In the region where two bulk phases meet, an intricate electric charge distribution arises due to charge transfer, ion adsorption, molecular orientation, and charge distortion. This complex distribution is commonly referred to as the electrical double layer.
When a solid electrode interfaces with ions in an electrolyte solution, the speed of electron transfer dictates the rates of oxidation and reduction. The electrode acquires a charge through the escape of atoms into the solution as cations or the attachment of ions to the surface. As the charge accumulates, an electrical potential difference develops, hindering the process. Equilibrium is eventually reached, leading to a characteristic potential difference between the electrode and the solution.
The charged electrode influences the surrounding electrolyte, causing ions with opposite charges to cluster nearby. To study electrode processes, a concentrated solution of supporting electrolyte is often used to maintain constant activity coefficients. Various models, such as the Helmholtz layer model and Gouy–Chapman model, attempt to describe the double-layer structure, but each has its limitations: the former overemphasizes solution rigidity, and the latter underemphasizes the structure of the double layer.
The Stern model combines aspects of both, introducing an inner Helmholtz plane to enhance accuracy. Stern proposed in 1924 that excess negative ions are adsorbed on the electrode, influencing the electric potential in the interphase region.
The Stern model, though likely correct, didn't explicitly consider the orientation of water dipoles at the electrode, where most of the surface is covered with adsorbed water molecules. This orientation, particularly when the electrode is positively charged, affects the electric potential in the interphase region.
The Galvani potential difference, Δϕ = ϕM − ϕS, represents the potential difference between the bulk metal and solution. Together, these models explain how potential changes across the interface.
The charge distribution across the interface between a solid electrode and an electrolyte solution forms an electrical double layer.
The primitive model considers two idealized charge sheets: a positive sheet on the electrode surface and a negative sheet in the solution.
In reality, ions in solution are solvated and cannot reside directly at the electrode surface. Helmholtz refined the model by introducing solvated ions at the electrode surface, held apart by hydration spheres, with the outer Helmholtz plane marking the ionic charge sheet.
Here, the electric potential linearly shifts from ϕM at the metal to ϕS at the OHP. However, it neglects the disruptive effect of thermal motion on the rigid charge plane.
Considering the thermal motion, the Gouy–Chapman model features a diffuse double layer where the oppositely charged ions cluster at the electrode by diffusing into the solution, causing a non-linear potential change.
Combining both ideas, Stern proposed that some negative ions adhere to the electrode, maintaining a fixed distance based on the ionic radius.
Simultaneously, thermal motion disperses the remaining excess negative ions throughout the interphase region, leading to a gradual change in electric potential.
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