Salting-out effect of ionic liquids on poly(propylene glycol) (PPG): Formation of PPG + ionic liquid aqueous two-phase systems

Article abstract of DOI:10.1021/je100604m

In the present work, aqueous poly(propylene glycol) (PPG) solution was separated into aqueous two-phase systems (ATPSs) in the presence of ionic liquid 1-allyl-3-methylimidazolium chloride ([Amim]Cl), 1-butyl-3-methylimidazolium acetate ([C₄mim]Ac), or 1-butyl-3-methylimidazolium chloride ([C ₄mim]Cl). The top phase was PPG-rich, whereas the bottom phase was ionic liquid (IL)-rich. Liquid-liquid equilibrium data for the ATPSs and the salting-out coefficients of the ILs have been determined at 298.15 K and atmospheric pressure. It was shown that the binodal curves and the tie-lines could be described by a three-parameter equation and the Othmer-Tobias and Bancroft equations, respectively. The salting-out ability of the ILs on PPG was found to follow the order: [Amim]Cl > [C₄mim]Ac > [C ₄mim]Cl > [C₄mim][BF₄]. Since PPG is a thermo-sensitive polymer and can be recovered simply by heating, these novel ATPSs are expected to have applications in the recycling and/or enrichment of hydrophilic ILs from aqueous solutions.

Full text of DOI:10.1021/je100604m

5
004
J. Chem. Eng. Data 2010, 55, 5004–5008
Salting-Out Effect of Ionic Liquids on Poly(propylene glycol) (PPG):
Formation of PPG + Ionic Liquid Aqueous Two-Phase Systems
†
,‡
,†
†
†
†
Changzeng Wu, Jianji Wang,* Yuanchao Pei, Huiyong Wang, and Zhiyong Li
School of Chemical and Environmental Sciences, Key Laboratory of Green Chemical Media and Reactions, Ministry of
Education, Henan Normal University, Xinxiang, Henan 453007, P. R. China, and School of Chemistry and Chemical
Engineering, Xuchang University, Xuchang, Henan 461000, P. R. China
In the present work, aqueous poly(propylene glycol) (PPG) solution was separated into aqueous two-phase
systems (ATPSs) in the presence of ionic liquid 1-allyl-3-methylimidazolium chloride ([Amim]Cl), 1-butyl-
3
4 4
-methylimidazolium acetate ([C mim]Ac), or 1-butyl-3-methylimidazolium chloride ([C mim]Cl). The top
phase was PPG-rich, whereas the bottom phase was ionic liquid (IL)-rich. Liquid-liquid equilibrium data
for the ATPSs and the salting-out coefficients of the ILs have been determined at 298.15 K and atmospheric
pressure. It was shown that the binodal curves and the tie-lines could be described by a three-parameter
equation and the Othmer-Tobias and Bancroft equations, respectively. The salting-out ability of the ILs on
PPG was found to follow the order: [Amim]Cl > [C
4
mim]Ac > [C
4
4 4
mim]Cl > [C mim][BF ]. Since PPG
is a thermo-sensitive polymer and can be recovered simply by heating, these novel ATPSs are expected to
have applications in the recycling and/or enrichment of hydrophilic ILs from aqueous solutions.
1
1-13
Introduction
tosterone, opium alkaloids, and bovine serum albumin,
and
their extraction efficiency is usually higher than the classical
Aqueous two-phase systems (ATPSs), used first in 1956 by
14
ATPSs.
1
Albertsson, can be formed spontaneously upon mixing two
Inorganic salts are always used in poly(ethylene glycol) (PEG)
aqueous solutions of structurally different components, such as
two polymers or one polymer and one salt, above a certain
concentration. In the two phases, the top phase is polymer-rich,
and the bottom phase is salt-rich (or the other polymer-rich).
Because the bulk of the two phases consists mainly of water
and no traditional volatile organic compounds are used, ATPSs
are considered to be environmentally friendly and provide a
biocompatible environment for biomaterials. Moreover, the
separation technique based on the ATPSs is relatively simple
and inexpensive, and the equipments traditionally used in
the liquid-liquid solvent extraction can be easily adapted
+
salt and IL + salt ATPSs. It is known from their phase
10,11,15,16
diagrams
that the concentration of the salt in the salt-
rich phase is quite high. This will produce a great amount of
highly salty water in the applications, thus causing environ-
mental problems. In this work, new kinds of APTSs were created
from poly(propylene glycol) (PPG) being salted-out by ILs.
Here, the thermo-sensitive polymer PPG was used to replace
the inorganic salts. This polymer is biodegradable and nontoxic
and can be recovered conveniently by heating. To understand the
phase behavior of these systems, the liquid-liquid equilibrium data
at 298.15 K were determined for the ATPSs of PPG400 + 1-allyl-
2
,3
for aqueous two-phase extraction. As a result, ATPSs have
been widely used for the recovery and purification of vari-
ous biomolecules such as proteins, enzymes, and nucleic
3
3
-methylimidazolium chloride ([Amim]Cl), PPG400 + 1-butyl-
-methylimidazolium acetate ([C mim]Ac), PPG400 + 1-bu-
mim]Cl), PPG1000 +
mim]Cl. The results have been
4
tyl-3-methylimidazolium chloride ([C
4
4
-6
acids.
[Amim]Cl, and PPG1000 + [C
4
In recent years, ionic liquids (ILs) have been used for
discussed from the competing interaction of PPG and ILs with water.
7
,8
analytical and separation processes owing to their “green”
characteristics such as negligible volatility, nonflammability,
large liquid range, high thermal and chemical stability, excellent
solvation power for organic and inorganic compounds, and a
number of possible variations in cation and anion features which
Experimental Section
Materials. PPG400 (> 99 %), PPG1000 (> 99 %), allyl
chloride, and AmberliteIRA-400 (Cl) ion-exchange resin were
obtained from Alfa. N-Methylimidazole (C. P.) purchased from
Linhai Kaile Chem. Co. was distilled before use. All other
reagents were of analytical grade, and double-distilled
deionized water was used in the experiments. The ILs
9
allow fine-tuning of the IL properties. In 2003, Rogers and
1
0
co-workers reported, for the first time, that a hydrophilic IL,
-butyl-3-methylimidazolium chloride ([C mim]Cl), could form
ATPSs in the presence of a high concentration of inorganic salt
such as K PO . These ATPSs have many advantages, such as
1
4
3
4
[C mim]Cl (g 99.0 %) and 1-butyl-3-methylimidazolium
4
lower viscosity, little emulsion formation, quick phase separa-
tion, and gentle biocompatible environment. Since then, IL-
based ATPSs have been used to separate testosterone, epites-
tetrafluoroborate ([C mim][BF ] (g 99.0 %) were purchased
4
4
from Henan Lihua Pharmaceutical Co., Ltd., and dried under
vacuum at 60 °C before use. The water content in the ILs
determined by Karl Fischer titration was less than 0.002 mass
fraction.
*
To whom correspondence should be addressed. E-mail: jwang@
henannu.edu.cn.
†
Preparation of [C
ylimidazolium acetate was prepared by neutralization: at first,
mim]Ac and [Amim]Cl. 1-Butyl-3-meth-
Henan Normal University.
Xuchang University.
4
‡
17
1
0.1021/je100604m  2010 American Chemical Society
Published on Web 09/15/2010

Journal of Chemical & Engineering Data, Vol. 55, No. 11, 2010 5005
the anion exchange resin was transformed from Cl-type into
OH-type by passing aqueous NaOH through the resin columns
Table 1. Binodal Data for PPG400 (1) + ILs (2) + H
at T ) 298.15 K
2
O (3) Systems
until Cl- could not be detected with aqueous AgNO
/HNO
solution. Excessive NaOH solution was washed out with
deionized water. [C mim]Cl was transformed into [C mim]OH
through the OH-type resin. The aqueous [C mim]OH solution
PPG400 + [Amim]Cl PPG400 + [C
4
mim]Ac PPG400 + [C
4
mim]Cl
3
3
w
1
w
2
w
1
w
2
w
1
w
2
4
4
0
.0934
0.4159
0.3704
0.3214
0.2725
0.2513
0.2366
0.2150
0.1851
0.1616
0.1449
0.1249
0.1021
0.0887
0.0768
0.0659
0.0571
0.1323
0.1405
0.1542
0.1685
0.1854
0.2037
0.2297
0.2453
0.2742
0.2960
0.3262
0.3600
0.3968
0.4422
0.4910
0.4401
0.3998
0.3467
0.3060
0.2653
0.2289
0.1937
0.1460
0.1335
0.1057
0.0939
0.0742
0.0564
0.0526
0.0317
0.2110
0.2229
0.2329
0.2398
0.2521
0.2679
0.2796
0.2944
0.3145
0.3359
0.3575
0.3781
0.4000
0.4228
0.445
0.5268
0.4927
0.4686
0.4429
0.4096
0.3763
0.3523
0.3201
0.2925
0.2664
0.2387
0.2151
0.1954
0.1798
0.1662
0.1509
0.1326
4
0.1040
0
0
0
0
.1196
.1435
.1559
.1611
was then neutralized with equimolar acetic acid. Then, water
was evaporated under reduced pressure, and the viscous liq-
uid was obtained through being thoroughly washed with diethyl
ether and then dried under vacuum for 48 h at 60 °C.
0.1745
0.1975
0
0
1
8
[
Amim]Cl was prepared as described in the literature.
.2167
.2340
Briefly, 1-methylimidazole and allyl chloride were added to a
round-bottomed flask fitted with a reflux condenser at a proper
mole ratio. The mixture was stirred for 8 h below 60 °C and
then distilled under reduced pressure. [Amim]Cl with a slightly
amber color was obtained and dried under vacuum for 48 h at
0.2552
0
0
0
.2833
.3027
.3221
0.3416
.3588
0
0.4677
0.5112
6
4
0 °C. The structure of [C mim]Ac and [Amim]Cl was con-
1
firmed by H NMR spectroscopy (AV-400 Bruker spectrometer),
and their purity was found to be greater than 0.990 mass fraction.
Table 2. Binodal Data for PPG1000 (1) + ILs (2) + H
Systems at T ) 298.15 K
2
O (3)
Methods
PPG1000 + [Amim]Cl
PPG1000 + [C
4
mim]Cl
w
1
w
2
w
1
w
2
The binodal curves were determined by the cloud-point
1
0
method. Stock aqueous IL solutions were prepared with
different mass fractions. A few grams of pure PPG were
weighed into a test tube, and a known mass of water was added
under stirring. An aqueous IL solution of known mass fraction
was added dropwise to the test tube until the mixture became
cloudy. Then water was added to get a clear one-phase system
again, and the above procedure was repeated. The test tubes
were immersed in a jacketed glass vessel, and the temperature
of the system was maintained at 298.15 ( 0.05 K by a DC-
0.0465
0.0392
0.0619
0.0748
0.0868
0.0995
0.1110
0.1261
0.1368
0.1468
0.1514
0.1627
0.1701
0.1759
0.1811
0.1875
0.1934
0.1999
0.2068
0.2110
0.2409
0.0414
0.0334
0.0302
0.0271
0.0242
0.0210
0.0190
0.0173
0.0160
0.0148
0.0132
0.0120
0.0109
0.0100
0.0094
0.0085
0.0079
0.0827
0.1146
0.1239
0.1387
0.1513
0.1668
0.1776
0.1883
0.1973
0.2044
0.2145
0.2236
0.2325
0.2389
0.2452
0.2524
0.2603
0
0
0
0
.0408
.0360
.0322
.0281
0.0247
0.0214
.0192
.0174
.0159
.0141
0
0
0
0
2
006 water thermostat (Shanghai Hengping Instrument Factory).
0.0130
0
0
0
0
.0121
.0113
.0104
.0097
0.0088
0.0082
The concentration of the phase components was determined by
mass quantification of all of the components added within an
uncertainty of ( 1.0·10- kg.
7
For the determination of tie-lines, a series of ATPSs with
different total compositions were prepared in graduated glass
tubes which were placed in a thermostatted bath. The system
was vigorously stirred for at least 1 h to attain equilibrium. The
phase separation occurred quickly after cessation of the stirring.
Then, a XYJ-802 centrifuge (Jiangsu Medical Instrument
Factory) operated at 2000 rpm was still used for a period of 10
min to ensure a complete phase separation. Then, the systems
were placed again into the thermostatted bath. After phase
equilibrium, visual estimates of the top and bottom volumes
were made. Concentrations of the IL in both the top and the
bottom phases were determined by spectrophotometry at 211
nm after dilution. The quantification of water content in the
top and bottom phases was determined according to the
0
0
.0076
.0063
Results and Discussion
Binodal CurWes. The binodal data determined at 298.15 K
for PPG400 + ILs ([Amim]Cl, [C mim]Ac, or [C mim]Cl) +
O and PPG1000 + ILs ([Amim]Cl or [C mim]Cl) + H
systems are reported in Tables 1 and 2. For the sake of easy
understanding of the phase behavior, the binodal curves are
plotted in Figures 1 and 2. These curves provide information
about the concentration of phase-forming components required
to form two phases. The closer the binodal curve is to the origin,
the lower the IL concentration required for the formation of
two phases. It can be seen from Figure 1 that the IL concentra-
tion required for the formation of two phases increases in the
order: [Amim]Cl < [C
ability of these ILs for the phase separation follows the order:
Amim]Cl > [C mim]Ac > [C mim]Cl. However, it was found
that phase separation cannot be observed in the presence of
mim][BF ] IL. This order follows the Hofmeister series for
the strength of the water structure-making anions. Considering
the fact that [C mim]Ac, [C mim]Cl, and [C mim][BF ] share
4
4
H
2
4
2
O
1
9
procedure described in our previous work. A mass balance
check was made between the initial mass of each component
and the amounts in the top and bottom phases on the basis of
equilibrium compositions. Then the concentrations of PPG in
the top and bottom phases were calculated by mass balance.
The tie-line length (TLL) at different compositions were
4
mim]Ac < [C
4
mim]Cl. Therefore, the
[
4
4
1
9,20
calculated by using eq 1:
[
C
4
4
t
b 2
t
b 2
TLL ) (w - w ) + (w - w )
(1)
4
4
4
4
√
1
1
2
2
a common imidazolium cation but contain different anions, it
is easy to see that the anion with a higher salting-out ability has a
more negative value of Gibbs energy of hydration (∆Ghyd):
t, w b
, w
t, and w
b
represent the equilibrium mass
21
where w
1
1
2
2
-
-1
-
fraction of PPG (1) and ILs (2) in the top and bottom phases,
respectively.
CH
3
CO
2
(∆Ghyd ) -365 kJ·mol ) > Cl (∆Ghyd ) -340
- -1
-
1
kJ·mol ) > BF
4
(∆Ghyd ) -190 kJ·mol ). The hydration

5
006 Journal of Chemical & Engineering Data, Vol. 55, No. 11, 2010
Figure 1. Binodal curves for PPG400 (1) + IL (2) ATPSs at 298.15 K: 9,
Amim]Cl; b, [C mim]Ac; 2, [C mim]Cl.
[
4
4
Figure 2. Binodal curves for PPG1000 (1) + IL (2) ATPSs at 298.15 K:
, [Amim]Cl; b, [C mim]Cl.
9
4
of BF -
On the other hand, [Amim]Cl and [C
is not strong enough to induce the formation of ATPSs.
mim]Cl have the same
4
4
anion but different cations. Their binodal data can be used to
compare the phase separation ability of the cations. It is obvious
from Figures 1 and 2 that the salting-out ability of the cations
decreases in the order [Amim]+ > [C
mim] . The possible
+
4
+
reason is that, compared with [C
4
mim] , the shorter alkyl chain
length and the plain configuration of double bond in the alkyl
+
chain make [Amim] to have stronger interaction with water.
Compared with IL + salt and PEG + salt ATPSs, the novel
ATPSs developed in the present work are much more environ-
mentally friendly due to the use of thermo-sensitive PPG which
can be recovered simply by heating.
Figure 3. Phase diagram for the PPG400 (1) + ILs (2) ATPSs at 298.15
K: 9, binodal curve; 0, tie line; (a) [Amim]Cl; (b) [C mim]Ac; (c)
mim]Cl.
4
[C
4
In fact, the formation of ATPSs is the result of competing
hydration of the two phase-forming components such as the
polymer and IL. In the two phase-forming components, one is
water structure-making, while another one is water structure-
breaking in nature. In the traditional polymer + salt ATPSs,
polymer was salted-out by salt; here the salt is a water structure-
making component because of its stronger hydration ability
PPG was salted-out by the ILs due to their stronger hydration
ability than PPG. Therefore, in ILs + PPG ATPSs, ILs are water
structure-making components, while PPG is the water structure-
breaking component. This can be understood from the content
of water in the two phases. It was known that for the IL + salts
systems, the content of water in the top IL-rich phase was lower
22,23
compared with the polymer.
Similarly, in IL + salt ATPSs,
11,19,24
19
the IL was salted-out by inorganic salts,
and the salt is
than that in the bottom salt-rich phase. On the contrary, it
also a water structure-making component, while IL is a water
structure-breaking component. However, in the present work,
can be seen from Table 4 that the content of water in the bottom
IL-rich phase was higher than that in the top PPG-rich phase

Journal of Chemical & Engineering Data, Vol. 55, No. 11, 2010 5007
Table 5. Parameter Values for Equations 3 and 4 at T ) 298.15 K
Table 3. Parameters Obtained by Equation 2 for the Binodal
Curves of the PPG + ILs ATPSs at T ) 298.15 K
a
a
system
k
n
σ
k
1
r
σ
a
R2
system
a
b
c
σ
PPG400-[Amim]Cl 0.0148 2.6617 0.009 3.6458 0.2622 0.004
PPG400-[Amim]Cl
-0.1729 -0.1521 -0.0558 0.002 0.9995
PPG400-[C
PPG400-[C
4
mim]Ac 0.1796 1.4550 0.003 2.8820 0.5031 0.002
mim]Cl 0.5847 0.8434 0.101 1.7072 0.6714 0.010
PPG400-[C
PPG400-[C
4
mim]Ac -0.1458 -0.1247 -0.1126 0.011 0.9916
4
4
mim]Cl
-0.2105 -0.1012 -0.1627 0.006 0.9976
a
σ, standard deviation.
PPG1000-[Amim]Cl -0.1076 -0.2772
0.0027 0.004 0.9948
0.0095 0.001 0.9996
PPG1000-[C mim]Cl -0.1619 -0.3979
4
2
6
a
PPG through H atoms of the imidazolium ring, so the
imidazolium-based ILs can dissolve PPG400.
σ, standard deviation.
2
7
The empirical equations proposed by Othmer-Tobias (eq
for the PPG + ILs systems studied here. As a result, ILs are
water structure-breaking components in IL + inorganic salts
ATPSs, but they are water structure-making components in IL
20
3
) and Bancroft (eq 4):
t
b n
+
PPG ATPSs. Therefore, ILs can be either water structure-
1 - w1
1 - w2
)
k
(3)
(4)
making or water structure-breaking components in the formation
of ATPSs, depending on their relative hydration ability to the
other phase-forming component.
The binodal data for the PPG + ILs ATPSs can be fitted by
the equation:
t
b
(
)
)
(
)
w1
w2
b
b
t
t
r
w3
w3
1
2
) k1
(
(
)
w2
w1
w ) a ln(w + c) + b
(2)
1
2
for the assessment and correlation of the tie-lines for PEG +
2
7-31
salt ATPSs
have been used to correlate the tie-line data
which relates the mass fraction of PPG (w
of the ILs (w
nonlinear regression as described in our previous work, and
the result was given in Table 3.
1
) to the mass fraction
t
t
determined in the present work. In these equations, w
1 3
and w
2
). The parameters a, b, and c were obtained by
represent the mass fraction of PPG and water in the top phase,
1
9
b
b
while w
2
and w
3
stand for the mass fraction of ILs and water
, and r represent
in the bottom phase, respectively. The k, n, k
1
Tie Lines. The phase compositions determined for the
PPG400 + ILs + water systems are collected in Table 4
and illustrated in Figure 3, parts a, b, and c. The tie lines for
PPG1000 + ILs + water systems were very difficult to be
determined because of the low solubility of PPG1000. Therefore,
no reliable data have been reported in the present paper.
Generally, in traditional PEG + salt + water systems, the top
and bottom phases mainly consist of water, and the top polymer-
rich phase contains a small quantity of salt and the bottom salt-
rich phase a small quantity of polymer. However, in the PPG400
the fitting parameters which can be calculated by linear least-
squares regression. The TLL values and the parameters
calculated by eqs 3 and 4 were given in Tables 4 and 5,
respectively.
Salting-Out Coefficients. The salting-out ability of the ILs
for the PPG400 in aqueous solutions could be expressed
19
quantitatively by the salting-out coefficient k
the following equation:
s
calculated from
32,33
t
+
ILs ATPSs studied here, the top PPG400-rich phase contains
about (10 to 40) % water, and the bottom IL-rich phase contains
7 to 17) % PPG400. These unusual results can be explained
by the two following reasons: (i) PPG is more hydrophobic than
cp
b
t
b
s
t
s
ln
) k (c - c ) + k (c - c )
(5)
p
p
p
s
(
b
)
cp
(
2
5
PEG due to methyl group of the side chain; (ii) the imida-
zolium cation can form hydrogen bonds with oxygen atom of
where k
p
is a constant relating the activity coefficient of PPG
to its concentrations, k
s
represents the salting-out coefficient of
Table 4. Tie-Line Compositions (in Mass Fraction) Determined for the Feed, Top, and Bottom Phases and the Corresponding Tie-Line Length
at T ) 298.15 K
feed composition
top phase
bottom phase
IL
PPG400
IL
PPG400
IL
PPG400
TLL
PPG400-[Amim]Cl
0.6821
0
0
0
0
0
.1832
.1816
.2006
.2285
.2136
0.2606
0.2711
0.2784
0.2803
0.2829
0.0498
0.0474
0.0399
0.0282
0.0321
0.2123
0.2196
0.2484
0.2896
0.2721
0.1734
0.1619
0.1354
0.0973
0.1089
0.5340
0.5641
0.6646
0.8158
0.7532
0.6991
0.7665
0.8701
0.8229
4
PPG400-[C mim]Ac
0
0
0
0
0
.2340
.2206
.2357
.2462
.2489
0.2304
0.2981
0.317
0.3347
0.3562
0.1054
0.0911
0.0856
0.0836
0.0827
0.5593
0.6191
0.6755
0.7065
0.7212
0.2708
0.2973
0.3303
0.3569
0.3786
0.1350
0.1143
0.0869
0.0811
0.0706
0.4554
0.5452
0.6374
0.6825
0.7147
4
PPG400-[C mim]Cl
0
0
0
0
0
.3045
.3777
.3339
.3501
.3604
0.4422
0.3785
0.3574
0.3632
0.3774
0.1905
0.1876
0.1924
0.1911
0.1875
0.6231
0.6721
0.5963
0.6162
0.6467
0.4906
0.5466
0.4514
0.4871
0.5258
0.1401
0.1208
0.1546
0.1373
0.1225
0.5686
0.6579
0.5120
0.5630
0.6239

5
008 Journal of Chemical & Engineering Data, Vol. 55, No. 11, 2010
Table 6.
k
s
Values Calculated by Equation 5 at T ) 298.15 K
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k
s
kg·mol-1
R2
system
PPG400-[Amim]Cl
PPG400-[C
PPG400-[C
8.28
7.00
3.75
0.9985
0.9856
0.9903
4
mim]Ac
mim]Cl
4
(
15) Li, C.; Bai, J. H.; Li, W.; Cai, Z. L.; Ouyang, F. Optimization of
Conditions for Bacteriocin Extraction in PEG/Salt Aqueous Two-Phase
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p s
ILs, c and c stand for the molality of PPG400 and the ILs,
2
001, 17, 366–368.
and superscripts “t” and “b” refer to the top polymer-rich phase
and the bottom IL-rich phase, respectively. The values of salting-
out coefficients of the ILs were included in Table 6. It can be
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seen that k
s
values of the three ILs decrease in the order:
mim]Cl. This order is in agree-
[
Amim]Cl > [C
4
mim]Ac > [C
4
(
ment with that determined from the binodal curves.
Conclusions
(
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In the present work, new kinds of ATPSs were reported.
These ATPSs were formed from PPG which was salted-out by
ILs. The phase-forming ability of the studied ILs follows the
2
031.
(
20) Tubio, G.; Pellegrini, L.; Nerli, B. B.; Pico, G. A. Liquid-Liquid
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order: [Amim]Cl > [C
4
mim]Ac > [C
4
mim]Cl > [C
4
4 4
mim]BF ,
is not strong enough
and the salting-out ability of [C
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mim]BF
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(
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Received for review May 31, 2010. Accepted August 10, 2010.
JE100604M