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Liquid−Liquid Equilibrium of Alcohols + Ammonium/Potassium/
Sodium Acetate + Water Systems: Experimental and Correlation
Sewn Cen Lo,† Ramakrishnan Nagasundara Ramanan,†,‡ Beng Ti Tey,†,‡ Tau Chuan Ling,§
Pau Loke Show,∥ and Chien Wei Ooi*,†,‡
†
Chemical Engineering Discipline, School of Engineering and ‡Advanced Engineering Platform, School of Engineering,
Monash University Malaysia, Jalan Lagoon Selatan, 47500 Bandar Sunway, Selangor, Malaysia
§
Institute of Biological Sciences, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia
∥
Manufacturing and Industrial Processes Division, Faculty of Engineering, Centre for Food and Bioproduct Processing,
University of Nottingham Malaysia Campus, Jalan Broga, 43500 Semenyih, Selangor Darul Ehsan, Malaysia
S Supporting Information
*
ABSTRACT: Liquid−liquid equilibra of alcohol + acetate salt + water
systemsnamely, 2-butanol + ammonium acetate + water, 1-propanol +
sodium/potassium acetate + water, and 2-propanol + sodium/potassium acetate +
water systemswere successfully determined. The ability of acetate salt to form
aqueous two-phase system (ATPS) with alcohol was found to be mainly driven
by a high Gibbs energy (i.e., greater than −1000 kJ/mol). The fitting of the
experimental binodal data was done by using the Merchuk equations. The tieline data were satisfactorily correlated by the Othmer−Tobias and Bancroft
equations. The salting-out strength of the acetate salts was evaluated based on
the effective excluded volume theory and the Setschenow-type equation.
The phase-forming abilities of the investigated alcohols were in the order of
2-butanol > 1-propanol > 2-propanol. For 2-propanol + acetate salt + water
systems, the salting-out strength of sodium acetate was greater than that of
potassium acetate. In contrast, for 1-propanol + acetate salt + water systems,
the salting-out strength of potassium acetate was greater than that of sodium acetate. Ethanol was unable to form ATPS with all
the investigated acetate salts, whereas ammonium acetate could not form ATPS with either 1-propanol or 2-propanol.
1. INTRODUCTION
Aqueous two-phase systems (ATPSs) have been successfully
applied to the partitioning and purification of proteins, human
antibodies, enzymes, plant cells, and nanoparticles.1−7 The formation of ATPS entails the mixing of phase-forming components above a threshold concentration.8 Unlike the traditional
liquid−liquid extraction which utilizes harmful organic solvents,
ATPS is made up of a high percentage of water and therefore
the nature of biomolecules can be well preserved in the twophase system. ATPS can overcome the limitations in chromatographic techniques, such as the low throughput, low capability
in process integration, tedious processing cycles, difficulty in
scaling up, and barrier in diffusional transfer.9,10
In general, there are two major categories of ATPSs, namely,
polymer-based ATPSs11,12 and nonpolymer-based ATPSs.13,14
Dual-polymer ATPSs and polymer + salt ATPSs have been well
studied in the past 50 years. In the past decade, the use of ionic
liquid (IL) or alcohol as a phase-forming component in ATPS
was greatly explored. Nevertheless, the IL-based ATPS poses
restriction in practical application owing to the high synthesis
cost of IL and the difficulty in recycling IL.15 In contrast, ATPS
composed of short-chain aliphatic alcohol and salts has been
perceived as an inexpensive and sustainable variant of ATPS.
© 2015 American Chemical Society
Alcohol + salt ATPS offers the advantages such as inexpensiveness and wide availability of phase-forming constituents,
ease of solvent recovery and reutilization, low viscosity as
well as rapid phase-separation.13 The recycling of both phaseforming components in alcohol + salt ATPS is feasible. Alcohol
can be easily recovered by evaporation process, whereas the salt
can be recovered through an extractive crystallization utilizing
alcohol.15−17
The liquid−liquid equilibrium (LLE) data is crucial in the
design of ATPS as a purification technique. The LLE data
provide information regarding the phase behavior and the
physicochemical properties of ATPSs,18 which are useful in the
study of the partitioning of molecules between the two phases.
Previously, numerous LLE data on alcohol + salt ATPSs were
established.1,15−17,19−21 In particular, the application of biodegradable salts (e.g., citrate, carbonate and acetate) in the
formation of ATPS is more favorable because the discharge of
wastewater containing these salts will not raise serious environmental concerns. LLE of sodium/potassium/ammonium
Received: March 4, 2015
Accepted: August 19, 2015
Published: September 1, 2015
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DOI: 10.1021/acs.jced.5b00200
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Article
citrate + alcohol + water systems20,22−24 and sodium/potassium/
cesium carbonate + alcohol + water systems16,25,26 also have
been reported.
To the best of our knowledge, there is no LLE data available
for ATPSs composed of alcohols and acetate salts. In view of
this, the ability of acetate salts to form two-phase systems
with aliphatic alcohols was studied and the respective LLE data
of alcohol + acetate salts were generated. The experimental
binodal curves were correlated by the Merchuk equations,18
whereas the obtained TLL data were fitted using Othmer−
Tobias27 and Bancroft equations.28 Furthermore, the saltingout abilities of acetate salts were evaluated by using effective
excluded volume (EEV) theory21,29 and Setschenow-type
equation.29−31
Table 2. Binodal Data in Unit of Mass Fraction for the
Alcohol (1) + Acetate Salt (2) + Water (3) Systems at T =
297.15 Ka and p = 0.1 MPa
w1
0.1574
0.1552
0.1543
0.1278
0.7165
0.7012
0.6854
0.6693
0.5825
0.4819
2. MATERIALS AND METHODS
2.1. Materials. The source and purity of chemical reagents
used in this work are shown in Table 1. All the chemicals were
used as received. Deionized water was used in all experiments.
0.7091
0.6829
0.6760
0.6549
0.5102
0.4771
Table 1. Source and Purity of Chemical Reagents Used in
This Work
chemical name
ethanol (EtOH)
1-propanol (1-PrOH)
2-propanol (2-PrOH)
2-butanol (2-BuOH)
sodium acetate
(NaCH3COO)
potassium acetate
(KCH3COO)
ammonium acetate
(NH4CH3COO)
source
Merck
Darmstadt,
Germany
Merck
Darmstadt,
Germany
Merck
Darmstadt,
Germany
Merck
Darmstadt,
Germany
Sigma-Aldrich,
U.S.A.
Sigma-Aldrich,
U.S.A.
Sigma-Aldrich,
U.S.A.
mass
fraction
purity
analysis method
≥ 0.999
gas chromatography
≥ 0.995
gas chromatography
≥ 0.998
gas chromatography
≥ 0.990
gas chromatography
≥ 0.990
titration
≥ 0.990
titration
≥ 0.990
calculated on dry
substance
0.8714
0.8640
0.8322
0.7941
0.7897
0.8863
0.8610
0.8321
0.7968
0.6573
w2
w1
w2
w1
2-BuOH + NH4CH3COO + water
0.0045
0.1241
0.0496
0.0537
0.0133
0.1210
0.0545
0.0497
0.0088
0.0977
0.1075
0.0424
0.0426
0.0845
0.1776
0.0401
1-PrOH + NaCH3COO + water
0.0423
0.3233
0.1167
0.1760
0.0420
0.2817
0.1315
0.1617
0.0450
0.2128
0.1560
0.1612
0.0476
0.2000
0.1600
0.1279
0.0583
0.1908
0.1679
0.1158
0.0803
0.1783
0.1765
1-PrOH + KCH3COO + water
0.0382
0.4752
0.0950
0.2128
0.0420
0.3063
0.1378
0.2104
0.0432
0.2896
0.1448
0.1668
0.0466
0.2712
0.1492
0.1539
0.0850
0.2562
0.1537
0.1533
0.0954
0.2131
0.1705
0.1347
2-PrOH + NaCH3COO + water
0.0396
0.7679
0.0555
0.2290
0.0401
0.6433
0.0792
0.1910
0.0436
0.4988
0.1197
0.1794
0.0496
0.4439
0.1381
0.1664
0.0528
0.2938
0.1958
0.1442
2-PrOH + KCH3COO + water
0.0577
0.6315
0.1122
0.1726
0.0586
0.4959
0.1622
0.1430
0.0605
0.2679
0.2624
0.1315
0.0638
0.2347
0.2813
0.1242
0.1013
0.1942
0.3006
w2
0.3719
0.3977
0.4330
0.4579
0.1760
0.1860
0.1917
0.2139
0.2315
0.1702
0.1731
0.1895
0.2014
0.1992
0.2271
0.2290
0.2503
0.2584
0.2659
0.2825
0.3212
0.3444
0.3612
0.3725
a
Standard uncertainty of temperature, u(T) = 1 K. Expanded
uncertainty: for 2-BuOH + NH4CH3COO + water system, Uc are
Uc(2-BuOH) = Uc(NH4CH3COO) = 0.0006 (95% level of confidence); for 1-PrOH + NaCH3COO + water system, Uc(1-PrOH) =
Uc(NaCH3COO) = 0.0035 (95% level of confidence); for 1-PrOH +
KCH3COO + water system, Uc(1-PrOH) = Uc(KCH3COO) = 0.0022
(95% level of confidence); for 2-PrOH + NaCH3COO + water system,
Uc(2-PrOH) = Uc(NaCH3COO) = 0.0093 (95% level of confidence); for 2-PrOH + KCH3COO + water system, Uc(2-PrOH) =
Uc(KCH3COO) = 0.0116 (95% level of confidence).
2.2. Construction of Binodal Curve. The binodal data
were determined experimentally by using turbidimetric titration
method.32 In brief, a 10 g sample of ATPS was first prepared
by adding known mass fractions of alcohol, salt, and deionized
water in a centrifuge tube to produce a turbid mixture, which is
an indication of the formation of two-phase solution. Then,
the deionized water was added dropwise to the turbid solution
followed by a gentle mixing until the solution turned clear and
transparent. The mass of the water added to the mixture was
measured by an analytical balance with a precision of ± 0.0001 g,
and was used in the calculation of the concentrations of alcohol
and salt in the final mixture. The procedure was repeated until
there were sufficient points for plotting the binodal curve. All the
experiments were conducted at 297.15 K.
2.3. Determination of Tie-Line. The tie-line data were
obtained from the measurement of the concentrations of phaseforming components in both top and bottom phases of ATPSs
at room temperature. First, an ATPS sample at a final weight
of 20 g was prepared in a 50 ml centrifuge tube. After the
equilibrium and separation stages, the aliquots of samples were
carefully withdrawn from both phases by using a syringe fitted
with needle. The salt concentration was determined by conductivity measurement33 based on the equation in the following
form:
κ = b0 + b1w2
(1)
−1
where κ is the conductivity (S·m ); b0 and b1 are the fitting
parameters; w2 is the mass fraction of salt. The conductivity
predicted by eq 1 is valid only when the salt mass fraction is
≤ 0.15. The relative standard uncertainty of the conductivity
measurement was found to be 0.20. The concentrations of
the standard ternary solutions and the measured conductivity
data are given in Supporting Information Table S1. The values
of fitting parameters for eq 1 are reported in Supporting
Information Table S2.
The concentration of alcohol was determined by the refractive index measurement performed using a refractometer.19 The
uncertainty in the measurement of the refractive index was
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Figure 1. LLE data for the 2-BuOH (1) + NH4CH3COO (2) + water (3) system: □, experimental binodal data; ---, binodal curve reproduced
by eq 3; , tie-line; +, total compositions of tie-line.
Figure 2. LLE data for the 1-PrOH (1) + NaCH3COO (2) + water (3) system: □, experimental binodal curve; ---, binodal curve reproduced
by eq 3; , tie-line; +, total composition of tie-line.
found to be ± 0.0001. For dilute aqueous solution containing
alcohol and salt, the relation between refractive index (nD),
mass fractions of alcohol (w1), and mass fraction of salt (w2) is
as follows:
nD = a0 + a1w1 + a 2w2
3. RESULTS AND DISCUSSION
3.1. Binodal Data and Correlation. The experimental
binodal data of 2-BuOH + NH4CH3COO + water, 1-PrOH +
NaCH3COO + water, 1-PrOH + KCH3COO + water, 2-PrOH +
NaCH3COO + water, and 2-PrOH + KCH3COO + water
systems are given in Table 2, and their respective binodal curves
are presented in Figures 1−5. For the correlation of experimental
binodal data, the empirical nonlinear equation developed by
Merchuk et al.18 was adopted
(2)
where a0 is the refractive index of pure water (i.e., 1.3325
at 298.15 K). a1 and a2 are the constants of alcohol and salt,
respectively. It should be noted that, eq 2 is valid only under
the condition in which w1 ≤ 0.1 and w2 ≤ 0.05. The values of
coefficients for eq 2 are reported in Supporting Information
Table S3.
w1 = a + bw20.5 + cw2 + dw22
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Figure 3. LLE data for the 1-PrOH (1) + KCH3COO (2) + water (3) system: □, experimental binodal data; ---, binodal curve reproduced by
eq 3; , tie-line; +, total compositions of tie-line.
Figure 4. LLE data for the 2-PrOH (1) + NaCH3COO (2) + water (3) system: □, experimental binodal data; ---, binodal curve reproduced by
eq 3; , tie-line; +, total compositions of tie-line.
3.2. Tie-Line Data and Correlation. The tie-line data of
the investigated systems are shown in Table 4 and the plots of
tie-lines are presented in Figures 1−5. The Othmer−Tobias
equation (eq 4) and the Bancroft equation (eq 5) were used to
evaluate the reliability of the tie-line data obtained from the
experimental results
where w1 is the mass fraction of aliphatic alcohols; w2 is the mass
fraction of salts; and a, b, c, and d are the fitting parameters.
The eq 3 has been successfully used in the fitting of binodal
data of ATPSs such as alcohol + salt + water, IL + salt + water,
and polymer + salt + water systems.23,34,35 The fitting parameters in eq 3 was solved by least-squares regression method
using the Solver function in Microsoft Excel.36 The fitting
parameters, correlation coefficient (R2), and standard deviations
(SD) obtained from the correlation of binodal data are listed in
Table 3. The reliability of eq 3 in the fitting of the investigated
ternary systems was confirmed by the obtained R2 values
approaching unity and the SD values ranging from 0.0026 to
0.0138 (as shown in Table 3).
⎛ 1 − w b ⎞n
1 − w1t
2
⎟⎟
=
k
⎜
1⎜
b
w1t
w
⎝
⎠
2
(4)
⎛ w3t ⎞r
= k 2⎜ t ⎟
w2b
⎝ w1 ⎠
(5)
w3b
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Figure 5. LLE data for the 2-PrOH (1) + KCH3COO (2) + water (3) system: □, experimental binodal data; ---, binodal curve reproduced by
eq 3; , tie-line; +, total compositions of tie-line.
Table 3. Values of Parameters of Eq 3 for the Alcohol + Acetate Salt + Water Systems
systems
a
b
c
d
R2
SDa
2-BuOH + NH4CH3COO + water
1-PrOH + NaCH3COO + water
1-PrOH + KCH3COO + water
2-PrOH + NaCH3COO + water
2-PrOH + KCH3COO + water
0.1800
1.4291
0.4118
1.4913
1.1640
−0.2850
−3.7278
4.3343
−3.4986
−3.3603
0.1461
0.8006
−15.6576
1.7939
1.5834
−0.0630
5.5216
23.4319
0.0411
0.6404
0.9996
0.9999
0.9991
0.9999
0.9999
0.0026
0.0057
0.0057
0.0042
0.0138
a
exp 2
exp
0.5
SD = (Σi N= 1(wcal
1 −w1 ) /N) , where w1 and N represent the mass fraction of alcohol and the number of binodal data, respectively. w1 is the
is
the
corresponding
data
calculated
using
eq
3.
experimental mass fraction of alcohol, and wcal
1
the salt’s kosmotropicity.38 In an aqueous solution containing
a mixture of salt and water-miscible alcohol, a salt’s ion possessing a stronger intermolecular interaction with water will be
able to attract more water molecules, which in turn weakens
the alcohol−water intermolecular interactions but promotes the
interaction between alcohol molecules in the solution. If the
concentration of kosmotropic salt exceeds a specific threshold,
the alcohol will be excluded from the solution and an alcoholrich phase will be formed.
Figure 6 presents the compilation of binodal curves plotted
in unit of modified molality for all the obtained alcohol +
acetate salt + water ternary systems. The equations for modified
molality are as follows:
where wt1, wb1, wt2, wb2,wt3 and wb3 represent the equilibrium
compositions (in mass fraction) of alcohol (1), salt (2), and
water (3) in the top (t) and bottom (b) phases, respectively. k1,
k2, n, and r are the fitting parameters. The Othmer−Tobias
equation27 and Bancroft equation28 have been widely used in the
correlation of tie-line compositions of poly(ethylene glycol) +
salt + water systems, IL + salt + water systems, and hydrophilic
alcohol + salt + water systems.26,34,37 The fitting parameters of
eqs 4 and 5 were calculated along with the R2 and SD values (as
shown in Table 5). For all of the investigated ternary systems,
the obtained R2 values were higher than 0.97, indicating a good
fitting of the experimental results with both Othmer−Tobias and
Bancroft equations. The tie-line length (TLL) and tie-line slope
(TLS) were calculated using eqs 6 and 7
TLL =
2
ΔX + ΔY
TLS = ΔY /ΔX
2
(6)
Alcohol = 100w1/M1
(8)
Salt = 100w2/M 2
(9)
where w1, w2, M1, and M2 represent the mass fraction of alcohol,
the mass fraction of salt, the molecular weight of alcohol and
the molecular weight of salt, respectively. The phase-forming
ability of the investigated alcohols could be postulated from the
position of the binodal curves in the phase diagram. A binodal
curve located closer to the origin in the phase diagram implies a
larger biphasic region and therefore the formation of ATPS
could be achieved at the lower concentrations of the phaseforming components. Judging from the position of binodal curves
for 1-PrOH + NaCH3COO/KCH3COO + water systems and
2-PrOH + NaCH3COO/KCH3COO + water systems shown in
(7)
where ΔX and ΔY represent the difference (in mass fraction) of
acetate salt (2) and alcohol (1), respectively, in both top (t) and
bottom (b) phases (i.e., ΔX = wt2 − wb2; ΔY = wt1 − wb1). The tielines of 2-PrOH + NaCH3COO/KCH3COO + water systems
are parallel, whereas the TLSs for 2-BuOH + NH4CH3COO +
water, and 1-PrOH + NaCH3COO/KCH3COO + water systems
become less steep as the TLLs increase.
3.3. Phase-Forming Ability of Alcohols and Acetate
Salts. The phase-forming ability of an alcohol can be related to
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Table 4. Tie-Line Data in Unit of Mass Fraction for the Alcohol (1) + Acetate Salt (2) + Water (3) Systems at T = 297.15 Ka
and p = 0.1 MPa
total compositions
w1
w2
top phase
bottom phase
w1
w2
w1
w2
TLL
TLS
0.3719
0.3977
0.4330
0.4579
0.7904
0.8013
0.8211
0.8302
−1.9781
−1.8538
−1.7156
−1.6198
0.1765
0.1917
0.2139
0.2315
0.5076
0.5443
0.5985
0.6298
−3.8089
−3.5712
−3.3355
−3.1748
0.1895
0.1731
0.2014
0.2271
0.4621
0.5298
0.5525
0.6047
−3.5162
−3.4810
−3.3184
−3.0408
0.2503
0.2584
0.2659
0.2825
0.6089
0.6440
0.7019
0.7595
−2.9615
−2.9684
−2.9951
−2.9695
0.3212
0.3444
0.3612
0.3725
0.6752
0.7453
0.7898
0.8246
−2.4250
−2.4273
−2.4108
−2.4209
2-BuOH + NH4CH3COO + water
0.4957
0.5094
0.5154
0.5156
0.1485
0.1497
0.1573
0.1643
0.7591
0.7549
0.7518
0.7465
0.3987
0.4091
0.4223
0.4386
0.1187
0.1223
0.1256
0.1298
0.6693
0.6854
0.7012
0.7165
0.4128
0.3973
0.4147
0.4317
0.1188
0.1163
0.1228
0.1294
0.6760
0.6549
0.6829
0.7091
0.5818
0.6125
0.6233
0.6523
0.1183
0.1125
0.1133
0.1114
0.7679
0.7897
0.8322
0.8640
0.5080
0.5268
0.5505
0.5582
0.1829
0.1863
0.1874
0.1932
0.7968
0.8321
0.8610
0.8863
0.0153
0.0537
0.0173
0.0497
0.0195
0.0424
0.0218
0.0401
1-PrOH + NaCH3COO + water
0.0476
0.1783
0.0450
0.1612
0.0420
0.1279
0.0423
0.1158
1-PrOH + KCH3COO + water
0.0432
0.1668
0.0466
0.2104
0.0420
0.1539
0.0382
0.1347
2-PrOH + NaCH3COO + water
0.0555
0.1910
0.0528
0.1794
0.0436
0.1664
0.0401
0.1442
2-PrOH + KCH3COO + water
0.0638
0.1726
0.0605
0.1430
0.0586
0.1315
0.0577
0.1242
a
Standard uncertainty of temperature, u(T) = 1K. Expanded uncertainty: for 2-BuOH + NH4CH3COO + water system, Uc are Uc(2-BuOH) =
Uc(NH4CH3COO) = 0.0030 (95% level of confidence); for 1-PrOH + NaCH3COO + water system, Uc(1-PrOH) = Uc(NaCH3COO) = 0.0106
(95% level of confidence); for 1-PrOH + KCH3COO + water system, Uc(1-PrOH) = Uc(KCH3COO) = 0.0150 (95% level of confidence); for
2-PrOH + NaCH3COO + water system, Uc(2-PrOH) = Uc(NaCH3COO) = 0.0325 (95% level of confidence); for 2-PrOH + KCH3COO + water,
Uc(2-PrOH) = Uc(KCH3COO) = 0.0024 (95% level of confidence).
Table 5. Values of Parameters of Eqs 4 and 5 for the Alcohol + Acetate Salt + Water Systems
Othmer−Tobias equation
2
system
k1
n
R
2-BuOH + NH4CH3COO + water
1-PrOH + NaCH3COO + water
1-PrOH + KCH3COO + water
2-PrOH + NaCH3COO + water
2-PrOH + KCH3COO + water
1.030
0.611
0.791
7.858
4.303
−0.398
2.783
3.141
17.37
9.854
0.977
0.996
0.991
0.970
0.972
Bancroft equation
SD
k2
r
R2
SD
0.001
0.001
0.002
0.012
0.012
2.078
0.993
0.711
0.427
0.309
−4.696
1.151
0.484
0.124
0.152
0.971
0.998
0.971
0.965
0.978
0.006
0.002
0.007
0.003
0.007
For isomers like 2-PrOH, the branch chain in the molecular
structure will confer a steric hindrance resulting in a reduction
in the acting force.39 A higher acting force between 1-PrOH
molecules causes an easier exclusion of 1-PrOH from the
aqueous-rich phase in comparison with that of 2-PrOH. As
for the phase-forming ability of 2-BuOH, it is believed that
2-BuOH can easily form ATPS with acetate salts owing to its
hydrophobic nature. Unlike other short-chain alcohols,
2-BuOH has a longer carbon chain length which makes it partially miscible in water. An increase in the carbon chain length of
alcohol will decrease the solubility of alcohol in water because
the alcohol molecules are more tightly packed as the size and
mass of alcohol increase. Moreover, the self-intermolecular forces
of 2-BuOH are stronger than that of 1-PrOH and 2-PrOH. All in
all, the phase-forming ability of the alcohols is in the order of:
2-BuOH > 1-PrOH > 2-PrOH.
Figure 6, it is evident that the binodal curves for 1-PrOH +
NaCH3COO/KCH3COO + water systems are closer to the
origin in phase diagram as compared with that for 2-PrOH +
NaCH3COO/KCH3COO + water systems.
The fact that the phase-forming ability of 1-PrOH is greater
than that of 2-PrOH can be explained in terms of the alcohol’s
properties such as self-intermolecular forces and molecular
branching. As implied earlier, the formation of alcohol + salt
ATPS necessitates an exclusion of alcohol from the solution,
which is facilitated by a strong molecular interaction between
molecules of alcohol. The self-intermolecular forces (i.e.,
hydrogen bonding and van der Waals forces) between alcohol
molecules can be evaluated by the alcohol’s boiling point.
Because the boiling point of 1-PrOH (97 °C) is higher than
that of 2-PrOH (82.6 °C), the self-intermolecular forces in
1-PrOH are stronger and therefore 1-PrOH can form an ATPS
with salt at a relatively lower concentration than for 2-PrOH.
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Figure 6. Comparison of the binodal curves plotted in unit of modified molality for the obtained alcohol + acetate salt + water ternary systems: ●,
2-BuOH + NH4CH3COO + water; ◆, 1-PrOH + NaCH3COO + water; ▲, 1-PrOH + KCH3COO + water; ■, 2-PrOH + NaCH3COO + water; × ,
2-PrOH + KCH3COO + water.
There are five out of the ten systems in this study that were
not able to form the two-phase solution, that is, (i) 1-PrOH +
NH4CH3COO + water, (ii) 2-PrOH + NH4CH3COO + water,
(iii) EtOH + NH4CH3COO + water, (iv) EtOH + NaCH3COO + water, and (v) EtOH + KCH3COO + water. EtOH was
found unable to form ATPS with all the tested acetate salts (i.e.,
NH4CH3COO, NaCH3COO, and KCH3COO). Previous
studies22 reported that the formation of EtOH + salt ATPS
required a salt with high solubility in water. Such a salt must be
able to compete with EtOH for attracting more water molecules in order to salt-out the EtOH as a separate phase from the
solution. In contrast to this fact, the acetate salts (NH4CH3COO and KCH3COO) used in this study have a relatively high
solubility in water (See Table 6), and yet they could not form
an ATPS with EtOH. Thus, the ability of salts to form ATPS
should be inferred through the Gibbs hydration energy (ΔGhyd)
of salts. ΔGhyd is the change in free energy of an isolated ion
from an ideal gas phase to an aqueous solution, and a more
negative ΔGhyd signifies that the ion is more kosmotropic.40
Table 6 presents a compilation of ΔGhyd and solubility data of
salts used in the formation of several EtOH + salt + water
systems found in the literature.15,16,20,22,39 Regardless of the
level of solubility, salt possessing low negative values of ΔGhyd
(i.e., < −1000 kJ/mol) in both cation and anion components
cannot form an ATPS with EtOH. Contrastively, salt with at
least one of the ion pair having a high negative value of ΔGhyd
(i.e., > −1000 kJ/mol) is able to form an ATPS with EtOH. As
the absolute value of negative ΔGhyd for cation and anion in
acetate salts NH4CH3COO, NaCH3COO, and KCH3COO are
all below −1000 kJ/mol, it is justifiable that the acetate salts
could not form a two-phase solution with EtOH.
It was also observed that the NH4CH3COO could not form
ATPS with either 1-PrOH or 2-PrOH. On the basis of the
Hofmeister series, the strength of hydration of ions is in the
ascending order of NH4+ < Cs+ < K+ < Na+ < Ca2+ < Mg2+ <
Al3+. Because the NH4+ is a weakly hydrated cation, it is incapable of competing with alcohol for water molecules. Thus,
NH4CH3COO is unable to form ATPS with 1-PrOH or
2-PrOH.
3.4. Evaluation of Salting-out Strength of Acetate
Salts by EEV Theory and Setschenow-Type Equation.
The EEV theory and Setschenow-type equation were used in
this study to evaluate the salting-out abilities of acetate salts.
EEV theory is based on the concept that every composition of
the mixture on a binodal curve is a geometrically saturated
solution of one solute in the presence of another, and macroscopically, any molecule species in the solution is randomly
distributed.21,29 In the case of alcohol + salt + water systems,
the compatibility of both phase-forming components in a
system can be described by EEV, which reflects the smallest
spacing of alcohol where an individual salt can be accepted.39
This model was originally applied to polymer + polymer +
water systems, and it could also be applied to aliphatic alcohol +
salt + water systems15,16 in the following form:
⎛ w ⎞
w
ln⎜V1 2 ⎟ + V1 1 = 0
M1
⎝ M2 ⎠
(10)
where V1, w1, w2, M1, M2 are the EEV of salts, the mass fraction
of alcohol, the mass fraction of salt, the molecular weight of
alcohol, and the molecular weight of salt, respectively. The experimental binodal curves obtained in this study were correlated
by the EEV model eq 10. The V1 and the corresponding R2 are
listed in Table 7.
The Setschenow-type equation has previously been employed in the assessment of salting-out abilities of salts through
the correlation of tie-line data for systems like polymer +
salt ATPS, IL + salt ATPS and alcohol + salt ATPS.15,41,42 The
Setschenow-type equation is as follows:
⎛ m *t ⎞
ln⎜⎜ 2 b ⎟⎟ = β + k(m1*b − m1*t )
⎝ m2* ⎠
(11)
where k is the salting-out coefficient, β is the constant related to
the activity coefficient, m1* is the modified molality of alcohol,
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Table 6. ΔGhyd and Solubility Data of Salts Used in the Formation of EtOH + Salt + Water Systems
Gibbs hydration energy
(kJ/mol)
system
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
+
+
+
+
+
+
+
+
+
+
+
+
+
+
NH4CH3COO + water
KCH3COO + water
NaCH3COO + water
KH2PO4 + water
NH4Cl + water
NaCl + water
MgSO4 + water
ZnSO4 + water
(NH4)2SO4 + water
Na2CO3 + water
K3PO4 + water
K3C6H5O7 + water
Cs2SO4 + water
K2CO3 + water
two-phase formation
cation
anion
solubility of salt (%, w/v) at 298.15 K
reference
No
No
No
No
No
No
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
−285
−295
−365
−295
−285
−365
−1922
−2044
−285
−365
−295
−295
−250
−295
−365
−365
−365
−465
−340
−340
−1080
−1080
−1080
−1315
−2765
−2793
−1080
−1315
143
253
46.4
20
28.2
26.43
33.7
53.8
74.4
21.5
165
154
179
111
Present study
Present study
Present study
22
22
22
15
15
39
16
20
20
22
22
Table 7. Values of Parameters of Eqs 10 and 11 for the Alcohol + Acetate Salt + Water Systems
EEV
a
Setschenow-type equation
system
V1 (g mol−1)
R2
k
β
R2
SDa
2-BuOH + NH4CH3COO + water
1-PrOH + NaCH3COO + water
1-PrOH + KCH3COO + water
2-PrOH + NaCH3COO + water
2-PrOH + KCH3COO + water
1076.52
208.49
219.98
172.41
166.41
0.9939
0.9971
0.9998
0.9999
0.9998
4.7516
2.0948
2.1480
1.7354
1.3471
1.4404
0.3571
0.3028
0.1187
−0.1788
0.9991
0.9999
0.9990
0.9333
0.9946
0.05
0.02
0.03
0.02
0.01
exp 2
0.5
SD = (Σi N= 1(mcal
1 −m1 ) /N) , where N represents the number of tie-lines.
m*2 is the modified molality of salt, superscript t denotes the
alcohol-rich phase and superscript b refers to the salt-rich
phase. The fitting parameters of eq 11 along with R2 and SD are
given in Table 7.
In 2-PrOH + acetate salt + water systems, the V1 and k of
NaCH3COO were higher than that of KCH3COO. A salt with
higher V1 or k values has a greater salting-out ability, implying a
lower concentration of the salt is needed in the formation of
ATPS. On the basis of the fact that the investigated acetate salts
only vary in cation, it can be concluded that the salting-out
ability of Na+ is greater than that of K+ in 2-PrOH + acetate
salt + water systems. In contrast, in 1-PrOH + acetate salt +
water systems, the V1 and k of KCH3COO were higher than
that of NaCH3COO. The salting-out strength of a salt in the
presence of different alcohols varies due to the difference in
size, shape, interaction of unlike molecules, and molar masses of
hydrophilic alcohols and salts.
As 1-PrOH is more readily salted-out by KCH3COO or
NaCH3COO than 2-PrOH, the V1 and k values of KCH3COO
or NaCH3COO for 1-PrOH + acetate salt + water systems
were, therefore, higher than that for 2-PrOH + acetate salt +
water systems.
It was previously shown that NH4CH3COO can form ATPS
with 2-BuOH but not with 1-PrOH or 2-PrOH. Based on the
fact that the V1 or k values shown in Table 7, it can be seen that
the salting-out strength of NH4CH3COO in 2-BuOH + acetate
salt + water system is very high, indicating that the 2-BuOH can
be easily salted out even by a weakly hydrated ion like NH4+.
3.5. Conclusion. LLE data for 2-BuOH + NH4CH3COO +
water, 1-PrOH/2-PrOH + NaCH3COO + water, and 1-PrOH/
2-PrOH + KCH3COO + water systems were experimentally
obtained from this study. Experimental data of the binodal
curves and the tie-lines were satisfactorily correlated by using
the Merchuk equations, and the Othmer−Tobias and Bancroft
equations, respectively. The EEV theory and Setschenow-type
equation were used to evaluate the phase-forming abilities of
alcohols and the salting-out strength of salt. The phase-forming
abilities of alcohols are in the order of 2-BuOH > 1-PrOH >
2-PrOH. For 2-PrOH + acetate salt + water systems, the saltingout strength of NaCH3COO is greater than that of KCH3COO,
whereas for 1-PrOH + acetate salt + water systems, the saltingout strength of KCH3COO is greater than that of NaCH3COO.
■
ASSOCIATED CONTENT
S Supporting Information
*
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.jced.5b00200.
Conductivity data as a function of acetate salt mass fractions, values of parameters of eq 1, values of parameters
of eq 2. (PDF)
■
AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]. Tel.: +60 3 55146201.
Fax: +60 3 55146207.
Funding
The authors would like to acknowledge Ministry of Science,
Technology and Innovation (MOSTI) for the e-Science
funding (02-02-10-SF088) provided. The funding support
from Multidisciplinary Research Competitive Grant (AEP-15002), Advanced Engineering Platform, Monash University
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Notes
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