Development of Cation-Dependent Layer to Layer Intermolecular Correlations in 5-Bilayer Arachidic Acid Multilayers

Abstract

ABSTRACTWe have recorded two-dimensional X-ray diffraction patterns from fivebilayer Langmuir-Blodgett multilayer films. The films were deposited on alkylated glass substrates from monolayers of arachidic acid which were spread on barium or cadmium cation containing subphases: the pH was systematically varied from 4 to 9. The diffraction patterns were recorded using doubly-focusing X-ray optics and a two-dimensional position sensitive detector.Generalized Patterson function analysis of the meridional X-ray diffraction I (qxy = 0, qz), which arises from the profile structure of the multilayer, indicated that the bilayer to bilayer correlations improved with increasing pH value (i.e., with incorporation of divalent metal cations into the film). Experimentally determined electron density profiles have previously shown that the average in-plane density of the down-stroke monolayers was greater than that of the the up-stroke monolayers, in agreement with the observed transfer ratios.We observed Bragg rods at several qxy-values only for multilayers for which the subphase was of intermediate to high pH. The intensity and degree of sampling of the Bragg rods along qz increased with increasing pH value. At the highest pH values, the in-plane intermolecular correlation lengths were ∼600 Å and the bilayer to bilayer intermolecular correlation lengths were ∼165 Å or ∼2 1/2 bilayers. This indicates that for the fatty-acid salts the monolayers of low average in-plane density are “patched” with separated domains which have been deposited epitaxially on the underlying monolayers of high average inplane density.