Ion hydration under pressure

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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Solvent Dynamics of Aqueous Halides before and after Photoionization

Electron transfer reactions can be strongly influenced by solvent dynamics. We study the photoionization of halides in water as a model system for such reactions. There are no internal nuclear degrees of freedom in the solute, allowing the dynamics of the solvent to be uniquely identified. We simulate the equilibrium solvent dynamics for Cl^-, Br^-, I^-, and their respective neutral atoms in water, comparing quantum mechanical/molecular mechanical (QM/MM) and classical molecular dynamics (MD) methods. On the basis of the obtained configurations, we calculate the extended X-ray absorption fine structure (EXAFS) spectra rigorously based on the MD snapshots and compare them in detail with other theoretical and experimental results available in the literature. We find our EXAFS spectra based on QM/MM MD simulations in good agreement with their experimental counterparts for the ions. Classical MD simulations for the ions lead to EXAFS spectra that agree equally well with the experiment when it comes to the oscillatory period of the signal, even though they differ from the QM/MM radial distribution functions extracted from the MD. The amplitude is, however, considerably overestimated. This suggests that to judge the reliability of theoretical simulation methods or to elucidate fine details of the atomistic dynamics of the solvent based on EXAFS spectra, the amplitude as well as the oscillatory period need to be considered. If simulations fail qualitatively, as does the classical MD for the aqueous neutral halogen atoms, the resulting EXAFS will also be strongly affected in both oscillatory period and amplitude. The good reliability of QM/MM-based EXAFS simulations, together with clear qualitative differences in the EXAFS spectra found between halides and their atomic counterparts, suggests that a combined theory and experimental EXAFS approach is suitable for elucidating the nonequilibrium solvent dynamics in the photoionization of halides and possibly also for electron transfer reactions in more complex systems.

Ion association in hydrothermal aqueous NaCl solutions: implications for the microscopic structure of supercritical water

Knowledge of the microscopic structure of fluids and changes thereof with pressure and temperature is important for the understanding of chemistry and geochemical processes. In this work we investigate the influence of sodium chloride on the hydrogen-bond network in aqueous solution up to supercritical conditions. A combination of in situ X-ray Raman scattering and ab initio molecular dynamics simulations is used to probe the oxygen K-edge of the alkali halide aqueous solution in order to obtain unique information about the oxygen's local coordination around the ions, e.g. solvation-shell structure and the influence of ion pairing. The measured spectra exhibit systematic temperature dependent changes, which are entirely reproduced by calculations on the basis of structural snapshots obtained via ab initio molecular dynamics simulations. Analysis of the simulated trajectories allowed us to extract detailed structural information. This combined analysis reveals a net destabilizing effect of the dissolved ions which is reduced with rising temperature. The observed increased formation of contact ion pairs and occurrence of larger polyatomic clusters at higher temperatures can be identified as a driving force behind the increasing structural similarity between the salt solution and pure water at elevated temperatures and pressures with drawback on the role of hydrogen bonding in the hot fluid. We discuss our findings in view of recent results on hot NaOH and HCl aqueous fluids and emphasize the importance of ion pairing in the interpretation of the microscopic structure of water.

Ion Solvation and Water Structure in an Aqueous Sodium Chloride Solution in the Gigapascal Pressure Range

The structure of a 3 m (= mol/kg) NaCl aqueous solution at 1.3 and 1.7 GPa and 300 K, as well as at an ambient condition, is determined by synchrotron X-ray diffraction measurements combined with an empirical potential structure refinement (EPSR) modeling. When the solution is pressurized to the gigapascal pressure range, the ice-like hydrogen-bonded water network at 300 K/0.1 MPa is drastically perturbed to give rise to a simple, liquid-like water molecules arrangement retaining the hydrogen bonds. The coordination number of the chloride ion increases from around 6 at 0.1 MPa to about 16 at 1.7 GPa, accompanied by the extended solvation shells' evolution. On the other hand, the sodium ion's solvation structure does not change significantly with pressure and consists of 6-fold water molecules' coordination. We discuss a structure makers/breakers' concept for the ion solvation concerning the water structure in the gigapascal pressure range.

Solvation structure of the halides from x-ray absorption spectroscopy

Three-dimensional models for the aqueous solvation structures of chloride, bromide, and iodide are reported. K-edge extended X-ray absorption fine structure (EXAFS) and Minuit X-ray absorption near edge (MXAN) analyses found well-defined single shell solvation spheres for bromide and iodide. However, dissolved chloride proved structurally distinct, with two solvation shells needed to explain its strikingly different X-ray absorption near edge structure (XANES) spectrum. Final solvation models were as follows: iodide, 8 water molecules at 3.60 ± 0.13 Å and bromide, 8 water molecules at 3.40 ± 0.14 Å, while chloride solvation included 7 water molecules at 3.15 ± 0.10 Å, and a second shell of 7 water molecules at 4.14 ± 0.30 Å. Each of the three derived solvation shells is approximately uniformly disposed about the halides, with no global asymmetry. Time-dependent density functional theory calculations simulating the chloride XANES spectra following from alternative solvation spheres revealed surprising sensitivity of the electronic state to 6-, 7-, or 8-coordination, implying a strongly bounded phase space for the correct structure during an MXAN fit. MXAN analysis further showed that the asymmetric solvation predicted from molecular dynamics simulations using halide polarization can play no significant part in bulk solvation. Classical molecular dynamics used to explore chloride solvation found a 7-water solvation shell at 3.12 (-0.04/+0.3) Å, supporting the experimental result. These experiments provide the first fully three-dimensional structures presenting to atomic resolution the aqueous solvation spheres of the larger halide ions.

Octa-Coordination and the Aqueous Ba(2+) Ion

The hydration structure of Ba(2+) ion is important for understanding blocking mechanisms in potassium ion channels. Here, we combine statistical mechanical theory, ab initio molecular dynamics simulations, and electronic structure methods to calculate the hydration free energy and local hydration structure of Ba(2+)(aq). The predicted hydration free energy (-304 ± 1 kcal/mol) agrees with the experimental value (-303 kcal/mol) when a maximally occupied, unimodal inner solvation shell is treated. In the local environment defined by the first shell of hydrating waters, Ba(2+) is directly and stably coordinated by eight (8) waters. Octa-coordination resembles the crystal structure of Ba(2+) and K(+) bound in potassium ion channels, but differs from the local hydration structure of K(+)(aq) determined earlier.

Using a combined theoretical and experimental approach to understand the structure and dynamics of imidazolium-based ionic liquids/water mixtures. 2. EXAFS spectroscopy

Extended X-ray absorption fine structure (EXAFS) spectroscopy is employed, in conjunction with molecular dynamics (MD) simulations, to investigate the interaction of water with the Br(-) ion in an imidazolium-based ionic liquid (IL). 1-Butyl-3-methylimidazolium bromide/water mixtures with molar ratios ranging from 1:3 to 1:200 have been analyzed, and a clear picture of the structural arrangements of the water molecules inside the IL has been obtained from the synergic interpretation of the EXAFS and MD data. At the lowest investigated water content, the presence of water is mainly detected around the Br(-) anion. Upon increasing the water fraction, more water molecules enter the Br(-) first-coordination shell but always in a lower number than what is needed to saturate the inner sphere. This suggests that interactions also exist between water and the imidazolium cation. The existence of tight ion pairs has been evidenced, even when water is present in the mixtures in great excess.

Absolute hydration free energy scale for alkali and halide ions established from simulations with a polarizable force field

A polarizable potential function for the hydration of alkali and halide ions is developed on the basis of the recent SWM4-DP water model [Lamoureux, G.; MacKerell, A. D., Jr.; Roux, B. J. Chem. Phys. 2003, 119, 5185]. Induced polarization is incorporated using classical Drude oscillators that are treated as auxiliary dynamical degrees of freedom. The ions are represented as polarizable Lennard-Jones centers, whose parameters are optimized to reproduce the binding energies of gas-phase monohydrates and the hydration free energies in the bulk liquid. Systematic exploration of the parameters shows that the monohydrate binding energies can be consistent with a unique hydration free energy scale if the computed hydration free energies incorporate the contribution from the air/water interfacial electrostatic potential (-540 mV for SWM4-DP). The final model, which can satisfyingly reproduce both gas and bulk-phase properties, corresponds to an absolute scale in which the intrinsic hydration free energy of the proton is -247 kcal/mol.

Ions in water: The microscopic structure of concentrated hydroxide solutions

Neutron-diffraction data on aqueous solutions of hydroxides, at solute concentrations ranging from 1 solute per 12 water molecules to 1 solute per 3 water molecules, are analyzed by means of a Monte Carlo simulation (empirical potential structure refinement), in order to determine the hydration shell of the OH- in the presence of the smaller alkali metal ions. It is demonstrated that the symmetry argument between H+ and OH- cannot be used, at least in the liquid phase at such high concentrations, for determining the hydroxide hydration shell. Water molecules in the hydration shell of K+ orient their dipole moment at about 45 degrees from the K+-water oxygen director, instead of radially as in the case of the Li+ and Na+ hydration shells. The K+-water oxygen radial distribution function shows a shallower first minimum compared to the other cation-water oxygen functions. The influence of the solutes on the water-water radial distribution functions is shown to have an effect on the water structure equivalent to an increase in the pressure of the water, depending on both ion concentration and ionic radius. The changes of the water structure in the presence of charged solutes and the differences among the hydration shells of the different cations are used to present a qualitative explanation of the observed cation mobility.

Structural and electronic evolution of the As(OH)3 molecule in high temperature aqueous solutions: An x-ray absorption investigation

The geometrical and electronic structure of the arsenious acid molecule As(OH)(3) in aqueous solutions has been investigated by x-ray absorption spectroscopy (XAS) within extended x-ray absorption spectroscopy (EXAFS) and x-ray absorption near edge structure (XANES), using realistic first-principle calculations in the latter case. This investigation was performed on aqueous solutions of arsenious acid from ambient to supercritical conditions (P = 250 and 600 bars, T <or= 500 degrees C) using a new optical cell. The analysis of the XAS spectra is consistent with (1) a constant As-O distance, (2) an opening of the O-As-O angles within the C(3V) pyramidal structure in the range 30-200 degrees C. This structural evolution comes along with a small decrease of the partial charges of the atoms in the As(OH)(3) molecule. The explanation invoked for both structural and electronic modifications observed is the weakening of the interactions, through hydrogen bonds, between the As(OH)(3) complex and water molecules. This is a fingerprint of the similar weakening of hydrogen bonding interactions in the solvent itself.