Octanol/water partition coefficients of pyridinium-based ionic liquids

Article abstract of DOI:10.14233/ajchem.2015.18088

Nine pyridinium-based ionic liquids were synthesised, which include 1-butylpyridinium bromide ([BPYR] [Br]), 1-hexylpyridinium bromide ([HPYR][Br]), 1-octylpyridinium bromide ([OPYR][Br]), 1-hexylpyridinium bis(trifluoromethylsulfonyl)imide ([HPYR] [NTf₂]), 1-octylpyridinium bis(trifluoromethylsulfonyl)imide ([OPYR][NTf₂]), 1-hexylpyridinium trifluoromethanesulfonate ([HPYR] [TFO]), 1-octylpyridinium trifluoromethanesulfonate ([OPYR][TFO]), 1-hexylpyridinium tetrafluoroborate ([HPYR] [BF₄]), 1-octylpyridinium I tetrafluoroborate ([OPYR][BF₄]). The octanol/water partition coefficients (K_OWs) of these pyridinium-based ionic liquids were determined by shake-flask method. It is found that the pyridinium-based ionic liquids measured in this work are extremely hydrophilic except for [OPYR] [NTf₂]. It is also found that K_OWs of [NTf₂] ionic liquids were dependent on the concentration and high ionic liquids concentration in water leads to big K_OW.

Full text of DOI:10.14233/ajchem.2015.18088

Asian Journal of Chemistry; Vol. 27, No. 8 (2015), 2819-2822
A
SIAN
J
OURNAL OF HEMISTRY
C
http://dx.doi.org/10.14233/ajchem.2015.18088
Octanol/Water Partition Coefficients of Pyridinium-Based Ionic Liquids
*
TENG ZHANG and YAQUAN WANG
Key Laboratory for Green Chemical Technology of the Ministry of Education, School of Chemical Engineering and Technology, Tianjin
University, Tianjin 300072, P.R. China
*
Corresponding author: Tel/Fax: +86 22 23507881; E-mail: yqwang@tju.edu.cn
Received: 27 May 2014; Accepted: 6 August 2014;
Published online: 27 April 2015;
AJC-17139
Nine pyridinium-based ionic liquids were synthesised, which include 1-butylpyridinium bromide ([BPYR] [Br]), 1-hexylpyridinium
bromide ([HPYR][Br]), 1-octylpyridinium bromide ([OPYR][Br]), 1-hexylpyridinium bis(trifluoromethylsulfonyl)imide ([HPYR] [NTf ]),
]), 1-hexylpyridinium trifluoromethanesulfonate ([HPYR] [TFO]),
-octylpyridinium trifluoromethanesulfonate ([OPYR][TFO]), 1-hexylpyridinium tetrafluoroborate ([HPYR] [BF ]), 1-octylpyridinium
]). The octanol/water partition coefficients (KOWs) of these pyridinium-based ionic liquids were determined
by shake-flask method. It is found that the pyridinium-based ionic liquids measured in this work are extremely hydrophilic except for
OPYR] [NTf ]. It is also found that KOWs of [NTf ] ionic liquids were dependent on the concentration and high ionic liquids concentration
in water leads to big KOW
2
1
-octylpyridinium bis(trifluoromethylsulfonyl)imide ([OPYR][NTf
2
1
4
tetrafluoroborate ([OPYR][BF
4
[
2
2
.
Keywords: Ionic liquids, Octanol-water partition coefficients, Pyridinium.
environments may lead to water pollution. 1-Octanol/water
INTRODUCTION
partition coefficient (KOW) is a key parameter in determining
the distribution and fate of compounds in the environment. It
basically gives direct information on the hydrophobicity of a
compound. Some papers have reported KOW of ionic liquids.
Ionic liquids (ILs) are a novel class of room temperature
molten salts with melting points near ambient temperature. In
recent years, ionic liquids have attracted an increasing attention
in the chemical industry for their attractive properties such as
negligible vapour pressure, thermomechanical and electroche-
mical stability, a wide temperature range for liquids, solvating
properties for diverse materials, wide electrochemical window.
Furthermore, the possible combinations between cations and
anions lead to an enormous number of viable ionic liquids.
As one kind of ionic liquids, pyridinium based ionic
liquids, whose structures are shown in Fig. 1, have been widely
researched for their multiplicate uses. Firstly, they have an
active role in chemical reactions and catalysis, which is one
of the most important applications of pyridinium based ionic
12
Ropel et al. reported the KOW of 12 imidazolium-based ionic
liquids and found that these ionic liquids will not accumulate
or concentrate in the environment for the KOW values are very
13
small. Lee et al. investigated the KOW of 1-butyl-3-methylimi-
dazolium ionic liquids containing either hexafluorophosphate
or bis(trifluoromethyl)sulfonylamide over a wide range of ionic
liquid concentrations and the log KOW value of [bmim][Tf N]
2
(2.06) was found to be similar to those of organic solvents such
as butyl ethyl ether (2.0), chloroform (2.0) and benzene (2.1).
14
Chapeaux et al. predicted KOW for 15 imidazolium, pyridinium
and quaternary ammonium ionic liquids using the nonrandom
two liquid (NRTL) and electrolyte nonrandom two liquid (eNRTL)
1
-6
liquids . For their capacity of dissolving a variety of solutes,
pyridinium based ionic liquids were found effective for extrac-
15
excess Gibbs energy models. de los Ríos et al. determined KOW
of 6 imidazolium-based ionic liquids and the KOW value was
used as a key parameter for the selective behaviour of the
supported liquid membranes. However, most of the researches
focus on imidazolium based ionic liquids and few KOWs data of
pyridinium-based ionic liquids were determined by experiment.
In this work, we have synthesised nine pyridinium-based
ionic liquids and determined the KOW of these ionic liquids by
shake-flask method.
7,8
9,10
tion processes and widely used as reaction solvent . For
the properties of low viscosity and high conductivities pyridi-
11
nium based ionic liquids were applied in electrolytic media .
Ionic liquids are called environmental friendly solvents
for their negligible vapour pressures, which may reduce the air
pollution with respect to the typical volatile organic solvents.
Although ionic liquids can lessen the risk of air pollution, a
release of ionic liquids from industrial processes into aquatic

2
820 Zhang et al.
Asian J. Chem.
(
2H, t, CHpyr-m), 4.620-4.550 (2H, t, N-CH
m, CH ), 1.422-1.234 (6H, m, C
OPYR][NTf
.810 (2H, d, CHpyr-o), 8.535-8.475 (H, t, CHpyr-p), 8.100-8.010
2H, t, CHpyr-m), 4.630-4.550 (2H, t, N-CH ), 2.071-1.948 (2H,
10), 0.800-0.825 (3H, t,
2
), 2.040-1.950 (2H,
2
3
H
6
), 0.895-0.805 (3H, t, CH
3
).
1
[
2
]: H NMR (500 MHz, CDCl , δ): 8.870-
3
8
(
Fig. 1. Structure of pyridinium cation
2
EXPERIMENTAL
m, CH
2
), 1.490-1.160 (10H, m, C H
5
CH
3
).
HPYR][TFO] and [OPYR][TFO]: An excess (10 %)
Details of the purities of chemicals and procurement are
[
given in Table-1.
All the ionic liquids are synthesized using reported
of potassium trifluoromethanesulfonate was reacted with 1-
alkylpyridinium halide ([HPYR][Br] or [OPYR][Br]) in
acetonitrile at room temperature for 12 h. After filtering and
removal of acetonitrile by rotary evaporation, the product was
washed with methylene chloride 1 time. After filtering, the
product was washed with methylene chloride and water (5:1)
until there was no silver bromide found when the product was
added in silver nitrate. After filtering and rotary evaporating,
the product was set under high vacuum for 24 h 1-alkyl-
pyridinium bis(trifluoromethylsulfonyl)imide ([HPYR][TFO]
1
6,17
method . The nine pyridinium-based ionic liquids are 1-
butylpyridinium bromide ([BPYR][Br]), 1-hexylpyridinium
bromide ([HPYR][Br]), 1-octylpyridinium bromide ([OPYR]-
[
(
Br]), 1-hexylpyridinium bis(trifluoromethylsulfonyl)imide
[HPYR][NTf ]), 1-octylpyridinium bis(trifluoromethyl-
]), 1-hexylpyridinium trifluoro-
2
sulfonyl)imide ([OPYR][NTf
2
methanesulfonate ([HPYR][TFO]), 1-octylpyridinium trifluoro-
methanesulfonate ([OPYR][TFO]), 1-hexylpyridinium tetra-
fluoroborate ([HPYR][BF
4
]) and 1-octylpyridinium tetrafluo-
or [OPYR]-[TFO]) was obtained as a white solid.
roborate ([OPYR][BF ]). The structures of these ionic liquids
4
1
[
HPYR][TFO]: H NMR (500 MHz, CDCl
3
, δ): 9.005-
1
were confirmed using H NMR. All the synthesis progresses
are as following:
8
(
.955 (2H, d, CHpyr-o), 8.520-8.460 (H, t, CHpyr-p), 8.105-8.035
2H, t, CHpyr-m), 4.715-4.635 (2H, t, N-CH ), 2.040-1.950 (2H,
), 0.944-0.795 (3H, t, CH ).
, δ): 8.870-
.810 (2H, d, CHpyr-o), 8.535-8.475 (H, t, CHpyr-p), 8.100-8.030
2H, t, CHpyr-m), 4.635-4.555 (2H, t, N-CH ), 2.054-1.942 (2H,
10), 0.905-0.815 (3H, t, CH ).
] and [OPYR][BF ]: An excess (10 %) of
2
[
BPYR][Br], [HPYR][Br] and [OPYR][Br]: Pyridine
m, CH
2
), 1.421-1.217 (6H, m, C
OPYR][TFO]: H NMR (500 MHz, CDCl
3
H
6
3
was reacted with an excess (10 %) of an alkyl halide (1-bromo-
butane, 1-bromohexane, or 1-bromooctane) in acetonitrile at
0 °C for 24 h. After cooling to room temperature, the solvent
was removed by rotary evaporation. The product was washed
with ethyl acetate 3 times and then washed with water and
ethyl acetate (1:1) 3 times. After filtered, ethyl acetate and
water were removed by rotary evaporation and the resulting
product was set under vacuum for 24 h. 1-Alkylpyridinium
halide ([BPYR][Br], [HPYR][Br] or [OPYR][Br]) was obtained
1
[
3
8
(
7
2
m, CH
2
), 1.436-1.177 (10H, m,C
5
H
3
[
HPYR][BF
4
4
sodium tetrafluoroborate was reacted with 1-alkylpyridinium
halide ([HPYR][Br] or [OPYR][Br]) in acetonitrile at room
temperature for 12 h.After filtering and removal of acetonitrile
by rotary evaporation, the product was washed with methylene
chloride 1 time. After filtering, the product was washed with
methylene chloride and water (5:1) until there was no silver
bromide found when the product was added in silver nitrate.
After filtering and rotary evaporating, the product was set under
high vacuum for 24 h 1-alkylpyridinium tetra-fluoroborate
as a white solid.
1
[BPYR][Br]: H NMR (500 MHz, CDCl
3
, δ): 9.574-9.538
(
2H, d, CHpyr-o), 8.525-8.465 (H, t, CHpyr-p), 8.145-8.085 (2H,
t, CHpyr-m), 4.965-4.910 (2H, t, N-CH ), 2.000-1.930 (2H, m,
CH ), 1.385-1.295 (2H, m, CH ), 0.896-0.844 (3H, t, CH ).
HPYR][Br]: H NMR (500 MHz, CDCl , δ): 9.635-
.565 (2H, d, CHpyr-o), 8.610-8.550 (H, t, CHpyr-p), 8.235-8.175
2H, t, CHpyr-m), 5.050-4.980 (2H, t, N-CH ), 2.115-2.018 (2H,
), 0.883-0.0.825 (3H, t,
2
2
2
3
1
[
3
(
[HPYR][BF
HPYR][TFO]: H NMR (500 MHz, CDCl
.880 (2H, d, CHpyr-o), 8.555-8.455 (H, t, CHpyr-p), 8.100-8.000
), 2.025-1.920 (2H,
), 0.883-0.771 (3H, t, CH ).
, d): 8.885-
.830 (2H, d, CHpyr-o), 8.540-8.480 (H, t, CHpyr-p), 8.110-8.040
2H, t, CHpyr-m), 4.675-4.585 (2H, t, N-CH ), 2.050-1.945 (2H,
10), 0.885-0.805 (3H, t,
4
] or [OPYR][BF
4
]) was obtained as a white solid.
9
(
1
[
3
, δ): 8.875-
2
8
m, CH
2
), 1.482-1.233 (6H, m,C
3
H
6
(2H, t, CHpyr-m), 4.650-4.570 (2H, t, N-CH
2
CH
3
).
OPYR][Br]: H NMR (500 MHz, CDCl
.532 (2H, d, CHpyr-o), 8.595-8.535 (H, t, CHpyr-p), 8.224-8.176
2H, t, CHpyr-m), 5.040-4.987 (2H, t, N-CH ), 2.110-2.020 (2H,
10), 0.890-0.830 (3H, t,
m, CH
2
), 1.410-1.199 (6H, m,C
OPYR][TFO]: H NMR (500 MHz, CDCl
3
H
6
3
1
[
3
, δ): 9.580-
1
[
3
9
(
8
(
2
2
m, CH
2
), 1.438-1.182 (10H, m, C H
5
m, CH
2
), 1.442-1.144 (10H, m,C H
5
CH
3
).
HPYR][NTf
of lithium bis(trifluoromethylsulfonyl)imide was reacted with
-alkylpyridinium halide ([HPYR][Br] or [OPYR][Br]) in water
CH
3
).
There are many methods to determine the KOW values,
[
2
] and [OPYR][NTf ]: An excess (10 %)
2
such as the shake-flask method, the slow-stirring method, the
generator-column method, the chromatograph method, etc.As
a tradition method the shake-flask method was used in this
1
at room temperature for 12 h. Then the product was washed
with methylene chloride and water (5:1) until there was no
silver bromide found when the product was added in silver
nitrate. After filtering and rotary evaporating, the product was
set under high vacuum for 24 h 1-alkylpyridinium bis(trifluoro-
18
work. This method was used for determining KOW of halogen
benzoic acids. A diagram of the experimental apparatus is
shown in Fig. 2. The apparatus consisted of a 40 mL, 90 mm
tall glass vial containing a 2 mm diameter glass tubing which
was used for taking sample from water-rich.
methylsulfonyl)imide ([HPYR][Tf
2
N] or [OPYR][Tf
2
N]) was
obtained as a light-yellow liquid.
1
-Octanol and water are not completely immiscible. At
1
[
HPYR][NTf
2
]: H NMR (500 MHz, CDCl
3
, δ): 8.860-
25 °C, the solubility of water in octanol is approximately 0.275
8
.805 (2H, d, CHpyr-o), 8.525-8.465 (H, t, CHpyr-p), 8.085-8.015
19
-5
mol/L and the solubility of 1-octanol in water is 7.5 × 10

Vol. 27, No. 8 (2015)
No.
Octanol/Water Partition Coefficients of Pyridinium-Based Ionic Liquids 2821
TABLE-1
PURITIES OF CHEMICALS USED IN THE EXPERIMENT
Name Purity (%)
Lithium bis(trifluoromethylsulfonyl)imide
Company
Alfa Aesar
Alfa Aesar
1
2
3
4
5
6
7
8
9
98
98
Potassium trifluoromethanesulfonate
1-Bromobutane
98
98
98
99.5
Reagent
99.5
98
Tianjin Guangfu Technology Development Co. Ltd, China
Tianjin Guangfu Technology Development Co. Ltd, China
Tianjin Guangfu Technology Development Co. Ltd, China
Tianjin Guangfu Technology Development Co. Ltd, China
Tianjin Guangfu Technology Development Co. Ltd, China
Tianjin Guangfu Technology Development Co. Ltd, China
Tianjin Guangfu Technology Development Co. Ltd, China
Tianjin Guangfu Technology Development Co. Ltd, China
1-Bromohexane
1-Bromooctane
Methylene chloride
Acetonitrile
Ethyl acetate
Sodium tetrafluoroborate
Octanol
10
Reagent
O
+
O
Water/IL
sampling
[
MX] +[M ]
KOW
=
W
+
W
(2)
[
MX] +[M ]
O
W
where [MX] and [MX] are concentrations of ionic liquid in
-octanol and water phases; [M ] and [M ] are concen-
+
O
+ W
1
trations of cation in 1-octanol and water phases.
In this work KOW is reported as:
O
0
O
W
+
W
W
[
MX] × V -([MX] +[M ] )× V
KOW
=
W
+
W
O
(3)
(
[MX] +[M ] )× V
W
O
Octanol-rich
phase
whereV andV are the volume of water and 1-octanol phases;
O
[MX
0
] is the concentration of ionic liquid in 1-octanol phase
when it was added to the vial.
W
O
In this work, because V = V = 10 mL, eqn. 3 can be
simplified as:
Water-rich
phase
O
0
W
+ W
[
MX] -([MX] +[M ] )
KOW
=
(4)
W
+ W
[MX] +[M ]
Fig. 2. Diagram of the experimental apparatus
RESULTS AND DISCUSSION
2
0
mol/L . Therefore the water was presaturated with 1-octanol
and 1-octanol was presaturated with water first. 10 mL 1-octanol-
ionic liquid "stock" solution, consisting of 1-octanol presatu-
rated with water and a known amount of ionic liquid was added
to the vial. 10 mL water that had been presaturated with 1-
octanol was added to the vial. Then the glass tubing was sealed
by teflon to prevent 1-octanol and water evaporation. The vial
was shook in constant temperature shaker (WE-1, Ounuo co.
Tianjin, China) slowly to prevent the emulsion at room
temperature 25 ± 0.1 °C for 12 h. Then the sample was stood
for 24 h at 25 ± 0.1 °C to separate the two phases. The sample
was withdrawn from the water-rich phase using a syringe.
Concentration of ionic liquid in water-rich phase was measured
using an ultraviolet spectrophotometer (U-3010, Hitachi Ltd.,
Tokyo, Japan), which has a sensitivity of ± 0.0001 absorbance
units. The above process was repeated 3 times for each ionic
liquid at a concentration and the average data was recorded as
the result. And for each ionic liquid KOW is determined at 3
concentrations.
The values of KOW for the nine ionic liquids are listed in
Table-2. The maximum absorbance (λmax) of the nine pyri-
dinium-based ionic liquids in water is 259 nm. The correlation
2
coefficients (R ) of standard curves are also given in Table-2
and the result shows that the standard curves are all quite
precise. KOW is determined at different concentrate, therefore
the concentrate ranges of ionic liquids in water/(mol/L) are
shown in Table-2. The KOW values were influenced by the
concentrate of ionic liquids and the KOW ranges of ionic liquids
are presented in Table-2 as well.
The results indicate that the pyridinium-based ionic liquids
measured in this work are extremely hydrophilic except for
[
OPYR] [NTf ]. The KOWs of ionic liquids are determined by
2
both cation and anion. For the cation influence: KOWs increase
with the carbon number of -R, which can be seen from Fig. 3.
For the anion influence: NTf
shown in Fig. 4.
2
> TFO > BF > Br, which is
4
It is found that the KOWs of [TNf
dependent upon concentration, which is shown clearly in
Table-3. The KOWs of [OPYR][NTf ] and [HPYR][NTf ] are
2
] ionic liquids were
KOW is defined as:
2
2
O
C
all changes with ionic liquids concentration in water. High
ionic liquids concentration in water leads to big KOW. For
KOW
=
(1)
W
C
O
W
where C and C are extremely dilute concentrations of the test
compound in water and 1-octanol phases. ionic liquids will have
a greater tendency to dissociate in the water-rich phase. Due to
[OPYR][NTf
concentration changes from 4.38 × 10 to 1.04 × 10 mol/L. For
[HPYR][NTf ], KOW changes from 0.32 to 0.72 when ionic liquids
concentration changes from 1.01 × 10 to 3.11 × 10 mol/L.
2
], KOW changes from 2.0 to 4.1 when ionic liquids
-5
-4
2
12
-4
-4
the difficulty of this measurement, KOW is reported as :

2
822 Zhang et al.
Asian J. Chem.
TABLE-2
RESULTS OF THE KOWs DETERMINATION
2
Compounds
KOW
KOW range
Concentration ranges of ILs in water (mol/L)
λmax in water (nm)
R
-
4
-4
-4
-4
-4
-4
-4
-4
-4
-4
[BPYR][Br]
0.014
0.087
0.10
0.098
0.19
0.12
0.40
0.50
3.0
0.011-0.018
0.073-0.102
0.10-0.11
0.093-0.1039
0.12-0.23
0.11-0.14
0.39-0.42
0.32-0.72
2.0-4.1
1.27 × 10 –5.08 × 10
259
259
259
259
259
259
259
259
259
0.9998
0.9999
0.9999
0.9995
0.9996
0.9998
1.0000
0.9998
1.0000
-
4
[
[
HPYR][Br]
OPYR][Br]
1.04 × 10 –4.17 × 10
-
5
9.04 × 10 –3.76 × 10
-
4
[
[
HPYR][BF4]
OPYR][BF4]
1.04 × 10 –4.32 × 10
-
5
9.79 × 10 –3.92 × 10
-
5
[
[
[
[
HPYR][TFO]
OPYR][TFO]
HPYR][NTf2]
OPYR][NTf2]
8.52 × 10 –3.56 × 10
-
4
5.28 × 10 –2.13 × 10
-
4
1.00 × 10 –3.12 × 10
-
5
4.34 × 10 –1.04 × 10
TABLE-3
INFLUENCE OF IONIC LIQUIDS CONCENTRATION IN WATER ON THE KOWs DETERMINATION
[
OPYR][NTf2]
[HPYR][NTf2]
Concentration ranges of ILs in water (mol/L)
KOW
2.0
2.9
Concentration ranges of ILs in water (mol/L)
KOW
0.32
0.45
-
05
-04
4
.38 × 10
1.01 × 10
-
a05
-04
6.70 × 10
1.84 × 10
0
0
0
0
.5
.4
.3
.2
the cation influence: KOWs increase with the carbon number
of -R. For the anion influence: NTf > TFO > BF > Br. It is
also found that the KOWs of [NTf ] ionic liquids were dependent
on concentration. High ionic liquids concentration in water
2
4
Br
2
leads to big KOW
.
ACKNOWLEDGEMENTS
This work has been supported by Natural Science
Foundation of China with Grant No. 21276183 and Program
of Introducing Talents of Discipline to Universities (No:
B06006).
0
.1
0
4
5
6
7
8
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Conclusion
In this work, nine pyridinium-based ionic liquids were
synthesized. Then the KOW of these nine pyridinium-based ionic
liquids were determined by shake-flask method. It is found
that pyridinium-based ionic liquids measured in this work are
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2