Polyelectrolyte Complexes in the Dispersed and Solid State I

Polyelectrolytes are macromolecules composed of charged monomers and exhibit unique properties due to the interplay of their flexibility and electrostatic interactions. In solution, they are attracted to oppositely charged surfaces and interfaces and exhibit a transition to an adsorbed state when certain conditions are met concerning the charge densities of the polymer and surface and the properties of the solution. In this review, we discuss two limiting cases for adsorption of flexible polyelectrolytes on curved surfaces: weak and strong adsorption. In the first case, adsorption is strongly influenced by the entropic degrees of freedom of a flexible polyelectrolyte. By contrast, in the strong adsorption limit, electrostatic interactions dominate, which leads to particular adsorption patterns, specifically on spherical surfaces. We discuss the corresponding theoretical approaches, applying a mean-field description for the polymer and the polymer–surface interaction. For weak adsorption, we discuss the critical adsorption behavior by exactly solvable models for planar and spherical geometries and a generic approximation scheme, which is additionally applied to cylindrical surfaces. For strong adsorption, we investigate various polyelectrolyte patterns on cylinders and spheres and evaluate their stability. The results are discussed in the light of experimental results, mostly of DNA adsorption experiments.

This chapter describes the progress made in understanding the mechanisms of ion conduction in polyelectrolyte complexes (PEC). Understanding of ion dynamics is based on frequency-dependent conductivity data obtained by impedance spectroscopy as a function of temperature, hydration, and composition. In most of the work, strong polyelectrolytes such as poly(alkali 4-styrene sulfonate) (AlkaliPSS) and poly(diallyldimethyl ammoniumchloride) (PDADMAC) are employed, forming complexes of type xAlkaliPSS · (1 − x) PDADMAC. The dc conductivity is always determined by the alkali ions, which exhibit a size-dependent mobility. This holds even in PEC with an excess of PDADMAC. The ion dynamics and transport mechanisms are different in PDADMAC-rich and in NaPSS-rich PEC. We review the treatment of the frequency-dependent shape of conductivity spectra by scaling concepts and by models involving forward–backward hopping motions of small ions as well as localized motions of charges. Thus, many quantitative concepts established in other disordered ion conductors can be transferred to PEC. In addition to the well-known time–temperature superposition principle (TTSP), the novel concept of time–humidity superposition (THSP) was established for PEC and describes the dependence of ion dynamics on water content.

Polyelectrolyte complex formation is a well-studied subject in colloid science. Several types of complex formation have been studied, including PEMs, macroscopic polyelectrolyte complexes, soluble complexes and polyelectrolyte complex micelles. The chemical nature of the complex-forming polyelectrolytes and the environmental conditions (e.g., pH, ionic strength and temperature) influence the final structural properties of these complexes. This chapter deals with the kinetics of polyelectrolyte complex formation and discusses how ionic strength, charge density and pH influence the dynamics of the complexes, which can range from glass-like (solid) precipitates to liquid-like phases. The switching between the glass-like and liquid-like phase as a function of the ionic strength has a strong analogy to the phase behaviour of polymer melts as function of temperature.

By performing calorimetry during complex formation it has been found that the enthalpy of complex formation of systems that form glass-like phases has an opposite sign to the enthalpy of systems that form liquid-like phases, i.e., the formation of glass-like phases is exothermic and the formation of liquid-like phases is endothermic. The free energy (Δ_f G), enthalpy (Δ_f H) and entropy (Δ_f S) of polyelectrolyte complex formation and how they vary as a function of the ionic strength will be discussed.

Results from dynamic light scattering (DLS) titrations, Atomic Force Microscopy (AFM), surface force measurements and rheology will be used to illustrate how differences in kinetics show up in experiments on colloidal micellar systems. In the section on DLS titrations, three-component systems containing two oppositely charged polyelectrolytes and protein molecules will be discussed. This chapter concludes with a section dedicated to the complex formation of oppositely charged protein molecules.

This review considers interpolyelectrolyte complexes, with a particular emphasis on advanced macromolecular co-assemblies based on polyionic species with nonlinear topology and on polymer–inorganic hybrids formed by interpolyelectrolyte complexes containing metal ions and/or metal nanoparticles.