92168.pdf

14 The colloidal state

Virtually all of the sulphur particles

are born within this short time period

Rapid relief of supersaturation

o"5

0_C^5

0-5-

d 3

T

Saturation concentration

Time

Figure 1.4 Formation of an approximately monodispersed sulphur sol by the slow

reaction between Na 2 S 2 O 3 and HCI in dilute aqueous solution

complexes formed in the presence of excess silver or halide ions. In

each case the concentration of the material of the dispersed phase

slowly passes the saturation point and attains a degree of supersatura-

tion at which nucleation becomes appreciable. Since the generation

of dispersed phase material is slow, the appearance of nuclei and the

accompanying relief of supersaturation is restricted to a relatively

short period and few new nuclei are formed after this initial outburst.

The nuclei then grow uniformly by a diffusion-controlled process and

a sol of monodispersed particles is formed.

Various methods are also available for the preparation of mono-

dispersed hydrous metal oxide sols^19 and silica sols^20 '^134.

Monodispersed polystyrene sols are used as calibration standards

for electron microscopes, light scattering photometers, Coulter

counters, particle sieves, etc. Monodispersed silica is used for

antireflection lens coatings. Monodispersity (even at a modest level)

can usefully be exploited in photographic film, magnetic devices,

pharmaceutical preparations and catalysis.

Macromolecular colloids

Macromolecular chemistry covers a particularly wide field which
includes natural polymeric material, such as proteins, cellulose, gums
and natural rubber; industrial derivatives of natural polymers, such as
sodium carboxymethyl cellulose, rayon and vulcanised rubber; and
the purely synthetic polymers, such as polythene (polyethylene),
Teflon (polytetrafluoroethylene), polystyrene, Perspex (poly (methyl