Nonadditivity of Polymeric and Charged Surface Interactions: Consequences for Doped Lamellar Phases

Abstract

We explore theoretically the modifications to the interactions between charged surfaces across an ionic solution caused by the presence of dielectric polymers. Although the chains are neutral, the polymer physics and the electrostatics are coupled; the intra-surface electric fields polarise any low permittivity species (e.g., polymer) dissolved in a high permittivity solvent (e.g., water). This coupling enhances the polymer depletion from the surfaces and increases the screening of electrostatic interactions with respect to a model which treats polymeric and electrostatic effects as independent. As a result, the range of the ionic contribution to the osmotic interaction between surfaces is decreased, while that of the polymeric contribution is increased. These changes modify the total interaction in a nonadditive manner. Building on the results for parallel surfaces, we investigate the effect of this coupling on the phase behaviour of polymer-doped smectics.