Impacts of surface chemistry and adsorbed ions on dynamics of water around detonation nanodiamond in aqueous salt solutions

TL;DR

This study uses molecular dynamics simulations to explore how surface chemistry and dissolved ions affect water dynamics around detonation nanodiamonds, revealing specific ion effects and surface charge influences on water behavior.

Contribution

It provides new insights into the combined effects of surface functional groups and ions on water dynamics near nanodiamonds, which was previously not well understood.

Findings

K+ has the least impact on water diffusion in the hydration shell.

Na+ slows water reorientation more than K+ near DND-COOH.

Charged surface groups and counterions cooperatively slow water reorientation.

Abstract

Water near detonation nanodiamonds (DNDs) forms a Hydrogen Bond (HB) network, whose strength influences DNDs' fluorescence intensity and colloidal stability in aqueous suspensions. However, effects of dissolved ions and DND's surface chemistry on dynamics of water that manifest in rupture and formation of HBs still remain to be elucidated. Thus, we carried out molecular dynamics simulations to investigate the aforementioned effects in the aqueous salt (any of KCl, NaCl, CaCl\_2, or MgCl\_2) solution of DND functionalized with any of -H, -NH\_2, -COOH, or -OH groups. We observed the specific cation effects on both translational and reorientational dynamics of water around the negatively charged DND-COOH. In the whole hydration shell of this DND, we obtained K\{+} < Na\{+} < Ca\{2+} < Mg\{2+} ordering for the impact of the cation on reducing the translational diffusion coefficient of water. In the immediate vicinity of the charged DND-COOH, the slowdown impacts of cations on the reorientational dynamics of dipole and OH vectors of water were according to Na\{+} < K\{+} < Ca\{2+} < Mg\{2+} and Na\{+} < Ca\{2+} < K\{+} < Mg\{2+} ordering, respectively. Furthermore, regardless of the type of dissolved ions, positively charged DND-NH\_2 and negatively charged DND-COOH induced, respectively, the slowest dipole and OH reorientational dynamics in the first hydration layer of DND. Our results led us to conclude that charged groups on the surface of DNDs on the one hand and the adsorbed counterions on the other hand cooperatively slow down the reorientational dynamics of water in multiple directions.