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Influence of the addition of Tween 20 on the phase behaviour of ionic liquids-based aqueous systems
J. Chem. Thermodynamics 79 (2014) 178–183
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Influence of the addition of Tween 20 on the phase behaviour of ionic liquids-based aqueous systems María S. Álvarez, Ana Mateo, Francisco J. Deive ⇑, M. Ángeles Sanromán, Ana Rodríguez ⇑ Department of Chemical Engineering, Universidade de Vigo, P.O. Box 36310, Vigo, Spain
a r t i c l e
i n f o
Article history: Received 21 July 2014 Received in revised form 1 August 2014 Accepted 2 August 2014 Available online 9 August 2014 Keywords: Ionic liquids Aqueous biphasic systems Tween 20 Imidazolium Potassium salts
a b s t r a c t The addition of a non-ionic surfactant (Tween 20) to 1-ethyl-3methyl imidazolium alkylsulfate (C2C1imCnSO4)-based aqueous biphasic systems was investigated in this work. The solubility curves of the systems {(C2C1imCnSO4 (n = 2 and 8) + Tween 20) + high charge density salt (K3PO4/K2CO3/ K2HPO4) + H2O} were carried out at T = 298.15 K. The obtained experimental data were correlated by using three empirical equations. The molar Gibbs free energy of hydration (DhydG) is a valuable parameter to analyse the segregation capacity provided by the inorganic salts. Additionally, the efficiency of the separation capacity was discussed in terms of the salting out potential of the selected salts and the presence of Tween 20. Othmer–Tobias and Bancroft equations have been used to correlate the experimental tie-line data. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Currently, the hunt for more competitive and greener processes has led to the application of ionic liquids, neoteric solvents with outstanding properties such as their negligible volatility and tunability [1,2], in a range of fields such as electrochemistry, analytical chemistry, or chemical engineering, to name a few [3]. In this sense, one of the areas that has benefitted from the presence of these molten salts is the separation of compounds with industrial interest by means of (liquid + liquid) equilibrium [4–6]. Among the existing alternatives, the segregation of phases in systems containing aqueous solutions of ionic liquids is a field with booming interest since 2003, when Gutowski and co-workers published the first work on the capacity of hydrophilic ionic liquids to form aqueous biphasic systems (ABS) [7]. This separation strategy generally consists in adding an inorganic salt to aqueous solutions of ionic liquids, thus triggering phase disengagement: an upper aqueous phase mainly composed of ionic liquid and a lower aqueous layer rich in inorganic salt [8]. In this work, 1-ethyl-3-methyl imidazolium ethylsulfate and octylsulfate have been selected as model ionic liquids to be salted out in aqueous solutions, since they are reasonably cheap, they can be easily synthesized in an atomefficient and halide-free way, they display relatively low viscosities and melting points [9], and they belong to one of the most
⇑ Corresponding authors. Tel.: +34 986 81 87 23. E-mail addresses: [email protected] (F.J. Deive), [email protected] (A. Rodríguez). http://dx.doi.org/10.1016/j.jct.2014.08.004 0021-9614/Ó 2014 Elsevier Ltd. All rights reserved.
representative families (1-ethyl-3methyl imidazolium) already synthesized at levels higher than one ton per year [10]. Different types of ABS have been described in the literature, such as those exclusively formed by a polymer and a salt, a polymer and a polymer, an ionic liquid and a salt, an ionic liquid and a polymer, and a surfactant and a salt [11–15]. The latter has been reported to be advantageous in terms of cost, availability at bulk quantities, biodegradability, lower interface tension, mild operation temperatures and wider immiscibility window. Thus, in this study, the phase segregation capacity of different potassium-based inorganic salts was studied in aqueous mixtures of ionic liquids and a model non-ionic surfactant (Tween 20). This family is commonly used in industrial biotechnology, for instance in enzyme production processes or bioremediation studies, and has been reported to act even as nutrient in culture media [16,17]. In recent research works, the suitability of this kind of non-ionic surfactants (Tween 20) to extract biomolecules [18], industrial dyes [19] and metals [20] has already demonstrated. Additionally, the applicability of C2C1imCnSO4 has also been concluded for enzyme separation [8,21]. Therefore, in this study we intend to shed light on the ABS behaviour of both compounds mixed (nonionic surfactants and ionic liquids at ratios 25:75 and 75:25, respectively) in the presence of the inorganic salts K3PO4, K2HPO4 and K2CO3. These salts are well-known salting out agents, as predicted by the Hofmeister series, so their segregation capacity was discussed in this work by analysing the solubility curves at T = 298.15 K and the molar Gibbs free energy of hydration (DhydG). Three empirical equations were employed to correlate the
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experimental solubility data. The segregation capacity was also evaluated in terms of tie-line data, tackling the slope (S) and the tie-line length (TLL). Othmer–Tobias and Bancroft models were used to evaluate the consistency of the tie-line data [22]. 2. Experimental 2.1. Chemicals The ionic liquids 1-ethyl-3-methyl imidazolium ethylsulfate (C2C1imC2SO4) and 1-ethyl-3-methyl imidazolium octylsulfate (C2C1imC8SO4) were provided by Merck. They were subjected to vacuum of P = 101 Pa and temperature T = 330.15 K for several days in order to remove moisture and possible traces of organic volatile compounds. The inorganic salts K3PO4, K2HPO4 and K2CO3 were supplied by Sigma–Aldrich and were used as received, without further purification. The non-ionic surfactant Tween 20, polyethoxylated sorbitan monolaurate was purchased from Sigma–Aldrich. All the data concerning chemicals purities and provenance are shown in table 1. 2.2. Experimental procedure The solubility curves were empirically ascertained by means of the cloud point titration method [23] in a jacketed glass vessel containing a magnetic stirrer at atmospheric pressure, 101.33 kPa. The temperature was controlled at T = 298.15 K with a F200 ASL digital thermometer with an uncertainty of T = ±0.01 K. Known amounts of Tween 20 and C2C1imCnSO4 in aqueous solution were introduced to a vessel, and the immiscibility window was mapped by sequential additions of inorganic salt (K3PO4, K2HPO4 and K2CO3) and water, until the detection of a turbid and clear solution, respectively. Additionally, a mixture with known mass fraction at the biphasic region was prepared to determine the tie-lines (TLs), in accordance with the protocol defined by Merchuk [24]. Briefly, after a vigorous stirring, the mixture was left to settle for 24 h to allow a complete phase separation. Top and bottom layers were then separated and weighted, and the level arm rule was used to determine each TL composition. The estimated uncertainty associated with the determination of the surfactant (top) and salt (bottom) phases mass compositions is ±2%. All the samples were weighed in an analytical Sartorius Cubis MSA balance (125P-100-DA, ±105 g).
are collected in tables 2 and 3, and they are plotted in figures 1 and 2. The data obtained can be analysed in the light of the surfactant addition, the alkyl chain length in the ionic liquid anion, and the salting out potential of the selected inorganic salt. These factors have proven to be crucial to achieve phase segregation. The formation of an upper (ionic liquid/surfactant)-rich phase is observed, in line with previous investigations by Wang et al. (2004), who concluded that ionic liquids and surfactants form an organised moiety: not only does the ionic liquid act as a specific solvent, but also as a co-surfactant [25]. Additionally, it becomes patent that the increase of the surfactant concentration from 25% to 75% entails immiscibility regions closer to the origin, no matter the ionic liquid or inorganic salt employed. The comparison of the obtained data with the systems containing the pure surfactant (100% Tween 20) and ionic liquid (100% C2C1imCnSO4), reported by Álvarez et al. [22] and Deive et al. [26], respectively, reveals a close agreement with the observed trends. The explanation of this behaviour may be related to the increased hydrophobicity provided by the non-ionic surfactant. Thus, the competition between the mixture ionic liquid/surfactant and the inorganic salt for the water molecules is easily won when the first is more hydrophobic, due to they establish weaker hydrogen-bonds with water. This is coincident with other types of ABS involving ionic liquids and PEG [14,27]. The observed trends pose undoubted advantages related to the process economy and supply logistics.
TABLE 2 Binodal data in mass fraction for {(C2C1imC2SO4 + Tween 20) (1) + salt (2) + H2O (3)} two-phase systems for different surfactants concentrations at T = 298.15 K, P = 101.33 kPa.a K3PO4 100 w2
3. Results and discussion 3.1. Phase diagrams determination and correlation Up to our knowledge, this is the first time that the ABS resulting from the combination of the non-ionic surfactant Tween 20 with an ionic liquid and a high charge density inorganic salt is investigated. Thus, (liquid + liquid) de-mixing was characterised for the systems {(C2C1imCnSO4 (n = 2 and 8) + Tween 20) + (K3PO4 or K2CO3 or K2HPO4) + H2O} at T = 298.15 K. The experimental solubility data TABLE 1 Purities and suppliers of chemicals.a
a
Chemical
Supplier
Mass fraction purity
K3PO4 K2CO3 K2HPO4 Tween 20 C2C1imCnSO4
Sigma–Aldrich
0.98 0.99 0.98 0.98 0.98
Merck
Deionised water was used in all the experiments.
a
K2HPO4 100 w1
2.83 3.53 4.32 4.84 5.40 5.92 6.24 6.74 7.06 7.49 8.00 8.13 8.50 8.89 9.17 9.31 9.66 9.97 10.02
51.43 46.06 42.48 38.73 35.58 32.41 28.34 25.66 22.72 20.79 17.14 15.28 13.39 10.87 9.37 8.00 5.66 3.45 2.54
10.36 10.57 10.80 10.95 11.02 11.01 11.03 11.05 11.09 11.00 10.94 11.01 11.37 11.71
25.96 23.03 20.14 17.57 15.32 13.41 8.21 6.99 5.61 11.32 9.74 13.41 3.17 1.96
100 w2
K2CO3 100 w1
100 w2
25% C2C1imC2SO4 + 75% Tween 20 2.21 53.74 4.07 6.95 19.35 4.98 7.63 15.85 5.96 8.06 12.32 6.45 8.43 10.21 6.93 8.65 8.36 7.33 8.89 6.56 7.43 9.24 4.75 7.69 9.44 3.58 8.01 3.44 47.75 7.16 4.33 41.93 7.92 4.98 37.26 8.64 5.48 33.48 8.31 5.84 31.17 8.87 6.15 28.62 9.20 6.43 26.21 7.00 22.15
75% C2C1imC2SO4 + 25% Tween 20 8.30 32.48 8.65 8.88 28.56 8.75 9.15 25.10 9.02 9.38 23.11 9.04 9.49 18.68 9.07 9.64 17.10 9.02 9.45 20.29 9.07 9.69 15.19 9.22 9.70 12.49 9.09 9.65 9.90 9.54 9.78 7.53 8.37 9.79 5.88 9.90 9.99 3.02
100 w1 46.42 38.90 33.71 28.43 24.77 20.48 18.54 16.22 13.26 23.22 10.99 6.40 9.09 3.24 1.80
29.98 26.16 22.55 20.32 17.37 15.02 13.42 10.76 8.73 4.83 33.38 2.58
Standard uncertainties are u(w) = ± 0.0002, u(T) = ± 0.01 K; u(P) = ± 0.03 kPa.
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0.16
TABLE 3 Binodal data in mass fraction for {(C2C1imC8SO4 + Tween 20) (1) + salt (2) + H2O (3)} two-phase systems for different surfactants concentrations at T = 298.15 K, P = 101.33 kPa.a
a
100 w1
1.09 1.36 1.65 2.35 3.26 4.18 4.90 5.59 6.21 6.58 6.99 7.21 7.65 8.17 8.65 8.97 9.11 9.56 9.93 14.86 17.11 17.14
51.60 49.93 48.32 45.21 42.12 37.57 32.91 29.49 26.88 23.79 21.66 19.78 16.58 13.78 11.34 9.30 7.29 5.25 3.13 0.27 0.02 0.02
2.56 2.99 3.94 3.98 4.65 5.32 6.53 6.84 7.89 8.55 9.27 10.18 10.71 11.10 12.30 12.73 13.42 13.96 16.53 18.20 19.52
51.46 49.10 45.15 44.21 40.84 37.87 31.61 29.03 25.48 23.07 19.13 16.60 13.88 11.90 9.60 7.10 4.37 2.58 2.55 1.14 0.54
100 w2
0.12
K2CO3 100 w1
25% C2C1imC8SO4 + 75% Tween 20 1.32 48.26 1.90 44.99 2.43 40.74 3.67 37.81 4.01 35.96 4.66 32.28 5.81 27.60 6.12 25.44 6.15 23.16 6.66 20.33 7.10 18.13 7.50 15.79 7.63 12.50 8.26 9.77 8.67 7.02 9.23 3.97
100 w2 2.91 3.44 4.22 4.81 5.58 5.76 6.22 6.75 7.13 7.56 7.95 8.38 8.67 8.99
75% C2C1imC8SO4 + 25% Tween 20 3.52 49.85 6.67 3.97 43.44 8.48 4.69 38.29 9.83 5.64 35.96 10.95 6.05 33.11 12.11 7.08 30.34 13.23 7.65 27.85 13.78 8.27 24.95 15.07 9.15 22.11 16.28 9.93 19.26 17.14 10.12 17.17 5.02 11.46 13.13 18.10 12.24 9.89 18.65 13.56 6.42 19.19 14.15 4.21 14.53 2.06
100 w1 45.23 41.97 36.77 31.65 27.73 24.79 22.39 18.48 15.74 12.89 9.47 6.85 4.80 2.58
w1/M1
100 w2
K2HPO4
0.08
0.04
0.00 0.00
0.08
0.16
0.24
0.32
w2/M2 FIGURE 1. Plot of experimental and correlated solubility data of {(C2C1imC2SO4 + Tween 20) (1) + salt (2) + H2O (3)} at T = 298.15 K, P = 101.33 kPa: (s), K3PO4; (4), K2HPO4; (h) K2CO3. Blue, 100% Tween 20, reference [22]; Black, 75% Tween 20; Red, 25% Tween 20; Cyan, 0% Tween 20, reference [26] (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).
0.15 41.61 35.27 31.72 29.29 25.99 22.61 21.22 17.44 13.59 9.58 47.75 7.87 4.87 3.11
Standard uncertainties are u(w) = ± 0.0002, u(T) = ± 0.01 K; u(P) = ± 0.03 kPa.
Secondly, the comparison of the immiscibility gap for the two selected ionic liquids shown in figures 1 and 2, makes it evident the higher tendency of the octylsulfate-based ionic liquids for ABS formation, in agreement with the data reported by Deive et al. [26]. Although generally speaking, the self-aggregation of ionic liquids has been more evident for long alkyl chains in the cation [28], the present data evidence the existence of similar phenomena for the anion, thus confirming analogous hydrophobicity effects to those discussed for the surfactants. The analysis of the salting out ability of the salts under study evidences the following sequence for the selected anions (given 2 2 that the cation is fixed): PO3 4 > HPO4 > CO3 . Seemingly, the obtained data follow the Hofmeister series, an ion classification on the basis of the salting out potential of salts. As expected, trivalent anions entail more interplay with water than the divalent anions, a fact that can also be interpreted in the light of the molar Gibbs free energy of hydration (DhydG), which confirms the observed trends. Thus, the lower values of this thermodynamic parameter are related with an easier capacity to trigger phase
0.10 w1/M1
K3PO4
0.05
0.00 0.00
0.05
0.10
0.15
0.20
w2/M2 FIGURE 2. Plot of experimental and correlated solubility data of {(C2C1imC8SO4 + Tween 20) (1) + salt (2) + H2O (3)} at T = 298.15 K, P = 101.33 kPa: (s), K3PO4; (4), K2HPO4; (h) K2CO3. Blue, 100% Tween 20, reference [22]; Black, 75% Tween 20; Red, 25% Tween 20; Cyan, 0% Tween 20, reference [26] (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).
1 disengagement. In this sense, the value for PO3 ) 4 (2765 kJ mol 2 1 is higher than that reported for HPO4 (1789 kJ mol ), and in turn, this is higher than the one concluded for CO2 (1315 kJ 3 mol1) [29,11]. Therefore, it is confirmed that trivalent phosphate anion will be more akin to interact with water molecules, thus leading to greater immiscibility window. The experimental data were correlated by means of the following empirical equations [22]:
0:5
½S þ IL ¼ a expðb½Salt ½S þ IL ¼ a þ b½Salt
0:5
3
c½Salt Þ;
ð1Þ 2
þ c½Salt þ d½Salt ;
½S þ IL ¼ expða þ b½Salt
0:5
ð2Þ 2
þ c½Salt þ d½Salt Þ
ð3Þ
being [S + IL] the mass fraction of the mixture surfactant and ionic liquid, [Salt] the mass fraction of the potassium-based inorganic
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M.S. Álvarez et al. / J. Chem. Thermodynamics 79 (2014) 178–183 TABLE 4 Parameters of equation (1) and standard deviation for {(C2C1imCnSO4 + Tween 20) (1) + salt (2) + H2O (3)}.a Ionic liquid and non ionic surfactant (1) 25% 25% 25% 25% 25% 25% 75% 75% 75% 75% 75% 75% a
C2C1imC2SO4 + 75% C2C1imC2SO4 + 75% C2C1imC2SO4 + 75% C2C1imC2SO4 + 75% C2C1imC2SO4 + 75% C2C1imC2SO4 + 75% C2C1imC2SO4 + 25% C2C1imC2SO4 + 25% C2C1imC2SO4 + 25% C2C1imC2SO4 + 25% C2C1imC2SO4 + 25% C2C1imC2SO4 + 25%
Tween Tween Tween Tween Tween Tween Tween Tween Tween Tween Tween Tween
20 20 20 20 20 20 20 20 20 20 20 20
Salt (2)
H2O (3)
K3PO4 K2HPO4 K2CO3 K3PO4 K2HPO4 K2CO3 K3PO4 K2HPO4 K2CO3 K3PO4 K2HPO4 K2CO3
a
H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O
b
0.5926 0.6438 0.7953 0.5043 0.5669 0.5403 3.62 104 3.39 104 1.39 105 0.9271 0.9458 0.6779
r
c
0.7042 0.9296 1.3934 0.6015 1.5655 0.6748 25.92 31.45 38.29 3.4794 3.6147 1.5453
3
2.19 10 2.65 103 2.70 103 2.15 103 2.15 103 3.08 103 3.05 103 3.06 103 2.97 103 6.43 102 5.40 102 2.5 102
0.0108 0.0101 0.0234 0.0088 0.0133 0.0096 0.0323 0.0370 0.0402 0.0083 0.0138 0.0137
Standard deviation (r) was calculated by means of equation (4).
salts, and a, b, c, and d the fitting parameters. The values of these parameters are collected in tables 4–6, together with the standard deviations (r), which were calculated by means of the following equation:
r¼
PnDAT i
ðzexp zadjust Þ2 nDAT
3.2. Tie-lines determination and correlation After having determined the phase behaviour of the systems under study, the composition of the top and bottom phases were determined by simple mass balances, using the correlation equation (2), since it turned out to be the most suitable one to describe the ABS. Two typical parameters used to describe the phase separation are the tie-line length (TLL) and the slope of the tie lines (S), which expressions are given below:
!1=2 ð4Þ
;
h i 2 2 0:5 TLL ¼ ðwI1 wII1 Þ þ ðwI2 wII2 Þ ;
where the experimental and adjustable solubility data are represented by zexp and zadjust, respectively and nDAT is the number of experimental data. From the deviations obtained (tables 4–6), it seems evident that the four parameters-based equation (2) is the one able to describe in a more suitable manner the experimental solubility data, which is also in agreement with previous works on surfactant-based ABS [22].
S¼
ð5Þ
wI1 wII1 ; wI2 wII2
ð6Þ
where the equilibrium mass fraction of the mixture surfactant and ionic liquid (1) and the inorganic salt (2), in the mixture surfactant
TABLE 5 Parameters of equation (2) and standard deviation for {(C2C1imCnSO4 + Tween 20) (1) + salt (2) + H2O (3)}.a
a
Ionic liquid and non ionic surfactant (1)
Salt (2)
H2O (3)
a
b
c
d
r
25% 25% 25% 25% 25% 25% 75% 75% 75% 75% 75% 75%
K3PO4 K2HPO4 K2CO3 K3PO4 K2HPO4 K2CO3 K3PO4 K2HPO4 K2CO3 K3PO4 K2HPO4 K2CO3
H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O
0.6733 0.5870 2.9120 0.4640 0.5994 0.6741 4.7343 3.6752 3.9760 0.9759 1.3973 1.3351
0.1377 1.0519 23.84 1.0872 1.1560 0.2456 31.50 10.58 11.72 2.8731 7.2962 6.0976
6.3151 8.8087 68.40 7.2829 1.2571 5.9496 79.36 2.2994 2.8037 1.2789 13.81 11.21
5.6113 6.3900 235.87 5.7882 39.4900 5.1655 238.9 2.9561 2.9755 2.7035 28.56 21.33
0.0055 0.0095 0.0162 0.0047 0.0100 0.0040 0.0321 0.0327 0.0358 0.0053 0.0097 0.0038
C2C1imC2SO4 + 75% C2C1imC2SO4 + 75% C2C1imC2SO4 + 75% C2C1imC2SO4 + 75% C2C1imC2SO4 + 75% C2C1imC2SO4 + 75% C2C1imC2SO4 + 25% C2C1imC2SO4 + 25% C2C1imC2SO4 + 25% C2C1imC2SO4 + 25% C2C1imC2SO4 + 25% C2C1imC2SO4 + 25%
Tween Tween Tween Tween Tween Tween Tween Tween Tween Tween Tween Tween
20 20 20 20 20 20 20 20 20 20 20 20
Standard deviation (r) was calculated by means of equation (4).
TABLE 6 Parameters of equation (3) and standard deviation for {(C2C1imCnSO4 + Tween 20) (1) + salt (2) + H2O (3)}.a
a
Ionic liquid and non ionic surfactant (1)
Salt (2)
H2O (3)
a
b
c
d
r
25% 25% 25% 25% 25% 25% 75% 75% 75% 75% 75% 75%
K3PO4 K2HPO4 K2CO3 K3PO4 K2HPO4 K2CO3 K3PO4 K2HPO4 K2CO3 K3PO4 K2HPO4 K2CO3
H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O
1.2464 0.6922 1.5000 2.1431 1.2992 2.9208 36.91 17.02 17.25 1.1674 1.8826 5.6250
26.48 19.88 7.7352 39.57 31.46 48.26 276.68 51.82 54.16 19.30 25.33 56.64
107.02 88.45 10.91 151.81 131.36 184.23 690.16 27.13 27.63 57.14 69.48 141.01
650.98 657.32 356.39 788.59 794.04 998.90 1919.2 37.49 37.23 259.74 254.56 294.99
0.0106 0.0091 0.0240 0.0079 0.0095 0.0090 0.0323 0.0375 0.0383 0.0085 0.0121 0.0091
C2C1imC2SO4 + 75% C2C1imC2SO4 + 75% C2C1imC2SO4 + 75% C2C1imC2SO4 + 75% C2C1imC2SO4 + 75% C2C1imC2SO4 + 75% C2C1imC2SO4 + 25% C2C1imC2SO4 + 25% C2C1imC2SO4 + 25% C2C1imC2SO4 + 25% C2C1imC2SO4 + 25% C2C1imC2SO4 + 25%
Tween Tween Tween Tween Tween Tween Tween Tween Tween Tween Tween Tween
20 20 20 20 20 20 20 20 20 20 20 20
Standard deviation (r) was calculated by means of equation (4).
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TABLE 7 Experimental tie-lines in mass fraction for {(C2C1imCnSO4 + Tween 20) (1) + salt (2) + H2O (3)} at T = 298.15 K, P = 101.33 kPa.
a
Surfactant and ionic liquid-rich phase
Inorganic salt-rich phase
100wI1
100wI2
100wII1
TLL
S
25.66 35.58 51.43
6.74 5.40 2.83
{(25% C2C1imC2SO4 + 75% Tween 20) (1) + K3PO4 (2) + H2O (3)} 1.49 12.81 0.50 14.19 0.12 15.71
25.78 36.50 52.88
3.62 3.77 4.01
31.17 51.52 41.93
5.84 2.70 4.33
{(25% C2C1imC2SO4 + 75% Tween 20) (1) + K2HPO4 (2) + H2O (3)} 1.75 11.04 0.02 14.14 0.83 12.42
32.68 52.76 43.18
3.99 4.24 4.04
24.77 33.71 46.42
6.93 5.96 4.07
{(25% C2C1imC2SO4 + 75% Tween 20) (1) + K2CO3 (2) + H2O (3)} 0.02 14.00 0.00 16.10 0.00 18.31
25.74 35.20 48.61
3.50 3.33 3.22
26.88 37.57 49.93
6.21 4.18 1.65
{(25% C2C1imC8SO4 + 75% Tween 20) (1) + K3PO4 (2) + H2O (3)} 1.49 12.81 0.27 14.86 0.02 17.11
26.24 38.80 50.71
3.85 3.49 3.12
42.57 53.03 32.28
2.49 0.05 4.66
{(25% C2C1imC8SO4 + 75% Tween 20) (1) + K2HPO4 (2) + H2O (3)} 0.06 14.04 0.02 16.23 0.17 12.42
44.05 55.46 33.03
3.68 3.27 4.14
27.73 36.77 45.23
5.58 4.22 2.91
{(25% C2C1imC8SO4 + 75% Tween 20) (1) + K2CO3 (2) + H2O (3)} 0.39 11.80 0.60 13.02 0.80 14.29
28.16 37.22 45.83
4.41 4.11 3.96
11.32 18.64 25.96
11.00 10.65 10.36
{(75% C2C1imC2SO4 + 25% Tween 20) (1) + K3PO4 (2) + H2O (3)} 0.53 12.94 0.18 13.69 0.06 14.37
11.55 18.71 26.23
5.93 6.07 6.23
15.19 20.29 25.10
9.69 9.45 9.15
{(75% C2C1imC2SO4 + 25% Tween 20) (1) + K2HPO4 (2) + H2O (3)} 0.41 11.96 0.28 12.84 0.06 14.16
15.78 20.45 25.54
4.36 4.70 4.99
17.37 22.55 29.98
9.07 9.02 8.65
{(75% C2C1imC2SO4 + 25% Tween 20) (1) + K2CO3 (2) + H2O (3)} 0.39 11.52 0.24 11.96 0.24 13.11
17.55 23.26 30.07
6.94 6.82 6.67
44.21 49.10 51.46
3.98 2.99 2.56
{(75% C2C1imC8SO4 + 25% Tween 20) (1) + K3PO4 (2) + H2O (3)} 2.55 16.53 1.14 18.20 0.54 19.52
43.51 50.31 53.67
3.32 3.15 3.00
24.95 35.96 49.09
8.27 5.64 3.25
{(75% C2C1imC8SO4 + 25% Tween 20) (1) + K2HPO4 (2) + H2O (3)} 0.16 19.00 0.17 20.50 0.17 22.52
27.23 38.47 52.31
2.20 2.54 2.68
25.99 35.27 47.75
12.11 8.48 5.02
{(75% C2C1imC8SO4 + 25% Tween 20) (1) + K2CO3 (2) + H2O (3)} 0.17 26.28 0.41 29.96 0.58 34.02
28.21 39.33 52.85
2.15 2.10 2.07
100wII2
Standard uncertainties are u(w) = ±0.0002, u(T) = ±0.01 K; u(P) = ±0.03 kPa.
TABLE 8 Parameters of Othmer–Tobias equation and correlation coefficient for {(C2C1imCnSO4 + Tween 20) (1) + salt (2) + H2O (3)}. Ionic liquid and non ionic surfactant (1) 25% 25% 25% 25% 25% 25% 75% 75% 75% 75% 75% 75%
C2C1imC2SO4 + 75% C2C1imC2SO4 + 75% C2C1imC2SO4 + 75% C2C1imC2SO4 + 75% C2C1imC2SO4 + 75% C2C1imC2SO4 + 75% C2C1imC2SO4 + 25% C2C1imC2SO4 + 25% C2C1imC2SO4 + 25% C2C1imC2SO4 + 25% C2C1imC2SO4 + 25% C2C1imC2SO4 + 25%
Tween Tween Tween Tween Tween Tween Tween Tween Tween Tween Tween Tween
20 20 20 20 20 20 20 20 20 20 20 20
Salt (2) K3PO4 K2HPO4 K2CO3 K3PO4 K2HPO4 K2CO3 K3PO4 K2HPO4 K2CO3 K3PO4 K2HPO4 K2CO3
H2O (3) H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O
a 4.8034 2.9881 3.0636 2.9468 2.7407 3.5071 6.4927 3.0714 4.4180 1.4434 4.9037 2.6085
R2
b 4
3.02 10 4.21 103 1.20 102 9.62 103 9.71 103 2.23 103 2.70 105 1.16 102 5.42 104 1.21 101 2.39 103 1.96 101
0.993 0.992 0.995 0.999 0.995 0.999 0.999 0.975 0.969 0.987 0.997 0.998
183
M.S. Álvarez et al. / J. Chem. Thermodynamics 79 (2014) 178–183 TABLE 9 Parameters of Bancroft equation and correlation coefficient for {(C2C1imCnSO4 + Tween 20) (1) + salt (2) + H2O (3)}. Ionic liquid and non ionic surfactant (1)
Salt (2)
H2O (3)
k
r
R2
25% 25% 25% 25% 25% 25% 75% 75% 75% 75% 75% 75%
K3PO4 K2HPO4 K2CO3 K3PO4 K2HPO4 K2CO3 K3PO4 K2HPO4 K2CO3 K3PO4 K2HPO4 K2CO3
H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O
5.4648 6.3676 4.3301 4.9035 5.4668 5.7246 5.4038 4.6320 5.7850 4.4427 2.4806 1.9311
0.2046 0.3131 0.3398 0.3414 0.3854 0.3015 0.1113 0.2908 0.1990 0.6359 0.2829 0.4172
0.995 0.988 0.993 0.999 0.990 0.998 0.996 0.966 0.950 0.977 0.985 0.996
C2C1imC2SO4 + 75% C2C1imC2SO4 + 75% C2C1imC2SO4 + 75% C2C1imC2SO4 + 75% C2C1imC2SO4 + 75% C2C1imC2SO4 + 75% C2C1imC2SO4 + 25% C2C1imC2SO4 + 25% C2C1imC2SO4 + 25% C2C1imC2SO4 + 25% C2C1imC2SO4 + 25% C2C1imC2SO4 + 25%
Tween Tween Tween Tween Tween Tween Tween Tween Tween Tween Tween Tween
20 20 20 20 20 20 20 20 20 20 20 20
and ionic liquid-rich phase (I) and potassium salt-rich phase (II), are represented. The TLL and S data obtained for each system are compiled in table 7, together with the layers compositions. From the data presented, it is evident that higher salt concentrations are correlated with greater TLL values, which is due to the fact that as more salt is added to the aqueous solution of Tween 20 and C2C1imCnSO4, more surfactant and ionic liquid is segregated to the upper phase. The analysis of the TL consistency was carried out by fitting the experimental data to Othmer–Tobias and Brancroft [30,22] equations:
a 1 wI1 1 wII2 ¼b ; I II w1 w2
ð7Þ
II I r w3 w3 ¼ k ; wII2 wI1
ð8Þ
being a, b, k and r the fitting parameters, w the mass fraction, subscripts 1, 2 and 3 the mixture (Tween 20 + C2C1imCnSO4), the potassium-based salt and water, respectively, and superscripts I and II the (ionic liquid and surfactant) rich-phase and salt-rich phase, respectively. In tables 8 and 9, all the parameters defining both models are collected, together with the correlation factor R2. In general terms, on the basis of the values of this correlation coefficient, it is possible to conclude that Othmer–Tobias model entails a better description of the experimental data for both ionic liquids, no matter the salt employed, in agreement with previous works on this topic [22].
4. Conclusions In this work, the suitability of a non-ionic surfactant to assist in the formation of ionic-liquid based ABS has been demonstrated. The increase of surfactant concentration has led to greater immiscibility windows, which is advantageous from both an extraction an economic point of view. It was confirmed that the Hofmeister series and the molar Gibbs free energy of hydration are valuable tools to predict the salting out potential of the selected inorganic salts in systems involving mixtures of ionic liquids and surfactants. All the experimental solubility data were suitably correlated by means of three and four parameter-based empirical equations. Additionally, Othmer–Tobias model served our goal to properly describe the TL data.
Acknowledgements This work has been supported by the Spanish Ministry of Economy and Competitiveness and EDERF funds (project CTM201231534). References [1] M.J. Earle, J.M.S.S. Esperança, M.A. Gilea, J.N. Canongia Lopes, L.P.N. Rebelo, J.W. Magee, K.R. Seddon, J.A. Widegren, Nature 439 (2006) 831–834. [2] E.F. Borra, O. Seddiki, R. Angel, D. Eisenstein, P. Hickson, K.R. Seddon, S.P. Worden, Nature 447 (2007) 979–981. [3] N.V. Plechkova, K.R. Seddon, Chem. Soc. Rev. 37 (2008) 123–150. [4] A. Rodríguez, J. Canosa, J. Tojo, J. Chem. Thermodyn. 33 (2001) 1383–1397. [5] A.B. Pereiro, A. Rodríguez, Green Chem. 11 (2009) 346–350. [6] U. Domanska, K. Walczak, M. Zawadzki, J. Chem. Thermodyn. 69 (2014) 27–35. [7] K.E. Gutowski, G.A. Broker, H.D. Willauer, J.G. Huddleston, R.P. Swatloski, J.D. Holbrey, R.D. Rogers, J. Am. Chem. Soc. 125 (2003) 6632–6633. [8] F.J. Deive, A. Rodríguez, A.B. Pereiro, J.M.M. Araújo, M.A. Longo, M.A.Z. Coelho, J.N. Canongia Lopes, J.M.S.S. Esperança, L.P.N. Rebelo, I.M. Marrucho, Green Chem. 13 (2011) 390–396. [9] M. Petkovic, K.R. Seddon, L.P.N. Rebelo, C.S. Pereira, Chem. Soc. Rev. 40 (2011) 1383–1403. [10] A.B. Pereiro, F.J. Deive, J.M.S.S. Esperança, A. Rodríguez, Fluid Phase Equilib. 291 (2010) 13–17. [11] X. Zhao, X. Xie, Y. Yan, Thermochim. Acta 516 (2011) 46–51. [12] M.G. Antov, D.M. Pericin, Enzyme Microb. Technol. 28 (2000) 467–472. [13] F.J. Deive, A. Rodríguez, J. Chem. Thermodyn. 54 (2012) 272–277. [14] M.G. Freire, J.F.B. Pereira, M. Francisco, H. Rodriguez, L.P.N. Rebelo, R.D. Rogers, J.A.P. Coutinho, Chem. Eur. J. 18 (2012) 1831–1839. [15] G. Ulloa, C. Coutens, M. Sánchez, J. Sineiro, A. Rodríguez, F.J. Deive, M.J. Núñez, J. Chem. Thermodyn. 47 (2012) 62–67. [16] L.F. Bautista, R. Sanz, M.C. Molina, N. González, D. Sánchez, Int. Biodeterior. Biodegrad. 63 (2009) 913–922. [17] M.A.O. da Silva, M.A.Z. Arruda, Talanta 77 (2009) 985–990. [18] G. Ulloa, C. Coutens, M. Sánchez, J. Sineiro, J. Fábregas, F.J. Deive, A. Rodríguez, M.J. Núñez, Green Chem. 14 (2012) 1044–1051. [19] M.S. Álvarez, F. Moscoso, A. Rodríguez, M.A. Sanromán, F.J. Deive, Bioresour. Technol. 146 (2013) 689–695. [20] M.S. Álvarez, E. Gutiérrez, A. Rodríguez, M.Á. Sanromán, F.J. Deive, Ind. Eng. Chem. Res. 53 (2014) 8615–8620. [21] F.J. Deive, A. Rodríguez, L.P.N. Rebelo, I.M. Marrucho, Sep. Purif. Technol. 97 (2012) 205–210. [22] M.S. Álvarez, F. Moscoso, F.J. Deive, M.Á. Sanromán, A. Rodríguez, J. Chem. Thermodyn. 55 (2012) 151–158. [23] P.A. Albertsson, Aqueous Polymer-Phase Systems, John Wiley and Sons, New York, 1986. [24] J.C. Merchuk, B.A. Andrews, J.A. Asenjo, J. Chromatogr. B 711 (1998) 285–293. [25] L. Wang, X. Chen, Y. Chai, J. Hao, Z. Sui, W. Zhuang, Z. Sun, Chem. Commun. (2004) 2840–2841. [26] F.J. Deive, A. Rodríguez, I.M. Marrucho, L.P.N. Rebelo, J. Chem. Thermodyn. 43 (2011) 1565–1572. [27] H. Rodríguez, M. Francisco, M. Rahman, N. Sun, R.D. Rogers, Phys. Chem. Chem. Phys. 11 (2009) 10916–10922. [28] M.G. Freire, A.F.M. Claudio, J.M.M. Araújo, J.A.P. Coutinho, I.M. Marrucho, J.N. Canongia Lopes, L.P.N. Rebelo, Chem. Soc. Rev. 41 (2012) 4966–4995. [29] Y. Marcus, J. Chem. Soc. Faraday Trans. 87 (1991) 2995–2999. [30] D.F. Othmer, P.E. Tobias, Ind. Eng. Chem. 34 (1942) 693–696.
JCT 14-412
Contents lists available at ScienceDirect
J. Chem. Thermodynamics journal homepage: www.elsevier.com/locate/jct
Influence of the addition of Tween 20 on the phase behaviour of ionic liquids-based aqueous systems María S. Álvarez, Ana Mateo, Francisco J. Deive ⇑, M. Ángeles Sanromán, Ana Rodríguez ⇑ Department of Chemical Engineering, Universidade de Vigo, P.O. Box 36310, Vigo, Spain
a r t i c l e
i n f o
Article history: Received 21 July 2014 Received in revised form 1 August 2014 Accepted 2 August 2014 Available online 9 August 2014 Keywords: Ionic liquids Aqueous biphasic systems Tween 20 Imidazolium Potassium salts
a b s t r a c t The addition of a non-ionic surfactant (Tween 20) to 1-ethyl-3methyl imidazolium alkylsulfate (C2C1imCnSO4)-based aqueous biphasic systems was investigated in this work. The solubility curves of the systems {(C2C1imCnSO4 (n = 2 and 8) + Tween 20) + high charge density salt (K3PO4/K2CO3/ K2HPO4) + H2O} were carried out at T = 298.15 K. The obtained experimental data were correlated by using three empirical equations. The molar Gibbs free energy of hydration (DhydG) is a valuable parameter to analyse the segregation capacity provided by the inorganic salts. Additionally, the efficiency of the separation capacity was discussed in terms of the salting out potential of the selected salts and the presence of Tween 20. Othmer–Tobias and Bancroft equations have been used to correlate the experimental tie-line data. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Currently, the hunt for more competitive and greener processes has led to the application of ionic liquids, neoteric solvents with outstanding properties such as their negligible volatility and tunability [1,2], in a range of fields such as electrochemistry, analytical chemistry, or chemical engineering, to name a few [3]. In this sense, one of the areas that has benefitted from the presence of these molten salts is the separation of compounds with industrial interest by means of (liquid + liquid) equilibrium [4–6]. Among the existing alternatives, the segregation of phases in systems containing aqueous solutions of ionic liquids is a field with booming interest since 2003, when Gutowski and co-workers published the first work on the capacity of hydrophilic ionic liquids to form aqueous biphasic systems (ABS) [7]. This separation strategy generally consists in adding an inorganic salt to aqueous solutions of ionic liquids, thus triggering phase disengagement: an upper aqueous phase mainly composed of ionic liquid and a lower aqueous layer rich in inorganic salt [8]. In this work, 1-ethyl-3-methyl imidazolium ethylsulfate and octylsulfate have been selected as model ionic liquids to be salted out in aqueous solutions, since they are reasonably cheap, they can be easily synthesized in an atomefficient and halide-free way, they display relatively low viscosities and melting points [9], and they belong to one of the most
⇑ Corresponding authors. Tel.: +34 986 81 87 23. E-mail addresses: [email protected] (F.J. Deive), [email protected] (A. Rodríguez). http://dx.doi.org/10.1016/j.jct.2014.08.004 0021-9614/Ó 2014 Elsevier Ltd. All rights reserved.
representative families (1-ethyl-3methyl imidazolium) already synthesized at levels higher than one ton per year [10]. Different types of ABS have been described in the literature, such as those exclusively formed by a polymer and a salt, a polymer and a polymer, an ionic liquid and a salt, an ionic liquid and a polymer, and a surfactant and a salt [11–15]. The latter has been reported to be advantageous in terms of cost, availability at bulk quantities, biodegradability, lower interface tension, mild operation temperatures and wider immiscibility window. Thus, in this study, the phase segregation capacity of different potassium-based inorganic salts was studied in aqueous mixtures of ionic liquids and a model non-ionic surfactant (Tween 20). This family is commonly used in industrial biotechnology, for instance in enzyme production processes or bioremediation studies, and has been reported to act even as nutrient in culture media [16,17]. In recent research works, the suitability of this kind of non-ionic surfactants (Tween 20) to extract biomolecules [18], industrial dyes [19] and metals [20] has already demonstrated. Additionally, the applicability of C2C1imCnSO4 has also been concluded for enzyme separation [8,21]. Therefore, in this study we intend to shed light on the ABS behaviour of both compounds mixed (nonionic surfactants and ionic liquids at ratios 25:75 and 75:25, respectively) in the presence of the inorganic salts K3PO4, K2HPO4 and K2CO3. These salts are well-known salting out agents, as predicted by the Hofmeister series, so their segregation capacity was discussed in this work by analysing the solubility curves at T = 298.15 K and the molar Gibbs free energy of hydration (DhydG). Three empirical equations were employed to correlate the
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experimental solubility data. The segregation capacity was also evaluated in terms of tie-line data, tackling the slope (S) and the tie-line length (TLL). Othmer–Tobias and Bancroft models were used to evaluate the consistency of the tie-line data [22]. 2. Experimental 2.1. Chemicals The ionic liquids 1-ethyl-3-methyl imidazolium ethylsulfate (C2C1imC2SO4) and 1-ethyl-3-methyl imidazolium octylsulfate (C2C1imC8SO4) were provided by Merck. They were subjected to vacuum of P = 101 Pa and temperature T = 330.15 K for several days in order to remove moisture and possible traces of organic volatile compounds. The inorganic salts K3PO4, K2HPO4 and K2CO3 were supplied by Sigma–Aldrich and were used as received, without further purification. The non-ionic surfactant Tween 20, polyethoxylated sorbitan monolaurate was purchased from Sigma–Aldrich. All the data concerning chemicals purities and provenance are shown in table 1. 2.2. Experimental procedure The solubility curves were empirically ascertained by means of the cloud point titration method [23] in a jacketed glass vessel containing a magnetic stirrer at atmospheric pressure, 101.33 kPa. The temperature was controlled at T = 298.15 K with a F200 ASL digital thermometer with an uncertainty of T = ±0.01 K. Known amounts of Tween 20 and C2C1imCnSO4 in aqueous solution were introduced to a vessel, and the immiscibility window was mapped by sequential additions of inorganic salt (K3PO4, K2HPO4 and K2CO3) and water, until the detection of a turbid and clear solution, respectively. Additionally, a mixture with known mass fraction at the biphasic region was prepared to determine the tie-lines (TLs), in accordance with the protocol defined by Merchuk [24]. Briefly, after a vigorous stirring, the mixture was left to settle for 24 h to allow a complete phase separation. Top and bottom layers were then separated and weighted, and the level arm rule was used to determine each TL composition. The estimated uncertainty associated with the determination of the surfactant (top) and salt (bottom) phases mass compositions is ±2%. All the samples were weighed in an analytical Sartorius Cubis MSA balance (125P-100-DA, ±105 g).
are collected in tables 2 and 3, and they are plotted in figures 1 and 2. The data obtained can be analysed in the light of the surfactant addition, the alkyl chain length in the ionic liquid anion, and the salting out potential of the selected inorganic salt. These factors have proven to be crucial to achieve phase segregation. The formation of an upper (ionic liquid/surfactant)-rich phase is observed, in line with previous investigations by Wang et al. (2004), who concluded that ionic liquids and surfactants form an organised moiety: not only does the ionic liquid act as a specific solvent, but also as a co-surfactant [25]. Additionally, it becomes patent that the increase of the surfactant concentration from 25% to 75% entails immiscibility regions closer to the origin, no matter the ionic liquid or inorganic salt employed. The comparison of the obtained data with the systems containing the pure surfactant (100% Tween 20) and ionic liquid (100% C2C1imCnSO4), reported by Álvarez et al. [22] and Deive et al. [26], respectively, reveals a close agreement with the observed trends. The explanation of this behaviour may be related to the increased hydrophobicity provided by the non-ionic surfactant. Thus, the competition between the mixture ionic liquid/surfactant and the inorganic salt for the water molecules is easily won when the first is more hydrophobic, due to they establish weaker hydrogen-bonds with water. This is coincident with other types of ABS involving ionic liquids and PEG [14,27]. The observed trends pose undoubted advantages related to the process economy and supply logistics.
TABLE 2 Binodal data in mass fraction for {(C2C1imC2SO4 + Tween 20) (1) + salt (2) + H2O (3)} two-phase systems for different surfactants concentrations at T = 298.15 K, P = 101.33 kPa.a K3PO4 100 w2
3. Results and discussion 3.1. Phase diagrams determination and correlation Up to our knowledge, this is the first time that the ABS resulting from the combination of the non-ionic surfactant Tween 20 with an ionic liquid and a high charge density inorganic salt is investigated. Thus, (liquid + liquid) de-mixing was characterised for the systems {(C2C1imCnSO4 (n = 2 and 8) + Tween 20) + (K3PO4 or K2CO3 or K2HPO4) + H2O} at T = 298.15 K. The experimental solubility data TABLE 1 Purities and suppliers of chemicals.a
a
Chemical
Supplier
Mass fraction purity
K3PO4 K2CO3 K2HPO4 Tween 20 C2C1imCnSO4
Sigma–Aldrich
0.98 0.99 0.98 0.98 0.98
Merck
Deionised water was used in all the experiments.
a
K2HPO4 100 w1
2.83 3.53 4.32 4.84 5.40 5.92 6.24 6.74 7.06 7.49 8.00 8.13 8.50 8.89 9.17 9.31 9.66 9.97 10.02
51.43 46.06 42.48 38.73 35.58 32.41 28.34 25.66 22.72 20.79 17.14 15.28 13.39 10.87 9.37 8.00 5.66 3.45 2.54
10.36 10.57 10.80 10.95 11.02 11.01 11.03 11.05 11.09 11.00 10.94 11.01 11.37 11.71
25.96 23.03 20.14 17.57 15.32 13.41 8.21 6.99 5.61 11.32 9.74 13.41 3.17 1.96
100 w2
K2CO3 100 w1
100 w2
25% C2C1imC2SO4 + 75% Tween 20 2.21 53.74 4.07 6.95 19.35 4.98 7.63 15.85 5.96 8.06 12.32 6.45 8.43 10.21 6.93 8.65 8.36 7.33 8.89 6.56 7.43 9.24 4.75 7.69 9.44 3.58 8.01 3.44 47.75 7.16 4.33 41.93 7.92 4.98 37.26 8.64 5.48 33.48 8.31 5.84 31.17 8.87 6.15 28.62 9.20 6.43 26.21 7.00 22.15
75% C2C1imC2SO4 + 25% Tween 20 8.30 32.48 8.65 8.88 28.56 8.75 9.15 25.10 9.02 9.38 23.11 9.04 9.49 18.68 9.07 9.64 17.10 9.02 9.45 20.29 9.07 9.69 15.19 9.22 9.70 12.49 9.09 9.65 9.90 9.54 9.78 7.53 8.37 9.79 5.88 9.90 9.99 3.02
100 w1 46.42 38.90 33.71 28.43 24.77 20.48 18.54 16.22 13.26 23.22 10.99 6.40 9.09 3.24 1.80
29.98 26.16 22.55 20.32 17.37 15.02 13.42 10.76 8.73 4.83 33.38 2.58
Standard uncertainties are u(w) = ± 0.0002, u(T) = ± 0.01 K; u(P) = ± 0.03 kPa.
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0.16
TABLE 3 Binodal data in mass fraction for {(C2C1imC8SO4 + Tween 20) (1) + salt (2) + H2O (3)} two-phase systems for different surfactants concentrations at T = 298.15 K, P = 101.33 kPa.a
a
100 w1
1.09 1.36 1.65 2.35 3.26 4.18 4.90 5.59 6.21 6.58 6.99 7.21 7.65 8.17 8.65 8.97 9.11 9.56 9.93 14.86 17.11 17.14
51.60 49.93 48.32 45.21 42.12 37.57 32.91 29.49 26.88 23.79 21.66 19.78 16.58 13.78 11.34 9.30 7.29 5.25 3.13 0.27 0.02 0.02
2.56 2.99 3.94 3.98 4.65 5.32 6.53 6.84 7.89 8.55 9.27 10.18 10.71 11.10 12.30 12.73 13.42 13.96 16.53 18.20 19.52
51.46 49.10 45.15 44.21 40.84 37.87 31.61 29.03 25.48 23.07 19.13 16.60 13.88 11.90 9.60 7.10 4.37 2.58 2.55 1.14 0.54
100 w2
0.12
K2CO3 100 w1
25% C2C1imC8SO4 + 75% Tween 20 1.32 48.26 1.90 44.99 2.43 40.74 3.67 37.81 4.01 35.96 4.66 32.28 5.81 27.60 6.12 25.44 6.15 23.16 6.66 20.33 7.10 18.13 7.50 15.79 7.63 12.50 8.26 9.77 8.67 7.02 9.23 3.97
100 w2 2.91 3.44 4.22 4.81 5.58 5.76 6.22 6.75 7.13 7.56 7.95 8.38 8.67 8.99
75% C2C1imC8SO4 + 25% Tween 20 3.52 49.85 6.67 3.97 43.44 8.48 4.69 38.29 9.83 5.64 35.96 10.95 6.05 33.11 12.11 7.08 30.34 13.23 7.65 27.85 13.78 8.27 24.95 15.07 9.15 22.11 16.28 9.93 19.26 17.14 10.12 17.17 5.02 11.46 13.13 18.10 12.24 9.89 18.65 13.56 6.42 19.19 14.15 4.21 14.53 2.06
100 w1 45.23 41.97 36.77 31.65 27.73 24.79 22.39 18.48 15.74 12.89 9.47 6.85 4.80 2.58
w1/M1
100 w2
K2HPO4
0.08
0.04
0.00 0.00
0.08
0.16
0.24
0.32
w2/M2 FIGURE 1. Plot of experimental and correlated solubility data of {(C2C1imC2SO4 + Tween 20) (1) + salt (2) + H2O (3)} at T = 298.15 K, P = 101.33 kPa: (s), K3PO4; (4), K2HPO4; (h) K2CO3. Blue, 100% Tween 20, reference [22]; Black, 75% Tween 20; Red, 25% Tween 20; Cyan, 0% Tween 20, reference [26] (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).
0.15 41.61 35.27 31.72 29.29 25.99 22.61 21.22 17.44 13.59 9.58 47.75 7.87 4.87 3.11
Standard uncertainties are u(w) = ± 0.0002, u(T) = ± 0.01 K; u(P) = ± 0.03 kPa.
Secondly, the comparison of the immiscibility gap for the two selected ionic liquids shown in figures 1 and 2, makes it evident the higher tendency of the octylsulfate-based ionic liquids for ABS formation, in agreement with the data reported by Deive et al. [26]. Although generally speaking, the self-aggregation of ionic liquids has been more evident for long alkyl chains in the cation [28], the present data evidence the existence of similar phenomena for the anion, thus confirming analogous hydrophobicity effects to those discussed for the surfactants. The analysis of the salting out ability of the salts under study evidences the following sequence for the selected anions (given 2 2 that the cation is fixed): PO3 4 > HPO4 > CO3 . Seemingly, the obtained data follow the Hofmeister series, an ion classification on the basis of the salting out potential of salts. As expected, trivalent anions entail more interplay with water than the divalent anions, a fact that can also be interpreted in the light of the molar Gibbs free energy of hydration (DhydG), which confirms the observed trends. Thus, the lower values of this thermodynamic parameter are related with an easier capacity to trigger phase
0.10 w1/M1
K3PO4
0.05
0.00 0.00
0.05
0.10
0.15
0.20
w2/M2 FIGURE 2. Plot of experimental and correlated solubility data of {(C2C1imC8SO4 + Tween 20) (1) + salt (2) + H2O (3)} at T = 298.15 K, P = 101.33 kPa: (s), K3PO4; (4), K2HPO4; (h) K2CO3. Blue, 100% Tween 20, reference [22]; Black, 75% Tween 20; Red, 25% Tween 20; Cyan, 0% Tween 20, reference [26] (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).
1 disengagement. In this sense, the value for PO3 ) 4 (2765 kJ mol 2 1 is higher than that reported for HPO4 (1789 kJ mol ), and in turn, this is higher than the one concluded for CO2 (1315 kJ 3 mol1) [29,11]. Therefore, it is confirmed that trivalent phosphate anion will be more akin to interact with water molecules, thus leading to greater immiscibility window. The experimental data were correlated by means of the following empirical equations [22]:
0:5
½S þ IL ¼ a expðb½Salt ½S þ IL ¼ a þ b½Salt
0:5
3
c½Salt Þ;
ð1Þ 2
þ c½Salt þ d½Salt ;
½S þ IL ¼ expða þ b½Salt
0:5
ð2Þ 2
þ c½Salt þ d½Salt Þ
ð3Þ
being [S + IL] the mass fraction of the mixture surfactant and ionic liquid, [Salt] the mass fraction of the potassium-based inorganic
181
M.S. Álvarez et al. / J. Chem. Thermodynamics 79 (2014) 178–183 TABLE 4 Parameters of equation (1) and standard deviation for {(C2C1imCnSO4 + Tween 20) (1) + salt (2) + H2O (3)}.a Ionic liquid and non ionic surfactant (1) 25% 25% 25% 25% 25% 25% 75% 75% 75% 75% 75% 75% a
C2C1imC2SO4 + 75% C2C1imC2SO4 + 75% C2C1imC2SO4 + 75% C2C1imC2SO4 + 75% C2C1imC2SO4 + 75% C2C1imC2SO4 + 75% C2C1imC2SO4 + 25% C2C1imC2SO4 + 25% C2C1imC2SO4 + 25% C2C1imC2SO4 + 25% C2C1imC2SO4 + 25% C2C1imC2SO4 + 25%
Tween Tween Tween Tween Tween Tween Tween Tween Tween Tween Tween Tween
20 20 20 20 20 20 20 20 20 20 20 20
Salt (2)
H2O (3)
K3PO4 K2HPO4 K2CO3 K3PO4 K2HPO4 K2CO3 K3PO4 K2HPO4 K2CO3 K3PO4 K2HPO4 K2CO3
a
H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O
b
0.5926 0.6438 0.7953 0.5043 0.5669 0.5403 3.62 104 3.39 104 1.39 105 0.9271 0.9458 0.6779
r
c
0.7042 0.9296 1.3934 0.6015 1.5655 0.6748 25.92 31.45 38.29 3.4794 3.6147 1.5453
3
2.19 10 2.65 103 2.70 103 2.15 103 2.15 103 3.08 103 3.05 103 3.06 103 2.97 103 6.43 102 5.40 102 2.5 102
0.0108 0.0101 0.0234 0.0088 0.0133 0.0096 0.0323 0.0370 0.0402 0.0083 0.0138 0.0137
Standard deviation (r) was calculated by means of equation (4).
salts, and a, b, c, and d the fitting parameters. The values of these parameters are collected in tables 4–6, together with the standard deviations (r), which were calculated by means of the following equation:
r¼
PnDAT i
ðzexp zadjust Þ2 nDAT
3.2. Tie-lines determination and correlation After having determined the phase behaviour of the systems under study, the composition of the top and bottom phases were determined by simple mass balances, using the correlation equation (2), since it turned out to be the most suitable one to describe the ABS. Two typical parameters used to describe the phase separation are the tie-line length (TLL) and the slope of the tie lines (S), which expressions are given below:
!1=2 ð4Þ
;
h i 2 2 0:5 TLL ¼ ðwI1 wII1 Þ þ ðwI2 wII2 Þ ;
where the experimental and adjustable solubility data are represented by zexp and zadjust, respectively and nDAT is the number of experimental data. From the deviations obtained (tables 4–6), it seems evident that the four parameters-based equation (2) is the one able to describe in a more suitable manner the experimental solubility data, which is also in agreement with previous works on surfactant-based ABS [22].
S¼
ð5Þ
wI1 wII1 ; wI2 wII2
ð6Þ
where the equilibrium mass fraction of the mixture surfactant and ionic liquid (1) and the inorganic salt (2), in the mixture surfactant
TABLE 5 Parameters of equation (2) and standard deviation for {(C2C1imCnSO4 + Tween 20) (1) + salt (2) + H2O (3)}.a
a
Ionic liquid and non ionic surfactant (1)
Salt (2)
H2O (3)
a
b
c
d
r
25% 25% 25% 25% 25% 25% 75% 75% 75% 75% 75% 75%
K3PO4 K2HPO4 K2CO3 K3PO4 K2HPO4 K2CO3 K3PO4 K2HPO4 K2CO3 K3PO4 K2HPO4 K2CO3
H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O
0.6733 0.5870 2.9120 0.4640 0.5994 0.6741 4.7343 3.6752 3.9760 0.9759 1.3973 1.3351
0.1377 1.0519 23.84 1.0872 1.1560 0.2456 31.50 10.58 11.72 2.8731 7.2962 6.0976
6.3151 8.8087 68.40 7.2829 1.2571 5.9496 79.36 2.2994 2.8037 1.2789 13.81 11.21
5.6113 6.3900 235.87 5.7882 39.4900 5.1655 238.9 2.9561 2.9755 2.7035 28.56 21.33
0.0055 0.0095 0.0162 0.0047 0.0100 0.0040 0.0321 0.0327 0.0358 0.0053 0.0097 0.0038
C2C1imC2SO4 + 75% C2C1imC2SO4 + 75% C2C1imC2SO4 + 75% C2C1imC2SO4 + 75% C2C1imC2SO4 + 75% C2C1imC2SO4 + 75% C2C1imC2SO4 + 25% C2C1imC2SO4 + 25% C2C1imC2SO4 + 25% C2C1imC2SO4 + 25% C2C1imC2SO4 + 25% C2C1imC2SO4 + 25%
Tween Tween Tween Tween Tween Tween Tween Tween Tween Tween Tween Tween
20 20 20 20 20 20 20 20 20 20 20 20
Standard deviation (r) was calculated by means of equation (4).
TABLE 6 Parameters of equation (3) and standard deviation for {(C2C1imCnSO4 + Tween 20) (1) + salt (2) + H2O (3)}.a
a
Ionic liquid and non ionic surfactant (1)
Salt (2)
H2O (3)
a
b
c
d
r
25% 25% 25% 25% 25% 25% 75% 75% 75% 75% 75% 75%
K3PO4 K2HPO4 K2CO3 K3PO4 K2HPO4 K2CO3 K3PO4 K2HPO4 K2CO3 K3PO4 K2HPO4 K2CO3
H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O
1.2464 0.6922 1.5000 2.1431 1.2992 2.9208 36.91 17.02 17.25 1.1674 1.8826 5.6250
26.48 19.88 7.7352 39.57 31.46 48.26 276.68 51.82 54.16 19.30 25.33 56.64
107.02 88.45 10.91 151.81 131.36 184.23 690.16 27.13 27.63 57.14 69.48 141.01
650.98 657.32 356.39 788.59 794.04 998.90 1919.2 37.49 37.23 259.74 254.56 294.99
0.0106 0.0091 0.0240 0.0079 0.0095 0.0090 0.0323 0.0375 0.0383 0.0085 0.0121 0.0091
C2C1imC2SO4 + 75% C2C1imC2SO4 + 75% C2C1imC2SO4 + 75% C2C1imC2SO4 + 75% C2C1imC2SO4 + 75% C2C1imC2SO4 + 75% C2C1imC2SO4 + 25% C2C1imC2SO4 + 25% C2C1imC2SO4 + 25% C2C1imC2SO4 + 25% C2C1imC2SO4 + 25% C2C1imC2SO4 + 25%
Tween Tween Tween Tween Tween Tween Tween Tween Tween Tween Tween Tween
20 20 20 20 20 20 20 20 20 20 20 20
Standard deviation (r) was calculated by means of equation (4).
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M.S. Álvarez et al. / J. Chem. Thermodynamics 79 (2014) 178–183
TABLE 7 Experimental tie-lines in mass fraction for {(C2C1imCnSO4 + Tween 20) (1) + salt (2) + H2O (3)} at T = 298.15 K, P = 101.33 kPa.
a
Surfactant and ionic liquid-rich phase
Inorganic salt-rich phase
100wI1
100wI2
100wII1
TLL
S
25.66 35.58 51.43
6.74 5.40 2.83
{(25% C2C1imC2SO4 + 75% Tween 20) (1) + K3PO4 (2) + H2O (3)} 1.49 12.81 0.50 14.19 0.12 15.71
25.78 36.50 52.88
3.62 3.77 4.01
31.17 51.52 41.93
5.84 2.70 4.33
{(25% C2C1imC2SO4 + 75% Tween 20) (1) + K2HPO4 (2) + H2O (3)} 1.75 11.04 0.02 14.14 0.83 12.42
32.68 52.76 43.18
3.99 4.24 4.04
24.77 33.71 46.42
6.93 5.96 4.07
{(25% C2C1imC2SO4 + 75% Tween 20) (1) + K2CO3 (2) + H2O (3)} 0.02 14.00 0.00 16.10 0.00 18.31
25.74 35.20 48.61
3.50 3.33 3.22
26.88 37.57 49.93
6.21 4.18 1.65
{(25% C2C1imC8SO4 + 75% Tween 20) (1) + K3PO4 (2) + H2O (3)} 1.49 12.81 0.27 14.86 0.02 17.11
26.24 38.80 50.71
3.85 3.49 3.12
42.57 53.03 32.28
2.49 0.05 4.66
{(25% C2C1imC8SO4 + 75% Tween 20) (1) + K2HPO4 (2) + H2O (3)} 0.06 14.04 0.02 16.23 0.17 12.42
44.05 55.46 33.03
3.68 3.27 4.14
27.73 36.77 45.23
5.58 4.22 2.91
{(25% C2C1imC8SO4 + 75% Tween 20) (1) + K2CO3 (2) + H2O (3)} 0.39 11.80 0.60 13.02 0.80 14.29
28.16 37.22 45.83
4.41 4.11 3.96
11.32 18.64 25.96
11.00 10.65 10.36
{(75% C2C1imC2SO4 + 25% Tween 20) (1) + K3PO4 (2) + H2O (3)} 0.53 12.94 0.18 13.69 0.06 14.37
11.55 18.71 26.23
5.93 6.07 6.23
15.19 20.29 25.10
9.69 9.45 9.15
{(75% C2C1imC2SO4 + 25% Tween 20) (1) + K2HPO4 (2) + H2O (3)} 0.41 11.96 0.28 12.84 0.06 14.16
15.78 20.45 25.54
4.36 4.70 4.99
17.37 22.55 29.98
9.07 9.02 8.65
{(75% C2C1imC2SO4 + 25% Tween 20) (1) + K2CO3 (2) + H2O (3)} 0.39 11.52 0.24 11.96 0.24 13.11
17.55 23.26 30.07
6.94 6.82 6.67
44.21 49.10 51.46
3.98 2.99 2.56
{(75% C2C1imC8SO4 + 25% Tween 20) (1) + K3PO4 (2) + H2O (3)} 2.55 16.53 1.14 18.20 0.54 19.52
43.51 50.31 53.67
3.32 3.15 3.00
24.95 35.96 49.09
8.27 5.64 3.25
{(75% C2C1imC8SO4 + 25% Tween 20) (1) + K2HPO4 (2) + H2O (3)} 0.16 19.00 0.17 20.50 0.17 22.52
27.23 38.47 52.31
2.20 2.54 2.68
25.99 35.27 47.75
12.11 8.48 5.02
{(75% C2C1imC8SO4 + 25% Tween 20) (1) + K2CO3 (2) + H2O (3)} 0.17 26.28 0.41 29.96 0.58 34.02
28.21 39.33 52.85
2.15 2.10 2.07
100wII2
Standard uncertainties are u(w) = ±0.0002, u(T) = ±0.01 K; u(P) = ±0.03 kPa.
TABLE 8 Parameters of Othmer–Tobias equation and correlation coefficient for {(C2C1imCnSO4 + Tween 20) (1) + salt (2) + H2O (3)}. Ionic liquid and non ionic surfactant (1) 25% 25% 25% 25% 25% 25% 75% 75% 75% 75% 75% 75%
C2C1imC2SO4 + 75% C2C1imC2SO4 + 75% C2C1imC2SO4 + 75% C2C1imC2SO4 + 75% C2C1imC2SO4 + 75% C2C1imC2SO4 + 75% C2C1imC2SO4 + 25% C2C1imC2SO4 + 25% C2C1imC2SO4 + 25% C2C1imC2SO4 + 25% C2C1imC2SO4 + 25% C2C1imC2SO4 + 25%
Tween Tween Tween Tween Tween Tween Tween Tween Tween Tween Tween Tween
20 20 20 20 20 20 20 20 20 20 20 20
Salt (2) K3PO4 K2HPO4 K2CO3 K3PO4 K2HPO4 K2CO3 K3PO4 K2HPO4 K2CO3 K3PO4 K2HPO4 K2CO3
H2O (3) H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O
a 4.8034 2.9881 3.0636 2.9468 2.7407 3.5071 6.4927 3.0714 4.4180 1.4434 4.9037 2.6085
R2
b 4
3.02 10 4.21 103 1.20 102 9.62 103 9.71 103 2.23 103 2.70 105 1.16 102 5.42 104 1.21 101 2.39 103 1.96 101
0.993 0.992 0.995 0.999 0.995 0.999 0.999 0.975 0.969 0.987 0.997 0.998
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M.S. Álvarez et al. / J. Chem. Thermodynamics 79 (2014) 178–183 TABLE 9 Parameters of Bancroft equation and correlation coefficient for {(C2C1imCnSO4 + Tween 20) (1) + salt (2) + H2O (3)}. Ionic liquid and non ionic surfactant (1)
Salt (2)
H2O (3)
k
r
R2
25% 25% 25% 25% 25% 25% 75% 75% 75% 75% 75% 75%
K3PO4 K2HPO4 K2CO3 K3PO4 K2HPO4 K2CO3 K3PO4 K2HPO4 K2CO3 K3PO4 K2HPO4 K2CO3
H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O
5.4648 6.3676 4.3301 4.9035 5.4668 5.7246 5.4038 4.6320 5.7850 4.4427 2.4806 1.9311
0.2046 0.3131 0.3398 0.3414 0.3854 0.3015 0.1113 0.2908 0.1990 0.6359 0.2829 0.4172
0.995 0.988 0.993 0.999 0.990 0.998 0.996 0.966 0.950 0.977 0.985 0.996
C2C1imC2SO4 + 75% C2C1imC2SO4 + 75% C2C1imC2SO4 + 75% C2C1imC2SO4 + 75% C2C1imC2SO4 + 75% C2C1imC2SO4 + 75% C2C1imC2SO4 + 25% C2C1imC2SO4 + 25% C2C1imC2SO4 + 25% C2C1imC2SO4 + 25% C2C1imC2SO4 + 25% C2C1imC2SO4 + 25%
Tween Tween Tween Tween Tween Tween Tween Tween Tween Tween Tween Tween
20 20 20 20 20 20 20 20 20 20 20 20
and ionic liquid-rich phase (I) and potassium salt-rich phase (II), are represented. The TLL and S data obtained for each system are compiled in table 7, together with the layers compositions. From the data presented, it is evident that higher salt concentrations are correlated with greater TLL values, which is due to the fact that as more salt is added to the aqueous solution of Tween 20 and C2C1imCnSO4, more surfactant and ionic liquid is segregated to the upper phase. The analysis of the TL consistency was carried out by fitting the experimental data to Othmer–Tobias and Brancroft [30,22] equations:
a 1 wI1 1 wII2 ¼b ; I II w1 w2
ð7Þ
II I r w3 w3 ¼ k ; wII2 wI1
ð8Þ
being a, b, k and r the fitting parameters, w the mass fraction, subscripts 1, 2 and 3 the mixture (Tween 20 + C2C1imCnSO4), the potassium-based salt and water, respectively, and superscripts I and II the (ionic liquid and surfactant) rich-phase and salt-rich phase, respectively. In tables 8 and 9, all the parameters defining both models are collected, together with the correlation factor R2. In general terms, on the basis of the values of this correlation coefficient, it is possible to conclude that Othmer–Tobias model entails a better description of the experimental data for both ionic liquids, no matter the salt employed, in agreement with previous works on this topic [22].
4. Conclusions In this work, the suitability of a non-ionic surfactant to assist in the formation of ionic-liquid based ABS has been demonstrated. The increase of surfactant concentration has led to greater immiscibility windows, which is advantageous from both an extraction an economic point of view. It was confirmed that the Hofmeister series and the molar Gibbs free energy of hydration are valuable tools to predict the salting out potential of the selected inorganic salts in systems involving mixtures of ionic liquids and surfactants. All the experimental solubility data were suitably correlated by means of three and four parameter-based empirical equations. Additionally, Othmer–Tobias model served our goal to properly describe the TL data.
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JCT 14-412