Ionic liquids/water distribution ratios of some polycyclic aromatic hydrocarbons

Article abstract of DOI:10.1021/je049879e

By using the shake-flask procedure, the distribution ratios (D) at infinite dilution and 298.1 K of 15 polycyclic aromatic hydrocarbons (PAHs) between room-temperature ionic liquids, l-alkyl-3-methylimiazolium hexafluorophosphates ([C_nMIM] [PF<

Full text of DOI:10.1021/je049879e

1422
J . Chem. Eng. Data 2004, 49, 1422-1424
Ion ic Liqu id s/Wa ter Distr ibu tion Ra tios of Som e P olycyclic
Ar om a tic Hyd r oca r bon s
J in g-fu Liu ,† Yu -gu a n g Ch i,† J in -fen g P en g,† Gu i-bin J ia n g,*,† a n d J a n A° k e J o1n sson ‡
Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for Eco-Environmental
Sciences, Chinese Academy of Sciences, P.O. Box 2871, Beijing 100085, China, and Department of Analytical
Chemistry, Lund University, P.O. Box 124, S-221 00 Lund, Sweden
By using the shake-flask procedure, the distribution ratios (D) at infinite dilution and 298.1 K of 15
polycyclic aromatic hydrocarbons (PAHs) between room-temperature ionic liquids, 1-alkyl-3-methylimi-
azolium hexafluorophosphates ([CnMIM][PF6], n ) 4 and 8), and water were determined. The log D values
are in the range of 3.34-4.36, which increased very slowly with the molar mass of PAHs.
In tr od u ction
[PF6] and determined the [C4MIM][PF6]/water and [C4-
MIM][PF6]/heptane distribution ratios for a set of 40
compounds, including organic acids, organic bases, amino
acids, antioxidants, and neutral compounds.
Room-temperature ionic liquids (ILs) are gaining wide
recognition as potential environmental benign solvents.
Some unique properties of ILs such as the negligible vapor
pressure, low flammability, tuneable viscosity, and the
miscibility in water and other organic solvents merit their
consideration as an alternative recyclable medium for
synthesis, catalysis, and separation.1,2 ILs were recently
considered as attractive water-immiscible phases for liquid-
liquid extraction of target substances; however, the very
limited physicochemical data such as the distribution ratios
of solutes between ILs and water has delayed their ap-
plication.
The D values of these above target compounds in the
ILs/water system reported in reference were determined
using the traditional procedure, i.e., aliquots of ILs (1-3
mL) were mixed with aqueous solutions and vortexed for
the prescribed time, then the mixtures were centrifuged
to separate the two phases, and the compounds in the two
phases were determined by radio analysis,9 atomic absorp-
tion,7 or high-performance liquid chromatography (HPLC)12
measurement. Finally the distribution ratio was calculated
according to the concentration ratio between the two
phases. However, this method suffered from the fact that
it is impossible to determine the distribution ratio of very
hydrophobic compounds such as polycyclic aromatic hy-
drocarbons (PAHs) and polychlorinated biphenyls (PCBs)
by this procedure.
Dai et al.3 studied the extraction of Sr2+ from an aqueous
phase into disubstituted imidazolium hexafluorophos-
phates and 1,1,1-trifluo-N-[(trifluoromethyl)sulfonyl]meth-
anesulfonamide ILs by dicyclohexano-18-crown-6, and it
was found that distribution ratios (D) values in ILs with
1,1,1-trifluo-N-[(trifluoromethyl)sulfonyl]methanesulfon-
amide are much larger than those with hexafluorophos-
phates. Later, the D values of Na+, Cs+, and Sr2+ in 1-alkyl-
3-methylimiazolium hexafluorophosphates ([CnMIM][PF6],
n ) 4, 6, and 8)/water with crown ethers as extractant were
Currently, the shake-flask and slow-stirring methods are
widely applied to determine the distribution coefficient in
octanol/water systems (Kow).13-16 The slow-stirring method
has been shown to be applicable for the determination of
reported.4,5 Heavy metal ions such as Cd2+, Co2+, Ni2+
,
K
ow values of very hydrophobic compounds, as slow stirring
Pb2+, Cu2+, Ag+, and Fe3+ and actinides in the [C4MIM]-
[PF6]/water system with various organic and inorganic
extractants6-8 were also determined. Visser et al.9 prepared
a series of task-specific ILs by appending urea-, thiourea-,
and thioether-substituted alkyl groups to imidazoles and
determined the D values of Hg2+ and Cd2+ between the
task-specific ILs and water.
can prevent the formation of microemulsion of octanol in
the water phase. In the shake-flask procedure, however,
microemulsion of octanol forms in the water phase and thus
prevents the reliable determination of compounds with
large Kow values.13
In this present study, the distribution ratio at infinite
dilution (D) of some PAHs between [CnMIM][PF6] (n ) 4
and 8) and water were determined. Since the IL phase and
the water phase can be separated easily after mixing and
shaking, the shake-flask procedure was adopted in this
study.
ILs have also been applied to extract organic compounds
from aqueous solutions. The D values of a series of organic
compounds in [C4MIM][PF6]/water system were deter-
mined, and it was reported that the D values in octanol/
water are in general an order of magnitude higher than
those in [C4MIM][PF6]/water.10 While Abraham et al.11
calculated and determined the D values for some organic
compounds in ILs/water systems, Carda-Broch et al.12
investigated in detail the solvent properties of [C4MIM]-
Exp er im en ta l Section
Ma ter ia ls. A mixed PAHs standard solution containing
the 16 American Environmental Protection Agency (EPA)
priority PAHs (shown in Table 1) was purchased from
AccuStandard Inc. (New Haven, CT). 1-Methylimidazole
(99%), 1-chlorobutane (99%), 1-chloroctane (99%), and
* To whom correspondence should be addressed. E-mail: gbjiang@
mail.rcees.ac.cn. Fax: 8610 62849339. Tel: 8610 62849334.
† Chinese Academy of Sciences.
‡ Lund University.
10.1021/je049879e CCC: $27.50 © 2004 American Chemical Society
Published on Web 08/19/2004

J ournal of Chemical and Engineering Data, Vol. 49, No. 5, 2004 1423
Ta ble 1. F lu or escen ce Detection Wa velen gth (F L) a n d th e Deter m in ed Ion ic Liqu id s/Wa ter Distr ibu tion Ra tios (log D)
(Mea n ( Sta n d a r d Devia tion , 3 Mea su r em en ts) a t In fin ite Dilu tion a t 298.1 K of Som e P AHs
FL (nm)
excitation
log D
a
compounds
naphthalene
acenaphthylene
acenaphthene
fluorene
phenanthrene
anthracene
fluoranthene
pyrene
benz[a]anthracene
chrysene
benzo[b]fluoranthene
benzo[k]fluoranthene
benzo[a]pyrene
dibenz[a,h]anthracene
benzo[ghi]perylene
indenol[1,2,3cd]pyrene
emission
log Kow
[C4MIM][PF6]/water
[C8MIM][PF6]/water
218
218
226
226
250
250
234
234
286
286
250
250
294
298
298
246
357
357
359
359
350
425
440
440
405
405
420
420
460
420
420
490
3.24 to 3.40
4.33
3.34 ( 0.06
NDb
3.51 ( 0.07
ND
4.07
4.18
3.71 ( 0.07
3.82 ( 0.15
4.06 ( 0.12
4.15 ( 0.10
4.20 ( 0.13
4.21 ( 0.11
4.22 ( 0.15
4.22 ( 0.14
4.31 ( 0.17
4.30 ( 0.20
4.32 ( 0.18
4.30 ( 0.21
4.31 ( 0.22
4.36 ( 0.17
3.86 ( 0.16
3.97 ( 0.19
4.14 ( 0.07
4.12 (0.33
4.23 ( 0.17
4.21 ( 0.20
4.17 ( 0.14
4.23 ( 0.18
4.26 ( 0.18
4.22 ( 0.19
4.32 ( 0.16
4.23 ( 0.18
4.30 ( 0.18
4.31 ( 0.18
4.46 to 4.64
4.55 to 4.79
5.12 to 5.31
5.0 to 5.18
5.74 to 6.04
5.63 to 5.94
5.78
5.86 to 6.28
5.91 to 6.28
6.5
5.95 to 6.38
7.04
a
b
References 16, 20, and 21. Not detected.
hexafluorophosphoric acid (60 (m/m)% solution in water)
were obtained from Acros Organics. HPLC-grade methanol
and acetonitrile were purchased from Scharlace Chemie
SA, Barcelona, Spain. All other chemicals were of analytical
grade (Beijing Chemicals Corporation, Beijing, China), and
ultrapure water (EASYpure LF) was used throughout.
[PF6]/water system and 1 mL for the [C8MIM][PF6] system,
respectively.
The PAH concentrations in IL and aqueous phases were
determined by direct injection of 20 µL of the above
prepared solutions into the HPLC system and quantified
by external standard method. The HPLC equipment used
was an Agilent 1100LC including a gradient (BinPump)
pump, a UV/Vis (VWD) detector set at 254 nm wavelength,
and a fluorescence (FLD) detector. The excitation and the
emission wavelength of the fluorescence detector were set
as shown in Table 1. HPLC separation of PAHs was
conducted by using a C18 analytical column (Inertsil ODS-
P, 250 mm × 4.6 mm inside diameter, 5 µm particles, GL
Sciences, Inc., J apan) and a mixture of acetonitrile and
water as mobile phase at a flow rate of 1.0 mL/min. The
gradient profile was 55 (v/v)% acetonitrile for (0-5) min,
increase to 60 (v/v) % acetonitrile during (5-13) min and
keep to 20 min, then increase to 80 (v/v)% acetonitrile
during (20-25) min, 90 (v/v) % acetonitrile during (25-
28) min, and 100 (v/v)% acetonitrile during (28-30) min.
The 100 (v/v)% acetonitrile was kept to 70 min and
decreased to 55 (v/v)% acetonitrile in 5 min.
Appr a tu s a n d P r oced u r e. The synthesis and some
physicochemical properties of [CnMIM][PF6] (n ) 4 and 8)
ILs were described in our previous study.17 Briefly, [Cn-
MIM]Cl (n ) 4 and 8) was prepared by adding equal
amounts (0.2 mol) of 1-methylimidazole and 1-chlorobutane
or 1-chloroctane to a round-bottomed flask fitted with a
reflux condenser and reacting for 48 h at 343 K. [CnMIM]-
[PF6] (n ) 4 and 8) mixtures were prepared by slowly
adding hexafluorophosphoric acid (0.13 mol) to each of their
corresponding chlorides (0.1 mol) in 100 mL of water. After
stirring for 12 h, the lower liquid portion was washed with
water until the washings were no longer acidic. The ILs
were cleaned up with Al2O3 and dried under a vacuum at
353 K. The obtained two ILs were colorless viscous liquids,
and the determined density (293.1 K) and viscosity (298.1
K) were 1.37 g/mL and 393 mPa‚s for [C4MIM][PF6] and
1.20 g/mL and 710 mPa‚s for [C8MIM][PF6], respectively.
The chloride and water content were 107 mg/L and 920
mg/kg for [C4MIM][PF6] and 102 mg/L and 366 mg/kg for
[C8MIM][PF6], respectively.
In HPLC determination, blank ILs gave many matrix
peaks when detected by a UV detector, but this was much
improved by using a fluorescence detector. As the concen-
trations of PAHs are relatively high, all the studied PAHs
(except acenaphthylene) can be easily identified and quan-
tified. The concentrations of naphthalene, acenaphthene,
and fluorine were quantified based on the peak area
obtained with the UV detection, while the other PAHs were
quantified based on the peak area of fluorescence detection.
ILs and water were presaturated with each other by
adding 50 mL of IL and 900 mL of water in a 1000-mL
flask and shaking occasionally for 48 h at (298.1 ( 0.2) K.
Then the two phases were separated and collected. Fol-
lowing the presaturation, the D value of PAHs in the IL-
water system were determined between the water-satu-
rated IL and the IL-saturated water. To a 500-mL flask
were added 4 mL of water-saturated IL and 0.02 mg each
of PAHs (100 µL of 200 mg/L mixed PAHs standard), and
then 400 mL of IL-saturated water was added. After
shaking and mixing, the mixtures were kept at (298.1 (
0.2) K with occasional shaking for 48 h. Then, the IL and
the aqueous phases were separated. The PAHs concentra-
tion in the IL phase can be determined directly. For
determining the PAHs concentration in the aqueous phase,
the aqueous phase was extracted twice and each time with
30 mL of dichloromethane. The extractants were combined,
the dichloromethane solvent was removed under a gentle
stream of nitrogen, and the residue was dissolved by adding
methanol to obtain a total volume of 5 mL for the [C4MIM]-
The distribution ratio at infinite dilution of a PAH
between [CnMIM][PF6] (n ) 4 and 8) and water (D,
dimensionless) was calculated from the ratio of its concen-
tration (mg/L) in the IL phase (CIL) to that in the aqueous
phase (Caq
)
D ) CIL/Caq
Three separate measurements for the D values were
made by using the above-described procedures. Reported
values of log D are the arithmetic average of the three
independent measurements, and the standard deviation of
these replicate measurements was taken to be the estimate
of the uncertainty in the reported mean value.

1424 J ournal of Chemical and Engineering Data, Vol. 49, No. 5, 2004
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Table 1 shows the measured log D values of 15 PAHs in
the [C4MIM][PF6]/water and the [C8MIM][PF6]/water sys-
tems at infinite dilution at 298.1 K as well as their log Kow
values. As can be seen, the log D values of low molar mass
PAHs, including naphthalene, acenaphthene, and fluorene
in the [C4MIM][PF6]/water system are lower than that in
the [C8MIM][PF6]/water system, while there are no sig-
nificant differences between the values in these two
systems for the other studied PAHs. The log D value of
fluoranthene in the [C4MIM][PF6]/water system deter-
mined in this study (4.20 ( 0.13) is very close to the value
(4.22) theoretically calculated by Abraham et al.11 Accord-
ing to their calculated result, the log D value of fluoran-
thene in the [C6MIM][PF6]/water system is 3.75. This is
lower than our experimentally determined values in the
[C4MIM][PF6]/water system (4.20 ( 0.13) and [C4MIM]-
[PF6]/water system (4.23 ( 0.17). This result is reasonable
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Table 1 also indicates that, in contrast to the log Kow
,
which increased significantly with the molar mass of PAHs,
the log D value for [CnMIM][PF6]/water (n ) 4 and 8)
increased very slowly with molar mass for the high
molecular PAHs. Thus, while the D value for [CnMIM]-
[PF6]/water (n ) 4 and 8) is almost the same as the Kow for
naphthalene, the D values are in general 1-3 orders of
magnitude lower than the Kow values for high molar mass
PAHs. This is not in agreement with the result of Hud-
dleston et al.,6 who reported that the distribution ratios in
the [C4MIM][PF6]/water system are in general an order of
magnitude lower than those in octanol/water. However, our
results are reasonable as the polarity of [C4MIM][PF6] was
reported to be in the same region of lower alcohols such as
methanol and ethanol,11,19 and generally the solubility of
PAHs in ethanol decreased with the increasing of molar
mass. For example, the solubility of anthracene in ethanol
(15 g/L) is much less than that of naphthalene (77 g/L).
Therefore, it is expected that the change of PAHs solubility
in [CnMIM][PF6] (n ) 4 and 8) should have a similar
tendency as in ethanol, and the increase of D in the [Cn-
MIM][PF6] (n ) 4, 8)/water system should not be as
significant as Kow
.
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Received for review March 29, 2004. Accepted J uly 14, 2004. This
work was jointly supported by the National Natural Science
Foundation of China (20377045, 20177026), and the National Key
Project for Basic Research (2002CB412308).
J E049879E