Surfactant Molecules Intercalated in Laponite as Studied by 13C and

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Surfactant Molecules Intercalated in Laponite as Studied by 13C and 29Si MAS NMR Dana Kubies,†,§ Robert Je´roˆme,† and Jean Grandjean*,‡ University of Liege, Institute of Chemistry B6a, CERM, COSM, Sart Tilman, B-4000 Liege, Belgium Received November 29, 2001. In Final Form: April 25, 2002

The structure and dynamics of surfactant molecules intercalated in Laponite have been characterized by solid-state 13C NMR relaxation times and 13C- and 29Si-detected proton relaxation times. These results are compared to similar data obtained for a surfactant intercalated in montmorillonite and for different mesoporous materials. As compared with montmorillonite clay, a higher mobility and a greater percentage of the gauche conformation of the surfactant molecules are found in Laponite. The 29Si-detected proton relaxation times in the rotating frame are sensitive to the amount of intercalated surfactant.

Introduction In the past few years, we have used nuclear magnetic resonance (NMR) methods to characterize the interaction of cationic,1 nonionic,2,3 and zwitterionic4 surfactants with synthetic clays in aqueous suspensions, showing how the structure and charge density of the clay modulate this interaction. The lamellar structure of clays, also named phyllosilicates, consists of layers formed by condensation of sheets of linked Si(O,OH)4 tetrahedra with those of linked M2-3(OH)6 octahedra, where M is either a divalent or a trivalent cation. Three-sheet clays result from 2:1 condensation, the octahedral layer being sandwiched between two tetrahedral layers. Trioctahedral clays show their three octahedral sites occupied by Mg(II), and dioctahedral phyllosilicates have two of three octahedral sites filled with Al(III). Clay platelets are negatively charged, as a result of cation isomorphous substitution either in the octahedral layer or in the tetrahedral layer. Exchangeable cations such as sodium occupy the interlamellar space in order to preserve electroneutrality.5 Alkylammonium surfactants have been recently used to prepare clay-polymer nanocomposites.6 These systems are currently characterized by the d001 basal spacing from X-ray diffraction, which may be related indirectly to the number of surfactant layer(s) and their mean orientation with respect to the clay basal plane.7 Fourier transform infrared spectroscopy provides a more direct way to define the surfactant molecule orientation and conformation. These results are based on small variation of vibrational * To whom correspondence should be addressed. E-mail: [email protected]. † CERM. ‡ COSM. § On leave from the Institute of Macromolecular Chemistry, Academy of Science of the Czech Republic, Heyrovsky sq. 2., 162 06, Prague, Czech Republic. (1) Grandjean, J.; Laszlo, P. J. Magn. Reson., Ser. A 1996, 118, 103. (2) Grandjean, J. Langmuir 1998, 14, 1037. (3) Gevers, C.; Grandjean, J. J. Colloid Interface Sci. 2001, 236, 290. (4) Grandjean, J. J. Colloid Interface Sci. 2001, 239, 27. (5) Theng, B. K. G. The Chemistry of Clay-Organic Reactions; J. Wiley: New York, 1974; Chapter 1. (6) 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. (7) Lagaly, G. Solid Conformation Ionics 1986, 22, 43.

frequencies assigned to stretching and bending modes of the methylene (CH2) groups.8 NMR is known to be a powerful technique for probing structure, conformation, and dynamics of molecules at interfaces.9 Thus, motion of the aromatic ring of phenethylammonium cation intercalated in hectorite and Laponite has been described by 13C cross-polarization magic-angle spinning (CP MAS) NMR spectra.10 Dynamics of n-octylammonium saponite11 and dimethyldistearylammonium montmorillonite12 have been investigated by 2 H, 1H NMR and 13C CP MAS NMR, respectively. The conformation heterogeneity and mobility of 1-octadecylamine intercalated in montmorillonite have been described by NMR relaxation techniques and 2D WISE NMR.13 Tetraethylammonium ions are used to aid crystallization during hectorite synthesis by hydrothermal treatment. 29 Si and 13C CP MAS NMR techniques provide adequate means to follow this process.14 In a nylon-6-clay nanocomposite, 15N CP MAS NMR methods have shown that the clay induces the γ crystal phase while maintaining the same percent of crystallinity.15 The clay dispersion quality and the organic-modifier stability of this system have been characterized by NMR means.16 The structure determination of clay-methyl methacrylate copolymer interlayer complexes has been defined by 13C solid-state NMR.17 7Li NMR has probed Li+ dynamics in a poly(ethyleneoxide) Li-montmorillonite nanocomposite.18 One of the critical steps in the preparation of polymer materials by using modified clays is the surface treatment of the mineral. Cationic surfactants are ion-exchanged (8) Vaia, R. A.; Teukolsky, R. K.; Giannelis, E. P. Chem. Mater. 1994, 6, 1017. (9) Grandjean, J. Annu. Rep. NMR Spectrosc. 1998, 35, 217. (10) Bank, S.; Ofori-Oki, G. Langmuir 1992, 8, 1688. (11) Yamauchi, M.; Ishimaru, S.; Ikeda, R. Mol. Cryst. Liq. Cryst. 2000, 341, 315. (12) Khatib, K.; Francois, M.; Tekely, P.; Michot, L. J.; Bottero, J. Y.; Baudin, I. J. Colloid Interface Sci. 1996, 183, 148. (13) Wang, L.-Q.; Liu, J.; Exharos, J.; Flanigan, K. Y.; Bordia, R. J. Phys. Chem. B 2000, 104, 2810. (14) Carrado, K. A.; Xu, L.; Gregory, D. M.; Song, K.; Seifert, S.; Botto, R. E. Chem. Mater. 2000, 12, 3052. (15) Mathias, L. J.; Davis, R. D.; Jarrett, W. L. Macromolecules 1999, 32, 7958. (16) Vanderhart, D. L.; Asano, A.; Gilma, J. W. Macromolecules 2001, 34, 3819. (17) Forte, C.; Geppi, M.; Giamberini, S.; Ruggeri, G.; Veracini, C. A.; Mendez, B. Polymer 1998, 39, 2651. (18) Yang, D.-K.; Zax, D. B. J. Chem. Phys. 1999, 110, 5325.

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with interlamellar sodium cations to form intercalated clay-surfactant hybrids. The surface treatment is to ensure the dispersion of the mineral within the polymer matrix. The location of cation isomorphous substitution and the charge density of the clay should modulate this interaction. Montmorillonite is a dioctahedral clay with cation replacement mainly in the octahedral layer. Hectorite and saponite, two trioctahedral clays, show cation substitution in the octahedral and tetrahedral layer, respectively.5,19 Among these clays, the charge density may fluctuate from 0.2 to 0.9 charge per half-unit cell.19 Therefore, the complete understanding of the claysurfactant interaction requires a more systematic approach. As a first part of our NMR studies on clay nanocomposites, we have investigated two cationic surfactants intercalated in Laponite, a synthetic hectorite with well-known properties.21-23 Different solid-state NMR techniques have been used to characterize these hybrid materials. Experimental Section Laponite RD (Laporte Industries, Ltd.) is a synthetic Na+ hectorite corresponding to the molecular formula Si8[Mg5.5Li0.4H4O24]7-Na0.70.7+.24 Its cation exchange capacity (CEC) is 7.3 × 10-4 mol/g. Octadecylamine (ODA) and hexadecyltrimethylammonium (HDTA) bromide (Aldrich Chemical Co.) were used as received. Hydrochloric acid, ethanol, and dioxane (Aldrich Chemical Co.) were used without any purification process. The deionized water was obtained by a Milli-Q UF Plus system (Millipore). Two grams of Laponite was swelled in 100 mL of deionized water at 80 °C for 2 h. Then, a hot solution of the modifying alkylammonium salt or protonated octadecylamine in 100 mL of deionized water was added dropwise to the clay dispersion, and the mixture was stirred vigorously for the next 2 h at 80 °C. Two-times excess of the ammonium cations to CEC of Laponite was used (14.6 × 10-4 mol/g) for a cation exchange reaction. The organically modified Laponite was recovered by filtration. To remove the unreacted ammonium salts, the product was washed twice with 200 mL of hot water and once by ethanol and dioxane. The presence of the free salts in the filtrate was determined by a silver nitrate test. The purified product was freeze-dried from dioxane and dried in a vacuum at 60 °C. The content of the organic phase in the cation-exchanged Laponite was determined from the weight loss during the decomposition of the remaining alkylammonium cations in the clay by means of thermogravimetric analysis (Thermal Analyst 2100, TA Instrument; heating temperature of 20-800 °C and heating rate of 10 °C/min, under N2 flow). The degree of the cation exchange (%) was calculated as a ratio between the organic content (mol/g) and CEC of Laponite (7.3 × 10-4 mol/g). The changes in the basal spacing d001 in the clay sheet organization after the cation exchange were determined by X-ray diffraction (XRD) measurement (Powder Diffractometer Siemens D 5000; Cu KR radiation λ ) 1.54 Å, Ni-filter, 25 °C). 29Si and 13C MAS NMR spectra were recorded with 4 mm zirconia rotors spinning at 3 kHz on a Bruker Avance DSX 400WB spectrometer (B0 ) 9.04 T) working at the Larmor frequency of 79.50 and 100.62 MHz, respectively. The cross-polarization (CP) experiments were performed under high-power proton decoupling (19) Ollis, A. C.; Douglas, L. A. In Proceedings of the International Clay Conference, Strasbourg, 1989; Farmer, V. C., Tardy, V., Eds.; Sciences Ge´ologiques Me´moire, Vol. 86; Institut de ge´ologie, Universite´ Louis Pasteur de Strasbourg et Centre de ge´ochimie de la surface, CNRS: Strasbourg, 1990; p 127. (20) Yui, T.; Yoshida, H.; Tachibana, H.; Tryk, D. A.; Inoue, H. Langmuir 2002, 18, 891. (21) To¨ro¨k, B.; Bala´zsik, K.; De´ka´ny, I.; Bartok, M. Mol. Cryst. 2000, 341, 339. (22) Ramsey, J. D. F.; Lindner, P. J. Chem. Soc., Faraday Trans. 1993, 89, 4207. (23) Saunders: J. M.; Goodwin, J. W.; Richardson, R. M.; Vincent, B. J. Phys. Chem. B 1999, 103, 9211. (24) Ramsey, J. D. F. J. Colloid Interface Sci. 1986, 109, 441.

Kubies et al. Chart 1

Table 1. Characteristics of the Organically Modified Laponite sample

ammonium salt

degree of cation exchange (%)

d001 (Å)

Laponite Lap/C16 Lap/C18

(CH3)3C16H33N+BrC18H37N+Cl-

91 70

13.9 14.7 18.3

(83 kHz) with a delay time of 5-10 ms and a contact time of 2 and 5 ms for 13C and 29Si, respectively. Carbon-13 longitudinal relaxation times (T1, T1F) were determined for the nanocomposites by application of usual CP pulse sequences (13C 90° pulse of 5.5 µs). The carbon (silicon)-detected proton relaxation times T1 and T1F were determined by varying the proton interpulse delay and the duration of the 1H spin-lock period, respectively, before a fixed contact period, and detection of the heteronuclear spin with proton decoupling. TCH was determined from CP MAS experiments, increasing progressively the contact time. All of the parameters were optimized, and the 13C resolution was checked on a glycine sample (signal-to-noise (S/N) g 50 for the methylene signal). The experiments (12-16 delays) were run with 1001300 scans.

Results and Discussion The modified Laponites with their characteristics are listed in Table 1. The X-ray diffractogram of the parent Laponite exhibits rather broad signals with a basal d001 spacing of 13.9 Å. A small increase of the interlamellar distance is observed after surfactant incorporation (Table 1). These basal spacing values are typical of a monolayer of HDTA and a bilayer of ODA organic cations with long alkyl chains lying down on the silicate surface.20,21 By subtraction of the intrinsic layer thickness (9.6 Å),20-23 the interlamellar space is 5.1 and 8.3 Å with HDTA and ODA cations, respectively. The cross-sectional area of the tetramethylammonium cation and the rigidly packed all-trans alkyl chain are 12 and 24 Å2, respectively.25 This is consistent with the above conclusions. Such alkyl chain orientation with respect to the Laponite basal plane is due to the low charge density of the clay (see Chart 1 for the all-trans conformation).21 The orientation of the incorporated surfactant molecules is mainly stabilized through cation-anion electrostatic interaction between the quaternary ammonium cation and the negative charge on the clay surface. Destabilization is brought from the electrostatic repulsion between the positive ammonium ions and steric repulsion between the ammonium substituents. The destabilization effects are expected to be weak with the low charge of Laponite. The average surface area per charge is estimated to ca. 170 Å2, and the mean distance between two neighboring charges is ca. 13 Å.1 The length of the HDTA cation in the all-trans conformation is ca. 22 Å, the usual conformation in the solid state. However, the 13C NMR spectra show (25) Fujii, M.; Li, B.; Fukuda, K.; Kato, T.; Seimiya, T. Langmuir 2001, 17, 1138.

Surfactant Molecules Intercalated in Laponite

Figure 1.

13C

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CP MAS NMR spectrum of HDTA bromide.

Table 2. 13C NMR Chemical Shifts δ (ppm) of HDTA Bromide and the Hexadecyltrimethylammonium Ion Exchanged on Laponite (LAPHDTA) carbon atom

HDTA bromide

C1 N-CH3 C14 C4-13 C2 C3,15 C16

64.46 56.8 36.66 34.55 31.64 26.54; 25.76 18.65

LAPHDTA 69.6 56.2 34.45 (t); 32.87 (g) 29.75 26.12; 25.31 16.54

Figure 2. 13C CP MAS NMR spectrum of the hexadecyltrimethylammonium ion exchanged on Laponite (LAPHDTA). Table 3. Chemical Shifts δ (ppm), 13C Spin-Lattice Relaxation Times in the Laboratory Frame T1(C) (s) and in the Rotating Frame T1G(C) (ms), Dipolar Relaxation Times TCH (ms), and 13C-Detected Proton Relaxation Times in the Laboratory Frame T1(H) (ms) and in the Rotating Frame T1G(H) (ms) of the Main Methylene Signal of the Octadecylammonium Ion Exchanged on Laponite (LAPODA) δ

T1(C)

T1F(C)

TCH

T1(H)

T1F(H)

34.43 (t) 32.66 (g)

0.846 0.797

8.52 6.86

0.65 0.53

66.1 66.1

4.15 3.25

that the gauche conformation of the alkyl chains becomes important upon intercalation into the clay platelets, decreasing the average length of the hydrocarbon chains and steric repulsion between them. The 13C CP MAS NMR spectrum of the HDTA bromide contains eight resolved peaks (Table 2 and Figure 1). The signal attributions have been taken from literature data on the chloride salt26 although the bromide spectrum is simpler, particularly in the upfield region (Figure 1). Indeed, four different methyl signals have been reported for the chloride salt, resulting from four conformations.26 In contrast, one methyl peak is observed with the used bromide salt. In the crystalline phase, the all-trans conformation of the hydrocarbon chain is dominant, and the inner methylenes occur in the 34.2-32.8 ppm range.27 The gauche conformation induces an upfield shift of this signal.28 The 13C NMR spectra of the modified Laponite (LAPHDTA and LAPODA) exhibit much narrower lines than those obtained with the ODA-exchanged montmorillonite (MONTODA).13 The characteristics of this montmorillonite have not been reported.13 From the properties of such clays, it appears that a higher charge density or charges originating partly from substitution in the tetrahedral layer19 might reduce the alkyl chain mobility. The smaller size of Laponite platelets, ca. 300 Å20-23 instead of more than ca. 1000 Å for natural montmorillonites,5,21 could also account for that. Two partly resolved signals are observed for the main methylene signal in the 32-34 ppm range, resulting from a mixture of all-trans (downfield peaks at 34.45 and 34.43 ppm for LAPHDTA and LAPODA, respectively) and gauche conformations (signals at 32.87 and 32.66 ppm for LAPHDTA and LAPODA, respectively) (Figure 2 and Tables 2 and 3).

Such heterogeneity has also been shown for MONTODA.13 The all-trans conformation is dominant in this modified montmorillonite at room temperature, whereas the gauche conformation becomes the most populated at high temperatures.13 At room temperature, the all-trans conformation was not found dominant in LAPHDTA (30% at 293 K and 16% at 360 K) and LAPODA (ca. 50% at 293 K). In contrast, a unique signal has been reported at room temperature in MCM-41 mesoporous materials26 and for ODA in mesoporous silicates.29 At similar resolution, higher mobility in the mesopores could result in an average signal, shifted upfield with an increasing population of the gauche conformer. Thus, the confined geometry in the modified clays could decrease the exchange rate between both conformations showing two partly resolved signals. However, two hexagonal and one mesolamellar aluminophosphate hybrid materials show a dominant gauche conformation, but a dominant all-trans conformation is found with two other lamellar systems.30 Thus, the conformation of the surfactant alkyl chain depends on the structure of the inorganic phase. On the other hand, incorporation of HDTA in Laponite decreases the line width of the trimethylammonium (Figures 1 and 2). This could correspond to a higher mobility of the N-methyl groups in the nanocomposite, in line with the rather large distance between the clay charges resulting in a significant separation between the ammonium cation polar heads. In contrast, the signal from the terminal methyl is broadened and its mobility has to be reduced between the clay platelets. However, factors other than mobility may affect the line width, and a more precise analysis comes from relaxation data (Table 4).

(26) Simonutti, R.; Comotti, A.; Bracco, S.; Sozzani, P. Chem. Mater. 2001, 13, 771. (27) Ishikawa, S.; Kurosu, H.; Ando, I. J. Mol. Struct. 1991, 248, 361. (28) Tonelli, A. E. Annu. Rep. NMR Spectrosc. 1997, 34, 185.

(29) Wang, L.-Q.; Liu, J.; Exarhos, G. J.; Bunker, B. C. Langmuir 1996, 12, 2663. (30) Khimyak, Y. Z.; Klinowski, J. Phys. Chem. Chem. Phys. 2001, 3, 616.

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Table 4. 13C Spin-Lattice Relaxation Times in the Laboratory Frame T1(C) (s) and in the Rotating Frame T1G(C) (ms), Dipolar Relaxation Times TCH (ms), 13C-Detected Proton Relaxation Times in the Laboratory Frame T1(H) (ms) and in the Rotating Frame T1G(H) (ms) of HDTA Bromide and the Hexadecyltrimethylammonium Ion Exchanged on Laponite (LAPHDTA) carbon atom

HDTA bromide T1(H) T1F(H)

C1 N-CH3 C14 C4-13

2.12 1.94 2.06

5.23 5.38 5.08 5.25

C2 C3 C15 C16

2.45 2.16 2.38 2.15

5.10 5.27 5.39 5.16

T1(C) 0.074 0.147 0.441 (t) 0.356 (g) 0.262 0.382 0.817 >1

The 13C spin-lattice relaxation times T1 (C) of the alkyl chain are mainly governed by the dipolar relaxation of the carbon-13 nuclei with the directly bonded hydrogens. In the crystalline phase (slow-motion regime), long relaxation times are observed (ca. 50 s), as indicated from theoretical models. Values shorter by ca. 2 orders of magnitude were determined for LAPHDTA. The surfactant molecules are less packed in the nanocomposite, and mobility is higher (Table 4). In the fast-motion regime, larger T1C values reflect higher mobility which increases from the polar headgroup to the terminal methyl. These values are close to those observed in MCM-41 mesoporous materials.26 On the basis of these relaxation times, the signals at 26.12 and 25.31 ppm are assigned to C3 and C15, respectively. During the cross-polarization sequence, the 13C magnetization M increases exponentially with the time constant TCH whereas the proton magnetization is governed by the relaxation in the rotating frame (T1F(H)). The variation of the carbon magnetization is ruled by the equation M(t) ) M0[exp(-t/T1F(H)) - exp(-t/TCH)]/[1 - TCH(T1F(H))-1]

where t is the contact time and M0 is the equilibrium magnetization. The cross-polarization is most efficient for static 13C1H dipolar interaction and less mobile carbon groups that exhibit smaller TCH values. The TCH and T1F(C) data vary similarly to the T1(C) values and particularly confirm the attributions of C3 and C15 (Table 4). Mobility of the alltrans and gauche conformations can be characterized from these three relaxation times. The values obtained for LAPHDTA (Table 4) and LAPODA (Table 3) indicate higher mobility of the all-trans conformation. Such a behavior may depend on the investigated system. Indeed, in mesoporous aluminophosphates, the T1 ratios of the two conformers are either greater or smaller than 1.30 In LAPODA, the all-trans and the gauche conformations are characterized by TCH values of 0.65 and 0.53 ms, respectively (Table 4). These values are 1 order of magnitude greater than those of MONTODA,13 confirming higher mobility of the hydrocarbon chain in LAPODA (and LAPHDTA). In solids, spin-diffusion evens out the differences in local 1 H magnetization via a flip-flop mechanism. In heterogeneous materials, this evening-out occurs only if enough time is left for nonequilibrium magnetization to flow within the material. The propagation of magnetization is correlated to the strong dipolar coupling between hydrogens. The covered distance is given by the formula

〈L 〉 ) nDτ 2

T1F(C)

LAPHDTA TCH

13.0 26.3

0.54

15.4 (t) 15.1 (g)

0.40 (t) 0.41 (g)

13.6 14.5 30.0

0.19 0.46 4.3

T1(H)

T1F(H)

83.3 106

8.94 7.16

95.2 (t) 90.4 (g) 79.8 116 97.0 89.6

6.91 (t) 5.75 (g) 7.19 6.88 6.01 6.12

Table 5. 29Si-Detected Proton Relaxation Times in the Laboratory Frame T1(H) (ms) and in the Rotating Frame T1G(H) (ms) of Laponite and LAPHDTA Laponite LAPHDTA

T1(H)

T1F(H)

63.4 85.6

29.9 8.17

where D is the spin-diffusion constant, n ) 2, 4, and 6 for diffusion in one, two (here), and three dimensions, and τ is the diffusion time. At very high MAS speeds, the dipolar coupling is in principle reduced to zero, and no spindiffusion is expected. However, the relatively slow spinning rate applied in our experiments (3 kHz) does not affect significantly the strong homonuclear coupling. For rigid systems, reliable values of the spin-diffusion coefficient are available. We have used the value measured for n-alkanes (similar proton density) which is equal to 6.2 × 10-16 m2/s in the laboratory frame.31 In the presence of a spin-lock field along the x axis, D was calculated to be half of that in the laboratory frame spin-diffusion which is proportional to (3 cos2 θ - 1).32 With LAPHDTA, T1(H) values of ca. 0.1 s allow a magnetization propagation over a distance of ca. 10 nm. On the other hand, T1F(H) values of ca. 6 ms are influenced by heterogeneity on the scale of 2.5-3 nm. In LAPHDTA, surfactant molecules are lying down to the clay basal plane, and the 1H-1H dipolar interaction responsible for spin-diffusion differs from that in the crystalline phase (Table 4). In 29Si-detected proton relaxation times, cross-polarization results from hydroxyl groups and adsorbed water in Laponite and mainly from surfactant protons in LAPHDTA. Adsorbed water is typically 8-10% in smectite clays and less than 1% in modified 2/1 phyllosilicates. Therefore, 13C- and 29Sidetected proton relaxation times T1(H) and T1F(H) in LAPHDTA are different from those of the crystalline surfactant and Laponite, respectively (Tables 4 and 5). Proton relaxation times measured from 13C and 29Si NMR are very close, as expected from similar 1H-1H interactions. The 13C-detected proton relaxation times T1(H) and T1F(H) of LAPODA (Table 3) are smaller than those of LAPHDTA (Table 4), which is consonant with a higher proton density in the interlamellar space as a result of the bilayer formation. More interestingly, the 29Sidetected proton relaxation time T1F(H) of LAPODA is clearly not described by a monoexponential law. A biexponential fitting results in two components of ca. 65% and ca. 35% with the relaxation times of 4.46 and 19.5 ms, respectively. Inorganic counterions of Laponite have been partly exchanged by the octadecylammonium cations with (31) Douglas, D. C.; Jones, G. P. J. Chem. Phys. 1966, 45, 956. (32) McBrierty, V. J.; Packer, K. J. Nuclear magnetic resonance in solid polymers; Cambridge University Press: Cambridge, U.K., 1993.

Surfactant Molecules Intercalated in Laponite

a yield of ca. 70% (Table 1). Therefore, we can tentatively attribute the former component to the exchanged sites with a high proton density from the intercalated surfactant molecules. A T1F(H) of 4.46 ms, close to the 13C-detected results (Table 4), supports this assumption. The nonexchanged sites are more distant from the surfactant molecules, and the obtained value of 19.4 ms, between 4.46 and 29.9 ms (Table 4), is in line with this attribution. The biexponential behavior of T1F(H) points to heterogeneity for domain sizes of ca. 2.5-3 nm (see above), and this is consistent with the calculated charge separation in Laponite. Conclusions The crystalline hexadecyltrimethylammonium bromide exhibits a simpler 13C NMR spectrum than a recently published spectrum of the chloride salt.26 Our signal assignments are consonant with those of the chloride salt,26 but 13C relaxation time analysis of the modified Laponite (LAPHDTA) can differentiate between the C3 and C15 signals. The chemical shift of the main methylene peaks in the 31-34 ppm range is related to the alkyl chain conformation, and different figures have been observed in surfactant-inorganic hybrids. All-trans and gauche conformations coexist in the two modified Laponites

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(LAPHDTA and LAPODA). Such heterogeneity has been recently observed with a montmorillonite exchanged with octadecylammonium cations.13 In contrast with this study, we have found a higher percentage of the gauche conformation. Different relaxation times indicate higher mobility of surfactant molecules intercalated in Laponite. The size and the structural properties of Laponite platelets as compared to montmorillonite can account for such behavior. Such a mobility difference could play a role in the formation of the resulting polymer nanocomposites. With LAPODA in which ca. 70% of clay counterions have been exchanged, the 29Si-detected proton relaxation process in the rotating frame is described by two components which have been associated with the structural Si(IV) ions of Laponite near the organic molecules and near the nonexchanged sites. Acknowledgment. D.K. thanks the “Services ge´ne´raux des affaires scientifiques, techniques et culturelles; coope´ration S&T avec l’Europe centrale et orientale (Bruxelles)” for a scholarship. J.G. and R.J. are grateful to the FNRS (Bruxelles) for a grant to purchase the solid-state NMR spectrometer. LA0117350