Dynamics of Methanol in Ionic Liquids: Validity of the Stokes–Einstein and Stokes–Einstein–Debye Relations

The validity of Stokes–Einstein (SE) and Stokes–Einstein–Debye (SED) relations for methanol in the physical environment of the ionic liquid (IL) 1‐ethyl‐3‐methylimidazolium bis(trifluoromethylsulfonyl)imide is studied by means of nuclear magnetic resonance (NMR) relaxation time experiments, viscosity measurements and molecular dynamics (MD) simulations. The reorientational correlation times of the hydroxyl groups of pure methanol and of methanol in the IL/methanol mixtures were determined. For that purpose an approach for estimating NMR deuteron quadrupole coupling constants, presented by Wendt and Farrar (Mol. Phys.­ 1998 , 95, 1077–1081), was confirmed. The self‐diffusion coefficients of methanol were taken from the MD simulations. The viscosities of all systems were then measured and the SE and SED relations validated. For pure methanol both relations are valid, whereas they become increasingly invalid with increasing IL concentration, as indicated by effective volumes and radii that are too low. The deviation from the SE and SED relations could be related to dynamical heterogeneities described by the non‐Gaussian parameter α(t) obtained from MD simulations. For pure methanol, α(t) is close to zero in accord with the validity of both relations. With increasing IL concentration the dynamical heterogeneities of methanol increase strongly. The times t* at the maximum of α(t) increase linearly with the relative number of methanol monomers in the mixtures. Thus, the dynamical heterogeneities are largest for single methanol molecules fully embedded in the IL environment. In their own environment methanol molecules are highly mobile, whereas in the IL‐rich region the mobility is strongly reduced leading to the non‐validity of SE and SED relations.     

Triphilic Ionic‐Liquid Mixtures: Fluorinated and Non‐fluorinated Aprotic Ionic‐Liquid Mixtures

We present here the possibility of forming triphilic mixtures from alkyl‐ and fluoroalkylimidazolium ionic liquids, thus, macroscopically homogeneous mixtures for which instead of the often observed two domains—polar and nonpolar—three stable microphases are present: polar, lipophilic, and fluorous ones. The fluorinated side chains of the cations indeed self‐associate and form domains that are segregated from those of the polar and alkyl domains. To enable miscibility, despite the generally preferred macroscopic separation between fluorous and alkyl moieties, the importance of strong hydrogen bonding is shown. As the long‐range structure in the alkyl and fluoroalkyl domains is dependent on the composition of the liquid, we propose that the heterogeneous, triphilic structure can be easily tuned by the molar ratio of the components. We believe that further development may allow the design of switchable, smart liquids that change their properties in a predictable way according to their composition or even their environment.     

Local hydrodynamics of solvent near diffusing dendrimers: A test of the new Stokes–Einstein relation

We have reported a new Stokes–Einstein relation (SER) for size determination and tested it by different nanoparticles. We assumed that the breakdown for SER results from local increases in viscosity. Here we investigate hydrodynamics of solvent near dendrimers to further test generality of our new theory. We discuss simulations of dendrimers in comparison to nanoparticles, experimental data on dendrimers from literature, and our theory. Local viscosity and local diffusivity of solvent near dendrimers are estimated by persistence times and exchange times, respectively. We find that the local dynamics of solvent near dendrimers of low density stay almost the same as that of bulk solvent. While the motions of solvent particles slow down near dendrimers of high density. This is similar with changes in local dynamics of solvent near nanoparticles. According to the causes we proposed for the deviation of SER, this is consistent with our findings that the SER works for the dendrimers of low density, while it fails for the dendrimers of high density. The new SER is then tested to predict size of the dendrimers accurately. Taking this together with the results for the nanoparticles, we believe that the new theory is general. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 1380–1392     

Non‐Ideal Mixing Behaviour of Hydrogen Bonding in Mixtures of Protic Ionic Liquids

Ionic liquids (ILs) attract interest in science and technology as a result of their unique properties. Binary and ternary mixtures of ILs significantly increase the number of possible cation/anion combinations, resulting in targeted physical and chemical properties. In this work, we study the mixing behaviour of two protic ILs: triethyl ammonium methylsulfonate [Et₃NH][CH₃SO₃] and triethylammonium triflate [Et₃NH][CF₃SO₃]. We find a characteristic deviation from ideal mixing by means of low‐frequency infrared spectroscopy. By using molecular dynamics simulations, we explain this behaviour as being the result of different strengths of anion/cation hydrogen bonding. This non‐ideality of non‐random H‐bond mixing is also reflected in macroscopic properties such as the viscosity. Mixing suitable ILs may, thus, result in new ILs with targeted physical properties.     

Liquid‐Crystalline Ionic Liquids as Ordered Reaction Media for the Diels–Alder Reaction

Liquid‐crystalline ionic liquids (LCILs) are ordered materials that have untapped potential to be used as reaction media for synthetic chemistry. This paper investigates the potential for the ordered structures of LCILs to influence the stereochemical outcome of the Diels–Alder reaction between cyclopentadiene and methyl acrylate. The ratio of endo‐ to exo‐product from this reaction was monitored for a range of ionic liquids (ILs) and LCILs. Comparison of the endo:exo ratios in these reactions as a function of cation, anion and liquid crystallinity of the reaction media, allowed for the effects of liquid crystallinity to be distinguished from anion effects or cation alkyl chain length effects. These data strongly suggest that the proportion of exo‐product increases as the reaction media is changed from an isotropic IL to a LCIL. A detailed molecular dynamics (MD) study suggests that this effect is related to different hydrogen bonding interactions between the reaction media and the exo‐ and endo‐transition states in solvents with layered, smectic ordering compared to those that are isotropic.     

Supramolecular Ionic‐Liquid Gels with High Ionic Conductivity

Supramolecular ionogels were prepared by the gelation of room‐temperature ionic liquid 1‐butyl‐3‐methylimidazolium tetrafluoroborate ([BMIm][BF₄]) with (S,S)‐bis(leucinol)oxalamide. Remarkably, the ionic conductivity of solutions and ionogels with low gelator concentrations is higher than that of neat [BMIm][BF₄]. On the basis of molecular dynamics simulations and quantum mechanical calculations, the origin of this phenomenon is attributed to the higher affinity of gelator molecules towards [BF₄]^? ions, which reduces the electrostatic attraction between [BMIm]⁺ and [BF₄]^? and thus increases their mobility. With increasing gelator concentration, the ionic conductivity decreases due to the formation of a denser gelator matrix, which hinders the pathways for ionic transport. However, even for very dense ionogels, this decrease is less than one order of magnitude relative to neat [BMIm][BF₄], and thus they can be classified as highly conductive materials with strong potential for application as functional electrolytes.     

Temperature‐Dependent Prediction of the Liquid Entropy of Ionic Liquids

Modeling of the temperature‐dependent liquid entropy of ionic liquids (ILs) with great accuracy using COSMO‐RS is demonstrated. The minimum structures of eight IL ion pairs are investigated and the entropy, calculated from ion pairs, is found to differ on average only 2 % from the available experimental values (119 data points). For calculations with single ions, the average error amounts to 2.6 % and stronger‐coordinating ions tend to give higher deviations. Additionally, the first parameterization of the standard liquid entropy for ILs is presented in the context of traditional volume‐based thermodynamics (S_l⁰=1.585 kJ mol^?1 K^?1 nm^?3?r_m³+14.09 J mol^?1 K^?1), which sheds light on the statistical treatment of ionic interactions. The findings provide the first direct access to accurate predictions of liquid entropies of ILs, which are tedious and time‐consuming to measure.     

Porphyrin/Ionic‐Liquid Co‐assembly Polymorphism Controlled by Liquid–Liquid Phase Separation

Understanding and controlling multicomponent co‐assembly is of primary importance in different fields, such as materials fabrication, pharmaceutical polymorphism, and supramolecular polymerization, but these aspects have been a long‐standing challenge. Herein, we discover that liquid–liquid phase separation (LLPS) into ion‐cluster‐rich and ion‐cluster‐poor liquid phases is the first step prior to co‐assembly nucleation based on a model system of water‐soluble porphyrin and ionic liquids. The LLPS‐formed droplets serve as the nucleation precursors, which determine the resulting structures and properties of co‐assemblies. Co‐assembly polymorphism and tunable supramolecular phase transition behaviors can be achieved by regulating the intermolecular interactions at the LLPS stage. These findings elucidate the key role of LLPS in multicomponent co‐assembly evolution and enable it to be an effective strategy to control co‐assembly polymorphism as well as supramolecular phase transitions.     

On the Dynamic Interaction of n‐Butane with Imidazolium‐Based Ionic Liquids

The impact of a reactant from the gas phase on the surface of a liquid and its transfer through this gas/liquid interface are crucial for various concepts applying ionic liquids (ILs) in catalysis. We investigated the first step of the adsorption dynamics of n‐butane on a series of 1‐alkyl‐3‐methylimidazolium bis(trifluoromethanesulfonyl)imide ILs ([C_nC₁Im][Tf₂N]; n=1, 2, 3, 8). Using a supersonic molecular beam in ultra‐high vacuum, the trapping of n‐butane on the frozen ILs was determined as a function of surface temperature, between 90 and 125 K. On the C₈‐ and C₃‐ILs, n‐butane adsorbs at 90 K with an initial trapping probability of ≈0.89. The adsorption energy increases with increasing length of the IL alkyl chain, whereas the ionic headgroups seem to interact only weakly with n‐butane. The absence of adsorption on the C₁‐ and C₂‐ILs is attributed to a too short residence time on the IL surface to form nuclei for condensation even at 90 K.     

Computer‐Aided Design of Ionic Liquids as CO2 Absorbents

Ionic liquids (ILs), vary strongly in their interaction with CO₂. We suggest simple theoretical approach to predict the CO₂ absorption behavior of ILs. Strong interaction of the CO₂ with the IL anions corresponds to chemical absorption whereas weak interaction indicates physical absorption. A predictive estimate with a clear distinction between physical and chemical absorption can be simply obtained according to geometries optimized in the presence of a solvation model instead of optimizing it only in gas phase as has been done to date. The resulting Gibbs free energies compare very well with experimental values and the energies were correlated with experimental capacities. Promising anions, for ionic liquids with reversible CO₂ absorption properties can be defined by a reaction Gibbs free energy of absorption in the range of ?30 to 16 kJ mol^?1.     

Ordered Mesoporous Organosilica with Ionic‐Liquid Framework: An Efficient and Reusable Support for the Palladium‐Catalyzed Suzuki–Miyaura Coupling Reaction in Water

The preparation of a novel palladium‐supported periodic mesoporous organosilica based on alkylimidazolium ionic liquid (Pd@PMO‐IL) in which imidazoilium ionic liquid is uniformly distributed in the silica mesoporous framework is described. Both Pd@PMO‐IL and the parent PMO‐IL were characterized by N₂‐adsorption–desorption, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), TEM, and solid‐state NMR spectroscopy. We have demonstrated that Pd@PMO‐IL is an efficient and reusable catalyst for the Suzuki–Miyaura coupling reaction of various types of iodo‐, bromo‐, and even deactivated aryl chlorides in water. It was also found that although the PMO‐IL nanostructure acts as reservoir for soluble Pd species, it can also operate as a nanoscaffold to recapture the Pd nanoparticles into the mesochannels thus preventing extensive agglomeration of Pd. This observation might be attributed to the isolated ionic liquid units that effectively control the reaction mechanism by preventing Pd agglomeration and releasing and recapturing Pd nanoparticles during the reaction process. The catalyst can be recovered and reused for at least four reaction cycles without significant loss of activity.     

Reversible Hydrophobic–Hydrophilic Transition of Ionic Liquids Driven by Carbon Dioxide

Ionic liquids (ILs) with a reversible hydrophobic–hydrophilic transition were developed, and they exhibited unique phase behavior with H₂O: monophase in the presence of CO₂, but biphase upon removal of CO₂ at room temperature and atmospheric pressure. Thus, coupling of reaction, separation, and recovery steps in sustainable chemical processes could be realized by a reversible liquid–liquid phase transition of such IL‐H₂O mixtures. Spectroscopic investigations and DFT calculations showed that the mechanism behind hydrophobic–hydrophilic transition involved reversible reaction of CO₂ with anion of the ILs and formation of hydrophilic ammonium salts. These unique IL‐H₂O systems were successfully utilized for facile one‐step synthesis of Au porous films by bubbling CO₂ under ambient conditions. The Au porous films and the ILs were then separated simultaneously from aqueous solutions by bubbling N₂, and recovered ILs could be directly reused in the next process.     

Light‐Controllable Ionic Conductivity in a Polymeric Ionic Liquid

Polymeric ionic liquids (PILs) have attracted considerable attention as electrolytes with high stability and mechanical durability. Light‐responsive materials are enabling for a variety of future technologies owing to their remote and noninvasive manipulation, spatiotemporal control, and low environmental impact. To address this potential, responsive PIL materials based on diarylethene units were designed to undergo light‐mediated conductivity changes. Key to this modulation is tuning of the cationic character of the imidazolium bridging unit upon photoswitching. Irradiation of these materials with UV light triggers a circa 70 % drop in conductivity in the solid state that can be recovered upon subsequent irradiation with visible light. This light‐responsive ionic conductivity enables spatiotemporal and reversible patterning of PIL films using light. This modulation of ionic conductivity allows for the development of light‐controlled electrical circuits and wearable photodetectors.     

Rapid Preparation of Silsesquioxane‐Based Ionic Liquids

Three new hybrid ionic liquids (ILs) based on cage silsesquioxane (SQ) were rapidly prepared in high yields from octa(mercaptopropyl)silsesquioxane and 1‐allyl‐3‐methylimidazolium salts (Br^?, BF₄^?, PF₆^?) through the photochemical thiol–ene reaction. These SQ‐based ILs exhibited low glass transition temperatures and good thermal stability. The unique amphiphilic nature of these hybrid ILs cause them to self‐assemble into perfect vesicles with “yolk–shell” structures, in which cages formed the “yolk” due to their aggregation and outer anions formed the “shell”.     

Borohydride Ionic Liquids and Borane/Ionic‐Liquid Solutions as Hypergolic Fuels with Superior Low Ignition‐Delay Times

In propellant systems, fuels of choice continue to be hydrazine and its derivatives, even though they comprise a class of acutely carcinogenic and toxic substances which exhibit rather high vapor pressures and require expensive handling procedures and costly safety precautions. Hypergolic ionic liquids tend to have low volatility and high thermal and chemical stability, and often exhibit wide liquid ranges, which could allow the use of these substances as bipropellant fuels under a variety of conditions. A new family of borohydride ionic liquids and borane–ionic‐liquid solutions is described which meets nearly all of the desired important criteria for well‐performing fuels. They exhibit ignition‐delay times that are superior to that of any known hypergolic ionic liquid and may thus be legitimate replacements for hydrazine and its derivatives.     

The Microwave‐Assisted Ionic‐Liquid Method: A Promising Methodology in Nanomaterials

In recent years, the microwave‐assisted ionic‐liquid method has been accepted as a promising methodology for the preparation of nanomaterials and cellulose‐based nanocomposites. Applications of this method in the preparation of cellulose‐based nanocomposites comply with the major principles of green chemistry, that is, they use an environmentally friendly method in environmentally preferable solvents to make use of renewable materials. This minireview focuses on the recent development of the synthesis of nanomaterials and cellulose‐based nanocomposites by means of the microwave‐assisted ionic‐liquid method. We first discuss the preparation of nanomaterials including noble metals, metal oxides, complex metal oxides, metal sulfides, and other nanomaterials by means of this method. Then we provide an overview of the synthesis of cellulose‐based nanocomposites by using this method. The emphasis is on the synthesis, microstructure, and properties of nanostructured materials obtained through this methodology. Our recent research on nanomaterials and cellulose‐based nanocomposites by this rapid method is summarized. In addition, the formation mechanisms involved in the microwave‐assisted ionic‐liquid synthesis of nanostructured materials are discussed briefly. Finally, the future perspectives of this methodology in the synthesis of nanostructured materials are proposed.     

Synthesis and Properties of Alkoxy‐ and Alkenyl‐Substituted Peralkylated Imidazolium Ionic Liquids

Novel peralkylated imidazolium ionic liquids bearing alkoxy and/or alkenyl side chains have been synthesized and studied. Different synthetic routes towards the imidazoles and the ionic liquids comprising bromide, iodide, methanesulfonate, bis(trifluoromethylsulfonyl)imide ([NTf₂]^?), and dicyanamide {[N(CN)₂]^?} as the anion were evaluated, and this led to a library of analogues, for which the melting points, viscosities, and electrochemical windows were determined. Incorporation of alkenyl moieties hindered solidification, except for cations with high symmetry. The alkoxy‐derivatized ionic liquids are often crystalline; however, room‐temperature ionic liquids (RTILs) were obtained with the weakly coordinating anions [NTf₂]^? and [N(CN)₂]^?. For the viscosities of the peralkylated RTILs, an opposite trend was found, that is, the alkoxy derivatives are less viscous than their alkenyl‐substituted analogues. Of the crystalline compounds, X‐ray diffraction data were recorded and related to their molecular properties. Upon alkoxy substitution, the electrochemical cathodic limit potential was found to be more positive, whereas the complete electrochemical window of the alkenyl‐substituted imidazolium salts was shifted to somewhat more positive potentials.     

Double Layer of Au(100)/Ionic Liquid Interface and Its Stability in Imidazolium‐Based Ionic Liquids

Ring any bells? The differential capacitance curve of Au(100) in neat [BMI]BF₄ (BMI=1‐butyl‐3‐methylimidazolium) ionic liquid has a bell‐shaped feature (see picture). The adsorption of BMI⁺ shows a disorder–order transition and depends on the structure of the surface. Ordered adsorption in a micelle‐like structure stabilizes the underlying Au surface.

     

Molecular Reorientational Dynamics of the Neat Ionic Liquid 1‐Butyl‐3‐methylimidazolium Hexafluorophosphate by Measurement of 13C Nuclear Magnetic Relaxation Data

The reorientational dynamics of the ionic liquid 1butyl‐3‐methylimidazolium hexafluorophosphate ([BMIM]PF₆) were studied over a wide range of temperatures by measurement of ¹³C spin–lattice relaxation rates and NOE factors. The reorientational dynamics were evaluated by performing fits to the experimental relaxation data. Thus, the overall reorientational motion was described by a Cole–Davidson spectral density with a Vogel–Fulcher–Tammann temperature dependence of the correlation times. The reorientational motion of the butyl chain was modelled by a combination of the latter model for the overall motion with a Bloembergen–Purcell–Pound spectral density and an Arrhenius temperature dependence for the internal motion. Except for C2 in the aromatic ring, an additional reduction of the spectral density by the Lipari–Szabo model had to be employed. This reduction is a consequence of fast molecular motions before the rotational diffusion process becomes effective. The C2 atom did not exhibit this reduction, because the librational motion of the corresponding C2? H vector is severely hindered due to hydrogen bonding with the hexafluorophosphate anion. The observed dynamic features of the [BMIM]⁺ cation confirm quantum‐chemical structures obtained in a former study.