Abstract
We report on the study of mixed monolayers consisting of two amphiphilic molecules spread at the air/water interface, one with a hydrocarbon chain, the other with a partially fluorinated chain. The mixing behaviour of this system is deduced from a thermodynamic analysis correlated to a direct observation of the monolayers with fluorescence microscopy and Brewster angle microscopy. We show that this mixture presents a complete phase separation over the entire range of molar fractions and surface pressures. Two-dimensional circular domains displaying an optical anisotropy related to the tilt of the molecules are observed. The domains are identified as being exclusively occupied by the hydrocarbon amphiphile, regardless of the molecular density and the mixture composition.