Adsorption, self-organization, and mechanical properties of different fatty acid layers under different confinement states have been investigated as a function of the presence and conformation of one unsaturation in their aliphatic chain. In situ characterization, at the molecular level, was performed with the ATLAS molecular tribometer, in terms of rheology, forces, and thickness of confined fluid. The results demonstrate that the fatty acids adsorb on the surfaces by weak interactions and form viscoelastic films with a thickness of about 15 Å on each surface. The adsorption kinetics, the packing of the self-assembled monolayers, and the coverage rate depend on the molecular architecture of the fatty acids and lead to various mechanical behaviors under confinement.
The presence or absence of unsaturation in the carbon chain of carboxylic acids was examined on the friction response of boundary films according to normal force. Friction laws were established as a function of the normal force at the nanoscale with a molecular tribometer derived from a surface force apparatus. While the shear plane is mainly composed of methyl groups in all the cases investigated, the molecular organization differs within the shear plane, as suggested by the simultaneous film thickness evolution. This gave rise to dissipation modes within the interface, resulting in a wide range of shear stress levels and dependence with contact pressure. The friction laws were discussed, highlighting the combined role of nanometric surface roughness and molecule unsaturation. Special issue dedicated to the Memory of Mark Robbins.
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