Phase transitions in mixed Langmuir films of 8CB and stearic acid

Abstract

Pure and mixed monolayers of 4-n-octyl-4′-cyanobiphenyl (8CB) and stearic acid (SA) at the air–water interface have been investigated by a combination of surface balance measurements and polarization sensitive, optical second harmonic spectroscopy. These measurements make it possible to carry out in situ studies of the formation of the monolayer and to identify different regimes upon compression of the film. These regimes cover the whole range from an isotropic monolayer, a highly oriented phase, up to phase separation at a critical surface pressure π*. It is shown that the polarization dependence of the second harmonic intensity is a very sensitive method to investigate the orientation of the optically nonlinear 8CB chromophores in the mixed film as a function of surface pressure as well as concentration. A comparison of monolayers of different mole fractions of 8CB and SA shows that the alignment of the 8CB molecules in the mixed film depends on the surface pressure and not on their concentration in the monolayer only. This behavior can be explained by assuming an ideal mixed monolayer, where the orientation of the 8CB chromophores is due only to the hydrophobic interaction with the alkyl chains of the SA. In addition, the critical surface pressure of the mixed film πmix* can be calculated as a function of the 8CB mole fraction x8CB.