GRAZING-INCIDENCE X-RAY-DIFFRACTION STUDY OF ARACHIDIC ACID MONOLAYER ON CYANINE DYE AQUEOUS-SOLUTION

The angle and direction of tilt of arachidic acid molecules on the surface of 10(-4) M cyanine dye aqueous solution were determined as a function of surface pressure from the intensity profile along the Bragg rod obtained in a grazing incidence diffraction experiment using synchrotron X-radiation. Experimental data were interpreted by a model in which molecules tilt toward next nearest neighbors in a two dimensional hexagonal lattice at low surface pressures and stand upright at high surface pressures above 35 mN/m. The dependence of lattice spacings of (1,0), (0,1) and (1, -1) molecular rows on the angle of molecular tilt was qualitatively explained by this model.