Insight into CO2/CH4 separation by ionic liquids confined in MXene membrane from molecular level

Preliminary in situ X-ray diffraction measurements of UV-induced photomechanical effects in a mesogenic material

During the last decade there has been an explosion of interest in ionic liquids. If the keywords “ionic liquid” or “ionic liquids” are used for a literature research in the Web of Science, more than seven thousand entries can be found for 2011. That is a tenfold increase in the number of publications dealing with ionic liquids over the past decade, and the field shows no signs of slowing down. Ionic liquids have changed from a lab curiosity to materials of tremendous academic and industrial interest. This new and remarkable liquid substance with unique and fascinating properties offers a phenomenal opportunity for new science and technology. The interest in these new materials is due to their wide range of possible applications as solvents for reaction and material processing, as extraction media or as working fluids in mechanical applications. Originating in the field of electrochemistry and based upon their wide electrochemical windows and good conductivities, ionic liquids have found useful applications in sensors, solar cells, solid-state photocells, batteries, separation devices and as thermal fluids, lubricants, hydraulic fluids, ionogels and fuels. The generally accepted definition of an ionic liquid is a salt composed solely of ions with melting points below 100 °C. New combinations of ions provide changing physical properties and thus novel potential applications for this class of liquid material. To a large extent, the structure and properties of ionic liquids are determined by the intermolecular interaction between anions and cations. In particular, the subtle balance between Coulomb forces, hydrogen bonds, π stacking and repulsion–dispersion forces is of great importance for understanding the properties of ionic liquids. All the important properties such as structure, diffusion, viscosity, conductivity or melting points depend on these interactions between cations and anions in ionic liquids. Thus, an important goal is to develop a fundamental understanding of their chemical and physical properties at the molecular level. Reliable structure–property relationships can help us to synthesize ionic liquid compositions with the desired set of properties. Since 2005 a number of research programs have been set up worldwide to advance ionic liquids research. Among those, the Japanese Priority Area “Science of Ionic Liquids” running from 2005 to 2009 and the priority programme SPP 1191 “Ionic Liquids” implemented by the German Science foundation (DFG) in 2006 and finished at the end of this year may be the most prominent ones in the academic arena. The efforts and developments achieved in ionic liquids research from these and other programmes are presented in this special issue. On the occasion of the International Bunsen Meeting 2012 in Leipzig, we put together a collection of almost forty articles that show the growing diversity within the field. In this special issue of ChemPhysChem, developments in experiment, theory and simulations for investigating ionic liquids are presented. The articles highlight the interaction within ionic liquids between anions and cations studied by a variety of experimental and theoretical methods including spectroscopy, ab initio simulations, classical molecular dynamics simulations and quantum molecular dynamics simulations. A particular focus is put on hydrogen bonding and dispersion forces, which are often assumed to be unimportant in Coulomb systems but are absolutely crucial to understanding ionic liquids. The critical review of theoretical and simulation approaches for describing the properties of ionic liquids is an important issue in this respect. Another topic is the interaction of ILs with water and organic solvents leading to modified local structures and polarities (Figure 1). In general, IL mixtures with water, alcohols and amines allow interesting tuning of important IL physical properties, including structural and transport properties such as diffusion coefficients, viscosities and conductivities. Walden plots, which compare the molar conductivity as a function of fluidity (inverse viscosity), show deviations from ideal, linear behaviour derived from well-understood aqueous solutions. Ionic liquids in its pure state (left) and in a mixture with an organic solvent (blue). Why not consider mixing ILs themselves to change properties? Thus another focus is put on mixtures of ILs including different anion–cation combinations. By varying the cation or the anion only, structure and interaction strength can be modified in the desired way. Several examples are given for structure–property relations in ionic liquids, which may allow predictions for physical properties for a set of specially synthesized ILs. Chemical reactions and coordination in ionic liquids are also attracting increasing interest. In several articles the effects on physical properties of other materials such as gels and micelles in IL environments are reported. ILs have also become increasing important in biology. The behaviour of biomolecules, such as the folding and unfolding of proteins, are developing into an important facet in ionic liquids research. Of course, after having an idea about the origin of the physical properties, new ionic liquids need to be synthesized and characterized. New and special ionic liquids and their properties are introduced in some of the contributions. An important aspect in and of itself is the liquid/gas or the liquid/liquid interface of ionic liquids (Figure 2). How do these interfaces look like with respect to the polar and hydrophobic parts of anions and cations? How do the IL properties change at the interfaces? Combinations of experimental and theoretical methods try to provide answers to these questions. Ion-pair evaporating from the surface of an imidazolium-based ionic liquid. All these fundamental issues are addressed in this special issue of ChemPhysChem. We hope that this compilation improves our understanding of the fascinating physical chemistry of ionic liquids. An understanding at the molecular level opens new potential for fundamentals and applications. Ralf Ludwig 1 Ed Maginn 1 Bala Sundaram 1

Ionic liquids (ILs) are organic salts with a melting temperature lower than the boiling point of water, while deep eutectic solvents (DESs) are usually composed of two or more components that can associate with each other through hydrogen bonds, and the resulting melting point of the mixture is lower than each of the components. These two classes of solvents share many remarkable properties, such as low volatility, non- flammability, wide liquidous range, and tunability; moreover, DESs are more inexpensive and environmentally friendlier. In addition to the fundamental scientific interest in these types of materials, our modeled systems are relevant to practical applications such as dye- sensitized solar cells and gas separations, as well as developing nanomaterials based on ILs. We first performed molecular dynamics (MD) simulations to study the structure and dynamics of DESs and ILs confined inside slit-like graphite and titania [rutile (110)] nanopores. Our results show that hydrogen bond donor (HBD) molecules of modeled DESs can form long-lived hydrogen bonds with the oxygen atoms in the rutile walls, consequently the rutile walls are coated by HBD layers that are almost depleted of ions, which exhibiting a liquid structure that departs from that of the bulk DES and with very slow dynamics. In contrast, for DESs inside graphitic pores, all species are present in the fluid layers near the carbon walls, the local liquid structure everywhere is similar to that of a bulk DES, and the overall dynamics are faster than those observed inside rutile pores of the same pore size. When the pore size is reduced, in general the hydrogen bonds formed can exist for longer time, a larger proportion of the hydrogen bonds involve the walls (in the case of a rutile pore), and the overall dynamics of the confined DES become slower. We then performed MD simulations of confined DESs or ILs in contact with gas phases containing CO2, and later a binary mixture of CO2 and CH4. When the amount of DES in the pore is varied, in the absence of DES, CO2 adsorbs to the pore walls, but increasing amounts of DES inside the pores quickly displace carbon dioxide into the gas/liquid interfaces, and into dissolution within the confined DES. As the amount of DES inside the pores increases, the diffusivity of CO2 reaches a maximum in partially filled pores and decays to the values observed in pores filled with DES, which are similar to the diffusion coefficients of CO2 in the bulk DES. The average number densities of CO2 near the confined DES (i.e., dissolved in the DES, and adsorbed at the pore walls and at the gas/liquid interface) are significantly larger than the corresponding value observed in the bulk DES. Furthermore, the solubility selectivity, diffusion selectivity and permselectivity of CO2 over CH4 inside the pore will alter with different pore materials, the properties of confined DESs or ILs, the loadings of solvents in the nanopores. Additionally, biased MD simulations have been performed to model the heterogeneous nucleation of IL [dmim+][Cl-] near a graphitic surface from its supercooled liquid phase. As nucleation is a rare event, a combination of techniques (i.e. the string method in collective variables and Markovian milestoning with Voronoi tessellations) has been used to obtain a minimum free energy path connecting the supercooled liquid and crystal phases, and determine the free energy and the rates involved in the nucleation process. It is found that when the subcooled liquid phase is in contact with a graphitic disk, the free energy barrier becomes about 42% smaller (~49 kcal/mol), and the critical nucleus formed is about 17% smaller (~3.0 nm) than the one observed for homogeneous nucleation.

We prepared a hydrated mica mineral material with larger layer spacing by oxidized/cobalt ion exchange. The structural evolution during the modification and the electrochemical properties of the final obtained hydrated Co-mica were investigated. Results showed that the layer spacing of black mica increased, and the electrochemical properties were enhanced. In the oxidation environment, the divalent iron ions in the octahedral layer of black mica are oxidized. The potassium ions bound by Coulomb force in the black mica layer structure can be detached cobalt ions support the large layer spacing in the form of hydrated ions. Electrochemical performance tests showed that the electrochemical properties of hydrated Comica hydrate were significantly improved after the increase in layer spacing. The first discharge specific capacity of black mica raw ore was 1.769 F/g, and the first discharge-specific capacitance of hydrated Co-mica prepared by the oxide/cobalt ion-exchange method reached 4.305 F/g, which was significantly enhanced.

Adsorption of TCDD with 1-butyl-3-methylimidazolium dicyanamide ionic liquid: A combined molecular dynamics simulation and quantum chemistry study

HighlightsAccording to the design of the experiment (DoE), multivariate analysis models were used to optimize the critical process parameters combined with multi-objective optimization.Based on the optimized operating conditions, the MILT-HD method not only enhances the extraction efficiency from Amomi fructus but also reduces energy demands and CO2 emissions.Based on the density functional theoretical (DFT) and molecular dynamics (MD) simulations, the mechanisms for ionic liquids (ILs) to improve the extraction efficiency of essential oil was comprehensively revealed.In this paper, Amomi fructus (Latin) was used to explore the mechanism of ionic liquids (ILs) in improving the extraction efficiency of essential oils. Microwave assisted ionic liquid treatment followed by a hydro-distillation (MILT-HD) process for isolating Amomi fructus essential oil was optimized by multi-objective optimization. Under optimum operating conditions, the IL-assisted extraction method not only enhances extraction efficiency but also reduces energy demands and CO2 emissions. Since the hydrogen bond structure network of cellulose in the cell wall is an important reason for hindering diffusion of essential oils, the mechanism of ILs was explored by density functional theoretical (DFT) and molecular dynamics (MD) simulations. According to DFT calculations, ILs can facilitate the cleavage of cellulose chains and have strong non-covalent interactions with cellulose. Based on the MD simulations, the degree of destruction of the cellulose hydrogen bond structure was explored. According to the DFT and MD simulations, the ILs can significantly destroy cellulose structure, thereby promoting essential oil release from the plant. These results were confirmed by scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR). This work is conducive to better understand the MILT-HD process for isolating essential oil and comprehensively understand the mechanism of ILs in the extraction process.

Anisotropic pairing interactions mediated by phonons are examined in layer systems. It is shown that the screening effects become weaker when the layer spacing increases. Then the anisotropic components of the pairing interactions increase with the screening length since the momentum dependence changes. As a result, various types of anisotropic superconductivity occur depending on the parameter region. For example, p-wave superconductivity occurs when the short-range part of Coulomb repulsion is strong and the layer spacing is large. Two kinds of interlayer pairing may occur when the layer spacing is not too large. Although the phonon contribution to the d-wave pairing interaction is weaker than the p-wave interaction, it increases with the layer spacing. The relevance of the present results to organic superconductors, high-${T}_{c}$ cuprates, and ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$ is discussed.

Application of Imidazolium-based polyionic liquids to separate the 1,3,5-Trioxane-Water/Ethanol-Water system based on experimental verification and molecular mechanism analysis

The solvation of the enzyme Candida antarctica lipase B (CAL-B) was studied in eight different ionic liquids (ILs). The influence of enzyme-ion interactions on the solvation of CAL-B and the structure of the enzyme-IL interface are analyzed. CAL-B and ILs are described with molecular dynamics (MD) simulations in combination with an atomistic empirical force field. The considered cations are based on imidazolium or guanidinium that are paired with nitrate, tetrafluoroborate or hexafluorophosphate anions. The interactions of CAL-B with ILs are dominated by Coulomb interactions with anions, while the second largest contribution stems from van der Waals interactions with cations. The enzyme-ion interaction strength is determined by the ion size and the magnitude of the ion surface charge. The solvation of CAL-B in ILs is unfavorable compared to water because of large formation energies for the CAL-B solute cages in ILs. The internal energy in the IL and of CAL-B increases linearly with the enzyme-ion interaction strength. The average electrostatic potential on the surface of CAL-B is larger in ILs than in water, due to a weaker screening of charged enzyme residues. Ion densities increased moderately in the vicinity of charged residues and decreased close to non-polar residues. An aggregation of long alkyl chains close to non-polar regions and the active site entrance of CAL-B are observed in one IL that involved long non-polar decyl groups. In ILs that contain 1-butyl-3-methylimidazolium cations, the diffusion of one or two cations into the active site of CAL-B occurs during MD simulations. This suggests a possible obstruction of the active site in these ILs. Overall, the results indicate that small ions lead to a stronger electrostatic screening within the solvent and stronger interactions with the enzyme. Also a large ion surface charge, when more hydrophilic ions are used, increases enzyme-IL interactions. An increase of these interactions destabilizes the enzyme and impedes enzyme solvation due to an increase in solute cage formation energies.

The sustainable development encompasses the search for new materials for energy storage, gas capture, separation, and solvents in industrial processes that can substitute conventional ones in an efficient and clean manner. Ionic liquids (ILs) emerged and have been advanced as alternative materials for such applications, but an obstacle is their hygroscopicity and the effects on their physical properties in the presence of humidity. Several industrial processes depend on the aqueous interfacial properties, and the main focus of this work is the water/IL interface. The behavior of the aqueous ionic liquids at the water-vacuum interface is representative for their water interfacial properties. Using X-ray photoelectron spectroscopy in combination with molecular dynamics simulations we investigate four aqueous IL systems, and provide molecular level insight on the interfacial behaviour of the ionic liquids, such as ion-pair formation, orientation and surface concentration. We find that ionic liquids containing a chloride anion have a lowered surface enrichment due to the low surface propensity of chloride. In contrast, the ionic liquids containing a bistriflimide anion are extremely surface-enriched due to cooperative surface propensity between the cations and anions, forming a two-dimensional ionic liquid on the water surface at low concentrations.

Molecular insight into the dissolution-degradation process of cellulose bunch in ionic liquids

Thegas–liquid interface of ionic liquids (ILs) is criticallyimportant in many applications, for example, in supported IL phase(SILP) catalysis. Methods to investigate the interfacial structurein these systems will allow their performance to be improved in arational way. In this study, reactive-atom scattering (RAS), surfacetension measurements, and molecular dynamics (MD) simulations wereused to study the vacuum interface of mixtures of partially fluorinatedand normal alkyl ILs. The underlying aim was to understand whetherfluorinated IL ions could be used as additives to modify the surfacestructure of one of the most widely used families of alkyl ILs. Theseries of ILs 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide([Cnmim][Tf2N]) with n = 4–12 were mixed with a fixed-length, semiperfluorinatedanalogue (1H,1H,2H,2H-perfluorooctyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)imide([C8mimF13][Tf2N]), forming [Cnmim](1–x)[C8mimF13]x[Tf2N] mixtures, where x is the bulk mole fractionof the fluorinated component. The RAS-LIF method combined O-atom projectileswith laser-induced fluorescence (LIF) detection of the product OHas a measure of surface exposure of the alkyl chains. For [C8mim](1–x)[C8mimF13]x[Tf2N] mixtures,RAS-LIF OH yields are below those expected from stoichiometry. Thereare quantitatively consistent negative deviations from linearity ofthe surface tension. Both results imply that the lower-surface-tensionfluoroalkyl material dominates the surface. A similar deficit is foundfor alkyl chain lengths n = 4, 6, 8, and 12 and forall (nonzero) x investigated by RAS-LIF. Accessible-surface-area(ASA) analyses of the MD simulations for [Cnmim](1–x)[C8mimF13]x[Tf2N] mixturesqualitatively reproduce the same primary effect of fluoro-chain predominanceof the surface over most of the range of n. However,there are significant quantitative discrepancies between MD ASA predictionsand experiment relating to the strength of any n-dependenceof the relative alkyl coverage at fixed x, and onthe x-dependence at fixed n. Thesediscrepancies are discussed in the context of detailed examinationsof the surface structures predicted in the MD simulations. Potentialexplanations, beyond experimental artifacts, include inadequaciesin the classical force fields used in the MD simulations or the inabilityof simple ASA algorithms to capture dynamical factors that influenceRAS-LIF yields.

In recent years, short peptide self-assembled materials, prepared under the control of the thermolysin catalyst, have been investigated extensively and shown to acquire various morphologies and fun...

Room temperature ionic liquids (RTILs) are useful as new solvents or materials owing to their favorable properties such as negligible vapor pressure, non-flammability, and wide electrochemical window. For further development of new RTILs, it is indispensable to understand macroscopic properties as solvent and/or liquids of RTILs at a molecular or atomistic level. Here, liquid structure and the ion-ion interactions of room temperature ionic liquid ethylammonium nitrate (EAN) were studied by means of large angle X-ray scattering (LAXS) experiment and molecular dynamics simulations. X-ray interference function for EAN shows a small peak of 0.62 Å–1 indicating nano-scale segregation in the ionic liquid. X-ray radial distribution function as the form of D(r) – 4πr2ρ0 evidently shows a peak of 3.4 and broad ones of 4.7, 8 and 12 Å, suggesting that EAN has long range ordering in the liquid state. The intra-molecular X-ray interference function was estimated on the basis of molecular geometries found in crystals to yield the inter-molecular X-ray pair correlation functionG_mathrm{inter}mathrm{LAXS}(r). In G_mathrm{inter}mathrm{LAXS}(r), peaks of 3.0, 3.4, and 4.7 Å were found as the atom-atom correlation. The peak of 3.0 Å can be assigned to the atom-atom correlation of N (C2H5NH3+)…O (NO3–) in the NH…O hydrogen bonding. In addition, the peak of 3.4 Å is also ascribable to C (C2H5NH3+)…O (NO3–) correlations in the CH…O interactions. Molecular dynamics simulations based on newly developed force fields to describe a series of primary ammonium cation were performed to ascribe the peaks found in G_mathrm{inter}mathrm{LAXS}(r). The X-ray interference function imathrm{MD}(s) and the pair correlation function Gmathrm{MD}(r) derived from simulations were reasonably in agreement with the experimental ones. According to the partial atom-atom correlation functions derived from the simulations, the experimentally observed peaks of 3.0 and 3.4 Å can be ascribable to the NH…O and the CH…O correlations in the closest ethylammonium and nitrate interaction, respectively. The CH…O interactions may play an important role in macroscopic properties of this kind of ionic liquids.

The mobility of radiocesium in the environment is largely mediated by cation exchange in micaceous clays, in particular Illite-a non-swelling clay mineral that naturally contains interlayer K+ and has high affinity for Cs+. Although exchange of interlayer K+ for Cs+ is nearly thermodynamically nonselective, recent experiments show that direct, anhydrous Cs+-K+ exchange is kinetically viable and leads to the formation of phase-separated interlayers through a mechanism that remains unclear. Here, using classical atomistic simulations and density functional theory calculations, we identify a molecular-scale positive feedback mechanism in which exchange of the larger Cs+ for the smaller K+ significantly lowers the migration barrier of neighboring K+, allowing exchange to propagate rapidly once initiated at the clay edge. Barrier lowering upon slight increase in layer spacing (∼0.7 Å) during Cs+ exchange is an example of "chemical-mechanical coupling" that likely explains the observed sharp exchange fronts leading to interstratification. Interestingly, we find that these features are thermodynamically favored even in the absence of a heterogeneous layer charge distribution.