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.

Hydration and Ion-Pair Formation of NaNO3(aq): A Vibrational Spectroscopic and Density Functional Theory Study

Qualitative and quantitative Raman and infrared measurements on sodium nitrate (NaNO₃) solutions have been carried out over a wide concentration range (5.56 × 10^-6-7.946 mol/L) in water and heavy water. The Raman spectra were measured from 4000 cm^-1 to low wavenumbers at 45 cm^-1. Band fit analysis on the profile of the 1047 cm^-1 band, ν₁(a1') NO3- measured at high resolution at 0.90 cm^-1 produced a small contribution at 1027 cm^-1 of the isotopomer N¹⁶O₂¹⁸O(aq). The effect of solute concentration on the Raman and infrared bands has been systematically recorded. Extrapolation of the experimental data resulted in values for all the nitrate bands of the "free", i.e., fully hydrated NO3-(aq). However, even in dilute solutions, the vibrational symmetry of the hydrated NO3-(aq) is broken and the antisymmetric N-O stretch, which is degenerate for the isolated anion, is split by 56 cm^-1. At concentrations >2.5 mol/L, direct contact between Na⁺ and NO3- was observed and accompanied by large band parameter changes. DFT calculations on NO3-(H₂O)_n (n = 1-3) led to optimized geometries and vibrational frequencies which reproduced the measured ones within an accuracy of 1%. A hydrated gas phase species Na⁺(H₂O)₁₀NO3- was optimized resulting in the geometry and symmetry of the nitrate, which is bound in an antisymmetric bidentate fashion with the nitrate possessing C₁. The ν₁ Na⁺(OH₂) breathing mode in aqueous solution appears at 189 cm^-1, whereas in heavy water, ν₁ Na⁺(OD₂) is shifted to 175.6 cm^-1 due to the isotope effect. DFT calculations on hydrated Na⁺(OH₂)_n gas phase clusters provided realistic Na⁺ hydrate structures with n = 4 and 5, which resembled the measured frequency of ν₁ Na⁺ OH₂ mode quite well. Quantitative Raman analysis employing the symmetric stretching band, ν₁(a1') NO3-, has been carried out down to concentrations as low as 5.56 × 10^-6mol/L. The in-plane deformation mode ν₄(e') in the Raman scattering at higher concentrations has been used as an indicator band for directly coordinated NO3-.

First-Principles Study on the Cation-Dependent Electrochemical Stabilities in Li/Na/K Hydrate-Melt Electrolytes

Aqueous alkali-ion batteries, particularly earth-abundant sodium- or potassium-based systems, are potentially safe and low-cost alternatives to nonaqueous Li-ion batteries. Recently, concentrated aqueous electrolytes with Na and K salts as well as Li ones have been extensively studied to increase the voltage of aqueous batteries; however, the potential windows become narrower in the order of Li > Na > K. Here, we study the difference in the potential windows of Li-, Na-, and K-salt concentrated aqueous electrolytes (hydrate melts) by first-principles molecular dynamics. As the Lewis acidity of alkali cations decreases (Li⁺ > Na⁺ > K⁺), the sacrificial reduction of counter anions is less active and water molecules are more aggregated. This situation is unfavorable for achieving stable anion-derived passivation on negative electrodes as well as for being stabilized to oxidation on positive electrodes. Hence, the Lewis acidity of alkali cations is essential to dominate the potential windows of hydrate-melt electrolytes.

Effects of copper ions on removal of nutrients from swine wastewater and on release of dissolved organic matter in duckweed systems

High concentration of Cu²⁺ in swine wastewater raises concerns about its potential adverse effects on nutrient removal by aquatic plants like duckweed. In this work, the effects of copper ions on nutrient removal and release of dissolved organic matter (DOM) were investigated in duckweed systems. Results showed that the removal performance of ammonia nitrogen (NH₃N) and total phosphorus (TP) increased at 0.1-1.0 mg/L of Cu²⁺, while dropped at 2.0-5.0 mg/L of Cu²⁺. A novel kinetic model in which Cu²⁺ was taken into account was then developed which was used to optimize Cu²⁺ concentration at 0.96 mg/L for nutrient removal in duckweed systems. NADH, detected in DOM by the parallel factor (PARAFAC) analysis, exhibited high capacities of binding copper ions, so it played an important role on the decrease of Cu²⁺ concentrations in duckweed systems. The principle component analysis (PCA) showed that the dominant DOM were lower molecular weight compounds at 1.0 mg/L of Cu²⁺ and higher molecular weight compounds at 2.0-5.0 mg/L of Cu²⁺. The bonds of CH (humic-like), NO (NO₃^-) and ArH (tyrosine) in DOM were responsible for not only the fastest binding with Cu²⁺ from the result of the two-dimensional Fourier transform infrared correlation spectroscopy (2D-FTIR-CoS) but also the variations of DOM conformations at a critical concentration of 0.5 mg/L Cu²⁺ from the perturbation correlation moving window two-dimensional (PCMW2D) analysis. These findings lead to a better understanding on the environmental behaviors and mechanisms of Cu²⁺ in duckweed systems.

Ionic radii of hydrated sodium cation from QTAIM

The sodium cation is ubiquitous in aqueous chemistry and biological systems. Yet, in spite of numerous studies, the (average) distance between the sodium cation and its water ligands, and the corresponding ionic radii, are still controversial. Recent experimental values in solution are notably smaller than those from previous X-ray studies and ab initio molecular dynamics. Here we adopt a "bottom-up" approach of obtaining these distances from quantum chemistry calculations [full MP2 with the 6-31++G(d,p) and cc-pVTZ basis-sets] of gas-phase Na⁺(H₂O)_n clusters, as a function of the sodium coordination number (CN = 2-6). The bulk limit is obtained by the polarizable continuum model, which acts to increase the interatomic distances at small CN, but has a diminishing effect as the CN increases. This extends the CN dependence of the sodium-water distances from crystal structures (CN = 4-12) to lower CN values, revealing a switch between two power laws, having a small exponent at small CNs and a larger one at large CNs. We utilize Bader's theory of atoms in molecules to bisect the Na⁺-O distances into Na⁺ and water radii. Contrary to common wisdom, the water radius is not constant, decreasing even more than that of Na⁺ as the CN decreases. We also find that the electron density at the bond critical point increases exponentially as the sodium radius decreases.

Raman spectroscopic studies and DFT calculations on NaCH3CO2 and NaCD3CO2 solutions in water and heavy water

Sodium acetate and acetate-d₃ solutions in water and heavy water were studied using Raman spectroscopy over a wide concentration range from 40–4200 cm⁻¹ and DFT calculations were performed on acetate–water clusters.

Vibrational spectroscopic studies and DFT calculations on NaCH3CO2(aq) and CH3COOH(aq)

NaCH₃CO₂(aq) and CH₃COOH(aq) were studied using Raman and infrared spectroscopy over a large concentration range, in the terahertz region and up to 4000 cm⁻¹. Band assignments for CH₃CO₂⁻(aq) and CH₃COOH(aq) were carried out under guidance of DFT frequencies.

Solution structure of NaNO3 in water: diffraction and molecular dynamics simulation study

The structure of a series of aqueous sodium nitrate solutions (1.9-7.6 M) was studied using a combination of experimental and theoretical methods. The results obtained from diffraction (X-ray, neutron) and molecular dynamics simulation have been compared and the capabilities and limitations of the methods in describing solution structure are discussed. For the solutions studied, diffraction methods were found to perform very well in description of hydration spheres of the sodium ion but do not yield detailed structural information on the anion's hydration structure. Molecular dynamics simulations proved to be a suitable tool in the detailed interpretation of the hydration sphere of ions, ion pair formation, and bulk structure of solutions.

Hydration of alkali ions from first principles molecular dynamics revisited

Structural and dynamical properties of the hydration of Li(+), Na(+), and K(+) in liquid water at ambient conditions were studied by first principles molecular dynamics. Our simulations successfully captured the different hydration behavior shown by the three alkali ions as observed in experiments. The present analyses of the dependence of the self-diffusion coefficient and rotational correlation time of water on the ion concentration suggest that Li(+) (K(+)) is certainly categorized as a structure maker (breaker), whereas Na(+) acts as a weak structure breaker. An analysis of the relevant electronic structures, based on maximally localized Wannier functions, revealed that the dipole moment of H(2)O molecules in the first solvation shell of Na(+) and K(+) decreases by about 0.1 D compared to that in the bulk, due to a contraction of the oxygen lone pair orbital pointing toward the metal ion.