8th Liquid Matter Conference September 6-10, 2011 Wien, Austria ...

Thu 811:10-14:00P7.65The effect of confined water on the interaction ofnanoparticles: a molecular dynamic simulation studySabine Leroch, 1 Silvia Pabisch, 2 Herwig Peterlik, 2 and Martin Wendland 11 University of Life Sciences (BOKU), Institute for Process engineering and energy,Muthgasse 107 1190, Vienna, Austria2 University of Vienna, Vienna, AustriaAdhesion forces between nanoparticles depend on the amount of adsorbed condensed water fromambient atmosphere. Liquid water forms bridges in the cavities separating the particles, giving riseto the so called capillary forces which, in most cases dominate the van der Waals and long rangeelectrostatic interactions. Capillary forces promote the undesirable agglomeration of particles tolarge clusters thereby, hindering the flowability of dry powders in process containers. Usuallymacroscopic theories based on the Laplace pressures are used to estimate the strength of the capillaryforces. However, especially for low relative humidity and when treating the wetting of roughparticles, those theories can fail. Since, in these cases the liquid film does not fill the whole contactarea, but rather the water meniscus appears where surface aspherities on the particles approacheach other. Moreover, the location and the shape of the water bridges will vary with the chemicalcomposition on the particle surfaces. Molecular dynamic simulations can help to give better insightinto the water particle interface and to complement interaction forces in regions not accessible toAFM experiments. Small (diameter 5 nm) silica nanoparticles in atomic resolution with differentdensities of surface silanol groups to steer their hydrophilicity will serve as first test system.The interaction potentials are provided on one side, by conventional force fields which have beenparametrized to reproduce experimentally measured contact angles of water on silica plates on theother side, by an empirical potential recently developed in the group of Ciacchi. Force versus distancecurves as well as adhesion energies between wetted silica particles for different amounts ofadsorbed liquid water will be discussed in comparison to experimental pull-off forces and adhesionenergies deduced from AFM measurements and small angle X-ray scattering experiments carriedout by our collaborators.65