AFM studies on the thickness and mechanical properties surfactant corrosion inhibitor films

Abstract

An atomic force microscopy (AFM) study is presented to investigate the adsorption on mica and gold of a corrosion inhibitor tall oil fatty acid (TOFA) imidazolium chloride in aqueous solution. The formation of a continuous flat layer was observed at half and twice the critical micelle concentration (CMC). The thickness of the corrosion inhibitor films were accurately measured by the height variation between the inhibitor surface and the bare surface where the inhibitor molecules were scratched away using the AFM tip. At half CMC, the film thickness was approximately equal to the length of the surfactant inhibitor molecule, while at twice CMC, the film thickness was twice the molecular length. The latter is consistent with a bi-molecular layer. The mechanical properties of the inhibitor films were studied by quantitatively measuring the force to penetrate the film in the vertical direction and the force to remove inhibitor molecules in the lateral direction. The interfacial inhibitor film significantly changed the surface properties, providing a protective, mechanical barrier, in both vertical (normal) and lateral directions. Calculations based on the radius of the AFM tip showed that 2 and 4 nm thick inhibitor films can provide mechanical resistances of the order of 1 to 10 MPa. The vertical force to penetrate inhibitor films increased with film thickness. The lateral force to remove inhibitor molecules from the surface was over one order of magnitude higher than the normal forces and appeared to be independent of film thickness. As well as the mechanical properties, the kinetics of inhibitor re-adsorption can be measured after scratching inhibitor molecules from the surface.