Molecular-Orientation Change in Langmuir Films of Stearic Acid and Cadmium Stearate upon Surface Compression, as Studied by Infrared External-Reflection Spectroscopy

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For interpretating the various phases appearing in a two-dimensional system, we have recorded in situ polarized FT-IR external reflection spectra of Langmuir films of stearic acid and cadmium stearate on the water surface at various surface areas, and evaluated the molecular orientation angles quantitatively. In the stearic acid monolayer, the orientation angle of the hydrocarbon chain from the surface normal decreased from 20° to almost 0° upon monolayer compression. In the cadmium stearate monolayer, on the other hand, the orientation angle (near 0°) did not change upon monolayer compression, though it was widely scattered at large surface areas, because of the formation of a rigid crystalline island. These results correspond well to the π–A isotherms.