Istituto di Biofisica     
Cappella B., Baschieri P., Ruffa M., Ascoli C., Relini A., Rolandi R. Structure and Nanomechanical Properties of Solvent Cast Stearic Acid Films in Liquid: An Atomic Force Microscopy Study. In: Langmuir, vol. 15 (6) pp. 2152 - 2157. American Chemical Society, 1999.
Solvent-cast stearic acid samples have been studied by using atomic force microscopy 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 carboxylic group to water. Force−distance curves performed in aqueous solutions at different pH values show repulsive forces arising at high pH values, when both the silicon nitride tip and the carboxylic groups become negatively charged. When the tip is withdrawn, after sample indentation, an adhesive force with attractive surges and detachments appears. We make the hypothesis that such detachments are due to stearic acid molecules adhering strongly to the tip and forming bridges between tip and film surface. These molecules detach from the tip at different forces and distances. The adhesive force depends on the liquid environment. Its dependence on the scanning rate indicates a viscoelastic behavior of the bridges. Also the film shows viscoelastic properties when compressed at different rates.
DOI: 10.1021/la980736y

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