The Electrical Double Layer and Colloid Stability

If lattice ions or other potential-determining ions are adsorbed on a solid surface, they may be regarded as belonging to the solid and imparting an electrical charge to it. For the sake of overall electrical neutrality, an equivalent number of oppositely charged ions (counterions) exist in solution, drawn to the charged surface by electrical attraction. The counterions and the adsorbed lattice ions form an electrical double layer. The closest counterions cannot be nearer the surface than a finite distance (inner Helmholtz plane ) that depends on the ionic radius. 

FIGURE 8 7 Potential as a function of distance from a charged surface. 

When an electric field is applied to a solution containing charged colloidal particles, the rate of motion of the particles is proportional to neither nor ij/f but to 

FIGURE 8-8 Schematic diagram of potential as a function of distance from a charged surface A, effect of surface charge B, effect of electrolyte concentration (Cl Cl C3) C, effect of charge of counterion (Zi Z2 Z3). 

Schulze first showed that inorganic colloids are especially sensitive to electrolytes of high charge, and Hardy pointed out that their stability is closely related to mobility in an electric field. The Schulze-Hardy rule states that the sensitivity of lyophobic colloids to coagulating electrolytes is governed by the charge of the ion opposite that of the colloid and that the sensitivity increases more rapidly than the charge of the ion. 

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