Definitions

Electrical double layer

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.

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

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