Understanding and Exploiting the Phase Behavior of Mixtures of Oppositely Charged Polymers and Surfactants in Water

Complexes of oppositely charged polymers and surfactants (OCPS) in water come in many varieties, including liquid-crystalline materials, soluble complexes, structured nanoparticles, and water-insoluble surface layers. The range of available structures and properties increases even further with the addition of other amphiphilic substances that may enter, or even dissolve, the complexes, depending on the nature of the additive. Simple operations may change the properties of OCPS systems dramatically. For instance, dilution with water can induce a phase separation in an initially stable OCPS solution. More complicated processes, involving chemical reactions, can be used to either create or disintegrate OCPS particles or surface layers. The richness of their properties has made OCPS mixtures ubiquitous in everyday household products, such as shampoos and laundry detergents, and also attractive ingredients in the design of new types of responsive particles, surfaces, and delivery agents of potential use in future applications. A challenge for the rational design of an OCPS system is, however, to obtain a good fundamental understanding of how to select molecular shapes and sizes and how to tune the hydrophobic and electrostatic interactions such that the desired properties are obtained. Recent studies of OCPS phase equilibria, using a strategy where the minimum number of components is always used to address a particular question, have brought out general rules and trends that can be used for such a rational design. Those fundamental studies are reviewed here, together with more application-oriented studies where fundamental learning has been put to use.