Why is the double layer so thin?

The electrode charge is screened by ions over a distance set by the ionic strength—often less than a nanometer in concentrated electrolytes—because mobile ions rapidly rearrange to neutralize the surface charge.

What is specific adsorption?

It is the direct chemical attachment of certain ions to the electrode surface, shedding part of their solvation shell to reach the inner Helmholtz plane; it strongly affects the inner-layer capacitance and the potential at which charge changes sign.

Electrical Double Layer

The electrical double layer is the nanometer-scale region of separated charge that forms at an electrode–electrolyte interface, storing charge and shaping the potential distribution that governs electrode reactions.

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Definition

The structured arrangement of charge at an electrode–electrolyte interface, consisting of charge on the electrode balanced by an oppositely charged layer of ions in solution distributed across a compact and a diffuse region.

Scope

This topic covers the structure and models of the electrical double layer: the rigid Helmholtz model, the diffuse Gouy–Chapman model, and the combined Gouy–Chapman–Stern model with its compact and diffuse parts. It addresses double-layer capacitance, the inner and outer Helmholtz planes, specific adsorption of ions, and the dependence of structure on potential and electrolyte concentration.

Core questions

How is charge arranged on each side of an electrode–electrolyte interface?
How do the Helmholtz, Gouy–Chapman, and Stern models successively refine the picture?
What determines double-layer capacitance and how does it vary with potential and concentration?
How does specific adsorption of ions modify the inner layer?

Key theories

Gouy–Chapman–Stern model: Combines a compact Helmholtz layer of ions in contact with the electrode and a diffuse layer of mobile ions described by Boltzmann statistics, treating the two as capacitors in series to reproduce the observed potential and concentration dependence of capacitance.
Inner and outer Helmholtz planes: Specifically adsorbed ions sit at the inner Helmholtz plane in direct contact with the electrode, while solvated, non-specifically adsorbed ions approach only to the outer Helmholtz plane, distinctions that explain double-layer capacitance behavior.

Clinical relevance

The double layer stores charge in supercapacitors, sets the background capacitance in voltammetry and impedance measurements, governs colloid and emulsion stability through overlapping diffuse layers, and influences the rate of interfacial electron transfer.

History

Helmholtz introduced the rigid double-layer model in 1879; Gouy (1910) and Chapman (1913) added the diffuse layer, Stern combined them in 1924, and Grahame's 1947 review and mercury-electrode measurements established the detailed inner-layer structure still used today.

Key figures

Hermann von Helmholtz
Louis Georges Gouy
David Leonard Chapman
David C. Grahame

Seminal works

grahame1947
bard2001
bockris2000

Frequently asked questions

Why is the double layer so thin?: The electrode charge is screened by ions over a distance set by the ionic strength—often less than a nanometer in concentrated electrolytes—because mobile ions rapidly rearrange to neutralize the surface charge.
What is specific adsorption?: It is the direct chemical attachment of certain ions to the electrode surface, shedding part of their solvation shell to reach the inner Helmholtz plane; it strongly affects the inner-layer capacitance and the potential at which charge changes sign.