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ARTICLES Conformation Heterogeneity and Mobility of Surfactant Molecules in Intercalated Clay Minerals Studied by Solid-State NMR Li-Qiong Wang,*,† Jun Liu,† Gregory J. Exarhos,† Kyle Y. Flanigan,‡ and R. Bordia‡ Pacific Northwest National Laboratory, Battelle BouleVard, Richland, Washington 99352, and UniVersity of Washington, Seattle, Washington 98125 ReceiVed: August 26, 1999; In Final Form: January 19, 2000
The conformation heterogeneity and mobility of surfactant molecules in intercalated montmorillonite clay minerals have been investigated using variable-temperature solid-state 13C NMR and two-dimensional proton wide-line separation (2D WISE) 1H-13C NMR spectroscopy. Previous FTIR studies by Vaia et al. for the first time revealed the existence of a gauche conformation of surfactant molecules in clay minerals and further illustrated the transition from an ordered conformation, to a liquid crystalline state, to completely liquidlike behavior. The NMR study reported in this paper clearly demonstrates the coexistence of order and disordered chain conformations. Two main resonance peaks are resolved and associated with the backbone alkyl chains: the resonance at 33 ppm corresponds to the ordered conformation (all-trans), and the resonance at 30 ppm corresponds to the disordered conformation (mixture of trans and gauche). The NMR technique allows detailed characterization and quantification of the conformational heterogeneity, which is difficult to determine by other techniques. Furthermore, variable-temperature NMR also directly provides quantitative information on the rigidity of the different conformations. Results from cross-polarization time constant TCH measurements along with 2D WISE NMR suggest that molecules in the ordered all-trans conformation are as rigid as those in solid crystalline materials and that molecules in the disordered conformation are similar to those in liquid crystalline materials. Upon heating, the molecules in the disordered conformation remain more or less unchanged, while the molecules in the ordered conformation become disordered. However, the intercalated molecules in the clay minerals never attained the complete liquidlike behavior even after all the bound surfactant assumed a disordered conformation.
Introduction Alkylammonium surfactants have been widely used in the textile industry, for environmental remediation, agriculture, metallurgy, and cosmetics, and in the medical and health industries.1 More recently, these surfactants have been used to prepare novel mesoporous ceramic materials2 and polymerceramic nanocomposites.3-5 In the polymer-ceramic nanocomposites, the constituent clay minerals consisting of twodimensional, 1 nm layers of aluminate sheets sandwiched between two silicate sheets6 are molecularly dispersed in a polymer matrix. The high degree of dispersion of the ceramic nanoplates in these composite materials drastically alters the physical and chemical properties of the polymer materials and greatly improves mechanical integrity and thermal stability. A wide range of polymer-clay nanocomposites,1,4,7 based upon nylon, rubber, epoxy, polyaniline, poly(ethylene oxide), and polyimide polymers, have been reported. One of the critical steps in the preparation of polymer-ceramic nanocomposites is the surface treatment of the clay minerals, typically with alkyammonium surfactants. During this treatment cationic surfactants are ion exchanged with sodium ions resident in the clays to † ‡
Pacific Northwest National Laboratory. University of Washington.
form intercalated clay-surfactant hybrids. The surface treatment is to ensure the compatibility of the clay surface with the polymer matrix and to promote the dispersion of the minerals. The most widely used technique to characterize the claypolymer nanocomposites is X-ray diffraction (XRD), which gives the basal D-spacing of the clay minerals in the hybrid and composite materials. On the basis of XRD results, various models have been proposed for the molecular conformation of the surfactant, including lateral monolayer, lateral bilayer, paraffin monolayer, and paraffin bilayer.8 However, XRD does not provide direct information on how the surfactant molecules orient in the clay and the local conformation of the surfactant molecules. This information is critical for understanding the surfactant role in the formation of the nanocomposites and the effectiveness of the surface treatment. The best information to date on the molecular conformation was obtained by Vaia et al.9 using Fourier transform infrared spectroscopy (FTIR), and the main results are summarized as follows: (1) Under most conditions, a disordered conformation (gauche) is more predominant than the ordered (all-trans) conformation. (2) Increasing the packing density, or the chain length, improves the ordering of the chains. (3) High temperature favors the disordered conformation. The FTIR results are based on small vibrational frequency variations assigned to stretching and
10.1021/jp993058c CCC: $19.00 © 2000 American Chemical Society Published on Web 03/10/2000
Surfactant Molecules in Intercalated Clay Minerals bending modes of the methylene (CH2) groups on the surfactants. The decrease in stretching mode frequency is characteristic of an all-trans conformation while a shift to high frequencies is characteristic of the gauche conformation. Nuclear magnetic resonance (NMR) spectroscopy has been proven to be one of the most powerful techniques for probing structure, conformation, and dynamics of surfactant molecules at interfaces.10,11 Our previous study12 has shown that the structures and dynamics of surfactant molecules in ordered hexagonal mesophase silicates can be probed using a combination of 13C NMR chemical shift, line-shape measurements, and relaxation time studies including TCH (cross-polarization time constant) analyses along with variable-temperature NMR. The 13C resonance for long carbon-chain surfactant molecules is sensitive to the difference in conformation and packing in addition to the chemical structure. The chemical shift difference has been used to characterize the chain conformations of ionic surfactants13 and self-assembled monolayers on silica.10 Chain dynamics of bilayer n-decylammonium chloride have also been studied using 2H NMR spectroscopy14 by selectively deuterating one segment of the chain. 7Li and 2H NMR methods have been used to examine the dynamics for both the cations and the polymer intercalated in a layered silicate.15,16 Recently twodimensional wide-line separation (2D WISE) NMR spectroscopy developed by Spiess and co-workers17,18 has been used to study the mobility and conformation of the flexible side chains in stiff macromolecules. Because of the limited chemical shift range for 1H, one-dimensional 1H NMR measurements alone are insufficient to distinguish the proton signals in different chemical environments in the solid state. In 2D WISE NMR, the proton line widths associated with the individual carbon sites can be resolved by acquiring the 1H spectrum in one dimension and the high-resolution 13C CP MAS spectrum in the other dimension. For rigid crystalline solids, the strong dipolar interactions due to the abundant proton spins give rise to broad proton line widths. However, the line widths of the proton are significantly reduced in semicrystalline solids or liquid environments due to the weak dipolar interactions caused by the increased molecular motion, indicating that the proton line widths obtained from 2D wide NMR are a sensitive probe of the local chain mobility. As compared with both the cross-polarization time constant TCH and 2D WISE NMR methods, 2H NMR spectroscopy is more chemically demanding because of the requirement of the labeling. In this paper, the conformation heterogeneity and mobility of surfactant molecules in intercalated montmorillonite clay minerals have been investigated using variable-temperature solid-state 13C NMR (including the 13C NMR chemical shift measurements and cross-polarization time constant TCH analyses) and two-dimensional proton wide-line separation (2D WISE) 1H-13C NMR spectroscopy. The NMR results obtained in this paper are not only complementary to the FTIR results but also give new insights to the conformational heterogeneity and the rigidity of the surfactants in the clay interlayers. Unlike FTIR, where different molecular conformations can contribute to the same peak, two main resonance peaks associated with backbone methylene groups are resolved in NMR: the resonance at 33 ppm corresponding to the ordered (all-trans) conformation and the resonance at 30 ppm corresponding to the disordered (mixture of trans and gauche) conformation. This clearly demonstrates the coexistence of ordered and disordered chain conformations. Therefore, NMR can be used to directly characterize the conformational heterogeneity, which is difficult with other techniques. Furthermore, variable-temperature NMR
J. Phys. Chem. B, Vol. 104, No. 13, 2000 2811 also provides direct and quantitative information on the rigidity of the different conformations. Measured cross-polarization time constants derived from the contact-time array experiments suggest that the molecules in the ordered conformation are as rigid as those in solid crystalline materials and that the molecules in the disordered conformation are similar to those in liquid crystalline materials. Upon heating, molecules in the disordered conformation remain more or less unchanged, while molecules in the ordered conformation become disordered. However, the molecules in the clay minerals never attain complete liquidlike behavior even after all surfactant assumes the disordered conformation. Experimental Section The montmorillonite clay was purchased from the University of Missouri Clay Source. Since natural montmorillonite may contain a variety of contaminant ions as well as a high particle size distribution, sedimentation and ion exchange procedures were employed to standardize the stock material. The clay (5 g) was added to 1 M NaCl and shaken for 4 h. The swollen clay was then centrifuged and dispersed in distilled water. This was repeated three times. The washed and Na+ ion exchanged montmorillonite was placed in a 500 mL graduated cylinder and allowed to stand overnight (12 h). The upper portion of the suspension was retained. This material was centrifuged and dried between 105 and 120 °C. It was crushed using mortar and pestle and then labeled the “stock clay”. The preceding preparation yielded clay particles of homogeneous size and uniform galleries loaded with sodium and water, thus making the stock clay receptive for intercalation and exfoliation using cationic exchange. ODA (1-octadecylamine, from Lancaster) treated clay was prepared by combining the surfactant and the stock clay in a 1/1 (molar ratio) mixture of EtOH/H2O stirred in a water bath from 70 to 75 °C overnight. A 0.05 M [H+] was maintained to generate protonated amines. The material was washed in water and then in EtOH. After drying overnight in a vacuum, the material was ready for NMR measurements. The 75.0 MHz 13C solid-state NMR experiments were carried out with a Chemagnetics spectrometer (300 MHz, 89 mm wide bore Oxford magnet) using a variable temperature doubleresonance probe. Both single-pulse (SP) Bloch-decay and crosspolarization (CP) methods were used with 1H decoupling. Crosspolarization time constant TCH values were obtained using variable-contact-time 13C CP NMR.12 Samples were loaded into 7 mm Zirconia PENCIL rotors and spun at 3-4 kHz. Spectra were collected by using a single-pulse (SP) excitation Blochdecay method with a 4.5 µs (90°) 13C pulse and a repetition delay of 10-60 s. For all experiments, a 40 ms acquisition time and a 50 kHz spectral window were employed. The number of transients was 1000-3000. The power levels of the carbon and proton channels were set so that the Hartmann-Hahn match was achieved at 55 kHz in CP experiments with a 3 ms contact time (used in most spectra) and a 5 s repetition delay. A Lorentzian line broadening of 24 Hz was used for all 13C spectra. The 13C chemical shifts were referenced to tetramethylsilane at 0 ppm. Wide-line separation 2D NMR spectroscopy (2D WISE) was also used in this study. In a 2D WISE pulse sequence,17 a 1H 90° pulse was applied and followed by an incremented proton evolution period t1. After each t1 period, cross-polarization (CP) was followed by a TOSS (total suppression of spinning sidebands) sequence.19 The subsequent carbon detection with proton decoupling gives a modulated 13C spectrum as a function of t1 due to the free induction decay of the associated protons.
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Wang et al.
Figure 1. Single-pulse (SP) Bloch-decay solid-state 13C NMR spectra with proton decoupling for ODA (1-octadecylamine) in (a) the fully ion exchanged clay with a basal D-spacing of 32 Å and (b) the partially ion exchanged clay with a basal D-spacing of 17 Å.
The two-dimensional Fourier transform gives a 2D spectrum with high-resolution 13C CP MAS spectra in one dimension and the proton wide-line spectra associated with each carbon in the other dimension. In our measurements, a 90° pulse width for both 1H and 13C pulses was 4.5 µs. The 13C CP MAS spectra were taken with the contact time of 0.15 ms and a pulse delay of 3 s. The 2D data had a size of 128 points in the t1 (1H) dimension and 1K data points in the t2 (13C) dimension. The spectrum width in t1 was 200 kHz (dwell time of 5 µs) and 20 kHz in t2 (dwell time 50 µs). Magnitude spectra from the Fourier transform were used for the proton dimension (1H). Samples used in this NMR study were also analyzed by X-ray diffraction (XRD) using a Philips X-ray diffractometer with CuKR radiation. Results and Discussion Two kinds of samples were analyzed: fully ion exchanged and partially ion exchanged. XRD data indicate that the fully ion exchanged sample has a basal D-spacing of 32 Å, and the partially ion exchanged sample has a basal D-spacing of 17 Å. These correspond to interlayer heights (gallery heights) of 22 and 7 Å, respectively, assuming the thickness of the clay layer to be 10 Å.4 Therefore, it is reasonable to assume in the partially ion exchanged sample the surfactants adopt a lateral orientation and in the fully exchanged sample the surfactants adopt a paraffin type orientation.8 Figure 1 shows single-pulse (SP) Bloch-decay 13C NMR spectra with proton decoupling for ODA in fully and partially ion exchanged clays. On the basis of the previous NMR assignment for CTAC/silicates12 and pure ODA NMR spectra (shown in the upper left inset of Figure 5), the resonances at 30-33 ppm are attributed to the interior methylenes. These resonances are the focus of this paper since the information on the chain conformations can be obtained through the analyses of the chemical shifts associated with these resonances in combination with relaxation measurements. Previous studies20,21 have shown that the carbon atoms of n-alkanes gave a resonance at 30 ppm in solution where equilibrium populations of trans and gauche conformations exist, but in the crystalline solid a downfield shift of about 3-4 ppm is observed for an all-trans conformation. Recently, Gao et al.10 have reported that well-oriented monolayers display an intense peak at 33 ppm which is a characteristic 13C chemical shift for the interior methylene carbons of the alkane chains in an all-trans conformation. Our previous study12 has reported the chemical shift of interior methylenes at ∼30.8 ppm for CTAC surfactants
in hexagonal mesophase silicates, indicating that surfactants have a significant degree of gauche conformation in addition to trans conformation. Therefore, the two-resolved resonances at 33 and 30 ppm shown in Figure 1a indicate that alkyl chains exhibit a significant amount of gauche conformation in addition to all-trans conformation for ODA in the fully ion exchanged clay. For the partially ion exchanged clay (shown in Figure 1b), the resonances associated with the alkyl chains are broader and not well resolved. The resonances associated with the alkyl chain in Figure 1a can be deconvoluted and quantified using two fitting peaks at 33 and 30 ppm. The area ratios of the ordered conformation at 33 ppm (all-trans) to the disordered conformation at 30 ppm (mixed gauche and trans) are 2:1 for the fully exchanged sample with an uncertainty of (10%. Furthermore, the total content of gauche conformation can be calculated approximately from the chemical shifts and the area ratios between 33 and 30 ppm resonances based on the so-called γ-gauche effect.20,21 In the solid state, the 13C chemical shift is determined by the conformation and the packing density of the alkyl chains in addition to the chemical structure. The chemical shift of the methylene groups in alkyl chains depends on the conformation of the two γ-positions: trans-trans, trans-gauche/ gauche-trans, or gauche-gauche. This so-called γ-gauche effect20,21 generally leads to a low-field chemical shift of approximately 4.5 ppm for a methylene unit in a trans-trans chain compared to that in a trans-gauche and approximately 9 ppm compared to that in a gauche-gauche environment. Chemical shifts have been used to calculate the total content of gauche conformation for stiff macromolecules with flexible side chains.17 Using a similar method, total contents of gauche conformation were calculated to be approximately 18% for ODA in fully ion exchanged clay. The temperature-dependent measurements for the fully exchanged ODA/clay sample are shown in Figure 2. The relative intensities for the resonances at 33 and 30 ppm change as a function of temperature. As the temperature increases, the number of all-trans conformation decreases. The maximum peak intensity shifts from 33 ppm at 25 °C to 30 ppm at 50 °C, indicating that ODA molecules are mostly ordered in clay at 25 °C and become mostly disordered at 50 °C. At 100 °C, only one sharp resonance at 30 ppm remains, indicating that all ODA assumed the disordered conformation, as shown in previous work20 where the carbon atoms of n-alkanes in solution have equilibrium populations of trans and gauche conformations. The
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Figure 3. Contact-time array spectra for ODA (1-octadecylamine) in the fully ion exchanged clay. Figure 2. Single-pulse (SP) Bloch-decay solid-state 13C NMR spectra with proton decoupling for ODA (1-octadecylamine) in the fully ion exchanged clay as a function of temperature.
temperature range for the interlayer transitions to a more disordered conformation is in agreement with the range observed by FTIR.9 The 13C spectra of surfactants in clay interlayers can be compared with similar surfactants in different environments: CTAC surfactant precipitated with amorphous silica,12 CTAC surfactant in ordered mesoporous silica,12 ODA surfactant adsorbed in 40 Å mesoporous silica, ODA and CTAC adsorbed on silica nanoparticles, and pure ODA powders. In preparing the ordered mesoporous silica, the CTAC surfactants were used as template agents where they form an ordered liquid crystalline structure. In addition, an opportunity to evaluate the effect of the geometrical confinement is afforded by the cylindrical pore shape in mesoporous silica which is different from the layered structures in clay minerals. The CTAC molecules precipitated with amorphous silica do not have any liquid crystalline structure and should be similar to the free surfactant molecules in solution. In pure ODA powders the surfactant behaves like a solid material in an all-crystalline state. We have found that, besides pure ODA powders, only ODA molecules in the clay interlayers have the all-trans conformation. Therefore, the surfactant molecules in the clay appear to be very special and different from other environments discussed in this paper. The ordered all-trans conformation observed in ODA/clay is largely attributed to the geometric constraining effect due to the physical presence of the silicate layers in addition to the packing density requirements that maintain charge neutrality. Although changes in chemical shift associated with resonances at 30-33 ppm provide conformational information about the alkyl chains, the chemical shifts alone cannot quantitatively describe the rigidity of the alkyl chain. The conformations and dynamics of the surfactant chain can be further studied using relaxation time measurements. Contact-time array spectra are given in Figures 3 and 4 for ODA in clay and in 40 Å mesoporous silicates, 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. Figure 3 shows that the cross-polarization rate for the resonance associated with the alkyl chain differs significantly between ODA in clay and ODA in mesoporous silicates. The rate of cross-polarization for ODA in clay is much faster than in mesoporous silicates. Since cross-polarization is most efficient for the static 13C-1H dipolar interactions, the less mobile carbon groups exhibit the faster cross-polarization
TABLE 1: TCH Values (ms) Derived from Contact Time Array Measurements TCH (ms) ODA in clay (25 °C) ODA in clay (100 °C) crystalline ODA (25 °C) ODA in mesopores (25 °C) CTAC in mesopores (25 °C) CTAC in amorphous (25 °C)
30 ppm
33 ppm
0.07 0.09
0.02 0.02
0.11 0.11a 0.38b
a For CTAC in ordered mesoporous silicates from ref 12. b For CTAC precipitated in amorphous silicates from ref 12.
rate. The faster cross-polarization rate associated with the 33 ppm resonance (corresponding to an all-trans conformation) for ODA in clay suggests that ODA surfactants are more rigid in the confined geometry of the clay than ODA in 40 Å mesoporous silicates. Cross-polarization time constants (TCH) derived from variablecontact-time measurements22 can give quantitative descriptions of cross-polarization and relaxation behavior. A shorter crosspolarization time constant (TCH) corresponds to a faster crosspolarization rate. Thus, the TCH value associated with the alkyl chain resonance can be used as a measure of the rigidity of the chain conformations. Table 1 illustrates the different relaxation behavior for ODA in clay at 25 and 100 °C, pure crystalline ODA, ODA in mesoporous silicates, CTAC in ordered mesoporous silicates, and CTAC precipitated in amorphous silicates. From Table 1, the TCH for ODA in clay with an all-trans conformation is 0.02 ms, which is same as the TCH of the surfactant in the solid crystalline materials. Therefore, based on the TCH values, the molecules in an all-trans conformation, even though mixed with a disordered conformation, have a similar rigidity as the molecules in solid crystalline materials. The disordered conformations for ODA in clays with mixture of trans and gauche conformations have a TCH of about 0.070.11 ms. This result is very close to the cross-polarization time constant obtained for the surfactant adsorbed in ordered mesoporous materials or in a liquid crystalline state. Therefore, the assumption of a liquid crystalline behavior in the literature for the disordered surfactants is correct. However, the NMR results for the first time illustrate that two conformations coexist in the same material (the surfactant clay hybrid), and the surfactants in the different conformations have very different relaxation behavior. Upon heating to 100 °C, the all-trans conformation disappears, and all the molecules become disordered. The previous FTIR study9 inferred from analysis of the stretching frequencies of CH2 suggests that the surfactant is liquidlike at this conformation. However, the cross-polarization time constant
2814 J. Phys. Chem. B, Vol. 104, No. 13, 2000
Figure 4. Contact-time array spectra for ODA (1-octadecylamine) in mesoporous silicates.
TCH derived from the ODA/clay samples at 100 °C is 0.09 ms, not too much different from the result of 0.07 ms for the ODA/ clay sample at 25 °C. On the other hand, surfactant molecules in amorphous materials (resembling the liquid conformation) show a TCH of 0.38 ms. Therefore, the TCH values suggest that the surfactant molecules in the clay hybrid, even in the disordered conformation, are still highly constrained and do not have the motional freedom as do liquid surfactants. The surfactants in the clay hybrid never achieve the liquid conformation freedom because of the specific constrains of the clay gallery. Since proton line widths from 2D WISE NMR measurements can directly reflect the mobility of the surfactant chain without further data manipulation, 2D WISE NMR spectroscopy is used here to further quantify chain mobility in order to confirm the results obtained from the cross-polarization time constant TCH measurements. Proton line widths obtained from our 2D WISE NMR measurements are compared with other published results. In crystalline solids, the proton line widths are largely increased due to the strong dipolar interactions among the abundant proton spins. These line widths for crystalline solids typically range from 50 to 70 kHz. In semicrystalline or liquid environments, the proton line widths are decreased significantly due to the molecular motion which weakens the dipolar interaction. Therefore, the degree of chain mobility associated with a specific conformation can be revealed simply by comparing proton line widths obtained from 2D WISE spectra from different systems. 2D WISE NMR spectra for pure solid ODA (1-octadecylamine) are given in Figure 5. The upper-right inset displays the slice in the 1H dimension at 33 ppm of the 13C resonance corresponding to the all-trans chain conformation, while the singlepulse (SP) Bloch-decay solid-state 13C NMR spectrum with proton decoupling for pure solid ODA (1-octadecylamine) is plotted in the upper-left inset. The appearance of a single resonance at 33 ppm indicates the high degree of conformational order in the crystalline solid of ODA, in agreement with XRD data which show a 100% crystalline phase for pure solid ODA. The observation of a broad 1H NMR line width of 80 kHz at fwhh (full width at half-height) associated with the narrow resonance at 33 ppm in the 13C NMR spectrum suggests that the densely packed all-trans chain in the crystalline ODA solid has a high degree of rigidity, in agreement with the results obtained from the TCH measurements. 2D WISE NMR spectra for ODA in the fully ion exchanged clay are displayed in Figure 6. The single-pulse (SP) Blochdecay solid-state 13C NMR spectrum with proton decoupling for ODA in the fully ion exchanged clay shown in the upper-
Wang et al. left inset of Figure 6 exhibits a dominant all-trans conformation (at 33 ppm) in addition to the mixed gauche and trans conformation (30 ppm). The upper-right inset displays a slice in the 1H dimension for the methylene carbons in all-trans conformation at 33 ppm. The broad 1H NMR line width of 60 kHz at fwhh associated with the 13C resonance at 33 ppm is in the range for most of crystalline solids, indicating that the ordered ODA in the fully ion exchanged clay is as rigid as most of crystalline solids, in agreement with our TCH measurements. Therefore, the results from our 2D WISE NMR experiments further confirm that the cross-polarization time constant TCH can be used to estimate the rigidity of the alkyl chains. A previous study23 has reported the proton line widths at fwhh for an all-trans conformation (33 ppm) of 52, 20, and 23 kHz for ODPA (octadecylphosphonic acid) on Al2O3, TiO2, and ZrO2, respectively. The large proton line width of 52 kHz at fwhh for ODPA-Al2O3 suggested rigid crystalline packing of the all-trans chains, while for ODPA on TiO2 and ZrO2, the significantly reduced 1H line widths associated with the extended all-trans segments at 33 ppm indicated that the oriented chains were undergoing motion about the chain axes. Previous work concluded that the all-trans chains of self-assembled monolayers such as ODPA on TiO2 and ZrO2 are not as closely packed as in a crystalline solid. Although a high degree of conformation order was suggested by the observation of only one narrow resonance at 33 ppm in the high-resolution 13C NMR MAS spectra for ODPA on ZrO2, the relatively narrow proton line width of 23 kHz at fwhh from the 2D WISE NMR revealed considerable chain mobility. This suggests that the high conformational order does not necessarily reflect high rigidity. The presence of considerable motion about the chain axes in these systems makes the chain mobility more comparable with that of alkyl chains in stiff macromolecules with flexible side chains,17 rather than that of the rigid crystalline solids. Thus, the intercalated ODA molecules in an all-trans conformation have higher rigidity as compared with stiff macromolecules with flexible side chains17 and ODPA on TiO2 and ZrO2.23 To obtain more detailed dynamic information on ODA in clays requires future studies involving the characterization of the types of chain motions present using 2H NMR by selectively labeling segments of the chain. Speiss and co-workers in their studies17,18 have reported the observation of ordered and disordered conformations in stiff macromolecules with long flexible alkyl side chains. The size of the corresponding conformation of ODA in clay can be probed in the future using 2D WISE NMR with inclusion of spin diffusion measurements.18 However, it is difficult to obtain the accurate spin diffusion coefficients required for the domain size calculations of surfactant-clay systems because the spin diffusion coefficients are derived from comparison with the diffusion coefficient established in systems with well-known conformation sizes and similar proton line widths.18 Conclusions In this paper, the conformation heterogeneity and mobility of surfactant molecules in intercalated montmorillonite clay minerals have been studied using variable-temperature solidstate 13C NMR and two-dimensional proton wide-line separation (2D WISE) 1H-13C NMR spectroscopy. The NMR results reported here not only are complementary to the previous FTIR results9 but also give new insights into the conformational heterogeneity and the rigidity of the surfactants in the clay interlayers. Two main resonance peaks are resolved from the backbone methylene groups in NMR: the resonance at 33 ppm
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Figure 5. 2D WISE-NMR spectra for pure solid ODA (1-octadecylamine). The upper-right inset displays the slice in the 1H dimension for the methylene carbons in an all-trans conformation at 33 ppm, while the single-pulse (SP) Bloch-decay solid-state 13C NMR spectra with proton decoupling for pure solid ODA (1-octadecylamine) is plotted in the upper-left inset.
Figure 6. 2D WISE-NMR spectra for ODA (1-octadecylamine) in the fully ion exchanged clay. The upper-right inset displays the slice in the 1H dimension for the methylene carbons in an all-trans conformation at 33 ppm, while the single-pulse (SP) Bloch-decay solid-state 13C NMR spectrum with proton decoupling for ODA (1-octadecylamine) in the fully ion exchanged clay is shown in the upper-left inset.
corresponding to the ordered (all-trans) conformation and the resonance at 30 ppm corresponding to the disordered (mixture of all-trans and gauche) conformation. The coexistence of two conformations clearly suggests the conformational heterogeneity
of surfactants in the clays. Furthermore, variable-temperature NMR directly provides quantitative information on the conformational rigidity of different conformations. The cross-polarization time constants derived from the contact-time array experi-
2816 J. Phys. Chem. B, Vol. 104, No. 13, 2000 ments along with 2D WISE NMR suggest that the molecules in the ordered conformation are as rigid as those in most solid crystalline materials and that the molecules in the disordered conformation is similar to those in liquid crystalline materials. Upon heating, the molecules in the disordered conformation remain more or less unchanged, while the molecules in the ordered conformation become disordered. However, the intercalated molecules in the clay minerals never attained the complete liquidlike behavior even after all ODA assumed a disordered conformation. Acknowledgment. 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. References and Notes (1) Rosen, M. J. Surfactants and Interfacial Phenomena; John Wiley & Sons: New York, 1978. (2) (a) Beck, J. S.; Vartuli, J. C.; Roth, R. J.; Leonowicz, M. E.; Kresgec, C. T.; Chu, T-W.; Olson, D. H. J. Am. Chem. Soc. 1992, 114, 10834. (b) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (3) Okada, A.; Usuki, A.; Kurauchi, T.; Kamigaito, O. In Hybrid Organic-Inorganic Composites; Mark, J. E., Lee, C. Y.-C., Bianconi, P. A., Eds.; American Chemical Society: Washington, DC, 1995; p 55. (4) Lan, T.; Pinnavaia, T. J. Chem. Mater. 1994, 6, 2216.
Wang et al. (5) Vaia, R. A.; Ishii, H.; Giannelis, E. P. Chem. Mater. 1993, 5, 1694. (6) Grimshaw, R. W. In The Chemistry and Physics of Clays and AdVanced Ceramic Materials; TechBooks: Fairfax, VA, 1971; pp 124157. (7) Okada, A.; Usuki, A.; Kurauchi, T.; Kamigaito, O. In Hybrid Organic-Inorganic Composites; Mark, J. E., Lee, C. Y.-C., Bianconi, P. A., Eds.; American Chemical Society: Washington, DC, 1995; p 55. (8) Legaly, G. Solid Conformation Ionics 1986, 22, 43. (9) Vaia, R. A.; Teukolsky, R. K.; Giannelis, E. P. Chem. Mater. 1994, 6, 1017. (10) Gao, W.; Reven, L. Langmuir 1995, 11, 1860. (11) Badia, A.; Gao, W.; Singh, S.; Demers, L.; Cuccia, L.; Reven, L. Langmuir 1996, 12, 1262. (12) Wang, L.-Q.; Liu, J.; Exarhos, G. J.; Bunker, B. C. Langmuir 1996, 12, 2663. (13) Soderlind, E.; Stilbs, P. Langmuir 1993, 9, 1678. (14) Jurga, S.; Macho, V.; Huser, B.; Spiess, H. W. Z. Phys. B: Condens. Matter 1991, 84, 43. (15) Wong, S.; Vasudevan, S.; Vaia, R. A.; Giannelis, E. P.; Zax, D. B. J. Am. Chem. Soc. 1995, 117, 7568. (16) Wong, S.; Vaia, R. A.; Giannelis, E. P.; Zax, D. B. Solid Conformation Ionics 1996, 86-88, 547. (17) Clauss, J.; Schmidt-Rohr, K.; Adam, A.; Boeffel, C.; Spiess, H. W. Macromolecules 1992, 25, 5208. (18) Schmidt-Rohr, K.; Clauss, J.; Spiess, H. W. Macromolecules 1992, 25, 3237. (19) Dixon, W. T. J. Chem. Phys. 1982, 77, 1800. (20) Earl, W. L.; VanderHart, D. L. Macromolecules 1979, 12, 762. (21) Tonelli, A. E.; Schilling, F. C. Acc. Chem. Res. 1981, 14, 3. (22) Demco, D. E.; Tegenfeldt, J.; Waugh, J. S. Phys. ReV. B 1974, 11, 4133. (23) Gao, W.; Dickinson, L.; Grozinger, C.; Morin, F. G.; Reven, L. Langmuir 1996, 12, 6429.
