Polyelectrolyte–Surfactant Complexes As a Formulation Tool for Drug Delivery

Aqueous polyelectrolyte-surfactant complexes (PESCs) are very rich with respect to their properties and the structures formed by them. By design they normally contain hydrophobic micellar surfactant aggregates complexed by long polyelectrolyte chains, thereby combining the formation of small hydrophobic domains given by the surfactant with large-scale structuring due to the presence of the polyelectrolyte chain. In addition, they contain highly polar regions of surfactant head groups in contact with polyelectrolyte, forming a shell around the micellar aggregates, which often also possesses a certain hydrophobic character. Accordingly, the ability for solubilization of water-insoluble compounds of different sorts is particularly versatile in PESCs. Their solubilization sites with very different polarities and hydrophobic characters make them very flexible in adapting to the requirements of a given drug molecule. This renders them attractive for potential applications in drug delivery. In addition, modification of the rheological properties via self-assembly and network formation can be very important in PESC applications. In the following, we discuss the structures of PESCs and their properties, with a focus on the solubilization properties. Subsequently, examples are described where PESCs have been employed in the context of drug solubilization and delivery. These comprise examples with individual aggregates, cross-linked hydrogels, and ones taking advantage of the high solubilization capacity of microemulsions.