Octanol-Water Partition Coefficients of
(Imidazolium-based) Ionic Liquids
PATh
R. L. Gardas, M. G. Freire, I. M. Marrucho, J. A. P. Coutinho
CICECO, Departamento de Química, Universidade de Aveiro, Aveiro, Portugal
4. Results
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 investigated1-3 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 (KOW) 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
The KOW at room temperature was determined for several imidazolium based ILs, using the [bmim][Tf2N] to validate the
experimental method:
[hmim][PF6]
[bmim][Tf2N]
-
+
+
1-butyl-3-methylimidazolium bis(trifluoromethansulfonyl)imide
1-hexyl-3-methyl-imidazolium hexafluorophosphate
[bmmim][PF6]
+
+
-
3-methyl-1-octyl-imidazolium hexafluorophosphate
[omim][BF4]
+
-
1-butyl-2,3-dimethyl-imidazolium hexafluorophosphate
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][Tf2N]
4239 (4407±205)
4728 (4582±1145)
[hmim][PF6]
4228
4769
[omim][PF6]
4322
4220
[bmmim][PF6]
5548
6704
[omim][BF4]
4242
4149
Table 2. Octanol–water partition coefficients, KOW, of the investigated Ionic Liquids and the
concentration range studied
[omim][PF6]
-
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)
-
3-methyl-1-octyl-imidazolium tetrafluoroborate
The [bmim][Tf2N] and [omim][BF4] were aquired at IoLiTec with purities >99%.The [hmim][PF6] was acquired at Merck with a
purity ≥99 % and chloride content ≤100 ppm. The [omim][PF6] and [bdmim][PF6] 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.
Ionic Liquid
Kow
Starting conc. of IL in
octanol saturated with
water / (L∙mol-1)
[bmim][Tf2N]
0.43-0.66
1.23×10-3 -2.46×10-3
[bmim][Tf2N]11
0.11-0.625
(1.5×10-4 -2.2×10-3)5
[hmim][PF6]
0.12-0.34
3.07×10-4-1.38×10-3
[omim][PF6]
1.21
3.24×10-3
[bmmim][PF6]
0.61-1.16
3.25×10-4 -1.67×10-3
[omim][BF4]
0.58-1.22
3.44×10-4-1.77×10-3
Figure 3 and 4 present a general comparison between the obtained KOW for all the ILs
studied in the most and less concentrated starting solutions, respectively.
1.40
1.20
1.00
KOW
K OW
C

C
O
IL
W
IL
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 KOW 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][Tf2N]5 to validate our experimental
technique when measuring the KOW, being in good agreement with the reported
literature. The obtained KOW values measured in this work are reported in Table 2.
0.80
0.60
3. Experimental
In this study, the slow-stirring
0.40
0.20
method4,5
0.00
was used because it is a direct method for measuring
bmimTF2N
hmimPF6
omimPF6
bmmimPF6
omimBF4
KOW 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
Figure 3. Octanol–water partition coefficients, KOW, of the investigated Ionic Liquids in the concentration
order of 10-3 mol∙L-1
height, glass vial containing a 1 cm Teflon coated magnetic stirrer. The glass vials contain two
0.70
caps, one on top and another at side bottom, were screw caps sealed with a septum made of
0.60
90 mil silicone covering 10 ml Teflon. A figure of the experimental apparatus is shown in
0.50
Figure 1.
KOW
0.40
Figure 1. Cell used for the slow-stirring method
0.30
Approximately 45 mL of distilled, deionized water presaturated with 1-octanol was
0.20
introduced in the vial and an equal volume of water saturated with octanol containing a
0.10
known amount of IL, was carefully added to the vial to avoid emulsification. The caps are
0.00
then tightened to prevent octanol or water evaporation. The vials were stirred slowly to
hmimPF6
bmmimPF6
omimBF4
prevent emulsification and were maintained at room temperature (24±2 oC).
Figure 4. Octanol–water partition coefficients, KOW, of the investigated Ionic Liquids in the concentration
order of 10-4 mol∙L-1
To check for the influence of the initial ionic liquid concentration on the measured
KOWmultiple samples with different initial concentrations were used. Three vials for the
5. Conclusions
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
Figure 2. SHIMADZU UV-1700
Pharma-Spec Spectrometer
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.
References
1.
2.
3.
4.
5.
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, 396.
F. Stock, J. Hoffmann, J. Ranke, R. Stormann, B. Ondruschka and B. Jastorff, Green Chem., 2004, 6, 286.
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, 136.
D. N. Brooke, A. J. Dobbs and N. Williams, Ecotoxicol. Environ. Saf., 1986, 11, 251.
L. Ropel, L. S. Belveze, S. N. V. K. Aki, M. A. Stadtherr and J. F. Brennecke, Green Chem., 2005, 7, 83.
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 KOW values are very small, we can
conclude that these ILs will not accumulate or concentrate in the biota.
The KOWs of other ionic liquids of the imidazolium family and other families are
currently under study in our lab.
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.