Colloidal system interaction energy curve

Fig. 2. Total interaction energy curves of a colloidal system.

The overall stability of a colloid will depend on the net form of the interaction energy curve for the system—the sum of the attractive and repulsive energy terms as a function of the distance of separation of the particles. For the moment, we will consider only two contributing factors the attractive van der Waals term and the repulsive double-layer term, leaving aside any consideration of entropic or steric stabilization. 

FIGURE 10.16. In many colloidal systems, the interaction energy curve will have a small minimum, the secondary minimum, M , that allows the particles to undergo a lose, reversible flocculation. In some systems of relatively large, monodisperse particles, the secondary minimmn may lead to an optical phenomenon called opalescence in which a very regular structure is developed (similar to a crystal structure) that produces beautiful and interesting patterns with incident light. 

Draw a potential energy-interparticle distance curve (y H) for the interaction between two particles, in the case when the electrolyte concentration is (a) C = 3 X 10 mol and (b) C = 5 X 10 mol Do you expect the V-H curves to be qualitatively the same or different for the two different salts In which of the above cases would you describe the colloidal system as stable ... 

FIGURE 4.45 The free energy of interaction F (i.e., AOf) and the colloid stability (represented by the change in turbidity) in comparable macroscopic and nanoscopic systems lyophilized by a gradual increase of the water content. The addition of electrolyte promotes coagulation (transition from curve 1 to curve 3) hut does not have any significant impact on the value of F. (From Shchukin, E.D. and Yaminskiy, V.V., Colloids Surf., 32, 19, 1985.)... 

The forces between molecules are a balance of repulsive interactions at short distances and attractive interactions that predominate over larger length-scales. This is illustrated by the curve of potential energy as a function of intermolecular separation in Fig. 1.1. We will now consider the origin of the repulsive and attractive forces. Then we consider Coulombic forces since ions are present in solution in many colloid and surfactant systems, and in this case interactions between charged species predominate. 

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