Synthesis,physical, and thermodynamic properties of 1-alkyl-cyanopyridinium bis{(trifluoromethyl)sulfonyl}imide ionic liquids

Synthesis of new ionic liquids (ILs) viz. 1-butyl-3-cyanopyridinium bis{(trifluoromethyl)sulfonyl}imide, [BCN³Py][NTf₂], 1-hexyl-3-cyanopyridinium bis{(trifluoromethyl)sulfonyl}imide, [HCN³Py][NTf₂], 1-hexyl-4-cyanopyridinium bis{(trifluoromethyl)sulfonyl}imide, [HCN⁴Py][NTf₂], and 1-octyl-3-cyanopyridinium bis{(trifluoromethyl)sulfonyl}imide, [OCN³Py][NTf₂] were performed. The specific basic characterization of new compounds by NMR spectra, elementary analysis, water content and glass transition temperature as well as melting temperature, enthalpy of fusion and decomposition of compounds TG/DTA determined by the differential scanning calorimetry, DSC is presented. The heat capacity was measured at three temperatures (298.15, 323.15, and 353.15) K and at pressure 0.1 MPa. The effect of temperature on the density and viscosity is reported over the temperature range from (293.15 to 363.15) K and at 0.1 MPa. The density and viscosity correlation for these systems was provided by an empirical polynomial. From the density–temperature dependence, the isothermal expansion coefficient (volume expansivity), α, was calculated. The surface tension of pure ionic liquids was measured at 0.1 MPa at five temperatures (298.15, 308.15, 318.15, 328.15, and 338.15) K. The surface thermodynamic functions such as surface entropy and enthalpy, critical temperatures according to the Eötvös and Guggenheim definition and the total surface energy of the ILs studied were derived from the temperature dependence of the surface tension values. The parachor and speed of sound for pure ionic liquids were described within a range of temperature from (298.15 to 338.15) K. A qualitative analysis on these quantities in terms of molecular interactions is reported.     

1-Alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ionic liquids as solvents in the separation of azeotropic mixtures

(Liquid + liquid) equilibrium data for the ionic liquids 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [EMim][NTf₂], 1-propyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [PMim][NTf₂], 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [BMim][NTf₂], and 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [HMim][NTf₂], mixed with ethanol and heptane were studied at T = 298.15 K and atmospheric pressure. The ability of these ionic liquids as solvents for the extraction of ethanol from heptane was evaluated in terms of selectivity and solute distribution ratio. Moreover, density and refractive index values over the miscible region for the ternary mixtures were also measured at T = 313.15 K. Finally, the experimental data were correlated with the Non Random Two Liquids (NRTL) and UNIversal QUAsi Chemical (UNIQUAC) thermodynamic models, and an exhaustive comparison with available literature data of the studied systems was carried out.     

Liquid–liquid miscibility and volumetric properties of aqueous solutions of ionic liquids as a function of temperature

The volumetric properties of seven {water + ionic liquid} binary mixtures have been studied as a function of temperature from (293 to 343) K. The phase behaviour of the systems was first investigated using a nephelometric method and excess molar volumes were calculated from densities measured using an Anton Paar densimeter and fitted using a Redlich–Kister type equation. Two ionic liquids fully miscible with water (1-butyl-3-methylimidazolium tetrafluoroborate ([C₁C₄Im][BF₄]) and 1-ethyl-3-methylimidazolium ethylsulfate ([C₁C₂Im][EtSO₄])) and five ionic liquids only partially miscible with water (1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C₁C₂Im][NTf₂]), 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C₁C₄Im][NTf₂]), 1-butyl-3-methylimidazolium hexafluorophosphate ([C₁C₄Im][PF₆]), 1-butyl-3-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([C₁C₄Pyrro][NTf₂]), and butyltrimethylammonium bis(trifluoromethylsulfonyl)imide ([N₄₁₁₁][NTf₂])) were chosen. Small excess volumes (less than 0.5 cm³ · mol^?1 at 298 K) are obtained compared with the molar volumes of the pure components (less than 0.3% of the molar volume of the pure ionic liquid). For all the considered systems, except for {[C₁C₂Im][EtSO₄] + water}, positive excess molar volumes were calculated. Finally, an increase of the non-ideality character is observed for all the systems as temperature increases.     

Ethanol extraction from its azeotropic mixture with hexane employing different ionic liquids as solvents

In this work, the ionic liquids 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [EMim][NTf₂], 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [BMim][NTf₂], 1-butyl-3-methylpyridinium bis(trifluoromethylsulfonyl)imide, [BMpy][NTf₂], 1-butyl-3-methylpyridinium trifluoromethanesulfonate, [BMpy][TfO], have been investigated for their use as solvents in extraction processes for the ethanol removal from its azeotropic mixture with hexane. Therefore, the experimental determination of the liquid + liquid equilibrium for the ternary systems {hexane (1) + ethanol (2) + [EMim][NTf₂] (3)}, {hexane (1) + ethanol (2) + [BMim][NTf₂] (3)}, {hexane (1) + ethanol (2) + [BMpy][NTf₂] (3)} and {hexane (1) + ethanol (2) + [BMpy][TfO] (3)} was carried out at T = 298.15 K and atmospheric pressure. Classical parameters such as selectivity and solute distribution ratio, derived from the tie-line data, were calculated and afterwards, the structural influence of the ionic liquids on the extraction process was analyzed. Finally, the experimental LLE data were correlated by means of the NRTL and UNIQUAC models.     

Thermodynamic properties of the N-butylisoquinolinium bis(trifluoromethylsulfonyl)imide

A new isoquinolinium ionic liquid (IL) has been synthesised as a continuation of our work with quinolinium-based ionic liquids (ILs). The work includes specific basic characterization of synthesized compounds: N-isobutylquinolinium bromide, [BiQuin][Br] and N-isobutylquinolinium bis{(trifluoromethyl)sulfonyl}imide [BiQuin][NTf₂] by NMR spectra, elementary analysis and water content. The basic thermal properties of the pure [BiQuin][NTf₂], i.e. melting and glass-transition temperatures, the enthalpy of fusion as well as heat capacity at glass transition have been measured using a differential scanning microcalorimetry technique (DSC). Densities and viscosities were determined as a function of temperature. The temperature-composition phase diagrams of 8 binary mixtures composed of organic solvent dissolved in the IL: {[BiQuin][NTf₂] + aromatic hydrocarbon (benzene, or toluene, or ethylbenzene, or n-propylbenzene), or an alcohol (1-butanol, or 1-hexanol, or 1-octanol, or 1-decanol)} were measured at ambient pressure. A dynamic method was used over a broad range of mole fraction and temperature from (270 to 320) K. For all the binary systems with benzene and alkylbenzenes, the eutectic diagrams were observed with an immiscibility gap in the liquid phase existing at low mole fraction of the IL with a very high upper critical solution temperature (UCST). For mixtures with alcohols, complete miscibility was observed for 1-butanol and also an immiscibility gap with UCST in the liquid phase for the remaining alcohols. The typical dependence was observed that with increasing chain length of an alcohol, the solubility decreases. The well-known NRTL equation was used to correlate experimental (solid + liquid), SLE and (liquid + liquid), LLE phase equilibrium data sets.     

Phase equilibria study of the binary systems (ionic liquid + thiophene): Desulphurization process

(Solid + liquid) equilibria (SLE) and (liquid + liquid) equilibria (LLE) for the binary systems: {ionic liquid (IL) N-butyl-4-methylpyridinium tosylate (p-toluenesulfonate) [BM⁴Py][TOS], or N-butyl-3-methylpyridinium tosylate [BM³Py][TOS], or N-hexyl-3-methylpyridinium tosylate [HM³Py][TOS], or N-butyl-4-methylpyridinium bis{(trifluoromethyl)sulfonyl}imide [BM⁴Py][NTf₂], or 1,4-dimethylpyridinium tosylate [M^1,4Py][TOS], or 2,4,6-collidine tosylate [M^2,4,6Py][TOS], or 1-ethyl-3-methylimidazolium thiocyanate [EMIM][SCN], or 1-butyl-3-methylimidazolium thiocyanate [BMIM][SCN], or 1-hexyl-3-methylimidazolium thiocyanate [HMIM][SCN], or triethylsulphonium bis(trifluoromethylsulfonyl)imide [Et₃S][NTf₂] + thiophene} have been determined at ambient pressure. A dynamic method was used over a broad range of mole fractions and temperatures from (270 to 390) K. In the case of systems (pyridinium IL, or sulphonium IL + thiophene) the mutual immiscibility with an upper critical solution temperature (UCST) was detected at the very narrow and low mole fraction of the IL. For the binary systems containing (imidazolium thiocyanate IL + thiophene), the mutual immiscibility with the lower critical solution temperature (LCST) was detected at the higher mole fraction range of the IL. The basic thermal properties of the pure ILs, i.e. melting and glass-transition temperatures as well as the enthalpy of fusion have been measured using a differential scanning microcalorimetry technique (DSC). The well-known NRTL equation has been used to correlate experimental SLE/LLE data sets.     

Solubilities of hydrofluorocarbons in ionic liquids: Experimental and modelling study

In this work, experimental data on the gas solubility of hydrofluorocarbons (CHF₃, CH₂F₂ and CH₃F) in four room-temperature ionic liquids (RTILs) were determined within the temperature range 288 K to 308 K and at atmospheric pressure. The RTILs used were 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide([C₂mim][NTf₂]), (trihexyl)tetradecyl-phosphoniumbis(trifluoro-methylsulfonyl)imide ([P_6,6,6,14][NTf₂]), and N-methyl-2-hydroxyethylammoniumpropionate ([m-2-HEA][Pr]) and pentanoate ([m-2-HEA][P]). Two modelling approaches, which we denote as predictive and correlative, were compared. In the former, the cubic plus association equation of state (CPA EoS) is used as a predictive model to estimate the solubilities using only pure components physical properties. In the latter, the regular-solution theory is the basis to build an empirical model whose parameters are obtained through least-squares fitting of experimental values.     

PVT properties for binary ionic liquids of 1-methyl-1-propylpiperidinium bis(trifluoromethylsulfonyl)imide with anisole or acetophenone at pressures up to 50 MPa

Densities of pure 1-methyl-1-propylpiperidinium bis(trifluoromethylsulfonyl)imide, [C₃mpip][NTf₂], and its mixtures with anisole or acetophenone were measured with a high-pressure densimeter at temperatures from 298.15 K to 348.15 K and pressures up to 50 MPa. The Tait equation was employed to represent pressure effect on the isothermal densities. The experimental results reveal that the excess volumes of (anisole + [C₃mpip][NTf₂]) and (acetophenone + [C₃mpip][NTf₂]) are all negative over the entire experimental conditions. In addition to an empirical generalized equation, the density data were also correlated quantitatively with the Flory–Orwoll–Vrij (FOV) and the Schotte equations of state.     

Isotope effects on miscibility of 1-alkyl-3-methylimidazolium bis(trifluoromethyl)sulfonyl imides with aromatic hydrocarbons

The liquid–liquid miscibility temperatures as a function of composition and deuterium substitution have been experimentally determined for the binary mixtures of 1-decyl-3-methylimidazolium bis(trifluoromethyl)sulfonyl imide, [C₁₀MIM][NTf₂], and 1-dodecyl-3-methylimidazolium bis(trifluoromethyl)sulfonyl imide, [C₁₂MIM][NTf₂] with benzene and toluene and their deuterated forms. All systems exhibit upper critical solution temperatures (UCSTs) and better miscibility for ionic liquids with longer alkyl chain. The interchange of benzene for toluene results in worse miscibility for [C₁₀MIM][NTf₂] and better for [C₁₂MIM][NTf₂]. Deuteration of the aromatics leads to better miscibility in each case. The sign of the isotope effect is in agreement with expectations and its value seems to be seriously affected by the neighborhood of the hypercritical state.     

Ultrasonic-electrodeposition of gold–platinum alloy nanoparticles on multi-walled carbon nanotubes – ionic liquid composite film and their electrocatalysis towards the oxidation of nitrite

We report here for the first time on the electrochemical co-deposition of gold–platinum (AuPt) nanoparticles on multi-walled carbon nanotubes (MWNT) – ionic liquid (i.e. trihexyltetradecylphosphonium bis(trifluoromethylsulfonyl)imide, [P_6,6,6,14][NTf₂]) composite surface in ultrasonic field. The obtained AuPt nanoparticles were characterized by scanning electron microscope, X-ray diffraction and X-ray photoelectron spectroscopy. It was found that they were well-dispersed on the composite surface, with particle size about 10 nm. Furthermore, they exhibited alloy features. Electrochemical impedance spectroscopy and voltammetric experiments showed that the resulted AuPt-MWNT-[P_6,6,6,14][NTf₂] modified electrode possessed rather small electron transfer resistance and good catalytic activity towards nitrite oxidation. Under the optimized conditions, the oxidation current of nitrite was linear to its concentration in the range of 5–200 nM and the detection limit was down to 1.0 nM (S/N = 3).     

Effect of the temperature on the physical properties of pure 1-propyl 3-methylimidazolium bis(trifluoromethylsulfonyl)imide and characterization of its binary mixtures with alcohols

In this paper, physical properties of a high purity sample of the ionic liquid 1-propyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [PMim][NTf₂], and its binary mixtures with methanol, ethanol, 1-propanol, and 2-propanol were measured at atmospheric pressure. The temperature dependence of density, refractive index and speed of sound (293.15 to 343.15) K and dynamic viscosity (298.15 to 343.15) K were studied at atmospheric pressure by conventional techniques for the pure ionic liquid. For its mixtures with alcohols, density, speed of sound, and refractive index were measured at T = 298.15 K over the whole composition range. The thermal expansion coefficient of the [PMim][NTf₂] was calculated from the experimental results using an empirical equation, and values of the excess molar volume, excess refractive index, and excess molar isentropic compressibility for the binary systems at the above mentioned temperature, were calculated and fitted to the Redlich–Kister equation. The heat capacity of the pure ionic liquid at T = 298.15 K was determined using DSC.     

Solubility,volumetric properties and viscosity of the sustainable systems of liquid poly(ethylene glycol) 200 with imidazolium- and phosphonium-based ionic liquids: Cation and anion effects

In this work, solubility, volumetric and viscosity behavior were studied for the systems containing the environmentally acceptable compounds: liquid poly(ethylene glycol) (PEG200) and three ionic liquids: 1-butyl-3-methylimidazolium dicyanamide ([C₄mim][dca]), trihexyltetradecyl phosphonium dicyanamide ([P_6,6,6,14][dca]) and 1-hexyl-3-methylimidazolium bis{(trifluoromethyl)sulfonyl}amide ([C₆mim][NTf₂]). The studies were performed in a temperature range (288.15 to 328.15) K and at a pressure of 0.1 MPa. For the only system that evidenced limited miscibility, namely (PEG200 + [P_6,6,6,14][dca]), the temperature-composition phase diagram at 0.1 MPa was determined, mapping the existing one- and two-phase regions. In the homogeneous region of this diagram, densities and viscosities were measured and the excess molar volumes, as well as deviations in viscosity were calculated. For the other two systems, as they are always homogeneous in the temperature ranges of the present work, these measurements and calculations were performed in the full range of compositions. The molecular interactions in the studied systems were scrutinized using the obtained excess molar volumes, deviations of viscosity, as well as Kamlet–Taft parameters of PEG200 and the ionic liquids. In addition, the excess molar Gibbs free energies of activation of viscous flow and the related enthalpies and entropies were calculated and introduced to take into consideration the differences in size of the molecules.     

Densities and refractive indices of imidazolium- and phosphonium-based ionic liquids: Effect of temperature,alkyl chain length,and anion

A systematic study of densities and refractive indices of 17 room temperature ionic liquids is presented at four different temperatures ranging from 293 K to 333 K. The ionic liquids are grouped into four families: 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide, [C_nmim][Ntf₂], ionic liquids (with n = 2, 4, 6, 8, 10, 12, and 14); 1-alkyl-3-methylimidazolium hexafluorophosphate, [C_nmim][PF₆], ionic liquids (with n = 4, 6, 8); ionic liquids based on the trihexyl(tetradecyl)phosphonium cation, [P_{6 6 6 14}], combined with the anions bis(trifluoromethylsulfonyl)amide, [Ntf₂], acetate, [OAc], and triflate, [OTf]; and [C₄mim]-based ionic liquids combined with the anions [OAc], [OTf], methylsulfate [MeSO₄], and tetrafluoroborate [BF₄]. The data obtained were analysed to determine the effect of (i) temperature, (ii) the alkyl chain length of the 1-alkyl-3-methylimidazolium cation, and (iii) the nature of the anion. Different empirical models for the calculation of the densities of the ionic liquids were tested. Molar refractions were also calculated from the volumetric and refractive index data and the values were discussed with the aim of checking their utility in obtaining insights on the intermolecular forces and behaviour in solution of the different ionic liquids.     

Pressure–volume–temperature measurements of phosphonium-based ionic liquids and analysis with simple equations of state

In spite of the great importance of the (P, V, T) data of phosphonium–based ionic liquids, only limited information on these data seems to be available in the open literature. In this work, we present the results for the density measurements of the trihexyltetradecylphosphonium chloride, [(C₆H₁₃)₃P(C₁₄H₂₉)][Cl] and trihexyltetradecylphosphonium dicyanamide, [(C₆H₁₃)₃P(C₁₄H₂₉)][N(CN₂)] with an estimated uncertainty of ±0.5 kg · m^?3. The ranges of temperature and pressure are T = (273.15 to 318.15) K and p = (0.1 to 25) MPa for [(C₆H₁₃)₃P(C₁₄H₂₉)][Cl] and T = (273.15 to 318.15) K and p = (0.1 to 35) MPa for [(C₆H₁₃)₃P(C₁₄H₂₉)][N(CN₂)]. The high consistency of our data for [(C₆H₁₃)₃P(C₁₄H₂₉)][Cl] compared with those measured by other authors allowed all the experimental data for this IL to be combined and correlated using the Goharshadi–Morsali–Abbaspour equation of state over a wide range of temperature and pressure. From this equation, thermomechanical coefficients as the isothermal compressibility, thermal expansivity, thermal pressure, and internal pressure were calculated for the two ILs. The Sanchez–Lacombe equation of state was used also for (P, V, T) correlation and the estimation of the free volume in these phosphonium ionic liquids. Finally ionic volumes for trihexyltetradecylphosphonium cation and several anions available in the literature made possible the calculation of the free (hole) volume.     

(Liquid + liquid) equilibria for the ternary mixtures (alkane + toluene + ionic liquid) at T = 298.15 K: Influence of the anion on the phase equilibria

(Liquid + liquid) equilibrium data for the ionic liquids 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide, [BMpyr][NTf₂], and 1-butyl-1-methylpyrrolidinium trifluoromethanesulfonate, [BMpyr][TFO], with toluene, and heptane or cyclohexane were determined at T = 298.15 K and atmospheric pressure. In order to check if these ILs can be used as potential solvents for the extraction of toluene from aliphatic compounds, the ability of the ILs as solvents was evaluated in terms of selectivity and solute distribution ratio. The experimental data were correlated accurately with the Non Random Two-Liquid model.     

Thermodynamic properties and polymorphism of 1-alkyl-3-methylimidazolium bis(triflamides)

Heat capacities in a range of temperatures of (5 to 370) K, enthalpies and temperatures of phase transitions for 1-ethyl-3-methylimidazolium bis(triflamide) ([C₂mim][NTf₂]) and 1-octyl-3-methylimidazolium bis(triflamide) ([C₈mim][NTf₂]) have been determined by adiabatic calorimetry. [C₂mim][NTf₂] has been found to form four crystalline phases with different fusion temperatures. Formation of the phases can be controlled by the temperature of annealing during crystallization. [C₈mim][NTf₂] forms three sequences of crystalline modifications, each including two polymorphs. Based on results of the measurements, thermodynamic functions for the compounds under study have been calculated.A heat-capacity anomaly near T = 230 K reported earlier for [C₄mim][NTf₂] and [C₆mim][NTf₂] have been found in some crystalline modifications of both the studied compounds. The position of the anomaly depends on the temperature of annealing of the crystals.     

Highly mesoporous and high surface area carbon: A high capacitance electrode material for EDLCs with various electrolytes

Activated carbon fibers (ACFs) with high surface area and highly mesoporous structure for electrochemical double layer capacitors (EDLCs) have been prepared from polyacrylonitrile fibers by NaOH activation. Their unique microstructural features enable the ACFs to present outstanding high specific capacitance in aqueous, non-aqueous and novel ionic liquid electrolytes, i.e. 371 F g⁻¹ in 6 mol L⁻¹ KOH, 213 F g⁻¹ in 1 mol L⁻¹ LiClO₄/PC and 188 F g⁻¹ in ionic liquid composed of lithium bis(trifluoromethane sulfonyl)imide (LiN(SO₂CF₃)₂, LiTFSI) and 2-oxazolidinone (C₃H₅NO₂, OZO), suggesting that the ACF is a promising electrode material for high performance EDLCs.     

Thermodynamic properties of the N-octylquinolinium bis{(trifluoromethyl)sulfonyl}imide

This work is a continuation of our wide ranging investigation on quinolinium based ionic liquids (ILs). The study includes specific basic characterisation of the synthesized compounds N-octylquinolinium bromide, [OQuin][Br] and N-octylquinolinium bis{(trifluoromethyl)sulfonyl}imide [OQuin][NTf₂] by NMR spectra, elementary analysis and water content. Differential scanning calorimetry (DSC) measurements gave us properties of the pure [OQuin][NTf₂] i.e. melting and glass-transition temperatures, the enthalpy of fusion as well as heat capacity at the glass transition. Densities and viscosities were determined as a function of temperature. The temperature-composition phase diagrams of 10 binary mixtures composed of organic solvent dissolved in the IL: {[OQuin][NTf₂] + aromatic hydrocarbon (benzene, or thiophene, or toluene, or ethylbenzene, or n-propylbenzene), or an alcohol (1-butanol, or 1-hexanol, or 1-octanol, or 1-decanol, or 1-dodecanol)} were measured at ambient pressure. A dynamic method was used over a broad range of mole fractions and temperatures from (250 to 370) K. For mixtures with benzene and alkylbenzenes, the immiscibility gap in the liquid phase in a low mole fraction of the IL was observed with upper critical solution temperature (UCST) higher than the boiling point of the solvent. In the system with thiophene, the immiscibility gap is lower and UCST was measured. For binary mixtures with alcohols, complete miscibility in the liquid phase was observed for 1-butanol and 1-hexanol. In the systems with longer chain alcohols, the immiscibility gap with UCST was noted. Typical behaviour for ILs was observed with an increase of the chain length of an alcohol the solubility decreases. The well-known NRTL equation was used to correlate experimental (solid + liquid), SLE and (liquid + liquid), LLE phase equilibrium data sets.     

Radiation-induced darkening of ionic liquid [C4mim][NTf2] and its decoloration

The radiation effect on a hydrophobic room-temperature ionic liquid (RTIL), 1-butyl-3-methyl-imidazolium bis[(trifluoromethyl)sulfonyl]imide ([C₄mim][NTf₂]), was studied by γ-irradiation under nitrogen atmosphere. Accompanied by color darkening and increase of light absorbance in a wide wavelength range, a distinct absorption peak at around 290 nm for irradiated [C₄mim][NTf₂] appeared when acetonitrile was used as solvent, and the intensity of the peak enhanced with increasing dose. The spectrophotometric study on the irradiated RTILs containing 1,3-dialkylimidazolium cations associated with different inorganic anions revealed that the peak is ascribed to the radiolysis products of the [C₄mim]⁺. And the wavelength of the peak was affected by alkyl chain length on imidazolium cation, while the intensity of the peak was influenced by anions. With incorporating a little amounts of oxidants, such as KMnO₄ and HNO₃ into irradiated [C₄mim][NTf₂], the intensity of the peak at 290 nm decreased obviously and the decoloration of [C₄mim][NTf₂] occurred, suggesting that the peak at 290 nm is assigned to the colored species and the species can be oxidized.