Emulsification is the metastable single phase of two insoluble liquids. All emulsions are inherently unstable with the exception of spontaneously forming microemulsions. In the case of emulsions, all that formulators can do is delay instability.
Emulsions can be o/w or w/o, known as an emulsion or invert emulsion, respectively. Most cosmetic chemists are familiar with these systems. Emulsions typically have a surfactant added to stabilize them. Emulsions are, however, only one type of system known as a colloid.
A colloid is a chemical mixture where one substance is dispersed evenly throughout another.1 These systems typically consist of two separate phases (see Figure 1): a dispersed phase (or internal phase) and a continuous phase (or dispersion medium). A colloidal system may be solid, liquid or gaseous.
1. Encylopedia Britannica, www.britannica.com/EBchecked/topic/125898/colloid (Accessed on Feb 9, 2010)