No abstract

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 vibrational 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.

We report on the variation of the dissociation of a Langmuir monolayer of arachidic acid at the air/water interface as a function of the subphase pH and for several cations (Cd 2+ , Ca 2+ , Mg 2+ , and Na + ) with the help of the polarization-modulated infrared reflection-absorption spectroscopy (PM-IRRAS) method. The infrared spectra give access to the relative concentration of acid and salt molecules and allow us to determine the influence of the subphase pH on the acid dissociation reaction for each cation. It is shown that Na + obeys the purely electrostatic Gouy-Chapman theory quite well, whereas the behavior of Mg 2+ , and even more so that of Ca 2+ and Cd 2+ , requires the introduction of some complexation constant to be understood.

Anisotropic optical constants (index of refraction and extinction coefficient) of Langmuir−Blodgett (LB) cadmium arachidate (CdAr) monolayers have been determined from experimental FTIR spectra. The in-plane and out-of-plane complex refractive indexes were calculated using an iterative procedure from a normalized transmittance spectrum at normal incidence and a p-polarized reflectance spectrum at grazing incidence, respectively. The in-plane refractive index (n̂ x = n̂ y ) has been obtained for 8 and 10 CdAr monolayers deposited on calcium fluoride, zinc selenide, and silicon substrates. On the other hand, the out-of-plane refractive index (n̂ z ) has been determined for 7 CdAr monolayers deposited on a gold substrate. Orientation of CdAr molecules has been evaluated from the anisotropic extinction coefficients of the νaCH2 and νsCH2 modes; a tilt angle of about 12° has been found for the CdAr alkyl chains, which is in good agreement with the values available in the literature. Finally, based on these optical constants, a PM-IRRAS spectrum of a CdAr monolayer at the air/water interface has been simulated by using an anisotropic multilayer formalism. The simulated and experimental spectra have been compared in order to reveal the differences in both the orientation and the organization of the CdAr molecules deposited on solid and liquid substrates.