James H. Davis

Energy applications of ionic liquids

Ionic liquids offer a unique suite of properties that make them important candidates for a number of energy related applications. Cation–anion combinations that exhibit low volatility coupled with high electrochemical and thermal stability, as well as ionic conductivity, create the possibility of designing ideal electrolytes for batteries, super-capacitors, actuators, dye sensitised solar cells and thermo-electrochemical cells. In the field of water splitting to produce hydrogen they have been used to synthesize some of the best performing water oxidation catalysts and some members of the protic ionic liquid family co-catalyse an unusual, very high energy efficiency water oxidation process. As fuel cell electrolytes, the high proton conductivity of some of the protic ionic liquid family offers the potential of fuel cells operating in the optimum temperature region above 100 °C. Beyond electrochemical applications, the low vapour pressure of these liquids, along with their ability to offer tuneable functionality, also makes them ideal as CO2 absorbents for post-combustion CO2 capture. Similarly, the tuneable phase properties of the many members of this large family of salts are also allowing the creation of phase-change thermal energy storage materials having melting points tuned to the application. This perspective article provides an overview of these developing energy related applications of ionic liquids and offers some thoughts on the emerging challenges and opportunities.

Sweet success: ionic liquids derived from non-nutritive sweetenersElectronic supplementary information (ESI) available: experimental details; IR spectra. See http://www.rsc.org/suppdata/cc/b3/b313068a/

The anions of the sweeteners saccharin and acesulfame form ionic liquids when paired with a variety of organic cations.

The Fluid-Mosaic Model, Homeoviscous Adaptation, and Ionic Liquids: Dramatic Lowering of the Melting Point by Side-Chain Unsaturation†

Proposed by Singer and Nicolson in 1972, the fluid-mosaic model holds that the phospholipid bilayer is a dynamic twodimensional solvent milieu. Its proper function is closely tied to its “fluidity”, and that is often quantified by reference to the melting point, Tm (increased fluidity corresponds to a lower Tm value). The fluid-mosaic model is highly evocative of the emerging picture of nanoscale structuring in ionic liquids (ILs), and just as the function of phospholipid bilayers is tied to the Tm value, so too is the utility of ILs. Whereas the former often have low Tm values despite being composed of charged species with long aliphatic appendages, the fluidity of ILs generally decreases when progressively longer aliphatic appendages are used. It is a challenge to design imidazolium ILs (the most common IL class) that incorporate progressively more lipophilic structural elements while keeping their melting points below room temperature (Figure 1). Indeed, the Tm values of these ILs begin to rise dramatically once an appended Nalkyl group exceeds seven carbon atoms in length. Herein we report that by using an approach modeled on homeoviscous adaptation (HVA), ILs with very long alkyl appendages and very low Tm values can be prepared. This discovery may have significant implications for IL use in enzymatic catalysis, lubricants, heat-transfer fluids, and gas storage and separation, among other applications. Widely accepted as a mechanism by which the melting temperature of cell membranes is modulated, HVA is the incorporation into cell membranes of phospholipids with “kinked” tail structures. It is argued that the packing efficiency of the collective membrane hydrophobic components is diminished by the presence of these phospholipids and that increased fluidity results. A comparison of the Tm value of distearoylphosphatidylcholine with that of dioleylphosphatidylcholine provides a dramatic example of how much impact this seemingly trivial difference can have. The former, with its linear, saturated C18 tails has a Tm value of 58 8C; the latter, with its “kinked” C18 tails (each of which incorporates a cis-alkenyl group), has a Tm value of 22 8C. This effect is also at the heart of the Tm difference between the solid triacyl glycerols called fats, and those that are liquid at room temperature known as oils. In both instances, the effect is probably entropic in nature, as in the case of anthracene (“linear”, Tm = 217 8C) and phenanthrene (“kinked”, Tm = 99 8C). Accordingly, we hypothesized that ILs with long, unsaturated, aliphatic tail structures would, like the corresponding phospholipids, have significantly lower Tm values than their counterparts with saturated appendages. To test the validity of our hypothesis by measuring their Tm values, we prepared a series of lipid-inspired ILs in a threestep process from high-purity (99 + %) fatty-alcohol mesylates, 1-methylimidazole, NaI, and NaTf2N. [12] Each of the ILs (Scheme 1) had a long alkyl appendage identical to that in a natural fatty acid. Compounds 1, 3, and 8 feature fully saturated C16, C18, and C20 side chains, respectively, and their Figure 1. Influence of alkyl-chain length on Tm in N-alkyl N-methylimidazolium salts and the corresponding n-alkanes. The graph includes data from the literature and newly synthesized ionic liquids.

Simultaneous membrane transport of two active pharmaceutical ingredients by charge assisted hydrogen bond complex formation

Using permeation through a model membrane in a Franz diffusion cell, we have demonstrated that acidic and basic active pharmaceutical ingredients (APIs) in deep eutectic ‘liquid co-crystal’ form can be held tightly together, even in solution, via strong hydrogen bonds or partially ionized interactions, providing simultaneous transport at rates much higher than solutions of their corresponding, commercially available crystalline salts, albeit at rates that are lower than the neutral forms of the individual molecules. It was also shown that the deep eutectic APIs do not have to be premade, but hydrogen-bonded complexes can be formed in situ by mixing the corresponding API–solvent solutions. To understand the behavior, we have extensively studied a range of nonstoichiometric mixtures of lidocaine and ibuprofen spectroscopically and via membrane transport. The data demonstrates the nature of the interactions between the acid and base and provides a route to tune the rate of membrane transport.

Building a bridge between aprotic and protic ionic liquids

ILs of dications with linked protic and aprotic centres have been prepared. They have rich H-bonding and a Bronsted acid–base character like small ΔpKa protic ILs, but very low apparent vapour pressures, more akin to aprotic ILs.

On the Formation of a Protic Ionic Liquid in Nature

The practical utility of ionic liquids (ILs) makes the absence (heretofore) of reported examples from nature quite puzzling, given the facility with which nature produces many other types of exotic but utilitarian substances. In that vein, we report here the identification and characterization of a naturally occurring protic IL. It can be formed during confrontations between the ants S. invicta and N. fulva. After being sprayed with alkaloid-based S. invicta venom, N. fulva detoxifies by grooming with its own venom, formic acid. The mixture is a viscous liquid manifestly different from either of the constituents. Further, we find that the change results as a consequence of formic acid protonation of the N centers of the S. invicta venom alkaloids. The resulting mixed-cation ammonium formate milieu has properties consistent with its classification as a protic IL.

Thermally stable bis(trifluoromethylsulfonyl)imide salts and their mixtures

We show that both tetraphenylphosphonium bis(trifluoromethylsulfonyl)imide ([PPh4][NTf2]) and Cs[NTf2] are low melting salts of exceptionally high and also very similar thermal stability. This similarity indicates that the thermal stability is dominated by the anion. Moreover, eutectic mixtures of [PPh4][NTf2] and Cs[NTf2] with melting points below 100 °C are presented. Surface analysis of the latter in the liquid state reveals a surprising depletion of [PPh4]+ ions from the surface.

A simple and rapid route to novel tetra(4-thiaalkyl)ammonium bromides

A simple approach for the preparation of symmetrical quaternary ammonium bromides employing thiol–ene click chemistry is used to synthesize tetra(4-thiaalkyl)ammonium bromides. This approach allows the incorporation of a variety of alkyl moieties onto the nitrogen center with a one-step synthesis involving easy work-up, no side reactions and environmentally friendly reagents. To elucidate information regarding the behaviour of this novel class of compounds, comparisons to tetraalkylammonium analogues have been made. These include melting points, activity as phase-transfer catalysts, and conformational predictions from computational modelling. All results are consistent in indicating stronger bonding between the quaternary cation and the anion for the salts with 4-thiaalkyl chains as compared to those with n-alkyl chains.

Thermally robust: triarylsulfonium ionic liquids stable in air for 90 days at 300 °C

Select triarylsulfonium salts constitute ionic liquids with outstanding long-term, high-temperature aerobic stability (no mass loss in 90 days at 300 °C in air), making them among the most thermally stable organic materials known. A detailed analysis of their thermophysical properties reveals that lowering melting points in these salts by increasing ion size or lowering ion symmetry cannot be assumed, but remains an iterative process.

An evaluation of anion suitability for use in ionic liquids with long-term, high-temperature thermal stability

The stability of fourteen different PPN+ salts has been studied in 96 hour tests, in air, at temperatures of 200 °C, 250 °C, and 300 °C. The results have enabled generation of a ranking of their stabilities, and by extension, that of their anionic components. This data should prove especially useful in ongoing efforts to formulate ionic liquids for high-temperature applications.