Ion association in concentrated NaCI brines from ambient to supercritical conditions: results from classical molecular dynamics simulations

Accounting for Ion Pairing Effects on Sulfate Salt Sorption in Cation Exchange Membranes

Ion exchange membranes (IEMs) are frequently used in water treatment and electrochemical applications, with their ion separation properties largely governed by equilibrium ion partitioning between a membrane and contiguous solution. Despite an expansive literature on IEMs, the influence of electrolyte association (i.e., ion pairing) on ion sorption remains relatively unexplored. In this study, salt sorption in two commercial cation exchange membranes equilibrated with 0.01-1.0 M MgSO₄ and Na₂SO₄ is investigated experimentally and theoretically. Association measurements of salt solutions using conductometric experiments and the Stokes-Einstein approximation show significant concentrations of ion pairs in MgSO₄ and Na₂SO₄ relative to those in simple electrolytes (i.e., NaCl), which is consistent with prior studies of sulfate salts. The Manning/Donnan model, developed and validated for halide salts in previous studies, substantially underpredicts sulfate sorption measurements, presumably due to ion pairing effects not accounted for in this established theory. These findings suggest that ion pairing can enhance salt sorption in IEMs due to partitioning of reduced valence species. By reformulating the Donnan and Manning models, a theoretical framework for predicting salt sorption in IEMs that explicitly considers electrolyte association is developed. Remarkably, theoretical predictions of sulfate sorption are improved by over an order of magnitude by accounting for ion speciation. In some cases, good quantitative agreement is observed between theoretical and experimental values for external salt concentrations between 0.1 and 1.0 M using no adjustable parameters.

Sodium site occupancy and phosphate speciation in natrophosphate are invariant to changes in NaF and Na3PO4 concentration

Analysis of multimodal characterization of Natrophosphate suggests that the crystalline structure is preserved across a range of synthesis conditions.

Ion association in hydrothermal aqueous NaCl solutions: implications for the microscopic structure of supercritical water

Knowledge of the microscopic structure of fluids and changes thereof with pressure and temperature is important for the understanding of chemistry and geochemical processes. In this work we investigate the influence of sodium chloride on the hydrogen-bond network in aqueous solution up to supercritical conditions. A combination of in situ X-ray Raman scattering and ab initio molecular dynamics simulations is used to probe the oxygen K-edge of the alkali halide aqueous solution in order to obtain unique information about the oxygen's local coordination around the ions, e.g. solvation-shell structure and the influence of ion pairing. The measured spectra exhibit systematic temperature dependent changes, which are entirely reproduced by calculations on the basis of structural snapshots obtained via ab initio molecular dynamics simulations. Analysis of the simulated trajectories allowed us to extract detailed structural information. This combined analysis reveals a net destabilizing effect of the dissolved ions which is reduced with rising temperature. The observed increased formation of contact ion pairs and occurrence of larger polyatomic clusters at higher temperatures can be identified as a driving force behind the increasing structural similarity between the salt solution and pure water at elevated temperatures and pressures with drawback on the role of hydrogen bonding in the hot fluid. We discuss our findings in view of recent results on hot NaOH and HCl aqueous fluids and emphasize the importance of ion pairing in the interpretation of the microscopic structure of water.

Solvation of simple ions in water at extreme conditions

The interaction of ions and water at high pressure and temperature plays a critical role in Earth and planetary science yet remains poorly understood. Aqueous fluids affect geochemical properties ranging from water phase stability to mineral solubility and reactivity. Here, we report first-principles molecular dynamics simulations of mono-valent ions (Li⁺, K⁺, Cl^-) as well as NaCl in liquid water at temperatures and pressures relevant to the Earth's upper mantle (11 GPa, 1000 K) and concentrations in the dilute limit (0.44-0.88 m), in the regime of ocean salinity. We find that, at extreme conditions, the average structural and vibrational properties of water are weakly affected by the presence of ions, beyond the first solvation shell, similar to what was observed at ambient conditions. We also find that the ionic conductivity of the liquid increases in the presence of ions by less than an order of magnitude and that the dielectric constant is moderately reduced by at most ∼10% at these conditions. Our findings may aid in the parameterization of deep earth water models developed to describe water-rock reactions.

NaCl aggregation in water at elevated temperatures and pressures: Comparison of classical force fields

The properties of water vary dramatically with temperature and density. This can be exploited to control its effectiveness as a solvent. Thus, supercritical water is of keen interest as solvent in many extraction processes. The low solubility of salts in lower density supercritical water has even been suggested as a means of desalination. The high temperatures and pressures required to reach supercritical conditions can present experimental challenges during collection of required physical property and phase equilibria data, especially in salt-containing systems. Molecular simulations have the potential to be a valuable tool for examining the behavior of solvated ions at these high temperatures and pressures. However, the accuracy of classical force fields under these conditions is unclear. We have, therefore, undertaken a parametric study of NaCl in water, comparing several salt and water models at 200 bar-600 bar and 450 K-750 K for a range of salt concentrations. We report a comparison of structural properties including ion aggregation, hydrogen bonding, density, and static dielectric constants. All of the force fields qualitatively reproduce the trends in the liquid phase density. An increase in ion aggregation with decreasing density holds true for all of the force fields. The propensity to aggregate is primarily determined by the salt force field rather than the water force field. This coincides with a decrease in the water static dielectric constant and reduced charge screening. While a decrease in the static dielectric constant with increasing NaCl concentration is consistent across all model combinations, the salt force fields that exhibit more ionic aggregation yield a slightly smaller dielectric decrement.

Surface Energies and Structure of Salt-Brine Interfaces

Permeability of salt formations is controlled by the equilibrium between the salt-brine and salt-salt interfaces described by the dihedral angle, which can change with the composition of the intergranular brine. Here, classical molecular dynamics (MD) simulations were used to investigate the structure and properties of the salt-brine interface to provide insight into the stability of salt systems. Mixed NaCl-KCl brines were investigated to explore differences in ion size on the surface energy and interface structure. Nonlinearity was noted in the salt-brine surface energy with increasing KCl concentration, and the addition of 10% KCl increased surface energies by 2-3 times (5.0 M systems). Size differences in Na⁺ and K⁺ ions altered the packing of dissolved ions and water molecules at the interface, impacting the surface energy. Additionally, ions at the interface had lower numbers of coordinating water molecules than those in the bulk and increased hydration for ions in systems with 100% NaCl or 100% KCl brines. Ultimately, small changes in brine composition away from pure NaCl altered the structure of the salt-brine interface, impacting the dihedral angle and the predicted equilibrium permeability of salt formations.

Effect of interionic interactions on the structure and dynamics of ionic solvation shells in aqueous electrolyte solutions

Molecular dynamics (MD) simulations to explore the structure and dynamics of the ionic solvation shell of alkali ions and halide ions in aqueous solution.

Mean ionic activity coefficients in aqueous NaCl solutions from molecular dynamics simulations

The mean ionic activity coefficients of aqueous NaCl solutions of varying concentrations at 298.15 K and 1 bar have been obtained from molecular dynamics simulations by gradually turning on the interactions of an ion pair inserted into the solution. Several common non-polarizable water and ion models have been used in the simulations. Gibbs-Duhem equation calculations of the thermodynamic activity of water are used to confirm the thermodynamic consistency of the mean ionic activity coefficients. While the majority of model combinations predict the correct trends in mean ionic activity coefficients, they overestimate their values at high salt concentrations. The solubility predictions also suffer from inaccuracies, with all models underpredicting the experimental values, some by large factors. These results point to the need for further ion and water model development.

Charge and electric field fluctuations in aqueous NaCl electrolytes

Crystalloluminescence, the long-lived emission of visible light during the crystallization of certain salts, was first observed over 200 years ago; however, the origin of this luminescence is still not well understood. The observations suggest that the process of crystallization may not be purely classical but also involves an essential electronic structure component. Strong electric field fluctuations may play an important role in this process by providing the necessary driving force for the observed electronic structure changes. The main objective of this work is to provide a basic understanding of the fluctuations in charge, electric potentials, and electric fields for concentrated aqueous NaCl electrolytes. Our charge analysis reveals that the water molecules in the first solvation shell of the ions serve as a sink for electron density originating on Cl(-). We find that the electric fields inside aqueous electrolytes are extremely large (up to several V/Å) and thus may alter the ground and excited electronic states in the condensed phase. Furthermore, our results show that the potential and field distributions are largely independent of concentration. We also find the field component distributions to be Gaussian for the ions and non-Gaussian for the O and H sites (computed in the lab frame of reference), however, these non-Gaussian distributions are readily modeled via an orientationally averaged nonzero mean Gaussian plus a zero mean Gaussian. These calculations and analyses provide the first steps toward understanding the magnitude and fluctuations of charge, electric potentials, and fields in aqueous electrolytes and what role these fields may play in driving charge redistribution/transfer during crystalloluminescence.

Hydration water and bulk water in proteins have distinct properties in radial distributions calculated from 105 atomic resolution crystal structures

Water plays a critical role in the structure and function of proteins, although the experimental properties of water around protein structures are not well understood. The water can be classified by the separation from the protein surface into bulk water and hydration water. Hydration water interacts closely with the protein and contributes to protein folding, stability, and dynamics, as well as interacting with the bulk water. Water potential functions are often parametrized to fit bulk water properties because of the limited experimental data for hydration water. Therefore, the structural and energetic properties of the hydration water were assessed for 105 atomic resolution ( Collapse

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MESH Headings

• Crystallization
• Crystallography, X-Ray
• Proteins/chemistry
• Water/chemistry

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Grants

• P20 GM065762 NIGMS NIH HHS
• U01 GM062920 NIGMS NIH HHS
• R01 GM062920 NIGMS NIH HHS
• P20 GM065762-03 NIGMS NIH HHS
• R01 GM062920-09 NIGMS NIH HHS
• GM062920 NIGMS NIH HHS
• GM065762 NIGMS NIH HHS

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Affiliation(s)

Xianfeng Chen Department of Biology, Georgia State University, Atlanta, Georgia 30302–4010, USA
Irene Weber Department of Biology, Georgia State University, Atlanta, Georgia 30302–4010, USA
Robert W. Harrison Department of Biology, Georgia State University, Atlanta, Georgia 30302–4010, USA Department of Computer Science, Georgia State University, Atlanta, Georgia 30302–4010, USA

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Computer simulation of ion cluster speciation in concentrated aqueous solutions at ambient conditions

Dynamic simulations are used to investigate ion cluster formation in unsaturated aqueous NaCl at 25 degrees C. Statistical, structural, and dynamic properties are reported. An effort is made to identify general behaviors that are expected to hold beyond the limitations of the force field. Above approximately 1 M, clusters with more than ten ions begin to form after approximately 10-20 ns of simulation time, but no evidence of irreversible ion aggregation is observed. Cluster survival times are estimated, showing that the kinetics become increasingly complex as salt is added, leading to multiple decay rates. Cluster dipole moment distributions show characteristic peaks that reflect the preferred conformations of clusters in solution. These are modulated by electrostatic and liquid-structure forces and are described in detail for clusters of up to five ions. For a given size and charge, the cluster morphology is independent of salt concentration. Below approximately 2 M, clusters affect the structure of water in their first hydration shells, so dipole moments parallel to the cluster macrodipoles are induced. These effects show a weak dependence with concentration below approximately 2 M, but vanish in the 2-3 M range. A possible connection with the structural transition recently suggested by NMR data in concentrated electrolytes is discussed. The effects of electrostatics on cluster speciation and morphology are discussed based on results from a set of simulations carried out with the ionic charges removed.

Morphology of ion clusters in aqueous electrolytes

Formation of ion clusters in aqueous NaCl solutions at 25 degrees C is investigated with dynamics simulations in the 0.1-3M concentration range. The medium is found to be highly structured even at moderate concentrations, and clusters of over 20 ions are observed above approximately 2M . The medium can be viewed as a multicomponent fluid, composed of reacting particles with well-defined average populations, shapes, sizes, and charge distributions. Clusters show substantial morphological variations that are here characterized by their hydrodynamic radii and internal charge distributions. Clusters can be described as prolate spheroids in the subnanometer length scale. The hydrodynamic radius and the radius of gyration follow simple power laws with the number of ions in the cluster. Dipole moment distributions show characteristic peaks in the approximately 10-60 debye range that reflect conformational preferences modulated by electrostatic and liquid-structure forces.

Structure and hydrogen bonding in liquid and supercritical aqueous NaCl solutions at a pressure of 1000 bar and temperatures up to 500 degrees C: A comprehensive experimental and computational study

The behavior of aqueous 1.1 M NaCl solution at a constant pressure of 1000 bar in the temperature range 25-500 degrees C has been studied with the use of IR absorption, Raman scattering, X-ray diffraction, and molecular dynamics (MD) simulations. The results are compared with the data for pure water under identical external conditions. The main purpose of the experimental and theoretical studies was to understand in what way an electrolyte dissolved in water influences the hydrogen bonding and structural features of water. As was found, the vibrational spectra show no essential difference between the properties of solution and pure water. However, the experimental pair correlation functions and the results of MD simulations present an evidence for very different nature of these substances. A characteristic feature of the structure of NaCl solution is a considerable contribution of strong O-H...Cl- bonds. As the temperature increases, the number of such bonds decreases partially due to a phenomenon of ion pairing, so that at high temperatures the properties of the solution become closer to the properties of water.

Quantitative characterization of ion pairing and cluster formation in strong 1:1 electrolytes

Aqueous solutions of 1:1 strong electrolytes are considered to be the prototype for complete ionic dissociation. Nonetheless, clustering of strong 1:1 electrolytes has been widely reported in all atom molecular dynamics simulations, and their presence is indirectly implicated in a diverse range of experimental results. Is there a physical basis for nonidealities such as ion pairing and cluster formation in aqueous solutions of strong 1:1 electrolytes? We attempt to answer this question by direct comparison of results from detailed molecular dynamics simulations to experimentally observed properties of 1:1 electrolytes. We report the analysis of a series of lengthy molecular dynamics simulations of alkali-halide solutions carried out over a wide range of physiologically relevant concentrations using explicit representations of water molecules. We find evidence for pronounced nonideal behavior of ions at all concentrations in the form of ion pairs and clusters which are in rapid equilibrium with dissociated ions. The phenomenology for ion pairing seen in these simulations is congruent with the multistep scheme proposed by Eigen and Tamm based on data from ultrasonic absorption experiments. For a given electrolyte, we show that the dependence of cluster populations on concentration can be described through a single set of equilibrium constants. We assess the accuracy of calculated ion pairing constants by favorable comparison to estimates obtained by Fuoss and co-workers and based on conductometric experiments. Ion pairs and clusters form on length scales where the size of individual water molecules is as important as the hard core radius of ions. Ion pairing results as a balance between the favorable Coulomb interactions and the unfavorable partial desolvation of ions needed to form a pair.

Kinetics of NaCl nucleation in supercritical water investigated by molecular dynamics simulations

At system pressures between 17 MPa and 25 MPa the nucleation and growth of NaCl nanoparticles in water at supercritical conditions was investigated by molecular dynamics simulations at different system temperatures and system densities. Our results show that particle formation takes place within a few hundred picoseconds after the jump from ambient to supercritical conditions. After nucleation a phase of growth by adding monomers is followed by growth via cluster-cluster collisions. We present results on the time development of distributions of cluster sizes, cluster compositions, and cluster temperatures as well as radial distribution functions and nucleation rates.

Local Order in Aqueous NaCl Solutions and Pure Water: X-ray Scattering and Molecular Dynamics Simulations Study

The microstructures of pure water and aqueous NaCl solutions over a wide range of salt concentrations (0-4 m) under ambient conditions are characterized by X-ray scattering and molecular dynamics (MD) simulations. MD simulations are performed with the rigid SPC water model as a solvent, while the ions are treated as charged Lennard-Jones particles. Simulated data show that the first peaks in the O...O and O...H pair correlation functions clearly decrease in height with increasing salt concentration. Simultaneously, the location of the second O...O peak, the signature of the so-called tetrahedral structure of water, gradually disappears. Consequently, the degree of hydrogen bonding in liquid water decreases when compared to pure fluid. MD results also show that the hydration number around the cation decreases as the salt concentration increases, which is most likely because some water molecules in the first hydration shell are occasionally substituted by chlorine. In addition, the fraction of contact ion pairs increases and that of solvent-separated ion pairs decreases. Experimental data are analyzed to deduce the structure factors and the pair correlation functions of each system. X-ray results clearly show a perturbation of the association structure of the solvent and highlight the appearance of new interactions between ions and water. A model of intermolecular arrangement via MD results is then proposed to describe the local order in each system, as deduced from X-ray scattering data.