Controlling the Formation of Ionic-Liquid-based Aqueous Biphasic Systems by Changing the Hydrogen-Bonding Ability of Polyethylene Glycol End Groups.

The formation of aqueous biphasic systems (ABS) when mixing aqueous solutions of polyethylene glycol (PEG) and an ionic liquid (IL) can be controlled by modifying the hydrogen-bond-donating/-accepting ability of the polymer end groups. It is shown that the miscibility/immiscibility in these systems stems from both the solvation of the ether groups in the oxygen chain and the ability of the PEG terminal groups to preferably hydrogen bond with water or the anion of the salt. The removal of even one hydrogen bond in PEG can noticeably affect the phase behavior, especially in the region of the phase diagram in which all the ethylene oxide (EO) units of the polymeric chain are completely solvated. In this region, removing or weakening the hydrogen-bond-donating ability of PEG results in greater immiscibility, and thus, in a higher ability to form ABS, as a result of the much weaker interactions between the IL anion and the PEG end groups.

"Washing-out" ionic liquids from polyethylene glycol to form aqueous biphasic systems.

The molecular-level mechanisms behind the formation of aqueous biphasic systems (ABS) composed of ionic liquids (ILs) and polymers are hitherto not completely understood. For the first time, it is herein shown that polymer-IL-based ABS are a result of a "washing-out" phenomenon, and not of a salting-out effect of the IL over the polymer as assumed in the past few years. Novel evidence is herein provided by experimental results combined with molecular dynamics (MD) simulations and density functional theory (DFT) calculations.

Ionic liquid processing of cellulose.

Utilization of natural polymers has attracted increasing attention because of the consumption and over-exploitation of non-renewable resources, such as coal and oil. The development of green processing of cellulose, the most abundant biorenewable material on Earth, is urgent from the viewpoints of both sustainability and environmental protection. The discovery of the dissolution of cellulose in ionic liquids (ILs, salts which melt below 100 °C) provides new opportunities for the processing of this biopolymer, however, many fundamental and practical questions need to be answered in order to determine if this will ultimately be a green or sustainable strategy. In this critical review, the open fundamental questions regarding the interactions of cellulose with both the IL cations and anions in the dissolution process are discussed. Investigations have shown that the interactions between the anion and cellulose play an important role in the solvation of cellulose, however, opinions on the role of the cation are conflicting. Some researchers have concluded that the cations are hydrogen bonding to this biopolymer, while others suggest they are not. Our review of the available data has led us to urge the use of more chemical units of solubility, such as 'g cellulose per mole of IL' or 'mol IL per mol hydroxyl in cellulose' to provide more consistency in data reporting and more insight into the dissolution mechanism. This review will also assess the greenness and sustainability of IL processing of biomass, where it would seem that the choices of cation and anion are critical not only to the science of the dissolution, but to the ultimate 'greenness' of any process (142 references).

Coagulation of chitin and cellulose from 1-ethyl-3-methylimidazolium acetate ionic-liquid solutions using carbon dioxide.

Chemisorption of carbon dioxide by 1-ethyl-3-methylimidazolium acetate ([C2 mim][OAc]) provides a route to coagulate chitin and cellulose from [C2 mim][OAc] solutions without the use of high-boiling antisolvents (e.g., water or ethanol). The use of CO2 chemisorption as an alternative coagulating process has the potential to provide an economical and energy-efficient method for recycling the ionic liquid.

Biphasic liquid mixtures of ionic liquids and polyethylene glycols.

We have found that 1-alkyl-3-methylimidazolium chloride ionic liquids (ILs) can form immiscible liquid mixtures with some polyethylene glycols (PEGs). Binary mixtures of 1-ethyl-3-methylimidazolium chloride with PEG of molecular weight 1500, 2000, or 3400 g mol(-1), or of 1-butyl-3-methylimidazolium chloride with PEG of molecular weight 2000 or 3400 g mol(-1), have been found to give rise to entirely liquid, stable biphasic systems over a significant temperature range (from 333.15 K to 413.15 K), while mixtures of 1-ethyl-3-methylimidazolium chloride with PEG-1000 and 1-butyl-3-methylimidazolium chloride with PEG-1000 and PEG-1500 are miscible. The mutual immiscibility of the IL and the PEG increases as the temperature increases. The evolution of the composition of the phases in equilibrium with the molecular weight of the PEG, or with the variation of the length of the alkyl substituent chain of the imidazolium cation of the IL, has been explored. The trends observed are explained through the complexity of interactions present within the binary system. A thermodynamic analysis of the liquid-liquid equilibrium data indicates negative values for the change of enthalpy and entropy of mixing. The potential application of these biphasic, entirely liquid systems, with low volatility and good solvation properties, for the dissolution and separation of cellulose and lignin at elevated temperature has been preliminarily explored, although only modest results have been achieved to date.

Advances in Processing Chitin as a Promising Biomaterial from Ionic Liquids.

Chitin isolated through microwave-assisted dissolution using ionic liquids is a high molecular weight (MW) polymer that can be manufactured into materials of different architectures (e.g., fibers, films, microspheres, nanostructured materials) to be used as wound care dressings, drug delivery devices, scaffolds, etc. However, because of differences from traditional isolation methods and, thus, differences in polymer length and degree of deacetylation, it could exhibit bio-related properties that differ from those of traditionally 'pulped' chitin. Here we present the initial assessments of bio-related chitin properties in order to provide a useful scientific basis for clinical applications: biocompatibility, cytotoxicity (intracutaneous reactivity), wound healing efficacy, histological evaluation of the wounds treated with chitin dressing, and antibacterial activity. We also provide the studies that outline potential applications of chitin as a raw polymer for preparation of biomaterials. Graphical Abstract.

Ionic liquid-based preparation of cellulose-dendrimer films as solid supports for enzyme immobilization.

Surface-active cellulose films for covalent attachment of bioactive moieties were achieved by codissolution of cellulose with polyamidoamine (PAMAM) dendrimers in an ionic liquid followed by regeneration of the composite as a film. Different generations of PAMAM were used for the formation of cellulose-dendrimer composites, as well as films with the dendrimer covalently bonded to the cellulose by means of the linker 1,3-phenylene diisocyanate. Surface characterization, thermal stability, and utility for immobilization of laccase were determined. The presence of the dendrimer amino groups was confirmed by detailed characterization of the films' surfaces. These modified films exhibit acceptable thermal stability, comparable to that of other regenerated cellulose films, but the number of active functional groups on the surface is much smaller than the theoretical amount expected. Films made with 1,3-phenylene diisocyanate as linker for covalently bound cellulose and dendrimers exhibit a better performance for immobilization of laccase than those prepared by simple mixing of the cellulose and dendrimer. In general, a linear correspondence between the dendrimer generation within the films and the specific activity of immobilized laccase in such films was not observed.

Sensor technologies based on a cellulose supported platform.

A simple approach to sensor development based on encapsulating a probe molecule in a cellulose support followed by regeneration from an ionic liquid solution is demonstrated here by the codissolution of cellulose and 1-(2-pyridylazo)-2-naphthol in 1-butyl-3-methylimidazolium chloride followed by regeneration with water to form strips which exhibit a proportionate (1 : 1) response to Hg(II) in aqueous solution.

Mechanism of cellulose dissolution in the ionic liquid 1-n-butyl-3-methylimidazolium chloride: a 13C and 35/37Cl NMR relaxation study on model systems.

13C and 35/37Cl NMR relaxation measurements on several model systems demonstrate that the solvation of cellulose by the ionic liquid (IL) 1-n-butyl-3-methylimidazolium chloride ([C4mim]Cl) involves hydrogen-bonding between the carbohydrate hydroxyl protons and the IL chloride ions in a 1 ratio 1 stoichiometry.

The opposite effect of temperature on polyethylene glycol-based aqueous biphasic systems versus aqueous biphasic extraction chromatographic resins.

Variation in operational temperatures has revealed differences in the partitioning behavior of probe solutes between the phases in aqueous biphasic systems (ABS) and the related aqueous biphasic extraction chromatographic resin (ABEC). This difference has been studied using the hydrophobic anion, 99TcO4-, as a probe and (NH4)2SO4 as the kosmotropic salt. Distribution of the hydrophobic anion 99TcO4- to the PEG-rich phase in a MePEG-5000/(NH4)2SO4 ABS increases with increasing temperature, but decreases are observed in batch uptakes of this anion to ABEC resins from (NH4)2SO4 solutions. Phase diagrams were constructed at five different temperatures from 10 to 50 degreesC using cloud point titration for the ABS and a correlation between the phase divergence, measured in terms of tie line length (TLL), and the temperature of the partitioning system was verified. Thermodynamic parameters (deltaHdegrees,deltaSdegrees, deltaGdegrees, ) as a function of temperature were calculated for the various systems studied and the results imply thermodynamic differences between partitioning in ABS versus ABEC.

High-resolution 13C NMR studies of cellulose and cellulose oligomers in ionic liquid solutions.

High-resolution 13C NMR studies of cellulose and cellulose oligomers dissolved in the ionic liquid (IL) 1-butyl-3-methylimidazolium chloride ([C4mim]Cl) show that the beta-(1-->4)-linked glucose oligomers are disordered in this medium and have a conformational behavior which parallels the one observed in water, and thus, reveal that the polymer is disordered in IL solution as well.

Polar, non-coordinating ionic liquids as solvents for the alternating copolymerization of styrene and CO catalyzed by cationic palladium catalysts.

The palladium-catalyzed copolymerization of styrene and CO in an ionic liquid solvent, 1-hexylpyridinium bis(trifluoromethanesulfonyl)imide, gave improved yields and increased molecular weights compared to polymerizations run in methanol.

Conventional free radical polymerization in room temperature ionic liquids: a green approach to commodity polymers with practical advantages.

Free-radical polymerization of methyl methacrylate and styrene using conventional organic initiators in the room temperature ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate ([C4mim][PF6]) is rapid and produces polymers with molecular weights up to 10x higher than from benzene; both polymerization and isolation of products were achieved without using VOCs, offering economic as well as environmental advantages.

Application of ionic liquids as plasticizers for poly(methyl methacrylate).

The room temperature ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate, [C4mim][PF6] was found to be an efficient plasticizer for poly(methyl methacrylate), prepared by in situ radical polymerization in the ionic liquid medium; the polymers have physical characteristics comparable with those containing traditional plasticizers and retain greater thermal stability.

Effects of speciation on partitioning of iodine in aqueous biphasic systems and onto ABEC resins.

Polyethylene glycol (PEG)-aqueous biphasic systems (ABS) and PEG-grafted aqueous biphasic extraction chromatographic (ABEC) resins have been shown to remove inorganic species from environmental and nuclear wastes. The partitioning behavior of several iodide species (iodide, iodine, triiodide, iodate, and 4-iodo-2,6-dimethylphenol (I-DMP)) have been studied for PEG (MW 2000)-salt systems and ABEC resins. Iodide partitioning to PEG-rich phases or onto ABEC resins can be enhanced by derivatization with 2,6-dimethylphenol to form 4-iodo-2,6-dimethylphenol or by addition of I(2) to form triiodide. Conversely, iodide partitioning to the PEG-rich phase or onto ABEC resins is reduced by oxidation of iodide to IO(3)(-). Partitioning studies of iodide, iodate, and iodine in a PEG-ABS are compared to results using ABEC resins.

Application of polyethylene glycol-based aqueous biphasic reactive extraction to the catalytic oxidation of cyclic olefins.

Glutaric acid and 1,2,3,4-butanetetracarboxylic acid (BTCA) have been synthesized by sodium tungstate catalyzed oxidation of the cyclic olefins: cyclopentene and 1,2,3,6-tetrahydrophthalic anhydride (THPA), using hydrogen peroxide in a polyethylene glycol (PEG)-2000/NaHSO(4) aqueous biphasic system (PEG-ABS). The production of glutaric acid and BTCA was found to increase from the monophasic to the biphasic regimes, and was found to be greatest at short tie-line lengths (TLLs), close to the system's critical point, yielding glutaric acid and BTCA in 73.1 and 82.5% yield, respectively. The results imply that mutual mixing or contact of the components is important, because the product dicarboxylic acids were found to increase from the monophasic side to the critical point and decrease from the critical point to more divergent regimes. The two reactions were compared with adipic acid synthesis from cyclohexene in terms of the cyclic olefin structure, and the partitioning of the dicarboxylic acid product in the ABS.

Facile pulping of lignocellulosic biomass using choline acetate.

Treating ground bagasse or Southern yellow pine in the biodegradable ionic liquid (IL), choline acetate ([Cho][OAc]), at 100°C for 24h led to dissolution of hemicellulose and lignin, while leaving the cellulose pulp undissolved, with a 54.3% (bagasse) or 34.3% (pine) reduction in lignin content. The IL solution of the dissolved biopolymers can be separated from the undissolved particles either by addition of water (20 wt% of IL) followed by filtration or by centrifugation. Hemicellulose (19.0 wt% of original bagasse, 10.2 wt% of original pine, containing 14-18 wt% lignin) and lignin (5.0 wt% of original bagasse, 6.0 wt% of original pine) could be subsequently precipitated. The pulp obtained from [Cho][OAc] treatment can be rapidly dissolved in 1-ethyl-3-methylimidazolium acetate (e.g., 17 h for raw bagasse vs. 7h for pulp), and precipitated as cellulose-rich material (CRM) with a lower lignin content (e.g., 23.6% for raw bagasse vs. 10.6% for CRM).

Evidence for the interactions occurring between ionic liquids and tetraethylene glycol in binary mixtures and aqueous biphasic systems.

The well-recognized advantageous properties of poly(ethylene glycol)s (PEGs) and ionic liquids (ILs) in the context of an increasing demand for safe and efficient biotechnological processes has led to a growing interest in the study of their combinations for a wide range of procedures within the framework of green chemistry. Recently, one of the most promising and attractive applications has been the novel IL/polymer-based aqueous biphasic systems (ABS) for the extraction and purification of biomolecules. There still lacks, however, a comprehensive picture of the molecular phenomena that control the phase behavior of these systems. In order to further delve into the interactions that govern the mutual solubilities between ILs and PEGs and the formation of PEG/IL-based ABS, (1)H NMR spectroscopy in combination with classical molecular dynamics (MD) simulations performed for binary mixtures of tetraethylene glycol (TEG) and 1-alkyl-3-methylimidazolium-chloride-based ILs and for the corresponding ternary TEG/IL/water solutions, at T = 298.15 K, were employed in this work. The results of the simulations show that the mutual solubilities of the ILs and TEG are mainly governed by the hydrogen bonds established between the chloride anion and the -OH group of the polymer in the binary systems. Additionally, the formation of IL/PEG-based ABS is shown to be controlled by a competition between water and chloride for the interactions with the hydroxyl group of TEG.

Microwave-assisted dissolution and delignification of wood in 1-ethyl-3-methylimidazolium acetate.

Microwave irradiation can facilitate the dissolution and delignification of lignocelluloses in ionic liquids compared to simple oil bath heating as demonstrated here where 92.5% of 0.5 g ground southern yellow pine was dissolved in 10 g 1-ethyl-3-methylimidazolium acetate using microwave irradiation in only 4 min. Cellulose-rich material (pulp) regenerated from the wood/ionic liquid solution had a lignin content of ~10%; significantly lower than the lignin content of the original wood (31.9%) or that of pulp obtained from the same experiment but using 16 h of oil bath heating (16-24%). The 10% lignin content obtained with the microwave method was close to that of pulp obtained from the oil bath heating method when polyoxometalate catalysts were used (5-9%).

Where are ionic liquid strategies most suited in the pursuit of chemicals and energy from lignocellulosic biomass?

Certain ionic liquids have been shown to dissolve cellulose, other biopolymers, and even raw biomass under relatively mild conditions. This particular ability of some ionic liquids, accompanied by a series of concurrent advantages, enables the development of improved processing strategies for the manufacturing of a plethora of biopolymer-based advanced materials. The more recent discoveries of dissolution of lignocellulosic materials (e.g., wood) in ionic liquids, with at least partial separation of the major constituent biopolymers, suggest further paths towards the achievement of a truly sustainable chemical and energy economy based on the concept of a biorefinery which provides chemicals, materials, and energy. Nonetheless, questions remain about the use of ionic liquids and the advisability of introducing any new process which utilizes bulk synthetic chemicals which have to be made, disposed of, and prevented from entering the environment. In this article, we discuss our own journey from the discovery of the dissolution of cellulose in ionic liquids to the cusp of an enabling technology for a true biorefinery and consider some of the key questions which remain.