Tailoring Pore Size of Ordered Mesoporous Silicas Using One or Two

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Tailoring Pore Size of Ordered Mesoporous Silicas Using One or Two Organic Auxiliaries as Expanders J. L. Blin and B. L. Su* Laboratoire de Chimie des Mate´ riaux Inorganiques, I.S.I.S., The University of Namur, 61, rue de Bruxelles, 5000 Namur, Belgium Received January 14, 2002. In Final Form: April 9, 2002 Large-pore mesoporous materials were synthesized by incorporating alkanes as swelling agents during micellar solution preparation. The effect of the number of carbon atoms of the alkane on the properties of the final products has been investigated. The optimal swelling effect is obtained with decane, and the largest pore diameter that can be achieved with the latter is 5.0 nm. To further expand the pore size, we have incorporated jointly decane and 1,3,5-trimethylbenzene (TMB). The adding sequence of the two swelling agents has been studied and appears to strongly affect the pore size. This study led us to propose two different mechanisms, describing the swelling effect of decane and TMB molecules, which depend on the preparation conditions.

1. Introduction Since their discovery in the early 1990s, mesoporous molecular sieves have attracted much research attention due to a number of remarkable properties such as adjustable pore size, high surface area and pore volume, good thermal stability, and ease of surface modification. The synthesis of such molecular sieves is based on the use of assemblies of surfactant molecules as framework templates. Surfactants are large organic molecules with a hydrophilic head and a long hydrophobic tail of variable length. In aqueous solution, these molecules pack together to form first isolated spherical and then cylindrical micelles and finally higher-order phases, depending mainly on concentration. The synthesis of pure silica mesoporous molecular sieves consists of the condensation and polymerization of an inorganic source of silicium around the micelles of surfactant. Electrostatic pathways, based on a supramolecular assembly of charged surfactants (S+ or S-) with charged inorganic precursors (I- or I+), S+X-I+ (X- ) Cl-, Br-), S-M+I- (M+ ) Na+, K+),1-3 or neutral pathways S0I0 4-6 and N0I0 7-9 (N is a polyoxyethylene alkyl ether), can be used for preparation of the mesoporous materials. The wall thickness, the pore diameter, and the nature of the final structure (hexagonal, cubic, or lamellar) are affected by several physicochemical factors such as kind of surfactant,1 silicium/surfactant molar ratio,1 pH value of micellar solution preparation,10 and heating duration and temperature.11 * Corresponding author. Phone: 32-81-72-45-31. Fax: 32-81-7254-14. E-mail: [email protected]. (1) 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.; Schlender, J. L. J. Am. Chem. Soc. 1992, 114, 10834. (2) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (3) Huo, Q.; Margolese, D. I.; Ciesla, U.; Feng, P.; Gier, T. E.; Sieger, P.; Leon, R.; Petroff, P. M.; Schu¨th, F.; Stucky, G. D. Nature 1994, 368, 317. (4) Sayari, A. Stud. Surf. Sci. Catal. 1996, 102, 1. (5) Tanev, P. T.; Pinnavaia, T. J. Science 1995, 267, 865. (6) Tanev, P. T.; Pinnavaia, T. J. Chem. Mater. 1996, 8, 2068. (7) Bagshaw, S. A.; Pinnavaia, T. J. Angew. Chem., Int. Ed. Engl. 1996, 10, 1102. (8) Prouzet, E.; Pinnavaia, T. J. Angew. Chem., Int. Ed. Engl. 1997, 36, 516. (9) Blin, J. L.; Le´onard, A.; Su, B. L. Chem. Mater. 2001, 13 (10), 3542.

An important effort has been devoted to enlarge the pore size of MCM-41 as these large-pore silica supports with high surface area and high thermal stability are of particular interest in the preparation of catalysts,5,12-14 drug delivery systems,15 and sensors.16 Indeed, some technologically important catalytic treatments of organic molecules such as the ammoxidation of hydroxyacetophenones,12 the selective oxidation of aromatics,5 the hydroxylation of phenols,13 and vinyl acetate production14 require supports that exhibit quite large pore diameters in addition to the high surface area. This need has attracted much attention of research groups to investigate some pathways leading to the formation of large-pore mesoporous materials such as for example postsynthesis treatments,17,18 surfactants of different chain length,19 polymers such as triblock copolymers20 used as templates or swelling agents incorporated in the formed micelles. In the literature, 1,3,5-trimethylbenzene (TMB),21-23 triisopropylbenzene,24 amines,25 and tetraalkylammonium cations (TAA+)26 have been used as expanders, and materials (10) Le´onard, A.; Blin, J. L.; Su, B. L. Chem. Mater., submitted for publication. (11) Blin, J. L.; Otjacques, C.; Herrier, G.; Su, B. L. Int. J. Inorg. Mater. 2001, 3 (1), 75. (12) Clerici, M. G. Appl. Catal. 1991, 68, 249. (13) Clerici, M. G.; Ingallina, P. J. Catal. 1993, 140, 71. (14) Nakamura, S.; Yasui, T. J. Catal. 1970, 17, 336. (15) Vallet-Regi, M.; Ramila, A.; Del Real, R. P.; Pe´rez-Pariente, J. Chem. Mater. 2001, 13, 308. (16) Walcarius, A.; Lu¨thi, N.; Blin, J. L.; Su, B. L.; Lamberts, L. Electrochim. Acta 1999, 44, 4601. (17) Huo, Q.; Margolese, D. I.; Stucky, G. D. Chem. Mater. 1996, 8, 1147. (18) Sayari, A.; Liu, P.; Kruk, M.; Jaroniec, M. Chem. Mater. 1997, 9, 2499. (19) Sayari, A.; Karra, V. R.; Sudhakar Reddy, J. Symposium on Synthesis of Zeolites, Layered Compounds and Other Microporous Solids, 209th National Meeting of the American Chemical Society, Anaheim, CA, 1995. (20) Zhao, D.; Feng, J.; Huo, Q.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; Stucky, G. D. Science 1995, 279, 548. (21) Beck, J. S. U.S. Patent 5,057,296, 1991. (22) Branton, J.; Dougherty, J.; Lockhart, G.; White, J. W. Charact. Porous Solids, IV 1997, 668. (23) Desplantier-Giscard, D.; Galarneau, A.; Di Renzo, F.; Fajula, F. 15e`me re´union du Groupe Franc¸ ais des Ze´olithes, Carry le Rouet, France, 1999. (24) Kimura, T.; Sugahara, Y.; Kuroda, K. Chem. Commun. 1998, 559. (25) Sayari, A.; Kruk, M.; Jaroniec, M.; Moudrakovski, I. L. Adv. Mater. 1998, 10, 1376.

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with pore sizes superior to 8.0 nm have been obtained. However, the enlargement of pore size cannot be performed without affecting the mesoporous structure and the reproduction is also difficult. In the present study, alkanes and jointly decane and 1,3,5-trimethylbenzene have been used as swelling agents in order to obtain large-pore silica mesoporous materials. We try to understand how these organics solubilize into the micelles and to shed some light on the effect of using simultaneously different swelling agents on the texture of the obtained mesoporous molecular sieves. 2. Experimental Section 2.1. Syntheses. 2.1.1. Incorporation of Alkanes (from Pentane to Dodecane) as Swelling Agents. Cetyltrimethylammonium bromide (CTMABr) was first dissolved in water with stirring at 40 °C to obtain a transparent micellar solution. Then the swelling alkane was added at this temperature, and after homogenization, sodium silicate was introduced dropwise. The pH value of the obtained gel was adjusted with sulfuric acid. After stirring for several minutes at room temperature, the homogeneous gel with the molar composition of 1 CTMABr, 1 CnH2n+2, 0.63 SiO2, 102 H2O was sealed in Teflon autoclaves. The final products were obtained after ethanol extraction with a Soxhlet apparatus for 30 h. After 30 h solvent extraction, a small quantity of surfactant still remained in the pores of the materials. That is why a supplementary calcination under nitrogen and then an air atmosphere at 500 °C for 18 h was made. 2.1.2. Incorporation of Decane and Trimethylbenzene. The different steps of the synthesis are the same as described above. In the above procedure, only one swelling agent is used. Here, both decane and TMB are used, and the adding sequence of decane and TMB is studied in detail. The chemical composition of the gel is 1.0 CTMABr, 0.63 SiO2, x C10H22, y TMB, 102 H2O; x and y are the amounts of decane and TMB, respectively, in moles. Different decane/TMB molar ratios have been used and vary in a range from 0.40 to 1.33. 2.2. Characterization. Powder X-ray diffraction patterns of the obtained materials were recorded on a Philips PW 170 diffractometer, using Cu KR (1.54178 Å) radiation, equipped with a thermostatic unit. The transmission electron micrographs were taken using a Philips EM 301 microscope equipped with a tungsten gun at an accelerating voltage of 100 kV. Nitrogen adsorption-desorption isotherms were obtained at -196 °C over a wide relative pressure range from 0.01 to 0.995 with a volumetric adsorption analyzer ASAP 2010 manufactured by Micromeritics. The samples were degassed further under vacuum for several hours at 320 °C before nitrogen adsorption measurements. The specific surface area was determined by the BET (Brunauer-Emmett-Teller) method in the relative pressure range from p/p0 ) 0.05-0.25. The pore diameter and the pore size distribution were determined by the BJH (Barret-JoynerHalenda) method,27a although it is well-known that this method gives underestimated pore size values and some new interesting methods have been developed recently by Jaroniec et al.27b From our knowledge, some methods can give the approximative determination for materials with pore sizes larger than 5 nm. We use the BJH method here for the sake of simplicity, and this mathematical algorithm will not affect significantly our systematic comparison.

3. Results 3.1. Effect of the Alkane Chain Length on the Structure and the Pore Diameter of the Samples. Different alkanes from pentane to dodecane were incorporated in the micellar solution of CTMABr. According to (26) Corma, A.; Kan, K.; Navarro, M. T.; Pe´rez-Pariente, J.; Rey, F. Chem. Mater. 1997, 9, 2123. (27) (a) Barret, E. P.; Joyner, L. G.; Halenda, P. P. J. Am. Chem. Soc. 1951, 73, 37. (b) Jaroniec, M.; Kruk, M.; Sayari, A. Stud. Surf. Sci. Catal. 2000, 129, 587.

Figure 1. Variation of the XRD patterns of the samples synthesized with different alkanes: (a) C5H12 and (b) C10H22.

Figure 2. TEM micrographs (longitudinal view) of samples synthesized using undecane (a) and dodecane (b) as expanders.

the literature,28 an alkane/CTMABr surfactant molar ratio of 1.0 was used to investigate the influence of the alkane chain length on the structural and textural properties of the formed mesoporous materials. The hydrothermal treatment was performed for 8 days at 100 °C. 3.1.1. Structural Characteristics. For the compounds obtained with pentane and hexane as swelling agents, in addition to a sharp peak at 2θ ) 2.05° (4.3 nm), two peaks at 2θ ) 3.54° (2.5 nm) and 2θ ) 4.02° (2.2 nm) are detected (Figure 1a). The presence of these two last peaks is suggestive of a hexagonal organization of the channels in our materials. Indeed, it was reported29 that X-ray diffractograms of powdery hexagonal mesoporous materials exhibit a typical four-peak pattern with a very strong feature at a low angle (100 reflection line) and three other weaker peaks at higher angles (110, 200, and 210 reflection lines). These four reflection lines can be indexed on a (28) Ulagappan, N.; Rao, C. N. R. Chem. Commun. 1996, 2759. (29) Chen, C. Y.; Xiao, S. O.; Davis, M. E. Microporous Mater. 1995, 4, 20.

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Figure 3. Nitrogen adsorption-desorption isotherms (A) and pore size distributions (B) of the samples obtained by incorporation of different alkanes as expanders: (a) C5, (b) C6, (c) C7, (d) C8, (e) C9, (f) C10, (g) C11, and (h) C12.

hexagonal unit cell (a0 ) 2d100/(3)1/2) which is the sum of the pore diameter and the thickness of the pore wall. According to Bragg’s rule, the unit cell dimension (a0) can be deduced and is about 5.0 nm. If the alkane chain length is further increased till decane, no secondary reflections are detected anymore (Figure 1b), and thus the regular channel array of the channel is lost and the presence of a single reflection indicates the formation of the disordered structure. In this case, the recovered mesoporous molecular sieves exhibit a wormhole-like channel system, analogous to MSU-type materials. The broad peak that is observed on the XRD pattern is an indication of the average pore-to-pore separation in the disordered wormhole framework, which presents a lack of long-range crystallographic order. We can thus conclude that incorporation of alkanes from heptane to decane induces a reorganization of the micellar array. For undecane and dodecane, the regular channel arrangement appears again as shown by the TEM micrographs (Figure 2). 3.1.2. Textural Characteristics. Figure 3A shows the isotherms of the samples using different alkanes as expanders. With the exception of the sample obtained with nonane (Figure 3A, curve e), the adsorption-desorption isotherms of all compounds recovered are type IV, characteristic of mesoporous materials according to the BDDT (Brunauer-Deming-Deming-Teller) classification.30 Isotherms can be decomposed in three parts: the formation of the monolayer, a sharp increase characteristic (30) Brunauer, S.; Deming, L. S.; Deming, W. S.; Teller, E. J. Am. Chem. Soc. 1940, 62, 1723.

of the capillary condensation of nitrogen within the mesopores. From pentane to decane, the relative pressure at which the capillary condensation occurs increases significantly from 0.30 to 0.60, indicating that the value of the pore diameter increases sharply with the alkane chain length since the p/p0 position of the inflection point is related to the pore diameter. From undecane, this value decreases and reaches 0.40 for dodecane. The pore diameter attains thus a maximum for decane and drops again if a longer-chained alkane is used. This is contrary to expectations. In the case of nonane, the sharp increase due to the capillary condensation was never observed even after several reproducibility tests. The material is not a structured compound, and no homogeneous pore size distribution can be evidenced (Figure 3B, curve e). Some complementary studies will be done in order to understand this phenomenon. This compound will thus not be taken into account in the following analyses. The sharp increase in the adsorbed volume of nitrogen due to capillary condensation for the sample obtained with decane is relatively steep, reflecting homogeneity in pore sizes of the sample which has a specific surface area of 750 m2/g. For other compounds, the capillary condensation is less pronounced, meaning that only a part of the material is well “crystallized”. This is confirmed by the low value of nitrogen adsorbed and the low specific surface areas (500 m2/g for heptane, 405 m2/g for undecane). Using the BJH method, we can determine the pore size distribution and the mean value of pore diameter from the adsorption branch of the isotherm. The higher the

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Figure 4. Variation of the mean pore diameter with the number of carbon atoms of the alkane incorporated as the expander.

pore diameter, the broader the pore size distribution (Figure 3B). The variation of the mean diameter with the number of carbon atoms of the alkane is represented in Figure 4, and a linear relationship can be deduced from hexane to decane. 3.1.3. Discussion. From Figures 3B and 4, it is clear that the swelling effect is observed from heptane to dodecane. For pentane and hexane, the pore diameter is similar to the one obtained without a swelling agent.11 The boiling temperatures of pentane and hexane are 36 and 69 °C, respectively. These values are very close to the temperature of micellar solution preparation (40 °C), and so their evaporation probably occurs during the preparation. However, the samples synthesized with pentane or hexane give a highly well resolved X-ray diffraction (XRD) pattern even better than that without pentane or hexane, significative of the beneficial effect of pentane and hexane in the formation of highly ordered mesoporous materials. The real role will be further studied. The optimum and reproducible swelling effect is observed for decane. For undecane and dodecane, the swelling effect is less important, and we can assume that only a part of the added alkane is incorporated in the core of the formed micelles. The value of the mean pore diameter is only 4.1 nm for undecane and 3.9 nm for dodecane instead of 5.3 and 5.8 nm (values obtained by extrapolation of the line presented in Figure 4 if all the added molecules of alkane are considered to be incorporated in the core of the micelles). For these two alkanes, the penetration effect described by Kunieda et al.31 is the most important. Undecane and dodecane molecules are placed between the alkyl chains of the surfactants, the volume of the micelle does not increase, and only the effective crosssectional area of one surfactant molecule is modified but no swelling effect occurs. Two mechanisms can be used to explain the swelling effect. The first one is proposed by Ulagappan et al.28 In their study dealing with the incorporation and the organization of alkane and surfactant molecules in the process of forming mesoporous silica, the authors indicated that from pentane to octane, alkanes and the surfactant molecules can be described as molecular dispersions of the swelling auxiliaries between the tails of surfactant molecules (Figure 5A, structure a). Whereas for higher alkanes such as decane the alkane molecules form a core which is then surrounded by a layer of the cationic surfactant molecules, this swelling mechanism involves one molecule of surfactant for one molecule of expander (Figure 5A, structure b). The authors concluded that the optimal alkane/surfactant molar ratio is 1. In the present work, we have also kept a molar ratio of alkane/surfactant (31) Kunieda, H.; Ozawa, K.; Huang, K. L. J. Phys. Chem. B 1998, 102, 831.

Figure 5. Swelling mechanism proposed by (A) Ulagappan et al. (a) from pentane to octane and (b) for higher alkanes and (B) Kunieda et al. for decane.

Figure 6. TEM micrograph (A), XRD pattern (B), and nitrogen adsorption-desorption isotherm (C) of the compound synthesized with a decane/surfactant molar ratio of 0.5.

equal to 1.0; however, our results are quite different from those of Ulagappan. The mechanism proposed by Kunieda et al.,31 who justify the size expansion of micelle size of the polyoxyethylene dodecyl ether-water system by decane and squalane, can also be used here to explain the swelling effect of alkanes. During the micellar solution preparation, the alkane molecules are incorporated in the core of the micelles to form aggregates (Figure 5B). The volume of the micelle is increased; the effective cross-sectional area of one surfactant molecule remains constant, contrary to the case of the penetration effect. As observed by the XRD analysis, when the molar ratio of alkane/surfactant is equal to 1.0, the incorporation of the organic auxiliary disturbs the preformed micellar array in aqueous solution and a disordered material with a wormhole-like channel array is recovered, instead of a MCM-41-type compound. However, if the molar ratio of decane/surfactant is decreased to 0.5, high quality of MCM-41 materials can be obtained

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Figure 8. N2 adsorption isotherms at -196 °C (A) and the pore size distribution (B) obtained from the BJH method for 1.0 CTMABr, 0.63 SiO2, 1 C10H22, 1.5 TMB, 102 H2O: (a) decane added before TMB, (b) decane added after TMB, and (c) decane and TMB added simultaneously.

Figure 7. N2 adsorption isotherms at -196 °C (A) and the pore size distribution (B) obtained from the BJH method for 1.0 CTMABr, 0.63 SiO2, x C10H22, y TMB, 102 H2O: (a) y ) 0, (b) x ) 0, (c) decane added before TMB (x ) 2, y ) 1.5), (d) decane added after TMB (x ) 2, y ) 1.5), and (e) decane and TMB added simultaneously (x ) 2, y ) 1.5).

using same synthesis conditions. Figure 6 shows the transmission electron microscopy (TEM) image, the XRD pattern, and the nitrogen adsorption-desorption isotherm of one sample synthesized with a molar ratio of decane/ surfactant equal to 0.5. This indicates that with a molar ratio of 1.0, we can also synthesize a high-quality MCM41-type compound if we adjust the synthesis conditions. 3.2. Joint Incorporation of Decane and Trimethylbenzene. As just mentioned above, the optimal swelling effect was obtained with decane. The obtained compound is disordered with a molar ratio of decane/surfactant equal to 1.0, but it has been reported that wormhole-like structures with three-dimensional access to the reaction centers could be very beneficial for catalytic purposes.32 To further expand the pore size, in this part we have incorporated jointly decane and TMB. It is conceivable that the introduction of the second swelling agent will either affect or be affected by the presence of the first one. In this way, the first step in the syntheses was naturally to investigate the effect of the introduction sequence of the two swelling agents on the final formed structures. For this goal, decane was added (32) Pauly, T. R.; Liu, Y.; Pinnavaia, T. J.; Billinge, S. J. L.; Riecker, T. P. J. Am. Chem. Soc. 1999, 121, 8835.

before or after TMB, and we have also incorporated simultaneously the two expanders. This study was performed for a molar ratio of decane/TMB equal to 1.33 (x ) 2 and y ) 1.5) and 0.66 (x ) 1 and y ) 1.5). Figure 7 depicts the nitrogen adsorption isotherms (A) and the pore size distributions (B) of compounds prepared at the composition 1.0 CTMABr, 0.63 SiO2, x C10H22, y TMB, 102 H2O with either decane or TMB alone (x ) 2, y ) 0, and decane/surfactant ) 2.0; or x ) 0, y ) 1.5, and TMB/ surfactant ) 1.5) and with a decane/TMB molar ratio of 1.33 (x ) 2 and y ) 1.5) for different addition sequences. Except the sample synthesized when decane and TMB are simultaneously incorporated to the synthesis mixture (Figure 7A, curve e), all the synthesized materials exhibit a type IV isotherm (Figure 7A, curves a-d) according to the BDDT classification.29 It is clear that the introduction of either decane or TMB can expand the pore size. However, the expansion effect is much more pronounced when decane and TMB are jointly incorporated, since the sharp increase of the adsorbed volume of nitrogen due to capillary condensation occurs at higher relative pressures when the two swelling agents are jointly added and in particular when decane is introduced prior to the addition of TMB (p/p0 ) 0.65; Figure 7A, curve c). This part of the results indicates that the optimal expansion effect can be obtained when decane is introduced in the aqueous solution prior to TMB. This conclusion is further confirmed even if the molar ratio of decane/TMB is decreased to 0.66 (x ) 1.0 and y ) 1.5) (Figure 8). When the two swelling agents are simultaneously added to the micellar solution, the nitrogen adsorption isotherm is not type IV anymore but rather type II (Figure 8A, curve c), and the pore size distribution becomes very broad and bimodal (Figure 8B, curve c). If decane and TMB are not mixed before introduction into the micellar solution, from Figures 7B and 8B it is obvious that the joint introduction of decane and TMB has a beneficial effect. Indeed, the pore diameter is increased from 4.8 nm (x ) 2, y ) 0) or 3.8 nm (x ) 0, y

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Figure 9. Proposed mechanism for the incorporation of decane and TMB: (A) decane added before TMB and (B) decane added after TMB.

) 1.5) to a value located in the range of 5.5-7.6 nm for 1.0 CTMABr, 0.63 SiO2, 2 C10H22, 1.5 TMB, 102 H2O, depending on the introduction sequence of the two swelling agents. From this, we can conclude that the introduction of TMB in the micellar solution has an additional swelling effect compared to the addition of decane alone in the core of the micelles as reported above. If the introduction sequence is properly chosen, it appears that TMB also solubilizes in the preformed micelles. The largest pore sizes are obtained if decane is added first during the preparation of the micellar solution. TMB molecules can interact with the surfactant through electrostatic interactions between the π electrons of the benzenic core and the positively charged headgroup of the template. When decane molecules are first incorporated into the micellar solution, they provoke an expansion of the micelles as mentioned above. When TMB is added afterward, some molecules remain at the surface of the micelles, but some others move toward the inner core of the micelles and further expansion occurs (Figure 9A). If TMB is first introduced into the micellar solution, due to the interaction between the TMB molecules and the template, though some benzenic rings move into the hydrophobic micellar core, part of the TMB molecules remain at the outer hydrophilic shell of the micelles and act as a barrier. The subsequent addition of decane is then less efficient as only a part of the molecules can be incorporated in the core of the micelles resulting thus in a less pronounced swelling effect than the one observed when decane is the first expander (Figure 9B). The swelling agents should be introduced during the micellar solution preparation according to the following sequence: first decane and then TMB. The effect of the variation in the decane/TMB molar

ratio on the structural and textural properties of the synthesized materials will be reported elsewhere. 4. Conclusion The present work reveals that alkanes can be used as a swelling agent to enlarge the pore size of mesoporous materials. For pentane and hexane, no significant swelling effect is observed. Their boiling point is too close to the temperature of the micellar solution preparation, and evaporation occurs. From hexane to decane, a linear relationship is shown between the pore diameter and the number of carbon atoms of the alkane added as the expander. For the samples synthesized with undecane and dodecane, probably only a part of the alkane is incorporated in the core of the formed micelles. The maximum swelling effect is obtained with decane. The upper limit of obtained pore size is around 5.0 nm. The swelling mechanism was explained with the mechanism proposed by Kunieda et al. in the case of the expansion of the micelle size in the presence of oil in the polyoxyethylene dodecyl ether-water systems. Jointly incorporating decane and 1,3,5-trimethylbenzene during the micellar solution preparation allows us to further expand the pore sizes, which can be adjusted up to 9.0 nm. We have also shown that the best way to incorporate the two swelling agents during the micellar solution preparation is according to the following sequence: first decane and then TMB. Acknowledgment. This work has been performed within the framework of PAI/IUAP 4-10. The helpful assistance of Mr. A. Le´onard is acknowledged. LA020042W