Does the tail-to-head molecular arrangement exist in asymmetrically deposited fatty acid alkylester Langmuir-Blodgett multilayers?

Abstract

A major problem in the X-ray diffraction investigations of asymmetrically deposited fatty acid alkylester Langmuir-Blodgett (LB) films is whether the molecular overturning to a symmetrical multi-bilayer arrangement could always be detected. The conclusion for the occurrence of molecular overturning is related to the specificity of the electron density profiles (EDP) of the ester multilayers. The problem arises from the similarity of the EDP of the “tail-to-head” and the “head-to-head” arranged alkylester films. To establish the true structural arrangement, two approaches are applied in the present study for modification of fatty acid alkylester film EDP: (i) by using a ω-bromopentadecanoic acid methyl ester (BP) as a “label” of the molecular arrangement in propyl stearate (PS) and methyl arachidate (MA) multilayers, and (ii) by mixing of PS and MA. The structural arrangements, normal to the solid substrate, of X-type deposited pure and mixed LB films of PS, MA, and BP are studied by means of low-angle X-ray diffraction. The pure PS and MA films are characterized by a same single-layer periodicity. The X-ray patterns of mixed MA/PS films and of PS films, labeled by relatively low BP molar fractions, evidence a bilayer periodicity (head-to-head arrangement) of these fatty acid alkylester multilayers. Therefore, the multilayers studied possess symmetrical multi-bilayer structures, which could be detected by means of X-ray diffraction only under particular conditions.