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Chapter 2

2. Emulsion and emulsion stability

This chapter briefly introduces the basic definitions and concepts of emulsion,
summarizes the conventional knowledge on emulsion stability.

2.1. Basic definitions

Emulsion is a relatively stable dispersion of a liquid within another liquid with
which it exhibits limited miscibility (IUPAC, 1972). The dispersed phase is
commonly present in an emulsion in the form of spherical drops. Due to the large
surface area per droplet, the excess Gibbs energy per droplet is high to make the
emulsion thermodynamically unstable. The stability of emulsion is characterized
by a time evolution of its basic parameters, for instance, the volume fraction and
drop size distribution of the dispersed phase in the medium ^[1]. Notwithstanding
their thermodynamic instability, many emulsions are kinetically stable and do not
change appreciably for a prolonged period. The surface active agents at the
interfaces can delay the separation tendency of emulsion. Such agents are
molecules with polar and non-polar chemical groups in their structure, usually
referred to as surfactants, or finely divided solids ^[2].

In general, emulsions contain an organic liquid (oil phase) and an aqueous
solution (water phase). Emulsions of droplets of an oil phase in an aqueous
solution are indicated by the symbol O∕W and emulsions of aqueous droplets in

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