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Polarization-Modulated FT-IR Spectroscopy of a Spread Monolayer at the Air/Water Interface

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This study devoted to the FT-IR spectroscopy of monolayers spread at the air/water interface is, to our knowledge, the first report presenting complete mid-infrared monolayer spectra perfectly extracted from the strong water vapor bands. This has been possible with the use of the polarization-modulated IRRAS method, which is not sensitive to the isotropic absorptions of the sample environment. On the basis of theoretical modeling and experiments, the best angle of incidence has been found near 76° for detection of intraplane as well as out-of-plane oriented monolayer absorptions. With the use of such experimental conditions, on the normalized difference (covered vs. uncovered water) PM-IRRAS spectra, monolayer vibrational bands come out upward or downward, depending on the orientation of their transition moment with respect to the interface. Application to the study of deuterated arachidic acid and arachidate monolayers allows observation of the vibrational modes of the polar head groups interacting with the liquid water molecules and provides some evidence of their symmetrical anchoring. The virbrational modes of the liquid water subphase contribute to these difference spectra as broad dips that certainly contain information on a possible restructuring of the water molecules at the interface.

Keywords: FT-IR spectroscopy; LB monolayers; Polarization modulation; Water surface

Document Type: Rapid Communication


Affiliations: 1: Centre de Physique Moléculaire Optique et Hertzienne, URA 283 du CNRS 2: Laboratoire de Spectroscopie Moléculaire et Cristalline, URA 124 du CNRS 3: Université de Bordeaux I, 33405 Talence Cedex, France; and Centre de Recherche en Sciences et Ingéniérie des Macromolécules, Université Laval, Quebec G1K 7P4, Canada

Publication date: July 1, 1993

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