Regular Stripe Patterns in Skeletonized Langmuir−Blodgett Films of Arachidic Acid

Abstract

Regular stripe patterns are formed in skeletonized LB films of two arachidic acid monolayers deposited on Si wafers from Cd2+-containing aqueous subphase after monolayer transfer at pH 5.7. These stripe patterns are very different from those less regular patterns observed at lower or higher pH values of the aqueous subphase. Atomic force microscopy (AFM) and phase shift interference microscopy (PSIM) studies have shown that these stripes are grooves aligned in definite distances along the meniscus and perpendicular to the dipping direction of the monolayer transfer. There exist deep straight grooves of about 6 nm depth in a regular distance and between them less marked stripelike defect lines. These regular stripe patterns occur only in the skeletonized LB films, but they are already preformed during the monolayer transfer onto the solid substrate. The formation of the regular stripe patterns is decisively affected by the arachidic acid/cadmium arachidate ratio but also by the dipping rate. The direct correlation to the autooscillations of the meniscus suggests an electrohydrodynamic instability mechanism as cause for the nonuniform ordering and composition of the LB film.

Concentration Polarization at the Langmuir Monolayer Deposition: Theoretical Considerations

“…The concentration polarization leads to local variation of surface charge density and surface potential, to a local change of the monolayer composition, and, therefore, to changes of composition, morphology, and local ordering of the deposited LB monolayer (for example, stripe formation within the deposited monolayer 27 ). According to the model described, both the electrodiffusion fluxes and the electrochemical potential gradients should increase as the withdrawal velocity increases.…”

Ion redistribution and meniscus stability at Langmuir monolayer deposition

“…The withdrawal velocities in Ref. [31] were in the range of 0.005-0.013 (cm/s), i.e., smaller than the maximum deposition rate determined in Refs. [23,24,65] in the presence of Cd 2+ ions and larger than in the absence of Cd 2+ ions (see above).…”

“…Recently Mahnke et al [31] observed meniscus oscillations (so-called bslip-stickQ behaviour) during deposition of arachidic acid monolayers which have been explained by the meniscus instability due to concentration polarization near the contact line [28]. During the withdrawal the threephase contact line follows the substrate sticking on it at a certain position and slips down periodically to other sticking position.…”

“…[31]. Thus, even small local changes of ionic composition in the subphase near the contact line during the monolayer deposition can lead to appreciable changes in the transferred monolayer composition when the pH is kept close to 5.7 in the bulk solution.…”

“…At some specific experimental conditions (pH and bivalent ions concentration in solution) this effect can lead to meniscus instability and to a stripelike composition of the deposited fatty acid monolayer [31]. The concentration polarization is determined by the peculiarities of coions and counterions distributions and the velocity field within the meniscus region.…”