Concentration Polarization at Langmuir Monolayer Deposition: The Role of Indifferent Electrolytes

Concentration polarization effect at the deposition of charged Langmuir monolayers

The review summarizes the results of the recent studies of the electrokinetic relaxation process within the meniscus region during the deposition of charged Langmuir monolayers. Such electrokinetic relaxation is the consequence of the initial misbalance of partial ion fluxes within a small region near the contact line, where the diffuse parts of electric double layers, formed at the monolayer and the substrate surface, overlap. The concentration polarization within the solution near the three-phase contact line should lead to long-term relaxations of the meniscus after beginning and stopping the deposition process, to changes of the ionic composition within the deposited films, to change of the interaction of the monolayer with the substrate, and to dependence of the maximum deposition rate on the subphase composition.

Ions redistribution and meniscus relaxation during Langmuir wetting process

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.