Thin Solid Films, 210/211 (1992) 583 585 583

Fourier transform infrared metal overlayer attenuated total reflection spectra of Langmuir-Blodgett films of 12-hydroxystearic acid and its cadmium salt

Takeshi Hasegawa, Junzo Umemura and Tohru Takenaka Institute for Chemical Research, Kyoto University, Uji, Kyoto-Fu 611 (Japan)

Abstract

dl-12-Hydroxystearic acid (12HSA) exhibits a wide plateau region in its surface pressure area isotherm. LB films of 12HSA were prepared at various stages of surface compression on silver-coated slide glasses. FTIR metal overlayer ATR spectra of these LB films were recorded and intensity ratios of the symmetric CH3 stretching band to the symmetric CH2 stretching band were analyzed as well as frequencies of the latter band. The results indicate that the average orientation of the methylene chains toward the surface normal upon compression is improved slightly in the plateau region, but greatly at the solid-condensed state. The cadmium salt of 12HSA which has no plateau region showed higher perpendicular orientation even at very low surface pressure.

I. Introduct ion

Hydroxystearic acids are known to exhibit wide plateau regions in their rt-A isotherms [1-4]. From various view points including the result of the surface dipole moment measurement, this plateau region has been attributed to the structural transition from an initial contact of both hydroxyl and carboxyl groups with the aqueous subphase to a contact only of the carboxyl group, where the hydroxyl group is forced out of the interface during surface compression [1, 2, 4]. Since, however, spectroscopic examination of structural changes during this process has not been performed, we tried to make these points clear by preparing LB films of dl-12-hydroxystearic acid (12HSA) at various stages of surface compression on a flat metal surface and measuring FTIR metal overlayer ATR spectra which has been recently proposed by Ishida et al. [5-7] as a high-sensitivity tool for studying the molecular structure and orientation in ultrathin organic films. For compari- son, the study was also done of cadmium salt of 12HSA which has no plateau region in its n -A isotherm. Although the present technique is not in situ, it nicely reflects the molecular state on the water surface.

2. E x p e r i m e n t a l detai ls

By combining both vertical (the first monolayer only) and horizontal lifting (succeeding monolayers) meth- ods, multilayer LB films of 12HSA were fabricated on silver-evaporated (100 nm thick) slide glasses from

monolayers on the water subphase controlled to pH 4.5 by HC1 at various stages of surface compression includ- ing the plateau region (see points a - h in the rc-A isotherm shown in Fig. 1A). LB films of the cadmium salt of 12HSA were similarly made from monolayers on the water subphases at pH 6.0 with 3 × 10 - 4 M CdC12 and 3 × 10 - 4 M NaHCO3. (For the rc-A isotherm and preparation points, see Fig. lB.) The transfer ratios were 1.0 in the vertical lifting process and 1.9-2.0 in the horizontal lifting process. The number of monolayers in

5O

A

"7 ~= 40

Z E

3O L _

~ h - ~, 2o

50

2.0

1.5 ̧ v

J

0,5.

• a e a i'--_Z2_ 0 . 5 1 . 0 1.

E 40 .2.0

Z E ~

30 t-k ,1.5 >-

Q' 20 • .1.0

~ J

c/~ 10 .0.5

,'J i o ~,J o

0 0.5

2 ,1 Area I n m • mo lecu le

Fig. 1. Plot of the surface pressure ( ) and the intensity ratio of the symmetric CH 3 stretching band to the symmetric CH 2 stretching band ( e ) against the surface area for 12HSA (A) and its cadmium salt (B).

0040-6090/92/$5.00 ,~ 1992 - - Elsevier Sequoia. All rights reserved

584 T. llasegawa c/ al. ,/ I.B fil.l.v q/ 12-hvdro.xv.<tcaric acid and its cadmium salt

~ ~ L B film

I slide glass

Fig. 2. Sample setup for the metal overlayer ATR measurement of a LB film.

the LB films were 5 (points c h in Fig. 1A and points j, k in Fig. 1B) or 9 (points a, b in Fig. 1A and point i in Fig. 1B). As shown in Fig. 2, these samples were tightly attached to a flat surface of a hemicylindrical germanium prism, and subjected to FTIR ATR mea- surements with an angle of incidence of 7Y' by a Nicolet Model 6000C spectrophotometer equipped with a Harrick variable-angle reflection accessory, the Seag- ull '~' [8]. To secure a good contact between the germa- nium prism and the samples, the original prism holder and its tightening screws were replaced by stronger o n e s .

3. Results and discussion

FTIR metal overlayer ATR spectra of 12HSA at various stages of surface compression are shown in Fig. 3. The ordinate scales were normalized to the same surface density (reciprocal surface area) and the same number of monolayers. Although the S/N ratios are not high enough, they are satisfactory if we consider that the LB films prepared at the points a - e are in expanded states. The four CH stretching vibration bands are seen at around 2960 (asymmetric CH3), 2920 (antisymmetric C H 2 ) , 2875 (symmetric C H 3 ) , and 2850 cm -1 (symmet- ric CH2). The absorbance of the bands in the metal overlayer ATR spectra changed appreciably ( + 30°/,,), depending upon the tightening pressure of the prism holder of the Seagull ~". In Fig. 3, however, we can find a general trend that the antisymmetric and symmetric CH2 stretching bands keep their intensities almost un- changed up to the point e, but diminish upon further monolayer compression (the points f -h) . The symmet- ric C H 3 stretching band, on the other hand, gradually increases its intensity upon compression. The selection rule of the metal overlayer ATR spectra is such that only the transition moment which has the component normal to the film surface is observed [5], as in reflec- tion absorption spectroscopy [6,9]. Therefore, the above trends of the CH2 and symmetric CH 3 stretching bands can be ascribed to the improved perpendicular orientation of the hydrocarbon chain axis. Since the CH2 stretching bands and the symmetric CH 3 stretching band present opposite intensity changes upon compres-

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L.

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x2.88

x4.19 ~-!

x3.35 ~

I

x 2 . 2 3 ~

x1.14

x1.05

x1.0

3000 2950 2900 2850 2800

Wavenumber / cm -j

Fig. 3. Metal overlayer ATR spectra of LB films of 12HSA at various stages of surface compression. The ordinate scale was multi- plied by the factors indicated to compensate for the differences in surface density and number of monolayers.

sion, the intensity ratio of the symmetric CH 3 stretching band to the symmetric CH 2 stretching band can be a sensitive measure of the degree of the perpendicular orientation of the hydrocarbon chain. Thus, these in- tensity ratios were plotted against surface area in Fig. 1A, and compared to the ~ A isotherm. A good corre- spondence can be seen between the intensity ratio and ~ - A curve. Upon surface compression, the ratio in- creases slightly in the plateau region, while drastically at the solid condensed state. This result indicates that the average orientation of the hydrocarbon chain changes only slightly in the plateau region. If, however, the chains are gradually forced out of the water surface in this region as has been pointed out by Tachibana and others [1, 2, 4], a larger change of the intensity ratio will be expected here. A key to this discrepancy can be found in their symmetric CH2 stretching fre- quencies. It was 2855.7 + 0.5 cm- l at the points a - e in Fig. 1A, indicating that the methylene chain includes disordered gauche conformations [ 10, 11]. If the methy- lene chain is disordered, then the average orientation of the chain will not become so high as expected from the forced-out model of the all-trans methylene chains. A marked improvement of the average orientation was found at the solid condensed state where the symmetric CH2 stretching frequency shifted to 2850.5 ___ 0.3 cm -~,

T. Hasegawa et al. / LB films of 12-hydroxystearic acid and its cadmium salt 585

4O

30

L ,

~ 20

°

0 0 .5 1 . 0 1 .5 2 I -1

A r e a / n m . m o l e c u l e

e0OH-- H eoo_~.--

Fig. 4. Schematic illustration for structural changes o f 12HSA during surface compression.

i.e., the typical value of the all-trans conformation of the methylene chain [9, 11]. The estimated overall features of the structural changes upon surface com- pression of 12HSA are schematically illustrated in Fig. 4.

In the case of cadmium salt of 12HSA, there was no plateau region in its 1t-A isotherm (Fig. 1B) as in the case of cadmium stearate [12]. Metal overlayer ATR spectra of these LB films showed only higher intensity ratios of the symmetric CH 3 stretching band to the symmetric CH2 stretching band, indicating the all-trans conformation of the methylene chain and the presence of islands even at large surface areas, as in the case of cadmium stearate monolayers [12]. In accord with this, the symmetric CH2 stretching frequencies were 2850.3- 2850.4 cm 1, which is typical of the all-trans conforma- tion of the methylene chain.

Acknowledgments

This work was partly supported by Grant-in-Aid for Scientific Research 01540373 and Grant-in-Aid for Sci-

entific Research on Priority Areas 02205072 from the Ministry of Education, Science and Culture, Japan.

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