Hydration Shells Of Ions Research Articles

DEHP- extractant binding to trivalent lanthanide Er3+: Fast binding accompanied by concerted angular motions of hydration water.

Solvent extraction of trivalent rare earth metal ions by organophosphorus extractants proceeds via binding of phosphoric acid headgroups to the metal ion. Water molecules in the tightly bound first hydration shell of the metal ions must be displaced by oxygen atoms from phosphoric acid headgroups. Here, we use classical molecular dynamics simulations to explore the event in which a fully hydrated Er3+ binds to its first phosphoric acid headgroup. Approach of the headgroup into the region between the first and second hydration shells leads to a fast ejection of a water molecule that is accompanied by reordering of the hydration water molecules, including discretization of their angular positions and collective rotation about the metal ion. The water molecule ejected from the first shell is located diametrically opposite from the binding oxygen. Headgroup binding places a headgroup oxygen closer to Er3+ than its first hydration shell and creates a loosely bound water that subsequently exchanges between the first shell and its environment. This second exchange of water also occurs at discrete angular positions. This geometrical aspect of binding may be of relevance to understanding the binding and transport of ion-extractant complexes that are expected to occur at the organic-aqueous liquid-liquid interface used in solvent extraction processes.

Effect of water solubility in organic solvents on the standard Gibbs energy of ion transfer across a water/organic solvent interface

Ion transfer voltammetry is used to study the transfer of alkali metal and proton cations and the Cl− anion from the aqueous solution of MCl (M+ = H+, Li+, Na+, K+, Rb+, Cs+) to the solution of bis(triphenylphosphoranylidene)ammonium tetrakis(pentafluorophenyl)borate in α,α.α-trifluorotoluene (TFT) or 1,2-dichloroethane (DCE). The scale of the applied potentials is converted to the scale of the Galvani potential differences on the basis of the voltammetric measurements of the standard ion transfer potential for the tetraethylammonium (TEA+) ion used as a reference ion in situ. The interfacial tension measurements at the water/TFT interface yield the zero-charge potential difference, which is close to the expected zero value. The standard Gibbs energies of ion transfer ΔwoGi0 are evaluated from the voltammetric data on considering both the effect of the association between the transferred ion and the counter-ion of the organic solvent phase, and the effect of the ion migration in the aqueous phase. The values of ΔwoGi0 for the ion transfer to TFT are found to be considerably higher than those obtained for the ion transfer to DCE. This difference is proposed to be related to the significantly lower solubility of water in TFT, possibly requiring the removal of the ion hydration shell in the course of the ion transfer from water to TFT. Such conclusion is supported by the calculations of ΔwoGi0 by using an advanced Born-type model of ion solvation.

Structural Mechanism of Ionic Conductivity of the TRPV1 Channel

The so-called “hydrophobic gating” is widely discussed as a putative mechanism to control water and ion conduction via ion channels. This effect can occur in narrow areas of the channels pore lined by non-polar residues. In the closed state of the channel, such regions may spontaneously transit to a dehydrated state to block water and ions transport without full pore occlusion. In the open state, the hydrophobic gate is wide enough to provide sustainable hydration and conduction. Apparently, the transport through the open hydrophobic gate may by facilitated by some polar residues that assist polar/charged substances to overcome the energy barrier created by nonpolar environment. In this work, we investigated the behavior of Na+ ions and their hydration shells in the open pore of the rat TRPV1 ion channel by molecular dynamics simulations. We show that polar protein groups coordinate water molecules in such a way as to restore the hydration shell of ions in the hydrophobic gate that ensures ion transport through the gate in a fully hydrated state.

Bidirectional Transport Phenomenon of Ions in Electric Fields Due to the Cluster Formation in Two-Dimensional Graphene Channels

In aqueous solution, cations and anions are known to be moving oppositely and unidirectionally in electric fields. In this work, we observe an abnormal bidirectional ion transport phenomenon in electric fields because of the formation of ion clusters in two-dimensional graphene channels. A series of molecular dynamic simulations reveal that the separated ions within two-dimensional graphene channels will rapidly assemble into individual clusters under the electric field, originating from the confinement-induced effective ionic interactions as well as the strongly compressed hydration shells of ions. Notably, owing to the competition between ionic interaction and electric field, the structure of ion clusters changes from a square-unit pattern to chain-like with the increase in electric field, and the cluster size displays a maximum behavior. As a result, the motion of cations and anions in these clusters will be strongly coupled to each other, leading to the bidirectional transport phenomenon. Furthermore, these ion clusters will disappear abruptly with the increase in graphene layer distance, and thus the ion transport becomes unidirectional as normal. Our findings shed light on the unusual ion dynamics in two-dimensional channels and provide a potential avenue for the design of novel nanofluidic devices.

On the temporal selectivity of desalination for a porous composite graphene-copper membrane (GCuM): A molecular dynamics study

The concept of the temporal selectivity breaks the spatial selectivity-based paradigm of permeability and selectivity in membrane separation technology, and its realization can provide simultaneous efficient salt rejection and high permeability with large pores. In this work, a rotating centrifuge model made of porous copper foil covered with porous graphene is designed to explore the possibility to realize the temporal selectivity on a porous composite graphene‑copper membrane (GCuM) in a molecular dynamics study. The results show that the permeability of porous rotating GCuM increases up to 156 L/cm2/day/MPa with a salt rejection larger than 90 %, which is 2.6 to 1000 times larger than the commercial and other advanced reverse osmosis membranes. The boundary slip velocity between the seawater-GCuM interface inhibits the passage of salt ions, resulting in high salt rejection even in the case of 2–4 nm diameter pores. The stability of the ionic hydration shell and the energy barrier of the ions passing through the pores increase with the increase of the angular velocity, which explains the influence of the boundary slip velocity on the salt rejection from the molecular mechanism. The geometric relation that governs the temporal selectivity principle on porous rotating GCuM has been further derived to explain pore size, membrane thickness, and hydrated ion mass on high permeability and salt rejection. The findings further verify and expand the application scope of the temporal selectivity mechanism, and will promote the application of graphene-based nanoporous centrifuges.

Physical insight into the entropy-driven ion association.

The ion association is widely believed to be dominated by the favorable entropy change arising from the release of water molecules from ion hydration shells. However, no direct thermodynamic evidence exists to validate the reliability and suitability of this view. Herein, we employ complicated free energy calculations to rigorously split the free energy including its entropic and enthalpic components into the water-induced contributions and ion-ion interaction terms for several ion pairs from monatomic to polyatomic ions, spanning the size range from small kosmotropes to large chaotropes (Na+ , Cs+ , Ca2+ , F- , I- , CO3 2- , and HPO4 2- ). Our results successfully reveal that though ion associations are indeed determined by a delicate balance between the favorable entropy variation and the repulsive enthalpy change, the entropy gain dominated by the solvent occurs only for the monatomic ion pairing. The water-induced entropic contribution significantly goes against the ion pairing between polyatomic anion and cation, which is, alternatively, dominated by the favorable entropy from the ion-ion interaction term, due to the configurational arrangement of polyatomic anions involved in ion association. The structural and dynamic analysis demonstrates that the entropy penalty from the water phase is primarily ascribed to the enhanced stability of water molecules around the cation imposed by the incoming anion. Our study successfully provides a fundamental understanding of water-mediated ion associations and highlights disparate lengthscale dependencies of the dehydration thermodynamics on the specific types of ions.

Investigation of scale inhibition effect and mechanism of S-HGMF in the clean recirculating cooling water system

The formation of scales in a recirculating water system is a common problem in industrial water treatment; it seriously affects the production in various industries and pollutes the environment. Although conventional scale inhibition methods are effective, they are expensive and harm the environment. Herein, an advanced method is proposed to solve the scaling issue in recirculating cooling water systems using the superconducting high-gradient magnetic field (S-HGMF) treatment. The scale inhibition performance could be improved by changing the magnetic flux density, operation time, and flow rate. The results showed that S-HGMF could increase the number of hydrogen bonds in the recirculating cooling water, enhance molecular interaction, increase the thickness of the ion hydration shell, reduce the nucleation rate, stabilize the water quality, improve the solubility of scale-forming ions, and inhibit scale formation. The scale inhibition performance reached 8.10%. Interestingly, S-HGMF had a memory effect in that it could maintain the scale inhibition effect for some period after treatment completion. Moreover, S-HGMF changed the crystal structure of the scale and promoted the transformation of the scale to a metastable phase. Ultimately, calcite was transformed to aragonite to reduce the precipitation of hard scale (calcite), achieving the purpose of scale inhibition. As a physical method, the application of S-HGMF to inhibit scaling has great potential for industrial applications.

Where Lennard-Jones Potentials Fail: Iterative Optimization of Ion-Water Pair Potentials Based on Ab Initio Molecular Dynamics Data.

The use of the Lennard-Jones (LJ) potential in computer simulations of aqueous electrolyte solutions is widespread. The standard approach is to parametrize LJ potential parameters against thermodynamic solution properties, but problems in representing the local structural and dynamic properties of ion hydration shells remain. The r-12-term in the LJ potential is responsible for this as it leads to overly repulsive ion-water interactions at short range. As a result, the LJ potential predicts blue-shifted vibrational peaks of the cations' rattling mode and too large negative ion hydration entropies. We demonstrate that cation-water effective pair potentials derived from ab initio MD data have softer short-range repulsions and represent hydration shell properties significantly better. Our findings indicate that replacing the LJ potential with these effective pair potentials offers a promising route to represent thermodynamic solution properties and local interactions of specific ions with nonpolarizable force field models.

Derivation of the solvation effect-incorporated Poisson-Boltzmann equation

Based on molecular kinetic consideration relating to dynamic properties for both the ionic atmosphere and ion solvation shell, solvation effect-incorporated PB (SEI-PB) equation was derived.Consequently, there appeared a solvated ionic screening thickness containing not only the ionic atmosphere thickness (Debye length), but also a new screening thickness of the solvation shell in both the linearized SEI-PB equation and its solution. The thickness of solvated ionic atmosphere has physical meaning of the screening thickness taking into account the volumetric effect of ions together with ionic solvation effect.As a result of the solution of linear SEI-PB equation, the total electric potential was divided into two terms: the first one was derived from the ions, which were affected by the solvation shells and the second one was derived from the solvation shells which were affected by the ions. An analytical equation for ion–solvent molecular dipole moment interaction (ISMDI) potential energy was established based on the solution of linear SEI-PB equation.Also by using the linear SEI-PB equation, analytical expressions of the activity coefficients for ions and electrolytes as well as solvent were derived and an equation for estimating the mean lifetimes of solvation shells according to ionic species was proposed and applied to estimate those of ionic hydration shells. Estimated mean lifetimes for ionic hydration shells according to ionic species were compatible with the previous XRD, NMR data and MD simulations.We confirmed that the SEI-PB equation has peculiarity containing not only solvation effect, but also dielectric saturation and that the dielectric saturation is connected implicitly with ionic solvation effect.

Extension of the primitive model by hydration shells and its impact on the reversible heat production during the buildup of the electric double layer.

Recently, the reversible heat production during the electric double layer (EDL) buildup in a sodium chloride solution was measured experimentally [Janssen et al., Phys. Rev. Lett. 119, 166002 (2017)] and matched with theoretical predictions from density functional theory and molecular dynamics simulations [Glatzel et al., J. Chem. Phys. 154, 064901 (2021)]. In the latter, it was found that steric interactions of ions with the electrode's walls, which result in the so-called Stern layer, are sufficient to explain the experimental results. As only symmetric ion sizes in a restricted primitive model were examined, it is instructive to investigate systems of unequal ion sizes that lead to modified Stern layers. In this work, we explore the impact of ion asymmetry on the reversible heat production for each electrode separately. In this context, we further study an extension of the primitive model where hydration shells of ions can evade in the vicinity of electrode's walls. We find a strong dependence on system parameters such as particle sizes and the total volume taken by particles. Here, we even found situations where one electrode was heated and the other electrode was cooled at the same time during charging, while, in sum, both electrodes together behaved very similarly to the already mentioned experimental results. Thus, heat production should also be measured in experiments for each electrode separately. By this, the importance of certain ingredients that we proposed to model electrolytes could be confirmed or ruled out experimentally, finally leading to a deeper understanding of the physics of EDLs.

Confinement induces stable calcium carbonate formation in silica nanopores.

Scalable efforts to remove anthropogenic CO2via the formation of durable carbonates require us to harness siliceous nanoporous geologic materials for carbon storage. While calcium carbonate formation has been extensively reported in bulk fluids, there is a limited understanding of the influence of nanoconfined fluids on the formation of specific stable and metastable polymorphs of calcium carbonates in siliceous materials that are abundant in subsurface environments. To address this challenge, silica nanochannels with diameters of 3.7 nm are architected and the formation of specific calcium carbonate phases is investigated using X-ray diffraction (XRD), and molecular dynamics (MD) simulations. The formation of stable calcium carbonate (or calcite) is noted in silica nanochannels. The presence of fewer water molecules in the first hydration shell of calcium ions in confinement compared to in bulk fluids contributes to stable calcium carbonate formation. These studies show that nanoporous siliceous environments favor the formation of stable calcium carbonate formation.

Stripping away ion hydration shells in electrical double-layer formation: Water networks matter

The double layer at the solid/electrolyte interface is a key concept in electrochemistry. Here, we present an experimental study combined with simulations, which provides a molecular picture of the double-layer formation under applied voltage. By THz spectroscopy we are able to follow the stripping away of the cation/anion hydration shells for an NaCl electrolyte at the Au surface when decreasing/increasing the bias potential. While Na+ is attracted toward the electrode at the smallest applied negative potentials, stripping of the Cl- hydration shell is observed only at higher potential values. These phenomena are directly measured by THz spectroscopy with ultrabright synchrotron light as a source and rationalized by accompanying molecular dynamics simulations and electronic-structure calculations.

A flexible GAS belt responds to pore mutations changing the ion selectivity of proton-gated channels.

Proton-gated ion channels conduct mainly Na+ to induce postsynaptic membrane depolarization. Finding the determinants of ion selectivity requires knowledge of the pore structure in the open conformation, but such information is not yet available. Here, the open conformation of the hASIC1a channel was computationally modeled, and functional effects of pore mutations were analyzed in light of the predicted structures. The open pore structure shows two constrictions of similar diameter formed by the backbone of the GAS belt and, right beneath it, by the side chains of H28 from the reentrant loop. Models of nonselective mutant channels, but not those that maintain ion selectivity, predict enlargement of the GAS belt, suggesting that this motif is quite flexible and that the loss of stabilizing interactions in the central pore leads to changes in size/shape of the belt. Our results are consistent with the "close-fit" mechanism governing selectivity of hASIC1a, wherein the backbone of the GAS substitutes at least part of the hydration shell of a permeant ion to enable crossing the pore constriction.

Two-dimensional monolayer salt nanostructures can spontaneously aggregate rather than dissolve in dilute aqueous solutions

It is well known that NaCl salt crystals can easily dissolve in dilute aqueous solutions at room temperature. Herein, we reported the first computational evidence of a novel salt nucleation behavior at room temperature, i.e., the spontaneous formation of two-dimensional (2D) alkali chloride crystalline/non-crystalline nanostructures in dilute aqueous solution under nanoscale confinement. Microsecond-scale classical molecular dynamics (MD) simulations showed that NaCl or LiCl, initially fully dissolved in confined water, can spontaneously nucleate into 2D monolayer nanostructures with either ordered or disordered morphologies. Notably, the NaCl nanostructures exhibited a 2D crystalline square-unit pattern, whereas the LiCl nanostructures adopted non-crystalline 2D hexagonal ring and/or zigzag chain patterns. These structural patterns appeared to be quite generic, regardless of the water and ion models used in the MD simulations. The generic patterns formed by 2D monolayer NaCl and LiCl nanostructures were also confirmed by ab initio MD simulations. The formation of 2D salt structures in dilute aqueous solution at room temperature is counterintuitive. Free energy calculations indicated that the unexpected spontaneous salt nucleation behavior can be attributed to the nanoscale confinement and strongly compressed hydration shells of ions.

Origin and control of ionic hydration patterns in nanopores

In order to permeate a nanopore, an ion must overcome a dehydration energy barrier caused by the redistribution of surrounding water molecules. The redistribution is inhomogeneous, anisotropic and strongly position-dependent, resulting in complex patterns that are routinely observed in molecular dynamics simulations. Here, we study the physical origin of these patterns and of how they can be predicted and controlled. We introduce an analytic model able to predict the patterns in a graphene nanopore in terms of experimentally accessible radial distribution functions, giving results that agree well with molecular dynamics simulations. The patterns are attributable to a complex interplay of ionic hydration shells with water layers adjacent to the graphene membrane and with the hydration cloud of the nanopore rim atoms, and we discuss ways of controlling them. Our findings pave the way to designing required transport properties into nanoionic devices by optimising the structure of the hydration patterns.

Heavy metal ion separation from industrial wastewater using stacked graphene Membranes: A molecular dynamics simulation study

In recent years, fluid transport through stacked graphene membranes has gained considerable attention due to its potential applications in water purification and desalination. Here, we are resorting to molecular dynamics (MD) simulations to elucidate the metal ion separation efficiency and mechanism of ion transport in stacked graphene membranes of varying interlayer spacing (d). Our simulation results show that metal ions, including Cd2+, Cu2+, Hg2+, and Pb2+, can permeate through wide membranes (d = 1.0 nm), but metal ion rejection is close to complete for the narrower nanochannels, irrespectively of the metal ion type. The increase of the interlayer spacing positively influences the permeance of the metal ions and water. The impact of the first hydration shell of the metal ions on ion rejection from membranes is significantly high, whereas the impact of the second hydration shell is considerably low. Even if the second hydration radius of the metal ions is greater than the half-width of the channel, the ions can pass through the membranes because of the loosely bonded water molecules in that hydration shell. Moreover, free energy analysis using steered molecular dynamics (SMD) simulation techniques show that free energy profiles of a metal atom in wider channels are almost the same, and free energy for the metal atom within the membrane channels is always higher compared to that for water. The free energy of the metal atom within the narrower channel is noticeably high, and that is the reason for complete ion rejection. To sum up, our results suggest that stacked graphene membranes with an interlayer spacing of 0.8 nm are the best for complete metal ion rejection and considerable permeation of water.

Selectivity of ion transport in narrow carbon nanotubes depends on the driving force due to drag or drive nature of their active hydration shells.

Molecular dynamics simulations have revealed the important roles of hydration shells of ions transported through ultrathin carbon nanotubes (CNTs). In particular, ions driven by electric fields tend to drag their hydration shells behind them, while for ions transported by pressure, their hydration shells can actively drive them. Given the different binding strengths of hydration shells to ions of different sizes, these active roles of hydration shells affect the relative entry rates and driving speeds of ions in CNTs.

Wettability alteration by Smart Water multi-ion exchange in carbonates: A molecular dynamics simulation study

Smart Water ions (Ca2+, Mg2+, and SO42−) have been found to alter the wettability of oil-wet calcite surfaces, however, the widely proposed multi-ion exchange mechanism has never been understood on a molecular scale. In this work, a new model calcite surface with steps and locally charged sites was developed, and its properties were studied by quantum chemical calculations and molecular dynamics simulations to investigate the origin of the oil-wetness and its reversal by the Smart Water ions at temperatures ranging from 300 to 400 K. The simulation results using the proposed surface model are consistent with experimental results reported in the literature, including the Smart Water ions acting as the potential determining ions, and the oil carboxylate adsorption causing oil-wetness. The wettability alteration involves initial surface binding of Ca2+, which facilitates adsorption of SO42−. The latter enables the detachment of the Ca2+-carboxylate complexes at elevated temperatures (>350 K) where the ionic hydration shells are weakened as suggested by the increased interaction energy from less than −100 to over −400 kJ/mol between Ca2+ and SO42−, rationalizing the observed temperature dependence of the Smart Water effect. Overall, the presented simulations with the newly developed model calcite surface provide important new insights into wettability alteration effect of Smart Water ions, which has important implications for the optimization of the injection brines for enhanced oil recovery.

How increasing pressure affects the ion hydration structure and shell properties at ambient temperature

This work deals with the effect of increasing pressure at 298.15 K on the structure and hydration shell properties of ions in infinitely diluted solutions. Results were obtained from NPT Monte Carlo simulations at various pressures, from 1 atm up to 8000 atm, for some alkali metal, alkaline earth and halide ions in TIP4P water. As pressure increases, the ion-O and the ion-H radial distribution functions (rdfs) are subjected to changes to differing extents depending on both the charge and the size of the ion. The first peak of the ion-O rdf is shifted to a shorter distance from the ion for Cs+, Br− and I−, while in the other cases there is no evidence of shortening. In contrast, for the alkaline earth ions, the most prominent effect is the decrease in the height of the first peak. Minima positions of the ion-O rdfs were used to define the first and the second hydration shells at a given pressure. Water dipole orientation with respect to the radial direction was examined, showing that at higher pressures the ion-dipole interaction becomes less attractive than at 1 atm. A much less favorable orientation was found for waters in the first shell of halide ions. Shell properties were computed from definite integrals of the ion-O rdfs, such as the coordination number and the shell contribution to the excess volume. For the first hydration shell, apart from Ca2+, there is a significant increase in the coordination number upon increasing pressure. This effect becomes more important the larger the ion size is and this is very significant for alkali metal and halide ions. In the case of I− the gap observed between 4 katm and 5 katm reflects the striking effect of increasing pressure on the shell definition. The coordination number remains almost constant when using an alternative boundary for the shell. This was suggested by the radial distribution of water-dipole orientations. Shell excess volume contributions are discussed by examining their dependence on pressure. Electrostriction is shown for the first shell, while the second shell's contribution to the excess volume is positive. At a higher pressure, the shell electrostrictive volume per water molecule is always less than at 1 atm. The greatest effect is shown for the first shell of alkaline earth. The effect of a different shell boundary is examined on the shell quantities of halide ions.

Binary structure and dynamics of the hydrogen bonds in the hydration shells of ions.

Ion-specific effects of cations (Li+, Na+, K+, Mg2+, Ca2+) and anions (F-, Cl-) on the hydrogen bond structure and dynamics of the coordination waters in the hydration shells have been studied using molecular dynamics simulations. Our simulations indicate that the hydrogen bonds between the first and second hydration shell waters show binary structural and dynamic properties. The hydrogen bond with a first shell water as the donor (HD) is strengthened, while those with a first shell water as the acceptor (HA) are weakened. For a hydrated anion, this binary effect reverses, but is less significant. This ion-specific binary effect correlates with the size and the valence of the ion, and is more significant for the strong kosmotropic ions of high charge density.