Origin of Charge on Colloidal Particles

The colloidal particles are known to carry positive or negative charge and all the particles of same sol carry same kind of charge. Various views have been put forward to explain the charge of colloidal particles.
(i). Frictional Electrification
The origin of charge on the colloidal particles may be due to frictional electrification. It is believed that frictional electrification due to rubbing the dispersed phase particles with those of dispersion medium results in some type of charge on colloidal particles of the sol. This view is not satisfactory.
(ii). Electron Capture by Colloidal Particles
It is believed that the colloidal solutions prepared by Bredig’s Arc Dispersion Method acquire a charge by electron capture. This view is not valid in all cases.
(iii). Dissociation Of Surface Molecules
The dissociation of surface molecules leads to electric charge on colloidal particles of the sol. For example, Consider an aqueous solution of soap which undergoes dissociation into ions.

Herein, the cation (Na⁺) passes into the solvent while the anions (C₁₅H₃₁ COO^-) have a tendency to form negatively charged aggregates due to weak attractive forces present in the long hydrocarbon chains. These aggregates are of colloidal dimensions and are negatively charged. This is not valid for colloidal solutions of non-electrolytes such as clay, smoke etc.
(iv). Presence of Some Acidic or Basic Groups in Colloidal Solution
Colloidal particles may acquire electric charge due to the presence of certain acidic or basic groups in colloidal solution. For instance, protein molecules give rise to formation of colloidal solutions. Thus the particles of protein sol either have positive charge or negative charge depending upon the PH of medium. A molecule of protein contains a carboxylic acid (COOH) group and also a basic amino (–NH2) group, it will form a positively charged particle in acidic medium and a negatively charged particle in basic or alkaline medium. This can be illustrated as follows:

At an intermediate PH point referred to as Iso-electric point, the protein will exist as uncharged molecules.
(v). Selective Adsorption of ions from Solutions
Colloidal particles adsorb preferentially positive or negative ions present in the dispersion medium. When two or more ions are present in the dispersion medium, the selective adsorption of ions common to the colloidal particles takes place resulting in the formation of positively charged or negatively charged particles in a colloidal solution. In simple words, the electric charge on the colloidal particles essentially originates by the selective adsorption of ions common to the colloidal particles from the dispersion medium.
For illustration, consider some typical examples of selective adsorption of ions.
(i). Positively charged Ferric hydroxide sol.
Fe(OH)₃ sol is prepared by shaking Ferric hydroxide precipitate with dilute solution of Ferric Chloride.

(ii). Positively charged silver chloride sol.
It is obtained by shaking silver chloride precipitate with dilute solution of silver nitrate.

(iii). Negatively charged Arsenious Sulphide Sol.
It is prepared by passing H₂S gas slowly through solution of AS₂O₃.

(iv). Negatively charged Silver Chloride Sol.
It is prepared by shaking silver chloride precipitate with dilute solution of hydrochloric acid.