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 e xtremely well with recent, calculated values of the Hamaker function ( including retardation) for gold/water/gold. The best fit to the experi mental data yields a value of 2.5+/-0.5x10(-19) J for the unretarded H amaker constant. In the presence of cetyltrimethyl ammonium bromide (C TAB) monolayers, electrostatic repulsion is observed at all distances for gold sphere (radius 3.3 mu m) interactions with a flat gold surfac e. However, an attractive force is observed at very small separations for gold-coated silicon nitride tips (effective radius 0.1 mu m), whic h is attributed to penetration of the CTAB monolayers by the sharper t ip.