Browsing by Author "Bozna, Bianca L."
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- ItemDynamic effects in friction and adhesion through cooperative rupture and formation of supramolecular bonds(Cambridge : Royal Society of Chemistry, 2015) Blass, Johanna; Albrecht, Marcel; Bozna, Bianca L.; Wenz, Gerhard; Bennewitz, RolandWe introduce a molecular toolkit for studying the dynamics in friction and adhesion from the single molecule level to effects of multivalency. As experimental model system we use supramolecular bonds established by the inclusion of ditopic adamantane connector molecules into two surface-bound cyclodextrin molecules, attached to a tip of an atomic force microscope (AFM) and to a flat silicon surface. The rupture force of a single bond does not depend on the pulling rate, indicating that the fast complexation kinetics of adamantane and cyclodextrin are probed in thermal equilibrium. In contrast, the pull-off force for a group of supramolecular bonds depends on the unloading rate revealing a non-equilibrium situation, an effect discussed as the combined action of multivalency and cantilever inertia effects. Friction forces exhibit a stick-slip characteristic which is explained by the cooperative rupture of groups of host-guest bonds and their rebinding. No dependence of friction on the sliding velocity has been observed in the accessible range of velocities due to fast rebinding and the negligible delay of cantilever response in AFM lateral force measurements.
- ItemFriction mediated by redox-active supramolecular connector molecules(Washington D.C. : American Chemical Society, 2015) Bozna, Bianca L.; Blass, Johanna; Albrecht, Marcel; Hausen, Florian; Wenz, Gerhard; Bennewitz, RolandWe report on a friction study at the nanometer scale using atomic force microscopy under electrochemical control. Friction arises from the interaction between two surfaces functionalized with cyclodextrin molecules. The interaction is mediated by connector molecules with (ferrocenylmethyl)ammonium end groups forming supramolecular complexes with the cyclodextrin molecules. With ferrocene connector molecules in solution, the friction increases by a factor of up to 12 compared to control experiments without connector molecules. The electrochemical oxidation of ferrocene to ferrocenium causes a decrease in friction owing to the lower stability of ferrocenium−cyclodextrin complex. Upon switching between oxidative and reduction potentials, a change in friction by a factor of 1.2−1.8 is observed. Isothermal titration calorimetry reveals fast dissociation and rebinding kinetics and thus an equilibrium regime for the friction experiments.