Ionic liquids (ILs) are organic salts with melting points near room temperature (or by convention below 100 degrees C). Recently, their unique materials and solvent properties and the growing interest in a sustainable, "green" chemistry has led to an amazing increase in interest in such salts. A huge number of potential cation and anion families and their many substitution patterns allows the desired properties for specific applications to be selected. Because it is impossible to experimentally investigate even a small fraction of the potential cation-anion combinations, a molecular-based understanding of their properties is crucial. However, the unusual complexity of their intermolecular interactions renders molecular-based interpretations difficult, and gives rise to many controversies, speculations, and even myths about the properties that ILs allegedly possess. Herein the current knowledge about the molecular foundations of IL behavior is discussed.

Understanding Ionic Liquids at the Molecular Level: Facts, Problems, and Controversies

Ionic liquids (ILs), considered to be a relatively recent magical chemical due their unique properties, have a large variety of applications in all areas of the chemical industries. The areas of application include electrolyte in batteries, lubricants, plasticizers, solvents and catalysis in synthesis, matrices for mass spectroscopy, solvents to manufacture nano-materials, extraction, gas absorption agents, etc. Non-volatility and non-flammability are their common characteristics giving them an advantageous edge in various applications. This common advantage, when considered with the possibility of tuning the chemical and physical properties of ILs by changing anion–cation combination is a great opportunity to obtain task-specific ILs for a multitude of specific applications. There are numerous studies in the related literature concerning the unique properties, preparation methods, and different applications of ILs in the literature. In this review, a general description of ILs and historical background are given; basic properties of ILs such as solvent properties, polarity, toxicology, air and moisture stability are discussed; structure of ILs, cation, anion types and synthesis methods in the related literature are briefly summarized. However, the main focus of this paper is how ILs may be used in the chemicals processing industries. Thus, the main application areas are searched and the basic applications such as solvent replacement, purification of gases, homogenous and heterogeneous catalysis, biological reactions media and removal of metal ions are discussed in detail. Not only the advantages of ILs but also the essential challenges and potentials for using ILs in the chemical industries are also addressed. ILs have become the partner of scCO 2 in many applications and most of the reported studies in the literature focus on the interaction of these two green solvents, i.e. ILs and scCO 2 . The chemistry of the ILs has been reviewed in numerous papers earlier. Therefore, the major purpose of this review paper is to provide an overview for the specific chemical and physical properties of ILs and to investigate IL–scCO 2 systems in some detail. Recovery of solutes from ILs with CO 2 , separation of ILs from organic solvents by CO 2 , high-pressure phase behavior of IL–scCO 2 systems, solubility of ILs in CO 2 phase, and the interaction of the IL–scCO 2 system at molecular level are also included.

A review of ionic liquids towards supercritical fluid applications

Ionic liquids (ILs) are ambient temperature molten salts and have attracted considerable attention because of their unique properties such as nonflammability, negligible volatility, high ionic conductivity, and thermal stability. Here, we show that the combinations of macromolecules with ILs as solvents and additives could offer new progress, challenges, and opportunities in polymer materials science. Compatible binary systems, where ILs are solidified (gelled) by polymers, can be used as ion-conducting polymer electrolytes (ion gels). The ion transport in ion gels is found to be decoupled from the segmental motion of the polymers, leading to relatively high ionic conductivities even at their glass transition temperatures. The flexibility of the molecular design of ILs allows us to prepare many new ILs having electrochemical functionalities such as proton, electron, and lithium ion transport. The new ILs and their ion gels appear to be promising candidates as new materials for electrochemical applications...

Macromolecules in Ionic Liquids: Progress, Challenges, and Opportunities

The preparation of ionic liquids derived from amino acids, and their properties, are outlined Since amino acids have both a carboxylic acid residue and an amino group in a single molecule, they can be used as either anions or cations These groups are also useful in their ability to introduce functional group(s) Twenty different natural amino acids were used as anions, to couple with the 1-ethyl-3-methylimidazolium cation The salts obtained were all liquid at room temperature The properties of the resulting ionic liquids (AAILs) depend on the side groups of the amino acids involved These AAILs, composed of an amino acid with some functional groups such as a hydrogen bonding group, a charged group, or an aromatic ring, had an increased glass transition (or melting) temperature and/or higher viscosity as a result of additional interactions among the ions Viscosity is reduced and the decomposition temperature of imidazolium-type salts is improved by using the tetrabutylphosphonium cation The chirality of AAILs was maintained even upon heating to 150 degrees C after acetylation of the free amino group The amino group was also modified to introduce a strong acid group so as to form hydrophobic and chiral ionic liquids Unique phase behavior of the resulting hydrophobic ionic liquids and water mixture is found; the mixture is clearly phase separated at room temperature, but the solubility of water in this IL increases upon cooling, to give a homogeneous solution This phase change is reversible, and separation occurs again by raising the temperature a few degrees It is extraordinary for an IL/water mixture to display such behavior with a lower critical solution temperature Some likely applications are proposed for these amino acid derived ionic liquids

Amino acid ionic liquids.

Physical properties of 4 room-temperature ionic liquids consisting of the 1-butyl-3-methylimidazolium cation with various perfluorinated anions and the bis(trifluoromethylsulfonyl)imide (Tf2N-) anion with 12 pyrrolidinium-, ammonium-, and hydroxyl-containing cations are reported. Electronic structure methods are used to calculate properties related to the size, shape, and dipole moment of individual ions. Experimental measurements of phase-transition temperatures, densities, refractive indices, surface tensions, solvatochromic polarities based on absorption of Nile Red, 19F chemical shifts of the Tf2N- anion, temperature-dependent viscosities, conductivities, and cation diffusion coefficients are reported. Correlations among the measured quantities as well as the use of surface tension and molar volume for estimating Hildebrand solubility parameters of ionic liquids are also discussed.

Physical properties of ionic liquids consisting of the 1-butyl-3-methylimidazolium cation with various anions and the bis(trifluoromethylsulfonyl)imide anion with various cations.

A systematic study of liquid–liquid phase transitions in mixtures of room temperature ionic liquids (RTIL) with nonionic solvents of different dielectric constant is presented. We report phase diagrams and the location of the critical points in the binary systems. The salts are 1-alkyl-3-methylimidazolium tetrafluoroborates and hexafluorophosphates with butyl-, hexyl-, octyl- side chains. The solvents are water, n-alcohols (n = 1 − 16), branched alcohols, and dialcohols. A corresponding state analysis of the data is carried out in terms of the variables of the restricted primitive model (RPM), which considers ionic fluids as equal-sized charged hard spheres in a dielectric continuum. The reduced critical temperatures are found to depend almost linearly on the dielectric constant of the solvent. The data of all investigated systems are represented by a master plot, which indicates a continuous crossover from a Coulomb driven phase transition, reasonably described by the RPM, towards a phase transition determined by hydrophobic interactions.

Corresponding states analysis of the critical points in binary solutions of room temperature ionic liquids

Precise coexistence curves are reported for the liquid–liquid phase transition of binary solutions of the room temperature ionic liquid (RTIL) 1-methyl-3-hexylimidazolium tetrafluoroborate (C6mim+BF4−) in a series of alcohols (1-butanol, 1-pentanol, 2-butanol, and 2-pentanol). The phase diagrams are determined by measuring the temperature dependence of the refractive index in the two phases of samples of critical composition. The critical data of the systems are in the region predicted for the model fluid of equal-sized, charged, hard spheres in a dielectric continuum, the so-called restricted primitive model (RPM). Therefore, the phase transition can be classified as essentially driven by Coulomb interactions. The effective exponents βeff determined are close to the universal Ising value, where the deviations are found to be negative, when the volume fraction or the mass fraction are chosen as concentration variable. The negative values of the first Wegner correction indicate non-uniform crossover from Ising to mean-field criticality. The diameter of the coexistence curves shows the non-analytic temperature dependence typical for Ising systems.

The liquid–liquid coexistence of binary mixtures of the room temperature ionic liquid 1-methyl-3-hexylimidazolium tetrafluoroborate with alcohols

The existence of a tricritical point was postulated for the ternary system water/3-methylpyridine/NaBr (J. Jacob et al., Phys. Rev. E, 1998, 58, 2188) because light-scattering measurements yielded the mean-field critical exponent γ = 1 of the susceptibility at high concentrations of the salt, while the usual Ising value γ = 1.24 was found at low salt concentrations. However, in the preceding papers we have reported measurements of the viscosities and of the coexistence curves, which for all salt concentrations showed normal Ising criticality with corrections to scaling at large separations from the critical temperatures. In this paper, we continue the research on this system and report now measurements of the scattering intensity in the homogeneous phase approaching the critical temperature from below. Micro-heterogeneities causing a background were observed, which required waiting times up to 8 days to decay. The data are corrected for multiple scattering using a Monte Carlo simulation of the photon-paths in the sample and also by using the 3D cross-correlation method that enables to separate the singly scattered light from the other contributions. The scattering intensities of the equilibrated samples are described by the Ornstein–Zernike expression. For all salt concentrations, (10–17 mass%) the critical exponents are consistent with Ising critical behaviour. No systematic variation with the concentration of the salt is found for the various properties determined as the effective values of the critical exponents γ and ν for the susceptibility and the correlation length, respectively, and the corresponding amplitudes.

Tricriticality in the ternary system 3-methylpyridine/water/NaBr? The light-scattering intensity

In this work, we report measurements of viscosity and density near the liquid–liquid phase transition of the binary solution of the room temperature ionic liquid (RTIL) 1-methyl-3-hexylimidazolium tetrafluoroborate (C6mim+BF4−) in 1-pentanol. The density and the shear viscosity have been determined as function of the temperature and of the concentration. The critical point is located at the temperature Tc = 314.91 K and the mass fraction wc = 0.31034 of the salt. For the critical concentration, the viscosity measurements yield an enhancement as expected for Ising criticality with a crossover to regular behaviour. The regular background is obtained by interpolation from the measurements at non-critical concentrations. The critical enhancement is well described by the mode-coupling crossover function of Bhattacharjee et al. (Phys. Rev. A, 1981, 24, 1469) .The closed form result of the limiting case qD/qc → 0 matches the data almost perfectly.

Critical viscosity near the liquid–liquid phase transition in the solution of the ionic liquid 1-methyl-3-hexylimidazolium tetrafluoroborate in 1-pentanol

The existence of a tricritical point was postulated recently for the ternary system water/3-methylpyridine/NaBr (J. Jacob et al., Phys. Rev. E, 1998, 58, 2188) because light-scattering measurements showed at high concentrations of the salt purely mean field behaviour with a critical exponent of the susceptibility γ=1 instead of the Ising value γ=1.24, which is found at low salt concentrations. However, in a preceding paper (Wagner et al., Phys. Chem. Chem. Phys., 2002, 4, 5300) we have reported measurements of the viscosities, which for all salt concentrations showed normal Ising criticality with a crossover to non-critical behaviour at large separations from the critical temperatures. In this paper we continue the research on this system and report refractive index measurements of the coexisting phases. For all salt concentrations (10–17 mass%) the critical exponents are consistent with Ising critical behaviour where the Lorenz–Lorentz function is taken as order parameter. The effective value of the critical exponent β is found to depend on the concentration of the salt. In the region of high salt concentration mean field tricriticality was expected with an exponent β=1 while a value somewhat below the Ising value is found. The effective value of β is reduced at lower salt content. Essentially the same results are obtained when the mole, mass or volume fractions of the different components are taken as order parameter, where the compositions in the two phases are estimated on the basis of the separation temperatures determined for previously given concentrations.

Olimpiu Stanga

Papers

Corresponding states analysis of the critical points in binary solutions of room temperature ionic liquids

The liquid–liquid coexistence of binary mixtures of the room temperature ionic liquid 1-methyl-3-hexylimidazolium tetrafluoroborate with alcohols

Tricriticality in the ternary system 3-methylpyridine/water/NaBr? The light-scattering intensity

Critical viscosity near the liquid–liquid phase transition in the solution of the ionic liquid 1-methyl-3-hexylimidazolium tetrafluoroborate in 1-pentanol

Tricriticality in the ternary system 3-methylpyridine/water/NaBr? The coexistence curves