In
this work we present a comprehensive experimental study to determine
the contact forces between individual metal oxide nanoparticles in
the gas-phase using atomic force microscopy. In addition, we determined
the amount of physisorbed water for each type of particle surface.
By comparing our results with mathematical models of the interaction
forces, we could demonstrate that classical continuum models of van
der Waals and capillary forces alone cannot sufficiently describe
the experimental findings. Rather, the discrete nature of the molecules
has to be considered, which leads to ordering at the interface and
the occurrence of solvation forces. We demonstrate that inclusion
of solvation forces in the model leads to quantitative agreement with
experimental data and that tuning of the molecular order by addition
of isopropanol vapor allows us to control the interaction forces between
the nanoparticles.