MOLECULAR-ORIENTATION CHANGE IN LANGMUIR FILMS OF STEARIC-ACID AND CADMIUM STEARATE UPON SURFACE COMPRESSION, AS STUDIED BY INFRARED EXTERNAL-REFLECTION SPECTROSCOPY

For interpretating the various phases appearing in a two-dimensional s ystem, we have recorded in situ polarized FT-IR external reflection sp ectra of Langmuir films of stearic acid and cadmium stearate on the wa ter surface at various surface areas, and evaluated the molecular orie ntation angles quantitatively. In the stearic acid monolayer, the orie ntation angle of the hydrocarbon chain from the surface normal decreas ed from 20 degrees to almost 0 degrees upon monolayer compression, In the cadmium stearate monolayer, on the other hand, the orientation ang le (near 0 degrees) did not change upon monolayer compression, though it was widely scattered at large surface areas, because of the formati on of a rigid crystalline island. These results correspond well to the pi-A isotherms.