New insights into the fundamental chemical nature of ionic liquid film formation on magnesium alloy surfaces

Ionic liquids (ILs) based on trihexyltetradecylphosphonium coupled with either diphenylphosphate or bis(trifluoromethanesulfonyl)amide have been shown to react with magnesium alloy surfaces, leading to the formation a surface film that can improve the corrosion resistance of the alloy. The morphology and microstructure of the magnesium surface seems critical in determining the nature of the interphase, with grain boundary phases and intermetallics within the grain, rich in zirconium and zinc, showing almost no interaction with the IL and thereby resulting in a heterogeneous surface film. This has been explained, on the basis of solid-state NMR evidence, as being due to the extremely low reactivity of the native oxide films on the intermetallics (ZrO₂ and ZnO) with the IL as compared with the magnesium-rich matrix where a magnesium hydroxide and/or carbonate inorganic surface is likely. Solid-state NMR characterization of the ZE41 alloy surface treated with the IL based on (Tf)₂N− indicates that this anion reacts to form a metal fluoride rich surface in addition to an organic component. The diphenylphosphate anion also seems to undergo an additional chemical process on the metal surface, indicating that film formation on the metal is not a simple chemical interaction between the components of the IL and the substrate but may involve electrochemical processes.

Ionic conductivity in poly(diethylene glycol-carbonate)/sodium triflate complexes

New polymer electrolytes were synthesized and characterized based on a new polymer host. The motivation was to produce a host polymer with a high dielectric constant which should reduce ion clustering with an attendant increased conductivity. The new polymer host, poly(diethylene glycol carbonate) and its sodium triflate complexes were characterized by thermal analysis and AC impedance measurements. The polycarbonate backbone appears less flexible than the polyether hosts as evidenced by the higher glass transition temperatures. The conductivity for the sodium triflate complexes was measured as ~ 10⁻⁵ S cm⁻¹ at 55 °C and the dielectric constant of the host polymer was found to be 3.6 at 3 GHz. The low conductivity is attributed to rigidity of the polycarbonate.

N-methyl-N-alkylpyrrolidinium tetrafluoroborate salts : ionic solvents and solid electrolytes

A series of N-methyl-N-alkylpyrrolidinium tetrafluoroborate salts were synthesised. The spectroscopic, physical and electrochemical characteristics of this family of salts have been investigated with respect to potential usage as ionic solvents and electrolytes. The lowest melting point among the family is 64°C for the N-methyl-N-propylpyrrolidinium tetrafluoroborate (P₁₃BF₄). This is sufficiently low to enable this salt to be useful as an ionic liquid in chemical synthesis involving reactions above 70°C. Most of the compounds exhibit one or more solid–solid transitions below the melting point, this behaviour is thought to indicate the existence of plastic crystal phases.

N-methyl-N-alkylpyrrolidinium nonafluoro-1-butanesulfonate salts : Ionic liquid properties and plastic crystal behaviour

A series of N-methyl-N-alkylpyrrolidinium nonafluoro-1-butanesulfonate salts were synthesised and characterised. The thermophysical characteristics of this family of salts have been investigated with respect to potential use as ionic liquids and solid electrolytes. N-Methyl-N-butylpyrrolidinium nonafluoro-1-butanesulfonate (p_1,4NfO) has the lowest melting point of the family, at 94 °C. Electrochemical analysis of p_1,4 NfO in the liquid state shows an electrochemical window of ~6 V. All compounds exhibit one or more solid–solid transitions at sub-ambient temperatures, indicating the existence of plastic crystal phases.

Liquids intermediate between 'molecular' and 'ionic' liquids : liquid ion pairs?

Ionic liquids comprised of tetradecyltrihexyl- and tetrabutyl- phosphonium cations paired with chloride or sulfonyl amide anions exhibit properties that reflect strong ion association, including comparatively low viscosity as well as a degree of volatility, and hence exemplify an interesting intermediate state between true ionic and true molecular liquids.

Protein solubilising and stabilising ionic liquids

We report a family of biocompatible ionic liquids (ILs) which are able to dissolve significant amounts of proteins such as cytochrome c and in which ATR-FTIR spectroscopy results show retention of secondary structure to extreme temperatures.

Solubility and Stability of cytochrome c in hydrated ionic liquids : effect of oxo acid residues and kosmotropicity

Hydrated ionic liquids (ILs) were prepared by adding appropriate amounts of water to hydrophilic ILs. Some hydrated ILs show excellent solubilizing ability for proteins, keeping the basic properties of ILs. The solubility of cytochrome c (cyt c) depended on the structure of the component ions. When component anions have oxo acid residues, the resulting hydrated ILs solubilize cyt c quite well. In such hydrated ILs, the structure and activity of cyt c is influenced by the kosmotropicity of the component ions. We synthesized ILs from various ions having different kosmotropicity, including dihydrogen phosphate (dhp), dibutylphosphate, acetate, lactate, and methanesulfonate as anions. The activity of the dissolved cyt c depends on the permutations of kosmotropicity of the component ions. cyt c shows no structural change and retains its activity when dissolved in the hydrated choline dhp, which is an excellent combination of chaotropic cation and kosmotropic anion. Furthermore, cyt c dissolved in the hydrated choline dhp remained in a native state and was active after 18 months of storage at room temperature.

Methanesulfonate and p-toluenesulfonate salts of the N-methyl-N-alkylpyrrolidinium and quaternary ammonium cations : novel low cost ionic liquids

The preparation and characterization of a series of novel salts, based on the N-methyl-N-alkylpyrrolidinium or quaternary ammonium organic cations coupled with sulfonate type anions, namely the mesylate (CH₃SO₃⁻) and tosylate (CH₃C₆H₄SO₃⁻) anions are reported. These salts are analogues of the previously described organic cation bis(trifluoromethanesulfonyl)amide (TFSA) salts that form useful ionic liquids of interest in “Green” synthesis. Several of the salts are liquid below 50 °C, e.g. tributylhexylammonium tosylate and ethylmethylpyrrolidinium mesylate and one is liquid at and below room temperature (tributylhexylammonium mesylate). These new salts have a cost advantage over salts of the TFSA⁻, PF₆⁻ and CF₃SO₃⁻ anions. Electrochemical and thermal properties have been investigated. The salts are stable to beyond 100 °C and exhibit electrochemical potential windows of at least ±2 V vs. Ag/Ag+. Some of the salts exhibit multiple crystalline phases below their melting points, potentially indicative of plastic crystal behaviour, whilst others showed more simple solid–liquid behaviour. Many of the salts were found to be glass forming.

An investigation of a phosphinate-based ionic liquid for corrosion protection of magnesium alloy AZ31

This work reports a preliminary exploration of the potential of the ionic liquid trihexyl(tetradecyl)phosphonium bis(2,4,4-trimethylpentyl)phosphinate (P_6,6,6,14M₃PPh) for use as a conversion coating agent for corrosion protection of magnesium alloy AZ31. Results obtained for the as received IL did not indicate any measureable improvement in protection. However, when the IL was allowed to reach equilibrium/saturation with moisture from the atmosphere, treatment with this ‘wet’ solution resulted in a substantial improvement in corrosion resistance. Preliminary electrochemical, optical, and spectroscopic characterization of the film will be presented along with a possible mechanism for film formation.

Towards a better understanding of 'delocalized charge' in ionic liquid anions

One of the main characteristics that are attributed to ionic liquids (especially those with a low melting point) is that the anions comprising the ionic liquids possess a certain degree of charge delocalization as compared to anions in traditional molten salts. Based on the proton affinity equilibrium we proposed a new energetic criterion that can be used as a measure of charge delocalization. The proposed proton affinity comparison quantifies the extent to which ionic liquid anions are delocalized. Thus it should lead to a better understanding towards the design of task-specific ionic liquids. Therefore, this criterion can be applied to newly designed anions to assure that the extent of charge delocalization falls within the same range of values on the proton affinity scale as other commonly used ionic liquid anions.

On the components of the dielectric constants of ionic liquids : ionic polarization?

According to dielectric spectroscopy measurements, ionic liquids (ILs) have rather modest dielectric constants that reflect contributions from distortion and electronic polarization caused by the molecular polarizability as well as the orientation polarization caused by the permanent dipole moment of the ions. To understand the relative importance of these various contributions, the electronic polarizabilities of 27 routinely used ionic liquid ions of different symmetry and size were calculated using ab initio-based methods such as HF and MP2. Using the Clausius–Mossotti equation, these polarizabilities were then used to obtain the electronic polarization contribution (ε_op) to the dielectric constants of six ionic liquids, [C₂mim][BF₄], [C₂mpyr][N(CN)₂], [C₂mim][CF₃SO₃], [EtNH₃][NO₃], [C₂mim][NTf₂] and [C₂mim][EtSO₄]. Theoretical ε_op values were compared to experimental refractive indices of these ionic liquids as well as to those of traditional molecular solvents such as water, tetrahydrofuran (THF), dimethylsulfoxide (DMSO) and formamide. The dipole moments of the ions were also calculated, and from these it is shown that the molecular reorientation component of the dielectric constants of the ionic liquids consisting of ions with small or negligible dipole moments is quite small. Thus it is concluded that a contribution from a form of “ionic polarization” must be present.

Protic ionic liquids based on the dimeric and oligomeric anions: [(AcO)xHx−1]−

We describe a fluidity and conductivity study as a function of composition in N-methylpyrrolidine–acetic acid mixtures. The simple 1:1 acid–base mixture appears to form an ionic liquid, but its degree of ionicity is quite low and such liquids are better thought of as poorly dissociated mixtures of acid and base. The composition consisting of 3 moles acetic acid and 1 mole N-methylpyrrolidine is shown to form the highest ionicity mixture in this binary due to the presence of oligomeric anionic species [(AcO)_xH_x−1]− stabilised by hydrogen bonds. These oligomeric species, being weaker bases than the acetate anion, shift the proton transfer equilibrium towards formation of ionic species, thus generating a higher degree of ionicity than is present at the 1:1 composition. A Walden plot analysis, thermogravimetric behaviour and proton NMR data, as well as ab initio calculations of the oligomeric species, all support this conclusion.

Cycling performance of lithium metal polymer cells assembled with ionic liquid and poly(3-methyl thiophene)/carbon nanotube composite cathode

A poly(3-methylthiophene) (PMT)/multi-walled carbon nanotube (CNT) composite is synthesized by in situ chemical polymerization. The PMT/CNT composite is used as an active cathode material in lithium metal polymer cells assembled with ionic liquid (IL) electrolytes. The IL electrolyte consists of 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF4) and LiBF4. A small amount of vinylene carbonate is added to the IL electrolyte to prevent the reductive decomposition of the imidazolium cation in EMIBF4. A porous poly(vinylidene fluoride-co-hexafluoropropylene) (P(VdF-co-HFP)) film is used as a polymer membrane for assembling the cells. Electrochemical properties of the PMT/CNT composite electrode in the IL electrolyte are evaluated and the effect of vinylene carbonate on the cycling performance of the lithium metal polymer cells is investigated. The cells assembled with a non-flammable IL electrolyte and a PMT/CNT composite cathode are promising candidates for high-voltage–power sources with enhanced safety.