section 1.6
A dispersion of colloidal particles in a solvent is not always stable. In many cases, the strong van der Waals forces between colloidal particles cause them to aggregate and sediment out of solution [8]. This phenomenon is called flocculation. In order for a colloidal suspension to be stable towards flocculation, the interaction potential must have a repulsive barrier to prevent the particles getting at the distances where the strong van der Waals attraction takes over. In general this is achieved by the charges on the surface on the colloidal particle. In a solvent with added salt, this induces a double layer repulsion, which counteracts the van der Waals attraction arising from the dielectric properties of the colloidal particle and the solvent. The total interaction potential shows a repulsion at short distances followed by an attraction further away [18]. Another possibility to prevent flocculation is steric stabilization of the particles by grafting the surface with short polymer chains [19]. These polymers cause a very short ranged repulsive steric interaction which prevents the irreversible flocculation.
As colloidal dispersions can, to a first approximation, be described as simple fluids, it is natural to assume that spherical colloids will exhibit the same phase behavior as atomic substances. That is, one would expect that these colloidal suspensions can occur in the solid, liquid and vapor phase. However, although the fluid-solid transition is found in systems of charge- or sterically stabilized colloids, the liquid-vapor transition is not, even if there is an attractive part in the potential. In the absence of attraction, the fluid-solid transition is similar to the freezing transition of hard spheres. As attraction becomes stronger (or the temperature is lowered), the fluid-solid density gap broadens (see figure 1.2b). Clearly, there must be a fundamental difference between these colloidal systems and molecular systems that obey the van der Waals equation (see section 1.3). We recall that the van der Waals approach becomes exact in the limit of infinitely long-ranged, infinitely weak attractive forces. However, in charge- or sterically stabilized colloids, the range of the attraction is quite small compared to the diameter of the particles (quite unlike the attractive interaction in the Lennard-Jones model that is often used to model simple liquids, such as argon [2]). As we discussed in section 1.3, the stability of the liquid phase is very sensitive on the potential range. The liquid region is large in the van der Waals limit, or for the Lennard-Jones model, but decreases as the range of attraction shrinks. When this range becomes less than about one third of the hard-core diameter of the particles, the triple point approaches the critical point and the liquid-vapor transition will disappear completely .
In order to study the effect of the interaction potential on the phase behavior, one would need to be able to ``tune'' the range of the attractive interaction between particles, while keeping their hard-core repulsion fixed. For molecular systems, it is not possible to tune the attractive van der Waals forces in this way. However, in colloidal dispersions there is a neat way to control the strength and range of the interparticle attraction: adding polymer.
section 1.6