The surface of colloidal particles acquires a positive or negative charge by selective adsorption of a layer of positive and negative ions around it. This layer attracts counter ions from the medium which form a second layer of opposite charges. The combination of two layers of positive and negative charges around the sol particles is called double layer. This layer was observed by Helmholtz, hence it is also called Helmholtz double layer. According to Helmholtz, the charges next to the particle surface were fixed while the compensating charges along the medium were mobile. Recent considerations have shown that the double layer is made up of (a) A compact layer of positive and negative charges which are fixed firmly onto the particle surface.(b) A diffused layer of counter ions containing opposite ions. The combination of the compact and diffused layer is called the stern double layer. Because of the distribution of the charge around the particle, there is a difference in potential between the compact layer and the bulk of the solution across the diffused layer. This is called Electro kinetic or Zeta potential. 

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