COLLOIDS 191Classification of colloids
Colloids do not only involve solids and liquids. The classification of common types
of colloid is shown in Table 11.7.
Emulsions
Dispersions of liquids in liquids are important commercially, and occur in foods and
in living organisms.
Suppose we violently shake 1 cm^3 of oil with 10 cm^3 of water for a few seconds.
Upon stopping shaking, the oil remains in the form of tiny droplets for a fraction of a
second before the mixture reverts to two layers again. The purpose of emulsification
(also called solubilization) is to ensure that the solute (here the oil) remains dispersed
permanently in the form of tiny droplets. To achieve this, we add an emulsifying
agent.
Milk is an emulsion of butterfat droplets in an aqueous mixture of sugars, mineral
salts and proteins. Ice cream is another emulsion with a similar composition. The
emulsifying agents in both emulsions are proteins with smaller amounts of phos-
pholipids, mainly lecithin. In egg yolk, the emulsifying agents are phospholipids and
cholesterol.
Emulsions play an important role in the digestion of fats in the intestines. In the
small intestine, fats are emulsified by the alkaline secretions of the pancreas and the
liver. Why is it necessary to emulsify fats before they can be digested? The answer is
that the rate at which fats are digested depends upon the surface area of fat ‘solution’
exposed to the intestines. For example, if 1 cm^3 of oil is dispersed to make oil droplets
of 5 nm diameter, the total area of all the drops is about 1200 m^2! Emulsification
spreads out the oil so effectively that enzymes can break down fats more rapidly.
Soap
The colloidal particles of soap and other detergents are called ionic micelles. Such
micelles typically contain 50–100 ions stuck together (see page 175). Ionic micelles
are important in explaining the emulsification of fats and oils by soaps and deter-
gents. Apart from its use in washing clothes, soaps are used as emulsifying agents in
many cosmetics.
Table 11.7Classification of common types of colloid
Colloid type Composition Examples
Sol solid particles dispersed in a most paints, starch and tea in water,
liquid inks, soapy water, many proteins in water
Gel (or solid continuous network of solid jellies, gelatine
emulsion) particles dispersed in a liquid
Aerosol solid or liquid dispersed in a gas smoke in a room, fog
Foam gas dispersed in liquid beer foam, soapsuds, whipped cream
and shaving foam
Emulsion liquid dispersed in liquid milk, mayonnaise, butter