Why charge is present on colloidal particles?Also explain what is electrical double layer ?Explain electrical double layer in very very simple words.
The various theories for the origin of electrical charge on the colloidal particles are as follows:
(i) Frictional Electrification caused by the mutual rubbing of the colloidal particles with molecules of the dispersion medium.
(ii) Electron capture by particles from air.
(iii) Preferential adsorption of ions from solutions. An ionic colloid adsorb ions common to its own lattice during the preparation of the colloidal sol. For example, if colloidal sol of AgI is prepared by adding KI solution to AgNO3 solution till KI is in little excess, iodide ions (I-) will be adsorbed on the surface of AgI particles thereby giving a negative sol.
AgI + I- AgI : I-
negative sol
​Electrical Double Layer
Out of the various theories of the origin of charge discussed above, preferential adsorption of ions is considered as best explanation.
When one type of the ions of electrolyte are adsorbed on the surface of the colloidal particles, it forms a fixed layer. It attracts the counter ions from the medium forming a second layer which is mobile and is called diffused layer.The double layer of opposite charges thus formed is called as Helmholtz electrical double layer.
(i) Frictional Electrification caused by the mutual rubbing of the colloidal particles with molecules of the dispersion medium.
(ii) Electron capture by particles from air.
(iii) Preferential adsorption of ions from solutions. An ionic colloid adsorb ions common to its own lattice during the preparation of the colloidal sol. For example, if colloidal sol of AgI is prepared by adding KI solution to AgNO3 solution till KI is in little excess, iodide ions (I-) will be adsorbed on the surface of AgI particles thereby giving a negative sol.
AgI + I- AgI : I-
negative sol
​Electrical Double Layer
Out of the various theories of the origin of charge discussed above, preferential adsorption of ions is considered as best explanation.
When one type of the ions of electrolyte are adsorbed on the surface of the colloidal particles, it forms a fixed layer. It attracts the counter ions from the medium forming a second layer which is mobile and is called diffused layer.The double layer of opposite charges thus formed is called as Helmholtz electrical double layer.