Influence of water on the surface of hydrophilic and hydrophobic room-temperature ionic liquids

Vibrational spectroscopy, sum frequency generation, has been used to study the gas-liquid interface of imidazolium-based, room-temperature ionic liquids. The ionic liquids are divided into two categories on the basis of their miscibility with water, as hydrophobic (immiscible) and hydrophilic (miscible). The spectroscopy results indicate that water will reorient the cations of the hydrophobic ionic liquid, while in the hydrophilic ones they remain unaffected. Thus, under low water conditions the plane of the imidazolium ring remains parallel to the surface for both types of ionic liquid. However, at high water content the ring will reorient to along the surface normal for the hydrophobic ionic liquid but remain parallel to the surface for the hydrophilic one. This is a reversible process.     

A comparative study of solvation dynamics in room-temperature ionic liquids

The solvation dynamics of ionic liquids have been the subject of many experimental and theoretical studies but remain poorly understood. We analyze these dynamics by modeling the time-resolved fluorescence response of coumarin 153 in two room-temperature ionic liquids: 1-butyl-1-methylpyrrolidinium bromide and 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide. Our results demonstrate that phenomena such as electrostatic screening operate significantly differently in the two liquids, and the relative importance of translational and rovibrational components of the ionic response depends significantly on the character of the ions involved. However, collective motion dominates the response of both ionic liquids, and the qualitative features of this collective behavior are strikingly similar in both cases.     

Sonochemistry and sonoluminescence in ionic liquids, molten salts, and concentrated electrolyte solutions

Ionic liquids have favorable intrinsic properties that make them of interest as solvents for various chemical reactions. The same properties that make the liquids effective solvents also make them interesting liquids for studies involving sonochemistry, acoustic cavitation, and sonoluminescence. Recent interest in using ultrasound to accelerate chemical reactions conducted in ionic liquids necessitates an understanding of the effects of acoustic cavitation on these solvents. Here, we review our previous results on the effects of cavitation on some room-temperature ionic liquids, including the sonoluminescence spectra of molten salt eutectics and concentrated aqueous electrolyte solutions. In all cases, regardless of the essentially nonexistent vapor pressure of the solution atomic and small molecule emitters are observed in the spectra which arise from sonolysis of the ionic liquids.     

Sum frequency generation spectroscopy of dicyanamide based room-temperature ionic liquids. Orientation of the cation and the anion at the gas-liquid interface

The orientation of the cation and the anion of room-temperature ionic liquids using sum frequency generation vibrational spectroscopy is reported. The ionic liquids are based on butyl-methyl imidazolium [BMIM]+ and hexyl-tributyl ammonium [N6444]+ together with dicyanamide [DCA]- as the anion. The tilt angle of the C3 axis of the methyl group from the alkyl chain in the cations was found to vary from 52 degrees to 80 degrees as a function of the distribution width sigma (which ranges from 0 degrees to 30 degrees with respect to the surface normal) for [BMIM][DCA] and similarly for [N6444][DCA]. The orientation of the C2 axis in the dicyanamide anion as a function of the twist angle phi, varied between 46 degrees and 90 degrees for [BMIM][DCA] and from 53 degrees to 90 degrees for [N6444][DCA]. These results suggest the presence of both ionic species at the gas-liquid interface and help describe the behavior of a simple inorganic anion at the surface.     

室温离子液体在色谱分离中的应用进展

室温离子液体作为21世纪的新型溶剂,近几年在分析领域应用广泛.本文重点综述了离子液体在气相色谱、液相色谱、毛细管电泳及薄层色谱等方面的应用进展,并对其在色谱分离中的研究进行了展望.     

The interfaces of Au(111) and Au(100) in a hexaalkyl-substituted guanidinium ionic liquid: an electrochemical and in situ STM study

Five hexaalkylguanidinium-based ionic liquids have been synthesised, and based on their cyclic voltammograms the most suited one, N,N-dibutyl-N',N'-diethyl-N',N'-dimethylguanidinium bis(trifluoromethylsulfonyl)imide, has been chosen for electrochemical studies. The surface interaction of this room-temperature ionic liquid with single crystalline gold surfaces (Au(100) and Au(111)) has been investigated using cyclic voltammetry, impedance spectroscopy and in situ scanning tunnelling microscopy (STM). The interfacial capacitance was found to be very low; STM measurements revealed the hex-reconstruction and herringbone reconstruction for Au(100) and for Au(111), respectively, at negative potentials; that is, at these potentials no hints for ad-structures of the cation could be found.     

咪唑基离子液体固定分离CO2的研究进展

以咪唑基离子液体为代表,综述了近期普通咪唑基离子液体、功能咪唑基离子液体、支撑咪唑基离子液体和聚合咪唑基离子液体在分离固定CO2方面的研究进展,说明了各类咪唑基离子液体分离固定CO2的可行性及优缺点,并总结了离子液体固定CO2的影响因素和分离机制.     

Thermophysic comparative study of two isomeric pyridinium-based ionic liquids

A comprehensive thermophysical study of isomeric room-temperature ionic liquids n-butyl-3-methyl-pyridinium tetrafluoroborate and n-butyl-4-methyl-pyridinium tetrafluoroborate has been performed. This paper reports various experimental data including density, speed of sound, refractive index, surface tension, isobaric molar heat capacity, and kinematic viscosity. From the experimental results, coefficients of thermal expansion, dynamic viscosities and molar refractions of the studied ionic liquids have been determined. Results have been analyzed paying special attention to the different features of the isomers and their structural differences. Several theories and empirical relations have been applied in order to predict physical properties of ionic liquids. A good agreement between experimental and calculated data has been found. Furthermore, a study about the versatility and application of the different relationships has been carried out finding that in general density and coefficients of thermal expansion can be estimated with relatively good accuracy.     

One-Pot Wittig and Knoevenagel Reactions in Ionic Liquid as Convenient Methods for the Synthesis of Coumarin Derivatives

Efficient synthesis of a variety of 3-substituted coumarins via NaOMe-catalyzed Knoevenagel condensation and one-pot preparation of 3,4-unsubstituted coumarins in the presence of NaOMe via Wittig reaction in room-temperature ionic liquids are described. Knoevenagel condensation of 2-hydroxybenzaldehyde with dimethyl- and diethylmalonate was performed with excellent yields in room-temperature ionic liquids. Although diethyl- and dimethylchloromalonates were mostly recovered unchanged in Knoevenagel condensation, higher conversions were observed via Wittig reaction of these compounds with 2-hydroxybenzaldehyde derivatives and triphenylphosphine. Other 2-hydroxybenzaldehyde derivatives, methyl- and ethylchloroacetates, were reacted in ionic liquids to afford simple coumarins in good yields. These reactions widen the applicability of ionic liquid in organic synthesis.     

Picosecond time-resolved fluorescence study on solute-solvent interaction of 2-aminoquinoline in room-temperature ionic liquids: aromaticity of imidazolium-based ionic liquids

Time-resolved fluorescence spectra and fluorescence anisotropy decay of 2-aminoquinoline (2AQ) have been measured in eight room-temperature ionic liquids, including five imidazolium-based aromatic ionic liquids and three nonaromatic ionic liquids. The same experiments have also been carried out in several ordinary molecular liquids for comparison. The observed time-resolved fluorescence spectra indicate the formation of pi-pi aromatic complexes of 2AQ in some of the aromatic ionic liquids but not in the nonaromatic ionic liquids. The fluorescence anisotropy decay data show unusually slow rotational diffusion of 2AQ in the aromatic ionic liquids, suggesting the formation of solute-solvent complexes. The probe 2AQ molecule is likely to be incorporated in the possible local structure of ionic liquids, and hence the anisotropy decays only through the rotation of the whole local structure, making the apparent rotational diffusion of 2AQ slow. The rotational diffusion time decreases rapidly by adding a small amount of acetonitrile to the solution. This observation is interpreted in terms of the local structure formation in the aromatic ionic liquids and its destruction by acetonitrile. No unusual behavior upon addition of acetonitrile has been found for the nonaromatic ionic liquids. It is argued that the aromaticity of the imidazolium cation plays a key role in the local structure formation in imidazolium-based ionic liquids.     

Ionic Liquids for and by Analytical Spectroscopy

Abstract

In this mini‐review we highlight two aspects of ionic liquids that to date have either no or limited studies. They are (1) exploitation of unique features of ionic liquids to develop novel spectroscopic methods and (2) development of novel spectroscopic methods, for the sensitive and accurate determination of thermal physical properties of ionic liquids. In the first category, we will describe several novel methods which were developed utilizing unique properties of ionic liquids for measurements which are not possible otherwise. They include the sensitive and accurate method to determine enantiomeric compositions of a variety of pharmaceutical products with different size, shape, and functional groups. This method is based on the use of a chiral IL which serves both as a solvent and also as a chiral selector. Ionic liquids have also been successfully used to substantially enhance the sensitivity of thermal lens measurements. In the second category, we will describe recent development in which transient grating technique and thermal lens technique have been successfully used for the sensitive, accurate, nondestructive determination of thermal physical properties of ILs.     

Ionic liquid electrolytes for dye-sensitized solar cells

The potential of room-temperature molten salts (ionic liquids) as solvents for electrolytes for dye-sensitized solar cells has been investigated during the last decade. The non-volatility, good solvent properties and high electrochemical stability of ionic liquids make them attractive solvents in contrast to volatile organic solvents. Despite this, the relatively high viscosity of ionic liquids leads to mass-transport limitations. Here we review recent developments in the application of different ionic liquids as solvents or components of liquid and quasi-solid electrolytes for dye-sensitized solar cells.     

Activity coefficients at infinite dilution of organic compounds in 1-(meth)acryloyloxyalkyl-3-methylimidazolium bromide using inverse gas chromatography

Activity coefficients at infinite dilution, gammainfinity, of organic compounds in two new room-temperature ionic liquids (n-methacryloyloxyhexyl-N-methylimidazolium bromide (C10H17O2MIM)(Br) at 313.15 and 323.15 K and n-acryloyloxypropyl-N-methylimidazolium bromide(C6H11O2MIM)(Br)) were determined using inverse gas chromatography. Phase loading studies of the net retention volume per gram of packing as a function of the percent phase loading were used to estimate the influence of concurrent retention mechanisms on the accuracy of activity coefficients at infinite dilution of solutes in both ionic liquids. It was found that most of the solutes were retained largely by partition with a small contribution from adsorption and that n-alkanes were retained predominantly by interfacial adsorption on ionic liquids studied in this work. The solvation characteristics of the two ionic liquids were evaluated using the Abraham solvation parameter model.     

What causes extended layering of ionic liquids on the mica surface?

Extended layering of ionic liquids (ILs) on the mica surface has been reported by several groups previously and it is generally accepted that the electrostatic interaction at the IL/mica interface is critical to the observed extended layering. Here we report that, indeed, water adsorption on the mica surface is the key to the extended layering of ionic liquids. The atomic force microscopy (AFM), attenuated total reflectance-fourier transform infrared spectroscopy (ATR-FTIR) and contact angle (CA) results show that ionic liquids form extended layering on a mica surface under ambient conditions when water is adsorbed on the mica surface under such conditions. However, when airborne hydrocarbon contaminants replace the water on the mica surface at the elevated temperatures, instead of layering, ionic liquids exhibit droplet structure, i.e., dewetting. Based on the experimental results, we propose that water enables ion exchange between K+ and the cations of ILs on the mica surface and thus triggers the ordered packing of cations/anions in ILs, resulting in extended layering.     

Selective separation of aromatic hydrocarbons through supported liquid membranes based on ionic liquids

Ionic liquids are emerging as alternative solvents for volatile organic compounds traditionally used in liquid–liquid extraction and liquid membrane separation. In this paper, we examine whether room-temperature ionic liquids as a membrane solution can be utilized for hydrocarbon separation by using a supported liquid membrane. Aromatic hydrocarbons, benzene, toluene and p-xylene were successfully transported through the membrane based on the ionic liquids. Although the permeation rates through the membrane based on the ionic liquids were less than those of water, the selectivity of aromatic hydrocarbons was greatly improved. The maximum selectivity to heptane was obtained using benzene in the aromatic permeation and 1-n-butyl-3-methylimidazolium hexafluorophosphate in the liquid membrane phase.     

PEG-linked geminal dicationic ionic liquids as selective, high-stability gas chromatographic stationary phases

It is known that room-temperature ionic liquids (RTILs) have wide applicability in many scientific and technological fields. In this work, a series of three new dicationic room-temperature ionic liquids functionalized with poly(ethylene glycol) (PEG) linkages were synthesized and characterized via a linear solvation model. The application of these ILs as new GC stationary phases was studied. The efficient separation of several mixtures containing compounds of different polarities and 24 components of a flavor and fragrance mixture indicated comparable or higher resolving power for the new IL stationary phases compared to the commercial polysiloxane and poly(ethylene glycol)-based stationary phases. In addition, the selectivities of the IL stationary phases could be quite unique. The separation of a homologous alkane and alcohol mixture displayed the “dual nature” of these ionic liquids as GC stationary phases. The thermal stability study showed the column robustness up to 350 °C. The high separation power, unique selectivity, high efficiency and high thermal stability of the new dicationic ionic liquids indicate that they may be applicable as a new type of robust GC stationary phase.     

Supported Ionic Liquid Catalysis

Supported ionic liquid catalysis is a concept which combines the advantages of ionic liquids with those of heterogeneous support materials. The viability of this concept has been confirmed by several studies which have successfully confined various ionic phases to the surface of support materials and explored their potential catalytic applications. Although the majority of the evaluated supports were silica based, several studies focused on polymeric materials including membranes. The preparation of these materials was achieved by using two different immobilization approaches. The first approach involves the covalent attachment of ionic liquids to the support surface whereas the second simply deposits the ionic liquid phases containing catalytically active species on the surface of the support. Herein recent advances made in this area are described.     

Supported ionic liquid catalysis

Supported ionic liquid catalysis is a concept which combines the advantages of ionic liquids with those of heterogeneous support materials. The viability of this concept has been confirmed by several studies which have successfully confined various ionic phases to the surface of support materials and explored their potential catalytic applications. Although the majority of the evaluated supports were silica based, several studies focused on polymeric materials including membranes. The preparation of these materials was achieved by using two different immobilization approaches. The first approach involves the covalent attachment of ionic liquids to the support surface whereas the second simply deposits the ionic liquid phases containing catalytically active species on the surface of the support. Herein recent advances made in this area are described.     

Physical properties of ionic liquids consisting of the 1-butyl-3-methylimidazolium cation with various anions and the bis(trifluoromethylsulfonyl)imide anion with various cations

Physical properties of 4 room-temperature ionic liquids consisting of the 1-butyl-3-methylimidazolium cation with various perfluorinated anions and the bis(trifluoromethylsulfonyl)imide (Tf2N-) anion with 12 pyrrolidinium-, ammonium-, and hydroxyl-containing cations are reported. Electronic structure methods are used to calculate properties related to the size, shape, and dipole moment of individual ions. Experimental measurements of phase-transition temperatures, densities, refractive indices, surface tensions, solvatochromic polarities based on absorption of Nile Red, 19F chemical shifts of the Tf2N- anion, temperature-dependent viscosities, conductivities, and cation diffusion coefficients are reported. Correlations among the measured quantities as well as the use of surface tension and molar volume for estimating Hildebrand solubility parameters of ionic liquids are also discussed.     

Design and synthesis of pyridinium chiral ionic liquids tethered to a urea functionality

Nine chiral room-temperature ionic liquids (RTILs), which contain a chiral moiety and a urea functionality bonded to a pyridinium ring, have been designed and synthesized. The synthesis of these ionic liquids is concise and practical due to the commercial availability of the starting materials. These novel RTILs were readily prepared from 2-(aminomethyl)pyridine and amino acid ester derived isocyanates. We envision that these new chiral RTILs can serve as effective reaction media as well as chiral catalysts for asymmetric reactions, which are presently being investigated in our laboratory.