Dipolarity versus Polarizability and Acidity versus Basicity of Ionic Liquids as a Function of Their Molecular Structures

The Simulation of Solvent Polarizabilities and Dipolarities with Polarizable Continuum Model

The ability of polarizable continuum models (PCM) to simulate nonspecific solvent effects (dipolarity and polarizability) was evaluated by calculating the transition energies of 1,1,10,10-tetrabutyldecanonaene (ttbp9) and 2-N,N-dimethylamino-7-nitrofluorene (DMANF), basis of Catalán's polarizability (SP) and dipolarity (SdP) solvent scales, respectively. Time-dependent density-functional theory (TD-DFT) calculations were performed at different levels of theory, employing four basis sets in 10 different solvents, covering the full range of the normalized SP and SdP scales. Transition energies were calculated using linear response (LR) and corrected linear response (cLR2) schemes. Although these methods yielded variable mean absolute errors, the LR-PCM calculations reproduced medium polarizability and dipolarity trends. While calculated ttbp9 transition energies correlated with SP and Laurence's dispersion-induced (DI) scales, the DMANF transition energies correlated poorly with SdP or Laurence's ES dipolarity scales. This result agrees with the fact that DMANF solvatochromism is "contaminated" by solvent polarizability and HB acidity. The incorporation of SP or DI contributions led to much better (r2 > 0.95) correlations with the DMANF-calculated transitions. The results offer a clearer picture of the limitations of continuum models in simulating the behavior of solvatochromic dyes in solution by pointing out their poor performance when specific solvent effects, such as hydrogen-bond interactions, play a significant role in their solvatochromism.

Do Electrostatics Control the Diffusive Dynamics of Solitary Water? NMR and MD Studies of Water Translation and Rotation in Dipolar and Ionic Solvents

NMR-based measurements of the diffusion coefficients and rotation times of solitary water and benzene at 300 K are reported in a diverse collection of 13 conventional organic solvents and 10 imidazolium ionic liquids. Proton chemical shifts of water are found to be correlated to water OH-stretching frequencies, confirming the importance of electrostatic interactions in these shifts. However, the influence of magnetic interactions in aromatic solvents renders chemical shifts a less reliable indicator of electrostatics. Diffusion coefficients (DB) and rotational correlation times (τB) of benzene in the solvents examined are accurately described as functions of viscosity (η) by DB ∝ η-0.81 and τB ∝ η0.64. Literature values of DB and τB in alkane and normal alcohols, which were not included among the solvents studied here, are systematically faster than predicted by these correlations, indicating that factors beyond solvent viscosity play a role in determining the friction on benzene. In contrast to benzene, water diffusion and rotation are poorly described in terms of viscosity alone, even in the dipolar and ionic solvents measured here. The present data and the substantial literature data already available on dilute water diffusion show a systematic dependence of DW on solvent polarity among isoviscous solvents. The aspect of solvent polarity most relevant to water dynamics is the ability of a solvent to accept hydrogen bonds from water, as conveniently quantified by the frequency of water's OH stretching band, ΔνOH. The friction on translation, ζtr = kBT/DW, and rotation, ζrot = kBTτW, are both well correlated by functions of the form ζ(η, ΔνOH) = a1ηa2 exp (a3ΔνOH), where the ai are adjustable parameters. Molecular dynamics simulations reveal a strong coupling between electrostatic and nonelectrostatic water-solvent interactions, which makes it impossible to dissect the friction on water into additive dielectric and hydrodynamic components. Simulations also provide a tentative explanation for the unusual form of the correlating function ζ(η, ΔνOH), at least in the case of ζrot.

Synthesis and Physicochemical Properties of Acrylate Anion Based Ionic Liquids

Two polymerizable ionic liquids (or monomeric ionic liquids, mILs) namely 1-butyl-3-methylimidazolium and choline acrylates ([C4mim]A and ChA, respectively) were synthesized using the modified Fukumoto method from corresponding chlorides. The chemical structure of the prepared mILs was confirmed with FTIR and NMR study. Investigation of the thermal properties with DSC demonstrates that both mILs have a Tg temperature of about 180 K and a melting point around 310 K. It was shown that the temperature dependence of FTIR confirm the Tg to be below 200. Both mILs exhibited non-Newtonian shear thinning rheological behavior at shear rates >4 s−1. It was shown that [C4mim]A is able to dissolve bacterial cellulose (BC) leading to a decrease in its degree of polymerization and recrystallisation upon regeneration with water; although in the ChA, the crystalline structure and nanofibrous morphology of BC was preserved. It was demonstrated that the thixotropic and rheological properties of cellulose dispersion in ChA at room temperature makes this system a prospective ink for 3D printing with subsequent UV-curing. The 3D printed filaments based on ChA, containing 2 wt% of BC, and 1% of N,N′-methylenebisacrylamide after radical polymerization induced with 1% 2-hydroxy-2-methylpropiophenone, demonstrated Young’s modulus 7.1 ± 1.0 MPa with 1.2 ± 0.1 MPa and 40 ± 5% of strength and ultimate elongation, respectively.

OH Stretching and Libration Bands of Solitary Water in Ionic Liquids and Dipolar Solvents Share a Single Dependence on Solvent Polarity

Ionic liquids are an emerging class of materials which are finding application in a variety of technologically important areas. Because of their hydrophilic character, at least a small concentration of water is often present when ionic liquids are used in practical applications. This study employs infrared spectroscopy in the OH stretching and libration regions together with DFT calculations to better characterize the state of dilute water in ionic liquids. Water mole fractions (x_w ∼ 0.1) are chosen such that nearly all water occurs in monomeric form and spectra probe the solvation structure and dynamics of solitary water molecules. New data are reported for a series of 1-ethyl-3-methylimidazolium liquids [Im₂₁][X] with X^- = (C₂F₅)₃F₃P^-, (CF₃SO₂)₂N^-, BF₄^-, B(CN)₄^-, CF₃SO₃^-, C₂H₅SO₄^-, NO₃^-, SCN^-, and CH₃CO₂^-, as well as for the two 1-hexyl-3-methylimidazolium liquids [Im₆₁][Cl] and [Im₆₁][I]. For comparison, spectra are also recorded in a variety of dipolar solvents, and much of the available literature data are summarized, providing a comprehensive perspective on monomeric water in homogeneous solution. Most prior studies of dilute water in ionic liquids interpreted OH stretching spectra only in terms of water being specifically bonded to two anions in A^-···H-O-H···A^- type solvates. The more detailed analysis presented here indicates the additional presence of asymmetrically solvated water, which in some cases includes both singly solvated (A^-···H-O-H) and more subtle forms of asymmetric solvation. The same pattern of solvation also pertains to dipolar solvents capable of accepting hydrogen bonds from water. No clear distinction is found between OH spectra in high-polarity conventional solvents and ionic liquids. In all solvents, OH frequencies are strongly correlated to measures of solvent basicity or hydrogen bond accepting ability. Far-infrared spectra of the water libration band also show common trends in ionic and dipolar solvents. Despite the different character of the libration and OH modes, the frequencies of these vibrations show virtually the same solvent dependence (apart from sign) except in weakly polar or nonpolar solvents.

Thermal Properties and Solvent Polarities of Mixed-Valence Ionic Liquids Containing Cationic Biferrocenylene Derivatives

Ionic liquids (ILs) containing cationic mixed-valence biferrocenylene derivatives were synthesized with an octanoyl or octyl substituent in each cation. Their melting points ranged between 25 and 39 °C, and the octanoyl derivatives exhibited higher melting points than the octyl derivatives. In addition, each IL exhibited a glass transition in the temperature ranging from -66 to -45 °C after melting. Their melting points were ∼10 °C higher than those of mononuclear octamethylferrocenium salts bearing the same substituents. The solvent polarity (E_T^N) and Kamlet-Taft parameters (π*, α, and β) of these dinuclear and mononuclear ILs were then examined. The dinuclear ILs bearing octanoyl substituents exhibited significant increases in E_T^N and π* and a decrease in α with the decreasing temperature, whereas the other ILs exhibited a significantly less pronounced temperature dependence. Finally, the intervalence charge-transfer (or charge-resonance) bands of the octanoyl dinuclear ILs exhibited red shifts with the decreasing temperature, which can be regarded as self-thermosolvatochromism.

The influence of the cation structure on the basicity-related polarity of ionic liquids

UV/Vis absorption data of (E)-4-(2-[5-{4,4,5,5-tetramethyl-1,3,2-dioxaborolane-2-yl}thiene-2-yl]vinyl)-2-(dicyano-methylene)-3-cyano-5,5-dimethyl-2,5-dihydrofuran (ThTCF) as a solvatochromic probe is applied to examine the anion coordination strength (e.g. of N(CN)₂, BF₄, PF₆, N(Tf)₂, CF₃COO) as a function of the cation structure of ionic liquids. Several 1-n-alky-3-methylimidazolium- and tetraalkylammonium CH₃-NR₃⁺-based ILs with different n-alkyl chain lengths (R = -C₄H₉, -C₆H₁₁, -C₈H₁₇, -C₁₀H₂₁) are considered. UV/Vis absorption data of ThTCF show subtle correlations with hydrogen bond accepting (HBA) ability-related measurands such as Kamlet-Taft β, Freire's E_HB, and Laurence β₁ parameter as a function of anion and cation structure. The different influence of the n-alkyl chain length of imidazolium- and tetraalkylammonium-based ILs on the dipolarity and HBA strength is confirmed by comparison with the ¹⁴N isotropic hyperfine coupling constants (A_iso) of a positively (CATI) and negatively charged spin probe (TSKCr) of TEMPO-type [(2,2,6,6-tetramethylpiperidin-1-yl)oxyl] and quantum chemically derived dipoles of the cations. The A_iso values correlate with the absorption energy of ThTCF and E_HB, but in different ways depending on the anion or charge of the spin probe. In a final discussion of the β, E_HB, and β₁ scales in relation to ThTCF, the importance of the molar concentration N of ionic liquids for the physical significance of the respective parameters is discussed.

The physical significance of the Kamlet–Taft π* parameter of ionic liquids

The Kamlet–Taft dipolarity/polarizability parameters π* for various ionic liquids were determined using 4-tert-butyl-2-((dicyanomethylene)-5-[4-N,N-diethylamino)-benzylidene]-Δ3-thiazoline and 5-(N,N-dimethylamino)-5′-nitro-2,2′-bithiophene as solvatochromic probes.

Complementary interpretation of ET(30) polarity parameters of ionic liquids

Reichardt's empirical ET(30) polarity parameter has been established as appropriate polarity scale for ionic liquids. In this study, the relationships of ET(30) of ionic liquids with the empirical Kamlet-Taft polarity parameters α (hydrogen bond donating ability), β (hydrogen bond accepting ability) and π* (dipolarity/polarizability) as well as Catalán's parameter set SA (solvent acidity), SB (solvent basicity), SP (solvent polarizability) and SdP (solvent dipolarity) are examined by means of multiple square correlation analyses. Several subtasks were carried out to address this main concern. First, the influence of anion structure on ET(30) polarity parameters for various ionic liquids are investigated by use of nine differently substituted pyridinio phenolate betaine dyes of the Reichardt type. It is assumed that halide anions can have an effect on the ET(30) parameter. In the second part, the Kamlet-Taft π* parameters have been independently determined for several protic ionic liquids using 4-tert-butyl-2-(dicyanomethylene)-5-[4-(diethylamino)-benzylidene]-Δ3-thiazoline (Th) and N,N-diethyl-4-nitroaniline (DENA) to show the impact of the hydrogen bond donating ability of the IL on the actual π* values as function of probe. α and SA values have been measured using the dicyano-bis(1,10-phenanthroline) iron(ii) complex (Fe) as HBD probe. Finally, the newly determined Reichardt ET(30), Kamlet-Taft and Catalán parameters of ionic liquids were used in addition to literature data to prove correlations of ET(30) with α and π* as well as of ET(30) with SA and SdP. Linear correlations of SdP with the molar concentration of the ionic liquid are highlighted.

Understanding the efficiency of ionic liquids–DMSO as solvents for carbohydrates: use of solvatochromic- and related physicochemical properties

The quantification of interactions of solvatochromic probes with ionic liquids/DMSO serves as an expedient approach for predicting the solvent efficiency in dissolving carbohydrates

Distribution of N-Methylimidazole in Ionic Liquids/Organic Solvents Systems

The partition coefficients, Kmim, of N-methylimidazole (mim) in two-component systems composed of ionic liquid (IL) and a series of organic solvents immiscible with ILs (butyl acetate, ethyl acetate, tert-butyl methyl ether, diethyl ether and cyclohexane) were determined by a shake-flask method. The influence of different factors such as temperature, solvent polarity, mim concentration, and water content on Kmim by using 1-butyl-3-methylimidazolium chloride {[C4C1im]Cl} as a model compound was comprehensively studied. The calculated thermodynamic functions of transfer (∆trG0, ∆trH0, ∆trS0) showed that the mim migration (IL→organic phase) is a thermodynamically unfavorable and enthalpy-determined process in the temperature range of 298.15 to 328.15K; however, the results suggested that mim partition toward the organic phase can be enhanced by the rational manipulation of the extraction conditions. Experiments conducted with other 1-alkyl-3-methylimidazolim chlorides (CnC1im]Cl (n = 6, 8, 10) revealed that mim possesses similar behavior and can be successfully washed out from the ILs by extraction with organic solvents. The results obtained in this study give some clues toward the choice of an appropriate solvent and conditions to be employed for the purification of halide-based ILs by means of a liquid-liquid extraction.

Kinetics and Energetics of Thermal Cis-Trans Isomerization of a Resonance-Activated Azobenzene in BMIM-Based Ionic Liquids for PF₆-/Tf₂N- Comparison

BMIM PF₆ (1-butyl-3-methylimidazolium hexafluorophosphate) and BMIM Tf₂N (1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide) are two conventional room-temperature ionic liquids widely employed and investigated as reaction media. Despite the presence of the same imidazolium ring in their structure they are different in many chemical and physical properties due to the nature of the anions. The thermal cis-trans isomerization of an electronically activated azobenzene have been used as reaction model to compare the behavior of PF₆- and Tf₂N-. Rotation is the mechanism by which the investigated azobenzene is converted into the trans isomer spontaneously in the dark both in BMIM PF₆ and in BMIM Tf₂N. The kinetic rate constants of the process have been determined at different temperatures and the activation energies of the reaction have been calculated according to the Arrhenius and Eyring equations. The results presented herein highlight different solute-solvent interactions involving the PF₆- and Tf₂N- anions during the cis-trans isomerization.

Polarity of tetraalkylammonium-based ionic liquids and related low temperature molten salts

Solvatochromic measurements and ¹H NMR spectroscopy have been used to investigate the overall polarity of tetraalkylammonium based ionic liquids and low temperature molten salts.

[N-Methyl-2-pyrrolidone][C1-C4 carboxylic acid]: a novel solvent system with exceptional lignin solubility

Novel solvent systems composed of N-methyl-2-pyrrolidone and C1-C4 carboxylic acid exhibit unique physicochemical properties, e.g. large polarity, low viscosity and excellent hydrogen bonding capacity, which have demonstrated excellent lignin solubility that outperforms conventional solvents and ionic liquids.

Hydrogen-bond acidity of ionic liquids: an extended scale

One of the main drawbacks comprising an appropriate selection of ionic liquids (ILs) for a target application is related to the lack of an extended and well-established polarity scale for these neoteric fluids. Albeit considerable progress has been made on identifying chemical structures and factors that influence the polarity of ILs, there still exists a high inconsistency in the experimental values reported by different authors. Furthermore, due to the extremely large number of possible ILs that can be synthesized, the experimental characterization of their polarity is a major limitation when envisaging the choice of an IL with a desired polarity. Therefore, it is of crucial relevance to develop correlation schemes and a priori predictive methods able to forecast the polarity of new (or not yet synthesized) fluids. In this context, and aiming at broadening the experimental polarity scale available for ILs, the solvatochromic Kamlet-Taft parameters of a broad range of bis(trifluoromethylsulfonyl)imide-([NTf2](-))-based fluids were determined. The impact of the IL cation structure on the hydrogen-bond donating ability of the fluid was comprehensively addressed. Based on the large amount of novel experimental values obtained, we then evaluated COSMO-RS, COnductor-like Screening MOdel for Real Solvents, as an alternative tool to estimate the hydrogen-bond acidity of ILs. A three-parameter model based on the cation-anion interaction energies was found to adequately describe the experimental hydrogen-bond acidity or hydrogen-bond donating ability of ILs. The proposed three-parameter model is also shown to present a predictive capacity and to provide novel molecular-level insights into the chemical structure characteristics that influence the acidity of a given IL. It is shown that although the equimolar cation-anion hydrogen-bonding energies (EHB) play the major role, the electrostatic-misfit interactions (EMF) and van der Waals forces (EvdW) also contribute, admittedly in a lower extent, towards the hydrogen-bond acidity of ILs. The new extended scale provided for the hydrogen-bond acidity of ILs is of high value for the design of new ILs for task-specific applications.

Fluorosolvatochromism of furanyl- and thiophenyl-substituted acetophenones

Fluorosolvatochromism of four furanyl- or thiophenyl-substituted acetophenone derivatives is suitable to measure environmental effects.

Task specific ionic liquids as polarity shifting additives of common organic solvents

The effect of the addition of RTILs to tetrahydrofuran, chloroform, acetonitrile, isopropanol and ethanol on ENT values.