Liquid–Liquid–Solid Triple-Phase Data for Aqueous Two-Phase

Article pubs.acs.org/jced

Liquid−Liquid−Solid Triple-Phase Data for Aqueous Two-Phase Systems comprising Ethanol−1-Propanol2-Propanol−Acetone and Salts Er-long Nan,† Gareth R. Williams,‡ Heng-huan Song,† Jing Quan,† Hua-li Nie,*,†,§ and Li-min Zhu*,† †

College of Chemistry, Chemical Engineering and Biotechnology, Donghua University, Shanghai, 201620, P. R. China School of Human Sciences, Faculty of Life Sciences and Computing, London Metropolitan University, 166-220 Holloway Road, London, N7 8DB, United Kingdom § Key Laboratory of Textile Science & Technology, Ministry of Education, Donghua University, Shanghai 201620, P. R. China ‡

ABSTRACT: In this paper, the liquid−liquid−solid triple-phase data of some aqueous two-phase systems (ATPSs) containing hydrophilic organic solvents (HOS) and simple salts were explored. The systems studied comprise aqueous solutions containing ethanol, 1-propanol, 2-propanol, or acetone with (NH4)2SO4 and solutions containing 1-propanol with (NH4)2SO4, NaCl, or KCl. The Gibbs phase rule predicts that there is a linear relationship existing at such a triple-phase boundary. The linear liquid−liquid−solid boundaries were determined, and the effects of temperature, solvent, and salt on the boundary were investigated. The tie line length (TLL) of the systems distributed on the triple-phase boundary was invariant. Phase equilibrium experiments determined that the average TLL of ethanol−(NH4)2SO4 ATPSs at 298.15 K was 62.93 % with a standard deviation of 2.13 %. The linear liquid−liquid−solid triple-phase boundary was used to elucidate the two-phase region and determine the content of organic solvent or salt in an unknown sample. These results increase our understanding of HOS−salt−water aqueous two phase system (ATPS) and will be useful for those looking to develop new systems for separation science.

1.2. The Liquid−Liquid−Solid Triple-Phase Boundary. Gibbs’ phase rule25 was obtained by considering the general properties of the fundamental equation and equilibrium conditions of a thermodynamic system. Given a system with C components and P coexistent phases, the degree of freedom F is F = C + 2 − P. This equation is given by solving a system of equations and using the relationship between the number of equations and variables. For the triple-phase equilibrium in the ternary HOS−salt−water system, the number of degrees of freedom (or alternatives), with no account taken of the pressure, is equal to F = C + 1 − P = 3 + 1 − 3 = 1. As a rule, temperature is such a variable. The temperature will certainly have an effect on the liquid−liquid−solid boundary. However, at a constant pressure and temperature, this condition of invariance (F = 0) dictates that the state of the system would be completely fixed over the entire region. That is, top, bottom, and solid phase compositions are invariant. From the lever rule, it is clear that the liquid−liquid−solid triple-phase boundary (expressed as mass fraction of total composition) is actually a straight line. Moreover, as the phase rule predicts, the linear boundary only exists in the case where the ATPS undergoes a separation from two phases into three. Therefore, the linear

1. INTRODUCTION 1.1. Aqueous Two-Phase Systems. Aqueous two-phase systems (ATPSs) are commonly used for liquid−liquid extraction and have shown great potential for the purification of high value biological products.1 The main types of ATPS currently in use are based on aqueous polymer/polymer (e.g., polyethylene glycol/dextran) systems or polymer/salt solutions (e.g., polyethylene glycol/phosphate).2,3 The components are separately dissolved in water and later mixed for separation purposes. Although these traditional ATPSs can be efficiently used for the separation and purification of biomacromolecules, their application at a large scale has been hampered by the high cost of polymers and difficulties in back-extraction from the polymer-containing phases.4−7 Recently, attention has been devoted to aqueous two-phase systems containing hydrophilic organic solvents (HOS) such as ethanol, 1-propanol, 2propanol, and acetone.8−10 These have advantages over standard ATPSs including lower cost, reduced viscosity, and easy product recovery by evaporation.5,11 They have been successfully employed to isolate amino acids, proteins, and other natural products.12−15 Over the last two decades, a number of researchers have investigated liquid−liquid equilibrium data for HOS-containing ATPSs.16−24 However, all of these investigations have only focused on the liquid/liquid phase boundary and phase diagram: no attention has been paid to the liquid−liquid− solid triple-phase boundary in such systems. © 2013 American Chemical Society

Received: April 17, 2013 Accepted: October 9, 2013 Published: October 31, 2013 3314

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temperature of 513.15 K, with n-butyl alcohol as an internal standard. The tie lines were measured in accordance with the literature procedure.16 The concentrations of ethanol and salt in the two phases were determined as mentioned above. The tie-line length (TLL) of the HOS-based ATPSs can be calculated by the equation:

liquid−liquid−solid boundary could be used to characterize the HOS−salt−water ATPSs. 1.3. Research Aims. Based on the predictions of Gibbs’ phase rule, the work described in this paper aimed to determine the linear liquid−liquid−solid boundaries of some HOS−salt− water systems and explored the influence of temperature, different organic solvents, and salts. The liquid−liquid−solid phase boundary was furthermore investigated through phase equilibrium experiments. Finally, the utility of the triple-phase boundary in determining the compositions of unknown samples has been studied. The results reported here offer increased understanding of HOS-containing ATPSs and the potential to design better separation systems in future.

TLL = [(ω1t − ω1b)2 + (ω2t − ω2b)2 ]0.5

(1)

where ωt1, ωb1, ωt2, and ωb2 represent the equilibrium mass fractions of the ethanol (1) and salt (2) in the top (t) and bottom (b) phases. 2.2.3. Application of the Liquid−Liquid−Solid Triple-Phase Boundary. A small amount (about 0.05 g) of ethanol was added to approximately 10 g of an aqueous solution of (NH4)2SO4 with known concentration until precipitation occurred. The mass of the mixture was measured. The experiment was repeated with different salt concentrations and the mass fraction of ethanol plotted against the mass fraction of (NH4)2SO4 to give a calibration curve. To demonstrate the accuracy of the method, ten aqueous solutions with known concentrations of (NH4)2SO4 were prepared at 298.15 K. They were titrated with ethanol until precipitation appeared. The mass fractions of ethanol and (NH4)2SO4 were calculated and compared to the calibration curve to evaluate the accuracy of this method.

2. EXPERIMENTAL METHODS 2.1. Materials. All organic solvents and inorganic salts were of analytical grade and used without further purification. Ethanol, 1-propanol, 2-propanol, acetone, 1-butanol, ammonium sulfate, sodium chloride, and potassium chloride were purchased from the National Pharmaceutical Group Corp., China with a minimum purity of 99.0 %. Distilled water was used throughout. 2.2. Methods. 2.2.1. Measurements of Liquid−Liquid− Solid Triple-Phase Boundaries and Binodal Curves. The liquid−liquid−solid triple-phase boundary of HOS−salt−water ATPSs was determined by the following procedure. ATPSs were prepared by mixing a hydrophilic organic solvent and an aqueous salt solution. The mixture was initially turbid, indicating that two phases would eventually form. Organic solvent was then added drop by drop, with each addition followed by thorough mixing, until salt precipitation occurred. The temperature was maintained within ± 0.05 K with a DK8D electrothermostatic water bath (Shanghai Sumsung Laboratory Instrument Co. Ltd., China). The mass of the mixture was measured using an analytical balance (Mettler Toledo, USA) with a precision of 0.1 mg. The concentrations of the phase-forming components in the final system were calculated. The mass fraction of organic solvent was plotted against the mass fraction of salt, resulting in the liquid−liquid− solid triple-phase boundary. The binodal curve for the ethanol/(NH4)2SO4/water system was obtained by the titration (cloud point) method. An aqueous (NH4)2SO4 solution of known concentration in a 50 mL conical flask was titrated with ethanol until the clear solution turned turbid. The temperature was maintained at 298.15 ± 0.05 K in a water bath. The mass of the mixture was measured on an analytical balance. 2.2.2. Phase Equilibrium Experiments. Phase equilibrium studies were carried out in 20 mL glass vessels maintained at 298.15 ± 0.05 K in a water bath. Samples were prepared by mixing a suitable amount of ethanol, (NH4)2SO4, and water, and the mass of each composition determined. Each sample was mixed using a vortex shaker (WH-2, Shanghai Huxi Analysis Instrument Factory Co. Ltd., China) for 20 min and then allowed to settle to reach equilibrium. Some 5 mL and 3 mL aliquots were carefully extracted by pipet from the top and bottom phases for further analysis. (NH4)2SO4 was concentrated by evaporation and then dried in an air oven at 341.15 K until the mass was constant. The content of ethanol was analyzed by gas chromatography (GC7980, Techcomp Ltd., China) with a flame ionization detector. Gas chromatography was performed on an Agilent capillary column (30 mm × 0.250 mm) at a column temperature of 373.15 K and a detector

3. RESULTS AND DISCUSSION 3.1. Determination of the Linear Liquid−Liquid−Solid Boundaries. Experimental data and the linear relationships observed in the liquid−liquid−solid boundaries are given in Tables 1 and 2 and Figures 1 to 3. In agreement with the theoretical analysis above, all of the liquid−liquid−solid triplephase boundaries exhibited highly linear relationships (R2 > 0.999). The slope k and intercept b obtained are listed in listed Table 2. It is important to note that this linear relationship only occurs in the case where the ATPS undergoes a separation from two phases into three, as the phase rule predicted. Thus, the linear liquid−liquid−solid phase boundary could be used to characterize a given ATPS. The slope k, which is negative, denotes that it requires increasing amounts of HOS to reach the triple-phase boundary as the amount of salt added rises. A smaller slope means that the HOS is more easily salted-out and more easily forms a two-phase system. This characteristic could be used to compare the phase-separation ability of different HOSs. 3.2. Properties of the Linear Liquid−Liquid−Solid Boundary. 3.2.3. Influence of Temperature. As detailed in the phase rule, temperature comprises a degree of freedom in the triple-phase equilibrium of the ternary HOS−salt−water system. The liquid−liquid−solid boundaries were explored in the ethanol/(NH4)2SO4/water system at 288.15 K, 293.15 K, and 298.15 K. The results are plotted in Figure 1. Over the investigated range, the slope varies from −1.3238 to −1.4679, and the intercept varies from 0.5241 to 0.5936. The liquid− liquid−solid boundary shifted away from the coordinate origin as the temperature increased. This could be explained in the following manner. An increase of temperature will destroy intermolecular hydrogen bonds between the ethanol and water.23 Concomitantly, a temperature rise increases the solubility of (NH4)2SO4 in water. The two factors mean that more ethanol or (NH4)2SO4 are required to 3315

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Table 1. Total Composition Data Detailing Liquid−Liquid− Solid Boundaries for HOS (1) + Salt (2) + Water (3) ATPSs at T = 288.15, 293.15, and 298.15 K and Pressure p = 0.1 MPaa 100 ω1 288.15 45.98 39.16 30.43 24.39 15.84 6.86

100 ω2 K 4.94 9.94 16.21 21.56 27.88 34.16

2-Propanal− (NH4)2SO4 288.15 K 33.39 24.31 42.60 19.11 53.38 13.34 62.23 8.79 73.46 2.69 22.62 30.12 13.37 34.83 1-Propanal− (NH4)2SO4 298.15 K 66.84 9.49 57.02 14.28 39.13 23.39 29.26 28.45 82.10 1.60 49.68 18.19

100 ω1

100 ω2

Ethanol−(NH4)2SO4 293.15 K 51.82 4.38 45.58 9.05 36.20 14.68 28.22 20.45 19.75 26.66 8.89 34.13

100 ω1

100 ω2

Acetone−(NH4)2SO4

298.15 K 52.66 4.37 45.24 9.81 37.76 14.75 31.60 19.07 20.11 26.53 15.14 30.17 9.38 34.05 1-Propanal−(NH4)2SO4

288.15 K 7.56 37.20 16.24 32.57 34.61 21.81 26.16 26.60 46.75 15.06 61.86 6.36 68.77 2.93 1-Propanal−NaCl

288.15 K 24.51 30.08 53.93 15.38 65.48 9.82 81.73 1.66 44.26 20.25 35.69 24.71 18.52 33.38 1-Propanal−KCl

298.15 K 41.51 13.45 46.66 11.79 63.66 7.10 74.37 4.16 78.53 2.76 81.55 1.78

298.15 K 26.42 16.99 49.83 10.08 60.24 6.77 67.77 4.50 72.77 2.99 75.63 2.19

Figure 1. Liquid−liquid−solid boundaries of the ethanol/(NH4)2SO4/ water system at 288.15 K, 293.15 K, and 298.15 K.

288.15 K (see Table 1 and Figure 2). It can be seen from Table 2 that the slope k decreases in the order: ethanol > acetone > 2-

a

Figure 2. Liquid−liquid−solid boundaries of ethanol−1-propanol−2propanol−acetone, (NH4)2SO4 and water systems at 288.15 K.

reach the liquid−liquid−solid boundary. Therefore, the liquid/ liquid region extends to the side of liquid−liquid−solid region with the temperature increases. This effect is visible in Figure 1, although over the temperature range explored here the change is relatively small. Experiments were also undertaken with 1propanol and ammonium sulfate at different temperatures, and the same trend was seen as the temperature increased (see Table 2). 3.2.1. Effect of Hydrophilic Organic Solvents. The liquid− liquid−solid triple-phase boundaries for ethanol, 1-propanol, 2propanol, or acetone with (NH4)2SO4 were determined at

propanol > 1-propanol. This order is closely correlated with the polarity of the organic solvents (polarity order: ethanol > acetone > 1-propanol > 2-propanol).26 This is expected, because the formation of HOS-based ATPSs results from competition between hydration of the HOS and the salt.27 Increased polarity of the organic solvent leads to a stronger affinity for water. Therefore, less organic solvent is required to reach the liquid−liquid−solid boundary for a given concentration of salt. This means the slope becomes less negative as the polarity of the organic solvent increases. However, it should be noted that this is not the only factor involved, as the slope k of 2-propanol is found to be less negative than for 1-propanol despite the greater polarity of the latter.

Standard uncertainties u are u(ω) = 0.0002, u(T) = 0.05 K, and u(p) = 10 kPa, where ω1 and ω2 represent the mass fraction of HOS (1) and salt (2) in the total system.

Table 2. Linear Relationship on the Liquid−Liquid−Solid Boundaries of HOS−Salt−Water ATPSs

a

ATPS

T/K

slope k

intercept b

Na

R2

100 sdb

ethanol−(NH4)2SO4 ethanol−(NH4)2SO4 ethanol−(NH4)2SO4 2-propanol−(NH4)2SO4 acetone−(NH4)2SO4 1-propanol−(NH4)2SO4 1-propanol−(NH4)2SO4 1-propanol−NaCl 1-propanol−KCl

288.15 293.15 298.15 288.15 288.15 288.15 298.15 298.15 298.15

−1.3238 −1.4461 −1.4679 −1.8624 −1.7710 −1.9997 −1.9714 −3.4878 −3.3072

0.5241 0.5810 0.5936 0.7844 0.7348 0.8495 0.8535 0.8824 0.8277

6 6 7 7 7 7 6 6 6

0.9993 0.9993 0.9998 0.9999 0.9998 0.9999 0.9999 0.9994 0.9999

0.446 0.481 0.252 0.242 0.365 0.265 0.177 0.476 0.25

N is the number of data points. bsd = standard deviation. 3316

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3.4. Applications of the Liquid−Liquid−Solid TriplePhase Boundary. 3.4.1. The Complete Two-Phase Region. The binodal curve and triple-phase boundary data of the aqueous ethanol and (NH4)2SO4 system at 298.15 K are presented in Table 3, and the complete two-phase region is

For a given salt in different component solvents, an increase in the slope of the boundary indicates that the corresponding organic solvent combines more easily with water and is more difficult to “salt-out” to form a separate phase through the addition of salt. In other words, the phase-separation ability of hydrophilic organic solvents decreases with an increase in the slope of the liquid−liquid−solid boundary. Hence the phaseseparation ability of the organic solvents is in the order: 1propanol > 2-propanol > acetone > ethanol. Similar conclusions have been drawn by Wang et al.19 3.2.2. Effect of Salt Variation. The liquid−liquid−solid triple-phase boundaries for 1-propanol with (NH4)2SO4, NaCl, and KCl were studied at 298.15 K and the results are presented in Figure 3. The location of the boundary is mainly dictated by

Table 3. Binodal Data and Total Composition Data Detailing the Liquid−Liquid−Solid Boundary for the Ethanol (1) + (NH4)2SO4 (2) + Water (3) ATPS at T = 298.15 K and Pressure p = 0.1 MPaa 100 ω1

100 ω2

100 ω1

100 ω2

100 ω1

100 ω2

Binodal Data 5.50 37.80 20.54 19.58 37.46 8.42 5.69 37.19 21.06 19.18 38.39 7.99 6.07 36.21 22.09 18.38 39.80 7.32 6.38 35.56 23.13 17.63 40.96 6.80 7.43 33.69 24.09 16.91 42.26 6.28 8.51 31.80 25.06 16.24 43.45 5.81 9.07 30.94 26.29 15.34 44.64 5.35 10.30 29.22 27.09 14.77 46.20 4.83 11.42 27.86 28.27 13.97 47.57 4.43 12.55 26.67 29.23 13.32 48.63 4.08 13.10 26.14 30.63 12.38 50.84 3.51 14.34 24.86 31.50 11.83 51.84 3.27 15.57 23.67 32.60 11.13 52.85 3.02 16.15 23.16 33.59 10.55 53.59 2.84 17.40 22.11 34.58 9.96 56.01 2.35 18.56 21.08 35.51 9.46 58.05 1.98 19.32 20.52 36.44 8.92 59.34 1.73 Total Composition Data Detailing Liquid−Liquid−Solid Boundary 52.66 4.37 31.60 19.07 9.38 34.05 45.24 9.81 20.11 26.53 37.76 14.75 15.14 30.17

Figure 3. Liquid−liquid−solid boundaries of 1-propanol, (NH4)2SO4−NaCl−KCl, and water systems at 298.15 K.

the solubility of the salts. The lower the solubility is, the closer to the coordinate origin the boundary is. Evidence previously reported suggests that the ability of salts (or salt ions) to induce the formation of ATPS follows the Hofmeister series.28 However, this effect is not very significant among the triplephase boundaries investigated. The systems distributing on the liquid−liquid−solid boundary are actually a salt saturated solution containing water and 1-propanol. Since the salt is insoluble in 1-propanol, its solubility in water must play a major role in the triple-phase boundary. 3.3. Phase Equilibrium Experiments. Experimental liquid−liquid equilibria for six ethanol−(NH4)2SO4 ATPSs distributed on the liquid−liquid−solid triple-phase boundary were determined at 298.15 K. Based on the phase rule, there exist only one tie line, this being the edge of the triple-phase system at fixed temperature and pressure. The tie-line length (TLL) is a thermodynamic parameter that provides useful information on the properties of aqueous two-phase systems. As the TLL increases, the top and bottom phases show increasing differences in composition. The TLLs (and hence equilibrium compositions of the top and bottom phases) of ATPSs distributed on the liquid−liquid−solid triple-phase boundary are all observed to be very similar with a standard deviation of 2.13 %, as the phase rule predicts. When the salt content reaches saturation point in the ethanol−water solvent mixture, the further addition of salt will have no effect on the liquid−liquid equilibrium of the ATPS. In the saturated systems of ethanol−(NH4)2SO4 ATPSs, the average TLL investigated was 62.93 %, and the average concentrations of ethanol and (NH4)2SO4 in the top and bottom phases were 58.81 %, 2.15 %, 5.88 %, and 36.18 %, respectively.

a Standard uncertainties u are u(ω) = 0.0002, u(T) = 0.05 K, and u(p) = 10 kPa, where ω1 and ω2 represent the mass fraction of ethanol (1) and (NH4)2SO4 (2) in the total system.

Figure 4. The complete two-phase region for the ethanol/ (NH4)2SO4/water system at 298.15 K. L+L = two coexisting liquid phases.

displayed in Figure 4. The letters L denote liquid phases. In the L+L region, two liquid phases are in equilibrium: a top phase rich in ethanol and a bottom phase rich in (NH4)2SO4. The liquid−liquid two phase region begins at the binodal curve and ends at the triple-phase boundary. This region occupies a small portion of the total phase diagram, although the coexisting 3317

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phases are quite different in composition. Research conducted in this two-phase region could facilitate separation process design. 3.4.2. Determination of the Contents of Organic Solvent or Salt in an Unknown Sample. The liquid−liquid−solid triple-phase boundary can be used as a calibration curve to determine the concentration of salt or ethanol in an unknown sample. Given the mass fraction of ethanol (ω1) added, we can calculate the concentration of (NH4)2SO4 (ωcal 2 ) in a sample from the triple-phase boundary. If the concentration of ethanol is unknown in a sample, we can determine it using an analogous procedure. A number of samples of known concentration were prepared, and their compositions calculated using the phase boundary to evaluate the accuracy of this approach. The experimental and calculated results are listed in Table 4. The absolute deviations are less than 1 % at 298.15 K; the maximum absolute deviation is only 0.94 %, which shows the accuracy of this approach.

salt sample

*Tel.: +86-21-67792748. Fax: +86-21-62372655. E-mail: lzhu@ dhu.edu.cn. *E-mail: [email protected]. Funding

This work was supported by the National Natural Science Foundation of China (21006010). Notes

The authors declare no competing financial interest.

■

ωcal 1

ADs

ωexp 2

ωcal 2

ADs

1 2 3 4 5 6 7 8 9 10 ADDs

10.07 13.50 14.98 19.97 19.34 24.97 29.81 35.01 36.06 36.70

10.12 13.49 14.93 19.03 19.70 24.43 29.26 34.44 36.13 36.61

0.05 0.01 0.05 0.94 0.36 0.54 0.55 0.57 0.07 0.09 0.23

54.91 52.95 52.01 49.14 48.15 43.52 36.52 25.01 19.57 17.93

54.94 52.94 51.96 48.27 48.49 42.93 35.79 24.04 19.69 17.76

0.03 0.01 0.05 0.87 0.34 0.59 0.73 0.97 0.12 0.17 0.29

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ethanol sample

ωexp 1

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Corresponding Authors

Table 4. Absolute Deviations (ADs) and Average Absolute Deviations (AADs) between Experimental and Calculated Data for the Ethanol−(NH4)2SO4−Water ATPS at 298.15 Ka sample

Article

cal exp cal ωexp 1 , ω1 , ω2 and ω2 represent the experimental and calculated mass fractions of ethanol (1) and (NH4)2SO4 (2). The results are means of triple independent measurements. a

4. CONCLUSIONS In this paper, the liquid−liquid−solid triple-phase data of several HOS−salt−water ATPSs were investigated. The systems studied comprise aqueous solutions containing ethanol, 1-propanol, 2-propanol, or acetone with (NH4)2SO4 and solutions containing 1-propanol with (NH4)2SO4, NaCl, or KCl. The triple-phase boundary was influenced by temperature, solvent, and salt. The two-phase region expands with an increase of temperature. For a given salt, the slope of the liquid−liquid−solid boundary could be used to compare the phase-separation ability of organic solvents. This slope k increased with decreasing phase-separation ability in the order: k (1-propanol) < k (2-propanol) < k (acetone) < k (ethanol), which is similar to the polarity of these solvents. For a given HOS, the triple-phase boundary is mainly related to the solubility of the salt. Phase equilibrium experiments determined that the average TLL of ethanol-(NH4)2SO4 ATPSs at 298.15 K was 62.93 % with a standard deviation of 2.13 %. In addition, the liquid−liquid−solid triple-phase boundary was used to elucidate the two-phase region and determine the content of organic solvent or salt in an unknown sample. 3318

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dx.doi.org/10.1021/je400364b | J. Chem. Eng. Data 2013, 58, 3314−3319