Measuring electrostatic double-layer forces at high surface potentials with the atomic force microscope

The aim of this study was to measure interaction forces between surfaces with high electric potentials in aqueous electrolyte solutions. Therefore, the force between a platinum or gold sample, which served as the working electrode, and a silicon nitride tip of an atomic force microscope was measured. Various potentials were applied between the sample and a reference electrode. Experimental results were compared to forces calculated with the Poisson-Boltzmann equation. As predicted by theory, the electrostatic double-layer force changed only in a narrow potential range of about 300 mV and saturated below and above this range. Within this range the repulsion grew with more negative sample potentials. This was expected, since the tip was negatively charged at the high pH chosen. At strong negative sample potentials this saturation was not complete and the force continued to rise slightly when lowering the potential. Another surprising and yet unexplained observation was a weak long-range attraction at positive sample potentials. This attraction decayed with a decay length of typically 50 nm. In parallel, the structure of Au(111) was imaged. We confirmed a (root 3 x p, p > 10) reconstruction at potentials below about -0.3 V-SHE and the normal (1 x 1) hexagonal packing above this potential. Above about +0.8 V-SHE the (1 x 1) structure disappeared and no crystalline packing was observed anymore.