POLARIZATION-MODULATED FT-IR SPECTROSCOPY OF A SPREAD MONOLAYER AT THE AIR-WATER-INTERFACE

This study devoted to the FT-IR spectroscopy of monolayers spread at t he air/water interface is, to our knowledge, the first report presenti ng complete mid-infrared monolayer spectra perfectly extracted from th e strong water vapor bands. This has been possible with the use of the polarization-modulated IRRAS method, which is not sensitive to the is otropic absorptions of the sample environment. On the basis of theoret ical modeling and experiments, the best angle of incidence has been fo und near 76-degrees for detection of intraplane as well as out-of-plan e oriented monolayer absorptions. With the use of such experimental co nditions, on the normalized difference (covered vs. uncovered water) P M-IRRAS spectra, monolayer vibrational bands come out upward or downwa rd, depending on the orientation of their transition moment with respe ct to the interface. Application to the study of deuterated arachidic acid and arachidate monolayers allows observation of the vibrational m odes of the polar head groups interacting with the liquid water molecu les and provides some evidence of their symmetrical anchoring. The vib rational modes of the liquid water subphase contribute to these differ ence spectra as broad dips that certainly contain information on a pos sible restructuring of the water molecules at the interface.