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August 1995 - present
Publications
Publications (293)
Alchemical free energy calculations via molecular dynamics have been applied to obtain thermodynamic properties related to solid–liquid equilibrium conditions, such as melting points. In recent years, the pseudo-supercritical path (PSCP) method has proved to be an important approach to melting point prediction due to its flexibility and applicabili...
Hydrofluorocarbon (HFC) refrigerants with zero ozone-depleting potential have replaced chlorofluorocarbons and are now ubiquitous. However, some HFCs have high global warming potential, which has led to calls by governments to phase out these HFCs. Technologies to recycle and repurpose these HFCs need to be developed. Therefore, thermophysical prop...
This work explores the use of thermodynamics‐informed Gaussian processes (GPs) and active learning (AL) to model activity coefficients and construct phase diagrams. Relying on synthetic data generated from an excess Gibbs energy model, GPs were found to accurately describe the activity coefficients of several binary mixtures across large compositio...
Ionic liquids (ILs) have shown promise for applications that leverage differential gas solubility in an IL solvent, e.g., gas separations. Although most available literature provides Henry's law constants, the ability to efficiently estimate full isotherms is important for engineering design calculations. Molecular simulation can be used as a tool...
We present the results of molecular dynamics simulations of the ionic liquid (IL) 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [C2C1im][NTf2] in the presence of external electric fields (EEFs) of varying strengths to understand the effects of EEFs on the glass transition temperature Tg. We compute Tg with an automated and objective...
An ab initio quantum chemical approach for the modeling of propellant degradation is presented. Using state-of-the-art bonding analysis techniques and composite methods, a series of potential degradation reactions are devised for a sample hydroxyl-terminated-polybutadiene (HTPB) type solid fuel. By applying thermochemical procedures and isodesmic r...
The physical properties of four ionic liquids (ILs), including 1-n-butyl-3-methylimidazolium tetrafluoroborate ([C4C1im][BF4]), 1-n-butyl-3-methylimidazolium hexafluorophosphate ([C4C1im][PF6]), 1-n-butyl-3-methylimidazolium thiocyanate ([C4C1im][SCN]), and 1-n-hexyl-3-methylimidazolium chloride ([C6C1im][Cl]), and their mixtures with hydrofluoroca...
Experimental measurements and classical molecular dynamics (MD) simulations were carried out to study electrolytes containing CuCl2 and CuCl salts in mixtures of choline chloride (ChCl) and ethylene glycol (EG). The study focused on the concentration of 100 mM of both CuCl2 and CuCl with the ratio of ChCl/EG varied from 1:2, 1:3, 1:4, to 1:5. It wa...
The solvation structure and transport properties of Li+ in ionic liquid (IL) electrolytes based on n-methyl-n-butylpyrrolidinium cyano(trifluoromethanesulfonyl)imide [PYR14][CTFSI] and [Li][CTFSI] (0 ≤ xLi ≤ 0.7) were studied by Raman and Nuclear Magnetic Resonance (NMR) diffusometry, and molecular dynamics (MD) simulations. At xLi < 0.3, Li+ coord...
Superconcentrated electrolytes have emerged as a promising class of materials for energy storage devices, with evidence that high voltage performance is possible even with water as the solvent. Here, we study the changes in the water hydrogen bonding network induced by the dissolution of lithium bis(trifluoromethane sulfonyl)imide (LiTFSI) in conce...
Understanding the mechanisms of charge transport in batteries is important for the rational design of new electrolyte formulations. Persistent questions about ion transport mechanisms in battery electrolytes are often framed in terms of vehicular diffusion by persistent ion-solvent complexes versus structural diffusion through the breaking and refo...
Aqueous rechargeable Zn metal batteries (RZMBs) are promising candidates for coupling with intermittent renewable energy sources to realize a carbon-neutral energy transition. However, irreversible issues of Zn metal anodes and a poor understanding of the interphasial chemistry severely limit the viability of RZMBs. Here, we demonstrate that the ad...
Deep eutectic solvents (DESs) are an emerging class of non-aqueous solvents that are potentially scalable, easy to prepare and functionalize for many applications ranging from biomass processing to energy storage technologies. Predictive understanding of the fundamental correlations between local structure and macroscopic properties is needed to ex...
In this work surrogate assisted optimization is utilized to calibrate predictive molecular models, called force fields, used in molecular simulations to reproduce the liquid density of a hydrofluorocarbon refrigerant molecule. A previous calibration workflow which relied on Gaussian process regression models and large Latin hypercube samples to scr...
The low melting point of room temperature ionic liquids is usually explained in terms of the presence of bulky, low symmetry and flexible ions, with the first two factors related to the lattice energy while an entropic effect is attributed to the latter. By means of molecular dynamics simulations, the melting points of 1-ethyl-3-methylimidazolium h...
The concept of water-in-salt electrolytes was introduced recently, and these systems have been successfully applied to yield extended operation voltage and hence significantly improved energy density in aqueous Li-ion batteries. In the present work, results of X-ray scattering and Fourier-transform infrared spectra measurements over a wide range of...
We introduce a new Python interface for the Cassandra Monte Carlo software, molecular simulation design framework (MoSDeF) Cassandra. MoSDeF Cassandra provides a simplified user interface, offers broader interoperability with other molecular simulation codes, enables the construction of programmatic and reproducible molecular simulation workflows,...
Quantitatively accurate molecular models, called force fields, are necessary for predictive molecular simulations. However, optimizing force fields to accurately reproduce experimental properties is a challenging endeavor. Here we present a machine learning directed workflow for force field optimization. Surrogate-assisted optimization is used to e...
The nitroxide radical redox organic molecule, 2-phenyl-4,4,5,5-tetrame- thylimidazoline-1-oxyl-3-oxide (PTIO), was investigated for the first time in a deep eutectic solvent (DES)-like system consisting of a 1:4 molar ratio of choline chloride and ethylene glycol (Ch1EG4) as a redox flow battery electrolyte. PTIO is a single molecule with three oxi...
The addition of phosphonium-based supporting salts and a subtle difference in their ligand chemistry remarkably improves the reversibility of aqueous Zn electrolytes. By promoting interphase formation, P444(2O1)-TFSI supports a Zn coulombic efficiency above 99 % even at 1.17 mA cm⁻² with 20 % Zn utilization per cycle, and a stable Zn∥Na2V6O16⋅1.63...
Deep eutectic solvents (DESs) are an emerging class of mixtures characterized by significant depressions in melting points compared to those of the neat constituent components. These materials are promising for applications as inexpensive "designer" solvents exhibiting a host of tunable physicochemical properties. A detailed review of the current l...
Results from extensive molecular dynamics simulations of molten LiCl, NaCl, KCl, and RbCl over a wide range of temperatures are reported. Comparison is made between the “Polarizable Ion Model” (PIM) and the non-polarizable “Rigid Ion Model” (RIM). Densities, self-diffusivities, shear viscosities, ionic conductivities, and thermal conductivities are...
A range of techniques including physical property measurements, neutron scattering experiments, ab initio molecular dynamics and classical molecular dynamics simulations are used to probe the structural, thermodynamic, and transport properties of a deep eutectic solvent comprised of a 1:2 molar ratio of choline chloride and ethylene glycol. This mi...
There is a long history of models that to different extents reproduce structural and dynamical properties of high-temperature molten salts. Whereas rigid ion models can work fairly well for some of the monovalent salts, polarizability is fundamentally important when small divalent or multivalent ions are combined with significantly polarizable anio...
The low melting point of room temperature ionic liquids is usually explained in terms of the presence of bulky, low-symmetry, and flexible ions, with the first two factors related to the lattice energy while an entropic effect is attributed to the latter. By means of molecular dynamics simulations, the melting points of 1-ethyl-3-methyl-imidazolium...
Accurate molecular models of pure alkali halides are a prerequisite for developing transferable models of molten salts that can predict the properties of complex salt mixtures, such as those including dissolved actinide species and metal ions. Predicting the melting point of a substance represents a rigorous test of model quality. To this end, we c...
The migration behaviour of an ionic liquid (IL) nano-droplet on a graphene surface under temperature gradients was studied using non-equilibrium molecular dynamics (MD) simulations. 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF4]) was used in the study. The migration of the IL nano-droplet from the hot end to the cold end of the graphene...
A range of techniques including physical property measurements, neutron scatter- ing experiments, ab initio molecular dynamics and classical molecular dynamics simu- lations are used to probe the structural, thermodynamic, and transport properties of a deep eutectic solvent comprised of a 1:2 molar ratio of choline chloride and ethylene glycol. Thi...
This paper presents an assessment on the use of Monte Carlo simulations and equations of state to calculate fluid phase equilibrium of natural gas containing CO2 and H2O at extreme pressures. Standard Gibbs ensemble Monte Carlo simulations in the canonical ensemble were used to study the coexistence properties of carbon dioxide and methane at tempe...
A coarse-grained Brownian dynamics model was used to simulate two proteins of similar sizes inside model membrane pores of varying size and hydrophobicity. The two proteins, which have radii of gyration of approximately 9.5 Å in their native states, are a 36-residue hydrophilic villin head piece (HP-36) and a 40-residue hydrophobic amyloid beta (Aβ...
The applicability of an atomistic Class II force field to capture the properties of the orthorhombic crystal phase of ammonium perchlorate was investigated. Structural and dynamical behaviors including density, lattice parameters, bulk modulus, infrared spectrum, and rotational dynamics were calculated from the trajectories of molecular dynamics (M...
The structural and dynamical changes in the solvation shell surrounding Li⁺ in a multi-anion environment are studied by Raman spectroscopy and molecular dynamics (MD) simulations. The ternary electrolyte is composed of a mixture of two ionic liquids (ILs) n-methyl-n-propylpyrrolidinium bis(trifluoromethylsulfonyl) imide ([PYR13][TFSI]), 1-ethyl-3-m...
The performance of the general AMBER force field (GAFF) was evaluated by computing the density (ρ), dielectric constant (ε), viscosity (η), and enthalpy of vaporization (ΔHvap) for 19 different organic solvents at 298 K and 1 atm. The force field performed very well for liquid densities, with deviations from experiment averaging around 3%. The perf...
The absorption of CO2 into an aprotic heterocyclic anion ionic liquid (IL) is modeled using reaction ensemble Monte Carlo (RxMC) with the semi-grand reaction move. RxMC has previously been unable to sample chemical equilibrium involving molecular ions in nanostructured liquids due to the high free energy requirements to open and close cavities and...
Classical molecular dynamics simulations were performed on twelve different ionic liquids containing aprotic heterocyclic anions doped with Li⁺. These ionic liquids have been shown to be promising electrolytes for lithium ion batteries. Self-diffusivities, lithium transference numbers, densities, and free volumes were computed as a function of lith...
A combination of ab initio calculations and classical molecular dynamics simulations was used to calculate the free energy of reacting an aprotic heterocyclic anion ionic liquid with CO2. The overall reaction was broken into a series of steps using a thermodynamic cycle to calculate the free energy of the gas phase reaction and the free energy cont...
Ionic liquids (ILs) have gained considerable attention in recent years as CO2-reactive solvents that could be used to improve the economic efficiency of industrial-scale CO2 separations. Researchers have demonstrated that IL physical and chemical properties can be optimized for a given application through chemical functionalization of both cations...
We present a newly-developed Monte Carlo scheme to predict bulk surfactant concentrations and surface tensions at the air-water interface for various surfactant interfacial coverages. Since the concentration regime of these systems of interest are typically very dilute (< 10^-5 mol. frac.), Monte Carlo simulations with the use of insertion/deletion...
The original reaction move for the reaction ensemble Monte Carlo (RxMC) method is adapted to align both the position and orientation of inserted product molecules and deleted reactant molecules. The accuracy and efficiency of this move is demonstrated for xylene isomerization in vapor, liquid, and supercritical phases. Classical RxMC requires the i...
In this work we use computational design to examine fifteen new electrolyte salt anions by performing chemical variations and mutations on the bis-(trifluoromethane)sulfonamide (TFSI) anion. Based on our calculations, we propose two new anions as potential candidates for magnesium energy storage systems, which are evolved from TFSI with the substit...
An all-atom force field consistent with the general AMBER force field (GAFF) format for poly(ethylene glycol) dimethyl ether (diglyme or G2) was developed by fitting to experimental liquid densities and dielectric constants. Not surprisingly, the new force field gives excellent agreement with experimental liquid phase densities and dielectric const...
Molecular dynamics and Gibbs ensemble Monte Carlo simulations were used to compute the self-diffusion coefficients and solubilities of CO2, CH4 and H4 in model membranes consisting of slit pores with diameters of 2 nm and 5 nm. Solubility selectivities, diffusion selectivities and permselectivities of CO$_2$ for binary gas mixtures of CO2/CH4 and C...
Cassandra is an open source atomistic Monte Carlo software package that is effective in simulating the thermodynamic properties of fluids and solids. The different features and algorithms used in Cassandra are described, along with implementation details and theoretical underpinnings to various methods used. Benchmark and example calculations are s...
The influence of three sodium salts, covering a wide range of the Hofmeister series, on the conformation of three proline-based peptide models in aqueous solution is examined using a combination of nuclear magnetic resonance spectroscopy and molecular dynamics simulations. The anions preferentially interact with the cis conformers of the peptide mo...
Molecular dynamics simulations were used to compare water solubilities and the effects of water on the structure and dynamics of ionic liquids (ILs) comprised of phosphonium cations paired with azolide and phenolate anions. The addition of water decreases ordering of the ions compared to the dry ILs with the exception of anion-anion ordering in the...
A combination of X-ray scattering experiments and molecular dynamics simulations were conducted to investigate the structure of ionic liquids (ILs) which chemically bind CO2. The structure functions were measured and computed for four different ILs consisting of two different phosphonium cations, triethyloctylphosphonium ([P2228]⁺) and trihexyltetr...
The binary system of menadione in explicit supercritical carbon dioxide (SC-CO2) was studied using molecular dynamics (MD) simulations, with the objective to understand the nature of interactions between menadione and SC-CO2 at different temperatures and pressures in order to complement experimental solubility measurements. A force field was develo...
Molecular dynamics simulations were carried out to study the self-diffusion coefficients of CO2, methane, propane, n-hexane, n-hexadecane, and various poly(ethylene glycol) dimethyl ethers (glymes in short, CH3O–(CH2CH2O)n–CH3 with n = 1, 2, 3, and 4, labeled as G1, G2, G3, and G4, respectively) at different conditions. Various system sizes were ex...
Among different classes of ionic liquids (ILs), those with cyano-based anions have been of special interest due to their low viscosity and enhanced solvation ability for a large variety of compounds. Experimental results from this work reveal that the solubility of glucose in some of these ionic liquids may be higher than in water - a well-known so...
The application of ionic liquids (ILs) in many industrially relevant processes provides an urgent need to better understand their molecular interactions with biological systems. A detailed understanding in the cytotoxicity mechanism of ILs can be helpful in facilitating the molecular design of non-toxic ILs. Using coarse-grained molecular dynamics...
The viscosity and density of the ionic liquid (IL) 1-n-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ( ), the molecular solvent tetraethylene glycol dimethyl ether (tetraglyme or G4) and their binary mixtures were measured experimentally as a function of temperature. The same systems were also studied using classical molecular dynamic...
The viscosities of ionic liquids (ILs) that chemically react with CO2 and contain an aprotic heterocyclic anion (AHA) change very little after CO2 absorption, whereas the viscosities of other kinds of reactive ILs increase dramatically after CO2 absorption. This unique property has overcome a major problem with IL-based facilitated transport membra...
Ionic liquids (ILs) are salts that remain liquid down to low temperatures, and sometimes well below room temperature. ILs have been called “green solvents” because of their extraordinarily low vapor pressure and excellent solvation power, but ecotoxicology studies have shown that some ILs exhibit greater toxicity than traditional solvents. A fundam...
Glucose is an important carbohydrate, relevant both for its biological functions and as a raw material for industrial processes. As a monomer of cellulose, the most abundant biopolymer, it is an alternative feedstock for fuels and chemicals in the biorefinery framework. Since glucose is often used and processed in aqueous solutions, it is important...
A series of branched ionic liquids (ILs) based on the 1-(iso-alkyl)-3-methylimidazolium cation from 1-(1-methylethyl)-3-methylimidazolium bistriflimide to 1-(5-methylhexyl)-3-methylimidazolium bistriflimide and linear ILs based on the 1-(n-alkyl)-3-imidazolium cation from 1-propyl-3-methylimidazolium bistriflimide to 1-heptyl-3-methylimidazolum bis...
Equilibrium molecular dynamics is often used in conjunction with a Green-Kubo integral of the pressure tensor autocorrelation function to compute the shear viscosity of fluids. This approach is computationally expensive and is subject to a large amount of variability because the plateau region of the Green-Kubo integral is difficult to identify una...
Gibbs ensemble Monte Carlo simulations were used to calculate absorption isotherms of water in three imidazolium-based ionic liquids (ILs) with different degrees of hydrophobicity: 1-n-butyl-3-methylimidazolium hexafluorophosphate ([C4mim][PF6]), 1-n-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide ([C4mim][Tf2N]) and 1-n-butyl-3-methyl...
We present a high-throughput infrastructure for the automated calculation of molecular properties with a focus on battery electrolytes. The infrastructure is largely open-source and handles both practical aspects (input file generation, output file parsing, and information management) as well as more complex problems (structure matching, salt compl...
Ionic liquid active pharmaceutical ingredients (IL APIs) are novel materials in which the ions themselves are APIs, but the pure salt is a liquid under ambient conditions. It has been found that IL APIs can have superior performance relative to their conventional salt analogues, but the mechanism for this is unclear. We have used molecular simulati...
Ionic liquids are being investigated as potential solvents for pre- and post-combustion CO2 capture. In pre-combustion applications, knowledge of the selectivity of CO2 over H2 in high-pressure and high-temperature gas mixtures is crucial. In the present work, the solubility of carbon dioxide, hydrogen, and their mixture in the ionic liquids 1-alky...
We report experiments and simulations to understand the factors that control chromium (Cr3+) electrodeposition from ionic liquid solutions. Speciation, conductivities and diffusivities in mixtures of trivalent chromium chloride, water and choline chloride (CrCl3/ x H2O / y ChCl) were computed from molecular dynamics simulations and compared to meas...
Understanding fundamental solvation phenomena and mixture thermodynamic properties for organic molecules in ionic liquids is essential to the development of ionic liquids in many application areas. In the present work, molecular simulations were used to compute a wide range of properties for the pure ionic liquid trimethylbutylammonium bis(trifluor...
Self-diffusivities as a function of temperature were computed for 29 different ionic liquids (ILs) covering a wide variety of cation and anion classes. Ideal ionic conductivities (σNE) were estimated from the self-diffusivities via the Nernst-Einstein relation. The ion pair (IP) lifetimes (τIP) and ion cage (IC) lifetimes (τIC) of each IL were also...
Atomistic molecular dynamics simulations of small clusters and nanodroplets of the ionic liquid 1-ethyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide [EMIM-Tf2N] subject to an external electric field were performed. A 125-ion-pair droplet was found to be nearly spherical with an isotropic distribution of cations and anions under vacuum co...
High field diffusion and exchange NMR were used in combination with kinetic Monte Carlo simulations to study diffusion in a mixture of an amine-functionalized ionic liquid and carbon dioxide. It was found that carbon dioxide exhibits a time-dependent effective diffusivity on a millisecond time scale due to a chemical exchange between the reacted an...
Most gas separation processes for which ionic liquids have been evaluated have focused on traditional contacting devices in which the solubility of the gases in the bulk phase of the ionic liquid is of most relevance. Recently, hybrid devices based on “Supported Ionic Liquid Phase” or SILP concepts have gained popularity due to improved mass transf...