A Conformation and Orientation Model of the Carboxylic Group of Fatty Acids Dependent on Chain Length in a Langmuir Monolayer Film Studied by Polarization-Modulation Infrared Reflection Absorption Spectroscopy
The conformation of the carboxylic group of fatty acids in a Langmuir (L) monolayer film on water is described
in relation to the aggregation property of the hydrocarbon chain. Polarization-modulation infrared reflection
absorption spectra (PM-IRRAS) of L films of heptadecanoic acid (C17), octadecanoic acid (C18), and
nonadecanoic acid (C19) exhibit systematic spectral changes in both the CsH and CdO stretching vibration
regions. Through a stabilization analysis of the L films at a high surface pressure, the C19 L film has been
found outstandingly stable exhibiting no film shrink, while the other two compounds exhibit a large shrink
at high surface pressure. By taking into account the uniquely high aggregation property of the hydrocarbon
chains of C19, the three major bands arising from the CdO stretching vibration mode propose three types of
molecular conformations about the carboxylic group, which are elucidated by a balance of the hydration of
the carboxylic group, the chain length of the hydrocarbon chain, and the surface pressure.
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