MESOPHASES IN MONOLAYERS OF FATTY-ACIDS AND PHOSPHOLIPIDS

Langmuir monolayers at the air/water interface are distinguished by th eir richness of phases. These can be assigned to smectic categories, a lthough the dominant competing interactions are not yet well elaborate d. For single chain compounds, there are van der Waals attractions of the chains and repulsive forces of the head groups competing in determ ining the local structure. For larger molecules like phospholipids the re appear also to be important dipolar interactions of the head groups . In order to study these interactions, we report surface X-ray diffra ction studies of films of fatty acids and phospholipids at the air/wat er interface, with varying environmental parameters, as well as head g roup size and shape. We observe a wealth of mesophases and also the in fluence of molecular chirality on the structure. The defined control o f the monolayer structure can be made use of to control crystal format ion by diffusion from the subphase and nucleation at the interface. Th is is studied spectroscopically, microscopically and via electron diff raction of transferred films of cyanine dyes. In that case one observe s epitaxy between the dye crystal and surfactant layer, and the relati ve lattices can be varied via suitable dye mixtures. A very peculiar i nterfacial influence results from the orientation of molecular dipoles at the interface. This causes long-range repulsive interactions leadi ng to periodic superlattices in two phase coexistence regions. These f orces are observed and quantified via fluorescence microscopy.