PATh Octanol-Water Partition Coefficients of (Imidazolium-based) Ionic Liquids R. L. Gardas, M. G. Freire, I. M. Marrucho, J. A. P. Coutinho CICECO, Departamento de Química, Universidade de Aveiro, Aveiro, Portugal 1. Introduction Ionic liquids (ILs) are a novel class of chemical compounds that are driving a lot of research in several fields due to their unique and inherent properties. Since they present negligible vapour pressures they cannot contribute to air pollution and an increasing attention is being given to ILs as possible substitutes for volatile organic solvents. However their relatively solubility in common fluids could lead to their dispersion into the environment through liquid effluents and then into soils and seawater. Besides the hydrophobic ILs even the known “hydrophobic” ILs present reasonably solubilities in water and their effect on aquatic organisms is starting to be investigated 1-3 and therefore it is important to understand how ILs will influence the aquatic ecosystems. A key parameter in the assessment of environmental risk and in the prediction of the fate of chemicals in the environment is the octanol–water partition coefficient (K OW ) that is a measure of the bioconcentration tendency of a chemical in a hydrologic cycle. Kow describes the hydrophobicity or hydrophilicity tendency of a certain compound and it is the basis of correlations to calculate bioaccumulation and toxicity in fish, as well as sorption to soils and sediments. In this work, we have measured octanol-water partition coefficients of several imidazolium based ionic liquids at room temperature by the slow-stirring method. 4-5 The octanol–water partition coefficient commonly reported is the total concentration of salt in the octanol phase divided by the total concentration in the water phase: 2. Materials 3. Experimental The K OW at room temperature was determined for several imidazolium based ILs, using the [bmim][Tf 2 N] to validate the experimental method: The [bmim][Tf 2 N] and [omim][BF 4 ] were aquired at IoLiTec with purities >99%.The [hmim][PF 6 ] was acquired at Merck with a purity ≥99 % and chloride content ≤100 ppm. The [omim][PF 6 ] and [bdmim][PF 6 ] were acquired at Solchemar with purities >99%. The chloride content in both ILs is <80 ppm. The 1-octanol was acquired from Fluka with a purity ≥99.5 % (GC). The water used was double distilled, passed by a reverse osmosis system and further treated with a Milli-Q plus 185 water purification apparatus. It has a resistivity of 18.2 MΩ∙cm, a TOC smaller than 5µg∙L -1 and it is free of particles greater than 0.22 µm. In this study, the slow-stirring method 4,5 was used because it is a direct method for measuring K OW with accurate results over a wide range of values without the need for complex equipment. The apparatus consisted of a 120 mL, with a 4.4 cm diameter and 9.0 cm in height, glass vial containing a 1 cm Teflon coated magnetic stirrer. The glass vials contain two caps, one on top and another at side bottom, were screw caps sealed with a septum made of 90 mil silicone covering 10 ml Teflon. A figure of the experimental apparatus is shown in Figure 1. Figure 1. Cell used for the slow-stirring method Figure 2. SHIMADZU UV-1700 Pharma-Spec Spectrometer Approximately 45 mL of distilled, deionized water presaturated with 1-octanol was introduced in the vial and an equal volume of water saturated with octanol containing a known amount of IL, was carefully added to the vial to avoid emulsification. The caps are then tightened to prevent octanol or water evaporation. The vials were stirred slowly to prevent emulsification and were maintained at room temperature (24±2 o C). To check for the influence of the initial ionic liquid concentration on the measured K OW multiple samples with different initial concentrations were used. Three vials for the same initial concentration of IL were used for each measurement. Samples were collected from the octanol-rich phase with a syringe from top cap. Similarly samples from the water-rich phase were collected from side bottom cap. Each phase was sampled from all vials during at least three sampling events occurring over a 15 to 30 day period. Sampling ceased when the concentrations in both phases stabilized. Concentrations of IL in each phase were measured at 211 nm wavelength using UV-vis spectroscopy (SHIMADZU UV-1700 Pharma-Spec Spectrometer) shown in Figure 2. If necessary the samples taken from the vials were diluted so that the measured absorbance was below 1.0. Ionic Liquid ε in water saturated with octanol / (L∙mol -1 ∙cm -1 ) ε in octanol saturated with water / (L∙mol -1 ∙cm -1 ) [bmim][Tf 2 N]4239 (4407±20 5 )4728 (4582±114 5 ) [hmim][PF 6 ] [omim][PF 6 ] [bmmim][PF 6 ] [omim][BF 4 ] Table 1. Extinction coefficients, ε /(L∙mol -1 ∙cm -1 ) of imidazolium based ionic liquids in water and octanol saturated rich phases (at λ=211 nm) Table 2. Octanol–water partition coefficients, K OW, of the investigated Ionic Liquids and the concentration range studied Ionic LiquidKow Starting conc. of IL in octanol saturated with water / (L∙mol -1 ) [bmim][Tf 2 N] × ×10 -3 [bmim][Tf 2 N] (1.5× ×10 -3 ) 5 [hmim][PF 6 ] × ×10 -3 [omim][PF 6 ] ×10 -3 [bmmim][PF 6 ] × ×10 -3 [omim][BF 4 ] × × Conclusions  The octanol–water partition coefficients of the investigated imidazolium based ionic liquids range between 0.12 and 1.22 at room temperature and seem to be concentration dependent for the ionic liquids studied.  The values are lowest for the most hydrophilic ionic liquids and increase with the cation alkyl chain length increase. Since all of the K OW values are very small, we can conclude that these ILs will not accumulate or concentrate in the biota.  The K OW s of other ionic liquids of the imidazolium family and other families are currently under study in our lab. References 1.J. Ranke, K. Molter, F. Stock, U. Bottin-Weber, J. Poczobutt, J. Hoffmann, B. Ondruschka, J. Filser and B. Jastorff, Ecotoxicol. Environ. Saf., 2004, 58, 3, F. Stock, J. Hoffmann, J. Ranke, R. Stormann, B. Ondruschka and B. Jastorff, Green Chem., 2004, 6, B. Jastorff, R. Stormann, J. Ranke, K. Molter, F. Stock, B. Oberheitmann, W. Hoffmann, J. Hoffmann, M. Nuchter, B. Ondruschka and J. Filser, Green Chem., 2003, 5, D. N. Brooke, A. J. Dobbs and N. Williams, Ecotoxicol. Environ. Saf., 1986, 11, L. Ropel, L. S. Belveze, S. N. V. K. Aki, M. A. Stadtherr and J. F. Brennecke, Green Chem., 2005, 7, 83. Acknowledgements This work was supported by Fundação para a Ciência e a Tecnologia (Project POCI/EQU/58152/2004). R. L. Gardas and M. G. Freire acknowledge the financial support from Fundação para a Ciência e a Tecnologia through, respectively, their post-doctoral (SFRH/BPD/23246/2005) and PhD. (SFRH/BD/14134/2003) scholarships. Extinction coefficients, ε, of the ILs in octanol and water are required to determine the concentration of IL in each rich phase by interpolation with calibration curves, and therefore the resulting K OW values. So, extinction coefficients, ε /(L∙mol -1 ∙cm -1 ), of each studied IL were measured and are reported in Table 1. Also shown in Table 1 are the extinction coefficients of [bmim][Tf 2 N] 5 to validate our experimental technique when measuring the K OW, being in good agreement with the reported literature. The obtained K OW values measured in this work are reported in Table Results 1-butyl-3-methylimidazolium bis(trifluoromethansulfonyl)imide 3-methyl-1-octyl-imidazolium hexafluorophosphate - + [omim][PF 6 ] [omim][BF 4 ] 3-methyl-1-octyl-imidazolium tetrafluoroborate butyl-2,3-dimethyl-imidazolium hexafluorophosphate [bmmim][PF 6 ] hexyl-3-methyl-imidazolium hexafluorophosphate [hmim][PF 6 ] +- [bmim][Tf 2 N] Figure 3. Octanol–water partition coefficients, K OW, of the investigated Ionic Liquids in the concentration order of mol∙L -1 Figure 3 and 4 present a general comparison between the obtained K OW for all the ILs studied in the most and less concentrated starting solutions, respectively. Figure 4. Octanol–water partition coefficients, K OW, of the investigated Ionic Liquids in the concentration order of mol∙L -1