PATh

Octanol-Water Partition Coefficients of (Imidazolium-

based) Ionic LiquidsR. L. Gardas, M. G. Freire, I. M. Marrucho, J. A. P. Coutinho

CICECO, Departamento de Química, Universidade de Aveiro, Aveiro, Portugal

1. IntroductionIonic 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

3. Experimental

The KOW at room temperature was determined for several imidazolium based ILs, using the [bmim][Tf2N] to

validate the experimental method:

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.

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 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 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.

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 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, KOW, of the investigated Ionic

Liquids and the concentration range studied

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

5. ConclusionsThe 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.References1. 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, 396.2. F. Stock, J. Hoffmann, J. Ranke, R. Stormann, B. Ondruschka and B. Jastorff, Green Chem., 2004, 6, 286.3. 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,

136.4. D. N. Brooke, A. J. Dobbs and N. Williams, Ecotoxicol. Environ. Saf., 1986, 11, 251.5. L. Ropel, L. S. Belveze, S. N. V. K. Aki, M. A. Stadtherr and J. F. Brennecke, Green Chem., 2005, 7, 83.

AcknowledgementsThis 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 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.

4. Results

1-butyl-3-methylimidazolium bis(trifluoromethansulfonyl)imide 3-methyl-1-octyl-imidazolium hexafluorophosphate

-+

[omim][PF6]

[omim][BF4]

3-methyl-1-octyl-imidazolium tetrafluoroborate

+ -

1-butyl-2,3-dimethyl-imidazolium hexafluorophosphate

[bmmim][PF6]

+ -

1-hexyl-3-methyl-imidazolium hexafluorophosphate

[hmim][PF6]

+ -+ -+ -[bmim][Tf2N]

WIL

OIL

OW C

CK

Figure 3. Octanol–water partition coefficients, KOW, of the investigated Ionic Liquids in the

concentration order of 10-3 mol∙L-1

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.

Figure 4. Octanol–water partition coefficients, KOW, of the investigated Ionic Liquids in the

concentration order of 10-4 mol∙L-1

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

bmimTF2N hmimPF6 omimPF6 bmmimPF6 omimBF4

KOW

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

hmimPF6 bmmimPF6 omimBF4

KOW