Ions Redistribution and Meniscus Relaxation during Langmuir Wetting Process

Abstract

Nonstationary kinetics of the ion redistribution within the meniscus region during deposition of a charged Langmuir monolayer after beginning or stopping of the substrate motion is analyzed on the basis of the results of numerical simulations. The time evolution of the ions concentration profiles forming at the contact line and propagating toward the bulk solution is considered. It is shown that the diffusion front propagates much slower within the region of overlapping diffuse layers than outside of this region. At the beginning of the deposition process a region characterized by quasi-stationary behavior of the ion concentration and electric potential distributions is formed in close vicinity to the contact line. A stationary deposition regime is established when the region of quasi-stationary distributions reaches the external boundary of the Nernst layer provided that the substrate motion is not very fast. For the substrate velocities higher than the critical one the concentration near the contact line can decrease to such small values which do not allow a stable deposition process. The developed mathematical model allows addressing to transient regimes of the monolayer deposition which are very important for understanding the mechanisms leading to meniscus instability.

Concentration Polarization During Langmuir–Blodgett Films’ Deposition

“…We assume here that the substrate surface remains not charged being in contact with the solution, i.e., only one meniscus surface is charged. This situation is different from the previously considered case [8,9] of symmetrically charged meniscus surfaces shown in Fig. 1, b, where the substrate surface is covered by a similar amphiphilic monolayer film with the same surface charge density.…”

“…During the fabrication of such films by deposition of charged monolayers, the ions transfer processes between the bulk solution and the three-phase contact line region play an important role [5][6][7][8][9]. In particular, insufficient rate of such ions transfer sets a limit for the maximum possible rate, at which a stable homogeneous film is deposited on the substrate surface [2,8,9].…”

“…In Refs. [8,9], we performed a detailed simulation of the process of a film transfer to the surface of a substrate covered with a previously transferred monolayer. The presence of an already deposited monolayer at the substrate surface allowed assuming the charges of both meniscus surfaces to be the same, and the potential distribution to be symmetrical relative to the meniscus middle plane.…”