Investigation of the Structure and Dynamics of Surfactant Molecules in

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Langmuir 1996, 12, 2663-2669

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Investigation of the Structure and Dynamics of Surfactant Molecules in Mesophase Silicates Using Solid-State 13C NMR Li-Qiong Wang,* Jun Liu, Gregory J. Exarhos, and Bruce C. Bunker Department of Materials Science, Pacific Northwest National Laboratory, Richland, Washington 99352 Received December 6, 1995. In Final Form: February 22, 1996X The structure and dynamics of surfactant molecule reorganization in mesophase silicates have been investigated using variable-temperature 13C solid-state nuclear magnetic resonance (NMR) spectroscopy. Functional groups and side groups of the surfactant CTAC (cetyltrimethylammonium chloride) were identified from high-resolution 13C MAS (magic angle spinning) NMR spectra obtained using high-power 1H decoupling. We also obtained information on surfactant organization and relaxation in mesophase silicates using a combination of NMR line-shape and relaxation-time analyses with variable-temperature NMR. The behavior of the surfactant in the ordered mesophase silicate was compared with that of the surfactant solution (CTAC-water) and that of the surfactant in the disordered silicate, which was precipitated in solution during an early stage of the reaction. The electrostatic binding between the electropositive end of the surfactant and the silicate substrate causes a ∼1 ppm downfield shift for the NMR resonance associated with the methyl groups next to the head group and substantial broadening for the peak corresponding to the methylene group adjacent to the head group. The splitting of the resonance associated with the N-methyl groups suggests that the methyl groups next to the head group lose their stereochemical symmetry due to the intermolecular interaction in the ordered mesophase silicates. Each segment of the surfactant associated with an ordered silicate is less mobile than the corresponding segment associated with a disordered silicate precursor. For both ordered and disordered silicates, the methylene group adjacent to the head group exhibits a marked lack of motion relative to other segments of the surfactant. Variable-temperature NMR studies show motional narrowing as temperature increases. The NMR results obtained from this study provide insight into the formation mechanism of mesophase materials.

Introduction There has been great interest in the design and study of new open-structured inorganic materials since the recent discovery of the M41S series of mesoporous materials.1,2 Several proposed mechanisms for the formation of mesoporous materials have already appeared in the literature. Beck et al. proposed that mesoporous materials are produced by a liquid crystal templating mechanism in which an inorganic phase nucleates and grows within an ordered organic liquid crystal phase.1,2 Monnier et al. later proposed a cooperative mode for the synthesis of mesostructures.3 They suggested that three processes are involved in mesostructure formation: multidentate binding of silicate oligomers to the cationic surfactant, preferred silicate polymerization in the interfacial region, and charge density matching between the surfactant and the silicate. Recently, Liu et al. reported that colloidal silica and colloidal titania particles promote the formation of mesoporous materials.4 Under similar conditions no ordered structure is observed without the colloidal particles. A heterogeneous nucleation mechanism was proposed. Although the formation of mesoporous materials is generally attributed to the “templating” effect of the surfactant, the silicate species in the solution actually X

play an important role in the organization of the surfactant molecules.3-5 The key issues for understanding this kind of cooperative interactions involve surfactant organization at the silicate solution interface and the nature of the interaction between the surfactant and the silicate surface. Detailed information regarding the structure and dynamics of surfactant molecules during nucleation and growth is required to provide insight into this problem. Nuclear magnetic resonance (NMR) spectroscopy experiments provide one route to understanding the three major phenomena involved in the synthesis of mesoporous ceramics: (1) the assembly of surfactant systems into a liquid crystal array, (2) the formation of ceramic phases via nucleation and growth of soluble inorganic precursors, and (3) the interaction chemistry between the surfactant, inorganic precursors, and both liquid crystal and ceramic phases. NMR techniques have been used extensively to characterize the structure and dynamics of liquid crystal surfactants used in the synthesis of mesoporous ceramics as well as their phase diagrams.6-9 However, few studies have been reported regarding the nature of the interactions between surfactants and silicate precursors which evolve mesoporous silicates. Phase transformations in surfactant-silicate systems have been studied using 2H and 15N NMR,5,10 while 29Si NMR has been used to study the degree of silicate condensation and the structure of the

Abstract published in Advance ACS Abstracts, May 1, 1996.

(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.; Sheppard, E. W.; McCullen, S. B.; Hihhins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834. (2) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (3) Monnier, A.; Schuth, F.; Huo, Q.; Kumar, D.; Margolese, D.; Mawell, R. S.; Stucky, G. D.; Krishnamurty, M.; Petroff, P.; Firouzi, A.; Janicke, M.; Chmelka, B. F. Science 1993, 261, 1299. (4) Liu, J.; Kim, A. Y.; Virden, J. W.; Bunker, B. C. Langmuir 1995, 11, 689.

S0743-7463(95)01515-0 CCC: $12.00

(5) 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.; Stucky, G. D.; Chmelka, B. F. Science 1995, 267, 1138. (6) Tiddy, G. J. T. Phys. Lett. 1980, 57, 1. (7) Blackmore, E. S.; Tiddy, G. J. T. Liq. Cryst. 1990, 8, 131. (8) Belmajdoub, A.; Boubel, J. C.; Canet, D. J. Phys. Chem. 1989, 93, 4844. (9) Henriksson, U.; Blackmore, E. S.; Tiddy, G. J. T.; Soderman, O. J. Phys. Chem. 1992, 96, 3894. (10) Steel, A.; Carr, S. W.; Anderson, M. W. J. Chem. Soc., Chem. Commun. 1994, 1571.

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mesophase silicates.1,5,11,12 The 13C spectra have been reported for surfactants in as-synthesized mesoporous materials.1,11 There has been no detailed study reported using 13C NMR to probe the structures and dynamics of surfactant molecules in mesophase silicates. This paper focuses on applying variable-temperature 13 C NMR to investigate the structure and dynamics of the surfactant molecule CTAC (cetyltrimethylammonium chloride) in mesoporous silicates. We have selected three systems: surfactant solutions (CTAC-water), CTACdisordered silicates formed in the early stage of the reaction, and CTAC-ordered mesophase silicates (assynthesized mesoporous materials) to study the surfactant structures and reorganization at different reaction stages during the formation of mesoporous silicates. Information on surfactant organization and relaxation in both ordered and disordered silicates was obtained using a combination of NMR line-shape, TCH (cross-polarization time constant), and T1FΗ (the proton spin-lattice relaxation time constant in the rotating frame) analyses with variable-temperature NMR. The behavior of the surfactant in the ordered mesophase silicate was compared with that of the surfactant solution and that of the surfactant in the disordered silicate. Measurements obtained at 300 K were compared with those obtained at 170 K for a given system. The results obtained from this study provide insight into the formation mechanism of mesophase materials. Experimental Section The detailed procedure for the preparation of CTAC-mesoporous silicates has been reported previously.1,2,4 The basic ingredients include CTAC, sodium aluminate, silica, and tetramethylammonium silicate (TMA-silicate). A typical composition is 6.416 g of CTAC, 2.786 g of CTAC/OH, 0.190 g of sodium aluminate, 1.127 g of Hi-Sil 233, and 4.593 g of TMA. The molar chemical composition of the sample is 1 Al2O3, 26 SiO2, 9 CTAC, and 3.76 TMA. The ratio of Si:Al is 13:1. The chemicals were reacted at 105 °C in a closed container. Some of the material was removed from the solution after 30 min of reaction. The reaction products were quenched with 25 °C DI water, and then the whole suspension was run through a vacuum filter and washed with additional DI water to yield a whitish powder. At this stage, the majority of the material was still amorphous (disordered) phase, as observed by transmission electron microscopy (TEM). The reaction was completed in 24 h. TEM and X-ray diffraction results showed that the final reaction product had a highly ordered hexagonal structure with a pore size of ∼25 Å supported by both TEM and pore size distribution measurements. Samples for the solid-state NMR analysis were prepared by drying the above disordered and ordered materials with vacuum filtration, while the surfactant solution (29 wt % CTAC in water) at 25 °C was used for the liquid-state NMR analysis. The 75.0 MHz 13C solid-state NMR experiments were carried out with a Chemagnetics spectrometer (300 MHzs89 mm wide bore Oxford magnet) using a variable-temperature doubleresonance probe. For both ordered and disordered materials, single-pulse (SP) Bloch-decay, and cross-polarization (CP) methods were used with and without 1H decoupling. Each experiment was performed at two different temperatures, 170 and 300 K. T1F and TCH were obtained using variable-contact-time 13C CP NMR. The dried samples were loaded into 7 mm Zirconia PENCIL rotors and spun at 3-4 kHz. Spectra were collected by using a singlepulse (SP) excitation Bloch-decay method with a 4.5 µs (90°) 13C pulse and a 5 s repetition delay. For all experiments, 40 ms acquisition times and a 50 kHz spectral window were employed. The number of transients was 500-1000. The power levels of the carbon and proton channels were set so that the HartmannHahn match was achieved at 55 kHz in CP experiments. A Lorentzian line broadening of 24 Hz was used for all spectra. (11) Kolodziejski, W.; Corma, A.; Navarro, M. T.; Pariente, J. Solid State Nucl. Magn. Reson. 1993, 2, 253. (12) Steel, A.; Carr, S. W.; Anderson, M. W. Chem. Mater. 1995, 7, 1829.

Figure 1. (a) 13C SP NMR spectrum with 1H decoupling at 300 K for the CTAC-ordered mesophase silicate. (b) 13C SP NMR spectrum with 1H decoupling at 300 K for the CTAC-disordered silicate. (c) 13C SP NMR spectrum with 1H decoupling at 300 K for the surfactant solution (29 wt % CTAC in water). The liquid-state NMR spectra for the surfactant solution were collected on a Varian VXR-300 spectrometer using a Doty Scientific, Inc. probe. The 13C chemical shifts were referenced to tetramethylsilane at 0 ppm.

Results Single-pulse (SP) 13C NMR spectra along with the peak assignments are given in Figure 1 for the CTAC-ordered mesophase silicates, the CTAC-disordered silicates, and the surfactant solution, respectively. Several features of the 13C MAS spectra of CTAC on silicates are noteworthy in Figure 1a and b. The peak at 14.9 ppm is assigned to the terminal methyl group (C17) while the R and β methylenes (C16 and C15) are found at 23.7 and 33.2 ppm, respectively. The relatively broad and partially resolved peak at 27.4 ppm is attributed to the methylene group (C3). The weak and broad peak at 67.8 ppm is assigned to the N-methylene group (C2) while the 54.6 ppm peak is associated with the N-methyl group (C1). The remainder of the internal methylenes (C4-14) are lumped together in the broad peak centered at 31.1 ppm. Spectra for ordered and disordered silicates (see Figure 1) appear to be similar except an additional peak (C1′) appeared at ∼57.4 ppm for the ordered mesophase silicate. This additional peak is also absent in Figure 1c for the surfactant solution. However there are marked differences between the spectra of surfactant-silicates and those of the surfactant solution (CTAC-water). All peaks in fIgure 1c for the surfactant solution are much narrower than those in Figure 1a and b for the surfactant-silicates. Peaks C3 and C15 are completely resolved from the main peak C4-C14 in surfactant solutions. The electrostatic

Surfactant Molecules in Mesophase Silicates

Figure 2. (a) 13C SP NMR spectrum with 1H decoupling at 300 K for the CTAC-ordered mesophase silicate. (b) 13C CP NMR spectrum (5 ms contact time) with 1H decoupling at 300 K for the CTAC-ordered mesophase silicate. (c) 13C SP NMR spectrum with 1H decoupling at 170 K for the CTAC-ordered mesophase silicate. (d) 13C CP NMR spectrum (3 ms contact time) with 1H decoupling at 170 K for the CTAC-ordered mesophase silicate.

binding between the surfactant and the silicate causes a ∼1 ppm downfield shift for the methyl groups next to the head group (C1) and substantial broadening for all peaks. The peak corresponding to the methylene group adjacent to the head group exhibits the largest broadening among all carbon groups. Single-pulse and cross-polarization 13C MAS spectra taken at two temperatures (170 and 300 K) with 1H decoupling are shown in Figures 2 and 3 for ordered mesophase silicates and disordered silicates, respectively. Since cross-polarization is a measure of the efficiency of magnetization transfer by the dipolar coupling from 1H to 13C, the relative peak intensities for individual carbon groups in the CP spectra reflect both the magnitude of the dipolar coupling and motions of the C-H vector.13,14 The SP and CP 13C spectra in Figure 2a and b show that the peak intensity for the terminal methyl group (C17) is significantly decreased in the CP spectrum as compared with the SP spectrum at 300 K, indicating that the terminal methyl groups are extremely mobile. As the temperature decreases, the corresponding carbon peaks are broadened compared to those at 300 K. The relative intensity of the terminal methyl group and the C4-C14 peak remained similar in both the CP and SP spectra at 170 K, indicating that the mobility of the methyl group is decreased so significantly at 170 K that no obvious (13) Pines, A.; Gibby, M. G.; Waugh, J. S. J. Chem. Phys. 1973, 59, 569. (14) Voelkel, R. Angew Chem., Int. Ed. Engl. 1988, 27, 1468.

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Figure 3. (a) 13C SP NMR spectrum with 1H decoupling at 300 K for the CTAC-disordered silicate. (b) 13C CP NMR spectrum (5 ms contact time) with 1H decoupling at 300 K for the CTACdisordered silicate. (c) 13C SP NMR spectrum with 1H decoupling at 170 K for the CTAC-disordered silicate. (d) 13C CP NMR spectrum (2 ms contact time) with 1H decoupling at 170 K for the CTAC-disordered silicate.

difference was found under the CP and SP conditions. The additional peak (C1′) shown in Figure 2c has almost the same intensity as the C1 peak of the N-methyl group at 170 K. The chemical shift difference of ∼2.7 ppm between the C1 and C1′ peaks at 300 K is the same as that at 170 K. This additional peak is more prominent as the temperature decreases. However, the N-methyl group in the CTAC-disordered silicates (Figure 3) gives a symmetric peak at 300 K. At low temperature, this peak becomes very broad but does not split, in contrast to the spectrum shown in Figure 2c for the ordered mesophase silicates. For both CTAC-silicate systems, the main peak of C4-C14 at 170 K has ∼2-3 ppm downfield shifts relative to the corresponding peak at 300 K. Figure 4 displays the single-pulse 13C MAS spectra with 1H coupling for both ordered and disordered silicates at two temperatures, 170 and 300 K. Further information about the 1H-13C dipolar interactions can be obtained from the NMR spectra taken with 1H coupling. With 1H coupling, the terminal methyl group (C17) in the disordered silicate exhibits an obvious quartet pattern (see Figure 4b) caused by the JCH coupling while a less wellresolved triplet is observed for the methylene group next to the terminal methyl group (C16) for the same reason. Furthermore, a poorly resolved quartet is discernible in Figure 4b for the N-methyl group (C1) but is absent in Figure 4a. Similar splitting is seen for the C17 and C16 peaks in the ordered mesophase silicates, as shown in Figure 4a, but the splitting is not as well resolved as in Figure 4b. The JCH coupling associated with the terminal methyl C17 carbon is ∼120 Hz, obtained directly from the quartet splitting, in agreement with the reported value

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Figure 6. Contact-time array experiment for the CTACordered mesophase silicate at 170 K.

Figure 4. (a) 13C SP NMR spectrum with no 1H decoupling at 300 K for the CTAC-ordered mesophase silicate. (b) 13C SP NMR spectrum with no 1H decoupling at 300 K for the CTACdisordered silicate. (c) 13C SP with no 1H decoupling at 170 K for the CTAC-ordered mesophase silicate. (d) 13C SP NMR spectrum with no 1H decoupling at 170 K for the CTACdisordered silicate.

Figure 7. Contact-time array experiment for the CTACdisordered silicate at 300 K

Figure 5. Contact-time array experiment for the CTACordered mesophase silicate at 300 K.

for ethane.15 Figure 4a and b shows that the C1 and C4C14 peaks for the ordered silicate with 1H coupling are broader than those for the disordered silicate. The strong 13 C-1H dipolar interaction may broaden the C2 peak too much to be observed in Figure 4a for an ordered silicate, while the C2 peak is almost absent in Figure 5b for a disordered phase. The 170 K spectra shown in Figure 4c and d for both ordered and disordered silicates are very (15) Harris, R. K.; Mann, B. E. NMR and the Periodic Table; Academic Press: New York, 1978.

different from the 300 K spectra shown in Figure 4a and b. Only a very broad peak associated with the C1 carbon is observed, suggesting that cooling is sufficient to freeze the motions for almost all the carbons. Contact-time array spectra are given in Figures 5 and 6 for the ordered mesophase silicates at 300 and 170 K, respectively, while Figures 7 and 8 show the contact-time array spectra for disordered silicates at 300 and 170 K, respectively. The cross-polarization rate can be roughly viewed as how fast the resonance intensity of a given carbon group reaches the maximum from the contacttime array spectra. Figures 5-8 show that the crosspolarization rate varies among all individual carbon groups at a given temperature and differs between two different temperatures for a given carbon group. For CTAC-ordered mesophase silicates at 300 K, the rate of cross-polarization (Figure 5) for the N-methylene group (C2) is much faster than that for the terminal methyl group (C17). As shown in Figure 5, the rate for the N-methylene group (C2) is the fastest among all carbon groups while the rate for the terminal methyl group (C17) is the slowest among all carbon groups. This is also true for the disordered silicates, as shown in Figure 7.

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Figure 8. Contact-time array experiment for the CTACdisordered silicates at 170 K. Table 1. TCH Values (ms) for the CTAC-Ordered and CTAC-Disordered Silicates at the Temperatures 170 and 300 Ka samples ordered (300 K) disordered (300 K) ordered (170 K) disordered (170 K)

C1

C1′

C2

C3

C4-C14 C15 C16

C17

0.44 0.76b 0.06 0.11

0.11

0.14 0.09

0.89

0.15 0.16

0.38

1.00 0.21 13.0

0.16 0.16b 0.06 0.07

0.09

nac

0.06

0.15

0.03

nac

0.02

0.10

0.18

0.02 0.04

3.50

a Measured by 13C NMR via variable-contact-time crosspolarization experiments. Estimated standard deviation 10%.b The C1′ peak is an additional peak for the ordered silicate. c The C3 peak is not resolved from the main peak C4-C14 at 170 K.

However, the cross-polarization rates for all given carbon groups at 300 K are slower for CTAC-disordered silicates than those of the corresponding carbon groups for CTACordered mesophase silicates (Figures 5 and 7). For both CTAC-ordered and CTAC-disordered silicates, all carbon groups are cross-polarized quickly at 170 K (Figures 6 and 8). Figures 6 and 8 exhibit similar cross-polarization rates among all carbon groups for CTAC at 170 K in both ordered and disordered silicates and no obvious difference in cross-polarization rates between these two systems at 170 K for a given carbon group. The cross-polarization time constant (TCH) and the proton spin-lattice relaxation time constant in the rotating frame (T1FΗ) were derived from the variablecontact-time measurements, giving rise to more quantitative descriptions of cross polarization and relaxation behaviors for the CTAC-silicates. Table 1 further illustrates that for both the CTAC-silicate systems crosspolarization behaviors varied among all carbon groups and that the variation for a given system is more at 300 K than at 170 K. For example, TCH values shown in Table 1 for the N-methylene group (C2) and for the terminal

methyl group (C17) at 300 K are 0.06 and 3.5 ms for the CTAC-ordered silicates and 0.15 and 13 ms for the CTAC-disordered silicates, respectively. Since a shorter cross-polarization time corresponds to a faster crosspolarization rate, the results from Table 1 show that the cross-polarization rate for the N-methylene head group is twice as large for the ordered silicate as for the disordered silicate at 300 K and that the rate for the terminal methyl group for the ordered silicate is about four times that for the disordered silicate at 300 K. Table 2 shows that values of T1FΗ at 170 K are much smaller than those at 300 K for CTAC in both ordered and disordered silicates. Since the molecular motion at 170 K is significantly reduced, the smaller relative values indicate less molecular motion in the system. The terminal methyl groups C17 exhibit much larger T1FΗ values than other carbon groups in both CTAC-silicate systems at 300 K, indicating the largest mobility for the terminal methyl group. There are less obvious differences in T1FΗ values between ordered and disordered phases for a given temperature than in TCH values. At 170K both ordered and disordered phases have similarly small T1FΗ values among all carbons, indicating that molecular motion is sufficiently frozen at 170 K. Discussion The surfactants are organized as spherical micelles in the surfactant solution (29 wt % CTAC in water)6 used in this study. The extremely narrow peaks shown in Figure 1c for all carbon groups in the surfactant solution indicate that surfactants as spherical micelles are highly mobile in the solution, similar to free-moving molecules in liquid. Upon adsorption onto the silicates, the surfactant molecules become less mobile, resulting in the broader peaks. The broader and weaker peak C2 in the surfactantsilicates relative to those of the other carbon groups indicates that reorientational motion of the surfactant head group adsorbed onto a solid surface was strongly hindered. The mobility and relaxation behavior of the surfactants in silicates varied among all carbon groups for a given system and depended on the kind of CTACsilicate systems formed at a certain reaction stage in the synthesis of mesoporous silicates. A consistent and detailed picture for the mobility of all segments of the surfactant was obtained in this study using a combination of line-shape analysis and relaxation measurements described in the previous section. Each segment of surfactant associated with an ordered mesoporous silicate is less mobile than the corresponding segment associated with a disordered silicate precursor. For both ordered and disordered silicates, the methylene group adjacent to the head group is less mobile relative to the other segments of the surfactant, while the terminal methyl group is highly mobile. Variable-temperature NMR studies show motional narrowing as temperature increases. Variable-contact-time measurements (Figures 5-8, Table 1 and 2) provided more detailed information on the mobility for each segment of the surfactant, in agreement with the line-shape analysis obtained from 13C MAS spectra with and without 1H decoupling. Since crosspolarization is most efficient for the static 13C-1H dipolar

Table 2. T1GH Values (s) for the CTAC-Ordered and CTAC-Disordered Silicates at the Temperature 170 and 300 Ka samples

C1

C1′

C2

C3

C4-C14

C15

C16

C17

ordered (300 K) disordered (300 K) ordered (170 K) disordered (170 K)

0.034 0.020 0.0035 0.0017

0.022b

0.0073 0.0049 0.0018 0.0015

0.0038 0.0063 0.0025 0.0016

0.020 0.020 0.0026 0.0023

0.023 0.011 nac nac

0.029 0.146 0.0023 0.0016

>0.2 >0.2 0.0041 0.0023

0.0019b

a Measured by 13C NMR via variable-contact-time cross-polarization experiments. Estimated standard deviation 10%. b The C1′ peak is an additional peak for the ordered silicate. c The C3 peak is not resolved from the main peak C4-C14 at 170 K.

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interactions, the less mobile carbon groups exhibit the faster cross-polarization rate or the shorter TCH. Table 1 shows that a disordered silicate at 300 K has a longer TCH than an ordered mesophase silicate, indicating that each segment of the surfactant on the ordered mesophase was less mobile than on the disordered silicate phase. The N-methylene group (C2) is the most rigid of the carbon groups, indicating that the head group of CTAC is anchored onto the silicate surface. The N-methyl group exhibits a larger TCH than the N-methylene group because of the CH3 rotation. The terminal methyl group C17 for both ordered and disordered phases has a much larger TCH than other segments of the carbon chain, indicating that the largest motion occurs in the tail. Furthermore, from the relative intensities in the 1H decoupled CP MAS spectra for both ordered and disordered silicates (Figures 2 and 3), we concluded that the terminal methyl group C17 is extremely mobile with respect to other segments of the surfactant and is more mobile for the surfactant in disordered silicates than in ordered silicates. This agrees with the results from the contact-time measurements. Similar cross-polarization behavior is also evidenced in 1H coupled MAS spectra where the peak width is directly related to the motion of the 13C-1H vector. A better resolved JCH splitting for both C16 and C17 carbon groups in disordered silicates than in ordered silicates suggested that the carbon groups at the tail in disordered silicates have larger mobility (Figure 4). The broader line width for the C4-C14 carbon groups in ordered silicates further indicates that the mobility of the CTAC chain in the ordered silicates is less than that in the disordered silicates. The line broadening may also be attributed to the difference in inhomogeneous broadening of the peaks possibly resulting from the distribution of sites with comparable mobility. However, the consistent results for the mobility of CTAC in silicates obtained from both lineshape analysis and relaxation measurements suggest that the line broadening in a 1H-coupled spectrum is most likely from the mobility of the surfactant rather than from the distribution of sites. The assignment of the additional peak observed at both 300 and 170 K (Figure 2) to the splitting of the resonance associated with the methyl groups next to the head group is partly based on the proximity of this peak to the known head group peak C1, which has been assigned according to the solution-state NMR spectra. In addition, comparison of integrated peak areas in SP spectra supported the above assignment. The combined areas associated with peaks C1 and C1′ (attributed to the N(CH3)3 head group) together are three times the peak area associated with C17 for the ordered mesophase silicate at 300 K, while for the disordered silicate at 300 K the peak area for C1 alone is also three times the peak area for C17. At 170 K, the area ratio between the combined peak (C1 and C1′) and the C17 peak is the same for the ordered silicate as the area ratio between the C1 and C17 peaks for the disordered silicate. Furthermore, the intensity of the additional peak C1′ relative to the C1 peak intensity at 300 K (area ratio C1′:C1 of ∼1:6) is increased significantly at 170 K (area ratio C1′:C1 of ∼1:1.3), while the chemical shift difference of ∼2.7 ppm for the C1 and C1′ peak splitting remained constant with temperature. All of the above evidence suggests that the additional peak observed at both temperatures for the ordered silicates is most likely due to the splitting of the resonance associated with the methyl groups. Moreover, the absence of splitting for the disordered phase and pure surfactant solution suggests that the additional peak is not from contaminations. A previous study reported that TMA ions were incorporated in the as-synthesized zeolites and that TMA ions in zeolites

Wang et al.

Figure 9. (a) All-trans molecular conformation of CTAC illustrated here only for the purpose of the explanation for the split resonance associated with the N-methyl groups. (b) Proposed structure for the surfactant CTAC on the surface of ordered mesophase silicates.

and in the aqueous solution have a chemical shift of ca. 56 ppm,16 which is in between the C1′ and C1 peaks. However, the peak area analysis and temperaturedependent studies described earlier exclude the possibility of the TMA contribution to the additional peak. The relaxation data in conjunction with the observed splitting of the resonance associated with the methyl group next to the head group into C1′ and C1 features provide some insight into the surfactant organization at the silicate-surfactant interface. Figure 9 provides a simple illustration for the proposed structure of surfactant adsorbed onto the ordered silicate surface along with the all-trans molecular conformation for CTAC (the all-trans molecular conformation drawn here is only for the purpose of the explanation for the peak splitting). Figure 9b exhibits various molecular conformations for surfactant molecules adsorbed on the ordered silicate surface. The head groups are relatively aligned (less mobile) as compared with the tail groups in the ordered silicates. On the basis of the earlier discussion, the splitting of the resonance associated with the N-methyl groups is most likely resulted from the nonequivalent methyl groups due to the restrained conformation. As shown in Figure 9a, the three methyl groups are symmetrical with respect to C2 but asymmetrical with respect to the C3 group because of the tetrahedral bonding constraint imposed by SP3 hybridization along the chain. In surfactant solutions, the three methyl groups bound to nitrogen are equivalent because the tail groups exhibit unrestricted rotational motion. Even after the surfactant is electrostatically bound to the silicate precursor, the tail group can still rotate and the time-averaged effect on the three methyl groups makes them equivalent. Therefore in the surfactant solution and in the disordered phase only C1 is observed. However in the ordered phase, the surfactant molecules are close to one another. The tail groups (C3) close to the head group cannot rotate easily due to the intermolecular interaction of the surfactant, causing the three methyl groups to lose their symmetry. There may (16) Engelhardt, E.; Michel, D. High Resolution Solid-State NMR of Silicates and Zeolites; Wiley: New York, 1987.

Surfactant Molecules in Mesophase Silicates

be other reasons for the C1 peak splitting. For example, the strong binding between the surfactant and the ordered silicates may cause the peak to shift downfield. However, this cannot explain why the intensity of the shifted peak is increased upon lowering the temperature. Chemical inhomogeneity may also cause the peak splitting, but this cannot explain the temperature dependence either. If an additional peak results from the Al related adsorption site rather than an Si related site, the peak area ratio between C1′ and C1 would be very small and would remain small at decreasing temperature, since the Al:Si ratio is 1:13 (assuming equal probability for occupying Si and Al sites). However, the low-temperature results showed almost equal integrated peak areas for the C1 and C1′ peaks (1.3:1), further indicating that the splitting is unlikely due to the inhomogeneity resulting from a distribution of different sites. Detailed information about the molecular conformation requires further study involving measurement of 2H NMR line shapes, as reported for the C18-silica study.17 According to the above discussion, a possible reaction mechanism can be proposed: First, the surfactant molecules electrostatically bind to the silicate precursors, causing a ∼1 ppm downfield shift for the NMR resonance associated with the methyl groups adjacent to the head group. At this stage the silicate is already partially condensed (as evidenced by the Q3 and Q4 distribution in 29Si NMR). The tail groups next to the head group become significantly less mobile, but rotational motion of the surfactant molecule chains is still allowed. Further silicate condensation causes the surfactant molecules to interact with one another. Eventually the head groups are closely spaced and may be partially ordered. The rotation of the tail groups next to the head group becomes restricted. The asymmetrically arranged tail groups next to the head group cause the observed splitting of C1 into C1 and C1′. The rotational motion will be even more hindered at lower temperatures, and C1′ will increase with decreasing temperature. In all the systems studied, the tail groups away from the head group remain highly mobile. Therefore, the NMR results discussed in this paper support the cooperative mechanism proposed by Monnier et al.3,5 Conclusions Detailed information regarding the structure and dynamics of surfactant molecules in the synthesis of (17) Zeigler, R. C.; Maciel, G. E. J. Am. Chem.. Soc. 1991, 113, 6349.

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mesophase silicates has been obtained using a combination of 13C NMR methods. Functional groups and side groups in the surfactant (CTAC) were identified from highresolution 13C MAS NMR spectra obtained using highpower 1H decoupling, while information on surfactant organization and relaxation in both ordered and disordered silicates was obtained using a combination of NMR lineshape and relaxation-time analyses with variable-temperature NMR. The behavior of the surfactant in the ordered mesophase silicate was compared with that of the surfactant solution and that of the surfactant in the disordered silicate. The electrostatic binding between the electropositive group of the surfactant and the silicate substrate causes a ∼1 ppm downfield shift for the NMR resonance associated with methyl groups next to the head group and substantial broadening for the peak corresponding to the methylene group adjacent to the head group. Different NMR measurements provide a consistent picture about the mobility of the surfactant on silicates. Each segment of surfactant associated with an ordered mesoporous silicate is less mobile than the corresponding segment associated with a disordered silicate precursor. For both ordered and disordered silicates, the methylene group adjacent to the head group exhibits a marked lack of motion relative to other segments of the surfactant. The splitting of the resonance associated with the Nmethyl groups suggests that methyl groups next to the head group lose their stereochemical symmetry due to the intermolecular interaction in the ordered phase. Variable-temperature NMR studies show motional narrowing as temperature increases. The NMR results obtained from this study provide insight into the formation mechanism of mesophase materials. Acknowledgment. The authors would like to thank Dr. R. A. Santos for the help of setting up the lowtemperature NMR measurement. L.-Q.W. would like to thank Mr. A. Kim for some sample preparation, Dr. J. A. Franz for taking the liquid NMR spectrum and helpful discussions, Dr. G. E. Fryxell, and Dr. J. S. Frye for helpful discussions. This work has been supported by the Division of Materials Sciences, Office of Basic Energy Sciences, U. S. Department of Energy (USDOE). Pacific Northwest National Laboratory is a multiprogram national laboratory operated for the USDOE by Battelle Memorial Institute under Contract DE-AC06-76RLO 1830. LA951515K