Protic ionic liquids: physicochemical properties and behavior as amphiphile self-assembly solvents

The physicochemical properties of 22 protic ionic liquids (PILs) and 6 protic molten salts, and the self-assembly behavior of 3 amphiphiles in the PILs, are reported. Structure-property relationships have been explored for the PILs, including the effect of increasing the substitution of ammonium cations and the presence of methoxy and hydroxyl moieties in the cation. Anion choices included the formate, pivalate, trifluoroacetate, nitrate, and hydrogen sulfate anions. This series of PILs had a diverse range of physicochemical properties, with ionic conductivities up to 51.10 mS/cm, viscosities down to 5.4 mPa.s, surface tensions between 38.3 and 82.1 mN/m, and densities between 0.990 and 1.558 g/cm3. PILs were designed with various levels of solvent cohesiveness, as quantified by the Gordon parameter. Fourteen PILs were found to promote the self-assembly of amphiphiles. High-throughput polarized optical microscopy was used to identify lamellar, hexagonal, and bicontinuous cubic amphiphile self-assembly phases. The presence and extent of amphiphile self-assembly have been discussed in terms of the Gordon parameter.     

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

The phase behavior, including glass, devitrification, solid crystal melting, and liquid boiling transitions, and physicochemical properties, including density, refractive index, viscosity, conductivity, and air-liquid surface tension, of a series of 25 protic ionic liquids and protic fused salts are presented along with structure-property comparisons. The protic fused salts were mostly liquid at room temperature, and many exhibited a glass transition occurring at low temperatures between -114 and -44 degrees C, and high fragility, with many having low viscosities, down to as low as 17 mPa.s at 25 degrees C, and ionic conductivities up to 43.8 S/cm at 25 degrees C. These protic solvents are easily prepared through the stoichiometric combination of a primary amine and Br?nsted acid. They have poor ionic behavior when compared to the far more studied aprotic ionic liquids. However, some of the other physicochemical properties possessed by these solvents are highly promising and it is anticipated that these, or analogous protic solvents, will find applications beyond those already identified for aprotic ionic liquids. This series of protic fused salts was employed to determine the effect of structural changes on the physicochemical properties, including the effect of hydroxyl groups, increasing alkyl chain lengths, branching, and the differences between inorganic and organic anions. It was found that simple structural modifications provide a mechanism to manipulate, over a wide range, the temperature at which phase transitions occur and to specifically tailor physicochemical properties for potential end-use applications.     

Alkyltrioctylphosphonium chloride ionic liquids: synthesis and physicochemical properties

A series of alkyltrioctylphosphonium chloride ionic liquids, prepared from trioctylphosphine, and the respective 1-chloroalkane (C(n)H(2n+1)Cl), where n = 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12 or 14, is presented. The cynosure of this work is the manner in which the variable chain length impacts the physical properties, such as melting points/glass transitions, thermal stability, density and viscosity. Experimental density and viscosity data were interpreted using QPSR correlations and group contribution methods. We present the first example of an empirical alternation effect for ionic liquids.     

Eutectic-based ionic liquids with metal-containing anions and cations

Eutectic mixtures of zinc chloride and donor molecules such as urea and acetamide are described and it is proposed that these constitute a new class of ionic liquids. FAB-MS analysis shows that the liquids are made up of metal-containing anions and cations in which the donor is coordinated to the cation. Data on the viscosity, conductivity, density, phase behaviour and surface tension are presented and these are shown to be significantly different to other related ionic liquids that incorporate quaternary ammonium salts. The conductivity and viscosity are comparable with other ionic liquids and the data fit well to the Hole theory model recently proposed.     

Review of ionic liquids with fluorine-containing anions

Interest in ionic liquids has grown markedly in recent years. The syntheses, characterization and properties of quaternary alkyl-substituted ammonium, imidazolium, triazolium and pyridinium salts with a variety of fluorine-containing anions have been studied in detail; however, ionic liquids with other cations and anions continue to be discovered. This review is limited to the literature from 2000 to 2005 dealing with ionic liquids with fluorine-containing anions.     

Protic ionic liquids as electrolytes for lithium-ion batteries

In this work we report for the first time about the use of protic ionic liquids (PILs) as electrolyte for lithium-ion batteries. The electrolyte 1 M LiTFSI in Et₃NHTFSI displays a conductivity comparable to that of aprotic ionic liquids, and electrochemical stability window large enough to allow the realization of LIBs containing LFP as cathode and LTO as anode. The use of this PIL as electrolyte in LIBs allows the realization of devices able to deliver good capacity and promising cycling stability.     

Hydrogen bonds in protic ionic liquids and their correlation with physicochemical properties

The temperature dependence of the N-H proton chemical shift in protic ionic liquids (PILs) and FT-IR spectra of the N-H bonds indicated the presence of strong hydrogen bonds between the protonated cation and the anion, depending on the ΔpK(a) of the constituent acid and base, and their successive breaking with temperature, which may explain the characteristic properties of PILs such as relatively low ionicity and its decrease with temperature.     

Protic ionic liquids based on phosphonium cations: comparison with ammonium analogues

Novel protic ionic liquids (PILs) based on a tributyl phosphonium cation have been synthesised and characterised, revealing that the phosphonium based ILs show high thermal stability, high ionic conductivity and facile proton reduction compared to the corresponding ammonium based ILs.     

Alkyltributylphosphonium chloride ionic liquids: synthesis, physicochemical properties and crystal structure

A series of alkyltributylphosphonium chloride ionic liquids, prepared from tributylphosphine and the respective 1-chloroalkane, C(n)H(2n+1)Cl (where n = 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12 or 14), is reported. This work is a continuation of an extended series of tetraalkylphosphonium ionic liquids, where the focus is on the variability of n and its impact on the physical properties, such as melting points/glass transitions, thermal stability, density and viscosity. Experimental density and viscosity data were interpreted using QPSR and group contribution methods and the crystal structure of propyl(tributyl)phosphonium chloride is detailed.     

Protic ionic liquids based on the dimeric and oligomeric anions: [(AcO)xH(x-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)(x)H(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.     

Superbase-derived protic ionic liquids with chelating fluorinated anions

Eighteen new protic ionic liquids were synthesized in one step from five organic superbases and five commercially available fluorinated β-diketones. Physical properties of the ionic liquids, including thermal decomposition temperature were determined. Nine of the ionic liquids were examined as extraction media for La³⁺, with some very large distribution coefficients obtained.     

Protic cationic oligomeric ionic liquids of the urethane type

Protic oligomeric cationic ionic liquids of the oligo(ether urethane) type are synthesized via the reaction of an isocyanate prepolymer based on oligo(oxy ethylene)glycol with M = 1000 with hexamethylene-diisocyanate followed by blocking of the terminal isocyanate groups with the use of amine derivatives of imidazole, pyridine, and 3-methylpyridine and neutralization of heterocycles with ethanesulfonic acid and p-toluenesulfonic acid. The structures and properties of the synthesized oligomeric ionic liquids substantially depend on the structures of the ionic groups. They are amorphous at room temperature, but ethanesulfonate imidazolium and pyridinium oligomeric ionic liquids form a low melting crystalline phase. The proton conductivities of the oligomeric ionic liquids are determined by the type of cation in the temperature range 80–120°C under anhydrous conditions and vary within five orders of magnitude. The resulting compounds are thermally stable up to 200–270°C.     

Amphiphilicity determines nanostructure in protic ionic liquids

The bulk structure of the two oldest ionic liquids (ILs), ethylammonium nitrate (EAN) and ethanolammonium nitrate (EtAN), is elucidated using neutron diffraction. The spectra were modelled using empirical potential structure refinement (EPSR). The results demonstrate that EAN exhibits a long-range structure of solvophobic origin, similar to a bicontinuous microemulsion or disordered L(3)-sponge phase, but with a domain size of only 1 nm. The alcohol (-OH) moiety in EtAN interferes with solvophobic association between cation alkyl chains resulting in small clusters of ions, rather than an extended network.     

Novel guanidinium zwitterion and derived ionic liquids: physicochemical properties and DFT theoretical studies

A new zwitterion containing 1,1,3,3-tetramethylguanidine was synthesized using a simple method, and the physicochemical properties as well as the crystal structure of the compound were also studied. Two new acidic ionic liquids (ILs) were synthesized using the zwitterion as the precursor and the physicochemical properties including density, viscosity, thermal property, and acidic scale of the ILs were investigated. A density functional theory investigation of the geometrical structures and electronic properties of the guanidinium triflate ionic liquid was also presented for better understanding the new ILs.     

Protic metal-containing ionic liquids as catalysts: Cooperative effects between anion and cation

HCo(CO)₄ is known to be the active species in the cobalt-catalyzed hydroformylation reaction. Although it is known that the anion [Co(CO)₄]⁻ is catalytically inactive, some cobalt carbonyl-containing ionic liquids are surprisingly able to catalyze hydroformylation reactions. However, only ionic liquids with protic cations demonstrate activity, whilst aprotic cations such as BMIM⁺ result in a completely inactive compound. The four applied cobalt-containing ionic liquids differ only by the cation component. Their different performance in catalytic activity allows the presumption of cooperative effects between the cation and the anion. These fundamental influences of the cation on the hydroformylation kinetics give hints for the reaction mechanism of biphasic hydroformylation reactions as well as on the reaction pathways of the conventional hydroformylation reaction under different reaction conditions.     

The physicochemical properties of some imidazolium-based ionic liquids and their binary mixtures

The density, viscosity and conductivity of ionic liquids (ILs), 1-octyl-3-methylimidazolium tetrafluoroborate ([omim][BF4]), 1-octyl-3-methylimidazolium chloride ([omim][Cl]), 1-hexyl-3-methylimidazolium tetrafluoroborate ([hmim] BF4]), 1-hexyl- 3-methylimidazolium chloride ([hmim][Cl]), 1-hexyl-3-methylimidazolium hexafluorophosphate ([hmim][PF6]), and the [omim][BF4] + [omim][Cl], [hmim][BF4] + [hmim][Cl], and [hmim][PF6] + [hmim][Cl] binary mixtures were studied at dif- ferent temperatures. It was demonstrated that the densities of both the neat ILs and their mixtures varied linearly with temper- ature. The density sensitivity of a binary mixture is between those of the two components. The excess molar volumes (VE) of [hmim][BF4] + [hmim][Cl] and [hmim][PF6] + [hmim][Cl] mixtures are positive in the whole composition range. For [omim][BF4] + [omim][Cl], the VE is also positive in the [omim][Cl]-rich region, but is negative in the [omim][BF4]-rich re- gion. The viscosity or conductivity of a mixture is in the intermediate of those of the two neat ILs. For all the neat ILs and the binary mixtures studied, the order of conductivity is opposite to that of the viscosity. The Vogel-Tammann-Fulcher (VTF) equations can be used to fit the viscosity and conductivity of all the neat ILs and the binary mixtures. The neat ILs and their mixtures obey the Fractional Walden Rule very well, and the values of the Walden slopes are all smaller than unit, indicating obvious ion associations in the neat ILs and the binary mixtures.     

Electrogenerated radical anions in room-temperature ionic liquids

The sequential two-electron reduction of benzaldehyde to the radical anion and dianion species in 1-butyl-3-methylimidazolium triflimide and 1-butyl-1-methylpyrrolidinium triflimide is reported. In 1-butyl-1-methylpyrrolidinium triflimide, the heterogeneous electrochemistry and follow-up chemical reactivity are essentially equivalent to that in conventional molecular-solvent-based electrolytes where no interaction with the media is observed. In 1-butyl-3-methylimmidazolium triflimide, reduction occurs via the same two heterogeneous processes; however, the apparent heterogeneous rate constants are smaller by ca. 1 order of magnitude which leads to quasi-reversible electrochemical behavior. Since the bulk viscosities of the liquids are similar, the slower heterogeneous kinetics are attributed to local interfacial viscosity due to local ordering in the imidazolium-based medium. Also, a dramatic anodic shift in the reduction potentials is observed in 1-butyl-3-methylimidazolium triflimide media that is attributed to a stabilizing interaction of the radical anion and dianion species with the imidazolium cation.     

Novel Lewis-base ionic liquids replacing typical anions

We have synthesized two kinds of new Lewis-base ionic liquids (ILs); one is based on the relatively strong Lewis basic acetate anion, and the other is a salt composed of a mono-alkylated diamine such that the Lewis base site is incorporated in the cation. 1-Octyl-4-aza-1-azonia-bicyclo[2.2.2]octane bis(trifluoromethanesulfonyl)amide, [C₈dabco]TFSA, and N-butyl-N-methylpyrrolidinium acetate, [p_1,4]OAc, melted into fluid liquids at 26 and 81 °C, respectively. The thermal decomposition of [p_1,4]OAc started at around 150 °C, whereas the thermal stability of [C₈dabco]TFSA was almost equal to that of typical TFSA-based ILs in spite of the Lewis base site. This suggests that if the Lewis base site is incorporated into the cation the IL can maintain higher thermal stability. In addition, as a further result of the presence of the basic nitrogen, [C₈dabco]TFSA can dissolve hydrated Cu(NO₃)₂ whereas the other TFSA-based ILs cannot.     

Protic ionic liquids: preparation, characterization, and proton free energy level representation

We give a perspective on the relations between inorganic and organic cation ionic liquids (ILs), including members with melting points that overlap around the borderline 100 degrees C. We then present data on the synthesis and properties (melting, boiling, glass temperatures, etc.) of a large number of an intermediate group of liquids that cover the ground between equimolar molecular mixtures and ILs, depending on the energetics of transfer of a proton from one member of the pair to the other. These proton-transfer ILs have interesting properties, including the ability to serve as electrolytes in solvent-free fuel cell systems. We provide a basis for assessing their relation to aprotic ILs by means of a Gurney-type proton-transfer free energy level diagram, with approximate values of the energy levels based on free energy of formation and pK(a) data. The energy level scheme allows us to verify the relation between solvent-free acidic and basic electrolytes, and the familiar aqueous variety, and to identify neutral protic electrolytes that are unavailable in the case of aqueous systems.