Here, εr,DL is the relative permittivity of the diffuse layer, which is generally considered to be the same as the bulk permittivity εr. This is because the electric field in the diffuse layer is at least one order of magnitude lower than that in the Stern layer. In addition, lDL is the approximate diffuse layer thickness of the equivalent parallel plate capacitor, also known as the Debye length, and can be estimated from equation 8.