Electric-Field Effects on Ionic Hydration: A Molecular Dynamics Study

Abstract

In this work, we report the electric-field effects on ionic hydration of Cl, Na, and Pb using molecular dynamics simulations. It is found that the effect of weak fields on ionic hydration can be neglected. Strong fields greatly disturb the water orientation in the hydration shells of ions, though ion coordination number remains almost unchanged. Under strong fields, the first hydration shell of ions is significantly weakened and the ion-water interaction energy is dramatically reduced; surprisingly, the second hydration shells of Cl and Na are slightly structured because of the optimal water orientation; moreover, ionic hydration structures become asymmetrical along the field direction because of the uniformly aligned water dipoles. Compared with Na and Pb, the hydration of Cl is less disturbed by external fields, probably ascribed to the different water reorientation around anions and cations as well as the different structure-maker/breaker nature of the ions. Additionally, strong fields significantly enhance ion mobility and remarkably shorten the water residence time in the hydration shell. This work demonstrates that applying strong fields is an effective way to weaken ion hydration.

Discovering the chloride pathway in the CFTR channel

“…This small diameter difference may not be sufficient to allow the passage of the chloride ion surrounded by a hydrate shell. Under physiological conditions, this shell can be weakened by the negative membrane potential, which cannot be modeled in our MD simulations [57]. Although the protein structure was not constrained in an open conformation during our metadynamics simulation, the chloride ion visited both sides of the bottleneck region.…”

Li+ Selectivity of Carboxylate Graphene Nanopores Inspired by Electric Field and Nanoconfinement

“…Under an appropriate electric field, the fast-migrating cations will partially dehydrate near the space-confined nanopore, facilitating the association of Mg 2+ /Li + with the fast-migrating Cl − in the opposite direction, and thus the degree of association of cation with Cl − increases. The work of He et al [26] also showed the ion hydration may be affected as the electric field is greater than 1.0 V nm −1 , especially for Cl − , which may facilitate ion association. Our results indicate that Mg 2+ ions form stable clusters with Cl − ions under a certain electric field while Li + ions are not easy to form stable clusters with Cl − ions, enabling Li + pass through COO − nanopores more easily under a certain electric field.…”

“…Similarly, compared with 0.2 V nm −1 , the inner (1st and 2nd) shells around Mg 2+ or Li + become more flexible and expanding under the electric fields of 0.6−1.0 V nm −1 (Figure 4e,f), which may be attributed to the fast transporting of ions and the dynamics of the hydrogen bonding network. [26] For 1.0 V nm −1 , the number of water molecules in the inner shell is relatively small. Since the dehydration is conducive to the association of Mg 2+ and Cl − , they are easier to form ion associated structures under the electric field of 1.0 V nm −1 compared with other electric fields.…”

“…Ji et al [25] proposed that a voltage of 6 V is more desirable for Li + recovery from high Mg 2+ /Li + brines by selective electrodialysis, SO 4 2− has greater electrostatic interaction to bivalent Mg 2+ , which will make it easier for Li + to pass through the exchange membrane. MD study of He et al [26] found that the effect of weak fields on ionic hydration can be neglected, while strong fields can greatly interfere the water orientation in the hydration shells of ions. MD simulations of Azamat et al [27] showed that as the electric fields increase, the residence time of Mg 2+ and Li + inside nanotubes embedded in a SiN membrane is decreased and the ionic current is increased.…”

Structure and Transport Properties of Water and Hydrated Ions in Nano‐Confined Channels

“…They concluded that the ion coordination number increased with the increase of the ionic radius, but the number of hydrated water molecules decreased. Furthermore, He et al reported that ion hydration can be affected by strong electric field. The water orientation in the hydration shell of ion would be disturbed under strong fields, though the ion coordination number remained almost unchanged.…”