By a comparison of infrared reflection-absorption (RA) and transmission intensities, a method for the quantitative evaluation of molecular orientation in thin Langmuir-Blodgett (LB) films has been developed. The enhancement factors for the RA to transmission absorption intensities of hypothetical isotropic films were theoretically calculated by using Hansen's optical formulas for thin multilayer films. By combination of these values with the experimentally determined intensity ratios of the RA to transmission spectra for uniaxially oriented LB films, the orientation angles of the transition moments of major infrared bands were evaluated. Application of this method to the 7-monolayer LB film of cadmium stearate deposited on Ag (for RA measurements) and on ZnSe (for transmission measurements) substrates gave reasonable tilt angles for the molecular chain as compared to those reported by other investigators and to the data from the X-ray analysis. This method will be discussed in detail and the various factors which influence the accuracy of the orientation analysis will also be covered.

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In analyzing optical properties of multilayered LB films with the uniaxial anisotropy around the surface normal, a new method for calculating infrared reflection-absorbances was developed, by extending Drude's anisotropic calculation theory for two-phase system to Hansen's optical theory for thin isotropic multilayers. With this method, infrared external reflection spectra of a 9-monolayer LB film of cadmium stearate prepared on a gallium arsenide substrate were analyzed to obtain molecular orientations. The result was in fair agreement with that obtained by X-ray diffractometry. Reflection-absorbance spectra of the same LB film on a silver-evaporated slide glass at various temperatures were also analyzed by the same method and the orientation angle of each molecular group was quantitatively obtained, clarifying the process of disordering with the increase of temperature. Further, the dependence of the degree of disordering on the monolayer location in LB films were discussed in light of the isotope substitution experiment.

Several hydrophobic α-helical peptides containing a disulfide group were synthesized, and the formation of oriented self-assembled monolayers (SAMs) on gold surface was investigated. The orientation of helices in the SAMs was determined by Fourier transform infrared reflection−absorption spectroscopy measurements. The tilt angle of the helix axis from the surface normal was sensitively affected by the choice of solvent used in the preparation of SAMs, the nature of component amino acids, the molecular structure (either one-helix or two-helix peptide), and the length of peptide chain. The tilt angle was smaller when ethanol was used in the preparation of SAMs rather than N,N-dimethylformamide. Lipo-(Ala-Aib)8-OBzl (Lipo and OBzl represent a lipoic acid group and a benzyl ester group, respectively) showed a smaller tilt angle than Lipo-(Lys(Z)-Aib)8-OCH3. A one-helix peptide with a lipoic acid group at the N-terminal showed a smaller tilt angle than a two-helix peptide, in which the two helix peptides were connected by a disulfide linkage. The longest peptide, Lipo-(Ala-Aib)12-OBzl, showed the smallest tilt angle of 30° when the SAM was prepared in an ethanol solution. Taken together, helix−helix interaction should play a more important role in regulating the orientation of helical peptides in the SAM than the Au−disulfide interaction. Lipo-(Ala-Aib)12-OBzl formed a nearly vertically oriented SAM with a parallel orientation of helices due to the highly self-assembling properties of the peptide.

Fourier transform infrared (FTIR)-attenuated total reflection (ATR) spectra have been recorded of Langmuir-Blodgett (LB) films of stearic acid deposited on a germanium plate with 1, 2, 3, 5, and 9 monolayers. Examination of the CH2 scissoring band suggests that the hydrocarbon chain of stearic acid in the first monolayer is in a hexagonal or pseudohexagonal subcell packing where each hydrocarbon chain is freely rotated around its axis oriented approximately perpendicular to the surface. In the LB films thicker than 2 monolayers, on the other hand, the molecules in the upper monolayers other than the first monolayer crystallize with the monoclinic form where the hydrocarbon chains are packed alternately and are inclined at an angle of about 30°with respect to the surface normal, showing a tendency to align their a crystal axes parallel to the direction of the withdrawal of the germanium plate in the film preparation. It is also concluded from frequencies and intensities of the progression bands due to the CH2 wagging vibrations that stearic acid occurs as the cis configuration for the C=0 and Ca-€ß bonds in the 1-monolayer film but the trans configuration starts to appear in the 3-monolayer film. A striking feature in this study is the absence of the C-O stretching band of stearic acid in the first monolayer on the germanium plate. This phenomenon may be interpreted by the short-range image field model for its oscillating dipole parallel to the germanium surface.

adsorption solutions. All the buffers were 0.05 M, with the exception of the pH 11 buffer, which was 0.1 M. The following buffers were used for the titrations under cyclooctane:malonic acid (pH = 2, 3), sodium phosphate (4, 12), acetic acid (5), maleic acid (6), HEPES (7, 8), boric acid (9,10) l,3-diamino-2propanol (11), and KC1 (13). The pH of the buffers was within 0.1 unit of the values stated.Acknowledgment. We thank S. R. Holmes-Farley and Ernest B. Troughton (Lord Corporation) and H. Biebuyck for valuable discussions during the course of this work.

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