Electrical double layer

The electrical double layer is a structure that describes the variation of electric potential near a surface, and has a large bearing on the behaviour of colloids and other surfaces in contact with solutions and solid state fast ion conductors. The analogue in a plasma is the double layer.

Additional recommended knowledge

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Historical development of double layer models

The earliest model of the electrical double layer is usually attributed to Helmholtz (1879). Helmholtz treated the double layer mathematically as a simple capacitor, based on a physical model in which a single layer of ions is adsorbed at the surface.

Later Gouy and Chapman (1910-1913) made significant improvements by introducing a diffuse model of the electrical double layer, in which the potential at a surface decreases exponentially due to adsorbed counter-ions from the solution.

The current classical electrical double layer is the Gouy-Chapman-Stern model, which combines the Helmholtz single adsorbed layer with the Gouy-Chapman diffuse layer. Important approximations used in this model are:

• Ions are effectively point charges
• The only significant interactions are coulombic
• Electrical permittivity is constant throughout the double layer
• The solvent is uniform at the atomic scale.

Classical models of electrical double layer are not suited for the fast ion conductor/metal interface as the concentration of mobile ions in solid ion conductor can be extremely high, Ni~10²²/cm³.

Distribution of cations and anions next to clay surface is continuous

• Some cations are held tightly right next to the surface
• Others held more loosely further away
• Anion exclusion zone next to surface
• Soluble cations are reached only when [cations] = [anions]

See also

• The Electrical Double Layer
• Electrical double layer: revisit based on boundary conditions