Properties Of Room Temperature Ionic Liquids Research Articles

Flexible Aluminum-Air Batteries

Wearable and flexible electronics have a wide range of applications. Making a flexible power source for them is essential. Aluminum-air batteries are attractive for powering portable, lightweight devices due to their inexpensive, light, and powerful energy source. In this project, the Al-air battery was made using activated carbon and aluminum electrodes, chloride-based electrolytes with ionic liquid additives. Various mixtures of 1-ethyl-3-methylimidazolium chloride, aluminum chloride, sodium hydroxide, and sodium chloride were studied. Fundamental properties of room-temperature ionic liquids, such as lower conductivity and density, improved the performance of aluminum-air batteries. Cell capacity, cyclic voltammetry behavior, and cell interfacial impedance with various electrolytes were optimized using the design of experiments. The battery's electrical power output and cell capacity were lower than those obtained using an activated carbon air cathode. The cell capacity over repeated electrochemical reactions was stable. The Al-air battery had regular cyclic voltammetry behavior. Aluminum hydroxide was not found on the anode electrode when an ionic liquid electrolyte was used. The interfacial cell impedance did not decrease after electrochemical reactions over a long time.

Ionic liquid-containing non-aqueous Pickering emulsions prepared with sterically-stabilized polymer nanoparticles: A highly efficient platform for Knoevenagel reaction

Room temperature ionic liquids (RTILs)-containing Pickering emulsions exhibit promising application potentials because of the integration of the “green” quality and the tunable applications of RTILs with the advantages of Pickering emulsions, such as the high stability and adjustable droplet size. In the present study, sterically-stabilized polymer nanoparticles (SSPNs) were first prepared via the reversible addition-fragmentation chain transfer (RAFT)-mediated non-aqueous dispersion polymerization, in which poly(lauryl methacrylate) (PLMA) and poly(4-vinyl pyridine) (P4VP) acted as the stabilizer block and the core-forming block, respectively. Kinetically-trapped PLMA-b-P4VP spheres and mixed spherical/worm-like morphologies could be achieved at a target copolymer composition. Subsequently, core cross-linked SSPNs (CSSPNs) were obtained by means of chemical cross-linking of P4VP cores of SSPNs with 1,4-dibromobutane in a diluted solution of SSPNs in toluene (0.4 mg/mL). Stable 1-butyl-3-methylimidazolium tetrafluoroborate-in-toluene ([Bmim][BF4]-in-toluene) non-aqueous Pickering emulsions could be formed at a low emulsifier content of 0.005 wt% adopting CSSPNs with mixed spherical/worm-like morphologies as Pickering emulsifiers. Benefiting from large surface areas of Pickering emulsions and unique chemical/physical properties of RTILs, [Bmim][BF4]-in-toluene Pickering emulsions provided readily available platforms for many organic reactions. As a proof of the concept, proline catalyzed Knoevenagel reaction of 4-nitrobenzaldehyde with malononitrile was chosen as a model here. Both high reaction efficiency and good recyclability were observed.

Efficient Parametrization of Force Field for the Quantitative Prediction of the Physical Properties of Ionic Liquid Electrolytes.

The prediction of transport properties of room-temperature ionic liquids from nonpolarizable force field-based simulations has long been a challenge. The uniform charge scaling method has been widely used to improve the agreement with the experiment by incorporating the polarizability and charge transfer effects in an effective manner. While this method improves the performance of the force fields, this prescription is ad hoc in character; further, a quantitative prediction is still not guaranteed. In such cases, the nonbonded interaction parameters too need to be refined, which requires significant effort. In this work, we propose a three-step semiautomated refinement procedure based on (1) atomic site charges obtained from quantum calculations of the bulk condensed phase; (2) quenched Monte Carlo optimizer to shortlist suitable force field candidates, which are then tested using pilot simulations; and (3) manual refinement to further improve the accuracy of the force field. The strategy is designed in a sequential manner with each step improving the accuracy over the previous step, allowing the users to invest the effort commensurate with the desired accuracy of the refined force field. The refinement procedure is applied on N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(trifluoromethanesulfonyl)imide (DEME-TFSI), a front-runner as an electrolyte for electric double-layer capacitors and single-molecule-based devices. The transferability of the refined force field is tested on N,N-dimethyl-N-ethyl-N-methoxyethoxyethylammonium bis(trifluoromethanesulfonyl)imide (N112,2O2O1-TFSI). The refined force field is found to be better at predicting both structural and transport properties compared to the uniform charge scaling procedure, which showed a discrepancy in the X-ray structure factor. The refined force field showed quantitative agreement with structural (density and X-ray structure factor) and transport properties-diffusion coefficients, ionic conductivity, and shear viscosity over a wide temperature range, building a case for the wide adoption of the procedure.

15N NMR studies provide insights into physico-chemical properties of room-temperature ionic liquids.

Ionic liquids (ILs) have gained a lot of attention as alternative solvents in many fields of science in the last two decades. It is known that the type of anion has a significant influence on the macroscopic properties of the IL. To gain insights into the molecular mechanisms responsible for these effects it is important to characterize these systems at the microscopic level. Such information can be obtained from nuclear spin-relaxation studies which for compounds with natural isotope abundance are typically performed using direct 1H or 13C measurements. Here we used direct 15N measurements to characterize spin relaxation of non-protonated nitrogens in imidazolium-based ILs which are liquid at ambient temperature. We report heteronuclear 1H-15N scalar coupling constants (nJHN) and 15N relaxation parameters for non-protonated nitrogens in ten 1-ethyl-3-methylimidazolium ([C2C1IM]+)-based ILs containing a broad range of anions. The 15N relaxation rates and steady-state heteronuclear 15N-{1H} NOEs were measured using direct 15N detection at 293.2 K and two magnetic field strengths, 9.4 T and 16.4 T. The experimental data were analyzed to determine hydrodynamic characteristics of ILs and to assess the contributions to 15N relaxation from 15N chemical shift anisotropy and from 1H-15N dipolar interactions with non-bonded protons. We found that the rotational correlation times of the [C2C1IM]+ cation determined from 15N relaxation measurements at room temperature correlate linearly with the macroscopic viscosity of the ILs. Depending on the selected anion, the 15N relaxation characteristics of [C2C1IM]+ differ considerably reflecting the influence of the anion on the physicochemical properties of the IL.

Understanding the Properties of Ionic Liquids: Electrostatics, Structure Factors, and Their Sum Rules.

The properties of room-temperature ionic liquids (ILs) may be viewed as resulting from a balance of electrostatic interactions that can be tuned at short range but constrained to satisfy universal, asymptotic screening conditions. Short-range interactions and ion packing provide ample opportunity for chemical tunability, while asymptotic sum rules dictate that the long-range structure and charge oscillation be similar to those of molten alkali halide salts. In this work, we study the structure factors and long-range electrostatic interactions in six ILs. The cation in all cases is 1-butyl-3-methylimidazolium (BMIM+), and we study six anions, namely, tetrafluoroborate (BF4-), hexafluorophosphate (PF6-), nitrate (NO3-), triflate (CF3SO3-), bisfluorosulfonylimide [(FSO2)2N-], and bistriflimide [(CF3SO2)2N-]. To gain insight, we perform similar computer simulations of a primitive molten salt model with and without electronic polarization. We emphasize universal similarities among ionic liquids and molten salts in the long-range ion ordering and the influence of electronic polarization on the screening conditions while also characterizing important differences in the short-range electrostatic interactions. We show that polarization systematically reduces charge oscillations by as much as ∼0.5-1 ion per radial shell, which we argue is general to all room-temperature ILs as well as molten salts. We suggest that a fundamentally important distinction among BMIM-based ionic liquids (with different anions) is the nature of the midrange, ∼1 Å-1 peak in the charge-correlation structure factor; while this correlation is straightforward to analyze in computer simulations, it may often be hidden in X-ray and/or neutron scattering structure factors.

Physicochemical and Electrochemical Properties of Room-Temperature Ionic Liquids Based on Quaternary Phosphonium Cations and Tetracyanoborate Anion

The physicochemical and electrochemical properties of room-temperature ionic liquids based quaternary phosphonium cations together with tetracyanoborate anion are presented in this report. It was found that the tetracyanoborate anion based phosphonium ionic liquids showed relatively low viscosity, high conductivity and considerably high thermal stability when compared to those of the corresponding ammonium and the conventional phosphonium ionic liquids. The introduction of a methoxy group into the phosphonium cation tended to reduce the thermal stability and the electrochemical stability of the ionic liquids.

Predictions of Physicochemical Properties of Ionic Liquids with DFT

Nowadays, density functional theory (DFT)-based high-throughput computational approach is becoming more efficient and, thus, attractive for finding advanced materials for electrochemical applications. In this work, we illustrate how theoretical models, computational methods, and informatics techniques can be put together to form a simple DFT-based throughput computational workflow for predicting physicochemical properties of room-temperature ionic liquids. The developed workflow has been used for screening a set of 48 ionic pairs and for analyzing the gathered data. The predicted relative electrochemical stabilities, ionic charges and dynamic properties of the investigated ionic liquids are discussed in the light of their potential practical applications.

Chemical Analysis of Ionic Liquids Using Photoelectron Spectroscopy

The feasibility of utilizing X‐ray photoelectron spectroscopy (XPS) to analyze room‐temperature ionic liquids (RTILs) was investigated in this study. Conventionally, the chemical structure of organic compounds is identified by nuclear magnetic resonance (NMR) spectroscopy. The properties of RTILs, especially their low vapor pressure, make it possible to analyze RTILs by using XPS. The usefulness of XPS on RTILs was confirmed by commercial RTILs. All atoms in RTILs were detected in survey XPS spectra, and the calculated atomic percentages matched well with theoretical values. After the verification of commercial RTILs by XPS, we synthesized three RTILs and investigated them with XPS. The atomic ratio and chemical environment of carbon in RTILs were verified by XPS. By adapting XPS to the investigation of RTILs, carbon atoms in different chemical environments were distinguishable by the binding energy shift, and the atomic ratio of the constituent atoms was identifiable after peak deconvolution. In addition, inorganic constituents were detected by XPS unlike in the case of NMR spectroscopy.

Physical and Electrochemical Properties of Room-Temperature Ionic Liquids Containing Allyl-Based Phosphonium Cations and Bis(fluorosulfonyl)Amide Anion

The physical and electrochemical properties of room-temperature ionic liquids based on quaternary phosphonium cations containing an allyl group in combination with bis(fluorosulfonyl)amide anion. It was found that the allyl-based phosphonium RTILs showed lower melting points, lower viscosities and higher conductivities than those of the unsubstituted phosphonium and the corresponding ammonium RTILs. The allyl-based phosphonium RTILs with bis(trifluoromethyl- sulfonyl)amide anion exhibited relatively high thermal stability when compared to the corresponding bis(fluorosulfonyl)amide anion based RTILs. The electrochemical stability tended to be more or less reduced by introducing the allyl group.

Water sorption properties of room-temperature ionic liquids over the whole range of water activity and molecular states of water in these media

The water sorption behavior for various RTILs ([C4C1im][BF4], [C4C1im][PF6], [C6C1im][PF6], [C4im][DBP], [C4im][BEHP] and [Et3HN][CF3SO3]) was studied over the whole range of water activity using a continuous gravimetric method and ATR-IR measurements.

Polarization versus temperature in pyridinium ionic liquids.

Electronic polarization and charge transfer effects play a crucial role in thermodynamic, structural, and transport properties of room-temperature ionic liquids (RTILs). These nonadditive interactions constitute a useful tool for tuning physical chemical behavior of RTILs. Polarization and charge transfer generally decay as temperature increases, although their presence should be expected over an entire condensed state temperature range. For the first time, we use three popular pyridinium-based RTILs to investigate temperature dependence of electronic polarization in RTILs, based on a nonperiodic electronic density description for a cation-anion pair. Atom-centered density matrix propagation molecular dynamics, supplemented by a weak coupling to an external bath, is used to simulate the temperature impact on system properties. We show that, quite surprisingly, nonadditivity in the cation-anion interactions changes negligibly between 300 and 900 K, while the average dipole moment increases due to thermal fluctuations of geometries. Our results contribute to the fundamental understanding of electronic effects in the condensed phase of ionic systems and foster progress in physical chemistry and engineering.

Solvent extraction of U(VI) by trioctylphosphine oxide using a room-temperature ionic liquid

The unique physical and chemical properties of room-temperature ionic liquids (RTILs) have recently received increasing attention as solvent alternatives for possible application in the field of nuclear industry, particularly in liquid-liquid separations of radioactive nuclides. We investigated solvent extraction of U(VI) from aqueous solutions into a commonly used ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C(4)mim][NTf2]) using trioctylphosphine oxide (TOPO) as an extractant. The effects of contact time, TOPO concentration, acidity, and nitrate ions on the U(VI) extraction are discussed in detail. The extraction mechanism was proposed based on slope analysis and UV-Vis measurement. The results clearly show that TOPO/[C(4)mim][NTf2] provides a highly efficient extraction of U(VI) from aqueous solution under near-neutral conditions. When the TOPO concentration was 10 mmol/L, the extraction of 1 mmol/L U(VI) was almost complete (> 97%). Both the extraction efficiency and distribution coefficient were much larger than in conventional organic solvents such as dichloromethane. Slope analysis confirmed that three TOPO molecules in [C(4)mim][NTf2] bound with one U(VI) ion and one nitrate ion was also involved in the complexation and formed the final extracted species of [UO2(NO3)(TOPO)(3)](+). Such a complex suggests that extraction occurs by a cation-exchange mode, which was subsequently evidenced by the fact that the concentration of C(4)mim(+) in the aqueous phase increased linearly with the extraction percent of U(VI) recorded by UV-Vis measurement.

Static and Transport Properties of Alkyltrimethylammonium Cation-Based Room-Temperature Ionic Liquids

We have measured physicochemical properties of five alkyltrimethylammonium cation-based room-temperature ionic liquids and compared them with those obtained from computational methods. We have found that static properties (density and refractive index) and transport properties (ionic conductivity, self-diffusion coefficient, and viscosity) of these ionic liquids show close relations with the length of the alkyl chain. In particular, static properties obtained by experimental methods exhibit a trend complementary to that by computational methods (refractive index ∝ [polarizability/molar volume]). Moreover, the self-diffusion coefficient obtained by molecular dynamics (MD) simulation was consistent with the data obtained by the pulsed-gradient spin-echo nuclear magnetic resonance technique, which suggests that computational methods can be supplemental tools to predict physicochemical properties of room-temperature ionic liquids.

Influence of Introducing a Benzyl Group on Physical and Electrochemical Properties of Bis(fluorosulfonyl)amide-Based Phosphonium Ionic Liquids

The physical and electrochemical properties of room-temperature ionic liquids based on quaternary phosphonium cations containing a benzyl group in combination with bis(fluorosulfonyl)amide anion. It was found that the benzylphosphonium RTILs showed lower melting points, lower viscosities and higher conductivities than those of the corresponding TFSA-based benzylphosphonium and ammonium RTILs. The thermal stability of the benzyl-phosphonium RTILs was significantly improved by introducing a phenyl group into the phosphonium cations. The introduction of a phenyl group tended to reduce the electrochemical stability of the benzylphosphonium RTILs.

Dynamic and Structural Properties of Room-Temperature Ionic Liquids near Silica and Carbon Surfaces

The dynamic and structural properties of a room-temperature ionic liquid (RTIL) 1-butyl-3-methyl-imidazolium(trifluoromethanesulfonimide) ([C4mim][Tf2N]) confined in silica and carbon mesopores were investigated by molecular dynamics (MD) simulations and nuclear magnetic resonance (NMR) experiments. The complex interfacial microstructures of confined [C4mim][Tf2N] are attributed to the distinctive surface features of the silica mesopore. The temperature-dependent diffusion coefficients of [C4mim][Tf2N] confined in the silica or carbon mesopore exhibit divergent behavior. The loading fraction (f = 1.0, 0.5, and 0.25) has a large effect on the magnitude of the diffusion coefficient in the silica pore and displays weaker temperature dependence as the loading fraction decreases. The diffusion coefficients of mesoporous carbon-confined [C4mim][Tf2N] are relatively insensitive to the loading faction and exhibit a temperature dependence that is similar to the bulk dependence at all loading levels. Such phenomena can be attributed to the unique surface heterogeneity, dissimilar interfacial microstructures, and interaction potential profile of RTILs near silica and carbon walls.

MD Simulations of the Formation of Stable Clusters in Mixtures of Alkaline Salts and Imidazolium-Based Ionic Liquids

Structural and dynamical properties of room-temperature ionic liquids containing the cation 1-butyl-3-methylimidazolium ([BMIM](+)) and three different anions (hexafluorophosphate, [PF6](-), tetrafluoroborate, [BF4](-), and bis(trifluoromethylsulfonyl)imide, [NTf2](-)) doped with several molar fractions of lithium salts with a common anion at 298.15 K and 1 atm were investigated by means of molecular dynamics simulations. The effect of the size of the salt cation was also analyzed by comparing these results with those for mixtures of [BMIM][PF6] with NaPF6. Lithium/sodium solvation and ionic mobilities were analyzed via the study of radial distribution functions, coordination numbers, cage autocorrelation functions, mean-square displacements (including the analysis of both ballistic and diffusive regimes), self-diffusion coefficients of all the ionic species, velocity and current autocorrelation functions, and ionic conductivity in all the ionic liquid/salt systems. We found that lithium and sodium cations are strongly coordinated in two different positions with the anion present in the mixture. Moreover, [Li](+) and [Na](+) cations were found to form bonded-like, long-lived aggregates with the anions in their first solvation shell, which act as very stable kinetic entities within which a marked rattling motion of salt ions takes place. With very long MD simulation runs, this phenomenon is proved to be on the basis of the decrease of self-diffusion coefficients and ionic conductivities previously reported in experimental and computational results.

Transport properties of room-temperature ionic liquids from classical molecular dynamics

Room-temperature ionic liquids (RTILs) have attracted much attention in the scientific community in the past decade due their novel and highly customizable properties. Nonetheless, their high viscosities pose serious limitations to the use of RTILs in practical applications. To elucidate some of the physical aspects behind transport properties of RTILs, extensive classical molecular dynamics calculations are reported. Here, in particular, bulk viscosities and ionic conductivities of butyl-methyl-imidazole based RTILs are presented over a wide range of temperatures. The dependence of the properties of the liquids on simulation parameters, e.g., system-size effects or the choice of the interaction potential, is analyzed in detail.

HYDROGEN BOND NETWORK OF 1-ALKYL-3-METHYLIMIDAZOLIUM IONIC LIQUIDS: A NETWORK THEORY ANALYSIS

Hydrogen bond is a key factor in the determination of structures and properties of room-temperature ionic liquids. Connections of these hydrogen bonds form a network. In this work, we analyzed the hydrogen bond network of 1-alkyl-3-methylimidazolium ionic liquids using network theory. A two-dimensional view of the hydrogen bond network has been generated, the connection pattern shown that the average length of line shape connection is 2.44 to 2.77 for six 1-alkyl-3-methylimidazolium ionic liquids, and the connection patterns are different for short and long alkyl side chain length. The degree of each ion was calculated and analyzed. The nodes with zero degree were adopted to detect the boundary of the clusters in the ionic liquids, which have no hydrogen bond connected with neighbor ions. This work indicates that the network analysis method is useful for understanding and predicting the structure and function of RTILs.

The thermodynamic properties prediction of aqueous ionic liquid ([Emim][X]) solutions by Monte Carlo simulation

Unrestricted Primitive Model (UPM) has been applied to predict various properties of room-temperature ionic liquids (RTILs) systems. Five RTILs with different characteristics are selected for this study: 1-etyl-3-methylimidazolium ([Emim][X], X=iodide, chloride, tri-fluoromethanesulfonate, trifluoroacetate, and ethyl sulfate) aqueous solutions. Monte Carlo (MC) simulations are carried out in the NVT ensemble to calculate the thermodynamic properties. Binary mixtures i.e. [Emim][Cl]+water are studied in three temperatures (298.15K, 308.15K and 318.15K) to calculate internal energy and osmotic coefficient. Moreover, activity of solvent, osmotic coefficient and activity coefficient for [Emim][Otf]+water and [Emim][Tfa]+water are reported at 298.15K. Available experimental values of osmotic coefficients for [Emim][EtSO4] are compared with simulation results. In agreement with previous experimental studies, our results support the validity of the UPM to investigate aqueous solutions of RTILs.

Application of ionic liquids in nuclear fuel reprocessing

The nuclear fuel reprocessing, as the essential part of nuclear fuel cycle, is of great significance from the point of view of both nuclear safety and sustainable development of nuclear energy. Room temperature ionic liquids (RTILs) regarded as “new generation green solvents” have recently received an ever-increasing amount of interest in nuclear fuel reprocessing due to their unique physical and chemical properties. They can be used in aqueous reprocessing as environmentally benign alternatives to volatile organic solvents for traditional liquid-liquid extraction of high level radioactive nuclides. They are also applicable in non-aqueous reprocessing by substituting caustic molten salts for electro-deposition of metal ions. Herein, we reviewed the recent basic researches on the utility of RTILs in nuclear fuel reprocessing, from which the key scientific issues on their practical application were summarized. In addition, it is well known that nuclear fuel reprocessing involves high-level radioactive matter, and full realization of the potential application of RTILs in nuclear fuel reprocessing requires a comprehensive knowledge of stability and properties of RTILs under radiation. Therefore, the recent investigations of radiation effects on RTILs were also selectively described in this paper, and the feasibility of RTILs as alternative media for separations of highly radioactive nuclides from spent nuclear fuel was assessed from the radiation chemistry aspect. Finally, further basic studies on the application of RTILs in the nuclear fuel reprocessing were prospected.