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Microemulsion Templates for Mesoporous Silica Patrick Schmidt-Winkel,†,‡ Charles J. Glinka,§ and Galen D. Stucky*,†,‡ Department of Chemistry and Materials Research Laboratory, University of California, Santa Barbara, California 93106, and Cold Neutron Research Facility, National Institute of Standards and Technology, Gaithersburg, Maryland 20899 Received June 1, 1999. In Final Form: August 25, 1999 Small-angle neutron scattering (SANS) studies indicate that oil-in-water microemulsions, consisting of aqueous HCl, the nonionic block copolymer surfactant Pluronic P123 (poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide), EO20-PO70-EO20, Mav ) 5800), 1,3,5-trimethylbenzene (TMB, oil), and ethanol (cosurfactant), are novel colloidal templates that direct the synthesis of mesoporous silica with well-defined ultralarge pores. The sizes of the microemulsion droplets can be controlled by the TMB concentration and by temperature. The microemulsion droplet sizes and the cell sizes of the mesostructured cellular foam (MCF) materials increase linearly with the cube root of the TMB concentration. Increasing the temperature from 40 to 80 °C expands the droplet sizes, which is similar to micellar solutions of Pluronic surfactants in the absence of oil. Ethanol acts as a cosurfactant, increases the TMB solubility of the P123 micelles, and enables swelling of the P123 micelles. Low concentrations of NH4F (8 × 10-3 mol/L) show no significant effect upon the nature of the microemulsions. The polydispersities of the droplet sizes range from 11% to 21%. The microemulsion templates reported in this paper are considered as a valuable addition to existing colloidal templates that direct the synthesis of porous materials. The benefits of the microemulsion templates are (i) their easy preparation by simply mixing water, surfactant, oil, and a cosurfactant, and (ii) the synthesis of ultralarge-pore mesoporous materials with narrow pore size distributions without the need for further processing.
Introduction Molecular sieves with uniform and well-defined pores are promising materials for a variety of possible applications such as catalysts, supports for catalysts, hosts for chemical reactions, use in separation, immobilization or encapsulation involving large molecules, etc.1 Mesoporous materials with pore sizes from 2 to 50 nm appear potentially useful for these purposes.2 Most procedures for preparing mesoporous materials are based on the selfaggregation properties of various kinds of surfactants.2 Amphiphilic block copolymers have turned out to be valuable supramolecular templates for mesostructured materials possessing long-range order.3 Aside from employing different types of surfactants,2 the pore sizes of * To whom correspondence should be addressed. Telephone: (805) 893-4872. Fax: (805) 893-4120. E-mail:
[email protected]. † Department of Chemistry, UCSB. ‡ Materials Research Laboratory, UCSB. § Center for Cold Neutron Research, NIST. (1) (a) Perry, R. H.; Green, D. In Perry’s Chemical Engineers’ Handbook; Perry, R. H., Green, D., Eds.; McGraw-Hill: New York, 1984. (b) Hearn, M. T. W. In HPLC of Proteins, Peptides and Polynucleotides; VCH: New York, 1991. (c) Davis, M. E. Chem. Ind. 1992, 4, 137. (2) For recent reviews, see: (a) Ying, J. Y.; Mehnert, C. P.; Wong, M. S. Angew. Chem Int. Ed. 1999, 38, 56. (b) Ciesla, U.; Schu¨th, F. Microporous Mesoporous Mater. 1999, 27, 131. (3) (a) Bagshaw, S. A.; Prouzet, E.; Pinnavaia, T. J. Science 1995, 269, 1242. (b) Templin, M.; Franck, A.; Du Chesne, A.; Leist, H.; Zhang, Y.; Ulrich, R.; Scha¨dler, V.; Wiesner, U. Science 1997, 278, 1795. (c) Zhao, D.; Feng, J.; Huo, Q.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; Stucky, G. D. Science 1998, 279, 548. (d) Zhao, D.; Huo, Q.; Feng, J.; Chmelka, B. F.; Stucky, G. D. J. Am. Chem. Soc. 1998, 120, 6024. (e) Go¨ltner, C. G.; Henke, S.; Weissenberger, M. C.; Antonietti, M. Angew. Chem. Int. Ed. 1998, 37, 613. (f) Kra¨mer, E.; Fo¨rster, S.; Go¨ltner, C.; Antonietti, M. Langmuir 1998, 14, 2027. (g) Go¨ltner, C. G.; Berton, B.; Kra¨mer, E.; Antonietti, M. Chem. Commun. 1998, 2287. (h) Zhao, D.; Yang, P.; Melosh, N.; Feng, J.; Chmelka, B. F.; Stucky, G. D. Adv. Mater. 1998, 10, 1380. (i) Yang, P.; Zhao, D.; Chmelka, B. F.; Stucky, G. D. Chem. Mater. 1998, 10, 2033. (j) Ulrich, R.; Du Chesne, A.; Templin, M.; Wiesner, U. Adv. Mater. 1999, 11, 141. (k) Schmidt-Winkel, P.; Yang, P.; Margolese, D. I.; Chmelka, B. F.; Stucky, G. D. Adv. Mater. 1999, 11, 303.
mesoporous materials have been controlled by changing the hydrophobic volumes of the templates, which may be altered by changing the reaction temperature or by adding organic cosolvents as swelling agents.3c,d,4 More recently developed strategies for obtaining well-defined macroporous and large-pore mesoporous materials use colloidal templating methods, in which either emulsions5 or polymer latex spheres6 are utilized as templates. Our goal was to come up with a simple and efficient route to prepare mesoporous materials with well-defined ultralarge pores (30-50 nm), which are difficult to obtain by existing synthesis methods.3f,6c We were inspired by the promising developments in the preparation of well-defined, mainly macroporous materials obtained by colloidal templating techniques.5b,c,6 Despite their great templating capabilities, the colloids used to prepare porous materials with narrow size distributions are rather difficult or time-consuming to prepare.5b,c,6 For instance, emulsion droplets can be used as templates for the syntheses of macroporous oxides and organic polymers.5 However, to produce porous materials with narrow pore size distributions, the metastable emulsion droplets must be fractionated. Supramolecular (4) (a) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T. W.; Olson, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834. (b) Huo, Q.; Leon, R.; Petroff, P. M.; Stucky, G. D. Science 1995, 268, 1324. (c) Prouzet, E.; Pinnavaia, T. J. Angew. Chem., Int. Ed. Engl. 1997, 36, 516. (5) (a) Schacht, S.; Huo, Q.; Voigt-Martin, I. G.; Stucky, G. D.; Schu¨th, F. Science 1996, 273, 768. (b) Imhof, A.; Pine, D. J. Nature 1997, 389, 948. (c) Imhof, A.; Pine, D. J. Adv. Mater. 1998, 10, 697. (d) Imhof, A.; Pine, D. J. Adv. Mater. 1999, 11, 311. (6) (a) Velev, O. D.; Jede, T. A.; Lobo, R. F.; Lenhoff, A. M. Nature 1997, 389, 447. (b) Velev, O. D.; Jede, T. A.; Lobo, R. F.; Lenhoff, A. M. Chem. Mater. 1998, 10, 3597. (c) Antonietti, M.; Berton, B.; Go¨ltner, C.; Hentze, H.-P. Adv. Mater. 1998, 10, 154. (d) Holland, B. T.; Blanford, C. F.; Stein, A. Science 1998, 281, 538. (e) Yang, P.; Deng, T.; Zhao, D.; Feng, P.; Pine, D.; Chmelka, B. F.; Whitesides, G. M.; Stucky, G. D. Science 1998, 282, 2244. (f) Davey, R. J.; Alison, H.; Cilliers, J. J.; Garside, J. Chem. Commun. 1998, 2581. (g) Holland, B. T.; Blanford, C. F.; Do, T.; Stein, A. Chem. Mater. 1999, 11, 795. (h) Holland, B. T.; Abrams, L.; Stein, A. J. Am. Chem. Soc. 1999, 121, 4308.
10.1021/la9906774 CCC: $19.00 © 2000 American Chemical Society Published on Web 10/27/1999
Microemulsion Templates for Mesoporous Silica
templating techniques allow the formation of thermally and mechanically robust, highly ordered, periodic mesoporous materials with pore sizes greater than 25 nm if nonionic block copolymer surfactants are used.3c,d To achieve our stated goal, we set out to combine the supramolecular templating route based on commercially available and environmentally benign block copolymer surfactants3a,c,d,k and the colloidal templating approach using emulsion droplets5 to create mesoporous materials with well-defined pores in the size range from 25 to 50 nm via a simple synthesis. We have recently extended the colloidal templating techniques for porous solids to include thermodynamically stable microemulsions7 as templates for ultralarge-pore mesoporous silica with well-defined pores.8 This method has enabled the simple, time- and cost-efficient preparation of mesostructured cellular foam (MCF) materials that represent a new class of aerogellike, three-dimensional, continuous, large-pore mesoporous materials, which have the additional benefit of uniformly sized and shaped pores whose size can be easily controlled.8 Here we wish to describe the nature of the templating oil-in-water (o/w) microemulsions that have been studied by small-angle neutron scattering (SANS). Experimental Section Measurements. The SANS measurements were carried out on the 30-m SANS instrument on neutron guide NG7 at NIST’s Center for Neutron Research.9 This instrument utilizes a mechanical velocity selector as a monochromator, circular pinhole collimation, and a twodimensional position-sensitive detector (65 × 65 cm2) to record data over a range of angles simultaneously. Data were taken with λ ) 6 Å neutrons and sample-to-detector distances of 2, 4, and 11 m, which covered a range of scattering vectors Q from 0.04 to 3.0 nm-1, where Q ) 4π/λ sin(θ/2) and θ is the scattering angle. The data were corrected for background, empty cell scattering, and sample transmission, as described in ref 9. The low-Q data were analyzed according to the Guinier approximation.10,11 In addition, least-squares fits over the entire measured Q-range were carried out using a model of the scattering calculated for an ensemble of polydispersed spherical particles with hard-sphere interactions (PHS).10b,11,19 The volume fractions calculated for each microemulsion system were used for fitting the scattering curves. Materials. All materials were used as received from the chemical vendors without further purification. Microemulsion Preparation for SANS Measurements. The o/w microemulsions for our SANS studies were prepared with D2O in place of H2O that has been used in the syntheses of the MCFs.8 The nonionic block copolymer Pluronic P123 (poly(ethylene oxide)-block(7) (a) Langevin, D. Acc. Chem. Res. 1988, 21, 255. (b) Huang, J. S. J. Surf. Sci. Technol. 1989, 5, 83. (c) Kabalnov, A.; Lindman, B.; Olsson, U.; Piculell, L.; Thuresson, K.; Wennerstro¨m, H. Colloid Polym. Sci. 1996, 274, 297. (d) Kegel, W. K.; Reiss, H. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 300. (8) Schmidt-Winkel, P.; Lukens, W. W., Jr.; Zhao, D.; Yang, P.; Chmelka, B. F.; Stucky, G. D. J. Am. Chem. Soc. 1999, 121, 254. (9) Glinka, C. J.; Barker, J. G.; Hammouda, B.; Krueger, S.; Moyer, J. J.; Orts, W. J. J. Appl. Crystallogr. 1998, 31, 430. (10) (a) Guinier, A., Fournet, G. Small-Angle Scattering of X-rays; Wiley: New York, 1955. (b) Feigin, L. A.; Svergun, D. I. In Structure Analysis by Small-Angle X-ray and Neutron Scattering; Taylor, G. W., Ed.; Plenum Press: New York, 1987. (c) Cabane, B. In Surfactant Solutions: New Methods of Investigation; Surfactant Science Series; Zana, R., Ed.; Marcel Dekker: New York, 1987; Vol. 22, p 57. (11) (a) Griffith, W. L.; Triolo, R.; Compere, A. L. Phys. Rev. A 1987, 35, 2200. (b) The software is available from NIST via the Internet at http://www.ncnr.nist.gov (1999).
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poly(propylene oxide)-block-poly(ethylene oxide), EO20PO70-EO20, Mav ) 5800) was used as surfactant, and 1,3,5trimethylbenzene (TMB) served as organic swelling agent (oil). Control over the droplet sizes was achieved by varying the TMB concentration while the amount of P123 was kept constant.8 Thus, the amount of TMB used has been expressed in terms of TMB/P123 weight ratios. Solutions of 1.6 M HCl in D2O were prepared by mixing concentrated HCl (36.5%) with D2O and provided the aqueous, acidic media for the microemulsions. The conditions were adjusted to the point that the molar amounts of D2O matched those of H2O, which was originally used for the syntheses of MCFs.8 The microemulsions with TMB/P123 weight ratios of 0, 0.25, 0.50, 1.0, and 1.5 were prepared by mixing 1.6 M HCl with appropriate amounts of P123, TMB, NH4F (under the conditions outlined in ref 8), and ethanol (200 proof), which is generated during hydrolysis of TEOS.12 For example, in 8.2 g of 1.6 M HCl were dissolved 0.20 g (0.04 mmol) of Pluronic P123 at room temperature. After this mixture was heated to 37-40 °C for 45 min, 0.20 (1.7 mmol) of TMB was added (TMB/P123 ) 1.00). After 45 min at 37-40 °C, 0.196 g (4.25 mmol) of ethanol (2 TEOS equiv)12 was added. After mixing for another 45 min at 37-40 °C, the microemulsions were transferred to preheated (40 °C) 1 mm path length quartz cells and placed in a temperature-controlled sample changer in the SANS instrument for data collection. Here they were allowed to thermally equilibrate for at least 2.5-3 h before the SANS data were collected at 40, 60, and 80 °C. Results and Discussion The starting point for our MCF syntheses is a combination of the nonionic block copolymer-based, supramolecular templating route described by Zhao et al.3c,d and the colloidal templating approach using emulsion droplets.5 We have chosen the nonionic Pluronic surfactant P123 (EO20-PO70-EO20) since it forms expandable aggregates that can template periodic SBA-15-type mesoporous silica in acidic media,3c,d,h,k and is well-suited for stabilizing oilin-water and oil-in-formamide emulsions.5b-d,13 These properties are probably related to the composition and molecular weight of P123.14 This surfactant possesses a long poly(propylene oxide) segment and two medium length poly(ethylene oxide) blocks and has a relatively high molecular weight (Mav ) 5800). Such a combination favors the formation of micelles, which have dehydrated poly(propylene oxide) blocks in their cores and coronas of hydrated poly(ethylene oxide) segments at the micellar surface, at low surfactant concentrations (low critical micellization concentration, CMC), and at low temperatures (low critical micellization temperature, CMT).14 This (12) Hydrolysis of 1 mol of TEOS gives 4 mol of ethanol. Thus, 2 equivsexpressed in molesscorresponds to 2× (mol of TEOS), and 4 equivsexpressed in molesscorrespond to 4× (mol of TEOS). (13) Imhof, A.; Pine, D. J. J. Colloid Interface Sci. 1997, 192, 368. (14) (a) For a comprehensive review, see: Alexandridis, P.; Hatton, T. A. Colloids Surf. A 1995, 96, 1. (b) Nace, V. M. In Nonionic Surfactants, Polyoxyalkylene Block Copolymers; Nace, V. M., Ed.; Surfactant Science Series; Marcel Dekker: New York, 1996; Vol. 60, p 145. (c) Pluronic and Tetronic Surfactants, Technical Brochure; BASF Corporation: Parsippany, NJ, 1989. (15) Nagarajan, R.; Barry, M.; Ruckenstein, E. Langmuir 1986, 2, 210. (16) Bahadur, P.; Pandya, K.; Almgren, M.; Li, P.; Stilbs, P. Colloid Polym. Sci. 1993, 271, 657. (17) (a) Tanford, C. In The Hydrophobic Effect: Formation of Micelles and Biological Membranes, 2nd ed.; Wiley: New York, 1980. (b) BenNaim, A. Hydrophobic Interactions; Plenum Press: New York, 1980. (18) Wanka, G.; Hoffmann, H.; Ulbricht, W. Macromulecules 1994, 27, 4145. (19) Mortensen, K.; Brown, W. Macromolecules 1993, 26, 4128.
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Table 1. Droplet Sizes of Microemulsions Consisting of 1.6 M HCl in D2O, P123,a and TMBb TMB/P123, w/wc
DGuinier at 40 °C without NH4F, nmd
DGuinier at 40 °C with NH4F,e nmd
DPHSat 40 °C without NH4F, nmf (PD)g
DPHS at 40 °C with NH4F,e nmf (PD)g
0 0.25 0.50 0.75 1.00 1.50
16.6 ( 0.1h 21.0 ( 0.2 24.2 ( 0.2 24.4 ( 0.2 24.8 ( 0.2 24.6 ( 0.2
16.6 ( 0.1h 21.6 ( 0.2 25.2 ( 0.2 25.6 ( 0.2 25.6 ( 0.2 24.8 ( 0.2
12.9 ( 0.1h (0.132) 17.4 ( 0.1 (0.120) 19.4 ( 0.1 (0.146) 19.4 ( 0.1 (0.125) 19.8 ( 0.1 (0.127) 19.4 ( 0.1 (0.127)
13.0 ( 0.1h (0.127) 17.2 ( 0.1 (0.117) 19.2 ( 0.1 (0.107) 19.3 ( 0.1 (0.110) 19.6 ( 0.1 (0.110) 19.0 ( 0.1 (0.111)
a Nonionic Pluronic surfactant (EO -PO -EO ). b 1,3,5-Trimethylbenzene. c Weight ratio using a fixed amount of P123. d Droplet 20 70 20 diameter according to Guinier approximation10 of SANS data: DGuinier ) 2RGuinier with Rg2 ) 3/5RGuinier2 and Rg being the radius of gyration.10b e NH4F concentration: 8 × 10-3 mol/L. f Droplet diameter derived from a complete fit of SANS data, assuming interaction of polydisperse hard spheres (PHS).10d,11,19 g In parentheses: Polydispersities PD obtained from a complete fit of SANS data assuming interaction of polydisperse hard spheres.10d,11 PD is the ratio of the standard deviation to the mean for droplet radii distributed according to a Schulz distribution. h The relative standard uncertainties in the fitted values are reported as 1 standard deviation from the mean value.
combination also produces a relatively high cloud point, the lower critical solution temperature at which a surfactant phase separates from aqueous solution, of 90 °C for a 1% aqueous P123 solution.14 In addition, P123 solubilizes organics well.14a These features suggest that P123 would be a suitable choice for our purposes. To prepare the o/w microemulsions, different amounts of 1,3,5-trimethylbenzene (TMB) were added to dilute solutions containing a fixed amount of P123 in aqueous HCl. The addition of TMB resulted in cloudy mixtures that appeared blue-white in daylight. Aromatic TMB was used as the organic cosolvent because this compound has proven to be an effective swelling agent for the preparation of large-pore mesoporous materials.2,3c,d,4a Also, aromatic hydrocarbons are more soluble than aliphatic hydrocarbons in aqueous solutions of poly(alkylene oxide) block copolymer surfactants.15 To investigate the nature of the microemulsions and to determine their droplet sizes and droplet size distributions, we carried out SANS measurements. During our first set of experiments, we examined microemulsions that were composed of aqueous HCl, P123, and TMB. These SANS data were analyzed according to the Guinier approximation10 and a model assuming polydisperse spheres interacting as hard spheres (PHS)10b,11,19 and are given in Table 1. The droplet diameters, DGuinier, determined at 40 °C according to the Guinier approximation,10,11 increase from 16.6 to 21.0 nm for TMB/P123 weight ratios of 0 and 0.25, respectively, but level off at a TMB/P123 weight ratio of 0.5 to give droplet diameters of 24.2-24.8 nm. Larger TMB/P123 weight ratios of 0.75, 1.00, and 1.50 do not expand the droplet size significantly beyond the value already reached at a TMB/P123 weight ratio of 0.5 (see Figure 1). This observation is in marked contrast to the behavior of the cell diameters in MCF materials,8 which show a linear relationship with the cube root of the amount of added TMB up to TMB/P123 weight ratios of at least 2.5 (see Figure 2). The leveling off of the droplet sizes for the aqueous HCl, P123, and TMB containing microemulsions was unanticipated, but not very surprising. Shortchain alcohols added to microemulsions act as cosurfactants, whose presence renders the surfactant films at the oil-water interfaces more flexible by lowering the films’ bending moduli and makes possible a reduction of the films’ spontaneous curvature.7 Consequently, the surfactant micelles can absorb more oil than in the absence of the cosurfactant and therefore grow in size. Critical scattering at very low Q-values in SANS data for TMB/ P123 > 0.5 systems in the absence of ethanol suggests that the observed leveling off in droplet size may have been caused by the formation of a two-phase equilibrium, in which a microemulsion with spherical surfactant micelles swollen with TMB coexists with excess TMB that cannot dissolve in the micelles.7c Because the MCF
Figure 1. Effect of ethanol (EtOH) as a cosurfactant for the microemulsions. The droplet diameters determined according to the Guinier approximation10 are plotted vs the cube root of the concentration of 1,3,5-trimethylbenzene (TMB, oil) that is expressed in terms of the TMB/P123 weight ratio with a fixed amount of P123 (P123, Pluronic surfactant). This diagram shows the droplet sizes of microemulsions that contain (i) no EtOH, (ii) 2 equiv of EtOH, and (iii) 4 equiv of EtOH.12 The effect of EtOH is insignificant for TMB/P123 e 0.5, but substantial for TMB/P123 > 0.5. Two equivalents of EtOH are sufficient to induce this effect. SANS data derived from a model for polydisperse spherical particles with hard-sphere interactions (PHS)10b,11,19 show the same results.
synthesis mixtures contain tetraethyl orthosilicate (TEOS) as a source of silica,8 we were concerned that ethanol, produced by hydrolysis of TEOS,12 was acting as a cosurfactant during the microemulsion templating of MCF materials. To check this hypothesis, we conducted a second set of SANS experiments on microemulsions that contained aqueous HCl, P123, TMB, and either 2 or 4 equiv12 of ethanol. These data are summarized in Table 2. Added ethanol indeed serves as a cosurfactant in the anticipated fashion and 2 equiv of ethanol are sufficient to trigger this effect. This is illustrated in Figure 1. Comparison of the DGuinier and DPHS data given in Tables 1 and 2 for 40 °C shows that the effect of ethanol upon the droplet sizes is insignificant up to TMB/P123 ) 0.5, where the droplet sizes level off in the absence of ethanol, but is substantial at larger TMB concentrations, as expected. Figures 1 and 2 show that, in the presence of ethanol, the droplet sizes of the microemulsions do not level off as observed in the absence of ethanol. On the basis of these results, we believe that ethanol acts as a cosurfactant to add the required flexibility to the surfactant films in the microemulsions and increases the solubility of TMB within the micelles, thereby expanding the droplet sizes. Figure 2 shows that both the droplet sizes and the MCF cell sizes increased linearly with the cube root of the TMB concentration, which is expected for spherical microemulsion droplets leading to spherical MCF cells (Vsphere ) 4/3
Microemulsion Templates for Mesoporous Silica
Figure 2. Correlation of the diameters of the microemulsion droplets in the presence of 4 equiv of EtOH12 and MCF cell sizes (nitrogen sorption and SAXS data)8 as functions of the concentration of TMB. Both the droplet sizes and the cells sizes increase linearly with the cube root of the TMB concentration, which is in agreement with a spherical model for the droplets and cells (Vsphere ) 4/3πr3). Please note the similar slopes of the linear fits. The close correlation between the droplet sizes and the cell sizes corroborates the proposal that MCFs are templated by microemulsions.8
πr3).8 Despite the difference in droplet sizes and cell sizes, the slopes of the linear fits are very similar. This correlation strongly supports the premise that microemulsion droplets are the templates for MCF materials.8 The droplet sizes are larger than the MCF cell sizes. This observation is typical and in agreement with other templating procedures, leading to porous materials in which shrinking of the pores occurs upon polymerization and solidification of the solid precursors around the templates.2,5,6 From characterization of the MCF materials, we have learned that adding small amounts of NH4F (F/Si molar ratio: 0.03) during the MCF syntheses selectively increases the sizes of the windows that interconnect the cells.8 To obtain more information regarding this remarkable effect, we have examined the influence of NH4F on the nature of the microemulsions. SANS measurements (see Table 1) show that adding small amounts of NH4F (8 × 10-3 mol/L) do not appreciably affect the droplet sizes of the microemulsions. This is expected due to the small concentration of NH4F in the reaction mixtures. Poly(ethylene oxide)-based surfactants generally form highly flexible surfactant films in microemulsions because of the high degree of solvent penetration and low electrostatic repulsion between the surfactant headgroups.7c Therefore, the screening effect of electrostatic headgroup repulsion, mediated by inorganic salts and commonly observed in microemulsions prepared with ionic surfactants and leading to surfactant film bending toward water,7 is anticipated to be small in our case. However, we want to mention that inorganic salts in concentrations of 0.5 mol/L or greater have strong effects upon the cloud points and micelle behavior of Pluronic surfactants.16 From these results, we conclude that the NH4F-mediated enlargement of the windows in siliceous MCFs8 is not due to a change in structure of the microemulsion droplets. This view is supported by the observation that NH4F does not appreciably affect the sizes of the spherical cells in the MCF materials.8 As evident from Figure 3 and the data in Table 2, the nature of the microemulsions is sensitive to temperature, a behavior that is anticipated due to use of the Pluronic surfactant. Both the hydrophilic poly(ethylene oxide) blocks and the hydrophobic poly(propylene oxide) segments decrease in polarity and become more hydrophobic
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with increasing temperature because of increased dehydration that is driven by an entropy gain in water (hydrophobic effect17).14 As a result, the phase diagrams and aggregation behavior of Pluronic surfactants and aqueous solutions thereof are sensitive to both temperature and surfactant composition.14,18 The microemulsion droplet sizes of P123 micelles derived from SANS data were analyzed according to the Guinier approximation10 and a model assuming polydispersed spheres interacting as hard spheres (PHS).10b,11,19 For systems with TMB/P123 ) 0, these data compare well with light-scattering and SANS data reported for aqueous solutions of P123.18 In Figure 3, we plot the variation in microemulsion droplet size with temperature for various TMB concentrations. The droplet sizes increase slightly between 40 and 60 °C, and grow significantly between 60 and 80 °C, which is near the cloud point of 90 °C for an aqueous 1% solution of P123.14 The same trend is observed for the PHS modelderived data. Similar temperature effects have been reported for solutions of other Pluronic surfactants.16,19-21 In the absence of swelling agents and far from the cloud points, the sizes of Pluronic micelles are almost independent of temperature.14,18 This characteristic behavior of Pluronics is explained by the two effects of increasing the temperature: (i) increased aggregation numbers in micelles (micelles grow) and (ii) decreased micellar hydration (micelles contract). As a result, the net change in micelle size is close to zero.14,18 However, near the cloud points of Pluronic surfactants, micelle contraction due to dehydration is difficult and increasing aggregation numbers lead to larger micelles.16,20,21 This picture explains the greatly increased micelle sizes when the temperature approaches the cloud points. In the case of the microemulsions described in this paper, similar effects may account for the observed temperature behavior. We suggest that hydrophobic TMB is the preferred and selective solvent for the hydrophobic poly(propylene oxide) blocks and thus swells the micelle cores, while the hydrophilic poly(ethylene oxide) blocks are preferentially solvated by water. Increasing the temperature may enhance the aggregation numbers of the micelles, and the hydrated poly(ethylene oxide) segments become less polar and may prefer to interact with TMB dissolved in the micelle cores rather than with surrounding water. Because of the presence of TMB, a good solvent for the poly(propylene oxide) blocks, the micelle cores are expected to be completely dehydrated, so no water may be squeezed out of the cores and core contraction may not occur. Stronger hydrophobic interactions of the poly(ethylene oxide) segments with TMB upon temperature increase would increase the hydrophobic volumes of the micelles. The hydrophilic coronas of Pluronic micelles are estimated to measure ∼1 nm,18 which is small compared to the size of the hydrophobic cores. When TMB is dissolved in the cores of the Pluronic micelles, the hydrophilic poly(ethylene oxide) blocks must stabilize larger surface areas at the organic-inorganic interfaces than in the absence of oil, which reduces the density of poly(ethylene oxide) segments at the micelle surfaces. This dilution effect may thin the shells of the TMB swollen micelles even more. When the poly(ethylene oxide) segments become less polar with increasing temperature and interact more strongly with the hydrophobic micelle cores, the resulting volume loss of the already thin hydrophilic shell may be small compared to the gain in hydrophobic volume. All these (20) Al-Saden, A. A.; Whateley, T. L.; Florence, A. T. J. Colloid Interface Sci. 1982, 90, 303. (21) Pandya, K.; Bahadur, P.; Nagar, T. N.; Bahadur, A. Colloids Surf. 1993, 70, 219.
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Table 2. Droplet Sizes and Polydispersities of Microemulsions Consisting of 1.6 M HCl in D2O, P123,a TMB,b and Ethanolc TMB/P123, w/wd
DGuinier at 40 °C, nme
DGuinier at 60 °C, nme
DGuinier at 80 °C, nme
0 0.25 0.50 1.00 1.50
16.2 ( 0.1i 21.2 ( 0.2i 23.0 ( 0.2i 36.2 ( 0.7i 37.0 ( 0.7i
18.6 ( 0.2i 22.8 ( 0.2i 25.8 ( 0.6i 36.6 ( 0.3i 37.8 ( 0.3i
0 0.25 0.50 1.00 1.50
16.0 ( 0.1i 21.2 ( 0.2i 23.8 ( 0.2i 35.4 ( 0.6i 36.4 ( 0.7i
18.0 ( 0.2i 23.6 ( 0.3i 26.4 ( 0.7i 36.2 ( 0.7i 37.2 ( 0.3i
DPHS at 40 °C, nmf (PD)g
DPHS at 60 °C, nmf (PD)g
DPHS at 80 °C, nmf (PD)g
2 equiv of Ethanol12 34.4 ( 1.0i 13.4 ( 0.1i (0.125) 36.6 ( 1.0i 17.1 ( 0.1i (0.136) 41.6 ( 1.6i 18.8 ( 0.2i (0.150) 52.6 ( 0.6i 30.6 ( 0.4i (0.177) 53.0 ( 1.0i 31.4 ( 0.6i (0.208)
14.9 ( 0.2i (0.170) 18.5 ( 0.1i (0.175) 19.6 ( 0.2i (0.149) 31.2 ( 0.3i (0.155) 31.8 ( 0.4i (0.172)
n/ah n/ah n/ah 50.4 ( 0.5i (0.185) 51.0 ( 0.5i (0.184)
4 equiv of Ethanol12 33.8 ( 2.0i 13.3 ( 0.1i (0.137) 37.8 ( 1.2i 17.2 ( 0.2i (0.144) 46.2 ( 2.0i 19.3 ( 0.2i (0.157) 55.2 ( 1.0i 29.8 ( 0.4i (0.177) 55.6 ( 1.0i 30.0 ( 0.6i (0.210)
14.0 ( 0.2i (0.174) 18.8 ( 0.3i (0.195) 20.2 ( 0.3i (0.158) 30.6 ( 0.3i (0.153) 31.4 ( 0.4i (0.174)
n/ah 38.8 ( 0.3i (0.173) n/ah 50.6 ( 0.6i (0.195) 51.8 ( 0.6i (0.186)
a Nonionic Pluronic surfactant (EO -PO -EO ). b 1,3,5-Trimethylbenzene. c Amount of ethanol added corresponded to the amount 20 70 20 of ethanol generated by hydrolysis of TEOS in MCF syntheses.12 d Weight ratio using a fixed amount of P123. e Droplet diameter according 10 to Guinier approximation of SANS data: DGuinier ) 2RGuinier with Rg2 ) 3/5RGuinier2 and Rg being the radius of gyration.10b f Droplet diameter derived from a complete fit of SANS data, assuming the interaction of polydisperse hard spheres (PHS).10d,11,19 g In parentheses: Polydispersities PD obtained from a complete fit of SANS data, assuming interaction of polydisperse hard spheres.10d,11 PD is the ratio of the standard deviation to the mean for droplet radii distributed according to a Schulz distribution. h Polydisperse hard-sphere fit to SANS data not successful. i The relative standard uncertainty in the fitted values is reported as 1 standard deviation from the mean value.
Figure 3. Variation of microemulsion droplet sizes with temperature for different TMB concentrations in the presence of 4 equiv of EtOH.12 The temperature behavior is similar for all TMB concentrations investigated and is in line with the observed temperature behavior of Pluronic micelles in the absence of organic oils when approaching the cloud points.
effects would lead to the observed growth of the microemulsion droplets with temperature (see Figure 3). When the microemulsion droplet diameters listed in Tables 1 and 2, which were derived from SANS data according to either Guinier approximation10 or complete curve fits assuming a PHS interaction model10d,11 (see Figure 4) are compared, it is evident that the Guinier micelle sizes, DGuinier, are larger than the sizes obtained from PHS fits, DPHS. However, the changes in the microemulsion droplet sizes with TMB concentration and with temperature are very similar for both data sets and follow the same trend, which is illustrated in Figure 5 for the data collected in Table 2. We have established a close correlation between the microemulsion droplet sizes, the MCF cell sizes, and the concentration of TMB, as shown in Figure 2, by using either set of droplet size data from Table 2. The DPHS values are 4-19% smaller than the corresponding DGuinier values (see Table 2). One reason for this observation is that the Guinier method is biased toward larger scatterers in the case of polydispersed particle sizes.10a,22 However, since the difference between DGuinier and DPHS becomes smaller with increasing micelle sizes, that is, with larger TMB concentrations and at (22) Glinka, C. J. SANS from Dilute Particle Systems; NCNR Summer School, NIST, June 1-5, 1998.
Figure 4. Representative SANS data and the corresponding complete curve fit assuming an ensemble of polydispersed spherical particles with hard-sphere interactions (PHS)10b,11,19 for a microemulsion measured at 80 °C. This system was prepared with TMB/P123 ) 1.5 in the presence of 4 equiv of ethanol. The first two data points are affected by the beam stop and are therefore not included in the fit. Please note that no signs of critical scattering indicative of phase separation are observed.
higher temperatures, while the polydispersities, PD, are not reduced at larger droplet sizes (see Table 2), other factors might be involved, too. It may be possible that the PHS interaction model is more likely to give the dimensions of the “hard parts” of the Pluronic micelles as opposed to the sizes of the entire micelles (“hard” core and “soft” shell) and may not accurately reflect the existence of the soft hydrophilic shells or soft organic-inorganic interfaces that may be pushed aside and neglected when the hard parts of the micelles interact. The flexibility of the hydrophilic shell and the organic-inorganic interface of the micelles is assumed to be large due to the presence of ethanol as a cosurfactant (see above). With increasing micelle diameters, the sizes of the soft parts of the micelles may become smaller, whereas the hard hydrophobic parts of the micelles would grow (see above). Consequently, less soft material may be pushed aside by the hard parts of the micelles upon interaction. This picture would explain why the difference between DGuinier and DPHS decreases with increasing micelle size.
Microemulsion Templates for Mesoporous Silica
Langmuir, Vol. 16, No. 2, 2000 361
Conclusions
Figure 5. Comparison of microemulsion droplet sizes as functions of the TMB concentration determined in the presence of 4 equiv of EtOH.12 The SANS data were analyzed according to the Guinier approximation10 and by complete curve fits based on a model of the scattering calculated for an ensemble of polydispersed spherical particles with hard-sphere interactions (PHS).10b,11,19 Despite the differences in the absolute droplet diameters derived from the two models, both data sets follow the same trend.
The polydispersities, PD, of the microemulsions are obtained from PHS fits and are given in Tables 1 and 2. They vary between 0.11 and 0.21, which is typical for microemulsions.7 From X-ray simulation experiments and nitrogen sorption of MCFs, we found that the spherical cells are fairly uniform in size.8 This suggests that the polydispersities of the templates become smaller at some stage during the MCF formation after TEOS is added. It is not unreasonable to assume that repulsive interactions between silica-coated TMB/P123 droplets, which are still flexible in the earlier stages of the MCF synthesis, may narrow the droplet size distribution upon packing of the droplets by narrowing the potential energy curve of the droplets. The narrowed size distributions may then be locked in during the subsequent aging and rigidifying stage of the MCF syntheses and lead to uniformly sized pores. SANS studies of MCF synthesis mixtures after the addition of TEOS are currently underway.
We have investigated the nature of the colloidal systems that direct the syntheses of MCFs8 as ultralarge-pore mesoporous silicas. SANS measurements show that, prior to the addition of TEOS, the MCF synthesis mixtures consist of o/w microemulsions. The sizes of the microemulsion droplets can be controlled by the oil concentration and by temperature. Both the droplet sizes and the MCF cell sizes increase linearly with the cube root of the oil concentration, and the slopes of the linear fits are very similar. This close correlation between the microemulsion droplet sizes and the MCF cells sizes indicates that the MCFs are templated by the o/w microemulsions. The temperature behavior of the microemulsions is similar to the temperature behavior of micelles formed from Pluronic surfactants in the absence of oil. We believe that the microemulsion templates reported in this paper represent a valuable addition to the existing class of colloidal templates5,6 that direct the synthesis of porous materials. Compared to other colloidal templates, the microemulsion templates are easier to prepare by simply mixing the water, surfactant, oil, and a cosurfactant. In addition, microemulsion templating8 leads to mesoporous materials with narrow pore size distributions without further processing. Acknowledgment. Funding for these studies was provided by the National Science Foundation under Grants DMR-9634396, the Materials Research Laboratory program of the National Science Foundation under Award DMR-9632716, and the U.S. Army Research Office under Grant DAAH 04-96-1-0443. We thank Professors B. F. Chmelka (UCSB), J. Israelachvili (UCSB), D. J. Pine (UCSB), and J. Y. Ying (MIT), and Dr. W. W. Lukens (LBNL) for helpful discussions, and BASF (Mt. Olive, NJ) for providing Pluronic P123. The names of commercial products, which appear in the text, are for information only and do not constitute or imply endorsement by NIST for any purpose. LA9906774
