  Prior studies on the effects of surface chemistry of detonation nanodiamonds
(DNDs) on Hydrogen Bonds (HBs) of water have not unambiguously distinguished
among three broad categories of HBs: water-site HBs (HB$^{(s)}$), water-water
HBs in hydration layers of DNDs (HB$^{(h)}$), and water-water HBs in the bulk
region (HB$^{(b)}$). Furthermore, the effects of dissolved ions in the solution
on the aforementioned HBs have not been studied yet. In this study, we
addressed these issues via Molecular Dynamics simulations. Each system under
study contained a single DND that was functionalized either with purely -H or
with a mixture of -H and one of -NH$_2$, -COOH, or -OH groups. Then, the
product was dissolved in 0.1M aqueous solution of any of KCl, NaCl, CaCl$_2$,
or MgCl$_2$ salts. All DNDs except for DND-OH, which was charge neutral,
existed as either a charged or a neutral particle in the solution. Whereas
DND-H with +56 net charges induced the strongest HB$^{(h)}$, the addition of
+28 charges to DND-H led to a drop in HB$^{(h)}$ strength and became identical
to what we observed for HB$^{(h)}$ strength around DND-OH. Considering only
neutral DNDs, HB$^{(h)}$ strength increased in the sequence as DND-H $\approx$
DND-COOH < DND-OH < DND-NH$_2$. For the case of the negatively charged
DND-COOH, while HB$^{(h)}$ in either NaCl or KCl solution was slightly weaker
than HB$^{(b)}$, HB$^{(h)}$ strength corresponding to CaCl$_2$ or MgCl$_2$
solutions were enhanced compared to HB$^{(b)}$. Interestingly, Mg$^{2+}$ and to
a lesser extent Ca$^{2+}$ around the charged DND-COOH were associated with the
strongest HB$^{(s)}$in this study.
