Spectroscopic Signatures of Hydrogen-Bonding Motifs in Protonic Ionic Liquid Systems: Insights from Diethylammonium Nitrate in the Solid State.

Diethylammonium nitrate, [N0 0 2 2][NO3], and its perdeuterated analogue, [N D D 2 2] [NO3], were structurally characterized and studied by infrared, Raman, and inelastic neutron scattering (INS) spectroscopy. Using these experimental data along with state-of-the-art computational materials modeling, we report unambiguous spectroscopic signatures of hydrogen-bonding interactions between the two counterions. An exhaustive assignment of the spectral features observed with each technique has been provided, and a number of distinct modes related to NH···O dynamics have been identified. We put a particular emphasis on a detailed interpretation of the high-resolution, broadband INS experiments. In particular, the INS data highlight the importance of conformational degrees of freedom within the alkyl chains, a ubiquitous feature of ionic liquid (IL) systems. These findings also enable an in-depth physicochemical understanding of protonic IL systems, a first and necessary step to the tailoring of hydrogen-bonding networks in this important class of materials.

Characterization of ionic liquid cytotoxicity mechanisms in human keratinocytes compared with conventional biocides.

The ability to chemically modify ionic liquids (ILs) has led to an expansion in interest in their use in a diversity of applications, not least as antimicrobials and biocides. Relatively little is known about cytotoxicity mechanisms of ILs in comparison to other biocides currently in widespread use, as well as their practical significance for the ecological environment and human health. Using NCTC 2544 and HaCat human keratinocyte cells, this study aimed to characterize cytotoxicity rates and mechanisms of a range of ILs. Using both lactate dehydrogenase (LDH) and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) based cytotoxicity assays, it was confirmed that at biocide-relevant concentrations, ILs with longer alkyl chains exhibited greater biocidal activity than those with shorter alkyl chains, with comparable activity to the commonly used biocides chlorhexidine, benzalkonium chloride and cetylpyridinium chloride, at relevant in-use biocide concentrations. Mode of cell death, measured using fluorescence-activated cell sorting (FACS) and caspase 3/7 activity, determined necrosis to be the primary cytotoxic mechanism at higher concentrations of the biocides stated above, and with ILs [C14MIM]Cl and [C14quin]Br, with apoptosis observed at borderline necrotic concentrations. Perhaps most interestingly, modification of anion had a significant effect on cytotoxicity. The use of N[SO2CF3] as an anion to [C16MIM] attenuated cytotoxicity 10-fold in comparison to other anions, suggesting cytotoxicity may also be a tuneable property when using ILs as biocides.

Induced Protic Behaviour in Aprotonic Ionic Liquids by Anion Basicity for Efficient Carbon Dioxide Capture.

The interactions between aprotonic tetrabutylphosphonium carboxylate ionic liquids (ILs), [P4 4 4 4 ][Cn COO] (n=1, 2 and 7), and water were investigated. The cation-anion interactions occur via the α-1 H on [P4 4 4 4 ]+ and the carboxylate headgroup of the anion. Upon addition, H2 O localises around the carboxylate headgroups, inducing an electron inductive effect towards the oxygens, leading to ion-pair separation. Studies with D2 O and [P4 4 4 4 ][Cn COO] revealed protic behaviour of the systems, with proton/deuterium exchange occurring at the α-1 H of the cation, promoted by the basicity of the anion, forming an intermediate ylide. The greater influence of van der Waals forces of the [P4 4 4 4 ][C7 COO] system allows for re-orientation of the ions through larger interdigitation. The protic behaviour of the neat ILs allows for CO2 to be chemically absorbed on the ylide intermediate, forming a phosphonium-carboxylate zwitterion, signifying proton exchange occurs even in the absence of H2 O. The absorption of CO2 in equimolar IL-H2 O mixtures forms a hydrogen carbonate, through a proposed reaction of the CO2 with an intermediate hydroxide, and carboxylic acid.

High-throughput toxicity screening of novel azepanium and 3-methylpiperidinium ionic liquids.

Ionic liquids (ILs) have been employed as potentially environmentally friendly replacements for harmful organic solvents, but have also been studied for their use in bioelectrochemical applications, such as in microbial electrochemistry for bioenergy production, or in industrial biocatalysis. For these processes, low microbial toxicity is important and there is a growing need for microbial toxicology studies for novel ILs. In this study, we report initial toxicity data for novel ILs, based on azepanium and 3-methylpiperidinium cations. Agar disc diffusion assays are used, along with minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) determinations, to obtain rapid and inexpensive initial toxicity data for these novel ILs against Escherichia coli and Staphylococcus epidermidis. Many of the novel ILs characterised possess low microbial toxicity relative to well-studied ILs, highlighting their potential for further study in applications where this is a desirable property.

Halogen-bond mediated efficient storage of extremely volatile perfluoroiodides in ionic liquids.

Highly volatile and valuable reagents, perfluoroiodides were shown to complex with ionic liquids with the aid of a halogen-bond between a diverse set of anions and the electrophilic iodine atoms in perfluorinated iodides. The affinity of anions in various ionic liquids towards perfluoroiodides was examined by 19F NMR. Making use of the non-volatility of ionic liquids, we demonstrate here that perfluoroiodides can be stored and released controllably, on demand, from an ionic liquid matrix.

Anomalous and Not-So-Common Behavior in Common Ionic Liquids and Ionic Liquid-Containing Systems.

This work highlights unexpected, not so well known responses of ionic liquids and ionic liquid-containing systems, which are reported in a collective manner, as a short review. Examples include: (i) Minima in the temperature dependence of the isobaric thermal expansion coefficient of some ILs; (ii) Viscosity Minima in binary mixtures of IL + Molecular solvents; (iii) Anomalies in the surface tension within a family of ILs; (iv) The constancy among IL substitution of Cp/Vm at and around room temperature; (v) ILs as glass forming liquids; (vi) Alternate odd-even side alkyl chain length effects; (vii) Absolute negative pressures in ILs and IL-containing systems; (viii) Reversed-charged ionic liquid pairs; (ix) LCST immiscibility behavior in IL + solvent systems.

Sum frequency generation spectroscopy of tetraalkylphosphonium ionic liquids at the air-liquid interface.

Sum frequency generation (SFG) spectroscopy is a nonlinear vibrational spectroscopic technique used in the study of interfaces, due to its unique ability to distinguish surface molecules that have preferential ordering compared to the isotropic bulk. Here, a series of alkyltrioctylphosphonium chloride ionic liquids, systematically varied by cation structure, were characterized at the air-liquid interface by SFG. The effect on surface structure resulting from molecular variation (i.e., addition of cyano- and methoxy-functional groups) of the cation alkyl chain was investigated. SFG spectra in the C-H stretching region (2750-3100 cm-1) for [P8 8 8 n ][Cl], where n = 4, 5, 8, 10, 12, or 14, showed characteristic changes as the alkyl chain length was increased. Spectral profiles for n = 4, 5, 8, or 10 appeared similar; however, when the fourth alkyl chain was sufficiently long (as in the case of n = 12 or n = 14), abrupt changes occurred in the spectra. Molecular dynamics (MD) simulation of a slab of each ionic liquid (with n = 8, 10, or 12) confirmed gauche defects, with enhancement for the long alkyl chain and an abrupt increase of gauche occurrence from n = 8 to n = 10. A comparison of the tilt angle distribution from the simulation and the SFG analysis show a broad distribution of angles. Using experimental SFG spectra in conjunction with MD simulations, a comprehensive molecular picture at the surface of this unique class of liquids is presented.

Ionic Liquid-Liquid Chromatography: A New General Purpose Separation Methodology.

Ionic liquids can form biphasic solvent systems with many organic solvents and water, and these solvent systems can be used in liquid-liquid separations and countercurrent chromatography. The wide range of ionic liquids that can by synthesised, with specifically tailored properties, represents a new philosophy for the separation of organic, inorganic and bio-based materials. A customised countercurrent chromatograph has been designed and constructed specifically to allow the more viscous character of ionic liquid-based solvent systems to be used in a wide variety of separations (including transition metal salts, arenes, alkenes, alkanes, bio-oils and sugars).

X-ray and molecular dynamics studies of butylammonium butanoate-water binary mixtures.

The nanostructural organisation of mixtures of the ionic liquid (butylammonium butanoate) and water at several mole fractions of water has been investigated using small and wide angle X-ray scattering (S-WAXS) and molecular dynamics (MD) simulations. The presence of a first sharp diffraction peak (FSDP) in the pure ionic liquid has been observed, experimentally and theoretically, suggesting the possibility of segregation of domains of different polarity in the system. With increasing dilution in water, the prepeak is shifted towards smaller Q values, and becomes very weak, while the principal peak moves towards larger Q values. These phenomena suggest the disruption of the hydrogen-bonded network of the ionic liquid, primarily through hydrogen bonding of the anion to water, a conclusion supported by MD simulations.

Ionic Liquid-Based Microemulsions in Catalysis.

The design and properties of surface-active ionic liquids that are able to form stable microemulsions with heptane and water are presented, and their promise as reaction media for thermomorphic palladium-catalyzed cross-coupling reactions is demonstrated.

Mercury capture on a supported chlorocuprate(ii) ionic liquid adsorbent studied using operando synchrotron X-ray absorption spectroscopy.

Mercury scrubbing from gas streams using a supported 1-butyl-3-methylimidazolium chlorocuprate(ii) ionic liquid ([C4mim]2[Cu2Cl6]) has been studied using operando EXAFS. Initial oxidative capture as [HgCl3]- anions was confirmed, this was then followed by the unanticipated generation of mercury(i) chloride through comproportionation with additional mercury from the gas stream. Combining these two mechanisms leads to net one electron oxidative extraction of mercury from the gas with increased potential capacity and efficiency for supported ionic liquid mercury scrubbers.

A new insight into the nanostructure of alkylammonium alkanoates based ionic liquids in water.

In this paper, small angle X-ray scattering has been used to study a series of ionic liquids, alkylammonium alkanoates ([N0 0 0 n][CmCO2]), with varying alkyl chain lengths in the cation and the anion. We investigate the behaviour and the structure of such ionic liquids in their neat state, and in binary mixtures with water. To the best of our knowledge, this is the first structural study dealing with the behaviour of propylammonium propanoate [N0 0 0 3][C2CO2], butylammonium propanoate [N0 0 0 4][C2CO2], propylammonium butanoate [N0 0 0 3][C3CO2] and butylammonium butanoate [N0 0 0 4][C3CO2] when mixed in water. Generally, in the ionic liquids containing alkyl chains on both the cation and the anion, the correlation distance and the resulting scattering peak, which signal intermediate range order, are affected equally by both of the chains. The interesting result obtained regarding these systems is that the shift of the pre- and principal peaks with the addition of water depends on the overall molar concentration of the chains and is generally cumulative. Although, for a given sum of cation and anion chain lengths, the shift is reliant on the cation-anion combination in the neat state, it is not the case in their mixtures with water. In some recent papers, it has been reported that with addition of water, the pre-peak position remains constant, but our results show a shift in both pre- and principal Q peaks.

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.

Pro-fragrant ionic liquids with stable hemiacetal motifs: water-triggered release of fragrances.

Stable liquid and solid salts in the form of elusive hemiacetals, appended with fragrant alcohols, have been synthesised as pro-fragrances, and the controlled release of these fragrances, triggered by water, is demonstrated.

An ionic liquid process for mercury removal from natural gas.

Efficient scrubbing of mercury vapour from natural gas streams has been demonstrated both in the laboratory and on an industrial scale, using chlorocuprate(II) ionic liquids impregnated on high surface area porous solid supports, resulting in the effective removal of mercury vapour from natural gas streams. This material has been commercialised for use within the petroleum gas production industry, and has currently been running continuously for three years on a natural gas plant in Malaysia. Here we report on the chemistry underlying this process, and demonstrate the transfer of this technology from gram to ton scale.

3-Methylpiperidinium ionic liquids.

A wide range of room temperature ionic liquids based on the 3-methylpiperdinium cation core were produced from 3-methylpiperidine, which is a derivative of DYTEK® A amine. First, reaction with 1-bromoalkanes or 1-bromoalkoxyalkanes generated the corresponding tertiary amines (Rmßpip, R = alkyl or alkoxyalkyl); further quaternisation reactions with the appropriate methylating agents yielded the quaternary [Rmmßpip]X salts (X(-) = I(-), [CF3CO2](-) or [OTf](-); Tf = -SO2CF3), and [Rmmßpip][NTf2] were prepared by anion metathesis from the corresponding iodides. All [NTf2](-) salts are liquids at room temperature. [Rmmßpip]X (X(-) = I(-), [CF3CO2](-) or [OTf](-)) are low-melting solids when R = alkyl, but room temperature liquids upon introduction of ether functionalities on R. Neither of the 3-methylpiperdinium ionic liquids showed any signs of crystallisation, even well below 0 °C. Some related non-C-substituted piperidinium and pyrrolidinium analogues were prepared and studied for comparison. Crystal structures of 1-hexyl-1,3-dimethylpiperidinium tetraphenylborate, 1-butyl-3-methylpiperidinium bromide, 1-(2-methoxyethyl)-1-methylpiperidinium chloride and 1-(2-methoxyethyl)-1-methylpyrrolidinium bromide are reported. Extensive structural and physical data are collected and compared to literature data, with special emphasis on the systematic study of the cation ring size and/or asymmetry effects on density, viscosity and ionic conductivity, allowing general trends to be outlined. Cyclic voltammetry shows that 3-methylpiperidinium ionic liquids, similarly to azepanium, piperidinium or pyrrolidinium counterparts, are extremely electrochemically stable; the portfolio of useful alternatives for safe and high-performing electrolytes is thus greatly extended.

Brønsted acids in ionic liquids: how acidity depends on the liquid structure.

Gutmann Acceptor Number (AN) values have been determined for Brønsted acid-ionic liquid mixtures, over a wide compositional range. Four systems of general formula [C2mim][A]-HA (A(-) = bistriflamide, [NTf2](-); triflate, [OTf](-); mesylate, [OMs](-); or acetate, [OAc](-), [C2mim](+) = 1-ethyl-3-methylimidazolium cation) were studied. A library of Brønsted acidic systems of varying acidity was constructed and the AN parameter was found to be a convenient approach for quantifying their acidity. HOAc, HOMs and HOTf, when dissolved in ionic liquids, were found to associate with the respective anions to form hydrogen-bonded anionic clusters, [A(HA)x](-). In contrast, HNTf2 was solubilised as a discrete, undissociated molecule. AN values were sensitive to the presence of anionic clusters; acidity could be buffered to a particular AN by binding the solubilised acid in the anionic cluster form. Overall, a simple way to manipulate and quantify the Brønsted acidity of acid-ionic liquid mixtures was demonstrated, and measured AN values were related to liquid speciation.

Electrostatic and non-covalent interactions in dicationic imidazolium-sulfonium salts with mixed anions.

A series of thioether-functionalised imidazolium salts have been prepared and characterized. Subsequent reaction of the thioether-functionalised imidazolium salts with iodomethane affords imidazolium-sulfonium salts composed of doubly charged cations and two different anions. Imidazolium-sulfonium salts containing a single anion type are obtained either by a solvent extraction method or by anion exchange. The imidazolium-sulfonium salts undergo a methyl-transfer reaction on exposure to water, giving rise to a new, singly charged imidazolium salt with iodide introduced at the 2-position of the imidazolium ring. Crystal structures of some of the imidazolium-sulfonium salts were determined by X-ray crystallography providing the topology of the interactions between the dications and the anions. Electrospray ionization mass spectrometry and quantum-chemical calculations were used to rationalise the relative strength of these interactions.

The alternation effect in ionic liquid homologous series.

A recent report on a density odd-even alternation effect in a homologous series of ionic liquids (alkyltrioctylphosphonium chlorides, with the linear alkyl group ranging from ethyl to decyl) led to the detection of a similar trend in another ionic liquid family based on a different cation (1-alkyl-3-methylimidazolium). Ab initio calculations and Molecular Dynamics simulations of the corresponding ions confirmed that the charge distribution along the alkyl side chains and the conformations adopted by them are not the direct cause of the odd-even effect. The simulations also showed that all cation side chains tend to adopt transoid conformations that pack head-to-head in the liquid phase. Such types of conformations/packing lead to odd-even alternation effects on properties involving solid phases of different molecular compounds containing linear alkyl chains. The surprising results obtained for the ionic liquid series enabled us to unveil similar trends in the liquid phases of linear alkanes and alkanols via the application of a simple corresponding states principle.

Estimation of the refractive indices of imidazolium-based ionic liquids using their polarisability values.

Statistical models have been used to estimate the refractive index of 72 imidazolium-based ionic liquids using the electronic polarisability of their ions as the data for two different mathematical approaches: artificial neural networks, in the form of multi-layer perceptrons, and multiple linear regression models. Although the artificial neural networks and linear models have been able to accomplish this task, the multi-layer perceptron model has been shown to be a more accurate method, thanks to its ability of determining non-linear relationships between different dependent variables. Additionally, it is clear that the multiple linear regression presents a systematic deviation in the estimated refractive index values, which confirms that it is an inappropriate model for this system.