Ion transport through polyelectrolyte multilayers under steady-state conditions V. Garc  ıa-Morales a , T.H. Silva b , C. Moura b , J.A. Manzanares a, * , F. Silva b a Departament de Termodin amica, Universitat de Val encia, E-46100 Burjassot, Spain b LEQA – Laborat orio de Electroqu  ımica e Qu  ımica Anal  ıtica CIQ-L4 Departamento de Qu  ımica, Faculdade de Ci^ encias do Porto, Rua do Campo Alegre, 687, 4169-007 Porto, Portugal Received 2 October 2003; received in revised form 12 February 2004; accepted 13 February 2004 Available online 9 April 2004 Abstract The permeability of a self-assembled polyelectrolyte multilayer to small ions under the influence of an applied potential difference is studied as a function of the number of layers and the nature of the supporting electrolyte. The multilayer is described as a series of homogeneously charged membranes with alternating sign of their fixed charge. Ion transport is described on the basis of the dif- fusion equation and the assumption of (Donnan) electrochemical equilibrium at the boundaries between layers. The calculated steady-state current–voltage curves are found to be in good agreement with experimental linear sweep voltammograms (at low sweep rate). The permeability of polystyrenesulfonate/polyallylamine multilayers to ferro/ferricyanide ions is found to decrease mono- tonically with increasing number of layers when sodium perchlorate is used as the supporting electrolyte. However, an interesting non-monotonic dependence on the number of layers is observed when the supporting electrolyte is barium perchlorate. Both types of behaviour are accounted for satisfactorily by the theoretical model. Ó 2004 Elsevier B.V. All rights reserved. Keywords: Membrane-covered electrode; Polyelectrolyte multilayers; Linear sweep voltammetry; Supporting electrolyte; Theory 1. Introduction Self-assembled polyelectrolyte multilayers (PEMUs) can be obtained by sequential adsorption of polymers of two or more types in a solid substrate [1]. These novel materials have proven applications in several areas in- cluding light-emitting diodes [2], nonlinear optical devices [3–5], biosensors [6–8], gas separation [9], ethanol–water pervaporation [10], electrochromics [11], conductive coatings [12] and patterning [13,14]. PEMUs have also shown promise as selective membranes for the immobi- lization of macromolecules or particles [15–20] and for the controlled transport and release of small molecules [21,22]. The build-up of PEMUs is driven by electrostatic interactions between each new layer added and that previous already deposited having a net charge of op- posite sign (mainly distributed at the surface). There- fore, each layer of polymer being added reverses the net charge on the surface leaving it primed for the next [1,23]. Several issues concerning this build-up process are currently under debate. For instance, it is not well known to what extent the excess of charge is distributed on the surface. Some studies attribute it to the first few layers under the surface [23,24], while others attribute it to molecule–molecule and molecule–surface charge overcompensation [25,26]. Another aspect that remains to be clarified concerns whether the compensation of the internal charge inside the multilayer ‘‘as formed’’ is due to a 1:1 stoichiometry of the alternating polymers (‘‘intrinsic compensation’’) or to the presence of mobile salt ions (‘‘extrinsic com- pensation’’); see [27] and references therein. It is gener- ally accepted, however, that if the multilayer is immersed in an external solution with a significant salt concentration, the chemical potential of the salt forces some swelling of the multilayer thus establishing some degree of extrinsic compensation [24,28,29]. The * Corresponding author. Tel.: +34-96-354-3119; fax: +34-96-354- 3385. E-mail address: manzanar@uv.es (J.A. Manzanares). 0022-0728/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2004.02.018 Journal of Electroanalytical Chemistry 569 (2004) 111–119 www.elsevier.com/locate/jelechem Journal of Electroanalytical Chemistry