Phase diagram data for several salt + salt aqueous biphasic systems at 298.15 K

Article abstract of DOI:10.1021/je7000478

Phase diagrams corresponding to aqueous biphasic systems of salt (the organic ionic liquid of salts [C₄min]Cl, [C₆mim]Cl, and [C₈mim]Cl) + salt (K₃PO₄, K₂CO ₃) + water were deter

Full text of DOI:10.1021/je7000478

1332
J. Chem. Eng. Data 2007, 52, 1332-1335
Phase Diagram Data for Several Salt + Salt Aqueous Biphasic Systems at
298.15 K
Yuefeng Deng, Ji Chen,* and Dongli Zhang
Key Laboratory of Rare Earth Chemistry and Physics, Changchun Institute of Applied Chemistry, Chinese Academy of Science,
Changchun, 130022, People’s Republic of China
Phase diagrams corresponding to aqueous biphasic systems of salt (the organic ionic liquid of salts [C4mim]Cl,
[C6mim]Cl, and [C8mim]Cl) + salt (K3PO4, K2CO3) + water were determined at 298.15 K. The binodal curve
was fitted to the Merchuk equation. Tie lines assigned from mass phase ratios according to the lever arm rule
were satisfactorily described using the Othmer-Tobias and Bancroft equations.
(1) 1-Octyl-3-methylimidazolium chloride ([C8mim]Cl). Freshly
distilled 1-chlorooctane (5 L) and N-methylimidazole (2.32 L)
were added in a 20 L reactor (Shensheng, Shanghai, China)
with a mechanical stirrer and a condenser. The solution was
stirred for 70 h at 80 °C. The [C8mim]Cl was easily separated
from the small amount of unreacted compounds by separating
funnels and washed three times with ethyl acetate (3 × 3 L).
The ethyl acetate was finally removed using a vacuum line.
Thus, the pure [C8mim]Cl was obtained (5.7 L, yield 84.2 %).
The purity of the compound was verified through 1H NMR and
13C NMR (Bruker AV 400 NMR). The water content was
determined by a Karl Fischer titration using a WA-1C moisture
meter (Jiangfen, Jiangsu, China).
(2) 1-Hexyl-3-methylimidazolium chloride ([C6mim]Cl). The
same procedure was used, except that 1-chlorohexane was
substituted for 1-chlorooctane.
(3) 1-Butyl-3-methylimidazolium chloride ([C4mim]Cl). The
same procedure was used, except that 1-chlorobutane was
substituted for 1-chlorooctane.
Methods. The binodal curve was determined by the cloud
point method at room temperature.1 From the stock, an IL
solution of known concentration was taken into the vessel. A
salt solution of known mass fraction was then added dropwise
to the vessel until the mixture became turbid or cloudy. The
composition of this mixture was noted. Then, water was added
dropwise to the vessel to get a clear one-phase system, and more
salt solution was added again to form the ABS, and the above
procedure was repeated and so on. The binodal curve was fitted
by least-squares regression to an empirical relationship devel-
oped by Merchuk18 and shown as eq 1:
Introduction
Aqueous biphasic systems (ABS) are formed when two
mutually incompatible polymers/polymer, polymer/salt, or
salt/salt are dissolved in water above a certain critical con-
centration. These systems comprise mass fraction of water
of (80 to 90)
% in equilibrium phases, each phase
containing mainly one of the compounds and a small amount
of the other. ABS have been widely used in the separation,
concentration, and fractionation of biological solutes and
particles such as cells and proteins.1,2 ABS have also been
studied for applications including the selective distribution and
separation of metal ion species,3 small organic molecules,4,5
nano- and micro-solid particulates,6,7 and as green reaction
media.8-10 In recent years, ionic liquids (ILs) have been gaining
great exposure due to their potential use as green solvents and
as possible replacements for traditional volatile organic com-
pounds (VOCs). A new type of ABS consisting of ILs and salt
was reported for recycle or concentration hydrophilic ILs from
aqueous solution. Rogers et al.11 first reported that 1-butyl-3-
methylimidazolium chloride ([C4mim]Cl) + K3PO4 + water can
be used to form an ABS. Li and co-workers12,13 studied the
extraction of biochemical compounds and drugs using [C4mim]-
Cl + K2HPO4 + water ABS. Akama et al.14,15 used a
tetrabutylammonium bromide + (NH4)2SO4 + water ABS to
separate metal ions. Bridges et al.16 described linear solvent free-
energy relationships and Gibbs free energy of methylene transfer
in salt + salt + water ABS. However, data for the phase diagram
of the salt + salt + water systems are not available at the
present.
In our previous work, ILs were concentrated from aqueous
solutions by aqueous biphasic systems.17 The goal of this study
is to obtain salt (imidazolium ILs of [C4mim]Cl, [C6mim]Cl,
and [C8mim]Cl) + salt (K3PO4, K2CO3) + water ABS phase
diagrams at 298.15 K.
3
YA ) a exp(bX0A.5 - cXA )
(1)
where YA and XA are the concentrations of IL and salt. The
constants a, b, and c were obtained by least-squares regression.
The tie line was determined by application of the lever arm
rule to the relationship between the mass phase composition
and the overall system composition.18 The mathematical meth-
ods are shown in eqs 2 and 3:
Experimental Section
Materials. K3PO4 and K2CO3 were analytical grade reagents
(Beijing Chemical, Beijing, China), and all the solutions were
prepared from deionized water. The ILs used in this study were
prepared by the following procedures:
YT ) (YM/R) - ((1 - R)/R)YB
XT ) (XM/R) - ((1 - R)/R)XB
(2)
(3)
* Corresponding author. Tel: +86-431-8526-2646. Fax: +86-431-8526-
2646. E-mail: jchen@ciac.jl.cn.
10.1021/je7000478 CCC: $37.00 © 2007 American Chemical Society
Published on Web 05/08/2007

Journal of Chemical and Engineering Data, Vol. 52, No. 4, 2007 1333
Table 1. Binodal Data for Salt (1) + Salt (2) + Water (3) ABS at 298.15 K
[C4mim]Cl + K3PO4 [C4mim]Cl + K2CO3 [C6mim]Cl + K3PO4 [C6mim]Cl + K2CO3
[C8mim]Cl + K3PO4
[C8mim]Cl + K2CO3
100 w2
100 w1
100 w2
100 w1
100 w2
100 w1
100 w2
100 w1
100 w2
100 w1
100 w2
100 w1
15.48
16.68
17.84
19.12
20.37
21.39
22.36
22.65
23.76
24.41
24.85
25.49
25.98
26.37
27.03
27.33
27.87
28.08
28.70
29.34
29.70
29.99
30.65
30.96
31.88
18.73
16.68
15.38
13.72
12.19
10.99
9.97
9.57
8.53
7.84
7.46
6.81
6.32
6.02
5.51
5.29
4.87
4.69
4.27
3.88
3.62
3.46
3.11
2.95
2.51
13.69
16.62
18.95
20.73
21.98
23.77
25.18
26.49
27.64
28.65
29.52
30.38
31.74
33.28
25.48
20.89
17.67
15.19
13.34
11.19
9.69
8.34
7.25
6.35
5.61
7.41
9.44
41.94
35.63
25.27
22.60
19.47
17.05
15.17
13.53
11.89
10.98
10.23
9.57
8.63
7.80
7.34
6.90
6.52
5.55
5.25
4.71
8.41
10.44
13.55
14.52
16.91
18.50
19.53
21.16
22.31
23.35
23.77
24.54
25.30
26.04
26.80
27.14
27.70
28.72
29.63
30.44
31.18
32.06
32.77
33.67
44.51
36.57
29.66
26.25
21.97
19.09
16.86
14.57
12.77
11.32
10.42
9.45
8.55
7.76
7.03
6.61
6.11
5.23
4.51
3.96
13.55
14.66
16.87
18.37
20.04
21.26
22.15
23.33
24.38
25.66
26.52
27.16
27.87
28.73
29.36
29.90
30.74
31.65
32.56
34.31
35.53
28.83
25.24
21.63
18.84
16.03
14.39
12.95
11.43
10.12
8.75
7.78
7.07
6.39
5.70
5.16
4.72
4.16
3.61
9.22
11.56
13.34
15.51
17.09
18.28
19.61
20.16
21.29
22.23
22.94
23.5
23.84
24.52
24.93
25.13
25.62
26.07
26.94
27.453
28.13
30.42
30.76
40.9
34.23
29.27
24.74
21.53
19.04
16.97
15.98
14.20
12.80
11.69
10.72
10.29
9.4
8.77
8.48
7.88
7.36
6.51
6.05
5.51
3.76
13.22
14.02
15.84
17.20
18.04
19.24
20.41
21.21
21.60
21.96
22.89
23.99
24.50
24.84
25.12
26.15
26.34
26.84
27.66
28.07
4.95
4.08
3.19
3.13
2.31
1.78
4.12
3.78
3.45
2.96
2.58
2.12
3.56
Table 2. Parameters of Equation 1 for Salt (1) + Salt (2) + Water
Literature results confirmed that the method can satisfactorily
correlate binodal curves and determined tie lines in [C4mim]Cl
+ K3PO4 ABS.11 The coefficients of eq 1 along with the
corresponding correlation coefficient values (R) and standard
deviations (σa) are given in Table 2, it indicated that the
empirical equation (eq 1) gave a rather good fit to the data for
descriptive purposes. Tie lines for these cloud point curves were
fitted by a least-squares method based on the lever arm rule
described previously. Numerical data for these tie lines are given
in Table 3. The average slopes of the tie lines are shown in
Table 3. As may be seen from these data, the slopes of the tie
lines were approximately constant at short tie line lengths but
showed significant change as tie line length increased. This was
probably due to the fact that the salt phase was virtually free of
IL at long tie line lengths.19 These data are also in agreement
with that for [C4mim]Cl + K3PO4 ABS initially reported by
Gutowski et al.11
(3) ABS
system
a
b
c
R
σa
1 [C4mim]Cl/K3PO4
77.19 -0.3132 5.123 × 10-5 0.9998 0.065
2 [C4mim]Cl/K2CO3 83.80 -0.2914 4.347 × 10-5 0.9999 0.069
3 [C6mim]Cl/K3PO4 152.47 -0.4635 5.385 × 10-5 0.9993 0.26
4 [C6mim]Cl/K2CO3 149.52 -0.4118 4.802 × 10-5 0.9993 0.29
5 [C8mim]Cl/K3PO4 144.80 -0.4141 4.275 × 10-5 0.9994 0.19
6 [C8mim]Cl/K2CO3 132.49 -0.3741 5.399 × 10-5 0.9997 0.16
where M, T, and B represent the mixture, the top phase, and
the bottom phase, respectively; Y and X are the concentration
of IL and salt; and R is the following measured ratio:
R ) (weight of the top phase)/(weight of the mixture) (4)
This is a system of four equations with four unknowns that can
easily be solved, and the equation for the tie lines can be
obtained:
We can generalize the formation of ABS of this type by
considering the use of other water structuring salts to salt-out
the IL ions from aqueous solution.9 Figure 1 illustrates the
binodal curves for [C4mim]Cl, [C6mim]Cl, and [C8mim]Cl with
K3PO4 and K2CO3 ABS. It shows that the binodal curves for
Y ) Y0 + S
(5)
where the slope (S) is given by
S ) (YT - YB)/(XT - XB)
(6)
and Y0 is the point corresponding to X ) 0. The tie line lengths
(TLL) for the different compositions were calculated by the
equivalent expression:
TLL ) [(YT - YB)2 + (XT - XB)2]1/2
(7)
Results and Discussion
The binodal data for salt + salt + water systems are given
in Table 1. The binodal curves for the various salt + salt systems
are represented in Figure 1. The data were fitted according to
the empirical relationship of Merchuk, and the parameters for
this equation as determined by least-squares regression of the
cloud point data are shown in Table 2. Huddleston et al. have
studied phase diagram data for several PEG + salt + water
ABS at 25 °C using this approach and obtained good results.19
Figure 1. Phase diagram for [Cnmim]Cl (1) + salts (2) + water
(3) ABS at 298.15 K: O, [C4mim]Cl + K3PO4 + water; 4, [C6mim]Cl +
K3PO4 + water; 0, [C8mim]Cl + K3PO4 + water; b, [C4mim]Cl + K2-
CO3 + water; 2, [C6mim]Cl + K2CO3 + water; 9, [C8mim]Cl + K2CO3
+ water.

1334 Journal of Chemical and Engineering Data, Vol. 52, No. 4, 2007
Table 3. Tie Line Composition of Salt (1) + Salt (2) + Water (3)
the same IL with different salts ABS, and K3PO4 results in the
binodal curve closer than K2CO3 to the origin, meaning less
salt is needed to form an ABS. It indicated that the salting-out
ability of the anions follows the ordering PO43- > CO32-. This
Assigned by the Lever Arm Rule
[C4mimCl] (1) + K3PO4 (2) + Water (3)
tie 1
tie 2
tie 3
tie 4
ordering directly follows the anions calculated ∆Ghyd values
100 w1(mixture)
100 w2(mixture)
100 w1(top)
100 w1(bottom)
100 w2(top)
100 w2(bottom)
R
15.22
25.04
44.49
1.19
16.63
27.89
51.76
0.40
18.12
30.08
60.16
0.19
22.16
39.99
75.63
0.0007
0.0042
56.56
0.293
94.44
-1.34
(all ∆Ghyd date from ref 3, PO43-, -2835 kJ‚mol-1 > CO3
,
2-
-1300 kJ‚mol-1).
The correlating equations proposed by Othmer-Tobias (eq
8) and Bancroft (eq 9) have been used to correlate the tie line
compositions:
3.08
1.63
0.63
35.57
0.324
54.13
-1.33
40.02
0.316
64.12
-1.34
42.64
0.299
73.22
-1.43
TLL
S
t
b n
1 - W1
1 - W2
[C4mimCl] (1) + K2CO3 (2) + Water (3)
) K
(8)
(9)
t
b
(
)
(
)
tie 1
tie 2
tie 3
tie 4
tie5
W1
W2
100 w1(mixture)
100 w2(mixture)
100 w1(top)
100 w1(bottom)
100 w2(top)
100 w2(bottom)
R
15.08
23.01
33.61
4.66
15.99
24.86
42.75
2.39
16.98
30.12
59.47
0.62
18.14
34.84
68.13
0.12
18.94
39.85
76.63
0.016
0.094
52.89
0.247
93.04
-1.45
b
b
t
r
W3
W2
W3
W1
) K1
t
(
)
(
)
9.13
5.24
1.38
0.50
30.82
0.360
36.17
-1.33
34.83
0.337
50.04
-1.36
41.18
0.278
71.04
-1.47
47.22
0.265
82.51
1.45
t
b
where W1 is the mass fraction of ILs in the top phase; W2 is
TLL
b
t
S
the mass fraction of salt in the bottom phase; W3 and W3 are
the mass fractions of water in the bottom and top phases,
respectively; and K, n, K1, and r are the fit parameters. Equations
8 and 9 are linearized by taking the logarithm on both sides of
the equations to determine the fit parameters. The values of the
parameters are given in Table 4.
[C6mimCl] (1) + K2CO3 (2) + Water (3)
tie 1
tie 2
tie 3
tie 4
100 w1(mixture)
100 w2(mixture)
100 w1(top)
100 w1(bottom)
100 w2(top)
100 w2(bottom)
R
15.18
25.33
36.47
0.69
17.24
29.98
60.72
0.25
19.00
34.84
72.16
0.031
2.60
46.35
0.263
84.36
-1.65
21.09
39.87
83.03
0.0018
1.72
52.86
0.254
97.51
-1.62
Recently, eqs 8 and 9 have been successfully used for the
correlation of the tie line compositions of the PEG + sodium
citrate + water and PEG + potassium citrate + water
9.01
3.92
36.44
0.405
45.08
-1.30
40.17
0.281
70.50
-1.67
TLL
t
systems.20,21 A linear dependency of the plots log[(1 - W1 )/
S
t
b
b
b
b
W1 ] against log[(1 - W2 )/W2 ] and log(W3 /W2 ) against
[C6mimCl] (1) + K2CO3 (2) + Water (3)
t
t
log(W3 /W1 ) indicated an acceptable consistency of the results.
The corresponding correlation coefficient values (R) are also
given in Table 4. Furthermore, on the basis of the standard
deviations (σa) given in Table 4, eqs 8 and 9 can be satisfactorily
used to correlate the tie line data of the investigated systems.
In the new salt + salt ABS, there are four different ions and
speciation of the two different cations and anions. Upon phase
separation these ions partition to one of the two phases while
maintaining electroneutrality. The most chaotropic ion of each
type partitions to the upper phase. Increased kosmotropic salts
concentrations enhance phase separation. As the TLL increases,
the bottom phase becomes increasingly kosmotropic. The phase
is becoming more structured, and thus forcing the chaotropic
ion to the upper phase with concurrent transfer of chloride
anions. The increased water structuring of the kosmotropic phase
decreases the difference in the distribution ration between [Cn-
mim]+ and Cl-.16
tie 1
tie 2
tie 3
tie 4
tie5
100 w1(mixture)
100 w2(mixture)
100 w1(top)
100 w1(bottom)
100 w2(top)
100 w2(bottom)
R
15.12
25.13
46.89
3.19
16.04
27.92
61.44
1.55
17.07
30.08
69.26
0.77
18.10
32.01
75.44
0.39
19.81
37.79
87.91
0.039
1.66
48.28
0.225
99.47
-1.88
7.64
4.62
3.48
2.75
31.70
0.273
49.89
-1.82
35.36
0.242
67.32
-1.95
38.39
0.238
76.87
-1.96
41.05
0.236
84.26
-1.96
TLL
S
[C8mimCl] (1) + K3PO4 (2) + Water (3)
tie 1
tie 2
tie 3
tie 4
100 w1(mixture)
100 w2(mixture)
100 w1(top)
100 w1(bottom)
100 w2(top)
100 w2(bottom)
R
16.13
25.49
52.93
3.20
17.95
30.03
67.48
0.82
19.52
35.18
79.56
0.14
20.46
38.71
86.95
0.035
1.52
50.16
0.235
99.60
-1.79
5.81
3.38
2.09
32.40
0.260
56.39
-1.87
39.25
0.257
75.70
-1.85
45.86
0.244
90.68
-1.81
TLL
S
[C8mimCl] (1) + K2CO3 (2) + Water (3)
Conclusions
tie 1
tie 2
tie 3
tie 4
tie 5
100 w1(mixture)
100 w2(mixture)
100 w1(top)
100 w1(bottom)
100 w2(top)
100 w2(bottom)
R
18.31
27.11
67.91
1.33
19.19
31.00
74.91
0.32
19.64
34.69
78.06
0.062
2.00
45.65
0.251
89.38
-1.79
20.10
37.88
85.84
0.017
1.35
49.04
0.234
98.18
-1.79
21.01
41.43
90.17
0.002
1.06
53.69
0.233
104.40
-1.71
For salt ([C4mim]Cl, [C6mim]Cl, and [C8mim]Cl) + salt (K3-
PO4, K2CO3) + water ABS, binodal data and tie line data were
measured at 298.15 K. The binodal data was fit with the
Merchuk equation relating the concentration of the ILs and salt.
The tie line data for these systems were correlated satisfactorily
by the Othmer-Tobias and Bancroft equations.
3.18
2.32
35.30
0.255
73.92
-2.07
40.71
0.253
83.89
-1.94
TLL
S
Table 4. Values of the Parameters of Equations 8 and 9
systems
K
n
R
σa
K1
r
R
σa
1 [C4mim]Cl/K3PO4
2 [C4mim]Cl/K2CO3
3 [C6mim]Cl/K3PO4
4 [C6mim]Cl/K2CO3
5 [C8mim]Cl/K3PO4
6 [C8mim]Cl/K2CO3
0.4715
0.3695
0.2614
0.1104
0.1630
0.1601
1.5649
2.0253
2.9862
2.9714
2.3668
1.9375
0.9898
0.9965
0.9696
0.9988
0.9964
0.9737
0.044
0.032
0.11
0.019
0.035
0.067
1.5691
1.5443
1.3897
1.9301
2.0153
2.3747
0.6008
0.4329
0.2613
0.3051
0.4012
0.4806
0.9866
0.9909
0.9375
0.9957
0.9963
0.9686
0.030
0.022
0.046
0.011
0.014
0.035

Journal of Chemical and Engineering Data, Vol. 52, No. 4, 2007 1335
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Received for review January 30, 2007. Accepted April 1, 2007. This
work was supported by the Natural Science Foundation of China (No.
50574080), Hundred of Talent Program of Chinese Academy of
Sciences, and the Distinguished Young Scholar Foundation of Jilin
Province (No. 20060114).
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