92168.pdf

236 Colloid stability

electrolyte concentration is often sufficiently high to render electro-

static stabilisation ineffective.

Effect on electric double layer interactions

If the stabilising agent is ionised and carries a charge of the same sign

as that on the particles (e.g. anionic surfactant adsorbed on

negatively charged particles), then electric double layer repulsion will

be enhanced. The adsorbed stabilising agent (even if non-ionic) will

influence electrostatic interactions by causing a displacement of the

Stern plane away from the particle surface; this will increase the

range of electric double layer repulsion and, as such, enhance

stability.

Effect on van der Waals interactions

Adsorbed layers of stabilising agent may cause a significant lowering

of the effective Hamaker constant and, therefore, a weakening of the

interparticle van der Waals attraction. This effect has been considered

by Void^207 and by Vincent and co-workers^208 in terms of the

Hamaker microscopic treatment of dispersion forces.

For the interaction depicted in Figure 8.10,

— 12V'A = Fit (Aii — A*)->\ + F-y?(Ay> — A-IT.)

Figure 8.10