Grazing Incidence X-ray diffraction on Langmuir films: toward atomic resolution.

We have analyzed grazing incidence diffration (GIXD) data from condensed phases of Langmuir films of long-chain fatty acids at the air-water interface by using a new method consisting of a careful extraction of the structure factors followed by fitting of molecular parameters. We show that, contrary to the general belief, the information contained in GIXD spectra is enough to obtain near-atomic structural information. In particular, we directly determine for the first time the orientation of the chain backbone planes and of the carboxylic headgroups and we evaluate chain conformation defects. This new method allowed us to evidence a new phase of symmetry p2gm at high pressure, corresponding to a minimum in lattice energy, but never observed.