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 surfac es with high electric potentials in aqueous electrolyte solutions. The refore, the force between a platinum or gold sample, which served as t he working electrode, and a silicon nitride tip of an atomic force mic roscope was measured. Various potentials were applied between the samp le and a reference electrode. Experimental results were compared to fo rces calculated with the Poisson-Boltzmann equation. As predicted by t heory, the electrostatic double-layer force changed only in a narrow p otential range of about 300 mV and saturated below and above this rang e. Within this range the repulsion grew with more negative sample pote ntials. 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 lowerin g the potential. Another surprising and yet unexplained observation wa s a weak long-range attraction at positive sample potentials. This att raction decayed with a decay length of typically 50 nm. In parallel, t he 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 obser ved anymore.