Transport across poly(o-aminophenol) modified electrodes in contact with media containing redox active couples.: A study using rotating disc electrode voltammetry

The electrochemistry of poly(o-aminophenol) modified electrodes in the presence of different electroactive solutes (Fe(CN)(6)(4-/3-), hydroquinone/benzoquinone (HQ/Q), sn(2+)) able to diffuse through the polymer film was studied by rotating disc electrode voltammetry. Experimental results relating to the diffusion of these electroactive species through the polymer were interpreted on the basis of the membrane-diffusion theory for the rotating disc electrode. The electron hopping model was invoked in order to obtain a diffusion constant for the electron transport. External variables such as thickness and acid concentration in solution affect both the permeation process of electroactive species and the electron motion within the film. The effect of film thickness on the permeability of the electroactive species was attributed to changes in the polymer morphology as the film thickness varies. The influence of the acid concentration in solution on the electron transport was ascribed to a different degree of protonation of the polymer redox centers, which affects the electron hopping process. Permeation of electroactive species across the polymer decreases as the acid concentration in solution decreases. This was attributed to a progressive polymer oxidation as the pH increased which would affect the swelling of the polymer. For high film thickness and a given acid concentration in solution (pH 1), the following sequence of permeation rates for the different electroactive species through the polymer is obtained kappa D-s(Sn2+) < kappa D-s(Fe(CN)(6)(4-/3-)) < kappa D-s(HQ/Q). These permeabilities are compared with the diffusion rate of electroinactive anions required to maintain electroneutrality of the polymer. Low permeability for highly charged species could be due to ion-pairing effects and formation of a coordination complex inside the film. Formation of hydrogen bonds inside the film could affect the diffusion rate of neutral species. Also, different partitioning (kappa) of charged and undercharged redox species at the polymer \ solution interface during the oxidation process of the polymer could influence the permeation process of these electroactive species. (C) 1999 Elsevier Science S.A. All rights reserved.