Relaxation phenomena during polyelectrolyte complex formation

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 (Δf G), enthalpy (Δf H) and entropy (Δf 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.