Insights on the solubility of CO2 in 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide from the microscopic point of view

Size Matters: A Computational Study of Hydrogen Absorption in Ionic Liquids

We combined both density functional theory and classical molecular dynamics simulations to investigate the molecular mechanisms governing hydrogen solvation in a total of 12 ionic liquids. Overall, the analysis of the structural properties under high temperature and pressure conditions revealed weak interactions between hydrogen and the ionic liquids, with a slight preference of this gas to be placed at the apolar domains. Interestingly, those ionic liquids comprising nitrate anions allow the accommodation of hydrogen molecules also in the polar areas. The study of the hydrogen velocity autocorrelation functions supports this observation. In addition, the structure of all of the tested ionic liquids was almost insensitive to the addition of hydrogen, so the available free volume and cavity formation are presumably the most important factors affecting solubility.

Energy Applications of Ionic Liquids: Recent Developments and Future Prospects

Ionic liquids (ILs) consisting entirely of ions exhibit many fascinating and tunable properties, making them promising functional materials for a large number of energy-related applications. For example, ILs have been employed as electrolytes for electrochemical energy storage and conversion, as heat transfer fluids and phase-change materials for thermal energy transfer and storage, as solvents and/or catalysts for CO2 capture, CO2 conversion, biomass treatment and biofuel extraction, and as high-energy propellants for aerospace applications. This paper provides an extensive overview on the various energy applications of ILs and offers some thinking and viewpoints on the current challenges and emerging opportunities in each area. The basic fundamentals (structures and properties) of ILs are first introduced. Then, motivations and successful applications of ILs in the energy field are concisely outlined. Later, a detailed review of recent representative works in each area is provided. For each application, the role of ILs and their associated benefits are elaborated. Research trends and insights into the selection of ILs to achieve improved performance are analyzed as well. Challenges and future opportunities are pointed out before the paper is concluded.

Morphology and Dynamics in Hydrated Graphene Oxide/Branched Poly(ethyleneimine) Nanocomposites: An In Silico Investigation

Graphene oxide (GO)-branched poly(ethyleneimine) (BPEI) hydrated mixtures were studied by means of fully atomistic molecular dynamics simulations to assess the effects of the size of polymers and the composition on the morphology of the complexes, the energetics of the systems and the dynamics of water and ions within composites. The presence of cationic polymers of both generations hindered the formation of stacked GO conformations, leading to a disordered porous structure. The smaller polymer was found to be more efficient at separating the GO flakes due to its more efficient packing. The variation in the relative content of the polymeric and the GO moieties provided indications for the existence of an optimal composition in which interaction between the two components was more favorable, implying more stable structures. The large number of hydrogen-bonding donors afforded by the branched molecules resulted in a preferential association with water and hindered its access to the surface of the GO flakes, particularly in polymer-rich systems. The mapping of water translational dynamics revealed the existence of populations with distinctly different mobilities, depending upon the state of their association. The average rate of water transport was found to depend sensitively on the mobility of the freely to move molecules, which was varied strongly with composition. The rate of ionic transport was found to be very limited below a threshold in terms of polymer content. Both, water diffusivity and ionic transport were enhanced in the systems with the larger branched polymers, particularly with a lower polymer content, due to the higher availability of free volume for the respective moieties. The detail afforded in the present work provides a new insight for the fabrication of BPEI/GO composites with a controlled microstructure, enhanced stability and adjustable water transport and ionic mobility.

CO2 capture using dicationic ionic liquids (DILs): Molecular dynamics and DFT-IR studies on the role of cations

Dicationic ionic liquids (DILs) have been shown to be useful as an effective solvent for the absorption of CO₂. However, compared to monocationic ionic liquids (MILs), they have been less investigated for this application. Previous studies on MIL-CO₂ systems have shown that anions play the main role in tuning CO₂ capture, but the partial negative charge on the oxygens of CO₂ may interact with cation centers and, especially, for DILs with two charge centers, the role of cations can be significant. Therefore, the current work focuses on how cation symmetry and the length of side chains affect interactions and also the dynamical and structural properties of DIL-CO₂ systems using molecular dynamics simulation. In addition, the effect of CO₂ on the infrared vibrational spectra of isolated ions and ion triplet (DIL molecules) was studied using density functional theory calculations and the observed red and blue shifts have been interpreted. The results indicated that symmetric cation with longer side chains tend to interact more strongly with CO₂ molecules. It seems that increasing the length of the side chains causes more bending of the middle chain, and in addition to increasing the free fraction volume, it weakens the interaction between cations and anions, and as a result more interaction between gas and cation. The results of this work may contribute to the rational molecular design of DILs for CO₂ capture, DIL-based gas sensors, etc.

Highly water-stable bimetallic organic framework MgCu-MOF74 for inhibiting bacterial infection and promoting bone regeneration

As a typical metal-organic framework (MOF), Mg-MOF74 can release biocompatible Mg²⁺when the framework is degraded, and it has the potential to be used as filler in the field of bone tissue engineering. However, Mg-MOF74 has poor stability in aqueous environment and limited antibacterial ability, which limit its further development and applications. In this work, MgCu-MOF74 particles with different Cu content were synthesized through a facile one-step hydrothermal method. The physicochemical properties and water stability of the synthesized powders were characterized. The osteogenic potential of the MgCu-MOF74 particles on human osteogenic sarcoma cells (SaOS-2) was evaluated. The hybrid MgCu-MOF74 exhibited favorable water stability. These results indicated that MgCu-MOF74 enhanced cellular viability, alkaline phosphatase levels, collagen (COL) synthesis and osteogenesis-related gene expression. Moreover, the samples doped with Cu²⁺were more sensitive to the acidic microenvironment produced by bacteria, and exhibited stronger antibacterial ability than Mg-MOF74. In conclusion, MgCu-MOF-74 with good water stability, osteogenic ability and antibacterial ability, which could be attributed to the doping of Cu²⁺. Hence, MgCu-MOF74 shows great potential as a novel medical bio-functional fillers for the treatment of bone defects.

The structure of CO2 and CH4 at the interface of a poly(urethane urea) oligomer model from the microscopic point of view

The world desperately needs new technologies and solutions for gas capture and separation. To make this possible, molecular modeling is applied here to investigate the structural, thermodynamic, and dynamical properties of a model for the poly(urethane urea) (PUU) oligomer model to selectively capture CO₂ in the presence of CH₄. In this work, we applied a well-known approach to derive atomic partial charges for atoms in a polymer chain based on self-consistent sampling using quantum chemistry and stochastic dynamics. The interactions of the gases with the PUU model were studied in a pure gas based system as well as in a gas mixture. A detailed structure characterization revealed high interaction of CO₂ molecules with the hard segments of the PUU. Therefore, the structural and energy properties explain the reasons for the greater CO₂ sorption than CH₄. We find that the CO₂ sorption is higher than the CH₄ with a selectivity of 7.5 at 298 K for the gas mixture. We characterized the Gibbs dividing surface for each system, and the CO₂ is confined for a long time at the gas-oligomer model interface. The simulated oligomer model showed performance above the 2008 Robeson's upper bound and may be a potential material for CO₂/CH₄ separation. Further computational and experimental studies are needed to evaluate the material.

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.

Free energy profile analysis to identify factors activating the aggregation-induced emission of a cyanostilbene derivative

An approach to quantitatively analyze the factors contributing to the activation of aggregation-induced emission (AIE) of a molecule is proposed using molecular simulations. A cyanostilbene derivative, 1-cyano-1,2-bis-(4'-methylbiphenyl)ethylene (CN-MBE), has two isomers, E and Z forms. The E-form of CN-MBE exhibits AIE, and is non-emissive in dilute solutions but becomes highly emissive in aggregated states. The Z-form is non-emissive, even in the solid state, that is, the E-form of CN-MBE is AIE-active, while its Z-form is AIE-inactive. In this study, the free energy profiles of the AIE processes of the E and Z forms of CN-MBE are investigated using the free energy perturbation method at the quantum mechanics/molecular mechanics level. The free energy profiles reveal significant differences in the extent to which steric hindrance from surrounding molecules restricts the intramolecular motions of the E and Z forms in the aggregated states. The structural features of the E and Z forms are characterized based on the conformational changes in the excited state relaxation process to reach the conical intersections and the free volume space around the molecules in the aggregated states. This study determines the contributing factors that cause the AIE activity of the molecule by identifying characteristic differences in the free energy profiles of the AIE processes of the AIE-active E-form of CN-MBE and the inactive Z-form. The approach used in this study can be applied to the rational design of highly efficient AIE luminogens utilizing computer modeling.

Understanding the interactions between the bis(trifluoromethylsulfonyl)imide anion and absorbed CO2 using X-ray diffraction analysis of a soft crystal surrogate

The selective carbon dioxide (CO2) absorption properties of ionic liquids (ILs) are highly pertinent to the development of methods to capture CO2. Although it has been reported that fluorinated components give ILs enhanced CO2 solubilities, it has been challenging to gain a deep understanding of the interactions occurring between ILs and CO2. In this investigation, we have utilized the soft crystalline material [Cu(NTf2)2(bpp)2] (NTf2‒ = bis(trifluoromethylsulfonyl)imide, bpp = 1,3-bis-(4-pyridyl)propane) as a surrogate for single-crystal X-ray diffraction analysis to visualize interactions occurring between CO2 and NTf2‒, the fluorinated IL component that is responsible for high CO2 solubility. Analysis of the structure of a CO2-loaded crystal reveals that CO2 interacts with both fluorine and oxygen atoms of NTf2‒ anions in a trans rather than cis conformation about the S-N bond. Theoretical analysis of the structure of the CO2-loaded crystal indicates that dispersion and electrostatic interactions exist between CO2 and the framework. The overall results provide important insight into understanding and improving the CO2 absorption properties of ILs.

A potential for molecular simulation of compounds with linear moieties

The harmonic angle bending potential is used in many force fields for (bio)molecular simulation. The force associated with this potential is discontinuous at angles close to 180°, which can lead to numeric instabilities. Angle bending of linear groups, such as alkynes or nitriles, or linear molecules, such as carbon dioxide, can be treated by a simple harmonic potential if we describe the fluctuations as a deviation from a reference position of the central atom, the position of which is determined by the flanking atoms. The force constant for the linear angle potential can be derived analytically from the corresponding force constant in the traditional potential. The new potential is tested on the properties of alkynes, nitriles, and carbon dioxide. We find that the angles of the linear groups remain about 2° closer to 180° using the new potential. The bond and angle force constants for carbon dioxide were tuned to reproduce the experimentally determined frequencies. An interesting finding was that simulations of liquid carbon dioxide under pressure with the new flexible model were stable only when explicitly modeling the long-range Lennard-Jones (LJ) interactions due to the very long-range nature of the LJ interactions (>1.7 nm). In the other tested liquids, we find that a Lennard-Jones cutoff of 1.1 nm yields similar results as the particle mesh Ewald algorithm for LJ interactions. Algorithmic factors influencing the stability of liquid simulations are discussed as well. Finally, we demonstrate that the linear angle potential can be used in free energy perturbation calculations.

Role of Ionic Liquid [EMIM]+[SCN]- in the Adsorption and Diffusion of Gases in Metal-Organic Frameworks

We study the adsorption performance of metal-organic frameworks (MOFs) impregnated of ionic liquids (ILs). To this aim we calculated adsorption and diffusion of light gases (CO₂, CH₄, N₂) and their mixtures in hybrid composites using molecular simulations. The hybrid composites consist of 1-ethyl-3-methylimidazolium thiocyanate impregnated in IRMOF-1, HMOF-1, MIL-47, and MOF-1. We found that the increase of the amount of IL enhances the adsorption selectivity in favor of carbon dioxide for the mixtures CO₂/CH₄ and CO₂/N₂ and in favor of methane in the mixture CH₄/N₂. We also provide detailed analysis of the microscopic organization of ILs and adsorbates via radial distribution functions and average occupation profiles and study the impact of the ILs in the diffusion of the adsorbates inside the pores of the MOFs. Based on our findings, we discuss the advantages of using IL/MOF composites for gas adsorption to increase the adsorption of gases and to control the pore sizes of the structures to foster selective adsorption.

Systems Analysis of Physical Absorption of CO2 in Ionic Liquids for Pre-Combustion Carbon Capture

This study develops an integrated technical and economic modeling framework to investigate the feasibility of ionic liquids (ILs) for precombustion carbon capture. The IL 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide is modeled as a potential physical solvent for CO₂ capture at integrated gasification combined cycle (IGCC) power plants. The analysis reveals that the energy penalty of the IL-based capture system comes mainly from the process and product streams compression and solvent pumping, while the major capital cost components are the compressors and absorbers. On the basis of the plant-level analysis, the cost of CO₂ avoided by the IL-based capture and storage system is estimated to be $63 per tonne of CO₂. Technical and economic comparisons between IL- and Selexol-based capture systems at the plant level show that an IL-based system could be a feasible option for CO₂ capture. Improving the CO₂ solubility of ILs can simplify the capture process configuration and lower the process energy and cost penalties to further enhance the viability of this technology.

Molecular Dynamics Simulations for Loading-Dependent Diffusion of CO2, SO2, CH4, and Their Binary Mixtures in ZIF-10: The Role of Hydrogen Bond

The loading-dependent diffusion behavior of CH₄, CO₂, SO₂, and their binary mixtures in ZIF-10 has been investigated in detail by using classical molecular dynamics simulations. Our simulation results demonstrate that the self-diffusion coefficient D_i of CH₄ molecules decreases sharply and monotonically with the loading while those of both CO₂ and SO₂ molecules initially display a slight increase at low uptakes and follow a slow decrease at high uptakes. Accordingly, the interaction energies between CH₄ molecules and ZIF-10 remain nearly constant regardless of the loading due to the absence of hydrogen bonds (HBs), while the interaction energies between CO₂ (or SO₂) and ZIF-10 decease rapidly with the loading, especially at small amounts of gas molecules. Such different loading-dependent diffusion and interaction mechanisms can be attributed to the relevant HB behavior between gas molecules and ZIF-10. At low loadings, both the number and strength of HBs between CO₂ (or SO₂) molecules and ZIF-10 decrease obviously as the loading increases, which is responsible for the slight increase of their diffusion coefficients. However, at high loadings, their HB strength increases with the loading. Similar loading-dependent phenomena of diffusion, interaction, and HB behavior can be observed for CH_4, CO₂, and SO₂ binary mixtures in ZIF-10, only associated with some HB competition between CO₂ and SO₂ molecules in the case of the CO₂/SO₂ mixture.

Natural polyprenylated benzophenone: keto-enol tautomerism from density functional calculations and the AIM theory

The quantum theory of atoms in molecules (QTAIM) and density functional theory (DFT) calculations were employed to investigate the structure and tautomeric equilibrium of epiclusianone, a polyisoprenylated benzophenone with interesting biological activities. Two different exchange-correlation functionals were employed, namely ωB97x-D and M06-2x, including implicit solvent models (benzene and DMSO). Our results for the thermodynamic properties show that the isomer in which the H atom is bonded to the oxygen away from the benzene ring is the most stable tautomer form of the epiclusianone, thus confirming previous charge density analysis from X-ray diffraction data (Martins et al. J Braz Chem Soc 18(8):1515-1523, 22).

Local environment structure and dynamics of CO2 in the 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide and related ionic liquids

Despite the innumerous papers regarding the study of the ionic liquids as a potential candidate for CO₂ capture, many details concerning the structure and dynamics of CO₂ in the system are still to be revealed, i.e., the correlation between the local environment structure and the dynamic properties of the substance. This present work relied on the performance of molecular dynamics both for the neat [C₂mim][Tf₂N] and [C₂mim][Tf₂N]/CO₂ mixtures in an attempt to elucidate the local environment of CO₂ and their effects on the dynamic properties of [C₂mim][Tf₂N]. A slight change in the orientation of the cation and anion could be observed, which was correlated to the cation and anion moving away from each other in order to receive the carbon dioxide. The gas molecules pushed both the cation and the anion away to create sufficient void to its accommodation. The diffusion coefficient of [C₂mim]⁺ is higher than [Tf₂N]^- regardless the increase of the CO₂ concentration. The addition of CO₂ in the ionic liquid has shown an increase of 4-5 times for the diffusivity of ions, which was related to the decrease of cation-anion interaction strength. The transport properties' results showed that the addition of CO₂ in the ionic liquid generates the fluidization of the system, decreasing the viscosity as a consequence of the local environment structure changing. Likewise, the effect of the type of anion and cation on the system properties was studied considering [Ac]^- and [BMpyr]⁺ ions, showing large effects by the change of anion to [Ac]^- which rise from the strong [C₂mim]⁺-[Ac]^- interaction, which conditions the solvation of ions by CO₂ molecules.

Molecular dynamics and a spectroscopic study of sulfur dioxide absorption by an ionic liquid and its mixtures with PEO

An investigation comprising experimental techniques (absorption capacity of SO₂ and vibrational spectroscopy) and molecular simulations (thermodynamics, structure, and dynamics) has been performed for the polymer poly(ethylene oxide) (PEO), the ionic liquid butyltrimethylammonium bis(trifluoromethylsulfonyl)imide ([N₄₁₁₁][Tf₂N]) and their mixtures as sulfur dioxide (SO₂) absorbing materials. The polymer PEO has higher capacity to absorb SO₂ than the neat ionic liquid, whereas the mixtures presented intermediary absorption capacities. The band assigned to the symmetric stretching band of SO₂ at ca. 1140 cm^-1, which is considered a spectroscopic probe for the strength of SO₂ interactions with its neighborhood, shifts to lower wavenumbers as more negative total interaction energy values of SO₂ were evaluated from the simulations. The solvation free energy of SO₂, ΔG_sol, correlates linearly with the absorption capacity of SO₂. The negative values of ΔG_sol are due to negative and positive values of enthalpy and entropy, respectively. In the ionic liquid, SO₂ weakens the cation-anion interactions, whereas in the mixture with a high content of PEO these interactions are slightly increased. Such effects were correlated with the relative population of cisoid and transoid conformers of Tf₂N anions as revealed by Raman spectroscopy. Moreover, the presence of SO₂ in the systems provokes the increase of diffusion coefficients of the absorbing species in comparison with the systems without the gas. Proper to the slow dynamics of the polymer, the diffusion coefficient of ions and SO₂ diminishes with the increase of the PEO content.

Water diffusion in amorphous hydrophilic systems: a stop and go process

The diffusion of H2O in three amorphous polymer-H2O systems is studied as a function of H2O content using molecular dynamics. A picture of H2O molecule motion comprising alternating steps of being bound at an adsorption site ("stop") and moving ("go") emerges. This picture is made quantitative. The bound time, frequency of stop-go steps, and tortuosity all decrease with H2O content. Fourier analysis of particle motion during bound time segments provides a measure of an attempt frequency that is connected quantitatively to the bound time and an activation energy of a hydrogen bond. For increasing H2O content, the polymer-H2O systems swell, leading to an increase in the diffusion coefficient and porosity and a decrease in activation energy.

Novel Guanidinium-Based Ionic Liquids for Highly Efficient SO2 Capture

The application of ionic liquids (ILs) for acidic gas absorption has long been an interesting and challenging issue. In this work, the ethyl sulfate ([C2OSO3](-)) anion has been introduced into the structure of guanidinium-based ILs to form two novel low-cost ethyl sulfate ILs, namely 2-ethyl-1,1,3,3-tetramethylguanidinium ethyl sulfate ([C2(2)(C1)2(C1)2(3)gu][C2OSO3]) and 2,2-diethyl-1,1,3,3-tetramethylguanidinium ethyl sulfate ([(C2)2(2)(C1)2(C1)2(3)gu][C2OSO3]). The ethyl sulfate ILs, together with 2-ethyl-1,1,3,3-tetramethylguanidinium bis(trifluoromethylsulfonyl)imide ([C2(2)(C1)2(C1)2(3)gu][NTf2]) and 2,2-diethyl-1,1,3,3-tetramethylguanidinium bis(trifluoromethylsulfonyl)imide ([(C2)2(2)(C1)2(C1)2(3)gu][NTf2]), are employed to evaluate the SO2 absorption and desorption performance. The recyclable ethyl sulfate ILs demonstrate high absorption capacities of SO2. At a low pressure of 0.1 bar and at 20 °C, 0.71 and 1.08 mol SO2 per mole of IL can be captured by [C2(2)(C1)2(C1)2(3)gu][C2OSO3] and [(C2)2(2)(C1)2(C1)2(3)gu][C2OSO3], respectively. The absorption enthalpy for SO2 absorption with [C2(2)(C1)2(C1)2(3)gu][C2OSO3] and [(C2)2(2)(C1)2(C1)2(3)gu][C2OSO3] are -3.98 and -3.43 kcal mol(-1), respectively. While those by [C2(2)(C1)2(C1)2(3)gu][NTf2] and [(C2)2(2)(C1)2(C1)2(3)gu][NTf2] turn out to be only 0.17 and 0.24 mol SO2 per mole of IL under the same conditions. It can be concluded that the guanidinium ethyl sulfate ILs show good performance for SO2 capture. Quantum chemistry calculations reveal nonbonded weak interactions between the ILs and SO2. The anionic moieties of the ILs play an important role in SO2 capture on the basis of the consistently experimental and computational results.

Water Effect on Acid-Gas Capture Using Choline Lactate: A DFT Insight beyond Molecule-Molecule Pair Simulations

The suitability of CO2 and SO2 capture by using choline lactate ionic liquid as a sorbent and the effect of water content for acid-gas absorption were investigated through density functional theory (DFT) simulations in this work. Simulations that contain model systems considering up to four molecules (cholinium, lactate, water, and CO2/SO2) have been analyzed, and compositional effects on small cluster(s) formed by four ionic pairs and variable number of water molecules have been studied in this work. Assessment of the effect of water content on acid-gas capture that uses exotic ionic liquids is a rare study, and our results showed that water presence hinders CO2/SO2 affinity and solubility dramatically, mainly due to the dominated affinity between the ionic pair and water molecule rather than the CO2/SO2 molecule. Moreover, our studies also showed that affinity between ionic liquid and CO2 is hindered by more than ionic liquid and SO2 rich system with the presence of water in the environment.

Computational approaches to understanding reaction outcomes of organic processes in ionic liquids

The utility of using a combined experimental and computational approach for understanding ionic liquid media, and their effect on reaction outcome, is highlighted through a number of case studies.