Concentration polarization at Langmuir monolayer deposition: the role of indifferent electrolytes.

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.