Ion Pairing in Alkali Nitrate Electrolyte Solutions

Effects of Ionic Interferents on Electrocatalytic Nitrate Reduction: Mechanistic Insight

Nitrate, a prevalent water pollutant, poses substantial public health concerns and environmental risks. Electrochemical reduction of nitrate (eNO3RR) has emerged as an effective alternative to conventional biological treatments. While extensive lab work has focused on designing efficient electrocatalysts, implementation of eNO3RR in practical wastewater settings requires careful consideration of the effects of various constituents in real wastewater. In this critical review, we examine the interference of ionic species commonly encountered in electrocatalytic systems and universally present in wastewater, such as halogen ions, alkali metal cations, and other divalent/trivalent ions (Ca2+, Mg2+, HCO3-/CO32-, SO42-, and PO43-). Notably, we categorize and discuss the interfering mechanisms into four groups: (1) loss of active catalytic sites caused by competitive adsorption and precipitation, (2) electrostatic interactions in the electric double layer (EDL), including ion pairs and the shielding effect, (3) effects on the selectivity of N intermediates and final products (N2 or NH3), and (4) complications by the hydrogen evolution reaction (HER) and localized pH on the cathode surface. Finally, we summarize the competition among different mechanisms and propose future directions for a deeper mechanistic understanding of ionic impacts on eNO3RR.

Anionic Effects on Concentrated Aqueous Lithium Ion Dynamics

The dynamics, orientational anisotropy, diffusivity, viscosity, and density were measured for concentrated lithium salt solutions, including lithium chloride (LiCl), lithium bromide (LiBr), lithium nitrite (LiNO2), and lithium nitrate (LiNO3), with methyl thiocyanate as an infrared vibrational probe molecule, using two-dimensional infrared spectroscopy (2D IR), nuclear magnetic resonance (NMR) spectroscopy, and viscometry. The 2D IR, NMR, and viscosity results show that LiNO2 exhibits longer correlation times, lower diffusivity, and nearly 4 times greater viscosity compared to those of the other lithium salt solutions of the same concentration, suggesting that nitrite anions may strongly facilitate structure formation via strengthening water-ion network interactions, directly impacting bulk solution properties at sufficiently high concentrations. Additionally, the LiNO2 and LiNO3 solutions show significantly weakened chemical interactions between the lithium cations and the methyl thiocyanate when compared with those of the lithium halide salts.

New Electrical Conductivity Model for Electrolyte Solutions Based on the Debye-Hückel-Onsager Theory

A new electrical conductivity model is developed for unassociated electrolyte solutions based on the Debye-Hückel-Onsager theory. In this model, we assume that a single cation and a single anion with their crystallographic ionic radii are in a continuum medium of the solvent(s). We compare the predictions of the developed model with the experimental measurements of binary 1:1, 2:1, 1:2, 2:2, 1:3, 3:1, 2:3, 3:2, 3:3, 1:4, and 2:4 aqueous solutions in the temperature range 273.15-373.15 K. Our results are in good agreement with the experimental data. An extension of the model was formulated to incorporate ion pairing, and its effectiveness was evaluated across three essential systems: 2:2 aqueous sulfate solutions, ionic liquid-co-solvent systems, and NaCl-water-1,4-dioxane solutions. This adaptation demonstrated a strong correlation with experimental data, highlighting the broad applicability.

Molecular modeling and simulation of aqueous solutions of alkali nitrates

A set of molecular models for the alkali nitrates (LiNO3, NaNO3, KNO3, RbNO3, and CsNO3) in aqueous solutions is presented and used for predicting the thermophysical properties of these solutions with molecular dynamics simulations. The set of models is obtained from a combination of a model for the nitrate anion from the literature with a set of models for the alkali cations developed in previous works of our group. The water model is SPC/E and the Lorentz–Berthelot combining rules are used for describing the unlike interactions. This combination is shown to yield fair predictions of thermophysical and structural properties of the studied aqueous solutions, namely the density, the water activity and the mean ionic activity coefficient, the self-diffusion coefficients of the ions, and radial distribution functions, which were studied at 298 K and 1 bar; except for the density of the solutions of all five nitrates and the activity properties of solutions of NaNO3, which were also studied at 333 K. For calculating the water the activity and the mean ionic activity coefficient, the OPAS ( osmotic pressure for the activity of selvents) method was applied. The new models extend an ion model family for the alkali halides developed in previous works of our group in a consistent way.

Gamma Radiation-Induced Degradation of Acetohydroxamic Acid (AHA) in Aqueous Nitrate and Nitric Acid Solutions Evaluated by Multiscale Modelling

Acetohydroxamic acid (AHA) has been proposed for inclusion in advanced, single-cycle, used nuclear fuel reprocessing solvent systems for the reduction and complexation of plutonium and neptunium ions. For this application, a detailed description of the fundamental degradation of AHA in dilute aqueous nitric acid is required. To this end, we present a comprehensive, multiscale computer model for the coupled radiolytic and hydrolytic degradation of AHA in aqueous sodium nitrate and nitric acid solutions. Rate coefficients for the reactions of AHA and hydroxylamine (HA) with the oxidizing nitrate radical were measured for the first time using electron pulse radiolysis and used as inputs for the kinetic model. The computer model results are validated by comparison to experimental data from steady-state gamma ray irradiations, for which the agreement is excellent. The presented model accurately predicts the yields of the major degradation products of AHA: acetic acid, HA, nitrous oxide, and molecular hydrogen.

Accurate Description of Solvent-Exposed Salt Bridges with a Non-polarizable Force Field Incorporating Solvent Effects

The strength of salt bridges resulting from the interaction of cations and anions is modulated by their environment. However, polarization of the solvent molecules by the charged moieties makes the accurate description of cation-anion interactions in an aqueous solution by means of a pairwise additive potential energy function and classical combination rules particularly challenging. In this contribution, aiming at improving the representation of solvent-exposed salt-bridge interactions with an all-atom non-polarizable force field, we put forth here a parametrization strategy. First, the interaction of a cation and an anion is characterized by hybrid quantum mechanical/molecular mechanics (QM/MM) potential of mean force (PMF) calculations, whereby constantly exchanging solvent molecules around the ions are treated at the quantum mechanical level. The Lennard-Jones (LJ) parameters describing the salt-bridge ion pairs are then optimized to match the reference QM/MM PMFs through the so-called nonbonded FIX, or NBFIX, feature of the CHARMM force field. We apply the new set of parameters, coined CHARMM36m-SBFIX, to the calculation of association constants for the ammonium-acetate and guanidinium-acetate complexes, the osmotic pressures for glycine zwitterions, guanidinium, and acetate ions, and to the simulation of both folded and intrinsically disordered proteins. Our findings indicate that CHARMM36m-SBFIX improves the description of solvent-exposed salt-bridge interactions, both structurally and thermodynamically. However, application of this force field to the standard binding free-energy calculation of a protein-ligand complex featuring solvent-excluded salt-bridge interactions leads to a poor reproduction of the experimental value, suggesting that the parameters optimized in an aqueous solution cannot be readily transferred to describe solvent-excluded salt-bridge interactions. Put together, owing to their sensitivity to the environment, modeling salt-bridge interactions by means of a single, universal set of LJ parameters remains a daunting theoretical challenge.

Solubilities in aqueous nitrate solutions that appear to reverse the law of mass action

Non-ideal aqueous electrolyte solutions have been studied since the start of the application of thermodynamics to chemistry in the late 19th century. The present study examines some of the most extreme non-ideal behavior ever observed: solubilities of alkali and NH₄⁺ nitrate salts in water that appear to behave the opposite of how the Law of Mass Action would predict. A literature review discovered that the solubilities of NH₄NO₃ and many alkali nitrate salts increases when another nitrate-bearing electrolyte is added to solution. These occurrences were in concentrated solutions with insufficient water to provide all ions their preferred hydration number without sharing waters between ions. This water deficit results in the formation of contact ion-pairs as well as larger ion-clusters. These ion-clusters may be favored when there is more than one type of monovalent cation present.

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.

The unexpected effect of aqueous ion pairs on the forbidden n →π* transition in nitrate

Aqueous nitrate is ubiquitous in the environment, found for example in stratospheric clouds, tropospheric particulate matter, rain and snow, fertilized fields, rivers and the ocean. Its photolysis is initiated by absorption into the strongly forbidden n →π* transition. Photolysis reactivates deposited nitrate, releasing nitrogen oxides, and UV light is commonly used to break down nitrate pollution. The transition is doubly forbidden unless its symmetry is broken, giving a powerful means of probing the interactions of nitrate with its environment and of using experiment to validate the results of theory. In this study we demonstrate the remarkably different effects of the addition of a series of mono- and di-valent metal chlorides on the nitrate UV transition. While they all shift the transition to shorter wavelengths, the shift changes significantly from one to another. For the monovalent series Li⁺, Na⁺, K⁺, the blue shift decreases down the column being strongest for Li⁺ and weakest for K⁺. For the divalent series Mg²⁺, Ca²⁺, Ba²⁺, the opposite effect is observed with the energy shift of Ba²⁺ being an order of magnitude larger than for Mg²⁺. The absorption intensity also changes; the addition of Na⁺ and K⁺ decrease intensity whereas Li⁺ increases intensity. For the divalent cations an increase is seen for all three members of the series Mg²⁺, Ca²⁺ and Ba²⁺. Paradoxically, the effect of addition of CaCl₂ to the solution is to decrease the environmental photolysis rate of nitrate; despite the increase in intensity, Ca²⁺ blue shifts the peak position above the tropospheric photolysis threshold around 300 nm. Using computational chemistry we conclude that the effects are due to the microscopic interactions of the nitrate anion and not continuum effects. Two microscopic mechanisms are investigated in detail, the formation of a nitrate monohydrate cluster and a contact ion pair. The contact ion pair shows the potential for significant impact on the energy and intensity of the transition.

Improved Cation Binding to Lipid Bilayers with Negatively Charged POPS by Effective Inclusion of Electronic Polarization

Phosphatidylserine (PS) lipids are important signaling molecules and the most common negatively charged lipids in eukaryotic membranes. The signaling can be often regulated by calcium, but its interactions with PS headgroups are not fully understood. Classical molecular dynamics (MD) simulations can potentially give detailed description of lipid-ion interactions, but the results strongly depend on the used force field. Here, we apply the electronic continuum correction (ECC) to the Amber Lipid17 parameters of 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-l-serine (POPS) lipid to improve its interactions with K⁺, Na⁺, and Ca²⁺ ions. The partial charges of the headgroup, glycerol backbone, and carbonyls of POPS, bearing a unit negative charge, were scaled with a factor of 0.75, derived for monovalent ions, and the Lennard-Jones σ parameters of the same segments were scaled with a factor of 0.89. The resulting ECC-POPS model gives more realistic interactions with Na⁺ and Ca²⁺ cations than the original Amber Lipid17 parameters when validated using headgroup order parameters and the "electrometer concept". In ECC-lipids simulations, populations of complexes of Ca²⁺ cations with more than two PS lipids are negligible, and interactions of Ca²⁺ cations with only carboxylate groups are twice more likely than with only phosphate groups, while interactions with carbonyls almost entirely involve other groups as well. Our results pave the way for more realistic MD simulations of biomolecular systems with anionic membranes, allowing signaling processes involving PS and Ca²⁺ to be elucidated.

Water in Rechargeable Multivalent-Ion Batteries: An Electrochemical Pandora's Box

Multivalent-ion batteries built on water-based electrolytes represent energy storage at suitable price points, competitive performance, and enhanced safety. However, to comply with modern energy-density requirements, the battery must be reversible within an operating voltage window greater than 1.23 V or the electrochemical stability limits of free water. Taking advantage of its powerful solvation and catalytic activities, adding water to electrolyte preparations can unlock a wider gamut of liquid mixtures compared with strictly nonaqueous systems. However, a point-by-point sweep of all potential formulations is arduous and ineffective without some form of systematic rationalization. The present Review consolidates recent progress, pitfalls, limits, and insights critical to expediting aqueous electrolyte designs to boost multivalent-ion battery outputs.

Decoding Oxyanion Aqueous Solvation Structure: A Potassium Nitrate Example at Saturation

The ability to probe the structure of a salt solution at the atomic scale is fundamentally important for our understanding of many chemical reactions and their mechanisms. The capability of neutron diffraction to "see" hydrogen (or deuterium) and other light isotopes is exceptional for resolving the structural complexity around the dissolved solutes in aqueous electrolytes. We have made measurements using oxygen isotopes on aqueous nitrate to reveal a small hydrogen-bonded water coordination number (3.9 ± 1.2) around a nitrate oxyanion. This is compared to estimates made using the existing method of nitrogen isotope substitution and those of computational simulations (>5-6 water molecules). The low water coordination number, combined with a comparison to classical molecular dynamics simulations, suggests that ion-pair formation is significant. This insight demonstrates the utility of experimental diffraction data for benchmarking atomistic computer simulations, enabling the development of more accurate intermolecular potentials.