doublelayer

A double layer, in physics and chemistry, is a structure that forms at an interface between two phases (typically a solid electrode and an electrolyte, or a plasma and a surface) where charge accumulates in two parallel layers of opposite sign, producing a potential difference across the boundary.

In electrochemistry, the electrode–electrolyte interface develops an inner layer of fixed or immobile charge (the Helmholtz layer) and a surrounding diffuse layer of mobile counterions (the Gouy-Chapman layer). Together they form the electrical double layer, commonly described by the Gouy-Chapman-Stern model. The thickness is characterized by the Debye length and depends on the electrolyte concentration and temperature.

In plasmas, double layers are localized regions where electrons and ions arrange into two adjacent regions of net charge, creating a potential drop. They are observed in space and laboratory plasmas and can influence particle acceleration, energy transport, and shock formation.

Applications include electric double-layer capacitors, or EDLCs, which store energy in the electric double layer at electrode–electrolyte interfaces and can offer high power density. The concept also informs corrosion science, electrochemical sensing, and the electrical behavior of biological membranes, where a surface charge and ionic environment form a similar structure around cells.

Historically, the idea traces to Helmholtz, with refinements by Gouy and Chapman; modern treatments commonly combine fixed and diffuse layers in the Stern model to describe potential profiles at interfaces.