The structure and layering transitions of ultrathin films of the 4-(5-pentamethyldisiloxypentoxy), $4^{\prime }$-cyanobiphenyl smectic liquid crystal $\mathrm{}\left(5AB\right)\mathrm{}$ at the air-water interface has been studied by x-ray reflectivity. By adjusting the molecular area in a Langmuir trough the reversible, layer-by-layer growth starting from a single monolayer was controlled and analyzed. The films consist of a monolayer in direct contact with the aqueous subphase and bilayers on top of it. The thickness of the monolayer $(\cong 12$ Å) suggests a molecular tilt of about 58$°$. The bilayers are about 36 Å thick, which is approximately the thickness of the bulk smectic $A_d$ phase. Hence, the molecules are presumably in the same interdigitated arrangement of the aromatic cores as in the bulk. A careful analysis of the reflectivity data further indicates a slightly different molecular ordering in the successive bilayers. The interfacial contrast was optionally enhanced, via salt addition, in order to increase the information content and the reliability of the analysis. In these cases ion depletion layers between the bulk subphase and the film were detected. The aromatic parts of the monolayer are found to be partially immersed in water. The bilayer structure itself is not influenced significantly by the addition of ions.

• Received 20 January 1997

DOI:https://doi.org/10.1103/PhysRevE.56.1844