are bound together to formÜbrils,
which have great strength and lim-
ited elasticity. Collagen accounts for
over 30% of the total body protein of
mammals.
collective oscillationAn oscilla-
tion in a many-body system in which
all the particles in the system take
part in a cooperative. Plasma oscilla-
tions provide an example of collec-
tive oscillations. In systems described
by quantum mechanics, collective os-
cillations are quantized to give collec-
tive excitations.colligationThe combination of
two free radicals to form a covalent
bond, as in H 3 C•+ Cl•→CH 3 Cl. It is
the reverse of *homolyticÜssion.colligative properties Properties
that depend on the concentration of
particles (molecules, ions, etc.) pre-
sent in a solution, and not on the na-
ture of the particles. Examples of
colligative properties are osmotic
pressure (see osmosis), *lowering of
vapour pressure, *depression of
freezing point, and *elevation of boil-
ing point.collision densityThe number of
collisions that occur in unit volume
in unit time when a given particle
Ûux passes through matter.collision quenchingSee external
conversion.collodionA thinÜlm of cellulose
nitrate made by dissolving the cellu-
lose nitrate in ethanol or ethoxy-
ethane, coating the surface, and
evaporating the solvent.colloid millsMachines used to
grind aggregates into veryÜne parti-
cles or to apply very high shearing
forces within aÛuid to produce col-
loid suspensions or emulsions in
which the particle sizes are less than
1 micrometer. One type of colloid
mill is called a disc mill, in which amixture of a solid and liquid (or two
liquids) is passed between two discs a
small distance apart, which rotate
very rapidly relative to each other.
Other types of colloid mills are the
valveand oriÜcetypes, in which the
mixture is forced through an oriÜce
at a very high speed and then strikes
a breaker ring. Applications of colloid
mills occur in food processing, in
paint manufacture, and in the phar-
maceutical industry.colloidsColloids were originally
deÜned by Thomas *Grahamin 1861
as substances, such as starch or
gelatin, which will not diffuse
through a membrane. He distin-
guished them from crystalloids(e.g.
inorganic salts), which would pass
through membranes. Later it was rec-
ognized that colloids were distin-
guished from true solutions by the
presence of particles that were too
small to be observed with a normal
microscope yet were much larger
than normal molecules. Colloids are
now regarded as systems in which
there are two or more phases, with
one (the dispersed phase) distributed
in the other (the continuous phase).
Moreover, at least one of the phases
has small dimensions (in the range
10 –9–10–6m). Colloids are classiÜed in
various ways.
Solsare dispersions of small solid
particles in a liquid. The particles
may be macromolecules or may be
clusters of small molecules. Lyopho-
bic sols are those in which there is
no afÜnity between the dispersed
phase and the liquid. An example is
silver chloride dispersed in water. In
such colloids the solid particles have
a surface charge, which tends to stop
them coming together. Lyophobic
sols are inherently unstable and in
time the particles aggregate and
form a precipitate. Lyophilic sols, on
the other hand, are more like true so-collective oscillation 134c