Helmholtz Double Layer (HDL)
Definition - What does Helmholtz Double Layer (HDL) mean?
A Helmholtz double layer (HDL) is a structural representation of charge accumulation and separation that occurs at the meeting boundary of an electrode and an electrolyte. It exists primarily in fluid-based mixture systems, such as paints, and ceramic slurry used as a coating layer for corrosion prevention.
Corrosionpedia explains Helmholtz Double Layer (HDL)
A Helmholtz double layer is made up of two layers with opposite charges, one negative and the other positive. When an electronic conductor is contacted with an conductor (electrolyte), a common boundary between the two phases is created.
Hermann von Helmholtz first identified that two layers of opposite polarity at the junction interface between an electrode and electrolyte. In 1853, it was observed that an electrical double layer is a molecular dielectric stored charge. This double layer was further named the "Helmholtz double layer".
An an example, a HDL is in general composed of adsorbed water molecules at the surface of the electrode, followed by a layer of electrolytic ions. The water forms the inner HDL, while the solvated ions form the external HDL. Both layers jointly form a planar conductor when examined in unison.