Zeta Potential

The repulsive property of colloid particles is due to electrical forces that they possess. The characteristic of these forces is indicated in the upper half of Figure 12.1b. At a short distance from the surface of the particle, the force is very high. It dwindles down to zero at infinite distance from the surface.

FIGURE 12.1 (a) Hydrophilic colloid encased in bound water; (b) interparticle forces as a function of interparticle distance.

The electrical forces are produced due to the charges that the particles possess at their surfaces. These charges called primary charges are, in turn, produced from one or both of two phenomena: the dissociation of the polar groups and preferential adsorption of ions from the dispersion medium. The primary charges on hydrophobic colloids are due to preferential adsorption of ions from the dispersion medium.

The primary charges on hydrophilic colloids are due chiefly to the polar groups such as the carboxylic and amine groups. The process by which the charges on these types of colloids are produced is indicated in Figure 12.2. The symbol R represents the colloid body. First, the colloid is represented at the top of the drawing, without the effect of pH. Then by a proper combination of the H+ and OH- being added to the solution, the colloid attains ionization of both carboxylic and the amine groups. At this point, both ionized groups neutralize each other and the particle is neutral. This point is called the isoelectric point, and the corresponding ion of the colloid is called the zwitter ion. Increasing the pH by adding a base cause the added OH- to neutralize the acid end of the zwitter ion (the NH+); the zwitter ion disappears, and the whole particle becomes negatively charged. The reverse is true when the pH is reduced by the addition of an acid. The added H+ neutralizes the base end of the zwitter ion (the COO-); the zwitter ion disappears, and the whole particle becomes positively charged. From this discussion, a hydrophilic colloid can attain a primary charge of either negative or positive depending upon the pH.

The primary charges on a colloid which, as we have seen, could either be positive or negative, attract ions of opposite charges from the solution. These opposite charges are called counterions. This is indicated in Figure 12.3. If the primary charges are r

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