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Preparation and characterization of silylated-magadiites
Applied Clay Science 15 Ž1999. 253–264
Preparation and characterization of silylated-magadiites Shinobu Okutomo a , Kazuyuki Kuroda
a,b,)
, Makoto Ogawa
c,d
a
Department of Applied Chemistry, Waseda UniÕersity, Ohkubo 3-4-1, Shinjuku-ku, Tokyo 169-8555, Japan b Kagami Memorial Laboratory for Materials Science and Technology, Waseda UniÕersity, Nishiwaseda 2-8-26, Shinjuku-ku, Tokyo 169-0051, Japan c PRESTO, Japan Science and Technology Corporation, Nishiwaseda 1-6-1, Shinjuku-ku, Tokyo 169-8050, Japan d Department of Earth Sciences, Waseda UniÕersity, Nishiwaseda 1-6-1, Shinjuku-ku, Tokyo 169-8050, Japan Received 19 August 1998; received in revised form 26 February 1999; accepted 2 March 1999
Abstract Organic modification of a layered silicate, magadiite Žthe ideal formula is Na 2 Si 14O 29 P nH 2 O., was conducted by the reaction between the interlayer hydroxyl groups of magadiite and organochlorosilanes Žtrimethylchrolosilane triethylchlorosilane, triisopropylchlorosilane, butyldimethylchlorosilane, octyldimethylchlorosilane, and octadecyldimethylchlorosilane.. By utilizing the dodecyltrimethylammonium-exchanged magadiite as the intermediate, bulky organosilyl groups have successfully been introduced into the interlayer space of magadiite. The silylation of the interlayer silanol groups of layered silicates may lead to novel functional inorganic–organic supramolecular systems alternative to the intercalation compounds formed by ion exchange reactions. q 1999 Elsevier Science B.V. All rights reserved. Keywords: layered silicate; intercalation; silylation; surface modification
1. Introduction Intercalation of organic guest species into layered inorganic solids is a way to construct ordered inorganic–organic assemblies with unique microstructures )
Corresponding author. Department of Applied Chemistry, Waseda University, Ohkubo 3-4-1, Shinjuku-ku, Tokyo 169-8551, Japan 0169-1317r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 1 3 1 7 Ž 9 9 . 0 0 0 1 0 - 1
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controlled by host–guest and guest–guest interactions ŽWhittingham and Jacobson, 1982; Muller-Warmuth and Schollhorn, 1994. . Besides the scientific impor¨ ¨ tance of intercalation, the application of the intercalation compounds has attracted increasing interests. For example, organoammonium clays have been studied as a precursor for the pillared clay Ž Galarneau et al., 1995. , selective adsorbates ŽBoyd et al., 1988. , membranes Ž Okahata and Shimizu, 1989. , supports for catalysts ŽHu and Rusling, 1991. and photoactive species Ž Seki and Ichimura, 1990; Ogawa et al., 1991, 1992a, 1993, 1995; Takagi et al., 1991; Tomioka and Itoh, 1991; Ahmadi and Rusling, 1995. , and so on Ž Lagaly, 1981; Ogawa and Kuroda, 1997. . Magadiite Žthe ideal formula is Na 2 Si 14O 29 P nH 2 O. is a layered silicate ŽEugster, 1967; Lagaly et al., 1975a,b; Rojo et al., 1988. , which is capable of incorporating guest species in the interlayer space to form intercalation compounds ŽLagaly, 1979; Landis et al., 1991; Dailey and Pinnavaia, 1992. . Compared with smectites Ž Theng, 1974. , magadiite possesses some unique properties for organizing guest species. Ž 1. The density of the cation exchange sites on the layer surface is expected to be higher than those of smectites. Ž2. It can be conveniently prepared in a laboratory by hydrothermal synthesis. Ž 3. Reactive silanol groups are located in the interlayer surface. Organoammoniumexchanged magadiites have recently been applied to adsorbents for environmental pollutants ŽKim et al., 1997. , and polymer–inorganic nanocomposites Ž Shi et al., 1996. . In this paper, surface modification of magadiite by the reactions of alkyldimethylchlorosilanes with variable alkyl chain length and surface hydroxyl groups was investigated. The reactions between organochlorosilanes and hydroxylated surfaces create organically modified surfaces where organic moieties are covalently attached. The modification of silicas with variable geometry such as porous silica gels for chromatographic stationary phase Ž Wirth et al., 1997. and flat substrates for constructing molecular devices Ž so-called ‘‘selfassembly’’. ŽUlman, 1991. has extensively been investigated. The silylation of the interlayer silanol groups of layered silicates may lead to novel functional inorganic–organic supramolecular systems alternative to the intercalation compounds formed by ion exchange reactions. The trimethylsilylation of the interlayer silanol groups of protonated magadiite ŽH-magadiite. was achieved by using intercalation compounds with polar organic molecules ŽRuiz-Hitzky and Rojo, 1980; Ruiz-Hitzky et al., 1985. . However, bulkier organosilyl groups cannot be introduced into the interlayer space by this method. We have succeeded in the trimethylsilylation Ž Yanagisawa et al., 1988a., diphenylmethylsilylation Ž Yanagisawa et al., 1988b. and octyldimethylsilylation Ž Ogawa et al., 1998. of magadiite by utilizing the dodecyltrimethylammonium Ž abbreviated as C 12TMA. -exchanged form as an intermediate. The expanded interlayer space of the C 12TMA-magadiite made it possible to introduce bulky organosilyl groups in the interlayer space.
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2. Materials and methods 2.1. Materials Na-magadiite was hydrothermally synthesized by the method described by Kosuge et al. Ž1992. from colloidal silica, sodium hydroxide, and water at the molar ratio of 1:0.23:18.5 ŽSiO 2 :NaOH:H 2 O. . The mixture was sealed in a Teflon-lined autoclave at 423 K for 48 h. The product was washed with a dilute aqueous NaOH solution and dried in air. C 12TMA chloride Ž Tokyo Kasei. was used without further purification. Silylating reagents, trimethylchlorosilane ŽŽ CH 3 . 3 SiCl; abbreviated as TMSCl., triethylchlorosilane ŽŽC 2 H 5 . 3 SiCl; abbreviated as TESCl. , triisopropylchlorosilane ŽŽC 3 H 7 . 3 SiCl; abbreviated as TPSCl., butyldimethylchlorosilane ŽŽ C 4 H 9 . ŽCH 3 . 2 SiCl abbreviated as C 4 2C 1SiCl., octyldimethylchlorosilane ŽŽC 8 H 17 .ŽCH 3 . 2 SiCl; abbreviated as C 8 2C 1SiCl. , and octadecyldimethylchlorosilane ŽŽ C 18 H 37 .ŽCH 3 . 2 SiCl; abbreviated as C 18 2C 1SiCl. were purchased from Chisso and used as received. 2.2. Sample preparation The C 12TMA-magadiite was prepared by an ion exchange reaction between Na-magadiite Ž3 g. and a 0.1-M aqueous C 12TMA chloride solution Ž 200 ml. at room temperature for 4 days. During the reaction, the pH of the solution was kept at 9–10. The white precipitate was centrifuged and washed with acetone. The dried C 12TMA-magadiite Ž 1.5 g. was suspended and refluxed in a mixture of an organochlorosilane Ž 50–100 ml. and dried toluene Ž 30 ml. under nitrogen flow for 48 h. The products were separated by centrifugation and washed with toluene and acetone. 2.3. Characterization X-ray powder diffraction was performed on a Mac Science MXP 3 diffractometer using monochromated Cu K a radiation. Infrared spectra of KBr disks were recorded on a Perkin Elmer FT-1640 Fourier-transform spectrophotometer. TG curves were recorded on a Mac Science 2000S instrument at the heating rate of 108C miny1. Solid-state 29 Si nuclear magnetic resonance ŽNMR. spectra were recorded on a JEOL GSX-400 spectrometer at 79.3 MHz with magic angle spinning. A pr4 pulse was used, and the chemical shifts were reported with respect to external tetramethylsilane. The composition of the products was determined by CHN analysis. Nitrogen adsorptionrdesorption isotherms were obtained at 77 K using a BELSORP 28SA Ž Japan Bell. instrument. The samples were dried at 1008C under vacuum for 3 h prior to the measurements.
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3. Results and discussion The synthesis of Na-magadiite was confirmed by the X-ray diffraction ŽXRD., infrared ŽIR., scanning electron microscopy ŽSEM. and 29 Si-solid state NMR spectroscopy. The basal spacing of Na-magadiite was 1.55 nm, and no diffraction peaks due to the by-products were detected in the XRD pattern. The product forms spherical nodules resembling rosettes, which is characteristic of Na-magadiite. 29 Si-solid state NMR spectroscopy showed signals due to Q 3 and Q 4 environments of silicon. All these observations are consistent with those reported for Na-magadiite. Fig. 1 shows the XRD patterns of Na-magadiite Ž Fig. 1a. and the C 12TMAmagadiite Ž Fig. 1b. . The basal spacing of the C 12TMA-magadiite was 2.79 nm, showing the expansion of the interlayer space. The IR spectrum of the product showed absorption bands characteristic of C 12TMA, such as CH stretching vibration at around 2900 cmy1. The composition of the product was determined by elementary analysis to be C: 24.4%, N: 1.8%. From the elemental analysis and the TG result, the amount of the intercalated C 12TMA was determined to be 1.8 mol for a formula unit of magadiite Ž C 12TMA. 1.8 H 0.2 P Si 14O 29 P nH 2 O4 . The XRD patterns of TMS, TES and TPS derivatives are shown in Fig. 1c, d and e, respectively. The basal spacings decreased upon the silylation from 2.79 nm to 1.85, 1.98 and 2.08 nm for TMS, TES and TPS derivatives, respectively.
Fig. 1. X-ray powder diffraction patterns of Ža. Na-magadiite, Žb. C 12TMA-magadiite Žc. TMS-magadiite, Žd. TES-magadiite, and Že. TPS-magadiite.
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Table 1 Compounds of silylated-magadiites Silylated derivatives
C Ž%.
H Ž%.
N Ž%.
SiO 2 Ž%.
Amounts of silyl groups Žper 14SiO 2 .
TMS-magadiite TES-magadiite T i PrS-magadiite C 4 DMS-magadiite C 8 DMS-magadiite C 18 DMS-magadiite
7.0 13.2 16.3 12.6 18.7 30.8
1.2 2.3 3.3 2.4 3.3 6.0
0.0 0.1 0.1 0.0 0.0 0.1
90.6 87.7 81.7 86.6 80.2 65.8
2.1 2.0 1.7 1.9 1.8 1.9
There is a tendency towards larger basal spacings with the increase in the size of organosilyl group. The composition of the products determined by the CHN analysis is summarized in Table 1. The absence of N in the elementary analysis of the silylated products showed the deintercalation of C 12TMA during the
Fig. 2. IR spectra of Ža. C 12TMA-magadiite, Žb. TMS-magadiite, Žc. TES-magadiite, and Žd. TPS-magadiite.
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silylation. The amounts of the attached organosilyl groups were determined from the CHN analysis to be 2.1, 2.0 and 1.7 per 14SiO 2 Ž molar ratio. for TMS-, TES- and TPS-systems, respectively. Fourier transform infrared spectra of the silylated-magadiites are shown in Fig. 2 together with that of the C 12TMA-magadiite. The sharp band at 3650 cmy1 due to OH stretching vibration of isolated silanol groups disappeared and the new bands due to Si–C bonds of organosilyl groups appeared upon the silylation, indicating the successful grafting of the isolated silanol groups in the interlayer space of magadiite with organochlorosilanes. 29 Si-magic angle spinning ŽMAS. NMR spectra of the Na-magadiite and C 12TMA-magadiite showed similar signals due to Q 3 and Q 4 environments of silicon, showing that the structural regularity did not change during the ion exchange process. On the other hand, in 29 Si-MAS NMR spectra, the silylated products Ž Fig. 3. showed the appearance of new signals due to the M 1 environment of silicon Ž R 3SiŽ OSi.. at around 20 ppm and a decrease in the relative intensity of the Q 3 peak at y100 ppm compared with the Q 4 peak at around y105–y 115 ppm. These results showed that the silanol groups present in the interlayer surface of magadiite were modified by grafting the organosilyl groups to yield both Q 4 and M 1 environments of silicon. The absorption band at
Fig. 3. 29 Si MAS NMR spectra Ža. C 12TMA-magadiite, Žb. TMS-magadiite, Žc. TES-magadiite, and Že. TPS-magadiite.
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3650 cmy1 due to the isolated silanol groups disappeared almost completely in the IR spectra of the silylated products ŽFig. 2. while the Q 3 signal remained to some extent even after the silylation. This observation suggests that hydrogenbonded silanol groups remained even after the silylation. In the 29 Si-MAS NMR spectrum of the TMS-magadiite, two signals ascribable to M 1 environment of silicon appeared at 21 and 15 ppm. The splitting of the signal was also observed for the 29 Si-MAS NMR spectrum of the TMS-octosilicate ŽEndo et al., 1994. . Two different states of the attached TMS groups in the interlayer space of magadiite were estimated from the NMR result; however, the origin of the different chemical environments is not clear at present. Silylated derivatives were also obtained with alkyldimethylchlorosilanes Ž abbreviated as C n 2C 1 Si-, where n denotes the carbon number in the alkyl chain. with variable alkyl chain length Ž n s 4, 8 and 18. . The silylation was confirmed by the 29 Si-NMR and IR spectra of the products. The basal spacing changed from 2.79 nm Ž C 12TMA-magadiite. to 1.95, 2.33 and 3.25 nm for the C 4 2C 1Si-, C 8 2C 1Si- and C 18 2C 1Si-magadiite, respectively ŽFig. 4.. There is a linear relationship between the carbon number in the alkyl chain and the basal spacing
Fig. 4. X-ray powder diffraction patterns of Ža. C 4 2C 1Si-, Žb. C 8 2C 1Si- and Žc. C 18 2C 1Si-magadiite. Inset: the variation of the basal spacings as a function of alkyl chain length of organosilyl groups.
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of the products ŽFig. 4, inset.. From the slope Ž 0.093 nmrCH 2 ., two arrangements of the intercalated alkyldimethylsilyl groups can be considered; one is a
Fig. 5. SEM of Ža. Na-magadiite, Žb. C 12TMA-magadiite and Žc. C 8 2C 1Si-magadiite.
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monomolecular coverage with their alkyl chains inclined to the silicate sheets at ca. 478 and the other is a bimolecular coverage with the alkyl chains inclined to the silicate sheet at ca. 218. This small angle is unrealistic from geomechanical constraints. Thus, the bimolecular coverage is not very probable. The composition of the silylated products determined from the CHN analysis Žfor organic content. and TG Ž for SiO 2 content. is shown in Table 1. The amounts of the attached organosilyl groups were determined to be 1.9, 1.8 and 1.9 per 14SiO 2 Žmolar ratio. for the C 4 2C 1Si-, C 8 2C 1Si- and C 18 2C 1Si-magadiites, respectively. The composition of the derivatives, as well as the linear relationship between the alkyl chain length and the basal spacing, indicate that the alkyldimethylsilyl groups are grafted at the interlayer surface of magadiite. The SEM of the C 8 2C 1Si-magadiite is shown in Fig. 5c as a typical example together with those of Na-magadiite ŽFig. 5a. and the C 12TMA-magadiite Ž Fig. 5b.. Na-magadiite forms spherical nodules resembling rosettes. Although the rosette morphology is lost during the ion exchange with C 12TMA, the morphology of each platelet is not changed significantly. The platy morphology of magadiite is still preserved after the silylation as revealed by the SEM image ŽFig. 5c. of C 8 2C 1Si-magadiite. The nitrogen adsorptionrdesorption isotherm of the TMS-magadiite ŽFig. 6. yielded a Brunauer–Emmet–Teller ŽBET. surface area of 135 m 2 gy1 and an internal surface of 68 m2 gy1 determined by the t-plot. Thus, the TMS-magadiite was microporous where the interlayer TMS group acts as pillars to create
Fig. 6. Nitrogen adsorptionrdesorption isotherms of Ž`, v . TMS-magadiite and Ž^, '. TES-magadiite.
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micropores in the interlayer space. Microporous organoammonium-pillared smectites have long been recognized as adsorbents ŽBarrer, 1978.. The possible applications of the organoammonium-pillared materials as photonics materials ŽOgawa et al., 1992b, 1994. and sensors ŽYan and Bein, 1993. have been reported recently. Well-confined micropores of the pillared clays played the dominant role for the unique physicochemical properties. Selective adsorption of toxic organic compounds on organoammonium-pillared clays is also a current topic Ž Lee et al., 1990. . However, there is a possibility of deintercalation of the organoammonium ions during the adsorption of organic compounds. The TMSmagadiite is an alternative material for these purposes, since the host–pillar binding and the surface geometry of the TMS-magadiite are different from those of the organoammonium-pillared clays. On the contrary, other silylated derivatives were nonporous as evidenced by the nitrogen adsorptionrdesorption isotherms. The nitrogen adsorptionrdesorption isotherms of the TES-magadiite are shown in Fig. 6 as a typical example. The attached bulky organic groups occupy the interlayer space densely, so that nitrogen molecules cannot be adsorbed into the interlayer space. In other words, spatially controlled supramolecular assemblies have been obtained by the introduction of organosilyl groups into the interlayer space of magadiite. Since organosilyl groups were covalently bound to the interlayer surface, the organosilylation of layered polysilicates is an effective way to prepare functional inorganic–organic nanostructured materials.
Acknowledgements The authors are grateful to Nissan Chem. for providing us colloidal silica ŽSnowtex..
References Ahmadi, M., Rusling, J., 1995. Fluorescence studies of solute microenvironment in composite clay-surfactant films. Langmuir 11, 94–100. Barrer, B.M., 1978. Zeolites and Clay Minerals as Sorbents and Molecular Sieves. Academic Press, London. Boyd, S.A., Lee, J.-F., Mortland, M.M., 1988. Attenuating organic contaminant mobility by soil modification. Nature 333, 345–347. Dailey, J.S., Pinnavaia, T.J., 1992. Silica pillared derivatives of Hq-magadiite, a crystalline hydrated silica. Chem. Mater. 4, 855–863. Endo, K., Sugahara, Y., Kuroda, K., 1994. Formation of intercalation compounds of a layered sodium octosilicate with n-alkyltrimethylammonium ions and the application to organic derivatization. Bull. Chem. Soc. Jpn. 67, 3352–3355. Eugster, H.P., 1967. Hydrous sodium silicates from Lake Magadii, Kenya: precursors of bedded chert. Science 157, 1177–1180.
S. Okutomo et al.r Applied Clay Science 15 (1999) 253–264
263
Galarneau, A., Barodawalla, A., Pinnavaia, T.J., 1995. Porous clay heterostructures formed by gallery-templated synthesis. Nature 374, 529–531. Hu, N., Rusling, J.F., 1991. Surfactant-intercalated clay films for electrochemical catalysis. Reduction of trichloroacetic acid. Anal. Chem. 63, 2163–2168. Kim, C.S., Yates, D.M., Heanet, P.J., 1997. The layered sodium silicate magadiite: an analog to smectite for benzene sorption from water. Clays Clay Miner. 45, 881–885. Kosuge, K., Yamazaki, A., Tsunashima, A., Otsuka, R., 1992. Hydrothermal synthesis of magadiite and kenyaite. J. Ceram. Soc. Jpn. 100, 326–331, in Japanese. Lagaly, G., 1979. Crystalline silicic acids and their interface reactions. Adv. Colloid Interface Sci. 11, 105–148. Lagaly, G., 1981. Characterization of clays by organic compounds. Clay Miner. 16, 1–21. Lagaly, G., Beneke, K., Weiss, A., 1975a. Magadiite and H-magadiite: I. Sodium magadiite and some of its derivatives. Am. Miner. 60, 642–649. Lagaly, G., Beneke, K., Weiss, A., 1975b. Magadiite and H-magadiite: II. H-magadiite and its intercalation compounds. Am. Miner. 60, 650–658. Landis, M.E., Aufdembrink, B.A., Chu, P., Johnson, I.D., Kirker, G.W., Rubin, M.K., 1991. Preparation of molecular sieves from dense, layered metal oxides. J. Am. Chem. Soc. 113, 3189–3190. Lee, J.-F., Mortland, M.M., Chiou, C.T., Kile, D.E., Boyd, S.A., 1990. Adsorption of benzene, toluene, and xylene by two tetramethylammonium-smectites having different charge densities. Clays Clay Miner. 38, 113–120. Muller-Warmuth, W., Schollhorn, R. ŽEds.., 1994. Progress in Intercalation Research. Kluwer ¨ ¨ Academic Publishers, Dordrecht. Ogawa, M., Kuroda, K., 1997. Preparation of inorganic–organic nanocomposites through intercalation of organoammonium ions into layered silicates. Bull. Chem. Soc. Jpn. 70, 2593–2618. Ogawa, M., Fujii, K., Kuroda, K., Kato, C., 1991. Preparation of montmorillonite-p-aminoazobenzene intercalation compounds and their photochemical behavior. Mater. Res. Soc. Symp. Proc. 233, 89–94. Ogawa, M., Shirai, H., Kuroda, K., Kato, C., 1992a. Solid-state intercalation of naphthalene and anthracene into alkylammonium-montmorillonites. Clays Clay Miner. 40, 485–490. Ogawa, M., Handa, T., Kuroda, K., Kato, C., Tani, T., 1992b. Photochemical hole burning of 1,4-dihydroxyanthraquinone intercalated in a pillared layered clay mineral. J. Phys. Chem. 96, 8116–8119. Ogawa, M., Aono, T., Kuroda, K., Kato, C., 1993. Photophysical probe study of alkylammonium-montmorillonites. Langmuir 9, 1529–1533. Ogawa, M., Takahashi, M., Kuroda, K., 1994. Intercalation of p-nitroaniline into tetramethylammonium-saponite film under electric field and its optical second harmonic generation. Chem. Mater. 6, 715–717. Ogawa, M., Wada, T., Kuroda, K., 1995. Intercalation of pyrene into alkylammonium-exchanged swelling layered silicates: the effects of the arrangements of the interlayer alkylammonium ions on the states of adsorbates. Langmuir 11, 4598–4600. Ogawa, M., Okutomo, S., Kuroda, K., 1998. Control of interlayer microstructures of a layered silicate by surface modification with organochlorosilanes. J. Am. Chem. Soc. 120, 7361–7362. Okahata, Y., Shimizu, A., 1989. Preparation of bilayer-intercalated clay films and permeation control responding to temperature, electric field, and ambient pH changes. Langmuir 5, 954–959. Rojo, J.M., Ruiz-Hitzky, E., Sanz, J., 1988. Proton–sodium exchange in magadiite. Spectroscopic study ŽNMR, IR. of the evolution of interlayer OH groups. Inorg. Chem. 27, 2785–2790. Ruiz-Hitzky, E., Rojo, M., 1980. Intracrystalline grafting on layer silicic acid. Nature 287, 28–30. Ruiz-Hitzky, E., Rojo, M., Lagaly, G., 1985. Mechanism of the grafting of organosilanes on mineral surfaces. Colloid Polym. Sci. 263, 1025–1030.
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S. Okutomo et al.r Applied Clay Science 15 (1999) 253–264
Seki, T., Ichimura, K., 1990. Thermal isomerization behaviors of a spiropyran in bilayers immobilized with a linear polymer and a smectic clay. Macromolecules 23, 31–35. Shi, H., Lan, T., Pinnavaia, T.J., 1996. Hybrid organic–inorganic nanocomposites formed from an epoxy polymer and a layered silicic acid Žmagadiite.. Chem. Mater. 8, 1584–1587. Takagi, K., Kurematsu, T., Sawaki, Y., 1991. Intercalation and photochromism of spiropyrans on clay interlayers. J. Chem. Soc. Perkin Trans. 2, 1517–1522. Theng, B.K.G., 1974. The Chemistry of Clay–Organic Reactions. Adam Hilger, London. Tomioka, H., Itoh, T., 1991. Photochromism of spiropyrans in organized molecular assemblies. Formation of J- and H-aggregates of photomerocyanines in bilayer-clay matrices. J. Chem. Soc., Chem. Commun. 532–533. Ulman, A., 1991. An Introduction of Ultrathin Organic Films from Langmuir–Blodgett to Self-Assembly. Academic Press, San Diego. Whittingham, M.S., Jacobson, A.J. ŽEds.., 1982. Intercalation Chemistry. Academic Press, New York. Wirth, M.J., Fairbank, R.W.P., Fatunmbi, H.O., 1997. Mixed self-assembled monolayer in chemical separations. Science 275, 44–47. Yan, Y., Bein, T., 1993. Molecular recognition through intercalation chemistry. Chem. Mater. 5, 905–907. Yanagisawa, T., Kuroda, K., Kato, C., 1988a. Organic derivatives of layered polysilicates: I. Trimethysilylation of magadiite and kenyaite. React. Solids 5, 167–175. Yanagisawa, T., Kuroda, K., Kato, C., 1988b. Organic derivatives of layered polysilicates II. Bull. Chem. Soc. Jpn. 61, 3743–3745.
Preparation and characterization of silylated-magadiites Shinobu Okutomo a , Kazuyuki Kuroda
a,b,)
, Makoto Ogawa
c,d
a
Department of Applied Chemistry, Waseda UniÕersity, Ohkubo 3-4-1, Shinjuku-ku, Tokyo 169-8555, Japan b Kagami Memorial Laboratory for Materials Science and Technology, Waseda UniÕersity, Nishiwaseda 2-8-26, Shinjuku-ku, Tokyo 169-0051, Japan c PRESTO, Japan Science and Technology Corporation, Nishiwaseda 1-6-1, Shinjuku-ku, Tokyo 169-8050, Japan d Department of Earth Sciences, Waseda UniÕersity, Nishiwaseda 1-6-1, Shinjuku-ku, Tokyo 169-8050, Japan Received 19 August 1998; received in revised form 26 February 1999; accepted 2 March 1999
Abstract Organic modification of a layered silicate, magadiite Žthe ideal formula is Na 2 Si 14O 29 P nH 2 O., was conducted by the reaction between the interlayer hydroxyl groups of magadiite and organochlorosilanes Žtrimethylchrolosilane triethylchlorosilane, triisopropylchlorosilane, butyldimethylchlorosilane, octyldimethylchlorosilane, and octadecyldimethylchlorosilane.. By utilizing the dodecyltrimethylammonium-exchanged magadiite as the intermediate, bulky organosilyl groups have successfully been introduced into the interlayer space of magadiite. The silylation of the interlayer silanol groups of layered silicates may lead to novel functional inorganic–organic supramolecular systems alternative to the intercalation compounds formed by ion exchange reactions. q 1999 Elsevier Science B.V. All rights reserved. Keywords: layered silicate; intercalation; silylation; surface modification
1. Introduction Intercalation of organic guest species into layered inorganic solids is a way to construct ordered inorganic–organic assemblies with unique microstructures )
Corresponding author. Department of Applied Chemistry, Waseda University, Ohkubo 3-4-1, Shinjuku-ku, Tokyo 169-8551, Japan 0169-1317r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 1 3 1 7 Ž 9 9 . 0 0 0 1 0 - 1
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controlled by host–guest and guest–guest interactions ŽWhittingham and Jacobson, 1982; Muller-Warmuth and Schollhorn, 1994. . Besides the scientific impor¨ ¨ tance of intercalation, the application of the intercalation compounds has attracted increasing interests. For example, organoammonium clays have been studied as a precursor for the pillared clay Ž Galarneau et al., 1995. , selective adsorbates ŽBoyd et al., 1988. , membranes Ž Okahata and Shimizu, 1989. , supports for catalysts ŽHu and Rusling, 1991. and photoactive species Ž Seki and Ichimura, 1990; Ogawa et al., 1991, 1992a, 1993, 1995; Takagi et al., 1991; Tomioka and Itoh, 1991; Ahmadi and Rusling, 1995. , and so on Ž Lagaly, 1981; Ogawa and Kuroda, 1997. . Magadiite Žthe ideal formula is Na 2 Si 14O 29 P nH 2 O. is a layered silicate ŽEugster, 1967; Lagaly et al., 1975a,b; Rojo et al., 1988. , which is capable of incorporating guest species in the interlayer space to form intercalation compounds ŽLagaly, 1979; Landis et al., 1991; Dailey and Pinnavaia, 1992. . Compared with smectites Ž Theng, 1974. , magadiite possesses some unique properties for organizing guest species. Ž 1. The density of the cation exchange sites on the layer surface is expected to be higher than those of smectites. Ž2. It can be conveniently prepared in a laboratory by hydrothermal synthesis. Ž 3. Reactive silanol groups are located in the interlayer surface. Organoammoniumexchanged magadiites have recently been applied to adsorbents for environmental pollutants ŽKim et al., 1997. , and polymer–inorganic nanocomposites Ž Shi et al., 1996. . In this paper, surface modification of magadiite by the reactions of alkyldimethylchlorosilanes with variable alkyl chain length and surface hydroxyl groups was investigated. The reactions between organochlorosilanes and hydroxylated surfaces create organically modified surfaces where organic moieties are covalently attached. The modification of silicas with variable geometry such as porous silica gels for chromatographic stationary phase Ž Wirth et al., 1997. and flat substrates for constructing molecular devices Ž so-called ‘‘selfassembly’’. ŽUlman, 1991. has extensively been investigated. The silylation of the interlayer silanol groups of layered silicates may lead to novel functional inorganic–organic supramolecular systems alternative to the intercalation compounds formed by ion exchange reactions. The trimethylsilylation of the interlayer silanol groups of protonated magadiite ŽH-magadiite. was achieved by using intercalation compounds with polar organic molecules ŽRuiz-Hitzky and Rojo, 1980; Ruiz-Hitzky et al., 1985. . However, bulkier organosilyl groups cannot be introduced into the interlayer space by this method. We have succeeded in the trimethylsilylation Ž Yanagisawa et al., 1988a., diphenylmethylsilylation Ž Yanagisawa et al., 1988b. and octyldimethylsilylation Ž Ogawa et al., 1998. of magadiite by utilizing the dodecyltrimethylammonium Ž abbreviated as C 12TMA. -exchanged form as an intermediate. The expanded interlayer space of the C 12TMA-magadiite made it possible to introduce bulky organosilyl groups in the interlayer space.
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2. Materials and methods 2.1. Materials Na-magadiite was hydrothermally synthesized by the method described by Kosuge et al. Ž1992. from colloidal silica, sodium hydroxide, and water at the molar ratio of 1:0.23:18.5 ŽSiO 2 :NaOH:H 2 O. . The mixture was sealed in a Teflon-lined autoclave at 423 K for 48 h. The product was washed with a dilute aqueous NaOH solution and dried in air. C 12TMA chloride Ž Tokyo Kasei. was used without further purification. Silylating reagents, trimethylchlorosilane ŽŽ CH 3 . 3 SiCl; abbreviated as TMSCl., triethylchlorosilane ŽŽC 2 H 5 . 3 SiCl; abbreviated as TESCl. , triisopropylchlorosilane ŽŽC 3 H 7 . 3 SiCl; abbreviated as TPSCl., butyldimethylchlorosilane ŽŽ C 4 H 9 . ŽCH 3 . 2 SiCl abbreviated as C 4 2C 1SiCl., octyldimethylchlorosilane ŽŽC 8 H 17 .ŽCH 3 . 2 SiCl; abbreviated as C 8 2C 1SiCl. , and octadecyldimethylchlorosilane ŽŽ C 18 H 37 .ŽCH 3 . 2 SiCl; abbreviated as C 18 2C 1SiCl. were purchased from Chisso and used as received. 2.2. Sample preparation The C 12TMA-magadiite was prepared by an ion exchange reaction between Na-magadiite Ž3 g. and a 0.1-M aqueous C 12TMA chloride solution Ž 200 ml. at room temperature for 4 days. During the reaction, the pH of the solution was kept at 9–10. The white precipitate was centrifuged and washed with acetone. The dried C 12TMA-magadiite Ž 1.5 g. was suspended and refluxed in a mixture of an organochlorosilane Ž 50–100 ml. and dried toluene Ž 30 ml. under nitrogen flow for 48 h. The products were separated by centrifugation and washed with toluene and acetone. 2.3. Characterization X-ray powder diffraction was performed on a Mac Science MXP 3 diffractometer using monochromated Cu K a radiation. Infrared spectra of KBr disks were recorded on a Perkin Elmer FT-1640 Fourier-transform spectrophotometer. TG curves were recorded on a Mac Science 2000S instrument at the heating rate of 108C miny1. Solid-state 29 Si nuclear magnetic resonance ŽNMR. spectra were recorded on a JEOL GSX-400 spectrometer at 79.3 MHz with magic angle spinning. A pr4 pulse was used, and the chemical shifts were reported with respect to external tetramethylsilane. The composition of the products was determined by CHN analysis. Nitrogen adsorptionrdesorption isotherms were obtained at 77 K using a BELSORP 28SA Ž Japan Bell. instrument. The samples were dried at 1008C under vacuum for 3 h prior to the measurements.
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3. Results and discussion The synthesis of Na-magadiite was confirmed by the X-ray diffraction ŽXRD., infrared ŽIR., scanning electron microscopy ŽSEM. and 29 Si-solid state NMR spectroscopy. The basal spacing of Na-magadiite was 1.55 nm, and no diffraction peaks due to the by-products were detected in the XRD pattern. The product forms spherical nodules resembling rosettes, which is characteristic of Na-magadiite. 29 Si-solid state NMR spectroscopy showed signals due to Q 3 and Q 4 environments of silicon. All these observations are consistent with those reported for Na-magadiite. Fig. 1 shows the XRD patterns of Na-magadiite Ž Fig. 1a. and the C 12TMAmagadiite Ž Fig. 1b. . The basal spacing of the C 12TMA-magadiite was 2.79 nm, showing the expansion of the interlayer space. The IR spectrum of the product showed absorption bands characteristic of C 12TMA, such as CH stretching vibration at around 2900 cmy1. The composition of the product was determined by elementary analysis to be C: 24.4%, N: 1.8%. From the elemental analysis and the TG result, the amount of the intercalated C 12TMA was determined to be 1.8 mol for a formula unit of magadiite Ž C 12TMA. 1.8 H 0.2 P Si 14O 29 P nH 2 O4 . The XRD patterns of TMS, TES and TPS derivatives are shown in Fig. 1c, d and e, respectively. The basal spacings decreased upon the silylation from 2.79 nm to 1.85, 1.98 and 2.08 nm for TMS, TES and TPS derivatives, respectively.
Fig. 1. X-ray powder diffraction patterns of Ža. Na-magadiite, Žb. C 12TMA-magadiite Žc. TMS-magadiite, Žd. TES-magadiite, and Že. TPS-magadiite.
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Table 1 Compounds of silylated-magadiites Silylated derivatives
C Ž%.
H Ž%.
N Ž%.
SiO 2 Ž%.
Amounts of silyl groups Žper 14SiO 2 .
TMS-magadiite TES-magadiite T i PrS-magadiite C 4 DMS-magadiite C 8 DMS-magadiite C 18 DMS-magadiite
7.0 13.2 16.3 12.6 18.7 30.8
1.2 2.3 3.3 2.4 3.3 6.0
0.0 0.1 0.1 0.0 0.0 0.1
90.6 87.7 81.7 86.6 80.2 65.8
2.1 2.0 1.7 1.9 1.8 1.9
There is a tendency towards larger basal spacings with the increase in the size of organosilyl group. The composition of the products determined by the CHN analysis is summarized in Table 1. The absence of N in the elementary analysis of the silylated products showed the deintercalation of C 12TMA during the
Fig. 2. IR spectra of Ža. C 12TMA-magadiite, Žb. TMS-magadiite, Žc. TES-magadiite, and Žd. TPS-magadiite.
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silylation. The amounts of the attached organosilyl groups were determined from the CHN analysis to be 2.1, 2.0 and 1.7 per 14SiO 2 Ž molar ratio. for TMS-, TES- and TPS-systems, respectively. Fourier transform infrared spectra of the silylated-magadiites are shown in Fig. 2 together with that of the C 12TMA-magadiite. The sharp band at 3650 cmy1 due to OH stretching vibration of isolated silanol groups disappeared and the new bands due to Si–C bonds of organosilyl groups appeared upon the silylation, indicating the successful grafting of the isolated silanol groups in the interlayer space of magadiite with organochlorosilanes. 29 Si-magic angle spinning ŽMAS. NMR spectra of the Na-magadiite and C 12TMA-magadiite showed similar signals due to Q 3 and Q 4 environments of silicon, showing that the structural regularity did not change during the ion exchange process. On the other hand, in 29 Si-MAS NMR spectra, the silylated products Ž Fig. 3. showed the appearance of new signals due to the M 1 environment of silicon Ž R 3SiŽ OSi.. at around 20 ppm and a decrease in the relative intensity of the Q 3 peak at y100 ppm compared with the Q 4 peak at around y105–y 115 ppm. These results showed that the silanol groups present in the interlayer surface of magadiite were modified by grafting the organosilyl groups to yield both Q 4 and M 1 environments of silicon. The absorption band at
Fig. 3. 29 Si MAS NMR spectra Ža. C 12TMA-magadiite, Žb. TMS-magadiite, Žc. TES-magadiite, and Že. TPS-magadiite.
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3650 cmy1 due to the isolated silanol groups disappeared almost completely in the IR spectra of the silylated products ŽFig. 2. while the Q 3 signal remained to some extent even after the silylation. This observation suggests that hydrogenbonded silanol groups remained even after the silylation. In the 29 Si-MAS NMR spectrum of the TMS-magadiite, two signals ascribable to M 1 environment of silicon appeared at 21 and 15 ppm. The splitting of the signal was also observed for the 29 Si-MAS NMR spectrum of the TMS-octosilicate ŽEndo et al., 1994. . Two different states of the attached TMS groups in the interlayer space of magadiite were estimated from the NMR result; however, the origin of the different chemical environments is not clear at present. Silylated derivatives were also obtained with alkyldimethylchlorosilanes Ž abbreviated as C n 2C 1 Si-, where n denotes the carbon number in the alkyl chain. with variable alkyl chain length Ž n s 4, 8 and 18. . The silylation was confirmed by the 29 Si-NMR and IR spectra of the products. The basal spacing changed from 2.79 nm Ž C 12TMA-magadiite. to 1.95, 2.33 and 3.25 nm for the C 4 2C 1Si-, C 8 2C 1Si- and C 18 2C 1Si-magadiite, respectively ŽFig. 4.. There is a linear relationship between the carbon number in the alkyl chain and the basal spacing
Fig. 4. X-ray powder diffraction patterns of Ža. C 4 2C 1Si-, Žb. C 8 2C 1Si- and Žc. C 18 2C 1Si-magadiite. Inset: the variation of the basal spacings as a function of alkyl chain length of organosilyl groups.
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of the products ŽFig. 4, inset.. From the slope Ž 0.093 nmrCH 2 ., two arrangements of the intercalated alkyldimethylsilyl groups can be considered; one is a
Fig. 5. SEM of Ža. Na-magadiite, Žb. C 12TMA-magadiite and Žc. C 8 2C 1Si-magadiite.
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monomolecular coverage with their alkyl chains inclined to the silicate sheets at ca. 478 and the other is a bimolecular coverage with the alkyl chains inclined to the silicate sheet at ca. 218. This small angle is unrealistic from geomechanical constraints. Thus, the bimolecular coverage is not very probable. The composition of the silylated products determined from the CHN analysis Žfor organic content. and TG Ž for SiO 2 content. is shown in Table 1. The amounts of the attached organosilyl groups were determined to be 1.9, 1.8 and 1.9 per 14SiO 2 Žmolar ratio. for the C 4 2C 1Si-, C 8 2C 1Si- and C 18 2C 1Si-magadiites, respectively. The composition of the derivatives, as well as the linear relationship between the alkyl chain length and the basal spacing, indicate that the alkyldimethylsilyl groups are grafted at the interlayer surface of magadiite. The SEM of the C 8 2C 1Si-magadiite is shown in Fig. 5c as a typical example together with those of Na-magadiite ŽFig. 5a. and the C 12TMA-magadiite Ž Fig. 5b.. Na-magadiite forms spherical nodules resembling rosettes. Although the rosette morphology is lost during the ion exchange with C 12TMA, the morphology of each platelet is not changed significantly. The platy morphology of magadiite is still preserved after the silylation as revealed by the SEM image ŽFig. 5c. of C 8 2C 1Si-magadiite. The nitrogen adsorptionrdesorption isotherm of the TMS-magadiite ŽFig. 6. yielded a Brunauer–Emmet–Teller ŽBET. surface area of 135 m 2 gy1 and an internal surface of 68 m2 gy1 determined by the t-plot. Thus, the TMS-magadiite was microporous where the interlayer TMS group acts as pillars to create
Fig. 6. Nitrogen adsorptionrdesorption isotherms of Ž`, v . TMS-magadiite and Ž^, '. TES-magadiite.
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micropores in the interlayer space. Microporous organoammonium-pillared smectites have long been recognized as adsorbents ŽBarrer, 1978.. The possible applications of the organoammonium-pillared materials as photonics materials ŽOgawa et al., 1992b, 1994. and sensors ŽYan and Bein, 1993. have been reported recently. Well-confined micropores of the pillared clays played the dominant role for the unique physicochemical properties. Selective adsorption of toxic organic compounds on organoammonium-pillared clays is also a current topic Ž Lee et al., 1990. . However, there is a possibility of deintercalation of the organoammonium ions during the adsorption of organic compounds. The TMSmagadiite is an alternative material for these purposes, since the host–pillar binding and the surface geometry of the TMS-magadiite are different from those of the organoammonium-pillared clays. On the contrary, other silylated derivatives were nonporous as evidenced by the nitrogen adsorptionrdesorption isotherms. The nitrogen adsorptionrdesorption isotherms of the TES-magadiite are shown in Fig. 6 as a typical example. The attached bulky organic groups occupy the interlayer space densely, so that nitrogen molecules cannot be adsorbed into the interlayer space. In other words, spatially controlled supramolecular assemblies have been obtained by the introduction of organosilyl groups into the interlayer space of magadiite. Since organosilyl groups were covalently bound to the interlayer surface, the organosilylation of layered polysilicates is an effective way to prepare functional inorganic–organic nanostructured materials.
Acknowledgements The authors are grateful to Nissan Chem. for providing us colloidal silica ŽSnowtex..
References Ahmadi, M., Rusling, J., 1995. Fluorescence studies of solute microenvironment in composite clay-surfactant films. Langmuir 11, 94–100. Barrer, B.M., 1978. Zeolites and Clay Minerals as Sorbents and Molecular Sieves. Academic Press, London. Boyd, S.A., Lee, J.-F., Mortland, M.M., 1988. Attenuating organic contaminant mobility by soil modification. Nature 333, 345–347. Dailey, J.S., Pinnavaia, T.J., 1992. Silica pillared derivatives of Hq-magadiite, a crystalline hydrated silica. Chem. Mater. 4, 855–863. Endo, K., Sugahara, Y., Kuroda, K., 1994. Formation of intercalation compounds of a layered sodium octosilicate with n-alkyltrimethylammonium ions and the application to organic derivatization. Bull. Chem. Soc. Jpn. 67, 3352–3355. Eugster, H.P., 1967. Hydrous sodium silicates from Lake Magadii, Kenya: precursors of bedded chert. Science 157, 1177–1180.
S. Okutomo et al.r Applied Clay Science 15 (1999) 253–264
263
Galarneau, A., Barodawalla, A., Pinnavaia, T.J., 1995. Porous clay heterostructures formed by gallery-templated synthesis. Nature 374, 529–531. Hu, N., Rusling, J.F., 1991. Surfactant-intercalated clay films for electrochemical catalysis. Reduction of trichloroacetic acid. Anal. Chem. 63, 2163–2168. Kim, C.S., Yates, D.M., Heanet, P.J., 1997. The layered sodium silicate magadiite: an analog to smectite for benzene sorption from water. Clays Clay Miner. 45, 881–885. Kosuge, K., Yamazaki, A., Tsunashima, A., Otsuka, R., 1992. Hydrothermal synthesis of magadiite and kenyaite. J. Ceram. Soc. Jpn. 100, 326–331, in Japanese. Lagaly, G., 1979. Crystalline silicic acids and their interface reactions. Adv. Colloid Interface Sci. 11, 105–148. Lagaly, G., 1981. Characterization of clays by organic compounds. Clay Miner. 16, 1–21. Lagaly, G., Beneke, K., Weiss, A., 1975a. Magadiite and H-magadiite: I. Sodium magadiite and some of its derivatives. Am. Miner. 60, 642–649. Lagaly, G., Beneke, K., Weiss, A., 1975b. Magadiite and H-magadiite: II. H-magadiite and its intercalation compounds. Am. Miner. 60, 650–658. Landis, M.E., Aufdembrink, B.A., Chu, P., Johnson, I.D., Kirker, G.W., Rubin, M.K., 1991. Preparation of molecular sieves from dense, layered metal oxides. J. Am. Chem. Soc. 113, 3189–3190. Lee, J.-F., Mortland, M.M., Chiou, C.T., Kile, D.E., Boyd, S.A., 1990. Adsorption of benzene, toluene, and xylene by two tetramethylammonium-smectites having different charge densities. Clays Clay Miner. 38, 113–120. Muller-Warmuth, W., Schollhorn, R. ŽEds.., 1994. Progress in Intercalation Research. Kluwer ¨ ¨ Academic Publishers, Dordrecht. Ogawa, M., Kuroda, K., 1997. Preparation of inorganic–organic nanocomposites through intercalation of organoammonium ions into layered silicates. Bull. Chem. Soc. Jpn. 70, 2593–2618. Ogawa, M., Fujii, K., Kuroda, K., Kato, C., 1991. Preparation of montmorillonite-p-aminoazobenzene intercalation compounds and their photochemical behavior. Mater. Res. Soc. Symp. Proc. 233, 89–94. Ogawa, M., Shirai, H., Kuroda, K., Kato, C., 1992a. Solid-state intercalation of naphthalene and anthracene into alkylammonium-montmorillonites. Clays Clay Miner. 40, 485–490. Ogawa, M., Handa, T., Kuroda, K., Kato, C., Tani, T., 1992b. Photochemical hole burning of 1,4-dihydroxyanthraquinone intercalated in a pillared layered clay mineral. J. Phys. Chem. 96, 8116–8119. Ogawa, M., Aono, T., Kuroda, K., Kato, C., 1993. Photophysical probe study of alkylammonium-montmorillonites. Langmuir 9, 1529–1533. Ogawa, M., Takahashi, M., Kuroda, K., 1994. Intercalation of p-nitroaniline into tetramethylammonium-saponite film under electric field and its optical second harmonic generation. Chem. Mater. 6, 715–717. Ogawa, M., Wada, T., Kuroda, K., 1995. Intercalation of pyrene into alkylammonium-exchanged swelling layered silicates: the effects of the arrangements of the interlayer alkylammonium ions on the states of adsorbates. Langmuir 11, 4598–4600. Ogawa, M., Okutomo, S., Kuroda, K., 1998. Control of interlayer microstructures of a layered silicate by surface modification with organochlorosilanes. J. Am. Chem. Soc. 120, 7361–7362. Okahata, Y., Shimizu, A., 1989. Preparation of bilayer-intercalated clay films and permeation control responding to temperature, electric field, and ambient pH changes. Langmuir 5, 954–959. Rojo, J.M., Ruiz-Hitzky, E., Sanz, J., 1988. Proton–sodium exchange in magadiite. Spectroscopic study ŽNMR, IR. of the evolution of interlayer OH groups. Inorg. Chem. 27, 2785–2790. Ruiz-Hitzky, E., Rojo, M., 1980. Intracrystalline grafting on layer silicic acid. Nature 287, 28–30. Ruiz-Hitzky, E., Rojo, M., Lagaly, G., 1985. Mechanism of the grafting of organosilanes on mineral surfaces. Colloid Polym. Sci. 263, 1025–1030.
264
S. Okutomo et al.r Applied Clay Science 15 (1999) 253–264
Seki, T., Ichimura, K., 1990. Thermal isomerization behaviors of a spiropyran in bilayers immobilized with a linear polymer and a smectic clay. Macromolecules 23, 31–35. Shi, H., Lan, T., Pinnavaia, T.J., 1996. Hybrid organic–inorganic nanocomposites formed from an epoxy polymer and a layered silicic acid Žmagadiite.. Chem. Mater. 8, 1584–1587. Takagi, K., Kurematsu, T., Sawaki, Y., 1991. Intercalation and photochromism of spiropyrans on clay interlayers. J. Chem. Soc. Perkin Trans. 2, 1517–1522. Theng, B.K.G., 1974. The Chemistry of Clay–Organic Reactions. Adam Hilger, London. Tomioka, H., Itoh, T., 1991. Photochromism of spiropyrans in organized molecular assemblies. Formation of J- and H-aggregates of photomerocyanines in bilayer-clay matrices. J. Chem. Soc., Chem. Commun. 532–533. Ulman, A., 1991. An Introduction of Ultrathin Organic Films from Langmuir–Blodgett to Self-Assembly. Academic Press, San Diego. Whittingham, M.S., Jacobson, A.J. ŽEds.., 1982. Intercalation Chemistry. Academic Press, New York. Wirth, M.J., Fairbank, R.W.P., Fatunmbi, H.O., 1997. Mixed self-assembled monolayer in chemical separations. Science 275, 44–47. Yan, Y., Bein, T., 1993. Molecular recognition through intercalation chemistry. Chem. Mater. 5, 905–907. Yanagisawa, T., Kuroda, K., Kato, C., 1988a. Organic derivatives of layered polysilicates: I. Trimethysilylation of magadiite and kenyaite. React. Solids 5, 167–175. Yanagisawa, T., Kuroda, K., Kato, C., 1988b. Organic derivatives of layered polysilicates II. Bull. Chem. Soc. Jpn. 61, 3743–3745.