FIGURE 12.2 Primary charges of a hydrophilic colloid as a function of pH.

sufficiently large, the attracted counterions can form a compact layer around the primary charges. This layer is called the Stern layer. The counterions, in turn, can attract their own counterions, the coions of the primary charges, forming another layer. Since these coions form a continuous distribution of ions into the bulk of the solution, they tend to be diffused and form a diffused layer. The second layer is called the Gouy layer. Thus, the Stern and Gouy layers form an envelope of electric double layer around the primary charges.

All of the charges in the Stern layer move with the colloid; thus, this layer is a fixed layer. In the Gouy layer, part of the layer may move with the colloid particle by shearing at a shear plane. This layer may shear off beyond the boundary of the fixed Stern layer measured from the surface of the colloid. Thus, some of the charges in the layer move with the particle, while others do not. This plane is indicated in Figure 12.3.

The charges are electric, so they possess electrostatic potential. As indicated on the right-hand side of Figure 12.3, this potential is greatest at the surface and decreases to zero at the bulk of the solution. The potential at a distance from the surface at the location of the shear plane is called the zeta potential. Zeta potential meters are calibrated to read the value of this potential. The greater this potential, the greater is the force of repulsion and the more stable the colloid.

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