that microstructural changes in self-assembly through the addition of phenols affect the final structure of mesoporous silica. For wormlike micelles, the case is not clear. However, there is a tendency for wormlike micelles that are closer to spherical micelles to template toward hexagonal structures, while wormlike micelles closer to forming vesicles would lead to lamellar structured silicas. Figure 5 illustrates the same trends with phenol and with 4-noctylphenol. However, the onset of the lamellar silica mesophase occurs at a lower m value for n-octylphenol doping compared to that for phenol doping. The entire set of data for all phenols is listed in Table 1 which also lists the characteristic d-spacings of the crystalline mesophases. With progress toward longer alkyl chain lengths, the effective tail volume (V) increases. This contributes to a larger packing parameter. Thus, we observe the transition from hexagonal to lamellar silicas at smaller m values. Furthermore, we observe a small, but noticeable, increase in d-spacings with increasing chain length of the dopant. This increase in d-spacing is most noticeable in the lamellar crystalline mesophases increasing from about 3.5 nm in the case of the phenolic dopant to almost 4.5 nm with the n-octylphenol dopant. Again, we attribute this increase in d-spacings to the templating effect of vesicles, where we expect the bilayer of phenol-doped vesicles to be somewhat more compact than the bilayer structure of octylphenol-doped vesicles. The tighter filling of interlamellar pore volumes with the higher molecular weight dopant might contribute to the observed increase in d-spacings.
Figure 5. Comparison of the ordered nanostructured composites of (a) phenol and (b) 4-n-octylphenol at different [phenolic compound]/[CTAB] ) m ratios. The 4-n-octylphenol silica composites attain a fully developed lamellar structure at comparatively lower m values. (h) ) hexagonal; (l) ) lamellar; (wh) ) weakly hexagonal; (wl) ) weakly lamellar.
Surface areas and pore diameters of mesoporous silica can be determined by the Brunauer-Emmet-Teller (BET)34 and the Barret-Joyner-Halenda (BJH)35 methods, respectively. Uncalcined silicas templated by surfactants have negligible porosity since the pores are filled with the templating organic species. Therefore, a calcination procedure is performed to remove organic residues and surfactant. Upon calcination, nitrogen adsorption experiments indicate that the mesoporous silica formed without the dopant exhibits a large surface area (1288 m2/g) and high pore volume (0.84 cm3/g) (Table 2), consistent with that reported for MCM-41.19 Calcination aids condensation of hydrolyzed silica precursors, which solidifies the silica network. This causes shrinkage in the pore diameters to 2.6 nm, as compared to the diameter of CTAB spherical micelles of approximately 5-6 nm, as reported in cryo-TEM and small-angle neutron scattering studies.16,18,36,37 Table 2 also indicates that, with a particular dopant, the surface area drops upon calcination of the lamellar structures, evidently through collapse of the structures. Adsorption-desorption isotherms for silicas synthesized with low dopant (34) Brunauer, S.; Emmet, P. H.; Teller, E. J. Am. Chem. Soc. 1938, 60, 309. (35) Barret, E. P.; Joyner, L. G.; Halenda, P. P. J. Am. Chem. Soc. 1951, 73, 373. (36) Fritz, G.; Bergmann, A.; Glatter, O. J. Chem. Phys. 2000, 113, 9733. (37) Lin, Z.; Davis, H. T.; Scriven, L. E. Langmuir 1996, 12, 5489.
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Table 1. d-Spacings of Phenolic Silica Composites Formed from Different Structural Templates Obtained through Incorporation of Phenolic Solubilizates to a 50 mM CTAB Solution solubilizate phenol
p-cresol
p-ethylphenol
4-n-butylphenol
4-n-octylphenol
[solubilizate]/[CTAB]
d-spacing(hex) (Å)
0 0.17 0.33 0.67 1 3 0.17 0.33 0.67 1 3 0.17 0.33 0.67 1 3 0.17 0.33 0.67 1 3 0.17 0.33 0.67 1 3
contents (m < 0.33) where the structure is hexagonal show characteristics of the Type IV isotherms (Figure 6a). Pore sizes display a narrow distribution and the BJH adsorption pore
d-spacing(lam) (Å)
mesophase
37.83 37.91 37.05 37.24
hexagonal hexagonal hexagonal hexagonal weakly lamellar lamellar hexagonal hexagonal weakly lamellar weakly lamellar lamellar hexagonal hexagonal coexistence lamellar lamellar hexagonal coexistence coexistence lamellar lamellar hexagonal weakly hexagonal lamellar lamellar lamellar
35.05 30.99 39.96 39.40 32.36 32.33 33.48 38.53 39.05 39.57
31.87 34.08 33.68
37.33 40.35 39.41
34.22 35.83 37.64 35.07
39.21 38.26 40.06 43.44 44.90
Table 2. Comparison of BET Surface Area, Pore Volume, and BJH Adsorption Pore Diameter for Calcined p-Ethylphenol Silica Composites
[p-ethylphenol]/[CTAB]
BET surface area (m2/g)
pore volume (cm3/g)
BJH adsorption pore diameter (nm)
0 0.33 1 3
1288 1187 759 841
0.8 0.9 0.7 1.1
2.6 2.9 5.3 6.2
Table 3. BET Surface Area, Pore Volume, and BJH Adsorption Pore Diameter of Phenolic Silica Composites; [solubilizate]/ [CTAB] ) 0.33
solubilizate
BET surface area (m2/g)
pore volume (cm3/g)
BJH adsorption pore diameter (nm)
phenol p-ethylphenol 4-n-octylphenol
1185 1187 1241
0.8 0.9 1.1
2.7 2.9 4.6
diameters range from 2.6 to 2.9 nm with a broader distribution seen with longer alkylphenols (Figure 6b and Table 3). The efficiency of phenol encapsulation was determined by measuring the phenol content remaining in the supernatant after silica encapsulation, through UV absorbance measurements at 277 nm. With 4-ethylphenol as the model contaminant, over 85% of the phenol was removed from solution through encapsulation at m values of unity or less. The encapsulation efficiency dropped to 70% with high concentrations of ethylphenol (m ) 3).
Conclusions Figure 6. (a) Nitrogen adsorption-desorption isotherms for calcined phenol, 4-ethylphenol, and 4-n-octylphenol silica composites at m ) 0.33. (b) Average pore size distribution of the calcined phenol, 4-ethylphenol, and 4-n-octylphenol silica composites at m ) 0.33.
The results indicate the potential to remove phenol-based contaminants in highly concentrated form through partitioning into surfactant microstructures which are further trapped in
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mesoporous silicas. The direct entrapment of phenolic compounds in mesoporous silica provides a route to synthesize useful nanostructured composite materials, which transition from hexagonal to lamellar with increasing solubilizate content. This transition is gradual and can be correlated roughly with the varying micellar template shapes upon increasing the dopant concentration. The spherical CTAB micelles originally elongate onedimensionally forming wormlike micelles up to a certain point before a two-dimensional growth occurs to form multilamellar vesicles. These vesicles template multilamellar structures in silica. The technique of incorporating phenols in surfactants followed by materials synthesis provides a feasible and easy method to separate toxic phenolic pollutants from aqueous waste streams. Silica precipitation of contaminant-containing micelles is almost instantaneous, following which the supernatant can be removed from the retentate stream. Streams with high-surfactant and highcontaminants concentrations are usually the most difficult to treat using the stripping technique. Silica encapsulation may offer a fast and efficient method of removing the contaminant-
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containing surfactant micelles. A second potential application is based on the concept that phenolic compounds in CTAB micelles create confined environments for the enzymatic synthesis of polyphenolics.38,39 Coupling enzymatic polymerization with mesoporous silica synthesis introduces the possibility to create such novel polymer-ceramic nanocomposites. Thus, the concepts presented here have applications to the sequestration of pollutants and the generation of useful materials. Acknowledgment. Funding from the National Science Foundation (Grant 0438463), NASA (NAG-1-02070), and the Environmental Protection Agency (EPA-GR 832374) is gratefully acknowledged. LA702900T (38) Pang, J.; Ford, C.; Tan, G.; McPherson, G.; John, V. T.; Lu, Y. Microporous Mesoporous Mater. 2005, 85, 293. (39) Zheng, T.; Zhan, J.; Pang, J.; Tan, G.; He, J.; McPherson, G.; Lu, Y.; John, V. T. AdV. Mater. 2006, 18, 2735.
