Stability of colloid–polymer dispersions

To describe the stabilization of colloid particles on addition of polymers, the osmotic pressure of a colloid, relative to its supernatant, has been studied. The traditional way to calculate the osmotic pressure, is to use a truncated virial expansion. The problem with such an approach, however, is the fact that the virial expansion is valid only at very low volume fractions of colloid and polymer. To extend the range of validity of the theory to finite volume fractions, we use an established theory of the liquid state. In the approach adopted it is assumed that repulsive forces exist at short distances and attractive forces exist at longer distances between the constituents of the colloid–polymer mixture. The phase behaviour of the system has been calculated as a function of the volume fractions of the colloid particles and of the polymer particles, and as a function of the degree of attraction. Various characteristic phase behaviours have been obtained: (i) depletion flocculation, (ii) bridging flocculation at low polymer concentration followed by restabilisation at higher polymer concentration, and (iii) conditions where the mixture is stable at all volume fractions of colloid and polymer. The theoretical analyses forwarded here reveal that the three-body interaction colloid–polymer–colloid plays an important role in the phase behaviour.