Page 252 - Introduction to Colloid and Surface Chemistry

Colloid stability 241

3. Long-chain additives, such as gelatin, can sometimes bring about a
rather loose flocculation by a bridging mechanism in which the
molecules are adsorbed with part of their length on two or more
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particles ' ~ . Such flocculation normally occurs over a
narrow range of additive concentrations; at high concentrations
protective action is obtained, since bridging can occur only
through particle collisions under conditions where further adsorp-
tion of the additive is possible.

Dispersions of hydrophilic particles in oil media can be flocculated
by traces of water, which form thin interconnecting films between the
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particles .
Another interesting phenomenon is that of depletion flocculation.
This can be observed with dispersions (e.g. lattices) which contain
inert additives, such as free polymer, non-ionic surfactant or even
small (e.g. silica) particles. As the latex particles approach one
another, the gaps between them become too small to accommodate
the above additives, but the kinetic energy of the particles may be
sufficient to enable them to be expelled from the gap; i.e. a 'de-mix'
occurs, for which AC is positive. When this 'de-mix' has been
achieved, an osmotic situation exists in which the remaining pure
dispersion medium will tend to flow out from the gap between the
particles in order to dilute the bulk dispersion medium, thus causing
the particles to flocculate.

Stability control

Particle aggregation and sedimentation volume are important in
many practical situations, as illustrated by the following examples.

1. Agricultural soil. It is necessary to maintain agricultural soil in a
reasonably aggregated state in order to achieve good aeration and
drainage, and treatment with coagulants, such as calcium salts
(lime or gypsum) or organic polyelectrolytes (so-called soil
conditioners), is common practice. An extreme example of the
effect of soil peptisation occurs when agricultural land is flooded
with sea-water. The calcium ions of the naturally occurring clay
minerals in the soil exchange with the sodium ions of the sea-
water. Subsequent leaching of the sodium ions from the soil by