Ion Transport and Equilibria in Polyelectrolyte Multilayers

Abstract

Transport of redox-active probe ions through multilayers made from highly charged polyelectrolytes is described. Because of intrinsic charge balance between polyelectrolyte segment ion pairs, there are no sites for the exchange of small ions within the bulk of as-made multilayers. In the presence of salt in the external bathing solution, sites for small ion exchange are forced into these “reluctant” amphoteric exchangers. Equilibrium considerations lead to the conclusion that, under conditions of high salt concentration and strong polymer ion pairing, the exchanger site concentration is proportional to the solution salt concentration, while the population of redox ions within the multilayer remains constant. Rotating disk electrodes coated with multilayers yield cyclic voltammograms which retain their classical sigmoidal shape, indicating well-behaved membrane transport. Limiting currents are strongly attenuated by membranes of thickness on the order of a few hundred angstroms. Layer-by-layer buildup reveals enhanced transport of negative ions through multilayers bearing a positive surface charge, attributed to Donnan-enhanced membrane inclusion of negative ions. Good matching of internal charge, efficiently excluding ions, is a distinctic feature of membranes made from polyelectrolyte multilayers. The flux through membranes is a strongly nonlinear function of salt concentration for the multiply charged ferro- and ferricyanide ions. This leads to significant transport selectivity between ions, favoring species with lower charge. With the membrane concentration of redox ion, charge n, constant, membrane flux is proportional to membrane diffusion coefficient, D̄, which depends on solution salt concentration in the manner D̄ ∼ k [salt] n . The implications of this for membrane transport control of multiply charged biomolecules are discussed.