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., PF 6 2 and N(SO 2 CF 3 ) 2 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 PF 6 2 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 PF 6 2 ILs, contact with an aqueous phase may result in slow hydrolysis of the PF 6 2 with the concomitant release of HF and other species.

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.

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.

The crown ethers 18-crown-6 (18C6), dicyclohexano-18-crown-6 (DCH18C6), and 4,4‘-(5‘)-di-(tert-butylcyclohexano)-18-crown-6 (Dtb18C6) were dissolved in 1-alkyl-3-methylimidazolium hexafluorophosphate ([C n mim][PF6], n = 4, 6, 8) room-temperature ionic liquids (RTILs) and studied for the extraction of Na+, Cs+, and Sr2+ from aqueous solutions. In the absence of extractant, the distribution ratios for the metal ions indicate a strong preference for the aqueous phase. With the crown ethers as extractants in RTIL-based liquid/liquid separations, the resulting metal ion partitioning depends on the hydrophobicity of the crown ether and also on the composition of the aqueous phase (e.g., concentration of HNO3 vs Al(NO3)3). Aqueous solutions of HCl, Na3 citrate, NaNO3, and HNO3 (the latter at low concentrations) decrease the metal ion distribution ratios and also decrease the water content of the RTIL phase. High concentrations of HNO3 decompose PF6 - and increase both the water content and the water solubility of the RTIL phase. Highly hydrated salts such as Al(NO3)3 and LiNO3 salt out both the RTIL ions and the crown ethers; thus, when the aqueous phase contains Al(NO3)3, the trend more closely resembles traditional solvent extraction behavior where D Sr > D Cs and the most hydrophobic extracting phase produces the highest partitioning. When [C8mim][PF6] is used as the extracting phase, the metal ions can be loaded from Al(NO3)3 and stripped using water. Dtb18C6 forms 1:1 complexes with Cs+ and Sr2+ and also yields the highest distribution ratios out of the three crowns examined. In comparison to traditional solvent extraction behavior, the metal ion partitioning in these systems exhibits exceptional behavior and, in certain instances, suggests a complicated partitioning mechanism, which necessitates a more thorough understanding of RTILs as solvents before interpretation of the results.

A series of hydrophobic task-specific ionic liquids designed to extract Hg2+ and Cd2+ from water were prepared by appending urea-, thiourea-, and thioether-substituted alkyl groups to imidazoles and combining the resulting cationic species with PF6-. The new ionic liquids were characterized and investigated for their metal ion extraction capabilities. When used in liquid/liquid extraction of Hg2+ and Cd2+ from aqueous solutions, the metal ion distribution ratios increased several orders of magnitude, regardless of whether the ionic liquids were used as the sole extracting phase or doped into a series of [1-alkyl-3-methylimidazolium][PF6] (alkyl = n-C4-C8) ionic liquids to form a 1:1 solution. In the 1:1 mixtures, as the length of the alkyl chain increased from butyl to hexyl to octyl, the metal ion distribution ratios increased. Increasing the ratio TSIL/[C4mim][PF6] resulted in higher distribution ratios for both Hg2+ and Cd2+. Overall, the thiourea- and urea-derivatized cations yielded the highest distribution ratios, and those for Hg2+ were higher than those for Cd2+; however, a change in aqueous-phase pH does not promote the stripping of metal ions from the extracting phase. The combination of these imidazolium cations and PF6- produced ionic liquids with decreased thermal stability in comparison to [C(n)mim]-[PF6]. Gaussian98 restricted Hartree-Fock geometry optimizations for one of the thiourea-appended cations shows the charge delocalization around the ring and suggests that the thiourea group may aid in deprotonating the imidazolium ring and may be responsible for the lowered thermal stability of these cations.