Figure 6.3. Schematic potential energy curve describing the interactions between colloidal particles. The overall potential is a sum of an electrostatic repulsive term which arises due to any charged groups on the surface of the particle and the attractive van der Waals term.
to an 'electric double layer' consisting essentially of two distinct regions; an inner region of adsorbed ions called the Stern layer and a more diffuse region. When two such diffuse layers overlap, a repulsive interaction is introduced. For typical ionic strengths, e.g. 10~3moldm~3, the thickness of the double layer is about 10 nm. If the ionic strength is substantially higher, the double-layer interaction is sufficiently reduced and it can no longer provide stabilisation against the van der Waals driven aggregation. In contrast to the van der Waals interaction which falls off reciprocally with distance, the electrostatic repulsion falls off exponentially with distance. Consequently, the van der Waals interaction dominates at small and large distances, whilst the double-layer interaction dominates at intermediate distances. The interparticle interaction has the form shown in Figure 6.3.
The maximum in the potential corresponds to the barrier to aggregation -the inherent stability of the dispersion. If this barrier is larger than the thermal energy kT, the dispersion will be stable.
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