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.     

Temperature-Dependent Surface Enrichment Effects in Binary Mixtures of Fluorinated and Non-Fluorinated Ionic Liquids

Using angle-resolved X-ray photoelectron spectroscopy (ARXPS), we investigate the topmost nanometers of various binary ionic liquid (IL) mixtures at different temperatures in the liquid state. The mixtures consist of ILs with the same [PF₆]⁻ anion but two different cations, namely 3-methyl-1-(3,3,4,4,4-pentafluorobutyl)imidazolium hexafluorophosphate, [PFBMIm][PF₆], and 1-butyl-3-methylimidazolium hexafluorophosphate, [C₄C₁Im][PF₆], with 10, 25, 50 and 75 mol % content of [PFBMIm][PF₆]. We observe a preferential enrichment of the fluorinated chain in the topmost layer, relative to the bulk composition, which is most pronounced for the lowest content of [PFBMIm][PF₆]. Upon cooling the mixtures stepwise from 95 °C until surface charging effects in XPS indicate solidification, we observe a pronounced increase in surface enrichment of the fluorinated chain with decreasing temperature in the liquid state. In contrast to the mixtures with lower [PFBMIm][PF₆] contents, cooling the 75 mol % mixture additionally shows an abrupt decrease of the fluorinated chain signal before complete solidification occurs, which is assigned to partial precipitation effects.     

Influence of Substituents and Functional Groups on the Surface Composition of Ionic Liquids

We have performed a systematic study addressing the surface behavior of a variety of functionalized and non‐functionalized ionic liquids (ILs). From angle‐resolved X‐ray photoelectron spectroscopy, detailed conclusions on the surface enrichment of the functional groups and the molecular orientation of the cations and anions is derived. The systems include imidazolium‐based ILs methylated at the C2 position, a phenyl‐functionalized IL, an alkoxysilane‐functionalized IL, halo‐functionalized ILs, thioether‐functionalized ILs, and amine‐functionalized ILs. The results are compared with the results for corresponding non‐functionalized ILs where available. Generally, enrichment of the functional group at the surface is only observed for systems that have very weak interaction between the functional group and the ionic head groups.     

Surface tension of four binary systems containing (1-ethyl-3-methyl imidazolium alkyl sulphate ionic liquid + water or + ethanol)

In this work, we present surface tension experimental measurements for eight binary systems containing water or ethanol and an ionic liquid (IL) of the 1-ethyl-3-methyl imidazolium alkyl sulphate family, being the alkyl chain of the anion: ethyl, butyl, hexyl and octyl. Measurements were performed at the temperature of 25.0 °C and atmospheric pressure. All four ILs are completely miscible with water and ethanol, but for a concentration range of the octyl sulphate IL aqueous system the mixture jellifies, and so it is not possible to measure its surface tension. These measurements allow us to study the influence of the anion size on the surface tension for the pure IL compounds, and the role of the two different solvents in the surface tension behaviour. Thus, we observe that it is completely different when mixed with water or with ethanol, as also happens in other mixtures with different ionic liquids. From the experimental data, we extract surface tension deviations using the most popular definition. The calculated deviations for the ethanol based system are fitted using the Redlich–Kister equation and a novel one previously reported by us. Furthermore, we have also calculated the reduced surface pressure for the aqueous mixtures, which is fitted with good agreement using a theoretical equation obtained from the Bahe–Varela pseudo-lattice model.     

Angle-resolved X-ray photoelectron spectroscopy of the surface of imidazolium ionic liquids

The surfaces of three imidazolium based ionic liquids with a common anion were studied with angle-resolved X-ray photoelectron spectroscopy (XPS). The room temperature ionic liquids (RTILs): 1-butyl-3-methylimidazolium (bmim), 1-hexyl-3-methylimidazolium (hmim), and 1-octyl-3-methylimidazolium (omim) tetrafluoroborates, were meticulously purified and dried under vacuum. Survey and high-resolution spectra were obtained at different take-off angles (0-84 degrees ), thus increasing the surface sensitivity of the measurement. No impurities were detected and the survey spectra at normal emission (0 degrees ) confirmed the stoichiometric composition of the liquids. However, the spectra at take-off angles of 60, 70, 80 and 84 degrees indicated a higher amount of carbon. High resolution spectra of C1s, at these angles, showed an increased amount of aliphatic carbon when compared to the spectra at normal emission. The longer the side chain (R) of the imidazolium cation (Rmim), the larger was the amount of aliphatic carbon detected. Previous studies with other surface sensitive techniques have yielded contradictory conclusions about the surface orientation of the Rmim. We conclude unequivocally that the alkyl chain of the imidazolium ring of the investigated RTILs is oriented away from the liquid. Our study demonstrates the ability of XPS to probe the structure, along with the composition, of the free liquid surface by comparing signals from different penetration depths.     

Electrodeposition of silicon from three different ionic liquids: possible influence of the anion on the deposition process

The electrodeposition of silicon was investigated from three different ionic liquids with the cation 1-butyl-1-methylpyrrolidinium ([Py_1,4]⁺) and three different anions, namely, trifluoromethylsulfonate (TfO^?), bis(trifluoromethylsulfonyl)amide (TFSA^?) and tris(pentafluoroethyl)-trifluorophosphate (FAP^?) at room temperature and at 100 °C, respectively. The electrodeposition was performed on gold and on copper substrates. Cyclic voltammetry was used to evaluate the possible influence of anions on the deposition process. In situ STM studies were also carried out to examine the interfacial behaviour of the SiCl₄/[Py_1,4]TFSA and SiCl₄/[Py_1,4]FAP on Au(111) at room temperature. In situ STM measurements revealed that an underpotential deposition of Si in [Py_1,4]FAP occurred on Au (111) at ~ -0.5 V (vs. Fc/Fc⁺). In comparison, only adsorption of ionic liquid and gold surface reconstruction was found to occur in the potential regime between -0.3 and ?1.8 V (vs. Fc/Fc⁺), respectively, in the case of [Py_1,4]TFSA. In situ STM investigations reveal an effect of the anion on the interfacial processes. In situ I/U tunnelling spectroscopy shows that the band gap of the electrodeposits is ~1.1 eV, indicating that semiconducting silicon has been electrodeposited. Potentiostatic electrolysis was performed to deposit Si from the employed electrolytes at room temperature and at 100 °C. The deposits were characterised using scanning electron microscopy and X-ray diffraction. Thin films of Si could be obtained from the employed ionic liquids and the quality of the deposits was significantly improved at 100 °C.     

Unexpected decomposition of the bis (trifluoromethylsulfonyl) amide anion during electrochemical copper oxidation in an ionic liquid

In this letter we report on the decomposition of the bis (trifluoromethylsulfonyl) amide (TFSA) anion under quite mild electrochemical conditions. The results show clearly that the TFSA anion can easily be decomposed during anodic oxidation of copper in the ionic liquid 1-butyl-1-methylpyrrolidinium bis (trifluoromethylsulfonyl) amide [BMP] TFSA at 70 °C leading to the formation of CuF₂. At room temperature, however, no significant decomposition was obtained. Therefore, one has to be very careful in applying ionic liquids based on TFSA anions under anodic conditions at elevated temperature as the TFSA anion might decompose, depending on the anode material.     

Influence of the anion on the surface tension of 1-ethyl-3-methylimidazolium-based ionic liquids

The air–liquid interfacial tensions of eight ionic liquids, from (298 to 343) K, are presented in this work. The studied ionic liquids are formed by the fixed 1-ethyl-3-methylimidazolium cation combined with the anions acetate, dicyanamide, dimethylphosphate, methylphosphonate, methanesulfonate, thiocyanate, tosylate, and trifluoromethanesulfonate. The selected ionic liquids allowed a comprehensive study through the influence of the anion nature on the surface tension and on their surface ordering. A slight dependence of the surface tension with the ionic liquid molar volume was identified. The surface thermodynamic functions are mainly controlled by the anion which constitutes a given ionic liquid. The hypothetical critical temperatures of all ionic liquids were estimated by means of the Eötvos and Guggenheim correlations and are presented.     

Comparison Between Two Different Synthetic Routes of Pyrrolidinium Functional Polymeric Ionic Liquids

Summary: Two separate synthetic routes of preparation of new pyrrolidinium-based polymeric ionic liquids were compared. The first method utilized the modification of a commercial poly(diallyldimethylammonium) chloride via a direct anion exchange with several anions. The second route involved the anion exchange leading to several diallyldimethylammonium ionic liquid monomers having different anions and a subsequent polymerization. The latter method is particularly interesting for attaining tunable copolymers according to the desired properties of PILs. Both methods successfully resulted in ionic liquid polymers that are finding their way in a wide range of emerging applications.     

Surface characterization of functionalized imidazolium-based ionic liquids

The surface composition of oligo(ethylene glycol) ether functionalized bis(trifluoromethylsulfonyl)imide ionic liquids has been studied by means of X-ray photoelectron spectroscopy (XPS). For [Me(EG)MIM][Tf 2N], [Et(EG) 2MIM][Tf 2N], and [Me(EG) 3MIM][Tf 2N], which vary by the number of ethylene glycol (EG) units (from 1 to 3), we have shown that the stoichiometry of the surface near region is in excellent agreement with the bulk stoichiometry, which confirms the high purity of the ionic liquid samples investigated and rules out pronounced surface orientation effects. This has been deduced from the experimental observation that the angle-resolved XP spectra of all elements present in the IL anions and cations (C, N, O, F, S) show identical signals in the bulk and surfaces sensitive geometry, i.e., at 0 degrees and 70 degrees emission angle, respectively. The relative intensity ratios of all elements were found to be in nearly perfect agreement with the nominal values for the individual ILs. In contrast to these findings, we identified surface-active impurities in [Me(EG)MIM]I, which is the starting material for the final anion exchange step to synthesize [Me(EG)MIM][Tf 2N]. Sputtering of the surface led to a depletion of this layer, which however recovered with time. The buildup of this contamination is attributed to a surface enrichment of a minor bulk contamination that shows surface activity in the iodide melt.     

Thermal, rheological, and ion-transport properties of phosphonium-based ionic liquids

Phosphonium-based ionic liquids with varying counteranions from commercially available ionic liquid precursors enabled tunable viscosity, ionic conductivity, and thermal stability. Thermogravimetric analysis revealed a relationship between thermal stability and anion composition where anions with lower basicity remained stable to higher temperatures. Determination of glass transition temperatures and melting temperatures using differential scanning calorimetry revealed supercooling, crystallization, and dependence on anion composition. Rheological and ionic conductivity measurements determined the temperature-dependence of the viscosity and ionic conductivity of the phosphonium-based ionic liquids. Arrhenius analyses of conductivity and viscosity provided activation energies, which showed a decrease toward larger, more delocalized anions. An assessment according to the Walden plot displayed their efficacy relative to other ionic liquids.     

Significant changes of the charge distribution at the surface of an ionic liquid due to the presence of small amounts of water

The influence of small amounts of water dissolved in 1-hexyl-3-methylimidazolium chloride ([C(6)mim][Cl]) on the composition of the surface of the ionic liquid is investigated with the depth profiling technique neutral impact collision ion scattering spectroscopy. The concentration depth profiles of the elements in the sample were determined at three different water concentrations and show that small amounts of water affect the charge distribution in the ionic liquid along the surface normal. At low water concentrations (2500 ppm) the cation shows a strong presence at the surface with the alkyl chains oriented towards the gas phase, followed by a layer of anions below the alkyl chains of the cation. At higher water content (6000 to 10,000 ppm) the chloride anion shows an increased concentration at the ionic liquid surface while the alkyl chains move towards the bulk showing that the surface charge becomes more negative with increasing water content. The effect is attributed to the influence of water on the hydrogen bonding network in the ionic liquid.     

Surface tension of concentrated cellulose solutions in 1-ethyl-3-methylimidazolium acetate

Four sources of cellulose with different molecular weights were dissolved in the ionic liquid 1-ethyl-3-methylimidazolium acetate at 100 °C over a 10 h period. The solution densities were determined and these results were subsequently utilised to access the influence of dissolved cellulose on surface tension properties of cellulose/ionic liquid solutions. Surface tension measurements revealed increasing molecular weight and concentration reduced surface tension while temperature increases showed the opposite effect. These results are consistent with that of repulsive polymer-wall interactions near the interface in good solvent conditions. The semi-flexible nature of this carbohydrate in solution can help explain deviations of these results when compared to ideal flexible chains.     

Extraction of Polyoxometallate Anions Containing Tungsten Towards Ionic Liquids

Removal of tungstate \(\left( {{{\text{WO}}_{4}^{2-}}} \right)\) and metatungstate \(\left( {{{\text{W}}_{12} {\text{O}}_{39}^{6 - }} } \right)\) anions from aqueous solutions was studied by precipitation and liquid–liquid extraction using ionic liquids and without requiring any additional extraction agent. Hydrophilic ionic liquids were found to be very efficient at precipitating metatungstate anion and not to precipitate tungstate anion. Similarly, the large metatungstate anion was quantitatively removed from water at low pH towards a hydrophobic ionic liquid. Tungstate anions, in contrast, are not removed from water. The mechanism of extraction and influence of the charge density of anions on the extraction of W(VI) based anions are discussed.     

Bioelectrocatalysis in ionic liquids. Examining specific cation and anion effects on electrode-immobilized cytochrome c

Cytochrome c immobilized on alkylthiol self-assembled monolayers exhibits a characteristic Fe(II)/Fe(III) redox signal that is lost when exposed to ionic liquids composed of a butylimidazolium cation combined with either hexafluorophosphate or bis(trifluoromethylsulfonyl)imide anion. In this study it was shown that exposure to the aqueous solubilized ionic liquid components, butyl-, hexyl-, and octyl-imidazolium cations and hexafluorophosphate, tetrafluoroborate, and bis(trifluoromethylsulfonyl)imide anions, resulted in partial electrochemical signal loss. Absorbance and fluorescence measurements showed that signal loss due to the cationic ionic liquid component followed a different mechanism than that of the anionic component. Although a portion of the signal was recoverable, irreversible signal loss also occurred in both cases. The source of the irreversible component is suggested to be the loss of protein secondary structure through complexation between the ionic liquid components and the protein surface residues. The reversible electrochemical signal loss is likely due to interfacial interactions imposed between the electrode and the cytochrome heme group. The influence of the amount of exposed surface residues was explored with a simplified model protein, microperoxidase-11.     

Concentration Dependence of Ion Pairing in Imidazolium-Based Ionic Liquid Solutions

Infrared vibrational spectroscopy was used to probe concentration-dependent ion pair dissociation of imidazolium-based ionic liquids with three different halide anions (I⁻, Br⁻, and Cl⁻) in deuterated chloroform. Dissociation of the ion pairs at low concentrations of ionic liquids was found to be the easiest for ionic liquid with Cl⁻ anion, the most electronegative anion among the three investigated. This anomalous trend of ion pair dissociation was explained in terms of varying interaction strength between the solvent (CDCl₃) and the anions investigated.     

Probing the neutral graphene-ionic liquid interface: insights from molecular dynamics simulations

We study basic mechanisms of the interfacial layer formation at the neutral graphite monolayer (graphene)-ionic liquid (1,3-dimethylimidazolium chloride, [dmim][Cl]) interface by fully atomistic molecular dynamics simulations. We probe the interface area by a spherical probe varying the charge (-1e, 0, +1e) as well as the size of the probe (diameter 0.50 nm and 0.38 nm). The molecular modelling results suggest that: there is a significant enrichment of ionic liquid cations at the surface. This cationic layer attracts Cl(-) anions that leads to the formation of several distinct ionic liquid layers at the surface. There is strong asymmetry in cationic/anionic probe interactions with the graphene wall due to the preferential adsorption of the ionic liquid cations at the graphene surface. The high density of ionic liquid cations at the interface adds an additional high energy barrier for the cationic probe to come to the wall compared to the anionic probe. Qualitatively the results from probes with diameter 0.50 nm and 0.38 nm are similar although the smaller probe can approach closer to the wall. We discuss the simulation results in light of available experimental data on the interfacial structure in ionic liquids.     

四甲基铵Roussin红盐(Me_4N)_2-〔Fe_2S_2(NO)_4〕的分子和晶体结构

四甲基铵Roussin红盐晶体属于单斜晶系,空间群为C_(2h)~5—P2_1/a,晶胞参数为:a=14.804,b=14.086,c=9.382,13=92.2°和Do=1.519g/cm~3,Z=4(Dc=1.508g/cm~3)。在Philips PW1100型四圆衍射仪上用MoKα射线收集了强度数据,共得I_0>2σ(I_0)的独立衍射点2681个。晶体结构用重原子法解出,所有非H原子的坐标和各向异性温度因子经最小二乘对角矩阵修正,最后偏离因子R=O.065。晶胞中4个(Me_4N)_2Fe_2S_2(NO)_4分子分布于两套独立坐标上。阴离子形成双硫桥联的双核Fe络离子。在Fe<>Fe构成的“四元环”菱形平面上,∠S-Fe-S=105.6°,∠Fe-S-Fe=74.4°,Fe—S键长分别为2.247和2.242。此外还有两个NO基与Fe成端基配位且几乎处于与菱形平面垂直的平面上,使Fe原子周围的键大约成四面体分布。Fe—Fe间距为2.714,表明在[Fe_2S_2(NO)_4)~(2-)中的Fe—Fe之间存在着直接的M—M作用。{Fe(NO)_2}体系的Fe—N—O角偏离直线形约15°,彼此相向弯成“亲合”形。阳离子(Me_4N)~+中4个CH_3基绕中心N原子成四面体形配置。     

Anion configuration at the air/liquid interface of ionic liquid [bmim]OTf studied by sum-frequency generation spectroscopy

The air/liquid interface of a room temperature ionic liquid, 1-butyl-3-methylimidazolium trifluoromethanesulfonate ([bmim]OTf), is investigated using infrared-visible sum frequency generation (SFG) spectroscopy. The SFG spectra clearly show low-frequency modes [CF 3-symmetric stretching (ss) mode and SO 3-symmetric stretching (ss) mode] of the OTf anion, demonstrating the existence of anions polar oriented at the interface. The amplitude of the CF 3-ss peak of the OTf anion has the opposite sign with respect to that of the SO 3-ss peak, indicating that OTf anions at the surface have polar ordering where the nonpolar CF 3 group points away from the bulk into the air, whereas the SO 3 group points toward the bulk liquid. The line width of the SFG peak from the submerged SO 3 group is appreciably narrower than that from IR absorption, suggesting the environment of the surface OTf anions is much more homogeneous than that of the bulk. The vibrational calculations also suggest that the anions and the cations form a more specific aggregated configuration at the surface as compared to the bulk.     

Imidazolium 2‐Substituted 4,5‐Dicyanoimidazolate Ionic Liquids: Synthesis,Crystal Structures and Structure–Thermal Property Relationships

Thirty six novel ionic liquids (ILs) with 1‐butyl‐3‐methylimidazolium and 3‐methyl‐1‐octylimidazolium cations paired with 2‐substitited 4,5‐dicyanoimidazolate anions (substituent at C2=chloro, bromo, methoxy, vinyl, amino, methyl, ethyl, propyl, isopropyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, undecyl and phenyl) have been synthesized and characterized by using differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and single‐crystal X‐ray crystallography. The effects of cation and anion type and structure on the thermal properties of the resulting ionic liquids, including several room temperature ionic liquids (RTILs) are examined and discussed. ILs exhibited large liquid and crystallization ranges and formed glasses on cooling with glass transition temperatures in the range of ?22 to ?68 °C. The effects of alkyl substituents of the imidazolate anion reflected the crystallization, melting points and thermal decomposition of the ILs. The Coulombic packing force, van der Waals forces and size of the anions can be considered for altering the thermal transitions. Three crystal structures of the ILs were determined and the effects of changes to the cations and anions on the packing of the structure were investigated.