Measuring electrostatic double-layer forces on HOPG at high surface potentials

Abstract

The aim of this study was to investigate surface forces in aqueous electrolyte solutions between surfaces with high electric potentials. Therefore the force between a surface of highly oriented pyrolytic graphite (HOPG) and a silicon nitride tip of an atomic force microscope was measured. Various electric potentials by a counter electrode were applied to the HOPG, which served as working electrode. As predicted by the Poisson–Boltzmann theory the electrostatic double-layer force changed only in a narrow potential range of ≈300 mV. At high negative sample potentials, where the negatively charged tip was repelled from the sample, the force saturated. At positive potentials an attractive force with a decay length longer than the Debye-length was observed. These observations agree with results of previous experiments with gold and platinum samples. Hence, the effect observed with the hydrophobic HOPG seems to be relatively universal and is not restricted to noble metals.