Polarization Modulation Infrared Reflection Absorption Spectroscopy of a Cadmium Arachidate Monolayer at the Air/Water Interface

A polarization modulation infrared reflection absorption spectrum (PM-IRRAS) of a Langmuir monolayer on the water surface was constructed. The optimal incidence angle, the surface selection rule, and the frequency-orientation correlation were determined. The validity of this theory depends on how the anisotropic complex refractive indexes of the Langmuir monolayer are obtained. It is recommended that these optical constants should be calculated from the infrared spectra of the deposited monolayer on solid substrates. Using the cadmium arachidate (CdA) monolayer on the aqueous Cd2+ subphase as an example, we demonstrated why the optical constants should be obtained from the single monolayer rather than the multilayer deposited on solid substrates. The simulated and experimental PM-IRRAS spectra of the CdA monolayer on the aqueous Cd2+ subphase had similar intensities of the νas(CH2), ν s(CH2), ν as(COO), and δ(CH2) bands, justifying the optical constants' determination procedure. They were dissimilar in the ν as(CH2) and ν as(COO) frequencies, and the δ(CH2) bandwidth, indicating the problem of LB transfer artifacts. Finally, the present simulation procedure was compared to the previous one in which the optical constants were determined from multilayer spectra. The interlayer headgroup interaction is found to be responsible for the anomalous carboxylate stretching intensities in the previously simulated PM-IRRAS spectrum.