What drives adsorption of ions on surface of nanodiamonds in aqueous solutions?

Abstract: 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.