Electrochemistry of Langmuir–Blodgett and self-organized monolayers of an azocrown ether, both pure and mixed with a phospholipid

Abstract

The electrochemical behavior of monolayers of an azocrown ether (L16), both pure and mixed with dioleoylphosphatidylcholine (DOPC) was investigated using a mercury electrode. The monolayers formed at the air ∣ water interface were transferred onto the electrode using the Langmuir–Blodgett approach. The reversibility of the electrode processes depends on the surface pressure during the transfer of the monolayer. The reduction mechanism of the azo to the hydrazo group was studied in acidic medium by cyclic voltammetry and potential-step chronocoulometry. The dependence of the faradaic charge due to azo group reduction at constant pH upon scan rate (for voltammetry) or upon electrolysis time (for chronocoulometry) was examined on the basis of a general kinetic approach. The same approach was used to interpret the dependence of the faradaic charge upon pH at constant scan rate or electrolysis time. The reduction of the azo to the hydrazo group takes place via the reversible uptake of one electron, followed by the rate determining protonation of the resulting radical anion. When L16 is in the form of a pure monolayer its electroreduction is accompanied by a 2D phase transition involving the passage from a liquid-like to a solid-like structure. No such phase transition is observed in mixed L16–DOPC monolayers. At intermediate compositions of this mixture, strong attractive interactions between L16 and DOPC molecules decrease the mean area per molecule with respect to the ideal behavior at the air ∣ water interface, and prevent complete electroreduction of the L16 molecules in the monolayer.