AN INVESTIGATION OF MISCIBILITY IN GUEST-HOST MONOLAYERS BY SURFACE PRESSURE AND FLUORESCENCE METHODS

The miscibility of the thermotropic liquid crystal dodecylcyanobiphenyl (12CB) in air-water monolayers of arachidic acid (AA) and oleic acid (OA) was studied by surface pressure and spectroscopic techniques. The mixing behavior was obtained from surface pressure-area isotherms by using the surface phase rule and excess area criterion. Results show that 12CB is immiscible in AA and miscible in OA over a wide range of compositions. These observations are confirmed by fluorescence emission spectroscopy. For the 12CB/AA system, the emission is due to 12CB excimer formation even at low mole fractions of 12CB, indicating immiscibility. In the case of 12CB/OA, the monomer emission is predominant at low mole fractions of 12CB, but diminishes with increasing mole fraction of 12CB. The presence of some contribution from the excimer in the emission of the 0.05 mole fraction 12CB/OA monolayer can be attributed to a dynamic process whereby the homogeneously mixed 12CB molecules diffuse to form an excimer within the lifetime of the excited state. The geometrical requirement for excimer formation supports other evidence that the orientation of 12CB in the monolayer is with the long axis of the 12CB molecule close to parallel with the air-water interface.