Ab Initio Molecular Dynamics Study of a Highly Concentrated LiCl Aqueous Solution

Low-Frequency Spectra of Hydrated Ionic Liquids with Kosmotropic and Chaotropic Anions

In this study, we investigated the water concentration dependence of the intermolecular vibrations of two hydrated ionic liquids (ILs), cholinium dihydrogen phosphate ([ch][dhp]) and cholinium bromide ([ch]Br), using femtosecond Raman-induced Kerr effect spectroscopy (fs-RIKES). The anions of the former and latter hydrated ILs are kosmotropic and chaotropic, respectively. We found that the spectral peak of ∼50 cm-1 shifted to the low-frequency side in hydrated [ch][dhp], indicating the weakening of its intermolecular interactions. In contrast, no change in the peak frequency of the low-frequency band at ∼50 cm-1 was observed with increasing water concentration in hydrated [ch]Br. The vibrational density of states (VDOS) spectra generated from molecular dynamics (MD) simulations were in qualitative agreement with the experimental results. Decomposition analysis of the VDOS spectra for each component revealed that the red shift of the low-frequency band in the hydrated [ch][dhp] upon water addition was essentially due to the contributions of anions and water rather than that of the cholinium cation. We also found from the low-frequency spectra of the two hydrated ILs that they differed in the concentration dependence of the 180 cm-1 band, which is assigned as a hindered translational motion of water molecules combined to form O···O stretching motions. From the relationship between the peak frequency of the low-frequency band and the bulk parameter, which is the square root of the surface tension divided by the density, we found that the peak frequency in the hydrated IL with kosmotropic [dhp]- depends on the bulk parameter, similar to the case for an aqueous solution of the typical deep eutectic solvent reline. However, the peak frequency of the hydrated IL with chaotropic Br- is constant with the bulk parameter.

Structural and dynamical properties of concentrated alkali- and alkaline-earth metal chloride aqueous solutions

Concentrated ionic aqueous electrolytes possess a diverse array of applications across various fields, particularly in the field of energy storage. Despite extensive examination, the intricate relationships and numerous physical mechanisms underpinning diverse phenomena remain incompletely understood. Molecular dynamics simulations are employed to probe the attributes of aqueous solutions containing LiCl, NaCl, KCl, MgCl2, and CaCl2, spanning various solute fractions. The primary emphasis of the simulations is on unraveling the intricate interplay between these attributes and the underlying physical mechanisms. The configurations of cation-Cl- and Cl--Cl- pairs within these solutions are disclosed. As the solute fraction increases, consistent trends manifest regardless of solute type: (i) the number of hydrogen bonds formed by the hydration water surrounding ions decreases, primarily attributed to the growing presence of counter ions in proximity to the hydration water; (ii) the hydration number of ions exhibits varying trends influenced by multiple factor; and (iii) the diffusion of ions slows down, attributed to the enhanced confinement and rebound of cations and Cl- ions from the surrounding atoms, concurrently coupled with the changes in ion vibration modes. In our analysis, we have, for the first time, clarified the reasons behind the slowing down of the diffusion of the ions with increasing solute fraction. Our research contributes to a better understanding and manipulation of the attributes of ionic aqueous solutions and may help designing high-performance electrolytes.

Raman Spectrum of the Li2SO4-MgSO4-H2O System: Excess Spectrum and Hydration Shell Spectrum

Lithium, as a green energy metal used to promote world development, is an important raw material for lithium-ion, lithium-air, and lithium-sulfur batteries. It is challenging to directly extract lithium resources from brine with a high Mg/Li mass ratio. The microstructure study of salt solutions provides an important theoretical basis for the separation of lithium and magnesium. The changes in the hydrogen bond network structure and ion association of the Li2SO4 aqueous solution and Li2SO4-MgSO4-H2O mixed aqueous solution were studied by Raman spectroscopy. The SO42- fully symmetric stretching vibration peak at 940~1020 cm-1 and the O-H stretching vibration peak at 2800~3800 cm-1 of the Li2SO4 aqueous solution at room temperature were studied by Raman spectroscopy and excess spectroscopy. According to the peak of the O-H stretching vibration spectrum, with an increase in the mass fraction of the Li2SO4 solution, the proportion of DAA-type and DDAA-type hydrogen bonds at low wavenumbers decreases gradually, while the proportion of DA-type hydrogen bonds at 3300 cm-1 increases. When the mass fraction is greater than 6.00%, this proportion increases sharply. Although the spectra of hydrated water molecules and bulk water molecules are different, the spectra of the two water molecules seriously overlap. The spectrum of the anion hydration shell in a solution can be extracted via spectrum division. By analyzing the spectra of these hydration shells, the interaction between the solute and water molecules, the structure of the hydration shell and the number of water molecules are obtained. For the same ionic strength solution, different cationic salts have different hydration numbers of anions, indicating that there is a strong interaction between ions in a strong electrolytic solution, which will lead to ion aggregation and the formation of ion pairs. When the concentration of salt solution increases, the hydration number decreases rapidly, indicating that the degree of ion aggregation increases with increasing concentration.

Nitrogen Electrocatalysis: Electrolyte Engineering Strategies to Boost Faradaic Efficiency

The electrochemical activation of dinitrogen at ambient temperature and pressure for the synthesis of ammonia has drawn increasing attention. The faradaic efficiency (FE) as well as ammonia yield in the electrochemical synthesis is far from reaching the requirement of industrial-scale production. In aqueous electrolytes, the competing electron-consuming hydrogen evolution reaction (HER) and poor solubility of nitrogen are the two major bottlenecks. As the electrochemical reduction of nitrogen involves proton-coupled electron transfer reaction, rationally engineered electrolytes are required to boost FE and ammonia yield. In this Review, we comprehensively summarize various electrolyte engineering strategies to boost the FE in aqueous and non-aqueous medium and suggest possible approaches to further improve the performance. In aqueous medium, the performance can be improved by altering the electrolyte pH, transport velocity of protons, and water activity. Other strategies involve the use of hybrid and water-in-salt electrolytes, ionic liquids, and non-aqueous electrolytes. Existing aqueous electrolytes are not ideal for industrial-scale production. Suppression of HER and enhanced nitrogen solubility have been observed with hybrid and non-aqueous electrolytes. The engineered electrolytes are very promising though the electrochemical activation has several challenges. The outcome of lithium-mediated nitrogen reduction reaction with engineered non-aqueous electrolyte is highly encouraging.

Self-Consistent Implementation of a Solvation Free Energy Framework to Predict the Salt Solubilities of Six Alkali Halides

To assess the salt solubilities of six alkali halides in aqueous systems, we proposed a thermodynamic cycle and an efficient molecular modeling methodology. The Gibbs free energy changes for vaporization, dissociation, and dissolution were calculated using the experimental data of ionic thermodynamic properties obtained from the NBS tables. Additionally, the Marcus' and Tissandier's solvation free energy data for Li+, Na+, K+, Cl-, and Br- ions were compared with the conventional solvation free energies by substituting into our self-consistent thermodynamic cycle. Furthermore, Tissandier's absolute solvation free energy data were used as the training set to refit the Lennard-Jones parameters of OPLS-AA force field for ions. To predict salt solubilities, an assumption of a pseudo-solvent was proposed to characterize the coupling work of a solute with its environment from infinitely diluted to saturated solutions, indicating that the Gibbs energy change of solvation process is a function of ionic strength. Following the self-consistency of the cycle, the newly derived formulas were used to determine the salt solubilities by interpolating the intersection of Gibbs free energy of dissolution and the zero free energy line. The refined ion parameters can also predict the structure and thermodynamic properties of aqueous electrolyte solutions, such as densities, pair correlation functions, hydration numbers, mean activity coefficients, vapor pressures, and the radial dependences of the net charge at 298.15 K and 1 bar. Our method can be used to characterize the solid-liquid equilibria of ions or charged particles in aqueous systems. Furthermore, for highly concentrated strong electrolyte systems, it is essential to introduce accurate water models and polarizable force fields.

Raman spectroscopy study for the systems (LiCl-H2O and LiCl-MgCl2-H2O): Excess spectra and hydration shell spectra

The micro-structure of hydration shell of solute in water is significant for understanding the properties of aqueous solutions. Raman spectroscopy has been employed for studying the hydration shell structure of the solute for decades, however, Raman imaging data is still seriously overlapped, making it challenging to obtain information on the spectrum of hydrated water molecules. In this paper, Raman spectroscopy was employed to study the O-H vibration peaks of LiCl aqueous solution and LiCl-MgCl₂-H₂O mixed aqueous solution. The changes of stretching vibration peak of 2800 ∼ 3800 cm^-1O-H and hydrogen bond network structure in aqueous solution were analyzed at room temperature and ion association. With the increase of magnesium salt ratio, the damage of solute to the bulk water gradually decreases in the mixed solution, which indicated that LiCl has a more significant influence on the bulk water molecules. It is mainly due to the intense hydration of Li⁺, which can not only affect the water molecules in the first hydration shell but also affect the water molecules in the second hydration shell. The number of water molecules in the first hydration shell were obtained by extracting the spectra of different solute first hydration shells from the solution spectra. Those spectra of the hydration shell were employed to study the micro-structures of the first hydration shells of anions, and the aggregation behavior of ions in the the mixed solution.

Dissolving salt is not equivalent to applying a pressure on water

Salt water is ubiquitous, playing crucial roles in geological and physiological processes. Despite centuries of investigations, whether or not water’s structure is drastically changed by dissolved ions is still debated. Based on density functional theory, we employ machine learning based molecular dynamics to model sodium chloride, potassium chloride, and sodium bromide solutions at different concentrations. The resulting reciprocal-space structure factors agree quantitatively with neutron diffraction data. Here we provide clear evidence that the ions in salt water do not distort the structure of water in the same way as neat water responds to elevated pressure. Rather, the computed structural changes are restricted to the ionic first solvation shells intruding into the hydrogen bond network, beyond which the oxygen radial-distribution function does not undergo major change relative to neat water. Our findings suggest that the widely cited pressure-like effect on the solvent in Hofmeister series ionic solutions should be carefully revisited.

By advanced machine learning techniques, first-principles simulations find that dissolving salt in water does not change water structure drastically. It is contrary to the notion of “pressure effect” which has been widely applied over past 25 years.

Salting-out effect promoting highly efficient ambient ammonia synthesis

The electroreduction of nitrogen to ammonia offers a promising alternative to the energy-intensive Haber-Bosch process. Unfortunately, the reaction suffers from low activity and selectivity, owing to competing hydrogen evolution and the poor accessibility of nitrogen to the electrocatalyst. Here, we report that deliberately triggering a salting-out effect in a highly concentrated electrolyte can simultaneously tackle the above challenges and achieve highly efficient ammonia synthesis. The solute ions exhibit strong affinity for the surrounding H₂O molecules, forming a hydration shell and limiting their efficacy as both proton sources and solvents. This not only effectively suppresses hydrogen evolution but also ensures considerable nitrogen flux at the reaction interface via heterogeneous nucleation of the precipitate, thus facilitating the subsequent reduction process in terms of both selectivity and activity. As expected, even when assembled with a metal-free electrocatalyst, a high Faradaic efficiency of 71 ± 1.9% is achieved with this proof-of-concept system.

Resolving the Equal Number Density Puzzle: Molecular Picture from Simulations of LiCl(aq) and NaCl(aq)

The change in number densities of aqueous solutions of alkali chlorides should be qualitatively predictable. Typically, as cations get larger, the number density of the solution decreases. However, aqueous solutions of lithium and sodium chloride exhibit at ambient conditions practically identical number densities at equal molalities despite different ionic sizes. Here, we provide an atomistic interpretation of this experimentally observed anomalous behavior using molecular dynamics simulations. The obtained results show that the rigidity of the Li⁺ first and second solvation shells and the associated compromised hydrogen bonding result in practically equal average water densities in the local hydration regions for Li⁺ and Na⁺ despite different sizes of the cations. In addition, in more distant regions from the cations, the water densities of these two solutions also coincide. These findings thus provide an atomistic interpretation for matching number densities of LiCl and NaCl solutions. In contrast, the number density differences between NaCl and KCl solutions as well as between LiCl and KCl solutions behave in a regular fashion with lower number densities of solutions observed for larger cations.

Ab Initio Molecular Dynamics Simulations of the Influence of Lithium Bromide on the Structure of the Aqueous Solution-Air Interface

We present the results of ab initio molecular dynamics simulations of the solution–air interface of aqueous lithium bromide (LiBr). We find that, in agreement with the experimental data and previous simulation results with empirical polarizable force field models, Br^– anions prefer to accumulate just below the first molecular water layer near the interface, whereas Li⁺ cations remain deeply buried several molecular layers from the interface, even at very high concentration. The separation of ions has a profound effect on the average orientation of water molecules in the vicinity of the interface. We also find that the hydration number of Li⁺ cations in the center of the slab N_{c,Li⁺–H2O} ≈ 4.7 ± 0.3, regardless of the salt concentration. This estimate is consistent with the recent experimental neutron scattering data, confirming that results from nonpolarizable empirical models, which consistently predict tetrahedral coordination of Li⁺ to four solvent molecules, are incorrect. Consequently, disruption of the hydrogen bond network caused by Li⁺ may be overestimated in nonpolarizable empirical models. Overall, our results suggest that empirical models, in particular nonpolarizable models, may not capture all of the properties of the solution–air interface necessary to fully understand the interfacial chemistry.

Unusual Influence of Fluorinated Anions on the Stretching Vibrations of Liquid Water

Infrared (IR) spectroscopy is a commonly used and invaluable tool in the studies of solvation phenomena in aqueous solutions. Concurrently, ab initio molecular dynamics (AIMD) simulations deliver the solvation shell picture at a molecular detail level and allow for a consistent decomposition of the theoretical IR spectrum into underlying spatial correlations. Here, we demonstrate how the novel spectral decomposition techniques can extract important information from the computed IR spectra of aqueous solutions of BF₄^- and PF₆^-, interesting weakly coordinating anions that have been known for a long time to alter the IR spectrum of water in an unusual manner. The distance-dependent spectra of both ions are analyzed using the spectral similarity method that provides a quantitative picture of both the spectrum of the solute-affected solvent and the number of solvent molecules thus altered. We find, in accordance with previous experiments, a considerable blue shift of the ν_OH stretching band of liquid water by 264 cm^-1 for BF₄^- and 306 cm^-1 for PF₆^-, with the affected numbers being 3.7 and 4.2, respectively. Considering also the additional information on solute-solvent dipolar couplings delivered by radially and spatially resolved IR spectra, the computational IR spectroscopy based on AIMD simulations is shown to be a viable predictive tool with strong interpretative power.

Sensitivity of Electron Transfer Mediated Decay to Ion Pairing

Ion pairing in electrolyte solutions remains a topic of discussion despite a long history of research. Very recently, nearest-neighbor mediated electronic de-excitation processes of core hole vacancies (electron transfer mediated decay, ETMD) were proposed to carry a spectral fingerprint of local solvation structure and in particular of contact ion pairs. Here, for the first time, we apply electron-electron coincidence detection to a liquid microjet, and record ETMD spectra of Li 1s vacancies in aqueous solutions of lithium chloride (LiCl) in direct comparison to lithium acetate (LiOAc). A change in the ETMD spectrum dependent on the electrolyte anion identity is observed for 4.5 M salt concentration. We discuss these findings within the framework of the formation and presence of contact ion pairs and the unique sensitivity of ETMD spectroscopy to ion pairing.

A continuum solvent model of ion-ion interactions in water

The calculation of ion-ion interactions in water is a problem of long standing importance. Modelling these interactions is a prerequisite to explaining Hofmeister (specific ion) effects. We here generalize our solvation model of ions to calculate the free energy of two ions in water as a function of separation. The same procedure has previously been applied to calculate ion interactions with the air-water interface successfully. The Conductor like Screening Model (COSMO) is used. This treats the ions on a quantum mechanical level and calculates numerically the electrostatic response of the surrounding solvent. Estimates of the change in the cavity formation energy and the change in the ion-water dispersion energy as the ions approach are included separately. The calculated interaction potentials are too attractive and this is a significant issue. However, they do reproduce the affinity of similarly sized ions for each other, which is a crucial property of these potentials. They are also oscillatory, another important property. We normalize the potentials to reduce the over-attraction, and good correlation with experimental values is achieved. We identify the driving contributions to this like-prefers-like behaviour. We then put forward a plausible hypothesis for the over-attraction of the potentials. An agreeable feature of our approach is that it does not rely on salt specific parameters deliberately adjusted to reproduce experimental values.

Solvation shell resolved THz spectra of simple aqua ions – distinct distance- and frequency-dependent contributions of solvation shells

Spatial decomposition schemes for infrared spectra reveal the importance of both dipolar couplings and correlations in particle motion in aqueous solutions of Na⁺and Cl⁻.

Aqueous solutions: state of the art in ab initio molecular dynamics

The simulation of liquids by ab initio molecular dynamics (AIMD) has been a subject of intense activity over the last two decades. The significant increase in computational resources as well as the development of new and efficient algorithms has elevated this method to the status of a standard quantum mechanical tool that is used by both experimentalists and theoreticians. As AIMD computes the electronic structure from first principles, it is free of ad hoc parametrizations and has thus been applied to a large variety of physical and chemical problems. In particular, AIMD has provided microscopic insight into the structural and dynamical properties of aqueous solutions which are often challenging to probe experimentally. In this review, after a brief theoretical description of the Born-Oppenheimer and Car-Parrinello molecular dynamics formalisms, we show how AIMD has enhanced our understanding of the properties of liquid water and its constituent ions: the proton and the hydroxide ion. Thereafter, a broad overview of the application of AIMD to other aqueous systems, such as solvated organic molecules and inorganic ions, is presented. We also briefly describe the latest theoretical developments made in AIMD, such as methods for enhanced sampling and the inclusion of nuclear quantum effects.