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

The electrochemical behavior of monolayers of an azocrown ether (L16), both pure and mixed with dioleoylphosphatidylcholine (DOPC) was investigated us ing a mercury electrode. The monolayers formed at the air I water interface were transferred onto the electrode using the Langmuir- Blodgett approach. The reversibility of the electrode processes depends on the surface pressu re during the transfer of the monolayer. The reduction mechanism of the azo to the hydrazo group was studied in acidic medium by cyclic voltammetry an d potential-step chronocoulometry. The dependence of the faradaic charge du e 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 de pendence of the faradaic charge upon pH at constant scan rate or electrolys is time. The reduction of the azo to the hydrazo group takes place via the reversible uptake of one electron, followed by the rate determining protona tion of the resulting radical anion. When L16 is in the form of a pure mono layer its electroreduction is accompanied by a 2D phase transition involvin g the passage from a liquid-like to a solid-like structure. No such phase t ransition is observed in mixed L16-DOPC monolayers. At intermediate composi tions of this mixture, strong attractive interactions between L16 and DOPC molecules decrease the mean area per molecule with respect to the ideal beh avior at the air I water interface, and prevent complete electroreduction o f the L16 molecules in the monolayer. (C) 2001 Elsevier Science B.V. All ri ghts reserved.