MOLECULAR-ORIENTATION IN LANGMUIR FILMS OF 12-HYDROXYSTEARIC ACID STUDIED BY INFRARED EXTERNAL-REFLECTION SPECTROSCOPY

12-Hydroxystearic acid is known to exhibit a wide plateau region betwe en 0.3 and 0.95 nm(2/)molecule in the pi-A isotherm. In situ polarized FT-IR external-reflection spectra of Langmuir (L) films of 12-hydroxy stearic acid on the water surface were recorded under various surface areas, and the molecular-orientation angles were quantitatively evalua ted. At the final stage of the plateau region (0.4 nm(2)/molecule), th e orientation angle of the hydrocarbon chain was about 55 degrees from the surface normal. Upon monolayer compression, however, the angle de creased to about 28 degrees in the solid-phase region. This change cor responds well to the result in Langmuir-Blodgett films that was report ed earlier. In addition, it was elucidated that the hydrocarbon chain tended to become random during the surface pressure relaxation.