Beyond the Helmholtz layer, the interface transitions into a compact layer where ion concentration reaches a maximum, C max , constrained by steric effects and determined by the hydration radii of the ions (more information is provided in SI section 1). The permittivity in this region follows a similar trend to that of the Helmholtz layer, increasing as the electric field weakens. Following the compact layer, a diffuse layer extends into the electrolyte, where ion concentration progressively decreases toward the bulk solution. In this interfacial regions, the electric field declines sharply, and the solvent permittivity recovers to its unsaturated bulk value. Moving away from the interfacial region, the bulk electrolyte is present, where ion concentrations stabilize at their bulk equilibrium value, C 0 .