Anomalous Helmholtz-capacitance on stepped surfaces of silver and gold

We have studied the capacitance of the solid/electrolyte interface on Ag(11n) and Au(11n) surfaces in KClO4 and HClO4 as function of the electrolyte concentration and the step density. We find that the inner layer capacitance (Helmholtz-capacitance) at the potential of (total) zero charge is dramatically reduced on stepped surfaces. Standard theories which describe the Helmholtz-capacitance by properties of the liquid, a hard wall boundary condition and the polarizability of the electron gas at the metal surface fail to describe this behavior. We propose that the different polarizability of water bonded to the surface need be taken into account and attribute the reduced capacitance at steps to the lower polarizability of water molecules bonded to step edges.