Liquid-liquid interfaces: In isolation and in interaction

Although liquid-liquid interfaces are as important as liquid-vapor interfaces in many fields, including biology and technology, they have received much less attention in terms of systematic experimental studies. Many techniques are, in principle, relevant to both types of interface; likewise similar theories can often be developed for both. The basic physical chemistry of isolated interfaces, i.e. interfaces between two bulk liquids in mutual contact, is introduced first in this review. The interfacial tension, the forces acting at interfaces (i.e. van der Waals, Coulombic and steric forces), and the thermodynamic treatment of such systems are each considered, and the experimental techniques and some of the more important results are summarized. Next, the problem of three-phase contact (in which two or three of the phases are liquid) is introduced, and the concept of wetting and spreading considered. This leads to a discussion of systems in which two bulk phases (either, or both, of which are liquids) are separated by a liquid film; the mutual interaction of the two interfaces now becomes relevant. The stability of such systems is discussed in terms of the various forces acting within the systems, plus any external forces, such as gravity. The thermodynamics of liquid films is briefly introduced, and some discussion of the magnitude of the two interfacial tensions given. Finally, it is shown that the factors governing the formation and stability of liquid droplets and emulsion systems are directly related to the consideration of the earlier sections.