Polyelectrolyte complexes

This chapter presents selected ideas concerning complexes that are formed either by oppositely charged polyelectrolytes or by polyelectrolytes and surfactants of opposite charge. The polyelectrolyte complexes (PECs), which are surfactant-free, form typical structures of a low degree of order such as the ladder- and scrambled-egg structures. In contrast, polyelectrolyte-surfactant complexes (PE-surfs) show a large variety of highly ordered mesomorphous structures in the solid state. The latter have many similarities to liquid-crystals. However, as a result of their ionic character, mesophases of PE-surfs are thermally more stable. Both, PECs and PE-surfs can be prepared as water-soluble and water-insoluble systems, as dispersions and nanoparticles. A stoichiometry of 1:1 with respect to their charges are found frequently for both. Structures and properties of PECs and PE-surfs can be tuned to a large extent by varying composition, temperature, salt-concentration etc. Drug-carrier systems based on PECs and PE-surfs are discussed. Examples are complexes of retinoic acid (PE-surfs) and DNA (PECs). A brief overview is given concerning some theoretical approaches to PECs and PE-surfs such as the formation of polyelectrolyte multilayers.