The mechanism of the meniscus oscillations and the stripes formation within the deposited fatty acid monolayer is theoretically analyzed on the basis of a supposition of concentration polarization within the solution during the deposition process. The concentration polarization can lead to decrease of adhesion work, dynamic contact angle, and maximum deposition speed under dynamic conditions resulting in meniscus instability. The adhesion work is evaluated from the disjoining pressure isotherm at a given subphase composition taking into account the charge regulation for a fatty acid monolayer. The relation of the proposed mechanism to the known experimental facts and observations is discussed.

A mathematical model describing the concentration polarization in the three-phase contact region during the Langmuir-Blodgett deposition process is developed. It is shown that the stationary deposition is only possible when, in additional to convective fluxes, electrodiffusion ionic fluxes and corresponding concentration gradients are developed in the system. At a sufficiently low withdrawal speed, the occurring diffusion and migration ionic fluxes restore the steady-state ionic balance. As well, electric charge is accumulated in close vicinity to the three-phase contact line to produce a stationary electric field. The concentration polarization affects the parameters of the deposition process (dynamic contact angle, work of adhesion, maximum deposition rate) as well as morphology, composition, and structure of the deposited monolayer. When the withdrawal speed exceeds a critical value, the transport of counterions becomes insufficient to compensate interfacial charge in close vicinity to the three-phase contact line. Consequently, the electrostatic repulsion between the monolayers becomes sufficiently strong to disrupt the deposition process. The latter can result in meniscus instability. The proposed mechanism correlates with some experimental observations.

A mathematical problem is formulated and numerically solved for addressing the electric field and ionic concentration distributions developing around the three-phase contact line during the Langmuir-Blodgett deposition of charged monolayers. Compared to a previous paper dealing with the same effect (J. Phys. Chem. B 2004, 108, 13449), the present analysis is not restricted to the case of low deposition rates and small concentration changes. The obtained results show that, for sufficiently high deposition rates, the subphase composition substantially changes in the immediate vicinity of the three-phase contact line. It is shown that the predicted changes in the subphase composition can drastically affect the adhesion work and the dynamic contact angle. On this basis, the influence of the concentration polarization effect on meniscus behavior is discussed.

The theoretical analysis of electrostatic interactions and ion redistribution in the close vicinity of the three-phase contact line shows their important role in the Langmuir wetting process. To provide a sufficient rate for the ion transfer, which is intended to neutralize the interfacial charge, the concentration and potential distributions deviate from the equilibrium. As a consequence, during the deposition process the adhesion work, and hence the contact angle, are defined by the local ionic concentrations near the three-phase contact line. The concentration profiles and the electro-diffusion ion fluxes induced during the Langmuir wetting process are strongly dependent on the subphase composition and on the monolayer properties. The results of the analysis are in a good agreement with the experiments.

Under dynamic conditions of the charged Langmuir monolayer deposition onto a substrate surface, ion concentration and electric potential profiles are induced in the subphase around the three-phase contact line. Such local changes in the subphase influence the deposition process, particularly the monolayer adhesion work and the maximum deposition rate. If indifferent electrolytes (not interacting chemically with interfacial groups) are present in the solutions, they can affect electric potential distributions and therefore the monolayer charge and the deposition process as a whole. With increasing deposition rate, the indifferent electrolyte counterions replace gradually the potential-determining counterions in a close vicinity to the contact line. This leads to increasing monolayer ionization and increasing electrostatic repulsion between the monolayer and substrate. When the deposition rate approaches the critical one, the charge of the monolayer increases dramatically and the stationary monolayer deposition becomes impossible. Such a significant increase of the monolayer charge is not observed in the absence of indifferent electrolytes.