Thermochemistry of mixing imidazole-based ionic liquids with water and organic solvents

The results of the calorimetric studies of the enthalpies of mixing of some ionic liquids, 1-butyl-3-methylimidazolium chloride and acetate, with water, pentanol, dimethyl sulfoxide and dimethylformamide are presented. Protic solvents interact with the ionic liquid anion, wheras aprotic, with the cation. The mixing enthalpy depends on the dissociation degree of the ionic liquid.     

Structure and interaction between the [BMIM][Ala] alanine anion and the 1-butyl-3-methylimidazolium cation in ion pairs

The equilibrium geometries and vibrational frequencies of the ionic liquid 1-butyl-3-methylimidazolium cation and the alanine anion [BMIM][Ala] are studied using density functional theory (DFT) at the B3PW91/6-311+G(d,p) leve1. The most stable structures of the anion, the cation, and the ion pairs are obtained and characterized, and the geometry parameters of the ion pairs confirm the presence of a hydrogen bonding interaction between the anion and the cation. Natural bond orbital (NBO) analysis is also performed to analyze the atomic charge distribution and charge transfer in the [BMIM]⁺ cation and [BMIM][Ala] ionic liquids. The results show that there are the electrostatic interaction and multiple hydrogen bond interactions between the cation and the anion of the ionic liquids, and the stability of the ground state of the ion pairs mostly results from the hydrogen bonding between the lone pairs of O atoms in the anion and H in the imidazole cycle of the cation. There are some changes in microstructures and the charge distribution during the formation of the ion pairs.     

[EPy][BF4]和[EPy][PF6]离子对的气相及液相阴阳离子相互作用研究

采用从头算HF/6-31G和密度泛函理论B3LYP/6-31+G(d,p)方法, 对乙基吡啶四氟硼酸盐([EPy][BF4])和乙基吡啶六氟磷酸盐([EPy][PF6])的离子对进行了结构优化和频率分析, 并利用自洽反应场(SCRF)的导体极化连续模型(CPCM)考察了离子对液态下的结构及相互作用, 得到了两种离子对的红外光谱及气相、液相下最稳定结构. 由两种离子对的几何参数可知, 阴阳离子通过氢键相互作用, 两种离子液体的红外光谱特征值与实验值比较吻合. 应用自然键轨道(NBO)理论分析了吡啶阳离子及离子对中的原子电荷分布和电荷转移情况, 结果表明两种离子对中阴阳离子间存在静电相互作用和氢键作用. 通过几何参数、相互作用能及NBO分析研究发现, 液相下由于周围电荷的中和作用, 离子对中阴阳离子的相互作用明显降低.     

Benzene Solubility in Ionic Liquids: Working Toward an Understanding of Liquid Clathrate Formation

The solubility of benzene in 15 imidazolium, pyrrolidinium, pyridinium, and piperidinium ionic liquids has been determined; the resulting, benzene‐saturated ionic liquid solutions, also known as liquid clathrates, were examined with ¹H and ¹⁹F nuclear magnetic resonance spectroscopy to try and understand the molecular interactions that control liquid clathrate formation. The results suggest that benzene interacts primarily with the cation of the ionic liquid, and that liquid clathrate formation (and benzene solubility) is controlled by the strength of the cation–anion interactions, that is, the stronger the cation–anion interaction, the lower the benzene solubility. Other factors that were determined to be important in the final amount of benzene in any given liquid clathrate phase included attractive interactions between the anion and benzene (when significant), and larger steric or free volume demands of the ions, both of which lead to greater benzene solubility.     

[EPy]Cl和[EPy]Br离子对的气相和液相结构及阴阳离子间的相互作用

采用密度泛函理论B3LYP/6-31+G(d,p)方法对氯化乙基吡啶([EPy]Cl)、溴化乙基吡啶([EPy]Br)的离子对进行了结构优化和频率分析, 并利用自洽反应场(SCRF)的导体极化连续模型(CPCM)考察了离子液体液相下的结构及相互作用. 得到了两种离子液体的离子对在气相、液相下最稳定结构及气相红外光谱特征值, 两种离子液体的离子对结构存在相似性, 红外光谱特征值与文献值比较吻合. 应用自然键轨道(NBO)理论分析了离子对中原子电荷分布及电荷转移情况, 结果证明两种离子液体中阴阳离子间除了静电相互作用外还存在着氢键作用. 通过对比气相及液相下的几何参数、相互作用能及NBO分析结果, 发现液相下阴阳离子的相互作用明显降低. 液相环境抵消了大部分阴阳离子间的静电作用, 导致液相下阴阳离子间相互作用的减小.     

Structure and dynamics of N-methyl-N-propylpyrrolidinium bis(trifluoromethanesulfonyl)imide ionic liquid from molecular dynamics simulations

Molecular dynamics (MD) simulations were performed on N-methyl-N-propylpyrrolidinium bis(trifluoromethanesulfonyl)imide (mppy+TFSI-) from 303 to 393 K to improve understanding of the structure and ion transport of this ionic liquid. The density, ion self-diffusion coefficients, conductivity, and viscosity of mppy+TFSI- predicted from MD simulations are in good agreement with experimental measurements. The time-dependent shear modulus of the ionic liquids was calculated and compared with that for nonionic liquids. On average each mppy+ cation was found to be coordinated by four TFSI- anions. The angular distributions of N(TFSI-)-N(mppy+)-N(TFSI-) and N(mppy+)-N(TFSI-)-N(mppy+) exhibit a maximum at 80-90 degrees and a second maximum at 180 degrees . Correlation of ion motion was found to lower ionic conductivity by approximately one-third from the expected value based upon ion self-diffusion coefficients. Rotational motion of the cation and anion are anisotropic with the degree of anisotropy increasing with decreasing temperature. Electrostatic interactions are responsible for slowing down the dynamics of the ionic liquid by more than an order of magnitude and a dramatic decrease of the time-dependent shear modulus.     

Mass Distribution and Diffusion of [1‐Butyl‐3‐methylimidazolium][Y] Ionic Liquids Adsorbed on the Graphite Surface at 300–800 K

The structure and diffusion behavior of 1‐butyl‐3‐methylimidazolium ([bmim]⁺) ionic liquids with [Cl]^?, [PF₆]^?, and [Tf₂N]^? counterions near a hydrophobic graphite surface are investigated by molecular dynamics simulation over the temperature range of 300–800 K. Near the graphite surface the structure of the ionic liquid differs from that in the bulk and it forms a well‐ordered region extending over 30 Å from the surface. The bottom layer of the ionic liquid is stable over the investigated temperature range due to the inherent slow dynamics of the ionic liquid and the strong Coulombic interactions between cation and anion. In the bottom layer, diffusion is strongly anisotropic and predominantly occurs along the graphite surface. Diffusion perpendicular to the interface (interfacial mass transfer rate k_t) is very slow due to strong ion–substrate interaction. The diffusion behaviors of the three ionic liquids in the two directions all follow an Arrhenius relation, and the activation barrier increases with decreasing anion size. Such an Arrhenius relation is applied to surface‐adsorbed ionic liquids for the first time. The ion size and the surface electrical charge density of the anions are the major factors determining the diffusion behavior of the ionic liquid adjacent to the graphite surface.     

Thermodynamic modeling of CO2 solubility in ionic liquid with heterosegmented statistical associating fluid theory

Heterosegmented statistical associating fluid theory is used to represent the CO₂ solubility in ionic liquids. As in our previous work, ionic liquid molecule is divided into several groups representing the alkyls, cation head, and anion. The cation of ionic liquid is modeled as a chain molecule that consists of one spherical segment representing the cation head and groups of segments of different types representing different substituents (alkyls). The anion of ionic liquid is modeled as a spherical segment of different type. To account for the electrostatic/polar interaction between the cation and anion, the spherical segments representing cation head and anion each have one association site, which can only cross associate. Carbon dioxide is modeled as a molecule with three association sites, two sites of type O and one site of type C, where sites of the same type do not associate with each other. The parameters of CO₂ are obtained from the fitting of the density and the saturation vapor pressure of CO₂. For the CO₂-ionic liquid systems, cross association between site of type C in CO₂ and another association site in anion is allowed to occur to account for the Lewis acid–base interaction. The parameters for cross association interactions and the binary interaction parameters used to adjust the dispersive interactions between unlike segments are obtained from the fitting of the available CO₂ solubility in ionic liquids. The model is found to well represent the CO₂ solubility in the imidazolium ionic liquids from 283 to 415 K and up to 200 bar.     

From dysfunction to bis-function: on the design and applications of functionalised ionic liquids

Some of the recent developments concerning the synthesis, properties and applications of functionalised ionic liquids are highlighted. Various strategies are presented, including functionalisation of the cation, anion or both cation and anion in the same ionic liquid, leading to what has been termed dual-functionalised ionic liquids. Particular attention is given to the application of functionalised ionic liquids as reaction media, to stabilise nanoparticles/modify surfaces and to generate porous materials.     

Effect of structure of ionic liquids on their dissolving power toward natural polymers

Experimental and calculated data on the effect of anion and cation structure on the dissolving power of ionic liquids based on imidazole toward the natural polymers cellulose, fibroin and keratin are presented.     

Theoretical analysis of the hydrogen bond of imidazolium C(2)-H with anions

The intermolecular interaction energies of ion pairs of imidazolium-based ionic liquids were studied by MP2/6-311G level ab initio calculations. Although the hydrogen bond between the C(2) hydrogen atom of an imidazolium cation and anion has been regarded as an important interaction in controlling the structures and physical properties of ionic liquids as in the cases of conventional hydrogen bonds, the calculations show that the nature of the C(2)-H...X interaction is considerably different from that of conventional hydrogen bonds. The interaction energies of the imidazolium cation with neighboring anions in the four crystals of ionic liquids were calculated. The size of the interaction is determined mainly by the distance between the imidazolium ring and anion. The calculated interaction energy is nearly inversely proportional to the distance, which shows that the charge-charge interaction is the dominant interaction in the attraction. The orientation of the anion relative to the C(2)-H bond does not greatly affect the size of the interaction energy. Calculated interaction energy potentials of 1,3-dimethylimidazolium tetrafluoroborate ([dmim][BF(4)]) complexes show that the C(2)-H bond does not prefer to point toward a fluorine atom of the BF(4). This shows that the C(2)-H...X hydrogen bond is not essential for the attraction.     

The Double‐Faced Nature of Hydrogen Bonding in Hydroxy‐Functionalized Ionic Liquids Shown by Neutron Diffraction and Molecular Dynamics Simulations

We characterize the double‐faced nature of hydrogen bonding in hydroxy‐functionalized ionic liquids by means of neutron diffraction with isotopic substitution (NDIS), molecular dynamics (MD) simulations, and quantum chemical calculations. NDIS data are fit using the empirical potential structure refinement technique (EPSR) to elucidate the nearest neighbor H???O and O???O pair distribution functions for hydrogen bonds between ions of opposite charge and the same charge. Despite the presence of repulsive Coulomb forces, the cation–cation interaction is stronger than the cation–anion interaction. We compare the hydrogen‐bond geometries of both “doubly charged hydrogen bonds” with those reported for molecular liquids, such as water and alcohols. In combination, the NDIS measurements and MD simulations reveal the subtle balance between the two types of hydrogen bonds: The small transition enthalpy suggests that the elusive like‐charge attraction is almost competitive with conventional ion‐pair formation.     

Effects of water content on the dissolution behavior of wool keratin using 1-ethyl-3-methylimidazolium dimethylphosphate

Ionic liquids(ILs) are eco-friend and recyclable solvents for dissolution of wool keratin, and water is often used as antisolvent to regenerate keratin from IL solution. To recycle the ILs, removing water is the necessary step. However, complete removal of the water is energy-intensive and costly. The water in ionic liquids would change the physicochemical properties and cluster structures of the IL and further affect its dissolution behavior on keratin. Here, 1-ethyl-3-methylimidazolium dimethylphosphate([Emim]DMP) was used for experiments due to its good performance on dissolving keratin. The experimental and simulation results showed that the dissolving capability of [Emim]DMP was decreased and the interactions between cation and anion became weaker with water concentration increasing. Furthermore, the dissolution time of wool keratin in [Emim]DMP increased with water content rising. At the same time, the effect of water in ILs on the secondary structure distribution and thermal stability of regenerated keratin was not obvious. In this work, by taking the structures of [Emim]DMP, keratin dissolution time and properties of the regenerated keratin into consideration, a balanced range of water content in [Emim]DMP was determined, which could not only reduce recycling cost but also not affect the dissolution behavior of the IL.     

Potential energy landscape of bis(fluorosulfonyl)amide

The conformational landscape of the bis(fluorosulfonyl)amide, [FSI]-, anion was analyzed using data obtained from Raman spectroscopy, molecular dynamics (MD), and ab initio studies. The plotting of three-dimensional potential energy surfaces and the corresponding MD simulation conformer-population histograms show the existence of two stable isomers, C2 (trans) and C1 (cis) conformers, and confirm the nature of the anion as a flexible molecule capable of interconversion between conformers in the liquid state. In ionic liquids, the two [FSI]- conformers coexist in equilibrium, a result confirmed by the Raman data. The implications of the conformational behavior of the ion [FSI]- are discussed in terms of the solvation properties of the corresponding ionic liquids.     

Solvent effects in ionic liquids: empirical linear energy-density relationships

Multiparameter linear energy-density relationships to model solvent effects in room temperature ionic liquids (RTILs) are introduced and tested. The model incorporates two solvent dependent and two specific solute-solvent parameters represented by a set of electronic indexes derived from the conceptual density functional theory. The specific solute-solvent interactions are described in terms of the electronic chemical potential for proton migration between the anion or cation and the transition state structure of a specific reaction. These indexes provide a quantitative estimation of the hydrogen bond (HB) acceptor basicity and the hydrogen bond donor acidity of the ionic solvent, respectively. A sound quantitative scale of HB strength is thereby obtained. The solvent dependent contributions are described by the global electrophilicity of the cation and nucleophilicity of the anion forming the ionic liquid. The model is illustrated for the kinetics of cycloaddition of cyclopentadiene towards acrolein. In general, cation HB acidity outweighs the remaining parameters for this reaction.     

Thermodynamical and structural properties of imidazolium based ionic liquids from molecular simulation

We have performed molecular dynamics simulations to determine the densities and heat of vaporization as well as structural information for the 1-alkyl-3-methyl-imidazolium based ionic liquids [amim][Cl] and [amim][BF(4)] in the temperature range from 298 to 363 K. In this simulation study, we used an united atom model of Liu et al. [Phys. Chem. Chem. Phys. 8, 1096 (2006)] for the [emim(+)] and [bmim(+)] cations, which we have extended for simulation in [hmim]-ILs and combined with parameters of Canongia Lopes et al. [J. Phys. Chem. B 108, 2038 (2004)] for the [Cl(-)] anion. Our simulation results prove that both the original united atoms approach by Liu et al. and our extension yield reasonable predictions for the ionic liquid with a considerably reduced computational expense than that required for all atoms models. Radial distribution functions and spatial distribution functions where employed to analyze the local structure of this ionic liquid, and in which way it is influenced by the type of the anion, the size of the cation, and the temperature. Our simulations give evidence for the occurrence of tail aggregations in these ionic liquids with increasing length of the side chain and also increasing temperature.     

Predicting properties of new ionic liquids: density functional theory and experimental studies of tetra-alkylammonium salts of (thio)carboxylate anions, RCO2⁻, RCOS⁻ and RCS2⁻

Density functional theory calculations of alkyl-carboxylate anions and their sulfur substituted variants are presented here as an aid for the development of new ionic liquids. Electron transfer both within the anion, and between the anion and cation of an ion pair, are described using natural bond order analysis, using tetraethylammonium as a common cation. The overall stabilising effect of this electron transfer is quantified for the series of anions, and is found to correlate with clear trends in ion-pair binding energy. These and other electronic properties determine which compounds are synthesised, and experimental results validate the computational results. In combination with tetraethylammonium, a carboxylate with an unsaturated alkyl chain produces an ionic liquid at room temperature. However, computations suggest that sulfur substituted anions will produce a lower melting point and perhaps more fluid ionic liquid, but one which would be less stable against oxidation.     

Theoretical study for pyridinium-based ionic liquid 1-ethylpyridinium trifluoroacetate: synthesis mechanism, electronic structure, and catalytic reactivity

By performing density functional theory calculations, we have studied the synthesis mechanism, electronic structure, and catalytic reactivity of a pyridinium-based ionic liquid, 1-ethylpyridinium trifluoroacetate ([epy](+)[CF(3)COO](-)). It is found that the synthesis of the pyridinium salt follows a S(N)2 mechanism. The electronic structural analyses show that multiple H bonds are generally involved in the pyridinium-based ionic liquid, which may play a decisive role for stabilizing the ionic liquid. The cation-anion interaction mainly involves electron transfer between the lone pair of the oxygen atom in the anion and the antibonding orbital of the C*-H bond (C* denotes the carbon atom at the ortho-position of nitrogen atom in the cation). This present work has also given clearly the catalytic mechanism of [epy](+)[CF(3)COO](-) toward to the Diels-Alder (D-A) reaction of acrylonitrile with 2-methyl-1,3-butadiene. Both the cation and anion are shown to play important roles in promoting the D-A reaction. The cation [epy](+), as a Lewis acid, associates the C≡N group by C≡N···H H bond to increase the polarity of the C═C double bond in acrylonitrile, while the anion CF(3)COO(-) links with the methyl group in 2-methyl-1,3-butadiene by C-H···O H bond, which weakens the electron-donating capability of methyl and thereby lowers the energy barrier of the D-A reaction. The present results are expected to provide valuable information for the design and application of pyridinium-based ionic liquids.     

Highly Luminescent and Color‐Tunable Salicylate Ionic Liquids

High quantum yields of up to 40.5 % can be achieved in salicylate‐bearing ionic liquids. A range of these ionic liquids have been synthesized and their photoluminescent properties studied in detail. The differences noted can be related back to the structure of the ionic liquid cation and possible interionic interactions. It is found that shifts of emission, particularly in the pyridinium‐based ionic liquids, can be related to cation–anion pairing interactions. Facile and controlled emission color mixing is demonstrated through combining different ILs, with emission colors ranging from blue to yellow.     

The effect of hydrogen bond acceptor properties of ionic liquids on their cellulose solubility

It is nowadays well-known that ionic liquids can dissolve cellulose. However, little systematic data has been published that shed light onto the influence of the ionic liquid structure on the dissolution of cellulose. We have conducted 1H NMR spectroscopy of ethanol in a large number of ionic liquids, and found an excellent correlation of the data obtained with the hydrogen acceptor properties (β-values). With this tool in hand, it is possible to distinguish between cellulose-dissolving and non-dissolving ionic liquids. A modulating effect of both, the anion of the non-dissolving ionic liquid and its cation was found in solubility studies with binary ionic liquid mixtures. The study was extended to other non-dissolving liquids, namely water and dimethylsulfoxide, and the effect of the cation was also investigated.