Reversible swelling of polyanion/polycation multilayer films in solutions of different ionic strength

Immersion of consecutively adsorbed polyanion/polycation multilayer films in solutions of different ionic strength causes reversible thickness changes. Swelling of the films up to 18% was observed after soaking the films in aqueous solutions of NaCl at concentrations up to 1.0 mol/l. X-ray reflectivity clearly shows that the thickness changes are reversible within experimental error and that the films seem to get smoother after several cycles of immersion in salt solutions and in pure water. The extent of swelling depends on the internal film structure, namely an the number of polyanion/polycation interfaces in a film of a given thickness. Interestingly, the swelling levels off at salt concentrations above 0.1 mol/l and no additional changes were observed even at NaCl concentrations of 3.0 mol/l over periods of several days. It is believed that Na+ and Cl- ions penetrate reversibly into the films and are responsible for the observed effects.