Molecular-based Modeling and Simulation

Goals and General Comments: Our main thrust is the development and application of statistical mechanical tools and molecular simulation protocols to advance our microscopic understanding of solvation phenomena, solution nonidealities, and their fundamental connections. Much of our simulation effort comprises molecular dynamics and Monte Carlo simulations performed in serial and parallel architectures (Beowulf clusters and/or supercomputers from the CCS at ORNL). Toward that end we develop specialized simulation codes designed to answer specific questions posed by the phenomenon/a under investigation.

General Areas of Interest:
a)  Ionic solvation at extreme conditions, i.e., from superheated vapors to gas hydrates and supercooled ices
b)  Solvation effects on reaction kinetics in highly compressible solvents
c)  Gas hydrates formation in 'dirty' natural environments
d)  Force-field developments for classical simulation of aqueous systems
e)  Molecular-based interpretation of scattering structural information
f)   Field effects on phase equilibria
g)  Isotopic effects on fluid mixture non-idealities

Current External Collaborations:
Ivo Nezbeda, Institute of Chemical Process Fundamentals, Czech Academy of Sciences, Prague (http://home.icpf.cas.cz/ivonez/theory/)
On the effect of the range of the intermolecular interactions on the thermodynamics and phase equilibria of molecular fluids

Milan Predota, Institute of Chemical Process Fundamentals, Czech Academy of Sciences, Prague.
Simulation of aqueous solution in contact with atomistically-detailed rutile surfaces

Andras Baranyai, Department of Theoretical Chemistry. Eötvös Lorand University, Budapest (http://teo.elte.hu/fs/chemdirect.html)
Simulation studies of water and ice toward the developments of more accurate force field parameterization

Current Simulation Activities:
A. Molecular-level Structure and Dynamics of Aqueous Solutions

1) Basic Aqueous Chemistry to High Temperatures and Pressures:
Objective and Approach: Investigate atomic-level interactions between ions in solution at steam-like conditions through molecular-level determination of the ion-pair association constant of model NaCl and other aqueous electrolyte solutions. Link the thermodynamic and related microscopic behavior of the ion-pair formation at steam-like conditions, including the zero solvent-density limit along near-critical isotherms, to the model-system results to understand the large changes in solvent speciation with density near the solvent critical point.

Results: Aqueous NaCl can range from a moderately strong to a relatively week electrolyte in dilute aqueous solutions at near critical temperatures. The density dependence of the dielectric constant of water, both in bulk solvent and in locally dense regions near ions in solution, has a strong effect on ion association. The resulting partial dielectric screening of the electrostatic charges of the ion pair allows for a wide range of degree of dissociation as water goes from steam-like to liquid-like densities.

Significance: Current modeling approaches that describe dilute solutions in steam rely on conjectures regarding the microscopic behavior of water around ions. Molecular simulation provides a sound fundamental basis to link experimental results at high densities with the limiting behavior of ions in vacuum. This guide for modeling ion association in steam will permit more reliable modeling of processes including corrosion and fouling in power plants, solute transport in water and steam at high temperatures, and the performance of geothermal power systems.

Publications:
Chialvo A. A. and Cummings P. T. (1999) Molecular-based Modeling of Water and Aqueous Solutions at Supercritical Conditions. In Advances in Chemical Physics, Vol. 109 (ed. S. A. Rice), pp. 105-205. Wiley & Sons.

Chialvo A. A., Cummings P. T., and Simonson J. M. (2000) H3O+/Cl- Ion Pair Formation in High-Temperature Aqueous Solutions. Journal of Chemical Physics 113, 8093-8100.

Chialvo A. A., Cummings P. T., Simonson J. M., and Mesmer R. E. (1999) Solvation in High-Temperature Electrolyte Solutions. I. Hydration Shell Behavior from Molecular Simulation. Journal of Chemical Physics 110, 1064-1074.

Chialvo A. A., Cummings P. T., Simonson J. M., and Mesmer R. E. (2000) Ion Association in High-Temperature Aqueous HCl Solutions. A Molecular Simulation Study. In Steam, Water, and Hydrothermal Systems: Physics and Chemistry Meeting the Needs of Industry (ed. P. G. H. P. Tremaine, D. Irish, P.V. Balakrishnan), pp. 409-417. NRC Research Press.

Chialvo A. A., Cummings P. T., Simonson J. M., and Mesmer R. E. (2000) Solvation in High-Temperature Aqueous Electrolyte Solutions. Journal of Molecular Liquids 87, 233-242.

Chialvo A. A., Ho P. C., Palmer D. A., Gruszkiewicz M. S., Cummings P. T., and Simonson J. M. (2002) H3O+/Cl- Association in High-Temperature Aqueous Solutions over a Wide Range of State Conditions. A Direct Comparison between Simulation and Electrical Conductance Experiment. Journal of Physical Chemistry B 106, 2041-2046.

Chialvo A. A. and Simonson J. M. (2003) Aqueous Na+Cl- Pair Association from Liquid-like to Steam-like Densities along Near-critical Isotherms. Journal of Chemical Physics 118, 7921-7929.

2) Microstructural Studies of Aqueous Electrolyte Solutions
Objective and Approach: The aim of this effort is the interpretation of the hydration behavior of ions and coions in concentrated aqueous electrolyte solution, where the relevant microscopic information come from both neutron diffraction with isotope substitution (NDIS) and molecular-based simulation. To that end, molecular simulation is used to test the adequacy of the methodologies used in the extraction and interpretation of the structural information from scattering experiment.
Results: The simulation results indicate significant ion-pair effects on the Ca+2 and Cl- neutron-weighted distribution functions gCa(r) and gCl(r). The penetration of Ca+2 into the Cl- hydration shell superimposes and/or overlaps the H-Ca+2 and/or H-Cl- contributions to gCa(r) and gCl(r), respectively, depending on the strength of the Ca+2…Cl- interactions. This behavior appears to be a common occurrence in many other MxXm aqueous systems, making difficult the interpretation of the raw data at high concentrations due to the lack of additional information regarding the ever-present likelihood of Mm+…Xx- ion pair formation.

Fig: Concentration dependence of the first coordination number of species in calcium chloride solutions at ambient conditions. Note the unexpected behavior for O-Cl caused by the Ca-Cl pair formation.

Fig: Partial contributions to the normalized Ca+2 neutron-weighted distribution functions of a 6.4 m calcium chloride solution. The arrow highlights the incipient formation of a peak due to Ca-Cl pair formation, which becomes a rather challenging feature to detect experimentally.

Significance: Molecular simulation can help resolve features in the structural information from diffraction experiments, further enhancing the importance of those experimental methods in additional developments toward more realistic intermolecular potential models, and simultaneously, can provide insight on the effects of ion association on structural results, which may be particularly helpful as experimental conditions change (e.g., to higher temperatures) to favor ion pairing. Molecular-based simulation can provide powerful assistance and guidance for the accurate interpretation of the diffraction raw data, because it provides the two ends of the analysis and their rigorous connection, i.e., all details of the system structure under study and the corresponding weighted distribution functions. Thus, simulation offers a route for the interpretation of structural information from diffraction experiments, and insight into structural features that may be poorly resolved in experiments.

Publications:
Chialvo A. A. and Simonson J. M. (2002) The Structure of Concentrated NiCl2 Aqueous Solutions. What is Molecular Simulation Revealing about the Neutron Scattering Methodologies? Molecular Physics 100, 2307-2315.

Chialvo A. A. and Simonson J. M. (2003) The Effect of Salt Concentration on the Structure of Water in CaCl2 Aqueous Solutions. Journal of Molecular Liquids (In press).

Chialvo A. A. and Simonson J. M. (2003) The Structure of CaCl2 Aqueous Solutions over a Wide Range of Concentrations. Interpretation of Diffraction Experiments Via Molecular Simulation. Journal of Chemical Physics (Submitted for publication).

3) Molecular-based Formalisms for Dilute Solutes in Highly Compressible Solvents
Objective and Approach: Systems involving dilute solutes in highly compressible solvents are frequently encountered in natural and industrial processes. The description of their thermophysical properties is usually hampered by the disparity of the length scales of the involved phenomena, i.e., long-range compressibility-driven correlation lengths and short-range solvation-driven microstructural perturbations. Our goal is to develop molecular-based formalisms to sort out the despaired length-scale phenomena as a prerequisite to the interpretation of the available experimental results and their successful modeling.
Results: The partial molar volume of dilute solutes in near critical fluids has been the property most frequently studied, and the primary target for modeling purposes. The coexistence of compressibility- and solvation-driven contributions to any mechanical partial molar property of the solute at infinite dilution imposes stringent, but unnecessary, constraints to its modeling. We have demonstrated, through fundamental statistical mechanical arguments, that a judicious segregation between solvation and compressibility driven phenomena allows us to describe the thermodynamics of solvation of these systems excluding the computationally annoying compressibility-driven divergencies.

Fig: Comparison between integral equation predictions and the actual behavior of the solvation (SR) and compressibility-driven (LR) contributions to the infinite dilution partial molar volume of aqueous CsBr (For details see Journal of Physics. Condensed Matter 12, 3585-3593)

Fig: Comparison between integral equation predictions and the actual behavior of the 'solvation number' and the corresponding difference in direct correlation function integrals for infinite dilute aqueous CsBr solutions (For details see Journal of Physics. Condensed Matter 12, 3585-3593)

Significance: By segregating the solvation- from the compressibility-driven contributions to any mechanical partial molar property of an infinitely dilute solute, we have been able to provide the microscopic interpretation of the origin of their nonidealities, and consequently, to guide the developments of well-behaved representations in terms of state conditions which can be used to successfully regress the available experimental data. The resulting solvation formalism for ionic and non-ionic solutions involves no assumptions regarding either the size of the solvation shell or the type of intermolecular forces involved, and offers a rigorous tool to study solubility enhancement in highly compressible solvents.

Publications:
A. A. Chialvo, P. T. Cummings, J. M. Simonson, and R. E. Mesmer, Journal of Chemical Physics 110, 1075 (1999).

A. A. Chialvo, P. G. Kusalik, P. T. Cummings, and J. M. Simonson, Journal of Physics. Condensed Matter 12, 3585 (2000)

A. A. Chialvo, P. T. Cummings, P. G. Kusalik, J. M. Simonson, and R. E. Mesmer, in Steam, Water, and Hydrothermal Systems: Physics and Chemistry Meeting the Needs of Industry, edited by P. G. H. P. Tremaine, D. Irish, P.V. Balakrishnan (NRC Research Press, Ottawa, 2000), pp. 517

A. A. Chialvo, P. T. Cummings, J. M. Simonson, and R. E. Mesmer, Journal of Molecular Liquids 87, 233 (2000)

A. A. Chialvo, P. G. Kusalik, P. T. Cummings, and J. M. Simonson, Journal of Chemical Physics 114, 3575 (2001)

B. Stable Isotope Behavior in Mineral-fluid-gas Systems:
Objective and Approach: Isotopic mixtures do not follow classical but quantum (statistical) mechanical laws, even though their deviations from the classical behavior are rather small. This special situation makes it possible for us to analyze the quantum mechanically driven non-idealities from a classical mechanical point of view, and thus, to take advantage of existing molecular-based tools such as statistical mechanical perturbation expansions. Our studies encompass physicochemical aspects associated with isotopic fluids and their mixtures, and our main interest here is to develop the simulation methodology for the study of isotope partitioning in simple atomic and molecular systems and their mixtures.

Results: Comparisons between simulation predictions and the most accurate measurements of isotopic fractionation for noble gases and their mixtures have indicated the realism of the intermolecular potentials and the accuracy of the Kirkwood-Wigner perturbation expansion. The success of this approach involving atomic fluids lends support to its application to molecular fluids with more than one type of isotopic site.

Fig: Comparison between simulation predictions and experimental data of the orthobaric temperature-dependence of the vapor-liquid isotopic fractionation factors for 36Ar/40Ar (for details see Journal of Chemical Physics 119(8))

Fig: Comparison between simulation predictions and experimental data of the isothermal composition dependence of the vapor-liquid isotopic fractionation factors for 36Ar/40Ar and 80Kr/84Kr (for details see Journal of Chemical Physics 119(8))

Significance: Due to the small size of the isotopic effects and the experimental difficulties behind their measurements, the simulation route appears as a very viable approach for the determination and/or prediction of isotope partitioning of noble gases and their mixtures. Because the methodology is based on a classical statistical mechanical perturbation approach, the Kirkwood-Wigner expansion, it provides a direct and accurate route to the thermodynamic properties associated with isotopic contributions. The development of increasingly more accurate intermolecular potentials for atomic and molecular fluid models, in conjunction with GEMC and Gibbs-Duhem integration methods, allows us to assess isotopic effects in a wide range of state conditions, from solid to gas like densities, and predict the properties of systems difficult to study by experiment.

Publications:
Chialvo A. A. and Horita J. (2003) Isotopic Effect on Phase Equilibria of Atomic Fluids and their Mixtures: A Direct Comparison between Molecular Simulation and Experiment. Journal of Chemical Physics 119(8).

C. Nanoscale Complexity at the Oxide-water Interface
Objective and Approach: The main thrust behind our effort is the comprehensive microscopic characterization of the structure of aqueous electrolyte solutions in the neighborhood of the solid-fluid interface, the formation of the electric double layer, and their connection to the protonation behavior of the mineral surface. This effort has been divided according to the degree of complexity into the analysis of interfaces around neutral and charged structureless flat walls, as well as atomistically detailed walls, both in contact with electrolyte solutions. Ideally, the simulation of mineral-aqueous interface would require a detailed knowledge of the surface protonation behavior, i.e., the complete scheme of the acid-base reactions and water dissociation at the mineral surface. As a result of the formidable complexity underlying the surface chemistry, the ideal approach becomes clearly an unrealistic simulation scenario, and therefore, we chose a reasonable compromise between the realism (atomistic description) of the metal-oxide surface and the aqueous electrolyte phase, and the practicality of classical simulations using well-characterized intermolecular potential models, together with either pedestrian but insightful or ab initio described surfaces.

Results: An important determining factor in the electric double layer formation is the magnitude of the static dielectric permitivity of the solid surface (and its temperature dependence) relative to that of the electrolyte solution.

Significance: A mineral surface in contact with an aqueous solution undergoes physicochemical changes, such as protonation and consequent surface charging, which modify the thermophysical properties of the neighboring fluid phase. This phenomenon is of common occurrence including natural geochemical environments and industrial processes, and has been traditionally analyzed in terms of physico-chemical models that incorporated ad hoc descriptions for the wall and the fluid phases, in conjunction to rather detailed descriptions of the complexation process. Molecular simulation of aqueous electrolytes is currently able to provide increasingly more accurate/realistic description of the microstructure and dynamics of the species in the vicinity of and away from the interface. In turn, this allows the assessment of the reliability of the assumptions embedded into current complexation protocols such as the MUSIC model.

Publications: Work in progress

D. Self-assembly of Polyelectrolyte Structures in Solution
Objective and Approach: Here we pursue a systematic investigation of the nanoscale structures formed by polyelectrolytes in aqueous solution using realistic models to elucidate the extent and importance of the range of interactions from atomic to nanoscale dimensions, including inter- and intramolecular interactions between polyelectrolyte chains, and the interactions between polyelectrolytes, counterions, and solvent. For this purpose we use an explicit and realistic description of water and its interaction with ionomers and other species in solution, as opposed to the traditional continuum dielectric, that allows us to analyze specific solvation factors affecting the structure and dynamics of dissolved polyelectrolytes. Moreover, we use realistic representations of the salt in solution, based on the accurate parameterization for aqueous alkali halides, as opposed to simple equal-sized charged spheres. And finally, we make direct contact between structural information from simulation and from neutron diffraction to facilitate the interpretation of raw scattering data

Results: Work in progress

Significance: Polyion-counterion interactions play an essential role in determining the stability and solubility of polyelectrolytes in aqueous solutions. These interactions are particularly strong for multivalent counterions, common in biological systems, where the negatively charged biopolymer interacts with divalent metal ions. The nature of the counterion, including its electrostatic charge, polarizability, and short-range (non-electrostatic) interaction with the binding site of the polyelectrolyte, appears to be as important as the location of the binding site. Consequently, the binding between polyelectrolyte and counterion should exhibit a marked ion selectivity resulting from a delicate balance between short-range (solvation) and long-range (electrostatic) forces that define the local environment. This local environment depends on the solvent's properties, the ionic strength, and the state conditions (e.g., temperature), and cannot be described quantitatively in terms of bulk properties. This points out the need for a more detailed understanding, beyond the macroscopic treatments, of the polyion-counterion interaction. In particular, and understanding of the mechanism underlying the ion-selectivity of highly charged polyelectrolytes is needed to interpret quantitatively a variety of experimental measurements of physical and chemical properties.

Publications: Work in progress