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Can someone explain the origin of charge on colloidal particles??

Can someone explain the origin of charge on colloidal particles??

Grade:11

2 Answers

Aarti Gupta
askIITians Faculty 300 Points
8 years ago
Colloidal particles posses a definite type of electrical charge and this is the one of the most important properties of colloidal solutions.In a particular colloidal solution,all the colloidal particles carry the same type of charge,while the dispersion medium has an equal but opposite charge as a result colloidal solution as a whole is electrically neutral. There are several views regarding the origin of charge on colloidal particles and these are as follows--
1).Due to dissociation of the adsorbed molecular electrolytes:- Colloidal particles have a strong tendency to adsorb reactant or product molecule which may undergo dissociation/ionization and may impart charge to them.For ex- during the preparation of sulphide sols(e.g., As2S3sol),H2S molecules get adsorbed on colloidal particles which undergo ionization and release H+ions into the medium thereby colloidal particles are left with negative charge.
2).Due to the dissociation of molecules forming colloidal aggregates:-The molecules responsible for the formation of aggregates of colloidal dimensions may themselves undergo dissociation/ionisation resulting in the development of charge on the colloidal particles formed by their aggregation.For ex- the soap molecules (RCOONa) dissociate to give RCOO-and Na+ions.RCOO-ions aggregate together to form micelles which carry negative charge.
3).Due to preferential adsorption of ions from solutions:- The colloidal particles have a tendency to preferentially adsorb a particular type of ions from the solution.A colloidal particle usually adsorbs those ions which are in excess and are common to its own lattice and such preferential adsorption of a particular type of ions imparts a particular type of charge to colloidal particles.For ex- when a ferric hydroxide sol is prepared by the hydrolysis of ferric chloride, the colloidal particles of Fe(OH)3formed have a tendency to adsorb preferentially the Fe3+ions present in the solution as Fe3+ions are common to the lattice of Fe(OH)3particle which impart positive charge to the colloidal particles present in the sol.
deapan
131 Points
8 years ago
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 Methodacquire 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.
 
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