Kow of Imidazolium based ILs

1
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
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
octanolwater 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
octanolwater partition coefficient commonly
reported is the total concentration of salt in
the octanol phase divided by the total
concentration in the water phase
Extinction coefficients, e, 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, e /(Lmol-1cm-1), of
each studied IL were measured and are reported in
Table 1. Also shown in Table 1 are the extinction
coefficients of bmimTf2N5 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.
Table 1. Extinction coefficients, e
/(Lmol-1cm-1) of imidazolium based ionic
liquids in water and octanol saturated rich
phases (at ?211 nm)
2. Materials
The KOW at room temperature was determined for
several imidazolium based ILs, using the
bmimTf2N to validate the experimental method
Table 2. Octanolwater partition coefficients,
KOW, of the investigated Ionic Liquids and the
concentration range studied
bmimTf2N
1-butyl-3-methylimidazolium bis(trifluoromethansul
fonyl)imide
3-methyl-1-octyl-imidazolium hexafluorophosphate
1-hexyl-3-methyl-imidazolium hexafluorophosphate
1-butyl-2,3-dimethyl-imidazolium
hexafluorophosphate
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.
The bmimTf2N and omimBF4 were aquired at
IoLiTec with purities gt99.The hmimPF6 was
acquired at Merck with a purity 99 and
chloride content 100 ppm. The omimPF6 and
bdmimPF6 were acquired at Solchemar with
purities gt99. The chloride content in both ILs
is lt80 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 MOcm, a TOC smaller than
5µgL-1 and it is free of particles greater than
0.22 µm.
3. Experimental
In this study, the slow-stirring method4,5 was
used because it is a direct method for measuring
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 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 3. Octanolwater partition coefficients,
KOW, of the investigated Ionic Liquids in the
concentration order of 10-3 molL-1
Figure 1. Cell used for the slow-stirring method
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 (242 oC). 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 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.
Figure 4. Octanolwater partition coefficients,
KOW, of the investigated Ionic Liquids in the
concentration order of 10-4 molL-1
5. Conclusions

• The octanolwater 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.

Figure 2. SHIMADZU UV-1700 Pharma-Spec
Spectrometer
References
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.

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