  It is not yet clear what drives the adsorption of ions on detonation
nanodiamonds (DNDs), which plays a critical role on the loading (unloading) of
chemotherapeutic drugs on (from) the surface of DNDs in their targeted therapy
applications. Furthermore, effects of adsorbed ions on the hydration layers of
water around DNDs with different surface chemistries have not been studied yet.
Through a series of Molecular Dynamics simulations, we found out that the law
of matching water affinity generally explains well the adsorption patterns of
ions onto the surface functional groups of DNDs. Depending on whether the water
affinity of the ion matches with that of the surface functional group or not,
the former predominantly forms either Contact Ion-Pair (CIP) or Solvent-shared
Ion-Pair (SIP) with the latter. In this regard, Na$^{+}$ and Mg$^{2+}$ have the
highest tendencies to form, respectively, CIP and SIP associations with
${-}$COO$^{-}$ functional groups. In the extreme case of 84 ${-}$COO$^{-}$
groups on DND${-}$COOH, however, we observed few Mg$^{2+}$${-}$COO$^{-}$ CIP
associations, for which we have proposed a hypothesis based on the entropy
gains. Furthermore, Mg$^{2+}$ and to a lesser extent Ca$^{2+}$ in cooperation
with ${-}$COO$^{-}$ functional groups on the surface of charged DND${-}$COOH
lead to relatively high residence times of water in the first hydration layer
of DND. This study also provides a firsthand molecular level insight about the
preferential orientation of water in the vicinity of positively charged
DND${-}$H, on which prior experimental studies have not yet reached a
consensus.

Computational Physics