Phase Behavior Of Ionic Liquids Research Articles

Investigation of Nonidealities through an Extended Flory–Huggins Model of Ionic Liquid Mixtures with Ammonia

The phase behavior of ionic liquids and ammonia is critical due to their application in various fields such as sorption-based heating/cooling cycles and direct ammonia capture. To date, various ionic liquids have been developed and investigated to enhance the solubility of ammonia without compromising the reversibility due to recycling concerns and various thermodynamic models have been used to correlate the experimental vapor–liquid equilibria data. Surely, high ammonia sorption capacities are required and understanding the interaction between ammonia and ionic liquids can lead to synthesizing targeted ionic liquids for ammonia. In this study, we developed an extended Flory–Huggins model on ammonia and imidazolium-based ionic liquid systems, where both combinatorial and residual terms are considered, to understand the interaction between ammonia and ionic liquids. The results showed that the nonidealities in ammonia and imidazolium-based IL mixtures are due to both entropic and enthalpic contributions. Therefore, increasing the size of the IL would not be enough to achieve higher solubilities. A deeper understanding of the molecular-level interaction between ionic liquids and ammonia can be investigated via molecular simulations.

Ionic-Liquid-Based Aqueous Two-Phase Systems Induced by Intra- and Intermolecular Hydrogen Bonds.

In recent years, aqueous two-phase systems (ATPSs) have been widely used in different fields and have become an increasingly attractive subject due to their application in the separation and purification of biomolecules. In this work, the aqueous phase behavior of ionic liquids (ILs) was modulated by changing the cis-trans structure of the anion in ILs. With the same tetra-butyl-phosphine as the cation, the cis-anion exhibited upper critical solution temperature (UCST) phenomena. In contrast, the trans-anion exhibited lower critical solution temperature (LCST) phenomena. The proposed mechanism shows that the main factors responsible for these phenomena include variations in the dissociation degree with temperature and the steric hindrance of the ILs. This phase behavior combines the chemical equilibrium in a solution with the microstructure of the molecule and is useful for constructing new chemical dynamic equilibria in ATPS. As an example of its application, aqueous solutions of both ILs can be used for the efficient separation and extraction of specific amino acids. The two ATPS systems reported in this work highlight a simple, effective, and environmentally friendly method for separating small biological molecules.

Does Explicit Polarizability Improve Simulations of Phase Behavior of Ionic Liquids?

Molecular dynamics simulations are performed for a test set of 20 aprotic ionic liquids to investigate whether including an explicit polarizability model in the force field leads to higher accuracy and reliability of the calculated phase behavior properties, especially the enthalpy of fusion. A classical nonpolarizable all-atom optimized potentials for liquid simulations (OPLS) force-field model developed by Canongia Lopes and Pádua (CL&P) serves as a reference level of theory. Polarizability is included either in the form of Drude oscillators, resulting in the CL&P-D models, or in the framework of the atomic multipole optimized energetics for biomolecular application (AMOEBA) force field with polarizable atomic sites. Benchmarking of the calculated fusion enthalpy values against the experimental data reveals that overall the nonpolarizable CL&P model and polarizable CL&P-D models perform similarly with average deviations of about 30%. However, fusion enthalpies from the CL&P-D models exhibit a stronger correlation with their experimental counterparts. The least successful predictions are interestingly obtained from AMOEBA (deviation ca. 60%), which may indicate that a reparametrization of this force-field model is needed to achieve improved predictions of the fusion enthalpy. In general, all FF models tend to underestimate the fusion enthalpies. In addition, quantum chemical calculations are used to compute the electronic cohesive energies of the crystalline phases of the ionic liquids and of the interaction energies within the ion pair. Significant positive correlations are found between the fusion enthalpy and the cohesive energies. The character of the present anions predetermines the magnitude of individual mechanistic components of the interaction energy and related enthalpic and cohesive properties.

A modified UNIQUAC model for calculating phase-equilibrium of ionic liquid containing systems

Since ionic liquids are increasingly employed as separation media in separation technologies, the development of thermodynamic model for design and optimization of the corresponding separation operations is very urgent. At present, the widely used thermodynamic models treat ionic liquid as a non-electrolyte or strong electrolyte. Although the calculation results are close to the experimental results, the treatment of ionic liquid does not consistent with the physical fact that ionic liquids ionize in solution, even in pure ionic liquid. In this work, we proposed a modified model for the calculation of the phase behavior of ionic liquid containing systems. The model combines a term of the modified Debye-Hückel type with a UNIQUAC equation and treats ionic liquid as a weak electrolyte, using an empirical expression to describe the ionization of ionic liquid. The new model has been utilized to correlate the vapor-liquid equilibrium, liquid-liquid equilibrium, solid-liquid equilibrium data as well as infinite dilution activity coefficient for ionic liquid containing systems with high accuracy. Besides, the model can be used to quantitatively describe the ionization behavior of ionic liquid.

The temperature influence on the phase behavior of ionic liquid based aqueous two-phase systems and its extraction efficiency of 2-chlorophenol

With the increasing research on the aqueous two-phase systems (ATPS), the ATPS containing ionic liquid (IL) and salt have been widely used in the extraction and purification process. In this work, the partitioning of 2-chlorophenol in novel ATPS composed of n-butylpyridinium trifluoromethanesulfonate and salts (ammonium sulfate, sodium dihydrogen phosphate) is evaluated. Firstly, the cloud point titration method was used to determine the binodal curves data experimentally at four temperatures and atmospheric pressure. And Merchuk equation was used to fit the binodal curves. Then, the gravimetric method was used to initially determine the tie-lines data and the concentration of all the ions in the top and bottom phases were determined by ion chromatography method to study the ion exchange in the ATPS. The reliability of tie-lines was evaluated by Othmer-Tobias and Bancroft equations. Finally, the influence of types of salts and temperature on the phase diagrams were studied carefully. Besides, in order to analyze the application of ATPS in the extraction, the partition coefficient of 2-chlorophenol was calculated at the same total composition while at different temperature. The ATPS formed by n-butylpyridinium trifluoromethanesulfonate, ammonium sulfate, and water at 298.15 K was found to have the maximum amount of partition coefficient and extraction efficiency.

Cationic clustering influences the phase behaviour of ionic liquids

“Unlike charges attract, but like charges repel”. This conventional wisdom has been recently challenged for ionic liquids. It could be shown that like-charged ions attract each other despite the powerful opposing electrostatic forces. In principle, cooperative hydrogen bonding between ions of like-charge can overcome the repulsive Coulomb interaction while pushing the limits of chemical bonding. The key challenge of this solvation phenomenon is to establish design principles for the efficient formation of clusters of like-charged ions in ionic liquids. This is realised here for a set of well-suited ionic liquids including the same hydrophobic anion but different cations all equipped with hydroxyethyl groups for possible H-bonding. The formation of H-bonded cationic clusters can be controlled by the delocalization of the positive charge on the cations. Strongly localized charge results in cation-anion interaction, delocalized charge leads to the formation of cationic clusters. For the first time we can show, that the cationic clusters influence the properties of ILs. ILs comprising these clusters can be supercooled and form glasses. Crystalline structures are obtained only, if the ILs are dominantly characterized by the attraction between opposite-charged ions resulting in conventional ion pairs. That may open a new path for controlling glass formation and crystallization. The glass temperatures and the phase transitions of the ILs are observed by differential scanning calorimetry (DSC) and infrared (IR) spectroscopy.

Physicochemical features of short-chain 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)-imide ionic liquids containing equilibrium water absorbed from air

In the present research we studied ionic liquids of 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [CnMIm][TFSI], where n = 2 (ethyl, C2), 3 (propyl, C3) and 4 (butyl, C4) with small water impurity absorbed though the contact with air. IR spectra, thermodynamic, electrochemical and the bulk properties, such as density and viscosity, were determined by means of FT-IR spectroscopy, conductometry, densimetry, viscosimetry, TG and DSC methods. The influence of temperature, absorbed water and elongation of the cation alkyl chain on the physicochemical properties was studied. It was found that the absorbed water (approximately 0.24–0.25% by weight) is frozen at a temperature from −40.5 to −46.3 °C, has little effect on thermal stability and significantly affects the phase behavior of ionic liquids. The linearly dependence of the density on the temperature was observed. The conductivity increases and the viscosity decreases with increasing temperature according with the VFT equation. Physicochemical properties depend on alkyl chain length, but not linearly, so that C3 is close to the transition between C2 and C4. Such tendency was explained in terms of Van der Waals forces and hydrogen bonds.

Effect of Fluorinated Anion on the Physicochemical, Rheological and Solvatochromic Properties of Protic and Aprotic Ionic Liquids: Experimental and Computational Study

AbstractThe essential properties of ionic liquids (ILs) for various applications can be tuned by changing the combination of cations or anions in ILs. This work demonstrates how the structure variations effect the physicochemical, rheological and solvatochromic properties of protic and aprotic ionic liquids. For this, a set of four ionic liquids have been synthesized and characterized through spectral techniques. Thermal stability and phase behaviour of ILs have been analyzed by TGA and DSC. The experimental density, speed of sound and viscosity have been measured as a function of temperature from 293.15 to 343.15 K and an attempt has been made to distinguish the crucial role of counter ions on measured properties. Further, the fluidity and polarity behaviour of hydroxyl ammonium based protic or 1‐alkyl‐1,2,4‐triazolium based aprotic cation and fluorinated anions were investigated through their rheological and polarity parameters at 298.15 K and discussion have been made on the basis of columbic and hydrogen bonding interactions. Interestingly, we found that anions have dominant effect on studied physicochemical properties irrespective of protic/aprotic nature of ionic liquid while the contribution of cations were vital for solvatochromic and rheological properties. Additionally, the ion‐pair interaction energy, dipole moment and hydrogen bonding interactions of studied ILs have been analysed using density functional theory (DFT) calculations to support the experimental findings.

Modification of the Extended UNIQUAC Model to Study the Phase Behavior of the [Bmim]Cl–K2HPO4 Aqueous Two Phase System at 298.15 K

Aqueous two phase systems (ATPS), consisting of two incompatible aqueous solutions, are widely used in biological extraction. In recent years, many experimental studies on the phase behavior of ionic liquid(IL)–inorganic salt ATPS systems have been reported but few investigations of the thermodynamics of the IL-based ATPS have been reported. In this work, the Extended UNIQUAC model is modified to calculate the equilibrium of [Bmim]Cl–K2HPO4–H2O ATPS at 298.15 K. All the interaction parameters of the model are salt specific. The results show that the experimental compositions of tie lines are correlated successfully using the proposed model. The root mean square deviation of the mass fraction is 0.0256.

Representation of phase behavior of ionic liquids and their mixtures using various forms of cubic-two-state equation of state

In this work, in addition of Soave-Redlich-Kwong (SRK) equation of state, two other alternative cubic models such as Peng- Robinson (PR) and Cubic Square Well (CSW) EoSs are combined with the Two-State Association Model (TSAM) and the new forms of the Cubic-Two-State equation of state (CTS EoS) as PR-TS and CSW-TS are presented. In the beginning, the present models are used to correlate the saturated vapor pressure and liquid density of pure water, alcohols and phenol. The models are also employed to predict vapor molar volume and second virial coefficients of pure compounds to explore the predictability of the different type of the CTS models. Also, the liquid density of different pure ionic liquids (ILs) at various temperatures from 298.15 K up to 500 K and pressure from 0.1 MPa up to 59.59 MPa is correlated and the parameters of the CTS models are obtained. Following successful application of the models for the pure components, using one temperature independent binary interaction parameter, the SRK-TS and PR-TS models are applied to predict the vapor-liquid equilibrium of the several binary mixtures consists of IL with non-association and association components at various temperatures from 298.15 K up to 363.15 K and pressure from 0.09 kPa up to 216 kPa. The results of the SRK-TS and PR-TS models for pure and binary systems containing ILs are in very good agreement with experiments and also are compared with various equations of states that show better results. In overall for the systems that are presented in this work, the PR-TS model shows slightly better results with respect to the SRK-TS model specially in prediction, and PR-TS and SRK-TS models present superiority with respect to the CSW-TS EoS.

Lower critical solution temperature (LCST) phase behaviour of an ionic liquid and its control by supramolecular host-guest interactions.

Lower critical solution temperature (LCST) phase behaviour of an imidazolium-based ionic liquid is reported, which can be controlled by concentration, the choice of cation, anion and solvent, and by supramolecular host-guest complex formation. Molecular dynamics simulations provide insight into the molecular basis of this LCST phenomenon. This thermo-responsive system has potential applications in cloud point extraction processes.

Successive disorder to disorder phase transitions in ionic liquid [HMIM][BF4] under high pressure

In situ high-pressure Raman spectroscopy and synchrotron X-ray diffraction have been employed to investigate the phase behavior of ionic liquid, 1-hexyl-3-methylimidazolium tetrafluoroborate ([HMIM][BF4]) under high pressure up to 20 GPa at room temperature. With increasing pressure, some characteristic bands of [HMIM][BF4] disappear, and some characteristic bands of [HMIM][BF4] display non-monotonic pressure-induced frequency shift and non-monotonic variation of full width at half-maximum. Two successive phase transitions at ∼1.7 GPa and 7.3 GPa have been corroborated by the results above. The glass transition pressure (Pg) of [HMIM][BF4] at ∼7.3 GPa has been obtained by ruby R1 line broadening measurements and analysis of synchrotron X-ray diffraction patterns, and its glass transition mechanism is also analyzed in detail. These facts are suggestive of two successive disorder to disorder phase transitions induced by compression, that is, [HMIM][BF4] serves as a superpressurized glass under the pressure above 7.3 GPa, which is similar to the glassy state at low temperature, and a compression-induced liquid to liquid phase transition in [HMIM][BF4] occurs at ∼1.7 GPa. Besides, the conformational equilibrium of the GAAA conformer and AAAA conformer was converted easily in liquid [HMIM][BF4], while it was difficult to be influenced in glassy state.

Pressure-induced amorphization of ionic liquid [HMIM][PF6

Phase behavior of ionic liquid [HMIM][PF6] has been investigated under high pressure up to 5.6GPa at room temperature. The results indicated that [HMIM][PF6] might experience a phase transition at about 3.4GPa upon compression, which could be identified as solidification to superpressurized glass by ruby R1 line broadening measurement and synchrotron X-ray diffraction (XRD) patterns. For conformational equilibrium, the fraction of all-anti (AAAA) conformer increased upon compression, while the conformational change was independent of temperature. These facts indicated that there were large differences of the structure in response to the extreme conditions, especially in the structure of the cation.

Azeotropy Breaking in Refrigerant Blends with Ionic Liquids

The principal aim of this work is to study azeotropy breaking in the refrigerant blends with ionic liquids (IL). The global phase diagram approach is applied to correlate azeotropic data for binary mixtures based only on critical properties and acentric factor of the individual components in mixtures. Analytical expressions to predict azeotropy phenomena in terms of critical parameters of pure components and binary interaction parameters are given. The eventual azeotropy appearance in the refrigerant - IL blends is discussed and conclusion about highly improbable azeotropic blend formation for these systems is given. Global phase behavior of ionic liquid - industrial refrigerant blends is analyzed and possible III, IV and/or V types according to the classification scheme of Scott and van Konynenburg [1] are established. The azeotropy breaking in binary refrigerant mixtures with ionic liquid adding is predicted.

Phase Equilibria of Binary and Ternary Systems Containing ILs, Dodecane, and Cyclohexanecarboxylic Acid

This paper reports both the binary and ternary phase behavior of ionic liquids for extracting cyclohexanecarboxylic acid (CCA) from dodecane. This system is a model for the extraction of acids representative of naphthenic acids found in crude oils. In order to develop an effective ternary liquid-liquid extraction system the preliminary selection of ionic liquids was based on CCA miscibility and the dodecane immiscibility with selected ILs. A wide range of ILs based on different cations, anions, cation alkyl-chain length, as well as the effect of temperature on the overall fluid phase behavior is reported. Factors such as variation of cation group, anion effect, alkyl-chain length, and temperature all impact the extraction to various degrees. The largest effects were found to be the lipophilicity of the IL cation and the co-ordination ability of the anion. While CCA capacity increased with lipophilicity of the cation, as did the dodecane. Highly coordinating anions such as trifluoroacetate and triflate demonstrated that highly efficient extraction could be obtained producing favorable tie-lines in the ternary phase diagram. Overall, this study demonstrates that ILs can selectively extract acids from hydrocarbon streams and offers possible treatment solutions for problems associated with the processing of high acid crude oils. Supplemental materials are available for this article. Go to the publisher's online edition of Separation Science and Technology to view the free supplemental file.

Polyacrylic acid polymer modulates the UCST-type phase behavior of ionic liquid and water

Upper critical solution temperature (UCST)-type phase behavior was observed in the critical mixture of water (W) and hydrophobic ionic liquid, 1-hexyl-3-methylimidazolium tetrafluoroborate (IL), by refractive index and fluorescence measurements. The phase separation temperature depends on the composition of IL as well as water. Furthermore, for the first time, the behavior of high molecular weight polymer polyacrylic acid, PAA (Mw = 450 000 g mol−1), as an impurity in the UCST mixture was investigated by simultaneous refractive index (n) measurements for both the coexisting phases of ILW. The refractive index in each phase of ILW as well as three different PAA concentrations (C = 0.5, 1.0, and 1.5 mg cm−3) in the near critical composition of ILW have been measured at temperatures below the system's upper critical point. We observed that the polymer significantly affected on the critical region of ILW. The phase-transition region of coexisting phases of ILW significantly shifts down with an increase in the concentrations of PAA. The Tc values decreases linearly from 58.256 (free of PAA) to 55.168 °C (PAA in ILW) with increasing PAA concentrations in ILW. This indicates that the polymer chain entangles with the coexisting phases, thereby the polymer monomers strongly interact with neighbor solvent particles. At temperatures T close enough to Tc, the critical exponent (β) values were obtained from the measured n values of both the coexisting liquid phases and was found to increase from 0.332 to 0.379, when the PAA concentration changes from 0.5 to 1.5 mg cm−3. The obtained 3D Ising values are modified in the presence of PAA and obviously belong to the renormalized class. These values are higher than that of 0.326 ± 0.002 of pure ILW, which is compatible with the 3D Ising value β = 0.325. In addition, we have performed the fluorescence measurement for the determination of Tc for ILW and in the presence of PAA as an impurity. The observed increase in the fluorescence intensities with temperature predicts unambiguously the formation of the solvation structure at Tc for the critical mixture in presence of the polymer.

The role of long-range interactions in the phase behavior of ionic liquids

The COSMO-type liquid activity coefficient models are among the most successful methods for the prediction of the phase behaviours of fluids containing ionic liquids. However, these models consider only interactions between species that are in close contact; in other words, the long-range Coulomb interactions, which are known to be important for systems containing charged species, are completely ignored. Here we show that the inclusion of long-range terms (with, e.g., the Pitzer-Debye-Hückel model) is indeed critical to obtain the correct osmotic coefficients and the asymptotic behaviours in the infinite dilution of IL solutions. However, the direct combination of PDH and COSMO-type models does not produce accurate activity coefficient over the whole concentration range because both models contain short range interactions. We show that a recent revision of the COSMO-SAC model, which includes the PDH contributions and a specific treatment of solvent-ion and ion-ion interactions, is capable of providing an accurate description of the properties (phase behaviour, osmotic coefficients) of aqueous IL solutions over the whole concentration range.

Ionic liquids and oligomer electrolytes based on the B(CN)4− anion; ion association, physical and electrochemical properties

The role of B(CN)(4)(-) (Bison) as a component of battery electrolytes is addressed by investigating the ionic conductivity and phase behaviour of ionic liquids (ILs), ion association mechanisms, and the electrochemical stability and cycling properties of LiBison based electrochemical cells. For C(4)mpyrBison and C(2)mimBison ILs, and mixtures thereof, high ionic conductivities (3.4 ≤σ(ion)≤ 18 mS cm(-1)) are measured, which together with the glass transition temperatures (-80 ≤T(g)≤-76 °C) are found to shift systematically for most compositions. Unfortunately, poor solubility of LiBison in these ILs hinders their use as solvents for lithium salts, although good NaBison solubility offers an alternative application in Na(+) conducting electrolytes. The poor IL solubility of LiBison is predicted to be a result of a preferred monodentate ion association, according to first principles modelling, supported by Raman spectroscopy. The solubility is much improved in strongly Li(+) coordinating oligomers, for example polyethylene glycol dimethyl ether (PEGDME), with the practical performance tested in electrochemical cells. The electrolyte is found to be stable in Li/LiFePO(4) coin cells up to 4 V vs. Li and shows promising cycling performance, with a capacity retention of 99% over 22 cycles.

Mutual Solubility of Water and Structural/Positional Isomers of N-Alkylpyridinium-Based Ionic Liquids

Despite many previous important contributions to the characterization of the liquid-liquid phase behavior of ionic liquids (ILs) plus water systems, a gap still exists as far as the effect of isomers (of ILs) is concerned. Therefore, in this work, a comprehensive study of the liquid-liquid equilibria between water and isomeric pyridinium-based ionic liquids has been performed. Atmospheric pressure mutual solubilities between water and pyridinium-based ionic liquids combined with the common anion bis[(trifluoromethyl)sulfonyl]imide were experimentally determined between (288.15 and 318.15) K. The main goal of this work is to study the isomeric effects on the pyridinium-based cation, namely, the structural and positional isomerism, as well as the alkyl side chain length. To the best of our knowledge, the influence of both structural and positional isomerism on the liquid-liquid behavior in ionic-liquid-water-containing systems is an unexplored field and is here assessed for the first time. Moreover, from the experimental solubility data, several infinite dilution molar thermodynamic functions of solution, namely, the Gibbs energy, the enthalpy, and the entropy, were estimated and discussed. In addition, aiming at gathering a broader picture of the underlying thermodynamic solvation phenomenon, molecular dynamics simulations were also carried out for the same experimental systems.

Tuning the phase behavior of ionic liquids in organically functionalized silica ionogels

We have synthesized mesoporous silica monoliths functionalized with 2-(4-pyridylethyl)triethoxysilane 1 and N,N-dimethyl-pyridine-4-yl-(3-triethoxysilyl-propyl)-ammonium iodide 2. The organically modified silica monoliths were characterized via IR spectroscopy, nitrogen sorption, small angle X-ray scattering (SAXS), thermogravimetric analysis-differential thermal analysis (TGA-DTA), and acid-base titration. The degree of functionalization can be changed by the ratio of the functional silane to the silica precursor tetramethyl orthosilicate (TMOS). The functionalized silica monoliths were filled with 1-ethyl-3-methyl imidazolium [Emim]-X (X = dicyanamide [N(CN)2] or triflate [TfO]) ionic liquids (ILs) using an established methanol-IL exchange technique. The phase behavior of the resulting ionogels was investigated via differential scanning calorimetry (DSC). DSC curves show that the modification of the silica pore walls with organic groups strongly affects the phase behavior of the confined ILs. Modification with silane 1 completely suppresses the glassy state of [Emim][TfO] previously observed in unmodified silica monoliths (Göbel et al., Phys. Chem. Chem. Phys. 2009, 11, 3653). In contrast, modification with silane 2 leads to the appearance and disappearance, respectively, of a presumed additional phase in [Emim][TfO] and [Emim][N(CN)2] with varying degree of monolith functionalization. The data thus show that organic modification of silica matrix materials could be a viable approach for the tuning of ionogel properties.