Ion transport through polyelectrolyte multilayers under steady-state conditions

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 diffusion 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 monotonically 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.