Page 19 - Introduction to Colloid and Surface Chemistry

10 The colloidal state
each particle is proportional to its mass (as in light scattering), a
mass-average relative molecular mass or particle mass is given:

(mass average) = —'—— (1.2)
M r
For any polydispersed system, Af r (mass average) > M r (number
average), and only when the system is monodispersed will these
averages coincide. The ratio M T (mass average)/Af r (number average)
is a measure of the degree of polydispersity.

Preparation and purification of colloidal systems

Colloidal dispersions

Basically, the formation of colloidal material involves either degrada-
tion of bulk matter or aggregation of small molecules or ions.
Dispersion of bulk material by simple grinding in a colloid mill or
by ultrasonics does not, in general, lead to extensive subdivision,
owing to the tendency of smaller particles to reunite (a) under the
influence of the mechanical forces involved and (6) by virtue of the
attractive forces between the particles. After prolonged grinding
the distribution of particle sizes reaches an equilibrium. Somewhat
finer dispersions can be obtained by incorporating an inert diluent
to reduce the chances of the particles in question encountering one
another during the grinding, or by wet-milling in the presence of
surface-active material. As an example of the first of these
techniques, a sulphur sol in the upper colloidal range can be prepared
by grinding a mixture of sulphur and glucose, dispersing the resulting
powder in water and then removing the dissolved glucose from the sol
by dialysis.
A higher degree of dispersion is usually obtainable when a sol is
prepared by an aggregation method. Aggregation methods involve
the formation of a molecularly dispersed supersaturated solution
from which the material in question precipitates in a suitably divided
form. A variety of methods, such as the substitution of a poor solvent
for a good one, cooling and various chemical reactions, can be
utilised to achieve this end.