2D Phase Transitions in Langmuir Monolayers of the Aromatic 4‘,5‘-Diphenyl-1,1‘:3‘,1‘ ‘-terphenyl-4-carboxylic Acid

Langmuir monolayers of 4‘,5‘-diphenyl-1,1‘:3‘,1‘ ‘-terphenyl-4-carboxylic acid (in short DTCA) were studied under different experimental conditions. The π− A isotherms displayed a broad plateau on a pure water subphase and two plateaus on a neutral aqueous subphase containing Cd 2+ ions. DTCA monolayers were transferred as LB multilayer films and characterized by FTIR, X-ray diffraction, and electric surface potential measurements. From the analysis of surface potentials for LB films transferred at surface pressures corresponding to different parts of the π− A isotherm it was possible to infer that molecular orientational changes are the probable cause for the plateau in the isotherm.