Accelerated Oxidative Degradation of Phosphonium-Type Ionic Liquid with l-Prolinate Anion: Degradation Mechanism and CO2 Separation Performance.

Amino acid ionic liquids (AAILs) are regarded as green alternatives to existing CO2-sorptive materials because amino acids are readily available from renewable sources in large quantities. For widespread applications of AAILs, including direct air capture, the relationship between the stability of AAILs, especially toward O2, and the CO2 separation performance is of particular importance. In the present study, the accelerated oxidative degradation of tetra-n-butylphosphonium l-prolinate ([P4444][Pro]), a model AAIL that has been widely investigated as a CO2-chemsorptive IL, is performed using a flow-type reactor system. Upon heating at 120-150 °C and O2 gas bubbling to [P4444][Pro], both the cationic and anionic parts undergo oxidative degradation. The kinetic evaluation of the oxidative degradation of [P4444][Pro] is performed by tracing the decrease in the [Pro]- concentration. Supported IL membranes composed of degraded [P4444][Pro] are fabricated, and the membranes retain CO2 permeability and CO2/N2 selectivity values in spite of the partial degradation of [P4444][Pro].

Ionic Liquid Mixtures for Direct Air Capture: High CO2 Permeation Driven by Superior CO2 Absorption with Lower Absolute Enthalpy.

This paper reports a series of liquid materials suitable for use as high-performance separation membranes in direct air capture. Upon mixing two ionic liquids (ILs), namely N-(2-aminoethyl)ethanolamine-based IL ([AEEA][X]) and 1-ethyl-3-methylimidazolium acetate ([emim][AcO]), the resulting mixtures with a specific range of their composition showed higher CO2 absorption rates, larger CO2 solubilities, and lower absolute enthalpies of CO2 absorption compared to those of single ILs. NMR spectroscopy of the IL mixture after exposure to 13CO2 allowed elucidation of the chemisorbed species, wherein [AEEA][X] reacts with CO2 to form CO2-[AEEA]+ complexes stabilized by hydrogen bonding with acetate anions. Supported IL membranes composed of [AEEA][X]/[emim][AcO] mixtures were then fabricated, and the membrane with a suitable mixing ratio showed a CO2 permeability of 25,983 Barrer and a CO2/N2 selectivity of 10,059 at 313.2 K and an applied CO2 partial pressure of 40 Pa without water vapor. These values are higher than those reported for known facilitated transport membranes.

Design and properties of functional zwitterions derived from ionic liquids.

A zwitterion, an ion pair where cation and anion are covalently tethered, is known to be a type of salt. These ions have not been recognised as interesting, but they are physicochemically unique and fascinating ions. In the present review, some functional zwitterions derived from ionic liquids are mentioned to emphasise the usefulness of the tethering of the component cations and anions of ionic liquids. Basic properties, advantages and disadvantages after the functional design of zwitterions, and some applications are summarised.

Zwitterion/Brønsted Acid Mixtures Showing Controlled Lower Critical Solution Temperature-Type Phase Changes with Water.

A new ammonium-type zwitterion (ZI), N,N-dihexyl-N-monopentyl-3-sulfonyl-1-propaneammonium (N665 C3S) with adequate hydrophobicity showed reversible and highly temperature-sensitive lower critical solution temperature (LCST)-type phase transitions after being mixed with pure water. Generally for such compounds, those with longer alkyl chains were immiscible with water and those with shorter chains were miscible with water, regardless of temperature. A slightly more hydrophobic ZI than N665 C3S showed LCST-type phase behavior with water when it was mixed with equimolar amounts of a Brønsted acid such as trifluoromethanesulfonic acid (HTfO). The phase-transition temperature of the ZI/Brønsted acid mixed aqueous solution was controllable by water content.

A thermoresponsive poly(ionic liquid) membrane enables concentration of proteins from aqueous media.

A new type of poly(ionic liquid) membrane, which shows switchable hydrated states via lower critical solution temperature-type phase behaviour, enables concentration of some water-soluble proteins from aqueous media.

Imidazolium-Based Poly(ionic liquid)/Ionic Liquid Ion-Gels with High Ionic Conductivity Prepared from a Curable Poly(ionic liquid).

Ionic liquid (IL)-based ion-gel membranes were prepared from a curable poly(IL)-based materials platform with the free ILs 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI]), bis(fluorosulfonyl)imide ([EMIM][FSI]), 1-butylimidazolium bis(trifluoromethylsulfonyl)imide ([C4 IMH][TFSI]), and ethylmethylammonium nitrate [EAN][NO3 ] and evaluated for their ionic conductivity performance at ambient and elevated temperatures. The resulting cross-linked, free-standing ion-gel membranes were found to have less than 1 wt% water (with the exception of [EAN][NO3 ] which contained ≈20 wt% water). Increasing free IL content from 50 to 80 wt% produces materials with ionic conductivity values ≥10(-2) S cm(-1) at 25 °C and ≈10(-1) S cm(-1) at 110 °C. Additionally, ion-gels containing 70 wt% of the protic ILs [C4 IMH][TFSI] and [EMIM][FSI] display ionic conductivity values of ≈10(-3) to 10(-2) S cm(-1) over the temperature range of 25-110 °C.

Thermoresponsive Poly(Ionic Liquid)s in Aqueous Salt Solutions: Salting-Out Effect on Their Phase Behavior and Water Absorption/Desorption Properties.

Here, a thermoresponsive phase behavior of polymerized ionic liquids (PILs) composed of poly([tri-n-alkyl(vinylbenzyl)phosphonium]chloride) (poly([Pnnn VB ]Cl) is reported, where n (the number of carbon atoms of an alkyl chain) = 4, 5, or 6 after mixing with aqueous sodium chloride solutions. Both monomeric [P555VB ]Cl and the resulting poly([P555VB ]Cl) linear homopolymer show a lower critical solution temperature (LCST)-type phase behavior in aq. NaCl solutions. The phase transition temperature of the PIL shifts to lower value by increasing concentration of NaCl. Also the swelling degree of cross-linked poly([P555VB ]Cl) gel decreases by increasing NaCl concentration, clearly suggesting the "salting-out" effect of NaCl results in a significant dehydration of the poly([P555VB ]Cl) gel. The absorbed water in the PIL gel is desorbed by moderate heating via the LCST behavior, and the absolute absorption/desorption amount is improved by copolymerization of [P555VB ]Cl with more hydrophilic [P444VB ]Cl monomer.

Reversible water uptake/release by thermoresponsive polyelectrolyte hydrogels derived from ionic liquids.

Thermoresponsive polyelectrolyte hydrogels, derived from tetra-n-alkylphosphonium 3-sulfopropyl methacrylate-type ionic liquid monomers, show reversible water uptake/release, in which the gels absorb/desorb water for at least ten cycles via a lower critical solution temperature-type phase transition.

Is seven the minimum number of water molecules per ion pair for assured biological activity in ionic liquid-water mixtures?

Ionic liquids (ILs) containing small amounts of water are called hydrated ILs and they show diverse physico-chemical properties that are strongly dependent on their water content. Some properties of hydrated ILs, such as biological activity and phase transition behaviour, were found to change non-linearly, with an inflection at a water molecule to ion pair ratio of around 7:1. This critical hydration number of ILs has been discussed in this paper with respect to the state of solvated water molecules.

High ethene/ethane selectivity in 2,2'-bipyridine-based silver(i) complexes by removal of coordinated solvent.

Following removal of coordinated CH3 CN, the resulting complexes [Ag(I) (2,2'-bipyridine)][BF4 ] (1) and [Ag(I) (6,6'-dimethyl-2,2'-bipyridine)][OTf] (2) show ethene/ethane sorption selectivities of 390 and 340, respectively, and corresponding ethene sorption capacities of 2.38 and 2.18 mmol g(-1) when tested at an applied gas pressure of 90 kPa and a temperature of (20±1) °C. These ethene/ethane selectivities are 13 times higher than those reported for known solid sorbents for ethene/ethane separation. For 2, ethene sorption reached 90 % of equilibrium capacity within 15 minutes, and this equilibrium capacity was maintained over the three sorption/desorption cycles tested. The rates of ethene sorption were also measured. To our knowledge, these are the first complexes, designed for olefin/paraffin separations, which have open silver(I) sites. The high selectivities arise from these open silver(I) sites and the relatively low molecular surface areas of the complexes.

Ammonium based zwitterions showing both LCST- and UCST-type phase transitions after mixing with water in a very narrow temperature range.

Through the study to analyse the required factors for zwitterion (ZI)-type ionic liquids to show LCST-type phase transition with water, we found that ammonium based ZIs having adequate hydrophilicity showed both LCST- and UCST-type phase transitions in a very narrow range of temperature after mixing with water.

A cobalt(II) bis(salicylate)-based ionic liquid that shows thermoresponsive and selective water coordination.

A metal-containing ionic liquid (MCIL) has been prepared in which the [Co(II)(salicylate)2](2-) anion is able to selectively coordinate two water molecules with a visible colour change, even in the presence of alcohols. Upon moderate heating or placement in vacuo, the hydrated MCIL undergoes reversible thermochromism by releasing the bound water molecules.

Design of phosphonium-type zwitterion as an additive to improve saturated water content of phase-separated ionic liquid from aqueous phase toward reversible extraction of proteins.

We designed phosphonium-type zwitterion (ZI) to control the saturated water content of separated ionic liquid (IL) phase in the hydrophobic IL/water biphasic systems. The saturated water content of separated IL phase, 1-butyl-3-methyimidazolium bis(trifluoromethanesulfonyl)imide, was considerably improved from 0.4 wt% to 62.8 wt% by adding N,N,N-tripentyl-4-sulfonyl-1-butanephosphonium-type ZI (P555C4S). In addition, the maximum water content decreased from 62.8 wt% to 34.1 wt% by increasing KH2PO4/K2HPO4 salt content in upper aqueous phosphate buffer phase. Horse heart cytochrome c (cyt.c) was dissolved selectively in IL phase by improving the water content of IL phase, and spectroscopic analysis revealed that the dissolved cyt.c retained its higher ordered structure. Furthermore, cyt. c dissolved in IL phase was re-extracted again from IL phase to aqueous phase by increasing the concentration of inorganic salts of the buffer solution.

Introduction of hydrophilic groups onto the ortho-position of benzoate anions induced phase separation of the corresponding ionic liquids with water.

Ionic liquids (ILs) composed of tetrabutylphosphonium cations and benzoate anion derivatives with hydrophilic hydroxyl or carboxyl groups introduced onto the ortho-position were found to be less hydrophilic and showed phase separation with water, whereas unmodified benzoate-type IL was freely miscible with water.

Ionic liquids showing phase separation with water prepared by mixing hydrophilic and polar amino acid ionic liquids.

Both water soluble lysine- and aspartic acid-based amino acid ionic liquids (AAILs) were mixed to prepare polar AAILs showing phase separation with water.

Detection of small differences in the hydrophilicity of ions using the LCST-type phase transition of an ionic liquid-water mixture.

The phase separation temperature of the tetrabutylphosphonium trifluoroacetate-water mixture, which undergoes a lower critical solution temperature-type phase transition, is highly sensitive to the hydrophilicity of added salts; this system can be used to compare the hydrophilicity of the added ions.

Ionic liquid-derived charged polymers to show highly thermoresponsive LCST-type transition with water at desired temperatures.

A polymerised ionic liquid (poly(IL)) having suitable hydrophobicity undergoes a strongly temperature-sensitive LCST-type phase transition with pure water and also with aqueous salt solution, and the resulting hydrated poly(IL) further undergoes a sharp and reversible liquid-to-gel transition at ambient temperatures.

Addition of suitably-designed zwitterions improves the saturated water content of hydrophobic ionic liquids.

The saturated water content in a hydrophobic ionic liquid, 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)-imide, was improved from 0.4 wt% to 17.8 wt% by adding a 3-(1-butyl-3-imidazolio)propanesulfonate-type zwitterion in appropriate amounts. The mixture containing 17.8 wt% water successfully dissolved horse heart cytochrome c without significant change of the higher ordered structure.

Ionic liquid/water mixtures: from hostility to conciliation.

Water was originally inimical to ionic liquids (ILs) especially in the analysis of their detailed properties. Various data on the properties of ILs indicate that there are two ways to design functions of ionic liquids. The first is to change the structure of component ions, to provide "task-specific ILs". The second is to mix ILs with other components, such as other ILs, organic solvents or water. Mixing makes it easy to control the properties of the solution. In this strategy, water is now a very important partner. Below, we summarise our recent results on the properties of IL/water mixtures. Stable phase separation is an effective method in some separation processes. Conversely, a dynamic phase change between a homogeneous mixture and separation of phases is important in many fields. Analysis of the relation between phase behaviour and the hydration state of the component ions indicates that the pattern of phase separation is governed by the hydrophilicity of the ions. Sufficiently hydrophilic ions yielded ILs that are miscible with water, and hydrophobic ions gave stable phase separation with water. ILs composed of hydrophobic but hydrated ions undergo a dynamic phase change between a homogeneous mixture and separate phases according to temperature. ILs having more than seven water molecules per ion pair undergo this phase transition. These dynamic phase changes are considered, with some examples, and application is made to the separation of water-soluble proteins.

Temperature-responsive ionic liquid/water interfaces: relation between hydrophilicity of ions and dynamic phase change.

Phase separation between ionic liquids (ILs) and molecular liquids is of interest physico-chemically, and also has industrial relevance. IL/water mixtures are of great interest in many fields. Unlike static phase separation between IL and water, dynamic shifts of IL/water mixtures between a homogeneous mixture and separate phases have a wide variety of applications. The miscibility of ILs with water generally increases upon heating, and a few ILs undergo a lower critical solution temperature (LCST)-type phase transition with water in which the separated biphases become miscible upon cooling. As the phase transition is controlled by changing the temperature by a few degrees, the LCST-type phase response of IL/water mixtures makes it possible to use ILs as solvents in various energy-saving processes. Since many hydrophilic ILs do not undergo phase separation with water, we aim to determine the necessary conditions under which hydrophobic ILs undergo the phase transition. Based on physico-chemical analysis of many hydrophobic ILs that undergo a phase separation after mixing with water, we find there is a particular range of "hydrophilicity" of these hydrophobic ILs within which the LCST-type phase transition is possible. Accordingly, a hydrophilicity index (HI) of ILs is proposed, in terms of the number of water molecules in the separated IL phase. The HI value proves to be a good indicator of the phase behaviour of IL/water mixtures, as well as their phase transition temperature. Potential application of the LCST-type phase change to the selective extraction of water-soluble proteins is also summarised.