Polymerized ionic liquids: Effects of counter‐anions on ion conduction and polymerization kinetics

A novel imidazolium‐containing monomer, 1‐[ω‐methacryloyloxydecyl]‐3‐(n‐butyl)‐imidazolium (1BDIMA), was synthesized and polymerized using free radical and controlled free radical polymerization followed by post‐polymerization ion exchange with bromide (Br), tetrafluoroborate (BF₄), hexafluorophosphate (PF₆), or bis(trifluoromethylsulfonyl)imide (Tf₂N). The thermal properties and ionic conductivity of the polymers showed a strong dependence on the counter‐ions and had glass transition temperatures (T_g) and ion conductivities at room temperature ranging from 10 °C to −42 °C and 2.09 × 10⁻⁷ S cm⁻¹ to 2.45 × 10⁻⁵ S cm⁻¹. In particular, PILs with Tf₂N counter‐ions showed excellent ion conductivity of 2.45 × 10⁻⁵ S cm⁻¹ at room temperature without additional ionic liquids (ILs) being added to the system, making them suitable for further study as electro‐responsive materials. In addition to the counter‐ions, solvent was found to have a significant effect on the reversible addition‐fragmentation chain‐transfer polymerization (RAFT) for 1BDIMA with different counter‐ions. For example, 1BDIMATf₂N would not polymerize in acetonitrile (MeCN) at 65 °C and only achieved low monomer conversion (< 5%) at 75 °C. However, 1BDIMA‐Tf₂N proceeded to high conversion in dimethylformamide (DMF) at 65 °C and 1BDIMABr polymerized significantly faster in DMF compared to MeCN. NMR diffusometry was used to investigate the kinetic differences by probing the diffusion coefficients for each monomer and counter‐ion in MeCN and DMF. These results indicate that the reaction rates are not diffusion limited, and point to a need for deeper understanding of the role electrostatics plays in the kinetics of free radical polymerizations. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 1346–1357     

One-dimensional ion transport in self-organized columnar ionic liquids

New fan-shaped ionic liquids forming columnar liquid crystalline phases have been prepared to obtain one-dimensional ion-transporting materials. The ionic liquids consist of two incompatible parts: an imidazolium-based ionic part as an ion-conducting part and tris(alkyloxy)phenyl parts as insulating parts. Two compounds having octyl and dodecyl chains have been synthesized. Self-assembly of these materials leads to the formation of thermotropic hexagonal columnar liquid crystalline states at room temperature. Anisotropic one-dimensional ionic conductivities have been successfully measured by the cells having comb-shaped gold electrodes. The self-organized columns have been aligned macroscopically in two directions by shearing perpendicular and parallel to the electrodes. The ionic conductivities parallel to the column axis are higher than those perpendicular to the axis. The incorporation of lithium salts in these columnar materials leads to the enhancement of the ionic conductivities and their anisotropy. These materials would be useful for anisotropic transportation of ions at the nanometer level.     

Alkylation of the ambident indole ion in ionic liquids

Alkylation of the ambident indole anion in ionic liquids has been investigated. The reaction rate is greater in ionic liquids than in organic solvents. The polarity of certain ionic liquids has been determined to be located between methanol and acetonitrile. __________ Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 5, pp. 676–690. May, 2008.     

Dispersion of carbon nanotubes by carbazole-tailed amphiphilic imidazolium ionic liquids in aqueous solutions

Spectrophotometric, kinetic, and transmission electron microscopic (TEM) data for the formation of Ag-nanoparticles using aspartic acid (Asp) as reductant are reported for the first time. In the formation of transparent silver sols, an alkaline medium is required. The silver nanoparticles are spherical, uniform particle size, and strongly depend on the [Asp]. The apparent rate constant decreases with [Asp] (from 4.0 to 24.0×10(-4)moldm(-3), the rate constants decreased from 2.6×10(-4) to 0.3×10(-4)s(-1)). For a certain reaction time, i.e., 30min, the absorbance of the silver sol first increased until it reached a maximum, and then decreased with [Asp]. Kinetic and TEM results indicate that the size of the Ag-nanoparticles depends on the [Asp]. It is proposed that the oxidation of Asp occurs by the adsorbed Ag(+) ions on the surface of Ag(2)O particles.     

Molecular ions of ionic liquids in the gas phase

Ionic liquids form neutral ion pairs (CA) upon evaporation. The softness of the gas-phase ionization of field ionization has been used to generate "molecular ions," CA(+*), of ionic liquids, most probably by neutralization of the anion. In detail, 1-ethyl-3-methylimidazolium-thiocyanate, [C(6)H(11)N(2)](+) [SCN](-), 1-butyl-3-methylimidazolium-tricyanomethide, [C(8)H(15)N(2)](+) [C(4)N(3)](-), N-butyl-3-methylpyridinium-dicyanamide, [C(10)H(16)N](+) [C(2)N(3)](-), and 1-butyl-1-methylpyrrolidinium-bis[(trifluormethyl)sulfonyl]amide, [C(9)H(20)N](+) [C(2)F(6)NO(4)S(2)](-) were used. The assignment as CA(+*) ions, which has been confirmed by accurate mass measurements and misassignments due to thermal decomposition of the ionic liquids, has been ruled out by field desorption and electrospray ionization mass spectrometry of the residues.     

The nature of ionic liquids in the gas phase

Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS) experiments showed that when aprotic ionic liquids vaporize under pressure and temperature conditions similar to those of a reduced-pressure distillation, the gas phase is composed of discrete anion-cation pairs. The evolution of the mass spectrometric signals recorded during fractional distillations of binary ionic liquid mixtures allowed us to monitor the changes of the gas-phase composition and the relative volatility of the components. In addition, we have studied a protic ionic liquid, and demonstrated that it exists as separated neutral molecules in the gas phase.     

Reversed phase liquid chromatography of alkyl-imidazolium ionic liquids

Eleven 1-alkyl-3-methyl imidazolium ionic liquid (IL) salts were analyzed in reversed phase mode with a Kromasil C18 column. The mobile phases were water-rich acetonitrile solutions (water content > or =70%, v/v) without any added salts. It is shown that it is possible to separate different ILs sharing the same cation and differing by the anion when salt-free mobile phases are used. When a buffer, acetate or phosphate salt, or any salt, such as sodium chloride or sodium tetrafluorobarate, is added to the mobile phase, the ILs differing only by their anions cannot be separated. ILs with different alkyl chains in the imidazolium cation are separated by mobile phases with or without added salts following a hydrophobic interaction behavior: log k is proportional to nC, the carbon number of the alkyl chain. Important differences in ion/stationary phase interactions are observed depending on the ionic content of the mobile phase. With salt-free mobile phases, the IL/C18 stationary phase interactions correspond to concave isotherms associated with fronting peaks for all ILs. With mobile phase containing 0.01 M of salt, tailing IL peaks correspond to convex adsorption isotherms. Also, the IL retention factor depends on the concentration and nature of the added salt. Hexafluorophosphate chaotropic anions can adsorb on the Kromasil C18 surface dramatically increasing the imidazolium cation retention factors.     

Anisotropic ionic liquids built from nonmesogenic cation surfactants and Keggin-type polyoxoanions

Non-mesogenic unit bearing Keggin-type polyoxoanions with tetra-n-octylammonium counterions exhibit an ionic liquid crystalline structure via ionic self-assembly in a certain temperature range.     

Deviations from ideality in mixtures of two ionic liquids containing a common ion

Excess molar volumes of six binary mixtures composed of two ionic liquids of the 1-methyl-3-alkyl-imidazolium bis(trifluoromethylsulfonyl)amide family -- ([C(m)()mim] + [C(n)()mim])[NTf(2)] with n and m ranging from 2 to 10 -- were measured for several compositions at 298 and 333 K. Similarly, three other binary systems containing [C(4)mim](+) (1-methyl-3-butylimidazolium) as a common cation have been studied: [C(4)mim]([NTf(2)] + [PF(6)]), [C(4)mim]([NTf(2)] + [BF(4)]) and [C(4)mim]([BF(4)] + [PF(6)]). Thus, the mixing process of two distinct ions of equal sign embedded in a constant field network of a given counterion was analyzed. All systems exhibit small, positive V(E) values of the order of a few tenths of cm(3) mol(-1), which are essentially temperature and pressure independent. The results show additive trends, for instance, the V(E) values become larger as the difference between the alkyl chain lengths of the two cations increases. The Flory theory of mixtures provides a useful, semiquantitative correlation between the excess volumes and excess enthalpies in these systems, allowing for comparison with analogous results obtained within the linear primary alcohols.     

Chirality transfer in mandelate ionic liquids through ion pairing effects

Ion pairing in [N-(3′-oxohexyl)-N-methylimidazolium][(R)-mandelate] was probed as a function of its concentration in ethanol and compared to the corresponding [(S)-camphorsulfonate] ionic liquid. The applied methodologies comprised asymmetric hydrogenation with ee monitoring as well as independent diffusion-ordered NMR and conductivity measurements.     

Thermophysical properties and thermodynamic phase behavior of ionic liquids

The thermal stability of many tested ionic liquids (ILs) was investigated by the TGA and DTA curves over the wide temperature range from 200 to 780 K. The TGA curves have mainly a sigmoid shape, which can be split into three segments. The thermal decomposition of the samples was higher than 500 K. For the ammonium salts, C₂BF₄, or C₂PF₆, or C₂N(CN)₂, or C₄Br, the temperatures of the decompositions were 583.5, 556.1, 545.1 and 525.3 K, respectively. Generally, it was found that the temperature of decomposition of investigated ionic liquid is strongly depended on the type of cation and the anion. Phase equilibria and thermophysical constants were measured also for the dialkoxy-imidazolium ILs, [(C₄H₉OCH₂)₂IM][BF₄], [(C₈H₁₇OCH₂)₂IM][Tf₂N], [(C₁₀H₂₁OCH₂)₂IM][Tf₂N] and for pyridinium IL, [Pyr][BF₄].The characterization and purity of the compounds were obtained by the elemental analysis, water content (Fisher method) and differential scanning microcalorimetry (DSC) analysis. From (DSC) method, the melting points, the enthalpies of fusion, the temperatures and enthalpies of solid-solid phase transitions and the half C_p temperatures of glass transition of all investigated ionic liquids were measured.The phase equilibria of these salts with common popular solvents: water, or alcohols or n-alkanes, or aromatic hydrocarbons have been measured by a dynamic method from 290 K to the melting point of IL, or to the boiling point of the solvent in the whole mole fraction range, x from 0 to 1.These salts mainly exhibit simple eutectic systems with immiscibility in the liquid phase with upper critical solution temperatures (UCST), not only with aromatic hydrocarbons, cycloalkanes and n-alkanes but also with longer chain alcohols. For example the C₂BF₄ salt show simple eutectic system with water and simple eutectic systems with immiscibility in the liquid phase with upper critical solution temperature with alcohols.The solid-liquid phase equilibria, SLE curves were correlated by means of the different G^Ex models utilizing parameters derived from the SLE. The root-mean-square deviations of the solubility temperatures for all calculated data depend on the particular system and the equation used.     

Physico-chemical properties and phase behaviour of piperidinium-based ionic liquids

The sufficient review of the existing literature of the 1-alkyl-1-methylppiperidinium-based ionic liquids has been presented. The phase diagrams for the binary systems of {1-butyl-1-methylpiperidinium thiocyanate [BMPIP][SCN] + an alcohol (1-hexanol, 1-heptanol, 1-octanol, 1-nonanol, 1-decanol, 1-dodecanol), or + water, or + aliphatic hydrocarbons (n-hexane, n-heptane, n-octane), or + cyclohexane, or, + cycloheptane, or + aromatic hydrocarbons (benzene, toluene, ethylbenzene)} and for the binary systems of {1-ethyl-1-methylpiperidinium bis{(trifluoromethyl)sulfonyl}imide [EMPIP][NTf₂] + an alcohol (ethanol, 1-propanol, 1-butanol, 1-hexanol, 1-heptanol, 1-octanol, 1-nonanol), or + water} have been determined at atmospheric pressure using a dynamic method. The influence of an alcohol chain length was discussed for these ionic liquids. A systematic decrease in the solubility was observed with an increase of the alkyl chain length of an alcohol. (Solid + liquid) phase equilibria with complete miscibility in the liquid phase region were observed for the systems involving water and the alcohols for the thiocyanate-based ionic liquid. Opposite, the bis{(trifluoromethyl)sulfonyl}imide-based ionic liquid reveal the immiscibility gap in the liquid phase. The correlation of the experimental data has been carried out using the NRTL equation. The phase diagrams reported here have been compared to the systems published earlier with the 1-alkyl-1-methylpiperidinium-based ionic liquids. The influence of the cation and anion on the phase behaviour has been discussed. The basic thermal properties of pure ILs, i.e. melting temperature and the enthalpy of fusion, the solid-solid phase transition temperature and enthalpy have been measured using a differential scanning microcalorimetry technique.     

From colloidal stability in ionic liquids to advanced soft materials using unique media

Owing to their fascinating properties, ionic liquids (ILs) are now receiving a great deal of attention as alternatives to organic solvents and electrolyte solutions and as synthetic and dispersion media for colloidal systems. Colloidal stability is an essential factor in determining the properties and performance of colloidal systems combined with ILs. The remarkable properties of ILs primarily originate from their highly ionic nature. Although such high ionic strength often causes colloidal aggregation in aqueous and organic suspensions, some colloidal particles can be well suspended in ILs without any stabilizers. In the first part of this article, we focus on recent experiments conducted to investigate the colloidal stability of bare and polymer-grafted silica nanoparticles and on the surface force between silica substrates and ILs. Three different repulsions between colloidal particles (i.e., electrostatic, steric, and solvation forces) are also highlighted, after which a possible interpretation of the results in terms of the stabilization mechanism in ILs both in the presence and in the absence of stabilizers is proposed. The latter part of this article provides an overview of our recent studies on colloidal soft materials with ILs. On the basis of the dispersed states of the silica colloids in ILs, two different soft materials, a colloidal gel and a colloidal glass in ILs, were fabricated. The relationship between their functional properties, such as ionic transport, rheological properties, and optical properties, and the microstructure of the colloidal materials is also described.     

Magnitude and directionality of interaction in ion pairs of ionic liquids: relationship with ionic conductivity

The intermolecular interaction energies of nine ion pairs of room temperature ionic liquids were studied by MP2/6-311G level ab initio calculations. The magnitude of the interaction energies of 1-ethyl-3-methylimidazolium (emim) complexes follows the trend CF(3)CO(2)(-) > BF(4)(-) > CF(3)SO(3)(-) > (CF(3)SO(2))(2)N(-) approximately PF(6)(-) (-89.8, -85.2, -82.6, -78.8, and -78.4 kcal/mol, respectively). The interaction energies of BF(4)(-) complexes with emim, ethylpyridinium (epy), N-ethyl-N,N,N-trimethylammonium ((C(2)H(5))(CH(3))(3)N), and N-ethyl-N-methylpyrrolidinium (empro) are not very different (-85.2, -82.8, -84.6, and -84.4 kcal/mol, respectively), while the size of the orientation dependence of the interaction energies follows the trend emim > epy approximately (C(2)H(5))(CH(3))(3)N > empro. Comparison with the experimental ionic conductivities shows that the magnitude and directionality of the interaction energy of the ion pairs play a crucial role in determining the ionic dissociation/association dynamics in the ionic liquids. The electrostatic interaction is the major source of attraction between ions. The induction contribution is small but not negligible. The hydrogen bonding with the C(2)-H of imidazolium is not essential for the attraction in the ion pair. The interaction energy of the BF(4)(-) complex with 1-ethyl-2,3-dimethylimidazolium (em2im) (-81.8 kcal/mol) is only 4% smaller than that of the emim complex.     

A re-entrant smectic C phase in a sequence smectic C-oblique columnar-smectic C-nematic

A new six-ring double-swallow-tailed compound is presented which exhibits a re-entrant smectic C phase in the sequence smectic C-oblique columnar-smectic C-nematic. This unusual sequence detected by observation of the optical textures and by X-ray investigations is also confirmed by the phase diagrams with homologous compounds.     

Prediction of phase equilibrium in mixtures containing ionic liquids using UNIFAC

The UNIFAC model is extended to mixtures of ionic liquids consisting of the imidazolium cation and the hexafluorophosphate anion with alkanes, cycloalkanes, alcohols and water. Two new main groups, the imidazolium and the hexafluorophosphate groups, are introduced in UNIFAC. The required group interaction parameters between these groups and the existing UNIFAC main groups, CH₂, OH and H₂O, are determined by fitting binary liquid–liquid equilibrium and infinite dilution activity coefficient experimental data. The predictive capability of the extended UNIFAC model is examined against experimental data for vapour–liquid equilibrium, liquid–liquid equilibrium and activity coefficients at infinite dilution of binary and ternary systems containing 1-alkyl-3-alkyl′-imidazolium hexafluorophosphate ionic liquids, alkanes, cycloalkanes, alcohols and water. The results indicate that UNIFAC is a reliable model for phase equilibrium predictions in mixtures containing this type of ionic liquids.     

Orientational order in smectic liquid-crystalline phases of amphiphilic diols

The thermotropic smectic phases of amphiphilic 2-(trans-4-n-alkylcyclohexyl)-propane-1,3-diols were investigated by means of small- and wide-angle x-ray scattering and values of the smectic (bi-)layer spacing, the orientational order parameters P(2) and P(4), the orientational distribution function as well as the intralayer correlation length were extracted from the scattering profiles. The results for the octyl homolog indicate that these smectic phases combine a very high degree of smectic one-dimensional-translational order with remarkably low orientational order, the order parameter of which (P(2) approximately 0.56) is far below those values typically found in nonamphiphilic smectics. This combination, quite exceptional in thermotropic smectics, most likely originates from the intermolecular hydrogen bonding between the terminal diol groups which seems to be the specific driving force in the formation of the thermotropic smectic structure in these amphiphiles and leads to a type of microphase segregation. Even in the absence of a solvent, the liquid-crystalline ordering of the amphiphilic mesogens comes close to the structure of the so-called neat soaps, found in lyotropic liquid crystals.     

Tribological properties of self-assembled monolayers of catecholic imidazolium and the spin-coated films of ionic liquids

A novel compound of an imidazolium type of ionic liquid (IL) containing a biomimetic catecholic functional group normally seen in mussel adhesive proteins was synthesized. The IL can be immobilized on a silicon surface and a variety of other engineering material surfaces via the catecholic anchor, allowing the tribological protection of these substrates for engineering applications. The surface wetting and adhesive properties and the tribological property of the synthesized self-assembled monolayers (SAMs) are successfully modulated by altering the counteranions. The chemical composition and wettability of the IL SAMs were characterized by means of X-ray photoelectron spectroscopy (XPS) and contact angle (CA) measurements. The adhesive and friction forces were measured with an atomic force microscope (AFM) on the nanometer scale. IL composite films were prepared by spin coating thin IL films on top of the SAMs. The macrotribological properties of these IL composite films were investigated with a pin-on-disk tribometer. The results indicate that the presence of IL SAMs on a surface can improve the wettability of spin-coated ionic liquids and thus the film quality and the tribological properties. These films registered a reduced friction coefficient and a significantly enhanced durability and load-carrying capacity. The tribological properties of the composite films are better than those of pure IL films because the presence of the monolayers improves the adhesion and compatibility of spin-coated IL films with substrates.