Application of emulsion theory to complex and real systems

Synopsis Theories of emulsion stability are discussed in relation to dilute, concentrated and semisolid emulsions. It is shown that whilst a unified theory cannot be applied quantitatively to emulsion systems, a better understanding of the various mechanisms of stabilization and breakdown is emerging. Established concepts, eg, the HLB concept, are now given a less empirical gloss. The application of colloid stability theory, even to very dilute emulsions, is difficult. This is because of the polydispersity of the systems and because surfactant and stabilizers may be free to move on the globule surfaces, as well as dissolve in the dispersed or continuous phases. Further difficulties arise because flocculation is not necessarily a sign of emulsion instability in the practical sense, as creamed or flocculated emulsions often may be redispersed by shaking. Crucial to emulsion stability is the prevention of coalescence, and this is governed by the nature of the adsorbed emulsifier film. The flocculation and coalescence that can be understood in dilute emulsions from the DVLO theory and the metastability of thin films against rupture do not address themselves to flocculation and coalescence in concentrated, structured or semi-solid emulsions. These may possess surfactant multilayers rather than monolayers at the oil/water interfaces and the bulk phases are frequently structured. Some emulsions invert at critical temperatures. In semi-solid emulsions, stability is controlled by the phase behaviour of the emulsifier components, rather than by the forces governing stability in dilute colloids and thin films. Stability is usually considered to be product stability during storage. The stability of an emulsion in use, eg, during and after application to the skin, is a topic of equal importance but little documented. This related problem is also discussed.