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Chemistry · Glossary

What is Electrical double layer?

Also known as: Helmholtz layer · diffuse layer · Debye length · double-layer capacitance

Definition 15.1 University Chemistry — Year 3 · Chapter 15 — Electrode Kinetics and Electroanalysis

The electrical double layer is the arrangement of charge at an electrode–solution interface: the charge on the metal and the compensating ionic charge in the solution. Its compact part, the Helmholtz layer, is the layer of ions and solvent molecules in contact with the surface; its diffuse layer is the region beyond, where thermal motion spreads the excess ions over a distance of the order of the Debye length κ−1\kappa^{-1}. The double-layer capacitance CdlC_{\mathrm{dl}} is the charge stored per unit area per volt of potential across the interface.

The electrical double layer (Gouy–Chapman–Stern model, schematic): a negatively charged metal, a compact layer of cations at the Helmholtz plane, and a diffuse layer where cations outnumber anions over a few Debye lengths. Below, the size of the potential (negative, like the charge of the metal) falls linearly across the compact layer, then exponentially in the diffuse layer.
The electrical double layer (Gouy–Chapman–Stern model, schematic): a negatively charged metal, a compact layer of cations at the Helmholtz plane, and a diffuse layer where cations outnumber anions over a few Debye lengths. Below, the size of the potential (negative, like the charge of the metal) falls linearly across the compact layer, then exponentially in the diffuse layer.

Examples

Example 15.3 (The thickness of the double layer)

In water at 298.15 K298.15\,\mathrm{K} (εr=78.4\varepsilon_r = 78.4) with 0.10 mol/L0.10\,\mathrm{mol}/\mathrm{L} of a 1:1 salt (I=100 mol m−3I = 100\,\mathrm{mol}\,\mathrm{m}^{-3}), κ−1=78.4×8.854×10−12×2479/(2×96 4852×100)=0.96 nm\kappa^{-1} = \sqrt{78.4 \times 8.854 \times 10^{-12} \times 2479/(2 \times 96\,485^2 \times 100)} = 0.96\,\mathrm{nm}; at 1.0 mmol/L1.0\,\mathrm{mmol}/\mathrm{L}, ten times more. A supporting electrolyte compresses the diffuse layer to a few molecular diameters.

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