Surface Chemistry Part - 8 | Class 12th

Ø  Stability of Colloidal Sols: All the dispersed particles in a colloid carry the same charge (either positive or negative) while the dispersion medium has an equal and opposite charge. The particles, therefore, repel one another and do not come close together to form large non-colloidal particles.

The charge on colloidal particles is due to preferential adsorption of ions, e.g., Fe(OH)₃ adsorbs Fe³⁺ ions from FeCl₃ solution and forms positively charged sol. Similarly, AgCl particles can adsorb Cl⁻ ions from chloride solutions or Ag⁺ ions from solutions having silver ions. The sol will be negatively charged in the first case and positively charged in the second case.

v    Negatively charged colloidal sols: Metallic particles like Cu, Au, Pt, Ag etc., starch, clay,   silicic acid, metal sulphides like As₂S₃, CdS, Sb₂S₃ etc., acidic dyes like congo red, eosin etc.

v    Positively charged colloidal sols: Metal hydroxides like Fe(OH)₃ , Al(OH)₃, Cr(OH)₃, Ca(OH)₂, oxides like TiO₂ etc., haemoglobin and basic dyes like methylene blue.

The charge on the sol particles is mainly due to preferential adsorption of ions from solution. When 2 or more ions are present in the dispersion medium, preferential adsorption of the ion common to the colloidal particle takes place. e.g.

When AgNO₃ is added to KI, AgI is precipitated, which adsorbs iodide ions from the dispersion medium and thus get a negative charge.

But when KI is added to AgNO₃, the precipitated AgI adsorbs Ag⁺ ions from the solution and thus get a positive charge.

Due to the positive or negative charge in the sol particles, they attract the counter ions (opposite ions) from the medium. Thus, a double layer of opposite charges is formed. This is known as Helmholtz electrical double layer. The layer in which the ions are directly adsorbed to the sol particles is termed as fixed layer. The second layer is mobile and is termed as diffused layer.

Due to the opposite charges on the fixed and diffused layers, there arises a potential difference between these layers. This potential difference between the fixed layer and the diffused layer of opposite charges is called the electrokinetic potential or zeta potential.

Ø Electrophoresis: Since colloidal particles carry charge, they move under the influence of an electric field. This movement of colloidal particles is called electrophoresis. The positively charged sol particles move towards cathode (cataphoresis) and the negatively charged particles move towards the anode (anaphoresis).