92168.pdf

Emulsions and foams 263

with large droplets, through blue-white, then gray-translucent, to

transparent, with small microemulsion droplets.

In nearly all emulsions, one of the phases is aqueous and the other

is (in the widest sense of the term) an oil. If the oil is the dispersed

phase, the emulsion is termed an oil-in-water (O/W) emulsion; if the

aqueous medium is the dispersed phase, the emulsion is termed a

water-in-oil (W/O) emulsion. There are several methods by which the

emulsion type may be identified.

1. In general, an O/W emulsion has a creamy texture and a W/O
   emulsion feels greasy.


2. The emulsion mixes readily with a liquid which is rniscible with its
   dispersion medium.


3. The emulsion is readily coloured by dyes which are soluble in the
   dispersion medium.


4. O/W emulsions usually have a much higher electrical conductivity
   than W/O emulsions.

Emulsifying agents and emulsion stability

Probably the most important physical property of an emulsion is its
stability. The term 'emulsion stability' can be used with reference to
three essentially different phenomena - creaming (or sedimentation),
coagulation and a breaking of the emulsion due to droplet coalescence.
Creaming results from a density difference between the two phases
and is not necessarily accompanied by droplet coagulation, although
it facilitates this process.
Droplet collisions may result in coagulation, which, in turn, may
lead to coalescence into larger globules. Eventually, the dispersed
phase may become a continuous phase, separated from the dispersion
medium by a single interface. The time taken for such phase
separation may be anything from seconds to years, depending on the
emulsion formulation and manufacturing conditions.
Assessment of the stability of an emulsion against coalescence
involves droplet counting^218. The most unequivocal method (but one
which is rather laborious) is to introduce a suitably diluted sample of
the emulsion into a haemocytometer cell and count the microscopic-
ally visible particles manually.
The Coulter counter affords a convenient indirect technique for