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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*,†,‡

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on September 9, 2015 | http://pubs.acs.org Publication Date (Web): September 1, 2015 | doi: 10.1021/acs.jced.5b00200

†

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. © XXXX 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

A

DOI: 10.1021/acs.jced.5b00200 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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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 B




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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 C

(3) DOI: 10.1021/acs.jced.5b00200 J. Chem. Eng. Data XXXX, XXX, XXX−XXX



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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

D




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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 E



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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


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. F




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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, G



Article

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


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.

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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)

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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 H



Article

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Malaysia is also greatly appreciated. S.C.L. acknowledges the Higher Degree by Research (HDR) scholarship and facilities provided by Monash University Malaysia. Notes

The authors declare no competing financial interest.


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