Structural Evolution during the Synthesis of Mesoporous Silica in Fatty

J. Phys. Chem. B 2004, 108, 20083-20089

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Structural Evolution during the Synthesis of Mesoporous Silica in Fatty Acid/ Aminoalkoxysilane/Water Systems Carlos Rodriguez-Abreu,† Teruki Izawa,† Kenji Aramaki,† Arturo Lopez-Quintela,‡ Kazutami Sakamoto,†,§ and Hironobu Kunieda*,† Graduate School of EnVironment and Information Sciences, Yokohama National UniVersity, Tokiwadai 79-7, Hodogaya-ku, Yokohama 240-8501, Japan, Departamento de Quı´mica Fı´sica, Facultad de Quı´mica, UniVersidad de Santiago, Santiago de Compostela, E-15782, Spain, and Shiseido Co., Ltd., Research Center, 2-2-1 Hayabuchi, Tsuzuki-ku, Yokohama 224-8558, Japan ReceiVed: July 23, 2004; In Final Form: October 13, 2004

We report on the phase behavior and structural evolution during the early stages of silica templating using surfactant liquid crystals in aminoalkoxysilane/lauric acid/water systems. Specific aminoalkoxysilanes are used so that hydrolysis and condensation reactions are slow enough to follow the changes in the early stages of mesoporous silica formation without the need for very sophisticated techniques. A lyotropic lamellar phase that swells with water is present in the early stages of the system and develops into kinetically correlated siliceous phases with preservation of morphology. Moreover, lyotropic and siliceous phases coexist at a certain time. Hydrolysis and condensation of alkoxysilane groups cause microstructural changes in the lyotropic lamellar phase, as monitored by in situ small-angle X-ray scattering and infrared spectroscopy measurements. Although the changes in the initial lamellar phase depend on the nature of the alkoxysilane groups, the final siliceous phases show similar correlation lengths, which are close to that of previously reported lamellar mesoporous materials.The results indicate three stages during liquid crystal templating, controlled respectively by self-assembly, hydrolysis, and condensation of alkoxysilane groups on the surface of aggregates. A mechanism, based on phase separation, is proposed for the formation of these hybrid organic-inorganic materials.

Introduction Mesoporous materials, containing narrow pore size distribution in the 1-10 nm range and with potential applications in separation processes and catalysis, have attracted enormous interest since their discovery in 1992.1 Several authors have investigated the building mechanism of mesoporous silica using surfactants as structure directors.1-8 For ionic surfactants at low surfactant concentrations, it has been proposed4 that oligomeric silicate polyanions act as multidentate ligands for surfactant headgroups, leading to a strongly interacting surfactant/silica interface. The polymerization of the silicate in the interface produces charge-density matching, leading to a phase transformation and to the formation of the silica-surfactant composite. Other authors have suggested a mechanism in which the key step is the formation of silica prepolymers.9 In micellar solutions of block copolymer nonionic surfactants, the following steps have been identified in the formation of hexagonal mesoporous silica:10 (a) hydrolysis of the silicon alkoxide, (b) adsorption of hydrolyzed silicate species to the surfactant hydrophilic groups, (c) clustering of poorly condensed silica-surfactant micelles into flocs, (d) elongation of the surfactant micelles, and (e) growth of the domain units. Studies at high surfactant concentration in nonionic systems11 have suggested that the surfactant liquid crystalline phases direct the * Corresponding author. Phone & Fax: +81-45-339-4190. E-mail: [email protected]. † Yokohama National University. ‡ Universidad de Santiago. § Shiseido Co., Ltd.

formation of mesoporous materials, so that multidentate binding and charge-density matching do not take place. There are some reports on the kinetics of mesoporous silica formation using cationic surfactants.12,13 In many cases, the reaction speed makes it difficult to perform a detailed study on the phase behavior and structural evolution during the early stages of the formation of mesoporous silica using surfactants as structure directors; therefore there is a need to use sophisticated techniques.14,15 Moreover, in most reports only little changes in the correlation lengths of the structures can be detected. Recently, an anionic surfactant templating route using fatty acids and aminosiloxanes as co-structure-directing agents (CSDAs) was discovered16,17 and mesoporous materials with several morphologies were prepared. To get insight on the mechanism of this route, here we report for the first time on the coupling of structural evolution and reaction kinetics in aminoalkoxysilane/lauric acid/water systems. First, we present the phase behavior of aminoalkoxysilane/lauric acid/water systems as a function of time. Next, small-angle X-ray scattering (SAXS) measurements are used to follow the structural evolution in the systems. Finally, Fourier transform infrared (FTIR) spectroscopy results are analyzed to relate compositional and structural changes. Materials and Methods Materials. Lauric acid (LA) and tetraethyl orthosilicate (TEOS) were supplied by Sigma-Aldrich (USA). 3-Aminopropyltriethoxysilane (APTES) and 3-aminopropyldiethoxy meth-

10.1021/jp0467245 CCC: $27.50 © 2004 American Chemical Society Published on Web 12/02/2004

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Figure 1. Phase maps of the APTES/LA/water (W) system at 25 °C for different times after sample preparation. I is an isotropic phase. S denotes amorphous solid. II is a region of two liquid phases. LR is a lyotropic lamellar liquid crystal. SLC is a region in which siliceous crystals are present. W denotes excess diluted aqueous solution.

Figure 2. Phase maps of the APDES/LA/W system at 25 °C for different times after sample preparation. The nomenclature is as in Figure 1. The arrow indicates the path along which SAXS measurements were performed.

ylsilane (APDES) were obtained from Tokyo Kasei Kogyo Co. Ltd. (Japan). LA can be regarded as an anionic amphiphile, whereas APTES and APDES are commonly used for coupling or modification of silica surfaces. TEOS is a widely known silica precursor. All chemicals were used without further purification. Millipore filtered deionized water was used in the preparation of the samples. Methods. Preparation of Samples. Samples were prepared by adding water to mixtures of lauric acid and aminopropylalkoxysilane (plus TEOS in some cases), with vigorous mixing at 25 °C. Phase Maps. Phase maps were constructed by visual observation of the samples through crossed polarizers and optical microscopy at 25 °C. Lamellar phases were additionally characterized by SAXS. Small-Angle X-ray Scattering. SAXS measurements were performed on a Rigaku Nanoviewer equipped with a CCD detector. The applied voltage and filament current were 40 kV and 20 mA, respectively. The samples were covered by plastic films for the measurement (Mylar method).

FTIR Measurements. A Shimadzu FTIR-8900 spectrometer equipped with a ZnSe internal reflection element was used to obtain infrared spectra at 25 °C. The change in the adsorption bands was measured as a function of time in a range of wavenumbers between 400 and 5000 cm-1. Results and Discussion Phase Behavior of Aminopropylalkoxysilane/LA/Water Systems. Figure 1 shows the phase maps of the APTES/LA/ water systems at 25 °C. The axis labels indicate only the initial components in the system since the composition changes with time due to the hydrolysis of the aminopropylalkoxysilane, as will be described later. There is a wide single phase, isotropic region adjacent to the APTES apex. The solubility of LA in APTES is 75 wt % and LA is insoluble in water; therefore a region in which precipitated solid is present extends along the LA-W apex. In the early stages of the system, namely, a few minutes after sample preparation, a single lyotropic lamellar liquid crystal (LR) phase is found between the isotropic and the

Synthesis of Mesoporous Silica

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Figure 3. SAXS data at 25 °C for (a) the APTES/LA/water system and (b) the APDES/LA/water system. θ is the scattering angle. In both panels a and b, the initial water concentration is 50 wt % and the LA/alkoxysilane weight ratio is 7/3. The initial SAXS patterns (corresponding to LR phase) are enlarged in the bottom of the figures. Arrows indicate the shifts in peak A (LR phase) and peak B (siliceous phase 1). Note the change in the time scale in panel b.

amorphous solid present regions, as confirmed by SAXS measurements and optical microscopy. This behavior is similar to that of aqueous catanionic systems, in which a lamellar phase is usually found for a mixture of anionic (fatty acid) and cationic (amine) components.18,19 The LR region expands toward the LA-APTES concentrated region with the course of time. Finally, near the end of the process a siliceous ordered structure containing hydrolyzed and/or condensed silica is formed, not only covering the region of the initial LR phase but also intruding into the isotropic phase, which indicates that hydrolysis of silica induces ordering in the system. After 1 day, still an isotropic solution covers a wide range of the phase map. It is known20 that the amino function causes both unhydrolyzed silane and its hydrolysis and condensation products to have excellent water solubility. A similar phase behavior was found for another CSDA, APDES, as is shown in Figure 2. Nevertheless, the phase changes are much slower than in the case of the APTES system. It is known that hydrolysis is delayed by a reduction in the number of reactive alkoxy groups or by steric hindrance effects of alkoxy and methyl groups.21 Microstructural Changes. Microstructural changes with time were also studied. Figure 3a shows representative SAXS patterns for APTES/LA/water systems. The initial water concentration is 50 wt %, and the LA/alkoxysilane weight ratio is 7/3. At short times, the system exhibits reflections with spacing ratios of 1:2:3, corresponding to the structure of a lyotropic lamellar liquid crystal phase LR (see the bottom of Figure 3a). With the course of time, these reflections first shift to higher angles (shorter interlayer spacings). Then, a new peak (B) associated with a siliceous phase appears. No high-order reflections were observed for this phase, but the textures observed by optical microscopy were lamellar-like, which means that the initial morphology is preserved during the hydrolysis/condensation of silica.

Figure 4. SAXS data at 25 °C for the APTES/LA/TEOS/water system (the initial water and TEOS concentrations are 45 and 5 wt %, respectively). θ is the scattering angle. The initial LA/APTES weight ratio is 7/3. The initial SAXS pattern (corresponding to LR phase) is enlarged in the bottom of the figure. Arrows indicate the shifts in peak A (LR phase), peak B (siliceous phase 1), and peak C (siliceous phase 2).

The intensity of peak B increases with time and its position slightly shifts to higher angles (shorter correlation lengths), whereas the main peak related to the LR phase (A) simulta-

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Figure 5. Interlayer spacing of peak A, dA (9, 0), peak B, dB (b, O), and peak C, dC (4) as a function of time in LA/APTES/water systems. The initial water concentration is 50 wt %. The initial LA/APTES weight ratio is 7/3. Filled symbols: no TEOS added. Open symbols: TEOS added (9 wt %).

Rodriguez-Abreu et al.

Figure 7. Change of interlayer spacing dB with the initial volume fraction of water (φw) for the siliceous phase of the APDES/LA/water system (open squares) and the APTES/LA/water system (filled squares). The initial LA/alkoxysilane weight ratio is 7/3.

structures are kinetically correlated. The coexistence of silicatropic phases has been previously reported,10,14,23 although the correlation lengths of these phases change little with time. The microstructural parameters of the lamellar phase are given by the following equations:24

Figure 6. Change of microstructural parameters with the initial volume fraction of water (φw) in the LR phase of the APDES/LA/water system. The initial LA/APDES ratio is 7/3. Squares represent the interlayer spacing dA, triangles the effective cross-sectional area per headgroup as, and circles the half thickness of the lipophilic layer in the LR phase.

neously shrinks, its position shifting largely toward smaller angles (longer correlation lengths) until it ultimately vanishes. These results indicate that one structure develops and other disappears, and moreover, the two structures coexist at a certain time. The same trend was observed in APDES systems (Figure 3b), although dA changes much more slowly due to slower reaction rates. As can be seen in Figure 4, similar SAXS results were obtained in the initial stages of the APTES/LA/water system when a typical ceramic precursor, TEOS, was added. However, a new peak (C) appears, and its final position corresponds to a correlation length similar to those found in lamellar mesoporous silica samples prepared from fatty acid/aminoalkoxysilane systems according to the procedure described in ref 16. The interlayer spacings associated with peaks A (dA), B (dB), and C (dC) as a function of time are shown in Figure 5. dA first decreases and then increases sharply with time, the change being somewhat faster for the system with added TEOS (the initial water content is different). On the other hand, dB and dC decrease only slightly, suggesting a little shrinkage of the siliceous structure.14,22 dC is shorter than dB, as a result of enhanced silica condensation. The time in which the sharp increase in dA starts coincides with the appearance of the siliceous peak B, namely, the two

dA )2dL/φL

(1)

as ) VL/dL

(2)

where dL, φL, and VL are the length, volume fraction, and molar volume of the amphiphile lipophilic part, respectively, and as is the effective surface area per surfactant molecule. Since lauric acid should reside inside the lipophilic layer of the lamellar phase due to its low solubility in water, the estimation of φL depends on the location of the aminoalkoxysilane in the aggregates. Assuming that the aminoalkoxysilane is located in the lipophilic layer, the calculation of as from eq 2 results in values that are smaller than the minimum theoretical value for as given by asmin ) VL/dLmax, where dLmax is the maximum length of the lipophilic chain (1.65 nm for LA25). This indicates that the aminoalkoxysilane does not actually reside in the lipophilic layer of the LR phase. Accordingly, we took φL as the volume fraction of LA in the system and using eqs 1 and 2, we calculated the parameters of the lamellar phase in the APDES/ LA/water system at the initial state (immediately after mixing components). As can be seen in Figure 6 (corresponding to samples along the arrow in Figure 2), the interlayer spacing increases with water content, indicating swelling of the layers.24 The effective cross-sectional area at the interface decreases with water concentration, indicating that the conformation of the lipophilic chains is affected by the swelling of the bilayers. The length of the lipophilic chain (lauric acid) is shorter than the fully extended length of the dodecyl chain (1.65 nm25). Similar results (not shown) were obtained for APTES/LA/water systems. In contrast to the interlayer spacing dA of the LR phase, in the siliceous phase dB remained almost constant with the initial water content for APTES and APDES at a given LA/aminoalkoxysilane ratio, as can be seen in Figure 7. dB values are close to the length of a LA double layer, indicating that the siliceous structure contains little trapped solvent. Figure 8 shows the changes with time of SAXS peak areas, which are related to changes in the fraction of the different crystal phases present in the system.26 Apparently, there are three

Synthesis of Mesoporous Silica

Figure 8. Areas of peak A, AA (0), peak B, AB (O), and peak C, AC (4), as a function of time in LA/APTES/TEOS/water systems. The initial water concentration is 50 wt %. The initial LA/APTES weight ratio is 7/3. The TEOS concentration in the system is 9 wt %.

Figure 9. Representative FTIR spectra for LA/APTES/water systems. The initial water concentration is 50 wt %. The initial LA/alkoxysilane weight ratio is 7/3. Thin line: initial state (t ) 0). Thick line: after 9 min.

stages during the evolution of the system. First, the system is in a lyotropic state. As the reaction develops, the lyotropic phase vanishes, and the siliceous phase start to appear, as indicated by the increase in the area of peak B. Finally, the condensation of silanetriols brings about structure C, with the simultaneous disappearance of peak B. Accordingly, structures seem to be kinetically correlated according to a mechanism of the type A f B f C. Analysis of FTIR Results. Hydrolysis and condensation of alkoxysilane groups cause changes in the composition of the systems and hence should affect its structure. A general equation for the hydrolysis of aminopropylalkoxysilanes, an autocatalyzed reaction, can be written as follows:27 where O-R is an alkoxy

group and X can be either an alkoxy or an alkyl group. The hydrolysis is followed by the condensation of silanepolyols. The stoichiometric coefficients n and m depend on R. As mentioned earlier, by changing R or X it is possible to control the reaction rate and the time window in which an appropriate structural SAXS analysis can be performed. FTIR spectra were used to determine the compositional change in the systems. Representative spectra are shown in Figure 9.

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Figure 10. Kinetics of hydrolysis in the APTES/LA/water system. The initial water concentration is 50 wt %, and the LA/APTES weight ratio is 7/3.

Figure 11. Absorbance (baseline subtracted) of FTIR peaks related to hydrolysis (b) and condensation (O) of alkoxysilanes as a function of time in LA/APTES/water systems. The initial water concentration is 50 wt %. The initial LA/alkoxysilane weight ratio is 7/3. The arrow indicates the time at which the peak at 1490 nm appeared.

The hydrolysis of alkoxysilanes has been reported to follow first- or second-order rate laws.28 If the hydrolysis is considered to be first order in both silane and water concentrations, the following expression results:28

k2t )

(

)(

)

[APTES]t[H2O]0 1 ln ) ln S [APTES]0[H2O]0 [APTES]0[H2O]t

where the subscripts 0 and t refer to the concentration of the species at times zero and t, respectively, and k2 is the secondorder rate constant. The concentration of ethanol produced in the hydrolysis was calculated using a calibration graph for the peak at 1050 cm-1, and from the stoichiometry of the reaction concentrations of APTES and H2O were estimated. As can be seen in Figure 10, a plot of ln S versus time gives a straight line, indicating that hydrolysis is first order in both alkoxysilane and water with k2 ) 1.3 M-1 s-1 derived from the slope of the line. The hydrolysis in the presence of LA is much faster than in pure aqueous solutions,28,29 which can be attributed to the different microenvironment that surrounds the alkoxy groups incorporated in the aggregates. When APTES is condensed at room temperature, a very strong band at 1489 cm-1 appears.30 For a qualitative analysis of FTIR data, we related this band to the condensation and the band at 1050 cm-1 (ethanol) to the hydrolysis of APTES. Figure 11 shows that the absorbance at 1050 cm-1 first increases and then reaches a plateau toward the end of the reaction. The

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Figure 12. 12. Scheme of the proposed mechanism for the structural evolution in LA/APTES/TEOS/water systems.

condensation starts after the hydrolysis and just before the sudden increase in dA and the appearance of peak B were detected (see Figure 5), which suggests that the structural changes are closely related to the condensation reactions. On the other hand, the initial decrease in dA seems to be caused by the effect of the hydrolysis reaction. According to eqs 1 and 2, the initial decrease in dA can be attributed to two factors: (a) an increase in φL and (b) an increase in as. Since there is no phase segregation at the initial stages, factor a is unlikely to occur. On the other hand, during the hydrolysis the conformation of aminosiloxanes becomes more bulky due to hydrogen bonding.30 This might cause a stronger repulsion between the heads of amphiphile molecules, and hence, the effective surface area per amphiphile increases. Therefore, factor b is probably the cause of the initial decrease in dA. When the condensation reaction starts, the siliceous phase separates out carrying some amphiphilic molecules; therefore the lamellar phase should keep a large amount of solvent (the siliceous phase contains very little) but with fewer amphiphiles (φL decreases). This process causes an increase in dA according to eq 1. The LR ultimately disappears when the number of amphiphile molecules is not enough to sustain the structure. According to the discussion above, the overall mechanism during the early stages of anionic templating of silica is sketched in Figure 12. It shows some analogies with the one previously reported for diluted cationic surfactant solutions.9 Conclusions During the anionic surfactant templating route using fatty acids, the reaction kinetics is modified by using different kinds

of aminoalkoxysilanes, so that several phase and structural changes can be detected. At the beginning, the system forms a lyotropic lamellar phase. The lamellar phase seems to contain hydrolyzed products that have some effect on the lipophilichydrophilic interface. As the condensation of silica starts, a new siliceous structure appears, and it coexists with the lamellar phase. However, as the fraction of this siliceous phase in the system increases, the initial lyotropic lamellar phase swells with solvent and it ultimately vanishes. The presence of a silica precursor such as TEOS imposes new conditions for condensation reactions, and a new siliceous structure is found toward the end of the process, with correlation lengths similar to that found in as-prepared mesoporous materials. Acknowledgment. C.R.A. is thankful to the Japan Society for the Promotion of Science (JSPS) for a research grant. This work has been supported by CREST of JST (Japan Science and Technology Corporation). References and Notes (1) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (2) Cieska, U.; Schu¨th, F. Microporous Mesoporous Mater. 1999, 27, 131. (3) Patarin, J.; Lebeau, B.; Zana, R. Curr. Opin. Colloid Interface Sci. 2002, 7, 107. (4) Monnier, A.; Schuth, F.; Quo, 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. (5) Ying, J. Y.; Mehnert, C. P.; Wong, M. S. Angew. Chem., Int. Ed. 1999, 38, 56. (6) Zhang, J.; Luz, Z.; Goldfarb, D. J. Phys. Chem. B 1997, 101, 7087.

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