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

The electrochemistry of poly(o-aminophenol) modified electrodes in the pres ence of different electroactive solutes (Fe(CN)(6)(4-/3-), hydroquinone/ben zoquinone (HQ/Q), sn(2+)) able to diffuse through the polymer film was stud ied by rotating disc electrode voltammetry. Experimental results relating t o the diffusion of these electroactive species through the polymer were int erpreted on the basis of the membrane-diffusion theory for the rotating dis c electrode. The electron hopping model was invoked in order to obtain a di ffusion constant for the electron transport. External variables such as thi ckness and acid concentration in solution affect both the permeation proces s of electroactive species and the electron motion within the film. The eff ect of film thickness on the permeability of the electroactive species was attributed to changes in the polymer morphology as the film thickness varie s. The influence of the acid concentration in solution on the electron tran sport was ascribed to a different degree of protonation of the polymer redo x centers, which affects the electron hopping process. Permeation of electr oactive species across the polymer decreases as the acid concentration in s olution decreases. This was attributed to a progressive polymer oxidation a s 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 foll owing 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 th e polymer. Low permeability for highly charged species could be due to ion- pairing effects and formation of a coordination complex inside the film. Fo rmation of hydrogen bonds inside the film could affect the diffusion rate o f neutral species. Also, different partitioning (kappa) of charged and unde rcharged redox species at the polymer \ solution interface during the oxida tion process of the polymer could influence the permeation process of these electroactive species. (C) 1999 Elsevier Science S.A. All rights reserved.