### Write a short note on Electrical Double Layer (Helmholtz and Stern Models).

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Helmholtz in 1879 introduced Concept of Electrical Double Layer, According to Helmholtz this double layer consists of two oppositely charged layers at a fixed distance apart so that it could be regarded as an electrical condenser with Parallel plates separated by a distance of the molecular size.

If the electrical double layer at the interface of motion between solid and liquid is regarded as an electrical condenser with parallel plates 'd cm' apart, each carrying a charge $$\sigma$$ per sq. cm. then according to electrostatics, $$\zeta \quad =\quad \frac { 4\pi \sigma d }{ D }$$ Where $$\zeta$$ = difference of potential between two plates and it is known as zetapotential or electro-kinetic potential. D = dielectric constant of the medium.

The concept of Helmholtzs electrical double layer involving a sharp potential gradient was modified by Gouy and Chapmann. They utilized the idea of diffused double layer: According to this view the solution side of the double layer is not merely one molecule or so in thickness but it extends some distance into the bulk of the solution. In this region thermal agitation permits the free movement of ions present in the solution. However, the electrostatic field arising from the charge of the solid results in a preferential attraction for particles of opposite charge.

Later on the theory was further modified by Stern in 1924. According to him double layer is of two types.

(i) The layer of the surface with single ion thickness remains constant (Helmholtz double layer).

(ii) The charge on the solid layer is fixed while an oppositely charged mobile layer nearby in the solution forms a diffused layer of charge. This may have the same or opposite charge from that of the adsorbed layer.

The diffused double layer concept makes an assumption that there is equilibrium between the adsorbed ion on the surface and those in the solution, similar to Langmuir adsorption isotherm. This gives a specific charge to the surface due to preferential adsorption of ions of opposite charge and thus the concentration goes on decreasing with the distance and becomes same as that of the bulk solution. The fall of potential is sharp in the fixed part of the double layer while that in the diffused part is gradual.

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