Thin Solid Films 284-285 (19%) 115-I 18

Adsorbability and structural characterization of Langmuir-Blodgett films of N,N’-w-p-xylene-bis- [ stearyldimethylammonium chloride]

Masashi Takahashi a, Koichi Kobayashi b, Kyo Takaoka b, Kazuo Tajima ay* a Department of Chemistry, Kanagawa University, Yokohama 221, Japan

’ Department of Chemistry, Musashi Institute of Technology, Setagaya, Tokyo 158, Japan

Abstract

The adsorbability of cr-naphthol orange (NO) onto the dicationic Langmui-Blodgett (LB) films of N&‘-o-p-xylene-bis-[stearyldi- methylammonium chloride] (XSAC) with octadecane was investigated. As a result, the specific adsorbability of mixed LB films was observed for NO molecules at the molar fraction of 0.25. The film structures before and after dye adsorption were also studied by using Fourier transform infrared spectroscopy, UV-Vis spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. It was found that NO molecules have the same molecular arrangements in both NO-adsorbed LB films and XSAC-NO-complexed LB films.

Keywok Langmuir-Blodgett films; Adsorption; Interdigitated structure; X-ray diffraction

1. Introduction

Recently, much attention has been focused on cationic Langmuir-Blodgett (LB) films containing long-chain alkyl- ammonium salts because of specific adsorbability which is not expected from the characteristics of anionic LB films. In previous studies, we have elucidated that the cationic mono- layers of long-chain quatemary ammonium salts (distearyl- dimethylammonium and tristearylmethylammonium chlo- rides) could be deposited on solid substrates under the surface pressure just below each collapse pressure [ 11, and that these cationic LB films have remarkable adsorbability for dye materials such as cw-naphthol orange and methyl orange due to the presence of ionic interactions [ 2,3]. Therefore, the LB film of a dicationic substance may be expected to have higher adsorbability for the various functional materials; for exam- ple, polymers, dispersed materials, proteins, and biological colloids.

In this study, we attempted to prepare the dicationic LB films of material having two ammonium groups in a molecule, and then investigated the adsorbability of dicationic LB films by using cY-naphthol orange molecules as an adsorbate. Fur- thermore, structural characterization of LB films with and without adsorption of cu-naphthol orange was carried out mainly by employing spectroscopic techniques.

* Corresponding author.

0040~6090/96/$15.00 0 1996 Elsevier Science S.A. All rights reserved SSDIOO40-6090(95)08284-O

2. Experimental details

NJV’-w-p-xylene-bis- [ stearyldimethylammonium chlo- ride] (XSAC), which was kindly supplied from Kao Co. Ltd., was used as a dicationic film material. Its structural formula is shown in the inset of Fig. 1. Dye material of (Y- naphthol orange (NO), commercially available, was used as an adsorbate without further purification. Octadecane of

60~ --. \ :

: MOLECULAR AREA I nm*~mdec-’

Fig. 1. n-A isotherms of XSAC/octadecaw mixed monolayers at 20 “C. The solid lines indicate the isotherms on distilled water (X-c: a, 1.0; b, 0.7; c, 0.5) and the dotted lines on 1.0X 10m4 M of aqueous NO solution (X,,: a’, 1.0; b’, 0.5; c’, 0.25). Inset shows the st~ctural formula of XSAC molecule.

116 M. Takahashi et al. /Thin Solid Films 284-285 (19%) 115-l 18

Table 1 Tilt angles of the alkyl chain and NO molecule in three types of LB films at different Xx,,,

&.%4c

0.25

1.0

Alkyl chain NO molecule Alkyl chain NO molecule

Tilt angle / degree

XSAC LB film

5.6

8.1

NO-adsorbed LB film XSAC-NO-complexed LB film

16.5 15.4 70 79 17.6 16.3 79 75

spectroscopic grade was mixed at various molar fractions with XSAC to enhance the lateral interaction between alkyl chains in XSAC film. The monolayer properties were exam- ined by surface pressure-area (T-A) isotherms. The XSAC monolayers with or without octadecane were transferred onto solid substrates (Au-plated brass for Fourier transform infra- red (FTIR) reflection absorption (RA) spectroscopy, CaF, plate for PTIR transmission spectroscopy, quartz plate and glass plate for W-Vis spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy) under the conditions of 35 mN m-i at 15 “C. Adsorption experiments were per- formed as follows. The LB film was dipped into aqueous NO solution ( 1 .O X 10e4 M) for a given period of time, and then removed from the solution after establishing an adsorption equilibrium. These LB films were hereafter referred to as “NO-adsorbed LB film”. The amount of NO molecules adsorbed on the LB film was evaluated by using a W-VIS- NIR scanning spectrophotometer (Shimadzu Co. Ltd., W- 3 1OOPC). The structures of LB films have been characterized using a FTIR spectrometer (JASCO FT/IR-8900) equipped with a MCT detector, an X-ray diffractometer (Rigaku Denki Co. Ltd., RAD-RIIC) with Cu Kcr as the X-ray source, and an X-ray photoelectron spectrometer (Surface Science Instruments, SSX 100).

3. Results and discussion

3.1. Adsorption of NO molecules on XSAC LBJilm

The r-A isotherms for mixed monolayers of XSAUocta- decane were measured at 20 “C as shown in Fig. 1. The apparent molecular area at constant surface pressure decreased proportional to molar fraction of XSAC (Xx& in the mixed monolayer. This indicates that XSAC molecules have mixed ideally with octadecane molecules in monolayers.

By using such monolayers, the fabrications of XSAC LB films were carried out at various Xx,,,. FTIR transmission spectra were measured for 2-S-layer XSAC LB films. Four main bands at 2 851 cm-‘, 2 919 cm-‘, 1 575 cm-‘, and 1469 cm-’ were observed. The former two bands were assigned to CH2 symmetric and antisymmetric stretching vibration bands, respectively, corresponding to the trans-zig- zag conformation of alkyl chain. The latter two bands were

assigned to the stretching vibration bands of a benzene ring in the XSAC molecule. The uniform and regular deposition of the XSAC monolayer on the substrate was confirmed by the linear relationship between the absorbance of each band and the number of layers.

FTIR reflection absorption (RA) spectra were also meas- ured for 2-8-layers of XSAC LB film. Compared with FTIR transmission spectra, the C-H symmetric and antisymmetric stretching vibration bands were weakly observed in RA spec- tra. The intensity ratio of transmission absorbance to RA absorbance allows us to estimate the orientation angle of alkyl chain [ 41. The results are cited in Table 1, together with other orientation angles. The tilt angle of the alkyl chain in the XSAC LB film was 5.6” at 0.25 =Xx,, and 8.1” at 1 .o = xx,,,. These values indicate that the alkyl chains in XSAC LB films are nearly perpendicular to the film surface, similarly to those observed for LB films of cadmium stearate

[51. Subsequent experiments were carried out for NO adsorp-

tion on the XSAC LB film. The amount of NO adsorbed on the XSAC LB film was evaluated from the absorption inten- sity of the W-Vis spectra. Fig. 2 shows the peak area of a NO absorption band at 474 nm adsorbed as a function of X XSAC- Although the first layer of XSAC LB film ought to ionically interact with the glass surface, the adsorbed amount of NO was appreciably observed on the monolayer, and more- over, the adsorbability of NO molecules indicates thedepend-

L I

0 0.5 1.0 XIVC

Fig. 2. Adsorption characteristics of NO on the XSAC LB film at various xx,,,. 0, monolayer; A, 5 layers; 0.9 layers.

M. Takahashi et al. /Thin Solid Films 284-285 (I 9%) 115-I 18 117

ency of film composition at 5 or 9 layers. In particular, the maximum adsorption of NO occurred at 0.25 =Xx,,. There- fore, the adsorption characteristics of NO molecules on the LB film were investigated for the distinctive state observed at 0.25 =Xx,,.

In order to clarify the molecular orientation of NO, polar- ized UV-Vis spectra were measured for the 9 layers of a NO- adsorbed LB film. Although there was no appreciable intensity difference between p- and s-polarized spectra at the normal incidence, the s-polarized spectrum was strongly observed in the measurement at the incident angle (cu) of 45”, and also, the peak maximum of the p-polarized spectrum showed a slightly blue shift. In this measurement, the absorp- tion maximum of NO at 474 nm is attributed to the transition moment of the NO chromophore, and the long axis of the NO molecule is considered to be almost parallel to transition moment of the chromophore. Under the condition of uniaxial orientation of the transition moment, the orientation angle of NO molecules can be estimated from dichroic ratio of these spectra by [ 63

APIA, = 1 + ( 1 ln*) sin* a( 2cot* 8 - 1) (1)

where A, and A, are an absorbance measured with the p- and s-polarized light, respectively, n is the refractive index of the LB film and 13 is the tilt angle of the transition moment from the surface normal. Assuming the n value to be 1.50, 8 can be calculated at 0.25 and 1 .O =Xx,,. These results are listed in Table 1. It was found that the transition moment, i.e. the long axis of the NO molecule, took a tilt angle of about 70- 80” from the surface normal, indicating that the long axis of the NO molecules distributed almost parallel to the LB film surface.

Similarly, the tilt angle of the alkyl chain was determined for NO-adsorbed LB films. The values obtained are also shown in Table 1. It can be seen that tilt angles of the alkyl chain in the XSAC LB films become about 10” larger than that before NO adsorption. These increases may be caused by the favorable extension of the intermolecular distance among XSAC molecules due to the specific interaction between XSAC and NO molecules.

In order to elucidate the atomic composition change before and after NO adsorption, X-ray photoelectron spectroscopy (XI’S) was applied to the XSAC LB films and NO-adsorbed LB films at 0.25 and 1 .O =Xx,, for both films. In XPS spectra, the peak coming from Cl 2p was observed for XSAC LB films, but not observed for NO-adsorbed LB films, while the peaks due to S 2s and S 2p were observed for NO-adsorbed LB films, but not observed for XSAC LB films. Accordingly, these results suggest that the XSAC molecule forms the complex with NO molecule by Coulomb interactions.

3.2. XSAC-NO-complexed LBjilm

Next, we tried to compare the adsorbability of NO mole- cules onto the LB film with that onto the XSAC monolayer at the air-water interface. The T-A isotherms of XSACI

0 1 2 3 4 5 6 7 6 9 10 NUMBER OF LAYERS

Fig. 3. Relationship between the peak area of absorption band at 474 nm and number of layers of a XSAC-NO-complexed LB film. X,,: 0,0.25; A, 1.0.

octadecane monolayers were measured on aqueous NO solu- tion as shown in Fig. 1. The surface pressure of XSAC mono- layers on aqueous NO solution increased steeply in T-A iso- therms, but the monolayer was more expanded and more stable than that on distilled water. Such a behavior was con- sidered to be due to the formation of a XSAC-NO complex.

In order to investigate the structural characterization by a spectroscopic method, the multilayer depositions were car- ried out for the monolayers of 0.25 and 1 .O = X,s, on an aqueous NO solution. These LB films are conventionally referred to as “XSAC-NO-complexed LB film”. Fig. 3 shows the relationship between the number of layers and the peak area of the UV-Vis spectra. It was seen that tbe peak area was proportional to the number of layers, and that there was no difference in peak area for the films of X,,,, = 0.25 and 1.0. This means that NO molecules adsorb uniformly onto any XSAC monolayers, and that XSAC-NO-complexed monolayer is regularly deposited on the substrate. Moreover, it might be mentioned that the adsorption of NO depends on the density of XSAC in monolayers irrespective of Xx,,,.

In addition, we examined the adsorbing state of NO mol- ecules in the XSAC-NO-complexed LB films by measuring polarized UV-Vis spectra, and obtained the similar spectral changes to those for NO-adsorbed LB films. Absorption max- ima, however, appeared at the 10 nm longer wavelength. This shift may be explained by the idea that the mobility of rr electrons in the NO molecule increases because NO mole- cules adsorb with less restriction. We estimated the orienta- tion angles of NO molecules from Eq. ( 1). As shown in Table 1, the tilt angles were calculated to be 79” and 75” for the LB films at 0.25 and 1.0 =XxsAc, respectively. These values were roughly equal to those of NO-adsorbed LB films within an experimental error.

The tilt angles of the alkyl chain in XSAC-NO-complexed LB films were also obtained. The results are shown in Table 1. These values were close to those calculated for NO-adsorbed LB films. Considering both orientation angles of the NO

118 M. Takahashi et al. /Thin Solid Films 284-28.5 (1996) 115-118

(a)XSAC LB film (b) NO-adsorbed LB film

Fig. 4. Schematic representation for the interdigitated structures of a XSAC LB film (a) and a NO-adsorbed LB film (b)

molecule and the alkyl chain, it was seen that the configura- tions of XSAC and NO molecules in XSAC-NO-complexed LB films were almost the same as those in NO-adsorbed LB films.

3.3. Adsorbing state of NO molecules

By taking into account the results in Table 1, we calculated the amounts of XSAC and NO molecules from the intensities of FTIR transmission spectra at 2 919 cm- ’ and those of UV-Vis spectra at 474 nm, respectively. Furthermore, using these values, we obtained the molecular ratio of XSAC to NO in the LB films. For the NO-adsorbed LB film, the molecular ratio was calculated to be 1: 1.07 at 0.25 =Xx,,, and 1:0.65 at 1 .O = Xxs*c, while molecular ratio was calculated to be 1:0.70-0.80 for XSAC-NO-complexed LB films at 0.25 and l.O=Xxs*c.

The repeat distances of three types of LB films; XSAC LB films, NO-adsorbed LB films, and XSAC-NO-complexed LB films, which had been prepared at 0.25 and 1 .O = Xx,,, were determined by X-ray diffraction measurements. The results were that XSAC LB films were 3.71 nm and 3.36 nm, NO-adsorbed LB films were 4.18 nm and 4.11 nm, and also XSAC-NO-complexed LB films were 3.91 nm and 3.92 nm for 0.25 and 1 .O = XxsAc, respectively. Since the repeat dis- tance was larger than the monolayer thickness expected from the XSAC molecule, it was confirmed that these LB films were deposited as Y-type multilayers. However, these bilayer thicknesses were incredibly shorter than two times the mon- olayer thickness. This may indicate that these bilayer films take an interdigitated structure as schematically shown in Fig. 4. The bilayer thickness of XSAC LB films was obvi- ously different, depending on the film compositions of X xsAc = 0.25 and 1 .O as deposited. On the other hand, the bilayer thicknesses of both NO-adsorbed LB films and XSAC-NO-complexed LB films were almost independent of

X xsAc. Furthermore, the bilayer thickness of NO-adsorbed LB films was about 0.2 nm larger than that of XSAC-NO- complexed LB films. This difference suggests the presence of somewhat distorted lamella in the NO-adsorbed LB film which is caused by the penetration of NO molecules into the XSAC LB film.

Assuming that the length of its alkyl chain was 2.26 nm and the thickness of the polar head moiety was 0.33 nm [ 71, the overlapping part of alkyl chains in the interdigitated struc- ture of the LB film was estimated to be 1.45 nm at 0.25 =Xx,,.,, and 1.77 nm at 1 .O = Xx,,,. This difference in the overlapping is supposed to influence the adsorbability of XSAC LB films, i.e. the relatively loose packing of alkyl chains at 0.25 =Xx,, contributes to enhancing the adsorb- ability of the LB film. Also, NO molecules adsorbed on the XSAC LB film are considered not to penetrate into the pari- sade layers of the XSAC film, but to be laid on the polar head moieties of LB film. This adsorbing state may attribute to the synergistic effects of not only dipole interaction among 7 electrons of the aromatic rings, but also the hydrogen bond- ing, in addition to ionic interaction between XSAC and NO molecules.

4. Conclusions

The monolayers of XSAC could be easily deposited onto a solid substrate by mixing with octadecane. These dicationic LB films of XSAC have a unique adsorbability due to the strong interaction with the anionic adsorbate such as NO molecules. NO molecules on the LB film were tilted about 70-80” from the normal of substrate surface. In the case of 0.25 = Xxs*c, NO molecules interacted with XSAC mole- cules at the ratio of XSAC:NO = 1:1.07. It was also found that NO molecules had the same molecular arrangements in both NO-adsorbed LB films and XSAC-NO-complexed LB films.

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