Facilitation of Hydrate Dissociation and Structural Evolution by Major Marine Anions under Static Electric Fields

Electric fields have been proven to be capable of significantly affecting the equilibrium state of hydrates. In this study, the thermodynamic properties and structural changes of methane hydrate (MH) in various anion solutions in an electric field at 0.7 V/nm were investigated by molecular dynamics simulations. The presence of anions significantly enhances the instability of methane hydrates under electric fields, leading to a staged dissociation process. First, the anions coexist with MH to form a temporary metastable structure under the action of an electric field. Then, the migration of anions causes the dissociation of nearby hydrates and the formation of flow channels in the hydrate layer, which leads to the complete dissociation of MH after a period. The promotive effects of F-, Br-, I-, and Cl- ions were close, while SO42- was relatively weak. The anions are still in hydration shells in the MH phase, but the structure of the hydration shells differs slightly from that in solution (the coordination numbers of I- and SO42- ions increased). The migration resistances of multiple anions to cross the surface of the hydrate layer are similar. However, inside the hydrate phase, the anions with a larger radius have a higher migration resistance. It is difficult for SO42- ions to migrate inside the hydrate phase, and they tend to form a metastable structure on the hydrate surface. Combining our previous studies, SrCl2 solution has the best hydrate promotion under an electric field environment.

Novel Computational Chemistry Infrastructure for Simulating Astatide in Water: From Basis Sets to Force Fields Using Particle Swarm Optimization

Using the example of astatine, the heaviest naturally occurring halogen whose isotope At-211 has promising medical applications, we propose a new infrastructure for large-scale computational models of heavy elements with strong relativistic effects. In particular, we focus on developing an accurate force field for At- in water based on reliable relativistic density functional theory (DFT) calculations. To ensure the reliability of such calculations, we design novel basis sets for relativistic DFT, via the particle swarm optimization algorithm to optimize the coefficients of the new basis sets and the polarization-consistent basis set idea's extension to heavy elements to eliminate the basis set error from DFT calculations. The resulting basis sets enable the well-grounded evaluation of relativistic DFT against "gold-standard" CCSD(T) results. Accounting for strong relativistic effects, including spin-orbit interaction, via our redesigned infrastructure, we elucidate a noticeable dissimilarity between At- and I- in halide-water force field parameters, radial distribution functions, diffusion coefficients, and hydration energies. This work establishes the framework for the systematic development of polarization-consistent basis sets for relativistic DFT and accurate force fields for molecular dynamics simulations to be used in large-scale models of complex molecular systems with elements from the bottom of the periodic table, including actinides and even superheavy elements.

Anion-Induced Interfacial Liquid Layers on LiCoO2 in Salt-in-Water Lithium-Ion Batteries

The incompatibility of lithium intercalation electrodes with water has impeded the development of aqueous Li-ion batteries. The key challenge is protons which are generated by water dissociation and deform the electrode structures through intercalation. Distinct from previous approaches utilizing large amounts of electrolyte salts or artificial solid-protective films, we developed liquid-phase protective layers on LiCoO₂ (LCO) using a moderate concentration of 0.5∼3 mol kg^-1 lithium sulfate. Sulfate ion strengthened the hydrogen-bond network and easily formed ion pairs with Li⁺, showing strong kosmotropic and hard base characteristics. Our quantum mechanics/molecular mechanics (QM/MM) simulations revealed that sulfate ion paired with Li⁺ helped stabilize the LCO surface and reduced the density of free water in the interface region below the point of zero charge (PZC) potential. In addition, in situ electrochemical surface-enhanced infrared absorption spectroscopy (SEIRAS) proved the appearance of inner-sphere sulfate complexes above the PZC potential, serving as the protective layers of LCO. The role of anions in stabilizing LCO was correlated with kosmotropic strength (sulfate > nitrate > perchlorate > bistriflimide (TFSI^-)) and explained better galvanostatic cyclability in LCO cells.

Nucleation Rate Theory for Coordination Number: Elucidating Water-Mediated Formation of a Zigzag Na2SO4 Morphology

Predicting and controlling nanostructure formation during nucleation can pave the way to synthesizing novel energy materials via crystallization. However, such control over nucleation and crystallization remains challenging due to an inadequate understanding of critical factors that govern evolving atomistic structures and dynamics. Herein, we utilize coordination number as a reaction coordinate and rate theory to investigate how sodium sulfate, commonly known as a phase-change energy material, nucleates in a supersaturated aqueous solution. In conjunction with ab initio and force field-based molecular dynamics simulation, the rate theoretical analysis reveals that sodium sulfate from an initially dissolved metastable state transits to a heterogeneous mixture of prenucleated clusters and finally to a large cylindrical zigzag morphology. Measurements of Raman spectra and their ab initio modeling confirm that this nucleated morphology contains a few waters for every sulfate. Rate processes such as solvent exchange and desolvation exhibit high sensitivity to the evolving prenucleation/nucleation structures, providing a means to distinguish between critical nucleation precursors. Desolvation and forming the first-shell interionic coordination structure via monomer-by-monomer addition around sulfates are found to explain the formation of large nuclei. Thus, a detailed understanding of the step-by-step structure formation across scales has been achieved. This can be leveraged to predict nucleation-related structures and dynamics and potentially control the synthesis of novel phase-change materials for energy applications.

Revitalizing Iron Redox by Anion-Insertion-Assisted Ferro- and Ferri-Hydroxides Conversion at Low Alkalinity

Iron hydroxides are desirable alkaline battery electrodes for low cost and environmental beneficence. However, hydrogen evolution on charging and Fe₃O₄ formation on discharging cause low storage capacity and poor cycling life. We report that green rust (GR) (Fe²⁺₄Fe³⁺₂ (HO^-)₁₂SO₄), formed via sulfate insertion, promotes Fe(OH)₂/FeOOH conversion and shows a discharge capacity of ∼211 mAh g^-1 in half-cells and Coulombic efficiency of 93% after 300 cycles in full-cells. Theoretical calculations show that Fe(OH)₂/FeOOH conversion is facilitated by intercalated sulfate anions. Classical molecular dynamics simulations reveal that electrolyte alkalinity strongly impacts the energetics of sulfate solvation, and low alkalinity ensures fast transport of sulfate ions. Anion-insertion-assisted Fe(OH)₂/FeOOH conversion, also achieved with Cl^- ion, paves a pathway toward efficient utilization of Fe-based electrodes for sustainable applications.

The "good," the "bad," and the "hidden" in neutron scattering and molecular dynamics of ionic aqueous solutions

We characterize a concentrated 7.3 m CaCl₂ solution, combining neutron diffraction with chloride isotopic substitution (Cl-NDIS) in null water and molecular dynamics (MD) simulations. We elucidate the solution structure, thermodynamic properties, and extent of ion pairing previously suggested as concentration-dependent and often not observed at lower concentrations. Our Cl-NDIS measurements designate the solvent-shared ion pairing as dominant and the contact ion pairing (CIP) as insignificant even under conditions close to the solubility limit. The MD models parameterized against neutron diffraction with calcium isotopic substitution (Ca-NDIS) overestimate CIP despite successfully reproducing most of the Cl-NDIS signal. This drawback originates from the fact that Ca²⁺-Cl^- interactions were primarily "hidden" in the Ca-NDIS signal due to overlapping with Ca²⁺-O_w and Ca²⁺-H_w contributions to the total scattering. Contrary, MD models with moderate CIP and possessing generally good performance at high concentrations fail to reproduce the NDIS measurements accurately. Therefore, the electronic polarization, introduced in most of the recent MD models via scaling ionic charges, resolves some but not all parameterization drawbacks. We conclude that despite improving the quality of MD models "on average," the question "which model is the best" has not been answered but replaced by the question "which model is better for a given research." An overall "good" model can still be inappropriate or, in some instances, "bad" and, unfortunately, produce erroneous results. The accurate interpretation of several NDIS datasets, complemented by MD simulations, can prevent such mistakes and help identify the strengths, weaknesses, and convenient applications for corresponding computational models.

Probing the Electrode-Electrolyte Interface of a Model K-Ion Battery Electrode─The Origin of Rate Capability Discrepancy between Aqueous and Non-Aqueous Electrolytes

Li-ion batteries are the electrochemical energy storage technology of choice of today's electrical vehicles and grid applications with a growing interest for Na-ion and K-ion systems based on either aqueous or non-aqueous electrolyte for power, cost, and sustainable reasons. The rate capability of alkali-metal-ion batteries is influenced by ion transport properties in the bulk of the electrolyte, as well as by diverse effects occurring at the vicinity of the electrode and electrolyte interface. Therefore, identification of the predominant factor affecting the rate capability of electrodes still remains a challenge and requires suitable experimental and computational methods. Herein, we investigate the mechanistic of the K⁺ insertion process in the Prussian blue phase, Fe4III[FeII(CN)6]3 in both aqueous and non-aqueous electrolytes, which reveals drastic differences. Through combined electrochemical characterizations, electrochemical-quartz-crystal-microbalance and ac-electrogravimetric analyses, we provide evidences that what matters the most for fast ion transport is the positioning of the partially solvated cations adsorbed at the material surface in aqueous as opposed to non-aqueous electrolytes. We rationalized such findings by molecular dynamics simulations that establish the K⁺ repartition profile within the electrochemical double layer. A similar trend was earlier reported by our group for the aqueous versus non-aqueous insertion of Li⁺ into LiFePO₄. Such a study unveils the critical but overlooked role of the electrode-electrolyte interface in ruling ion transport and insertion processes. Tailoring this interface structuring via the proper salt-solvent interaction is the key to enabling the best power performances in alkali-metal-ion batteries.

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.

Free Energy Prediction of Ion-Induced Nucleation of Aqueous Aerosols

Ion-induced nucleation (IIN) is thought to be an important nucleation pathway of atmospheric aerosols. We present a combined polarizable molecular dynamics (MD) simulation and the classic ion-induced nucleation theory (IINT) approach to predict the free energy profiles of the ion-induced nucleation of aqueous aerosols in a qualitative or semiquantitative way. The dependence of both cluster structure and thermodynamic properties on cluster sizes and ion species is also systemically studied. It is confirmed the ions can significantly enhance the cluster stability, and thereby increase the nucleation rate. The ability of the common atmospheric ions to enhance the nucleation rate follows the order SO₄^2- > H₃O⁺ > NH₄⁺ > NO₃^-, coinciding with the order of their solvation free energies. Therefore, the solvation energy can be employed as a rough index for evaluating the INN ability. Overall, the consistency between the present predictions and previous experimental and theoretical observations demonstrates the combination of MD simulation and the IINT appears to be a promising approach for exploring the IIN process and understanding the microscopic mechanism of atmospheric-related ions.

Ab Initio Molecular Dynamics Study of Aqueous Solutions of Magnesium and Calcium Nitrates: Hydration Shell Structure, Dynamics and Vibrational Echo Spectroscopy

Ab initio molecular dynamics simulations are performed to study the hydration shell structure, dynamics, and vibrational echo spectroscopy of aqueous Mg(NO₃)₂ and Ca(NO₃)₂ solutions. The hydration shell structure is probed through calculations of various ion-ion and ion-water radial and spatial distribution functions. On the dynamical side, calculations have been made for the hydrogen bond dynamics of hydration shells and also residence dynamics and lifetimes of water in different solvation environments. Subsequently, we looked at the dynamics of frequency fluctuations of OD modes of heavy water in different hydration environments. Specifically, the temporal decay of spectral observables of two-dimensional infrared (2DIR) spectroscopy, three pulse echo peak shift (3PEPS) measurements and also of time correlations of frequency fluctuations are calculated to investigate the dynamics of vibrational spectral diffusion of water in different hydration environments in these solutions. The OD stretch frequencies of water molecules in the vicinity of both divalent cations are found to be red-shifted and also fluctuating at a slower rate than other water molecules present in the solutions. The Mg²⁺ ions are found to be strongly hydrated which can be linked to their lower tendency to form contact ion-pairs and essentially no water exchange between the cationic hydration shells and bulk during the time scale of the current simulations. The stronger hydration of Mg²⁺ ions make their hydration shells structurally and dynamically more rigid and make the dynamics of hydrogen bonds and vibrational spectral diffusion, as revealed through spectral observables of 2DIR and 3PEPS slower than that for the Ca²⁺ ions. The structural and spectral dynamics of water molecules outside the cationic solvation shells in the Mg(NO₃)₂ solution are also found to be relatively slower than that of the Ca(NO₃)₂ solution and pure water which show the effects of stronger electric fields of Mg²⁺ ions extending beyond their first hydration shells. Also, water molecules in the hydration shells of the NO₃^- ions are found to relax at a slower rate in the Mg(NO₃)₂ solution which manifests the effect countercations have on anionic hydration shells for divalent metal nitrate solutions.

Engineering the ligand specificity of the human galectin-1 by incorporation of tryptophan analogs

Galectin-1 is a β-galactoside-binding lectin with manifold biological functions. A single tryptophan residue (W68) in its carbohydrate binding site plays a major role in ligand binding and is highly conserved among galectins. To fine tune galectin-1 specificity, we introduced several non-canonical tryptophan analogs at this position of human galectin-1 and analyzed the resulting variants using glycan microarrays. Two variants containing 7-azatryptophan and 7-fluorotryptophan showed a reduced affinity for 3'-sulfated oligosaccharides. Their interaction with different ligands was further analyzed by fluorescence polarization competition assay. Using molecular modeling we provide structural clues that the change in affinities comes from modulated interactions and solvation patterns. Thus, we show that the introduction of subtle atomic mutations in the ligand binding site of galectin-1 is an attractive approach for fine-tuning its interactions with different ligands.

Surfactant-induced wettability reversal on oil-wet calcite surfaces: Experimentation and molecular dynamics simulations with scaled-charges

HYPOTHESIS

Surfactant flooding is the leading approach for reversing the wettability of oil-wet carbonate reservoirs, which is critical for the recovery of the remaining oil. Combination of molecular dynamics (MD) simulations with experiments on simplified model systems can uncover the molecular mechanisms of wettability reversal and identify key molecular properties for systematic design of new, effective chemical formulations for the enhanced oil recovery.

EXPERIMENTS/SIMULATIONS

Wettability reversal by a series of surfactant solutions was studied experimentally using contact angle measurements on aged calcite chips, and a novel MD simulation methodology with scaled-charges that provides superior description of the ionic interactions in aqueous solutions.

FINDINGS

The MD simulation results were in excellent agreement with the experiments. Cationic surfactants were the most effective in reversing the calcite wettability, resulting in complete detachment of the oil from the surface. Some nonionic surfactants also altered the wettability, but to a lesser degree, while the amphoteric and anionic surfactants had no effect. From the tested cationic surfactants, the double-tailed one was the least effective, but the experiments were inconclusive due to its poor solubility. Contributions of specific interactions to the wettability reversal process and implications for the design and optimization of surfactants for the enhanced oil recovery are discussed.

A practical guide to biologically relevant molecular simulations with charge scaling for electronic polarization

Molecular simulations can elucidate atomistic-level mechanisms of key biological processes, which are often hardly accessible to experiment. However, the results of the simulations can only be as trustworthy as the underlying simulation model. In many of these processes, interactions between charged moieties play a critical role. Current empirical force fields tend to overestimate such interactions, often in a dramatic way, when polyvalent ions are involved. The source of this shortcoming is the missing electronic polarization in these models. Given the importance of such biomolecular systems, there is great interest in fixing this deficiency in a computationally inexpensive way without employing explicitly polarizable force fields. Here, we review the electronic continuum correction approach, which accounts for electronic polarization in a mean-field way, focusing on its charge scaling variant. We show that by pragmatically scaling only the charged molecular groups, we qualitatively improve the charge-charge interactions without extra computational costs and benefit from decades of force field development on biomolecular force fields.

Ion-specific interactions at calcite-brine interfaces: a nano-scale study of the surface charge development and preferential binding of polar hydrocarbons

This research provides an atomic-level insight into the synergic contribution of mono- and divalent ions to interfacial characteristics of calcite surfaces exposed to electrolyte solution containing organic compounds. The emphasis was placed on the ionic interactions responsible for charge developing mechanisms of calcite surfaces and also the capacity for adsorption of polar hydrocarbons, represented by benzoic acid (BA), at different brine compositions. For this purpose, molecular dynamics (MD) simulations were employed to explore the interplay of the main constituent ions of natural brines (Na+, Cl-, Mg2+, and SO42-) and BA at the interface of CaCO3. It was observed that surface accumulation of Na+ cations produces a positively charged layer immediate to the basal plane of calcite, validating the typical positive surface charge of carbonates reported by laboratory experiences. Meanwhile, a negatively charged layer appears beyond the sodium layer as a result of direct and solvent-mediated pairing of anions with Na+ cations lodged on the calcite substrate. In this process, sulfate adsorption severely diminishes surface charge to even a negative value in the case of a SO42--rich solution, providing an interpretation for the measurements reported in the literature. Our results revealed the inhibition of direct binding of BA molecules onto the calcite surface through complexation with protruding oxygen atoms of basal carbonates by the residing Na+ cations. Further, we noticed the sulfate-mediated pairing of BA molecules to the Na+ layer, which in effect intensifies surface adsorption of BA. However, BA-SO42- interaction is considerably reduced by magnesium cations shielding sulfate sites in the Mg2+-augmented brine. The findings presented in this study are of fundamental importance to advance our microscopic understanding of interfacial interactions in brine/oil/carbonate systems; with broad scientific and applied implications in the context of mobilizing organic contaminants trapped in aquifer sediments and enhancement of hydrophilicity of subsurface oil-bearing carbonate reservoirs by injecting ion-modified brine solutions.

Interfacial and bulk properties of concentrated solutions of ammonium nitrate

We conducted molecular dynamics (MD) simulations to calculate the density and surface tension of concentrated ammonium nitrate (AN) solutions up to the solubility limit of ammonium nitrate in water, by combining the SPC/E, SPCE/F and TIP4P/2005 water models with OPLS model for ammonium and nitrate ions. This is the first time that the properties of concentrated solutions of nitrates, especially AN, have been studied by molecular dynamics. We effectively account for the polarisation effects by the electronic continuum correction (ECC), practically realised via rescaling of the ionic charges. We found that, the full-charge force field MD simulations overestimate the experimental results, as the ions experience repulsion from the interface and prefer to remain in the subsurface layer and the bulk solution. In contrast, reducing the ionic charges results in the behaviour that fits well with the experimental data. The nitrate anions display a greater propensity for the interface than the ammonium cations. We accurately predict both the density and the rise in the surface tension of concentrated solutions of AN, recommending TIP4P/2005 for water and the scaled-charge OPLS model (OPLS/ECC) for the ions in the solutions. We observe that, the adsorption of anions to the interface accompanies their depletion in the subsurface layer, which is preferentially occupied by cations, resulting in the formation of the electric double layer. We demonstrate the ion deficiency for up to 3 Å below the surface and establish the requirement to include the polarisability effects in the OPLS model for AN. While these results confirmed the findings of the previous studies for dilute solutions, they are new in the solubility limit. Concentrated solutions exhibit a strong effect of the abundance of solute on the coordination numbers of ions and on the degree of ion pairing. Surprisingly, ion pairing decreases significantly at the interface compared with the bulk. The present study identifies OPLS/ECC, along with TIP4P/2005, to yield accurate predictions of physical properties of concentrated AN, with precision required for industrial applications, such as a formulation of emulsion and fuel-oil explosives that now predominate the civilian use of AN. An application of this model will allow one to predict the surface properties of supersaturated solutions of AN which fall outside the capability of the present laboratory experiments but are important industrially.

Effects of Solvent Polarity on Li-ion Diffusion in Polymer Electrolytes: An All-Atom Molecular Dynamics Study with Charge Scaling

We herein report an all-atom molecular dynamics study on the role of solvent polarity for Li+ diffusion in polymer electrolytes using PEO-LiTFSI (poly(ethylene oxide)-lithium bis(trifluoromethane)sulfonimide) as a model system. By separating the effect of Tg and the effect of solvent polarity in our simulations, we show that the maximum in the diffusion coefficient of Li+ with respect to the dielectric constant of polymer solvent εp is due to transitions in the transport mechanism. In particular, it is found that the frequent interchain hopping involves the coordination of both PEO and TFSI. This optimal solvating ability of PEO at an intermediate value of εp leads to the fast ion conduction. These findings highlight the synergetic effect of solvent polarity and bond polarity on Li-ion diffusion in solid polymer electrolytes.

Two-Dimensional Infrared Spectroscopy of Aqueous Solutions of Metal Nitrates: Slowdown of Spectral Diffusion in the Presence of Divalent Cations

The hydrogen-bonded network of water can be affected both structurally and dynamically by the presence of ions. In the present study, we have considered three aqueous solutions of metal nitrates to investigate the effects of divalent cations (Mg²⁺ and Ca²⁺), compared to that of monovalent Na⁺ ions, on hydrogen-bond fluctuations and vibrational spectral diffusion through calculations of linear and two-dimensional infrared spectra of these solutions at room temperature. We have employed the methods of molecular dynamics simulations using effective polarizable models of ions combined with quantum mechanical calculations of transition variables and statistical mechanical calculations of spectral response functions of vibrational spectroscopy. Divalent cations are found to have much stronger and longer-ranged effects on the structure and dynamics of the hydrogen-bonded network than that induced by the monovalent sodium ions. The blue shifts in the calculated linear spectra are found to follow the Hofmeister trend for the cations. The 2D-IR spectral lineshape and intensity corresponding to three-pulse echo peak shift (3PEPS) experiments are calculated. The timescales of these nonlinear spectral responses and also frequency-time correlations show significant slowing down of spectral diffusion for solutions containing divalent Mg²⁺ and Ca²⁺ ions compared to the corresponding dynamics of the solution containing Na⁺ ions. Unlike NaNO₃ solution, the relaxation of frequency and dipole orientational fluctuations of anion-bound water in Mg(NO₃)₂ and Ca(NO₃)₂ solutions are found to be somewhat slower than bulk water, which can be attributed to the presence of divalent cations whose effects go beyond their first solvation shells. This is also seen in the dynamics of bulk water in these solutions which is found to be notably slower for the solutions containing divalent cations than that in the NaNO₃ solution. Unlike Mg²⁺ and Ca²⁺ ions, no specific cationic effect is observed for the Na⁺ ions.

Pressure in Molecular Simulations with Scaled Charges. 1. Ionic Systems

Charge scaling, rationalized as MDEC (molecular dynamics in electronic continuum) or ECC (electronic continuum correction), has become a widely used simple approach to how to avoid self-consistent induced dipoles yet approximately take into account the effects of electronic polarizability. It has been assumed that the continuum permittivity does not depend on density; in turn, pressure is calculated by standard formulas. In this work, we elaborate a complementary approximation of density-independent molecular polarizability and derive formulas for pressure corrections within the MDEC framework; real behavior lies between these two extremes. The pressure corrections for test ionic systems are huge and negative, leading to sizable densities in constant-pressure MDEC simulations. A comparison of MDEC results with equivalent polarizable systems gives a good pressure match for a crystal but very low MDEC pressures for ionic liquids. These results witness about the importance of a correct density dependence not only of continuum permittivity in MDEC simulations but also of polarizability in polarizable simulations.

Molecular simulation of liquid-vapor coexistence for NaCl: Full-charge vs scaled-charge interaction models

Scaled-charge models have been recently introduced for molecular simulations of electrolyte solutions and molten salts to attempt to implicitly represent polarizability. Although these models have been found to accurately predict electrolyte solution dynamic properties, they have not been tested for coexistence properties, such as the vapor pressure of the melt. In this work, we evaluate the vapor pressure of a scaled-charge sodium chloride (NaCl) force field and compare the results against experiments and a non-polarizable full-charge force field. The scaled-charge force field predicts a higher vapor pressure than found in experiments, due to its overprediction of the liquid-phase chemical potential. Reanalyzing the trajectories generated from the scaled-charge model with full charges improves the estimation of the liquid-phase chemical potential but not the vapor pressure.

Molecular modeling of aqueous electrolytes at interfaces: Effects of long-range dispersion forces and of ionic charge rescaling

Molecular dynamics simulations of aqueous electrolytes generally rely on empirical force fields, combining dispersion interactions-described by a truncated Lennard-Jones (LJ) potential-and electrostatic interactions-described by a Coulomb potential computed with a long-range solver. Recently, force fields using rescaled ionic charges [electronic continuum correction (ECC)], possibly complemented with rescaling of LJ parameters [ECC rescaled (ECCR)], have shown promising results in bulk, but their performance at interfaces has been less explored. Here, we started by exploring the impact of the LJ potential truncation on the surface tension of a sodium chloride aqueous solution. We show a discrepancy between the numerical predictions for truncated LJ interactions with a large cutoff and for untruncated LJ interactions computed with a long-range solver, which can bias comparison of force field predictions with experiments. Using a long-range solver for LJ interactions, we then show that an ionic charge rescaling factor chosen to correct long-range electrostatic interactions in bulk accurately describes image charge repulsion at the liquid-vapor interface, and the rescaling of LJ parameters in ECCR models-aimed at capturing local ion-ion and ion-water interactions in bulk- describes well the formation of an ionic double layer at the liquid-vapor interface. Overall, these results suggest that the molecular modeling of aqueous electrolytes at interfaces would benefit from using long-range solvers for dispersion forces and from using ECCR models, where the charge rescaling factor should be chosen to correct long-range electrostatic interactions.

Restructuring a Deep Eutectic Solvent by Water: The Nanostructure of Hydrated Choline Chloride/Urea

Deep eutectic mixtures are a promising sustainable and diverse class of tunable solvents that hold great promise for various green chemical and technological processes. Many deep eutectic solvents (DES) are hygroscopic and find use in applications with varying extents of hydration, hence urging a profound understanding of changes in the nanostructure of DES with water content. Here, we report on molecular dynamics simulations of the quintessential choline chloride-urea mixture, using a newly parametrized force field with scaled charges to account for physical properties of hydrated DES mixtures. These simulations indicate that water changes the nanostructure of solution even at very low hydration. We present a novel approach that uses convex constrained analysis to dissect radial distribution functions into base components representing different modes of local association. Specifically, DES mixtures can be deconvoluted locally into two dominant competing nanostructures, whose relative prevalence (but not their salient structural features) change with added water over a wide concentration range, from dry up to ∼30 wt % hydration. Water is found to be associated strongly with several DES components but remarkably also forms linear bead-on-string clusters with chloride. At high water content (beyond ∼50 wt % of water), the solution changes into an aqueous electrolyte-like mixture. Finally, the structural evolution of the solution at the nanoscale with extent of hydration is echoed in the DES macroscopic material properties. These changes to structure, in turn, should prove important in the way DES acts as a solvent and to its interactions with additive components.

Dynamics of Water in the Solvation Shell of an Iodate Ion: A Born-Oppenheimer Molecular Dynamics Study

The iodate ion has an anisotropic structure and charge distribution. It has a pyramidal shape with the iodine atom located at the peak of the pyramid. The water molecules interact differently with the positively charged iodine and the negatively charged oxygen atoms of this anion, giving rise to two distinct solvation shells. In the present study, we have performed ab initio Born-Oppenheimer molecular dynamics simulations to investigate the dynamics of water molecules in the iodine and oxygen solvation shells of the iodate ion and compared the behavior with those of the bulk. The dynamics of water is calculated for both the BLYP and the dispersion-corrected BLYP-D3 functionals at room temperature. The dynamics of water in the solvation shells at higher temperatures of 353 and 330 K has also been investigated for the BLYP and BLYP-D3 functionals, respectively. The hydrogen bond dynamics, vibrational spectral diffusion, orientational and translational diffusion, and residence dynamics of water molecules in the two solvation shells are looked at in the current study. The ion-water hydrogen bond dynamics is found to be somewhat faster than that for water-water hydrogen bonds in the bulk, which can be attributed to a ring-like electron distribution on the iodate oxygens. The dynamical trends are connected to the water structure making/breaking properties of the positively charged iodine and negatively charged oxygen sites of the anion. Furthermore, orientational jumps of the iodate ion and also those of surrounding water molecules which are hydrogen bonded to the oxygen atoms of the iodate ion are also investigated. It is found that the nature of these orientational jumps can be different from those reported earlier for planar polyoxyanions such as the nitrate ion.

On the transferability of ion parameters to the TIP4P/2005 water model using molecular dynamics simulations

Countless molecular dynamics studies have relied on available ion and water force field parameters to model aqueous electrolyte solutions. The TIP4P/2005 model has proven itself to be among the best rigid water force fields, whereas many of the most successful ion parameters were optimized in combination with SPC/E, TIP3P, or TIP4P/Ew water. Many researchers have combined these ions with TIP4P/2005, hoping to leverage the strengths of both parameter sets. To assess if this widely used approach is justified and to provide a guide in selecting ion parameters, we investigated the transferability of various commonly used monovalent and multivalent ion parameters to the TIP4P/2005 water model. The transferability is evaluated in terms of ion hydration free energy, hydration radius, coordination number, and self-diffusion coefficient at infinite dilution. For selected ion parameters, we also investigated density, ion pairing, chemical potential, and mean ionic activity coefficients at finite concentrations. We found that not all ions are equally transferable to TIP4P/2005 without compromising their performance. In particular, ions optimized for TIP3P water were found to be poorly transferable to TIP4P/2005, whereas ions optimized for TIP4P/Ew water provided nearly perfect transferability. The latter ions also showed good overall agreement with experimental values. The one exception is that no combination of ion parameters and water model considered here was found to accurately reproduce experimental self-diffusion coefficients. Additionally, we found that cations optimized for SPC/E and TIP3P water displayed consistent underpredictions in the hydration free energy, whereas anions consistently overpredicted the hydration free energy.

Polarizable MD simulations of ionic liquids: How does additional charge transfer change the dynamics?

A new model for treating charge transfer in ionic liquids is developed and applied to 1-ethyl-3-methylimidazolium tetrafluoroborate. The model allows for us to examine the roles of charge transfer, polarizability, and charge scaling effects on the dynamics of ionic liquids.

Charge Scaling Manifesto: A Way of Reconciling the Inherently Macroscopic and Microscopic Natures of Molecular Simulations

Electronic polarization effects play an important role in the interactions of charged species in biologically relevant aqueous solutions, such as those involving salt ions, proteins, nucleic acids, or phospholipid membranes. Explicit inclusion of electronic polarization in molecular modeling is tedious both from the point of view of force field parametrization and actual performance of the simulations. Therefore, the vast majority of biomolecular simulations is performed using nonpolarizable force fields, which can lead to artifacts such as dramatically overestimated ion pairing, particularly when polyvalent ions are involved. Here, we show that many of these issues can be remedied without extra computational costs by including electronic polarization in a mean field way via charge rescaling. We also lay the solid physical foundations of this approach and reconcile from this perspective the microscopic versus macroscopic natures of nonpolarizable force fields.

Accurate Biomolecular Simulations Account for Electronic Polarization

In this perspective, we discuss where and how accounting for electronic many-body polarization affects the accuracy of classical molecular dynamics simulations of biomolecules. While the effects of electronic polarization are highly pronounced for molecules with an opposite total charge, they are also non-negligible for interactions with overall neutral molecules. For instance, neglecting these effects in important biomolecules like amino acids and phospholipids affects the structure of proteins and membranes having a large impact on interpreting experimental data as well as building coarse grained models. With the combined advances in theory, algorithms and computational power it is currently realistic to perform simulations with explicit polarizable dipoles on systems with relevant sizes and complexity. Alternatively, the effects of electronic polarization can also be included at zero additional computational cost compared to standard fixed-charge force fields using the electronic continuum correction, as was recently demonstrated for several classes of biomolecules.

Dielectric Decrement for Aqueous NaCl Solutions: Effect of Ionic Charge Scaling in Nonpolarizable Water Force Fields

We investigate the dielectric constant and the dielectric decrement of aqueous NaCl solutions by means of molecular dynamic simulations. We thereby compare the performance of four different force fields and focus on disentangling the origin of the dielectric decrement and the influence of scaled ionic charges, as often used in nonpolarizable force fields to account for the missing dynamic polarizability in the shielding of electrostatic ion interactions. Three of the force fields showed excessive contact ion pair formation, which correlates with a reduced dielectric decrement. In spite of the fact that the scaling of charges only weakly influenced the average polarization of water molecules around an ion, the rescaling of ionic charges did influence the dielectric decrement, and a close-to-linear relation of the slope of the dielectric constant as a function of concentration with the ionic charge was found.

Chemical equilibria of aqueous ammonium-carboxylate systems in aqueous bulk, close to and at the water-air interface

Previous studies have shown that the water-air interface and a number of water molecule layers just below it, the surface region, have significantly different physico-chemical properties, such as lower relative permittivity and density, than bulk water. The properties in the surface region of water favor weakly hydrated species as neutral molecules, while ions requiring strong hydration and shielding of their charge are disfavored. In this study the equilibria NH₄⁺(aq) + RCOO^-(aq) ⇌ NH₃(aq) + RCOOH(aq) are investigated for R = C_nH_2n+1, n = 0-8, as open systems, where ammonia and small carboxylic acids in the gas phase above the water surface are removed from the system by a gentle controlled flow of nitrogen to mimic the transport of volatile compounds from water droplets into air. It is shown that this non-equilibrium transport of chemicals can be sufficiently large to cause a change of the chemical content of the aqueous bulk. Furthermore, X-ray photoelectron spectroscopy (XPS) has been used to determine the relative concentration of alkyl carboxylic acids and their conjugated alkyl carboxylates in aqueous surfaces using a micro-jet. These studies confirm that neutral alkyl carboxylic acids are accumulated in the surface region, while charged species, as alkyl carboxylates, are depleted. The XPS studies show also that the hydrophobic alkyl chains are oriented upwards into regions with lower relative permittivity and density, thus perpendicular to the aqueous surface. These combined results show that there are several chemical equilibria between the aqueous bulk and the surface region. The analytical studies show that the release of mainly ammonia is dependent on its concentration in the surface region, as long as the solubility of the carboxylic acid in the surface region is sufficiently high to avoid a precipitation in/on the water-air interface. However, for n-octyl- and n-nonylcarboxylic acid the solubility is sufficiently low to cause precipitation. The combined analytical and surface speciation studies in this work show that the equilibria involving the surface region are fast. The results from this study increase the knowledge about the distribution of chemical species in the surface region at and close to the water-air interface, and the transport of chemicals from water to air in open systems.

195Pt NMR and Molecular Dynamics Simulation Study of the Solvation of [PtCl6]2- in Water-Methanol and Water-Dimethoxyethane Binary Mixtures

The experimental ¹⁹⁵Pt NMR chemical shift, δ(¹⁹⁵Pt), of the [PtCl₆]^2- anion dissolved in binary mixtures of water and a fully miscible organic solvent is extremely sensitive to the composition of the mixture at room temperature. Significantly nonlinear δ(¹⁹⁵Pt) trends as a function of solvent composition are observed in mixtures of water-methanol, or ethylene glycol, 2-methoxyethanol, and 1,2-dimethoxyethane (DME). The extent of the deviation from linearity of the δ(¹⁹⁵Pt) trend depends strongly on the nature of the organic component in these solutions, which broadly suggests preferential solvation of the [PtCl₆]^2- anion by the organic molecule. This simplistic interpretation is based on an accepted view pertaining to monovalent cations in similar binary solvent mixtures. To elucidate these phenomena in detail, classical molecular dynamics computer simulations were performed for [PtCl₆]^2- in water-methanol and water-DME mixtures using the anionic charge scaling approach to account for the effect of electronic dielectric screening. Our simulations suggest that the simplistic model of preferential solvation of [PtCl₆]^2- by the organic component as inferred from nonlinear δ(¹⁹⁵Pt) trends is not entirely accurate, particularly for water-DME mixtures. The δ(¹⁹⁵Pt) trend in these mixtures levels off for high DME mole fractions, which results from apparent preferential location of [PtCl₆]^2- anions at the borders of water-rich regions or clusters within these inherently micro-heterogeneous mixtures. By contrast in water-methanol mixtures, apparently less pronounced mixed solvent micro-heterogeneity is found, suggesting the experimental δ(¹⁹⁵Pt) trend is consistent with a more moderate preferential solvation of [PtCl₆]^2- anions. This finding underlines the important role of solvent-solvent interactions and micro-heterogeneity in determining the solvation environment of [PtCl₆]^2- anions in binary solvent mixtures, probed by highly sensitive ¹⁹⁵Pt NMR. The notion that preferential solvation of [PtCl₆]^2- results primarily from competing ion-solvent interactions as generally assumed for monatomic ions, may not be appropriate in general.

The influence of polarizability and charge transfer on specific ion effects in the dynamics of aqueous salt solutions

The diffusion rates for water molecules in salt solutions depend on the identity of the ions, as well as their concentration. Among the alkali metal ions, cesium and potassium increase and sodium strongly decreases the diffusion constant of water. The origin of the difference can be understood by examining the simulation results using different potential models. In this work, aqueous solutions of salts are simulated with a variety of models. Commonly used non-polarizable models, which otherwise reproduce many experimental properties, do not capture the trend in the diffusion constant, while models which include polarization and/or charge transfer interactions do. For the non-polarizable models, the diffusion constant decreases too strongly with salt concentration. The changes in the water diffusion constant with increasing salt concentration match the diffusion constant of the ion. The ion diffusion constant is dependent on the residence time for water in the ion solvation shell. The non-polarizable models over-estimate the residence time, relative to the translational diffusion constant and so tend to under-estimate the ion and water diffusion constants.

The solvent side of proteinaceous membrane-less organelles in light of aqueous two-phase systems

Water represents a common denominator for liquid-liquid phase transitions leading to the formation of the polymer-based aqueous two-phase systems (ATPSs) and a set of the proteinaceous membrane-less organelles (PMLOs). ATPSs have a broad range of biotechnological applications, whereas PMLOs play a number of crucial roles in cellular compartmentalization and often represent a cellular response to the stress. Since ATPSs and PMLOs contain high concentrations of polymers (such as polyethylene glycol (PEG), polypropylene glycol (PPG), Ucon, and polyvinylpyrrolidone (PVP), Dextran, or Ficoll) or biopolymers (peptides, proteins and nucleic acids), it is expected that the separated phases of these systems are characterized by the noticeable changes in the solvent properties of water. These changes in solvent properties can drive partitioning of various compounds (proteins, nucleic acids, organic low-molecular weight molecules, metal ions, etc.) between the phases of ATPSs or between the PMLOs and their surroundings. Although there is a sizable literature on the properties of the ATPS phases, much less is currently known about PMLOs. In this perspective article, we first represent liquid-liquid phase transitions in water, discuss different types of biphasic (or multiphasic) systems in water, and introduce various PMLOs and some of their properties. Then, some basic characteristics of polymer-based ATPSs are presented, with the major focus being on the current understanding of various properties of ATPS phases and solvent properties of water inside them. Finally, similarities and differences between the polymer-based ATPSs and biological PMLOs are discussed.

Interaction of biologically relevant ions and organic molecules with titanium oxide (rutile) surfaces: A review on molecular dynamics studies

The surface of a biomaterial can play a major role in its biological fate since the surface is the primary pathway for its interaction with the body. As the natural response of the body to a foreign material is to encapsulate it with a fibrous material, the interactions between the body and the biomaterial are mediated by this fibrous layer. Initial interactions occur between the biomaterial surface, water, ionic species and organic molecules, which then mediate further interactions with body tissues. Surface engineering can influence these interactions and hence, improve the biocompatibility of the biomaterial. Therefore, both experimental and computational studies have been interested in phenomena happening at the solid-solution interface as their mechanisms and driving forces can point to new directions for biomaterial design and evaluation. In this review, we summarize the computational work on the interaction of titanium oxide surfaces (mainly rutile) with solvated ions and organic molecules by means of molecular dynamics, with a certain relevance to bioactivity testing protocols. The primary goal of this review is to present the current state of the art and draw attention to points where further investigations are required.

Metal Ion Modeling Using Classical Mechanics

Metal ions play significant roles in numerous fields including chemistry, geochemistry, biochemistry, and materials science. With computational tools increasingly becoming important in chemical research, methods have emerged to effectively face the challenge of modeling metal ions in the gas, aqueous, and solid phases. Herein, we review both quantum and classical modeling strategies for metal ion-containing systems that have been developed over the past few decades. This Review focuses on classical metal ion modeling based on unpolarized models (including the nonbonded, bonded, cationic dummy atom, and combined models), polarizable models (e.g., the fluctuating charge, Drude oscillator, and the induced dipole models), the angular overlap model, and valence bond-based models. Quantum mechanical studies of metal ion-containing systems at the semiempirical, ab initio, and density functional levels of theory are reviewed as well with a particular focus on how these methods inform classical modeling efforts. Finally, conclusions and future prospects and directions are offered that will further enhance the classical modeling of metal ion-containing systems.

Relative Order of Sulfuric Acid, Bisulfate, Hydronium, and Cations at the Air-Water Interface

Sulfuric acid (H2SO4), bisulfate (HSO4(-)), and sulfate (SO4(2-)) are among the most abundant species in tropospheric and stratospheric aerosols due to high levels of atmospheric SO2 emitted from biomass burning and volcanic eruptions. The air/aqueous interfaces of sulfuric acid and bisulfate solutions play key roles in heterogeneous reactions, acid rain, radiative balance, and polar stratospheric cloud nucleation. Molecular-level knowledge about the interfacial distribution of these inorganic species and their perturbation of water organization facilitates a better understanding of the reactivity and growth of atmospheric aerosols and of the aerosol surface charge, thus shedding light on topics of air pollution, climate change, and thundercloud electrification. Here, the air/aqueous interface of NaHSO4, NH4HSO4, and Mg(HSO4)2 salt solutions as well as H2SO4 and HCl acid solutions are investigated by means of vibrational sum frequency generation (VSFG) and heterodyne-detected (HD) VSFG spectroscopy. VSFG spectra of all acid solutions show higher SFG response in the OH-bonded region relative to neat water, with 1.1 M H2SO4 being more enhanced than 1.1 M HCl. In addition, VSFG spectra of bisulfate salt solutions highly resemble that of the dilute H2SO4 solution (0.26 M) at a comparable pH. HD-VSFG (Im χ((2))) spectra of acid and bisulfate salt solutions further reveal that hydrogen-bonded water molecules are oriented preferentially toward the bulk liquid phase. General agreement between Im χ((2)) spectra of 1.1 M H2SO4 and 1.1 M HCl acid solutions indicate that HSO4(-) ions have a similar surface preference as that of chloride (Cl(-)) ions. By comparing the direction and magnitude of the electric fields arising from the interfacial ion distributions and the concentration of each species, the most reasonable relative surface preference that can be deduced from a simplified model follows the order H3O(+) > HSO4(-) > Na(+), NH4(+), Mg(2+) > SO4(2-). Interestingly, contrary to some other near-neutral salt solution interfaces (e.g., chlorides and nitrates), cation-specific effects are here overshadowed by hydronium ions.