Methods for stabilizing and activating enzymes in ionic liquids—a review

Ionic liquids (ILs) have evolved as a new type of non‐aqueous solvents for biocatalysis, mainly due to their unique and tunable physical properties. A number of recent review papers have described a variety of enzymatic reactions conducted in IL solutions; on the other hand, it is important to systematically analyze methods that have been developed for stabilizing and activating enzymes in ILs. This review discusses the biocatalysis in ILs from two unique aspects (1) factors that impact the enzyme's activity and stability, (2) methods that have been adopted or developed to activate and/or stabilize enzymes in ionic media. Factors that may influence the catalytic performance of enzymes include IL polarity, hydrogen‐bond basicity/anion nucleophilicity, IL network, ion kosmotropicity, viscosity, hydrophobicity, the enzyme dissolution, and surfactant effect. To improve the enzyme's activity and stability in ILs, major methods being explored include the enzyme immobilization (on solid support, sol–gel, or CLEA), physical or covalent attachment to PEG, rinsing with n‐propanol methods (PREP and EPRP), water‐in‐IL microemulsions, IL coating, and the design of enzyme‐compatible ionic solvents. It is exciting to notice that new ILs are being synthesized to be more compatible with enzymes. To utilize the full potential of ILs, it is necessary to further improve these methods for better enzyme compatibility. This is what has been accomplished in the field of biocatalysis in conventional organic solvents. Copyright © 2010 Society of Chemical Industry     

Antimicrobial properties and thermal stability of polycarbonate modified with 1‐alkyl‐3‐methylimidazolium tetrafluoroborate ionic liquids

Four water immiscible ionic liquids (ILs): 1‐hexyl‐3‐methylimidazolium tetrafluoroborate, 1‐heptyl‐3‐methylimidazolium tetrafluoroborate, 1‐octyl‐3‐methylimidazolium tetrafluoroborate and 1‐dodecyl‐3‐methylimidazolium tetrafluoroborate have been synthesized. Polycarbonate (PC) films containing ILs were prepared by solvent casting from methylene chloride solutions. Scanning electron microscopy measurements showed the high homogeneity of PC/IL films with the IL content up to 4 wt %. The tendency to IL aggregation was observed for polymeric films with higher IL content (5%). PC/IL composites were found to have the reduced thermal decomposition temperature under both an air and a nitrogen atmosphere in comparison with the neat PC. The effect of IL content on the antimicrobial activity of PC films against Escherichia coli bacteria was studied. Pronounced antimicrobial efficacy was revealed for PC/IL films for all studied ILs starting from 3 wt % of IL. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40050.     

Synergistic effects of two types of ionic liquids on the dispersion of multiwalled carbon nanotubes in ethylene–vinyl acetate elastomer: preparation and characterization of flexible conductive composites

Flexible conductive composites based on an elastomer, ethylene–vinyl acetate copolymer (EVM), and multiwalled carbon nanotubes (MWCNTs) were fabricated by melt‐blending and subsequent crosslinking. Two types of ionic liquids (ILs), tributylmethylammonium bis(trifluoromethanesulfonyl)imide and 1‐ethyl‐3‐methylimidazolium tetrafluoroborate, were used to modify the MWCNTs for the purpose of improving their dispersion in the polymer matrix. It was found that the MWCNTs modified by an individual IL form agglomerates in the EVM matrix, while the MWCNTs modified by the two types of ILs simultaneously disperse in the EVM matrix homogeneously. Moreover, nanocomposites based on the MWCNTs modified with the combination of the two ILs exhibit improved mechanical and electrical properties. The mechanism of such synergistic effects was investigated. It was found that EVM is miscible with tributylmethylammonium bis(trifluoromethanesulfonyl)imide and it is immiscible with 1‐ethyl‐3‐methylimidazolium tetrafluoroborate. On the other hand, MWCNTs show stronger interaction with 1‐ethyl‐3‐methylimidazolium tetrafluoroborate than tributylmethylammonium bis(trifluoromethanesulfonyl)imide. Therefore, the uniform dispersion of MWCNTs in the EVM matrix is attributed to ‘bridging’ effects of the two ILs due to the different interactions of the two ILs with MWCNTs and EVM. The application of MWCNTs modified with two types of ILs in a polymer matrix opens a new fabrication strategy for high‐performance polymer nanocomposites. © 2017 Society of Chemical Industry     

Computer‐aided design of ionic liquids as solvents for extractive desulfurization

Although ionic liquids (ILs) have been widely explored as solvents for extractive desulfurization (EDS) of fuel oils, systematic studying of the optimal design of ILs for this process is still scarce. The UNIFAC‐IL model is extended first to describe the EDS system based on exhaustive experimental data. Then, based on the obtained UNIFAC‐IL model and group contribution models for predicting the melting point and viscosity of ILs, a mixed‐integer nonlinear programming (MINLP) problem is formulated for the purpose of computer‐aided ionic liquid design (CAILD). The MINLP problem is solved to optimize the liquid‐liquid extraction performance of ILs in a given multicomponent model EDS system, under consideration of constraints regarding the IL structure, thermodynamic and physical properties. The top five IL candidates preidentified from CAILD are further evaluated by means of process simulation using ASPEN Plus. Thereby, [C₅MPy][C(CN)₃] is identified as the most suitable solvent for EDS. © 2017 American Institute of Chemical Engineers AIChE J, 64: 1013–1025, 2018     

Sulfonated polyisobutylene telechelic ionomers. XIV. Viscoelastic behavior of concentrated solutions in nonpolar solvents

The effect of molecular variables upon the dynamic viscoelastic behavior of solutions of sulfonated polyisobutylene telechelic ionomers in nonpolar solvents has been investigated. Intermolecular association of the ionic end groups in nonpolar media results in the formation of a transient ionic network which displays a viscous response at low frequencies and an elastic response at high frequencies. The frequency of the transition from viscous behavior to elastic behavior, as well as the plateau storage modulus, is dependent upon molecular variables such as architecture, molecular weight, neutralizing cation, and extent of neutralization. Variables which affect the strength of the ionic interactions, such as temperature and the type of solvent, also influence the viscoelastic response. Solutions of ionomers neutralized with cations of Groups IA and IIA, such as potassium and calcium, behave elastically over most of the experimentally accessible frequency range, while those neutralized with transition metals, such as zinc, display viscous flow over a rather wide range at low frequencies. As in previous studies of dilute solution viscosity behavior, the threearm star trifunctional species was found to form a more extensive network in nonpolar solvents than the linear difunctional species at equivalent concentrations. The failure of time–temperature superposition indicates that these solutions are thermoheologically complex.     

Ionic liquids containing an alkyl sulfate group as potential electrolytes

Various types of ionic liquid (IL) containing an alkyl sulfate group are synthesized and their physical and electrochemical properties are investigated. The temperature dependency of dynamic viscosity and ionic conductivity are measured for these ILs. The low temperature phase behaviour of the ethylsulfate salts is investigated using differential scanning calorimetry. Three ethylsulfate-containing ionic liquids exhibit wide electrochemical windows of about 5.0 V, and one pyrrolidinium-containing ionic liquid shows a conductivity of 3.8 mS cm⁻¹. The various cations of alkylsulfate-containing ionic liquids are shown to greatly influence viscosity, density, and conductivity. Absorbance solvatochromic probe, Nile Red is used to investigate the relative polarity of alkylsulfate base ionic liquids compared with several organic solvents. The electrochemical and thermal stabilities of these ILs make them promising electrolytes for use in electrochemical devices.     

Vapor Liquid Equilibria for Ionic Liquid/Ethanol/Water Systems and the Effect of Anion Hydrolysis

The performance of two tetrafluoroborate-based ionic liquids (ILs) as entrainers in the dehydration of water/ethanol azeotropic mixtures was evaluated. Isobaric vapor-liquid equilibrium (VLE) data were measured for the systems ethanol/water/1-butyl-3-methyl imidazolium tetrafluoroborate and ethanol/water /n-butylpyridinium tetrafluoroborate including the azeotropic region. VLE data for the ethanol/water, ethanol/IL, and water/IL binary mixtures were obtained at 100 kPa. The hydrolysis of the tetrafluoroborate anion was studied for both types of ILs by ¹⁹F NMR analysis. The hydrolysis of the tetrafluoroborate anion does not have much effect on the ethanol/water VLE. The ¹⁹F NMR analysis indicated that hydrolysis occurred at high mole fractions of water.     

Ionic liquids as plasticizers/lubricants for polylactic acid

In this study, two ionic liquids (IL) with different anions (decanoate, tetrafluoroborate) but with the same phosphonium‐based cation were evaluated as potential plasticizers and lubricants for polylactic acid (PLA). Both ILs at 5 wt% were well dispersible and partly miscible with PLA as evidenced by scanning electron microscopy, SEM, and glass transition temperature, T_g, characterization, as well as from solubility parameters calculations. However, phase separation in the PLA system containing the IL with tetrafluoroborate anions was observed 1 year after melt processing. The IL containing the tetrafluoroborate anion showed higher thermal stability compared with the IL containing decanoate as confirmed by thermogravimetric analysis. This was translated in corresponding behaviors of the PLA blends. The effects of the IL containing the tetrafluoroborate anion were more pronounced on lubrication as evidenced by lower values of coefficient of friction that could also be correlated with lower contact angle; this IL could then be considered as a more effective, overall, plasticizer than the IL containing the decanoate anion based on reduced flexural modulus and brittleness as well as T_g suppression results. The extent of plasticization, however, could also be related to the molecular weight reduction during melt processing that occurred in the presence of both ILs, but to different degrees. POLYM. ENG. SCI., 2010. © 2009 Society of Plastics Engineers     

Free radical polymerization in ionic liquids—Influence of the IL-concentration and temperature

The influence of the ionic liquid (IL) 1-ethyl-3-methylimidazoliumethylsulfate ([EMIM]EtSO₄) on the polymerization kinetics of methyl methacrylate was investigated. ILs are liquids with relatively high polarities and viscosities. These two characteristic properties are strongly correlated with the rate coefficients of propagation k_p and termination k_t of polymerizations carried out in ILs. The rate constant of termination k_t decreases when the concentration of ionic liquid, and thus the viscosity is increased, whereas the propagation rate coefficient k_p increases with increasing IL content. The viscosity of ILs can be varied by either working with mixtures of ILs with conventional organic solvents – here the IL [EMIM]EtSO₄ was mixed with dimethyl formamide (DMF) – or by variation of the temperature. The studies were carried out to determine the influence of the viscosity on the propagation and the termination reaction as well as the molecular weight distribution.     

Gas–liquid mass‐transfer properties in CO2 absorption system with ionic liquids

The deficiency of mass‐transfer properties in ionic liquids (ILs) has become a bottleneck in developing the novel IL‐based CO₂ capture processes. In this study, the liquid‐side mass‐transfer coefficients (k_L) were measured systematically in a stirred cell reactor by the decreasing pressure method at temperatures ranging from 303 to 323 K and over a wide range of IL concentrations from 0 to 100 wt %. Based on the data of k_L, the kinetics of chemical absorption of CO₂ with mixed solvents containing 30 wt % monoethanolamine (MEA) and 0–70 wt % ILs were investigated. The k_L in IL systems is influenced not only by the viscosity but also the molecular structures of ILs. The enhancement factors and the reaction activation energy were quantified. Considering both the mass‐transfer rates and the stability of IL in CO₂ absorption system, the new IL‐based system MEA + [bmim][NO₃] + H₂O is recommended. © 2014 American Institute of Chemical Engineers AIChE J, 60: 2929–2939, 2014     

Densities and viscosities for ionic liquids [BMIM][BF4] and [BMIM][Cl] and their binary mixtures at various temperatures and atmospheric pressure

The density and viscosity of 1-butyl-3-methylimidazolium tetrafluoroborate [BMIM][BF₄] and 1-butyl-3-methylimidazolium chloride [BMIM][Cl] and their binary mixtures within the temperatures from 303.15 K to 323.15 K and at ambient pressure were determined in this work. The temperature dependences of density and viscosity were satisfactorily described with the linear model and the Vogel-Tammann-Fulcher type equation, respectively. The molar volume and viscosity of binary IL mixtures were predicted through ideal mixing rules showing that almost null deviations for IL mixtures were observed and their mixing was remarkably close to linear ideal behavior in the molar volumes, while comparatively large errors in viscosity occurred. Additionally, the molar volume of the investigated pure ILs and their mixtures could well be predicted by a predictive model presented by Valderrama et al. (Fluid Phase Equilib., 275 (2009) 145).     

The structural organization in aqueous solutions of ionic liquids

The ¹H NMR combined with the local composition (LC) model has been employed to investigate the structural organization of two aqueous solutions of ionic liquids (ILs), namely 1‐ethyl‐3‐methylimidazolium tetrafluoroborate (EmimBF₄) and n‐butylammonium nitrate (N4NO₃). The correlation of chemical shifts using the LC model shows that the self‐association of IL plays the leading role, and water prefers to interact with IL rather than self‐association in IL‐rich region. Instead the network of water molecules is established in water‐rich region, because the self‐association of water predominates. Furthermore, the difference between the local and the bulk composition presents the turnover at x_(IL) (mole fraction of IL) close to 0.6 for EmimBF₄/water, which is in accordance with the change of excess function. Accordingly, it could be presumed that the excess properties for N4NO₃/water system should behave turnover at x_(IL) ≈ 0.55 since the local and the bulk exhibit maximal difference at this composition. © 2008 American Institute of Chemical Engineers AIChE J, 2009     

Science-guided data analytics for selecting ionic liquid solvents for aromatic extraction

Ionic liquids (ILs) are promising solvents for the aromatic extraction process. An attractive characteristic is the existence of hundreds of ILs that exhibit different properties. To identify key properties of IL solvents for an energy-optimum aromatic extraction, we use process simulation to generate the process datasets for multivariate data analytics with partial least squares, and use science-guided fundamentals to develop an IL heat load variable (HLV). We consider 16 well-studied ILs and correlate process steam duty and process variables affecting equipment size to the HLV for ethylene cracker feeds of low aromatic content. For such feeds in an IL aromatic extraction process, 11 of 16 ILs show energy advantage compared with sulfolane solvent with the lowest energy IL process requiring 57% of total energy required for an equivalent sulfolane process. Our results facilitate the IL solvent selection for pilot tests and subsequent commercialization of an IL aromatic extraction process.     

Mixtures of Ionic Liquids and Water as a Medium for Efficient Enantioselective Hydrogenation and Catalyst Recycling

In a screening of ligands, ionic liquids and reaction conditions in the Rh‐catalyzed hydrogenation of enamides, a novel multi‐phase reaction system consisting of an ionic liquid (IL) and water (wet ILs) was found to give the most promising results. In many cases such IL/water combinations were superior compared to conventional organic solvents and biphasic ILs/organic co‐solvents media with respect to catalytic performance as well as to catalyst separation and recycling. So far, the best results were obtained with Rh‐ferrocenyl‐diphosphine catalysts (>99% ee). Generally, somewhat lower ees were observed at higher pressure. However, this effect was less pronounced with wet ILs than with conventional solvents. It is shown that IL/water combination allow repeated catalyst recycling without significant loss of activity and that industrially relevant turnover numbers of >10,000 can be obtained.     

Photoinitiated polymerization in ionic liquids: Kinetics and viscosity effects

The photo-induced polymerization of poly(ethylene glycol) dimethacrylate and poly(ethylene glycol) monomethacrylate (crosslinking and linear, resp.) in four imidazolium-based ionic liquids (ILs) containing the same cation or the same anion in pairs is reported. The kinetic studies were accompanied by detailed viscosity measurements, which showed the occurrence of an interesting phenomenon - a viscosity synergism in monomer/IL mixtures (i.e. the viscosity of the mixture is higher than the simple additive combination of viscosities of the two components). Viscosity synergism, very important for kinetic considerations, is especially strong for ILs of low viscosity and its magnitude depends on the monomer structure. The polymerization conducted in ILs was considerably faster than in a reference solvent. The propagation rate coefficients were influenced mainly by the anion structure whereas the termination rate coefficients by viscosity of the initial monomer/IL mixture (taking into account the synergistic effect). FTIR studies showed the existence of specific interactions between the carbonyl group in the monomer and C₂-H of the imidazolium ring; the polymerization rates were directly related to the magnitude of the monomer/IL interaction.     

Effect of the medium on the stereostructure of poly(methyl methacrylate) synthesized in ionic liquids

Poly(methyl methacrylate) (PMMA) was synthesized in different ionic liquids (IL) by free radical polymerization. The average molecular weight and polydispersity of the PMMA were measured by gel permeation chromatography. It is found that ILs have effect not only on the molecular weights but also on the stereostructure of PMMA. From the FTIR and NMR analysis, it was confirmed that the chemical structure of PMMA synthesized in ILs was similar to the one obtained in conventional solvents while the stereostructure was influenced by the different IL used. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 2199–2202, 2006     

Synthesis of gold nanoparticles by laser ablation of an Au foil inside and outside ionic liquids

Stable gold nanoparticles (AuNPs) were prepared by simple laser ablation of an Au foil placed inside or outside four ionic liquids (ILs), without the addition of any external chemical reagent. Irregular spherical AuNPs with a diameter range of 5 to 20 nm were produced after laser ablation of an Au foil located inside or outside the ILs 1-n-butyl-3-methylimidazolium tetrafluoroborate (BMI·BF4), 1-n-butyl-3-methylimidazolium hexafluorophosphate (BMI·PF6) and 1-(3-cyanopropyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ((BCN)MI·NTf2). Additionally, whereas laser ablation inside the IL 1-n-butyl-3-methylimidazolium dicyanamide BMI·N(CN)2 produced flower-like shaped nanoparticles of about 50 nm in size, ablation outside this IL presented similar results to the others ILs studied, as determined by TEM and UV-Vis. The size and shape of the prepared NPs were related to where NP nucleation and growth occurred, i.e., at the IL surface or within the IL. Indeed, the chemical composition of the IL/air interface and surface ion orientation played important roles in the stabilization of the AuNPs formed by laser ablation outside the ILs.     

Synthesis and Characterization of Carbazole–Fluorene–Silole Copolymers as Efficient Green Light Emitting Diodes

A novel series of soluble polyfluorene–carbazole–silole-containing polymers (PF–Cz–S) were prepared through the Suzuki coupling reaction; and, the chemical structures were characterized by elemental analysis and NMR. Optoelectronic properties including UV absorption, electrochemistry, photoluminescence (PL), and electroluminescence (EL) of the copolymers were investigated. The copolymers exhibit main absorption peaks at 359?nm for solutions and 354?nm for films with combined contributions from the carbazole and fluorene units. Compared with the solution PL, complete PL excitation energy transfer from the wide band gap segment to a narrow band gap silole unit was achieved by film PL even at a lower silole content. The optical band gaps for PF–Cz–S are between 2.68 and 2.78?eV and the HOMO levels are ?5.65?eV, which are contributed by 3,6-carbazole and fluorene blocks. Light-emitting devices fabricated from these polymers display a maximum external quantum efficiency of 0.44?% with an indium–tin oxide (ITO)/poly(3,4-ethylenedioxythiophene) (PEDOT)/PF–Cz–S/CsF/Al configuration whereas an ITO/PEDOT/PCz–F–S/1,3,5-tris(N-phenylbenzimidizol-2-yl)benzene (TPBI)/CsF/Al with TPBI hole-blocking layer device configuration boosts the efficiency to 2.43?%. The TPBI hole-blocking layer largely improved the efficiency of exciton formation from which the highly efficient green EL from the silole units could be displayed.     

Towards Understanding Directed Evolution: More than Half of All Amino Acid Positions Contribute to Ionic Liquid Resistance of Bacillus subtilis Lipase A

Ionic liquids (ILs) are attractive (co‐)solvents for biocatalysis. However, in high concentration (>10 % IL), enzymes usually show decreased activity. No general principles have been discovered to improve IL resistance of enzymes by protein engineering. We present a systematic study to elucidate general engineering principles by site saturation mutagenesis on the complete gene bsla. Screening in presence of four [BMIM]‐based ILs revealed two unexpected lessons on directed evolution: 1) resistance improvement was obtainable at 50–69 % of all amino acid positions, thus explaining the success of small sized random mutant libraries; 2) 6–13 % of substitutions led to improved resistance. Among these, 66–95 % were substitutions by chemically different amino acids (e.g., aromatic to polar/aliphatic/charged amino acids), thus indicating that mutagenesis methods introducing such changes should, at least for lipases like BSLA, be favored to improve IL resistance.     

Spectroscopic investigations of the water pool in lecithin reverse micelles

The nature of the water pool formed in the reverse micellar system, lecithin/nonpolar solvent/water, has been investigated by means of near infrared, ultraviolet, fluorescence emission and visible spectroscopic techniques. The three nonpolar solvents chosen in this study were benzene, carbon tetrachloride and cyclohexane. Near infrared spectroscopic studies revealed that the amount of water present in the bulk organic phase is negligible at all water concentrations studied in all three solvents. The results of the polarity probe and 8-anilinonaphthalene sulfonic acid (ANSA) fluorescence emission maxima studies indicate that the polarity of the water pool is much lower than that of bulk water. The difference in polarity between the water pool and bulk water decreases with increasing water concentration in benzene and carbon tetrachloride systems. However, in the cyclohexane system, at a water content of 6 moles of water per mole of lecithin, where the system is known to change from isotropic reverse micelle to anisotropic liquid crystalline state, the polarity of the water pool is found to decrease.