[Show abstract][Hide abstract] ABSTRACT: 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
Full-text · Article · Jan 2016 · Pure and Applied Chemistry
[Show abstract][Hide abstract] ABSTRACT: We use the concept of statistical distance to define a measure of distinguishability between a pair of statistical mechanical systems, i.e., a model and its target, and show that its minimization leads to general convergence of the model’s static measurable properties to those of the target. We exploit this feature to define a rigorous basis for the development of accurate and robust effective molecular force fields that are inherently compatible with coarse-grained experimental data. The new model
optimization principles and their efficient implementation are illustrated through selected examples, whose outcome demonstrates the higher robustness and predictive accuracy of the approach compared to other currently used methods, such as force matching and relative entropy minimization. We also discuss relations between the newly developed principles and established thermodynamic concepts, which include the Gibbs-Bogoliubov inequality and the thermodynamic length.
Full-text · Article · Oct 2015 · The Journal of Chemical Physics
[Show abstract][Hide abstract] ABSTRACT: The importance of single-ion hydration thermodynamic properties for understanding the driving forces of aqueous electrolyte processes, along with the impossibility of their direct experimental measurement, have prompted a large number of experimental, theoretical, and computational studies aimed at separating the cation and anion contributions. Here we provide an overview of historical approaches based on extrathermodynamic assumptions and more recent computational studies of single-ion hydration in order to evaluate the approximations involved in these methods, quantify their accuracy, reliability, and limitations in the light of the latest developments. We also offer new insights into the factors that influence the accuracy of ion-water interaction models and our views on possible ways to fill this substantial knowledge gap in aqueous physical chemistry.
Full-text · Article · Jun 2015 · Fluid Phase Equilibria
[Show abstract][Hide abstract] ABSTRACT: We study the microstructural analysis of aqueous electrolytes and present a detailed account of the fundamentals underlying the neutron scattering with isotopic substitution (NDIS) approach for the experimental determination of ion coordination numbers in systems involving both halide anions and oxyanions. We place particular emphasis on the frequently overlooked ion-pairing phenomenon, identify its microstructural signature in the neutron-weighted distribution functions, and suggest novel techniques to deal with either the estimation of the ion-pairing magnitude or the correction of its effects on the experimentally measured coordination numbers. We illustrate the underlying ideas by applying these new developments to the interpretation of four NDIS test-cases via molecular simulation, as convenient dry runs for the actual scattering experiments, for representative aqueous electrolyte solutions at ambient conditions involving metal halides and nitrates.
Full-text · Article · May 2015 · Fluid Phase Equilibria
[Show abstract][Hide abstract] ABSTRACT: We developed the Gaussian charge-on-spring (GCOS) version of the original self-consistent field implementation of the Gaussian Charge Polarizable water model and test its accuracy to represent the polarization behavior of the original model involving smeared charges and induced dipole moments. For that purpose we adapted the recently proposed multiple-particle-move (MPM) within the Gibbs and isochoric-isothermal ensembles Monte Carlo methods for the efficient simulation of polarizable fluids. We assessed the accuracy of the GCOS representation by a direct comparison of the resulting vapor-liquid phase envelope, microstructure, and relevant microscopic descriptors of water polarization along the orthobaric curve against the corresponding quantities from the actual GCP water model.
No preview · Article · Mar 2015 · The Journal of Physical Chemistry B
[Show abstract][Hide abstract] ABSTRACT: We evaluate the ability of selected classical molecular models to describe the thermodynamic and structural aspects of gas-phase hydration of alkali halide ions and the formation of small water clusters. To understand the effect of many-body interactions (polarization) and charge penetration effects on the accuracy of a force field, we perform Monte Carlo simulations with three rigid water models using different functional forms to account for these effects: (i) point charge non-polarizable SPC/E, (ii) Drude point charge polarizable SWM4-DP, and (iii) Drude Gaussian charge polarizable BK3. Model predictions are compared with experimental Gibbs free energies and enthalpies of ion hydration, and with microscopic structural properties obtained from quantum DFT calculations. We find that all three models provide comparable predictions for pure water clusters and cation hydration, but differ significantly in their description of anion hydration. None of the investigated classical force fields can consistently and quantitatively reproduce the experimental gas phase hydration thermodynamics. The outcome of this study highlights the relation between the functional form that describes the effective intermolecular interactions and the accuracy of the resulting ion hydration properties.
Full-text · Article · Dec 2014 · The Journal of Physical Chemistry A
[Show abstract][Hide abstract] ABSTRACT: We explore the deconvolution of the water-nitrate correlations by the first-order difference approach involving neutron diffraction of heavy- and null-aqueous solutions of KNO3 under 14N/15N and natO/18O substitutions to achieve a full characterization of the first water coordination around the nitrate ion. For that purpose we performed isobaric-isothermal simulations of 3.5m KNO3 aqueous solutions at ambient conditions to generate the relevant radial distribution functions (RDF) 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 NO3-..K+ pair formation, (c) to test the accuracy of the NDIS-based coordination calculations and XRD-based assumptions, and (d) to describe the water coordination around both the nitrogen and oxygen sites of the nitrate ion.
No preview · Article · Dec 2014 · The Journal of Physical Chemistry B
[Show abstract][Hide abstract] ABSTRACT: We present a detailed derivation of the complete set of expressions required for the implementation of an Ewald summation approach to handle the long-range electrostatic interactions of polar and ionic model systems involving Gaussian charges and induced dipole moments with a particular application to the isobaric-isothermal molecular dynamics (NPT-MD) simulation of our Gaussian Charge Polarizable (GCP) water model and its extension to aqueous electrolytes solutions. The set comprises the individual components of the potential energy, electrostatic potential, electrostatic field and gradient, the electrostatic force and the corresponding virial. Moreover, we show how the derived expressions converge to known point-based electrostatic counterparts when the parameters, defining the Gaussian charge and induced-dipole distributions, are extrapolated to their limiting point values. Finally, we test the simulation outcomes from the Ewald implementation against the corresponding reaction-field (RF) approach at three contrasting hydrogen-bonded water environments, including thermodynamic quantities, polarization behavior and microstructural properties, where the simulated microstructures are compared with the available neutron scattering and x-ray diffraction data.
No preview · Article · Nov 2014 · The Journal of Physical Chemistry B
[Show abstract][Hide abstract] ABSTRACT: We studied via molecular dynamics the link between the strain-driven hydration free-energy changes in the association process involving finite-size graphene surfaces, the resulting water graphene interfacial tension, and the combined effect of the surface strain and confinement on the thermodynamic response functions and the dynamics of confined water. We found that an in-plane biaxial tensile strain epsilon = 10% enhances significantly not only the water graphene hydrophobicity with respect to that of the unstrained counterpart but also the confinement effect on the thermodynamic response functions and slowing down of the dynamics of water over those of the corresponding bulk counterpart. The interfacial behavior of water in contact with strained-graphene plates resembles that observed for "pp" corrugated-plate configuration, as reported earlier [Chialvo et al. J. Phys. Chem. C 2013, 117, 23875], exibiting a significant enhancement of the fluid surface hydrophobicity and response functions relative to those of the unstrained surface. In contrast, the slowing down of the dynamics of the confined water does not show any differentiation with respect to the type of surface.
No preview · Article · Aug 2014 · The Journal of Physical Chemistry C
[Show abstract][Hide abstract] ABSTRACT: Adsorption of supercritical CO 2 in nanoporous silica aerogel was investigated by a combination of experiments and molecular-level computer modeling. High-pressure gravimetric and vibrating tube densimetry techniques were used to measure the mean pore fluid density and excess sorption at 35 and 50 °C and pressures of 0−200 bar. Densification of the pore fluid was observed at bulk fluid densities below 0.7 g/cm 3 . Far above the bulk critical density, near-zero sorption or weak depletion effects were measured, while broad excess sorption maxima form in the vicinity of the bulk critical density. The CO 2 sorption properties are very similar for two aerogels with bulk densities of 0.1 and 0.2 g/cm 3 , respectively. The spatial distribution of the confined supercritical fluid was analyzed in terms of two nanodispersed phases with sorption-and bulk-phase densities and their volumes by means of the adsorbed phase model (APM), which used data from gravimetric sorption and small-angle neutron scattering experiments. To gain more detailed insight into supercritical fluid sorption, large-scale lattice gas GCMC simulations were utilized and tuned to resemble the experimental excess sorption data. The computed three-dimensional pore fluid density distributions show that the observed maximum of the excess sorption near the critical density originates from large density fluctuations pinned to the pore walls. At this maximum, the size of these fluctuations is comparable to the prevailing pore sizes.
Full-text · Article · Jun 2014 · The Journal of Physical Chemistry C
[Show abstract][Hide abstract] ABSTRACT: We studied the link between the water-mediated (tensile or compressive) strain-driven hydration free energy changes in the association process involving finite-size graphene surfaces, the resulting water-graphene interfacial behaviour, and the combined effect of surface strain and fluid confinement on the thermodynamic response functions and the dynamics of water. We found that either small surface corrugation (compressive strain) or surface stretching (tensile strain) is able to enhance significantly the water-graphene hydrophobicity relative to that of the unstrained surface, an effect that exacerbates the confinement impact on the isothermal compressibility and isobaric thermal expansivity of confined water, as well as on the slowdown of its dynamics that gives rise to anomalous diffusivity.
[Show abstract][Hide abstract] ABSTRACT: We carried out a systematic molecular simulation study of the behavior of a pair of finite-size graphene plates immersed in water at isobaric-isothermal conditions to provide insights into the nature of the water-graphene (corrugated) surface interactions. The goal was to address the link between the corrugation-driven hydration free energy changes in the association process involving graphene plates and the resulting water-graphene interfacial tension, to interrogate the effect of the surface corrugation and confinement on the thermodynamic response functions and the dynamics of confined water and to put the observed behavior in the context of Wenzel's modification of Young's equation. We found that graphene confinement induces a significant increase in the isothermal compressibility and isobaric thermal expansivity as well as a pronounced slowdown of the dynamics of water over that of the corresponding bulk counterpart, whose magnitudes depend on the type of surface corrugation involved. Our simulation results for different types of corrugated graphene plates involving identical surface areas do not support the meaning of the "r"-factor underlying Wenzel's equation for corrugated nanoscale surfaces.
No preview · Article · Nov 2013 · The Journal of Physical Chemistry C
[Show abstract][Hide abstract] ABSTRACT: At this point it is quite evident that we have to confront a significant knowledge gap in the area of thermodynamic and thermophysic properties of CO2-rich phases containing typical contaminants (including water), and consequently, it appears equally obvious that there is a crucial need for an integrated effort to bridge that gap in order to assure success in the design and implementation of the CCUS technology. The lack of experimental data (e.g., for the phase equilibrium of SO2-H2S, SO 2-NOx, H2S-NOx, the corresponding aqueous CO2 multicomponent mixtures, and the kinetics of their aqueous reactivity), and consequently, of suitable macroscopic correlations such as EoS formalisms and reliable mixing rules (including BIP's) to describe accurately the behavior of those fluid phases under proper conditions for pipeline transport and sub-surface storage, pose significant challenges to modelers whose reactive transport codes might not be able to provide realistic solubility calculations. For the same reasons, most reactive transport simulations of acid gas-bearing CO2 injections have typically treated the CO2 and acid gas co-injections as dissolved species in separate liquid brine streams, as opposed to the injection of a supercritical acid-gas bearing CO2 stream that will ultimately contact the subsurface liquid brines (Gunter et al. 2000; Xu et al. 2007; Xiao et al. 2009). This ad hoc approach cannot account for the actual partitioning of the CO2 contaminants between the CO2-rich and the aqueous (brine)-rich phases due to differential solubility of the contaminants in the two phases (Bachu and Bennion 2009; Pooladi-Darvish et al. 2009; Savary et al. 2012; Ji and Zhu 2013). A more detailed discussion of the limitations of current reactive transport modeling packages for the interested readers is given elsewhere (Jacquemet et al. 2009, 2011). Considering the expertize and capabilities in this area, we see no technical obstacles to broad our experimental effort by pursuing thermodynamics and phase equilibria measurements for the missing systems involving CO2-rich phases (e.g., where SO2, H 2S, NOx, and H2O are the minor components) at CCUS relevant conditions. These activities must be complemented by not only a modeling component aimed at developing accurate macroscopic correlations required in the reactive transport modeling packages but also, a molecular-based parameterization effort of the desired force fields to support the concomitant microscopic modeling. It has become also much clearer that we have a considerably precarious understanding of the thermochemical behavior of acid gas-bearing CO2 fluids under mineral confinement, due in part to the confluence of limited experimental data, difficult interpretation of raw data, and the involvement of indirect probes. This scenario advocates the need for a concerted effort to probe experimentally the physico-chemical interactions between acid gas-contaminated CO2 and (synthetic or otherwise) porous mineral substrates under diffusion- and reactioncontrolled conditions, within dry and wet environments. This effort should include volumetric/density determinations (Gruszkiewicz et al. 2012) involving acid gas-CO2 adsorption, in situ pH measurements, and chemical analysis to characterize the outcome of the fluid-mineral reactivity. The success of the CCUS technology depends on the caprock ability to prevent gas leakage, i.e., the capillary-sealing efficiency of the caprock that according to the Laplace law, depends on caprock surface wettability and becomes controlled by the fluid-gas interfacial tension (Israelachvili 1992). Therefore, it becomes crucial to understand how the CO2 contaminants can modify the fluid-mineral wettability, the corresponding fluid-gas interfacial tension, and consequently, the caprock's sealing efficiency. In particular, there is a wide knowledge gap regarding the pressure, temperature, and composition dependence of the interfacial tension of CO2-rich phases containing acid-gas contaminants, as well as their wetting behavior involving subsurface minerals. Moreover, the lack of experimental data for the thermodynamic and transport properties of CO2-X systems also limits the experimental probes via non-invasive approaches, to extract microscopic insights needed in the force-field parameterization of mineral-fluid interactions, and subsequently, the interpretation and characterization of sought after interfacial phenomena. Even though the non-polarizable force field parameterizations described in the section "Force fields for CO2-acid gas systems" (e.g., Tables 1 and 4) have been rather successful in the description of the pure component behavior, this success transfers only partially to their mixtures because the adjustment of the combining rules to define the unlike-pair interactions cannot always counter the effects of the large polarity asymmetry between CO2 and the solute species. In fact, as we have argued previously for the case of CO2-H2O mixtures (Vlcek et al. 2011), the adjustment of unlike-pair interaction parameters must be achieved with the simultaneous correction for over-polarization involving the fixed-charge non-polarizable species. The current molecular simulation effort involving the polar species H2O, SO2, H2S, and NO2 dissolved in CO2-rich phases, suggests the need for the explicit incorporation of polarizable interactions in order to account properly for the significant variation of polarity asymmetry in dilute solute solutions. This is by no means an easy task or a straightforward target but the soundest way to deal properly with these molecular asymmetries.
No preview · Article · Nov 2013 · Reviews in Mineralogy and Geochemistry
[Show abstract][Hide abstract] ABSTRACT: Simple non-polarizable models of aqueous electrolytes underlie the majority of current geochemical and biochemical molecular simulations. To evaluate the reliability and predictive qualities of such studies, it is necessary to understand the limitations of the molecular force fields. Here we present the results of our effort to identify and overcome some of the main limiting factors of the existing simple models.
First, we analyze relationships between experimental structural, dynamic, and thermodynamic properties and individual force field parameters. Our main focus is on the proper choice of reference experimental data for absolute single ion thermodynamics and utilizing known correlations between solvation entropies and tracer diffusivities. Subsequently, we use this information to construct empirical simple (Lennard-Jones potential and point charges) models of common mono- and divalent ions. These include alkali metal and alkaline earth cations paired with halide and oxo-anions. Finally, the limits of the resulting force field are evaluated and compared to other models.
Acknowledgements. This work was supported by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, U.S. Department of Energy.
[Show abstract][Hide abstract] ABSTRACT: 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 as 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, while 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 from 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.
Full-text · Article · Oct 2013 · The Journal of Physical Chemistry A
[Show abstract][Hide abstract] ABSTRACT: A balance between ion-ion and ion-water interactions plays a crucial role in the crystallization and dissolution of salts. Correct description of these interactions is therefore paramount for the understanding of natural processes leading to the precipitation of minerals, as well as for the design of new technologies, such as nuclear waste separation and storage. Our motivation is to investigate salt crystallization from mixed aqueous electrolyte solutions for the rational design of complexation agents used in selective crystallization of homologous series of oxoanions.
A reliable description of molecular and ionic interactions is needed to understand atomic-scale mechanisms underlying the thermodynamics and dynamics of electrolyte solutions. Thus, our goal is to develop a consistent force field parameterization for the study of oxoanions of general formula XO42- (X=S, Se, Cr, Mo, W). While several force fields already exist for the description of simple alkali metal and alkali-halide series , and models for individual oxoanions have been also published , a consistent force field for oxoions suitable for comparative studies is not yet available.
The choice of an appropriate potential model form depends on the intended applications, and must balance accuracy and computational efficiency. Since salt nucleation and crystallization occur over large time and length scales, the simplicity of the model is of high importance. While most of the above ions are highly polarizable, their mineral environment is consistently polar, justifying the use of effective pair potentials to account for those polarizable contributions. Therefore we consider pair potentials represented by a combination of point charges and Lennard-Jones interactions that are compatible with the SPC/E water model . We also test the limit of the effective description of the interactions and discuss ways to incorporate polarizability.
The resulting force field is optimized against experimental data including hydration free energy at infinite dilution , chemical potential at finite concentration based on the Kirkwood-Buff formalism , lattice constants and energies for selected crystals, and diffusion coefficient . The potential parameters (at least 5 for each oxoion) were determined using global optimization based on the coupling parameter technique .
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Acknowledgements. This work was supported as part of the “Center for Nanoscale Control of Geologic CO2”, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, and by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, U.S. Department of Energy.
[Show abstract][Hide abstract] ABSTRACT: We present a detailed molecular-based characterization via isobaric isothermal-molecular dynamics simulation of the microstructure and dynamics of water-rich aqueous CO2 solutions at silica surfaces and under extreme confinement between finite silica plates at state conditions relevant to geologic capture and sequestration of carbon dioxide. The study comprises three types of slit-pore plates to represent two extreme cases of surface polarity and a mismatched pair of plates to interrogate the fluid behavior at and confined between heterogeneous surfaces. We found layer formation of H2O and CO2 whose strength depends on the nature of the plate surface, i.e., stronger H2O layering at hydrophilic than at hydrophobic plates with simultaneous weaker water-mediated CO2/hydrophilic-surface interactions. We observed the opposite behavior with the hydrophobic plates in which the weaker water layering results from the CO2-mediated H2O/hydrophobic-surface interactions. Moreover, we illustrate how the interplay between these types of interactions and extreme fluid confinement, i.e., strong overlapping of interfacial structures, can induce a drying out of the pore environment whose immediate consequence is a significant CO, concentration enhancement relative to that of the bulk environment. Finally, we assessed the effect of the nature of the plate surfaces on the translational diffusion coefficient of water. We found that this property changes monotonically at purely interfacial regions but nonmonotonically under confinement.
No preview · Article · Jul 2012 · The Journal of Physical Chemistry C
[Show abstract][Hide abstract] ABSTRACT: The interactions of electrolyte fluids with carbon-based electrodes
control many complex interfacial processes encountered in
electrochemical energy storage systems. However, our knowledge of the
atomic/nanoscale reactivity at interfaces of electrolytes with
electrodes remain scares due to the incomplete understanding of
interfacial structures and processes in-situ and real-time encountered
in real operation conditions. In this talk, we will present our efforts
to obtain a molecular-scale perspective of the interactions of
electrolytes with carbon surfaces near ``real world'' conditions.
Structures of various electrolytes including slat aqueous and ionic
liquids on atomically flat graphene (epitaxially grown on a SiC
substrate), an ideal model fluid-solid interface system, were
investigated by coupling high-resolution interface X-ray scattering
techniques with molecular modeling-simulation approaches. These results
provide a base-line for understanding relevant electrolyte/carbon
interactions and will lead to fundamentally new insights and provide
unique tests of atomistic fluid-solid interface models for energy
[Show abstract][Hide abstract] ABSTRACT: The interaction of interfacial water with graphitic carbon at the atomic scale is studied as a function of the hydrophobicity of epitaxial graphene. High resolution x-ray reflectivity shows that the graphene-water contact angle is controlled by the average graphene thickness, due to the fraction of the film surface expressed as the epitaxial buffer layer whose contact angle (contact angle θc = 73°) is substantially smaller than that of multilayer graphene (θc = 93°). Classical and ab initio molecular dynamics simulations show that the reduced contact angle of the buffer layer is due to both its epitaxy with the SiC substrate and the presence of interfacial defects. This insight clarifies the relationship between interfacial water structure and hydrophobicity, in general, and suggests new routes to control interface properties of epitaxial graphene.
No preview · Article · Jan 2012 · Physical Review B