We review the work done on complexes between biopolyelectrolytes such as ionically modified cellulose or chitosan and oppositely charged surfactants. Around equimolarity of the charges one typically observes precipitation but for other mixing ratios one may form long-time stable complexes, where structure and rheology depend on the mixing ratio, total concentration and the molecular constitution of the components. In addition, it may be the case that the structures are formed under non-equilibrium situations and therefore depend on the preparation path. The binding is shown to occur cooperatively and the micelles present often retain their shape irrespective of the complexation. However, the rather stiff biopolyelectrolytes may lead to an interconnection between different aggregates thereby forming a network with the corresponding rheological properties. In general, the structure and the properties of the aggregates are rather versatile and correspondingly one can create a wide range of different surfactant-biopolyelectrolyte systems by appropriately choosing the composition. This is very interesting as it allows for formulations with a large range of tuneable properties with ecologically friendly polyelectrolytes for many relevant applications.

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In this study, the phase behavior, structure and properties of systems composed of the cationic, cellulose-based polycation JR 400 and the anionic surfactants sodium dodecylbenzenesulfonate (SDBS) or sodium dodecylethoxysulfate (SDES), mainly in the semidilute regime, were examined. This system shows the interesting feature of a very large viscosity increase by nearly 4 orders of magnitude as compared to the pure polymer solution already at very low concentrations of 1 wt%. By using rheology, dynamic light scattering (DLS), and small-angle neutron scattering (SANS), we are able to deduce systematic correlations between the molecular composition of the systems (characterized by the charge ratio Z=[+(polymer)]/[−(surfactant)]), their structural organization and the resulting macroscopic flow behavior. Mixtures in the semidilute regime with an excess of polycation charge form highly viscous network structures containing rodlike aggregates composed of surfactant and polyelectrolyte that are interconnected by the long JR 400 chains. Viscosity and storage modulus follow scaling laws as a function of surfactant concentration (η~c(s)(4); G(0)~c(s)(1.5)) and the very pronounced viscosity increase mainly arises from the strongly enhanced structural relaxation time of the systems. In contrast, mixtures with excess surfactant charges form solutions with viscosities even below those of the pure polymer solution. The combination of SANS, DLS, and rheology shows that the structural, dynamical, and rheological properties of these oppositely charged polyelectrolyte/surfactant systems can be controlled in a systematic fashion by appropriately choosing the systems composition.