Internal reflectance IR spectroscopy of Langmuir monolayer films at the air-water interface

We report here the vibrational spectroscopic study of monomolecular films at the air-water interface, by using internal reflection infrared spectroscopy. Unlike previous external reflectance IR methods developed in our laboratory, this method uses attenuated total reflectance (ATR) spectroscopy to study monolayer films. The internal reflectance approach offers several advantages for the study of Langmuir monolayers at the air-water interface, including iii improved SIN (signal-to-noise) ratio resulting from multiple reflections, and (ii) the ability to form supported planar bilayer films of defined chemical structure and orientation. One method under design in our laboratory uses a Ge internal reflection element derivatized with octadecyltrichlorosilane. Using this design, the study of spread Langmuir phospholipid monolayer films in this ATR geometry results in a supported, oriented, planar membrane bilayer with hydrated polar head groups. Preliminary studies reveal that this method of analyzing phospholipid Langmuir films at the air-water interface produces results consistent with previous ATR-IR studies of transferred Langmuir-Blodgett phospholipid monolayers.