Two-dimensional mixtures of stearic acid and partially fluorinated amphiphilic molecule: A grazing incidence X-ray diffraction study

We report a grazing incidence X-ray diffraction (GIXD) study df mixed Langm uir monolayers of stearic acid (ST) and a fluorinated amphiphilic molecule (FEP). The mixing behavior of the two components was investigated at two di fferent surface pressures, 5 and 30 mN/m: The ST molecules are found to be in the L-2 mesophase at 5 mN/m and in the LS mesophase at 30 mN/m, whereas the FEP molecules appear disordered. In the mixture, the GIXD measurements, associated with previous thermodynamical and optical microscopy results, s how that in the whole range of molar ratio, the two molecules are completel y phase-separated, Also, we observed at 5 mN/m, relaxation in the pure ST m onolayer and in the mixtures rich in ST; the tilt angle of the molecules an d the rectangular distortion of the 2D-lattice decrease. By comparing mixed films and pure ST films, this relaxation could be correlated to the size o f the domains.