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We establish a direct route for the accurate determination of the solvent effect on the Krichevskii parameter of a solute, based solely on the contrasting solvation behavior of the solute in the desired solvent relative to that of the reference solvent, i.e., in terms of the distinct solvation Gibbs free energies of the solute and the corresponding Krichevskii parameters of an ideal gas solute in the pair of solvents. First, we illustrate the proposed approach in the determination of the H/D−solvent effect on the Krichevskii parameter of gaseous solutes in aqueous solutions, when the solvents are different isotopic forms (isotopomers) of water, and then, by generalizing the approach to any pair of solvents. For that purpose, we (a) identify the links between the standard solvation Gibbs free energy of the i−solute in the two involved solvent environments and the resulting Krichevskii parameters, (b) discuss the fundamentally based linear behavior between the Krichevskii parameter and the standard solvation Gibbs free energy of the i−solute in an α−solvent, and interpret two emblematic cases of solutions involving either an ideal gas solute or an i−solute behaving identically as the solvating species, as well as (c) provide a novel microstructural interpretation of the solvent effect on the Krichevskii parameter according to a rigorous characterization of the critical solvation as described by a finite unambiguous structure making/breaking parameter Siα∞(SR) of the i−solute in the pair of α−solvents.

In this article, we raise awareness about the misuses of frequently invoked criteria for structure making/breaking phenomena, resulting from the absence of any explicit cause–effect relationship between the proposed markers and the microstructural perturbation of the solvent induced by the solute. First, we support our assessment with rigorous molecular-based foundations to determine, directly and quantitatively, the solute-induced perturbation of the solvent structure leading to an unambiguous definition of a structure making/breaking event. Then, we highlight and discuss the sources of concealed ambiguities in two of the most frequently invoked structure making/breaking criteria, i.e., Hepler’s thermal expansivity-based and Jones–Dole’s B coefficient-based markers. Finally, we illustrate how the implementation of rigorous molecular-based arguments, in conjunction with the available experimental evidence on a variety of aqueous species at infinite dilution, rule out the validity of these two criteria as structure making/breaking markers and suggest their discontinuation to avoid the perpetuation of myths.

This work deals with the solvation thermodynamics of systems containing extreme intermolecular interaction asymmetries, such as binary mixtures comprising ideal gas species plus other real species, and analyzes the composition dependence of relevant solvation properties from a rigorous molecular-based theoretical viewpoint. It focuses on the effect of large intermolecular interaction asymmetries, involving simple model species for which we have available rigorous results, to advance our understanding of the true molecular-based origin of their unconventional solvation behavior. This effort involves a statistical mechanics approach to identify the thermodynamic constraints and microstructural manifestation underlying the nonmonotonous composition behavior of the species' partial molar properties, as well as to describe formally the explicit link between the intermolecular asymmetries and the observed anomalous behavior. The outcome of the analysis provides sound support to the depiction of the solvation of a weakly interacting solute in a real solvent, a scenario that facilitates the molecular thermodynamic interpretation of gas solvation characterized by low solubility and thermodynamic stability gaps. Moreover, this work illustrates direct comparisons between real aqueous systems and the ideal gas-aqueous system counterparts to highlight and discuss the usefulness of invoking the ideal gas species as a reference solute to gain understanding of the solvation of real gas species.

We analyzed the consistency, or lack thereof, of the conjectured connection between the sign of Jones-Dole's B-coefficient, or its isobaric-temperature derivative, and the structure-making/breaking ability of a solute in a dilute solution. We sought to shed light on some crucial issues, including (i) whether Jones-Dole's B-coefficient contains any embedded microstructural information, (ii) whether we can either assign any definite foundation to the widely used assumption about the sign of the B-coefficient and its structure-making/breaking trend or provide a rational justification for its use as a structure-making/breaking marker, and (iii) whether we actually need Jones-Dole's B-coefficient and its isobaric-temperature derivative as markers for the interpretation of structure-making/breaking trends. Thus, we first addressed the fundamental (statistical mechanical) microscopic to (thermodynamic) macroscopic foundations of a rigorous approach to the structure-making/breaking ability of a solute, regardless of the type of solvent or nature of the solute-solvent intermolecular interaction asymmetries, and its exact relationships to the resulting solution thermodynamics. Then, we derived the required conditions for the signs of the B-coefficient and/or its isobaric-temperature derivative to describe either a structure-making or a structure-breaking event, according to the actual solute-induced perturbation of the solvent microstructure, and consequently, tested the rationale underlying the B-based conjectured structure-making/breaking markers. Moreover, we invoked a well-known transition-state (TS) interpretation of the B-coefficient to connect it explicitly to the derived molecular-based structure-making/breaking signature and to find under which TS conditions the signs of the B-coefficient and its temperature derivative could describe the actual solute-induced perturbation of the solvent microstructure. We illustrated the actual structure-making/breaking behavior for a series of aqueous solutes over a wide range of solute-solvent intermolecular interaction asymmetries and compared their behavior against that predicted by Jones-Dole's based markers. This comparison, supported by rigorous thermodynamic arguments, highlighted the lack of uniqueness (or one-to-one correspondence) in the response of the B-based markers to the solute-solvent intermolecular interaction asymmetry, and therefore, their inadequacy as structure-making/breaking descriptors. Finally, we discuss the findings and provide a cautionary outlook on the use of the viscosity-based structural markers.

This work addresses the thermodynamic limiting behavior of dilute species in binary solutions as described by their excess chemical, mechanical and thermal partial molar properties M ˆ i E ( T P x i ) as well as the corresponding composition slopes [ ∂ M ˆ i L R ( T P x i ) / ∂ x i ] T P , where M ˆ i L R ( T P x i ) comprises the partial molar properties of systems describing the excess Gibbs free energy and the corresponding temperature- and pressure-derivative quantities. These quantities include the species partial molar Gibbs free energies according to Lewis-Randall (LR) description, and their volumes, enthalpies, entropies, and isobaric heat capacities. The analysis builds upon the recently proposed solvation formalism for dilute systems [J. Chem. Phys., 128 (2008); J. Chem. Phys., 148 (2018); Fluid Phase Equilib., 472 (2018)] and its scope includes (a) the identification of the microstructural signatures, macroscopic manifestation, as well as the interrelations between the species partial molar quantities M ˆ i L R ( T P x i ) [ ∂ M ˆ i L R ( T P x i ) / ∂ x i ] T P , and the apparent molar counterpart M ϕ ( T P x i ) at finite composition as well as at infinite dilution; (b) the development and testing of a method to describe the actual limiting M ˆ i L R ( T P x i → 0 ) behavior in terms of the corresponding value at the condition of solute saturation, i.e., M ˆ i L R ( T P x i = x i s a t ) ; (c) the illustration and discussion of some successful approximations for the accurate estimation of the limiting M ˆ i L R ( T P x i → 0 ) , and consequently, (d) the discussion of potential drawbacks underlying the composition regression of experimental data and subsequent extrapolation to infinite dilution.

We analyzed the hydration behavior of alkyl-substituted urea osmolytes in terms of their deviation from Lewis-Randall solution ideality and characterized their structure-making/breaking tendency according to a proposed solvation formalism that provides a rigorous cause-effect connection between the system microstructure and its solution thermodynamic nonidealities. After a brief introduction of the rationale behind the use of Lewis-Randall over alternative solution ideality references, we (i) assessed the effect of the nature and type of alkyl substitution on the osmolyte-induced perturbation of the solution microstructure as a function of composition, (ii) analyzed their microstructural responses to changes in temperature and pressure, and (iii) demonstrated the structure-breaking nature of urea and the magnifying behavior of its alkyl-substituted osmolytes, whose trend follows the increasingly positive deviation of the osmolyte solutions from Lewis-Randall ideality. Then, we discussed the falsifiability of a pair of frequently used conjectured structure-making/breaking criteria, supported by the derived exact relationships between the structure-making/breaking parameter, the solution thermodynamic nonideality, and the osmotic second virial coefficient of the aqueous osmolytes. Finally, we provided an outlook on how the proposed approach could guide the quest for a truly (microstructural to free energy) causative explanation for the denaturation mechanism of proteins.

We studied the solute-induced perturbation of the solvent environment around a solute species from a microscopic viewpoint and propose a novel approach to the understanding of the structure-making/breaking process, regardless of the type and nature of the solute-solvent interactions. Based on the Kirkwood-Buff fluctuation formalism, we present a rigorous statistical mechanics description of the evolution of the solvent structure around the solute, analyze its response to small perturbations of the ( TP) state conditions and composition of the system, and make direct connections between a few equivalent micro- and macroscopic manifestations as probes for, and targets of, experimental measurements. We illustrate the analysis with theoretical results from integral equation calculations of model fluids and experimental evidence from available data for a variety of aqueous electrolyte and nonelectrolyte real fluid solutions. Finally, we provide a critical discussion about the inadequacy underlying a widely used de facto criterion for the classification of structure-making/breaking solutes.

Despite the long history behind the experimental determinations of gas solubility and the consequent wealth of experimental data for binary systems we are still confronted with lingering issues regarding their reliability, the approximations behind their modeling, and the consistency among resulting solvation quantities. In the present effort we discuss cautiously these issues through the thermodynamic scrutiny of the coexistence phase equilibrium equations involving sparingly soluble species according to the recently proposed rigorous molecular-based solvation formalism [Chialvo: Fluid Phase Equilibria https://doi.org/10.1016/j.fluid.2017.10.031 and Journal of Chemical Physics, 148 (2018)]. We address the fundamentals underlying the isothermal-isobaric phase equilibria associated specifically with the solubility of carbon dioxide in aqueous systems, their thermodynamic limit of stability, as well as the link between the latter and the microstructure of the pair of incipient phases. Moreover, we show how the resulting equilibrium phase composition can provide relevant solvation quantities to test the thermodynamic consistency of the experimental data. Finally, we illustrate the arguments explicitly through the analysis of experimental evidence from a few selected sources in the literature, and discuss the implications behind the findings.

We propose a thermodynamically consistent truncated composition expansion, around the infinite dilute quaternary reference system, to describe the thermodynamic behavior of dilute mixed solutes dissolved in mixed-solvents. The development involves the thermodynamically necessary second-order composition expansion of the species partial molar quantities, whose expansion coefficients encompass true molecular meaning based on their underlying links with the solvation properties of the corresponding infinite dilute reference system. Thus, we provide a rigorous molecular-based interpretation of the expansion coefficients in terms of precisely defined microscopic quantities that describe the mixed-solvent environment around all species in solution. Consequently, the proposed second-order truncated composition perturbation expansion applies to any type of quaternary system regardless of its aggregation state, in particular, it converges to the rigorous expressions for quaternary mixtures of imperfect gases obeying the truncated virial equation z = 1 + B P / k T , and it reduces naturally to its second-order truncated ternary and binary counterparts through the identification, and consequent manipulation, of the molecular asymmetries among species giving rise to the system nonidealities. Finally, we highlight the origin and key implications of the thermodynamic inconsistencies encountered in currently used first-order truncated expansions.

We present a novel molecular thermodynamic framework for the unambiguous assessment of the reliability of modeling approximations, their internal consistency, and the compliance with fundamental limiting behaviors in the description of solvation phenomena of species at infinite dilution in fluid systems over wide ranges of state conditions and solute-solvent intermolecular asymmetries. The proposed aprioristic approach does not rely on any type of regression technique; its accuracy rests on the theoretical validity and self-consistency of the underlying core solvation formalism and the first principles of chemical thermodynamics already established for model systems for which we have available exact formal results, and its application leads to the identification, as well as the isolation of the sources of modeling deficiencies. To that end, we (i) set the foundations for a rigorous description of the solvation behavior of solutes at infinite dilution at the microscopic and concomitant macroscopic levels, (ii) advance a set of thermodynamic limiting conditions as required constraints for any model of infinitely dilute solutions, (iii) identify the microscopic signatures of the solvation phenomena in terms of solute-induced perturbations of the solvent microstructure, and (iv) invoke an exact fluctuation formalism of solutions to assess the predictive capability of some current density-based models of aqueous non-electrolyte solutions. According to the findings of this analysis, we suggest a novel fully molecular-based alternative for the thermodynamic description of solutions at infinite dilution involving wide ranges of state conditions and solute-solvent intermolecular asymmetry, including the ideal gas solute and the solute behaving as another solvent species. This approach, which functions at a self-consistent integral plus derivative level, would automatically comply with all required thermodynamic constraints.

We investigate the microscopic mechanisms of the overlap of interfacial structures in confined fluids and attempt to answer the question whether the confined structures can be predicted from the original density profiles of individual solid–fluid interfaces. For that purpose, we perform (globally) isobaric–isothermal (locally, grand canonical) molecular dynamics simulations to extract not only the axial distribution functions of the water sites for the uncoupled graphene–water interfaces, but also those corresponding to the confined aqueous environments over the interplate range 8 ≤ h (A) ≤ 28 typically at ambient conditions. We have tested two (i.e., an arithmetic and a geometric) superposition approximations for the singlet density of confined water between flat graphene plates, as well as for a combination of flat and corrugated graphene plates. The outcome of this study suggests that the answer to the title’s question is a “yes”, provided that the interplate distance h is large enough to avoid fluid g...

In this work, we have derived rigorous expressions for the thermodynamic description of the solvation behavior of a model solute in highly-compressible environments of a real solvent, in order to address the question posed in the title of this manuscript. Toward that end, we first provide the essential statistical thermodynamic foundations to support a fundamentally-based approach for the determination of the Krichevskii parameter of a solute in any solvent, based solely on the knowledge of the corresponding Henry's law constant. Second, we analyze the actual orthobaric-density behavior of ln Q ( T , P σ ) and T σ ln Q ( T , P σ ) , when Q = K D ∞ and Q = ( H i , j I S / f j o ) , for an infinitely dilute ideal gas solute in an aqueous solution, a system for which we have complete knowledge of its microscopic and thermodynamic behaviors. Third, we invoke these results to test the assumptions behind the modeling of the orthobaric-density dependence of the Henry's law constant, infinite dilution vapor-liquid distribution, and Ostwald coefficients of real solutes. Fourth, we discuss the implications underlying the interpretation of the regressed orthobaric-density slopes, the accuracy of the resulting effective Krichevskii parameters, and the extension of the current analysis to the orthobaric-density behavior for the corresponding Ostwald coefficient, T σ ln L i , j ∞ ( T , P σ ) . Finally, we show how the fundamentally-based orthobaric-density linear expressions of ln Q ( T , P σ ) for the three Q − quantities converge to the corresponding, empirically-found, T σ ln Q ( T , P σ ) asymptotic linear representations for near-critical infinitely dilute solutions.

We present a novel approach for the calculation of the Krichevskii parameter A K r of solutes in dilute mixtures according to a universal thermodynamic expression that links the solute-solvent intermolecular asymmetry, as described by the Henry's law constant of the solute species in a pure solvent, to the desired A K r . The derived expression obeys two key thermodynamic constraints, i.e., corresponding to the behavior of either an ideal gas solute or a solute behaving as a solvent species representing a special case of Lewis-Randall ideal solution. Then, we provide a comparison between the thermodynamically consistent A K r values generated by this approach and those reported in the literature, from diverse experimental sources, and for a variety of aqueous solutes. Finally, we discuss how the A K r = 0 line divides the P − T plane of the pure solvent into two regions characterized by unambiguous solute-solvent intermolecular asymmetries, and provide additional microscopic as well as macroscopic interpretations for A K r .

We propose an orientation-averaged intermolecular potential model for the accurate computation of relevant thermodynamic and transport properties of steam at conditions typically encountered in extreme environments such as rocky exoplanetary atmospheres, aircraft and rocket engines, high temperature steam reforming and electrochemical reactors, as well as advanced ultra-supercritical steam power generators. We assess the reliability of the potential model to describe accurately the temperature dependence of relevant thermodynamic properties of rarified steam up to and beyond 3000K. Moreover, we discuss some fundamental issues including (i) the conditions at which an orientation-averaged intermolecular potential for a molecular fluid becomes an accurate representation of the fluid behaviour in extreme environments, (ii) the temperature range of validity of the approximations underlying an orientation averaging approach and, (iii) the methodology of force-field parameterisation to obtain an optimised representation, as well as the concomitant modelling of the thermodynamics and transport properties of rarified fluids. Finally, we illustrate how the simultaneous accurate description of the temperature dependence of the fluid’s collision integrals and second virial coefficient is crucial to the successful modelling of rarified steam in extreme environments.

We present an explicit molecular-based interpretation of the thermodynamic phase equilibrium underlying gas solubility in liquids, through rigorous links between the microstructure of the dilute systems and the relevant macroscopic quantities that characterize their solution thermodynamics. We apply the formal analysis to unravel and highlight the molecular-level nature of the approximations behind the widely used Krichevsky-Kasarnovsky [J. Am. Chem. Soc. 57, 2168 (1935)] and Krichevsky-Ilinskaya [Acta Physicochim. 20, 327 (1945)] equations for the modeling of gas solubility. Then, we implement a general molecular-based approach to gas solubility and illustrate it by studying Lennard-Jones binary systems whose microstructure and thermodynamic properties were consistently generated via integral equation calculations. Furthermore, guided by the molecular-based analysis, we propose a novel macroscopic modeling approach to gas solubility, emphasize some usually overlook modeling subtleties, and identify novel interdependences among relevant solubility quantities that can be used as either handy modeling constraints or tools for consistency tests.

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 i 2 + ⋯ S O 4 2 − $N{i^{2 + }} \cdots SO_4^{2 - }$ pair formation, identify its signature, suggest novel ways either for the direct probe of the contact ion pair (CIP) strength and the subsequent correction 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 i 2 + ⋯ S O 4 2 − $N{i^{2 + }} \cdots SO_4^{2 - }$ 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. 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.

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.

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.

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 electrolyte solutions. The set is comprised of the individual components of the potential energy, electrostatic potential, electrostatic field and gradient, electrostatic force, and 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.

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.

We present a novel molecular-based approach for the determination of the osmotic second virial coefficients of gaseous solutes in dilute binary solutions, according to a recently proposed molecular thermodynamic formalism of gas solubility [A. A. Chialvo, J. Chem. Phys. 148, 174502 (2018) and Fluid Phase Equilib. 472, 94 (2018)]. We discuss relevant solvation fundamentals and derive new expressions including (i) the relations among infinite-dilution solvation quantities leading to a novel self-consistent route to the calculation of the osmotic second virial coefficients, (ii) the new microstructural interpretation of the resulting osmotic second virial coefficients based on Kirkwood-Buff integrals, the unambiguous discrimination between short- and long-range contributions, and their limiting behavior as the solvent approaches its critical conditions, (iii) new rigorous expressions for the calculation of the osmotic second virial coefficients using standard reference thermodynamic data, and (iv) their underlying interdependence based on the constrained state variable invoked in the density expansion. We then invoke the proposed formalism to shed some light on the inaccuracies behind current calculations of osmotic second virial coefficients from molecular theory and simulation as well as macroscopic correlations. To advance the microscopic understanding and illustrate the functional relationship between the osmotic second virial coefficients, Henry’s law constant, and the solute-solvent intermolecular asymmetry as a source of solution non-ideality, we use data for the microstructural and thermodynamic behavior of infinitely dilute Lennard-Jones systems obtained self-consistently via integral equations calculations. The newly derived relationships leading to the proposed formalism offer novel routes for the accurate determination of osmotic second virial coefficients of any type of solutes in dilute solutions regardless of the type and nature of the intermolecular interactions. However, for illustration purposes in the current work, we dealt with aqueous solutions of simple gases to exploit the abundance of standard thermodynamic data for the orthobaric Henry’s law constant and solute distribution coefficients, as well as the availability of results from molecular-based calculations and macroscopic correlations.We present a novel molecular-based approach for the determination of the osmotic second virial coefficients of gaseous solutes in dilute binary solutions, according to a recently proposed molecular thermodynamic formalism of gas solubility [A. A. Chialvo, J. Chem. Phys. 148, 174502 (2018) and Fluid Phase Equilib. 472, 94 (2018)]. We discuss relevant solvation fundamentals and derive new expressions including (i) the relations among infinite-dilution solvation quantities leading to a novel self-consistent route to the calculation of the osmotic second virial coefficients, (ii) the new microstructural interpretation of the resulting osmotic second virial coefficients based on Kirkwood-Buff integrals, the unambiguous discrimination between short- and long-range contributions, and their limiting behavior as the solvent approaches its critical conditions, (iii) new rigorous expressions for the calculation of the osmotic second virial coefficients using standard reference thermodynamic data, and (iv) their unde...

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 ...

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 analysis of species behavior involving dilute fluid environments has been crucial for the advance of modern solvation thermodynamics through molecular-based formalisms to guide the development of macroscopic regression tools in the description of fluid behavior and correlation of experimental data (Chialvo 2013). Dilute fluid environments involving geologic formations are of great theoretical and practical relevance regardless of the thermodynamic state conditions. The most challenging systems are those involving highly compressible and reactive confined environments, i.e., where small perturbations of pressure and/or temperature can trigger considerable density changes. This in turn can alter significantly the species solvation, their preferential solvation, and consequently, their reactivity with one another and with the surrounding mineral surfaces whose outcome is the modification of the substrate porosity and permeability, and ultimately, the integrity of the mineral substrates. Considering that changes in porosity and permeability resulting from dissolution and precipitation phenomena in confined environments are at the core of the aqueous CO2-mineral interactions, and that caprock integrity (e.g., sealing capacity) depends on these key parameters, it is imperative to gain fundamental understanding of the mineral-fluid interfacial phenomena and fluid-fluid equilibria under mineral confinement at subsurface conditions. In order to understand the potential effects of acid gases as contaminants of supercritical CO2 streams, in the next section we will discuss the thermodynamic behavior of CO2 fluid systems by addressing two crucial issues in the context of carbon capture, utilization and sequestration (CCUS) technologies: (i) Why should we consider (acid gas) CO2 impurities? and (ii) Why are CO2 fluid - mineral interactions of paramount relevance? ### Background on flue gas sources, composition, and CO2 - acid gases co-injection The control of anthropogenic greenhouse gases emissions in the atmosphere by means of their capture and sequestration in deep geological reservoirs has emerged as a potential technology for the middle- to long-term mitigation of the greenhouse effect …

We present a detailed molecular-based characterization via isobaricisothermal 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 a simultaneous weaker water mediated CO2 /hydrophilic-surface interactions. We observed the opposite behavior with the hydrophobic plates where the weaker water layering results from the CO2 mediated H2O/hydrophobicsurface interactions. Moreover, we illustrated 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 CO2 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,more » where we found that this property changes 2 monotonically at purely interfacial regions, but non-monotonically under confinement.« less

This work uses molecular dynamics simulation to provide an atomistic view of the contrasting interfacial behavior between high-temperature dry air and wet (10–40 vol.% water) air in contact with stainless steels. A key finding was that H2O preferentially adsorbs and displaces oxygen at the metal–fluid interface. We also discuss how these findings are consistent with Ehlers et al. proposed competitive adsorption mechanism for the interpretation of the breakaway oxidation, and highlight their impact on other properties.

Injection of CO2 into subsurface geologic formations has been identified as a key strategy for mitigating the impact of anthropogenic emissions of CO2. A key aspect of this process is the prevention of leakage from the host formation by an effective cap or seal rock which has low porosity and permeability characteristics. Shales comprise the majority of cap rocks encountered in subsurface injection sites with pore sizes typically less than 100 nm and whose surface chemistries are dominated by quartz (SiO2) and clays. We report the behavior of pure CO2 interacting with simple substrates, i.e. SiO2 and muscovite, that act as proxies for more complex mineralogical systems. Modeling of small-angle neutron scattering (SANS) data taken from CO2–silica aerogel (95% porosity; ∼7 nm pores) interactions indicates the presence of fluid depletion for conditions above the critical density. A theoretical framework, i.e. integral equation approximation (IEA), is presented that describes the fundamental behavior of near-...

We discuss the interplay among theory, molecular simulations, and electric conductance experiments as an important tool for the extraction of ion-pair interaction potentials to make possible the bridging of the density gap between the lowest experimentally attainable conductance measurement and the theoretically acceptable zero-density limit of the ion-pair association constant. The density dependence of the Na+···Cl− pair association constant in ultrasupercritical (USC) steam environments is predicted by a constraint molecular dynamics simulation over state conditions relevant to the new generation of USC steam power plants. Finally, we draw attention to relevant modeling challenges associated with the behavior of these systems around the zero-density limit, discuss ways to overcome the drawbacks of traditional log Ka−log ρ models, and illustrate the case with an explicit quartic (ρ)1/2-polynomial representation for log Ka.

We analyzed the solvation behavior of aqueous lithium, nickel, and ytterbium sulfates at ambient conditions in terms of the relevant radial distributions functions and the corresponding first-order difference of the sulfur-site neutron-weighted distribution functions generated by isothermal-isobaric molecular dynamics simulation. We determined the partial contributions to the neutron-weighted distribution functions, to identify the main contributing peaks of the corresponding radial distribution functions, and the effect of the contact ion-pair configuration on the resulting water’s hydrogen coordination around the sulfate’s sulfur site. Finally, we assessed the extent of the ion-pair formation according to Poirier–DeLap formalism and highlighted the significant increase of the ion-pair association exhibited by these salts with cation charge.

We determined by molecular-based simulation the density profiles of the Na+ ⋅⋅⋅Cl− ion-pair association constant in steam environments along three supercritical isotherms to interrogate the behavior of ion speciation in dilute aqueous solutions at extreme conditions. Moreover, we describe a new ultra-sensitive, flow-through electrical-conductance apparatus designed to bridge the gap between the currently lowest steam-density conditions at which we are able experimentally to attain electrical conductance measurements and the theoretically reachable zero-density limit. Finally, we highlight important modeling challenges encountered near the zero-density limit and discuss ways to overcome them.

We present a detailed analysis of the behavior of aqueous electrolyte-polyelectrolyte systems in contact with neutral and charged graphene substrates, based on an extensive molecular dynamics simulation effort. Our study involves aqueous systems comprising short-chains of lithium-polystyrene sulfonate with explicit atomistic description of water, the chain backbones, and their interactions with all species in solution as well as with the graphene surface. We place special emphasis on the behavior of the axial profiles of species concentrations, local electrostatic charge density, electric field and corresponding surface-charge screening to provide a full characterization of the inhomogeneous environment at the solid-liquid interface, i.e., the electric double layer and the effect of the added salts (BaCl2 and LaCl3 ) on its structure. To complete the analysis, we assess the tendency toward ion pairing along planes parallel to the graphene surface and estimate, according to the axial distribution profiles, the strength of the adsorption of the polyelectrolyte, counterions, and other species in solution, in order to interpret the degree of surface-charge screening and the occurrence of surface-charge reversal. We present evidence of a recently reported new phenomenon of overcharging, and discuss the central role of the explicit description of the solvent on this occurrence. Moreover,more » to interpret the conformational behavior of the polyelectrolyte backbones we determine the axial profiles of the perpendicular and parallel components of the corresponding radius of gyration and end-to-end distance.« less

An extensive molecular-based simulation effort is conducted to analyze ion-pairing behavior in near-critical dilute HCl aqueous solutions along three near- (super- and sub-) critical isotherms over a wide density range, that is, from steam-like to liquid-like environments. The study encompasses the determination of the ion-pair association constant via potential of mean force calculations and the resulting degree of ion-pair association. The main goal behind this effort is to address a few relevant questions regarding the thermodynamic and corresponding microscopic behavior of the ion-pair formation in steamlike and highly compressible environments for which experimental data are extremely difficult to obtain accurately and in the process to illustrate the 'uneventful' behavior of the association constant along the critical isotherm of the solvent. Direct comparison is made between simulation, theoretical developments, and experiment to identify potential pitfalls in the interpretation of experimental data and subsequent macroscopic modeling.

The transferability of a few simple rigid non-polarizable water models were tested by Gibbs Ensemble Monte Carlo simulations to predict their vapor–liquid phase equilibria, and by isothermal–isobaric (Parrinello–Rahman) Monte Carlo simulations of the 13 known crystalline phases of ice. The temperature dependence of the corresponding second virial coefficients was also determined and then used to test the internal consistency of the simulated vapor-phase densities. The model predictions appear satisfactory for liquid water for ambient conditions, but they fail to mimic accurately the properties of the ice polymorphs and the orthobaric vapor phase. The major shortcomings of the models were in the overestimation by a factor of two (∼ 4–6 kJ/mol) of the internal energy difference between the high-pressure ice phases and the hexagonal phase. This unacceptable discrepancy is caused by the parameterization to reproduce the density of liquid water at ambient conditions, that accounts for the significant polarization effects in the condensed phases in terms of augmented dipole moments, with the consequent detrimental effect on the estimations of the vapor-phase properties.

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.

We perform extensive Gibbs Ensemble Monte Carlo simulations to study the capability of some recently re-parameterizations of the original TIP4P model intended to predict accurately the vapor-liquid coexistence envelope of water, its critical point, and its temperature dependence for the vapor pressure and second virial coefficient, and complement this analysis with the characterization of some specific crystalline faces of ice. We also disclose some trends between the resulting dipole moment of the models and the Lennard-Jones parameters, the location of the negative charge, as well as the estimated critical temperature. Finally, we discuss the inability of these models to predict accurately and simultaneously the melting temperature and the temperature of maximum density.

The vapor–liquid coexistence curve of the simple point charge heavy-water model (SPC-HW), [J. Chem. Phys., 114, 8064–8067 (2001)] is determined by Gibbs Ensemble Monte-Carlo (GEMC) simulation. The estimated critical conditions of the model based on the Wegner-type expansion for the order parameters and the rectilinear diameter are ρc = 0.300 g/cc, T c = 661 K and P c = 156 bars. The dielectric constant determined by isothermal–isochoric molecular dynamics is underpredicted along the coexistence curve by 29–44% in comparison with the experimental values. The analysis of the orthobaric temperature dependence of the system microstructure, in terms of the three site–site radial distribution functions, indicates that the first coordination numbers for the oxygen–oxygen and the oxygen–deuterium interactions are ∼4.3 ± 0.1 and ∼1.9 ± 0.1 at T = 300 K, and decrease by 15 and 55%, respectively, at criticality. The dipole–dipole correlation functions show that the orientational order in heavy water is quickly lost ...

We analyze the solvation behavior of short-chain polystyrene sulfonate (PSS) in aqueous electrolyte solutions by isothermal-isochoric molecular dynamics simulation to determine the solvation effects on the structure and conformation of the polyelectrolyte as a function of the aqueous environment. To that end, we study these aqueous systems including the explicit atomistic description of water, the PSS chain, and their interactions with all species in solution. In addition, we investigate the effect of the degree of sulfonation and its distribution along the PSS chain on the resulting conformation as well as solvation structure. Moreover, we assess the impact of added salts on the net charge of the PSS backbone, placing emphasis on the valence of the counterion and the extent of the ion-pair formation between the sulfonate group and the counterions. Finally, we present evidence for the so-called like-charge attraction between sulfonate groups through the formation of counterion-mediated interchain sulfonate-sulfonate and water-mediated intrachain sulfonate-sulfonate bridges, as well as between unlike counterion-counterion interactions.

Grand canonical Monte Carlo simulations are performed to study the adsorption of water in single-walled carbon nanotubes (SWCNs). At room temperature the resulting adsorption isotherms in (10:10) and wider SWCNs are characterized by negligible amount of water uptake at low pressures, sudden and complete pore filling once a threshold pressure is reached, and wide adsorption/desorption hysteresis loops. The width of these loops decreases as pore diameter narrows. Adsorption/desorption hysteresis loops are not observed for water adsorption in (6:6) SWCNs. When the nanotubes are doped with small amounts of oxygenated sites it is possible to obtain adsorption isotherms in which the water uptake increases gradually as the pressure increases. Simulated X-ray diffraction patterns for confined water are also reported.

The adsorption of water in graphitic carbons is usually simulated via a weighted average of the adsorption isotherms simulated in carbon-slit pore of different widths. By following this procedure, details about pore morphology and pore connectivity may be overlooked. Towards a better match between virtual and real experiments, we present simulated adsorption isotherms for SPC/E model water in porous carbons composed by interconnected carbon-slit pores. The pores are separated from each other by one graphene layer. Imperfections (lack of carbon atoms) in the graphene layers result in interconnections between pores. The grand canonical Monte Carlo algorithm is used here to simulate water adsorption. Our results show that while the qualitative features obtained in the simulation of independent slit-shaped pores are reproduced when interconnected pores are considered, the adsorption isotherms rise more gradually and the adsorption/desorption hysteresis loops are narrower in the latter case.

To complete the study on the effect of the long-range part of Coulombic interactions on properties of complex polar and associating fluids, we have investigated in detail three compounds with extreme features: acetonitrile for its unusually large dipole moment, hydrogen fluoride with very strong hydrogen bonding, and formic acid for its potential formation of different n-mers in liquid and gaseous phases. The effect of the long-range Coulombic interactions on both the structure and thermodynamics of the homogeneous phase, and on the vapor-liquid equilibria has been examined using the same decomposition of realistic potential models into a short-range part and a residual part as in the previous paper [Kettler, M.; et al. J. Phys. Chem. B 2002, 106, 7537-7546]. The present results fully confirm the previous findings that the properties of polar and associating systems are determined primarily by the short-range interactions regardless of their nature, i.e., contributions arising from the long-range interactions constitute only a small portion of the total properties, and thus that the short-range potential counterpart of full realistic models can be used as a convenient reference for a successful perturbation expansion.

The microstructure of water in aqueous CaCl2 solutions over a wide range of salt concentrations at T=298 K is characterized by molecular simulation to determine the effect of the ions on the structure of water. The structural results are subsequently used to test the validity of a recently proposed hypothesis, which equates the ion effect to a pressure or temperature effect on the structure of water in the aqueous solution. The simulation results indicate unequivocally that the changes of the water structure caused by the presence of ions in solution cannot be emulated as a pressure effect due to the local nature of such structure perturbation.

Water adsorption isotherms are calculated by grand canonical Monte Carlo simulations for the SPC/E water model in carbon nanopores at 298 K. The pores are of slit or cylindrical morphology. Carbon-slit pores are of widths 0.8, 1.0 and 1.6 nm. The simulated single-walled carbon nanotubes are of 1.4 and 2.7 nm diameter ((10:10) and (20:20) respectively). In all cases considered, the adsorption isotherms are characterized by negligible adsorption at low pressures, pore filling by a capillary-condensation-like mechanism and adsorption–desorption hysteresis loops. For both pore morphologies considered, the relative pressures at which pore filling occurs, and the width of the adsorption–desorption hysteresis loop decrease with decreasing pore size. Adsorption isotherms simulated for water in carbon nanotubes show pore filling at lower relative pressures and narrower adsorption–desorption hysteresis loops when compared to adsorption isotherms simulated in carbon-slit pores of similar sizes. By using representati...

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

With the use of the grand canonical Monte Carlo simulation, water adsorption isotherms were determined for SPC/E water in slit-shaped graphitic nanopores at 298 K. The pore widths considered were 0.6, 0.625, 0.7, 0.8, 1.0, 1.6, and 2.0 nm. The resulting adsorption isotherms indicated negligible adsorption at low pressures, pore-filling by a capillary-condensation-like mechanism, and adsorption/desorption hysteresis loops. For pore widths equal to or larger than 0.7 nm, the relative pressure at which pore filling occurs and the size of the hysteresis loop decrease with decreasing pore width. For 0.6-nm pores, pore filling occurs at pressures approaching saturation. Upon decreasing the pore width from 2.0 to 0.7 nm, the zero-coverage isosteric heat of adsorption increases from 6 to 14 kJ/mol. The limit at high coverage converges to the enthalpy of condensation for SPC/E water as the pore width increases. From the simulated adsorbed-water densities and distributions across the pores and the use of a mean-fie...