Polyelectrolytes in Solution and at Surfaces

Abstract

Abstract This chapter deals with charged polymers (polyelectrolytes) in solution and at surfaces. The behavior of polyelectrolytes (PEs) is markedly different from that of neutral polymers. In bulk solutions, that is, disregarding the surface effect, there are two unique features to charged polymers: first, owing to the presence of long‐ranged electrostatic repulsion between charged monomers, the polymer conformations are much more extended, giving rise to a very small overlap concentration and high solution viscosity. Second, the presence of a large number of counterions increases the osmotic pressure of PE solutions, making such polymers water soluble as this is of great importance to many applications. At surfaces, the same interplay between monomer–monomer repulsion and counterion degrees of freedom leads to a number of special properties. In particular, the adsorption behavior depends on both the concentration of polymers and the added salt in the bulk. We first describe the adsorption behavior of single PE molecules and discuss the necessary conditions to obtain an adsorbed layer and characterize its width. Depending on the stiffness of the PE, the layer can be flat and compressed or coiled and extended. We then proceed and discuss the adsorption of PEs from semidilute solutions. Mean‐field theory profiles of PE adsorption are calculated as a function of surface charge density (or surface potential), the amount of salt in the system, and the charge fraction on the chains. The phenomenon of charge inversion is reviewed and its relevance to the formation of multilayers is explained. The review ends with a short overview of the behavior of grafted PEs.