It is well-known that the interaction between passivated nanoparticles can be tuned by their complete immersion in a chosen solvent, such as water. What remains unclear on a molecular level is how nanoparticle interactions may be altered in the presence of solvent vapor where complete immersion is not achieved. In this paper, we report an all-atom molecular dynamics simulation study of the change in pair potential of mean force between dodecane thiol ligated gold nanoparticles (AuNPs) when exposed to water vapor. With the equilibrium vapor pressure of water at 25 degree C, there is very rapid condensation of water molecules onto the surface of the AuNPs in the form of mobile clusters of 100-2000 molecules that eventually coalesce into a few large clusters. When the distance between two AuNPs decreases, a water cluster bridging them provides an adhesive force that increases the depth and alters the shape of the pair-potential of mean force. That change of shape includes a decreased curvature near the minimum, consistent with experimental data showing that cyclic exposure to water vapor and its removal reversibly decreases and increases the Youngs modulus of a freely suspended self-assembled monolayer of these AuNPs.
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