HELMHOLTZ MODEL OF THE DOUBLE LAYER

"The simplest model of the electrical double layer. The excess ions in the solution side of the double layer line up in one plane (""Helmholtz plane"") very close to the electrode surface. In a somewhat more complex model there are two planes of closest approach of the ions. Ions in the ""outer Helmholtz plane"" are about two solvent-molecule diameters away from the electrode surface because both the ions and the electrode surface are solvated. Ions in the ""inner Helmholtz plane"" have shed their solvation layer (these are usually the weekly solvated, large anions) and penetrated the solvent layer on the electrode; these, so called contact adsorbed, ions are sitting directly on the electrode surface. The ionic portion of the Helmholtz model is often called the ""Helmholtz layer"" or ""compact layer."" "