  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 Young's modulus of a freely suspended self-assembled monolayer of
these AuNPs.
