Electronic structures of imidazolium-based ionic liquids

Electronic structures of ionic liquids formed by 1-buthyl-3-alkylimidazolium ion [C_nmim]⁺ (n = 4 and 8) with various inorganic and organic anions have been investigated by ultraviolet photoemission, X-ray photoemission, inverse photoemission and soft X-ray emission spectroscopies (SXES). The comparison of the calculated density of states with the observed spectra revealed that the molecular orbital energies of these ionic liquids are significantly affected by the electrostatic Madelung potential among the ions. The SXES results clearly show that the both highest occupied and lowest unoccupied states of [C₄mim]⁺PF₆⁻ are derived from the cation as a result of strong Madelung potential. On the other hand, the SXES results show the valence electronic structures of ionic liquids with larger anion molecules, [C_nmim]⁺Tf₂N⁻ and [C_nmim]⁺OTf⁻ are contributed from the both cation and anion.     

Molecular insight into structures of monocationic and dicationic ionic liquids in mica slits

Structures of ionic liquids (ILs) 1-decyl-3-methylimidazolium bis(trifluoromethanesulfonyl)azanide ([C₁₀mim][TFSA]) and 1-decyl-dimethylimidazolium bis(trifluoromethanesulfonyl)azanide ([C₁₀(mim)₂](TFSA)₂) in different-sized mica slits have been investigated using molecular dynamics simulations. Ion density and angular distributions for monocationic IL [C₁₀mim][TFSA] were analysed to elucidate the IL structures under different surface charges and especially their changes in the direction perpendicular to the surfaces. [C₁₀mim][TFSA] formes in bilayers, compatible with existing models of ILs with long alkyl chains. For dicationic IL [C₁₀(mim)₂](TFSA)₂, cations adjacent to the mica surface tend to stay parallel to the surface with both positively charged rings absorbed. While near the centre of the slit, dications show the weak tendency of orientation distribution, more random than [C₁₀mim]⁺ ions. Structures of [C₁₀(mim)₂](TFSA)₂ cannot be described by bilayer models. Additionally, the in-plane arrangement of [C₁₀mim][TFSA] is more ordered when K⁺ ions completely neutralise the negative charge of the mica surface, and [C₁₀mim]⁺ ions tend to be located in hexagonal mica lattices with two aluminium atoms in replacement of silicon atoms. [TFSA]^? ions are constrained by the neighbouring K⁺ ions absorbed onto mica lattices.     

Modification of Tao–Mason equation of state to ionic liquids

In our previous paper, we extended the Tao and Mason equation of state (TM EOS) to refrigerant fluids, using the speed of sound data. Here, we predict the equation of state for ionic liquids (ILs). The considered ILs are [Bmim][PF₆], [C₂mim][NtF₂], [C₃mim][NtF₂], [C₆mim][NtF₂], [C₇mim][NtF₂], [C₂mim][EtOSO₃], [Bmim][MeSO₄], [Bmim][OcSO₄], and [C₄mim][dca]. The equation of state consists of three temperature-dependent parameters: the second virial coefficient, a constant for scaling the softness of repulsive force, and an effective hard-sphere diameter equivalent to the van der Waals co-volume. The second virial coefficients of ILs are scare and there is no accurate potential energy function to allow their theoretical calculation. In this work, the second virial coefficient have been calculated using corresponding states correlation based on temperature and density at normal boiling point. The other two parameters of the equation of state can be calculated using a scaling rule. Analysis of our predicted results shows that the Tao?CMason equation of state is capable of accurately predicting the density of ILs at any temperature and pressure. The overall average absolute deviation densities for 1,633 data points are 2.05%. Also, the density of ILs obtained from the TM EOS has been compared with those calculated from vdW?CCS?C?? and Peng?CRobinson (PR) equation of state. Our results are in favor of the preference of the TM EOS over the two other equations of state. The overall average absolute deviation for 1,633 data points calculated by vdW?CCS?C?? and PR equation of state are 6.63% and 12.19%, respectively.     

Measuring the solubility of benzoic acid in room temperature ionic liquids using chronoamperometric techniques

The electrochemical reduction of benzoic acid (BZA) has been studied at platinum micro‐electrodes (10 and 2 µm diameters) in acetonitrile (MeCN) and six room temperature ionic liquids (RTILs): [C₂mim][NTf₂], [C₄mim][NTf₂], [C₄mpyrr][NTf₂], [C₄mim][BF₄], [C₄mim][NO₃] and [C₄mim][PF₆] (where [C_nmim]⁺ = 1‐alkyl‐3‐methylimidazolium, [NTf₂]^? = bis(trifluoromethylsulphonyl)imide, [C₄mpyrr]⁺ = N‐butyl‐N‐methylpyrrolidinium, [BF₄]^? = tetrafluoroborate, [NO₃]^? = nitrate and [PF₆]^? = hexafluorophosphate). Based on the theoretical fitting to experimental chronoamperometric transients in [C₄mpyrr][NTf₂] and MeCN at several concentrations and on different size electrodes, it is suggested that a fast chemical step preceeds the electron transfer step in a CE mechanism (given below) in both RTILs and MeCN, leading to the appearance of a simple one‐electron transfer mechanism. The six RTIL solvents and MeCN were saturated with BZA, and potential‐step chronoamperometry revealed diffusion coefficients of 170, 4.6, 3.2, 2.7, 1.8, 0.26 and 0.96 × 10^?11 m² s^?1 and solubilities of 850, 75, 78, 74, 220, 2850 and 48 mM in MeCN and the six ionic liquids, respectively, at 298 K. The high solubility of BZA in [C₄mim][NO₃] may suggest a strong interaction of the dissolved proton with the nitrate anion. Although there are relatively few literature reports of solubilities of organic solutes in RTILs at present, these results suggest the need for further studies on the solubilities of organic species (particularly acids) in RTILs, because of the contrasting interaction of dissolved species with the RTIL ions. Chronoamperometry is suggested as a convenient methodology for this purpose. Copyright © 2008 John Wiley & Sons, Ltd.     

Synthesis and Optical Properties of 1-Alkyl-3-Methylimidazolium Lauryl Sulfate Ionic Liquids

We report the synthesis of a new series of imidazolium-based halogen-free ionic liquids 1-alkyl-3-methylimidazolium lauryl sulfates. By reacting 1-methylimidazole (MIM) with butyl, hexyl, octyl, and decyl bromides and exchanging bromide ion with lauryl sulfate anion, a series of ionic liquids [RMIM][C₁₂H₂₅OSO₃] were produced. The high purity of these ionic liquids was verified with ¹H-NMR, ¹³C-NMR, FT-IR and mass spectrometry (MS), demonstrating the effectiveness of this synthetic approach. Solubility test of these ionic liquids showed that they are soluble in most organic solvents except nonpolar solvents such as hexane and cyclohexane. The optical properties of [BMIM]Br and [BMIM][C₁₂H₂₅OSO₃], where B refers to butyl, were examined. Both ionic liquids absorbed light in the UV region, yet essentially no absorption was recorded beyond 450 nm. Furthermore, both ionic liquids showed excitation wavelength-dependent fluorescence behavior. As an example, with an excitation wavelength of 360 nm, [BMIM][C₁₂H₂₅OSO₃] showed an emission band maximum at 447 nm. Increasing the excitation wavelength to 440 nm, the emission band maximum was shifted to ∼500 nm.     

Interaction mechanisms of ionic liquids [Cnmim]Br (n=4, 6, 8, 10) with bovine serum albumin

It is important to study the interaction of ionic liquids (ILs) with protein for the applications of ILs in biochemical process, and help the researchers to choose and design the better ILs to serve as a solvent. In this work, the interaction between 1-alkyl-3-methylimidazolium bromide [C_nmim]Br (n=4, 6, 8, 10) and bovine serum albumin (BSA) was systematically investigated for the first time by multi-spectroscopic approach (fluorescence, UV–vis and FT-IR spectroscopy) and density functional theory (DFT). [C_nmim]Br (n=4, 6, 8, 10) can bind to BSA by H-bond interaction between their cationic headgroups and Asp/Glu amino acid residue at the surface of BSA, and hydrophobic interaction between their hydrocarbon chains and the hydrophobic amino acid residues in the interior of BSA. On the basis of thermodynamic parameters and the similar structure of [C_nmim]Br (n=4, 6, 8, 10), it can be inferred that the hydrophobic interaction plays a major role in the interaction of [C₁₀mim]Br with BSA, while the hydrogen bond and van der Waals force play a major role in the interaction of [C_nmim]Br (n=4, 6, 8) with BSA. Synchronous fluorescence and FT-IR spectra indicate that [C₁₀mim]Br could markedly change the secondary structure of BSA, while [C_nmim]Br (n=4, 6, 8) could slightly change the secondary structure of BSA. The results allowed us to understand (i) the effect of the alkyl chain length of the cation on the mechanism of ILs–protein interaction and (ii) the effect of the alkyl chain length of the cation on the protein secondary structure.     

Predicting the viscosity and electrical conductivity of ionic liquids on the basis of theoretically calculated ionic volumes

Selected physical properties of the ionic liquids might be quantitatively predicted based on the volumes of the ions these systems are composed of. It is demonstrated that the ionic volumes calculated using relatively simple theoretical quantum chemistry methods can be utilised to estimate the viscosities and electrical conductivities of various commonly used ionic liquids. The fitting formulas of the exponential form are offered and their predictive usefulness is verified. The quality of such predictions is discussed on the basis of several ionic liquids involving [Tf₂N]^? and [BF₄]^? anions and 16 various cations. The dependence of the viscosity and electrical conductivity of the ionic liquids on the temperature is also investigated and the temperature-dependent equations are derived and compared to the experimentally measured values.     

温度对吡啶离子液体性质影响的分子动力学模拟研究

运用分子动力学模拟方法研究了温度对三种吡啶离子液体[BPy][BF_4]、[HPy][BF_4]、[OPy][BF_4]热力学性质的影响,得到了每个体系的密度、自扩散系数、电导率和黏度等.研究结果表明:随着温度升高,同一种离子液体的密度减小,阴阳离子的自扩散系数明显增大,电导率升高,而黏度降低.在同一温度下,随着阳离子上烷基链的增长,离子液体的密度减小,但热力学性质的变化规律并不完全同步.烷基链长最短的[BPy][BF_4]的自扩散系数和电导率在每个温度下均为最大,而黏度最小;但烷基碳链更长的[OPy][BF_4]和[HPy][BF_4]的各种性质相差不大,甚至当温度大于323 K时,烷基链较长的[OPy][BF_4]的自扩散系数比[HPy][BF_4]的大.     

温度对吡啶离子液体性质影响的分子动力学模拟研究

运用分子动力学模拟方法研究了温度对三种吡啶离子液体[BPy][BF4]、[HPy][BF4]、[OPy][BF4]热力学性质的影响, 得到了每个体系的密度、自扩散系数、电导率和黏度等. 研究结果表明: 随着温度升高, 同一种离子液体的密度减小, 阴阳离子的自扩散系数明显增大, 电导率升高, 而黏度降低. 在同一温度下, 随着阳离子上烷基链的增长, 离子液体的密度减小, 但热力学性质的变化规律并不完全同步. 烷基链长最短的[BPy][BF4]的自扩散系数和电导率在每个温度下均为最大, 而黏度最小; 但烷基碳链更长的[OPy][BF4]和[HPy][BF4]的各种性质相差不大,甚至当温度大于323K时, 烷基链较长的[OPy][BF4]的自扩散系数比[HPy][BF4]的大.     

Surface segregation in binary mixtures of imidazolium-based ionic liquids

Surface composition of binary mixtures of room-temperature ionic liquids has been investigated using time-of-flight secondary ion mass spectrometry at room temperature over a wide composition range. The imidazolium cations with longer aliphatic groups tend to segregate to the surface, and a bis(trifluoromethanesulfonyl)imide anion (Tf₂N^?) is enriched at the surface relative to hexafluorophosphate (PF₆^?). The surface of an equimolar mixture of Li[Tf₂N] and 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF₆]) has a nominal composition of [bmim][Tf₂N] because of surface segregation and ligand exchange. The surface segregation of cations and anions is likely to result from alignment of specific ligand-exchanged molecules at the topmost surface layer to exclude more hydrophobic part of the molecules.     

Study of alkyl chain length dependent characteristics of imidazolium based ionic liquids [CnMIM]+[TFSA]− by Brillouin and dielectric loss spectroscopy

This study examined the acoustic phonon mode of ionic liquids consisting of 1-alkyl-3-methyl-imidazolium family (C_nMIM) cations with n values ranging from 2 to 10 and bis(trifluoromethylsulfonyl)amide (TFSA) anion in the temperature range from 300 K to 100 K. [C_nMIM]⁺[TFSA]^? showed depolarized (VH) components of Brillouin peaks at temperatures below the glass transition temperature when n is larger than 4. On the other hand, in the case of ionic liquids with different anions, such as [C₄MIM]⁺[BF₄]^?, [C₄MIM]⁺[PF₆]^? and [C₈MIM]⁺[BF₄]^?, the VH component of Brillouin peaks was not observed in the temperature range investigated. The dielectric loss spectra showed that the temperature dependence of alkyl chain domain relaxation of all ionic liquids followed the Arrhenius law and showed an increase in activation energy at the temperature where the VH component of Brillouin peak appeared. These results suggest that the observed depolarized component of Brillouin peak might originate from uniquely induced polarization in the 2nd domain composed of head groups of cations and anions.     

Properties of Cellulose Regenerated from Powerful 1-Butyl-3-methylimidazolium Acetate/Dimethyl Sulfoxide Solvent

1-butyl-3-methylimidazolium?acetate C₄mim][CH₃COO]/dimethyl sulfoxide/DMSO can effectively dissolve cellulose at ambient temperature without any heating. However, the thermostability, morphology, and structure of cellulose regenerated from the [C₄mim][CH₃COO]/DMSO solvent is little known. Therefore, in this work, the regenerated cellulose was prepared and characterized by scanning electron microscope (SEM), Fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA), and the degree of polymerization (DP) measurements. The results indicated that the regenerated cellulose film displayed homogeneous structures; no chemical reaction occurred between cellulose and [C₄mim][CH₃COO] as well as DMSO during the dissolution and precipitation processes. The regenerated cellulose had good thermal stability, similar to the original cellulose, and the macromolecular chains of cellulose were hardly broken during the dissolution and precipitation processes.     

The electrochemistry of arylated anthraquinones in room temperature ionic liquids

Arylated anthraquinone derivatives of different sizes and different π‐basicities have been prepared, and the electrochemical behaviour of these substances has been studied on screen printed graphite electrodes in the three room temperature ionic liquids (RTILs), 1‐butyl‐3‐methylimidazolium hexafluorophosphate ([C₄MIM][PF₆]), 1‐hexyl‐3‐methylimidazolium hexafluorophosphate ([C₆MIM][PF₆]) and 1‐octyl‐3‐methylimidazolium hexafluorophosphate ([C₈MIM][PF₆]). Half redox potentials for the first and second one electron reduction waves were identified, and the diffusion coefficient values were estimated from cyclic voltammetry measurements. The influence of the nature of the RTIL and of the substitution pattern of the anthraquinone on the solvodynamic radii were studied. A correlation of the reductive potentials with the corresponding Hammett constants of the substituents was tested. Copyright © 2013 John Wiley & Sons, Ltd.     

An efficient enzymatic modification of cordycepin in ionic liquids under ultrasonic irradiation

A comparative study of the immobilized Candida antarctica lipase B (Novozym 435)-catalyzed acylation of cordycepin with vinyl acetate in ionic liquids (ILs) under ultrasonic irradiation and shaking was conducted. The application of ultrasonic irradiation instead of shaking during acylation resulted in an enhanced reaction rate and a higher level of substrate conversion. Among the various ILs examined, 1-butyl-3-methylimidazolium tetrafluorobrate ([C₄MIm][BF₄]) was the best medium for the reaction because it produced the highest substrate conversion. In [C₄MIm][BF₄], the optimal ultrasonic power, water activity, and reaction temperature were 120 W, 0.33, and 50 °C, respectively. The acylation of cordycepin in [C₄MIm][BF₄] proved to be regioselective under both conditions: the C5′-OH was acylated. Novozym 435 exhibited a much higher operational stability in [C₄MIm][BF₄], and 58.0% of its original activity was maintained after ten reuse cycles under ultrasonic irradiation. Compared with the cordycepin, the rate of adenosine deaminase-catalyzed deamination was greatly reduced when the 5′-OH was substituted by acetyl group. These results demonstrated that the combined application of ultrasonic irradiation and IL as a medium was an efficient approach for the enzymatic modification of cordycepin.     

Thermodynamic properties of some ionic liquids using a simple equation of state

In this work, we have used a simple equation of state (EoS) to predict the density and other thermodynamic properties such as isobaric expansion coefficient, α_P , isothermal compressibility, κ_T, and internal pressure, P_i, for nine ionic liquids including trihexyl (tetradecyl) phosphonium chloride ([(C₆H₁₃)₃P(C₁₄H₂₉)][Cl]), trihexyl (tetradecyl) phosphonium acetate ([(C₆H₁₃)₃P(C₁₄H₂₉)][Ac]), trihexyl (tetradecyl) phosphonium bis {(trifluoromethyl) sulfonyl} amide ([(C₆H₁₃)₃P(C₁₄H₂₉)][NTf₂]), 1-butyl-3-methylimidazolium bis {(trifluoromethyl) sulfonyl} amide ([bmim][NTf₂]), 1-hexyl-3-methylimidazolium bis {(trifluoromethyl) sulfonyl} amide ([hmim][NTf₂]), 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF₄]), 1-butyl-3-octylimidazolium tetrafluoroborate ([omim][BF₄]), 1-butyl-3-octylimidazolium hexafluorophosphate ([omim][PF₆]), and 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF₆]) at different temperatures and pressures. A wide comparison with experimental and literature data has been made. The results show that this EoS can be used to reproduce and predict different thermodynamic properties of ionic liquids within experimental errors.     

Surface structures of equimolar mixtures of imidazolium-based ionic liquids using high-resolution Rutherford backscattering spectroscopy

Surface structures of equimolar mixtures of imidazolium-based ionic liquids (ILs) having a common cation (1-butyl-3-methylimidazolium ([C₄MIM]) or 1-hexyl-3-methylimidazolium ([C₆MIM])) and different anions (bis(trifluoromethanesulfonyl)imide ([TFSI]), hexafluorophosphate ([PF₆]) or chlorine) are studied using high-resolution Rutherford backscattering spectroscopy (HRBS). Both cations and anions have the same preferential orientations at the surface as in the pure ILs. In the mixture, the larger anion is located shallower than the smaller anion. The [TFSI] anion is slightly enriched at the surface relative to [PF₆] with coverage of ~ 60% for the equimolar mixtures of [C₄₍₆₎MIM] [TFSI] and [C₄₍₆₎MIM] [PF₆]. No surface segregation is observed for [C₆MIM] [TFSI]_0.5[Cl]_0.5 and [C₆MIM] [PF₆]_0.5[Cl]_0.5. These results are different from the recent TOF-SIMS measurement where very strong surface segregation of [TFSI] was concluded for the mixture of [C₄MIM] [TFSI] and [C₄MIM] [PF₆].     

Influence of ultrasound-assisted ionic liquid pretreatments on the functional properties of soy protein hydrolysates

In this work, the effect of dual-frequency ultrasound-assisted ionic liquids (ILs) pretreatment on the functional properties of soy protein isolate (SPI) hydrolysates was investigated. The degree of hydrolysis (DH) of SPI pretreated by ultrasound and [BMIM][PF₆] increased by 12.53% as compared to control (P < 0.05). More peptides with low molecular weight were obtained, providing support for the changes in DH. The trichloroacetic acid-nitrogen soluble index presented an increase, suggesting a better protein hydrolysate property. The increase in the calcium-binding activity showed the ultrasound-assisted ILs pretreatment could potentially improve bone health. The foaming capacity and stability of SPI hydrolysates pretreated by ultrasound-assisted [BMIM][PF₆] always increased remarkably as compared to ultrasound-assisted [BDMIM][Cl] pretreatment. However, the synergistic effect of ultrasound-assisted [BMIM][PF₆] on the emulsifying activity and antioxidant activities (DPPH and hydroxyl radical scavenging activity) was not as ideal as ultrasound-assisted [BDMIM][Cl] pretreatment, which may be affected by the structure of peptide. In conclusion, these results indicated the combination of dual-frequency ultrasound and ionic liquids would be a promising method to improve the functional properties of SPI hydrolysates and broaden the application scope of compound modification in proteolysis industry.     

Ionic association and conductance of ionic liquids in dichloromethane at temperatures from 278.15 to 303.15 K

The electrical conductances of very dilute solutions of the ionic liquids 1-ethyl-3-methylimidazolium tetrafluoroborate [emim][BF₄] and 1-butyl-3-methylimidazolium tetrafluoroborate [bmim][BF₄] in the low-permittivity solvent dichloromethane have been measured in the temperature range from 278.15 to 303.15 K at 5 K intervals. The data was analyzed assuming the possible presence of contact (CIP) and solvent-separated (SSIP) ion pairs in the solution on the basis of lcCM model to obtain ionic association constants, K _A, and the limiting molar conductivities, Λ_o, of these electrolytes. The examined ionic liquids are strongly associated in dichloromethane over the whole temperature range. From the temperature dependence of the limiting molar conductivities, the Eyring’s activation enthalpy of charge transport was determined. The thermodynamic functions such as Gibbs energy, entropy, and enthalpy of the process of ion pair formation were calculated from the temperature dependence of the association constants.     

离子液体阳离子在空气/[bmim][BF4]水溶液界面的取向和结构

采用和频振动光谱研究了空气/[bmim][BF₄]低浓度水溶液界面的取向结构. 研究发现,在体相浓度非常低时,丁基链具有较大的旁式扭曲,表明此时的取向比较无序;阳离子咪唑环则采取一个较小的取向角. 随着浓度的升高,阳离子咪唑环趋向平躺在界面. 由于链-链相互作用,此时丁基链的旁式扭曲也减小,说明界面分子的取向变得有序. 进一步研究发现,PPP和SPS光谱上甲基反对称的峰存在位移,表明界面丁基链上的甲基存在不同取向或具有不同的化学环境.结果有助于从微观层次理解水溶性离子液体和基于咪唑的表面活性剂在界面上的物理化学行为.     

Quasi-solid state polymer electrolytes for dye-sensitized solar cells: Effect of the electrolyte components variation on the triiodide ion diffusion properties and charge-transfer resistance at platinum electrode

Quasi-solid state polymer electrolytes have been prepared from poly(vinylidenefluoride-co-hexafluoropropylene) (PVDF-HFP) as gelator for 1-ethyl-3-methylimidazolium based ionic liquids (with anions like trifluoromethanesulfonate [EMIM][TfO], bis(trifluoromethanesulfonyl)imide [EMIM][Tf₂N]) and polyacrylonitrile (PAN) for gelation of 1-ethyl-3-methylimidazolium dicyanamide [EMIM][DCA] as well as I⁻/I₃⁻ as the redox couple. All electrolytes exhibit high ionic conductivity in the range of 10^− 3 S/cm. The effect of gelation, redox couple concentration, I⁻/I₃⁻ ratio, choice of cations and additives on the triiodide diffusion and charge-transfer resistance of the platinum/electrolyte interface (R_ct) were studied. The apparent diffusion coefficient of triiodide ion (D(I₃⁻)) at various iodide/triiodide ratios in liquid and gelified electrolytes has been calculated from measurements of the diffusion limited current (I_lim) in electrochemical cell resembling the set-up of a dye-sensitized solar cell. The charge-transfer resistance of the platinum/electrolyte interface as well as the capacitance of the electrical double layer (C_dl) have been calculated from impedance measurements. Electrolytes with reduced content of polymer (2.5 wt.%) were doped with Al₂O₃ particles of different sizes (50 nm, 300 nm, 1 μm). The dispersion of the particles proceeds by speedy stirring of the hot electrolyte and the addition of PAN provides a homogeneous suspension. The addition of Al₂O₃ particles causes a slight increase of the triiodide diffusion constants. Furthermore the suggested enhancement of the charge transfer rate shows a dependence on the size of the particles.