DOUBLE-LAYER CAPACITANCE ON A ROUGH METAL-SURFACE - SURFACE-ROUGHNESSMEASURED BY DEBYE RULER

The properties of the double layer capacity on rough electrodes are di scussed in terms of the recently developed linear Poisson-Boltzmann th eory [L. I. daikhin, A. A. Kornyshev, M. Urbakh, Phys. Rev. E 53, 6192 (1996)]. This theory offers a concept of a ''Debye-length kappa depen dent roughness factor'', ie roughness function, which determines the d eviation of capacitance from the Gouy-Chapman result for a flat interf ace. Analytical expression for the roughness function is available for the case of weak Euclidean roughness. The two parameters-mean square height and correlation length-appear there. The way how the result cha nges in the case of moderate and strong roughness is analyzed on the b asis of a numerical solution obtained for a model roughness profile; a n efficient interpolation formula which covers the limits of weak and strong roughness is suggested. The role of anisotropy of the roughness profile is investigated. The predicted effects could be screened by t he crystallographic inhomogeneity of a rough surface not taken into ac count here. Tentatively ignoring such (often important) complications we discuss the results in the context of a possible method for in situ characterization of surface roughness of metal electrodes, based on t he double layer capacity measurements in solutions of variable concent ration. (C) 1997 Published by Elsevier Science Ltd.