Characterization of polyion complexed Langmuir-Blodgett films by electron spin resonance

Thin Solid Films, 243(1994) 634-637

634

Characterization of polyion complexed Langmuir-Blodgett films by electron spin resonance Kosaku Suga, Yutaka Iwamoto and Masamichi Fujihira Department qf Biomolecular Engineering, Tokyo Institute of Technology, Nagatsuta, Midori-ku, Yokohama 227 (Japan)

Abstract Electron spin resonance spectra of the Langrnuir-Blodgett films of 5-, 10-, and 16-doxylstearic acids (DXSTAs) and their mixtures with stearic acid complexed with polyallylamine, polyvinylmethylpyridinium iodide, and polyvinylbenzyltrimethylammonium chloride are measured at various angles between the magnetic field and the film normal. From the difference in the strength of Heisenberg spin exchange interaction among these three kinds of DXSTA films, it was indicated that the mobility of the methylene chain in these films increased with the distance from the head group. From the anisotropic properties of these spectra, it was also shown that the DXSTA molecules were well oriented with their methylene chain almost perpendicular to the film surface and were well arranged two dimensionally in the film plane.

I. Introduction

2. Experimental details

Langmuir-Blodgett (LB) films have been attracting much interest for their feasibility in constructing molecular electronic devices, artificial photosynthetic systems, and so on [1]. However, the problems of their stability or of their structures including defects still remain [2]. Polyion complexed LB films are expected to be improved with respect to stability and structural defects, because the stability of polyion complexed monolayer films on air-water interface and the ease of transferring them onto solid substrates are well known [3]. It is a current subject to characterize the microscopic structures of polyion complexed LB films. Electron spin resonance (ESR) spectroscopy including the spin probe method has been demonstrated to be a powerful means for this purpose, since it provides considerable information about the electronic states, the dynamic properties, and the conformational structures of many membrane systems containing paramagnetic molecules or incorporated with spin probe compounds which have some stable paramagnetic moieties such as nitroxides [4-7]. Furthermore, it has a high sensitivity, which is indispensable to the characterization of LB films. In this paper the dynamic properties and the orientation of molecules in polyion complexed LB films built up with doxylstearic acids (DXSTAs) and polymer cations are investigated by ESR and surface pressure-area n - A isotherms.

2.1. Materials

0040-6090/94/$7.00 SSD I 0040-6090(93)04125-C

Amphiphiles used in this study are 5-, 10-, and 16-doxylstearic acids (5-DXSTA, 10-DXSTA, and 16DXSTA) from Aldrich Chemicals Co. and stearic acid (STA) from Tokyo Kasei Co. and were used without further purification. Polyallylamine (PAA) from Nittobo Co., poly(4-vinyl-N-methylpyridinium iodide) (PVC~) and poly(4-vinylbenzyltrimethylammonium chloride) (PVB) from Mitsubishi Kasei Co. were used as polymers for polyion complex formation. Chemical structures of these materials are shown in Fig. 1.

o.

O--~-.N O 5-Doxyl slearic acid

OH

[ 5-DXSTA] 16-Doxyl stearic acid

[16-DXSTA]

o 10-OOxyl stearic acid

[SIAl

stearic acid

o [10-DXSTA]

,

GI *NH3 / n

' ~/

PAA

J

\\\ Cl" +N(CH ~H2 a)3/ /n

PVC1 PVB

Fig. 1. Chemical structures of amphiphiles and polymers used in this study.

c~(') 1994

Elsevier Sequoia. All rights reserved

K. Suga et al. / ESR study of polyion complexed films

2.2. Preparation of Langmuir-Blodgett films and measurement of rc-A &otherms Sample LB films were prepared by transferring the polyion complexed monolayer films formed on water subphase onto polyethyleneterephthalate (PETP) sheets (0.125mm thick) with a standard LB technique at a surface pressure of 30 mN m -~. Polyion complexed monolayers were formed by spreading chloroform solution containing DXSTA or a mixutre of DXSTA with STA on a water subphase containing cationic polymer. A Kyowa Kaimen Kagaku HBM-AP Langmuir trough was used for preparation of LB films and for the measurement of n - A isotherms. 2.3. Measurements of electron spin resonance spectra ESR spectra of LB films were collected by a JOELFE1X ESR spectrometer at room temperature. The sample PETP sheet (10mm x 30mm) coated with 60 monolayers was cut into several pieces and these were inserted into a quartz cell.

635

STA complexed with protonated PAA at the surface pressure of 30 mN m -1. The values for polyion complexed monolayers of pure DXSTAs decreased with increase in the distance between doxyl moiety and head COOH. This result suggests that doxyl moieties closer to the head group prevent more strongly the closest packing of methylene chains of DXSTAs.

3.2. Electron spin resonance spectra for Langmuir-Blodgett films of doxylstearic acids complexed with polyallylamine Figure 2 shows the ESR spectra of LB films of 5-DXSTA, 10-DXSTA, and 16-DXSTA and their mixed films with various amounts of STA complexed with PAA recorded at various angles 0 between the normal to the film plane and the direction of the static magnetic field (Fig. 3(a)). The ESR spectra except those for pure and 1:1 mixed LB films of 16-DXSTA have hyperfine structures (HFSs) due to the nitrogen nucleus. Disappearance of the HFS for these two spectra is ascribed to strong Heisenberg spin exchange

3. Results and discussion (A)

3.1. Properites of polyion complexed monolayers of doxylstearic acids and their mixtures with stearic acid Most of the rt-A isotherms for the polyion complexed monolayers of DXSTAs exhibited a typical transition region from expanded to condensed liquid films at surface pressures of 40-50 mN m -1. Collapse surface pressures for these monolayers were more than 50 mN m -~, and generally larger than those (less than 34 mN m-l) for pure monolayers of DXSTAs which are not complexed with polymer cations reported by Bonosi et al. [8]. This fact is attributable to the protection by the network of polymer cations against collapse of these monolayers. When surface pressures of these monolayers were controlled at constant values, areas of the monolayers were maintained constant. These facts indicate that the stability of the monolayers of DXSTAs was increased by complex formation with polymer cations. Table 1 shows the average molecular sectional areas for the monolayers of DXSTAs and their mixtures with

(B) 1O-DXSTA

~

~

-

~

~,~ 2OG

(c) 16-D

TABLE 1. Average molecular sectional areas for monolayers of doxylstearic acids and their mixtures with stearic aicd complexed with polyallyamine at the surface pressure of 30 mN m Amphiphile in DXSTA:STA

5-DXSTA 10-DXSTA 16-DXSTA

Molecular sectional areas for the following compositions (nm 2) Pure

1:1

1:3

1:9

0.400 0.383 0.262

0.325 0.299 0.218

0.270 0.274 0.223

0.242 0.238 0.241

Fig. 2. ESR spectra of LB films of 5-DXSTA, 10-DXSTA, and 16-DXSTA and their mixed films with various amounts of STA complexed with PAA recorded at various angles 0. The molar ratio was expressed as DXSTA:STA.

K. Suga et al. / ESR study of polyion complexed films

636

/-------

0=90 o 67.5 o 45 ° 22.50 0°

(a) 2OG i

(a) / O= 90 °

ff

//

--

67.5 °

45 °

× (b)

/

-----~/i

Fig. 3. (a) Definition of angle 0; (b) m o l e c u l a r axes for an oxazoline ring in a D X S T A .

interaction between doxyl moieties in the LB films of 16-DXSTA, because HFS appeared with the increase in mole fraction of STA. This is due to the increase in mean distance between the doxyl moieties in these films by dilution of DXSTA with STA. A similar but weaker spin exchange interaction can be observed in the LB films of 5- and 10-DXSTA, because the resolution of the HFS increased with similar dilution. From these observations, it is clear that the strength of the Heisenberg spin exchange interaction between doxyl moieties at the same mole fraction of DXSTA increases with the increase in the number of methylene groups between the doxyl moiety (oxazolidine ring) and the head COOH group in DXSTA molecules. This result indicates that the collision frequency between probe units (doxyl moieties) or the mobility of methylene groups to which the probe unit is attached increases as the probe unit is moved farther from COOH group. These spectra also reveal the strong dependence on the angle 0. The values of the apparent nitrogen hyperfine coupling constants for these polyion complexed LB films of DXSTA and the mixed films with STA, especially highly diluted mixed films, decreased clearly with increase in the value of 0. The hyperfine coupling tensor of the nitrogen nucleus for these DXSTAs has a large anisotropy and the z component of their principal value is about 5 times larger than the other two components (A= ~ 33 G, A,., ~ Ayy ~ 6 G) [5]. Molecular axes for an oxazoline ring in a DXSTA are illustrated schematically in Fig. 3(b). Since the z axis is parallel to the methylene chain of the molecule of DXSTA as shown in this figure, the 0 dependence of the ESR spectra of these polyion complexed LB films of DXSTA and the mixed films

~

~

/f--~--

X,

//--

....

-

22.5 ° 0 o

(b) Fig. 4. (a) O b s e r v e d E S R spectra of the 1:3 m i x e d LB films of 5 - D X S T A a n d STA c o m p l e x e d with P A A at various values o f angle 0; (b) c o m p u t e r - s i m u l a t e d spectra.

with STA indicates that the hydrocarbon chains of DXSTA molecules in these LB films are highly ordered and aligned almost perpendicular to the sheet plane. Figure 4 shows the observed and computer-simulated SER spectra of the 1:3 mixed LB films of 5-DXSTA and STA complexed with PAA at various values of the angle 0. The simulation was carried out on the assumption that the direction of the methylene chain of the DXSTA molecule was distributed around the film normal according to the gaussian distribution with a standard deviation of 30°. The simulated spectra are in good agreement with the observed spectra. Furthermore, the characteristic dependence of the peak-to-peak line width AHpp on the angle 0 was observed for the pure and 1:1 mixed LB films of 16DXSTA. When two-dimensionally arranged spins interact strongly with each other, the ESR spectrum of such system has only a single exchange-narrowed line. According to the paper by Richards and Salamon [9] the dependence of the value of AHpp on 0 for such a single-lined ESR spectrum obeys the following equation: A H p p oc

(3 cos20

-

1) 2 + c o n s t a n t

(I)

Figure 5 shows the plot of AHpp I)s. 0 for the pure polyion complexed LB films of 16-DXSTA and the theoretical curve calculated with the non-linear regression method according to eqn. (1). This result clearly indicates that the doxyl moieties of 16-DXSTA

K. Suga et al. / ESR study of polyion complexed films 20

I

•

r

I

I

I

[

I

I

15 -Q Q. {3_

-1-

10

5.0

b" . . . . . . .

I

20

I

t

I

-0

I

40 60 0 / degree

I

I

637

ascribed to low radical concentrations due to low transfer efficiency. The lower anisotropy of these ESR spectra is ascribed to the less order of molecular arrangement in these films with wide orientational distribution. Observed area-time curves for these monolayers at constant surface pressure, i.e. the decrease in area at constant surface pressure, also indicate that the monolayer of DXSTAs with PVCI or PVB was not as stable as that with PAA.

80

Fig. 5. ,~, plot of Anpp vs. 0 for the pure polyion complexed LB films of 16-DXSTA; - - - , theoretical curve calculated with the non-linear regression method according to eqn. (1).

molecules in these LB films are arranged regularly in a two-dimensional plane. Therefore, it also indicates the high regularity of the two-dimentsional arrangement of DXSTA molecules in these polyion complexed LB films. Bonosi et al. studied the ESR spectra of LB films of several DXSTA and mixed films with STA which were not complexed with polymer cations [8, 10]. ESR spectra obtained by them exhibited less anisotropy compared with our data. This fact indicates that DXSTA molecules in these polyion complexed LB films have more ordered molecular arrangement than those in non-complexed LB films.

3.3. Electron spin resonance spectra of the LangmuirBlodgett films of doxylstearic acids complexed with poly(4-vinyl-N-methylpyridinium iodide) or poly(4-vinylbenzyltrimethylammonium chloride) When PVC~ or PVB was used as the polymer cation, the ESR spectra of polyion complexed DXSTA LB films exhibited much weaker signals and much less anisotropy than those with PAA. The small signal-tonoise ratios observed in these ESR spectra can be

Acknowledgments The authors thank Professor Ken-ichi Aika, Tokyo Institute of Technology, for providing facilities for the ESR measurements. This work was partially supported by a Grant-in-Aid for Scientific Research (Grant 02650580) from the Ministry of Education, Science and Culture, Japan.

References 1 G. G. Roberts (ed.), Langmuir Blodgett Films, Plenum, New York, 1980. 2 H. G. Hansma, S. A. C. Gould, P. K. Hansma, H. E. Gaub, M. L. Longo and J. A. N. Zasadzinski, Langmuir, 7 (1991) 10511054. 3 M. Shimomura and T. Kunitake, Thin Solid Films, 132 (1985) 243. 4 H. M. McConell and B. G. McFarland, Q. Rev. Biophys., 3 (1970) 91-136. 5 S. Joachim, J. Am. Chem. Soc., 92 (1970) 3881-3887. 6 S. Kuroda, K. Ikegami, M. Sugi and S. Iizima, Solid State Commun., 58 (1986) 493 497. 7 K. Suga, S. Fujita and M. Fujihira, J. Phys. Chem., 93 (1989) 392-397. 8 F. Bonsoi, G. Gabrielli, G. Martini and M. F. Ottaviani, Langmuir, 5 (1989) 1037. 9 P. M. Richards and M. B. Salamon, Phys. Rev. B, 9(1974) 32. 10 G. Martini, F. Bonosi, M. F. Ottaviani and G. Gabrielli, Thin Solid Films, 178 (1989) 271.