Emulsion Instability and Breakdown

One can distinguish two different types of instabilities. There are instabilities resulting from thermodynamics, such as flocculation, coalescence, and Ost-wald ripening, and there are those resulting from gravity, such as sedimentation, creaming, and coacervation. 

Flocculation, by itself, can be reversible. Agitation may be sufficient to redisperse the droplets. However, when in close contact, the wall separating two droplets may break, allowing the droplets to merge into a single, bigger droplet. This phenomenon is referred to as coalescence and is not reversible. It results in a drift of the particle size distribution toward larger values, and may even lead to total phase separation. 

Flocculation and coalescence can be avoided by preventing droplets approaching each other. In fact, flocculation (which may lead to coalescence) is the result of the van der Waals forces between two droplets. These forces act at relatively short distances, and are always attractive. There are two ways to counteract these forces electrostatic repulsion and steric repulsion. 

Electrostatic repulsion has a limitation. It works only for systems that do not contain large quantities of electrolytes. Indeed, the presence of electrolytes reduces the so-called Debye length, which is basically the distance at which electrostatic repulsion is effective. Electrolytes also compress the electrical double layer. The result is a reduction of the electrostatic repulsion, which may become weaker than van der Waals attraction. 

The selection of polymer is critical. If too water soluble, the polymer will not adsorb very well on the droplet surface. If not hydrophilic enough, the polymer will lie flat on the surface, so that van der Waals attraction can again take place. 

---