Origin Of Charges On Colloidal Particles (Colloids)
- As we know that in Colloids the Dispersed phase remains suspended in Dispersion medium.
- The Colloidal system is a two phase system and very stable.
- It does not grow in size or lead to precipitation.
- This remarkable stability of Colloids is due to the presence of a small quantity of electrolyte in the medium and presence of charge on the surface of colloids.
- The colloidal particles carry an electric charge on it either positive or negative.
- Due to the presence of similar type of charges, the colloidal particles repel one another and do not combine together to form larger particles.
- This keeps them dispersed in the medium and the colloids remains stable.
- The type of charge present on the Colloidal particles can be detected by Electrophoresis experiment.
The various factors responsible for the origin of charges on the
colloidal particles are as follows :-
1) Dissociation of Molecules :-
The charge on the colloidal particles can be generated by the dissociation of molecules.
For eg :-
Soaps are sodium salts of fatty acids. In solutions they dissociate to give rise to ions as shown in the reaction below :-
R-COONa -----> RCOO- + Na+
The RCOO ions consists of two parts,long hydrocarbon chain ' R ' which is hydrophobic i.e water repelling and the polar group i.e ' COO ' which is hydrophilic i.e water loving.
As they form ions their hydrocarbon chain points at centre and COO remain outward.
This gives the colloids extra stability.
2) Medium :-
The charge present on the particles might be due to the medium in which they are present.
For eg:- Consider Proteins,
a) Amino acids are the building blocks of Proteins.
b) Amino acids have two functional groups i.e., Primary Amine & Carboxylic Acid.
c) In Acidic medium ---->The Primary Amine i.e the NH2 group accepts a proton from an acid and becomes positively(+) charged.
H2N - RCOOH --(H+)--> NH3 -RCOOH
Thus In Acidic Medium the proteins will move towards Negative(-) electrode.
d) In Alkaline medium ----> The Carboxylic Acid i.e the R-COOH group loses a proton from COOH group and becomes Negatively(-) charged.
RCOO + H2H2N - RCOOH --(OH-)--> H2N - O
Thus In Alkaline Medium the proteins will move towards Positive(+) electrode.
3) Surface Adsorption :-
The Colloidal particles have large surface area and are capable of adsorbing. Sometimes it happens that Certain kinds of ions from Dispersion medium get adsorbed on the colloidal particles and this can be the reason for the charge present on colloidal particles.
For eg:- Consider that,
AgI ' Sol ' is prepared by the reaction between Silver Nitrate
(AgNO3) and Potassium Iodide (KI).
Now, If excess of KI is added to AgNO3 , AgI sol absorbs I- ions preferably,forming a Negative(-) colloid, Whereas, If excess of AgNO3 is added to KI, then AgI sol absorb Ag+ ions preferably,forming a Positive(+) colloid.
4) Frictional Electrification :-
Sometimes it happens that the dispersed phase particles i.e the Colloidal particles gets rubbed with the particles of the dispersion medium giving rise to friction and this could be the reason for the charge present on the colloidal particles.