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 Cd²⁺ 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.