7102
J. Phys. Chem. C 2008, 112, 7102-7109
Evolution of Solution Structures during the Formation of the Cubic Mesoporous Material, KIT-6, Determined by Double Electron-Electron Resonance Sharon Ruthstein and Daniella Goldfarb* Department of Chemical Physics, Weizmann Institute of Science, RehoVot, 76100, Israel ReceiVed: January 8, 2008; In Final Form: February 13, 2008
Double electron-electron resonance (DEER) is employed to explore the evolution of solution structures during the formation of the bicontinuous cubic (Ia3hd) mesoporous material KIT-6. We focused on the early stages of the reaction, where micellar structures are not well-resolved in the micrographs of cryogenic transmission electron microscopy (cryo-TEM). KIT-6 is synthesized with Pluronic P123 block copolymers, PEO20PPO70PEO20, as a template. Initially, the aqueous solution, held at 40 °C, consists of Pluronic P123 micelles, which are characterized by a hydrophilic corona, comprising the poly(ethylene oxide) (PEO) blocks, and a hydrophobic core, consisting of the poly(propylene oxide) (PPO) block. The variations in the volume of the hydrophobic core of the micelles as a consequence of the addition of the silica source (TEOS, tetraethoxyorthosilane) were evaluated using a hydrophobic spin-probe, 4-hydroxy-tempobenzoate (4HTB), which is localized in the hydrophobic core of the micelles. The measurements were carried out on solutions that were freeze-quenched at different times after the addition of TEOS (tetraethoxyorthosilane). New details on the formation of KIT-6 were obtained, revealing swelling of the hydrophobic core during the first ∼10 min of the reaction followed by a contraction, close to the original size. The swelling was attributed to penetration of the TEOS and its hydrolysis products into the micelles.
Introduction Mesoporous materials synthesized using nonionic block copolymer surfactants, such as poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (Pluronics, PEOxPPOyPEOx), are characterized by a narrow range distribution of pores that can be tuned within the range 2-30 nm.1-4 The Pluronics copolymers exhibit atypical temperature behavior, namely, at low temperatures the polymer is soluble in water, but the solubility of the PPO block decreases when the temperature is raised, resulting in micelle formation. The micelles comprise a hydrophobic PPO core and a hydrophilic corona of hydrated PEO segments.5-7 The use of block copolymers as templates allows tuning the micellar structures by adjusting the solvent composition, molecular weight, or copolymer architecture. Moreover, at low solution concentrations, they permit the organization of structures considerably larger than those obtained with low-molecular-weight ionic surfactants.8 The first promising material that was synthesized using block copolymers was the 2D-hexagonal SBA-15 material, prepared with Pluronic P123 (PEO20PPO70PEO20).4 It has attracted considerable attention not only because of its high structure regularity, thick inorganic walls, and excellent thermal and hydrothermal stability, but also because the template is low cost and nontoxic, and the synthesis is simple and reproducible. Mesoporous materials can be organized also in a variety of cubic structures with Im3hm, Pm3n, and Ia3hd symmetries. These materials are expected to be superior to hexagonal structures for applications involving selectively tuned diffusion, immobilization of large molecules, or host-guest interactions in nanostructured materials.9,10 The synthesis of cubic mesoporous materials usually involves the use of additives such as inorganic salts or anionic surfactants,11 with or without a swelling agent.12 Recently, Ryoo and
co-workers have synthesized a cubic Ia3hd material, called KIT6, using Pluronic P123 and n-butanol at low acid conditions.13-15 Understanding the formation mechanism of mesoporous materials, in particular how a homogeneous micellar solution transforms into an ordered phase, is intriguing both fundamentally and practically, because it can assist synthetic control over the produced structures and direct efforts to improve properties required for specific applications. Several recent reviews focused on the formation mechanism of templated mesoporous materials.16-19 In general, the formation mechanism of mesoporous materials can be viewed at three length scales: (i) the molecular level, which involves the interaction between the organic and the inorganic precursors and the silica polymerization process; (ii) the mesoscopic scale, which involves the development of the micelles’ structure and the onset of the long-range order; and finally (iii) the macroscale, which is related to the shape of the final product. It is clear that processes at the molecular level form the driving force for the mesoscale structure, but the details of this relation are not fully comprehended. Most of the mechanistic studies reported so far have focused on one length scale, and only a few attempts to correlate different length scales using different techniques have been reported. Examples are the application of in situ 1H NMR, SAXS (small-angle X-ray scattering) and TEM (transmission electron microscopy) in the study of the formation mechanism of SBA15 by Flodstro¨m et al.20,21 In an earlier study, we combined in situ continuous wave (CW) EPR and freeze-quench electron spin-echo envelope modulation (ESEEM) spectroscopy of spinprobes with cryo-TEM (cryogenic-temperature TEM) to investigate the formation mechanisms of SBA-15 and KIT-6.22,23 The former provided information on the evolution of the tumbling rate of the organic molecules, reflecting local viscosity, and the polarity and water content of their close environment during
10.1021/jp8001367 CCC: $40.75 © 2008 American Chemical Society Published on Web 04/12/2008
Evolution of Solution Structures the reaction. The development of the nanostructures during the reaction was studied by cryo-TEM. In cryo-TEM, the structures in solution are observed by vitrifying a thin layer of a solution without staining or drying the sample. Our results revealed similar general features in the formation of SBA-15 and KIT6. It occurs through adsorption of silicate ions and their penetration into the corona. Hydrolysis and condensation of the silicate ions cause a change in the hydrophobicity of the PPO and PEO segments due to dehydration and thereby change the micelle curvature. This leads to rearrangement of the original micellar shape, mainly lengthening the micelles, followed by condensation of the silicate-covered micelles, which precipitate to form a disordered phase. This then transforms into an ordered hexagonal phase in the case of SBA-15, whereas for cubic KIT6, the hexagonal phase is further transformed into the cubic phase. The transformation occurs due to the presence of butanol added either at the beginning or after the formation of the hexagonal phase.23 We have identified five main stages in the formation of KIT6.23 The addition of butanol to the P123 micellar solution (before adding TEOS) causes a large increase in the water content in the corona, smearing the core/corona interface. Next, TEOS is added and depletion of water and butanol from the micelles takes place during the first 50 min of the reaction (first stage). Then, up to the precipitation time at t ∼140 min (second stage), the change in the water and butanol content is very mild, and a transition from spheroidal micelles to threadlike micelles (TLMs), that further aggregate, is observed toward the end of this stage. The third stage, the precipitation time (140-160 min), showed reorganization in the aggregate structure, and a change in the relative sizes of core and corona. The fourth stage ended after around 6 h and involved the formation of a hexagonal arrangement. During this stage, an accelerated condensation of silica oligomers in the corona, accompanied by a large depletion of water and butanol molecules was observed. The last stage corresponds to a transition from hexagonal to cubic phase occurring between 6 and 24 h. Although the reaction mechanism of KIT-6 was studied in detail, few questions remained open, especially the sizes of the micellar structures present during the early stages of the reaction (first 50 min), when the degree of silica polymerization is very low. In these early stages (t < 20 min), the micelles are not well-resolved in the cryo-TEM micrographs. In the present work, we show that the double electron-electron resonance (DEER) method, which determines distances between electron spins in the nanometer range, can be used to follow the formation of mesoporous materials and give information on variations in the micelles’ volumes, thus complementing the earlier EPR and cryo-TEM results. Milov and Tsvetkov carried out DEER experiments to study clustering of radicals and biradicals in glassy solutions.24,25 Later, Jeschke et al. used DEER to characterize an organic monolayer of surfactant spin-probes on organoclay particles with and without the presence of polystyrene in the monolayer.26 Recently, we demonstrated that DEER can be used to determine the volume of the core and the aggregation number of Pluronic micelles.27 This was achieved by adding minute amounts of the spin-probe 4-hydroxy-tempobenzoate (4HTB) to the micellar solution and determining the volume it occupies. Because 4HTB is hydrophobic and water-insoluble, it is preferably localized in the hydrophobic core of the micelles, and the volume it occupies is therefore proportional to the core volume.27 Here, we employ this approach to learn more about the evolution of solution structures during the formation of KIT-6, specifically
J. Phys. Chem. C, Vol. 112, No. 18, 2008 7103
Figure 1. (a) The spin-probe used: 4-hydroxy-tempobenzoate (4HTB). (b) The four-pulse DEER sequence.
at the early stages of the reaction. We observed that during the first 10 min of the reaction the hydrophobic volume of the micelles increases by a factor of about two, but then with the progression of the hydrolysis and dehydration, the micelle decreases back to its original size. Experimental Section Sample Preparation. The reagents we used for the synthesis were as follows: Pluronic P123 (PEO20PPO70PEO20, 〈M〉 ) 5800), a gift from BASF Corp. (USA), tetraethoxyorthosilane (TEOS, (CH2CH2O)4Si (Aldrich)), hydrochloric acid (HCl 32%, Frutarom), and butanol (99.7% Aldrich). The spin-probe employed in this study was 4-hydroxy-tempobenzoate (Aldrich) (Figure 1a). KIT-6 was synthesized according to the procedure reported by Kim et al.,15 except for the addition of the spin-probe as described earlier.23 The structure of the final material was characterized by SAXS and TEM, showing a clear cubic structure, and the addition of the spin-probe did not affect the final structure. The surface area and pore size distribution were obtained from nitrogen adsorption-desorption isotherms using conventional Braunauer-Emmett-Teller (BET) and BJH methods. All these results are shown in ref 23. Spectroscopic Measurements. EPR spectra were recorded using a X-band Bruker Elexsys 500 spectrometer (9.35 GHz). Gel and liquid samples were measured in flat cells. In situ EPR measurements were carried out as follows: The P123 solution was first prepared and then HCl + BuOH were added under stirring; after 1 h, TEOS was added under vigorous stirring.23 Ten minutes later, part of the mixture was quickly transferred into an EPR flat cell, which was kept in the EPR cavity until the end of the measurement at 40 °C. The solid formed during the experiment remained within the active part of the cavity throughout the measuring period. The remaining solution was left on the hot plate with no stirring. SAXS of the final material showed that the cubic structure had still formed, also without stirring. In addition, all visible effects, turbidity and precipitation, were observed at the same time, both in the flat cell and in the remaining solution. Constant-time DEER measurements were carried out at 9.4 GHz with a Bruker Elexsys E 580 spectrometer using the fourpulse DEER sequence (π/2)(νobs)-τ1-π(νobs)-t′-π(νpump)-(τ1+τ2t′)-π(νobs)-τ2-echo (Figure 1b).28 A two-step phase cycle was employed on the first pulse, and averaging over 25 increments of τ1 (τ1 ) 200 ns, ∆τ1 ) 8 ns) was carried out to suppress proton modulations.29 The echo was measured as a function of t′ (∆t′ ) 20 ns), while τ2 was kept constant at 2.4 µs to eliminate relaxation effects. The pump frequency, νpump, was set to the
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maximum of the nitroxide spectrum, and the observer frequency, νobs, was 60 MHz higher. The duration of the π/2 and π pulses was 40 ns. Typical numbers of shots per point and accumulations are 30 and 300, respectively. The measurement temperature was 15 K. The samples for the DEER measurements were prepared by taking a sample from the reaction mixture at different times and quickly transfering it into a Teflon tube and placing it in a 2-methyl butane bath cooled by liquid nitrogen. Theoretical Background. In spin-echo-based DEER experiments, the evolution of the spin-echo signal for a single pair (i, k) is30
V(t′) ) V0[1 - λk(1 - cos(ωikt′))]
(1)
t′ is the appropriate variable time interval, which depends on the chosen technique; λk is the probability to flip one of the two spins by the appropriate pulse, and its average is given by
〈λk〉g(∆ωk ) )
ω
2
∫-∞∞ ω 2 +1∆ω 2 × 1
k
(ω12 + ∆ωk2)1/2tp sin 2 g(∆ωk) d∆ωk (2) 2 where ω1 and tp are the amplitude and the nominal duration of the π pulse, respectively, ∆ωk is the off-resonance frequency, and g(∆ωk) is the probability of ∆ωk, which is given by the EPR spectrum. In eq 1, ωik is
ωik ) ωdd(ik)(3 cos2 θik - 1)
(3)
where ωdd(ik) is the so-called dipolar evolution frequency given by
ωdd(ik) )
µ0 gigk µB2 4π p r 3
(4)
ik
Spin i is the observer spin (A spin) and spin k is the pumped spin (B spin), gi and gk are the electron g values of the two spins, and θik is the angle between the external magnetic field, B0, and the vector connecting the loci of the two spins. Henceforth, we assume gi ) gk ) g for all i, k. In a multispin system (many B spins), the echo intensity due to any A spin is
V(t′) ) V0
[1 - λk(1 - cos(ωikt′))] ∏ i*k
(5)
and the total echo is the sum of the contributions from all A spins. This yields for N interacting spins
V(t′) ) V0
〈
N-1
[1 - λk(1 - cos(ωikt′))] ∏ i*k
〉
(6)
where 〈〉 denotes the relevant averaging. For an isotropic disordered system with a homogeneous distribution of spins, the dipolar time evolution exhibits a monoexponential decay, which depends on the spin concentration, C, according to31,32
V(t′) ) V0 exp( -t′/Thom) and the time constant, Thom, is
(7)
Thom )
9x3p 2πg2µB2µ0λC
(8)
Here, λ is the fraction of the excited B spins, given by eq 2. For S ) 1/2
Thom ) 1.0027
10-3 λC
(9)
where C is given in molar and Thom in µs. In a system that consists of aggregates, where the average inter-aggregate distance is larger than the average distance within the aggregate, the DEER decay is often presented as the following product:24,25,33,34
V(t′) ) Vintra(t′)Vinter(t′)
(10)
where Vintra(t′) presents the decay due to spins in the same aggregate, and Vinter(t′) is due to the interaction with spins in different aggregates. The latter can be approximated as a firstorder exponential decay, where Thom is given by eq 9, and C is equal to the total spin concentration in the system.24 For spin-probes dissolved in a micellar solution with an average number of spins per micelle given by Mav, Vintra(t′) (eq 6) is ∞
Vintra(t′) )
〈
M-1
(1 - λk(1 - cosωikt′)) ∑ P(M, Mav) ∏ M)1 i*k
〉
(11)
Here, the averaging is over all possible pair distances, rik, within the micelle and all possible orientations of rik with respect to the magnetic field. P(M, MaV) is the Poisson probability distribution for a given number of spins, M. The decay kinetics of a number of spins confined to a small volume with a homogeneous distribution within the volume should deviate significantly from a monoexponential decay.25 Our earlier study27 showed that the decay kinetics of 4HTB in Pluronic micelles was best modeled using the following: ∞
Vintra(t′) )
∑ P(M, MaV) ×
M)1 rmax
[∫ ∫ π
0
rmin
(1 - λ(1 - cos ωikt′))r2 dr sin θ dθ
]
M-1
(12)
In the above, rmax represents the maximum distance between spins and rmin their minimum approach. In case of spherical micelles, rmax is the diameter of the volume occupied by 4HTB. In eq 12, the possible distribution in sizes was negelected. For very large rmax values (>20 nm), this model represents an isotropic homogeneous distribution of spins.35 This value, however, depends on the spin concentration as shown in Figures S1 and S2 in the Supporting Information (SI). In all our calculations, rmin ) 1.5 nm was used because distances below 1.5 nm cannot be probed by DEER.28 Attempts to fit the DEER decays of 4HTB in P123 micelles according to eq 11, using a Monte Carlo approach, where all positions within the spherical volume have the same probability and the positions of the spins are uncorrelated, did not produce a satisfactory fit. Thus, here too, we excluded distances below 1.5 nm. The fact that data were best modeled with eq 12 suggests that the 4HTB molecules avoid close proximity, which is probably determined by the solubility characteristics of 4HTB in the PPO region and may also be a consequence of the freezing.
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J. Phys. Chem. C, Vol. 112, No. 18, 2008 7105
Figure 2. The relative amount of 4HTB in the sample, X(t), during the formation of KIT-6, as obtained from in situ CW-EPR measurements. The solid lines were drawn to guide the eyes and the vertical lines show the different stages. The different symbols represent two experiments.
When the phase memory time is not long enough, the maximum t′ is limited and the DEER kinetics within the time window available may be approximated by a mono-exponential decay. In such a simple model, the DEER trace can be approximated by
(
V(t′) ) Vintra(t′)Vinter(t′) ) exp -
λt′ (C C ) (13) 1.0027 inter + intra
)
where t′ is given in µs, Cinter represents the total spin concentration in mM, and Cintra is the effective concentration of the spins within the confined volume of the micelles in mM. In this work, we analyze the data in terms of these two models, referred to as model A (eq 12) and model B (eq 13). Results CW EPR Measurements. 4HTB is hydrophobic spin-probe, and therefore, it dissolves in hydrophobic regions, such as the core of the P123 micelles, or, because it is small, it can also dissolve in the hydrophobic region of single chains in solution.27 Thus, the DEER data can provide information regarding the core size of the micelles, once the partitioning of 4HTB between the two environments is known. Because the reaction takes place at low pH, part of the 4HTB free radicals transform during the reaction into the corresponding diamagnetic hydroxyl amine. Since the analysis of the DEER data requires the concentration of the spin-probe in the sample, its change during the reaction has to be quantified. This was obtained from the integral of the CW EPR spectrum. Figure 2 shows the relative amount of the radicals present in the reaction mixture, X(t), as a function of time, as determined from in situ CW EPR measurements.23 It shows four clear stages, distinguished according to their slopes: 0-50 min, 50-150 min, 150-200 min, and t > 200 min, where t ) 0 corresponds to the addition of TEOS to the P123 micellar solution + acid + BuOH. Interestingly, these stages are similar to those observed in the ESEEM experiments, which report different rates of water depletion from the corona.23 This water depletion was earlier ascribed to the hydrolysis and condensation of the silica at the corona.23 Hence, when the rate of the hydrolysis/condensation is fast, the transformation of 4HTB is faster. In addition, CW EPR measurements suggested that at the time of precipitation
(t ∼140 min), there is some reorganization in the shape of the micelle, which expels a significant amount of 4HTB into P123 single chains. This explains why at this time there is large transformation of 4HTB. 4HTB molecules that are no longer within an aggregate (micelle) become more exposed to acid attack compared to those remaining in the micelle aggregate. The reorganization of the micelles is also accompanied by large depletion of water from the corona of the micelles attributed to the condensation of the silica, which leads to a more hydrophobic environment in the micelle. With the progression in the silica condensation, the repulsion forces between the micelles decrease, aggregation of micelles occur, and at this point, the spin-probes within the aggregates coated with a silica layer are protected from the acid attack and the transformation is stopped. DEER Measurements. The EPR spectrum of 4HTB during the reaction of KIT-6 is characterized by two species, corresponding to 4HTB in micelles and single chains. The relative amount of these species (P(t)) varies during the reaction.23 Up to the precipitation time (∼140 min), most of the 4HTB are dissolved in micelles (∼77%), and the rest are dissolved in single chains, and contribute to the homogeneous background. After the precipitation time, it is harder to analyze the data because it is less reproducible. In addition, at this stage the solution is highly heterogeneous, containing several microstructures: single chains in solution, individual micelles, and clusters of micelles. Consequently, we limit the analysis of the DEER data up to t ) 120 min. Data Analysis According to Model B, Mono-exponential Decay. Figure 3a shows the DEER decay at some representative reaction times (solid line). It is clear that the decay rate varies with reaction time. The DEER decays can be well-reproduced by a monoexponential decay, as represented by the dotted traces in Figure 3a. The time dependence of the decay constant is shown in Figure 3b for three different experimental sets. Four different stages can be distinguished: 0-10 min, 10-70 min, 70-140 min, t > 140 min. In order to determine the change in the hydrophobic volume of the micelles during the first 2 h of the reaction, an apparent spin concentration in the reaction mixture, Cap(t), was extracted from the decay rate constant, according to
(
V(t′) ) exp -
λ C t′ 1.0027 ap
)
(14)
where λ ) 0.2 has been determined using toluene solutions of 4HTB at various concentration (see eq 7). Cap(t) determined from eq 14 includes contributions of Cintra and Cinter (eq 13). Moreover, the decrease in the concentration of 4HTB during the reaction, given by X(t), and the relative amount of 4HTB that are dissolved in micelles, P(t), should be taken into account. This yields
Cap(t) )
Ntotal(t ) -5)‚X(t)‚P(t) + Ctotal(t ) -5)‚X(t) ) n‚Vm Cintra(t) + Cinter(t) (15)
where Ntotal (t ) -5) and Ctotal (t ) -5) are the total spin number and concentration, respectively, at t ) -5 min, corresponding to a P123 solution without additives. X(t) is obtained from the in situ EPR spectra shown in Figure 2, and P(t) is obtained from ref 23. The number of micelles is n, and Vm is the volume of the hydrophobic core occupied by the spin-
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Figure 4. Cintra(t) vs reaction time, obtained from the analysis of the DEER decays according to eq 16.
Figure 3. (a) DEER decays for samples frozen at different times during the formation of KIT-6. The dotted lines correspond to a first-order exponential decay fit (model B). (b) The decay rate constant as a function of the reaction time, for three different experimental sets. The solid lines were drawn to guide the eyes. The different symbols represent three experiments.
probes. This yields an effective spin concentration in micelle, given by
Cintra )
Cap(t) X(t)P(t)
-
Ctotal (t ) -5) P(t)
)
Ntotal (t ) -5) n‚Vm
(16)
Cintra, as given in eq 16, takes into account the reduction in the total concentration of the spin-probes due to decomposition and the amount of spin-probes dissolved in single chains. Therefore, it reflects the change in the total micellar hydrophobic volume, given by nVm. At t ) -5 min, P(t) ) 0.85, and the addition of butanol and acid does not affect P(t). Ten minutes after the addition of TEOS, it reduces to 0.79, then up to 120 min is approximately constant P(t ) 120 min) ) 0.77. Figure 4 presents the time dependence of Cintra up to t ) 120 min. At t ) -5 min, Cintra ) 4.5 ( 0.2 mM, and after the addition of butanol and acid (t ) 0), it decreases to 3.0 ( 0.2 mM. Ten minutes after the addition of the TEOS, Cintra drops remarkably to 0.25 ( 0.15 mM. Between 10 to 70 min of the reaction, there is an increase in Cintra, and at 70 min, Cintra ) 2.3 ( 0.3 mM, which is close to the t ) 0 value. Then, between 70 and 120 min of the reaction, Cintra is practically invariant.
Figure 5. DEER decay for samples frozen at different times during the reaction of KIT-6. The dotted lines correspond to the simulations using model A. The values of Mav and rmax used in the simulations are given in Figure 6. The different symbols represent three experiments.
Data Analysis According to Model A, Distribution of Spins in a Confined Volume. The experimental results could also be fitted using eq 12, as represented by the dotted traces in Figure 5. The simulation was restricted to the first 70 min, where only individual spherical micelles are present.23 Equation 12 also takes into account that the spin numbers in a micelle are not identical, and there is a Poisson distribution around an average number of spins, Mav. This model, however, neglects the size distribution of micelles. We think that this simplification is justified because only the hydrophobic core volume is evaluated. When only one shape of micelle is present, the density of chains in the hydrophobic core is quite uniform, and a large distribution of core sizes is not expected.27,36-37 Figure 6a,b summarizes the fitting results. Figure 6a shows the time evolution of the normalized Mav relative to t ) 0, and taking into account the reduction in the total concentration of the spin-probes, X(t). It shows that Mav decreases up to t ) 70 min; the decrease rate, however, is larger during the first 10 min. From Mav, the concentration of the micelles, Cm, and the aggregation number, Nagg, can be calculated according to
Cm(t) )
Ctotal(t)‚P(t) Mav(t)
(17)
Evolution of Solution Structures
Nagg )
J. Phys. Chem. C, Vol. 112, No. 18, 2008 7107
Cp Cm(t)
(18)
where Cp is the P123 concentration. Mav (t ) -5 min) is 10.5 ( 1.5, which yields Nagg (t ) -5 min) ) 106 ( 15, Nagg (t ) 10 min) ) 86 ( 15, and Nagg (t ) 70 min) ) 54 ( 15. The value obtained at t ) -5 min is similar to that reported earlier in the literature.38 While we know the relative amount of 4HTB dissolved in single chains, we do not know the relative amount of the single chains in solution; therefore, this is neglected in eq 18. On the basis of earlier results using spin-labeled Pluronics, we estimate that the amount of free chains in solutions is small and does not exceed the relative amount of 4HTB in solution ( 140 min. The change in Cintra reflects the variation in the total micelle hydrophobic volume, nVm. The addition of butanol and acid to the P123 solution at t ) 0 min resulted in a decrease of Cintra by a factor of 1.5. For a given Cintra, and assuming spherical micelles, the average distance between spin-probes within the micelles is given by Cintra-1/3. Accordingly, assuming that the addition of the butanol and acid does not change the number of micelles, the change in Cintra yields an increase of a factor of 1.14 in the average distance between the probes. Model A also revealed a small increase in rmax by a factor of 1.1, and no change in Mav, which corresponds to no change in Nagg and the number of micelles. This small expansion of the hydrophobic core, suggested by both models, is consistent with the penetration of water and butanol into the corona smearing the core/corona interface, as observed previously in ESEEM experiments.23 During the first 10 min of the reaction, Cintra decreases by 10-45-fold compared to t ) -5 min (corresponding to 4HTB in P123 micellar solution). We exclude the possibility that this drastic change is brought about by the destruction of the micelles because the in situ CW EPR results show a spectrum with a hyperfine coupling and a correlation time typical of a micelle.23 A large depletion of spin-probes from the micelle’s core into single chains in solution was only observed at precipitation time (140-160 min).23 Therefore, the reduction must be associated with a change in Vm or Nagg or both. Usually, the radius of P123 micelles in aqueous solutions with up to 8 wt % is smaller than 10 nm, and the addition of hydrophobic additives up to 3 wt % leads only to an increase of 1-3 nm in the core radius.5,7 Butanol, which is water-soluble, will not significantly increase the micelle’s radius.39 Ganguly et al. have recently shown by small angle neutron scattering (SANS) that the addition of ethanol to P123 micelles at 30 °C causes a decrease in the P123
Figure 6. (a) The Mav values obtained from the DEER simulations, with respect to the Mav obtained at t ) 0, taking into account the decomposition of spin-probes, X(t), as a function of reaction time. (b) rmax, obtained from the simulations, as a function of the reaction time. The different symbols represent three experiments.
core volume but expands the P123 corona.40 A large concentration of trimethyl benzene or benzene (∼6 wt %) can increase the radius of the P123 micelle up to approximately 15 nm.4 The amount of TEOS added is 5 wt %. Prior to hydrolysis, the TEOS molecules are hydrophobic, and may dissolve in the hydrophobic core of the micelles and swell it. This was also suggested earlier for the synthesis of SBA-15.41 In addition, partly hydrolyzed TEOS molecules or ethanol, which is a TEOS hydrolysis product, can also contribute to this swelling by smearing the core/corona inteface.42 Model B yields a change in nVm by a factor of 10-45, consistent with Model A, which shows an increase in rmax by a factor of 2 (volume change by a factor of 8). This corresponds to an increase in the hydrophobic core radius from 4.5 to 9.0 nm. In addition, Model A shows a reduction of Nagg from 106 ( 15 to 86 ( 15, the difference, however, is small relative to the experimental error. It is known that an increase in the volume of the core leads to an increase of Nagg.43 Nevertheless, in our case, the hydrophobic core of the micelle increases owing to the presence of unhydrolyzed or partly hydrolyzed TEOS molecules and ethanol, and therefore, it is not obvious that the aggregation number should increase.
7108 J. Phys. Chem. C, Vol. 112, No. 18, 2008 Hence, we attribute the drastic reduction in Cintra in the first 10 min mostly to swelling and a small, less significant change in the number of micelles (e.g., decrease in aggregation number). Between 10 and 70 min, we observe an increase in Cintra to a value a little smaller than at t ) 0 min, indicating a decrease in nVm. We attribute this reduction to the progression of the hydrolysis/condensation of the silica at the micelle corona that is initially accompanied by a depletion of TEOS from the core due to hydrolysis, followed by depletion of water and butanol molecules from the core-corona interface due to condensation.23 At this time, Model A shows a large decrease between 10 and 20 min in rmax, reaching a value similar to t ) 0, namely, a contraction of the hydrophobic volume. At t ) 20-70 min, rmax remains constant within experimental error. In addition, Model A reports a continuous small decrease in Mav, which suggests a decrease in Nagg. This reduction is, however, not consistent with the appearance of the TLMs in a later stage.23 This may be a consequence of an experimental error or a sign of the initial appearance of various micelles with different sizes and shapes that Model A does not account for. At t ) 70-120 min, Cintra is constant. This is expected because at this time TLMs coexist with spheroidal micelles, as observed by cryo-TEM.23 The volume of the micelle, Vm, is expected to increase and the number of the micelles, n, to decrease, resulting in a fairly constant Cintra. Information on the changes in the volume/number of the micelles during the formation of mesoporous materials, as obtained from the DEER experiments, can, in principle, be derived also from SAXS and cryo-TEM. SAXS has the advantage that it can be acquired at ambient temperatures and measurements can be carried out under natural conditions, without interference such as the addition of probes, and more importantly, without freezing. However, the long acquisition time required makes it incompatible with time-resolved measurements. This can be overcome by conducting the experiments with a high-energy X-ray source, like synchrotron radiation, which makes this type of experiment quite expensive and difficult to access. Cryo-TEM is a very powerful tool for investigating the evolution of nanostructures in soft materials because it is based on quenching the reaction at different times and offering a resolution of a few seconds. However, sometimes, especially at the beginning of the reaction, P123 micelles do not provide the contrast required to observe the micelles. The latter is improved once the silica starts to coat the micelles.22 Here, DEER can serve as a complementary technique to the other techniques and can provide nanoscale information regarding the change in the sizes and the aggregation number of the structures formed during the reaction. In addition, DEER offers resolution of minutes owing to the fact that the samples can be quenched at different times during the reaction. The disadvantage of DEER is its complicated model-dependent data analysis. Conclusions We have shown that DEER measurements can provide new details on the formation of templated mesoporous materials. For KIT-6, variations in the hydrophobic core volume were detected. A new stage in the reaction, not observed earlier by EPR, ESEEM, and cryo-TEM, was resolved during the first 10 min. It corresponds to an increase in the average volume of the hydrophobic core and a small reduction in the aggregation number. This is attributed to the penetration of TEOS and hydrolysis products. Then, as hydrolysis and condensation proceed, the number of micelles and their size decrease and become close to their original size.
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