Phase Behavior of Aqueous Two-Phase Systems and Stabilization of Water-in-Water Emulsions

This chapter provides an overview of the current understanding of water-in-water (W/W) emulsions formed in aqueous two-phase systems (ATPS) that arise from the incompatibility between two solutes in aqueous mixtures. W/W emulsions refer to colloidal dispersions in which an aqueous solution (disperse phase) is dispersed within another aqueous solution (continuous phase). The chapter summarizes recent advances on the formation and properties of W/W emulsions, highlighting possible novel applications in the biomedical field. ATPS are very common, and are found in a great variety of aqueous systems, all of which are classified and categorized in this chapter. The interfaces of W/W emulsions have very low interfacial tensions, in comparison to conventional O/W emulsions. Consequently, small molecules do not adsorb and form monolayers on W/W interfaces. In recent years, many different strategies have been reported for the improvement of colloidal stability of W/W emulsions, as well as different methods for emulsion preparation. Consequently, novel applications such as carriers of microorganisms or 3D cell cultures are being proposed. In addition, microfluidic methods open a wide range of different possibilities for the accurate control of size and structure of multiple all-aqueous emulsions.