POLARIZATION MODULATION FTIR SPECTROSCOPY AT THE AIR-WATER-INTERFACE

Mid-infrared spectra of monolayers spread at the air-water interface h ave been obtained, completely devoid of strong water vapor absorptions , using polarization modulation infrared reflexion absorption spectros copy (PM-IRRAS). On normalized difference (covered vs. uncovered water ) PM-IRRAS spectra, the monolayer absorption bands appear upwards or d ownwards depending on the orientation of their transition moment with respect to the water surface. Vibrational modes of the water subphase contribute to these difference spectra as broad dips. Study of a monol ayer of cadmium arachidate has allowed observation of the vibrational modes of the polar heads and provides some evidence of their symmetric al anchoring at the water surface. Under surface compression, a monola yer of deuterated arachidic acid undergoes a molecular reorganization leading to a better ordering of the deuterated chains.