Electric charge on colloidal particles:- the stability of colloidal solution is due to the fact that the colloidal particles in the sol are electrically charged.
- Colloidal particles either carries the +ve or –ve charge and the dispersion medium have an equal and opposite charge.
- Ex. Ferric hydroxide particles are +vely charged. Where as dispersion medium is –vely charged.
Origin of electrical charge on colloidal particles:-
- Frictional electrification:- by mutual rubbing of colloidal particles with molecules of dispersion medium charge is developed.
- Preferential adsorption of ions from solution:- an ionic colloid adsorbs ions common to its own lattice during preparation of colloidal sol.
Forex- if colloidal sol of Ag I is prepared adding AgNO3 solution to KI, Ag+ aquires positive and I aquires –ve charge.
- Dissociation of molecules followed by aggregation of ions:-in case of soap the RCOO– groups get dissociated from Na+ ions and have the tendency to aggregate into a cluster carrying –ve charge.
Electro kinetic or Zeta Potential:- when one type of the ions of the electrolyte are adsorbed on the surface of colloidal particles it forms a “Fixed layer”. It attracts the opposite ions to form another layer called “diffused layer”.
The double layer of opposite charge thus formed is called Helmholtz electrical double layer.
As a result, a difference of potential exists between the fixed layer and the diffused layer. This potential difference is known as electro kinetic or zeta potential.
Electrophoresis or cataphoresis:- The movement of the colloidal particles under the influence of an electric field is called the Electrophoresis or the cataphoresis
Electro-osmosis:- it may be defined as a phenomenon in which the molecules of the dispersion medium are allowed to move under influence of an electric field where as colloidal particles are not allowed to move.
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