Structure and nanomechanical properties of solvent cast stearic acid filmsin liquid: An atomic force microscopy study

Solvent-cast stearic acid samples have been studied by using atomic force m icroscopy to inspect surface topography and measure tip-sample interactions . Differences between the topography in air and in water suggest that such films are likely to rearrange when dipped in water in order to expose the c arboxylic group to water. Force-distance curves performed in aqueous soluti ons at different pH values show repulsive forces arising at high pH values, when both the silicon nitride tip and the carboxylic groups become negativ ely charged. When the tip is withdrawn, after sample indentation, an adhesi ve force with attractive surges and detachments appears. We make the hypoth esis that such detachments are due to stearic acid molecules adhering stron gly to the tip and forming bridges between tip and film surface. These mole cules detach from the tip at different forces and distances. The adhesive f orce depends on the liquid environment. Its dependence on the scanning rate indicates a viscoelastic behavior of the bridges. Also the film shows visc oelastic properties when compressed at different rates.