Physicochemical Properties Of Ionic Liquids Research Articles

Functional ionic liquids for enhancement of Li-ion transfer: the effect of cation structure on the charge-discharge performance of the Li4Ti5O12 electrode.

As the development of high energy-density Li-ion batteries moves ahead, ensuring safety of the batteries has become increasingly important. Among the unique physicochemical properties of ionic liquids, thermal stability can be one of the answers to the challenge. The use of ionic liquids, however, causes critical issues concerning the kinetics of Li-ion transfer at the electrode-electrolyte interface. In the present study, ionic liquids consisting of 1-((2-methoxyethoxy)methyl)-1-methylpiperidinium (PP1MEM) or 1-hexyl-1-methylpiperidinium (PP16) and bis(trifluoromethanesulfonyl)amide (TFSA) were applied to an electrolyte for Li-ion batteries, and we investigated the effect of cation structure on interfacial Li-ion transfer using Li4Ti5O12 as a model electrode by means of Raman spectroscopy and electrochemical impedance spectroscopy. It was found that the ether functional group in the PP1MEM cation has the meaningful function; the cation structure reduces the electrostatic interaction between the Li ion and TFSA anions in an ionic liquid electrolyte. The solvation number of the TFSA anion per Li ion consequently became smaller than that in PP16-TFSA, and the lower solvation number in PP1MEM-TFSA allowed the facile Li-ion diffusion in the electrolyte bulk rather than the interfacial Li-ion transfer and significantly improved the rate performance. The results offer the prospect of utilization of PP1MEM-TFSA as an electrolyte solvent. The knowledge obtained from this study contributes to the development of next-generation Li-ion batteries having both high energy density and high safety.

Mixed anion effect on the ionic transport behavior, complexation and various physicochemical properties of ionic liquid based polymer gel electrolyte membranes

Incorporation of IL in PGEMs increases the amorphicity of the membranes, through reducing the intermolecular interaction between the polymer chains, and also increases the membrane ionic conductivity.

Synthesis and characterization of 2′,3′-epoxy propyl-N-methyl-2-oxopyrrolidinium salicylate ionic liquid and study of its interaction with water or methanol

Important physico-chemical properties of ionic liquids (ILs) can be manipulated by adjusting the nature of the cation or anion.

Quantification of the impact of water as an impurity on standard physico-chemical properties of ionic liquids

The objective of this work was to quantify the effect of the presence of water as impurity in ionic liquids. First, density and viscosity of five ionic liquids as well as their aqueous solutions were measured. For hydrophobic dried ionic liquids, traces of water (50ppm) have measurable impact neither on the density nor on the viscosity values. In the concentration range studied (up to 5000ppm), a linear evolution of the molar volume of the mixture with the mole fraction composition is observed. Practically, this allows to estimate the density of an neat ionic liquid provided (i) the water quantity and (ii) the density of the undried sample are known. This is particularly useful for hydrophilic ionic liquids that are difficult to dry. In the studied concentration range, a linear evolution of the relative viscosity was also depicted as a function of the mass fraction composition. It is thus possible to evaluate the viscosity of the pure ionic liquid knowing the water quantity and the viscosity of the undried sample. The comparison of the results obtained using two viscosimeters confirms that a Stabinger viscosimeter is appropriate to precisely measure ionic liquids viscosities. Second, NMR and IR spectroscopies were used to characterize the pure ionic liquids and their solutions with water. The sensitivity of IR spectroscopy does allow neither the quantification nor the detection of water below 1mol%. With NMR spectroscopy, water can be quantified using either the intensity or the chemical shift of the water proton peak for mole fractions as low as 200ppm. It is even possible to detect water in the dried hydrophobic ionic liquids, few ppm of water, as long as the water peak does not overlap the ionic liquid signal.

Probing Microenvironment in Ionic Liquids by Time-Resolved EPR of Photoexcited Triplets.

Unusual physicochemical properties of ionic liquids (ILs) open vistas for a variety of new applications. Herewith, we investigate the influence of microviscosity and nanostructuring of ILs on spin dynamics of the dissolved photoexcited molecules. We use two most common ILs [Bmim]PF6 and [Bmim]BF4 (with its close analogue [C10mim]BF4) as solvents and photoexcited Zn tetraphenylporphyrin (ZnTPP) as a probe. Time-resolved electron paramagnetic resonance (TR EPR) is employed to investigate spectra and kinetics of spin-polarized triplet ZnTPP in the temperature range 100-270 K. TR EPR data clearly indicate the presence of two microenvironments of ZnTPP in frozen ILs at 100-200 K, being manifested in different spectral shapes and different spin relaxation rates. For one of these microenvironments TR EPR data is quite similar to those obtained in common frozen organic solvents (toluene, glycerol, N-methyl-2-pyrrolidone). However, the second one favors the remarkably slow relaxation of spin polarization, being much longer than in the case of common solvents. Additional experiments using continuous wave EPR and stable nitroxide as a probe confirmed the formation of heterogeneities upon freezing of ILs and complemented TR EPR results. Thus, TR EPR of photoexcited triplets can be effectively used for probing heterogeneities and nanostructuring in frozen ILs. In addition, the increase of polarization lifetime in frozen ILs is an interesting finding that might allow investigation of short-lived intermediates inaccessible otherwise.

Ionic liquids effects on the permeability of photosynthetic membranes probed by the electrochromic shift of endogenous carotenoids

Ionic liquids (ILs) are promising materials exploited as solvents and media in many innovative applications, some already used at the industrial scale. The chemical structure and physicochemical properties of ILs can differ significantly according to the specific applications for which they have been synthesized. As a consequence, their interaction with biological entities and toxicity can vary substantially. To select highly effective and minimally harmful ILs, these properties need to be investigated. Here we use the so called chromatophores – protein-phospholipid membrane vesicles obtained from the photosynthetic bacterium Rhodobacter sphaeroides– to assess the effects of imidazolinium and pyrrolidinium ILs, with chloride or dicyanamide as counter anions, on the ionic permeability of a native biological membrane. The extent and modalities by which these ILs affect the ionic conductivity can be studied in chromatophores by analyzing the electrochromic response of endogenous carotenoids, acting as an intramembrane voltmeter at the molecular level. We show that chromatophores represent an in vitro experimental model suitable to probe permeability changes induced in cell membranes by ILs differing in chemical nature, degree of oxygenation of the cationic moiety and counter anion.

Studies on the Physicochemical Properties of Ionic Liquids Based On 1-Octyl-3-methylimidazolium Amino Acids

A series of new synthesized ionic liquids based on 1-octyl-3-methylimidazolium with glycinate, alaninate, serinate, prolinate, and asparaginate anions were prepared. Their thermophysical properties (density, viscosity, surface tension, and heat capacity) were measured at various temperatures and atmospheric pressures. Physicochemical properties such as thermal expansion coefficient values, molecular volume, standard molar entropy, and lattice energy standard were also determined using empirical methods. The effects of anions on the studied properties were further analyzed.

Theoretical evaluation on solubility of synthesized task specific ionic liquids in water

Task specific ionic liquids (TSILs) are synthesized and utilized for extraction of different metal atoms from their complexes. Since physico-chemical properties of ionic liquids (ILs) are modified by water (H2O) molecules, less hydrophilic ILs are suitable for the liquid–liquid metal extraction process. Recently, we have synthesized amino acid functionalized imidazolium based TSILs for extraction of rare earth metals. The synthesized ILs have a common functionalized cation [C20H28N3O3]+ and different anions namely, bromide-[Br]− and bis(trifluoromethylsulfonyl)imide-[NTF2]−. In this study, various first principles (Hatree–Fock and density functional theory) and molecular mechanics calculations are done to understand intermolecular interactions between H2O and the synthesized TSILs. The results of gas phase cluster calculations project the difference in solubility of these two TSILs in H2O.

Can the Cation Alkyl Side Chain Length Affect the Thermal and Transport Properties of Pyrrolidinium-TFSI Ionic Liquid Mixtures

There is growing interest in replacing the volatile and hazardous organic solvents, currently used in electrochemical devices, with ionic liquids (ILs), this pushing them far beyond the initially envisaged role of green solvents. However, the rational design of novel systems based on ILs requires a deep understanding of the intermolecular interactions and transport properties occurring in the ionic liquid bulk. The possibility of fine tuning the physicochemical properties of ionic liquids by forming IL blends is a very appealing characteristic [1]. For instance, the association and ordering of the ions in a mixture is more complicated than in the pure components, thus affecting the short-range aggregation motives and, consequently, the blend physicochemical properties. Ionic liquids are highly structured fluids with short-range aggregation motives, strongly correlated to the alkyl side chain length bounded to the nitrogen group of the cation [1].Our basic idea is to investigate the length effect of the cation main aliphatic side chain on the thermal and transport properties of binary ionic liquid mixtures formed by pyrrolidinium materials. Three sets of IL mixtures were prepared: i) PYR11TFSI-PYR14TFSI, ii) PYR11TFSI-PYR18TFSI and iii) PYR14TFSI-PYR18TFSI. The present study was performed by DSC, conductivity and self-diffusion coefficients (NMR measurements).The DSC heating trace of the ionic liquid blends is shown in Figure 1. The PYR11TFSI-PYR14TFSI samples (panel A) exhibit a well-defined melting peak. The progressive enrichment in PYR11TFSI was seen to hinder the crystallization process of the IL mixtures, as a result of the unfavorable ion packing likely ascribable to the different size of the (PYR11)+ and (PYR14)+ cations. Apart the x = 0.75 sample of panel B, the PYR11TFSI-PYR18TFSI (panel B) and PYR14TFSI-PYR18TFSI (panel C) mixtures do not display any feature, with exception of the glass transition one (Tg ), indicating that these IL blends are not able to be crystallized. This suggests that the “ionic confusion”, resulting from the contemporaneous presence of different symmetry pyrrolidinium cations, is able to prevent the crystallization of the mixtures which, instead, solidify in a glassy state at very low temperatures.Figure 1. DSC trace of PYR11TFSI-PYR14TFSI (panel A), PYR11TFSI-PYR18TFSI (panel B) and PYR14TFSI-PYR18TFSI (panel C) ionic liquid binary mixtures.In Figure 2 is reported the ionic conductivity vs. PYR14TFSI mole fraction dependence for (x)PYR14TFSI-(1-x)PYR18TFSI. At a first sight it is seen that the ionic conductivity decreases with the progressive impoverishment of the highest conductive material (PYR14TFSI) fraction in the binary mixture. However, a more careful observation shows that this trend is true only at temperatures above the melting point of PYR14FSI, e.g., -7 °C. At lower temperatures, the ionic conductivity for PYR14TFSI mole fractions ranging from 0.25 to 0.75, is from two to four orders of magnitude higher than the neat IL materials.Figure 2. Ionic conductivity vs. mole fraction behavior of (x)PYR14TFSI-(1-x)PYR18TFSI mixtures.AcknowledgementsJ.S.M. and G.B.A. thank the Italian Institute of Technology (IIT) for the financial support within the SEED Project “REALIST”.

Recent Advances in the Applications of Ionic Liquids in Protein Stability and Activity: A Review

Room temperatures ionic liquids are considered as miraculous solvents for biological system. Due to their inimitable properties and large variety of applications, they have been widely used in enzyme catalysis and protein stability and separation. The related information present in the current review is helpful to the researchers working in the field of biotechnology and biochemistry to design or choose an ionic liquid that can serve as a noble and selective solvent for any particular enzymatic reaction, protein preservation and other protein based applications. We have extensively analyzed the methods used for studying the protein-IL interaction which is useful in providing information about structural and conformational dynamics of protein. This can be helpful to develop and understanding about the effect of ionic liquids on stability and activity of proteins. In addition, the affect of physico-chemical properties of ionic liquids, viz. hydrogen bond capacity and hydrophobicity on protein stability are discussed.

Determination of pH Value and Acid-base Property of Ionic Liquid Aqueous Solutions

The ionic liquids (ILs) and their aqueous solutions have been considered as a new generation of absorption working substances for applications in gas separation and absorption refrigeration processes. It is important to understand the physicochemical properties of ionic liquids in aqueous solutions, such as chemical potential and acid-base properties before any industry application. In this study, we focused on the acid-base properties of the IL aqueous solutions by testing the pH values of the aqueous solutions of two ionic liquids, 1-ethyl-3-methylimidazolium acetate ([EMIM]Ac) and 1-hexyl-3-methylimidazolium chloride ([HMIM]Cl). All the tests were carried out at 25 and in a well controlled dry air °C surrounding. There were 16 water mass fractions for each of [EMIM]Ac and [HMIM]Cl aqueous solutions tested in range of 0.04 to 0.95 and 0.01 to 0.96, respectively. There, two methods, the sophisticated pH paper and the pH meter, were applied for the tests and the differences between the two test methods were detected. The test data from the two test methods and their differences were then evaluated with an aggregation film model and the activity theory for the film and aqueous solution phases. The results show that the difference of the pH values tested by the two methods is significant. The pH data from the sophisticated pH paper test present the bulk acid-base property of the IL aqueous solutions, while the pH meter can only reflect the hydrogen ion concentration in the IL aggregation film which was formed on the surface of the electrode of the pH meter. With analysis of acid-base reaction in the IL aqueous solution, a calculation model for the pH value of the bulk solution region was developed, and the data of the sophisticated pH paper test were correlated and the solution acid-base values were obtained. Based on analysis of the water species equilibrium in the IL aggregation film and the bulk aqueous solution phases and its effect on the concentration of hydrogen ions in the aggregation film, a method for calculation of the difference of pH values between the aggregation film and the bulk solution was proposed and the data from the pH meter were then fitted with the proposed calculation method. The results provide experimental data and a quantitative method for analysis of the acid-base property of IL aqueous solution systems.

Synthesis, characterization and thermal properties of thiosalicylate ionic liquids

In an attempt to produce new functionalized ionic liquids, a series of thiosalicylate ionic liquids based on imidazolium, ammonium, phosphonium, choline and pyrrolidinium cations were synthesized. The compounds were characterized by Infra Red (IR), Nuclear Magnetic Resonance (NMR) and mass spectra (ESI-MS). Their glass-transition temperatures, melting points and decomposition temperatures have been measured. Physicochemical properties of ionic liquids are influenced by alkyl chain length and nature of the cation of ionic liquids. A series of thiosalicylate ionic liquids based on imidazolium, ammonium, phosphonium, choline and pyrrolidinium cations were prepared. The compounds were characterized and their thermal properties were investigated. The physicochemical properties of ionic liquids are influenced by the alkyl chain length and the nature of the cation of ionic liquids.

Measurement of the Interfacial Tension in an Ionic Liquid–Dielectric Liquid System Using an Electrically Deformed Droplet

The unique chemical and physical properties of ionic liquids (ILs) are attracting an increasing amount of interest in energy and biological fields. However, some physicochemical properties of ILs are not well-known. Here, we present an alternative method for the measurement of the interfacial tension in an IL–dielectric liquid system by means of the small deformation of a charged IL droplet in a dielectric medium. Using the pendant drop method, the validity of the proposed measurement method is confirmed. The interfacial tension between ILs and silicone oil has been found to be lower than that between water and silicone oil, and the tension is negatively correlated to the size of the IL ion. Further analyses of Girifalco-Good’s interaction parameters of the two liquid–liquid systems demonstrate that the lower interfacial tension of IL can be attributed to the stronger intermolecular interaction between IL and silicone oil molecules. Due to a universal feature of the interaction parameter for a given class...

Structure and Physico-Chemical Properties of Ionic Liquids: What Mass Spectrometry is Telling Us

This review provides an overview of the use of mass spectrometry (MS) and its various ionization techniques and an evaluation of their suitability to investigate structures and physico-chemical properties of ionic liquids (IL) and the types of information that can be obtained from such experiments. We will focus on most representative and comprehensive work, and on the work that we are been performing in an collaboration involving our laboratories, one devoted to the synthesis and uses of IL and to the other to the development of new MS techniques and their applications. Keywords: Ionic liquids, Mass spectrometry, Ionic pairs, Reaction mechanisms, Hydrogen bonds, Supramolecules, Matrix asssisted laser desorption ionization (MALDI), Hydrodynamic diameters (Dh), Ion Mobility, Collision-induced dissociation (CID)

Surface adsorption and micelle formation of imidazolium-based zwitterionic surface active ionic liquids in aqueous solution

A novel class of zwitterionic surface active ionic liquids (SAILs), N-alkyl-N′-carboxymethyl imidazolium inner salts ([NCn, N′CO2Im], n=10, 12, 14), was synthesized. Their aggregation behavior in aqueous solution was investigated by surface tension, isothermal titration calorimetry, and steady-state fluorescence. Compared with the reported imidazolium-based cationic SAILs, 1-alkyl-3-methylimidazolium bromide ([Cnmim]Br) and zwitterionic betaine surfactants, (CnH2n+1N(CH3)2CH2COO−), [NCn, N′CO2Im] exhibits significantly lower critical micelle concentration (cmc) and surface tension at cmc (γcmc) values. It is attributed to the incorporation of a deprotonated carboxylic group into the head group, which weakens the electrostatic repulsion between head groups and favors micellization. The micellar aggregation number of [NCn, N′CO2Im] is larger than that of [Cnmim]Br, while less than that of CnH2n+1N(CH3)2CH2COO−. Similar to the traditional zwitterionic surfactants, the surface activity and adsorption properties of [NC12, N′CO2Im] at air/water interface have a slight variation with temperature, pH, and ionic strength. This indicates that the present zwitterionic SAILs display the aggregation behavior much similar to zwitterionic surfactants, distinctly different from imidazolium-based cationic SAILs. Sets in low sensitivity to the environmental conditions, superior surface activity and unique physicochemical properties of ionic liquids, [NCn, N′CO2Im] can be exploited for utilizing as a potential substitute for conventional surfactants in certain fields.

Physicochemical Characterization of 1-Butyl-3-methylimidazolium and 1-Butyl-1-methylpyrrolidinium Bis(trifluoromethylsulfonyl)imide

Densities of two selected ionic liquids (ILs), 1-butyl-3-methylimidazolium bis(trifluoro-methylsulfonyl)imide, [bmim][NTf2], and 1-butyl-1-methylpyrrolidinium bis(trifluoromethyl-sulfonyl)imide, [bmpyr][NTf2], were determined in the temperature range from (293.15 to 353.15) K. From the experimental density values, the thermal expansion coefficient was calculated. The specific conductivity was measured in the range from (303.15 to 353.15) K. Our experimental results were compared with available literature data. Also, viscosity of these ionic liquids was measured from room temperature up to 353.15 K. Using the obtained data, the temperature effect on the viscosity was analyzed. Since there are limited literature data on physicochemical properties of ionic liquids, data obtained in this study can provide valuable contributions for better understanding of ILs behavior.

Determination of LFER Descriptors of 30 Cations of Ionic Liquids—Progress in Understanding Their Molecular Interaction Potentials

In order to understand molecular interaction potentials of 30 cations of ionic liquids (ILs), the well-known linear free energy relationship concept (LFER) was applied. The LFER descriptors for the excess molar refractivity and the molar volume were calculated in silico and for hydrogen-bonding acidity and basicity, and the polarizability/dipolarity of IL cations were experimentally determined through high performance liquid chromatography (HPLC) measurements. For the study, three different columns (RP-select B, Cyan, and Diol) and buffered mobile phases, based on two organic solvents acetonitrile (ACN) and methanol (MeOH), were selectively combined to the HPLC separation systems RP-select B-ACN, RP-select B-MeOH, Cyan-MeOH, Diol-ACN, and Diol-MeOH. By measuring the retention factors of 45 neutral calibration compounds and calculating LFER descriptors of three cations in the HPLC systems, the system parameters, including an ionic z coefficient, were determined. Conversely, the LFER descriptors of 30 ionic liquid cations were determined, based on the parameters of five systems and their retention factors in the HPLC systems. The results showed that the type of head group, alkyl chain length and further substituents of the cation have a significant influence on the dipolarity/polarizability and the hydrogen-bonding acidity, and functionalized groups (hydroxyl, ether, and dimethylamino) lead to hydrogen-bonding basicity of the cation. The characterization of cationic LFER descriptors opens up the chance for a more quantitative understanding of molecular interaction potentials and physicochemical properties of ILs.

Thermodynamic Modeling of Imidazolium-Based Ionic Liquids with the [PF6]− Anion for Separation Purposes

There is an increased interest in developing accurate tools to relate the physicochemical properties of ionic liquids (ILs) to their microscopic structure as this information is needed to speed up the design of new ionic liquids for chemical and industrial processes. Molecular models can be used for this purpose. We explore here the extended capabilities of a model previously developed in the context of soft-SAFT, by Andreu and Vega in 2007 to reproduce the thermodynamic behavior of imidazolium hexafluorophosphate-based ([Cnmim][PF6]) ionic liquids. The molecular parameters optimized in the previous work have been used here in a transferable manner; some new members of the [Cnmim][PF6] family have also been added, as new recent experimental data has been published. The interfacial tensions have been calculated using a Density Gradient Approach and the results have been compared with available experimental data. The solubility of carbon monoxide and hydrogen in those ILs has been studied in the range of temperatures and pressures of application for separation processes. Binary mixtures with other imidazolium ionic liquids with different anions have been calculated, in a predictive manner. Finally, calculations of mixtures of ionic liquids with water also show very good agreement with experimental data. This work highlights the importance of using a simple but robust thermodynamic model, including the right level of interactions, to accurately describe the properties of these highly non-ideal systems.

Tuning the Physicochemical Properties of Diverse Phenolic Ionic Liquids for Equimolar CO2 Capture by the Substituent on the Anion

Phenolic ionic liquids for the efficient and reversible capture of CO(2) were designed and prepared from phosphonium hydroxide and substituted phenols. The electron-withdrawing or electron-donating ability, position, and number of the substituents on the anion of these ionic liquids were correlated with the physicochemical properties of the ionic liquids. The results show that the stability, viscosity, and CO(2)-capturing ability of these ionic liquids were significantly affected by the substituents. Furthermore, the relationship between the decomposition temperature, the CO(2)-absorption capacity, and the basicity of these ionic liquids was quantitatively correlated and further rationalized by theoretical calculation. Indeed, these ionic liquids showed good stability, high absorption capacity, and low absorption enthalpy for CO(2) capture. This method, which tunes the physicochemical properties by making use of substituent effects in the anion of the ionic liquid, is important for the design of highly efficient and reversible methods for CO(2)-capture. This CO(2) capture process using diverse phenolic ionic liquids is a promising potential method for CO(2) absorption with both high absorption capacity and good reversibility.

Sonochemical formation of iron oxide nanoparticles in ionic liquids for magnetic liquid marble

Ionic liquids (ILs)-stabilized iron oxide (Fe(2)O(3)) nanoparticles were synthesized by the ultrasonic decomposition of iron carbonyl precursors in [EMIm][BF(4)] without any stabilizing or capping agents. The Fe(2)O(3) nanoparticles were isolated and characterized by X-ray powder diffraction, transmission electron microscopy and susceptibility measurements. The physicochemical properties of ILs containing magnetic Fe(2)O(3) nanoparticles (denoted as Fe(2)O(3)@[EMIm][BF(4)]), including surface properties, density, viscosity and stability, were investigated in detail and compared with that of [EMIm][BF(4)]. The Fe(2)O(3)@[EMIm][BF(4)] can be directly used as magnetic ionic liquid marble by coating with hydrophobic and unreactive polytetrafluoroethylene (PTFE), for which the effective surface tension was determined by the puddle height method. The resulting magnetic ionic liquid marble can be transported under external magnetic actuation, without detachment of magnetic particles from the marble surface that is usually observed in water marble.