14500
J. Phys. Chem. C 2007, 111, 14500-14507
EPR Spin Probe Investigation into the Synthesis of Mesoporous Silica from the Water/Acetonitrile/n-Dodecylamine System Agneta Caragheorgheopol,*,† Florenta Savonea,† Duncan J. Macquarrie,‡ Rafael Luque,‡ Dan Donescu,§ and Mihai C. Corobea§ “Ilie Murgulescu” Institute of Physical Chemistry, Romanian Academy, 202 Splaiul Independentei, 060021 Bucharest, Romania, Green Chemistry Centre of Excellence, Department of Chemistry, UniVersity of York, Heslington, York, YO10 5DD, United Kindom, and Department of Polymer Synthesis, ICECHIM, 202 Splaiul Independentei, 060021 Bucharest, Romania ReceiVed: May 31, 2007; In Final Form: July 11, 2007
Mesoporous silica prepared in the dodecylamine/acetonitrile (ACN)/water/tetraethoxysilane (TEOS) system, has led, by functionalization, to a series of catalytically active materials with well-defined narrow pore size distributions. In this contribution, we follow the mechanism of templated synthesis of mesoporous silica in the nonfunctionalized parent system, for different ACN/water proportions. Electron paramagnetic resonance (EPR) of spin probes was employed to follow structural changes in the template structure during precipitation, while X-ray diffraction (XRD) and porosimetry determinations have in parallel reported on the time evolution of the mesoporous structure. Three solvent mixtures were studied, with largely different solvent compositions, and EPR data have shown that a 0.3 M dodecylamine (DDA) solution in (i) ACN/water ) 1/1 v/v was a true micellar solution, in (ii) ACN/water ) 2/8 v/v was a biphasic system with a lamellar liquid crystalline phase, and in (iii) ACN/water ) 8/2 was a molecular solution. In spite of these extreme differences, after 20 min reaction with TEOS, all compositions presented a well-developed mesoporous structure with similar pore dimensions. In case i, the mechanism of precipitation on preformed micelles is straightforward. In case ii, EPR of spin probes has provided evidence that tetraethoxysilane (TEOS) addition leads to an immediate phase transformation of the liquid crystalline phase to a micellar solution, on which silica precipitation occurs. Compared with i and ii, no DDA aggregates were found in solution before or during precipitation for the third solvent mixture iii, pointing to a true cooperative mechanism. XRD and nitrogen adsorption/desorption measurements entirely support these data. Interestingly, the precipitate formed in the first 5 min was nonstructured for all systems, even for i where preformed micelles were present.
Introduction The synthesis of mesoporous silicas has developed over the past decade and a half into a vigorously pursued area of research, with many variants on the synthetic route being reported.1 All of these have the theme of growing silica (or organo-silicas) around a micellar or liquid crystal template. Examples are known where the template exists prior to the addition of the silica precursor and also where the assembly process forms both the silica and the template around which it grows. We have actively studied the synthesis of silicas containing organic groups through the co-condensation of a silica precursor (typically tetraethoxysilane, TEOS) with trialkoxy organosilanes ((RO)3SiR′) using aqueous ethanol as solvent and n-dodecylamine (DDA) as structure directing agent.2 This method has led to a series of catalytically active materials with well-defined narrow pore size distributions. Acids,3 bases,4 and metal centered5 and bifunctional catalysts6 have been prepared and used. Template removal is straightforward, and recovery and reuse of the template is possible. We have recently switched from an aqueous ethanol solvent to aqueous acetonitrile (ACN), which has certain advantages * Corresponding author. † Romanian Academy. ‡ University of York. § ICECHIM.
SCHEME 1: Spin Probes Employed in the EPR Measurements
regarding the nucleophilicity of amino functional groups. The materials obtained by this method, described in a previous paper,7 have similar pore characteristics to our earlier aqueous ethanol systems but have significantly thicker walls and different morphology. Additionally, the relationship between pore size distribution and solvent composition is very different between the two solvent systems. For example, in the DDA/ethanol/water system, pore size increases as the water content increases. Beyond a critical water content, the system becomes biphasic, and two distinct materials are formed.8 In the DDA/ACN/water
10.1021/jp074202u CCC: $37.00 © 2007 American Chemical Society Published on Web 09/07/2007
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Figure 1. 5-DSA aN (A) and τC (B) measured in ACN/water mixtures and in the corresponding DDA (0.3 M) solutions as a function of the ACN weight fraction.
system, there is no discernible change in pore size, and there is no evidence for a second material, even at very high water contents. The use of electron paramagnetic resonance (EPR) of spin probes is a particularly well-suited technique for the study of the processes occurring in the synthesis of such materials. The tiny quantities of spin probes required, their ability to partition into the appropriate parts of a complex reaction mixture, and the rapid and easy accumulation of spectra from multiphasic systems without disruption of the system makes the use of this technique extremely powerful for the study of the synthesis of mesoporous silicas. Previous studies have already provided key insights into the processes occurring in several synthetic routes and have illustrated some key differences between the synthetic methods.9 Having already studied the synthesis of silica from DDA/ethanol/water using EPR spin probing,10 we decided to explore the new DDA/ACN/water system using the same technique. In the DDA/ACN/water system, unlike in DDA/ ethanol/water, the pore dimensions are practically uninfluenced by the solvent composition over a large composition range. Thus, this system attracted our attention as it seemed well-suited for relating starting solution characteristics with final product
Figure 2. EPR spectra in ACN/water ) 1:1 (S1) of (A) 5DSA, (B) 16-DSA, and (C) C12NO spin probes; (a) solvent, (b) DDA solution, and (c) silica precipitate after 18 h reaction.
properties and for following their influence on the synthesis mechanism of the mesoporous material. Experimental Section Materials. Solvents and chemicals were obtained from Aldrich and Fluka and used as received unless otherwise stated.
14502 J. Phys. Chem. C, Vol. 111, No. 39, 2007 Deionized water was used throughout. The spin probes 5-DSA (doxyl stearic acid) and 16-DSA were purchased from SIGMA, while TEMPO-laurate (C12-NO) was prepared according to published methods.10 Samples. A first series of solvent samples (C1-C9) were prepared for a preliminary screening of the starting solutions. The ACN fraction (g/g) in water was varied in 0.1 steps. 0.025 g/g DDA solutions were prepared from each of them. For synthetic purposes, three solvent mixtures were selected: S1 ) ACN/H2O ) 1:1 (v/v), S2 ) ACN/H2O ) 2:8 (v/v), and S3 ) ACN/H2O ) 8:2 (v/v) in which 0.3 M DDA were dissolved yielding three starting solutions, ST1, ST2, and ST3, respectively. TEOS was added to each solution (final concentration 0.1 M) and after 18 h stirring (700 rpm with a magnetic stirrer) at room temperature (RT), the corresponding P1, P2, and P3 precipitates were separated by centrifugation and washed with the solvent mixture used in the synthesis. After drying, initially at RT in air, and subsequently in an oven at 353 K for 2 h, the solids were extracted with ethanol in a Soxhlet instrument for 8 h. The extracted solids are named P1x, P2x, and P3x. In separate experiments, the reaction was stopped after different intermediate reaction times (0-5 min, 20 min, 1 h, 2 h, and 4 h), and the precipitates were treated as described above. Powder X-ray diffraction (XRD) measurements and nitrogen physisorption experiments were carried out to characterize the materials. XRD patterns were recorded on a Bruker AXS diffractometer with Cu KR (λ ) 1.5418 Å), over a 2θ range from 1 to 8° (low angle), using a step size of 0.01° and a counting time per step of 1 s. Nitrogen physisorption measurements were conducted on a Micromeritics ASAP 2010 instrument at 77 K. Samples were outgassed at 120 °C for 4 h under vacuum (P < 10-2 Pa) and subsequently analyzed. The linear part of the BrunauerEmmett-Teller (BET) equation (relative pressure between 0.05 and 0.22) was used for the determination of the specific surface area. The pore size distribution was calculated from the adsorption branch of the N2 physisorption isotherms and the Barret-Joyner-Halenda (BJH) formula.11 The cumulative mesopore volume VBJH was obtained from the pore size distribution (PSD) curve. EPR Measurements. Several stable nitroxide radicals (spin probes) were dissolved in the solvents, and the starting solutions (concentration 2 × 10-4 M) and their spectra were followed during the synthesis stages and in the wet, as-synthesized precipitates. These were washed with the corresponding solvents until no EPR signal was detected in the filtrate. The most relevant data were obtained with the spin probes 5-DSA and 16-DSA and TEMPO-laurate (C12NO) (Scheme 1), which were further used throughout the study. The EPR measurements were made on a FA-100 JEOL and a Bruker ESP 300E spectrometers. The relevant EPR spectral parameters are the 14N isotropic hyperfine splitting, aN, the polarity-sensitive parameter,12 and the rotational correlation time, τC, which was calculated according to the formula:13
τC ) (6.51 × 10-10)∆H(0){[h(0)/h(-1)]1/2 + [h(0)/h(1)]1/2 - 2} s where ∆H(0) is the line width (in Gauss) of the central line, and h(-1), h(0), and h(1) are the peak heights of the M ) -1, 0, and +1 derivative lines, respectively. τC is connected to the local viscosity, η, by the Debye-Stokes-Einstein equation:
Caragheorgheopol et al.
τC ) 4πηR3/3kT where R is the hydrodynamic radius of the tumbling entity. The τC values obtained were used to follow, in a qualitative manner, significant changes in the microenvironment of the probes. The order parameter, S, specific for doxyl probes is defined in terms of observed spectral parameters A|| and A⊥ as:14
S ) (A|| - A⊥)/[Azz - (Axx + Ayy)]/2 where Azz, Axx, and Ayy are the principal elements of the A tensor in absence of molecular motion and A|| and A⊥ are derived from experimental spectra. The order parameters, S, were calculated using the following parameters reported for doxyl probes:15 Azz ) 33.5 G, Axx ) 6.3 G, and Ayy ) 5.8 G. Results and Discussion Preliminary Description of the System. 5-DSA was dissolved in the series of solvents (C1-C9) with increasing ACN proportion (including water and ACN as limits) and in the corresponding DDA solutions. The results in Figure 1 represent the measured EPR parameters. The data show that at intermediate ACN proportions (0.3-0.6) aN is smaller and τC is larger in DDA solutions compared with the DDA-free solvent, meaning that the spin probe is in a different local environment, that is, in a micelle, while at a higher ACN proportion, the solution and solvent data converge, which is expected for a molecular solution. At the high water limit, the systems up to 0.2 ACN weight fraction are biphasic. Here, 5-DSA spectra with a measurable order degree, S, specific for lamellar liquid crystals (which is one of the phases) are observed, and the aN and τC values cannot be directly measured. It is worth mentioning that as the ACN proportion increases, the EPR parameters describing the micelles change gradually, indicating a progressive swelling of the micelles with ACN until complete dissolution. For the silica synthesis investigation, three systems have been chosen, one in each of the three regions described above, at ACN weight fractions 0.15, 0.41, and 0.71. These correspond to ACN/H2O v/v ) 1/1 (S1), 2/8 (S2), and 8/2 (S3). Starting Solutions and Final Precipitates. S1 System. As reported in Figure 2 and Table 1 in the S1 system, all probes have rapid isotropic rotation, with very low τC values. In the corresponding DDA solution (ST1), the correlation time for 5-DSA is considerably increased, as expected for the probe included in a micelle, and the spectrum reflects isotropic rotation. The absence of anisotropic features in the 5-DSA spectrum might originate in either lateral diffusion or overall micelle tumbling, two phenomena which affect EPR spectra when micelle radii are smaller than 2.5 nm, as is expected to be the case for DDA micelles. With 16-DSA and C12NO probes, which have the radical moiety located deeper in the micelle core, the changes observed in τC values were smaller. In order to check whether the probes are included in micelles or not, we have performed quenching experiments by adding CuCl2 or FeCl3 to the samples. Both these quenching agents will broaden the lines beyond the observation limit when having the possibility to collide with the probes. We observed that all spectra in solvents were indeed quenched, while those in the DDA solution (ST1) were not influenced. Thus, one can conclude that ST1 is a micellar solution. In the final, as-synthesized precipitate, P1, the dynamics of 5-DSA appeared significantly reduced to the so-called “slow motional regime”; 16-DSA and C12NO had their τC values more
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TABLE 1: EPR Spectral Parameters of 5-DSA, 16-DSA, and C12NO Spin Probes in ACN/H2O Solvent Mixtures (S1, S2, and S3), in the Corresponding 0.3 M DDA Solution (ST1,ST2, and ST3) and in the As-Synthesized Precipitates (P1, P2, and P3) 5-DSA sample/probe
aN, G
τC, 10-10 s
S1 ) ACN/H2O ) 1:1 (v/v) ST1 ) DDA/ACN/H2O P1
15.2 15.0
2.1 8.7
S2 ) ACN/H2O ) 2:8 (v/v) ST2 ) DDA/ACN/H2O P2
15.6
S3 ) ACN/H2O ) 8:2 (v/v) ST3 ) DDA/ACN/H2O P3
15.1 15.0
16-DSA S
2Azz, G
aN, G
τC, 10-10 s
52.0
15.2 14.8 14.4
0.7 1.1 10.4
15.6
1.4
50.0
14.3
14.1
63.0
14.9 14.9 14.8
1.1 1.1 10.9
2.9 0.49
aN, G
τC, 10-10 s
16.3 16.0 15.9
0.4 0.6 10.3
16.1 16.0
3.2 8.3
16.1 16.1
0.4 0.4
0.34
1.5 1.5
than 10 times increased. It is hard to distinguish whether the effects are due to increased viscosity in the core of the micelles or to the suppression of the lateral diffusion and of micellar tumbling or more probably to both; but whatever the cause, it relates to the silica surrounding the micelles. In the biphasic ST2 system (Figure 3), if the phases were left to separate, EPR spectra could only be observed in the upper part of the sample, where DDA is concentrated. In this case, the probes 5-DSA and 16-DSA show spectra specific for wellordered lamellar liquid crystalline phases (Figure 3). The fact that even 16-DSA, with its hydrocarbon chain longer than DDA, has this type of spectrum indicates a multilayered structure. The aN value of 13.8 G (estimated from aN ) 1/3(A|| + 2A⊥)) is typical of hydrocarbon solvents and represents a proof of the extended conformation of the 16-DSA spin probe and of the tight packing of the surfactant. The hydrophobic spin probe C12NO forms its own micelles in the solvent S2 (Figure 3C(a), the broad line) in equilibrium with a small quantity of dissolved molecules (three narrow lines). In the ST2, DDA solution dissolves in the aggregates. In the precipitate P2, most spin probes have spectra similar to those in P1. The 5-DSA spectrum seems to consist of a slow motional spectrum, somewhat more mobile compared to P1. ST3 spin probes have the same spectra as in S3. Considering also the gradual change of spectral parameters from micelle to molecular solution values observed in the preliminary screening of the starting solutions, we conclude that ST3 is a molecular solution. In the precipitate P3, 5-DSA has a “strongly immobilized” spectrum, while 16-DSA has a micelle-type spectrum, with the same τC value as in P1. One may presume that the stronger immobilization of 5-DSA in P3 as compared with those in P1 and P2 indicates a different situation: in P1 and P2, 5-DSA has reduced mobility being included in silica covered micelles; in S3, the acid headgroup of 5- and 16-DSA binds directly to the incipient inorganic phase, prior to surfactant aggregate formation (see below). This experimental fact points to the presumed cooperative mechanism of templated synthesis. This might also explain why C12NO, which has no headgroup binding directly to silica, was not found in the precipitate. Following the Precipitation in Time. Two different approaches have been used to follow the evolution of the silica formation in time. (1) The precipitation reaction was stopped at several intervals of time (typically at 0-5 min, 20 min, 1 h, 2 h, and 4 h). The precipitates were separated by centrifugation and treated in the same way as the final ones. (2) After TEOS addition to the starting solution, the reaction mixture was rapidly introduced into an EPR sample tube in the cavity of the EPR spectrometer, and the reaction was followed in situ for several hours. For this purpose, a Pasteur pipet was particularly useful as it possessed the right diameter. Measurements employing
C12NO S
melting point capillaries were not successful, the reaction progression being prevented by the small diameter of the tube. XRD and Porosity Measurements. For samples in procedure 1, XRD and porosity measurements (Table 2, Figures 4 and 5) have shown that a well-developed mesoporous structure is clearly formed as early as 20 min. From the increasing intensity of the signal with time, it is clear that the structure develops with time but does not change to any significant extent. Earlier samples were clearly characterized as nonstructured (see example in Figures 4 and 5). Pore volumes and surface area increased with time of synthesis, reaching almost constant values after a few hours, typically 1-2 h. Surface areas were close to or even higher than 1000 m2 g-1. Figure 5 summarizes the data, showing type IV isotherms for S1, typical of mesoporous micelle templated silica materials, with sharp inflections characteristic of capillary condensation within well-defined and uniform mesopores (p/p0 > 0.2) for all samples except A at 0 min (isotherm a) which is indicative of a microporous solid being formed at very early stages in the process. These sharp steps in the 0.2 < p/p0 < 0.5 region suggest a pore system of uniform size, and the relatively constant position of the inflection with time suggests that this micelle-derived pore system does not change significantly with time. Similar results were obtained for systems S2 and S3. Samples isolated at short periods of time revealed that a synthesis time of 1-2 h was enough to render materials with high quality textural and structural properties (Table 2, Figures 4 and 5). After the formation of the mesopores, the pore diameters did not significantly change in time and only the wall thickness continued to grow, especially in the latter time periods, in good agreement with XRD results in which a shift of the main peak at 2θ 2.2-2.3 in samples obtained at longer times (Figure 4) was found. Longer synthesis times not only did not improve the materials properties but also provided lower quality samples, especially after 18 h (Figure 5, d isotherms). These results were again in good agreement with the XRD patterns in which the intensity of the main diffraction line (ca. 2.2-2.3°) of samples obtained after 1-2 h was clearly similar to that of materials isolated at longer times (Figure 4b,c lines vs Figure 4d lines). A few samples have inflections at very high pressure, indicative of either cracks in the structure or the presence of a small amount of amorphous material. The main example of this is sample B after 1 h (isotherm c). Interestingly, this is not evident in the same sample after 18 h (isotherm d), suggesting that the amorphous (or cracked) material can be converted to the regular templated material over time. This also appears to be the case for sample A at t ) 0 min. Interestingly, the same trends were evident for all three systems examined, regardless of the very different nature of the starting solution.
14504 J. Phys. Chem. C, Vol. 111, No. 39, 2007
Caragheorgheopol et al. TABLE 2: Textural Properties [Pore Diameter (DBJH), Unit Cell Parameter (a0), Well Thickness, Pore Volume (VBJH), and Surface Area (SBET)] of the Different ACN/Water Systems material P1-0 h P1-20 min P1-1 h P1-2 h P1-4 h P1-18 h P2-0 h P2-20 min P2-1 h P2-2 h P2-4 h P2-18 h P3-0 h P3-20 min P3-1 h P3-2 h P3-4 h P3-18 h a
Figure 3. EPR spectra in ACN/water ) 2:8 (S2) of (A) 5DSA, (B) 16-DSA, and (C) C12NO spin probes; (a) solvent, (b) DDA solution, and (c) silica precipitate after 18 h reaction.
EPR Measurements. 5-DSA spectra in ST1 after 20 min have shown that the slow motion spectrum characteristic of the final precipitate is already present. 16-DSA and C12NO exhibited very similar spectra to the final ones after 20 min (results not shown), indicating little changes in time after precipitation of silica-on-template. It is worth mentioning that, although the existence of micelles in ST1 was clearly demonstrated, the precipitate which first appears (ca. 5 min) is nonstructured. The precipitate formation involves precipitation on preformed mi-
DBJH (des) a0 wall thickness VBJH SBET (nm) (nm) (cm3 g-1) (m2 g-1) (nm) a 2.06 2.23 2.27 2.27 2.26 a 2.39 2.54 2.59 2.59 2.57 a 2.06 2.16 2.18 2.17 2.15
4.18 4.40 4.42 4.46 4.48
2.12 2.17 2.15 2.19 2.22
0.159 0.631 0.630 0.756 0.780 0.82
271 940 1117 1194 1150 940
4.32 4.54 4.61 4.62 4.74
1.93 2.00 2.02 2.03 2.17
0.294 0.918 0.989 0.903 0.75
386 762 811 856 987
4.07 4.19 4.23 4.25 4.43
2.01 2.03 2.05 2.08 2.28
0.638 0.711 0.745 0.694 0.81
1040 1021 1112 1254 1002
No mesopores present in the material.
celles (ST1) but with a certain “induction” time which was found in all three systems. XRD and porosity measurements are unambiguous in describing a nonporous, nonordered material being formed in all three systems at the very beginning of the precipitation (less than 20 min). In our opinion, these data suggest that, in order for the precipitation-on-template to start, a certain hydrolysis product of TEOS (perhaps an oligomer) might be necessary, as the active intermediate to interact with the micelles. In many cases, prehydrolysis of TEOS is known to improve the quality of mesoporous materials, especially when using block copolymer templates.16 This appears not to have been described for amine templating. The data here suggest that hydrolysis is indeed a key stage in the development of both the precursor system itself and the condensation around the template. The role of silicate anions is known to be important in the ordering of template in MCM-41.17 In the case of ST3, although the starting solution has no preformed micelles, the precipitate formation followed the same steps: amorphous at the early stage turned out to be mesostructured after 20 min, as reported in XRD and porosity results. EPR examination of the precipitate after 20 min reveals the existence of an immobilized spectrum of 5-DSA in the solid, similar to the final one. The EPR spectrum of 16-DSA also corresponds to its inclusion in silica-surrounded micelles. The spectrum of the supernatant liquid is not changed compared to the solvent. Since here there is no intermediate micelle formation, a true co-operative precipitation mechanism should be responsible for the formation of templated mesoporous silica. Things are more complicated with ST2, the biphasic system. Here, the multilayered lamellar phase in the starting solution is dispersed, after stirring, into a monolayered structure. 5-DSA still shows ordered spectra, but 16-DSA has just a hindered isotropic movement. Materials isolated after 20 min and later exhibited a slow motional spectrum, similar to the final precipitate. The XRD results also show a nonstructured precipitate at the very beginning of the reaction, while with the precipitate obtained at 20 min, a clear signal is observed. Porosity measurements confirmed the presence of mesopores after 20 min of reaction. Since lamellar phases are very sensitive to additives, we have followed the changes in the starting solution upon addition of
Synthesis of Mesoporous Silica
Figure 4. XRD patterns of silica materials at different times (a, 0 min; b, 20 min; c, 1 h; d, 18 h); (A) ACN/water 1:1, P1x; (B) ACN/ water 2:8, P2x; (C) ACN/water 8:2, P3x.
ethanol in the quantity corresponding to hydrolysis of TEOS used in the recipe. The results are reported in Figure 6. The effect of adding half the ethanol quantity is to reduce the order in the lamellar phase, while the second half transforms the lamellar phase into a micellar one. Similar experiments with the other two systems did not lead to any significant changes in the 5-DSA spectra: micelles were not remarkably different in ST1 and neither was their formation observed in ST3. The systems were also studied by in situ EPR (procedure 2). We have introduced the reaction mixture, including the spin probe 5-DSA immediately after TEOS addition, into a sample tube and started immediately to record the spectra (Figure 7A). Unexpectedly, compared with the typical lamellar liquid crystal spectrum of ST2, the first recorded spectrum had already completely changed into a three line spectrum, in the rapid rotational region, characteristic of 5-DSA in micelles (Figure 7A, (5′). This implies the complete reorganization of the surfactant from the lamellar liquid crystal to a micelle solution. After a very short time, approximately 15 min, features of a slow motion spectrum gradually appeared and totally prevailed after approximately 45 min (Figure 7A). This can be seen by comparison with the spectrum of the precipitate separated after 20 min from another aliquot of the same reaction mixture. The
J. Phys. Chem. C, Vol. 111, No. 39, 2007 14505
Figure 5. Isotherm profiles of silica materials at different times (a, 0 min; b, 20 min; c, 1 h; d, 18 h); (A) ACN/water 1:1, P1x; (B) ACN/ water 2:8, P2x; (C) ACN/water 8:2, P3x.
Figure 6. Changes in the EPR spectrum of 5-DSA in ST2 (ACN/ water ) 2:8) upon addition of ethanol: (a) initial DDA solution, (b) with 0.2 M ethanol, and (c) with 0.4 M ethanol.
spectra obtained in situ at intermediate times could be reproduced by sums of the micelle and the precipitate spectra (Figure
14506 J. Phys. Chem. C, Vol. 111, No. 39, 2007
Caragheorgheopol et al. 7B,C) with the proportion of the micellar solution spectrum (5′) decreasing rapidly in time. Ethanol release, which is a gradual process during precipitation, does not seem to be the only cause of the aggregate reorganization. Therefore, we believe that TEOS, or some of its partly hydrolyzed products, may interact directly with the lamellar liquid crystal phase. Thus, the precipitation mechanism includes the rearrangement of the organic phase under the influence of the inorganic precursor to form the template of the mesoporous material. Conclusions We have shown that depending on the characteristics of the starting solutions, different chains of events take place during the templated silica precipitation, which finally lead to a similar organic/inorganic hybrid. In ST1 precipitation-on-(preformed) micelles occurs after an “induction” period. In ST3, a true cooperative precipitation occurs, since micelles are not present in the starting solution nor during precipitation. In the ST2 system, we have witnessed the sudden and total dissolution of the DDA lamellar liquid crystal phase into a micellar phase upon addition of TEOS and have witnessed the build up of silica on these micelles. We have also found that there is an initial precipitate formed in the first 0-5 min, which is nonstructured, while after only 20 min, a mesoporous structure is well-evidenced. This fact may indicate that a hydrolysis product of TEOS is the intermediate which interacts with the template. It can thus be concluded that, in assessing the mechanism of templated silica precipitation, one should take into consideration the complexity of the reaction mixture and, in this context, the fact that continuous changes of reagent concentrations may produce significant changes in the structure of the template aggregates and thus on the resulting silica structure. Acknowledgment. Reciprocal visits (of A.C., F.S., D.J.M., and R.L.) have been supported by an International Royal Society Joint Project. Support from the Romanian Academy, Grant 38/ 2006 is also acknowledged. References and Notes
Figure 7. (A) In situ recorded spectra of 5-DSA in ST2 solution at different times after addition of TEOS: (ST2) before addition of TEOS; (5′) immediately after TEOS addition to ST2 (15′) reaction mixture after 15 min; (20′) reaction mixture after 20 min; (pp) precipitate separated from sample (20′). (B,C) Simulation of the intermediate time spectra, exp 15′ and exp 20′, respectively, as sums of the precipitate spectrum (pp) and the solution spectrum after 5′(s), the proportion of (s) decreasing rapidly in time.
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