Helmholtz Double Layer (HDL)
Definition - What does Helmholtz Double Layer (HDL) mean?
A Helmholtz double layer (HDL) is an electrical double layer of positive and negative charges one molecule thick. This occurs at a surface where two bodies of different materials are in contact or at the surface of a metal or other substance capable of existing in a solution as ions and immersed in a dissociating solvent.
In the Helmholtz model, there is a layer of aligned molecules (or ions), which is one particle thick and then immediately next to that, free solution. A Helmholtz double layer is used for the purpose of determining the rate of corrosion.
The HDLs exist in practically all heterogeneous fluid-based systems, such as paint, ink and ceramic and cement slurry.
Corrosionpedia explains Helmholtz Double Layer (HDL)
A Helmholtz double layer constitutes two layers having opposite charges, one being negatively charged and the other positively charged. A separation of charges exists like in a capacitor.
When an electronic conductor is brought in contact with a solid or liquid ionic conductor (electrolyte), a common boundary (interface) among the two phases appears. Hermann von Helmholtz first realized that two layers of opposite polarity form at the interface between electrode and electrolyte. In 1853 he showed that an electrical double layer is essentially a molecular dielectric stored charge electrostatically. This double layer is known as a Helmholtz double layer in his honor.
The Helmholtz double layer is formed by two planar sheets of charges. One is due to the ions in solution adsorbed at the surface of the solid, the other is due to the ions of opposite charge attracted by adsorbed ions.
For example, a Helmholtz layer is in general composed of adsorbed water species (H+ and OH-) at the surface of the semiconductor or object, followed by a layer of solvated electrolyte ions. The water species form the so-called inner Helmholtz layer, while the solvated ions form the external Helmholtz layer. The two Helmholtz layers can be considered to form a planar capacitor.
In usual electrolytes, one of the two (H+ or OH-) water species dominates the inner Helmholtz layer, depending on the pH value of the solution. When the number of adsorbed H+ and OH- species is equal, then the effective charge on the surface is zero. In this case the voltage drop across the Helmholtz layer is zero.