Infrared spectroscopic studies of structure and phase transitions in a Langmuir monolayer of H(CH2)23COOH

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Abstract
We report the results of linked theoretical and experimental studies of external infrared reflectivity by a Langmuirmonolayer. The new feature in our analysis is the inclusion of the influence of the factor group splitting, arising from the presence of two molecules in the unit cell of a close packed two‐dimensional monolayer lattice (CS phase), on the polarized reflection spectra of the monolayer. Our experimental study of the polarized reflection spectra of water supported Langmuirmonolayers of tetracosanoic acid [H(CH2)23COOH] provide the first measurements of the factor group splittings of the CH2 symmetric and antisymmetric stretch vibrations in the CS phase of a monolayer. The existence of a nonzero factor group splitting is direct evidence for the presence of herringbonelike hydrocarbon chain packing in a two‐dimensional crystalline phase of the monolayer. We also report values for the collective tilt angle in the tetracosanoic acidmonolayer, as a function of surface area per molecule. The collective tilt angles for the unstrained I phase and the U phase of the tetracosanoic acidmonolayer found in this work agree with those obtained from grazing incidence x‐ray diffraction studies.

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