PHASE-TRANSITIONS IN MIXED LANGMUIR FILMS OF 8CB AND STEARIC-ACID

Pure and mixed monolayers of 4-n-octyl-4'-cyanobiphenyl (8CB) and stea ric acid (SA) at the air-water interface have been investigated by a c ombination of surface balance measurements and polarization sensitive, optical second harmonic spectroscopy. These measurements make it poss ible to carry out in situ studies of the formation of the monolayer an d to identify different regimes upon compression of the film. These re gimes cover the whole range from an isotropic monolayer, a highly orie nted phase, up to phase separation at a critical surface pressure pi. It is shown that the polarization dependence of the second harmonic i ntensity is a very sensitive method to investigate the orientation of the optically nonlinear 8CB chromophores in the mixed film as a functi on of surface pressure as well as concentration. A comparison of monol ayers of different mole fractions of 8CB and SA shows that the alignme nt of the 8CB molecules in the mixed film depends on the surface press ure and not on their concentration in the monolayer only. This behavio r can be explained by assuming an ideal mixed monolayer, where the ori entation of the 8CB chromophores is due only to the hydrophobic intera ction with the alkyl chains of the SA. In addition, the critical surfa ce pressure of the mixed film pi(mix) can be calculated as a function of the 8CB mole fraction X(8CB).