Can you please explain the HelmHoltz Double Lyer experiment

The double layer model is used to visualize the ionic environment in the vicinity of a charged surface. It can be either a metal under potential or due to ionic groups on the surface of a dielectric. It is easier to understand this model as a sequence of steps that would take place near the surface if its neutralizing ions were suddenly stripped away.

 

Helmholtz Double Layer

This theory is a simplest approximation that the surface charge is neutralized by opposite sign counterions placed at an increment of d away from the surface.

The surface charge potential is linearly dissipated from the surface to the contertions satisfying the charge. The distance, d, will be that to the center of the countertions, i.e. their radius.  The Helmholtz theoretical treatment does not adequately explain all the features, since it hypothesizes rigid layers of opposite charges.  This does not occur in nature.