12
Colloidal Interactions
In Chapter 3, bonds and interaction forces between species of molecular size
were briefly discussed. In this chapter, the same forces come into play when
they act between larger structural elements, mostly particles. Moreover,
Chapter 3 mostly concerns bond energy, whereas the interaction forces
discussed now can be in considerable part due to entropic effects: mixing,
conformational, and contact entropy can all play a part.
Colloidal interaction forces act primarily in a direction perpendicular
to the particle surface; forces primarily acting in a lateral direction were
discussed in Chapter 10. We merely consider ‘‘internal forces,’’ which find
their origin in the properties of the materials present. This excludes forces
due to an external field, such as gravitational, hydrodynamic, and external
electric forces; these are involved in some subjects of Chapter 13.
12.1 GENERAL INTRODUCTION
Consider two particles in a liquid, a large distance apart. Colloid scientists
try to predict the free energy needed to bring these particles from infinite
distance to a close distance between the particles’ surfacesh. If that energy is
positive, it means that the particles repel each other; if it is negative, they
attract each other. For particles of a size of the order of 1mm, the distance