Electrical double layer

When a metal electrode contacts an electrolyte such as extracellular fluid, charge redistributes at the interface to form an electrical double layer: a layer of charge on the metal balanced by a structured layer of counter-ions in the solution. The classic models build up in refinements — the Helmholtz model (a rigid capacitor-like plane of ions), the Gouy-Chapman model (a diffuse, thermally smeared ionic cloud), and the Stern model (a compact inner layer plus a diffuse outer layer) — and together they explain why the interface behaves largely as a capacitance, the double-layer capacitance, typically tens of microfarads per square centimeter of real (microscopic) area.

The double layer is why small electrodes have high impedance: capacitance scales with area, so shrinking a contact to improve spatial selectivity raises its interfacial impedance and thermal noise. It is also why coatings that increase effective surface area (roughening, nanostructuring, conducting polymers) lower impedance without enlarging the geometric footprint. Charge that crosses the double layer non-capacitively does so by Faradaic reactions, which for recording should be minimized and for stimulation must be controlled.