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

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.

Spectrin-like Repeats 11–15 of Human Dystrophin Show Adaptations to a Lipidic Environment

“…The PM-IRRAS spectra were recorded on a Nicolet 870 FT-IR spectrometer (Thermo Electron, Madison, WI). PM-IRRAS combines Fourier transform infrared reflection spectroscopy of an interface with fast modulation of the polarization of the infrared radiation between parallel (p) and perpendicular (s) directions with respect to the plane of incidence (19,20). The FT is calculated with data points every 1 cm Ϫ1 with a final spectral resolution of 8 cm Ϫ1 by co-adding 2400 scans.…”

Chitosan does not inhibit enzymatic action of human pancreatic lipase in Langmuir monolayers of 1,2-didecanoyl-glycerol (DDG)

“…1B shows PM-IRRA spectra in the 1500-1800 cm −1 region for DDG monolayers at 30 mN m −1 , featuring a broad band at 1737-1741 cm −1 due to C O stretching vibration [61] and a strong band assigned to H O H bending from surface water molecules at 1650-1720 cm −1 [61]. This latter band results from the difference in reflectivity between covered and uncovered aqueous subphases with the lipid monolayer [64]. For DDG monolayers on subphases containing chitosan, the spectra displayed two bands at 1546-1551 cm −1 and 1565-1582 cm −1 assigned respectively to protonated amines ( NH 3 + symmetric bend) of chitosan and NH bending from the acetylated glycosamine residues ( NH CO ) [44,65] (amide II).…”

Vibrational spectroscopy for probing molecular-level interactions in organic films mimicking biointerfaces

“…Blaudez and co-workers [60,61] realized the importance of PM-IRRAS and applied it to the characterization of Langmuir films. Water is not a perfect reflector and therefore both p and s polarizations can be simultaneously absorbed because the surface selection rule is not applicable for the air/water interface.…”