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Ion exchange membranes (IEMs) are frequently used in water treatment and electrochemical applications, with their ion separation properties largely governed by equilibrium ion partitioning between a membrane and contiguous solution. Despite an expansive literature on IEMs, the influence of electrolyte association (i.e., ion pairing) on ion sorption remains relatively unexplored. In this study, salt sorption in two commercial cation exchange membranes equilibrated with 0.01–1.0 M MgSO4and Na2SO4is investigated experimentally and theoretically. Association measurements of salt solutions using conductometric experiments and the Stokes–Einstein approximation show significant concentrations of ion pairs in MgSO4and Na2SO4relative to those in simple electrolytes (i.e., NaCl), which is consistent with prior studies of sulfate salts. The Manning/Donnan model, developed and validated for halide salts in previous studies, substantially underpredicts sulfate sorption measurements, presumably due to ion pairing effects not accounted for in this established theory. These findings suggest that ion pairing can enhance salt sorption in IEMs due to partitioning of reduced valence species. By reformulating the Donnan and Manning models, a theoretical framework for predicting salt sorption in IEMs that explicitly considers electrolyte association is developed. Remarkably, theoretical predictions of sulfate sorption are improved by over an order of magnitude by accounting for ion speciation. In some cases, good quantitative agreement is observed between theoretical and experimental values for external salt concentrations between 0.1 and 1.0 M using no adjustable parameters.

The transport properties, thermal properties, and CO2solubility for several ionic liquids (ILs) with triethyl(octyl)phosphonium cations and a variety of CO2-reactive aprotic N-heterocyclic anions (AHAs) are reported in this work. Eleven new ILs were designed and synthesized. They were characterized in terms of their melting points, glass transition temperatures, decomposition temperatures, viscosities and densities (where possible), as well as their CO2capacity as a function of pressure. Of the 11, 3 were solid at room temperature, 1 was a room-temperature liquid which remained liquid upon reaction with CO2, and 7 others were liquids that crystallized at room temperature upon reaction with CO2, so experimentation at elevated temperatures was required. The CO2uptake isotherms for seven of the ILs, at temperatures ranging from 49 to 64 °C and pressures from 0 to 80 kPa, were fit to a Langmuir model. The CO2solubility for several of these ILs was among the highest reported at these temperatures and pressures for AHA ILs in the literature, but they have lower thermal stability and higher viscosity than other promising AHA ILs.

Henry’s law constants of various gases in 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([bmim][Tf2N]), 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([emim][Tf2N]), 1-allyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([allylmim][Tf2N]), and 1-ethyl-3-methylimidazolium ethylsulfate ([emim][EtSO4]) were measured using an inverse gas chromatography (GC) method. The gases explored were carbon dioxide, ethane, ethylene, propane, and propylene at temperatures ranging from 298.15 to 373.15 K. The temperature dependence of the Henry’s law constants was used to determine the standard enthalpies of absorption of the gases in the ionic liquids (ILs). Comparison of our measured values of Henry’s law constants and enthalpies with the literature leads to the conclusion that the GC method is a fast technique for screening ILs for various gas separation applications.

The solubility of CO2in poly(ethylene oxide), poly(ethyl glycidyl ether), poly(iso-propyl glycidyl ether), poly(allyl glycidyl ether), poly(n-butyl glycidyl ether), and poly(ethyl vinyl ether) was measured at room temperature and 333.15 K and pressures up to 15 bar. CO2solubility, expressed as mole fraction in terms of molecular weight of the polymer repeat unit, was directly related to polymer repeat unit molecular weight regardless of pendant chain structure or ether oxygen placement in the backbone. The molality of CO2was highest in poly(ethyl vinyl ether) and was equal in all the poly(glycidyl ethers) and poly(ethylene oxide). The standard enthalpies and entropies of CO2absorption in poly(ethylene oxide), poly(ethyl glycidyl ether), and poly(ethyl vinyl ether) were calculated from the Henry’s constants obtained from three isotherms. CO2dissolution was slightly more favorable enthalpically in poly(ethylene oxide). However, the entropic penalty for absorption was lower in poly(ethyl glycidyl ether) and poly(ethyl vinyl ether). These results suggest that poly(glycidyl ethers) and poly(ethyl vinyl ether) are promising alternatives to poly(ethylene oxide) for CO2separation by absorption or membrane separation because they have similar CO2uptake capacity, low glass transition temperatures, are amorphous, and are more hydrophobic than poly(ethylene oxide).

Phosphonium-based ionic liquids (ILs) paired with aprotic heterocyclic anions (AHAs) are found to react with CO2to form both a carbamate product and a carboxyl product. The carboxyl product is formed primarily at elevated temperatures through the formation of a phosphonium ylide intermediate. The formation of the carboxyl product leads to the formation of the neutral azole, which can lead to an irreversible process if the neutral azole is sufficiently volatile. To understand how the ILs would behave in a CO2capture process operated at elevated temperatures, it was necessary to quantify the two reaction products. CO2was reacted with seven different AHA ILs to determine the equilibrium amounts of carbamate and carboxyl, the equilibrium constants for both reactions, and the rate of CO2absorption by each reaction path. The reactions were tracked and quantified in situ by using ATR-FTIR spectroscopy, while NMR spectroscopy was used after equilibrium was reached to determine the extent of each reaction at multiple temperatures and pressures. It was found that both the basicity and molecular size of the anion play key roles in the formation of the phosphonium ylide. In the extreme case of [P66614][4-Triaz] only half of the reacted product was the desired carbamate at 60 °C. Although there is a significant amount of the carboxyl product formed, the carbamate is kinetically favored.

The performance of an ionic liquid with an aprotic heterocyclic anion (AHA-IL), trihexyl(tetradecyl)phosphonium 2-cyanopyrrolide ([P66614][2-CNPyr]), for CO2capture has been evaluated considering both the thermodynamics and the kinetics of the phenomena. Absorption gravimetric measurements of the gas–liquid equilibrium isotherms of CO2–AHA-IL systems were carried out from 298 to 333 K and at pressures up to 15 bar, analyzing the role of both chemical and physical absorption phenomena in the overall CO2solubility in the AHA-IL, as has been done previously. In addition, the kinetics of the CO2chemical absorption process was evaluated by in situ Fourier transform infrared spectroscopy-attenuated total reflection, following the characteristic vibrational signals of the reactants and products over the reaction time. A chemical absorption model was used to describe the time-dependent concentration of species involved in the reactive absorption, obtaining kinetic parameters (such as chemical reaction kinetic constants and diffusion coefficients) as a function of temperatures and pressures. As expected, the results demonstrate that the CO2absorption rate is mass-transfer-controlled because of the relatively high viscosity of AHA-IL. The AHA-IL was encapsulated in a porous carbon sphere (Encapsulated Ionic Liquid, ENIL) to improve the kinetic performance of the AHA-IL for CO2capture. The newly synthesized AHA-ENIL material was evaluated as a CO2sorbent with gravimetric absorption measurements. AHA-ENIL systems preserve the good CO2absorption capacity of the AHA-IL but drastically enhance the CO2absorption rate because of the increased gas–liquid surface contact area achieved by solvent encapsulation.

Mixtures of ionic liquids (ILs) present intriguing possibilities for modifying both physical and chemical properties relative to the pure ILs. Here we investigate the solubility of CO2and N2in mixtures of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([emim][TFSI]) with 1-ethyl-3-methylimidazolium tetrafluoroborate ([emim][BF4]) at 313.2 K and 1-ethyl-3-methylimidazolium dicyanamide ([emim][DCA]) at 313.2 and 333.2 K. CO2solubilities are measured at pressures up to 15 bar, while N2solubilities were measured up to 140 bar. Both gases dissolve in these ILs by physical dissolution. For both mixtures the Henry’s Law constants for CO2and N2in the mixtures are lower (i.e., higher solubility) than a mole fraction weighted linear combination of the Henry’s Law constants in the pure ILs. In fact, the Henry’s Law constants for both gases in the mixtures are adequately fit with a mole fraction weighted linear combination of the natural logarithmof the pure IL Henry’s Law constants. However, in no case is the solubility of either gas in the IL mixtures greater than or less than the solubilities in the two pure ILs. As a result, the anticipated CO2/N2selectivity in the IL mixtures, based on pure gas solubility measurements in the IL mixtures, is always between the values of the CO2/N2selectivity for the pure ILs.

Aprotic-heterocyclic anion (AHA) ionic liquids (ILs) have the potential to be separation and reaction media for carbon capture and utilization (CCU) processes. While CO2capture may occur at moderate temperatures (e.g., between ambient and 50 °C), thermochemical conversion of the CO2requires good thermal stability of the ILs so that they can withstand reaction at elevated temperatures. ILs with noncoordinating anions such as bis(trifluoromethylsulfonyl)imide ([Tf2N]−) have shown improved thermal stability when the cation is modified to have 2+ charge, i.e., so-called dicationic ILs. To our knowledge, no previous studies have paired dications with AHAs. A series of dicationic ILs composed of di(tetraalkylphosphonium) cations with tributyl chains and linkers ranging from propyl to undecyl, paired with 2-cyanopyrrolide anions, do show improved thermal stability compared to their monocationic AHA IL equivalents. Interestingly, the dicationic ILs all have similar CO2uptake per anion, although they exhibit CO2capacity intermediate between those of monocationic ILs with the 2-cyanopyrrolide anion and tetraalkylphosphonium cations with longer (trihexyl(tetradecyl)) and shorter (triethyloctyl) chains. This is consistent with differences in the standard entropy of the reaction between the anion and CO2. Like their monocationic AHA counterparts, dicationic AHA ILs are sensitive to the presence of oxygen, which results in the formation of phosphine oxides, as determined by long-term isothermal stress tests with air purging.

Previously, we have shown that the dissociation of ions in ionic liquids (ILs) and IL solutions can be estimated from measurements of ionic conductivity, viscosity, and density. Here, we explore the limitations of this method by studying the effects of solvent polarity and hydrogen bonding on ion dissociation in mixtures of 1-ethyl-3-methylimidazolium ethyl sulfate ([emim][EtSO4]), trifluoroacetate ([emim][TFA]), and triflate ([emim][TfO]) with ethylene glycol, triethanolamine, and glycerol. New density, viscosity, and ionic conductivity measurements reveal significant reductions in viscosity at dilute IL concentrations due to the disruption of solvent OH–OH hydrogen bonding. This is apparent because the pure solvent viscosities are comparable to or greater than those of the pure ILs. As a result, we are able to show that macro-scale viscosity measurements taken experimentally do not accurately capture the micro-viscous forces acting on diffusing ions, leading to unrealistic dissociation values that exceed unity. Finally, both ion dissociation and the energetic favorability of mixing (as measured by HE) increase with anion hydrophilicity for all three solvents.

Ionic liquids with aprotic heterocyclic anions (AHAs) have been developed for postcombustion CO2capture applications. The anions of AHA ILs play a significant role in tuning anion–CO2complexation. In addition, AHAs are able to trigger the abstraction of acidic protons located at the α position of phosphonium cations by forming hydrogen bonds between cations and anions, eventually leading to cation-driven CO2complexation. Here we investigate the role of the anion in cation–anion hydrogen bonding and ylide formation. Using CO2uptake measurements, 31P nuclear magnetic resonance (NMR), attenuated total reflection-Fourier transform infrared (ATR-FTIR) deuterium exchange equilibrium and rates, two-dimensional nuclear Overhauser effect spectroscopy (2D NOESY), and density functional theory calculations, we show that the key is the proximity of the negatively charged nitrogen atoms on the anion to the α protons, which is governed not just by anion basicity but by sterics. Thus, we show that triethyl(octyl)phosphonium 3-methyl-5-trifluoromethylpyrazolide is much more effective in hydrogen-bonding with and deprotonating the cation than the equivalent [P2228] ILs with more basic 2-cyanopyrrolide and 3-trifluoromethylpyrazolide anions.

Four ionic liquids (ILs) consisting of the bis(trifluoromethylsulfonyl)imide anion paired with a phosphonium, pyridinium, imidazolium, or thiolanium cation were investigated as potential solvents to separate aromatic from aliphatic compounds in a liquid–liquid extraction process. The thiolanium IL was chosen due to its structural similarity to sulfolane, which is the most widely used organic solvent for aromatic/aliphatic separation in the industry. Interestingly, ternary liquid–liquid equilibrium data shows that 1-n-butylthiolanium bis(trifluoromethylsulfonyl)imide performs as well as the equivalent imidazolium IL despite the fact that it is not aromatic and cannot use π–π interactions (which are available to imidazolium ILs) to enhance aromatic solubility and selectivity. Moreover, we provide quantification of the IL solubility in the organic-rich phase, which is on the order of 10–4mole fraction. This quantity is important because it would represent IL loss and product contamination in a real extraction process; however, it is commonly reported to be nondetectable. The nonrandom two-liquid (NRTL), perturbed-chain statistical associating fluid theory (PC-SAFT), COSMO-RS, and COSMO-SAC models are appropriate for the mixtures explored, and each model’s strengths and weaknesses are discussed.

Lower critical solution behavior in binary blends of hydrophobic polyethers with 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([hmim][Tf2N]) exhibited a difference in lower critical solution temperature (LCST) greater than 80 °C between structurally homologous poly(isopropyl glycidyl ether) (PiPGE) and poly(n-butyl glycidyl ether) (PnBGE). Replacement of the acidic hydrogen on the imidazolium ring with a methyl group (i.e., 2,3-dimethyl-1-hexyl-imidazolium bis(trifluoromethylsulfonyl)imide ([hmmim][Tf2N])) significantly reduced the LCST of both the PnBGE/ionic liquid (IL) mixture and the PiPGE/IL mixture. Differing degrees of hydrogen bonding between the polymer and the cation cannot alone explain the observed behavior. Similar hydrogen bonding between the [hmim]+cation and both polymers from molecular dynamics simulations was consistent with this conclusion. However, stronger [hmim]+cation tail/polymer alkyl side-chain interactions for PnBGE, with consequently stronger cation/anion interactions, point to solvophobic interactions as the basis for the large LCST difference between the PnBGE/[hmim][Tf2N] and PiPGE/[hmim][Tf2N] blends.

Virtually, every investigation and application of ionic liquids (ILs) involves gas–liquid, liquid–liquid, and liquid–solid interactions. Therefore, understanding the behavior of ILs at those interfaces is critical. In this work, we studied the interfacial properties of protic and aprotic ILs with N-alkylimidazolium and 1-alkyl-3-methylimidazolium as cations and bis(trifluoromethylsulfonyl)imide, methanesulfonate, and trifluoromethanesulfonate as anions. The surface tension of these ILs is measured with the pendant drop method in a temperature range of 293.15–343.15 K and at atmospheric pressure. The contact angle measurements are performed at 293.15 K on three solid substrates: polytetrafluoroethylene, glassy carbon, and platinum. Dispersive and nondispersive components of the IL surface energy are determined from the experimental data using Fowkes theory. The most interesting result is that the protic ILs have lower surface tension and smaller contact angles than the equivalent aprotic ILs, despite the presence of high charge density on the proton associated with one of the nitrogens of the cation. Higher charge density on the anion results in a higher surface tension, and decreasing surface tension and contact angles are observed for increasing alkyl chain length on the cation.

Separation of aromatics from aliphatics following catalytic reforming or steam cracking processes is difficult and energy intensive, even with highly optimized extraction solvents like sulfolane. As a result, ionic liquids (ILs) have been studied extensively for this application. Here we review the new liquid–liquid equilibrium (LLE) data that have appeared in the literature for aromatic/aliphatic/IL systems since 2012, when the last comprehensive review appeared. We model recent systems with the conductor-like screening model for real solvents (COSMO-RS) and discuss the ability of this model to predict trends in the aromatic selectivity (S) and aromatic distribution ratio (D). With some exceptions, COSMO-RS provides good performance in predicting trends in Sand Dwith respect to changes in aliphatic, aromatic, IL cation and IL anion. However, COSMO-RS predictions generally do not show quantitative agreement with experimental data for aromatic/aliphatic/IL LLE. Previously known trends for increasing length of alkyl chains on the IL cation or anion, aliphatic length, alkyl chains on the aromatic, and cyclization were confirmed by both the new data and modeling. The new data with different anions show that Dand Sdo not correlate with the anion solvatochromic basicity parameter, β, as suggested previously. Rather, ILs with anions that have greater polarity or polarizability have higher S, whereas less-coordinating anions have greater capacity for the aromatic (higher D). Of all the trends, COSMO-RS has the greatest difficulty in predicting the effect of changing anion, and this is not limited to anions that interact more strongly with the cation. For new systems with mixed alkanes, one would expect Sand Dto vary between the values of the pure alkanes, as confirmed by the COSMO-RS predictions. On the basis of limited new mixed IL data, it appears that COSMO-RS may not be able to predict this effect well. Nonetheless, COSMO-RS is shown to be a useful tool in predicting many qualitative trends in Sand Dfor the extraction of aromatics from aliphatics for pure IL systems.

Pure gas permeabilities of propylene and propane through ionic liquids supported on porous alumina membranes were determined as a function of silver salt concentration from 0 to 1 M and feed pressure from 0.15 to 2.0 bar at 35 °C. Two ionic liquids were tested: 1-hexyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide ([hmmim][Tf2N]) and 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([hmim][Tf2N]). Propylene permeability increased with increasing silver content due to facilitated transport, but it decreased with increasing feed pressure. Permeability selectivity exhibited a nonlinear upward trend with silver concentration, indicating the formation of higher order silver-propylene complexes at higher silver concentrations. Solubility measurements were performed gravimetrically, which allowed for simultaneous determination of diffusion coefficients. Permeabilities were hindered by low diffusion coefficients due to increasing viscosity with increasing silver content. The most surprising result was the stability of the membrane after exposure to hydrogen gas, suggesting that the ionic liquids significantly slowed silver ion reduction.

We use multiple characterization methods, including viscosity, ionic conductivity, density, and surface tension measurements, to investigate the unusual transport behavior of multiple tetra-alkyl-phosphonium-2-cyano-pyrrolide ([Pnnnm][2CNPyr]) ILs in glycerol and triethanolamine solvents. Specifically, the viscosity goes through a maximum for all of the IL/glycerol and IL/triethanolamine mixtures, which cannot be explained by traditional mixing rules. Through conductivity and surface tension characterization we find substantial evidence of microstructure formation driven by the amphiphilic structure of the tetra-alkyl-phosphonium ([Pnnnm]+) cation accounting for the uncommon viscosity behavior. While micelle formation is well-known for IL+ water systems, the observation of these microstructures in nonaqueous systems is a significant discovery. The slope of the ionic conductivity as a function of IL concentration changes substantially at the critical aggregate concentration cac, at which point the ILs undergo solvophobic self-assembly; this phenomenon is observed in all IL/solvent mixtures investigated. The cac is observed at more dilute IL concentrations for ILs containing longer alkyl chains. Furthermore, we observe lower cac values in glycerol compared to that of triethanolamine, owing to its higher dielectric constant and larger cohesion energy, as measured by the Gordon parameter.

Separating mixtures that form azeotropes or have relative volatilities close to 1.0 has always been a challenging problem. Conventional distillation cannot yield high purity products for these systems, so separating them often becomes a capital- and energy-intensive process. One of the most popular alternative methods used to accomplish these separations is extractive distillation, in which an entrainer is added to increase the relative volatility of the mixture. Ionic liquids (ILs) show great promise as entrainers due to their extremely low volatility, thermal stability, and chemical diversity. In this work, 1-ethyl-3-methylimidazolium methanesulfonate ([EMIM][MeSO3]) and 1-ethyl-3-methylimidazolium methylsulfate ([EMIM][MeSO4]) were investigated for their ability to break the methyl acetate + methanol azeotrope at 313.15 K. [EMIM][MeSO3] was able to break the azeotrope at just 2.5 mol % IL, the lowest value currently recorded in the literature. [EMIM][MeSO4] also performed well, breaking the azeotrope at 6.0 mol %. Finally, the Non-Random Two-Liquid (NRTL) model was used to correlate the vapor–liquid equilibrium (VLE) data of the binary systems and then predict the ternary phase behavior of systems with ionic liquids.

The hydrophobic–hydrophilic character of a series of 15 tetra-alkylphosphonium ionic liquids with aprotic heterocyclic anions (AHAs) was studied by measuring their liquid–liquid equilibrium with water. Specifically, the solubility of water in the ionic-liquid-rich phase was determined at room temperature by Karl Fischer titration. The effect of the tetra-alkyl-phosphonium cation was studied by varying the alkyl chain length; as expected, hydrophobicity increases with increasing alkyl chain length. The effect of the anion was studied by pairing the tetra-alkyl-phosphonium cation with different AHAs and by adding different substituents on the anion. The anion nature, basicity, and electron withdrawing effect of different substituents proved to have a significant impact on the IL’s hydrophobicity. Changes in the alkyl chain length of the cation and the nature of the anion are able to tune the solubility of water in the ILs over an order of magnitude.

The solubility of O2was measured gravimetrically in 16 ionic liquids (ILs) containing piperidinium, pyrrolidinium, ammonium, tetrahydrothiophenium, methyltriazabicyclodecene, or guanidinium cations paired with bis(trifluoromethanesulfonyl)imide, bis(perfluoroethylsulfonyl)imide, or perfluoropropanoyl(trifluoromethylsulfonyl)imide anions. The temperature dependence of the solubility was determined for eight of the ILs. Henry’ law constants were determined from the solubility data as well as standard enthalpies and entropies of absorption for systems with temperature-dependent data. Knowledge of these thermodynamic properties is essential in guiding the structural design of ionic liquids for different applications involving O2. The O2solubility did not follow any obvious trend with regard to IL structure or molar volume; instead, the dipole–quadrupole interactions are the primary driving forces for O2dissolution in ionic liquids, as demonstrated by negative standard enthalpies of absorption for all of the ILs. In addition, diffusion coefficients of O2in the ILs as a function of temperature (293, 313, and 333 K) and applied O2pressure (1, 3, and 5 MPa) were obtained by fitting the time-dependent absorption data to a widely used mass diffusion model. The diffusion coefficients of O2in ionic liquids at 293 K and 1 MPa roughly correlate inversely with the pure IL viscosity, likely due to the relatively low and similar O2solubilities under these conditions. As expected, O2diffusivities increase with increasing temperature. However, they also increase with increasing pressure, likely due to decreasing solution viscosity with increasing gas solubility.

Ionic liquids with aprotic heterocyclic anions (AHAs) have been developed for CO2capture but have been considered for other applications as well. Previously, we have shown that AHA ILs, where the only site for reaction with CO2is the anion, the CO2capacity correlates with anion basicity. Moreover, we have shown that 1-ethyl-3-methylimidazolium ([Emim]+)AHA ILs can react with CO2in two ways. The first is with the anion to form a carbamate. The second is by reaction of the deprotonated C2 of the imidazolium cation to form a carboxylate. Here we show that the amount of carboxylate formed when [Emim]+AHA ILs are exposed to CO2is not proportional to the anion basicity, contrary to expectations. Rather, it is roughly the same for all AHA ILs investigated, as long as the anion is able to readily deprotonate the C2. Moreover, the strength of the hydrogen bond between C2–H and the anion is not proportional to the anion basicity, once again contrary to expectations. In addition, we discovered that deuterium exchange occurs not only with the acidic proton on the C2 of the cation but also with the less acidic C4 and C5 protons of the imidazolium cation, when the anions are sufficiently basic. These conclusions were reached based on quantification of cation–CO2and anion–CO2complex formation, IR spectra, 1H NMR chemical shifts, and deuterium exchange equilibrium and kinetics.

We encapsulated six solvents with novel physical and chemical properties for CO2sorption within gas-permeable polymer shells, creating Micro-Encapsulated CO2Sorbents (MECS), to improve the CO2absorption kinetics and handling of the solvents for postcombustion CO2capture from flue gas. The solvents were sodium carbonate (Na2CO3) solution, uncatalyzed and with two different promoters, two ionic liquid (IL) solvents, and one CO2-binding organic liquid (CO2BOL). We subjected each of the six MECS to multiple CO2absorption and regeneration cycles and measured the working CO2absorption capacity as a function of time to identify promising candidate MECS for large-scale carbon capture. We discovered that the uncatalyzed Na2CO3and Na2CO3-sarcosine MECS had lower CO2absorption rates relative to Na2CO3-cyclen MECS over 30 min of absorption, while the CO2BOL Koechanol appeared to permeate through the capsule shell and is thus unsuitable. We rigorously tested the most promising three MECS (Na2CO3-cyclen, IL NDIL0309, and IL NDIL0230) by subjecting each of them to a series of 10 absorption/stripping cycles. The CO2absorption curves were highly reproducible for these three MECS across 10 cycles, demonstrating successful absorption/regeneration without degradation. As the CO2absorption rate is dynamic in time and the CO2loading per mass varies among the three most promising MECS, the process design parameters will ultimately dictate the selection of MECS solvent.

Ionic liquids (ILs) with aprotic heterocyclic anions (AHAs) are promising candidates for post-combustion carbon capture technologies since they react with CO2stoichiometrically and reversibly. CO2solubilities in two AHA-ILs, triethyl(octyl)phosphonium 2-cyanopyrrolide ([P2228][2CNPyr]) and triethyl(octyl)phosphonium benzimidazolide ([P2228][BnIm]), are reported for multiple temperatures and a third, triethyl(octyl)phosphonium 6-bromobenzimidazolide ([P2228][6-BrBnIm]), at one temperature. Ionic liquid [P2228][2CNPyr] and phase-change ionic liquid (PCIL) tetraethylphosphonium benzimidazolide ([P2222][BnIm]) were encapsulated in a chemically compatible and CO2-permeable polydimethylsiloxane (PDMS) polymer shell in order to enhance absorption and desorption kinetics. Both the free and encapsulated [P2228][2CNPyr] and [P2222][BnIm] were subjected to thermodynamic testing. The CO2solubilities in the encapsulated IL and PCIL were in good agreement with the free IL and PCIL, meaning that the encapsulation of IL and PCIL greatly enhanced the kinetics of CO2absorption while maintaining the high CO2capture capacity. Recyclability testing was also performed on both the free and encapsulated [P2228][2CNPyr] and [P2222][BnIm]. The IL and PCIL materials, as well as the capsules, were stable upon cycling, with the CO2capacities for each cycle remaining unchanged. The IL and the PCIL showed no sign of degradation after cycling, which demonstrated excellent performance.

We employ precise measuring techniques to determine the densities, viscosities, and ionic conductivities of three aqueous 1-ethyl-3-methylimidazolium [emim]+ionic liquid (IL) systems with minimal experimental uncertainty. We simultaneously present a novel method for estimating ion dissociation relying only on these three measurements and the estimated Stokes radii of the ions based on the Stokes–Einstein and Nernst–Einstein equations. Ion dissociation values are estimated across a range of IL concentrations, emphasizing dilute IL regions, using ionic radii calculated from widely used UNIQUAC and UNIFAC values. With these approximations and assuming the presence of only ion pairs, the ion dissociation of all three ILs reaches a minimum value at a water mole fraction of about 0.98. Upon further dilution with water, the ion dissociation increases as the system approaches infinite dilution of the IL. We postulate that the apparent minimum in the ion dissociation is caused by the Stokes radii of the cation and anion increasing as the concentration becomes more dilute, due to the formation of ion triplets.

Protic ionic liquids (ILs) have been recently studied as a potential approach to enhance oxygen reduction reaction (ORR) activity of carbon supported platinum catalysts (Pt/C) for application in polymer electrolyte membrane fuel cells. The high oxygen solubility in the ILs was suggested as one of the main reasons for the accelerated reaction rates. Because the nature of the anion of the IL has been associated with increased oxygen solubility, in this work we survey a number of ionic liquids with various anions to study this effect. While the search for direct correlation between the ORR activities and the oxygen solubilities does not produce any conclusive results, by contrast, the specific activity showed dependence on the availability of oxygenated species free Pt sites. This finding indicates that the inhibition of Pt oxidation and less adsorption of non-reactive species may also play an important role in the enhanced ORR activity. Moreover, the degree of IL coverage on the Pt surface was estimated using (bi)sulfate ions as an indicator. The surface coverage not only affected the ORR activity, but also the Pt dissolution process. This suggests that an optimal balance between activity and stability can be achieved on a partially covered Pt surface.

Ionic liquids (ILs) have been evaluated extensively as post-combustion CO2capture solvents, with CO2solubility data available for a large number of ILs. However, data for the solubility of the less-soluble gas component, N2, are sparse. Therefore, the solubility of N2was measured gravimetrically at pressures up to 140 bar in 12 ILs containing imidazolium, ammonium, pyrrolidinium, and phosphonium cations paired with bis(trifluoromethanesulfonyl)imide ([TFSI]−), dicyanamide ([DCA]−), methanesulfonate ([MeSO3]−), tetrafluoroborate [BF4]−), and triflate ([TfO]−) anions. Increasing the alkyl chain length from ethyl to hexyl to decyl in alkylimidazolium bis(trifluoromethanesulfonyl)imide ILs increased the N2solubility. The same effect was observed with phosphonium cations paired with the bis(trifluoromethylsulfonyl)imide anion. Nitrogen solubility in ILs with the 1-ethyl-3-methylimidazolium cation followed the order [DCA]−< [MeSO3]−< [BF4]−< [TfO]−< [TFSI]−. Generally, regardless of the IL structure, the N2solubility increased with increasing IL molar volume. For ILs with similar chemical composition and molar volume (i.e., ILs with tetraalkylammonium and pyrrolidinium cations), N2was less soluble in the IL with the cyclic cation. The absorption of N2in bis(trifluoromethylsulfonyl)imide ILs did not depend significantly on temperature. However, [emim][BF4] had a significant positive enthalpy of N2absorption. These N2solubility data, along with the readily available CO2solubilities in ILs, is critical to evaluating ILs for post-combustion carbon capture processes.

We have previously studied a series of room temperature ionic liquids (RTILs) based on 1-ethyl-3-methylimidazolium ([emim]+) and tetra-alkylphosphonium ([P222n]+) cations with different aprotic heterocyclic anions (AHAs). It has been found that many of the AHA ILs exhibit very good conductivity for their viscosities. The results provide insight into the design of ILs with enhanced dynamics that may be suitable for electrolyte applications (1, 2). Lithium salts based on the bis(trifluoromethylsulfonyl)imide ([TFSI]-), bis(fluorosulfonyl)imide ([FSI]-) and tetrafluoroborate ([BF4]-) anions have been extensively studied in lithium salt/IL mixtures for such applications (3-18). Here we will further explore how the addition of lithium AHA salts, compared to the popular [TFSI]-, [FSI]- and [BF4]--based ones, affects the transport properties of the RTILs in order to evaluate the feasibility of using these AHA ILs for lithium-ion battery applications. In particular, we investigate the viscosity and ionic conductivity, as well as anion, cation and lithium diffusivity, as measured by Pulse-Gradient-Spin-Echo NMR spectroscopy.

Owing to their extremely high theoretical energy density, non-aqueous Li-air batteries have received remarkable attention in recent years. Ionic Liquids (ILs), molten salts that are liquid at room temperature, have been investigated as potential electrolytes in Li-air batteries due to their negligible vapor pressure, good thermal stability, and wide electrochemical windows. However, compared to conventional organic electrolytes, ILs generally have higher viscosity and, consequently, lower ionic conductivity and ion diffusivity. Therefore, we have selected eight low-viscosity ILs containing the bis(trifluoromethanesulfonyl)imide ([TFSI]-) anion, and their physical properties were characterized. The diffusivities of the cation and anion of the ILs were measured with pulsed-field-gradient spin-echo (PFG-SE) NMR Spectroscopy. In addition, the I-V performances of three characteristic ILs were tested in a Li-O2 battery. The solubility of O2 in the ILs and IL-Li[TFSI] mixtures, an additional property that can affect Li-air battery performance, was also measured using a gravimetric microbalance.

The molecular-level orientation and structure of ionic liquids (ILs) at liquid–solid interfaces are significantly different than in the bulk. The interfacial ordering influences both IL properties, such as dielectric constants and viscosity, and their efficacy in devices, such as fuel cells and electrical capacitors. Here, we report the layered structures of four ILs on unbiased, highly ordered pyrolytic graphite (HOPG) and Pt(111) surfaces, as determined by atomic force microscopy. The ILs investigated are 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([emim][Tf2N]), 1-ethyl-3-methylimidazolium perfluorobutylsulfonate ([emim][C4F9SO3]), 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene bis(trifluoromethylsulfonyl)imide ([MTBD][Tf2N]), and 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene perfluorobutylsulfonate ([MTBD][C4F9SO3]). Molecular dynamics simulations provide complementary information on the position and orientation of the ions. These ILs form a cation layer at the IL–solid interface, followed by a layer of anions. [Emim]+and [MTBD]+have similar orientations at the surface, but [MTBD]+forms a thinner layer compared to [emim]+on both HOPG and Pt(111). In addition, [Tf2N]−shows stronger interactions with Pt(111) surfaces than [C4F9SO3]−.

A series of novel ionic liquids (ILs) based on N, N'- dialkylimidazolium, tetra-alkylphosphonium and dialkylpyrrolidinium cations paired with two aprotic heterocyclic anions (AHAs) were synthesized in the present study. In addition, lithium salts of the corresponding anions were prepared. Physicochemical and electrochemical properties of the neat ILs and their mixtures with the lithium salts at various concentrations were systematically measured from 283.15K to 343.15K at atmospheric pressure, including density (r), viscosity (e) and electrical conductivity (s). The empirical Vogel-Fulcher-Tammann (VFT) was used to analyze the experimental viscosities and electrochemical conductivities. Furthermore, the Walden plot behavior of the pure ILs and ILs doped with lithium salts were investigated. The results provide some insight into the strength of the association between Li+ and the different anions. Consequently, the ILs with low viscosity and good conductivities are promising electrolytes candidates to be used in electrochemical applications.

A series of novel halogen-free ionic liquids (ILs) based on N, N'-dialkylimidazolium, tetra-alkylphosphonium, and di-alkylpyrrolidinium cations paired with two aprotic heterocyclic anions (AHAs) were synthesized and tested as potential electrolytes. In addition, the corresponding lithium salts were prepared to investigate the impacts of the Li salts on the physical properties of the IL-Li mixtures. Physicochemical and electrochemical properties (density (r), viscosity (e), electrical conductivity (s) and electrochemical window (EW) of the neat ILs and their mixtures with the lithium salts at various concentrations were measured at temperatures between 283.15 K and 343.15 K and at atmospheric pressure. The results indicate that ILs with both planar anions and cations exhibit lower viscosities, and better conductivities than those with more spherical geometries, while maintaining competitive EWs. The Walden plot behavior of the ILs provides qualitative insight into the suitability of these ILs as electrolytes. Based on systematic studies shown in this paper, it can be concluded that these new ILs are promising halogen-free electrolytes that deserve further investigation

Ionic liquids ILs are low melting organic salts that are being vigorously investigated as possible replacements for volatile organic solvents. While they cannot contribute to air pollution due to their negligible vapor pressure, they do have significant solubility in water. As a result, this is the most likely medium through which ILs will enter the environment. Therefore, it is important to understand how ILs will influence aquatic ecosystems. A simple thermodynamic measurement that has been extremely useful in estimating effects of chemical pollutants on aquatic environments is the octanol–water partition coefficient KOW. It is an extremely important quantity because it describes the hydrophobicity or hydrophilicity of a compound and has been correlated with bioaccumulation and toxicity in fish, as well as sorption to soils and sediments. Here we present measurements of the KOWof twelve imidazolium-based ILs at room temperature, using the slow-stirring method. For the butylmethylimidazolium cation, KOWvalues range from 0.003 to 11.1, depending on the choice of anion. In addition, we find that the KOWvalues increase with increasing alkyl chain length on the cation and that replacing the acidic hydrogen on the carbon between the two nitrogens in the imidazolium ring with a methyl group has negligible effect on the KOW. However, all of the KOWvalues measured, even for the most “hydrophobic” imidazolium-based ILs, are less than 15 so these ILs will not accumulate or concentrate in the environment.

Previously we have shown that organic solutes can be extracted from ionic liquids ILs with supercritical CO2and that ILs can be induced to separate from organic and aqueous mixtures by applying gaseous CO2pressure. Thus, we are interested in the solvent strength of ILCO2mixtures. Here we use 4-nitroaniline, N,N-diethyl-4-nitroaniline and Reichardt's dye 33 to determine the Kamlet–Taft parameters for four different imidazolium based ILs and their mixtures with CO2at 25 and 40 °C. The effect of temperature and carbon dioxide concentration on these parameters was determined. The polarizability parameter depends weakly on the CO2concentration. However, the hydrogen bond donating ability and the hydrogen bond accepting ability are virtually independent of CO2pressure. The results indicate that the strong interactions between ILs and probe molecules are not influenced by CO2.

A melting point at or below ambient temperature is an essential property of ionic liquids being considered as non-volatile replacement solvents. Here we use the Quantitative Structure-Property Relationship QSPR method to correlate and predict the melting points of organic salts based on the quaternary ammonium cation. For a set of 75 tetraalkyl-ammonium bromides, a correlation with R2 0.790 is created, and for a set of 34 n-hydroxyalkyl-trialkyl-ammonium bromides, two correlations are created with R2 0.716 and R2 0.766. Descriptors used in the correlations are analyzed to determine structural features that lower melting point, and melting points are predicted for salts that incorporate these features.

We report results from the first molecular simulation study of 1-n-butyl-3-methylimidazolium hexafluorophosphate [bmim][PF₆], a widely studied ionic liquid. Monte Carlo simulations are carried out in the isothermal–isobaric (NPT) ensemble to calculate the molar volume, cohesive energy density, isothermal compressibility, cubic expansion coefficient and liquid structure as a function of temperature and pressure. A united atom forcefield is developed using a combination of ab initio calculations and literature parameter values. Calculated molar volumes are within 5% of experimental values, and a reasonable agreement is obtained between calculated and experimental values of the isothermal compressibility and cubic expansion coefficient. PF₆ anions are found to preferentially cluster in two favorable regions near the cation.

No Abstract.

The determination of critical points of mixtures is important for both practical and theoretical reasons in modeling phase behavior, especially at high pressure. This article presents the first completely reliable method for locating all the critical points of a given mixture. The method also verifies the nonexistence of a critical point if a mixture of a given composition does not have one. The methodology used is based on interval analysis, in particular an interval Newton/generalized bisection algorithm providing a mathematical and computational guarantee that all mixture critical points are located. The procedure is initialization‐independent and thus requires no a priori knowledge of the number of mixture critical points or their approximate locations. The technique is illustrated using several example problems involving cubic equation‐of‐state models; however, the technique is for general purpose and can be applied in connection with other thermodynamic models.

To uniquely equip students with the active knowledge and the ability to implement pollution prevention technology, we have developed a design-oriented senior-level elective course on minimizing the environmental impact of chemical manufacturing processes. The objectives of the course are to educate students on the real costs of operating processes that release pollutants to the environment, to provide them with strategies to minimize or reduce the environmental impact of a given chemical proces, and to examine the design of processes using new technologies that totally eliminate pollutants at the source. Design-oriented projects form the core of the course, and bring together the students' skills in design, analysis and decision-making in the face of multiple and sometimes competing objectives. We believe this course contributes to the development of our graduates into chemical engineering professionals who are equipped with the awareness, knowledge, and ability to minimize the environmental impact of the chemical manufacturing processes that they oversee.

It is important to determine the existence and composition of homogeneous azeotropes in the analysis of phase behavior and in the synthesis and design of separation systems, from both theoretical and practical standpoints. A new method for reliably locating any and all homogeneous azeotropes for multicomponent mixtures is presented. The method also verifies the nonexistence of homogeneous azeotropes if none are present. The method is based on interval analysis, in particular an interval‐Newton/generalized‐bisection‐algorithm providing a mathematical and computational guarantee that all azeotropes are located. This general‐purpose technique can be applied in connection with any thermodynamic models. The technique is illustrated in several example problems using the Wilson, NRTL, and UNIQUAC activity‐coeficient models.

Carbon dioxide can be used to remove ammonium salts and zinc acetate from ionic liquid/organic mixtures at 25 °C. The amount of CO2required to achieve this separation is minimal.

The presence of CO2increases the solubility of O2and CH4in 1-hexyl-3-methylimidazolium bistrifluoromethylsulfonyl imide at 25 °C and pressures to 13 bar.