Protic ionic liquids: solvents with tunable phase behavior and physicochemical properties

How ionic species structure influences phase structure and transitions from protic ionic liquids to liquid crystals to crystals

Phase behaviour of n-alkylammonium (C6 to C16) nitrates and formates was characterised using synchrotron small angle and wide angle X-ray scattering, differential scanning calorimetry, cross polarised optical microscopy and FTIR.

The effect of alkyl chain length on the structure and thermodynamics of protic–aprotic ionic liquid mixtures: a molecular dynamics study

Mixtures of protic and aprotic ionic liquids exhibit non-ideal behavior with increasing alkyl chain length, caused by the formation of molecular complexes.

Amphoteric water as acid and base for protic ionic liquids and their electrochemical activity when used as fuel cell electrolytes

Amphoteric water was mixed with equimolar amounts of a super-strong acid, trifluoromethanesulfonic acid (TfOH), and a super-strong base, 1,8-diazabicyclo[5.4.0]-7-undecene (DBU). Bulk physicochemical and electrochemical properties of the mixtures were compared with those of the best ever reported protic ionic liquid (PIL), diethylmethylammonium trifluoromethanesulfonate ([dema][TfO]), which has excellent physicochemical properties as a fuel cell electrolyte. The acidic mixture ([H₃O][TfO]) behaved as a protic ionic liquid, while the basic mixture ([DBU]OH) showed incomplete proton transfer. The Walden plot indicated that [H₃O][TfO] behaves as a good PIL, similar to [dema][TfO], whereas [DBU]OH behaves as a poor PIL. [H₃O][TfO] showed excellent H₂/O₂ fuel cell performance at 80 °C; however, the performance deteriorated as the bulk water content increased, because of the retardation of the electrode kinetics due to the oxidation of Pt in the presence of bulk water. On the other hand, [DBU]OH exhibited very poor performance possibly because of the existence of neutral species in the system.

Time-of-flight secondary ion mass spectrometry using a new primary ion beam generated by vacuum electrospray of a protic ionic liquid, propylammonium nitrate

RATIONALE

Protic ionic liquids have the potential to be useful materials for primary ion beams in terms of protonation, since they have active protons. Selecting protic ionic liquids suitable for primary ion beams is of great importance to increase molecular secondary ion yields. Propylammonium nitrate ([C₃ H₇ NH₃ ][NO₃ ]) seems promising in view of its proton affinity. It is likely that [C₃ H₇ NH₃ ]⁺ cations can act as proton donors, and [NO₃ ]^- anions can work as proton acceptors.

METHODS

Time-of-flight secondary ion mass spectrometry (TOF-SIMS) experiments have been performed to verify the usefulness of [C₃ H₇ NH₃ ][NO₃ ]. A primary propylammonium nitrate cluster ion beam was generated by vacuum electrospray, and then used to analyze amino acids (arginine, glutamic acid, aspartic acid), angiotensin II and polyethylene glycol. Positive and negative secondary ion mass spectra were obtained to study both protonation and deprotonation.

RESULTS

The propylammonium nitrate cluster ion beam successfully generated protonated molecules [M + H]⁺ of all the analytes in positive ion mode. The primary ion beam also generated deprotonated molecules [M - H]^- of glutamic acid, aspartic acid and angiotensin II in negative ion mode. Additionally, adduct ions related to [C₃ H₇ NH₃ ][NO₃ ] were detected in the case of arginine and polyethylene glycol.

CONCLUSIONS

The TOF-SIMS experiments confirmed that the propylammonium nitrate cluster ion beam was useful in generating molecular secondary ions, demonstrating that it is well suited for a primary ion beam in TOF-SIMS.

Thermophysical and Electrochemical Properties of Ethereal Functionalised Cyclic Alkylammonium-based Ionic Liquids as Potential Electrolytes for Electrochemical Applications

A series of hydrophobic room temperature ionic liquids (ILs) based on ethereal functionalised pyrrolidinium, piperidinium and azepanium cations bearing the bis[(trifluoromethyl)sulfonyl]imide, [TFSI]- , anion were synthesized and characterized. Their physicochemical properties such as density, viscosity and electrolytic conductivity, and thermal properties including phase transition behaviour and decomposition temperature have been measured. All of the ILs showed low melting point, low viscosity and good conductivity and the latter properties have been discussed in terms of the IL fragility, an important electrolyte feature of the transport properties of glass-forming ILs. Furthermore, the studied [TFSI]- -based ILs generally exhibit good electrochemical stabilities and, by coupling electrochemical experiments and DFT calculations, the effect of ether functionalisation at the IL cation on the electrochemical stability of the IL is discussed. Preliminary investigations into the Li-redox chemistry at a Cu working electrode are also reported as a function of ether-functionality within the pyrrolidinium-based IL family. Overall, the results show that these ionic liquids are suitable for electrochemical devices such as battery systems, fuel cells or supercapacitors.

A Binary Mixture of Ionic Liquids as a New Approach for an Experimental Diffusion Correction in the Study of Activated Processes

A binary system of two ionic liquids, 1-ethyl-3-methylimidazolium and trioctylmethylammonium bis(trifluoromethylsulfonyl)imide ([EMIM][NTf₂ ] and [OMA][NTf₂ ], respectively), with varying molar fractions, is introduced. It allows the dynamic viscosity to remain constant over a range of almost 60 K; this means that any activated process can be studied independent of the temperature dependence of viscosity itself. This principle is proven upon reinvestigation of electron self-exchange kinetics of tetrathiafulvalene by continuous-wave ESR line-broadening experiments. From these results, it is also confirmed that this process is totally diffusion controlled.

Phase Behavior and Physical Properties of New Biobased Ionic Liquid Crystals

Protic ionic liquids (PILs) have emerged as promising compounds and attracted the interest of the industry and the academy community, due to their easy preparation and unique properties. In the context of green chemistry, the use of biocompounds, such as fatty acids, for their synthesis could disclose a possible alternative way to produce ILs with a low or nontoxic effect and, consequently, expanding their applicability in biobased processes or in the development of bioproducts. This work addressed efforts to a better comprehension of the complex solid-[liquid crystal]-liquid thermodynamic equilibrium of 20 new PILs synthesized by using fatty acids commonly found in vegetable oils, as well as their rheological profile and self-assembling ability. The work revealed that their phase equilibrium and physical properties are significantly impacted by the structure of the ions used for their synthesis. The use of unsaturated fatty acids and bis(2-hydroxyethyl)ammonium for the synthesis of these biobased ILs led to a drastic decreasing of their melting temperatures. Also, the longest alkyl chain fatty acids promoted higher self-assembling and more stable mesophases. Besides their sustainable appeal, the marked high viscosity, non-Newtonian profile, and very low critical micellar concentration values of the PIL crystals here disclosed make them interesting renewable compounds with potential applications as emulsifiers, stabilizers, thickeners, or biolubricants.

Kinetics, mechanisms and ionic liquids in the uptake of n-butylamine onto low molecular weight dicarboxylic acids

Atmospheric particles adversely affect visibility, health, and climate, yet the kinetics and mechanisms of particle formation and growth are poorly understood. Multiphase reactions between amines and dicarboxylic acids (diacids) have been suggested to contribute. In this study, the reactions of n-butylamine (BA) with solid C3-C8 diacids were studied at 296 ± 1 K using a Knudsen cell interfaced to a quadrupole mass spectrometer. Uptake coefficients for amines on the diacids with known geometric surface areas were measured at initial amine concentrations from (3-50) × 10¹¹ cm^-3. Uptake coefficients ranged from 0.7 ± 0.1 (2σ) for malonic acid (C3) to <10^-6 for suberic acid (C8), show an odd-even carbon number effect, and decrease with increasing chain length within each series. Butylaminium salts formed from evaporation of aqueous solutions of BA with C3, C5 and C7 diacids (as well as C8) were viscous liquids, suggesting that ionic liquids (ILs) form on the surface during the reactions of gas phase amine with the odd carbon diacids. Predictions from the kinetic multi-layer model of aerosol surface and bulk chemistry (KM-SUB) were quantitatively consistent with uptake occurring via dissolution of the underlying diacid into the IL layer and reaction with amine taken up from the gas phase. The butylaminium salts formed from the C4 and C6 diacids were solids, and their uptake coefficients were smaller. These experiments and kinetic modeling demonstrate the unexpected formation of ILs in a gas-solid reaction, and suggest that ILs should be considered under some circumstances in atmospheric processes.

Protic ammonium carboxylate ionic liquids: insight into structure, dynamics and thermophysical properties by alkyl group functionalization

This study is aimed at characterising the structure, dynamics and thermophysical properties of five alkylammonium carboxylate ionic liquids (ILs) from classical molecular dynamics simulations.

Effect of low water content in protic ionic liquid on ions electrosorption in porous carbon: application to electrochemical capacitors

We report the effect of low water content in [(C₂H₅)₃N⁺H] [TFSI⁻] on the Stern layer nanostructure in a porous carbon electrode at positive and negative potentials.

Micelle formation of a non-ionic surfactant in non-aqueous molecular solvents and protic ionic liquids (PILs)

Many ionic liquids and low molecular weight polar solvents have been reported to support amphiphile self-assembly, with most of these reported for the first time in the last decade. This phenomenon is attributed to the solvophobic effect (analogous to the hydrophobic effect in water). However, to date there has been no systematic study which evaluates micelle formation in a large library of non-aqueous solvents. Here we investigate micelle formation of a non-ionic amphiphile, hexa-ethyleneglycol mono n-dodecyl ether, C12E6, in a diverse range of molecular solvents and protic ionic liquids (PILs). Nine of the 19 non-aqueous molecular solvents investigated, and all four of the PILs, were found to support micelle formation. A link was investigated between the solvent cohesive energy density (as estimated using the Gordon parameter) and both the critical micelle concentration and the related free energy of micellization . In addition, the chemical structure and liquid mesostructure of the solvent were found to be important factors in the ability of the solvents to support micelle formation.

Tailored ionic liquid-based surfactants for the formation of microemulsions with water and a hydrophobic ionic liquid

Microemulsions (μe) with water and a hydrophobic ionic liquid (IL) usually require 45-60 wt% surfactant to solubilize equal amounts of water and IL. To increase the efficiency we designed a new class of surfactants by combining a hydrophilic but IL-ophobic carbohydrate-based part with a hydrophobic but IL-ophilic IL-based part. These surfactants allow formulating microemulsions with 20 wt% surfactant only which opens up a new arena for efficient water-IL μes.

From solutions to molecular emulsions

Concentration fluctuations play an important role in the statistical description of the stability of liquids, particularly in the neighborhood of phase transitions. Classical thermodynamics is blind to fluctuations, and statistical thermodynamics is required to fully understand quantities such as the isothermal compressibility or heat capacity, by linking them to fluctuations of appropriate statistical microscopic quantities and showing that they are response functions. This is illustrated by the seminal Kirkwood–Buff theory of solutions. However, the existence of micro-heterogeneous structures, particularly in aqueous mixtures, which leads to large Kirkwood–Buff integrals, suggest that micro-heterogeneity is a form of concentration fluctuation. This interpretation becomes difficult to accept when extrapolated to larger micro-heterogeneous structures such as micellar aggregates in micro-emulsions. By analyzing how different methods, experimental, computer experiments and theoretical approaches deal with the underlying duality behind these two physical manifestations, we put in evidence the need to reconsider the description of liquids by incorporating the description of emergent “objects”, such as the micro-heterogeneous structures from a molecular point of view. On this path, the concept of “molecular emulsion” allows to describe in a unified way all type of disordered liquids, from solutions to the organized liquids of soft matter.

(Eco)toxicity and biodegradability of protic ionic liquids

Ionic liquids (ILs) are often claimed to be "environmentally friendly" compounds however, the knowledge of their potential toxicity towards different organisms and trophic levels is still limited, in particular when protic ionic liquids (PILs) are addressed. This study aims to evaluate the toxicity against various microorganisms and the biodegradability of four PILs namely, N-methyl-2-hydroxyethylammonium acetate, m-2-HEAA; N-methyl-2-hydroxyethylammonium propionate, m-2-HEAPr; N-methyl-2-hydroxyethylammonium butyrate, m-2-HEAB; and N-methyl-2-hydroxyethylammonium pentanoate, m-2-HEAP. The antimicrobial activity was determined against the two bacteria, Sthaplylococcus aureus ATCC-6533 and Escherichia coli CCT-0355; the yeast Candida albicans ATCC-76645; and the fungi Fusarium sp. LM03. The toxicity of all PILs was tested against the aquatic luminescent marine bacterium Vibrio fischeri using the Microtox(®) test. The impact of the PILs was also studied regarding their effect on lettuce seeds (Lactuta sativa). The biodegradability of these PILs was evaluated using the ratio between the biochemical oxygen demand (BOD) and the chemical oxygen demand (COD). The results show that, in general, the elongation of the alkyl chain tends to increase the negative impact of the PILs towards the organisms and biological systems under study. According to these results, m-2-HEAA and m-2-HEAP are the less and most toxic PILs studied in this work, respectively. Additionally, all the PILs have demonstrated low biodegradability.

Protonic Ammonium Nitrate Ionic Liquids and Their Mixtures: Insights into Their Thermophysical Behavior

This study is centered on the thermophysical characterization of different families of alkylammonium nitrate ionic liquids and their binary mixtures, namely the determination at atmospheric pressure of densities, electric conductivities and viscosities in the 288.15 < T/K < 353.15 range. First, measurements focusing on ethylammonium, propylammonium and butylammonium nitrate systems, and their binary mixtures, were determined. These were followed by studies involving binary mixtures composed of ethylammonium nitrate (with three hydrogen bond donor groups) and different homologous ionic liquids with differing numbers of hydrogen bond donor groups: diethylammonium nitrate (two hydrogen bond donors), triethylammonium nitrate (one hydrogen bond donor) and tetraethylammonium nitrate (no hydrogen bond donors). Finally, the behavior of mixtures with different numbers of equivalent carbon atoms in the alkylammonium cations was analyzed. The results show a quasi-ideal behavior for all monoalkylammonium nitrate mixtures. In contrast, the other mixtures show deviations from ideality, namely when the difference in the number of carbon atoms present in the cations increases or the number of hydrogen bond donors present in the cation decreases. Overall, the results clearly show that, besides the length and distribution of alkyl chains present in a cation such as alkylammonium, there are other structural and interaction parameters that influence the thermophysical properties of both pure compounds and their mixtures.

Effects of a proton-conducting ionic liquid on secondary ion formation in time-of-flight secondary ion mass spectrometry

RATIONALE

A protic ionic liquid, diethylmethylammonium trifluoromethanesulfonate ([dema][TfO]), has low vapor pressure and high protonic conductivity even at room temperature. Since [dema][TfO] has a mobile proton in its salt structure, its primary beam is expected to enhance the formation of protonated molecular ions. However, mass spectrometric characteristics of [dema][TfO] are not well known. In order to develop an ionic-liquid primary beam source, it is necessary to investigate such characteristics.

METHODS

The first time-of-flight secondary ion mass spectrometry (TOF-SIMS) experiment using an Ar(+) primary ion beam was performed to analyze two samples: a neat [dema][TfO] sample and a mixed sample of arginine and [dema][TfO]. Beam characteristics of [dema][TfO] generated by vacuum electrospray were investigated using an apparatus for measuring transient responses of a beam current. The second TOF-SIMS experiment using a [dema][TfO] primary beam was performed to analyze three samples: arginine, a mixture of arginine and [dema][TfO], and poly(ethylene glycol) (PEG300).

RESULTS

The [dema][TfO] primary beam was useful in generating protonated arginine; however, it was not helpful in detecting PEG300. The results were explained by considering gas-phase basicities and proton affinities of analytes and [dema][TfO] constituents. Projectile energy per nucleon of the [dema][TfO] beam was examined; it would be necessary to reduce m/z values of ionic-liquid charged droplets. In addition, a screening method was proposed to select ionic liquids suitable for primary ion beams.

CONCLUSIONS

Since [dema][TfO] can act as a proton source, its primary beam can effectively generate protonated secondary ions of analytes. Consequently, proton-conducting ionic liquids such as [dema][TfO] are expected to have great potentials as primary ion beams in TOF-SIMS.

Physicochemical and electrochemical properties of a new series of protic ionic liquids with N-chloroalkyl functionalized cations

Six new protic ionic liquids (PILs) based on N-chloroalkyl functionalized morpholinium, piperidinium, pyrrolidinium and alkylammonium cations, with bis[(trifluoromethyl)sulfonyl]imide as counter-ion, were synthesized by a metathesis reaction.

Novel phosphorus-containing halogen-free ionic liquids: effect of sulfonate anion size on physical properties, biocompatibility, and flame retardancy

Phosphorus-containing sulfonate ILs with different anions size present different thermal stability, solubility, viscosity, melting points, and cytotoxicity. They show flame retardant effect for PA6 via accelerating decomposition of matrix.

Activity and conformation of lysozyme in molecular solvents, protic ionic liquids (PILs) and salt–water systems

The role of solvent features on enzyme stability was investigated by comparing the conformation and activity of lysozyme in molecular solvents, PILs and salt solutions. Electrostatic interactions and the solvophobic effect dominated at low and high solvent concentrations, respectively.