Peptization and Charge on Colloidal Particles

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Peptization or deflocculation is the process of converting precipitate into colloid by shaking it with a suitable electrolyte called peptizing agent.

This is particularly important in colloid chemistry or for precipitation reactions in an aqueous solution.

Peptization and Charge on Colloidal Particles

Origin of Charge on Colloidal Particles 

There are several views regarding the origin of charge on colloidal particles. According to these views, colloidal particles acquire charge due to the following reasons. 

  • Due to dissociation of the adsorbed molecular electrolytes: Colloidal particles have a strong tendency to adsorb reactant or product molecules. The molecules thus adsorbed on the surface of colloidal particles may undergo dissociation/ionization and may impart charge to them.
    For example, during the preparation of sulphide sols (e.g., As2S3 sol), H2S molecules get adsorbed on colloidal particles. H2S molecules thus adsorbed undergo ionization and release H+ ions into the medium. Consequently, colloidal particles are left with negative charge.
  • 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 example, the soap molecules (RCOONa) dissociate to give RCOO and Na+ ions. RCOO ions aggregate together to form micelles which carry negative charge as explained earlier. 
  • 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.
    This preferential adsorption of a particular type of ions imparts a particular type of charge to colloidal particles. 
    For example, when a ferric hydroxide sol is prepared by the hydrolysis of ferric chloride in warm water, the colloidal particles of Fe(OH)3 formed have a tendency to adsorb preferentially the Fe3+ ions present in the solution. This is because Fe3+ ions are common to the lattice of Fe(OH)3 particle. The Fe3+ ions thus adsorbed impart positive charge to the colloidal particles present in the sol. 
Fe(OH)3     +           Fe3+        →          Fe(OH)3    :      Fe3+ 
(colloidal            (ions common         preferential adsorption of Fe3+ ions
 particle)          to the lattice of           (colloidal particle acquires 
                        colloidal particle)                positive charge)

Similarly, during the preparation of AgCl sol using excess of KCl solution, the Cl ions are preferentially adsorbed and the colloidal particles acquire negative charge. However, if an excess of AgNO3 is used, Ag+ ions get preferentially adsorbed and the colloidal particles acquire positive charge. 
 

AgCI          +        CI                 →                        AgCI : CI
(colloidal)        (Chloride ions present              Preferential adsorption of CI ions 
particle)           in excess in the solution)       (Collodial particle acquires negative charge) 

AgCI          +                  Ag+                →                 AgCI : Ag+ 
(colloidal)             (Silver ions present in          Preferential adsorption of Ag+ions 
particle)              excess in the solution)     (Collodial particle acquires positive charge) 

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