Distance From Solids Surface C

FIG. 7.34.1 Model of electrical double layer for an elec-tronegatively charged surface.

tween the two surfaces. For surfaces with similar potentials, a repulsive force exists; for surfaces with opposite potentials, an attractive force exists. The magnitude of the repulsive or attractive force is related to the magnitude of their respective potentials and the distance of separation between the surfaces.

In addition to electrical forces between surfaces, van der Waals universal attractive forces between atoms are also significant. These attractive forces arise from interacting, fixed and induced atomic dipoles and fundamental dispersion forces.

Figure 7.34.2 shows the results of an interaction of charged surfaces according to the electrical double-layer model. Part A shows surfaces with a high charge concen-

FIG. 7.34.2 Typical electrical double layer interaction. Surface charge concentration is high in A and low in B.

tration. As two particles approach each other, the distance over which net repulsive forces exist is substantial. However, if the surface charge concentration is low, as in Part B, no net repulsive force exists, and a net attractive force can develop when the distance between the surfaces is reduced.

In typical wastewater, the diffuse electrical double layer extends only about 100A (10~8m) from the colloid surface into the bulk solution. Therefore, particle surfaces must approach each other to less than 200 A before their electric double layers interact and attractive or repulsive forces manifest themselves.

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