Study on Infrared Spectrophotometer for Langmuir monolayer

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1 Effect of infrared radiation on FT-IR external reflection spectra of Langmuir monolayers of stearic acid and zinc stearate were examined by changing the amount of radiation energy with a band path filter. If the radiation energy at the level of normal FT-IR measurements was used, the methylene stretching bands of the Langmuir monolayers of stearic acid at large surface areas could not be observed, owing to the temperature increase in the irradiated region and the enormous surface density decrease which follows it. On the contrary the bands of the Langmuir monolayers of zinc stearate were observed at any surface areas. The result means that the monolayer of zinc stearate is rigid and the molecules do not move easily. It is expected that the stearate molecules bind strongly each other through the zinc cations.2 Time dependence of the effect of infrared radiation on FT-IR external reflection spectra of Langmuir monolayers of stearic acid and stearyl alcohol were examined under various surface areas. At low surface pressures, the peak intensities of the bands of the Langmuir monolayers decreased with the time lapse, and finally the peak disappeared. Moreover the decrease occurred more quickly at larger surface areas, and in the Langmuir monolayer of stearic acid than that of stearyl alcohol. This difference of the decrease speed seems to depend on the difference of the viscosity of the Langmuir monolayers.3 FT-IR external reflection spectra of Langmuir mono layers of zinc stearate were recorded under various surface areas and under surface pressure relaxation, and the structural changes of the hydrocarbon chain and the carboxylate group were estimated. Moreover it was elucidated that the orientation of hydrocarbon chain in the zinc 12-hydroxystearate monolayer was different from that in the zinc stearate monolayer.