The Electrical Double Layer

An electrical double layer is the charge distribution that forms at the interface between a solid electrode and an electrolyte solution. It arises because the electrode acquires charge through electron transfer, causing ions with opposite charges to cluster nearby. This charge accumulation creates an electrical potential difference that eventually reaches equilibrium between the electrode and solution.

The Helmholtz model refines the primitive two-sheet model by accounting for solvated ions. It places the ionic charge sheet at the outer Helmholtz plane, separated from the electrode by hydration spheres. The electric potential changes linearly from the metal to the outer Helmholtz plane, though this model neglects thermal motion effects on the rigid charge arrangement.

The Gouy-Chapman model incorporates thermal motion, creating a diffuse double layer where oppositely charged ions cluster and diffuse into the solution rather than forming a rigid plane. This produces non-linear potential changes across the interface. Unlike the Helmholtz model's fixed charge sheet, the Gouy-Chapman approach accounts for the dynamic, temperature-dependent behavior of ions.

The Stern model combines the Helmholtz and Gouy-Chapman approaches by proposing that some negative ions adhere to the electrode at a fixed distance based on ionic radius, while thermal motion disperses remaining excess ions throughout the interphase region. This creates both an inner structured layer and a diffuse outer layer, providing a more accurate description of potential gradients across the electrode-solution interface.

Water molecules adsorb on the electrode surface and orient themselves based on the electrode's charge. When the electrode is positively charged, this orientation significantly affects the electric potential in the interphase region. The Stern model, though comprehensive, did not explicitly account for this water dipole orientation, which covers most of the electrode surface.

The Galvani potential difference, represented as Δϕ = ϕM − ϕS, is the potential difference between the bulk metal and the bulk solution. This potential arises from the charge distribution in the electrical double layer and represents the characteristic potential difference that develops when an electrode acquires charge through electron transfer and ion interactions at junction potentials in galvanic cells.

A concentrated solution of supporting electrolyte maintains constant activity coefficients during electrode studies. This stabilizes the ionic environment and allows researchers to focus on the electrode reaction kinetics without complications from changing electrolyte concentrations. Supporting electrolyte ensures reproducible conditions for examining how electron transfer rates dictate oxidation and reduction processes.