Definition - What does Double Layer mean?
A double layer is the interface between an electrode or a suspended particle and an electrolyte created by charge-charge interaction (charge separation) leading to an alignment of oppositely charged ions at the surface of the electrode or particle. It's related to electrical double layers such as the Helmholtz double layer (HDL) and works on the principle of the Nernst diffusion layer.
Corrosionpedia explains Double Layer
A double layer or Helmholtz double layer (HDL) is an electrical double layer of positive and negative charges with a thickness equal to one molecule. This occurs at a surface where two 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) that are one particle thick and then immediately next to that, a free solution. A Helmholtz double layer is used to determine the rate of corrosion. Double layers exist in practically all heterogeneous fluid-based systems, such as paint, ink and ceramic and cement slurry.