Emulsion
Principle
Emulsions are part of a more general class of two-phase
systems of matter, called colloids
(see Colloids).
Although the terms colloid and emulsion are sometimes used interchangeably, in emulsion
both the dispersed and the continuous
phase are liquid. Emulsions are also subclass of Suspensions.
An emulsion
is a mixture of two immiscible (unblendable) substances. One substance (the
dispersed phase) is dispersed in the other (the continuous phase).
Emulsions have a cloudy appearance, because the many phase
interfaces (the boundary between two phases) scatter light (see Light: scattering).
Emulsions are unstable and thus do not form spontaneously.
Energy input through stirring, etc., or spray processes are needed to form it.
Over time, emulsions tend to revert to the stable state of oil separated from
water. Surface active substances (surfactants, see Surface tension) can increase the
kinetic stability of emulsions greatly so that, once formed, the emulsion does
not change significantly over years of storage.
Emulsification is the
process by which emulsions are prepared.
Application
In medicine
Emulsions are frequently used in drugs.
In daily life
A large field of application is food and cosmetic
industry. Examples of emulsions include oil in water and butter. In butter and
margarine (are also suspensions), a continuous lipid phase surrounds droplets
of water (water-in-oil emulsion).
More Info
There are three types of emulsion instabilities:
· flocculation,
where the particles form clumps;
· creaming,
where the particles concentrate towards the surface (by buoyancy or by e.g.
centrifugation ) of the mixture while staying separated;
· breaking,
where the particles coalesce (recombination to form bigger ones) due to lack of
shaking and form a layer of liquid.
An emulsifier,
also known as an emulgent or surfactant, is a substance which
stabilizes an emulsion. An example of food emulsifiers is egg yolk (where the
main emulsifying chemical is the phospholipid lecithin). Proteins and
low-molecular weight emulsifiers are common as well.
Detergents, another class of surfactant, chemically
interact with both oil and water, thus stabilizing the interface between oil or
water droplets in suspension. This principle is exploited in soap to remove grease
for the purpose of cleaning. A wide variety of emulsifiers are used to prepare
emulsions such as creams and lotions.
Whether an emulsion turns into a water-in-oil emulsion or
an oil-in-water emulsion depends on the volume fraction of both phases and on
the type of emulsifier. Generally, the so-called Bancroft rule applies:
emulsifiers and emulsifying particles tend to promote dispersion of the phase
in which they do not dissolve very well. For example, proteins dissolve better
in water than in oil and so tend to form oil-in-water emulsions (that is they
promote the dispersion of oil droplets throughout a continuous phase of water).