Ann E. Visser

Characterization and comparison of hydrophilic and hydrophobic room temperature ionic liquids incorporating the imidazolium cation

A series of hydrophilic and hydrophobic 1-alkyl-3-methylimidazolium room temperature ionic liquids (RTILs) have been prepared and characterized to determine how water content, density, viscosity, surface tension, melting point, and thermal stability are affected by changes in alkyl chain length and anion. In the series of RTILs studied here, the choice of anion determines water miscibility and has the most dramatic effect on the properties. Hydrophilic anions (e.g., chloride and iodide) produce ionic liquids that are miscible in any proportion with water but, upon the removal of some water from the solution, illustrate how sensitive the physical properties are to a change in water content. In comparison, for ionic liquids containing more hydrophobic anions (e.g., PF6− and N(SO2CF3)2−), the removal of water has a smaller affect on the resulting properties. For a series of 1-alkyl-3-methylimidazolium cations, increasing the alkyl chain length from butyl to hexyl to octyl increases the hydrophobicity and the viscosities of the ionic liquids increase, whereas densities and surface tension values decrease. Thermal analyses indicate high temperatures are attainable prior to decomposition and DSC studies reveal a glass transition for several samples. ILs incorporating PF6− have been used in liquid/liquid partitioning of organic molecules from water and the results for two of these are also discussed here. On a cautionary note, the chemistry of the individual cations and anions of the ILs should not be overlooked as, in the case of certain conditions for PF6− ILs, contact with an aqueous phase may result in slow hydrolysis of the PF6− with the concomitant release of HF and other species.

Room temperature ionic liquids as novel media for ‘clean’ liquid–liquid extraction

The partitioning of simple, substituted-benzene derivatives between water and the room temperature ionic liquid, butylmethylimidazolium hexafluorophosphate, is based on the solutes’ charged state or relative hydrophobicity; room temperature ionic liquids thus may be suitable candidates for replacement of volatile organic solvents in liquid–liquid extraction processes.

Room-temperature ionic liquids: new solvents for f-element separations and associated solution chemistry

Abstract Ionic liquids (ILs) are composed of organic cations and either organic or inorganic anions that remain liquid over a wide temperature range, including room temperature. IL characteristics can be dramatically adjusted (e.g., hydrophobic vs. hydrophilic) by changing the anion type, or subtly altered by changing the length or number of alkyl groups appended to the cation. Changing alkyl chain lengths in the 1-alkyl-3-methylimidazolium cation, in combination with PF 6 − or N(SO 2 CF 3 ) 2 − anions, produces hydrophobic ILs with rheological properties suitable for their use in liquid/liquid separations. Actinides exhibit significant partitioning to these ILs from aqueous solutions with the addition of an extractant (e.g., octyl(phenyl)- N , N -diisobutylcarbamoylmethyl phosphine oxide) to the IL. Ionic liquids can, thus, be considered for actinide chemistry as a new class of materials with adjustable solvent characteristics, unique properties, and the potential for enhancing the principles of “green” chemistry in various chemical processes. Here we highlight the unique physical properties of some ILs and their use in liquid/liquid separations.

Task-specific ionic liquids for the extraction of metal ions from aqueous solutions

Imidazolium cations, such as those commonly used in preparing ionic liquids (ILs) can easily be derivatized to include task-specific functionality, such as metal ligating groups that when used as part of the solvent or doped into less expensive ILs, dramatically enhance the partitioning of targeted metal ions into the IL phase from water; the strategy of preparing task-specific ILs is applicable to a wide range of designer solvent needs.

LIQUID/LIQUID EXTRACTION OF METAL IONS IN ROOM TEMPERATURE IONIC LIQUIDS

The search for more environmentally-friendly reaction media has prompted the development of a wide array of alternative systems that will sustain biphasic separations with aqueous solutions without the use of volatile organic compounds (VOCs). We have begun to employ Room Temperature Ionic Liquids (RTIL), specifically 1-alkyl-3-methylimidazolium hexafluorophosphate ([Cnmim][PF6]), as VOC replacements in liquid/liquid separations of metal ions from aqueous solutions. Here we show that the partitioning of metal ions in these novel biphasic systems is consistent with traditional liquid/liquid separations: the metal ion affinity for the hydrophobic phase necessitates the presence of an extractant. In this report we explore the application of wellknown organic (1-(pyridylazo)-2-napthol, PAN, and (1-thiazolylazo)- 2-napthol, TAN) and inorganic (CN−, OCN−, SCN−, and halides) extractants for partitioning a variety of metal cations between [C4mim][PF6] or [C6mim][PF6] and an aqueous phase. PAN and TAN show pH dependent extraction of CD2+, Co+ Ni2+ and Fe3+ where their partitioning to the RTIL increases at least 2 orders of magnitude from pH 1 to 13. The effect of the halides on the partitioning of Hg2+ complexes increases F− < Cl− < Br− < I−. Pseudohalides, especially SCN−, had the greatest effect on enhancing the partitioning of Hg2+to the RTIL, whereas CN− and OCN− provided little benefit for the extraction of any of the metal ions examined.

Mercury(ii) partitioning from aqueous solutions with a new, hydrophobic ethylene-glycol functionalized bis-imidazolium ionic liquidThis work was presented at the Green Solvents for Catalysis Meeting held in Bruchsal, Germany, 13–16th October 2002.

A room temperature ionic liquid containing a bis-imidazolium cation incorporating a short ethylene-glycol spacer, 1,1′-[1,2-ethanediylbis(oxy-1,2-ethanediyl)] bis[3-methyl-1H-imidazolium-1-yl]bis(trifluoromethanesulfonyl)imide, has been prepared from the corresponding chloride salt, and the X-ray crystal structure of the low-melting hexafluorophosphate salt has been determined. The crystal structure reveals the ether linkage to be quite flexible and to participate in strong C2–H⋯O hydrogen bonds leading to asymmetry. The crystal structure of the bis-imidazolium salt incorporating a decyl-spacer, 1,1′-[1,10-decyl]bis[3-methyl-1H-imidazolium-1-yl] hexafluorophosphate, has also been determined and displays an all-trans (symmetric) conformation except at the beta carbon positions where a characteristic kink is observed. Introducing the ethylene-glycol functionality dramatically increases the distribution ratio of mercury ions, but not caesium, from aqueous solution to the hydrophobic ionic liquid, and from basic solution. This is the first example of pH dependent partitioning and stripping of mercury from ionic liquid/aqueous two-phase systems. The crystal structure of the related mercury(II) carbene complex, obtained from the reaction of mercury(II) acetate with 1,1′-[oxybis(2,1-ethanediyloxy-2,1-ethanediyl)]bis[3-methyl-1H-imidazolium-1-yl] tosylate, containing a three-ether spacer, in acetonitrile, reveals the possibility of a carbene extraction mechanism.

Hydrophobic ionic liquids incorporating N-alkylisoquinolinium cations and their utilization in liquid–liquid separations

The first examples of Room Temperature Ionic Liquids (RTIL) containing fused polycyclic N-alkylisoquinolinium cations ([Cnisoq]+) in combination with the bis(perfluoroethylsulfonyl)imide anion ([BETI]-) have been synthesized, characterized, and utilized in liquid-liquid partitioning from water; these salts have unexpectedly low melting points and give high distribution ratios for aromatic solutes, especially chlorobenzenes, between the RTIL and water.

Naphthol- and resorcinol-based azo dyes as metal ion complexants in aqueous biphasic systems

Aqueous biphasic systems (ABS) are comprised of both a polymer-rich phase (e.g., polyethylene glycol, PEG) and a salt-rich phase [e.g., (NH4)2SO4] such that both phases are 80% water on a molar basis. ABS have demonstrated applications as environmentally-friendly methods to separate relatively hydrophobic anionic species, such as pertechnetate and mercury halide anionic complexes, from high ionic strength solutions although partitioning of hydrated metal ions, such as Fe3+ and actinides, to the PEG-rich phase is negligible without the addition of a metal ion complexant to the system. Four naphthol- or resorcinol-based dyes; 1-(2-pyridylazo)-2-naphthol (PAN), 1-(thiazolylazo)-2-naphthol (TAN), 4-(2-pyridylazo)-resorcinol (PAR) and 4-(2-thiazolylazo)-resorcinol (TAR), each incorporating a naphthol or resorcinol with an ortho azo functional group, have been studied as metal ion extractants in ABS as a function of pH. In the PEG-2000/ (NH4)2SO4 ABS, the distribution ratios of Fe3+, Co2+ and Ni2- were enhanced by several orders of magnitude at high pH in contrast to the behavior of Cs+, Cd2+ and Eu3+ whose partitioning behavior was largely unaffected by the presence of these extractants at any pH. The three extracted metal ions, Fe3+, Co2+ and Ni2+, could be stripped by contact with a fresh salt phase at low pH.

Effects of gamma radiation on electrochemical properties of ionic liquids

Abstract The electrochemical properties of ionic liquids (ILs) make them attractive for possible replacement of inorganic salts in high temperature molten salt electrochemical processing of nuclear fuel. To be a feasible replacement solvent, ILs need to be stable in moderate and high doses of radiation without adverse chemical and physical effects. Here, we exposed seven different ILs to a 1.2 MGy dose of gamma radiation to investigate their physical and chemical properties as they related to radiological stability. The azolium-based ILs experienced the greatest change in appearance, but these ILs were chemically more stable to gamma radiation than some of the other classes of ILs tested, due to the presence of aromatic electrons in the azolium ring. All the ILs exhibited a decrease in their conductivity and electrochemical window (at least 1.1 V), both of which could affect the utility of ILs in electrochemical processing. The concentration of the irradiation decomposition products was less than 3 mol. %, with no impurities detectable using NMR techniques.

Room Temperature Ionic Liquids as Replacements for Traditional Organic Solvents and Their Applications Towards “Green Chemistry” in Separation Processes

The full effect of Green Chemistry will be realized when the words “environmentally friendly” and “chemistry” can be used in the same sentence without seeming to be a contradiction. In an effort to comply with governmental regulations and to spruce up the image of the chemical industry, one of the major goals of “green” chemistry is to prevent pollution and waste production at the source. In light of the vast usage of organic solvents in industry, we have investigated the use of Room Temperature Ionic Liquids (RTIL) as solvent alternatives in liquid/liquid separations. Starting from the initial study in which we examined the partitioning of simple benzene derivatives in liquid/liquid extraction systems, we have also studied how ionisable solutes partition in these systems. The knowledge of how organic solutes partition has facilitated the use of metal ion extractants in RTIL-based liquid/liquid separations. This report discusses our current results in the utilization of RTIL for liquid/liquid extraction and also highlights recent results from the literature (e.g., chromatography, supercritical fluid extraction) in which RTIL have been used for separations. The examples chosen serve as illustrations as to how RTIL can be easily used in separations, however, further research is needed to clarify where the use of RTIL is appropriate and before RTIL can be confirmed to be “green” solvent replacements.