Molecular orientation of a cyanobiphenyl liquid crystal in freely suspended films and at the air-water interface

Langmuir films and freely suspended (FS) films of 4'-n-octyl-4-cyanobiphenyl (8CB) in the smectic A phase have been studied by Fourier transform infrared spectroscopy. Examination of FS film transmission spectra gives a good estimation of molecular group orientation in the uniaxial him. At the air-water interface 8CB shows a first-order phase transition from a monolayer to a trilayer when the surface pressure increases. We have studied such films using polarisation modulated infrared reflection absorption spectroscopy. Specific surface selection rules on dielectrics allow an estimation of the orientation of the cyanobiphenyl core in the monolayer and in the trilayer and comparison of molecular organisations in the two types of films.