Comparison of Diluent Characteristics of Imidazolium Hexafluorophosphate Ionic Liquidwith n-Dodecane

Over the past ten years, ionic liquids (ILs) have emerged as possible alternative to volatile organic compounds that are used as solvents in various synthetic and pharmaceutical industries, owing to their environmentally benign nature and potential for use in a variety of areas. Recently, room temperature ionic liquids (RTILs) are gaining recognition in nuclear reprocessing industry, as well, due to their attractive extraction properties and stability towards radiation. Since ionic liquids comprise only of ions, they have negligible volatility, low flammability etc, which are the fundamental requirements for the solvent or diluent in industrial solvent extraction processes. Rogers and co-workers reported the possible application of ionic liquids as diluents for the separation of fission products and actinides from nuclear wastes. They have employed crown ether diluted with 1-butyl-3methylimidazolium hexaflurophosphate (bmimPF6) ionic liquids for extracting Sr, Cs, or actinide specific extractants diluted with bmim triflate imide ionic liquid for extracting fblock elements. 9 They observed that, these extractants in conjunction with ILs displayed unusual extraction of target metals from aqueous solution under conditions that gave negligible or meager extraction with customary diluents such as chloroform or dodecane (DD). Chun et al. have studied the structural variation in 1-alkyl-3-methylimidazolium hexafluorophosphate (amimPF6) ILs for investigating the variation in selectivity of alkali metal ion extraction. It was found that by increasing the chain length of alkyl group attached to the imidazole, the efficiency of extraction decreased but selectivity increased. Recently 13 we have also studied the extraction of uranium by TBP in 1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF6). It was reported that bmimPF6 as such extracts uranium and in contrast to the traditional extraction behavior exhibited by TBP/DD, the distribution ratios of uranium in 1.1 M TBP/bmimPF6 continued to increase even above 5 M nitric acid. In addition, we have also investigated the extraction behavior by substituting octyl (omimPF6) instead of butyl group in amimPF6. It was found that the distribution ratios of uranium were similar in both the cases at all nitric acid concentrations. But, the solubility of omimPF6 in aqueous nitric acid phase was lowered by a factor of 2 – 3 as compared to bmimPF6 and the viscosity of solutions of TBP/omimPF6 was higher by ~2 fold as compared to TBP/bmimPF6 solutions. While the investigation of ionic liquids as diluents for reprocessing has produced encouraging results, the possible chemical and radiolytic degradation, mechanistic aspects of extraction are yet to be studied in detail. The major factor, which raises concern over the use of IL, is the chemical stability in aqueous medium. Various authors have studied the physicochemical and thermodynamic properties of amimPF6. The solubility of bmimPF6 in water has been reported to be 1.8 wt%. The solubility was found to decrease by lengthening alkyl chain attached to imidazole or by replacing the anion with other bulkier anions (e.g. triflate imide). Nevertheless, the physical and chemical properties of ionic liquids need to be investigated from the reprocessing point of view for meeting the qualities of diluent in solvent extraction processes when ILs are put into use. This communication deals with some physical and chemical properties of the TBP/amimPF6 that are essential for the qualifying as diluent in reprocessing. The properties such as organic phase density, viscosity, phase disengagement time, and chemical degradation of amimPF6 and TBP/amimPF6 were investigated. The results are compared with the data on TBP/DD.