Page 245 - Introduction to Colloid and Surface Chemistry

234 Colloid stability

accumulate under the influence of gravity at the bottom or at the
8
surface. A sedimentation velocity (see Table 2.2) of up to c. 10~ m
1
s" is usually counteracted by the mixing tendencies of diffusion and
convection. Particle aggregation, of course, enhances sedimentation.
When sedimentation does take'place, the volume of the final
sediment depends upon the extent of aggregation. Relatively large
peptised particles pack efficiently, to give a dense sediment which is
difficult to redisperse, whereas aggregated particles bridge readily
and give a loose sediment which (provided that the particles are not
held together too strongly) is more easily dispersed (see Figure 8,9).
In extreme cases, the sedimentation volume may equal the whole
volume, and this can lead to the paradoxical situation where a small
amount of aggregating agent produces a sediment while a larger
amount does not. Gentle stirring usually reduces the sedimentation
volume.

Figure 8.9 Sedimentation volumes for (a) peptised and (b) aggregated particles

When the particles aggregate to form a continuous network
structure which extends throughout the available volume and
immobilises the dispersion medium, the resulting semi-solid system is
called a gel. The rigidity of a gel depends on the number and the
strength of the interparticle links in this continuous structure.

Systems containing lyophilic material

Polymer solutions

Macromolecular solutions are stabilised by a combination of electric
double layer interaction and solvation, and both of these stabilising