  Diamondoids are promising materials for applications in catalysis and
nanotechnology. Since many of their applications are in aqueous environments,
to understand their function it is essential to know the structure and dynamics
of the water molecules in their first hydration shells. In this study, we
develop an improved reactive force field (ReaxFF) parameter set for
atomistically resolved molecular dynamics simulations of hydrated diamondoids
to characterize their interfacial water structure. We parameterize the force
field and validate the water structure against geometry-optimized structures
from density functional theory. We compare the results to water structures
around diamondoids with all partial charges set to zero, and around charged
smooth spheres, and find qualitatively similar water structuring in all cases.
However, the response of the water molecules is most sensitive to the partial
charges in the atomistically resolved diamondoids. From the systematic
exclusion of atomistic detail we can draw generic conclusions about the nature
of the hydrophobic effect at nanoparticle interfaces and link it to the
interfacial water structure. The interactions between discrete partial charges
on short length scales affect the hydration structures strongly but the
hydrophobic effect seems to be stable against these short scale surface
perturbations. Our methods and the workflow we present are transferable to
other hydrocarbons and interfacial systems.
