Surfactant-Encapsulated Polyoxoanion: Structural Characterization of

Langmuir , 2002, 18 (16), pp 6398–6403 ... Publication Date (Web): July 12, 2002 ... The one-monolayer LB film of SEC-1 possesses a homogeneous and ...
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Langmuir 2002, 18, 6398-6403

Surfactant-Encapsulated Polyoxoanion: Structural Characterization of Its Langmuir Films and Langmuir-Blodgett Films Weifeng Bu,† Hailin Fan,‡ Lixin Wu,*,† Xueliang Hou,† Changwen Hu,‡ Gang Zhang,† and Xi Zhang† Key Laboratory for Supramolecular Structure and Spectroscopy of Ministry of Education, Jilin University, 130023, Changchun, People’s Republic of China, and Department of Chemistry, Northeast Normal University, 130024, Changchun, People’s Republic of China Received January 28, 2002. In Final Form: April 25, 2002 We synthesized a surfactant-encapsulated polyoxoanion, (DODA)16As4W30Cu4O112‚114H2O (denoted SEC-1), which possesses a hydrophobic dimethyl dioctadecylammonium (DODA) shell and an encapsulated hydrophilic polyoxoanion core in organic phases. The novel organic/inorganic composite can form stable Langmuir monolayers, but it is significantly distorted when spread onto an air/water interface although the general core-shell structure seems to remain. Langmuir-Blodgett (LB) films of SEC-1 are readily prepared by transferring the monolayers onto substrates with a transfer ratio near 1. It has been proposed that there exist well-orientated alkyl chains and highly defined layer structure in LB films. Fourier transformation infrared spectroscopy at different temperatures indicates that two obvious phase transitions appear near 34 and 47 °C for multilayer LB films of SEC-1. The polyoxoanion can be organized to some extent within LB films with its long axis parallel to the substrate by comparing the IR transmission spectrum with the reflection-absorption spectrum. The one-monolayer LB film of SEC-1 possesses a homogeneous and flat surface morphology, and a water contact angle of 104 ( 3° for odd-layer LB films shows a highly hydrophobic surface.

Introduction Supramolecular self-organization as a controllable technique for artificial supramolecular architecture at the molecular and/or nanosized level is showing fascinating perspectives in the field of molecular materials design, because organic and inorganic components can be facilely assembled together.1-3 Polyoxoanions are intriguing building blocks to form functional materials because they possess both well-defined molecular weight and nanostructure and furthermore display a lot of interesting conducting, magnetic, electronic, photonic, and catalytic properties.4,5 V. Luca and J. M. Hook studied the structure of a novel hexagonal mesostructured vanadium oxidesurfactant composite by a variety of spectroscopic characterization techniques and proposed a formation mechanism in which the vanadium mesophase is generated by cooperative formation of micellar entities and higher vanadate oligomers.6 By taking advantage of the electrostatic interaction between the heteropolytungstates of the Keggin type and a monolayer of dimethyldioctadecylammonium bromide (DODA‚Br) spread on a water subphase, E. Coronado and C. Mingotaud reported the fabrication of organic/inorganic hybrid Langmuir-Blodgett (LB) films, which show interesting magnetic properties.7-10 D. G. Kurth and D. Volkmer displayed for the first time * To whom correspondence should be addressed. Fax: (+86)431-8922331-3684. E-mail: [email protected]. † Jilin University. ‡ Northeast Normal University.

(1) Ringsdorf, H.; Schlarb, B.; Venzmer, J. Angew. Chem., Int. Ed. Engl. 1988, 27, 114. (2) Vo¨gtle, F. Supramolecular Chemistry; Wiley: New York, 1991. (3) Lehn, J. M. Supramolecular Chemistry; VCH: New York, 1995. (4) Wang, E. B.; Hu, C. W.; Xu, L. Introduction to polyoxometalate Chemistry; Chemical Industry Press: Beijing, 1998. (5) Chem. Rev. 1998, 98, 1. The entire issue is devoted to polyoxometalates. (6) Luca, V.; Hook, J. M. Chem. Mater. 1997, 9, 2731.

a facile and simple approach to modify the chemical properties of polyoxoanion cluster surfaces, which applies the replacement of the charge-balancing countercations from the outer sphere of the hydrated polyoxoanion with cationic surfactants.11-13 That is, the inorganic clusters are enwrapped with a water-insoluble shell composed of organic molecules so that the surfactant-encapsulated clusters (SECs) are soluble in organic media.11-17 These new supramolecular assemblies developed a new approach toward nanodevices. From the versatile investigations that were in excellent agreement with one another, a globular arrangement model was proposed, and one can clearly learn that these SECs can form stable Langmuir monolayers and will keep their structure when they are spread at an air/water interface.11-13 By employing similar methods, we successfully prepared an organic/inorganic composite (DODA)16As4W30Cu4O112‚ 114H2O, SEC-1, bearing a core-shell ellipsoid in an (7) Chemente-Le´on, M.; Agricole, B.; Mingotaud, C.; Go´mez-Garcı´a, C. J.; Coronado, E.; Delhaes, P. Langmuir 1997, 13, 2340. (8) Chemente-Le´on, M.; Agricole, B.; Mingotaud, C.; Go´mez-Garcı´a, C. J.; Coronado, E.; Delhaes, P. Angew. Chem., Int. Ed. Engl. 1997, 36, 1114. (9) Chemente-Le´on, M.; Mingotaud, C.; Go´mez-Garcı´a, C. J.; Coronado, E. Thin Solid Films 1998, 329, 439. (10) Coronado, E.; Mingotaud, C. Adv. Mater. 1999, 11, 869. (11) Kurth, D. G.; Lehmann, P.; Volkmer, D.; Co¨lfen, H.; Koop, M. J.; Mu¨ller, A.; Du Chesne, A. Chem.sEur. J. 2000, 6, 385. (12) Volkmer, D.; Du Chesne, A.; Kurth, D. G.; Schnablegger, H.; Lehmann, P.; Koop, M. J.; Mu¨ller, A. J. Am. Chem. Soc. 2000, 122, 1995. (13) Kurth, D. G.; Lehmann, P.; Volkmer, D.; Mu¨ller, A.; Schwahn, D. J. Chem. Soc., Dalton Trans. 2000, 3989. (14) Kimizuka, N.; Lee, S. H.; Kunitake, T. Angew. Chem., Int. Ed. 2000, 39, 389. (15) Kimizuka, N.; Oda, N.; Kunitake, T. Inorg. Chem. 2000, 39, 2684. (16) Kurth, D. G.; Lehmann, P.; Schu¨tte, M. Proc. Natl. Acad. Sci. U.S.A. 2000, 97, 5704. (17) Kurth, D. G.; Lehmann, P.; Lesser, C. J. Chem. Soc., Chem. Commun. 2000, 949.

10.1021/la020085c CCC: $22.00 © 2002 American Chemical Society Published on Web 07/12/2002

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organic solvent as schematically drawn in Figure 1 (inset). The [Cu4(H2O)2(As2W15O56)2]16- anion has a similar structure to that proposed earlier for [Cu4(H2O)2(P2W15O56)2]16-, a sandwich-type heteropolyoxoanion. Each of the two R-As2W15O5612- units share seven oxygen atoms, including the one on As, with a central set of four edge-sharing CuO6 octahedra. This heteropolyoxoanion possesses C2h symmetry. Similar structures will result in similar properties, for example, magnetic properties, catalytic activity, antiAIDS activity, and so forth.18 The encapsulated cluster has improved its existing state and may show tailored properties, compared with the naked heteropolyoxoanion. The related research of their properties is in progress. Herein, we show that this complex can form a stable monolayer, and the ellipsoid is significantly distorted at air/water interface although it is believed that the general core-shell structure seems to remain. This distortion is supported by various experimental investigations, including surface pressure-area isotherms, UV-vis absorption spectra, Fourier transformation infrared (FT-IR) spectra, wide-angle X-ray diffraction study, contact angle measurement, and friction-based scanning force microscopy (SFM) observations, and its LB films are also characterized in detail by these measurements.

the water subphase according to the consistent IR and UV-vis spectra between the polyoxoanion and LB films of SEC-1. Langmuir Monolayer and Deposition of LB Films. A Nima 622D Langmuir trough (double troughs) with a Wilhelmy balance was employed for the surface pressure-area (π-a) isotherm measurements and LB film preparation. A 60 µL quantity of chloroform solution (0.899 mg/mL) of SEC-1 was spread onto a distilled deionized water subphase (pH 6.3). The temperature of the subphase was kept at 18 °C. After evaporation of the solvent, the monolayer was compressed at a constant barrier rate of 20 cm2/min up to the surface pressure of 20 mN/m. After the films were allowed to equilibrate for 30 min at the given pressure, the condensed monolayers were transferred by the vertical dipping method onto CaF2 substrates for infrared and polarized infrared measurements, onto gold substrates for reflection-absorption IR measurements, onto quartz substrates for wide-angle X-ray diffraction and UV-vis absorption measurements, and onto freshly cleaved mica substrates for SFM measurements. Y-Type even-layer LB films were built up by passing the substrates with odd-layer LB films through air and down into trough A and then underwater and up from trough B. X-Type one-monolayer LB films were prepared as follows: a clean substrate was passed down from air and into trough A and then underwater and up from trough B.19 During these procedures, the samples were spread onto a water subphase in trough A, and only pure water is in trough B. In all cases, the dipping speed is 0.6 cm/min. Spectroscopic Measurements. Fourier transformation infrared measurements were performed on a Bruker IFS66V FTIR spectrometer equipped with a MCT detector. The spectra were recorded with a resolution of 4 cm-1. Polarized IR measurements were employed at the incident angles of 0° and 60°, together with a polarizer. In all cases, the spectra were the accumulation of 1500 scans. The IR reflection-absorption (RA) spectra were measured with the incidence angle of 80°. UV-vis absorption spectra of the LB films of SEC-1 were recorded on a Shimadzu UV-3100 spectrometer, and the slit width was set to 2 nm. General Measurements. The multilayer LB films of SEC-1 were characterized by wide-angle X-ray diffraction which was carried out on a Rigaku X-ray diffractometer (D/max rA, using Cu KR radiation of a wavelength of 1.542 Å). SFM images of LB films of SEC-1 and friction loops were obtained with commercial instruments (Digital Instruments, Nanoscope III, Dimension 3000 and multimode) which were operated in contact mode, under ambient conditions. Static contact angles for water were measured by a drop shape analysis system DSA20 MK2 (KRu¨SS), and every data point was collected reproducibly five times.

Experimental Section

Results and Discussion

Figure 1. Surface pressure area (π-a) isotherm of SEC-1 at the temperature of 18 °C. Inset: a schematic drawing of the chemical structure of SEC-1 that shows a core-shell structure in organic solvents.

Synthesis of SEC-1. A polyoxoanion Na16As4W30Cu4O112‚ (H2O)ca.200 (1) freshly prepared according to previous procedures18 was dissolved in aqueous solution and then was stirred with a chloroform solution of surfactant DODA‚Br according to the literature.11,12 The initial molar ratio of DODA‚Br to 1 was controlled at 16:1. The aqueous phase becoming colorless showed that the polyoxoanion was completely extracted and transferred into chloroform solution. The organic phase was separated, and SEC-1 was obtained by evaporating the chloroform to dryness. Then the sample was placed into a vacuum desiccator until the weight remained constant. IR (KBr): ν ) 2955, 2919, 2872, 2850, 1468, 943, 886, 851, 818, 773, 719, 524 cm-1. Elemental anal. calcd (%) for C608H1508N16As4W30Cu4O226 (18732): C, 38.98; H, 8.11; N, 1.20. Found: C, 38.98; H, 8.28; N, 1.24. The X-ray photoelectron spectroscopy (XPS) measurement proved the existence of oxygen and metal As, W, and Cu in the composite SEC-1. The fact that the polyoxoanion 1 is only soluble in water but the SEC-1 not soluble in water and it just dissolves in organic media such as chloroform, benzene, and toluene proved that the encapsulation with DODA‚Br was successful. So, based on the exact elemental analysis and the detailed characterization of the polyoxoanion, SEC-1 should correspond to the formula (DODA)16As4W30Cu4O112‚114H2O. This complex is stable in air, and the decomposition of the polyoxoanion is little process on (18) Bi, L. H.; Wang, E. B.; Peng, J.; Huang, R. D.; Wu, L.; Hu, C. W. Inorg. Chem. 2000, 39, 671 and references therein.

Langmuir Isotherms. The Langmuir technique19,20 offers a facile and elegant strategy to build up well-defined two-dimensional arrangements of SECs. The surface pressure-area (π-a) isotherm of SEC-1 on the subphase of pure water is shown in Figure 1. The isotherms are reproducible and display a sharp increasing with surface pressure upon compression until the film collapses at 63 mN/m, which is indicative of stable monolayer formation. The stability of the monolayer is further supported by the fact that the molecular area hardly shows any change at constant pressures (20 and 30 mN/m) with time. The monolayer of SEC-1 has a very typical condensed state, and its limiting area is determined to be 7.96 nm2 interpolated by extending the condensed state to zero pressure. DODA and the polyoxoanion have volumes of 1.0321 and 4.02 nm3,18 respectively, according to their crystal cell volumes. Sixteen DODAs occupy a volume of 16.48 nm3. Therefore, the total volume of SEC-1 is (19) Petty, M. C. Langmuir-Blodgette Films An Introduction; Cambridge Press: 1996. (20) Ulmann, A. An Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-Assembly; Academic Press: New York, 1991. (21) Okuyama, K.; Soboi, Y.; Iijima, N.; Hirabayashi, K.; Kunitake, T.; Kajiyama, T. Bull. Chem. Soc. Jpn. 1988, 61, 1485.

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Figure 2. UV-vis absorption spectra of multilayer LB films of SEC-1 transferred on quartz. The number of layers deposited is increased from 1 to 9 (bottom to top). Inset: the absorption intensity at 210 nm (triangle) and 282 nm (square) of LB films of SEC-1 as a function of the number of layers.

estimated to be 20.5 nm3, which corresponds to an ideal sphere with a diameter of 3.39 nm. With this diameter, the molecular area is estimated to be 9.06 nm2 approximately. This value is significantly larger than the limiting area of the encapsulated SEC-1 obtained from the Langmuir isotherms. There are two possibilities to explain this: one is that there may exist interdigitation of the alkyl chains between SEC-1’s, and the other is that SEC-1 is strongly distorted, or DODAs partially dissociate from the polyoxoanion.22 The latter should be attributed to the unfavorable energy when the hydrophobic SEC-1 was spread onto the air/water interface. The Langmuir monolayers are readily transferable as LB films onto solid substrates with a transfer ratio near 1 by Y-type deposition. However, when the SEC-1 monolayers are deposited as X-type LB films, the transfer ratio onto solid substrates is very poor, lower than 0.2. This result could not be suitably explained with the regular ellipsoid model, in which the transfer ratios should be consistent whatever deposition modes are applied, instead of showing that much difference. The large differences of transfer ratio between the two deposition modes also can be observed at low surface pressure, for instance, 0.1 mN/m. UV-Vis Spectra. UV-vis absorption spectra are applied to monitor the fabrication process of the LB films of SEC-1. Figure 2 depicts the UV-vis absorption spectra of Y-type LB multilayer films of SEC-1 deposited onto quartz substrates from one to nine layers (bottom to top). The band at 210 nm should be assigned to the charge transfer of Od f W, and the band at 282 nm should be due to that of Oc/Ob f W,18,23 which shows a slight shift, indicating the identical electronic structures between the polyoxoanion cluster and SEC-1. The slight shift should be attributed to the different environments (the water solution and LB films). The linear increase of absorbance at 210 nm (square) and 282 nm (triangle) with the increasing number of layers displays a uniform deposition with an identical transfer ratio in each dipping cycle. The absorbance at 210 and 282 nm of the one-monolayer LB film by Y-type deposition is about 0.018 and 0.005, (22) SEC-1 is completely immiscible in water and cannot undergo a structural reorganization at the air/water interface. Complete dissociation from the polyoxoanion leads to the geometric mismatch between the monolayer, which has a limiting area of 7.96 nm2, while the counterion has an area of about 3 nm2. This means that almost 2/3 of the DODA monolayer has no charge-balancing counterions. This will be confirmed by the water contact angle measurements of odd-monolayer and even-monolayer LB films. (23) Oa, each shared by AsO4 and three octahedral from the same W3O13. Ob, W-Ob-W bridges between different W3O13 groups. Oc, W-Oc-W bridges within same W3O13 group. Od, terminal oxygen atom.

Bu et al.

Figure 3. FT-IR spectrum of a multilayer LB film of SEC-1 deposited on a CaF2 plate. Inset: the growth of the absorbance at 2850 cm-1 (triangle) and 2918 cm-1 (square) of LB films as a function of the number of layers.

respectively. However, the absorption intensity of the onemonolayer LB film of SEC-1 deposited in the X-type mode is much weaker, lower than 0.002. This is in agreement with the aforementioned result that the transfer ratio of upward monolayer deposition is much larger than that of downward monolayer deposition onto solid plates, further supporting the strongly asymmetric structures of SEC-1 at the air/water interface. Fourier Transformation Infrared Spectra. Figure 3 depicts the FT-IR transmission spectrum of a multilayer LB film of SEC-1. Bands at 2918 and 2850 cm-1 are assigned to CH2 antisymmetric and symmetric stretching modes of DODA alkyl chains, respectively. The frequencies of the CH2 stretching bands are sensitive to the conformation of a hydrocarbon chain; low frequencies of the bands are characteristic of a highly ordered alkyl tail, while their upward shifts are indicative of the increase in conformational disorder, that is, gauche conformers, in the hydrocarbon chain.24-27 The fact that the CH2 stretching bands appear at 2918 and 2950 cm-1 in the infrared spectrum of Figure 3 suggests that the DODA alkyl chains of SEC-1 are well-ordered in both the mono- and multilayer LB films. IR linear dichroism was carried out by the polarized spectra to calculate the usual dichroic ratio R: R ) A|(i ) 60°)/A|(i ) 0°), where A(i) is the absorption coefficient and i denotes the angle between the planes of the LB film and the IR light electric vector. This ratio R is related to the degree of anisotropy out of the substrate plane and enables the evaluation of the angle Φ between the normal to the substrate and the dipole moment of a particular vibration.28 According to this equation, the average tilt angle of the DODA alkyl chains in the SEC-1 LB films is estimated to be close to 35°. The polyoxoanion is peanut-shaped and possesses the long to short axis ratio of 2:1. And the encapsulated SEC-1 is also an elongated ball as shown in Figure 1, which can reasonably explain the well-ordered alkyl chains and their mean orientation. Figure 3 (inset) presents a plot of the FT-IR transmission absorbance versus the number of monolayers of LB films of SEC-1. From the plot, we can clearly see the linear increase of the absorbance with increase of the number of layers, strongly suggesting that film growth is a uniform (24) Umemura, J.; Cameron, D. G.; Mantsch, H. H. Biochim. Biophys. Acta 1980, 602, 32. (25) Sapper, H.; Cameron, D. G.; Mantsch, H. H. Can. J. Chem. 1981, 59, 2543. (26) Maoz, R.; Sagiv, J. J. Colloid Interface Sci. 1984, 100, 465. (27) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (28) Vandevyver, M.; Barrau, A.; Raudel-Teixier, A.; Maillard, P.; Gianotti, C. J. Colloid Interface Sci. 1982, 85, 571.

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Figure 4. Transmission FT-IR spectrum (A) and reflectionabsorption IR spectrum (B) of 21-monolayer LB films of SEC-1 deposited on CaF2 and gold plates, respectively.

process. This result is in excellent agreement with the constant transfer ratio in each dipping cycle for both directions. This result is consistent with the conclusion exhibited in Figure 2 (inset). The peaks at 2954 and 1467 cm-1 should be attributed to the CH3 antisymmetric stretching mode and the CH2 deformation mode, respectively. The band at 1379 cm-1 is due to CH3 deformation. The strong bands are observed below 1000 cm-1 for the polyoxoanion vibrations.18 The assignments are as follows: 945 cm-1, W-Od asymmetric stretching; 883 cm-1, W-Ob-W asymmetric stretching; and 852 cm-1, As-Oa asymmetric stretching. The intensity of the W-Od asymmetric stretching mode is much stronger in the RA spectrum than in the IR transmission spectrum (Figure 4). According to the surface selection rule in IR-RA spectroscopy, vibrational modes with their transition moments perpendicular to surface are enhanced in a RA spectrum.29,30 The contrast of the W-Od asymmetric stretching mode between the transmission and RA spectra suggests that the transition moment of the W-Od asymmetric stretching band should be perpendicular to the substrate. The W-Od bond is vertical to the long-axis direction of the polyoxoanion from its crystal structure, and thus the long-axis orientation of the polyoxoanion seems parallel to the substrate. This result could be supported by X-ray diffraction of the LB film of SEC-1. The bands in the IR-RA spectrum shift slightly in comparison to those in the transmission spectrum: 883893 cm-1. The peak at 852 cm-1 disappeared in the RA spectrum. Kurth and Bein proposed that the anomalous dispersion of the sample layer contributes significantly to reflectance of the sample-substrate interface, and therefore peak positions in reflection spectra do not always correlate with their transmission reference data, because of the absence of those optical effects in the latter.31 The missing 852 cm-1 band in the RA-IR spectrum may be attributed to those optical effects. Temperature-dependent infrared transmission spectroscopy is a useful tool to study the phase transition of LB films. The bands at 2918 and 2850 cm-1 associated with CH2 stretching vibrational modes vary greatly with increasing temperature. To monitor the temperaturedependent spectral changes in the CH stretching region, the frequencies and bandwidths of the CH2 asymmetrical and symmetrical stretching band are plotted against temperature for a 17-layer LB film. The results are presented in Figure 5. Two clear spectral changes appear around 34 and 47 °C. The first transition may imply the (29) Greenler, R. G. J. Chem. Phys. 1966, 44, 310. (30) Chollet, P. A.; Messier, J.; Rosilio, C. J. Chem. Phys. 1976, 64, 1042. (31) Kurth, D. G.; Bein, T. Langmuir 1995, 11, 578.

Figure 5. Temperature dependence of the frequency and bandwidth at half the peak height of a CH2 antisymmetric stretching band (A) and a CH2 symmetric stretching band (B).

conversion of the hydrocarbon chain from one ordered state to another. The second transition should be due to an ordered to a disordered phase. The results above are in good agreement with those of the DODA‚Br aqueous solution at high concentration, which may correspond to an elongated vesicle shape.32 Synthetic bilayer membranes can be immobilized as molecular bilayer films by a simple casting.33,34 Kunitake et al. investigated the cast films of polymer-surfactant complexes (PSCs) such as DODA/PSS (poly(styrene sulfonate)) and clearly suggested that aqueous bilayers are immobilized in the form of polyion complexes with retention of fundamental bilayer characteristics such as phase transition and so on.35-38 The phase transition temperatures (Tm) reflect the extent of van der Waals interactions, which is dependent on the chain-packing density. Thus, the agreement among the Tm values of the DODA‚Br bilayer, the DODA/PSS complex, and the multilayer LB film of SEC-1 suggests that these orderdisorder processes are structurally related. X-ray Diffraction. The wide-angle X-ray diffraction pattern of a 15-layer LB film of SEC-1 is exhibited in Figure 6, in which one can obviously see three Bragg peaks. The three peaks can be clearly assigned to (001), (002), and (003) diffraction peaks, indicating that the layer structure of the LB films of SEC-1 is highly defined. From these data, the periodicity calculated by the Bragg (32) Blandamer, M. J.; Briggs, B.; Cullis, P. M.; Green, J. A.; Waters, M.; Soldl, G.; Engberts, J. B. F. N.; Hoekstra, D. J. Chem. Soc., Faraday Trans. 1992, 88, 3431. (33) Kunitake, T.; Okahata, Y.; Ando, R.; Kunitake, T. Ber. BunsenGes. Phys. Chem. 1981, 85, 789. (34) Kunitake, T.; Kimizuka, T.; Higashi, N.; Nakashima, N. J. Am. Chem. Soc. 1984, 106, 1978. (35) Kunitake, T.; Tsuge, A.; Nakashima, N. Chem. Lett. 1984, 1783. (36) Nakashima, N.; Eda, H.; Kunitake, M.; Manabe, O.; Nakano, K. J. Chem. Soc., Chem. Commun. 1990, 443. (37) Toko, K.; Nakashima, N.; Iiyama, S.; Yamafuji, K.; Kunitake, T. Chem. Lett. 1986, 1375. (38) Taguchi, K.; Yano, S.; Hiratani, K. Minoura, N.; Okahata, Y. Macromolecules 1988, 21, 3336.

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Figure 6. Wide-angle X-ray diffraction pattern of a 15monolayer LB film of SEC-1.

equation is close to 5.1 nm.39 As the long-axis orientation of the polyoxoanion was deduced to be parallel to the substrate based on RA-IR spectra, considering the diameter18 of the short axis of the polyoxoanion, the mean tilt angle, and the length (2.25 nm)7,8 of the DODA alkyl chain, the layer spacing can be estimated to be 5.03 nm. This value is in good agreement with the experimental result. Scanning Force Microscopy. A map of surface composition, for example, can be constructed if there are differences in topography that can be directly correlated with compositional and/or other structural expectations.40,41 The friction force measurements are performed with SFM so that a graph of torsion with respect to the movement of the sample when scanned against the major axis of the cantilever results in a “friction loop”. Parts a and b of Figure 7 show friction loops for one-monolayer and two-monolayer LB films, respectively. Obviously, both traces and retraces in the two friction loops are almost horizontal, which is indicative of the uniform surface compositions of both the one-monolayer and two-monolayer LB films, respectively. The magnitude of the traceretrace plot of the one-monolayer LB film of SEC-1 is smaller than 4 times that of the two-monolayer LB of SEC-1, demonstrating that the surface of the onemonolayer LB film possesses more lubricous surface properties than that of the two-monolayer LB film. This result also suggests an asymmetric structure forming at the air/water interface. An SFM image of the monolayer LB film of SEC-1 is presented in Figure 7c. One can clearly see that the surface morphology is very smooth and homogeneous. Contact Angle. The static contact angle of the oddmonolayer LB films of SEC-1 for water is 104 ( 3°, which shows a highly hydrophobic surface. The even-monolayer LB films also possess a water contact angle of 93 ( 3°. Some relative data are displayed in Table 1. This means that the encapsulated structure of SEC-1 still remains and demonstrates how efficiently the DODA shell encapsulates the internal hydrophilic polyoxoanion core. The wetting difference between the odd- and even-layer LB films is attributed to the strong distortion of SEC-1 at the air/water interface. (39) Both powder and cast film of SEC-1 by X-ray diffraction give a Bragg peak at 21.3°, corresponding to the distance of 0.42 nm between the alkyl chains. This means that the alkyl chains are close packed. (40) Ogletree, D. F.; Carpick, R. W.; Salmeron, M. Rev. Sci. Instrum. 1996, 67, 3298. (41) Takano, H.; Kenseth, J. R.; Wong, S. S.; O’Brien, J. C.; Porter, M. D. Chem. Rev. 1999, 99, 2845.

Figure 7. SFM friction loops of (a) one-monolayer and (b) twomonolayer LB films of SEC-1 Y-deposited onto freshly cleaved mica. The top line corresponds to the trace, and the bottom line corresponds to the retrace. (c) Atomic force microscope topographical image (1.25 µm × 1.25 µm) of Y-type monolayer LB film deposition on mica. Table 1. Static Contact Angles for Water as a Function of the Layer Number of LB Films of SEC-1 layer number 1

2

3

4

5

6

contact angle 104 ( 3 92 ( 3 105 ( 3 93 ( 3 104 ( 3 93 ( 3 (deg)

Conclusion Surfactant-encapsulated clusters based on a polyoxoanion cluster modified with surfactant with long hydrophobic chains present a novel class of organic/ inorganic composites and introduce new perspectives toward functional materials.11-13 The main driving force for this encapsulation is electrostatic interaction between the DODA ammonium and the polyoxoanions, which is also considered as a general approach for fabricating organic/inorganic hybrid materials.

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Stable Langmuir monolayers of SEC-1 can be formed and readily deposited onto substrates with a constant transfer ratio near 1. SEC-1 is strongly distorted and indicates an asymmetric structure at the air/water interface, which can be well supported by Langmuir isotherms, spectroscopic measurements, SFM measurements, and contact angle experiments. The contact angle measurements indicate that the encapsulated structure still remains. The LB films of SEC-1 possess highly defined layer structures and well-ordered DODA alkyl chains with a mean tilt angle of 35°. The long-axis orientation of the polyoxoanion seems parallel to the substrate by comparing the IR transmission spectrum with the RA spectrum. The variable-temperature FT-IR spectrum shows that two sharp phase transitions appear at 34 and 47 °C for multilayer LB films of SEC-1. The one-monolayer LB film

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of SEC-1 was directly imaged by SFM, which exhibited a flat and homogeneous surface morphology of the LB films. Acknowledgment. This work is supported from a Key Project of the National Natural Science Foundation of China (29992590-5), the Major State Basic Research Development Program (G200078102), and the Backbone Teacher Plan of the Ministry of Education of China. The authors thank Mr. Peng Miao from the Department of Chemistry of Jilin University for his help in the computer simulation. Thanks are also given to one of the reviewers for his help in fitting the molecular area of SEC-1. LA020085C