Journal of Electroanalytical Chemistry 509 (2001) 31 – 41 www.elsevier.com/locate/jelechem Electrochemistry of Langmuir – Blodgett and self-organized monolayers of an azocrown ether, both pure and mixed with a phospholipid Izabella Zawisza a , Renata Bilewicz a , Maria Rosa Moncelli b , Rolando Guidelli b, * a Department of Chemistry, Uniersity of Warsaw, Pasteur 1, 02093 Warsaw, Poland b Department of Chemistry, Uniersity of Florence, Via Gino Capponi 9, 50121 Florence, Italy Received 28 September 2000; received in revised form 6 January 2001; accepted 6 January 2001 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. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Self organized monolayers; Azocrown ether; Langmuir – Blodgett films; Chronocoulometry; Cyclic voltammetry; Kinetics 1. Introduction Lipophilic or amphiphilic electroactive molecules which are sparingly soluble in water can be readily adsorbed on mercury to form a pure submonolayer or monolayer. Alternatively, the monolayer can be pre- formed at the air  water interface and transferred onto the mercury surface using the Langmuir – Blodgett (L – B) approach [1,2]. L–B transfers have often been car- ried out on solid metals such as gold [3,4]. The electroactive molecules can be self-assembled in single- component monolayers or mixed with another com- pound acting as a diluent of the electroactive sites, e.g. a phospholipid or an alkanethiol [5–9]. Alkanethiols are not ideal diluents for electroactive amphiphilic molecules, due to the lack of flexibility. More flexible, liquid-crystalline monolayers are ex- pected to be better hosts, since they should not hinder any reorientations or conformational changes of the electroactive groups in compact monolayers. Phospho- lipids may be useful in terms of such flexibility, since they form flexible monolayers at the air  water interface [10,11]. Mixed monolayers of phospholipids with other compounds have been extensively studied at the air  water interface [11–13]. Dioleoylphosphatidyl- choline (DOPC) was reported to form a self-organized, liquid-crystalline monolayer of high stability on mer- cury [14]. Here, the term ‘self-organization’ will be used to denote the spontaneous organization of adsorbate * Corresponding author. Tel.: +39-55-2757540; fax: +39-55- 244102. E-mail address: guidelli@unifi.it (R. Guidelli). 0022-0728/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII:S0022-0728(01)00380-1