| Abstract
| - The nanomechanical properties of Langmuir−Blodgett monolayers of arachidic acid extracted at surfacepressures of 1, 15, and 35 mN/m and deposited on mica were investigated by atomic force microscopy, forcespectroscopy, and lateral force microscopy. It was experimentally demonstrated that the arachidic acid molecularorientation depends on the extraction pressure. According to this, tilting angles of 50, 34, and 22° with respectto the surface perpendicular were detected and identified as conformations that maximize van der Waalsinteractions between the arachidic acid alkyl chains. The vertical force needed to puncture the monolayerswith the AFM tip strongly depends on the molecular tilting angles attained at different monolayer extractionsurface pressures, obtaining values that range from 13.07 ± 3.24 nN for 50° to 22.94 ± 5.49 nN for 22°tilting angles. The different molecular interactions involved in the monolayer cohesion are discussed andquantitatively related to the experimental monolayer breakthrough forces. The friction measurements performedfrom low vertical forces up to monolayer disruption reveal the existence of three well-defined regimes: first,a low friction response due to the elastic deformation of the monolayer, which is followed by a sharp increasein the friction force due to the onset of a sudden plastic deformation. The last regime corresponds to themonolayer rupture and the contact between tip and substrate. The friction coefficient of the substrate is seento depend on the monolayer extraction pressure, a fact that is discussed in terms of the relationship betweenthe sample compactness and its rupture mechanism.
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