Polarization modulation of the incident electromagnetic field is used to increase the sensitivity and the in situ experiment ability of the well-known method of reflexion absorption FT-IR spectroscopy, for the characterization of surfaces and ultra-thin films. The experimental procedure and signal processing of the detected intensity are described and illustrated with the use of results obtained with Langmuir-Blodgett monolayers. The quantitative analysis of the spectra is then developed, and a linear behavior of the band intensities is found for ultra-thin films exhibiting no strong absorptions. This result is checked with the use of organic and inorganic ultra-thin films of increasing thicknesses.

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A calibration procedure is presented to relate Fourier transform infrared reflection-absorption spectroscopy (IRRAS) spectra obtained by polarization modulation (PM) and conventional techniques. This procedure concerns only PM-IRRAS measurements obtained with a lock-in amplifier. At the first stage, it consists in measuring two calibrated spectra, by adding a linear polarizer after the sample. The corresponding corrected PM-IRRAS spectra display band intensities close to those observed on IRRAS spectra (at about 6%), but the value of the baseline is lower. The calibration procedure can be performed at the second stage, by normalizing the corrected PM-IRRAS spectra of the sample and the bare substrate. In this case, the band intensities and the baseline of the normalized corrected PM-IRRAS and IRRAS spectra are similar.

Single planar bilayers of DMPC free-standing in air (black films) have been studied by confocal micro Raman spectroscopy in the 600-4000 cm -1 spectral range. The DMPC vibrational bands are well-defined. This allows the derivation of two parameters associated with the packing and mobility and with the fluidity of the alkyl chains and to quantify the rigidity enhancement accompanying the L R liquid crystalline to gel phase transition (T m ) 24 °C). The intensity of the water band allows to evaluate the thickness of the interstitial aqueous core which is 22 ( 4 Å in the L R phase. This water band exhibits a spectral change relative to bulk liquid water which is significant of the confinement of water molecules in the aqueous core.

The molecular organization in monolayers of deuterated cadmium arachidate deposited on solid metallic and dielectric substrates or spread at the air/water interface is investigated using different techniques of Fourier transform infrared spectroscopy. For metals, the polarization modulated reflectivity shows that the molecules have a high molecular order and are oriented quasi perpendicularly with respect to the surface. The in-plane organization is determined by complementary transmission experiments on CaF2 plates. The CD2 bending vibration in mono- and multilayer Langmuir–Blodgett stacks reveals the hexagonal packing of a single monolayer irrespective of the hydrophilic or hydrophobic character of the substrate. Probing the stack organization using selective monolayer deuteration in hydrogenated Langmuir–Blodgett films shows that the first deposited monolayer of an odd-numbered stack keeps its hexagonal structure while additional bilayers adopt an orthorhombic packing induced by the strong polar head/head interactions. However a monolayer may have an orthorhombic arrangement either if it is deposited on an orthorhombic commensurable substrate or if it is at a low temperature in which case intermolecular interactions are strengthened. Finally, differential reflectivity allows a single monolayer spread at the air/water interface to be studied. It is shown that such a Langmuir film is arranged in a two-dimensional hexagonal packing with its alkyl chains oriented perpendicularly with respect to the surface and its COO− polar groups symmetrically anchored at the interface.

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Water confined in the ultrathin interstitial core of black soap films has been studied by infrared and Raman spectroscopies. Lowering the water core thickness below 1 nm induces spectral modifications in the multicomponent O-H stretching band which can be associated with environment changes for water molecules. Differences observed between nonionic and ionic surfactants allow us to distinguish the sole effect of confinement from the combined effects of confinement and ionic strength. Both trends correlate with previous experimental observations done on different surfactant solutions and with results from numerical simulations.

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