Abstract: The Langmuir films of three quinone (ubiquinone or plastoquinone) molecules with long alkyl side chains, 2,3-dimethyl-5-nonadecyl-1,4-quinone(1), 2,3-dimethyl-5,6-dinonadecyl-1,4-quinone(2) and 6-methyl-2,3-dimethoxyl-5-nonadecyl-1,4-quinone(3) were investigated. The surface potential isotherms of these molecules have been studied for the first time by using Kelvin probe method. The phenomena presented by the surface pressure and surface potential isotherms have been well explained theoretically. By using the semi empirical molecular orbital theory, the simulation of molecular dipole moments were calculated and the packing orientation of different molecules on the air/water interface were deduced according to the results of surface potential measurements.

Key words: Ubiquinone, Plastoquinone, Langmuir film, Surface potential, Semi-empirical molecular orbital theory