Adsorption Behavior and Structural Characterization of Azo Dyes on a

Department of Chemistry, Kanagawa University, Yokohama 221-8686, Japan. Received November 30, 1999. In Final Form: May 23, 2000. The adsorbability of ...
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Langmuir 2000, 16, 6613-6621

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Adsorption Behavior and Structural Characterization of Azo Dyes on a Langmuir-Blodgett Film of Octadecylamine Masashi Takahashi, Koichi Kobayashi,* and Kyo Takaoka Department of Energy Science and Engineering, Musashi Institute of Technology, Setagaya, Tokyo 158-8557, Japan

Tatsuo Takada Department of Environmental and Information Studies, Musashi Institute of Technology, Yokohama 224-0015, Japan

Kazuo Tajima Department of Chemistry, Kanagawa University, Yokohama 221-8686, Japan Received November 30, 1999. In Final Form: May 23, 2000 The adsorbability of the Langmuir-Blodgett (LB) film of octadecylamine (ODA) has been investigated using R-naphthol orange (NO) and methyl orange (MO) as adsorbates. It was found that the specific adsorptions of these azo dyes occurred on the ODA LB film predominantly by ionic interaction, and also that the adsorption behavior depended on pH according to the ionic states of NO and MO as well as that of the ODA LB film. The structural characteristics of the LB films such as orientation, layer-layer distance, and topography of the film surface have been evaluated by means of FTIR and UV-vis spectroscopies, X-ray diffraction in the low-angle region, and atomic force microscopy (AFM), respectively. The results obtained indicated that the adsorption of dyes induced rearrangements of the layer structure, where the hydrocarbon chains took an interdigitated structure and the dye molecules in the LB film exhibited remarkable distinction in the adsorbing state, i.e., a lie-flat configuration of NO and an edge-on configuration of MO.

Introduction Among several techniques of producing organized molecular assemblies, a Langmuir-Blodgett (LB) method is one of the most versatile techniques of making wellordered ultrathin films wherein molecules are expected to maintain a highly ordered arrangement.1,2 Especially, the LB film’s immobilizing functional groups have great advantages for the practical application such as chemical sensors and devices.3-13 Simultaneously, studies of adsorption onto LB films are of increasing importance, because the adsorption phenomenon is largely affected by properties of the LB film, which are related to the film structural characteristics such as packing and orientation of the molecules. As for applications to the sensors, specific interactions between molecules and a sensing surface are * To whom correspondence should be addressed. (1) Ulman, A. An introduction to ultrathin organic films: from Langmuir-Blodgett to self-assembly; Academic Press: San Diego, 1991. (2) Roberts, G. Langmuir-Blodgett films; Plenum Press: New York, London, 1990. (3) Liu, M.; Ushida, K.; Kira, A.; Nakahara, H. J. Phys. Chem. B 1997, 101, 1101. (4) Tsuzuki, H.; Watanabe, T.; Okawa, Y.; Yoshida, S.; Yano, S.; Koumoto, K.; Komiyama, M.; Nihei, Y. Chem. Lett. 1988, 1265. (5) Owaku, K.; Goto, M.; Ikariyama, Y.; Aizawa, M. Sens. Actuators, B 1993, 13-14, 723. (6) Odashima, K.; Kotato, M.; Sugawara, M.; Umezawa, Y. Anal. Chem. 1993, 65, 927. (7) Moriizumi, T. Thin Solid Films 1988, 160, 413. (8) Barraud, A. Vacuum 1990, 41, 1624. (9) Beswick, R. B.; Pitt, C. W. J. Colloid Interface Sci. 1988, 124, 146. (10) Rella, R.; Serra, A.; Siciliano, P.; Tepore, A.; Valli, L.; Zocco, A. Langmuir 1997, 13, 6562. (11) Roberts, G. G.; Petty, M. C.; Baker, S.; Fowler, M. T.; Thomas, N. J. Thin Solid Films 1985, 132, 113. (12) Furuki, M.; Sunpu, L. Mol. Cryst. Liq. Cryst. 1993, 227, 325. (13) Souto, J.; Aroca, R.; DeSaja, J. A. J. Phys. Chem. 1994, 98, 8998.

employed for the selective adsorptions. Several types of interactions applied in the sensing process of exposing to gas or immersing in solutions have been reported, for example, complex formation of transition metal ions with a long-chain ligand molecule in the LB film,3 antigenantibody or enzymatic reactions of biological molecules,4,5 and inclusion of guest molecules in the films of cyclodextrin derivertives.6 In addition, for the purpose of the functionality of the LB films, a chemical modification can be made by means of adsorption to the floating monolayer from the subphase or to the LB film from the immersing solution, so that functional groups such as conjugated systems are incorporated without the long-chain substitution.14 In any case, molecular orientation in the LB film is an important factor for designing the function of the film, and therefore, information on the film structure is helpful for clarifying the properties of the LB film. In previous papers,15-20 we have reported on the adsorption behavior of azo dyes on cationic LB films with considerable interest in the features of azo dyes and cationic LB films. Owing to the functional properties of azo dyes such as cis-trans isomerization, interest in the (14) Lehmann, U. Thin Solid Films 1988, 160, 257. (15) Takahashi, M.; Tajima, K.; Kobayashi, K. Thin Solid Films 1992, 221, 298. (16) Takahashi, M.; Kobayashi, K.; Takaoka, K.; Tajima, K. Thin Solid Films 1996, 284-285, 115. (17) Takahashi, M.; Kobayashi, K.; Takaoka, K.; Tajima, K. Langmuir 1997, 13, 338. (18) Takahashi, M.; Kobayashi, K.; Takaoka, K.; Tajima, K. Thin Solid Films 1997, 307, 274. (19) Takahashi, M.; Kobayashi, K.; Takaoka, K.; Tajima, K. Bull. Chem. Soc. Jpn. 1998, 71, 1467. (20) Takahashi, M.; Kobayashi, K.; Takaoka, K.; Tajima, K. J. Colloid Interface Sci. 1998, 203, 311.

10.1021/la991554m CCC: $19.00 © 2000 American Chemical Society Published on Web 07/14/2000

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properties often leads to the possibilities of developing them as molecular switch and optical memory.21-23 Since the conjugated system of azo dyes usually takes the trans configuration, these azo dyes having a rodlike structure are most appropriate for estimating the molecular orientation by the spectroscopic techniques in both Langmuir monolayers and LB films as well as in stretched polymer films.24 On the other hand, cationic film materials such as longchain alkylammonium salt and alkylamine are considered to be interesting materials in view of their physical properties and their mechanisms of action in practical applications.25 That is because these cationic substances interact strongly with a variety of materials such as surfactants, polymers, dispersed materials, proteins, biological colloids, lipids, and so on. Several works on the behavior of the long-chain alkylamines have been reviewed and carried out dealing with the molecular interactions in their monolayers at the air/water interface.26-28 In the process of subsequent deposition, addition of appropriate anions in an aqueous subphase makes the formation of a suitable multilayer possible. Bardosova et al. have discussed the influence of various counterions on the structure of films of the long-chain alkylamine, making particular use of X-ray diffraction and IR absorption spectroscopy as diagnostic techniques.29 Anikin et al. studied a hydrophobically shielded porphyrin derivative in octadecylamine-based LB films and reported that the octadecylamine matrix controlled the aggregation state of porphyrin chromophores.30 Although the properties of the cationic film materials are analogues to those of fatty acids in many of their features, the cationic monolayer on the aqueous subphase is generally difficult to deposit on the solid substrate. Nevertheless, if the deposition of the monolayer could be successfully performed while retaining the cationic properties, the resulting LB films would be expected to possess the specific adsorbabilities different from those of ordinary anionic LB films. As we have shown in the previous papers,15-18 the LB films of quaternary long-chain alkylammonium salts exhibit remarkable adsorbability to the negatively charged adsorbates mainly by ionic interaction, and the adsorption takes place almost at a stoichiometric ratio. As for the long-chain alkylamine, the adsorbability of the LB film is expected to be controlled by changing the pH, and some reaction of amines would provide potential for modification of the LB films. In the present work, the adsorption behavior and structural characterizations of the octadecylamine (ODA) LB films are studied by spectroscopic methods and X-ray diffraction analysis. As adsorbates, water-soluble azo dyes of R-naphthol orange (NO) and methyl orange (MO) are used throughout the experiments because the distinction (21) Tachibana, H.; Goto, A.; Nakamura, T.; Matsumoto, M.; Maeda, E.; Niino, H.; Yabe, A.; Kawabata, Y. Thin Solid Films 1989, 179, 207. (22) Seki, T.; Sakuragi, M.; Kawanishi, Y.; Suzuki, Y.; Tamaki, T.; Fukuda, R.; Ichimura, K. Langmuir 1993, 9, 211. (23) Nishiyama, K.; Fujihira, M. Chem. Lett. 1988, 1257. (24) Fukuda, K.; Nakahara, H. J. Colloid Interface Sci. 1984, 98, 555. (25) Rubingh, D. N.; Holland, P. M., Eds. Cationic Surfactants: Physical Chemistry; Marcel Dekker: New York, 1991. (26) Gains, G. L. In Insoluble monolayers at liquid-gas interfaces; John Wiley & Sons: New York, 1966; pp 226-233. (27) Ganguly, P.; Paranjape, D. V.; Rondelez, F. Langmuir 1997, 13, 5433. (28) Sukhorukov, G. B.; Feigin, L. A.; Montrel, M. M.; Sukhorukov, B. I. Thin Solid Films 1995, 259, 79. (29) Bardosova, M.; Tredgold, R. H.; Ali-Adib, Z. Langmuir 1995, 11, 1273. (30) Anikin, M.; Tkachenko, N. V.; Lemmetyinen, H. Langmuir 1997, 13, 3002.

Takahashi et al. Chart 1

between their molecular structures, namely, the hydroxyl group of NO and the dimethyl amino group of MO (see Chart 1), is suitable for the orientation studies. We particularly focus on the differences in the adsorption behaviors between NO and MO, and discuss the influence of the molecular structures of dyes on the adsorption characteristics of the ODA LB films. Also, the study is extended to morphological characterizations of the surfaces of the ODA LB film by applying atomic force microscopy (AFM). Experimental Section The cationic film material of ODA (99% purity) was purchased from Aldrich Chemical Co., Inc. Dye materials of NO and MO used as adsorbates were supplied by Tokyo Chemical Industry and Wako Pure Chemical Industries, Ltd., respectively. The structural formulas are exhibited in Chart 1. These substances were used without further purification. All other chemicals used were of the highest grade available. A spreading solution was prepared by dissolving ODA in chloroform to a concentration of 1.0 × 10-3 mol dm-3. The triply distilled water and the aqueous 1.0 × 10-4 mol dm-3 NO solution were used as subphases for preparing the Langmuir monolayers of ODA. For the experiments of pH dependence, the pH in the subphase was systematically varied by adding hydrochloric acid or sodium hydroxide. Regardless of the incorporation of acid or base, the subphase without dye was referred to as “distilled water” to distinguish it from that containing NO. The measurements of surface pressure-area (π-A) isotherms and the multilayer depositions of the ODA monolayer were carried out by a HBM AP-type LB film balance of Kyowa Interface Science at a constant temperature (20 °C). The compression rate (the barrier speed) used was 5 mm min-1. Using the conventional vertical dipping (LB) method, the fabrication of the ODA LB film was performed under the conditions of an alkaline subphase (pH 10.1-10.5) and a surface pressure of 45 mN m-1. Under these conditions, the ODA monolayer was expected to be a typical condensed state and stable enough to withstand the high surface pressure (see Figure 1). The dipping and withdrawal speeds were kept at 7 mm min-1. The waiting time between dips was required to be at least 10 min to prevent respreading of the deposited layer. Solid substrates such as the gold-coated brass plate, calcium fluoride plate, quartz plate, glass plate, and silicon wafer were used for the most appropriate measurements. The surfaces of the former two plates were rendered hydrophobic by coating a few layers of a cadmium stearate LB film, whereas the latter three plates were cleaned by soaking in cleaning solutions; e.g., the quartz and glass plates were soaked in a chromic acid mixture for a few days, and the silicon wafer was soaked in an alkaline ethanol solution for 10 min. Adsorption of dye molecules was carried out by immersing the ODA LB film into the aqueous dye solution of 1.0 × 10-4 mol dm-3 for a given period of time. For the sake of convenience, we use the abbreviation for these LB films “dye (NO or MO)-adsorbed ODA LB film” in this study. UV-vis and FTIR spectra of the ODA LB films were measured with a Shimadzu UV-3100PC UV-vis-near-IR scanning spec-

Azo Dyes on a LB Film of ODA

Figure 1. π-A isotherms of the ODA monolayers on a distilled water (solid line) and an aqueous NO solution (dotted line) at 20 °C. pH: a, 3.0; b, 5.7; c, 10.1; d, 11.7; a′, 2.2; b′, 5.4; c′, 9.3; d′, 10.5; e′, 11.5. trophotometer and a JASCO FT/IR-8900 spectrometer equipped with an MCT detector, respectively. In the measurements of FTIR-reflection absorption (RA) spectra, we employed an incident angle of 80° and a p-polarized direction of the incident light. Low-angle diffraction patterns of the LB films were recorded using a Rigaku Denki RAD-RIIC X-ray diffractometer with Cu KR (λ ) 1.54 nm) as an X-ray source (40 kV, 50 mA). AFM images were taken with a Nanoscope III of Digital Instruments in tapping mode. The cantilever used was a commercially available etched silicon probe (Digital Instruments, Inc., type TESP) with a length of 125 µm and resonant frequencies of 303-383 kHz. All images were obtained under atmospheric conditions.

Results and Discussion Monolayer Properties of ODA on a Distilled Water and an Aqueous NO Solution. Prior to LB deposition, monolayer properties of ODA were examined by measuring π-A isotherms on a distilled water and an aqueous NO solution of 1.0 × 10-4 mol dm-3 at various pH values. As shown in Figure 1, ODA on the distilled water forms an expanded monolayer at pH 3.0 (curve a), whereas the ODA monolayer exhibits a typical condensed state above pH 10.1 (curves c and d) at which protonation of the amino group in the ODA molecule is practically forbidden. On the contrary, the effect of addition of NO is clearly presented in the figure. Namely, π-A isotherms below pH 9.3 (curves a′-c′) show rather steep rising of the surface pressure even at an acidic pH (curve a′), and the shapes of these isotherms resemble each other. Compared with the monolayers on the distilled water, the molecular areas are fairly expanded. These results indicate that the ODA monolayer is interactive with NO in this pH range. The limiting areas (A0) at zero pressure for these monolayers are estimated to be 0.56-0.57 nm2 molecule-1. Such behavior of the monolayer on the aqueous NO solution is similar to that observed on an aqueous MO solution except A0 at a corresponding pH (the values of A0 using the subphase containing MO were estimated to be ca. 0.50 nm2 molecule-1, being expected to form a rather compact complex). In addition, the monolayer on the aqueous NO solution reveals a decrease in molecular area above pH 10.5 (curves d′ and e′), resulting in the same condensed monolayer as that on the distilled water above pH 10.1. This can be understood by the equilibriums of both the protonation of ODA and the complexation between ODA cation and dye anion, as has already been pointed out in our previous paper.19 Adsorption Characteristics of NO and MO on the ODA LB Film. Subsequently, the LB deposition was

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Figure 2. Immersing-time dependence of UV-vis absorption for the NO and MO molecules adsorbed on the nine-layer ODA LB film: O, NO adsorption (at 475 nm); 4, MO adsorption (at 355 nm). The inset indicates the UV-vis spectra of the NOadsorbed ODA LB films at various immersing times.

applied to the ODA monolayers on the alkaline subphase without NO (curve c in Figure 1), and regular transfer of the monolayer on solid supports was successfully achieved at a surface pressure of 45 mN m-1. Similarly to the floating ODA monolayer, the resulting ODA LB film is expected to interact with NO anion. To clarify adsorption characteristics, we carried out adsorption experiments using azo dyes of NO and MO as adsorbates. After immersing the ODA LB film in the aqueous dye solution for a given period of time, we measured UV-vis spectra to evaluate an adsorbed amount of dye, which was estimated from an integrated area of absorbance (peak area) of UV-vis absorption in the spectrum. The inset in Figure 2 shows the UV-vis spectra of the nine-layer NO-adsorbed ODA LB films at various immersing times. The absorption band at around 475 nm is assigned to the π-π* transition moment of the NO chromophore, which is almost parallel to the long axis of the NO molecule.31 In Figure 2, we plotted the peak area of the absorption band against the immersing time, together with the results similarly obtained for the MO adsorption. If we take into account the difference in molecular extinction coefficients between NO and MO,32 almost the same adsorption behavior can be found in both curves. Since the peak area of the absorption band corresponds to the adsorbed amount of dye, it is clear that the adsorption gradually proceeds with immersing time, and reaches an apparent saturation approximately after 2 h. Figure 2 also shows that the adsorption rate on the ODA LB film is slower than that on the LB films of longchain alkylammonium salts in adsorptions of both NO and MO.15,18 This may be due to a feature of the ODA LB film, i.e., the less hydrophilic property of the polar headgroup (amino group) and the more closely packed hydrocarbon chains in the layer structure than those of the LB films of long-chain alkylammonium salts. As shown in Figure 3, the layer dependence of the adsorbed amount was checked for NO and MO at the immersing time of 2 h. In both adsorptions, the specific adsorbability, increasing proportionally with increasing number of layers, is observed for the first several layers. (31) The direction of the transition moment was confirmed with stretched films of poly(vinyl alcohol) containing NO. (32) The molecular extinction coefficient of NO is about 5-fold larger than that of MO in the dye-complexed films.

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Figure 3. Peak area obtained from UV-vis spectra against the number of layers of the ODA LB film at an immersing time of 2 h: O, NO adsorption; 4, MO adsorption.

Consequently, it is clear that the adsorption quantitatively occurs with the deposited amount of ODA on the substrate. The adsorbed amount of dyes, however, shows a constant value above ten layers for the NO adsorption and above seven layers for the MO adsorption. This means that the adsorptions of dyes are not completed above these numbers of layers and that the adsorption rate of NO is faster than that of MO. The knick point in the figure gives us the maximum number of layers with complete dye adsorption within a given immersing time. Besides, we carried out experiments with different immersing times and estimated the knick point as well. The results showed that the relation between the number of dye-adsorbed layers and the immersing time was not linear, but when the number of deposited layers increased, the immersing time required for complete adsorption sufficiently became longer. This proves that the penetration of dye into the LB film is a rate-determining step in the adsorption process. Namely, the outer layer resulting from adsorption is considered to disturb further penetration of dye into the inner layer, and thereby, despite the strong interaction between ODA and dyes, the penetration rate becomes quite smaller as the adsorption proceeds deeper from the surface of the LB film. Furthermore, when MO is compared with NO, a slower adsorption rate of MO implies smaller permeability of the MO-complexed ODA layer, which may be caused by the difference in the adsorbing state between MO and NO. In the following adsorption experiments, we, hence, employed an immersing time of 2 h and the number of layers under the knick point in Figure 3 (mainly five layers) so that the adsorption of dyes could be completed. Next, we measured adsorption isotherms of NO and MO on the five-layer ODA LB films at 20 °C, and obtained isotherms indicating the presence of high affinity between the ODA LB film and the dyes. That is, the isotherms showed appreciable adsorption of dye even at a quite dilute concentration, and the NO and MO adsorptions reached saturation at concentrations of 3.0 × 10-5 and 1.0 × 10-4 mol dm-3, respectively. In analogy with the LB film of long-chain alkylammonium salts, such a high adsorbability of the ODA LB film is attributable to the ionic interaction between the cationic LB film and the anionic dyes. It is, therefore, reasonable to predict that the ODA LB film is preferable for the adsorption of the other adsorbates with a negatively charged group. The adsorbability of the ODA LB film is, also, affected by the pH of the immersing solution. Figure 4 shows

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Figure 4. pH dependence on adsorbability of the five-layer ODA LB film: O, NO adsorption; 4, MO adsorption.

changes in the adsorbed amount of dyes as a function of pH. The difference in adsorption behavior between NO and MO is especially found in the acidic pH range; i.e., contrary to the adsorption of NO, the adsorption of MO is hardly admitted at pH below the color change interval. Since the adsorption of dyes is mainly caused by ionic interaction, different ionic states of dyes as well as that of ODA sufficiently influence the adsorption characteristics. In this case, protonation of the dimethylamino group in the MO molecule (the value of pKa is 3.5) would be responsible for the decrease in the adsorbability of MO. On the other hand, as pH increases, an abrupt drop in adsorbability is observed in both adsorptions, at around pH 7 in the NO adsorption and pH 9.5 in the MO adsorption. These drops are attributable to the change in the ionic state of ODA, that is, the decrease in the degree of ionization of the amino group. Also, the difference between their critical pH values may be explained by the distinctive molecular environments of ODA in each LB film, for example, the surface potential being related to the surface pH. As will be stated in the next section, structures of the NO- and MO-adsorbed ODA LB films show a remarkable distinction between them, and so it is reasonable to predict that the surface potentials at both film surfaces are different from each other. These critical pH values below 10.1 (pKa for long-chain alkylamines) are consistent with the relationship between the pH at the surface with a certain potential and that in the bulk. In addition, the critical pH values for the monolayers on the dye-containing subphases are also admitted in the π-A isotherms, e.g., the change from curve c′ to curve e′ in Figure 1 for NO. These pH values are larger than those for the corresponding LB films, and this would be analogously understood by the difference in the molecular environment between the floating monolayer and the deposited film. The pH dependence described above suggests that such an LB film is a favorable system to control adsorbability by changing the pH of the solution. Structural Characterizations of an As-Deposited ODA LB Film and a Dye-Adsorbed ODA LB Film. To get information on the structural changes with dye adsorption, spectroscopic measurements were carried out for the ODA LB film. Figure 5 gives FTIR-transmission and -RA spectra measured for eight layers of the ODA LB film before (a) and after (b) NO adsorption. As a reference, we also depicted a spectrum for the crystalline NO in the figure. In Figure 5b, the bands coming from NO are presented in addition to those from ODA; e.g., the

Azo Dyes on a LB Film of ODA

Langmuir, Vol. 16, No. 16, 2000 6617 Table 1. Tilt Angles of the Hydrocarbon Chains and the Dye Molecules in the LB Films tilt angle/deg MO-adsorbed NO-adsorbed ODA LB film ODALB film ODA LB film hydrocarbon chain dye molecule

4.5

12.4 32.5

13.3 78.2

Figure 6. Polarized UV-vis spectra of the NO-adsorbed ODA LB film at R of 0 and 45°. The solid and dotted lines indicate spectra of p-polarized light and s-polarized light, respectively.

Figure 5. FTIR-transmission and -RA spectra for the eightlayer ODA LB films before (a) and after (b) NO adsorption. In addition to the ODA spectra, the spectrum for the NO crystalline is presented in (a) as well.

bands at 1599 and 1542 cm-1 are due to the stretching vibration modes of the aromatic ring in the NO molecule. In the RA spectrum, an electrical field vector of the incident light is perpendicular to the substrate surface, and thus, the components of the transition dipole moment perpendicular to the substrate surface absorb strongly in this mode. Accordingly, the result that the above bands in the RA spectrum appear stronger than those in the transmission spectrum implies qualitatively that the transition dipole moments of these bands are oriented almost perpendicularly to the substrate surface. On the other hand, CH2 symmetric and antisymmetric stretching vibration bands in Figure 5b, coming from the hydrocarbon chain in the ODA molecule, appear at 2851 and 2920 cm-1, respectively. The latter wavenumber is 2 cm-1 larger than that observed for the transmission spectrum in Figure 5a. It suggests that most of the hydrocarbon chains take a trans zigzag conformation with a little gauche characteristic.33 Also, when NO is adsorbed on the ODA LB film, the intensities of transmission absorbances become smaller, while those of RA absorbances contrarily become larger. However, as shown in Figure 5b, the absorptions of the transmission spectrum are still stronger than those of the RA spectrum. The intensity ratio of transmission absorbance to RA absorbance allows us to estimate a tilt angle of the long axis along the hydrocarbon chain in the (33) Nuzzo, R. G.; Korenic, E. M.; Dubois, L. H. J. Chem. Phys. 1990, 93, 767.

LB film.34 The tilt angles calculated for the ODA LB films before and after the NO adsorption are listed in Table 1 together with the result for the MO-adsorbed ODA LB film. As seen in the table, the angle of the hydrocarbon chain before dye adsorption is found to be 4.5° from the surface normal, pointing to a nearly perpendicular orientation of the ODA molecule to the film surface. On the other hand, an enlargement of the tilt angle occurs as the adsorptions of the dyes proceed. In both cases of MO and NO, the increments of the angle are almost the same value of 8-9°. Such a change in the tilt angle is probably required to form another layer structure in the LB film. Furthermore, a tilt angle of the NO molecule adsorbed on the ODA LB films is estimated from polarized UV-vis spectra. Figure 6 exhibits the spectra measured for nine layers of the NO-adsorbed ODA LB film at incident angles (R) of 0 and 45°. Because the transition moment of the chromophore of NO is almost parallel to the long axis of the molecule, the result that there is no appreciable intensity difference between the p- and s-polarized spectra at the normal incidence reveals uniaxial distribution of the long axis of the NO molecules around the surface normal. On the other hand, anisotropic spectra are observed at R of 45°; i.e., the absorption band of the s-polarized spectrum appears stronger than that of the p-polarized spectrum. From a dichroic ratio of the absorbances at 475 nm, we estimated the tilt angle of the NO molecule in the LB film by assuming the refractive index of the LB film to be 1.50 (the calculated value is given in Table 1).35 Also, a blue-shifted band at 415 nm is found to appear on the p-polarized spectrum only, and it becomes stronger if the incident angle increases. This shows that the transition moment of the shoulder band distributes perpendicularly to the film surface. However, the shoulder band could not be observed in the experiments carried out by using the stretched film of poly(vinyl alcohol) (34) Umemura, J.; Kamata, T.; Kawai, T.; Takenaka, T. J. Phys. Chem. 1990, 94, 62. (35) Akutsu, H.; Kyogoku, Y.; Nakahara, H.; Fukuda, K. Chem. Phys. Lipids 1975, 15, 222.

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Figure 7. Adsorption-time dependence of X-ray diffraction patterns for NO adsorption on the nine-layer ODA LB films.

instead of the ODA LB film at the same conditions. Therefore, the band at 415 nm would be ascribed to the specific arrangement of NO molecules in the ODA LB film. Besides, such a blue-shifted shoulder was not observed for NO in water but ethanol, so the specific arrangement is supposed to be present in the microenvironment of rather small dielectric constant, probably the boundary between the polar headgroups and the hydrocarbon-chain moiety. Similarly to the adsorption of NO, we also calculated the tilt angle of the MO molecule adsorbed in the LB film, and listed it in Table 1. As far as the tilt angles are concerned for both dyes, it is to be emphasized that the adsorbing states of the dyes exhibit a remarkable distinction between MO and NO. That is, the tilt angle of 32.5° for MO with respect to the surface normal indicates an edge-on structure, whereas that of 78.2° for NO means a lie-flat structure. These spontaneous orientations of the dyes can be understood for the following reason. Since MO and NO have a sulfonato group at a terminal position of the molecules in common, such a difference in the molecular orientation is ascribed to the properties of each functional group at the opposite side in the molecules. Namely, the sulfonato group interacts with the amino group of the ODA molecule, and thereby, the edge of the dye molecule is connected at the polar head moieties of the ODA layer. For the MO molecule, the dimethylamino group at the opposite end in the molecule has the less hydrophilic property except in a particular range of acidic pH, so it is likely that MO adsorbs with leaning of the molecule toward the hydrophobic part in the LB film. Considering a rodlike structure of the MO molecule, the hydrocarbon chains surrounding MO seem to influence the orientation of MO so as to coordinate the arrangement of MO with the array of the hydrocarbon chains. As for the NO molecule, the hydroxyl group at the side opposite the sulfonato group shows a hydrophilic property and seems to be interactive with the amino group of ODA. Accordingly, NO in the ODA LB film is not supposed to exist in the parisade layers but to be laid in the polar head moieties of the film. Simultaneously, the above results imply the possibility of making LB films in which functional molecules are incorporated with a desired orientation by means of designing the molecular structure of the adsorbates. To know the periodic structure of the LB films, we next applied X-ray diffraction measurements to the ODA LB film with the dye adsorptions at various conditions. Figure 7 shows the immersing-time dependence of the small-

Takahashi et al.

angle reflection X-ray patterns. In this figure, the ODA LB film (before adsorption, 0 min) clearly shows a regular layer pattern with Bragg peaks representing the layerlayer distance of 5.2 nm. If the length of the ODA molecule in the fully stretched conformation is assumed to be 2.4 nm,36 the layer-layer distance represents a bilayer thickness. Furthermore, the value of 5.2 nm is almost the same as that obtained from the powder pattern of ODA, and therefore, it is most likely that the ODA LB film is deposited as a Y-type multilayer. From the result that the bilayer thickness is somewhat larger than twice the extended length of the ODA molecule, the presence of the water layer, so as to screen the mutual charge repulsion of the headgroups, is predicted in the bilayer structure. In addition, the phenomenon so-called the odd-even oscillation of the X-ray diffraction profiles can be found in the pattern,37,38 and hence, the thickness of an electrondeficient layer between the two hydrocarbon-chain ends in a juxtaposition should be taken into account as well. The diffraction pattern of the ODA LB film also shows several peaks in the region of 2-5°.39 The number of peaks in this region was found to increase with increasing thickness of the LB film, and consequently, these peaks are confirmed to be attributable to the interference between the reflections from the surface of the solid substrate and that of the LB film. In the case of the ninelayer ODA LB film (Figure 7), 2θ values of the peaks are identical to the calculated values for the thickness of 9 × 2.6 nm, which would be additional proof of the highly ordered layer structure with a uniform and flat surface of the LB film. On the other hand, new peaks appear in the diffraction patterns of the NO-adsorbed ODA LB films. As seen in the figure, the intensities of these peaks gradually increase with increasing immersing time, whereas the peaks which can be observed before the NO adsorption become smaller and disappear if the adsorption of NO is completed. This is due to rearrangement of the layer structure in the ODA LB film. With regard to the MO adsorption, we have recorded the immersing-time dependence of diffraction patterns, and obtained results similar to those of the NO adsorption. From the peaks appearing after dye adsorptions, the layer-layer distances of NO- and MO-adsorbed ODA LB films are found to be 4.6 and 3.7 nm, respectively. These values are also considered to correspond to the bilayer thickness in the LB film; nevertheless, they are smaller than twice the monolayer thickness even if the reorientation of the hydrocarbon chains is taken into account. To explain the decrease in the bilayer thickness accompanying the change in the layer structure of the LB film, an interdigitated structure should be proposed where the hydrocarbon-chain ends in the neighboring layers overlap each other. This arrangement of the hydrocarbon chains can be understood as follows. Similarly to the change in the π-A isotherms, showing expansion of the molecular area by means of the interaction with dyes, the adsorption of dye into the LB film probably gives rise to extension of the intermolecular distance between the ODA molecules (in-plane direction). Consequently, the LB film needs to compensate for the lack of lateral interaction between the hydrocarbon chains. (36) This length was calculated from the interatomic distances and bond angles. (37) Matsuda, A.; Sugi, M.; Fukui, T.; Iizima, S.; Miyahara, M.; Otsubo, Y. J. Appl. Phys. 1977, 48, 771. (38) Peltonen, J. P. K.; He, P.; Rosenholm, J. B. Langmuir 1993, 9, 2363. (39) In Figure 7, only three peaks corresponding to l ) 10-12 reflections in (00l) can be admitted above 3.5°, where the patterns are magnified 100 times.

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Figure 8. Schematic representation of probable structures for the ODA LB film (a), MO-adsorbed ODA LB film (b), and NOadsorbed ODA LB film (c).

On the basis of the orientation data of ODA and dyes as well as the results from the X-ray diffraction measurements, we schematically illustrate models for the ODA LB film and the MO- and NO-adsorbed ODA LB films in Figure 8. The ODA LB film (Figure 8a) is considered to have a compact and well-ordered Y-type multilayer structure, in which the hydrocarbon chains take an almost perpendicular orientation with respect to the surface normal. When the azo dyes adsorb on the LB film, the ODA multilayer changes to the interdigitated arrangement and the tilt angle of the hydrocarbon chains, at the same time, increases by 8-9° in both MO and NO adsorptions. As shown in Figure 8b, MO in the LB layer exists at the base of the hydrocarbon chains with the edgeon orientation. If the length of the long axis of the MO molecule is 1.3 nm,36 the thickness of a MO-adsorbing part in the LB films is calculated to be 1.1 nm. As the length of the ODA molecule with a trans zigzag conformation is 2.4 nm, the thicknesses of an overlapping part and a void space in the interdigitated structure are evaluated to be 1.0 and 1.3 nm, respectively. Considering that the latter value may be somewhat overestimated because the water layer extending the head-to-head distance is omitted from the calculation, it is most likely that MO fits into the void space around the base of the hydrocarbon chains. Such a molecular arrangement of MO is favorable for not only ionic interaction but also hydrophobic interaction. This compact configuration of the MO-adsorbing layer probably acts as a barrier which decreases the penetration rate of MO into the inner layer of the ODA LB film. In contrast, the spontaneous adsorption of NO results in the lie-flat orientation at around the polar headgroups in the LB film (Figure 8c); thereby the bilayer thickness becomes larger by 0.9 nm than that of the MO-adsorbed ODA LB film. However, it is difficult to estimate the contribution of the NO molecules to extension of the polar head moiety on the basis of the present data because the thickness of the overlapping part of the hydrocarbon chains is concurrently changeable. Compared with the MO-adsorbed ODA LB film, the NOadsorbed ODA LB film is considered to have a less compact layer structure, and this would be responsible for the faster adsorption rate of NO than that of MO. Taking the tilt angles into consideration, the amounts of ODA and dyes (MO and NO) in the dye-adsorbed ODA LB films were evaluated from the intensities of FTIR-

transmission spectra at 2920 cm-1 and UV-vis spectra at 355 and 475 nm, respectively. From these values, we subsequently estimated molecular-number ratios of ODA to dyes in the LB films, and obtained 1.00:0.54 for the MO-adsorbed ODA LB film and 1.00:0.13 for the NOadsorbed ODA LB film. Since ODA has a monocationic property and MO has a sulfonato group in the molecule, MO should interact with ODA at a ratio of ODA to MO of 1:1. In contrast, if the hydroxyl group in the NO molecule slightly ionizes in addition to the sulfonato group and behaves as a divalent anion, the adsorption of NO is expected to occur at half the molecular-number ratio of ODA to MO, i.e., at a ratio of ODA to NO of 1:0.5. Nevertheless, the above results of the molecular-number ratio are fairly smaller than these stoichiometrical values, which suggest that adsorption of neither dye has taken place completely. As pointed out by Gains,40 one of the reasons for such a smaller ratio is supposed to be formation of carbamate in the ODA LB film. Because the FTIRtransmission spectrum in Figure 5a shows an absorption band at 1569 cm-1, part of ODA molecules in the LB film can be interpreted as having reacted with carbon dioxide in the atmosphere during the deposition of the ODA monolayer from the alkaline subphase. Another possibility is some restrictions present in the adsorption process of dye such as steric hindrance. Particularly, in addition to the fairly flat and large structure of the NO molecule, the lie-flat orientation in the adsorbing state is probably inferior for the stoichiometrical adsorption; i.e., NO adsorbed primarily may widely cover the amino groups of ODA, so that it tends to prevent the following adsorption of NO. However, it should be noted that the NO molecules, as presumed from the shoulder peak in the polarized UVvis spectra, would be stacked at around the polar head moiety with particular orientation. In other words, the ODA LB film has an appropriate microenvironment for facilitating the aggregation of NO, which the stretched PVA film does not offer. Atomic Force Microscopy Studies of the Surface Structure of the ODA LB Film. To study morphological changes of the ODA LB film induced by the adsorption of dye, we have characterized surface images of the LB films by means of AFM. In the experiments, we employed a silicon wafer as a solid substrate because the bare surface (40) Gains, G. L. Nature 1982, 298, 544.

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Figure 9. AFM images (1 µm × 1 µm) of the ODA monolayer (a) and the five-layer ODA LB film (b) on a silicon wafer.

Figure 10. AFM images of the ODA monolayers with NO adsorption (a) and MO adsorption (b) on a scan area of 1 µm × 1 µm.

of the silicon wafer is flatter and less scratched than that of the other substrates such as a slide glass. Figure 9 exhibits typical AFM images of the ODA LB films deposited at a surface pressure of 45 mN m-1 from the alkaline subphase above pH 10.1. The image of the monolayer (Figure 9a) reveals almost a uniform layer except projections of dust, but strictly speaking, the monolayer has a somewhat undulating surface with a surface roughness of less than 1.5 nm. From the features of the image, the monolayer seems to consist of large domains with an average diameter of approximately 0.13 µm. Similarly to the long-chain fatty acids,41 ODA spread on the surface of an alkaline subphase at 20 °C is supposed to form islands due to the steady lateral interaction between molecules, and therefore, the texture of the gathering islands probably remains even after the deposi(41) Kajiyama, T.; Oishi, Y.; Uchida, M.; Tanimoto, Y.; Kozuru, H. Langmuir 1992, 8, 1563.

tion of the LB monolayer. Figure 9b is an image of the five-layer ODA LB film showing a fairly flat surface except several spots of holes. In the subsequent adsorption process, penetration of dyes is considered to take place partially from the holes as well as from the film surface. As evaluated by section analysis, the depth of the holes corresponds to the thickness of a double layer or 2 times a double layer. These defects of the ODA LB film are probably responsible for gaps presumed to be present in kind between the domains and/or shrinkage of the molecular area occurring in the drying process of the deposited layers. However, it will be possible to fabricate the defect-diminished film by improving the conditions of the preparation process of the LB film. On the other hand, when the dye adsorbs on the ODA LB film, a typical change in the layer structure is observed. Figure 10 shows the surface characteristics of the ODA monolayers with the adsorptions of NO (a) and MO (b).

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Figure 11. AFM images of the five-layer ODA LB films with NO adsorption (a) and MO adsorption (b) on a scan area of 1 µm × 1 µm.

As shown in the figure, the most striking feature is the distinctive growth of islands protruding out from the monolayer phase. These kinds of changes are expected in the cases where the adsorption of dye into the monolayer leads to area expansion and thus results in a failure to retain the perfect planar surface. In both adsorptions, the islands pushed out from the major phase appear to consist of a few patches in tiers, but the differences in shape between them are obvious. Namely, in the image of the MO-adsorbed ODA monolayer, we find that the mean size of the islands is much larger than that of the NO-adsorbed monolayer, and individual patches have a plane surface with linear-shaped fringes, implying higher crystallinity in plates. This may arise from the particular adsorbing state of MO as well as the interdigitated structure of the hydrocarbon chains, which enhances the lateral interaction between the ODA molecules. Besides, Figure 11 gives AFM images of the five-layer ODA LB films captured after adsorptions of NO (a) and MO (b), and they show the textures of randomly distributed patches, indicating reorganization of the layer structure of the LB film. Particularly, the MO-adsorbed ODA LB film exhibits a surface pattern like the bark of a pine tree, and the step height corresponds to the thickness of a double layer. In each adsorption, since the surface image inherits the morphological features from the corresponding image as shown in Figure 10, the multilayer film of ODA with dye adsorption would be regarded as a piled up film of the dye-adsorbed monolayers. Although the surface characteristics of the dye-adsorbed LB films reveal the geometry of a rather disordered distribution of domains, they confirm the interpretation of maintaining the layer structure in which the patches keep the arrangement parallel to the substrate surface.

adsorbability with a high affinity for these azo dyes due to ionic interaction, which similarly acted between the floating ODA monolayer and the dye molecules in the aqueous subphase. The adsorbability was influenced by pH, depending on the ionic characters of both the ODA LB film and the dyes. These results imply that the ODA LB film is also applicable as an adsorption matrix to the other negatively charged adsorbates, e.g., functionalized molecules, dispersed particles, etc. Furthermore, the structural changes attending the adsorption were examined mainly by the spectroscopic methods. The tilt angle of the hydrocarbon chains, indicating almost perpendicular orientation to the film surface, was enlarged by 8-9° after the adsorption of the dyes. On the other hand, distinction in the adsorbing state was obviously observed for NO and MO; i.e., NO took the lie-flat configuration with respect to the film surface, while MO exhibited the edge-on configuration. At the same time, the bilayer thickness in the film was decreased for both adsorptions, indicating the formation of rather compact layer structure in which the hydrocarbon chains took an interdigitated structure. This would be responsible for the smaller adsorption rate of the ODA LB film than that of the LB film of long-chain alkylammonium salts. AFM images gave us the direct indication concerning the rearrangement of the layer structure taking place in the adsorption process. In contrast to the ordered packing before adsorption, the surface of either dye-adsorbed ODA LB film exhibited a characteristic texture of the domain structure where many patches distributed parallel to the substrate surface. Information on the molecular arrangement obtained in this study is expected to be helpful for understanding the adsorption behavior of various adsorbates on cationic LB films.

Conclusions

Acknowledgment. K.K. is grateful for financial support from a Grant-in-Aid for science research (09650033) from the Ministry of Education, Science, Sports and Culture of the Japanese Government.

In this study, the adsorption behavior and structural characterization including topographic analysis were examined for the cationic LB film of ODA by using NO and MO as adsorbates. The ODA LB film exhibited

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