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I don't quite understand what is meant by the Helmholtz double layer. Textbooks states something along the lines of:

the colloidal particles attain charge by selective adsorption, due to this it attracts a layer of opposite charge,this layer is called the Helmholtz double layer and the potential between them is called Zeta Potential

What stops the oppositely charged layers from combining (neutralizing) with each other?

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    $\begingroup$ Imagine a NaCl crystal, why don't Na+ and Cl- neutralize each other? The answer is that the bulk crystal is electrically neutral yet the ions retain their charge at a microscopic level. In the same way bulk solution containing colloidal particles is always neutral. Helmholtz layer is a microscopic view of the interface of a solid surface and the solution containing ions. The ions adsorbed on the surface are rather big and cannot fully neutralize the surface charge, thus there is a net charge on a particle. (Note zeta potential is not the surface charge). Check Youtube for excellent lectures. $\endgroup$ – M. Farooq Mar 10 at 4:53
  • $\begingroup$ @M.Farooq Another follow up question,which is not quite related probably,but still... If the layers adsorb on the top of one other,then the distance b/w the charges would be tending to zero. This amounts to a very large potential gradient being set up,and hence a large amount of potential energy would be needed to separate the two layers. Yet we say the 2nd layer is "diffused" and is easily movable,and can be removed easily.. Why? $\endgroup$ – YUSUF HASAN Mar 10 at 8:36
  • $\begingroup$ @M.Farooq Thank you for the reply and suggestion of watching videos,actually I did try to watch some lectures but they were all college level and I am still in High School. I have the same doubt as YASUF HASAN, if a potential develops shouldn't there be a force opposing this potential in order to stop both of them from interacting? $\endgroup$ – Vaishakh Sreekanth Menon Mar 10 at 9:55
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    $\begingroup$ @Yusuf and Vaishakh, people have studied electrical double for almost hundred years which can give you an idea that it took a really long time to understand electrical double layer. Perhaps it is not fully understood yet. They used hardcore mathematics and electrostatics and developed expressions for potential changes near the surface of the electrode or colloid surface. There is a theory behind it called DLVO. See this link for a non mathematical explanation sciencedirect.com/topics/chemistry/electric-double-layer $\endgroup$ – M. Farooq Mar 10 at 14:37

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