X-RAY REFLECTIVITY STUDY OF LAYERING TRANSITIONS AND THE INTERNAL MULTILAYER STRUCTURE OF FILMS OF 3-BLOCK ORGANOSILOXANE AMPHIPHILIC SMECTIC LIQUID-CRYSTALS AT THE AIR-WATER-INTERFACE

The: structure and layering transitions of ultrathin films of the 4-(5 -pentamethyldisiloxypentoxy). 4'-cyanobiphenyl smectic liquid crystal (5AB) at the air-water interface hoc been studied by I-ra! reflectivit y. By adjusting the molecular area in a Langmuir trough the reversible , layer-by-layer growth starting from a single monolayer was controlle d and analyzed. The films consist of a monolayer in direct contact wit h the aqueous subphase and bilayers on top of it. The thickness of the monolayer (congruent to 12 Angstrom) suggests a molecular tilt of abo ut 58 degrees. The bilayers an about 36 Angstrom thick, which is appro ximately the thickness of the hulk smectic A(d) phase, Hence, the mole cules are presumably in the same interdigitated arrangement of the aro matic cores as in the bulk. A careful analysis of the reflectivity dat a further indicates a slightly different molecular ordering in the suc cessive bilayers. The interfacial contrast was optionally enhanced, vi a salt addition, in order to increase the information content and the reliability of the analysis. In these cases ion depletion layers betwe en the hulk subphase and the film were detected, The aromatic parts of the monolayer are found to be partially immersed in water, The bilaye r structure itself is not influenced significantly by the addition of ions.