Influence of the Ionic Strength on the Polyelectrolyte Multilayers' Permeability

Abstract

The influence of ionic strength (I) and pH on the permeability of polyelectrolyte multilayers (PEMs) was studied. Fluorescein crystals were coated with a sodium poly(styrenesulfonate) (PSS)/poly(allylamine hydrochloride) (PAH) multilayer shell at pH 2.0 and subsequently dissolved at pH 7.0 or 8.0. From the release profiles, permeability coefficients for fluorescein diffusion through PEMs were found to depend on pH and I. Permeability coefficients changed from 1.0 × 10-7 to 6.6 × 10-8 m/s at pH 7.0 and from 4.5 × 10-8 to 8.7 × 10-9 m/s at pH 8.0, while the salt concentration was varied from 500 to 10 mM. Decrease of the permeability coefficients while going from pH 7.0 to pH 8.0 occurred due to the increase of the repulsion between the fluorescein molecule and the PEM. The found dependencies indicated that permeation occurs through the water-filled cavities in the multilayers. The permeability is suggested to be dependent on the free energy of the interpolyelectrolyte interaction and to be the function of ionic strength.