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RESEARCH PRODUCT

Adhesion and adhesion hysteresis of mica surfaces covered with bola-amphipiles in dry and humid air

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Abstract Bola-amphiphiles with spherical hydrophilic chinuclidinium head groups form laterally ordered monolayers on mica by adsorption from solution. To measure the surface adhesion, JKR-type compression/decompression cycles have been performed with the surface forces apparatus (SFA). In an inert atmosphere, the adhesion energy on increasing the contact area is much lower than predicted by wetting experiments. Furthermore, the pronounced adhesion hysteresis varies from sample to sample in a wide range, indicating a defect-rich surface. On increase of the relative humidity (rh), the reproducibility improves and the values for the adhesion energy measured on compression and decompression converge to the value theoretically predicted for a partially wetting surface. At 80% rh, optically a water meniscus is observed. Obviously, mobile water molecules adsorb on top of the monolayer, and equilibrium is obtained quickly at the monolayer–monolayer interface. At 100% rh, molecules are pressed out of the contact zone and the annealed monolayers are more hydrophobic, indicating a loss of head-group order.