Inorganic Superlattices: Keggin

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Langmuir 1997, 13, 2340-2347

Toward New Organic/Inorganic Superlattices: Keggin Polyoxometalates in Langmuir and Langmuir-Blodgett Films M. Clemente-Leoń,† B. Agricole,‡ C. Mingotaud,*,‡ C. J. Go´mez-Garcıá,† E. Coronado,† and P. Delhaes‡ Centre de Recherche Paul PascalsCNRS, Avenue A. Schweitzer, F-33600 Pessac, France, and Departamento de Quı´mica Inorga´ nica, Facultad de Quı´mica, Universidad de Valencia, 46100 Burjassot, Spain Received June 12, 1996. In Final Form: December 30, 1996X The effect of Keggin heteropolyoxotungstates (XW12O40n- with X ) H2, P, Si, B or Co) on Langmuir films has been studied for monolayers of DODA (dimethyldioctadecylammonium) and DPPC (1,2-dipalmitoylsn-glycero-3-phosphorylcholine). Marked modifications of the compression isotherms have been observed when the Keggin anions were dissolved in the subphase: this demonstrates that the polyanions interact with the monolayer. Langmuir-Blodgett (LB) films have been readily obtained from these systems (even with DPPC) for a particular range in polyanion concentration. X-ray diffraction and infrared dichroism experiments have shown a well-defined lamellar structure for these built-up films as well as the presence within the LB films of polyoxometalates organized in monolayers. Control of the Keggin polyanion amount in the multilayers is made possible by mixing DODA with a negatively charged lipid, which modifies the global electrical charge of the Langmuir film. Such an organic-inorganic system leads to new ultrathin materials having various properties related to the selected polyanions.

Introduction Polyoxometalate anions are a distinctive class of inorganic compounds known for well over a century. Thanks to their topological and electronic versatilities, they are useful in fields as diverse as catalysis, biology, medicine and material science.1,2 Their structures can be depicted as molecular fragments of close-packed metal oxides of formula XaMbOcn- (M ) Mo, W, V, ...; X ) P, Si, B, Co, Fe, ...).3 The first and best known polyoxometalate structural type is the so-called Keggin structure.4 It consists in a central heteroatom (X ) P, Si, Co, Fe, B, Cu, Ge, As, ...) in a tetrahedral cavity surrounded by four M3O13 groups (M ) W, Mo) formed by three octahedra sharing edges (see Figure 1). One of the most important properties of these metal oxide clusters is their ability to accept various numbers of electrons giving rise to mixed-valency species (heteropolyblues and heteropolybrowns). Furthermore, they can accommodate d-transition metals at specific sites.3 These abilities, together with their solubility and stability in aqueous and nonaqueous solvents, make them very useful to build new materials with electronic, magnetic, and optical properties. Thus, we have used them as magnetic entities in hybrid organic-inorganic conducting radical salts.5-8 Their properties, such as electrochromism9 or electrocatalysis, combined to polypyrrole,10 polyaniline,11,12 polythiophene,13 or poly(3-methylthiophene)14,15 * To whom correspondence should be addressed. † Universidad de Valencia. ‡ Centre de Recherche Paul PascalsCNRS. X Abstract published in Advance ACS Abstracts, March 1, 1997. (1) Pope, M. T.; Mu¨ller, A. Angew. Chem., Int. Ed. Engl. 1991, 30, 34-48. (2) Polyoxometalates: from platonic solids to anti-retroviral activity; Pope, M. T., Mu¨ller, A., Eds.; Kluwer Academic Publishers: Dordrecht, 1994; Vol. 10. (3) Pope, M. T. Heteropoly and isopoly oxometalates; SpringerVerlag: Heidelberg, 1983. (4) Keggin, J. F. Nature 1933, 131, 908. (5) Go´mez-Garcıá, C. J.; Ouahab, L.; Gimeńez-Saiz, C.; Triki, S.; Coronado, E.; Delhae`s, P. Angew. Chem., Int. Ed. Engl. 1994, 33, 223. (6) Go´mez-Garcıá, C. J.; Gimeńez-Saitz, C.; Triki, S.; Coronado, E.; Le Magueres, P.; Ouahab, L.; Ducasse, L.; Sourisseau, C.; Delhaes, P. Inorg. Chem. 1995, 34, 4139.

S0743-7463(96)00576-8 CCC: $14.00

Figure 1. Molecular structure of Keggin polyanion and compounds used in this work.

have led to the preparation of conducting catalytic electrodes. More recently, the use of poly(N-methylpyrrole) with FeIII-substituted Keggin polyoxometalates has (7) Galań-Mascaro´s, J. R.; Gimeńez-Saiz, C.; Triki, S.; Go´mez-Garcıá, C. J.; Coronado, E.; Ouahab, L. Angew. Chem., Int. Ed. Engl. 1995, 34, 1460. (8) Coronado, E.; Go´mez-Garcıá, C. J. Comments Inorg. Chem. 1995, 17, 255. (9) Shimidzu, T.; Ohtani, A.; Aiba, M.; Honda, K. J. Chem. Soc., Faraday Trans. 1988, 84, 3941. (10) Bidan, G.; Genie`s, E. M.; Lapkowski, M. J. Electronal. Chem. 1988, 251, 297. (11) Bidan, G.; Genie`s, E. M.; Lapkowski, M. J. Chem. Soc., Chem. Commun. 1988, 533. (12) Hasik, M.; Pron, A.; Raynor, J. B.; Luzny, W. New J. Chem. 1995, 19, 1155. (13) Lapkowskie, M.; Bidan, G.; Fournier, M. Synth. Met. 1991, 4143, 407. (14) Bidan, G.; Genie`s, E. M.; Lapkowski, M. Synth. Met. 1989, 31, 327.

© 1997 American Chemical Society

Organic/Inorganic Superlattices

Langmuir, Vol. 13, No. 8, 1997 2341

provided a sensitive assembly for the electrochemical detection of nitrite.16 To improve the order and the properties of such molecular assemblies, the LangmuirBlodgett (LB) technique may be a useful tool. Indeed, this technique has been widely applied to create new materials which can be used as chemical sensors, modified electrodes, or for molecular electronic devices.17 By adsorption of ions along a Langmuir film, multilayers can be built with a specific organic/inorganic architecture. Metallic ions (Cd, Ba, Zn, etc.) are indeed easily adsorbed along a fatty acid monolayer.18,19 Manganese and octadecylphosphonate monolayers can be combined and lead to interesting magnetic LB films.20 In the case of positively charged lipids, specific interactions with halides have been already reported.21 For all these examples, “simple” ions were used. However, such strategy based on the adsorption of ionic species along a charged Langmuir film can clearly be extended to more complex systems such as large inorganic ions or metal complexes. This paper describes the elaboration of multilayers containing Keggin polyanions by the LB technique. A monolayer of a positively charged lipid (dimethyldioctadecylammonium, see structure in Figure 1) was used as a template for adsorption of various polyoxometalates. The LB multilayers obtained from such Langmuir films were characterized and the effect of the monolayer charge on such adsorption process was analyzed. Such a system can then lead to a large number of materials with various properties associated with the polyoxometalates.

least two measurements were done for each set of experimental conditions. Built-up films have been obtained by the vertical lifting method using a home-made LB trough at room temperature under a continuous dried nitrogen flow. In this Teflon rectangular trough (250 × 550 × 3 mm3), the subphase level is maintained constant during the experiment by adding some water when needed. The barrier is moved by a dc motor in order to perform a stepwise compression of the monolayer. After each increase of the surface pressure, a waiting time (20-30 min in average) allows the system to reach its equilibrium. Steps of 2 mN/m were usually chosen. The subphase was Millipore Q-grade water with a resistivity higher than 18 MΩ. cm. Dipping speed was generally set to 0.5 cm/min. Films were transferred onto optically polished calcium fluoride (precoated with three layers of behenic acid if necessary) or zinc selenide for infrared measurements and onto optically polished glass substrate (treated with dichlorodimethylsilane if needed) for low-angle X-ray experiments. Infrared (IR) spectra were recorded on a FTIR 750 Nicolet spectrometer. To determine the orientation of the molecules in LB films, the linear infrared dichroism was used. The absorption of a polarized IR beam is proportional to (µiE)2 where µi is the transition dipole moment of the studied vibration and E is the local IR electric field. By changing the orientation of the electric field, it was possible to determine the orientation of µi. In the first spectrum, the incident light was set parallel to the substrate normal (the E vector was then parallel to the dipping direction t); in the second one, the incident IR beam formed an angle of 60° with the substrate normal. The out-of-plane dichroic ratio β for each band is then defined as the ratio between both spectra

Experimental Section

where A is the absorption of the IR band. β is related to the angle φ (between the substrate normal and the transition dipole moment µi) through the following equation26

The following heteropolyoxometalates have been used in this study: H4SiW12O40 (Aldrich), H3PW12O40 (Prolabo) and Na2HPW12O40 (Fluka). K5HCoW12O40, K5BW12O40 and K8SiW11O39 were synthetized according to literature procedures.22-24 Their purities have been checked by chemical analysis. Anal. Calcd for K5HCoW12O40‚13H2O: K, 5.86; W, 66.14; Co, 1.77; H2O, 7.00. Found: K, 5.97; W, 64.63; Co, 1.74; H2O 6.89. Anal. Calcd for K5BW12O40‚7H2O: K, 6.15; W, 69.41; B, 0.34; H2O, 3.96. Found: K, 5.02; W, 66.53; B, 0.34; H2O, 3.77. Anal. Calcd for H2K6SiW11O39‚4H2O: K, 7.86; W, 67.79; Si, 0.94; H2O, 2.41. Found: K, 7.51; W, 65.23; Si, 0.97; H2O, 2.15. Hydrogen dihexadecylphosphate was obtained from Aldrich, dimethyldioctadecylammonium bromide from Kodak, and 1,2dipalmitoyl-sn-glycero-3-phosphorylcholine from Bale Biochimie. All commercial compounds (purity higher than 99%) were used without further purification. Chloroform (HPLC grade from Prolabo) was used as spreading solvent, and the lipid solutions (concentration ca 10-3 M) were kept at -18 °C during experiments in order to limit solvent evaporation. Hydrochloric acid and sodium hydroxide solutions were made from Carlo Erba Normex. Langmuir films were studied in a thermostated trough already described.25 The compression was performed at 20 ( 0.5 °C using a continuous speed for the barrier of ca. 6 (Å2/molecule)/min. At (15) Lapkowski, M.; Bidan, G.; Fournier, M. Synth. Met. 1991, 4143, 411. (16) Fabre, B.; Bidan, G.; Lapkowski, M. J. Chem. Soc., Chem. Commun. 1994, 1509. (17) Ulman, A. Introduction to ultrathin organic films, 1st ed.; Academic Press: Boston, MA, 1991; p 442. (18) Peng, X.; Chen, H.; Kan, S.; Bai, Y.; Li, T. Thin Solid Films 1994, 242, 118. (19) Bettorini, S.; Bonosi, F.; Gabrielli, G.; Martini, G.; Puggelli, M. Thin Solid Films 1992, 210/211, 42. (20) Byrd, H.; Pike, J. K.; Talham, D. R. J. Am. Chem. Soc. 1994, 116, 7903. (21) Ahuja, R. C.; Caruso, P.-L.; Mo¨bius, D. Thin Solid Films 1994, 242, 195. (22) Sinmons, V. C. Doctoral Dissertation Thesis, Boston University, 1963. (23) Tourme, C. M.; Tourne, G. F.; Malik, S. A.; Weakley, T. J. R. J. Inorg. Nucl. Chem. 1970, 32, 3875. (24) Teze, A.; Herve, G. J. Inorg. Nucl. Chem. 1977, 39, 999. (25) Jego, C.; Leroux, N.; Agricole, B.; Mauzac, M.; Mingotaud, C. J. Phys. Chem. 1994, 98, 13408.

out-of-plane dichroic ratio

β(60°) ) A|(60°)/A|(0°)

〈cos2 φ〉/〈sin2 φ〉 ) F(β,n1,n2,n3,i,r) where 〈〉 is the average over all possible orientations and F a function depending on refractive indices n1 of air, n2 of the LB film, and n3 of the substrate. The refraction angle r is related to the angle i through the Snell-Descartes relation. This relation allows one to calculate the mean angle φ between the normal to the substrate and the dipole moment of a particular vibration with a precision of ca. 5°. X-ray diffraction patterns were obtained using a conventional generator (Kristalloflex Siemens Ltd.) delivering non-monochromatized line-focused Cu KR radiation. This beam passes through the sample (49 layers deposited on glass) which is mounted vertically and is oscillated during exposure. The integrated intensities of the (001) Bragg reflections were collected by an INEL CPS 120 curved position-sensitive detector (with a resolution of 0.1° in 2θ) associated with an IBM computer for peak assignments.

Results and Discussion (1) DODA Monolayers at the Gas-Water Interface. As already described in the literature,27 the DODA isotherm obtained on pure water (see Figure 2) corresponds to a liquid expanded state of the lipid at the gas-water interface. This is clearly due to the strong repulsion between polar heads in the monolayer. When some sodium chloride (or hydrochloric acid) is added to the subphase, the ∏-A curve of DODA is progressively modified. In particular, the isotherm recorded for a 10-3 M concentration (see Figure 2) presents a change in the slope around 84 Å2/molecule. Such change could be related to a phase transition from an expanded to a condensed liquid phase corresponding to areas lower than 60 Å2/molecule. Unlike (26) Vandevyver, M.; Barraud, A.; Ruaudel-Teixier, A.; Maillard, P.; Gianotti, C. J. Colloid Interface Sci. 1982, 85, 571. (27) Mingotaud, A. F.; Mingotaud, C.; Patterson, L. K. Handbook of Monolayers, 1st ed.; Academic Press: San Diego, CA, 1993.

2342 Langmuir, Vol. 13, No. 8, 1997

Figure 2. Compression isotherm of DODA on pure water and on a 10-3 M NaCl or HCl solution at 20 °C.

Marra’s work,28 no difference at low surface pressure (i.e., before this phase transition) has been recorded when adding sodium chloride (or hydrochloric acid) in the subphase, suggesting that ions dissolved in the subphase do not modify the liquid expanded phase. As demonstrated in Figure 3, the effect of the polyoxometalates on DODA monolayer is much stronger. For a low concentration of H3PW12O40 ions (or Na2HPW12O40 ions for which close results were found), the isotherm is shifted toward smaller areas per molecule (see Figure 3a). At the same time, the isotherms are steeper at the end of the compression. When the Keggin polyanion concentration continues to increase, the effect on the isotherm is reversed. As shown in Figure 3b, the ∏-A curves are then shifted toward larger areas and the collapse pressure is highly modified. At 10-6 M, a slight change in the slope (around 32 mN/m) of the isotherm is observed (this corresponds in fact to a collapse as demonstrated by the instability of the monolayer versus time for surface pressures higher than 32 mN/m). The collapse pressure decreases and reaches a minimum value (ca. 16 mN/m for [H3PW12O40] ) 5.8 × 10-6 M) when the Keggin polyanion concentration is increased. Then, the collapse pressure increases continuously with the polyanion concentration. By analysis of these data in terms of area per molecule at a given surface pressure, the two different stages are clearly demonstrated as shown in Figure 4. At low concentration, the mean molecular area is decreasing and reaches a minimum (which depends on the kind of polyanions used in the experiment). Further increase in the concentration induced an expansion of the monolayer. This behavior is observed for all polyanions, whatever their exact chemical structures and, in particular, their charges may be. However, the extent of such isotherm shift and the variation of the collapse pressure highly depends on the exact formula of the Keggin ions used (see for example parts c and d of Figure 3). The behavior observed at low concentrations of polyoxometalate could be easily explained by the adsorption of polyanions along the interface: the interactions between polyanions and charged ammonium counterbalance, at least partially, the repulsion between the head groups. The lipid density at the gas-water interface could then be higher for a given surface pressure. Clearly, the number of polyanions adsorbed along the charged interface should increase with the Keggin ion concentration, explaining the continuous shift of the compression isotherm with the polyoxometalate concentration. Moreover, taking into account the size of the trough and of the polyanion, one can easily demonstrate that the number of polyanions dissolved in the subphase for a polyanion concentration lower than ca. (28) Marra, J. J. Phys. Chem. 1986, 90, 2145.

Clemente-Leo´ n et al.

10-7 M is too small to give a total coverage of the interface. For such a concentration and, eventually, for the concentration corresponding to the minimum of area in Figure 4, the adsorbed anions along the monolayer do not form a compact inorganic layer. In other words, the repulsion between anions could limit the surface density of the adsorbed anions. Concerning the behavior at higher anion concentration, different explanations can be advanced. Firstly, it could be argued that change in the ionic strength of the subphase could modify the interactions between ions and then the packing at the interface. However, the isotherm of DODA on a subphase containing 10-6 M H3PW12O40 and 3 × 10-3 M NaCl is very close to the one obtained on a 10-6 M H3PW12O40 subphase. Just an ionic strength effect can then be rejected. Secondly, depending on the concentration and on subphase pH (which is modified when dissolving the Keggin ions), the polyanion may be totally or partially ionized. Changes in the polyanion charge might certainly modify the adsorption process and the packing of the monolayers. At low polyanion concentration, the Keggin molecules may be totally ionized and the resulting anion should bear a maximum negative charge. When the concentration of polyanion is increased, the dissociation factor should be lower; the average charge of the polyanions is then smaller, inducing a decrease in the compactness of the DODA monolayer. To verify such a hypothesis, hydrochloric acid or sodium hydroxide was added to the subphase containing the Keggin polyanion (see Figure 5). Indeed, hydrochloric acid induces a large change in the area per molecule in the DODA isotherm. Hydrochloric acid alone could not be responsible for such an effect (see Figure 2). On the contrary, the addition of sodium hydroxide to the subphase induces a decrease in area of the DODA at the gas-water interface. In other words, the change occurring in the DODA isotherm when increasing the H3PW12O40 concentration from 10-6 to 10-4 M could be triggered by changing the pH of the subphase. However, such a pH effect can only partially explain the results obtained with the Keggin polyanions. Indeed, in the case of K5BW12O40, similar behavior has been observed (see Figure 3c,d): a decrease in the area for a given surface pressure is followed by an increase when the concentration of polyanions continues to increase. For this polyanion having no labile proton, no dissociation leading to a decrease in the subphase pH could explain such behavior. The third possible explanation is related to the chemical stability of Keggin ions. It is well-known that such stability in aqueous solution is highly dependent on various experimental parameters.3 When the acidity of the solutions is too low, hydrolysis reaction occurs leading to lacunary anions, XW11O39n- in particular. The change in the compression isotherm behavior could then be related to a modification of the chemical stability of the polyanions in the subphase. However, the LB film obtained with a synthetized lacunary silicotungstate polyanion presents an IR spectrum clearly different from the one corresponding to the LB film containing the Keggin polyanions (see below). This demonstrates that such chemical hydrolysis leading to XW11O39n- is a minor process at the interface in the time scale and conditions of these Langmuir experiments. As a fourth possible explanation, even if one layer of polyanions is already adsorbed along the interface, more adsorption could take place along such a modified Langmuir film. When the heteropolyoxometalate concentration is increased in the subphase, a double layer of adsorbed polyanions could then be built at the interface, changing the interactions between adsorbed ions and the lipid density at the gas-water interface. Surface potential and


Langmuir, Vol. 13, No. 8, 1997 2343

a

b

c

d

Figure 3. Compression isotherm of DODA on various subphases: concentration of H3PW12O40, (a) lower than 10-6 M (b) higher than 10-6 M; concentration of K5BW12O40, (c) lower than 10-6 M, (d) higher than 10-6 M.

Figure 4. Area of the DODA monolayer for a surface pressure of 10 mN/m versus the concentration of H3PW12O40 in the subphase.

electrochemical experiments are currently under progress in order to validate or invalidate such a model. Whatever the exact origin of the observed phenomenon, the adsorption of Keggin polyanions along a DODA monolayer should clearly depend on various factors such as repulsions between anions, local pH, dissociation of the Keggin polyanions, etc. At equilibrium, the electrical charge of the monolayer should be compensated by the charge of all the anions adsorbed along the interface. If only the polyanions are adsorbed along the monolayer, then the electroneutrality of the film can be expressed as

nL/AL + nK/AK ) 0 where nL and nK are the charge of the lipid and of the

Figure 5. Effect of NaOH or HCl on the compression isotherm of DODA on a subphase containing the H3PW12O40 polyanion.

adsorbed Keggin polyanion, respectively, having mean molecular areas AL and AK at the interface. For the polyanions used in this study, the charge nK should be found between -3 and -6 assuming total ionization of the ions. In the maximum compressed state of the DODA monolayer, AL is close to 45 Å2/molecule, leading to an AK value of 120-240 Å2/ion depending on the polyanion charge. Taking into account the cross section of such a heteropolyoxometalate (ca. 85 Å2), the calculated AK value suggests that only 35-70% of the monolayer surface is occupied by the anions adsorbed along the interface. Such an estimation demonstrates that the adsorbed polyanions are not in a close packing state. Moreover, cations of the subphase (H+, K+) could be interacting with the large polyoxometalate, which are adsorbed along the Langmuir film. The global neutrality of the monolayer is then

2344 Langmuir, Vol. 13, No. 8, 1997


nL/AL + nK/AK + d+ ) 0 where d+ is the surface density of cations nearby the polyanions. From this value, the average number of monocations around each polyanion, n+, can easily be calculated

n+ ) d+AK In the case of a hypothetical hexagonal close packing of the polyanions (AK ) 93 Å2), the value of n+ corresponds to one to four H+ or K+ cations nearby each polyanion adsorbed along the monolayer. These cations adsorbed with the polyanions along the monolayer reduce the global charge of the Keggin anion. Such a process could explain the lack of strong correlation between the expected polyanion charge and the modifications of the DODA compression isotherm. (2) Transfer onto Solid Substrates. (a) Effect of Keggin Polyanions. Well-organized LB films having more than one layer cannot be built up from a DODA monolayer spread on pure water or sodium chloride solution. This is surely related to the relatively poor lipid density of such a Langmuir film as well as the strong interaction between the lipid polar heads and the water surface. On the contrary, DODA monolayers spread on solutions containing Keggin polyanions lead to very good multilayers. Indeed, on a 10-6 M solution of H3PW12O40, DODA monolayers could effectively be deposited onto hydrophilic substrate with a maximum transfer ratio between 0.9 and 1 for a surface pressure of 30 mN/m and a dipping speed of 0.5 cm/min. Lowering the surface pressure set for the transfer or increasing the dipping speed above 3 cm/min induced a drop in the transfer ratio. The effect of the polyanion concentration on this transfer is considerable. For a lower concentration (typically 10-7 M), some deposition of the monolayer onto the solid substrate is still observed but the transfer ratio is very low. When the polyoxometalate concentration is equal or higher than 10-5 M, no transfer can be achieved whatever the surface pressure and dipping speed may be. The transfer is efficient only around the minimum molecular area evidenced by the curve of Figure 4. This fact was demonstrated for all the polyanions used in this study, which lead to Y-type and optically defect-free LB films noted DODA/XW12O40. These multilayers have been characterized by infrared dichroism and X-ray diffraction. Infrared Linear Dichroism. Besides the bands at 2920, 2851, and 1467 cm-1 associated to the stretching and scissoring modes of CH bonds of DODA alkyl chains, the infrared spectra of DODA/Keggin polyanion LB films are characterized by very strong bands below 1200 cm-1, due to the polyanions. For example, the infrared spectrum in the 1100-700 cm-1 region for the DODA/SiW12O40 LB film is given in Figure 6. Clearly, the bands in the LB films associated with the anions are narrower and generally slightly shifted when compared to the spectrum of the polyanions in a KBr pellet. This may be due to the different organization of the polyanions in the LB films or to the lower hydratation of the Keggin anions in the multilayers, compared to the crystalline state. Indeed, whereas strong bands around 3300-3500 and 1600-1650 cm-1 associated to the stretching and bending modes of water are observed for the pristine polyoxometalates in a KBr pellet, those bands are missing in the LB films. This demonstrates that the anions are in a less hydrated state within the multilayers. The shift observed for different peaks (see Table 1) is probably related to the organization and in particular to the presence of positively charged DODA in the film. Indeed, Rocchiccioli-Deltcheff

Figure 6. Infrared spectra of the H4SiW12O40 polyanion in KBr pellet and of a DODA/SiW12O40 LB film (20 layers deposited on zinc selenide). The absorbance is given in arbitrary units. To the asterisk corresponds the band associated with the CH2 rocking mode of DODA.

et al.29 observed a shift of the Keggin infrared bands depending on the size of the tetralkylammonium cations used as counterion in the solid state. The values reported for the bigger cation (namely tetrabutylammonium) used by these authors are in close agreement with those measured for the corresponding Keggin polyanion in LB films. According to the interpretation of such frequency shift,17 our results indicate that the interanionic distance within the LB films is large enough to neglect anionanion interactions. Another striking feature of the infrared spectra of DODA/XW12O40 LB films is the strong out-of-plane dichroism (see Figure 7). When the infrared electrical field is not parallel to the plane of the substrate, new peaks clearly appear (see Table 1). This result shows that some transition dipoles associated to vibrations of the polyanions are oriented more or less parallel to the normal of the substrate. Then, the Keggin polyanions are not randomly organized within the LB film but should have a particular orientation (or at least a distortion due to the electrostatic interaction with the charged lipids) in the multilayers. In powder, IR bands are difficult to assign because the IR spectrum contains mainly the envelopes of different bands.30 It is then not possible to determine precisely the polyoxometalate orientation within the LB films using the IR dichroism technique. However, these experiments demonstrated that the Keggin polyanions can be organized by the Langmuir-Blodgett technique. Finally, looking at the dichroism of bands associated with the alkyl chains, it is possible to evaluate the tilt angle of these chains, which are supposed to adopt a fully extended all-trans conformation. Table 2 gives the calculated values of this angle with respect to the substrate normal for various anions. For all the Keggin polyanions, the tilt angle of the DODA chains within the LB film is somewhat constant and close to 30°. X-ray Diffraction. Three or four Bragg peaks were clearly identified in the X-ray diffractogram on DODA/ XW12O40 LB films. This number of peaks shows that the lamellar structure of those LB films is well-defined. The periodicity deduced from these experiments is close to 49 Å (see Table 2) in all cases. From these results and an estimation of the DODA length,31 the thickness of one inorganic layer within the LB films can be evaluated to (29) Rocchiccioli-Deltcheff, C.; Fournier, M.; Franck, R.; Thouvenot, R. Inorg. Chem. 1983, 22, 207. (30) Thouvenot, R.; Rocchiccioli-Deltcheff, C.; Souchay, P. C. R. Acad. Sci. Paris 1974, 278 C, 455. (31) Lvov, Y.; Essler, F.; Decher, G. J. Phys. Chem. 1993, 97, 13773.


Langmuir, Vol. 13, No. 8, 1997 2345

Table 1. Infrared Spectra of LB Films DODA/XW12O40 and Lacunar SiW11O39a X ) Si KBrc

LB

1020 m 982 s

1013 w 977b m 969 m 937b s 917 s 885 w 821b vs 791 vs

930 s 885 s 792 vs

assignmentsd ν W-Od as ν Si-Oa as ν W-Ob-W ν W-Oc-W

X ) CO KBre 1017 w 940 s 870 vs 760 vs

LB 940b s 926 s 874b sh 863 s 775b vs 748 vs

assignmentse ν Si-Oa as ν W-Od as ν W-Ob-W ν W-Oc-W

Figure 7. Infrared spectra of a DODA/SiW12O40 LB film (20 layers) on zinc selenide. The angle between the plane of the substrate and the electric field is either 0° (solid line) or 60° (dashed line). The absorbance is given in arbitrary units. The asterisk corresponds to the band associated with the CH2 rocking mode of DODA.

X)P KBrf

LB

assignmentsf

1080 s

1104 w 1080 w 1055 w 975b s 949 s 896 s 820b vs 815 vs

ν P-Oa as

985 s 887 s 807 vs

ν W-Od as ν W-Ob-W ν W-Oc-W

X)B KBrf

LB

1010 m 960 s

993 w 957b m 948 m 905b s 896 s 842b sh 816 vs

920 sh 820 vs

assignmentsf ν W-Od as ν B-Oa as ν W-Oc-W

X ) Si, W11 KBrg

LB

1000 952

989 w 948b m 943 m 896b s 886 s

885 870 797 725

811b sh 786 vs 721 s

assignmentsg ν W-Od as ν Si-Oa as ν W-Ob-W ν W-Oc-W ν W-Oc-W

a Key: vs ) very strong, s ) strong, m ) mean, w ) weak, sh ) shoulder. b Infrared band appearing when the angle between the plane of the substrate and the electric field is set to 60°. c From ref 30. d From reference 29. e From ref 34. f From ref 35. g From ref 36.

ca 10 Å. Compared to the radius of a Keggin polyanion29 (ca. 5.2 Å), such a value indicates that each inorganic layer is a monolayer of polyanions (and not a bilayer as supposed from the Y-type transfer). The transfer occurring during the upper stroke should then involve large changes concerning the ions adsorbed along the last monolayer deposited onto the solid substrate. (b) Effect of the Monolayer Nature. The adsorption of Keggin polyanions along a DODA monolayer must be induced by electrostatic interactions between the ion and the charge density at the interface. Correlation between the monolayer charge density and the amount of adsorbed polyoxometalates should then be found. For such a

Figure 8. Variation of the mean molecular area (at 10 mN/m) of a mixed DODA/HDHP monolayers spread on pure water with the mole fraction of DODA. Table 2. Tilt Angle (( 5°) of Alkyl Chains (Determined by Infrared Dichroism) and Periodicities ((1 Å) of the Layers (Deduced from X-ray Diffraction Experiments) in LB films DODA/Keggin Polyanions X)P

X ) Si

X)B

X ) Co

Φ (deg)

30

26

31

34

d (Å)

50

49

48

48

average angle, 30 ( 3° average thickness, 49 ( 2 Å

purpose, we used mixed HDHP/DODA Langmuir films and the polyanion K5HCoW12O40, which has the highest charge among all the anions used in this work. We have selected for these studies the HDHP lipid (see Figure 1) because it has two alkyl chains like DODA and bears a negative charge when spread at the gas-water interface. The miscibility of these two molecules at the gas-water interface has been checked by recording the compression isotherm of such a monolayer with various compositions.32 Figure 8 gives, for example, the change in mean molecular area at a surface pressure of 10 mN/m with the composition. This variation is not linear with the mole fraction (32) Gaines, G. L., Jr. Insoluble Monolayers at Liquid-Gas Interfaces, 1st ed.; Interscience Publishers: New York, 1966; p 386. (33) Peng, J. B.; Prakash, M.; Macdonald, R.; Dutta, P.; Ketterson, J. B. Langmuir 1987, 3, 1096. (34) Nomiya, K.; Miwa, M.; Kobayashi, R.; Aiso, M. Bull. Chem. Soc. Jpn. 1981, 54, 2983. (35) Rocchiccioli-Deltcheff, C.; Thouvenot, R.; Franck, R. Spectrochim. Acta 1976, 32A, 587. (36) Rocchiccioli-Deltcheff, C.; Thouvenot, R. J. Chem. Res. 1977, 46.

2346 Langmuir, Vol. 13, No. 8, 1997

Figure 9. Variation of the relative intensity I(ξ) of the ca. 750 cm-1 vibrational band in mixed LB films DODA,HDHP/ CoW12O40 with the excess of charge ξ of the monolayer (see text).

of DODA. Furthermore, the collapse pressure of the mixed film depends on the composition of the monolayer. All these results demonstrate that HDHP and DODA are miscible components in Langmuir film. The transfer of such mixtures spread on a 10-6 M subphase of K5HCoW12O40 has been performed at 30 mN/m with a transfer ratio approximately independent of the monolayer composition. The mean molecular area is very similar for all composition at the transfer surface pressure. Then assuming that the polyanions have similar organization in the built-up multilayers, the intensity (I) of infrared bands associated with the Keggin polyanion should be proportional to the concentration of the polyanion within the LB films. The relative concentration of such ions can then be calculated for a given composition of the monolayer. More precisely, instead of the usual mole fraction, the mixed monolayer can be characterized by its charge excess ξ

ξ ) x+ - x-

with x- ) 1 - x+

where x+ and x- are respectively the mole fraction of the positively and negatively charged lipid within the monolayer. This parameter is equal to 1 for a monolayer containing only the positively charged lipid, -1 for a pure negatively charged monolayer, and 0 for a mixture 1/1 for which the Langmuir film is globally neutral. Since the number of alkyl chains and the area per molecule at the surface pressure used for the transfer are similar for both HDHP and DODA, the parameter ξ is linked to the charge density of the monolayer. Figure 9 gives the variation of the relative intensity I(ξ)/I(ξ)1) with the charge excess ξ. For monolayers either neutral or having a partial negative charge, no adsorption of the polyanions is observed. However, when the monolayer is even slightly positively charged (ξ > 0), Keggin polyanion is transferred onto the solid substrate and the amount of anions within the LB film seems to be proportional to the charge of the film, as demonstrated by the experimental relation

I(ξ)/I(ξ)1) ) ξ

for ξ > 0

This result shows the possibility to control the amount of polyanions organized in LB films by simply changing the global charge of the monolayer used for the transfer. The adsorption of polyanions along the gas-water interface could be achieved only with positively charged monolayers. Then, a zwitterionic compound, though exhibiting a net neutral charge, should have a priori no effect. However, as demonstrated by Figure 10, K5HCoW12O40 polyanions dissolved in the subphase modify the compression isotherm


Figure 10. Effect of the concentration of K5HCoW12O40 dissolved in the subphase on the compression isotherm of DPPC.

Figure 11. Comparison between the infrared spectra of (A) a DPPC/CoW12O40 LB film (20 layers) on zinc selenide, (B) DPPC in KBr pellet, and (C) a DODA/CoW12O40 LB film (20 layers) on zinc selenide. The absorbance is given in arbitrary units.

of DPPC. Thus, some interactions between the lipid and the anions exist, suggesting that this zwitterionic lipid behaves like a partially positively charged compound. This was already pointed out by evaluation of a local pH close to a phosphatidylcholine monolayer.37 More precisely, the isotherm is shifted toward larger molecular areas when the Keggin polyanion concentration is increased. For concentrations higher than 10-5 M, the isotherm remains constant indicating some kind of saturation effect (Figure 10). Unlike DODA, for which the polyanions induced a higher molecular density at the gas-water interface, K5HCoW12O40 seems to increase the repulsions between DPPC molecules, perhaps by an induced change in the orientation of the lipid polar head along the interface. Furthermore, it is well-known that transferring more than one layer of DPPC onto a solid substrate is generally impossible. To our knowledge, only one paper reported the successful deposition of such lipid by dissolving uranyl ions in the subphase.33 Using a 10-4 M subphase of K5HCoW12O40, Y-type LB films of DPPC were easily obtained at a transfer surface pressure of 40 mN/m and with a transfer ratio close to 0.85. As demonstrated by the infrared spectra (Figure 11), the built-up multilayers contain both DPPC and polyanions. Comparing the intensities of the bands associated with the Keggin anions in DPPC/CoW12O40 and DODA/CoW12O40 LB films, the amount of Keggin polyanion transferred within the multilayers is clearly much smaller with the zwitterionic DPPC than with the ionic DODA. (37) Mingotaud, C.; Chauvet, J.-P.; Patterson, L. K. J. Phys. Chem. 1996, 100, 18554.


Conclusion This study has demonstrated that the adsorption properties of Keggin heteropolyoxometalates along a positively charged monolayer can be used to build new organic/inorganic superlattices. Whatever their charge and the heteroatom included in the Keggin structure, these polyanions can be organized by monolayers contained in a well-defined LB film. The effect of the size and shape of these polyoxometalates on the built-up multilayers is currently under study using Anderson and Dawson-Wells type anions. The preliminary results obtained with these compounds clearly indicate that the method described in this paper can be extended to all kinds of polyanions,

Langmuir, Vol. 13, No. 8, 1997 2347

leading to new lamellar materials. In fact, selecting the exact nature of the polyoxometalate should enable the elaboration of LB films having particular magnetic, optical, or electrochemical properties. Acknowledgment. The authors are indebted to J. Verge`s for the building of the LB trough. This work is supported by the European Union (network on new molecular conductors), the Ministerio de Educacioń y Cultura (CICYT), and the Generalitat de Catalunã (CIRIT) (Grant QFN93-4510). M.C.L. thanks the Generalitat Valenciana for a predoctoral grant. LA960576V

Inorganic Superlattices: Keggin

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