92168.pdf

234 Colloid stability

accumulate under the influence of gravity at the bottom or at the

surface. A sedimentation velocity (see Table 2.2) of up to c. 10~^8 m

s"^1 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