The colloidal state 13closeness, the barium sulphate particles will tend to link and the
dispersion will take the form of a translucent, semi-solid gel.
The ageing of dispersions is discussed on page 68.
Monodispersed solsAggregation methods usually lead to the formation of polydispersed
sols, mainly because the formation of new nuclei and the growth of
established nuclei occur simultaneously, and so the particles finally
formed are grown from nuclei formed at different times. In
experiments designed to test the validity of theories, however, there
are obvious advantages attached to the use of monodispersed
systems. The preparation of such systems requires conditions in
which nucleation is restricted to a relatively short period at the start
of the sol formation. This situation can sometimes be achieved either
by seeding a supersaturated solution with very small particles or
under conditions which lead to a short burst of homogeneous
nucleation.
An example of the seeding technique is based on that of
Zsigmondy (1906) for preparing approximately monodispersed gold
sols. A hot dilute aqueous solution of HAuCl 4 is neutralised with
potassium carbonate and a part of the solute is reduced with a small
amount of white phosphorus to give a highly dispersed gold sol with
an average particle radius of c. 1 nm. The remainder of the HAuCl 4
is then reduced relatively slowly with formaldehyde in the presence
of these small gold particles. Further nucleation is thus effectively
avoided and all of the gold produced in this second stage accumulates
on the seed particles. Since the absolute differences in the seed
particle sizes are not great, an approximately monodispersed sol is
formed. By regulating the amount of HAuCl 4 reduced in the second
stage and the number of seed particles produced in the first stage, the
gold particles can be grown to a desired size.
A similar seeding technique can be used to prepare monodispersed
polymer latex dispersions by emulsion polymerisation (see page 17).
Among the monodispersed sols which have been prepared under
conditions which lead to a short burst of homogeneous nucleation are
(a) sulphur sols^132 , formed by mixing very dilute aqueous solutions of
HC1 and Na2$2Oa; (&) silver bromide sols^133 , by controlled cooling of
hot saturated aqueous solutions of silver bromide; and (c) silver
bromide and silver iodide sols^133 , by diluting aqueous solutions of the