Emulsion
 |
A. Two immisicble liquids, not emulsified; B. An emulsion of Phase B dispersed in Phase A; C. The unstable emulsion progressively separates; D. The (purple) surfactant positions itself on the interfaces between Phase A and Phase B, stabilizing the emulsion |
An
emulsion is a mixture of two
immiscible (unblendable) substances. One substance (the dispersed phase) is
dispersed in the other (the continuous phase). Examples of emulsions include
butter and
margarine,
espresso,
mayonnaise, the photo-sensitive side of
Photographic film, and
cutting fluid for
metalworking. In butter and margarine, a continuous lipid phase surrounds droplets of water (water-in-oil emulsion).
Emulsification is the process by which emulsions are prepared.
Emulsions tend to have a cloudy appearance, because the many
phase interfaces (the boundary between the phases is called the interface)
scatter light that passes through the emulsion. Emulsions are unstable and thus do not form spontaneously. Energy input through shaking, stirring, homogenizers, or spray processes are needed to form an emulsion. Over time, emulsions tend to revert to the stable state of oil separated from water. Surface active substances (
surfactant) can increase the kinetic stability of emulsions greatly so that, once formed, the emulsion does not change significantly over years of storage. Homemade
oil and
vinegar salad dressing is an example of an unstable emulsion that will quickly separate unless shaken continuously. This phenomenon is called
coalescence, and happens when small droplets recombine to form bigger ones. Fluid emulsions can also suffer from
creaming, the migration of one of the substances to the top of the emulsion under the influence of
buoyancy or
centripetal force when a
centrifuge is used.
Emulsions are part of a more general class of two-phase systems of matter called
colloids. Although the terms colloid and emulsion are sometimes used interchangeably, emulsion tends to imply that both the dispersed and the continuous phase are
liquid.
There are three types of emulsion instability:
flocculation, where the particles form clumps,
creaming, where the particles concentrate towards the surface of the mixture while staying separated, or
breaking, where the particles coalesce and form a layer of liquid.
An Emulsifier
(also known as a
surfactant from
surface active material or
emulgent) is a substance which stabilizes an emulsion. Examples of food emulsifiers are
egg yolk (where the main emulsifying chemical is the
phospholipid lecithin), and
mustard, where a variety of chemicals in the
mucilage surrounding the seed hull act as emulsifiers;
proteins and low-molecular weight emulsifiers are common as well. In some cases, particles can stabilise emulsions as well through a mechanism called
Pickering stabilization. Both
mayonnaise and
Hollandaise sauce are oil-in-water emulsions stabilized with egg yolk lecithin.
Detergents are another class of surfactant, and will chemically interact with both
oil and
water, thus stabilising 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 in
pharmacy to prepare emulsions such as
creams and
lotions.
|
20 ml ampule of 1% propofol emulsion suitable for intravenous injection. The manufacturers emulsify the lipid soluble propofol in a mixture of water, soy oil and egg lecithin. |
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
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).
