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Structure and Dynamics of Cationic Surfactants Intercalated in Synthetic Clays R. Mu¨ller,† J. Hrobarikova,† C. Calberg,‡ R. Je´roˆme,‡ and J. Grandjean*,† University of Liege, Institute of Chemistry B6a, CERM, COSM, Sart Tilman, B-4000 Liege, Belgium Received November 4, 2003. In Final Form: January 5, 2004
Introduction To obtain clay-polymer nanocomposites, one of the critical steps is the surface treatment of the mineral. Cationic surfactants are ion-exchanged with interlamellar cations to form intercalated clay-surfactant hybrids. The surface treatment is to ensure the dispersion of the mineral within the polymer matrix. Such systems are currently characterized by macroscopic techniques such as X-ray diffraction (XRD), thermogravimetric analysis, or transmission electron microscopy. For instance, the d001 basal spacing from XRD may be related indirectly to the number of surfactant layer(s) and their mean orientation with respect to the clay basal plane.1 On another hand, nuclear magnetic resonance (NMR) provides a useful technique to characterize these hybrid materials at the molecular level, probing the structure, conformation, and dynamics of molecules at interfaces.2 Thus, these additional data complement XRD results. Three-sheet clays, such as hectorite and saponite, result from 2:1 condensation, the octahedral layer being sandwiched between two tetrahedral layers. Clay platelets are negatively charged, as a result of cation isomorphous substitution either in the octahedral layer [hectorite: Li(I) for Mg(II)] or in the tetrahedral layer [saponite: Al(III) for Si(IV)]. Exchangeable cations such as sodium occupy the interlamellar space to preserve electroneutrality.3 A first study dealt with the structure and dynamics of hexadecyltrimethylammonium (HDTA) and octadecylammonium cations intercalated in Laponite.4 Here, we have investigated the arrangement of HDTA and two closely related ammonium cations intercalated in synthetic saponites of a variable interlayer charge. NMR data can explain, at a molecular level, the variation of the basal spacing as a function of the saponite charge. Experimental Section Laponite RD (Laporte Industries, Ltd.) is a synthetic Nahectorite with well-known properties.5-7 Synthetic Na-saponites with a charge per half-unit cell in the 0.30-0.80 range were * To whom correspondence should be addressed. E-mail:
[email protected]. † COSM. ‡ CERM. (1) Lagaly, G. Solid State Ionics 1986, 22, 43. (2) Grandjean, J. Annu. Rep. NMR Spectrosc. 1998, 35, 217. (3) Theng, B. K. G. The Chemistry of Clay-Organic Reactions; J. Wiley: New York, 1974; Chapter 1. (4) Kubies, D.; Jerome, R.; Grandjean, J. Langmuir 2002, 18, 6159. (5) To¨ro¨k, B.; Bala´zsik, K.; De´ka´ny, I.; Bartok, M. Mol. Cryst. 2000, 341, 339. (6) Ramsey, J. D. F.; Lindner, P. J. Chem. Soc., Faraday Trans. 1993, 89, 4207. (7) Saunders, J. M.; Goodwin, J. W.; Richardson, R. M.; Vincent, B. J. Phys. Chem. B 1999, 103, 9211.
prepared as described previously.8 These minerals have been characterized by different techniques8,9 and particularly by NMR.10 HDTA bromide and ethanol (Aldrich Chemical Co.) were used as received. The deionized water was obtained by a Milli-Q UF Plus system (Millipore). (2-Hydroxyethyl)dimethylhexadecylammonium iodide (HEDMHA) and di-(2-hydroxyethyl)methylhexadecylammonium iodide (DHEMHA) were prepared, according to a published procedure,11 by quaternization reactions of respectively N,N-dimethylethanolamine (Aldrich, 99.5+%) and N,N-diethanolmethylamine (Acros, 99+%) with 1-iodohexadecane (Aldrich 95%). Organically modified clays were prepared and characterized accordingly to the published procedure.4 13C MAS NMR spectra were recorded with 4-mm zirconia rotors spinning at 4 kHz on a Bruker Avance DSX 400WB spectrometer (B0 ) 9.04 T) working at the Larmor frequency of 100.62 MHz. The cross polarization (CP) experiments were performed under high-power proton decoupling (83 kHz) with a delay time of 4 s and a contact time of 2 ms. The values of TCH were determined from the plots of line intensity versus contact time in CP MAS experiments. The experiments (13-16 delays) were run with 900-1600 scans. The trans conformer population of the surfactant hydrocarbon chain was obtained in two different ways, either from the trans/gauche conformer line intensities calculated from this plot or by line deconvolution of the two overlapping 13C signals assuming Lorentzian line shapes. All of the parameters were optimized, and the 13C resolution was checked on a glycine sample (signal-to-noise g 50 for the methylene signal). Fourier transform infrared spectra obtained from KBr pellets were collected using a Perkin-Elmer 16PC FT IR spectrometer.
Results and Discussion The location of cation isomorphous substitution modulates the intensity of counterion interaction with the clay surface, as shown experimentally12,13 and by Monte Carlo simulation.14-16 Outer-sphere complexes consisting of cations loosely bound to the solid surface are associated with octahedral replacement. In Laponite, a low-charge (0.28 per half-unit cell) synthetic hectorite, Li(I) atoms substitute partly for Mg(II) atoms in the octahedral layer of the clay platelets. A basal d001 spacing value of 14.7 Å is consistent with a monolayer of HDTA intercalated in Laponite with the mobile hydrocarbon chain lying down on the silicate surface.4 Partial cation replacement [Al(III) for Si(IV)] occurs in the tetrahedral layer of saponites, providing a stronger electrostatic interaction. This assertion is confirmed by Monte Carlo simulations showing inner sphere complexes in smectites with tetrahedral substitution16 and by NMR methods.13 Accordingly, that should strengthen the ordering and rigidity of the intercalated species. An XRD study of the synthetic saponites leads to d001 values between 12.5 and 12.3 Å.9 Surfactant intercalation at the level of the cation exchange capacity ((0.1) induces larger values (Table 1). Such a small variation of the (8) Bergaoui, L.; Lambert, J. F.; Frank, R.; Suquet, H.; Robert, J.-L. J. Chem. Soc., Faraday Trans. 1995, 91, 2229. (9) Michot, L. J.; Villie´ras, F. Clay Miner. 2002, 37, 39. (10) Delevoye, L.; Robert, J.-L.; Grandjean, J. Clay Miner. 2003, 38, 63. (11) Lepoittevin, B.; Pantousier, N.; Alexandre, M.; Calberg, C.; Je´roˆme, R.; Dubois, Ph. J. Mater. Chem. 2002, 12, 3528. (12) Sposito, G.; Prost, R. Chem. Rev. 1982, 82, 553. (13) Gevers, C.; Grandjean, J. J. Colloid Interface Sci. 2001, 236, 290. (14) Chang, F.-R. C.; Skipper, N. T.; Sposito, G. Langmuir 1997, 13, 2074. (15) Chang, F.-R. C.; Skipper, N. T.; Sposito, G. Langmuir 1998, 14, 1201. (16) Greathouse, J.; Sposito, G. J. Phys. Chem. B 1998, 102, 2406.
10.1021/la0304058 CCC: $27.50 © 2004 American Chemical Society Published on Web 03/04/2004
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Table 1. Basal d001 Spacing (Å) and trans Conformer (%) of the Surfactant Main Alkyl Chain of the Organically Modified Clays clay (charge)
ammonium salt
d001
trans conformer (quantitative analysis)
Laponite (0.28)a saponite (0.35) saponite (0.40) saponite (0.50) saponite (0.60) saponite (0.75) saponite (0.80) Laponite (0.28) saponite (0.30) saponite (0.40) saponite (0.50) saponite (0.60) Laponite (0.28) saponite (0.40) saponite (0.50) saponite (0.60)
HDTA HDTA HDTA HDTA HDTA HDTA HDTA HEDMHA HEDMHA HEDMHA HEDMHA HEDMHA DHEMHA DHEMHA DHEMHA DHEMHA
14.7 17.7 17.6 20.6 23.2 29.7 31.1 14.5 21.4 20.2 20.6 23.8 15.2 20.5 23.1 26.4
40 75 70 57 (58) 31 51 (47) 90 41 (48) 70 79 69 43 (49) 46 (49) 69 58 (51) 39 (41)
a
Ref 4. 17,18
incorporated cation does not perturb the d001 values, and their change results from the effect of the clay charge. These values, larger than those observed in Laponite systems, increase with the clay charge (Table 1). Because synthetic saponites are prepared from homogeneous gels, a uniform distribution of the substitution sites is expected.8 23Na two-dimensional 3QMAS NMR spectra have shown one mean Na+ environment for most saponites (charge e 0.60),10 supporting this assertion. To account for the most stringent conditions of modified saponites compared to the Laponite system, in the two lowest-charge saponites, surfactant cations adopt a bilayer structure17,18 (instead of a monolayer) with the long chains lying down on the clay surface. Basal spacings of 20 and 23 Å observed for saponites with intermediate charges (Table 1) may correspond to a (pseudo) trimolecular layer of surfactant,17,18 but the mean orientation of the alkyl chain bilayer may also be tilted with respect to the clay surface. The interlayer space of about 30 Å found with the high-charged saponites (Table 1) is consistent with a tilt angle of about 55° of the hydrocarbon tails with respect to the silicate surface (paraffin complex).18 Thus, to accommodate steric repulsion occurring from the shortening of the mean surface area per cationic site, the orientation of the hydrocarbon tail changes from parallel to radiating away from the silicate surface. We have also investigated the intercalation of HEDMHA and DHEMHA, two surfactants recently used to prepare polymer-clay nanocomposites.19 Variation of the d001 values follows the same trend, although the absolute values are very often greater than that measured with intercalated HDTA (Table 1). By contrast to the XRD data, which provide only indirect information on the surfactant arrangement, NMR techniques probe directly the structure and dynamics of the intercalated cations. In this section, we describe how the d001 values can be explained at a molecular level. Signal assignment of the 13C CP MAS spectrum of HDTA has been reported previously.4,20 For the solid surfactant, a single peak near 34 ppm from the C4-14 methylene groups (17) Lee, S. Y.; Kim, S. J. Clays Clay Miner. 2002, 50, 435. (18) Li, Y.; Ishida, H. Langmuir 2003, 19, 2479. (19) Lepoittevin, B.; Pantousier, N.; Devalckenaere, M.; Alexandre, M.; Kubies, D.; Calberg, C.; Je´roˆme, R.; Dubois, Ph. Macromolecules 2002, 35, 8385-8390. (20) Simonutti, R.; Comotti, A.; Braco, S.; Sozzani, P. Chem. Mater. 2001, 13, 771.
Figure 1. 13C CP MAS NMR spectra of HDTA-exchanged saponites with a layer charge of 0.60 (bottom) and 0.75 (top), respectively.
accounts for the all-trans conformation of the long hydrocarbon chain. After intercalation between the clay platelets, this signal is split and the upfield signal near 32 ppm is attributed to the gauche conformation of the CH2 groups. Saponite with a layer charge of 0.75 per halfunit cell deserves separate analysis. Indeed, this clay shares particular properties. High-resolution argon adsorption studies have shown that the length of the clay particles is multiplied by more than two compared to the lower-charge saponites. For a layer charge g0.65, each ditrigonal cavity formed by the arrangement of silicate tetrahedra contains at least one Al atom and shows the influence of the Al density on the crystal growth.9 More interestingly, as cation exchange is concerned here, one single mean cationic site is observed by 23Na twodimensional 3QMAS NMR for layer charges in the 0.350.60 range, whereas at least two sites are shown with the saponite of 0.75 charge per half-unit cell, probably an effect of increasing the intercationic repulsive interaction.10 In the 13C CP MAS NMR spectrum of the corresponding HDTA-exchanged clay, NMR lines are narrow giving rise to resolved signals for the trans and gauche conformations of the alkyl chain. Higher crystallinity of this material is probably responsible for it. By contrast, the spectrum of the other modified clays exhibits broader lines and the trans and gauche resonances overlap (Figure 1). Quantitative data require spectra recording as a function of the contact time.21 During the CP 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} (1) where t is the contact time and M0 is the equilibrium magnetization. The usual assumptions to apply this equation21 are fulfilled for our systems. From the M0 values, the trans conformer population of the saponite (0.75) accounts for (21) Kolodziejski, W.; Klinowski, J. Chem. Rev. 2002, 102, 613.
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47%. Such data analysis has been also performed when the trans/gauche conformer ratio was close to 1 (Table 1: data in parentheses). If one conformer is clearly dominant, the weaker component resonance merges into the strong signal of the other conformer, preventing any straightforward analysis. Therefore, we have defined a contact time (2 ms) that allows us to retain quantitative data (Table 1). The two sets of values are similar (within less than (7%), and deconvolution data under these experimental conditions provide valuable quantitative information. With the lowest-charge saponite, the trans conformation is dominant. The opposite observation was found with Laponite.4 The stronger electrostatic interaction from isomorphous substitution in the tetrahedral layers of saponites gives rise to more rigidity and favors interchain interaction and the more compact bilayer. That is consistent with the bilayer structure deduced from the XRD study. As the d001 spacing increases progressively with the layer charge from 0.35 to 0.6, one could speculate that the mean angle formed between hydrocarbon chains retaining the same conformation and the silicate surface increases similarly. Such an arrangement is not in agreement with NMR data showing a decrease of the trans conformer content (Table 1). The mean surface area per cation varies from 81.7 to 138 Å2, and the extended more rigid trans conformation of the surfactant alkyl chain (ca. 22 Å) is less easily formed as the clay charge increases. Thus, the less densely packed gauche conformation leads to the increase of the interlayer distance in agreement with the XRD results (Table 1). With the modified clay obtained from saponite (0.75), the fraction of the trans conformer increases compared to the 0.60 charge material. A d001 value of 29.7 Å is consistent with the so-called paraffin complex.18 In such an ideal structure, extended alkyl chains are parallel, favoring the trans conformation. Thus, the further shortening of the surface area per cation in the saponite (0.75) changes the balance between repulsive and attractive interactions leading to a different arrangement enhancing the effect of interchain van der Waals interactions and the trans conformation. Accordingly, with a charge of 0.8, the modified saponite data accounts for a trans/gauche conformational ratio of about 90%. For clay charges greater than about 0.9, smectite clays are named vermiculites.3 Neutron diffraction has been used to characterize a HDTA-exchanged vermiculite. Data analysis leads to an all-trans chain being tilted at 54.5° to the clay surface, the angle of the tilt that optimizes the binding of the headgroup to the clay surface.22 Such a paraffin complex has been also postulated for HDTA cations intercalated between sheets of cadmium thiophosphate CdPS3.23 Intercalation of HEDMHA or DHEMHA in Laponite and saponites leads to trans/gauche conformational ratios similar to that obtained with HDTA (Table 1). The IR bands at about 2920 and 2850 cm-1 that arise from the CH2 asymmetric and symmetric stretch, respectively, are also sensitive to surfactant intercalation. As the interlayer charge (packing density) increases, these bands shift to lower values.24 Accordingly, the wavenumber decreases from 2925 to 2918 cm-1 for HDTA-exchanged saponites with a layer charge varying from 0.35 to 0.75. (22) Williams-Daryn, S.; Thomas, R. K.; Castro, M. A.; Becerro, A. J. Colloid Interface Sci. 2002, 256, 314. (23) Suresh, R.; Vasudevan, S.; Ramanathan, K. V. Chem. Phys. Lett. 2003, 371, 118. (24) Vaia, R. A.; Teukolsky, R. K.; Giannelis, E. P. Chem. Mater. 1994, 6, 1017.
Notes
However, the wavenumber and the width of this band are also influenced by the gauche/trans conformational ratio, leading to higher values as the number of gauche conformers along the hydrocarbon chain increases.18,24 We have shown by NMR spectroscopy that the more populated trans conformers are found with the lowest-charge saponites, but the highest wavenumbers are observed. Thus, among these two antagonistic effects, the charge or packing density is dominant, and only NMR data are able to estimate unambiguously the trans/gauche conformational ratio in our systems. As noted previously, chain dynamics can be studied by NMR spectroscopy; in particular, short TCH values (1) are related to local rigidity. Accordingly, the greatest values are obtained for the mobile -NCH3 and terminal -CH3 groups. On the other hand, for modified clays in which the gauche conformer is dominant, the signal intensity of the trans conformer is the highest at short contact times t, suggesting a faster buildup of the more rigid trans conformer. NMR experiments on HEDMHA- and DHEMHA-exchanged clays lead to TCH values of -CH2CH2OH systematically shorter than those of -N-CH3, indicating higher motion of the last group. The opposite behavior is observed in the nonrestricted medium in which the methyl mobility in the ethyl groups is higher than that in the methyl ones. The resulting mobility restriction in the incorporated surfactant headgroup may be associated with the interaction of the hydroxyl group and the silicate layer or steric effect from the bulkier hydroxyethyl group. No data (NMR, IR) supports specific interaction, and this motional decrease is attributed to the steric effect. Furthermore, TCH are values systematically smaller for DHEMHA systems, indicating more rigidity of the headgroup in the presence of two bulkier groups. Conclusions The arrangement of the surfactants incorporated in the investigated clays is rather well-defined for the lowly and highly charged minerals. In modified Laponite, a lowcharge hectorite with cation isomorphous substitution in the octahedral layer, one single layer of surfactant is incorporated with the long alkyl chain lying down on the solid surface. The gauche conformation of the alkyl chain is important, indicating chain flexibility. Cation substitution in the tetrahedral layers of saponites gives rise to stronger interactions and less flexibility. Thus, the incorporated surfactant forms a bilayer with the alkyl chain lying down on the silicate surface. Mobility restriction is indicated by the higher content of the more densely packed trans conformer. With the highest-charged saponite, to accommodate steric repulsion occurring from the shortening of the mean surface area per cationic site, the orientation of the hydrocarbon tail changes from parallel to radiating away from the silicate surface. This arrangement maximizes the van der Waals interactions between the alkyl chains as supported by the high trans conformer content. Alkyl chains are shown more flexible after intercalation in saponites of intermediate charge, as shown by the enhancement of the gauche conformer content. Such a less densely packed conformation increases the basal spacings. Thus, evolution of the trans/gauche conformational ratio of the incorporated surfactant as a function of the saponite charge provides a deeper understanding of the d001 variation. The local mobility determined by TCH corroborates these conclusions on conformational flexibility and indicates motional change related to the nature of the headgroup.
Notes
Acknowledgment. J.G. and R.J. are grateful to the FNRS (Bruxelles) for a grant to purchase the solid-state NMR spectrometer and to support this study. J.H. thanks the FNRS for a postdoctoral fellowship (2.4503.02). V. Collard is acknowledged for X-ray and thermogravimetric
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measurements and A. Gertsmans for preparing the HEDMHA and DHEMHA surfactants. J.-L. Robert (Orle´ans) kindly supplies us with the synthetic saponite samples. LA0304058
