Molecular Dynamics Simulation of Limiting Conductance for Na2+, Cl2−, Na°, and Cl° in Supercritical Water

We report results of molecular dynamics (MD) simulations of the limiting conductances of MgCl2 and CaCl2 in supercritical water as a function of water density using the SPC/E model for water. The limiting conductances of Mg2+, Ca2+ and Cl- over the whole range of water density considered exhibits a linear dependence of the limiting conductance on the water density. In the cases of Mg2+ and Ca2+, a solventberg picture for the behavior of small divalent cation emerges from our studies. From the view of the solventberg picture, the ion and its shell moving together as an entity interacts with the second hydration shell water molecules, and its mobility is restricted mostly by the number of the second hydration shell water which is proportional to the water density of the whole system. In the case of Cl-, the range of water density considered in this study belongs to the higher-density region (above 0.45 g/cm3) in which the effect of the number of hydration water molecules around ions dominated. As the water density increases, the water molecules of the first hydration shell restrict the mobility of Cl- and the limiting conductance of Cl- decreases nearly linearly. Significant different dependence on the water density is observed between the calculated limiting conductances of MgCl2 and CaCl2 at 673 K and the experimental results over the water density of 0.60–0.90 g/cm3. Possible limitation of the extended simple point charge (SPC/E) model with regard to this difference should be pointed out and the use of a more precise model like the revised polarizable (RPOL) model is indispensable for a further MD study to gain a complete picture of the chemical circumstance around the ions.

We report molecular dynamics simulations of LiCl, NaBr, and CsBr in supercritical water in order to explain the experimental observations of the limiting conductances as a function of the density of water at supercritical state points. As was the case in our previous work on NaCl in supercritical water [Lee et al., Chem. Phys. Lett. 293, 289 (1998)], we find that the experimental trends in the limiting conductances as a function of water density are reproduced in our simulations—a clear change of slope from the assumed linear dependence of limiting conductances of LiCl, NaBr, and CsCl on the water density. We also found that the effect of the number of hydration water molecules around ions dominates in the higher-density region while the interaction strength between the ions and the hydration water molecules (as measured by the potential energy per hydration water molecule) dominates in the lower-density region. In the case of Cs+ and Br−, however, the latter factor in the lower-density region is not as dominant as in the cases of Na+ and Cl− since a clear difference between the potential energy per hydration water molecule at densities above and below 0.45 g/cm3 was not clearly observed in the cases. In the case of Li+, the interaction between the ions and the hydration water in the lower-density region is almost a nonfactor since the potential energy per hydration water molecule is monotonically decreased with decreasing water density, which is consistent with the linear increase of the limiting conductance for the Li+ ion with decreasing water density.

We report results of molecular dynamics simulations of the limiting conductance of Li + ion in ambient water and in supercritical water using polarizable models for water and Li + . The limiting conductances of Li + in ambient water calculated from mean square displacement (MSD) using four points transferable intermolecular potential model (TIP4P), extended simple point charge model (SPC/E), and revised polarizable model 1 (RPOL1) are larger than the experimental value. The behavior of the limiting conductance of Li + in supercritical water using the RPOL models results in good agreement with experimental results for the limiting conductance of LiCl. The agreement of the RPOL1 model with the experimental results is much better than the RPOL2 model in the higher-density regime, whereas that of the RPOL2 model is much better than the RPOL1 model in the lower-density regime. Using the RPOL models (in contrast to the SPC/E model), the number of hydration water molecules around Li + is the dominating contributor to the limiting conductance in the higher-density regime. In agreement with the SPC/E model, the interaction strength between Li + and the hydration water molecules is a non-factor in the lower-density region since the potential energy per hydration water molecule decreases with decreasing water density at the lowest water densities.

Molecular dynamics simulation of the limiting conductance of NaCl in supercritical water

We present results of molecular dynamics simulations for hydroxide ion in supercritical water of densities 0.22, 0.31, 0.40, 0.48, 0.61, and 0.74 g/cc using the SPC/E water potential with Ewald summation. The limiting molar conductance of OH − ion at 673 K monotonically increases with decreasing water density. It is also found that the hydration number of water molecules in the first hydration shells around the OH - ion decreases and the potential energy per hydrated water molecule also decreases in the whole water density region with decreasing water density. Unlike the case in our previous works on LiCl, NaCl, NaBr, and CsBr [Lee at al., Chem. Phys. Lett. 1998, 293, 289-294 and J. Chem. Phys. 2000, 112, 864-869], the number of hydrated water molecules around ions and the potential energy per hydrated water molecule give the same effect to cause a monotonically increasing of the diffusion coefficient with decreasing water density in the whole water density region. The decreasing residence times are consistent with the decreasing potential energy per hydrated water molecule.

At cryogenic temperature, a large number of hydration water molecules around proteins have been identified using X-ray crystal structure analyses, as described in Chap. 2. However, at a glance, it seems that there are no rules governing their distribution. In this chapter, we attempt to classify hydration water molecules with respect to their interaction modes with the protein atoms. Hydration water molecules having direct hydrogen bonds and/or van der Waals contacts are classified as the first-layer class, whereas those without direct interactions are classified as the second-layer class. Hydration water molecules in the first-layer class are the major components in protein hydration, and each water molecule occupies approximately 0.2 nm2 of accessible solvent area on the protein surface. An expected weight ratio of protein versus hydration water molecules range of 0.3–0.4 g-water/g-protein is consistent with the amount necessary for protein function. In addition, the distribution of hydration water molecules found in cryo-crystallography was evaluated by comparing the hydration sites from crystal structures with the solvent density calculated by averaging the trajectory in molecular dynamics simulations. The consistency between the results of crystallography and simulation is indicative of the hydration layer on the surface of the protein in solution.

The crystal structure analysis of the Fe-type nitrile hydratase from Rhodococcus sp. N-771 revealed the unique structure of the enzyme composed of the alpha- and beta-subunits and the unprecedented structure of the non-heme iron active center [Nagashima, S., et al. (1998) Nat. Struct. Biol. 5, 347-351]. A number of hydration water molecules were identified both in the interior and at the exterior of the enzyme. The study presented here investigated the roles of the hydration water molecules in stabilizing the tertiary and the quaternary structures of the enzyme, based on the crystal structure and the results from a laser light scattering experiment for the enzyme in solution. Seventy-six hydration water molecules between the two subunits significantly contribute to the alphabeta heterodimer formation by making up the surface shape, forming extensive networks of hydrogen bonds, and moderating the surface charge of the beta-subunit. In particular, 20 hydration water molecules form the extensive networks of hydrogen bonds stabilizing the unique structure of the active center. The amino acid residues hydrogen-bonded to those hydration water molecules are highly conserved among all known nitrile hydratases and even in the homologous enzyme, thiocyanate hydrolase, suggesting the structural conservation of the water molecules in the NHase family. The crystallographic asymmetric unit contained two heterodimers connected by 50 hydration water molecules. The heterotetramer formation in crystallization was clearly explained by the concentration-dependent aggregation state of NHase found in the light scattering measurement. The measurement proved that the dimer-tetramer equilibrium shifted toward the heterotetramer dominant state in the concentration range of 10(-2)-1.0 mg/mL. In the tetramer dominant state, 50 water molecules likely glue the two heterodimers together as observed in the crystal structure. Because NHase exhibits a high abundance in bacterial cells, the result suggests that the heterotetramer is physiologically relevant. In addition, it was revealed that the substrate specificity of this enzyme, recognizing small aliphatic substrates rather than aromatic ones, came from the narrowness of the entrance channel from the bulk solvent to the active center. This finding may give a clue for changing the substrate specificity of the enzyme. Under the crystallization condition described here, one 1,4-dioxane molecule plugged the channel. Through spectroscopic and crystallographic experiments, we found that the molecule prevented the dissociation of the endogenous NO molecule from the active center even when the crystal was exposed to light.

The kinetics of the water gas shift reaction was investigated in supercritical water using the Raman spectroscopic method for in situ quantitative measurement of reactants and relevant products in a stopped flow type reactor. The reaction proceeded according to the stoichiometric reaction as CO + H2O → CO2 + H2, whose rate was first order on the concentration of CO. The dependence on the reactant water was 1.5 ± 0.1 in the water density range from 4 to 25 mol dm–3 at a temperature of 380°C. The steady state concentration of formic acid was in the order of 10–4 against the initial concentration of CO and dependent on a 1.2 ± 0.3 order on the water density. These findings evidence that the rate determining step of the water gas shift reaction is the formation process of formic acid, which subsequently dissociates to CO2 + H2 at a much larger reaction rate. The slightly larger dependence than unity on the water density in the reaction rate as well as in the steady state concentration of the intermediate formic acid suggests that the transition state structure of the formic acid formation process resembles a certain kind of complex formed between a CO molecule and several water molecules such as suggested by Melius et al. (1990) by ab initio calculations.

Large-scale networks of hydration water molecules around bovine β-trypsin revealed by cryogenic X-ray crystal structure analysis

Monte Carlo simulations were used to predict the yield of primary specie • OH denoted as g ( • OH) that is formed from the radiolysis of pure, deaerat- ed supercritical water (SCW) (H 2 O) at 400 °C in the range of water density between ~0.15 and 0.6 g/ cm 3 . It is known that • OH, is one of the oxidizing species that significantly can increase the possibil- ity of various corrosion and material degradation as well. The thorough radiolysis processes in SCW - cooled reactor is not established currently, and it is believed to be a challenge in developing chemis- try control strategies for future Supercritical Water Reactor (SCWR). Since SCWR technology is now still under the conceptual design, hence there is only limited information published on the yields of radiolysis under these conditions. In this work, g ( • OH) was calculated at spur lifetime ( τ s / minimum time needed before the species within spur distributed homogeneously into the bulk solu- tion), 10 - 7 and 10 - 6 sec after the ionization event at all densities. From this work, it is shown that the data measured by other researcher at lower density ( 0.35 g/cm 3 ) is taken about near the spur lifetime. Finally, more experimental data are highly required in order to examine more thoroughly modeling calculation.

Contrast-variation small-angle neutron scattering (CV-SANS) is a widely used technique for quantifying hydration water in soft matter systems, but it is predominantly applied in the dilute regime or for systems with a well-defined structure factor. Here, CV-SANS was used to quantify the number of hydration water molecules associating with three water-soluble polymers with different critical solution temperatures and types of water-solute interactions in dilute, semidilute, and concentrated solution through the exploration of novel methods of data fitting and analysis. Multiple SANS fitting workflows with varying levels of model assumptions were evaluated and compared to give insight into SANS model selection. These fitting pathways ranged from general, model-free algorithms to more standard form and structure factor fitting. In addition, Monte Carlo bootstrapping was evaluated as a method to estimate parameter uncertainty through simulation of technical replicates. The most robust fitting workflow for dilute solutions was found to be form factor fitting without CV-SANS (i.e. polymer in 100% D2O). For semidilute and concentrated solutions, while the model-free approach can be mathematically defined for CV-SANS data, the addition of a structure factor imposes physical constraints on the optimization problem, suggesting that the optimal fitting pathway should include appropriate form and structure factor models. The measured hydration numbers were consistent with the number of tightly bound water molecules associated with each monomer unit, and the concentration dependence of the hydration number was largely governed by the chemistry-specific interactions between water and polymer. Polymers with weaker water-polymer interactions (i.e. those with fewer hydration water molecules) were found to have more bound water at higher concentrations than those with stronger water-polymer interactions due to the increase in the number of forced water-polymer contacts in the concentrated system. This SANS-based method to count hydration water molecules can be applied to polymers in any concentration regime, which will lead to improved understanding of water-polymer interactions and their impact on materials design.

Hitachi is advancing their designs for a conceptual reactor called the resource-renewable boiling water reactor (RBWR), a concept reactor similar to the advanced boiling water reactor with a harder neutron spectrum. This design aims to minimise construction costs and waste production as well as to utilise separated plutonium and minor actinide fuel. However, the axial heterogeneity of the design poses calculation difficulties. The aim of this work is to use a known method, reactivity-equivalent physical transformation (RPT), for calculating fuel with double heterogeneity and apply it to a BWR-type fuel pin. This could reduce the calculation time needed for optimisation of the design of the RBWR. The objective of the study is to use SCALE 6.2 to produce an equivalent axial pin model by comparison with the burnup and neutron spectra of a radial model of the fuel. This model can then be used for 2D burnup calculations, and in future work will be used for the generation of two-group and multigroup cross-sections for further deterministic calculations as part of a two-step approach for analysis of the RBWR. The RPT method has been extensively tested on spherical fuel, and SCALE is a standard industry code. The initial radial model is a hexagonal assembly with 20% enriched UO2 fuel in a zircaloy cladding, surrounded by light water moderator. The derived axial model has a water density distribution taken from Hitachi’s RBWR designs. Criticality over 70 GWd/tU burnup is estimated using the model. The application of the RPT to the BWR pin was shown to be possible, but to have limitations with the introduction of additional radial complexity. For a single pin, excellent agreement between the radial and axial models could be found across a range of water densities, but in the case of an assembly level calculation distinct equivalence models were required for each water density. In addition, the produced RPT model is validated using SCALE’s 3D Monte Carlo module, KENO.

The reaction pathways and kinetics of C1 aldehydes, formaldehyde (HCHO) and formic acid (HCOOH=HOCHO), are studied at 400 degrees C in neat condition and in supercritical water over a wide range of water density, 0.1-0.6 g/cm3. Formaldehyde exhibits four reactions: (i) the self-disproportionation of formaldehyde generating methanol and formic acid, (ii) the cross-disproportionation between formaldehyde and formic acid generating methanol and carbon dioxide, (iii) the water-independent self-disproportionation of formaldehyde generating methanol and carbon monoxide, and (iv) the decarbonylation of formaldehyde generating hydrogen and carbon monoxide. The self- and cross-disproportionations overwhelm the water-independent self-disproportionation and the formaldehyde decarbonylation. The rate constants of the self- and cross-disproportionations are determined in the water density range of 0.1-0.6 g/cm3. The rate constant of the cross-disproportionation is 2-3 orders of magnitude larger than that of the self-disproportionation, which indicates that formic acid is a stronger reductant than formaldehyde. Combining the kinetic results with our former computational study on the equilibrium constants of the self- and cross-disproportionations, the reaction mechanisms of these disproportionations are discussed within the framework of transition-state theory. The reaction path for methanol production can be controlled by tuning the water density and reactant concentrations. The methanol yield of approximately 80% is achieved by mixing formaldehyde with formic acid in the ratio of 1:2 at the water density of 0.4 g/cm3.

We present results of molecular dynamics simulations for diffusion of Na+ ion in water-filled carbon nanotubes (CNTs) at 25°C using the extended simple point charge water potential. Simulation results indicate the general trend that the diffusion coefficients of Na+ ion and water molecule in CNTs decrease with an increase in water density and are larger than those in the bulk solution. The average potential energies of ion–water and water–water, the radial distribution functions, the hydration numbers and the residence times of the hydrated water molecules are discussed. The classical solventberg picture describes Na+ ion in water adequately for systems with the small values of diffusion coefficients.

A highly stable ice monolayer with folded structural motifs is predicted by means of a novel tiling method augmented with ab initio calculations. This ice monolayer has every two neighboring water hexamers connected by a water square yet folded into two distinct planes, and is thus coined as a folded ice model. It is in the ground state in a range of water densities from 0.08 to 0.12 Å-2, with a stronger energy preference at a lower water density. Its stability shown by ab initio molecular dynamics simulations can sustain up to a temperature of 100 K. The tiling method also enables the prediction of a family of considerably stable ice monolayers with a variety of puckered structures. These results enrich our knowledge of low-dimensional water structures and pave a way to explore more exotic ice nanostructures under confinements.