Stability of Lyophobic Colloids against Aggregation

In Section 1.1, we defined colloidal systems as systems in which particles dispersed in a medium are snbjected to both thermal motion and motion due to external forces (e.g., gravity). This definition leads directly to the notion of stability of a colloidal dispersion. A colloidal dispersion is considered to be stable if no rapid phase separation occnrs through sedimentation (if the density of the particles is higher than that of the medium) or creaming (if the density of the particles is lower than that of the medinm). Thus, colloidal stability refers to the ability of a dispersion to resist aggregation into larger entities that then would segregate from the medium. 

Irrespective of the nature of the stabilizing forces, for the net force/between two surfaces at a separation distance h, we can write 

By convention, a repulsive force has a positive sign. For two surfaces approaching each other across a liquid medium, the liquid must leave the region between the two surfaces. Assuming noncompressibility of the liquid. Equation 16.1 can be rewritten as 

Based on the theory given in Sections 3.5 and 3.6, the right-hand term of Equation 16.2 equals the osmotic pressure k for the liquid between the surfaces. Thus, for two approaching surfaces, the force per unit area can be regarded as an osmotic pressure. This pressure is also referred to as the disjoining pressure. 

So far, we tacitly assumed that the colloidal particles are rigid, so that they do not deform upon approach. This assumption more or less holds for solid particles. However, for fluid particles, as present in emulsions and foams, deformations may easily occur and they invoke additional mechanisms affecting their stability. In this chapter, we restrict ourselves to nondeformable particles. Emulsions and foams are discussed in Chapter 18. 

---