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We explore the deconvolution of correlations for the interpretation of the microstructural behavior of aqueous electrolytes according to the neutron diffraction with isotopic substitution (NDIS) approach toward the experimental determination of ion coordination numbers of systems involving oxyanions, in particular, sulfate anions. We discuss the alluded interplay in the title of this presentation, emphasized the expectations, and highlight the significance of tackling the challenging NDIS experiments. Specifically, we focus on the potential occurrence of N$$2+\atop{i}$$ ...SO$$2-\atop{4}$$ pair formation, identify its signature, suggest novel ways either for the direct probe of the contact ion pair (CIP) strength and the subsequent correctionmore » of its effects on the measured coordination numbers, or for the determination of anion coordination numbers free of CIP contributions through the implementation of null-cation environments. For that purpose we perform simulations of NiSO₄ aqueous solutions at ambient conditions to generate the distribution functions required in the analysis (a) to identify the individual partial contributions to the total neutron-weighted distribution function, (b) to isolate and assess the contribution of N$$2+\atop{i}$$ ...SO$$2-\atop{4}$$ pair formation, (c) to test the accuracy of the neutron diffraction with isotope substitution based coordination calculations and X-ray diffraction based assumptions, and (d) to describe the water coordination around both the sulfur and oxygen sites of the sulfate anion. In conclusion, we finally discuss the strength of this interplay on the basis of the inherent molecular simulation ability to provide all pair correlation functions that fully characterize the system microstructure and allows us to “reconstruct” the eventual NDIS output, i.e., to take an atomistic “peek” (e.g., see Figure 1) at the local environment around the isotopically-labeled species before any experiment is ever attempted, and ultimately, to test the accuracy of the “measured” NDIS-based coordination numbers against the actual values by the “direct” counting.« less

We study the polarization behavior of water under geologically-relevantextreme aqueous environments along four equidistant supercriticalisotherms, 773 ! T (K) ! 1373, and over a wide pressure range,0 < P(GPa) ! 30 , by isobaric-isothermal molecular dynamics simulationsof the Gaussian Charge Polarizable (GCP ) water model, to unravel anddiscuss the underlying link between two precisely defined orientationalorder parameters and the magnitude of the average induced dipole momentof water. The predicted behavior indicates an isothermal linear dependence(a) between the magnitude of the average induced dipole moment μind andthe average system density ! , (b) between the magnitude of the averageinduced dipole μindmore » and that of the total dipole μtot , resulting from (c) acompensating (inverse) dependence between the permanent-to-induceddipolar angle ! and the magnitude of the average induced dipole momentμind . Moreover, we interpret this behavior in terms of the evolution of thestate dependent tetrahedral order parameter qT and the correspondingbond-order parameter Q6 , supplemented by the microstructural analysisbased on the three site-site radial distribution functions of water and thedistance-ranked nearest-neighbors distributions. Finally, we show thatwhile water exhibits a dramatic microstructural transformation from anopen four-coordinated hydrogen-bonded network at normal conditions to a quasi close-packed coordination, it still preserves a significant degree of hydrogen bonding.« less

The liquid-vapor equilibrium isotopic fractionation of water is determinedby molecular-based simulation, via Gibbs Ensemble Monte Carlo andisothermal-isochoric molecular dynamics involving two radically differentbut realistic models, the extended simple point charge (SPC/E) and theGaussian charge polarizable (GCP) models. The predicted temperaturedependence of the liquid-vapor equilibrium isotopic fractionation factorsfor H218O / H216O, H217O / H216O, and 2H 1H 16O / 1H216O are comparedagainst the most accurate experimental datasets to assess the ability ofthese intermolecular potential models to describe quantum effectsaccording to the Kirkwood-Wigner free energy perturbation!2!expansion. Predictions of the vapor pressure isotopic effect for theH218O / H216O and H217O / H216O pairsmore » are also presented in comparisonwith experimental data and two recently proposed thermodynamicmodeling approaches. Finally, the simulation results are used to discusssome approximations behind the microscopic interpretation of isotopicfractionation based on the underlying roto-translational coupling.« less

We derive 2nd-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 formalmore » 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+BP/kT, and (c) to discuss, and highlight with examples from the literature, the thermodynamic inconsistencies encountered in the currently available 1st-order truncated expansions, by pinpointing the mathematical origin and physical meaning of the inconsistencies that render the 1st-order truncated expansions invalid.« less

Liquid-vapor fractionation factors of molecular fluids are studied by molecular-based simulation, Gibbs ensemble Monte Carlo, and isothermal-isochoric molecular dynamics of realistic models for N{sub 2}, O{sub 2}, and CO. The temperature dependence of the fractionation factors for {sup 15}N {sup 14}N/{sup 14}N{sub 2}, {sup 15}N{sub 2}/{sup 14}N{sub 2}, {sup 18}O {sup 16}O/{sup 16}O{sub 2}, {sup 18}O{sub 2}/{sup 16}O{sub 2}, {sup 13}C {sup 16}O/{sup 12}C {sup 16}O, and {sup 12}C {sup 18}O/{sup 12}C {sup 16}O along the vapor-liquid coexistence curves as predicted by simulation is compared with the existing experimental data to assess the accuracy of {h_bar} 2-order Kirkwood-Wigner free energymore » expansion for specific model parametrizations. Predictions of the fractionation factors for other isotopologue pairs, including {sup 18}O {sup 17}O/{sup 16}O{sub 2}, {sup 16}O {sup 17}O/{sup 16}O{sub 2}, and {sup 17}O{sub 2}/{sup 16}O{sub 2}, as well as tests of some approximations behind the microscopic interpretation of the fractionation factors are also given.« less