Bijel Systems Based on the Phase Separation of Biological Macromolecules

In this chapter the aim is to focus on bijels where the two phases are aqueous solutions of biological macromolecules and the nature and types of particle that can maintain this bijel state, by adsorbing to the water–water (W/W) interface. Confusion concerning different terminologies in this area is discussed, such as the distinctions between true bijels, W/W emulsions, aqueous two-phase systems, sheared gels, interpenetrating polymer networks, Pickering and Mickering emulsions. We distinguish true biopolymer-based bijels as bicontinuous biopolymer solution phases where the bicontinuous structure is stabilized by the immobilization of particulate material at the interface between these two phases. It turns out that, so far, there seem to be few examples of such systems and, by reviewing the thermodynamic origins of biopolymer phase separation and the interactions of particles with such systems, we indicate several reasons why this might be the case. Primarily, these include the extremely low W/W interfacial tensions and therefore the need for large enough particles to adsorb to the interfaces. Added to this are the difficulties of maintaining particles that are neutrally wetting with regard to each phase, so that the particle contact angle is close to 90°, since biopolymers have strong tendencies to interact with all types of particle surface. Nevertheless, there do appear to be some possibilities of forming biopolymer phase-separated bijels, using particles such as inert inorganic solids, microgels, stable emulsion droplets or even microbial cells. These may lead to some fascinating new materials and applications.