The structure of aqueous sodium hydroxide solutions: A combined solution x-ray diffraction and simulation study

Dressing a Nonpolarizable Force Field for OH- in TIP4P/2005 Aqueous Solutions with Corrected Hirshfeld Charges

We present a rigid model for the OH- ion parametrized for binary mixtures with TIP4P/2005-type water molecules. Li+, Na+ and K+ were selected as counterions, hence mimicking the important and widely used solutions of soluble alkaline hydroxides. The optimized atomic charge distributions were obtained by scaling in a factor of 0.85 those derived from the atomic dipole corrected Hirshfeld approach. The agreement between experimental and Molecular Dynamics simulation results is remarkable for a set of properties, namely, the dependence of the density of the solutions on the hydroxide concentration and on temperature, the structure (i.e., positions of the atom-to-atom radial distribution functions and coordination numbers), the viscosity coefficients, the surface tension, or the freezing point depression. The proposed optimized potential parameters for OH- thus enlarge the set of models comprised within the Madrid-2019 force field and widen the potential applicability of the TIP4P/2005 water model in basic environments.

Alkali hydroxide (LiOH, NaOH, KOH) in water: Structural and vibrational properties, including neutron scattering results

Structural and vibrational properties of aqueous solutions of alkali hydroxides (LiOH, NaOH, and KOH) are computed using quantum molecular dynamics simulations for solute concentrations ranging between 1 and 10M. Element-resolved partial radial distribution functions, neutron and x-ray structure factors, and angular distribution functions are computed for the three hydroxide solutions as a function of concentration. The vibrational spectra and frequency-dependent conductivity are computed from the Fourier transforms of velocity autocorrelation and current autocorrelation functions. Our results for the structure are validated with the available neutron data for 17M concentration of NaOH in water [Semrouni et al., Phys. Chem. Chem. Phys. 21, 6828 (2019)]. We found that the larger ionic radius [rLi+ Collapse

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Affiliation(s)

Ruru Ma Collaboratory for Advanced Computing and Simulations, University of Southern California, Los Angeles, California 90007-0242, USA
Nitish Baradwaj Collaboratory for Advanced Computing and Simulations, University of Southern California, Los Angeles, California 90007-0242, USA
Ken-Ichi Nomura Collaboratory for Advanced Computing and Simulations, University of Southern California, Los Angeles, California 90007-0242, USA
Aravind Krishnamoorthy Department of Mechanical Engineering, Texas A&M University, College Station, Texas 77843-3123, USA
Rajiv K Kalia Collaboratory for Advanced Computing and Simulations, University of Southern California, Los Angeles, California 90007-0242, USA
Aiichiro Nakano Collaboratory for Advanced Computing and Simulations, University of Southern California, Los Angeles, California 90007-0242, USA
Priya Vashishta Collaboratory for Advanced Computing and Simulations, University of Southern California, Los Angeles, California 90007-0242, USA

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Computational study of the interactions of tetravalent actinides (An = Th-Pu) with the α-Fe13 Keggin cluster

In recent years, evidence has emerged that actinide (An) uptake at the enhanced actinide removal plant (EARP) at the UK's Sellafield nuclear site occurs via An interactions with an α-Fe13 Keggin molecular cluster during ferrihydrite formation. We here study theoretically the substitution of aquo complexes of the actinides Th-Pu onto a Na-decorated α-Fe13 Keggin cluster using DFT at the PBE0 level. The optimised Pu-O and Pu-Fe distances are in good agreement with experiment and Na/An substitutions are significantly favourable energetically, becoming more so across the early 5f series, with the smallest and largest ΔrG° being for Th and Pu at -335.7 kJ mol-1 and -396.0 kJ mol-1 respectively. There is strong correlation between the substitution reaction energy and the ionic radii of the actinides (Δrε0R2 = 0.97 and ΔrG° R2 = 0.91), suggesting that the principal An-Keggin binding mode is ionic. Notwithstanding this result, Mulliken and natural population analyses reveal that covalency increases from Th-Pu in these systems, supported by analysis of the occupied Kohn-Sham molecular orbitals where enhanced An(5f)-O(2p) overlap is observed in the Np and Pu systems. By contrast, quantum theory of atoms in molecules analysis shows that U-Keggin binding is the most covalent among the five actinides, in keeping with previous studies.

Comparative Investigation of the Microstructure of MgCl2 Aqueous Solutions Using Different X-ray Scattering Sources, Raman Spectroscopy, and Atomistic Simulations

Aqueous solutions of magnesium chloride (MgCl2(aq)) are often used to test advances in the theory of electrolyte solutions because they are considered an ideal strong 2:1 electrolyte. However, there is evidence that some ion association occurs in these solutions, even at low concentrations. Even a small ion-pairing constant can have a significant impact on the chemical speciation of ions, so it is important to determine whether ion pairing actually occurs. In this study, MgCl2(aq) with concentrations ranging from 1 to 35% was studied using three methods: X-ray scattering (XRS) with the Shanghai Synchrotron Radiation Facility (SSRF) and silver-anode laboratory sources, Raman spectroscopy, and molecular dynamics (MD) simulations with the COMPASS-II and Madrid force fields. XRS results were analyzed in the framework of PDF theory to obtain the reduced structure function F(Q) and the reduced pair distribution function G(r). The F(Q) values from synchrotron radiation and laboratory sources both showed that the tetrahedral hydrogen bonds in bulk water were destroyed with the increased MgCl2 concentration. The results of G(r) indicated that the main peaks centered at 2.05 and 2.80 Å can be ascribed to the interactions of Mg-O and O-O, respectively. The peak at 3.10 Å is attributed to the combined effect of O-O and Cl-O. By comparing the structural information on MgCl2 solution obtained from the two light sources, it was found that both SSRF and silver-anode laboratory sources can reflect the above-mentioned structural information on MgCl2 solution. The radial distribution function (RDF) obtained from MD simulations of MgCl2 solutions assigned the peaks at 2.0, 2.8, and 3.2 Å to the Mg-O, O-O, and Cl-O interatomic pairs, respectively. The decrease in the O-O coordination number confirms that the hydrogen-bonding network of water is disrupted by increasing MgCl2 observed by X-ray scattering. The proportion of Mg-Cl contact ion pairs gradually increases with MgCl2 concentration as does the coordination number. Raman spectroscopy results show that the bond type changes from double donor double acceptor (DDAA) to single donor-single acceptor (DA) with increasing concentration, providing explicit details of the hydrogen-bond evolution in the aqueous solution.

Exploring the impact of ions on oxygen K-edge X-ray absorption spectroscopy in NaCl solution using the GW-Bethe-Salpeter-equation approach

X-ray absorption spectroscopy (XAS) is a powerful experimental tool to probe the local structure in materials with the core hole excitations. Here, the oxygen K-edge XAS spectra of the NaCl solution and pure water are computed by using a recently developed GW-Bethe-Salpeter equation approach, based on configurations modeled by path-integral molecular dynamics with the deep-learning technique. The neural network is trained on ab initio data obtained with strongly constrained and appropriately normed density functional theory. The observed changes in the XAS features of the NaCl solution, compared to those of pure water, are in good agreement between experimental and theoretical results. We provided detailed explanations for these spectral changes that occur when NaCl is solvated in pure water. Specifically, the presence of solvating ion pairs leads to localization of electron-hole excitons. Our theoretical XAS results support the theory that the effects of the solvating ions on the H-bond network are mainly confined within the first hydration shell of ions, however beyond the shell the arrangement of water molecules remains to be comparable to that observed in pure water.

Molecular understanding of cation effects on double layers and their significance to CO-CO dimerization

Cation effects have been shown in numerous experiments to play a significant role in electrocatalysis. To understand these effects at the molecular level, we systematically investigate the structures and capacitances of electric double layers with a variety of cations as counter charges at Pt(111)-COad/water interfaces with ab initio molecular dynamics. It is encouraging to find that the computed Helmholtz capacitances for different cations are in quantitative agreement with experiments, and that the trend of cation effects on capacitances shows clear correlation with the structures of interface cations of differing sizes and hydration energies. More importantly, we demonstrate the Helmholtz capacitance as the key descriptor for measuring the activity of CO-CO dimerization, the rate-determining step for C2+ formation in electroreduction of CO and CO2. Our work provides atomistic insights into cation effects on electric double layers and electrocatalysis that are crucial for optimizing electrode and electrolyte materials.

Highly-efficient molten NaOH-KOH for organochlorine destruction: Performance and mechanism

Molten salt has been increasingly acknowledged to be useful in the destruction of chlorine-containing organic wastes (COWs), e.g., organochlorine. However, the operational temperatures are usually high, and local structure and thermodynamic property of the molten salt remain largely unclear. In this study, novel molten NaOH-KOH is developed for organochlorine destruction, and its eutectic point can be lowered to 453 K with 1:1 mol ratio of NaOH to KOH. Further experiment shows that this molten NaOH-KOH is highly-efficient towards the destructions of both trichlorobenzene and dichlorophenol, acquiring the final dechlorination efficiencies as 88.2% and 94.1%, respectively. The organochlorine destruction and chloride salt enrichment are verified by fourier-transform infrared spectrometer. Molten NaOH-KOH not only eliminates the C-Cl and CC bonds, but also traps generated CO₂, other acidic gases, and possibly particulate matters as a result of the high surface area and high viscosity. This makes it possibly advantageous over incineration for organic waste destruction for carbon neutrality. To sufficiently reveal the inherent mechanism for the temperature dependent performance, molecular dynamics simulation is further adopted. Results show that the radial distance between ions increases with temperature, causing larger molar volume and lower resistance to shear deformation. Moreover, thermal expansion coefficient, specific heat capacity, and ion self-diffusion coefficient of the molten NaOH-KOH are found to increase linearly with temperature. All these microscopic alterations contribute to the organochlorine destruction. This study benefits to develop highly-efficient molten system for COWs treatment via a low-carbon approach.

Solvation structures of calcium and magnesium ions in water with the presence of hydroxide: a study by deep potential molecular dynamics

The solvation structures of calcium (Ca²⁺) and magnesium (Mg²⁺) ions with the presence of hydroxide (OH^-) ion in water are essential for understanding their roles in biological and chemical processes but have not been fully explored. Ab initio molecular dynamics (AIMD) is an important tool to address this issue, but two challenges exist. First, an accurate description of OH^- from AIMD needs an appropriate exchange-correlation functional. Second, a long trajectory is needed to reach an equilibrium state for the Ca²⁺-OH^- and Mg²⁺-OH^- ion pairs in aqueous solutions. Herein, we adopt a deep potential molecular dynamics (DPMD) method to simulate 1 ns trajectories for the Ca²⁺-OH^- and Mg²⁺-OH^- ion pairs in water; the DPMD method provides efficient machine-learning-based models that have the accuracy of the SCAN exchange-correlation functional within the framework of density functional theory. The solvation structures of the cations and the OH^- in terms of three different species have been systematically investigated. On the one hand, we find that OH^- have more significant effects on the solvation structure of Ca²⁺ than that of Mg²⁺. We observe that the OH^- substantially affects the orientation angles of water molecules surrounding the cation. Through the time correlation functions, we conclude that the water molecules in the first solvation shell of Ca²⁺ change their preferred orientation faster than those of Mg²⁺. On the other hand, with the presence of the cation in the first solvation shell of OH^-, we find that the hydrogen bonds of OH^- are severely altered, and the adjacent water molecules of OH^- are squeezed. The two cations have substantially different effects on the solvation structure of OH^-. Our work provides new insight into the solvation structures of Ca²⁺ and Mg²⁺ in water with the presence of OH^-.

A New Force Field for OH- for Computing Thermodynamic and Transport Properties of H2 and O2 in Aqueous NaOH and KOH Solutions

The thermophysical properties of aqueous electrolyte solutions are of interest for applications such as water electrolyzers and fuel cells. Molecular dynamics (MD) and continuous fractional component Monte Carlo (CFCMC) simulations are used to calculate densities, transport properties (i.e., self-diffusivities and dynamic viscosities), and solubilities of H2 and O2 in aqueous sodium and potassium hydroxide (NaOH and KOH) solutions for a wide electrolyte concentration range (0-8 mol/kg). Simulations are carried out for a temperature and pressure range of 298-353 K and 1-100 bar, respectively. The TIP4P/2005 water model is used in combination with a newly parametrized OH- force field for NaOH and KOH. The computed dynamic viscosities at 298 K for NaOH and KOH solutions are within 5% from the reported experimental data up to an electrolyte concentration of 6 mol/kg. For most of the thermodynamic conditions (especially at high concentrations, pressures, and temperatures) experimental data are largely lacking. We present an extensive collection of new data and engineering equations for H2 and O2 self-diffusivities and solubilities in NaOH and KOH solutions, which can be used for process design and optimization of efficient alkaline electrolyzers and fuel cells.

Hydrated Anions: From Clusters to Bulk Solution with Quasi-Chemical Theory

The interactions of hydrated ions with molecular and macromolecular solution and interface partners are strong on a chemical energy scale. Here, we recount the foremost ab initio theory for the evaluation of the hydration free energies of ions, namely, quasi-chemical theory (QCT). We focus on anions, particularly halides but also the hydroxide anion, because they have been outstanding challenges for all theories. For example, this work supports understanding the high selectivity for F^- over Cl^- in fluoride-selective ion channels despite the identical charge and the size similarity of these ions. QCT is built by the identification of inner-shell clusters, separate treatment of those clusters, and then the integration of those results into the broader-scale solution environment. Recent work has focused on a close comparison with mass-spectrometric measurements of ion-hydration equilibria. We delineate how ab initio molecular dynamics (AIMD) calculations on ion-hydration clusters, elementary statistical thermodynamics, and electronic structure calculations on cluster structures sampled from the AIMD calculations obtain just the free energies extracted from the cluster experiments. That theory-experiment comparison has not been attempted before the work discussed here, but the agreement is excellent with moderate computational effort. This agreement reinforces both theory and experiment and provides a numerically accurate inner-shell contribution to QCT. The inner-shell complexes involving heavier halides display strikingly asymmetric hydration clusters. Asymmetric hydration structures can be problematic for the evaluation of the QCT outer-shell contribution with the polarizable continuum model (PCM). Nevertheless, QCT provides a favorable setting for the exploitation of PCM when the inner-shell material shields the ion from the outer solution environment. For the more asymmetrically hydrated, and thus less effectively shielded, heavier halide ions clustered with waters, the PCM is less satisfactory. We therefore investigate an inverse procedure in which the inner-shell structures are sampled from readily available AIMD calculations on the bulk solutions. This inverse procedure is a remarkable improvement; our final results are in close agreement with a standard tabulation of hydration free energies, and the final composite results are independent of the coordination number on the chemical energy scale of relevance, as they should be. Finally, a comparison of anion hydration structure in clusters and bulk solutions from AIMD simulations emphasize some differences: the asymmetries of bulk solution inner-shell structures are moderated compared with clusters but are still present, and inner hydration shells fill to slightly higher average coordination numbers in bulk solution than in clusters.

Behaviors of sodium and calcium ions at the borosilicate glass–water interface: Gaining new insights through an ab initio molecular dynamics study

We study reactivity and leaching at the calcium sodium borosilicate (CNBS)–water interface by means of a Car–Parrinello ab initio molecular dynamics simulation over a simulation time of 100 ps. With an emphasis on the comparison between the behaviors of Ca2+ and Na+ cations at the CNBS glass–water interface, different mechanism events during the trajectory are revealed, discussed, and correlated with other density functional theory calculations. We show that Na+ ions can be released in solution, while Ca2+ cannot leave the surface of CNBS glass. This release is correlated with the vacancy energy of Ca2+ and Na+ cations. Here, we found that the CNBS structure with the Na+ cation vacancy is energetically more favorable than the structure with the Ca2+ cation vacancy. The calcium adsorption site has been shown to have a greater affinity for water than can be found in the case of the sodium site, demonstrating that affinity may not be considered a major factor controlling the release of cations from the glass to the solution.

Super-ions of sodium cations with hydrated hydroxide anions: inorganic structure-directing agents in zeolite synthesis

In inorganic zeolite formation, a direct correspondence between liquid state species in the synthesis and the supramolecular decoration of the pores in the as-made final zeolite has never been reported. In this paper, a direct link between the sodium speciation in the synthesis mixture and the pore structure and content of the final zeolite is demonstrated in the example of hydroxysodalite. Super-ions with 4 sodium cations bound by mono- and bihydrated hydroxide are identified as structure-directing agents for the formation of this zeolite. This documentation of inorganic solution species acting as a templating agent in zeolite formation opens new horizons for zeolite synthesis by design.

Nanostructure of bone tissue probed with Ca 2p and O 1s NEXAFS spectroscopy

X-ray absorption spectroscopy is applied to investigate relationships between hierarchical organization of the skeleton and nanostructure of femoral bone in knee compartments and to understand the osteoarthritis (OA) related changes at the subcellular level. Our focus is on local electronic and atomic and molecular architectonics of the medial and lateral condyles of the femur resected during total knee arthroplasty in patients with medial compartmental knee OA. The element-specific and site-dependent peculiarities in spectral distributions of oscillator strength for core-to-valence transitions are revealed. The near Ca 2p and O 1s edges x-ray absorption fine structure (Ca 2p and O 1s NEXAFS) spectra of the saw cuts demonstrate substantial redistributions in intact and OA damaged areas on the proximal side, and on the proximal and distal sides of the samples. Examining the O 1s NEXAFS spectra new chemical bonds are revealed on the proximal surface in the OA areas. Strong intra-atomic intershell Ca2+ 2 p 3 / 2 , 1 / 2 5 3 d 1 interaction specifies the great similarity of the Ca 2p NEXAFS spectra. Their analysis performed in combination with the x-ray photoelectron data has demonstrated the formation of non-apatite calcium in the OA areas of the samples. It is shown that NEXAFS spectroscopy is a powerful tool for deeper understanding relationship between hierarchical skeletal organization and nanostructure of native bone. Perspectives for development of novel methods for medical imaging and diagnosis of subchondral bone at the nanolevel are discussed.

Hydroxide promotes ion pairing in the NaNO2-NaOH-H2O system

Nitrite (NO2-) is a prevalent nitrogen oxyanion in environmental and industrial processes, but its behavior in solution, including ion pair formation, is complex. This solution phase complexity impacts industries such as nuclear waste treatment, where NO2- significantly affects the solubility of other constituents present in sodium hydroxide (NaOH)-rich nuclear waste. This work provides molecular scale information into sodium nitrite (NaNO2) and NaOH ion-pairing processes to provide a physical basis for later development of thermodynamic models. Solubility isotherms of NaNO2 in aqueous mixtures with NaOH and total alkalinity were also measured. Spectroscopic characterization of these solutions utilized high-field nuclear magnetic resonance spectroscopy (NMR) and Raman spectroscopy, with additional solution structure detailed by X-ray total scattering pairwise distribution function analysis (X-ray PDF). Despite the NO2- deformation Raman band's insensitivity to added NaOH in saturated NaNO2 solutions, 23Na and 15N NMR studies indicated the Na+ and NO2- chemical environments change likely due to ion pairing. The ion pairing correlates with a decrease in diffusion coefficient of solution species as measured by pulsed field gradient 23Na and 1H NMR. Two-dimensional correlation analyses of the 2800-4000 cm-1 Raman region and X-ray PDF indicated that saturated NaNO2 and NaOH mixtures disrupt the hydrogen network of water into a new structure where the length of the OO correlations is contracted relative to the typical H2O structure. Beyond describing the solubility of NaNO2 in a multicomponent electrolyte mixture, these results also indicate that nitrite exhibits greater ion pairing in mixtures of concentrated NaNO2 and NaOH than in comparable solutions with only NaNO2.

Structure of aqueous sodium acetate solutions by X-Ray scattering and density functional theory

The structure of aq. sodium acetate solution (CH3COONa, NaOAc) was studied by X-ray scattering and density function theory (DFT). For the first hydrated layer of Na+, coordination number (CN) between Na+ and O(W, I) decreases from 5.02 ± 0.85 at 0.976 mol/L to 3.62 ± 1.21 at 4.453 mol/L. The hydration of carbonyl oxygen (OC) and hydroxyl oxygen (OOC) of CH3COO− were investigated separately and the OC shows a stronger hydration bonds comparing with OOC. With concentrations increasing, the hydration shell structures of CH3COO− are not affected by the presence of large number of ions, each CH3COO− group binds about 6.23 ± 2.01 to 7.35 ± 1.73 water molecules, which indicates a relatively strong interaction between CH3COO− and water molecules. The larger uncertainty of the CN of Na+ and OC(OOC) reflects the relative looseness of Na-OC and Na-OOC ion pairs in aq. NaOAc solutions, even at the highest concentration (4.453 mol/L), suggesting the lack of contact ion pair (CIP) formation. In aq. NaOAc solutions, the so called “structure breaking” property of Na+ and CH3COO− become effective only for the second hydration sphere of bulk water. The DFT calculations of CH3COONa (H2O)n=5–7 clusters suggest that the solvent-shared ion pair (SIP) structures appear at n = 6 and become dominant at n = 7, which is well consistent with the result from X-ray scattering.

ABEEM/MM OH- Models for OH-(H2O)n Clusters and Aqueous OH-: Structures, Charge Distributions, and Binding Energies

Based on the atom-bond electronegativity equalization method fused into molecular mechanics (ABEEM/MM), two fluctuating charge models of OH^--water system were proposed. The difference between these two models is whether there is charge transfer between OH^- and its first-shell water molecules. The structures, charge distributions, charge transfer, and binding energies of the OH^-(H₂O)_n (n = 1-8, 10, 15, 23) clusters were studied by these two ABEEM/MM models, the OPLS/AA force field, the OPLS-SMOOTH/AA force field, and the QM methods. The results demonstrate that two ABEEM/MM models can search out all stable structures just as the QM methods, and the structures and charge distributions agree well with those from the QM calculations. The structures, the charge transfer, and the strength of hydrogen bonds in the first hydration shell are closely related to the coordination number of OH^-. Molecular dynamics simulations on the aqueous OH^- solution are performed at 298 and 278 K using ABEEM/MM-I model. The MD results show that the populations of three-, four-, and five-coordinated OH^- are 29.6%, 67.1%, and 3.4% at 298 K, respectively, and those of two-, three-, four-, and five-coordinated OH^- are 10.8%, 44.9%, 39.2%, and 4.9% at 278 K, respectively; the average hydrogen bond lengths and the hydrogen bond angle in the first shell increase with the temperature decreasing.

Mechanical Stabilization of Deoxyribonucleic Acid Solid Films Based on Hydrated Ionic Liquid

Solid films of deoxyribonucleic acid (DNA) containing a hydrated ionic liquid, choline dihydrogen phosphate (CDP), were prepared by a solvent-casting method. Thermal properties, aggregation structure, thermal molecular motion, and tensile properties of CDP-containing DNA films were examined by thermogravimetry (TG), wide-angle X-ray diffraction (WAXD) measurement, dynamic mechanical analysis (DMA), and tensile tests, respectively. The water retentivity of the films at room temperature was much improved with CDP. The packing density of DNA helical chains clearly depended on the amount of CDP in the film. A small amount of CDP contributed to the suppression of the B_I → B_II conformational transition and the cooperative motion of the DNA duplex in the film. The tensile properties of the film drastically changed in the presence of CDP. When the amount of hydrated CDP in the film increased, the mechanical response of the film changed from glassy-like to rubbery-like via a semicrystalline-like state. The above results make it clear that CDP plays two major roles as a water absorber and plasticizer in the DNA film. Thus, it can be concluded that the use of an ionic liquid as an additive significantly increases the possibility of using a DNA solid film as a structural material.

Effect of confinement on the adsorption behavior of inorganic and organic ions at aqueous-cyclohexane interfaces: a molecular dynamics study

Molecular dynamics simulation was used to study the adsorption behavior of inorganic and organic ions at aqueous-cyclohexane interfaces, and that with such systems confined in a kaolinite nanopore. Four aqueous solutions were used, each containing one of the four solutes (NaCl, NaOH, CaCl₂ and Ca(OH)₂) at the concentration of 1.0 M. At the interface of each of the solutions with neat cyclohexane, there existed an ion depletion zone. The more strongly hydrated ions, such as calcium and hydroxide, were more intensely depleted from the interface as compared to sodium and chloride. Such surface exclusion led to interfacial tension increases, with greater increments for solutions containing calcium or hydroxide. Upon addition of sodium decanoate to the cyclohexane phase, they partly migrated to the aqueous-organic interface, and the remaining formed inverted micelle complexes. Also, a small fraction of the solvated cations in the aqueous phase drifted to the depletion zone to interact with the organic anions, with their affinity towards the interface still being controlled by their hydration-strength. When such systems were confined in a kaolinite nanopore, behavior of decanoate anions - which were found to be prone to strong adsorption to solid surfaces in the absence of any aqueous solution - was determined by the nature of the solvated ions in water. In these systems, the more weakly-hydrated ionic species exhibited preferential adsorption to the solid surface, while affinity of the more strongly-hydrated ones was towards remaining within the water layers or to the aqueous-organic interface. With the calcium chloride solution, almost all of the organic ions were detached from the surface and adsorbed at the aqueous-cyclohexane interface. This was caused by the release of double amount of inorganic anions by this 1 : 2 salt, the inner-sphere adsorption of majority of chloride anions to the octahedral surface, and the higher charge density of calcium cations.

Hydrated Sodium Ion Clusters [Na+(H2O)n (n = 1-6)]: An ab initio Study on Structures and Non-covalent Interaction

Structural, thermodynamic, and vibrational characteristics of water clusters up to six water molecules incorporating a single sodium ion [Na⁺(H₂O)_n (n = 1–6)] are calculated using a comprehensive genetic algorithm combined with density functional theory on global search, followed by high-level ab initio calculation. For n ≥ 4, the coordinated water molecules number for the global minimum of clusters is 4 and the outer water molecules connecting with coordinated water molecules by hydrogen bonds. The charge analysis reveals the electron transfer between sodium ions and water molecules, providing an insight into the variations of properties of O–H bonds in clusters. Moreover, the simulated infrared (IR) spectra with anharmonic correction are in good agreement with the experimental results. The O–H stretching vibration frequencies show redshifts comparing with a free water molecule, which is attributed to the non-covalent interactions, including the ion–water interaction, and hydrogen bonds. Our results exhibit the comprehensive geometries, energies, charge, and anharmonic vibrational properties of Na⁺(H₂O)_n (n = 1–6), and reveal a deeper insight of non-covalent interactions.

Investigation of the Structure of Concentrated NaOH Aqueous Solutions by Combining Molecular Dynamics and Wide-Angle X-ray Scattering

Classical molecular dynamics has been performed with explicit polarization on NaOH aqueous solutions from 0.5 mol L^-1 up to 9.7 mol L^-1. We adapted a force field of OH^- for polarizable simulation in order to reproduce the NaOH structural and thermodynamics properties in aqueous solutions. A good agreement between theoretical and experimental results has been found. Wide-angle X-ray scattering (WAXS) intensities issued from molecular dynamics are compared to experimental ones measured on Synchrotron facilities. The structure of the first coordination shell of Na⁺ has been studied to determine the variation of the oxygen number and hydroxide oxygen around the cation. In addition, Na⁺-OH^- McMillan-Mayer potential issued from molecular dynamics simulations has been calculated and allows for calculating Na⁺-OH^- pair association constant of 0.1 L mol^-1, which is in good agreement with the experiments.

Resolving local configurational contributions to X-ray and neutron radial distribution functions within solutions of concentrated electrolytes - a case study of concentrated NaOH

Extreme conditions of complex materials often lead to a manifold of local environments that challenge characterization and require new advances at the intersection of modern experimental and theoretical techniques. In this contribution, highly caustic and viscous aqueous NaOD solutions were characterized with a combination of X-ray and neutron radial distribution function (RDF) analyses, molecular dynamics simulations and sub-ensemble analysis. While this system has been the topic of some study, the current work expands upon the state of knowledge regarding the extent to which water is perturbed within this chemically extreme solution. Further, we introduce analyses that goes beyond merely identifying the different local environments (ion solvation and coordination environments) that are present, but toward understanding their relative contributions to the ensemble solution RDF. This integrated approach yields unique insight into the experimental sensitivity of RDFs to changes in local geometries, the composition of solvation environments about ions, and the challenge of experimentally differentiating the ensemble of all superimposed local environments-a feature of increasing importance within the extreme condition of high ionic strength.

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.

Local Activities of Hydroxide and Water Determine the Operation of Silver-Based Oxygen Depolarized Cathodes

Local ion activity changes in close proximity to the surface of an oxygen depolarized cathode (ODC) were measured by scanning electrochemical microscopy (SECM). While the operating ODC produces OH^- ions and consumes O₂ and H₂ O through the electrocatalytic oxygen reduction reaction (ORR), local changes in the activity of OH^- ions and H₂ O are detected by means of a positioned Pt microelectrode serving as an SECM tip. Sensing at the Pt tip is based on the pH-dependent reduction of PtO and obviates the need for prior electrode modification steps. It can be used to evaluate the coordination numbers of OH^- ions and H₂ O, and the method was exploited as a novel approach of catalyst activity assessment. We show that the electrochemical reaction on highly active catalysts can have a drastic influence on the reaction environment.

Aqueous sodium hydroxide (NaOH) solutions at high pressure and temperature: insights from in situ Raman spectroscopy and ab initio molecular dynamics simulations

Hydrothermal diamond anvil cell experiments in combination with Raman spectroscopy and first principles molecular dynamics simulations were performed to investigate the structure and dynamics of aqueous NaOH solutions for temperatures up to 700 °C, pressures up to 850 MPa and two different solute concentrations. The significant changes observed in the O-H stretching region of the Raman spectra between ambient and supercritical conditions are explained by both dynamic effects and structural differences. Especially important are a Grotthuss-like proton transport process and the decreasing network connectivity of the water molecules with increasing temperature. The observed transfer of Raman intensity towards lower wavenumbers by the proton transfer affects a wide range of frequencies and must be considered in the interpretation of Raman spectra of highly basic solutions. We suggest a deconvolution of the spectra using a model with four Gaussian functions, which are assigned to the molecular H2O and OH- vibrations, and one asymmetric exponentially modified Gaussian (EMG) function, which is assigned to [HO(H2O)n]- vibrations.

Structure of aqueous NaOH solutions: insights from neural-network-based molecular dynamics simulations

Sodium hydroxide, NaOH, is one of the most widely-used chemical reagents, but the structural properties of its aqueous solutions have only sparingly been characterized. Here, we automatically classify the cation coordination polyhedra obtained from molecular dynamics simulations. We find that, for example, with increasing concentration, octahedral coordination geometries become less favored, while the opposite is true for the trigonal prism. At high concentrations, the coordination polyhedra frequently deviate considerably from "ideal" polyhedra, because of an increased extent of interligand hydrogen-bonding, in which hydrogen bonds between two ligands, either OH₂ or OH^-, around the same Na⁺ are formed. In saturated solutions, with concentrations of about 19 mol L^-1, ligands are frequently shared between multiple Na⁺ ions as a result of the deficiency of solvent molecules. This results in more complex structural patterns involving certain "characteristic" polyhedron connectivities, such as octahedra sharing ligands with capped trigonal prisms, and tetrahedra sharing ligands with trigonal bipyramids. The simulations were performed using a density-functional-theory-based reactive high-dimensional neural network potential, that was extensively validated against available neutron and X-ray diffraction data from the literature.

Interaction of NaOH solutions with silica surfaces

HYPOTHESIS

Sodium adsorption on silica surfaces depends on the solution counter-ion. Here, we use NaOH solutions to investigate basic environments.

SIMULATIONS

Sodium adsorption on hydroxylated silica surfaces from NaOH solutions were investigated through molecular dynamics with a dissociative force field, allowing for the development of secondary molecular species.

FINDINGS

Across the NaOH concentrations (0.01 M - 1.0 M), ∼50% of the Na⁺ ions were concentrated in the surface region, developing silica surface charges between - 0.01 C/m² (0.01 M NaOH) and - 0.76 C/m² (1.0 M NaOH) due to surface site deprotonation. Five inner-sphere adsorption complexes were identified, including monodentate, bidentate, and tridentate configurations and two additional structures, with Na⁺ ions coordinated by bridging oxygen and hydroxyl groups or water molecules. Coordination of Na⁺ ions by bridging oxygen atoms indicates partial or complete incorporation of Na⁺ ions into the silica surface. Residence time analysis identified that Na⁺ ions coordinated by bridging oxygen atoms stayed adsorbed onto the surface four times longer than the mono/bi/tridentate species, indicating formation of relatively stable and persistent Na⁺ ion adsorption structures. Such inner-sphere complexes form only at NaOH concentrations of > 0.5 M. Na⁺ adsorption and lifetimes have implications for the stability of silica surfaces.

Interaction between Water and Alkali Metal Ions and Its Temperature Dependence Revealed by Oxygen K-Edge X-ray Absorption Spectroscopy

Interaction between water molecules and alkali metal ions in aqueous salt solutions has been studied by the oxygen K-edge soft X-ray absorption spectroscopy (XAS) in transmission mode. In the measurement of several alkali halide aqueous solutions with different alkali chlorides (Li, Na, and K) and different sodium halides (Cl, Br, and I), the pre-edge component arising from the hydration water molecules shows a blue shift in peak energy as strongly depending on cations but not on anions. In the temperature dependent measurement, the pre-edge component arising from water molecules beyond the first hydration shell shows the same behavior as that of pure liquid water. On the other hand, the pre-edge component arising from water molecules in the first hydration shell of Li⁺ ions is not evidently dependent on the temperature, indicating that the hydration water molecules are more strongly bound with Li⁺ ions than the other water molecules. These experimental results are supported by the results of radial distribution functions of the first hydration shell and their temperature dependence, evaluated by molecular dynamics simulations.

Ionic diffusion and proton transfer in aqueous solutions of alkali metal salts

We report on a series of ab initio molecular dynamics investigations on LiCl, NaCl, and KCl aqueous solutions under the effect of static electric fields.

Energetics and diffusion of liquid water and hydrated ions through nanopores in graphene: ab initio molecular dynamics simulation

The energetics and diffusion of water molecules and hydrated ions (Na⁺, Cl⁻) passing through nanopores in graphene are addressed by dispersion-corrected density functional theory calculations and ab initio molecular dynamics (MD) simulations.

Local electronic structure and nanolevel hierarchical organization of bone tissue: theory and NEXAFS study

Theoretical and experimental investigations of native bone are carried out to understand relationships between its hierarchical organization and local electronic and atomic structure of the mineralized phase. The 3D superlattice model of a coplanar assembly of the hydroxyapatite (HAP) nanocrystallites separated by the hydrated nanolayers is introduced to account the interplay of short-, long- and super-range order parameters in bone tissue. The model is applied to (i) predict and rationalize the HAP-to-bone spectral changes in the electronic structure and (ii) describe the mechanisms ensuring the link of the hierarchical organization with the electronic structure of the mineralized phase in bone. To check the predictions the near-edge x-ray absorption fine structure (NEXAFS) at the Ca 2p, P 2p and O 1s thresholds is measured for native bone and compared with NEXAFS for reference compounds. The NEXAFS analysis has demonstrated the essential hierarchy induced HAP-to-bone red shifts of the Ca and P 2p-to-valence transitions. The lowest O 1s excitation line at 532.2 eV in bone is assigned with superposition of core transitions in the hydroxide OH^-(H₂O) _m anions, Ca²⁺(H₂O) _n cations, the carboxyl groups inside the collagen and [PO₄]^2- and [PO₄]^- anions with unsaturated P-O bonds.