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 (C(17)), octadecanoic acid (C(18)), and nonadecanoic acid (C(19)) exhibit systematic spectral changes in both the C-H and C=O stretching vibration regions. Through a stabilization analysis of the L films at a high surface pressure, the C(19) 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 C(19), the three major bands arising from the C=O 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.
Muro
Maiko
M
Department of Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551, Japan.
Itoh
Yuki
Y
Hasegawa
Takeshi
T