The Origin of charge on the colloidal particles
The origin of the charge on the sol particles in most cases is due to the preferential adsorption of either positive or negative ions on their surface. The solution particles acquire the electrical charge in any one or more of the following given ways.
(1) Due to the dissociation of the surface molecules : Some colloidal particles develope electrical charge due to the dissociation / ionization of the surface molecules. Charge on the colloidal particles can be balanced by the oppositely charged ions in the sol. For instance, an aqueous solution of soap (sodium palmitate) which dissociates into ions as,
The cations (Na+) pass into the solution while the anions (C15H31COO) have a tendency to form aggregates due to weak attractive forces present in the hydrocarbon chains.
(2) Due to frictional electrification
(i) It is supposed that the frictional electrification due to the rubbing of the dispersed phase particles with that of dispersion medium results in some charge on the colloidal particles.
(ii) The dispersion medium must also get some charge, due to the friction. AS it does not carry any charge, theory does not appear to be correct.
(3) Due to selective adsorption of ions
(i) The particles having the dispersed phase adsorb only those ions preferentially which are common with their own lattice ions.
(ii) For example, when a small quantity of silver nitrate (AgNO3) solution is added to a large quantity of potassium iodide KIsolution, colloidal particles of the silver iodide adsorb I- from the solution to become negatively charged, (at this stage KI is in excess, and I-being common to AgI)
But, when a small quantity of potassium iodide (KI) solution is added to a large quantity of silver nitrate solution (AgNO3); the colloidal silver iodide particles adsorb Ag+ from the solution to become positively charged, (at this stage AgNO3 is in excess and Ag+ is common to AgI),
(iii) Depending upon the nature of charge on the particles of the dispersed phase, the colloidal solutions can be classified into positively charged and negatively charged colloids. Some of the typical examples of it are as follows
(a) Negatively charged
(b) Positively charged
- Metal dispersions : Ag, Au, Pt
- Acid dyes : Eosin, congo red
- Sols of starch, gums, gold,
- Basic dyes : Methylene blue
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