Asymmetry in the interior arachidic-acid bilayers within ultrathin multilayers fabricated via the Langmuir-Blodgett technique

Abstract

Relative electron density profiles derived from high-resolution meridional x-ray diffraction data for ultrathin multilayers, fabricated by repeating from one to five bilayers of arachidic acid deposited on alkylated glass substrates via the Langmuir-Blodgett technique, have consistently displayed an asymmetry within each bilayer in the multilayer. Although the last bilayer deposited systematically displays the most significant asymmetry [V. Skita et al., J. Phys. (Paris) 47, 1849 (1986)], in fact each interior bilayer profile retained some residual asymmetry. This asymmetry in the interior bilayer profiles, which may depend upon the particular deposition conditions utilized, concerns the relatively electron dense feature associated with the apposed carboxyl end groups not being centered between the adjacent relatively electron deficient features associated with the apposed methyl end groups: the carboxyl peak is systematically shifted by 2.5–3.0 Å away from the substrate surface in each interior bilayer profile. We have shown via model calculations that this asymmetry in the interior bilayer profiles is indeed accurately manifest in the experimental intensity functions and the derived multilayer profiles, and therefore indicates a necessary difference in the configurations of the hydrocarbon chains of alternate monolayers. Hence, the Langmuir-Blodgett technique need not necessarily result in the intended formation of an N bilayer multilayer composed of a symmetric bilayer profile repeated N times for such ultrathin multilayers.