Modification of the Electric Double Layer

This method of destabilizing colloidal suspensions modifies the electrical double layer. Reducing the surface or Stern potential or compressing the diffuse electrical double layer diminishes the repulsive interaction forces, which can result in destabilization.

In practice, many colloids have hydrogen- and hy-droxyl-potential-determining ions. Adjusting the solution pH alone reduces their surface potential, thus diminishing the repulsive electrical forces and resulting in destabiliza-tion. This condition is shown by both curves in Figure 7.34.3.

An indifferent electrolyte does not contain surface-charge-potential-determining ions. Adding an indifferent electrolyte to a stable colloidal suspension causes destabi-lization by compressing the electric double layer, which reduces or eliminates a net repulsive interaction barrier. Figure 7.34.3 shows the effect of indifferent electrolyte concentration changes where moving from point A to B requires the addition of an electrolyte and results in desta-bilization.

According to the Stern modification of the Gouy-Chapman electric, double-layer model, destabilizing a col

FIG. 7.34.3 Effect of pH and an indifferent electrolyte on colloidal suspension stability (for surfaces with hydrogen- and hydroxide-potential-determining ions). aAt a given separation distance where electric double layers interact.
FIG. 7.34.4 Effect of specific counter ion concentration on colloidal suspension stability. aAt some given separation distance where electric double layers usually interact.

loidal suspension by specific adsorption of specific counter ions at the colloid surface is possible. Here, the surface potential remains unchanged, but the Stern and zeta potential can be reduced or even reversed in charge. In this process, adsorptive rather than electrostatic, forces must be operative. Figure 7.34.4 shows the effect of Stern potential reduction and reversal and gives a range of specific counter ion bulk solution concentrations where destabi-lization occurs.

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