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The general principles of the colloidal stability of suspensions may be summarized as follows (Kruyt, 1952): A colloidally “stable” suspension (or “sol”) of submicroscopic particles is transparent, but it usually has a bluish milky appearance due to the scattering of light by the suspended particles. When observed in the ultramicroscope, the presence of particles is shown by specks of scattered light. The particles appear to be in vivid Brownian motion. A colloidally unstable suspension contains particle aggregates which are visible with the naked eye as “floes” or “coagulates”. Flocculation of a stable suspension can be induced by the addition of small amounts of electrolytes.

The classical DLVO theory by Derjaguin and Landau (1941) and Verwey and Overbeek (1948) explains the flocculation phenomena by considering the balance of two opposing interparticle forces: diffuse electrical double layer repulsion and van der Waals attraction. Both forces decay with increasing particle distance, but they are effective up to large distances. From plots of interaction energies as a function of particle distance, the net potential energy of interaction is obtained by summation of the curves. Such plots are shown in Figures 1a, 1b, and 1c for three different electrolyte concentrations. At low salt concentration, the net potential curve (Figure 1a) shows a deep attraction minimum at close approach of the particles and at larger distances an energy barrier which retards the association of the particles when they approaCh each other by diffusion. With increasing salt concentration, the diffuse electrical double layer is compressed and the

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