Formation of Thin Films of Clay−Organic Complexes with an

Sep 15, 1996 - at an air-water interface. The clay-organic complexes were prepared by reacting synthetic saponite with ... (1) Gosh, P. K.; Bard, A. J...
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Langmuir 1996, 12, 5195-5201

5195

Formation of Thin Films of Clay-Organic Complexes with an Application as an Electrode Modifier Yuji Hotta, Masahiro Taniguchi, Keiichi Inukai,† and Akihiko Yamagishi* Division of Biological Science, Hokkaido University, Sapporo 060, Japan, and National Industrial Research Institute of Nagoya, Nagoya 462, Japan Received August 18, 1995. In Final Form: June 19, 1996X A thin film was formed by spreading a chloroform dispersion of a hydrophobic clay-organic complex at an air-water interface. The clay-organic complexes were prepared by reacting synthetic saponite with 3-(aminopropyl)dimethylethoxysilane and thereafter with alkanoyl chlorides. From the analysis of surface pressure-area curves, it was concluded that the film were composed of 1-3 layers of clay crystallites. The film was repeatedly transferred onto a glassy carbon electrode to build up 30 layers by the LangmuirBlodgett method. An electrode modified with the clay multilayer was found to be electroactive when it was soaked in an aqueous solution of [Ru(bpy)3]Cl2 (1 × 10-5 M) and Na2SO4 (0.05 M).

Introduction The smectite group of clay minerals is a unique layered material which exhibits ion-exchange properties and intercalation into an interlayer space, etc. but is usually electrochemically inactive. An attempt to prepare claymodified electrodes (CMEs) was reported for the first time by Bard et al.1 They showed that the metal complexes incorporated into a clay film were electrochemically active. Since the report, many researchers have reported on CMEs.2-14 A large proportion of the reports have involved studies of the behavior of adsorbed metal complexes and cationic dyes.15 Clay is known to form an oriented film when its aqueous dispersion is cast on a solid surface, but the thickness of the clay film cannot be controlled accurately. Recently we have reported that a thin film of clay is formed at an air-water interface when a clay is ion-exchanged by an alkyl amine.16 By using the Langmuir-Blodgett method, it is however possible to prepare a clay film whose thickness is controlled on a molecular scale. Construction of Langmuir-Blodgett clay-organic complex films, which are hexadecylammonium and dioctadecylammonium hectorite clay-organic complexes, has been reported very recently by Kotov et al.17 The main purposes of the present work are as follows: (I) to synthesize a clay-organic complex with cation-

exchange properties and (II) to prepare a monolayer of the clay-organic complex as an electrode modifier whose thickness is controlled on a molecular scale. As for the first purpose, clay-organic complexes have been examined as functional materials and composites of a hydrophobic nature.18 One way to synthesize clayorganic complexes is to derivatize the hydroxyl groups in a clay structure directly with organic reagents to introduce substituents such as phenyl,19 acetyl,20 and silyl groups (for example, trimethylsilane).21

Si-OH + RX f Si-O-R + HX Si-OH + ClSi(CH3)3 f Si-O-Si(CH3)3 + HCl Another way is to chlorinate hydroxyl groups in the clay structure and then react them with organic reagents.

Si-OH + SOCl2 f Si-Cl + SO2 + HCl Si-Cl + HR f Si-R + HCl Recently Choudary et al. reported new triphase catalysis from montmorillonite.22 This report indicated that clay hydroxyl groups treated with (3-aminopropyl)triethoxysilane or (3-chloropropyl)trimethoxysilane reacted with methyl iodide or pyridine or tributhylamine as below:

* To whom correspondence should be addressed. † National Industrial Research Institute of Nagoya. X Abstract published in Advance ACS Abstracts, September 15, 1996.

Si-OH + NH2(CH2)3Si(OEt)3 f Si-O-Si(OH)2(CH2)3NH2

(1) Gosh, P. K.; Bard, A. J. J. Am. Chem. Soc. 1983, 105, 5691. (2) Fitch, A. Clays, Clay Miner. 1990, 38 (4), 391. (3) Fitch, A.; Lee, S. A. J. Electroanal. Chem. 1993, 344, 45. (4) Itaya, K.; Bard, A. J. J. Phys. Chem. 1985, 89, 5565. (5) Ege, D.; Ghosh, P. K.; White, J. R.; Equey, J. F.; Bard, A. J. J. Am. Chem. Soc. 1985, 107, 5644. (6) Kaviratna, P. D. S.; Pinnavaia, T. J. J. Electroanal. Chem. 1992, 332, 135. (7) Lee, S. A.; Fitch, A. J. Phys. Chem. 1990, 94, 4998. (8) Fitch, A.; Asina, L. F.; Lee, S. A.; Kirsh, M. T. J. Phys. Chem. 1988, 92, 6665. (9) Rong, D.; Mallouk, T. E. Inorg. Chem. 1993, 32, 1454. (10) Gobi, K. V.; Ramaraj, R. J. Electroanal. Chem. 1994, 368, 77. (11) Labbe, P.; Brahimi, B.; Reverdy, G.; Mousty, C.; Blankespoor, R.; Gautier, A.; Degrand, C. J. Electroanal. Chem. 1994, 379, 103. (12) Fitch, A.; Krzysik, R. J. J. Electroanal. Chem. 1994, 39, 129. (13) Xiang, Y.; Villemure, G. J. Electroanal. Chem. 1994, 370, 53. (14) Mousty, C.; Therias, S.; Forano, C.; Besse, J. P. J. Electroanal. Chem. 1994, 374, 63. (15) Shen, B.; Pheng, T.; Wang, H. Electrochim. Acta 1994, 39, 527. (16) Inukai, K.; Hotta, Y.; Taniguchi, M.; Tomura, S.; Yamagishi, A. J. Chem. Soc., Chem. Commun. 1994, 959. (17) Kotov, N. A.; Meldrum, F. C.; Fendler, J. H. Langmuir 1994, 10, 3797.

Si-O-Si(OH)2(CH2)3NH2 + 3MeI f

S0743-7463(95)00695-0 CCC: $12.00

Si-O-Si(OH)2(CH2)3N+(Me)3I- + 2HI Si-OH + Cl(CH2)3Si(MeO)3 f Si-O-Si(OH)2(CH2)3Cl Si-O-Si(OH)2(CH2)3Cl + (Bu)3N f Si-O-Si(OH)2(CH2)3N+(Bu)3ClMotivated by the above results, we synthesized saponite(3-aminopropyl)dimethylethoxysilane-alkanoyl chlorides as clay-organic complexes. These materials were found (18) Biasci, L.; Aglietto, M.; Ruggeri, G.; Ciardelli, F. Polymer 1994, 35, 3296. (19) Spencer, W. F.; Gieseking, J. E. J. Phys. Chem. 1952, 56, 751. (20) Spencer, W. F.; Gieseking, J. E. J. Phys. Chem. 1952, 56, 751. (21) Lentz, C. W. Inorg. Chem. 1964, 3, 574.

© 1996 American Chemical Society

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Table 1. CECs of Saponite and Saponite-Organic Complex clay and clay-organic complex

CEC (mmol/100 g)

saponitea saponite-(3-aminopropyl)triethoxysilaneb saponite-(3-aminopropyl)triethoxysilane-decanoyl chloridec saponite-(3-aminopropyl)triethoxysilane-myristoyl chloridec saponite-(3-aminopropyl)triethoxysilane-stearoyl chloridec saponite-(3-aminopropyl)dimethylethoxysilaneb saponite-(3-aminopropyl)dimethylethoxysilane-myristoyl chloridec saponite-(3-aminopropyl)dimethylethoxysilane-stearoyl chloridec

68.8 47.2 39.7 28.2

57.6 61.8

a

Saponite (0.01 g) was dispersed in 50 mL of crystal violet solution (0.057 g in 100 mL of water-methanol (1/1) (v/v)). b As these sampls did not disperse in chloroform and pure water, the measurement was impossible. c Crystal violet solution (0.057 g in 100 mL of water-methanol (1/1) (v/v)) was added to the dispersed solution (5 mL) of this clay-organic complex (0.01 g/100 g) dispersed in chloroform until the whole solution attained 50 mL.

Figure 1. Surface pressure-area (π-A) isotherms when the volume (V) of a chloroform dispersion of a saponite-(3aminopropyl)dimethylethoxysilane-stearoyl chloride (SA3ADM-ST) (0.01 g/100 mL) spread on an air-water interface was changed: V ) 0.5 mL (a), 1.0 mL (b), 1.5 mL (c), 2.0 mL (d), 2.5 mL (e), 3.0 mL (f). The horizontal axis denotes the area of the trough (A), and the vertical axis denotes the surface pressure (π). The temperature of the trough was 20 °C. The compression rate was 64.3 cm2/min.

to be dispersible in chloroform. We studied the thin film formation of the product and the electrochemical behaviors of a modified electrode made by the Langmuir-Blodgett method. Experimental Section Materials. Synthetic saponite (Smecton) was purchased from Kunimine Ind. Co., Japan. The counterions of the clay were magnesium and sodium. The CEC was 80 mequiv/100 g. The average size of clay particles was 0.19 µm.23 (3-Aminopropyl)dimethylethoxysilane, triethylamine, and alkanoyl chlorides were used as received. All solvents were used as received. Synthetic Method. Preparation of Saponite-(3-aminopropyl)dimethylethoxysaline. Saponite was dried at 100 °C in an air bath. Dehydrated saponite (1.5 g) was mixed with toluene (100 mL) and (3-aminopropyl)dimethylethoxysilane (0.4 g, 2.5 mmol) in a drybox and refluxed under nitrogen atmosphere for 48 h. After the product was washed with toluene and diethyl (22) Choudary, B. M.; Subba, R. Y. V.; Prasad, B. P. Clays, Clay Mineral. 1991, 39 (3), 329. (23) Nakanura, Y.; Yamagishi, A.; Iwamoto, T.; Kaga, M. Clays, Clay Miner. 1988, 36, 530.

Figure 2. Surface pressure-area (π-A) isotherms when the volume (V) of a chloroform dispersion of a saponite-(3aminopropyl)dimethylethoxysilane-myristoyl chloride (SA3ADM-MY) (0.01 g/100 mL) spread on an air-water interface was changed: V ) 0.5 mL (a), 1.0 mL (b), 1.5 mL (c), 2.0 mL (d), 2.5 mL (e), 3.0 mL (f), 4.0 mL (g), 4.5 mL (h). The horizontal axis denotes the area of the trough (A), and the vertical axis denotes the surface pressure (π). The temperature of the trough was 20 °C. The compression rate was 64.3 cm2/min.

Figure 3. Relationship between an amount of the spread solution (V) and the critical area, Ai (see the text): O saponite(3-aminopropyl)dimethylethoxysilane-stearoyl chloride (SA3ADM-ST); b, saponite-(3-aminopropyl)dimethylethoxysilanemyristoyl chloride (SA-3ADM-MY). The broken (a) and solid lines (a) are ideal lines when SA-3ADM-ST and SA-3-ADMMY are formed as a monolayer, respectively. The broken (b and c) and solid lines (b and c) are ideal lines when SA-3ADMST and SA-3ADM-MY are formed as double (b) and triple (c) layers, respectively. ether by decantation, it was dried in air. The product was analyzed with an X-ray diffractometer (RIGAKU, Japan) and an IR spectrometer (JASCO A-3, Japan). The chemical formula of the present adducts was assumed to be [OSi(CH3)2(CH2)3NH2]xNa0.33Mg3[Si11/3Al1/3]O10(OH)2. The results of the elemental analyses were the following: C, 3.84; N, 0.82; H, 2.04. From the elemental analysis values, x was determined to be 0.3; [OSi(CH3)2(CH2)3NH2]0.3Na0.33Mg3[Si11/3Al1/3]O10(OH)2 (calc C, 4.2; N, 1.0; H, 1.5). Preparation of Saponite-(3-Aminopropyl)dimethylethoxysilane-Stearoyl Chloride (SA-3ADM-ST). Saponite-(3-aminopropyl)dimethylethoxysilane (0.5 g) was added to triethylamine (2 g), dichloromethane (150 mL), and stearoyl chloride (2 g). After stirring for several hours, the product was washed three times with a small amount of basic water. The mixture was filtered under vacuum, and the precipitate was washed with n-hexane and dried in air. The product was analyzed by XRD and IR. The chemical formula of the present adducts was assumed to be [OSi(CH 3 ) 2 (CH 2 ) 3 NHCO(CH 2 ) 1 6 CH 3 ] x Na 0 . 3 3 Mg 3 [Si 1 1 / 3 Al 1 / 3 ]O10(OH)2. The elemental analysis gave the following: C, 17.8;

Formation of Thin Films of Clay-Organic Complexes

Figure 4. Surface pressure-area (π-A) isotherms: (A) saponite-(3-aminopropyl)triethoxysilane-decanoyl chloride (SA-3AM-DE) (V ) 0.5, 1, 2, 3, 4, and 5 mL); (B) saponite(3-aminopropyl)triethoxysilane-myristoyl chloride (SA-3AMMY) (V ) 0.5, 1, 2, 2.5, 3, 4, and 5 mL); (C) saponite-(3aminopropyl)triethoxysilane-stearoyl chloride (SA-3AM-ST) (V ) 0.5, 1, 2, 2.5, 3, 3.5, 4, and 4.5 mL). The temperature of the trough was 20 °C. The compression rate was 64.3 cm2/min. N, 1.34; H, 3.69. From the elemental analysis values, x was determined to be 0.3; [OSi(CH3)2(CH2)3NHCO(CH2)16CH3]0.3Na0.33Mg3[Si11/3Al1/3]O10(OH)2 (calc C, 16.4; N, 0.8; H, 3.2). Saponite-(3-aminopropyl)dimethylethoxysilane-myristoyl chloride (SA-3ADM-MY) was prepared by a similar method. Elemental analysis: C, 20.47; N, 1.23; H, 4.08. From the elemental analysis values, x was determined to be 0.5; [OSi(CH 3 ) 2 (CH 2 ) 3 NHCO(CH 2 ) 1 2 CH 3 ] 0 . 5 Na 0 . 3 3 Mg 3 [Si 1 1 / 3 Al 1 / 3 ]O10(OH)2 (calc C, 20.5; N, 1.3; H, 3.9). The other clay-organic complexes were prepared in a similar way. Measurements. CEC. The cationic ion exchange capacity (CEC) of the clay-organic complex was determined as follows: SA-3ADM-ST (0.01 g) was dispersed in chloroform (100 mL). Crystal violet (0.057 g) was dissolved in 100 mL of watermethanol (1/1 (v/v)). The crystal violet solution was added to 5 mL of the dispersed solution until the whole solution attained

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Figure 5. Relationship between the critical area, Ai, and the spread volume (V): (A) saponite-(3-aminopropyl)triethoxysilane-decanoyl chloride (SA-3AM-DE); (B) saponite-(3-aminopropyl)triethoxysilane-myristoyl chloride (SA-AM-MY); (C) saponite-(3-aminopropyl)triethoxysilane-stearoyl chloride (SA-3AM-ST). (a) Ideal line when a clay-organic complex spread on an air-water interface forms as a monolayer. (b) Experimental curve. 50 mL. After being left overnight, it was centrifuged (3500 rpm, 60 min). From the absorbance of a supernatant solution at 590 nm, the amount of crystal violet remaining in the solution was determined. The CECs of the other synthetic saponites were determined similarly. Measurements of the Surface Pressure-Area (π-A) for the Clay-Organic Material. A clay-organic complex was dispersed in chloroform (0.01 g/100 mL). The surface pressure-area (πA) curve for the product was recorded with Langmuir troughs (Joyce-Lobel, U.K., and U.S.I., Japan). The maximum and minimum developing areas of their troughs were 1022.8 and 122.3 cm2 (with Joyce-Lobel, U.K.) or 135 and 15 cm2 (with U.S.I., Japan), respectively. The measurement was started 30 min after the dispersion was introduced onto the interface. The compres-

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Figure 6. Surface pressure-area (π-A) isotherms of saponite(3-aminopropyl)dimethylethoxysilane-stearoyl chloride (SA3ADM-ST) when the temperature of the subphase was 10 °C (a), 20 °C (b), and 30 °C (c). The compression rate was 10 mm/ min. Table 2. Transferred Ratio (%)a number of 2 4 6 8 10 12 14 16 18 20 layers transferred 117.9 96.4 92.9 85.7 82.1 85.7 78.6 75.0 64.3 60.7 ratio (%) a

The style transferred to the substrate was Z film type.

sion rates were 64.3 cm2/min (with Joyce-Lobel) and 10 cm2/min (with U.S.I.), respectively. To observe reproducibility, a π-A curve was recorded again 30 min after the first measurement was finished. Formation of an LB Film of a Clay-Organic Complex. A multilayer clay film was prepared by transferring a thin film at the air-water interface onto a substrate (a glassy carbon). The XRD pattern of an LB film was recorded under the conditions of 40 kV, 30 mA, and Cu KR (1.540 56 Å) with a scan rate of 0.05 deg/min. Clay-Modified Electrodes of SA-3ADM-ST. The electrochemical measurements were performed with a potentiostat (TOHO TECHNICAL RESEARCH, Japan), a function generator (HOKUTO DENKO HB-111, Japan), and an X-Y recorder. A three-electrode cell was used for electrochemical measurements. The working electrode was a clay-modified electrode, the counter electrode a Pt net, and the reference electrode a saturated calomel electrode (SCE). The glassy carbon electrode was polished with 0.3 µm alumina. Saponite-(3-aminopropyl)dimethylethoxysilane-alkanoyl chloride (SA-3ADM-ST) was dispersed in chloroform (0.01 g/100 mL). The clay in chloroform dispersion (2.5 mL) was developed at an air-water interface. The thin film was transferred onto the glassy carbon electrode 30 times at a surface pressure of 20 mN/m. The area of the working electrode was 1.68 cm2. The electrolyte contained 1.0 × 10-5 M Ru(bpy)3Cl2 in 0.05 M Na2SO4 at pH 6.1. Cyclic voltammograms (CV) were obtained from 0.8 to 1.3 V (vs SCE) at the sweep rates of 100 and 200 mV/s.

Results and Discussion CECs of Clay-Organic Complexes. One of the purposes of the present work was to prepare a hydrophobic clay-organic complex with cation-exchange properties. We therefore measured the cation-exchange capacity (CEC) of the materials produced by use of the adsorption of methylene blue cation as described in the Experimental Section. The results of the CEC measurements are summarized in Table 1. In the table, no CEC values are given for saponite-(3-aminopropyl)triethoxysilane and saponite-(3-aminopropyl)dimethylethoxysilane because they could not be dispersed in chloroform. Other clayorganic complexes obtained by reacting with silyl reagents and long alkanoyl chlorides (C10, C14, C18) could be dispersed in chloroform. As is shown in Table 1, the clayorganic materials have CECs that are smaller than that

Figure 7. X-ray diffraction patterns for a bare glassy carbon (a), for saponite cast on a glassy carbon (b), for saponite-(3aminopropyl)dimethylethoxysilane-stearoyl chloride (SA3ADM-ST) cast on a glassy carbon (c), and for SA-3ADM-ST transferred on a glassy carbon (20 layers) (d).

of the original clay. The CEC decreases with increasing length of a carbon chain. Surface Pressure-Area (π-A) Curves. The monolayer formation of a clay-organic complex at an air-water interface was examined by measuring the surface pressure-area curves at constant temperature. An appropriate volume of chloroform dispersion of the material was spread on the water surface. Figure 1 shows an example of π-A curves in the case of SA-3ADM-ST. The horizontal axis denotes the area of the trough (A), and the vertical axis, the surface pressure (π). The figure includes the results when the volume of a spread chloroform dispersion was changed from 0.5 to 3.0 mL. On decreasing the area, π remained zero until the area attained a level-off point, Ai. π rose from zero smoothly when the area was compressed further from Ai. It was saturated at about 20 mN/m, showing a plateau at an area of about 50 cm2. After passing the plateau, π rose rapidly until it attained about 50 mN/m. These π-A curves were obtained with good reproducibility when the experiments were repeated two times. Figure 2 shows

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ST and SA-3ADM-MY. It is seen that Ai increases nearly in proportion to V for the latter case, while the curve deviates appreciably from the straight line for the former case. The total surface area is theoretically calculated in the following way. Assuming that a clay microcrystallite orients its silicate sheet parallel to the air-water interface, the total area covered by a clay monolayer is equated to half of the total area of silicate sheets. On the basis of the chemical formula given in the Experimental Section, the formula masses for SA-3ADM-ST and SA-3ADM-MY are 505.6 and 557.2, respectively. The unit cell of the silicate sheet in saponite has the surface area of ab ) 5.3 × 9.2 Å2.24 The average size of a clay particle (ca. 200 nm2) is much bigger than that of an area occupied by alkyl chains (2 nm), so that the edge part of the clay-organic complex is neglected. If clay particles occupy an airwater interface with no pinhole, the area occupied by clayorganic complex particles is represented by the following equation:

X)

Figure 8. (a) Cyclic voltammogram on a bare glassy carbon electrode as a working electrode with 0.05 M Na2SO4 as an electrolyte. (b) Cyclic voltammogram on an electrode modified by a cast film of saponite-(3-aminopropyl)dimethylethoxysilane-stearoyl chloride (SA-3ADM-ST) with 0.05 M Na2SO4 as an electrolyte. The counter electrode was a Pt net. The reference electrode was a saturated calomel electrode (SCE). Scan rates were 50, 100, and 200 mV/s.

ab(10-8)2(6.02 × 1023)m M

where X (cm2) is the occupied area (cm2) of clay-organic complex particles on an interface, ab is the unit cell area of the silicate sheet of saponite, m (g) is the weight of clay-organic complex particles spread on an interface, and M is the formula mass of a unit cell. In this experiment, the concentration of the suspension of the clay-organic complexes was 0.01 g/100 mL. Therefore m (g) in the above equation is represented by the spread volume (V mL) as follows:

m ) (0.01/100)V Inserting these values into the above equation, the following equation is obtained:

X ) cV

Figure 9. Cyclic voltammogram on a bare glassy carbon electrode as a working electrode in an aqueous solution of [Ru(bpy)3]Cl2 (1 × 10-5 M) and Na2SO4 (0.05 M). The other conditions were the same as those in Figure 8.

the π-A curve for saponite-(3-aminopropyl)dimethylethoxysilane-myristoyl chloride (SA-3ADM-MY). In this case, the surface pressure started to rise at a critical area (Ai) and continued to increase until it attained about 40 mN/m. No plateau region was observed in contrast to the results of SA-3ADM-ST, although there was a slight inflection point seen at about 20 mN/m. In interpreting the above results, we assume that each clay particle is completely dispersed on an air-water interface when a chloroform dispersion is spread on a water surface. On compressing the surface area, the clay particles approach each other and get in contact at the area Ai. Under this situation, the surface pressure increases from zero due to the repulsive interactions between the hydrophobic chains at the edges of the particles. On the basis of this assumption, Ai is expected to be equal to the total surface area occupied by clay particles. Figure 3 shows the plots of Ai against the volume, V, for the cases of SA-3ADM-

with c ) 580.6 and 526.8 for the monolayers of SA3ADM-ST and SA-3ADM-MY, respectively. We assume that the whole area occupied by clay-organic complex particles at an interface (X) is equal to the critical area (Ai), as obtained in the π-A curves. On the basis of these theoretical predictions, the theoretical dependencies of Ai on V are shown by a broken line (a) (SA-3ADM-ST) and a solid line (b) (SA-3ADMMY) assuming that a film was composed of a single clay layer in Figure 3. The theoretical plots based on double (b) and triple (c) layers are shown by broken lines (SA3ADM-ST) and solid lines (SA-3ADM-MY), respectively. The experimental plots are shown by O (SA3ADM-ST) and b (SA-3ADM-MY), respectively. The experimental plot for SA-3ADM-ST falls in the region between lines a and b. The experimental plot for SA3ADM-MY falls in the region between lines b and c. Therefore we conclude that these clay-organic complexes formed an ordered thin film composed of 1-3 layers. When the clay-organic complexes were spread for V < 3 mL, the relationship with V (mL) and A (cm2) of the experimental plots for SA-3ADM-ST and SA-3ADM-MY is expressed by a straight line. The deviation from the straight line at V > 3 mL for SA-3ADM-ST may be ascribed to the formation of an aggregate of clay particles at higher surface concentration. (24) Brindley, G. W.; Brown, G. Crystal Structures of Clay Minerals and Their X-ray Identification; Mineralogical Society: London, 1980.

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Figure 10. Cyclic voltammogram on a clay-modified electrode that was made by the Langmuir-Blodgett method for saponite(3-aminopropyl)dimethylethoxysilane-stearoyl chloride (SA-3ADM-ST). The conditions were the same as those in Figure 9.

Parts a-c of Figure 4 show the π-A curves in the case of saponite-(3-aminopropyl)triethoxysilane-decanoyl chloride (SA-3AM-DE), saponite-(3-aminopropyl)triethoxysilane-myristoyl chloride (SA-3AM-MY), and saponite-(3-aminopropyl)triethoxysilane-stearoyl chloride (SA-3AM-ST), respectively. These π-A curves were obtained with good reproducibility when the experiments were repeated two times. The shapes of the π-A curves were different from that of SA-3ADM-ST. After the surface pressure started to rise from zero at the initial area of Ai, the surface pressure rose gently without the plateau region of SA-3ADM-ST. Parts a-c of Figure 5 show the plots of Ai against the volume, V, for the cases of SA-3AM-DE, SA-3AM-MY, and SA-3AM-ST, respectively. It is seen that these clay-organic complexes form a thin film when they are spread at an air-water interface at the lower amount of V. The plateau in the π-A curves for the case of SA3ADM-ST indicates the presence of a phase transition in the thin film with the increase of surface pressure. Curves a-c in Figure 6 represent the π-A curves at T ) 10, 20, and 30 °C, respectively With the increase of temperature, the surface pressure at the phase transition increased to 14.5, 17.5, and 21.0 mN/m at 10, 20, and 30 °C, respectively. In these cases, too, the first curves coincided well with the second ones. One possibility is that the clay particles change the orientation of the layer surfaces to attain higher stacking at the smaller area per layer. X-ray Analyses of a Multilayer Film. A multilayer film of SA-3ADM-ST was formed at the surface pressure just before a plateau region, as described in the Experimental Section. As is shown in Table 2, the thin film was transferred onto a substrate only for the upward dipping direction. It was therefore concluded that the thin film formed a LB film of Z type. From the dependence of the transfer ratio on the layer number, the clay film was controlled on a molecular scale within at least the initial

16 layers, but some disorder occurred on increasing the number of layers. Figure 7 shows the results of the XRD measurements for various samples: (a) for a bare glassy carbon substrate, (b) for a sample prepared by casting a water dispersion of saponite on a glassy carbon, (c) for a sample prepared by casting a chloroform dispersion of SA-3ADM-ST on a glassy carbon, and (d) for a sample prepared by depositing a thin film of SA-3ADM-ST to 20 layers on a glassy carbon carbon, respectively. No diffraction peak was observed for a bare glassy carbon (Figure 7a). The diffraction peak was observed at 2θ ) 6.82° for a saponite film, leading to the basal spacing of d(001) ) 12.95 Å. The diffraction peak of a cast film of SA-3ADM-ST was observed at 2θ ) 1.34° leading to d(001) ) 65.88 Å. The height of the interlayer space was estimated to be 56.4 Å by subtracting the thickness of a saponite layer (9.5 Å) from d(001). The value corresponded well to the length of the alkyl chain of 3ADM-ST. Therefore the expansion of the basal spacing was ascribed to the intercalation of the alkyl chain of 3ADM-ST. The diffraction peak of a multilayer film of SA-3ADM-ST was observed at 2θ ) 1.38°, leading to d(001) ) 64.0 Å. Thus even in the multilayer film, the alkyl chain of 3ADMST was present between the layers. Electrode Modified by Saponite-(3-Aminopropyl)dimethylethoxysilane-Stearoyl Chloride. The electrochemical behavior of an electrode modified by a multilayer film of SA-3ADM-ST was examined. Figure 8a is a cyclic voltammogram on a bare glassy carbon electrode as a working electrode with 0.05 M Na2SO4 as an electrolyte. Figure 8b is a cyclic voltammogram on an electrode modified by a cast film of SA-3ADM-ST with 0.05 M Na2SO4 as an electrolyte. SA-3ADM-ST caused no redox reaction at this potential range electrochemically. Figure 9 is a cyclic voltammogram on a bare glassy carbon electrode at the scan rate of 100 mV/s in an aqueous solution of [Ru(bpy)3]Cl2 (1 × 10-5 M) and Na2SO4 (0.05

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Figure 11. Relationship of cathodic current Ipc (µA) and time (h) at the scan rates of 100 mV/s (b) and 200 mV/s (O).

M). The redox peaks of Ru2+/3+ and Ru3+/2+ appeared at 1.03 and 0.98 V (vs SCE), respectively. These values nearly coincided with those reported for a homogeneous system.25 Figure 10 is a cyclic voltammogram at the scan rate of 100 mV/s for the same solution on an electrode modified by the SA-3ADM-ST multilayer film that was made by the Langmuir-Blodgett method. The redox peaks of Ru2+/3+ and Ru3+/2+ appeared at 1.00 and 0.98 V (vs SCE), respectively. Figure 11 shows the dependence of the cathodic peak current, Ipc (µA), on time (h) at the scan rates of 100 and 200 mV/s after the electrode was soaked in the solution. Ipc increased with time until it was saturated after about 6 h. The saturated value of Ipc was about two times the initial value. It is assumed that the Ru complexes had permeated and concentrated in the clay-organic complexes during this time. After 6 h, the amount of Ru complexes was saturated in the film of clayorganic complexes. Because SA-3ADM-ST did not show the redox peak (Figure 8b), it is assumed that the Ru complex was adsorbed to the clay and/or was intercalated between the layers of the clay-organic complex. Figure 12 shows the Ipc dependence on the scan rate ν at the clay-modified electrode after the electrode had been soaked in the solution for 10 h. A plot of Ipc vs the scan rate ν gave a straight line. This observation indicates that the charge transfer process at a clay-organic complex film was not coupled with diffusion but that the Ru complexes behaved (25) Gobi, K. V.; Ramaraj, R. J. Electroanal. Chem. 1994, 368, 77.

Figure 12. Dependence of cathodic current Ipc (µA) on scan rate ν (mV/s) at the clay-modified electrode that was made by the Langmuir-Blodgett method for saponite-(3-aminopropyl)dimethylethoxysilane-stearoyl chloride (SA-3ADM-ST). The electrode was soaked for 10 h before measurements. The conditions were the same as those in Figure 9.

as surface-confined species.26,27 These situations are in contrast with the results on an electrode modified with a cast film of clay. In the latter electrode, Ipc is reported to increase with ν1/2, indicating that Ru complexes incorporated in a clay film diffuse to attain a film substrate interface.1/2 Conclusion A clay-organic complex (saponite-(3-aminopropyl)dimethylethoxysilane-stearoyl chloride) is concluded to form a monolayer or double-layer film at an air-water interface from the results of π-A curves. Cyclic voltammograms on an electrode modified with the controlled multilayer film on the clay-organic complex indicated that the Ru complexes were concentrated in a clay-organic complex and behaved as surface-confined species. Acknowledgment. We thank the instrumental analysis center at Hokkaido University for the elemental analysis. LA950695V (26) Oyama, N.; Anson, F. C. J. Electrochem. Soc. 1980, 127, 248. (27) Hurrel, H. C.; Mogstad, A. L.; Usifer, D. A.; Potts, K. T.; Abruna, H. D. Inorg. Chem. 1989, 28, 1080.