Surface interactions and adsorbate structures : An atomic force microscopy study

University dissertation from Stockholm : Kemi

Abstract: Atomic force microscopy, AFM, was used for studying somecurrent topics in the field of surface and colloid science. Themain topics addressed in this thesis were: the nature of thelong-range attraction between hydrophobic surfaces; slidingfriction behavior for microscopic contacts; and interfacialstructuring of polyelectrolyte-nanoparticle multilayerfilms.Surface interactions were measured in the sphere againstflat surface geometry, using the colloidal probe AFM technique.Composite multilayer films were produced by layer-by-layeradsorption of cationic polyelectrolyte and anionic silicaparticles. The adsorption was monitoredin situby ellipsometry. Dry films were characterizedusing Tapping Mode imaging.A novel method for calibrating the AFM cantilever responseto torque was presented. The torsional spring constantkøwas determined through a simple procedureinvolving an acute pivot, in the form of a cantilever tip lyingupside down.Surface interactions between hydrophilic silica and silicachemically grafted with alkylsilane (h-silica) were studied inaqueous solution in terms of force-distance and friction-loadrelations. The force-distance curves in the symmetrich-silica/h-silica and asymmetric silica/h-silica cases showthat the surfaces jump into contact from distances large enoughto rule out van der Waals interactions as the main source ofattraction. This attraction was instead, at least for thesymmetric hydrophobic case, inferred to be cavitation and theformation of a capillary vapor bridge. As the hydrophobicsurfaces were pulled apart, the entire force profile wascaptured from contact to zero force. The force profile measuredduring surface retraction exhibited step-like variations thatare suggested to originate from "stick-slip" motion of thethree-phase contact during stretching of the capillarybridge.A non-linear friction-load relation was observed forcellulose colloidal probes interacting with bare andhydrophobically modified silica surfaces in ambient air.Linearity was seen only at low loads close to the pull-offload. The cellulose-silica contact exhibited the strongestadhesion as well as the largest friction. The magnitude offriction is found to be a function of the polar contribution tothe surface energy.Multilayers formed at low electrolyte concentration provedmore stable than films formed at high electrolyteconcentration. Furthermore, the uniformity and particlecoverage of the dry films were dependent on the nanoparticlesize - the smallest particles formed the smoothest, mostuniform and best-covered films.Keywords:atomic force microscope, AFM, cantilevercalibration, colloidal probe, surface force, force curve,friction, hydrophobic, adhesion, roughness, multilayer, silica,cellulose, cationic polyacrylamide, MAPTAC

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