MEASUREMENT OF THE FORCES BETWEEN GOLD SURFACES IN WATER BY ATOMIC-FORCE MICROSCOPY

The forces between a flat gold surface and a gold-coated silica sphere have been measured in water using an atomic force microscope. A long-range attractive interaction is observed which is ascribed to the van der Waals interaction between the two surfaces. The force data agree extremely well with recent, calculated values of the Hamaker function (including retardation) for gold/water/gold. The best fit to the experimental data yields a value of 2.5+/-0.5x10(-19) J for the unretarded Hamaker constant. In the presence of cetyltrimethyl ammonium bromide (CTAB) monolayers, electrostatic repulsion is observed at all distances for gold sphere (radius 3.3 mu m) interactions with a flat gold surface. However, an attractive force is observed at very small separations for gold-coated silicon nitride tips (effective radius 0.1 mu m), which is attributed to penetration of the CTAB monolayers by the sharper tip.