Predicting Gaseous Solute Diffusion in Viscous Multivalent Ionic Liquid Solvents.

Calculating solute diffusion in dense, viscous solvents can be particularly challenging in molecular dynamics simulations due to the long time scales involved. Here, a new scaling approach is developed for predicting solute diffusion based on analyses of CO2 and SO2 diffusion in two different multivalent ionic liquid solvents. Various scaling approaches are initially evaluated, including single and separate thermostats for the solute and solvent, as well as the application of the Arrhenius relationship and the Speedy-Angell power law. A very strong logarithmic correlation is established between the solvent-accessible surface area and solute diffusion. This relationship, reflecting Danckwerts' surface renewal theory and the Vrentas-Duda free volume model, presents a valuable method for estimating diffusion behavior from short simulation trajectories at elevated temperatures. The approach may be beneficial for enhancing predictive modeling in similar challenging systems and should be more broadly evaluated.

Correction: Understanding liquid-liquid equilibria in binary mixtures of hydrocarbons with a thermally robust perarylphosphonium-based ionic liquid.

[This corrects the article DOI: 10.1039/D1RA06268A.].

Molecular simulation of glycerol-derived triether podands for lithium ion solvation.

Solvate ionic liquids (ILs) are promising candidates for several applications due to their stability, high coulombic efficiency, and low volatility. In this work, we investigate the solvation of lithium-bistriflimide by different glycerol-derived triether solvents, using molecular dynamics simulations. Very strong interactions between Li+ and the solvent oxygen sites are found, leading to significant conformational changes in the solvent. By comparing the conformation of the neat solvents with their IL mixtures at different concentrations and temperatures, we find that the presence of Li+ induces a distinct crown-like structure in the solvent molecules. The Li+ cations and the surrounding solvent form a podand complex, which is stable even at elevated temperatures. These glycerol-derived solvents exhibit distinct interactions with Li+ cations which may be exploited in electrolytic applications or lithium recovery processes.

Understanding Gas Solubility of Pure Component and Binary Mixtures within Multivalent Ionic Liquids from Molecular Simulations.

Understanding the molecular-level solubility of CO2 and its mixtures is essential to the progress of gas-treating technologies. Herein, we use grand canonical Monte Carlo simulations to study the single-component gas absorption of SO2, N2, CH4, and H2 and binary mixtures of CO2/SO2, CO2/N2, CO2/CH4, and CO2/H2 of varying mole fractions within multivalent ionic liquids (ILs). Our results highlight the importance of the free volume effect and the anion effect when interpreting the absorption behavior of these mixtures, similar to the behavior of CO2 found in our previous study (Phys. Chem. Chem. Phys. 2020, 22, 20618-20633). The deviation of gas solubility between the pure component absorption versus the binary absorption, as well as the solubility selectivity, highlights the importance of the relative affinity of gas species within a mixture to the different anions. The absorption selectivity within a specific IL system can be predicted based on the relative gas affinity to the anion.

Solubility Behavior of CO2 in Ionic Liquids Based on Ionic Polarity Index Analyses.

Ionic liquids (ILs) can serve as effective CO2 solvents with an appropriate selection of different anions and cations. However, due to the large library of potential IL compositions, rapid screening methods are needed for characterizing and ranking the expected properties. We have recently proposed the ionic polarity index (IPI) parameter, effectively connecting volume-based approaches and electrostatic potential analyses and providing a single metric that can potentially be used to rapidly screen for desirable IL properties. In this work, the corresponding anion and cation IPIs are used to generate correlations with respect to the CO2 volumetric solubility in ILs. The relationships are generally applicable to groups of ILs within a homologous ion series, and this can be particularly valuable for prescreening different ion pairings for maximizing gas solvation performance.

Screening Ionic Liquids Based on Ionic Volume and Electrostatic Potential Analyses.

Ionic liquids (ILs) are known to have tunable solvation properties, based on the pairing of different anions and cations, but the compositional landscape is vast and challenging to navigate efficiently. Some computational screening protocols are available, but they can be either time-consuming or difficult to implement. Herein, we perform a detailed investigation of the fundamental role of electrostatic interactions in these systems. We effectively develop a bridge between the previous volume-based approach with a quantum structure-property relationship approach to create fast, simple screening guidelines. We propose a new parameter that is applicable to both monovalent and multivalent ions, the ionic polarity index (IPI), which is defined as the ratio of the average electrostatic surface potential (V̅) of the ion to the net charge of the ion (q). The IPI correlation has been tested on a diverse data set of 121 ions, and reliable predictions can be obtained within a homologous series of IL compounds.

Understanding liquid-liquid equilibria in binary mixtures of hydrocarbons with a thermally robust perarylphosphonium-based ionic liquid.

Binary mixtures of hydrocarbons and a thermally robust ionic liquid (IL) incorporating a perarylphosphonium-based cation are investigated experimentally and computationally. Experimentally, it is seen that excess toluene added to the IL forms two distinct liquid phases, an "ion-rich" phase of fixed composition and a phase that is nearly pure toluene. Conversely, n-heptane is observed to be essentially immiscible in the neat IL. Molecular dynamics simulations capture both of these behaviours. Furthermore, the simulated composition of the toluene-rich IL phase is within 10% of the experimentally determined composition. Additional simulations are performed on the binary mixtures of the IL and ten other small hydrocarbons having mixed aromatic/aliphatic character. It is found that hydrocarbons with a predominant aliphatic character are largely immiscible with the IL, while those with a predominant aromatic character readily mix with the IL. A detailed analysis of the structure and energetic changes that occur on mixing reveals the nature of the ion-rich phase. The simulations show a bicontinuous phase with hydrocarbon uptake akin to absorption and swelling by a porous absorbent. Aromatic hydrocarbons are driven into the neat IL via dispersion forces with the IL cations and, to a lesser extent, the IL anions. The ion-ion network expands to accommodate the hydrocarbons, yet maintains a core connective structure. At a certain loading, this network becomes stretched to its limit. The energetic penalty associated with breaking the core connective network outweighs the gain from new hydrocarbon-IL interactions, leaving additional hydrocarbons in the neat phase. The spatially alternating charge of the expanded IL network is shown to interact favourably with the stacked aromatic subphase, something not possible for aliphatic hydrocarbons.

Molecular insight into the anion effect and free volume effect of CO2 solubility in multivalent ionic liquids.

For many years, experimental and theoretical studies have investigated the solubility of CO2 in a variety of ionic liquids (ILs), but the overarching absorption mechanism is still unclear. Currently, two different factors are believed to dominate the absorption performance: (a) the fractional free volume (FFV) accessible for absorption; and (b) the nature of the CO2 interactions with the anion species. The FFV is often more influential than the specific choice of the anion, but neither mechanism provides a complete picture. Herein, we have attempted to decouple these mechanisms in order to provide a more definitive molecular-level perspective of CO2 absorption in IL solvents. We simulate a series of nine different multivalent ILs comprised of imidazolium cations and sulfonate/sulfonimide anions tethered to benzene rings, along with a comprehensive analysis of the CO2 absorption and underlying molecular-level features. We find that the CO2 solubility has a very strong, linear correlation with respect to FFV, but only when comparisons are constrained to a common anion species. The choice of anion results in a fundamental remapping of the correlation between CO2 solubility and FFV. Overall, the free volume effect dominates in the ILs with smaller FFV values, while the choice of anion becomes more important in the systems with larger FFVs. Our proposed mechanistic map is intended to provide a more consistent framework for guiding further IL design for gas absorption applications.

Molecular Transport Behavior of CO2 in Ionic Polyimides and Ionic Liquid Composite Membrane Materials.

Ionic polyimides (i-PI) are a new class of polymer materials that are very promising for CO2 capture membranes, and recent experimental studies have demonstrated their enhanced separation performance with the addition of imidazolium-based ionic liquids (ILs). However, there is very little known about the molecular-level interactions in these systems, which give rise to interesting gas adsorption and diffusion characteristics. In this study, we use a combination of Monte Carlo and molecular dynamics simulations to analyze the equilibrium and transport properties of CO2 molecules in the i-PI and i-PI + IL composite materials. The addition of several different common ILs are modeled, which have a plasticization effect on the i-PI, lowering the glass transition temperature (Tg). The solubility of CO2 strongly correlates with the Tg, but the diffusion demonstrates more unpredictable behavior. At low concentrations, the IL has a blocking effect, leading to reduced diffusion rates. However, as the IL surpasses a threshold value, the relationship is inverted and the IL has a facilitating effect on the gas transport. This behavior is attributed to the simultaneous contributions of the increased i-PI plasticization at higher IL concentrations (facilitating gas hopping rates from cavity-to-cavity) and the increased IL continuity throughout the system, enabling more favorable transport pathways for CO2 diffusion.

The Nitric Oxide Dimer Reaction in Carbon Nanopores.

When confined within nanoporous carbons (activated carbon fibers or carbon nanotubes) having pore widths of about 1 nm, nitric oxide is found to react completely to form the dimer, (NO)2, even though almost no dimers are present in the bulk gas phase in equilibrium with the pore phase. Moreover, the yield of dimer is unchanged upon varying the temperature over the range studied in the experiments. Earlier molecular simulation studies showed a significant increase in dimer formation in carbon nanopores, but the dimer yield was considerably less than that found in the experiments, and decreased rapidly as the temperature was raised. Here, we report an ab initio and molecular simulation study of this reaction in both slit-shaped pores and single-walled carbon nanotubes. The ab initio calculations show that the nitric oxide dimer forms a weak chemical bond with the carbon, and the bonding energy is more than 20 times stronger than the van der Waals energy assumed in the previous studies. When this is accounted for, the predicted dimer yield is in good agreement with the experimental values, as is its temperature dependence. We also report results for the pressure tensor components for this confined reactive mixture. Local tangential pressures near the pore walls are as high as millions of bar, reflecting the strong nanoscale forces.

Molecular Simulation of Ionic Polyimides and Composites with Ionic Liquids as Gas-Separation Membranes.

Polyimides are at the forefront of advanced membrane materials for CO2 capture and gas-purification processes. Recently, ionic polyimides (i-PIs) have been reported as a new class of condensation polymers that combine structural components of both ionic liquids (ILs) and polyimides through covalent linkages. In this study, we report CO2 and CH4 adsorption and structural analyses of an i-PI and an i-PI + IL composite containing [C4mim][Tf2N]. The combination of molecular dynamics (MD) and grand canonical Monte Carlo (GCMC) simulations is used to compute the gas solubility and the adsorption performance with respect to the density, fractional free volume (FFV), and surface area of the materials. Our results highlight the polymer relaxation process and its correlation to the gas solubility. In particular, the surface area can provide meaningful guidance with respect to the gas solubility, and it tends to be a more sensitive indicator of the adsorption behavior versus only considering the system density and FFV. For instance, as the polymer continues to relax, the density, FFV, and pore-size distribution remain constant while the surface area can continue to increase, enabling more adsorption. Structural analyses are also conducted to identify the nature of the gas adsorption once the ionic liquid is added to the polymer. The presence of the IL significantly displaces the CO2 molecules from the ligand nitrogen sites in the neat i-PI to the imidazolium rings in the i-PI + IL composite. However, the CH4 molecules move from the imidazolium ring sites in the neat i-PI to the ligand nitrogen atoms in the i-PI + IL composite. These molecular details can provide critical information for the experimental design of highly selective i-PI materials as well as provide additional guidance for the interpretation of the simulated adsorption systems.

Confinement Effects on Carbon Dioxide Methanation: A Novel Mechanism for Abiotic Methane Formation.

An important scientific debate focuses on the possibility of abiotic synthesis of hydrocarbons during oceanic crust-seawater interactions. While on-site measurements near hydrothermal vents support this possibility, laboratory studies have provided data that are in some cases contradictory. At conditions relevant for sub-surface environments it has been shown that classic thermodynamics favour the production of CO2 from CH4, while abiotic methane synthesis would require the opposite. However, confinement effects are known to alter reaction equilibria. This report shows that indeed thermodynamic equilibrium can be shifted towards methane production, suggesting that thermal hydrocarbon synthesis near hydrothermal vents and deeper in the magma-hydrothermal system is possible. We report reactive ensemble Monte Carlo simulations for the CO2 methanation reaction. We compare the predicted equilibrium composition in the bulk gaseous phase to that expected in the presence of confinement. In the bulk phase we obtain excellent agreement with classic thermodynamic expectations. When the reactants can exchange between bulk and a confined phase our results show strong dependency of the reaction equilibrium conversions, [Formula: see text], on nanopore size, nanopore chemistry, and nanopore morphology. Some physical conditions that could shift significantly the equilibrium composition of the reactive system with respect to bulk observations are discussed.

Molecular Dynamics Simulation of Bismuth Telluride Exfoliation Mechanisms in Different Ionic Liquid Solvents.

Bismuth telluride (Bi2Te3) is a well-known thermoelectric material with potential applications in several different emerging technologies. The bulk structure is composed of stacks of quintuple sheets (with weak interactions between neighboring sheets), and the performance of the material can be significantly enhanced if exfoliated into two-dimensional nanosheets. In this study, eight different imidazolium-based ionic liquids are evaluated as solvents for the exfoliation and dispersion of Bi2Te3 at temperatures ranging from 350 to 550 K. Three distinct exfoliation mechanisms are evaluated (pulling, shearing, and peeling) using steered molecular dynamics simulations, and we predict that the peeling mechanism is thermodynamically the most favorable route. Furthermore, the [Tf2N-]-based ionic liquids are particularly effective at enhancing the exfoliation, and this performance can be correlated to the unique molecular-level solvation structures developed at the Bi2Te3 surfaces. This information helps provide insight into the molecular origins of exfoliation and solvation involving Bi2Te3 (and possibly other layered chalcogenide materials) and ionic liquid solvents.

Modeling the atomistic growth behavior of gold nanoparticles in solution.

The properties of gold nanoparticles strongly depend on their three-dimensional atomic structure, leading to an increased emphasis on controlling and predicting nanoparticle structural evolution during the synthesis process. In order to provide this atomistic-level insight and establish a link to the experimentally-observed growth behavior, a kinetic Monte Carlo simulation (KMC) approach is developed for capturing Au nanoparticle growth characteristics. The advantage of this approach is that, compared to traditional molecular dynamics simulations, the atomistic nanoparticle structural evolution can be tracked on time scales that approach the actual experiments. This has enabled several different comparisons against experimental benchmarks, and it has helped transition the KMC simulations from a hypothetical toy model into a more experimentally-relevant test-bed. The model is initially parameterized by performing a series of automated comparisons of Au nanoparticle growth curves versus the experimental observations, and then the refined model allows for detailed structural analysis of the nanoparticle growth behavior. Although the Au nanoparticles are roughly spherical, the maximum/minimum dimensions deviate from the average by approximately 12.5%, which is consistent with the corresponding experiments. Also, a surface texture analysis highlights the changes in the surface structure as a function of time. While the nanoparticles show similar surface structures throughout the growth process, there can be some significant differences during the initial growth at different synthesis conditions.

Mechanism of bismuth telluride exfoliation in an ionic liquid solvent.

Bismuth telluride (Bi2Te3) is a well-known thermoelectric material that has a layered crystal structure. Exfoliating Bi2Te3 to produce two-dimensional (2D) nanosheets is extremely important because the exfoliated nanosheets possess unique properties, which can potentially revolutionize several material technologies such as thermoelectrics, heterogeneous catalysts, and infrared detectors. In this work, ionic liquid (IL) 1-butyl-3-methylimidazolium chloride ([C4mim]Cl) is used to exfoliate Bi2Te3 nanoplatelets. In both experiments and in molecular dynamics (MD) simulations, the Bi2Te3 nanoplatelets yield a stable dispersion of 2D nanosheets in the IL solvent, and our MD simulations provide molecular-level insight into the kinetics and thermodynamics of the exfoliation process. An analysis of the dynamics of Bi2Te3 during exfoliation indicates that the relative translation (sliding apart) of adjacent layers caused by IL-induced forces plays an important role in the process. Moreover, an evaluation of the MD trajectories and electrostatic interactions indicates that the [C4mim](+) cation is primarily responsible for initiating Bi2Te3 layer sliding and separation, while the Cl(-) anion is less active. Overall, our combined experimental and computational investigation highlights the effectiveness of IL-assisted exfoliation, and the underlying molecular-level insights should accelerate the development of future exfoliation techniques for producing 2D chalcogenide materials.

Adsorption properties of nitrogen dioxide on hybrid carbon and boron-nitride nanotubes.

The properties of pristine carbon nanotubes (CNTs) can be modified in a number of different ways: covalent attachments, substitutional doping, induced defects, and non-covalent interactions with ligands. One unconventional approach is to combine CNTs with boron-nitride nanotubes (BNNTs) to form hybrid carbon and boron-nitride nanotube (CBNNT) materials. In this work, we perform a first-principles density functional theory study on the adsorption properties of NO2 on CBNNT heterostructures. It is found that the adsorption of NO2 is significantly increased on both zigzag CBNNT(8,0) and armchair CBNNT(6,6), as compared to either a pristine CNT or BNNT. For example, the chemisorption of NO2 on CNT(8,0) is found to be endothermic, while the chemisorption of NO2 on CBNNT(8,0) is an exothermic process with a very large binding energy of -27.74 kcal mol(-1). Furthermore, the binding of NO2 on both CBNNT(8,0) and CBNNT(6,6) induces an increase in the conductivity of the nanotube. These characteristics indicate that the CBNNT heterostructures may have significant potential as an NO2 sensor or as a catalyst for NO2 decomposition reactions. Our calculations provide critical information for further evaluation, such as molecular-level adsorption simulations and microkinetic studies.

Tuning the adsorption interactions of imidazole derivatives with specific metal cations.

In this work, we report a computational study of the interactions between metal cations and imidazole derivatives in the gas phase. We first performed a systematic assessment of various density functionals and basis sets for predicting the binding energies between metal cations and the imidazoles. We find that the M11L functional in combination with the 6-311++G(d,p) basis set provides the best compromise between accuracy and computational cost with our metal···imidazole complexes. We then evaluated the binding of a series of metal cations, including Li(+), Na(+), K(+), Co(2+), Ni(2+), Cu(2+), Zn(2+), Cd(2+), Ba(2+), Hg(2+), and Pb(2+), with several substituted imidazole derivatives. We find that electron-donating groups increase the metal-binding energy, whereas electron-withdrawing groups decrease the metal-binding energy. Furthermore, the binding energy trends can be rationalized by the hardness of the metal cations and imidazole derivatives, providing a quick way to estimate the metal···imidazole binding strength. This insight can enable efficient screening protocols for identifying effective imidazole-based solvents and membranes for metal adsorption and provide a framework for understanding metal···imidazole interactions in biological systems.

Oxygen adsorption characteristics on hybrid carbon and boron-nitride nanotubes.

In this work, first-principles density functional theory (DFT) is used to predict oxygen adsorption on two types of hybrid carbon and boron-nitride nanotubes (CBNNTs), zigzag (8,0), and armchair (6,6). Although the chemisorption of O2 on CBNNT(6,6) is calculated to be a thermodynamically unfavorable process, the binding of O2 on CBNNT(8,0) is found to be an exothermic process and can form both chemisorbed and physisorbed complexes. The CBNNT(8,0) has very different O2 adsorption properties compared with pristine carbon nanotubes (CNTs) and boron-nitride nanotube (BNNTs). For example, O2 chemisorption is significantly enhanced on CBNNTs, and O2 physisorption complexes also show stronger binding, as compared to pristine CNTs or BNNTs. Furthermore, it is found that the O2 adsorption is able to increase the conductivity of CBNNTs. Overall, these properties suggest that the CBNNT hybrid nanotubes may be useful as a gas sensor or as a catalyst for the oxygen reduction reaction.

Electrostatic potential within the free volume space of imidazole-based solvents: insights into gas absorption selectivity.

In this work, a variety of molecular simulation tools are used to help characterize the selective absorption of CO2 and CH4 in imidazole-based solvents. We focus our efforts on a series of 1-n-alkyl-2-methyl-imidazoles and ether-functionalized imidazoles, over a temperature range from 293 to 353 K, and we perform detailed analysis of the free volume. We find that the electrostatic potential within the solvent free volume cavities provides a useful indication of the selective absorption of CO2 and CH4. The electrostatic potential calculation is significantly faster than the direct calculation of the chemical potential, and tests with the 1-n-alkyl-2-methyl-imidazoles and the ether-functionalized imidazoles indicate that this may be a useful screening tool for other solvents.

Molecular simulation of the thermophysical properties of N-functionalized alkylimidazoles.

Molecular simulations are used to probe the thermophysical properties of a series of N-functionalized alkylimidazoles, ranging from N-methylimidazole to N-heptylimidazole. These compounds have been previously synthesized, and their solvation properties have been shown to be potentially useful for CO(2) capture from industrial sources. In this work, we use first-principles calculations to fit electrostatic charges to the molecular models, which are then used to perform a series of molecular dynamics simulations. Over a range of different temperatures, we benchmark the simulated densities and heat capacities against experimental measurements. Also, we predict the Henry's constants for CO(2) absorption and probe the solvents' structures using molecular simulation techniques, such as fractional free volume analysis and void distributions. We find that our simulations are able to closely reproduce the experimental benchmarks and add additional insight into the molecular structure of these fluids, with respect to their observed solvent properties.