In this chapter we describe briefly some of the main features of the colloidal state. The colloidal state is very widespread in biological systems and in many practical situations. Basically a colloid consists of two distinct phases, a continuous phase (the dispersion medium) and a particulate phase, where the particles generally have dimensions ranging between 20 and 2000 Å (2 and 200 nm). The two phases can be liquidin- liquid (milk), solid-in-liquid (paint), liquid-in-gas (aerosol) and other combinations. The basic problem with colloidal systems is their stability. Clearly the particles must not be too large otherwise gravity will produce ready sedimentation. However, if the density of the two phases is similar this tendency will be reduced. The other important factor is the attractive force between the particles: if this is too large the particles will cling together and separate as a cluster from the dispersion. To prevent this, various techniques are employed to reduce the attractive forces or to introduce repulsive forces.

van der Waals forces between macroscopic bodies

Before we approach the problem of colloid stability we need to know the van der Waals forces between the particles and how these are affected by the presence of a continuous phase (e.g. a liquid) between them.

The van der Waals force between macroscopic materials was quoted in Chapter 1 (equations (1.21) and (1.22)) in order to demonstrate the difference between normal and retarded forces. Here we indicate a simple derivation.