J. Phys. Chem. B 2000, 104, 279-285
279
The Formation of the Mesoporous Material MCM-41 as Studied by EPR Line Shape Analysis of Spin Probes Jingyan Zhang,† Zeev Luz,† Herbert Zimmermann,‡ and Daniella Goldfarb*,† Department of Chemical Physics, Weizmann Institute of Science, RehoVot 76100, Israel, and Max-Planck-Institute for Medical Research, D-69120 Heidelberg, Germany ReceiVed: June 3, 1999; In Final Form: October 20, 1999
The formation mechanism of the hexagonal mesoporous material MCM-41, prepared with tetraethyl-orthosilicon (TEOS) and cetyl-trimethylammonium chloride (bromide) (CTAC/B), was investigated through the motional characteristics of the spin probe 5-doxyl stearic acid (5DSA). Electron spin echo envelope modulation (ESEEM) experiments, carried out on the final product, showed that the spin probe is incorporated into the organic part and the nitroxide radical is located near the organic-inorganic interface. The EPR spectra of 5DSA were measured in situ during the formation of MCM-41 at 298 K. The spectra were analyzed by computer simulations that provide the time evolution of the rotational diffusion rates, R⊥ and R|, and of the ordering potential. As the reaction progresses, the spin probe, which reflects the behavior of the surfactant molecules, experiences an increasing order parameter, S, while its rotational diffusion rates decrease. From the time evolution of these parameters two stages were distinguished. During the first, which lasts about 12 min, S, R⊥ and R| change rapidly whereas during the second, which lasts about 1 h, R⊥ and R| remain essentially constant while S exhibits a mild increase. The fast stage is assigned to the onset of orientational ordering and silicate condensation, which occur simultaneously, while the slow process reflects the “hardening” of the silica wall.
Introduction The family of mesoporous materials, synthesized using molecular assemblies of organic molecules as structure-directors, has attracted considerable interest since their first synthesis in 19921,2 due to their many potential applications. Since then, an extensive volume of synthetic work has been carried out in the field of mesostructural materials, but little effort has been devoted to elucidating the formation mechanism of the various synthetic routes.3-5 The formation mechanism of these materials is complicated and involves specific interactions between the organic and inorganic phases that eventually result in a long range structural order. The following three essential principles for the formation of mesoporous materials using charged surfactants were pointed out by Monnier et al.:6 (i) multidentate binding of silicate oligomers, (ii) preferred polymerization of silicates at the surfactant-silicate interface, and (iii) charge density matching across the interface. The importance of (i) and (ii) were recently questioned by Zana et al.7 who showed using fluorescence measurements that only a small fraction of the micelle-bound bromide ions are exchanged with silicate ions. Since the formulation of these principles, mesoporous materials have also been synthesized with neutral template molecules8 and block copolymers,9,10 thus calling for a generalized description of the formation mechanism that will include nonelectrostatic interactions such as hydrogen bonding and van der Waals forces. We have recently shown that in situ EPR spectroscopy of spin probes is an effective technique for the investigation of the formation mechanism of mesoporous materials.11,12 The spin †
Department of Chemical Physics, Weizmann Institute of Science, Rehovot 76100, Israel. ‡ Max-Planck-Institute for Medical Research, D-69120 Heidelberg, Germany.
probe 4-(N,N-dimethyl-N-hexadecyl)ammonium-2,2,6,6,-tetramethyl piperidine-oxyl iodide (CAT16) was chosen because of its structural similarity to the surfactant molecules and the location of the paramagnetic site near the polar head region, where the interaction between the silicate precursors and the micellar aggregates occurs. Although CAT16 was found to be sensitive to changes in the microviscosity during the reaction, it was insensitive to the orientational order due to its molecular structure and conformational equilibria that average out the anisotropic magnetic interactions. In this work we investigate the formation mechanism of MCM-41 using a different spin probe, 5-doxyl stearic acid (5DSA), which is sensitive to both the microviscosity and the orienting potential of its environment. The polar head of this spin probe (a negatively charged carboxylate group) is, however, different from that of the surfactant molecules (a positively charged tertiary amine). Nevertheless, as we will show using electron spin echo envelope modulation (ESEEM) experiments, the nitroxide group is located close to the interface and can reflect the processes occurring in it. The line shapes obtained are analyzed by computer simulations that give the rotational diffusion rates, the order parameter, and their temporal evolution, thus providing a quantitative description of the formation process. The simulation results clearly show that the motion of the surfactant molecules is significantly slowed during the reaction, and concomitantly, the local ordering of the surfactant increases. Experimental Section Materials. The reagents used for the synthesis were tetraethyl-orthosilicon, (C2H5O)4Si, (TEOS, Aldrich 98%), cetyltrimethylammonium chloride, C16 H33N(CH3)3Cl (CTAC, 25 wt % aqueous solution, Aldrich), cetyl-trimethylammonium bro-
10.1021/jp9917998 CCC: $19.00 © 2000 American Chemical Society Published on Web 12/15/1999
280 J. Phys. Chem. B, Vol. 104, No. 2, 2000
Zhang et al.
mide, C16H33N(CH3)3Br (CTAB, Aldrich), NaOH (2 N aqueous solution). The spin probe 5-doxyl stearic acid (5DSA) was purchased from Molecular Probe Europe BV Company. All compounds were used without further purification. Deuterated CTAB in the R position (R-d2-CTAB) was synthesized as described in the literature.13 The synthesis procedure of MCM41 and the in situ EPR experiments were as described earlier.11 The MCM-41 containing 5DSA is referred to as MCM-41(5DSA). The composition of the reaction mixture was 1.0 TEOS:0.12 CTAC:0.52 NaOH:115 H2O:1.13 × 10-3 5DSA and the d-spacing of the final product was 40 Å. MCM-41(5DSA) with a pore size of ∼100 Å was prepared, using mesitylene as a swelling agent. A lyotropic silicate-surfactant liquid crystal exhibiting an hexagonal phase was prepared according to Firouzi et al.13 using Cab-O-Sil (Fluka) as a silica source. The composition of the final mixture was: 1.6 SiO2:1.32TMAOH:0.50CTAB:207 H2O: 15.6CH3OH. Spectroscopic Measurements. EPR spectra were recorded using either a Varian E-12 or a Bruker ER200D-SRC spectrometer operating at 9-9.5 GHz. Gel samples were measured in flat cells and solid samples in 3 mm o.d. quartz tubes. ESEEM experiments were carried out on a home-built pulsed EPR spectrometer using a Britt-Klein probe head design.14-16 The three-pulse sequence, π/2 - τ - π/2 - T - π/2 - τ - echo, was employed with the appropriate phase cycling,17 and the π/2 pulse length was 20 ns. Powder X-ray diffraction patterns of the final products were measured on a Rigaku D/Max-B diffractometer using Cu KR radiation (λ ) 1.54 Å). Simulations. Line shape simulations were carried out using the NLSL program developed by Budil et al.18,19 where the dynamic parameters characterizing the molecular motion are obtained by a nonlinear-least-squares fit of the experimental spectrum to model calculations based on the Stochastic Liouville equation.18 For the sake of clarity, we present a brief description of the parameters used in the simulations and a short summary of the motional models and their effects on the EPR spectrum. Several frames of reference are defined in the calculations. The first, (x,y,z), is the principal axis system (PAS) of the magnetic interactions, namely the g and hyperfine A interactions, where it is assumed that their principal axes coincide. By convention, the z axis is taken as lying along the nitrogen p orbital (or N-O π orbital), and the x axis is along the N-O bond.20 The second is the diffusion frame, assumed to be axially symmetric, with the unique axis z′ the long molecular axis (see Figure 1). The third, also assumed to be axially symmetric, is determined by the ordering potential with z′′ along the director. The last one is the laboratory frame, where Z is along the external magnetic field. We have used a Brownian diffusion model and based on the molecular geometry of 5DSA an axially symmetric diffusion tensor was adopted, thus defining two rotational diffusion rate constants R| and R⊥, corresponding respectively to a rotation about the long and short molecular axes. The transformation of a magnetic tensor, Tp, represented in its PAS, to the various frames described above for a rod-like molecule and an ordering potential with axial symmetry, is given by the following sequence with the indicated Euler angles: R,β,0
0,θ,0
0,ψ,0
Tp 98 Tdiff 98 Tdir 98 Tlab
(1)
Neglecting the small deviation from axial symmetry of the hyperfine tensor and the small g anisotropy, the transformation from the PAS to the diffusion frame can be described by the
Figure 1. (a) A schematic representation of a fully extended all-trans chain configuration of 5DSA, where z is parallel to z′ (β ) 0). (b) The same but with kinks in alkyl chain, such that the z is tilted relative to z′ (β * 0°).
single angle β between z and z′ (see Figure 1), which is referred to as the tilt angle.21 The microscopic order is expressed by the order parameter S:
S ) 〈D200(θ)〉 )
〈
〉
3 cos2 θ - 1 ) 2
(
)
∫P(θ) 3 cos 2θ - 1 sin θ dθ 2
(2)
where P(θ) is the orientational distribution function given by21
P(θ) )
1 exp λ(3 cos2 θ - 1) 2 1 exp λ(3 cos2 θ′ - 1) sin θ′ dθ′ 2
[
∫
[
]
]
(3)
and λ ) -U(θ)/kBT. U(θ) is the orienting potential, kB is the Boltzmann constant, and T is the temperature. For an hexagonal phase, or a rod-like micelle, the preferred orientation corresponds to θ ) 90°, whereas for disk-like micelles or lamellar phase θ ) 0°. We adopted the microscopically ordered and macroscopically disordered (MOMD) model, i.e., a case in which there are domains with local ordering but whose directors are isotropically distributed so that there is no macroscopic ordering. This corresponds to integration over ψ.21 A spin probe in a micellar system experiences several types of motions: (i) conformational dynamics that averages R and β, (ii) tumbling of the micelle as a whole which modulates ψ and is usually slow on the EPR time scale,22 (iii) the rotational and lateral diffusions of the molecule within the micelle which modulate θ. Fast lateral diffusion in spherical micelles results in an isotropic spectrum, whereas in rod-like micelles it leads to a scaled tensor which reflects the shape of the aggregate.22 The theory does not take into account combined motions and
Formation of Mesoporous Material MCM-41
Figure 2. EPR spectra of 5DSA in water and CTAB solutions with various concentrations recorded at 298 K (thick lines) and at 333 K (thin lines). The solid and dashed arrows mark the A′|(MI ) -1) and A′⊥(MI ) -1) singularities, respectively, in the spectrum of the 21 wt % solution recorded at 333 K.
is therefore applicable to cases where the spin probe is rigid. However, when the spin probe is not rigid, a fast conformational dynamics leads to a scaled tensor that is further modulated by lateral and rotational diffusions. This scaling can be taken into account in the simulations by introducing an effective local order parameter23 or an effective tilt angle, β.24 The simulations require the principal components of the g and A tensors. We have applied the often used approach where the g values are taken from the literature and the A values are obtained by fitting the rigid limit spectrum.25 The latter is required since the A tensor depends on the local polarity.26 We have used the principal g values, gxx ) 2.0027, gyy ) 2.0061, and gzz ) 2.0088, of 5DSA in lyophilized bovine serum albumin27 and Axx ) 4.9 G, Ayy ) 5.4 G, and Azz ) 34.2 G, were determined by fitting the rigid limit spectrum of MCM41(5DSA) recorded at 150 K. Although the local polarity may change during the reaction, the effect on the hyperfine coupling is expected to be negligible compared to the variations in the rotational diffusion rates and order parameter and therefore we took the above A values as constant for simplicity. Results The Location of the Spin Probe. 5DSA sparingly dissolves in water28 and at the concentration used (5 × 10-4 M) it forms an aqueous dispersion which exhibits a single broad EPR line (see Figure 2). As CTAB is added, the broad single line gradually transformes into a triplet structure due to dissolved 5DSA. At 0.5 wt % surfactant it becomes well resolved indicating that 5DSA is fully incorporated into the CTAB micelles. With further increase of the CTAB concentration, the EPR lines continue to narrow because the local spin probe concentration in each micelle decreases, up to a concentration of 5 wt % CTAB, where a limiting width is reached. To confirm that 5DSA is incorporated into the organic phase in the final product and that the nitroxide radical is located near the silica-surfactant interface, we performed ESEEM experiments on as-synthesized MCM-41(5DSA) and MCM-41(CAT16) which were prepared with R-d2-CTAB. Figure 3A shows the time domain wave forms and the corresponding FTESEEM spectra recorded at 30 K. The spectra exhibit peaks at the 1H and 2H Larmor frequencies, the intensities of which reflect the modulation depth in the corresponding time domain
J. Phys. Chem. B, Vol. 104, No. 2, 2000 281
Figure 3. Three-pulse ESEEM waveforms of MCM-41(CAT16) and MCM-41(5DSA) (left) and the corresponding Fourier transformed traces (absolute values) obtained after proper normalization (right). (A) τ ) 230 ns optimized for 2H modulation, and (B) τ ) 530 ns optimized for 14N modulation.
wave forms. For weak hyperfine interactions, as expected for the intermolecular dipolar interaction between the nitroxide radical and deuterons on nearby surfactant molecules, the modulation depth is a function of the electron-nuclei distance, the number of nuclei and their nuclear spin.29 Accordingly, the similar 2H peak intensities in the spectra of the two samples indicate that the distances of the radical to the R-deuterons of the neighboring surfactant molecules are comparable. The more intense 1H peak in MCM-41(CAT16) is due to intramolecular dipolar interactions with the protons of the nitrogen methyl groups.12 Further confirmation that the nitroxide radical is close to the interface is obtained from the appearance of a 14N signal at ∼1.0 MHz in the spectra of samples prepared with nonlabeled surfactant molecules as shown in Figure 3B (two lower traces). Similar experiments carried out on MCM-41(CAT16) showed somewhat deeper 14N modulation than in MCM-41(5DSA) (see Figure 3B). This is expected because in CAT16 the intramolecular dipolar interaction with 14N also contributes to the modulation. In addition, the nitroxide radical in CAT16 maybe closer to the 14N nucleus in the polar head of the surfactant than in 5DSA. We have also compared the 14N modulation depth of MCM-41(5DSA) with d ) 100 and 40 Å and found it to be independent of the pore size, which is consistent with the above results. The temperature dependence of the EPR spectrum of MCM41(5DSA), presented in Figure 4, shows it to be sensitive to the anisotropy of the environment. At 298 K the line shape is close to the rigid limit spectrum, while at 393 K it is characteristic of a partially averaged tensor. This is in contrast to MCM-41(CAT16) which at 373 K exhibits an almost isotropic spectrum. 5DSA, therefore, is an appropriate probe for investigating the interface region. In Situ EPR Measurements and Spectral Simulations. The time evolution of the EPR spectrum during the formation of MCM-41(5DSA) at 298 K, using CTAC, is presented in Figure 5. The same results were obtained with CTAB. All spectra, including that of the reaction mixture without TEOS, exhibit features characteristic of residual anisotropic magnetic interac-
282 J. Phys. Chem. B, Vol. 104, No. 2, 2000
Zhang et al. TABLE 1: Best-Fit Parameters Used in the Simulations Shown in Figure 5 Obtained with β ) 36° data set (reaction time, min) 0 3 12 18 45 60 120 product
R|/×108 s-1
R⊥/×108 s-1
line width, G
S
16.6 ( 0.8 11.7 ( 0.9 6.6 ( 1.1 6.5 ( 0.3 6.4 ( 1.0 6.3 ( 0.3 5.9 ( 0.6 2.1 ( 1.1
0.20 ( 0.05 0.16 ( 0.08 0.1 ( 0.08 0.09 ( 0.09 0.1 ( 0.09 0.1 ( 0.02 0.09 ( 0.09 0.04 ( 0.009
1.6 ( 0.6 1.4 ( 0.5 1.3 ( 0.2 1.2 ( 0.1 1.4 ( 0.4 1.3 ( 0.1 1.5 ( 0.3 2.3 ( 0.4
0.0 0.11 ( 0.03 0.27 ( 0.13 0.33 ( 0.08 0.45 ( 0.02 0.50 ( 0.06 0.56 ( 0.03 0.70 ( 0.01
a The range of the parameters were estimated from the simulations carried out with different β angles.
Figure 4. EPR spectra of as-synthesized MCM-41(5DSA) recorded at various temperatures.
Figure 6. The temporal evolution of R| and R⊥ as determined by EPR lineshape simulations.
Figure 5. Experimental EPR spectra of 5DSA in the reaction mixture recorded as a function of the reaction time (solid traces) at 298 K, and best-fit simulated EPR spectra calculated with the parameters listed in Table 1 (dotted traces). Zero time (t ) 0) corresponds to the mixture before the addition of TEOS. The arrow marks the A′|(MI ) -1) singularity.
tions. The major changes in the lineshape occur during the first 12 min after the addition of TEOS. The spectrum then undergoes small changes up to 60 min, thereafter it remains constant and is characteristic of a powder pattern scaled by motional averaging. The extent of the scaling depends on the water content in the sample as may be seen by comparison with the bottom trace in Figure 5, which corresponds to a sample dried at room temperature. The interpretation of the dynamic spectra requires first to determine the parameters of the spin probe prior to the TEOS addition (t ) 0). Although at this stage the reaction mixture contains only spherical micelles,30 the spectrum exhibits anisotropic features due to slow motion, as indicated by the high field A′|(MI ) -1) singularity marked by an arrow in Figure 5. By varying β, S, R⊥, and R| we found that the best fit was achieved with β ) 31-36° and S ) 0.31 The top trace in Figure 5 shows the spectrum calculated with β ) 36°, S ) 0 and the
parameters listed in Table 1. The fact that β * 0 reflects the presence of a fast conformation equilibrium and/or kinks.21 Similar deviations of β from zero were observed for 5DSA in solutions and swollen membranes of Nafion where β ) 12.527.5°.24 All simulations for t > 0, presented in Figure 5, were carried out with β ) 36°. We note, however, that simulations with β ) 31° generated the same overall quality of fit. The best fit parameters are listed in Table 1, where the range of the parameters was determined from the β ) 36° and β ) 31° sets of simulations. The temporal evolution of R| and R⊥ is plotted in Figure 6 and that of S is presented in Figure 7. R| and R⊥ show a fast initial decrease (up to 12 min) and are essentially constant thereafter. The order parameter exhibits a monotonic increase with a faster initial growth leveling off at longer time. The order parameter of fatty acid spin labels can also be estimated directly from the spectrum using the approximate relation:32
S)
〈A|〉 - [〈A⊥〉 + C] 〈A|〉 + 2[〈A⊥〉 + C]
1.66
(4)
C ) 4.06 - 0.053(〈A|〉 - 〈A⊥〉) MHz
(5)
where
〈A|〉 and 〈A⊥〉 are equal to half the separation of the outer and inner extrema, 〈2A|〉, 〈2A⊥〉, respectively, as indicated on Figure 5. The dotted curve in Figure 7 represents the time evolution of the order parameter as calculated from eq 4.
Formation of Mesoporous Material MCM-41
J. Phys. Chem. B, Vol. 104, No. 2, 2000 283
Figure 7. The temporal evolution of the order parameter S as determined by EPR line shape simulations (Table 1). The dotted line represents S calculated from the experimental spectra using the eqs 4 and 5.
Model Systems. For reference purposes, the spectra of 5DSA in spherical and rod-like micelles, and in the hexagonal mesophase, were measured. The line shapes of 5DSA in these systems can serve to distinguish spherical and rod-like micelles and to detect the formation of an hexagonal liquid crystalline phase. Earlier work reported that the transition between spherical to rod-like micelles occurs in CTAB water solutions at about 7-10 wt %30 and the hexagonal phase appears at about 21 wt %.33 Accordingly, aqueous solutions of 5DSA in 5, 13, and 21 wt % CTAB solutions were prepared and the EPR spectra were measured as a function of temperature. The spectra measured at 298 K and 333 K are shown in Figure 2. The differences between the spectra corresponding to spherical micellar, rodlike micellar, and hexagonal phases at 298 K are very subtle. The spectra of the 5 and 13 wt % solutions look similar also at 333 K, whereas that of the 21 wt % solution is significantly different, exhibiting a powder pattern in the high field component of the spectrum. This powder pattern is typical of a fast lateral diffusion in an hexagonal phase22 which corresponds to fast rotation about an axis perpendicular to the long molecular axis, z′ (see Figure 1). This rotation results in an new scaled tensor where the A′|(MI ) -1) and A′⊥(MI ) -1) singularities, marked by solid and dashed arrows in Figures 2, are reversed, namely, the splitting 〈2A⊥〉〉 . 〈2A|〉.22 In principle, a similar reversed powder pattern should be observed in the low field component (MI ) 1) but due to its narrower field spread22 it is not resolved. It has been demonstrated that under conditions of low reactivity of silicate species at high pH and relatively low temperatures, it is possible to stabilize lyotropic silicatesurfactant liquid crystals.13,34 Under these conditions, the silicate polymerization is suppressed and the mesoporous solid is not formed unless it is treated with HCl to initiate the silica condensation. A CTAB/silicate lyotropic hexagonal liquid crystalline phase was prepared according to Firouzi et al.13 in order to examine the EPR lineshapes of 5DSA in this phase. The mixture consists of two phases, a water rich phase and a liquid crystal phase which is highly viscous.13 Inspection of the a sample consisting primarily of the liquid crystal phase under a polarized microscope at 333-343 K revealed textures similar to those reported earlier at 323 K, confirming the presence of a liquid crystalline phase.13 Figure 8 shows the temperature dependence of the EPR spectrum of 5DSA in this sample. It
Figure 8. EPR spectra of 5DSA in the silicate-surfactant mesophase recorded at various temperatures. The solid and dashed arrows on the 383 K spectrum mark the A′|(MI ) -1) and A′⊥(MI ) -1) singularities, respectively. Those on the 313 K spectrum indicate both the A′|(MI ) (1) and A′⊥(MI ) (1) singularities.
was placed in the magnetic field at 298 K, and the spectra were recorded from low to high temperature. The powder patterns observed are characteristic of a non-aligned liquid crystal as expected for this heating procedure and the relatively low magnetic field (∼0.3 T) used. At 298 K, the spectrum exhibits a powder pattern with 〈2A|〉 similar to those of MCM-41(5DSA) at 393 K and the reaction mixture after ∼45 min. As the temperature increases, 〈2A|〉 decreases and at 373 K the spectrum becomes characteristic of an hexagonal phase (or rod-like micelles undergoing slow tumbling) where the spin probe experiences a fast lateral diffusion as observed for 5DSA in 21 wt % CTAB at 333 K (Figure 2). The powder-like appearance of the high field line at 383 K, which is well above the reported mesophase-isotropic phase transition in these systems (323363 K, determined using 2H NMR spectroscopy34) indicates the presence of rod-like micelles. The tumbling of these micelles is not fast enough at 383 K (on the EPR time scale) to fully average the anisotropic magnetic interaction. Complete averaging occurs only at 393 K where the spectrum becomes essentially isotropic. Discussion The anisotropic features of the 298 K spectrum of 5DSA in the micellar solution, prior to the addition of TEOS, indicate that the lateral diffusion of the spin probe is slow on the EPR time scale. This is supported by the small difference between the spectra of 5DSA in the spherical micelles, rod-like micelles, and the hexagonal phase (Figure 2). In contrast, at 333 K, the lateral diffusion rate becomes much faster as indicated by the appearance of a triplet in the spectra of the 5 and 13 wt % CTAB solutions and the scaled powder pattern of the high field component observed for the 21 wt % solution. In this pattern the A′|(MI ) -1) and A′⊥(MI ) -1) singularities are switched which is characteristic of the hexagonal phase with fast lateral diffusion.22 These results, and those obtained from the silicatesurfactant liquid crystal, indicate that due to the slow lateral
284 J. Phys. Chem. B, Vol. 104, No. 2, 2000 diffusion at room temperature it is difficult to detect changes in curvature of the aggregate that occur during the reaction. Simulations of the EPR lineshapes evolution show that the spin probe experiences an ordering potential already after 3 min and it keeps growing throughout the reaction. In addition, the time evolution of R⊥ and R| detects two stages: a short one lasting about 12 min followed by a longer one that continues for about an hour. The onset of an order parameter at the very beginning of the reaction is consistent with in situ X-ray measurements of the formation of MCM-41 at 303 K using TEOS showing that the hexagonal phase develops within 3 min.5 Before considering possible assignment of the fast and slow processes, we discuss the various steps that may occur during the synthesis and their effect on the rotational diffusion and the order parameter. Prior to the addition of TEOS the reaction mixture consists of micelles of surfactant molecules in which the spin probe 5DSA is dissolved. In addition, the mixture includes NaOH required for the hydrolysis of TEOS to form the silicate ions. Once TEOS is added the following steps may be visualized: (i) the interaction of the silicate ions oligomers with the surfactant head groups at the interface,35,36 which can change the packing and the microviscosity at interface and vary the lateral and rotational diffusion rates of the surfactant molecules; (ii) a transition from spherical to rod-like micelles occurring due to changes in the aggregate curvature induced by the interaction with the silicate ions,13,37 which should be associated with the introduction of an order parameter. In the case of a fast lateral diffusion and slow aggregate tumbling rate the transition should result in a pronounced change in the line shape.22 However, when the lateral diffusion is slow, as in our case, the line shape becomes rather insensitive to the shape of the aggregate; (iii) the packing of the rod-like silicate coated micelles to produce the hexagonal phase should, in principle, lead to an increase in S and a decrease in R⊥ and R|. Nonetheless, if the aggregate tumbling is very slow, this packing will not affect the lineshape; (iv) condensation of the silicate oligomers formed by the hydrolysis and polymerization of TEOS at the interface to generate the silica wall37 is expected to affect both R⊥, R|, and S. The four steps described above may occur sequentially or simultaneously. For example, it is most likely that (i) and (ii) occur concomitantly, whereas the slow lateral diffusion of 5DSA makes it difficult to probe (ii) and (iii). It maybe possible to monitor these processes using a smaller spin probe that exhibits faster lateral diffusion. We attribute the initial fast change in R⊥, R|, and S, to a simultaneous occurrence of any of the processes (i)-(iii), including initialization of (iv), resulting in a soft silica wall with many hydroxyl groups. The slow process, manifested primarily in a mild increase in S, is attributed to progress of process (iv), that is “hardening” of the silica wall by further dehydroxylation which reduce the silanol groups, converting Q2 and Q3 to Q4.38 This, in principle, can be verified by 29Si MAS NMR where the Q2/Q4 and Q3/Q4 is followed in situ. The difficulty in these experiments is the relatively long time needed to record the 29Si NMR spectrum of samples with natural abundance 29Si. NMR measurements were carried out on dry samples which were taken at various reaction times show that the Q3/Q4 ratio indeed decrease with reaction time.37 Recently, Galarneau et al.39 investigated the formation of MCM-41 synthesized with sodium silicate at 323 K, using three spin probes, 5DSA, CAT16, and 12DSA. The reaction with sodium silicate is significantly slower than with TEOS. From the simulations of the spectra of 5DSA they observed a rather unusual time evolution of S. After 3 min it is 0.4, it reduces to
Zhang et al. 0.31 after 3 h, and finally increases to 0.35 after 1.5 days. In these simulations β ) 0° and R⊥ is constant throughout the reaction. This irregular variation in S and the invariant R⊥ are rather surprising and different from our results. Conclusions The spin probe 5DSA is located in as-synthesized MCM-41 within the organic phase with the nitroxide radical close to the interface; therefore, it is sensitive to processes occurring at the interface. Lineshape simulations of the in situ EPR spectra of 5DSA recorded during the formation of MCM-41 with TEOS at 298 K show that, as the reaction progresses, the surfactant molecules experiences a growing ordering potential while the rotational diffusion rates of the molecules decrease. The time evolution of these two parameters indicates the presence of two stages. The first lasts about 12 min, during which both the order parameter and the rotational diffusion rates change rapidly. During the second process, which lasts about 1 h, the rotational diffusion rates remain constant while the order parameter exhibits a mild increase. The fast process is assigned to the initial stages of the silica condensation and the onset of orientational ordering, occurring simultaneously, while the slow process reflects the “hardening” of the silica wall. The simulations of the EPR line shapes in terms of the order parameter and the rotational diffusion rates provide means to “parameterize” the reaction and study its detailed kinetics. This can serve in the future as a basis for the comparison of different reaction conditions and pathways, producing different final products, thus leading to a better understanding of the formation mechanism of these materials. Acknowledgment. This work was supported by a grant from the Israeli Ministry of Science and Technology. We thank Prof. Eva Meirovitch for helpful discussions and Prof. David Budil for his help with the simulation program. References and Notes (1) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T-W.; Olson, D. H.; Shepard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834. (2) Beck, J. S.; Vartuli, J. C.; Kennedy, G. J.; Kresge, C. T.; Roth, W. J.; Schranm, S. E. Chem. Mater. 1994, 6, 1861. (3) Ying, Y. J.; Mehnert, C. P.; Wong, M. S. Angew. Chem., Int. Ed. 1999, 38, 56. (4) Calabro, D. C.; Valyocsik, E. W.; Ryan, F. X. Micropor. Mater. 1996, 7, 243. (5) (a) Linden, M.; Schunk, S.; Schu¨th, F. In Mesporous Molecular SieVes 1998, Studies in Surface and Catalysis; Bonneviot, L., Beland, F., Danumah, C., Giasson, S., Kaliaguine, S., Eds.; Elsevier Science B. V; Amsterdam, 1998; Vol. 117, p 45. (b) Linden, M.; Schunk, S.; Schu¨th, F. Angew. Chem., Int. Ed. Engl. 1998, 37, 821. (6) Monnier, A.; Schu¨th, F.; Huo, Q.; Kumar, D.; Margolese, D.; Maxwell, R. S.; Stucky, G. D.; Krishamurty, M.; Petroff, P.; Firouzi, A.; Janicke, M.; Chmelka, B. F. Science 1993, 261, 1299. (7) Zana, R.; Frasch, J.; Soulard, M.; Lebeau, B.; Patarin, J. Langmuir 1999, 15, 2603. (8) Bagshaw, S. A.; Prouzet, E.; Pinnavaia, T. J. Science 1995, 269, 1242. (9) Templin, M.; Franck, A.; Chesne, A. D.; Leist, H.; Zhang, Y.; Schajdler, V.; Wiesner, U. Science 1997, 278, 1795. (10) Zhao, D.; Feng, J.; Huo, Q.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; Stucky, G. D. Science 1998, 279, 548. (11) Zhang, J.; Luz, Z.; Goldfarb, D. J. Phys. Chem. B 1997, 101, 7087. (12) Zhang, J.; Zimmerman, H.; Luz, Z.; Goldfarb, D. In Mesoporous Molecular SieVes 1998, Studies in Surface and Catalysis; Bonneviot, L., Beland, F., Danumah, C., Giasson, S., Kaliaguine, S., Eds.; Elsevier Science B. V: Amsterdam, 1998; Vol. 117, p 535. (13) Firouzi, A.; Atef, F.; Oertli, A. G.; Stucky, G. D.; Chmelka, B. F. J. Am. Chem. Soc. 1997, 119, 3596. (14) Britt, R. D.; Klein, M. P. J. Magn. Reson. 1987, 74, 535.
Formation of Mesoporous Material MCM-41 (15) Goldfarb, D.; Fauth, J. M.; Tor, Y.; Shanzer, A. J. Am. Chem. Soc. 1991, 113, 1941. (16) Shane, J. J.; Gromov, I.; Vega, S.; Goldfarb, D. ReV. Sci. Instrum. 1998, 69, 3357. (17) Fauth, J. M.; Schweiger, A.; Braunschweiler, L.; Forrer J.; Ernst, R. R. J. Magn. Reson. 1986, 66, 74. (18) Budil, D. E.; Lee, S.; Saxena, S.; Freed, J. H. J. Magn. Reson. A 1996, 120, 155. (19) Schneider, D. J.; Freed, J. H. In Biologicl Magnetic Resonance. Spin Labeling; Berliner, L. J., Reuben, J., Eds.; Plenum: New York, 1989; Vol. 8, Chapter 1. (20) Freed, L. H. In Spin Labelling (Theory and Applications); Academic Press: New York, 1976; Chapter 3. (21) Meirovitch, E.; Nayeem, A.; Freed, J. H. J. Phys. Chem. 1984, 88, 3454. (22) Lasic, D. D.; Hauser, H. J. Phys. Chem. 1985, 89, 2648. (23) Wikander, G.; Eriksson, P.; Burnell, E. E.; Lindblom, G. J. Phys. Chem. 1990, 94, 5964. (24) Szajdzinska-Pietek, E.; Pilar, J. Schlick, S. J. Phys. Chem. 1995, 99, 313. (25) Ottaviani, M. F.; Daddi, R.; Brustolon, M.; Turro, N. J.; Tomalia, D. A. Langmuir 1999, 15, 1973. (26) Morrisett, J. D. In Spin Labelling (Theory and Applications); Academic Press: New York, 1976; Chapter 8. (27) Gaffney, B. J.; McConnell, H. M. J. Magn. Reson. 1974, 16, 1. (28) Hiff, T.; Kevan, L. J. Phys. Chem. 1989, 93, 1572.
J. Phys. Chem. B, Vol. 104, No. 2, 2000 285 (29) Kevan, L. In Modern Pulsed and Continues WaVe Electron Spin Resonance; Kevan, L., Bowman, M. K., Eds.; Wiley: New York, 1990. (30) Husson-Reiss, F.; Luzzati, V. J. Phys. Chem. 1964, 68, 3504. (31) This is in contrast to an earlier report of the spectrum of 5DSA in a CTAB (0.4%) solution with NaOH (0.14 M) which was nicely simulated with S ) 0, β ) 0, and R⊥ ) 7.74.39 We tried to simulate our spectrum with these parameters but the fit was still not satisfactory. We attribute this discrepancy to the different compositions of the solutions where we have [NaOH] ) 0.20 M and CTAB ) 2 wt %. (32) Gaffney, B. J. In Spin labeling (Theory and Applications); Berliner, L. J. , Eds.; New York, 1976; p 567. (33) Auray, X.; Petipas, C.; Anthore, R.; Rico, L.; Lattes, A. J. Phys. Chem. 1989, 93, 7458. (34) Tolbert, S. H.; Firouzi, A.; Stucky, G. D.; Chmelka, B. F. Science 1997, 278, 264. (35) 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. I.; Stucky, G. D.; Chmelka, B. F. Science 1995, 267, 1138. (36) Lee, Y. S.; Surjadi, D.; Rathman, J. F. Langmuir 1996, 12, 6202. (37) Huo, Q.; Margolese, D. I.; Ciesla, U.; Demuth, D. G.; Feng, P. Y.; Gier, T. E.; Sieger, P.; Firouzi, A.; Chmelka, B. F.; Schuth, F.; Stucky, G. D. Chem. Mater. 1994, 6, 1176. (38) Chen, C. Y.; Burkett, S. L.; Li, H. X.; and Davis, M. E. Microporous Mater. 1993, 2, 27. (39) Galarneau, A.; Renzo, F.; Fajula, F.; Mollo, L.; Fubini, B.; Ottaviani, M. F. J. Colloid Interface Sci. 1998, 201, 105.
