Solvation Shell Of Ions Research Articles

Elucidating the mechanism behind the infrared spectral features and dynamics observed in the carbonyl stretch region of organic carbonates interacting with lithium ions.

Ultrafast infrared spectroscopy has become a very important tool for studying the structure and ultrafast dynamics in solution. In particular, it has been recently applied to investigate the molecular interactions and motions of lithium salts in organic carbonates. However, there has been a discrepancy in the molecular interpretation of the spectral features and dynamics derived from these spectroscopies. Hence, the mechanism behind spectral features appearing in the carbonyl stretching region was further investigated using linear and nonlinear spectroscopic tools and the co-solvent dilution strategy. Lithium perchlorate in a binary mixture of dimethyl carbonate (DMC) and tetrahydrofuran was used as part of the dilution strategy to identify the changes of the spectral features with the number of carbonates in the first solvation shell since both solvents have similar interaction energetics with the lithium ion. Experiments showed that more than one carbonate is always participating in the lithium ion solvation structures, even at the low concentration of DMC. Moreover, temperature-dependent study revealed that the exchange of the solvent molecules coordinating the lithium ion is not thermally accessible at room temperature. Furthermore, time-resolved IR experiments confirmed the presence of vibrationally coupled carbonyl stretches among coordinated DMC molecules and demonstrated that this process is significantly altered by limiting the number of carbonate molecules in the lithium ion solvation shell. Overall, the presented experimental findings strongly support the vibrational energy transfer as the mechanism behind the off-diagonal features appearing on the 2DIR spectra of solutions of lithium salt in organic carbonates.

Effect of temperature on concentrated electrolytes for advanced lithium ion batteries.

Salt-concentrated electrolytes are emerging as promising electrolytes for advanced lithium ion batteries (LIBs) that can offer high energy density and improved cycle life. To further improve these electrolytes, it is essential to understand their inherent behavior at various operating conditions of LIBs. Molecular dynamics (MD) simulations are extensively used to study various properties of electrolytes and explain the associated molecular-level phenomena. In this study, we use classical MD simulations to probe the properties of the concentrated electrolyte solution of 3 mol/kg lithium hexafluorophosphate (LiPF6) salt in the propylene carbonate solvent at various temperatures ranging from 298 to 378K. Our results reveal that the properties such as ionic diffusivity and molar conductivity of a concentrated electrolyte are more sensitive to temperature compared to that of dilute electrolytes. The residence time analysis shows that temperature affects the Li+ ion solvation shell dynamics significantly. The effect of temperature on the transport and dynamic properties needs to be accounted carefully while designing better thermal management systems for batteries made with concentrated electrolytes to garner the advantages of these electrolytes.

Solvation shell dynamics explains charge transport characteristics of LIB electrolytes

Electrolytes made up of LiPF6 salt in various mixtures of carbonate solvents such as ethylene carbonate (EC) and dimethyl carbonate (DMC) are generally used in lithium ion batteries (LIBs). The electrolyte composed of EC and DMC in the ratio of 3:7 exhibits peak ionic conductivity. Despite its wide usage in LIBs, the reason for observed peak is not well established. We probe 1 M LiPF6 electrolytes of various EC:DMC solvent compositions ranging from 1:9 to 9:1 by moles using the molecular dynamics method to find that electrolyte of EC:DMC = 3:7 offers highest ionic conductivity. Though bulk properties such as viscosity and dielectric constant, and various structural properties vary with composition of the solvents, these properties do not explain the observed peak in ionic conductivity. However, the residence times of solvent molecules in ionic solvation shells show their minima for 3:7 electrolyte. It suggests that faster dynamics of solvation shell of ions (frequent exchange of solvent molecules) results in better ionic conductivity for 3:7 electrolyte. Insights gained in the present work, viz. the relation between ionic conductivity and solvation time of ions can be of help in designing novel high-performance electrolytes for next generation LIBs.

Investigations on the Improved Electrochemical Performance of Layered Fe-V-O Kazakhstanite Phase By Using Superconcentrated Electrolytes

In order to develop high energy-density lithium-ion batteries, multi-electron redox reaction based cathode materials are being actively researched across the globe, such as those observed in vanadium and molybdenum based compounds. The vanadium based cathode materials provide significantly higher specific capacity in comparison to the other state of the art materials such as over-lithiated layered oxides (OLO), NMC, NCA etc. However, their drawback of possessing a lower nominal voltage of operation brings their energy density values at the same level as those observed for OLO.In this presentation, we investigate the electrochemical performance of one of the vanadium containing compounds, namely the Kazakhstanite phase (Fe5V15O39(OH)9.9H2O), as a cathode material for lithium-ion batteries. Kazakhstanite is a naturally occuring mineral found in Kazakhstan and United States. Preliminary structural characterization is performed to identify the microstructural, compositional and structural features of the as-synthesized material. Electrochemical investigation with the conventional electrolyte (1M LiPF6 in EC:DMC = 1:1 v:v, Code named: PL) reveals that the material delivers a specific capacity of ~350mAhg-1 between 1.5-3.8V (Li+/Li). However, the cycleability of the material is observed to be extremely poor with the conventional electrolyte. The cause is identified to be elemental dissolution of Fe and V from the active material with the help of EDX analysis of the cycled separators. It is observed that the active material dissolution is arrested to a great extent by changing the electrolyte system to a solvent-in-salt based one, without any additional engineering done on the material. By reducing the amount of solvent in the electrolyte, a solubility limit can be attained which prevents dissolution of active material. The electrochemical performance of the Kazakhstanite phase is evaluated with a series of electrolytes containing LiTFSI as the salt and DOL:DME = 1:1 v:v as the solvent, with varying concentration of the LiTFSI salt ranging from 1M to 7M. The cycleability of the Kazakhstanite phase with 7M LiTFSI in DOL:DME=1:1 v:v (Code named: OL) is found to be the best amongst all the other electrolyte compositions tested. Electrochemical impedance spectroscopy reveals that there should be a bilayer diffusion mechanism operational over the Lithium counter electrode. This indicates that the dissolved products form a passivation layer over the Lithium electrode apart from the SEI. The lithium-ion diffusion across the bilayer of the SEI and the passivation layer is observed to follow equation (1).Zmt ∝ Δc1(0,s)/ΔJ1(0,s) = [Λtanh(λu)+tanh(u)]/[mu(Λtanh(u)tanh(λu)+1)] (1)where:D1, L1 : Diffusion coefficient of Li-ion through the passivation layer and thickness of the passivation layerD2, L2 : Diffusion coefficient of Li-ion through the SEI layer and thickness of the SEI layerλ = √(D1/D2) (L2/L1)Λ = √(D1/D2)m = -D1/L1 u = √(s/D1) L1 with s being the complex variable jωThe proposed bilayer diffusion model, along with the experimental results, explains the superiority of superconcentrated electrolytes over conventional organic electrolyte in tuning the electrochemical characteristics of the SEI and the passivation layer.Molecular Dynamics Simulations are carried out to understand the co-ordination environment of the dissolved vanadium ions in the superconcentrated and conventional electrolytes. The simulated physico-chemical properties such as density, diffusion coefficient of the ions, and ionic conductivity match well with the values reported in the literature. For the superconcentrated electrolyte OL, it is observed from the simulations that the fraction of TFSI- is higher in the first solvation shell of vanadium ions in comparison to the one observed for PL case. The magnitude of the solvation energy of the vanadium ions, calculated using Free Energy Perturbation (FEP) and Finite-Difference Thermodynamic Integration (FDTI) methods, is found to be lower in superconcentrated electrolytes. This indicates that the dissolution of the active material in superconcentrated electrolyte is unfavorable in comparison to the conventional organic electrolyte. Additional EMD simulations are carried out to correlate the motion of the vanadium ions with respect to Li+, TFSI- and solvent molecules.Figure Caption: (a) Co-ordination environment of Vanadium ions (0.16M) inside PL electrolyte. (b) Co-ordination environment of Vanadium ions (0.16M) inside OL electrolyte Figure 1

Ion Association and Electrolyte Structure at Surface Films in Lithium-Ion Batteries

During lithium-ion battery charging and discharging, carbonate electrolytes degrade from redox side reactions to produce electrode surface films. The composition of these films depends on the composition of the electrolyte layer at the electrode surface and the structure of the ion solvation shells found therein. However, both the composition and structure of an electrolyte at an interface can vary significantly from their counterparts in the bulk, as reported previously at air and mineral surfaces. Hence, a circular relationship holds in which the surface films formed depend on the electrolyte structure, which in turn is impacted by the presence of the surface film. In this work, three impacts from solid interfaces on carbonate electrolytes are considered: ion accumulation, ion pairing, and solvent exchange dynamics. By considering these effects at four different surfaces of varying solvent affinities (LiF, Li2CO3, Li2EDC, and graphite), we explore the impact of solvent–surface interactions and ion–surface interactions on these interfacial behaviors. Classical molecular dynamics provides a route to explore the molecular structure at the electrolyte boundary, and two different electrolytes are considered to investigate the role of ion association on the accumulation, pairing, and dynamics. By considering the changes as a result of switching between one electrolyte with mostly solvent-separated ions to another with contact ion pairs, we provide evidence that both bulk ion association and the solvent–surface interaction are key descriptors of ion aggregation at the electrode surface. The insights from these simulations inform not only about the impacts of battery interfaces on the surrounding electrolyte but also on the origins of differences reported between classical molecular dynamics simulations at these interfaces.

Solvation-Driven Electrochemical Actuation.

We demonstrate a novel principle of contactless actuation for ionic membranes in salt solution based on solvation. Actuation is driven by differential swelling of the sides of the membrane, due to comigrating water in the solvation shells of mobile ions. We validate our theory through a series of experiments, which unravel a strong dependence of membrane deflection on the hydration numbers of mobile ions in the external solution and membrane. Our study suggests a critical role of solvation in the chemoelectromechanics of natural and artificial selectively permeable membranes.

Hopping in High Concentration Electrolytes - Long Time Bulk and Single-Particle Signatures, Free Energy Barriers, and Structural Insights.

Although ion-hopping is believed to be a significant mode of transport for small ions in liquid high concentration electrolytes (HCE), its bulk signatures over sufficiently long time intervals are yet to be shown. We computationally establish the long and short time imprints of hopping in HCEs using LiBF4-in-sulfolane mixtures as models. The high viscosity of this electrolyte leads to significant dynamic heterogeneity in Li-ion transport. Li-ions exhibit a preference to transit to previously occupied Li-ion-sites, bridged through anion or solvent molecules. Hopping in the liquid matrix was found to be an activated process, whose free energy barrier and transition state structure have been determined. Evidence for nanoscale compositional heterogeneity at high salt concentrations is also presented. The simulations shed light on the composition, stiffness, and lifetime of the solvation shell of Li ions. The understanding of HCEs gleaned from this study will spearhead the choice, engineering and applicability of this class of electrolytes.

Molecular motions of acetonitrile molecules in the solvation shell of lithium ions.

Lithium ion solutions in organic solvents have become ubiquitous because of their use in energy storage technologies. The widespread use of lithium salts has prompted a large scientific interest in elucidating the molecular mechanisms, giving rise to their macroscopic properties. Due to the complexity of these molecular systems, only few studies have been able to unravel the molecular motions and underlying mechanisms of the lithium ion (Li+) solvation shell. Lately, the atomistic motions of these systems have become somewhat available via experiments using ultrafast laser spectroscopies, such as two-dimensional infrared spectroscopy. However, the molecular mechanism behind the experimentally observed dynamics is still unknown. To close this knowledge gap, this work investigated solutions of a highly dissociated salt [LiTFSI: lithium bis(trifluoromethanesulfonyl)imide] and a highly associated salt (LiSCN: lithium thiocyanate) in acetonitrile (ACN) using both experimental and theoretical methods. Linear and non-linear infrared spectroscopies showed that Li+ is found as free ions and contact ion pairs in ACN/LiTFSI and ACN/LiSCN systems, respectively. In addition, it was also observed from the non-linear spectroscopy experiments that the dynamics of the ACN molecules in the Li+ first solvation shell has a characteristic time of ∼1.6 ps irrespective of the ionic speciation of the cation. A similar characteristic time was deducted from ab initio molecular dynamics simulations and density functional theory computations. Moreover, the theoretical calculations showed that molecular mechanism is directly related to fluctuations in the angle between Li+ and the coordinated ACN molecule (Li+⋯N≡C), while other structural changes such as the change in the distance between the cation and the solvent molecule (Li+⋯N) play a minor role. Overall, this work uncovers the time scale of the solvent motions in the Li+ solvation shell and the underlying molecular mechanisms via a combination of experimental and theoretical tools.

Molecular dynamics and continuum analyses of the electrokinetic zeta potential in nanostructured slit channels

This work presents a theoretical and numerical study of electrokinetic flow and the zeta potential for the case of slit channels with nanoscale surface features of dimensions comparable to the Debye length, by employing molecular dynamics simulations and continuum-level analyses. A simple analytical model for considering the average effect of such surface features producing nanoscale roughness is proposed by employing matched asymptotic solutions for the charge density and fluid flow field, and matching conditions that satisfy electroneutrality and the Onsager reciprocal relation between the electroosmotic flow rate and streaming current. The proposed analytical model quantitatively accounts for results from molecular dynamics simulations that consider the presence of ion solvation shells and surface hydration layers. Our analysis indicates that a simultaneous knowledge of the electroosmotic and pressure-driven flow rate or streaming current can be instrumental to unambiguously determine the zeta potential in the presence of nanoscale surface features .

ReaxFF molecular dynamics simulations of electrolyte-water systems at supercritical temperature.

We have performed ReaxFF molecular dynamics simulations of alkali metal-chlorine pairs in different water densities at supercritical temperature (700 K) to elucidate the structural and dynamical properties of the system. The radial distribution function and the angular distribution function explain the inter-ionic structural and orientational arrangements of atoms during the simulation. The coordination number of water molecules in the solvation shell of ions increases with an increase in the radius of ions. We find that the self-diffusion coefficient of metal ions increases with a decrease in density under supercritical conditions due to the formation of voids within the system. The hydrogen bond dynamics has been interpreted by the residence time distribution of various ions, which shows Li+ having the highest water retaining capability. The void distribution within the system has been analyzed by using the Voronoi polyhedra algorithm providing an estimation of void formation within the system at high temperatures. We observe the formation of salt clusters of Na+ and K+ at low densities due to the loss of dielectric constants of ions. The diffusion of ions gets altered dramatically due to the formation of voids and nucleation of ions in the system.

Density functional theory based molecular dynamics study of solution composition effects on the solvation shell of metal ions.

We present an ab initio molecular dynamics study of the alkali metal ions Li+, Na+, K+ and Cs+, and of the alkaline earth metal ions Mg2+ and Ca2+ in both pure water and electrolyte solutions containing the counterions Cl- and SO42-. Simulations were conducted using different density functional theory methods (PBE, BLYP and revPBE), with and without the inclusion of dispersion interactions (-D3). Analysis of the ion-water structure and interaction strength, water exchange between the first and second hydration shell, and hydrogen bond network and low-frequency reorientation dynamics around the metal ions have been used to characterise the influence of solution composition on the ionic solvation shell. Counterions affect the properties of the hydration shell not only when they are directly coordinated to the metal ion, but also when they are at the second coordination shell. Chloride ions reduce the sodium hydration shell and expand the calcium hydration shell by stabilizing under-coordinated hydrated Na(H2O)5+ complexes and over-coordinated Ca(H2O)72+. The same behaviour is observed in CaSO4(aq), where Ca2+ and SO42- form almost exclusively solvent-shared ion pairs. Water exchange between the first and second hydration shell around Ca2+ in CaSO4(aq) is drastically decelerated compared with the simulations of the hydrated metal ion (single Ca2+, no counterions). Velocity autocorrelation function analysis, used to probe the strength of the local ion-water interaction, shows a smoother decay of Mg2+ in MgCl2(aq), which is a clear indication of a looser inter-hexahedral vibration in the presence of chloride ions located in the second coordination shell of Mg2+. The hydrogen bond statistics and orientational dynamics in the ionic solvation shell show that the influence on the water-water network cannot only be ascribed to the specific cation-water interaction, but also to the subtle interplay between the level of hydration of the ions, and the interactions between ions, especially those of opposite charge. As many reactive processes involving solvated metal ions occur in environments that are far from pure water but rich in ions, this computational study shows how the solution composition can result in significant differences in behaviour and function of the ionic solvation shell.

"Ion Solvation Spectra": Free Energy Analysis of Solvation Structures of Multivalent Cations in Aprotic Solvents.

Using advanced molecular dynamics free energy sampling techniques-both classical and ab initio-we analyze the solvation structures of multivalent cations in aprotic solvents. In contrast to previous studies of mono- and bivalent ions in organic solvents, mainly performed using hybrid cluster-continuum quantum chemistry calculations that rely on the assumption of uniqueness of ion solvation free energies, here we find that monatomic bivalent cations may have multiple well-defined minima, as previously reported only for water, or plateaus of free energy with respect to the ion-solvent coordination. These observations are generalized in the concept of the "ion solvation spectrum" to highlight the rich phenomenology related to ion solvation as opposed to the normally expected free energy profiles with a single coordination minimum. Specifically, we show that a single chemical species may exhibit a multiplicity of distinctly different electrochemical properties. Using one- and two-dimensional projections of the free energy landscape, we analyze the stability of ion solvation structures and reveal minimum free energy pathways for ion (de-)solvation with low-dimensional approximations to associated kinetic barriers. Unexpectedly, we show that in some cases the process of opening the first ion solvation shell, by removing a solvent molecule, may actually drive the ion into a free energy basin with a higher coordination number. Our study highlights some deficiencies of conventional methodologies for studying ion solvation as a path to determine redox potentials and provides experimentally testable predictions.

Solvation in ionic liquid-water mixtures: A computational study

Solvation in mixtures of 1-ethyl-3-methylimidazolium acetate ionic liquid with water in a molar concentration of 5% is studied by means of atomistic molecular dynamics (MD) simulations. We employed a spherical coarse-grained parameterization to describe a model solute and we performed MD simulations with positively charged, negatively charged and neutral model spheres and compare the results with those previously reported in pure ionic liquids. Our study reveals that water does not significantly perturb the structure of the electric double layer neither the orientation of ions in the neighborhood of the solute particle. Although water does not seem to play any significant role in the solvation of neutral solutes, there is an excess of water in the first and second ionic solvation shell of both charged solutes, respectively. We also studied the correlation between the electric field around the negatively charged model solute and the orientation of the dipole moment of water molecules, finding that, although the dynamic properties of water molecules are almost unaltered in the presence of the solutes, and the ionic cage around the central solutes is only slightly affected upon water addition, the mobility of the spheres is drastically enhanced.

Enhancing the Selectivity between Oxygen and Chlorine towards Chlorine during the Anodic Chlorine Evolution Reaction on a Dimensionally Stable Anode

AbstractThe selectivity of the chlorine evolution reaction over the oxygen evolution reaction during the electrolysis of aqueous NaCl is, despite being very high, still insufficient to prevent expensive separation of the formed Cl2 and O2 by means of liquefaction. We hypothesize that, by decreasing the local activity of H2O near the anode surface by substantially increasing the ionic strength of the electrolyte, the oxygen evolution reaction would be suppressed, leading concomitantly to a higher selectivity of Cl2 over O2 formation. Hence, the influence of the ionic strength on the competition between electrochemical evolution of O2 and Cl2 at dimensionally stable anodes (DSAs) was investigated. Addition of a high concentration of NaNO3, an inert electrolyte additive, increases the selectivity for chlorine at high current density, as determined by means of online electrochemical mass spectrometry and UV‐vis spectroscopy. We propose conditions in which free water is suppressed, owing to under‐coordination of the solvation shells of ions, as a general concept to modulate the selectivity of competing electrochemical reactions.

Probing Electric Double-Layer Composition via in Situ Vibrational Spectroscopy and Molecular Simulations.

At an electrode, ions and solvent accumulate to screen charge, leading to a nanometer-scale electric double layer (EDL). The EDL guides electrode passivation in batteries, while in (super)capacitors, it determines charge storage capacity. Despite its importance, quantification of the nanometer-scale and potential-dependent EDL remains a challenging problem. Here, we directly probe changes in the EDL composition with potential using in situ vibrational spectroscopy and molecular dynamics simulations for a Li-ion battery electrolyte (LiClO4 in dimethyl carbonate). The accumulation rate of Li+ ions at the negative surface and ClO4- ions at the positive surface from vibrational spectroscopy compares well to that predicted by simulations using a polarizable APPLE&P force field. The ion solvation shell structure and ion-pairing within the EDL differs significantly from the bulk, especially at the negative electrode, suggesting that the common rationalization of interfacial electrochemical processes in terms of bulk ion solvation should be applied with caution.

Vibration Spectral Dynamics of Weakly Coordinating Water Molecules near an Anion: FPMD Simulations of an Aqueous Solution of Tetrafluoroborate.

The extent to which the ions affect the nearby water molecules will decide the structure-making or breaking nature of those ions in aqueous solutions. The effects of a weakly coordinating anion on the structure, dynamics, and vibrational properties of water molecules are not so significant as compared to an anion capable of making strong ion-water hydrogen bonds. The present work deals with the first-principles molecular dynamics study of an aqueous solution of such a weakly coordinating anion, tetrafluoroborate (BF4-), using dispersion-corrected DFT-based first-principles molecular dynamics (FPMD) simulations. Various structural, dynamical, and spectral properties, such as radial distribution functions (RDFs), rotational dynamics, vibrational density of states (VDOS), hydrogen bond as well as dangling OH autocorrelation functions, and residence dynamics, were calculated to investigate the effects of the anion on nearby water molecules. The process of spectral diffusion was assessed through a time series wavelet transformation of trajectories obtained from FPMD simulations. The first ion-water solvation shell extends up to 5.5 Å, containing around 20 water molecules. The lifetime of the ion-water hydrogen bond is found to be 1.19 ps, whereas the water-water hydrogen bond lifetime is found to be 1.13 ps. Inside the solvation shell, the persistence time of dangling OH chromophores and the average frequency of OH modes inside the solvation shell are found to be more compared to bulk. Three time scales are found for solvation shell OH modes from the frequency-frequency correlation function. A very short time scale is found for the intact ion-water interaction; the short time scale is for the ion-water hydrogen bond, and the long time scale is for escape dynamics of water molecules from the ion solvation shell. From the mean squared displacement, it is found that solvation water molecules diffuse slower than the bulk. However, solvation shell water molecules show faster relaxation from the analysis of rotational anisotropy. Within the longer time scale of spectral diffusion, this process (which is related to various dynamics of the molecules) is not yet complete, as compared to fast anisotropic decay. This fact is similar to the experimental finding of spectral diffusion and anisotropy time scales in the aqueous solution of borohydride anion. The calculated results are also compared with available experimental data wherever possible.

Ionized water confined in graphene nanochannels.

When confined between graphene layers, water behaves differently from the bulk and exhibits unusual properties such as fast water flow and ordering into a crystal. The hydrogen-bonded network is affected by the limited space and by the characteristics of the confining walls. The presence of an extraordinary number of hydronium and hydroxide ions in narrow channels has the following effects: (i) they affect water permeation through the channel, (ii) they may interact with functional groups on the graphene oxide surface and on the edges, and (iii) they change the thermochemistry of water, which are fundamentally important to understand, especially when confined water is subjected to an external electric field. Here we study the physical properties of water when confined between two graphene sheets and containing hydronium and hydroxide. We found that: (i) there is a disruption in the solvation structure of the ions, which is also affected by the layered structure of confined water, (ii) hydronium and hydroxide occupy specific regions inside the nanochannel, with a prevalence of hydronium (hydroxide) ions at the edges (interior), and (iii) ions recombine more slowly in confined systems than in bulk water, with the recombination process depending on the channel height and commensurability between the size of the molecules and the nanochannel height - a decay of 20% (40%) in the number of ions in 8 ps is observed for a channel height of h = 7 Å (bulk water). Our work reveals distinctive properties of water confined in a nanocapillary in the presence of additional hydronium and hydroxide ions.

Renormalization of Ionic Solvation Shells in Nanochannels.

Recently, experimental studies on selective ion transport across nanoporous membranes or through single nanochannels have unveiled interesting behaviors of dissolved ions under nanoconfinement. However, the exploration was limited by the resolution of experimental characterization. In this work, we present an atomistic simulation-based study, showing how the nanoconfinement and surface functionalization of graphene and graphene oxide nanochannels renormalize the solvation of ions (Na+, K+, Mg2+, Ca2+, Cl-). We find that the spatial distribution of dissolved ions demonstrates a layered order in nanochannels. The 1st hydration shell structures of cations are well defined in channels with width beyond ∼1.0 nm, although the rotational degree of freedom is constrained, while the 2nd hydration shells could be destructed. In the graphene oxide nanochannels, oxygen-containing functional groups can participate in the hydration shells of univalent ions but not for the divalent ions, and the valence-dependent reduction in the ionic diffusivity offers good selectivity between the divalent and univalent ions with the interlayer spacing of ∼1.0 nm, which is absent in the graphene nanochannels. With these findings, we conclude that the assessment of permeability and selectivity of ions has to take the renormalized nature of ionic solvation shells into account in the design of nanoporous membranes or nanofluidic devices for energy and environmental applications.

Water and salt dynamics in multilayer graphene oxide (GO) membrane: Role of lateral sheet dimensions

Dependence of salt rejection efficiency and water permeability of layered graphene oxide (GO) membranes on the lateral dimension of constituting sheets are studied through equilibrium molecular dynamic (MD) simulation and experiments. This study suggests that with increasing sheets dimension permeability of the GO membranes decreases but its selectivity increases. The velocity and permeation time of the water molecules while permeating through the membrane are influenced to a greater extent by the pore offset distance (W) of the membranes. More over the larger pore offset distance increases the path length that the water molecules and ions have to traverse for permeating through the layered GO membranes. Based on the simple technique discussed in this work, one can construct GO membranes of required water permeability and salt rejection without the application of any foreign nanomaterials with the GO membrane, which retains the inherent selectivity of the GO membranes. This work also provides the effect of internal structure of GO membrane on the atomistic level details of the solvation shell of ions while they are permeating through the membrane.

Evidence for Reduced Hydrogen-Bond Cooperativity in Ionic Solvation Shells from Isotope-Dependent Dielectric Relaxation.

We find that the reduction in dielectric response (depolarization) of water caused by solvated ions is different for H_{2}O and D_{2}O. This isotope dependence allows us to reliably determine the kinetic contribution to the depolarization, which is found to be significantly smaller than predicted by existing theory. The discrepancy can be explained from a reduced hydrogen-bond cooperativity in the solvation shell: we obtain quantitative agreement between theory and experiment by reducing the Kirkwood correlation factor of the solvating water from 2.7 (the bulk value) to ∼1.6 for NaCl and ∼1 (corresponding to completely uncorrelated motion of water molecules) for CsCl.