J Michael {Mike} Simonson

The structure of CaCl2 aqueous solutions over a wide range of concentration. Interpretation of diffraction experiments via molecular simulation

A detailed analysis of the water structure around the ionic species in aqueous CaCl2 solutions is performed by molecular dynamics over a wide range of concentration (0<m⩽9.26) at ambient conditions. The goals behind the study are to address a long-standing controversy regarding the actual coordination number of aqueous Ca+2, to characterize the hydration structure of Ca+2, and consequently to interpret the origin of the disparity in the reported experimental data, to describe the additional information available from simulation to augment the first-order difference method in the determination of the species coordination numbers from neutron diffraction with isotopic substitution (NDIS) experiments, and to attain additional insights into the distribution of the “tilting” and “wagging” ion–water angles and their relation with the conventional configurational definitions.

Correspondence between Cluster-Ion and Bulk Solution Thermodynamic Properties: On the Validity of the Cluster-Pair-Based Approximation

Since the single-ion thermodynamic properties of bulk solutions are not directly accessible from experiments, extrapolations have been devised to estimate them from experimental measurements on small-clusters. Extrapolations based on the cluster-pair-based approximation (CPA) technique (Tissandier et al. J. Phys. Chem. A 1998, 102, 7787-7794) and its variants are currently considered one of the most reliable source of single-ion hydration thermodynamic data and have been used as a benchmark for the development of molecular and continuum solvation models. Despite its importance, the CPA has not been thoroughly tested and recent studies have indicated inconsistencies with molecular simulations. The present work challenges the key CPA assumptions that the hydration properties of single cations and anions in growing clusters rapidly converge to each other following a monotonous trend. Using a combination of simulation techniques to study the transition between alkali halide ions in small clusters and bulk solution, we show that this convergence is rather slow and involves a surprising change in trends, which can result in significant errors in the original estimated single-ion properties. When these cluster-size-dependent effects are taken into account, the inconsistencies between molecular models and experimental predictions disappear, and the value of the proton hydration enthalpy based on the CPA aligns with estimates based on other principles.

The structure of concentrated NiCl2 aqueous solutions: what is molecular simulation revealing about the neutron scattering methodologies?

An analysis is made of adequacy and limitations of the neutron weighted ion distribution function for the determination of the hydration ion structure in hydrothermal solutions. Our analysis indicates that the coordination number based on the O—Ni2+ interactions is unambiguously defined by the first peak of the neutron weighted cation distribution function G Ni(r), but that the corresponding H—Ni+2 and H—Cl− coordination numbers may be ill-defined due to the occurrence of Ni+2-Cl− ion pairing. For the system considered in this work, this effect contributes about 1.5 units to the H—Ni−2 and 0.85 units to the H—Cl− coordination numbers, respectively, for a 3.9 M NiCl2 aqueous solution under ambient conditions. A comparison under ambient conditions between the most reliable NDIS data on Ni2+ hydration and our simulation results suggests that the present intermolecular potential models underestimate the O-Ni+2 coordination numbers by about 1.5 units, and it might indicate the need for a reparametrization of the current ion-water intermolecular potentials. The hydration structure exhibits practically no temperature dependence for the isochore studied (1.356 g cm−3) and composition. The Ni+2-Cl− ion pair formation appears to affect the location of the shoulder in the neutron weighted distribution functions G Ni(r) and G Cl(r), although it does not affect the magnitude of the ion—water coordination.

Ion association in aqueous LiCl solutions at high concentration: Predicted results via molecular simulation

We perform molecular dynamics simulations to study the ionic solvation and association behavior in concentrated aqueous LiCl solutions at ambient conditions, including consideration of expected signatures of ion pairing that might be found in neutron diffraction experiments with isotopic substitution. The ten possible pair radial distribution functions that define the microstructure of the systems are determined and used to assess the first-order difference of the neutron-weighted correlation functions for these solutions in heavy and null water. Then, both sets of correlation functions are applied to the interpretation of the ions local environment in terms of the location of the relevant peaks and the penetration of ions into the counterion solvation shells as a signature of ion-pair formation. Finally, we illustrate how first-order difference experiments involving null and heavy water might be used to assess the magnitude of the M(v+) - X(v-) ion-pair formation for a salt M(v+)X(n) v- in an aqueous solution, provided the significant experimental challenges in these studies could be overcome.

The bound coherent neutron scattering lengths of the oxygen isotopes

The technique of neutron interferometry was used to measure the bound coherent neutron scattering length b(coh) of the oxygen isotopes (17)O and (18)O. From the measured difference in optical path between two water samples, either H(2)(17)O or H(2)(18)O versus H(2)(nat)O, where nat denotes the natural isotopic composition, we obtain b(coh,(17)O) = 5.867(4) fm and b(coh,(18)O) = 6.009(5) fm, based on the accurately known value of b(coh,(nat)O) = 5.805(4) fm which is equal to b(coh,(16)O) within the experimental uncertainty. Our results for b(coh,(17)O) and b(coh,(18)O) differ appreciably from the standard tabulated values of 5.6(5) fm and 5.84(7) fm, respectively. In particular, our measured scattering-length contrast of 0.204(3) fm between (18)O and (nat)O is nearly a factor of 6 greater than the tabulated value, which renders feasible neutron diffraction experiments using (18)O isotope substitution and thereby offers new possibilities for measuring the partial structure factors of oxygen-containing compounds, such as water.

Solvation phenomena in dilute multicomponent solutions I. Formal results and molecular outlook

We derive second-order thermodynamically consistent truncated composition expansions for the species residual partial molar properties--including volume, enthalpy, entropy, and Gibbs free energy--of dilute ternary systems aimed at the molecular account of solvation phenomena in compressible media. Then, we provide explicit microscopic interpretation of the expansion coefficients in terms of direct and total correlation function integrals over the microstructure of the corresponding infinite dilution reference system, as well as their pressure and temperature derivatives, allowing for the direct prediction of the species partial molar properties from the knowledge of the effective intermolecular interactions. Finally, we apply these formal results (a) to derive consistent expressions for the corresponding properties of the binary system counterparts, (b) to illustrate how the formal expressions converge, at the zero density limit, to those for multicomponent mixtures of imperfect gases obeying the virial equation of state Z = 1 + BPkT, and (c) to discuss, and highlight with examples from the literature, the thermodynamic inconsistencies encountered in the currently available first-order truncated expansions, by pinpointing the mathematical origin and physical meaning of the inconsistencies that render the first-order truncated expansions invalid.

Behavior of Aqueous Electrolytes in Steam Cycles - The Final Report on the Solubility and Volatility of copper(I) and Copper(II) Oxides

Measurements were completed on the solubility of cupric and cuprous oxides in liquid water and steam at controlled pH conditions from 25 to 400 C (77 to 752 F). The results of this study have been combined with those reported from this laboratory in two previous EPRI reports to provide a complete description of the solubility of these oxides and the speciation of copper dissolved in liquid water and steam as a function of oxidation state, temperature, pH, and in the case of steam, pressure. These constitute the first set of reliable data for cuprous oxide solubility over this range of conditions. For the more intensively studied CuO case, agreement was found between our results and those of previous studies of its solubility in steam, whereas only partial agreement was evident for its solubility in liquid water. For both oxides this disagreement often amounted to orders of magnitude. The solubility of cuprous oxide is somewhat lower than that of CuO at ambient conditions, except as very high pH. However, by 350 C (662 F), Cu{sub 2}O is the more soluble phase. At 100 C (212 F) and above, the logarithm of the solubility of both phases decreases linearly with increasing pH to a minimum value then sharply increases linearly with pH. In other words, above 100 C the solubility of both oxides become highly pH dependent. In fact at constant pH during startup, very high copper concentrations can be reached in the boiler water, more than an order of magnitude above those at ambient or operating temperatures. The enhancing effect of added ammonia on the solubility of both oxides is most significant at low temperatures and is much greater for cuprous oxide. Consequently, the mobility of copper is affected significantly under AVT startup conditions. The oxidation of copper metal and presumably cuprous oxide by addition of air-saturated makeup water can lead to much higher copper concentrations than equilibrium with cupric oxide would allow, but the presence of both copper metal and cuprous oxide provides an effective scavenger for oxygen, even at room temperature, with copper levels consistent with those in equilibrium with cuprous oxide. The solubilities of Cu{sub 2}O and CuO in steam are quite similar and are virtually temperature independent at the 1 to 2 ppb level, respectively, although at supercritical conditions, both solubilities increase with increasing pressure and temperature. The species that partition to the vapor are believed to be the neutrally charged molecules, Cu(OH){sup 0} and Cu(OH){sub 2}{sup 0}, for the reduced and oxidized forms, respectively, and their concentrations in the vapor are therefore independent of the pH of the liquid water phase from which they originate.