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

作者：Muro Maiko; Itoh Yuki; Hasegawa Takeshi^*

来源：Journal of Physical Chemistry B, 2010, 114(35): 11496-11501.

DOI：10.1021/jp105862q

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.

• 出版日期2010-9-9