Controlled Solubilization of Toluene by Silicate− Catanionic Surfactant

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Controlled Solubilization of Toluene by Silicate-Catanionic Surfactant Mesophases as Studied by in Situ and ex Situ XRD Anna Lind,† Jenny Andersson,† Stefan Karlsson,† Patrik A° gren,†,‡ Patrick Bussian,‡ Heinz Amenitsch,§ and Mika Linde´n*,† Department of Physical Chemistry, A° bo Akademi University, Porthansgatan 3-5, FIN-20500 Turku, Finland, Max-Plank Institut fu¨ r Kohlenforschung, Kaiser-Wilhelm-Platz 1, D-45470, Mu¨ lheim an der Ruhr, Germany, and Institute of Biophysics and X-ray Structure Research, Austrian Academy of Sciences, Schmiedlstrasse 6, A-8042, Graz, Austria Received August 6, 2001. In Final Form: November 9, 2001 Mesoscopically ordered silicate-surfactant composite materials of the M41S type synthesized in the presence of a swollen agent have been characterized by in situ and ex situ X-ray diffraction analysis. The key feature of the room-temperature synthesis is the use of a mixture of cationic and anionic surfactants as structure-directing agents. The lower interfacial charge density of the mixed surfactant aggregates stabilizes structures of lower interfacial curvature and therefore facilitates a more controlled solubilization of organic swelling agents. An increased solubilization capacity of the catanionic surfactant-silicate mesophase was observed close to an anionic/cationic surfactant ratio corresponding to a transition to the lamellar phase in the absence of toluene. In the presence of toluene, the catanionic template stabilizes microemulsion droplets that serve as building blocks for the final material. However, a fair amount of organic compound is solubilized in the silica-catanionic surfactant composite after the mesophase is formed. Although the present communication concerns mesoporous silica, the concept is a general one and may allow the synthesis of non-siliceous large-pore materials.

Introduction Silicate-surfactant composite structures that possess mesoscopic order have attracted large interest since the pioneering work of the Mobil Oil group published in 1992.1,2 The composite is synthesized in the presence of positively charged surfactant, typically CnTAB (n ) 8-18), at elevated pH where the silicate species are partially negatively charged. Stucky and co-workers3 concluded that the mesophase formation is a cooperative self-assembly process where multidentate binding of silicate oligomers, preferred polymerization of silicate at the surfactantsilicate interface, and charge density matching are key factors in the formation of a surfactant-silicate mesophase. While early stages of mesophase formation are determined mainly by the surfactant properties, further structural changes to lower energy conformations may be induced as changes in the charge density of the silicate occur upon silicate condensation. Recently, Zana and coworkers4,5 showed that some degree of prepolymerization of silica seems to be a prerequisite for the formation of the silicate-surfactant mesophase. When tetraethyl orthosilicate (TEOS) is used as the silica source at moderately alkaline pH, the hydrolysis of the precursor is the rateA° bo Akademi University. Max-Plank Institut fu¨r Kohlenforschung. § Austrian Academy of Sciences. † ‡

(1) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C. Nature 1992, 359, 710. (2) 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. (3) Monnier, A.; Schu¨th, F.; Huo, Kumar, Q. D.; Margolese, D.; Maxwell, R. S.; Stucky, G. D.; Krishnamurty, M.; Petroff, P.; Firouzi, A.; Janicke, M.; Chmelka, B. F. Science 1993, 261, 1299. (4) Zana, R.; Frasch, J.; Soulard, M.; Lebeau, B.; Patarin, J. Langmuir 1999, 15, 2603. (5) Frasch, J.; Lebeau, B.; Soulard, M.; Patarin, J. Langmuir 2000, 16, 9049.

limiting step for the formation and the event is the emulsification of the hydrophobic TEOS by the surfactant under stirring.6 Striking evidence for the lyotropic nature of the silicate-surfactant mesophase is the ability to swell the pores by solubilizing hydrophobic additives within the hydrophobic regions of the surfactant aggregates.1,2,7 Usually aromatic hydrocarbons, like trimethylbenzene, TMB, have been used for this purpose, although saturated hydrocarbons have been used as well.8 If block copolymers are used as templates, large-pore mesocellular foams are obtained.9 In situ small-angle X-ray scattering (SAXS)/ X-ray diffraction (XRD) has shown to be a powerful tool for the investigation of structural evolvement during the synthesis of ordered mesostructured materials.10-18 In a recent in situ study of the solubilization of toluene by a MCM-41 type surfactant-silicate mesophase, it was (6) 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. (7) Huo, Q.; Margolese, D. I.; Ciesla, U.; Demuth, D.; Feng, P.; Gier, T. E.; Sieger, P.; Firouzi, A.; Chmelka, B. F.; Schu¨th, F.; Stucky, G. D. Chem. Mater. 1994, 6, 1176. (8) Ulagappan, N.; Rao, C. N. R. Chem. Commun. 1996, 2759. (9) Schmidt-Winkel, P.; Lukens, W. W., Jr.; Zhao, D.; Yang, P.; Chmelka, B. F.; Stucky, G. D. J. Am. Chem. Soc. 1999, 121, 254. (10) O’Brien, S.; Francis, R. J.; Price, S. J.; O’Hare, D.; Clark, S. M.; Okazaki, N. Chem. Commun. 1995, 2423. (11) Linde´n, M.; Schunk, S. A.; Schu¨th, F. Angew. Chem., Int. Ed. 1998, 37, 821. (12) Rathousky´, J.; Schulz-Ekloff, G.; Had, J.; Zukal, A. Phys. Chem. Chem. Phys. 1999, 1, 3053. (13) Linde´n, M.; Blanchard, J.; Schacht, S.; Schunk, S. A.; Schu¨th, F. Chem. Mater. 1999, 11, 3002. (14) O’Brien, S.; Francis, R. J.; Fogg, A.; O’Hare, D.; Okazaki, N. Chem. Mater. 1999, 11, 1822. (15) Pevzner, S.; Regev, O. Microporous Mesoporous Mater. 2000, 38, 413. (16) Tiemann, M.; Fro¨ba, M.; Rapp, G.; Funari, S. S. Chem. Mater. 2000, 12, 1342. (17) Ruiz, E. J.; Tolbert, S. H. J. Phys. Chem. B 2000, 104, 5448. (18) Gross, A. F.; Le, V. H.; Kirsch, B. L.; Tolbert, S. H. Langmuir 2001, 17, 3496.

10.1021/la011240a CCC: $22.00 © 2002 American Chemical Society Published on Web 01/18/2002

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shown that a fair amount of the toluene was solubilized after the mesophase had formed.19 This effect clearly originates from a higher solubilization capacity of the formed surfactant-polymeric silica aggregates compared to that of the corresponding micellar surfactant solution void of silicate. Dilute, aqueous solutions of hexadecyltrimethylammonium bromide (CTAB) alone have a very limited hydrocarbon solubilization capacity, due to the preferred high interfacial curvature of charged micelles.20 Cosurfactants, that is, short-chain polar substances such as n-alcohols, are often used together with CTAB if a higher solubilization capacity is needed. The cosurfactants accumulate in the palisade layer of the micelle and decrease the preferred interfacial curvature. We have studied the influence of a large number of cosurfactants (anionic, cationic, and nonionic) on a standard MCM-41 synthesis.6,21,22 The addition of anionic or nonionic cosurfactants led to the formation of structures of lower curvature, normally the lamellar phase at room temperature. Furthermore, Li et al. have demonstrated the value of using mixtures of cationic and anionic surfactants in the synthesis of silica23,24 and alumina25 MCM-48 under hydrothermal conditions. The use of a mixture of cationic and anionic surfactants, often referred to as catanionic surfactants, is a simple means of adjusting the charge density at the micellar interface. The motivation for the present work is to use this concept to increase the toluene solubilization capacity of a silicate-surfactant mesophase in a controlled way and therefore allow for a more reproducible means of controlling the pore size of the calcined material. However, an increased solubilization capacity of the as-made material is a value of its own in potential applications such as drug release.26 This communication concerns the characterization of the solubilization process both ex situ and in situ. Experimental Section Materials. The following chemicals were used in the synthesis: Hexadecyltrimethylammonium bromide and tetraethyl orthosilicate were obtained from Aldrich. Ammonia (25%) was supplied by J.T. Baker. Hexanoic acid, octanoic acid, decanoic acid, dodecanoic acid, tetradecanoic acid, and toluene were supplied by Fluka. All chemicals were used as received. The water was purified by distillation and deionization. Synthesis. CTAB (m ) 2.4 g) and fatty acid, HA (hexanoic, octanoic, decanoic, dodecanoic or tetradecanoic acid), were dissolved in water (m ) 120 g) by stirring (500 rpm) at 30 °C. After complete dissolution of the CTAB and the fatty acid, 10 mL of 25% NH3 was added and the mixture was stirred for about 5 min. The pH was adjusted to 11.8 by addition of NaOH. The indicated amount of toluene was then added to the synthesis mixture, which was then stirred for another 5 min. TEOS (m ) 9.4 g) was rapidly added to the solution to give a final molar composition of 157/3/0.15/1/x/y H2O/NH3/CTAB/TEOS/HA/toluene. The samples characterized ex situ were isolated by filtering after 60 min of stirring. X-ray Analysis. The SAXS measurements were performed on a Kratky compact small-angle system. A Seifert ID-3003 X-ray generator, operating at a maximum intensity of 50 kV and 40 (19) Linde´n, M.; A° gren, P.; Karlsson, S.; Bussian, P.; Amenitsch, H. Langmuir 2000, 16, 5831. (20) Dam, Th.; Engberts, J. B. N. F.; Ka¨rtha¨user, J.; Karaborni, S.; van Os, N. M. Colloids Surf., A 1996, 118, 41. (21) A° gren, P.; Linde´n, M.; Rosenholm, J. B.; Blanchard, J.; Schu¨th, F.; Amenitsch, H. Langmuir 2000, 16, 8809. (22) Lind, A.; Andersson, J.; Karlsson, S.; Linde´n, M. Colloids Surf., A 2001, 183-185, 415. (23) Chen, F.; Huang, L.; Li, Q. Chem. Mater. 1997, 9, 2685. (24) Chen, F.; Yan, X.; Li, Q. Stud. Surf. Sci. Catal. 1998, 117, 273. (25) Chen, F.; Song, F.; Li, Q. Microporous Mesoporous Mater. 1999, 29, 315. (26) Vallet-Regi, M.; Ra´mila, A.; del Real, R. P.; Pe´rez-Pariente, J. Chem. Mater. 2001, 13, 308.

Langmuir, Vol. 18, No. 4, 2002 1381 mA, provided the Cu KR radiation of wavelength 1.542 Å. A Ni filter was used to remove the Kβ radiation, and a W filter was used to protect the detector from the primary beam. The system was equipped with a position-sensitive detector consisting of 1024 channels of 54.9 µm each. The sample-to-detector distance was 277 mm. To minimize the background scattering from air, the camera housing was evacuated during the measurements. The measurements were performed on wet samples, if not otherwise stated. In Situ X-ray Diffraction Analysis. The in situ XRD measurements were performed in an s-range from 0.035 to 0.85 nm-1, with an exposure time down to 0.3 s, at the Austrian highflux SAXS beam-line of the 2 GeV electron storage ring ELETTRA, Trieste, Italy.27 The experimental setup has been described in detail in a previous paper.6 The setup consists of a beaker containing the synthesis mixture, a peristaltic pump, tubing, and a quartz capillary. The reaction was started by initiating the mixing of the two-phase solution, and at the same time the pumping was started. The synthesis solution was pumped through a 1 mm diameter quartz capillary centered in the X-ray beam at a pumping rate of 25 mL/min. Both the batch reactor and the capillary were kept at 30 °C. The radiation wavelength was 1.542 Å. A linear position-sensitive Gabriel detector was used, which enabled simultaneous detection of the whole angular range studied.

Results CTAB/HA/TEOS/H2O/NH3. The influence of added fatty acid, HA, on the phase behavior of a CTAB-based, NH3-catalyzed synthesis of MCM-41 type silica was investigated as a function of fatty acid/CTAB molar ratio, m, and alkyl chain length of the acid. The acids investigated ranged from hexanoic to tetradecanoic acid. Assynthesized, wet samples isolated by filtration after a reaction time of 60 min were investigated by SAXS. Generally, a hexagonal to lamellar phase transition was observed with increasing m over an intermediate region where the two phases coexisted, as shown for the decanoic acid/CTAB system in Figure 1. The hexagonal-to-lamellar phase transition occurred at lower values of m with increasing fatty acid chain length. Interestingly, a relative increase in the intensity of the high-order reflections was observed upon addition of HA at amounts lower than that at which the lamellar phase is formed (results not shown). This behavior has been observed before also for corresponding benzoic acid/CTAB systems22 and probably corresponds to a change in the form factor of the formed structures. The d-spacing of the hexagonal phase increased with increasing m values for all fatty acids studied, except for hexanoic acid. The maximum d-spacing measured for the hexagonal phase is shown as a function of fatty acid chain length in Figure 2. A linear correlation between the d-spacing and the fatty acid chain length was obtained, as previously observed for CTAB/fatty alcohol mixtures.28 The d001-spacing of the lamellar phase also increased with increasing HA chain length as well as increasing HA/ CTAB ratio. These features clearly suggest that the organic portion of the composite material is a mixture of fatty acid and CTAB. This conclusion is also supported by IR measurements performed on thoroughly washed and dried powder, where a continuous increase in the intensity of the asymmetric stretching band of COO- at 1564 cm-1 was observed with increasing decanoate/CTAB ratio (results not shown). Solubilization of Toluene by CTAB/Decanoic Acid/ TEOS/H2O/NH3 Systems. The solubilization of toluene by the decanoic acid/CTAB templated silica was studied (27) Amenitsch, H.; Bernstorff, S.; Laggner, P. Rev. Sci. Instrum. 1995, 66, 1624. (28) Blanchard, J.; Kleitz, F.; Schu¨th, F.; A° gren, P.; Linde´n, M.; Rosenholm, J. B. Submitted.

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Figure 3. XRD patterns of decanoic acid/CTAB templated mesoscopic silica as a function of decanoic acid/CTAB molar ratio, m, at a constant toluene/CTAB molar ratio, o, of 6.6. Figure 1. XRD patterns of decanoic acid/CTAB templated mesoscopic silica as a function of decanoic acid/CTAB molar ratio, m.

Figure 2. Maximum observed d100-spacing for fatty acid/CTAB templated silica as a function of fatty acid chain length.

Figure 4. Observed d100-spacings of the hexagonal phase of decanoic acid/CTAB templated mesoscopic silica as a function of decanoic acid/CTAB molar ratio, m, and toluene/CTAB molar ratio, o.

as a function of toluene/CTAB and decanoic acid/CTAB molar ratio, o and m, respectively. Generally, the swelling of the hexagonal phase was increased with increasing decanoic acid/CTAB ratio, m, at any fixed toluene addition studied. An example of this behavior is shown in Figure 3 for o ) 6.6. Furthermore, the degree of swelling increased with increasing toluene concentration, as expected. In Figure 4, the observed d100-spacings of the hexagonal phase for fixed additions of toluene are plotted as a function of decanoic acid/CTAB ratio. An almost linear increase in the d100-spacing was observed for decanoic acid/CTAB molar ratios up to about 0.35. The swelling increased with increasing amounts of toluene added. However, at decanoic acid/CTAB molar ratios greater than 0.35 a transition to another linear region was observed with a higher degree of swelling compared to that observed at decanoic acid/ CTAB ratios equal to 0.35. This molar ratio very closely corresponds to that needed for the lamellar phase to be

dominant in the XRD diffractograms of samples synthesized in the absence of toluene, as also indicated in Figure 4. In Situ XRD Investigation of the Initial Stages of Composite Formation. In situ XRD measurements were undertaken in order to investigate the kinetics of solubilization of decanoic acid and toluene by the silicatesurfactant aggregates in more detail. The use of synchrotron irradiation allowed high-quality diffractograms to be collected at accumulation times less than 1 s. Decanoic Acid/CTAB/TEOS/NH3/H2O. An example of a time-resolved XRD measurement is shown in Figure 5 for an m value of 0.157 and without addition of toluene. This ratio corresponds to the region where only the hexagonal phase was observed in the conventional XRD measurements described above for samples isolated after a reaction time of 60 min. The diffractogram shown in Figure 5 is very similar to that measured for a corresponding system void of decanoic acid.6 The hexagonal mesophase was first

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Figure 5. Time-resolved in situ synchrotron SAXS pattern of decanoic acid/CTAB templated mesoscopic silica at a decanoic acid/CTAB molar ratio of 0.157.

Figure 7. Time-resolved in situ synchrotron SAXS pattern of decanoic acid/CTAB templated mesoscopic silica at a decanoic acid/CTAB molar ratio of 0.19 and a toluene/CTAB molar ratio of 6.6.

Figure 6. Time-resolved in situ synchrotron SAXS pattern of decanoic acid/CTAB templated mesoscopic silica at a decanoic acid/CTAB molar ratio of 0.35.

Figure 8. Time-resolved in situ synchrotron SAXS pattern of decanoic acid/CTAB templated mesoscopic silica at a decanoic acid/CTAB molar ratio of 0.35 and a toluene/CTAB molar ratio of 6.6.

observed after about 80 s, and it shrunk continuously (0.20.3 nm in d100) during the first minutes to finally reach a fairly constant value of d100 ) 4.3 nm. The reflection intensity also increased dramatically during this time. If the m value was increased to 0.35, a quite different evolution of the diffractogram was observed, as shown in Figure 6. The most obvious difference was the presence of a coexisting lamellar phase together with the hexagonal phase. However, several other differences could be observed. The shrinkage of the hexagonal phase with time was now much more pronounced, ∆d100 ) 0.5 nm. Furthermore, the scattered intensity in the diffuse region increased strongly with time, indicative of the formation of smaller particles. The d100-spacing of the hexagonal phase was about 4.53 nm, while the d001-spacing of the lamellar phase was about 3.72 nm at t ) 600 s. These values agree quite well with those measured for the wet samples after a reaction time of 60 min; see Figure 2. Toluene Added. A pronounced swelling of the hexagonal mesophase was observed if corresponding measurements were performed in the presence of toluene. The timeresolved diffractogram measured for m ) 0.19 and o ) 6.6 is shown in Figure 7. The hexagonal mesophase formed after about 75 s with an initial d100-spacing of 5.88 nm but immediately swelled to reach a d100 value of 6.32 nm at t ) 600 s. This behavior was similar to that observed in the absence of added decanoic acid,19 but in that case the degree of swelling was lower. In the absence of decanoic acid, the d100-spacing at t ) 600 s was 5.35 nm.19 The intense diffuse scattering at short reaction times originates from scattering from TEOS and toluene containing (micro)emulsion droplets, but the intensity of the diffuse scat-

tering decreased fast and remain fairly low throughout the experiment. Another mesophase is observed in the diffractograms at t ) 220 s with a d-spacing of 4.17 nm at t ) 600 s. Although the intensities of the reflections of this phase are low, there are clear indications that this phase is a lamellar phase. Increasing the decanoic acid/ CTAB ratio to 0.33 still keeping the toluene content at o ) 6.6 led to a much more swollen hexagonal phase with a d100 value of 7.95 nm at t ) 600 s, as shown in Figure 8. Again a pronounced swelling was observed after the mesophase had formed. The coexisting lamellar phase had a d001-spacing of 4.31 nm. The scattered intensity in the diffuse region remained high throughout the experiment, and as in the case with no added toluene, the high intensity at short reaction times originated from micelles and (micro)emulsion droplets while that at longer reaction times originated from the formation of small particles. The summary of the observed d100-spacings for different m and o values is shown in Figure 9 as a function of reaction time. Generally, the following observations can be made: (1) the final d-spacing increased with increasing m and o, (2) the extent of contraction of the hexagonal phase at o ) 0 increased with increasing m, (3) the time at which the first indication of the hexagonal phase was observed increased with increasing o, and (4) the time window within which swelling was observed in the presence of toluene widened with increasing o. For reasons of clarity, the lamellar phase is not included in Figure 9. However, the d001-spacing of the lamellar phase also increased slightly with increasing m and o.

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Figure 9. Position of the (100) reflection as a function of time for different decanoic acid/CTAB molar ratios, m, and toluene/ CTAB molar ratios, o.

Discussion Solution Behavior of Oppositely Charged Surfactants. The term catanionic surfactant was first introduced by Jokela et al.29 as a designation of a class of surfactants consisting of a cationic and an anionic surfactant in equimolecular ratio, where the inorganic counterions have been removed. When the two surfactants together with the counterions are present at mixing ratios other than 1:1, the term catanionic mixture is used. When the chain length of the two surfactants is similar, structures with a mean curvature close to zero, that is, lamellar and bicontinuous cubic phases, are often encountered in aqueous systems. However, if there is an asymmetry in the chain lengths a different phase behavior is observed. The borderline between catanionic surfactants and surfactants with an organic counterion is diffuse and will depend on the water solubility of the shorter surfactant. If the aqueous solubility of the countersurfactant is high, the phase behavior of the surfactant mixture will be similar to that of the longer surfactant.30 Furthermore, the isotropic solution phase is often stabilized in the presence of short and intermediate chain lengths of the countersurfactants,30,31 and the expected formation of the lamellar phase is observed when the difference in chain length between the two oppositely charged surfactants is small.31 The mixed micelles are often not spherical but rodlike, due to the decrease in the preferred interfacial curvature. Generally, we observe a similar effect for the catanionic CTAB/fatty acid mixtures used as templates for mesoscopically ordered silicate in the present study. The fatty acids have a pKa value of about 4.7 and are all completely dissociated at the synthesis pH of 11.8. It seems clear that the n-carboxylates form mixed micelles with the CTAB, as expected both from their opposite charge and from the amphiphilic nature of the fatty acid salts. In all cases except for hexanoic acid, the addition of fatty acid leads to a gradual swelling of the hexagonal mesophase and finally to a transition to the lamellar phase, as schematically shown in Figure 10. Furthermore, the fatty acid induced stabilization of longer surfactant-silicate rods. A similar effect has also recently been reported for benzoic acid.22 The sol viscosity is also greatly increased upon addition of larger amounts of fatty acid, especially decanoic acid, which also indicates the presence of long, rodlike micelles before addition of TEOS. (29) Jokela, P.; Jo¨nsson, B.; Khan, A. J. Phys. Chem. 1987, 91, 3291. (30) Eastoe, J.; Rogueda, P.; Shariatmadari, D.; Heenan, R. Colloids Surf., A 1996, 117, 215. (31) Persson, G.; Lindstro¨m, B. Prog. Colloid Interface Sci. 1997, 105, 317.

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Figure 10. The ion-pairing between CTAB and fatty carboxylate and the resulting decrease in the interfacial curvature of the silicate-surfactant materials (also in the presence of toluene) are schematically shown. The silicate is excluded for clarity.

Influence of Silicate Anions on the Solution Behavior of CTA+/A- Mixtures. While the special features of aqueous solutions of catanionic surfactant mixtures originate from both electrostatic and hydrophobic van der Waals interactions, it may be assumed that the electrostatic contribution to this interaction is smaller in systems containing negatively charged silicate monomers, oligomers, and polymers. It is therefore possible that the silicate (poly)anions could destabilize the fatty acid/CTAB complex. The magnitude of the attractive chain-chain interactions will increase with increasing chain length of the fatty acid. Therefore, it could be expected that the stability of the CTAB-fatty acid complex also increases with increasing fatty acid chain length. The fact that neither a transition to the lamellar phase nor a swelling, but rather a slight contraction, of the hexagonal phase was observed upon addition of hexanoic acid, while it was observed for all other fatty acids studied, could originate from this effect. The much higher water solubility of completely dissociated hexanoic acid compared to that of all other fatty acids studied, coupled with a destabilization of the CTAB-fatty acid complex in the presence of negatively charged silicate oligomers, could lower the fraction of hexanoic acid/CTAB in the composite structure compared to that of the mixed hexanoic acid/CTAB micelles. Swelling of the hexagonal mesophase in the presence of cosurfactants has previously been observed for both fatty n-alcohols (nC g 4) and n-amines (nC g 6) and was attributed to the increase in the hydrocarbon volume of the organic portion of the composite upon cosurfactant solubilization. This explanation also holds true for all the fatty acids studied here, except for hexanoic acid. The slight decrease in the d100-spacing is suggested to originate from an expulsion of some of the hexanoic acid into the bulk during silicate-surfactant mesophase formation, due to weakening of the hexanoic acid/CTAB complex because of competing electrostatic silicate/CTAB interactions. The in situ SAXS measurements on the CTAB/decanoic acid/TEOS system revealed another effect of the catanionic surfactant mixture. As already discussed earlier,6,32 the first step of the reaction is the emulsification of the TEOS by surfactant-stabilized droplets, which increases the hydrolysis rate of TEOS due to an increased TEOS contact (32) Linde´n, M.; Schunk, S.; Schu¨th, F. Angew. Chem., Int. Ed. 1998, 37, 821.

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area with water. The lower preferred interfacial curvature of decanoic acid/CTAB complexes compared to that of CTAB alone will lead to an increased TEOS solubilization capacity of the mixed surfactant aggregates. The mean diameter of the aggregates formed will therefore increase and this is seen as a higher initial d100-spacing of the mesophase formed. However, as the reaction proceeds, the aggregates will continuously shrink due to release of water-soluble, hydrolyzed silica species. The final dspacing will then depend only on the total amount of surfactant incorporated into the composite. Effect of the Addition of Toluene. The lower preferred interfacial curvature of the catanionic fatty acid/ CTAB complexes also gives an explanation for the high scattering intensity observed at low angles after introduction of the toluene, since it is obvious that microemulsion droplets containing a mixture of TEOS and toluene have formed in the sol. The hydrolysis rate of TEOS is decreased if toluene is added to a synthesis in the absence of cosurfactant,19 since the effective TEOS/ water contact area is decreased when TEOS is mixed with toluene. However, the presence of toluene did not slow the formation rate of the hexagonal mesophase in the presence of decanoic acid, which can be related to the increased stability of microemulsion droplets stabilized by catanionic surfactant and their high surface/volume ratio compared to those formed in the absence of decanoic acid. The lower preferred interfacial curvature of the catanionic fatty acid/CTAB/silica complexes also explains the observed increase in toluene solubilization capacity. We have previously observed that solubilization of both amphiphilic6,21 and hydrophobic solubilizates19 occurs also after the composite mesophase has formed. It has been shown that the transport of hydrocarbon between emulsion droplets or from emulsion droplets to micelles through the aqueous continuous phase may proceed at a significant rate if the hydrocarbon is slightly water soluble.33 Toluene, having comparatively high water solubility, is able to diffuse into the formed surfactant-silicate composite. It is also to be expected that the silicate-surfactant aggregates are kinetically more stable than the corresponding pure surfactant aggregates. The solubilization time of arenes by micelles has been shown to decrease with increasing micellar stability,34 and this effect may also play a role in our system. Freezing of the silicatesurfactant interface due to silicate condensation will eventually put a limit to the incorporation of more toluene into the interior of the aggregates. Although solubilization of hydrocarbon in the core of individual silicate-surfactant aggregates would promote a spherical aggregate shape, the silica-silica attractive forces will promote the transition from spherical to rod-shaped structures in order to maximize silica-silica interactions in the silicatropic mesophase.19 However, the clear transition observed in the d100 curve of the decanoic acid/CTAB plot in Figure 3 indicates that some structural change is occurring at m ≈ 0.35, although the diffraction pattern above the transi(33) Kabalnov, A.; Weers, J. Langmuir 1996, 12, 3442. (34) Lianos, P.; Viriot, M.-L.; Zana, R. J. Phys. Chem. 1984, 88, 1098.

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tion can still be indexed assuming a two-dimensional, hexagonal symmetry. If the incorporation of toluene into the CTAB-silicate aggregates is mainly controlled by molecular diffusion,19 the much more pronounced swelling of the mesophase with increasing decanoic acid content (see Figure 10) can hardly be explained solely on these grounds. We postulate that the incorporation of toluene in these systems occurs through CTAB-decanoic acid(-silicate) stabilized toluene droplets, which would allow for the faster toluene incorporation kinetics. The surfactant portion of the swollen hexagonal phase observed in the presence of decanoic acid is suggested to consist of CTAB/decanoic acid complexes together with CTAB, as indicated in Figure 10. The morphology on the nanometer scale will then most probably gradually change from a two-dimensional hexagonal arrangement of rods to a close packing of spherical silica-coated droplets. The extent of swelling of the hexagonal phase and the presence of the coexisting lamellar phase are crucially dependent on the stirring rate. The higher the stirring rate, the more swollen the hexagonal phase and the less intense the reflections originating from the lamellar phase in the diffractograms in the presence and absence of toluene, respectively. This indicates that the swollen hexagonal phase is kinetically stabilized and that the surfactant part of the two phases is most probably of similar composition. Interestingly, the aggregation of the primary particles formed is also affected by the presence of catanionic surfactants. As seen in Figures 6-9, the scattered intensity at low angles increases with the amount of decanoic acid added to the synthesis. This is a clear indication that apart from stabilizing microemulsion droplets in the initial sol, smaller silicate-surfactant particles and particle aggregates are formed in this system, compared to that of the pure CTAB synthesis. This is most probably due to the fact that more nucleation sites are being produced in the more well-dispersed and homogeneous catanionic system. This feature gives the material interesting sorptive properties when calcined, and this will be the focus of a forthcoming paper. Summary The use of a mixture of cationic and anionic surfactants as structure-directing agents in the synthesis of mesoscopically ordered silica in the presence of organic swelling agents has been investigated. The more controlled solubilization of the organic swelling agent observed is suggested to originate from ion-pairing between the oppositely charged surfactants leading to a decrease in the preferential interfacial curvature. The highly tolueneswollen hexagonal phase is in kinetic equilibrium with a lamellar phase, the extent of which is determined by the amount of toluene and countersurfactant added to the synthesis at a constant stirring rate. Acknowledgment. The Finnish National Technology Agency, TEKES, and the EU large-scale facility program are gratefully acknowledged for financial support. LA011240A