In situ measurement of the infrared spectra of insoluble monolayers at the air-water interface

Abstract

By utilizing the reflectance properties of water in the mid-infrared region of the electromagnetic spectrum, external reflection Fourier transform infrared spectroscopy is used to measure, in situ, the infrared spectrum of a monomolecular film at the air-water phase boundary. A monolayer of oleic acid (9-m-octadecenoic acid) was spread on water to its equilibrium spreading pressure of 30 dyn/cm. The absorbance spectrum that resulted from the ratioing of the single-beam reflectance spectrum of oleic acid on water to that of pure water was found to have negative absorbance bands due to the properties of the complex refractive index of the water substrate. Vibrations are identified for both the hydrocarbon and carboxylic acid segments of the long-chain fatty acid monolayer. It is shown that the carboxylic acid remains protonated when the oleic acid exists as a monolayer film on water.

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

“…ATR is now among the most useful tools to characterize biological films supported by solid crystals [55,56]. As a method to probe biointerfaces, IRRAS had its applicability largely expanded when it was adapted to Langmuir monolayers [57,58].…”

Surface FTIR Techniques to Analyze the Conformation of Proteins/ Peptides in H2O Environment

“…The technique of IRRAS for the Langmuir monolayer was developed in 1985 [33]. Subsequently, some proteins were shown to be also able to form a Lanmguir monolayer and IRRAS has been consequently used to analyze protein samples in the Langmuir monolayer [14,[34][35][36][37][38], which covers the surface of H 2 O. IRRAS can not only detect the FTIR signal of proteins, but also valuate the orientation of the proteins sample in the Langmuir monolayer due to the selection rules as discussed below [14,[34][35][36]38].…”

Elucidating the Effects of Cholesterol on the Molecular Packing of Double-chained Cationic Lipid Langmuir Monolayers by Infrared Reflection-absorption Spectroscopy

“…From the original IRRAS data, the reflectance-absorbance RA intensity data were obtained with an uncertainty of 2 10 4 for a major IR peak of alkyl chain, the antisymmetric methylene stretching vibration ν a -CH 2 at 2920 cm 1 35, 36 . Negative absorption bands of a monolayer at the air/water interface were observed in the spectra, and the bases of the negative absorbance have been explored and explained in the literature 36,37 . The IRRAS theory…”