Surfactant

© Copyright 2002 American Chemical Society

DECEMBER 24, 2002 VOLUME 18, NUMBER 26

Letters In-Situ SAXS Studies on the Formation of Silicate/ Surfactant Mesophases with Solubilized Benzene under Acidic Conditions Michael Tiemann,† Vale´rie Goletto,‡ Raphae¨l Blum,‡ Florence Babonneau,‡ Heinz Amenitsch,§ and Mika Linde´n*,† Department of Physical Chemistry, Åbo Akademi University, Porthansgatan 3-5, FIN-20500 Turku, Finland; Chimie de la Matie` re Condense´ e, Universite´ Pierre et Marie Curie, CNRS, 4 Place Jussieu, F-75252 Paris Cedex 05, France; and Institute of Biophysics and X-ray Structure Research, Austrian Academy of Sciences, Schmiedlstrasse 6, A-8042 Graz, Austria Received August 27, 2002. In Final Form: November 12, 2002 The formation of mesoscopically ordered silica/surfactant composites under acidic synthesis conditions is studied by time-resolved in-situ small-angle X-ray scattering (SAXS) using synchrotron radiation. Benzene is used a an additive which acts as a weak swelling agent although most of the benzene molecules are found to reside near the surfactant/silicate interface region rather than in the micelle cores. With increasing relative amounts of benzene, the curvature of the micellar aggregates decreases, which finally leads to a transition from (hexagonally) rodlike to lamellar; a mechanism for this change in curvature is suggested.

Introduction The concept of utilizing supramolecular arrays of surfactants as structure-directors for the synthesis of ordered mesoporous silica materials1 has been subject to many mechanistic studies. It is well-established that the formation of the mesostructure is usually a cooperative process which is governed to a large extent by interactions between the surfactant molecules and the reacting silicate species, that is, silica oligomers with various degrees of polymerization.2 The structural symmetry of this silicatropic mesophase depends on such factors as pH, temperature, or the presence of cosurfactants or cosolvents. * Corresponding author. E-mail: [email protected]. † Åbo Akademi University. ‡ Universite ´ Pierre et Marie Curie. § Austrian Academy of Sciences. (1) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710-712. (2) Monnier, A.; Schu¨th, F.; Huo, Q.; Kumar, D.; Margolese, D.; Maxwell, R. S.; Stucky, G. D.; Krishnamurty, M.; Petroff, P.; Firouzi, A.; Janicke, M.; Chmelka, B. F. Science 1993, 261, 1299-1303.

In the course of the synthesis the mesostructure becomes more and more rigid and the degree of silica polymerization may have a substantial impact on the curvature of the surfactant arrays and, thus, on the mesostructure of the final solid product. This effect is often expressed by phase transitions (e.g. from lamellar to tubular) during the reaction. Various in-situ techniques have been applied to monitor the phase behavior of such systems and to study the reaction mechanisms; a comprehensive review of this field has recently been published.3 In particular, timeresolved small-angle X-ray scattering (SAXS) has been shown to provide an effective technique in studying the syntheses of mesostructured silica,4-15 zirconia/titania,16 (3) Patarin, J.; Lebeau, B.; Zana, R. Curr. Opin. Colloid Interface Sci. 2002, 7, 107-115. (4) Firouzi, A.; Kumar, D.; Bull, L. M.; Besier, T.; Sieger, P.; Huo, Q.; Walker, S. A.; Zasadzinski, J. A.; Glinka, C.; Nicol, J.; Margolese, D.; Stucky, G. D.; Chmelka, B. F. Science 1995, 267, 1138-1143. (5) Regev, O. Langmuir 1996, 12, 4940-4944. (6) Linde´n, M.; Schunk, S. A.; Schu¨th, F. Angew. Chem., Int. Ed. 1998, 37, 821-823.

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or aluminum phosphates.17 Here we report on the SAXS investigation of the formation of hexagonal silica/surfactant mesophases under acidic reaction conditions (as in the synthesis of SBA-3 materials18), utilizing benzene as a swelling agent; the former studies mentioned above were all focused on the basic synthesis route. Experimental Section The measurements were conducted at the Austrian SAXS beamline19 at ELETTRA synchrotron (2 GeV electron storage ring), Trieste, Italy, utilizing the linear position-sensitive Gabriel detector. The SAXS camera was set to a length of 1.5 m and to a photon energy of 8 keV. The reactions were carried out in a batch reactor at room temperature (as described elsewhere7) by the following procedure: An aqueous solutions of hexadecyltrimethylammonium bromide (CTAB) and HCl was mixed with benzene under vigorous stirring. The data acquisition was triggered by the addition of tetraethyl orthosilicate (TEOS) (i.e. t ) 0 s); 1024 frames were recorded in total with a time resolution of 1 s each. The molar composition of the reaction mixtures was CTAB/benzene/TEOS/HCl/water ) 1/x/8.33/76.7/1083, 0 e x e 8.33; this corresponds to a concentration of CTAB in water of 0.051 mol‚L-1.

Results In all syntheses the formation of a mesostructured phase is observed at ∼120-240 s after the start of the reaction by the emergence of Bragg reflections in the SAXS patterns. In a synthesis without the addition of benzene, a single, hexagonal mesostructure forms (Figure 1); the d100 value decreases from 4.21 to 4.13 nm after 1024 s, corresponding to a slight shrinkage of the structure caused by increasing condensation of the inorganic domains. After 24 h the solid product (not dried) exhibits a d100 value of 3.95 nm (diffraction pattern not shown). During the reaction the intensity of all reflections increases, indicative of an increasing degree of structural ordering. A reaction with 1.67 equiv of benzene per CTAB (mol/mol) leads to a similar result (not shown), but the d100 value is slightly larger (initially 4.36 nm) than that in the absence of benzene, which shows that benzene acts as a swelling agent (see Discussion). Further swelling is observed when the molar amount of benzene is increased to 4.58 equiv per CTAB (Figure 2): The hexagonal mesophase has an initial d100 value of 4.49 nm. An additional weak reflection belonging to a second mesophase with a lamellar symmetry and an initial (7) A° gren, P.; Linde´n, M.; Rosenholm, J. B.; Schwarzenbacher, R.; Kriechbaum, M.; Amenitsch, H.; Laggner, P.; Blanchard, J.; Schu¨th, F. J. Phys. Chem B 1999, 103, 5943-5948. (8) O’Brien, S.; Francis, R. J.; Fogg, A.; O’Hare, D.; Okazaki, N.; Kuroda, K. Chem. Mater. 1999, 11, 1822-1832. (9) Rathousky´, J.; Schulz-Ekloff, G.; Had, J.; Zukal, A. Phys. Chem. Chem. Phys. 1999, 1, 3053-3057. (10) A° gren, P.; Linde´n, M.; Rosenholm, J. B.; Blanchard, J.; Schu¨th, F.; Amenitsch, H. Langmuir 2000, 16, 8809-8813. (11) Linde´n, M.; A° gren, P.; Karlsson, S.; Bussian, P.; Amentisch, H. Langmuir 2000, 16, 5831-5836. (12) Pevzner, S.; Regev, O. Microporous Mesoporous Mater. 2000, 38, 413-421. (13) Gross, A. F.; Ruiz, E. J.; Tolbert, S. H. J. Phys. Chem. B 2000, 104, 5448-5461. (14) Gross, A. F.; Le, V. H.; Kirsch, B. L.; Tolbert, S. H. J. Am. Chem. Soc. 2002, 124, 3713-3724. (15) Lind, A.; Andersson, J.; Karlsson, S.; A° gren, P.; Bussian, P.; Amenitsch, H.; Linde´n, M. Langmuir 2002, 18, 1380-1385. (16) Linde´n, M.; Blanchard, J.; Schacht, S.; Schunk, S. A.; Schu¨th, F. Chem. Mater. 1999, 11, 3002-3008. (17) Tiemann, M.; Fro¨ba, M.; Rapp, G.; Funari, S. S. Chem. Mater. 2000, 12, 1342-1348. (18) Huo, Q.; Margolese, D. I.; Stucky, G. D. Chem. Mater. 1996, 8, 1147-1160. (19) Amenitsch, H.; Rappolt, M.; Kriechbaum, M.; Mio, H.; Laggner, P.; Bernstorff, S. J. Synchrotron Radiat. 1998, 5, 506-508.

Figure 1. Temporal evolution of the SAXS pattern for a synthesis of mesostructured silicate/CTAB composites without the addition of benzene, showing the 100, 110, and 200 reflections of a single hexagonal phase.

Figure 2. Temporal evolution of the SAXS pattern for a synthesis of mesostructured silicate/CTAB composites with benzene as an additive and a molar ratio of benzene/CTAB ) 4.58, showing the 100, 110, and 200 reflections of a hexagonal phase as well as the 001 reflection of a lamellar phase.

d001 value of 4.24 nm emerges 170 s after the formation of the hexagonal structure. Initially, the d100 value of the hexagonal phase decreases but then, approximately during the first 90 s of the lamellar phase formation, increases (see Discussion). After that, both phases coexist for the remaining time of the measurement (1024 s), during which both the d100 value of the hexagonal phase and the d001 value of the lamellar phase slightly decrease, owing to the condensation of the inorganic domains. Syntheses with even larger amounts of benzene (6.58 and 8.33 mol per mol of CTAB) show a qualitatively different behavior: Again, a hexagonal phase is formed first, followed by an additional lamellar phase (Figure 3). However, both phases coexist only for a period of ∼90 s, during which the reflections of the hexagonal phase gradually become weaker and then disappear while those of the lamellar phase grow in intensity. This is a secondorder (i.e. continuous) phase transition from hexagonal to lamellar; a similar phase transition from hexagonal to

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Figure 5. Correlation of the initial d100 value of the hexagonal silica/CTAB mesophase with the relative amount of benzene used in the synthesis. Only a slight swelling effect is observed (ca. 11% for the highest molar benzene/CTAB ratio of 8.33); the correlation is approximately linear. (The bars correspond to an estimated error of (2 channels.) Figure 3. Temporal evolution of the SAXS pattern for a synthesis of mesostructured silicate/CTAB composites with benzene as an additive and a molar ratio of benzene/CTAB ) 6.58, showing a phase transition from hexagonal (100, 110, and 200 reflections) to lamellar (001 and 002 reflections).

the coexistence of the lamellar phase, increase (see Discussion). All d100 and d001 values are summarized in Figure 4. Discussion

Figure 4. d100 and d001 values of the hexagonal and lamellar silica/CTAB phases as a function of time for various molar benzene/CTAB ratios. The data were obtained by fitting Gauss profile functions to the diffraction patterns (after normalization and background correction). Low amounts of benzene (benzene/ CTAB e 1.67) lead to a single hexagonal phase, whereas higher benzene/CTAB ratios yield a hexagonal phase (formed first) and a second, lamellar phase; the hexagonal phase either coexists for the rest of the reaction (benzene/CTAB ) 4.58) or is replaced by the lamellar phase (benzene/CTAB g 6.58). All phases contract during the reaction, although the initial formation of the lamellar phase (temporarily) causes the hexagonal phase to expand (see text).

cubic has recently been reported20 (but not studied insitu) for an acidic synthesis of cubic mesoporous silica with trimethylbenzenes as additives. The initial d100 values of the hexagonal phase are 4.59 nm (for the synthesis with 6.58 mol benzene) and 4.67 (8.33 mol benzene), corresponding to further swelling as a function of the relative quantity of benzene (see below, Figure 5). Until the emergence of the lamellar phase, the d100 values of the hexagonal phase decrease and then, during (20) Che, S.; Kamiya, S.; Terasaki, O.; Tatsumi, T. J. Am. Chem. Soc. 2001, 123, 12089-12090.

General Considerations. The effect of additives with low molecular weight on the formation of mesostructured silica under basic conditions has been explored for various short-chain alcohols and amines,7,10,21,22 hexane,11 benzene,22 toluene,11,15 and trimethylbenzenes.1,23,24 The latter have also been shown20 to have an influence on the acidic synthesis of cubic mesoporous silica. Aromatic compounds exhibit significantly higher solubilities in micellar ionic surfactant/water systems (like CTAB/water) than those for saturated hydrocarbons of comparable molecular weight.25 A lower degree of alkyl substitution of the aromatic ring leads to a higher solubility in such micellar systems: 0.66 g of benzene per g of CTAB (3.08 mol/mol) has been reported to be solubilized in a CTAB/D2O system (with a concentration of 0.15 mol‚L-1);26 in comparison, the solubility of toluene in CTAB/H2O is only 0.20 g per g of CTAB (0.79 mol/mol).27 A similar trend applies to the water-solubility of simple aromatic compounds, which is 0.023 mol‚L-1 for benzene, compared to only 0.0072 mol‚L-1 for toluene.28 For the pure benzene/CTAB/water system (0.1 mol‚L-1 CTAB/water) it has been shown29 that up to a certain limit (benzene/CTAB e 1) most of the benzene is solubilized near the micellar interface; only when the relative amount of benzene is increased (benzene/ water g 2) is a significant fraction found to reside in the micellar core region. Likewise, for systems with a much higher CTAB concentration (0.5 mol‚L-1) benzene has been (21) Kleitz, F.; Blanchard, J.; Zibrowius, B.; Schu¨th, F.; Agren, P.; Linde´n, M. Langmuir 2002, 18, 4963-4971. (22) Firouzi, A.; Schaefer, D. J.; Tolbert, S. H.; Stucky, G. D.; Chmelka, B. F. J. Am. Chem. Soc. 1997, 119, 9466-9477. (23) Huo, Q.; Margolese, D. I.; Ciesla, U.; Demuth, D. G.; Feng, P.; Gier, T. E.; Sieger, P.; Firouzi, A.; Chmelka, B. F.; Schu¨th, F.; Stucky, G. D. Chem. Mater. 1994, 6, 1176-1191. (24) Desplantier-Giscard, D.; Galarneau, A.; Di Renzo, F.; Fajula, F. Stud. Surf. Sci. Catal. 2001, 135, 06-P-27. (25) Chaiko, M. A.; Nagarajan, R.; Ruckenstein, E. J. Colloid Interface Sci. 1984, 99, 168-182. (26) Duns, G. J.; Reeves, L. W.; Yang, D. W.; Williams, D. S. J. Colloid Interface Sci. 1995, 173, 261-264. (27) Dam, T.; Engberts, J. B. F. N.; Kartha¨user, J.; Karaborni, S.; van Os, N. M. Colloids Surf., A 1996, 118, 41-49. (28) American Petroleum Institute Technical Data Book; Washington, DC, 1979. (29) Hedin, N.; Sitnikov, R.; Furo´, I.; Henriksson, U.; Regev, O. J. Phys. Chem. B 1999, 103, 9631-9639.

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found to be solubilized preferentially near the micellar interface with a preferred orientation.30,31 In the system studied here the solubilization behavior may be quantitatively different from that of the pure additive/surfactant/water systems in the equilibrium state, since the various silicate species will have a substantial impact on the micellar interface. (In this context it should be kept in mind that the solubilization behavior in pure benzene/cetyltrimethylammonium systems strongly depends on the nature of the counterion of the surfactant.32) Also, due to the hydrolysis of TEOS, considerable amounts of ethanol are present in the system studied here, which will have an influence on the solubilization. Nevertheless, the considerations made above may serve as a rough, qualitative guideline here and it may be concluded that, compared to the case of the abovementioned alkyl-substituted aromatic additives which have been used in former syntheses of mesostructured silica, a higher amount of benzene can be (i) solubilized in the initial reaction mixture and (ii) incorporated in the micellar surfactant aggregates. (iii) Since benzene is less hydrophobic (and better water-soluble) than alkylsubstituted aromatic compounds, a larger fraction of benzene molecules is expected to be located near the surfactant headgroups and a smaller fraction in the cores of the micelles. Swelling Effect and Location of Benzene in the Hexagonal Phase. As mentioned above, the d100 value of the hexagonal phase, which is the first mesostructured phase to form in all syntheses, depends on the relative amount of benzene added, as shown in Figure 5. Two results should be stressed: (i) The swelling effect is relatively weak; 8.33 equiv of benzene per CTAB (mol/ mol) leads to an increase of the d100 value of less than 0.5 nm (as compared to the benzene-free synthesis), corresponding to a swelling by only 11%. (ii) An approximately linear dependence of the d100 value on the relative quantity of benzene is observed, although the first value (for the benzene-free synthesis) does not fully comply to this trend. These findings indicate that only a small fraction of the benzene molecules are located in the cores of the rodlike micelles; otherwise the swelling would have to be much more pronounced. Instead, the location of most of the benzene molecules that are attached to the micellar aggregates is more likely near the surfactant headgroups, which does not lead to any swelling of the micelle diameter.33 A significant fraction of the benzene may also be homogeneously dissolved in water (i.e. not attached to the micelles) because, as indicated above, a substantial increase in the solubility of benzene due to the subsequent release of ethanol by the hydrolysis of TEOS (up to a theoretical total of 25 mol of ethanol per mol of CTAB) is to be expected. This effect has been shown to increase the solubility of toluene by a factor of more than 3 in a similar system.11 On the other hand, the fact that a low degree of swelling is still observed indicates that a minor fraction of the benzene molecules do reside in the micellar cores. This is consistent with the considerations made above. The utilization of alkyl-substituted aromatic additives with the same surfactant (CTAB) in the synthesis of mesostructured silica has led to much stronger swelling (by up to 30% for toluene11,15 and 75-100% for mesitylene1,24), (30) Eriksson, C. J.; Gillberg, G. Acta Chem. Scand. 1966, 20, 2019. (31) Henriksson, U.; Klason, T.; O ¨ dberg, L.; Eriksson, J. C. Chem. Phys. Lett. 1977, 52, 554-558. (32) Cerichelli, G.; Mancini, G. Langmuir 2000, 16, 182-187. (33) Kunieda, H.; Ozawa, K.; Huang, K.-L. J. Phys. Chem. B 1998, 102, 831-838.

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and it was concluded that the preferred location of those molecules was in the micellar cores. In those cases the dependency of the d100 values on the relative amounts of the respective additive was usually marked by a steep increase followed by a plateau at fairly low relative quantities of additive; a maximum of swelling was reached, for example, at an approximate molar ratio of mesitylene/ CTAB ) 5.24 The fact that no plateau is observed in this study is another indication (in addition to the overall low degree of swelling) that most of the benzene is not located in the micellar cores, because the maximum of benzene molecules that can be accommodated there is apparently not yet reached even at a molar ratio of benzene/CTAB ) 8.33. Phase Transition and Lamellar Phase. Depending on the amount of benzene solubilized in the surfactant aggregates, a lamellar structure forms after the hexagonal phase. Similar results22 were obtained when benzene was used in a basic synthesis of mesostructured silica: Hexagonal products were obtained when the molar benzene/ CTAB ratio was lower than 2.3, whereas lamellar phases formed when the relative amount of benzene was larger. However, in that synthesis methanol was used as a cosolvent (and tetramethyl orthosilicate was used instead of TEOS); the reaction was not further investigated by in-situ techniques. In the system studied here, the hexagonal phase either coexists for the rest of the observation period (1024 s) or disappears as the result of a second-order hexagonal-to-lamellar phase transition (compare Figure 4). This corresponds to a gradual decrease of the curvature of the surfactant aggregates (changing from rodlike to planar), which is correlated with (i) increasing initial relative amounts of benzene (as no lamellar phase is observed without or with low quantities of benzene) and (ii) increasing duration of the reaction (because the lamellar phase always forms later than the hexagonal phase). Essentially, both factors appear to be one and the same when it is taken into account that the amount of benzene solubilized in the surfactant aggregates should increase in the course of the reaction: As TEOS is hydrolyzed, more and more ethanol is released to the reaction mixture (as mentioned above), which significantly enhances the solubility of benzene in the aqueous domains and, hence, the availability of benzene to be accommodated in the surfactant arrays. A second significant factor is the increasing degree of silicate polymerization, which progressively reduces the charge density at the silicate/ surfactant interface. In the absence of benzene (or other additives), this will usually increase the curvature of this interface, which is why lamellar-to-hexagonal phase transitions are frequently observed;2 in this case, however, a lower charge density also facilitates the incorporation of growing amounts of benzene into this region. For these two reasons it may be concluded that a larger amount of benzene solubilized in the surfactant aggregates is responsible for the reduction of the curvature and, hence, for the eventual formation of a lamellar structure. The finding that larger amounts of benzene make the transition from rodlike to planar more favorable may be explained in the light of the fact that, as discussed above, most of the benzene molecules are initially located near the surfactant headgroups rather than in the micellar cores. A possible mechanism is that, as more and more benzene is incorporated, a growing fraction of it will be “pushed” deeper into the micelles, that is, farther away (34) Monnier, A.; Schu¨th, F.; Huo, Q.; Kumar, D.; Margolese, D.; Maxwell, R. S.; Stucky, G. D.; Krishnamurty, M.; Petroff, P.; Firouzi, A.; Janicke, M.; Chmelka, B. F. Science 1993, 261, 1299-1303.

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is consistent with the data found for benzene solubilization in the pure CTAB/water system29 (see general considerations above). It also explains why the d100 value of the hexagonal phase actually increases during the formation of the lamellar phase (see Figures 3 and 4): The decrease in the curvature is continuous and therefore leads to a substantial increase in the diameter of the rodlike micelles before the transition to the lamellar arrangement takes place. This situation corresponds to a subsequent swelling of the rodlike micelles, which is exactly to be expected given the fact that the shift of the benzene molecules deeper into the micelles formally corresponds to an increasing degree of solubilization near the core of the micelle. Summary The utilization of benzene as an additive in the synthesis of mesoscopically ordered silica/CTAB composites under acidic aqueous conditions leads to hexagonal and/or lamellar phases, depending on the relative amount of benzene employed during the reaction. Only a small fraction of the benzene is solubilized in the core of the micellar aggregates, which explains why only a weak swelling effect is observed; the degree of swelling depends linearly on the benzene/CTAB ratio. Most of the benzene molecules are solubilized near the surfactant headgroups. In all syntheses a hexagonal phase forms first. When the benzene/CTAB ratio is high enough (4.58 mol/mol), a second, lamellar phase forms afterward, which either coexists with the hexagonal phase or (at even higher benzene/CTAB ratios of 5.58 mol/mol or more) replaces it as the result of a second-order phase transition. The phase behavior is explained by an increase of the curvature of the micellar aggregates with increasing amounts of solubilized benzene. Figure 6. Schematic drawing of the change in the curvature of the surfactant/silicate aggregates. As the reaction proceeds, an increasing amount of benzene is incorporated in the surfactant/silicate interface region and a growing fraction of the benzene molecules are forced deeper into the micelles, that is, farther away from the surfactant headgroups, which finally leads to a preferred planar (lamellar) arrangement.

from the headgroups, which will lead to a decrease in the curvature (and finally to a planar arrangement, i.e., to a lamellar phase), as depicted in Figure 6. This mechanism

Acknowledgment. M.T. would like to acknowledge the European Community for financial support of this work within the TMR network “NUCLEUS” (HPRNCT-1999-00025). M.L. would like to thank the Finnish National Technology Agency (TEKES) for financial support. F.B., V.G., and R.B. would like to acknowledge the European Community for financial support of this work within the TMR network “Oxycarbide glasses” (FMRXCT9801621). LA026473W