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(Solid + liquid) and (liquid + liquid) phase equilibria of (IL + water) binary systems. The influence of the ionic liquid structure on mutual solubility
Fluid Phase Equilibria 361 (2014) 273–281
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(Solid + liquid) and (liquid + liquid) phase equilibria of (IL + water) binary systems. The influence of the ionic liquid structure on mutual solubility Marta Królikowska ∗ Department of Physical Chemistry, Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland
a r t i c l e
i n f o
Article history: Received 6 September 2013 Received in revised form 4 November 2013 Accepted 5 November 2013 Available online 11 November 2013 Keywords: Ionic liquid Water Phase diagram UCST NRTL
a b s t r a c t In this work binary (solid + liquid) and (liquid + liquid) phase equilibria measurements for the new ionic liquids: 1-ethyl-3-methylimidazolium tricyanomethanide, [EMIM][TCM], 1-butyl-3-methylimidazolium tricyanomethanide, [BMIM][TCM], 1-butyl-1-methyl-pyrrolidinium tricyanomethanide, [BMPYR][TCM], 1-butyl-1-methylmorpholinium tricyanomethanide, [BMMOR][TCM], trihexyltertadecylphosphonium tricyanomethanide, [P6,6,6,14 ][TCM] with water over a wide temperature range and at atmospheric pressure have been determined using dynamic method. In order to discuss the influence of the ionic liquid structure on mutual miscibility with water the phase equilibria for 1-butyl1-methyl-pyrrolidinium trifluoromethanesulfonate, [BMPYR][CF3 SO3 ], 1-butyl-1-methylpyrrolidinium tetracyanoborate, [BMPYR][TCB] and 1-butyl-3-methylimidazolium dicyanamide, [BMIM] [DCA] were detected. For the tested binary systems with immiscibility gap the parameters of the LLE correlation have been derived using the NRTL equation. For the binary mixtures of water and [EMIM][TCM], [BMIM][DCA] and [BMPYR][CF3 SO3 ] the complete miscibility in the liquid phase over whole composition range was observed. For these systems the (solid + liquid) phase equilibria have been correlated by means of NRTL, UNIQUAC and Wilson equations. The influence of the ionic liquid structure on water solubility was discussed. Additionally the basic thermal characterization of pure ionic liquids that is: the glass transition temperature, the change of heat capacity at the glass-transition temperature, temperature and enthalpy of phase transition and temperature and enthalpy of melting were determined using DSC technique. Decomposition temperature was detected by thermograwimetry. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Due to the fascinating properties, room temperature ionic liquids (RTILs) have gained an increasing attention over recent decade. A numerous number of combinations of cations and anions allows of modification of the thermodynamic properties and design of ionic liquids for a clearly defined purpose [1–5]. In order to design any technological process with ionic liquids the knowledge of physical and thermodynamic properties, either for pure ILs or mixed with other solvents are required. Regarding to the study of physical properties for binary mixtures of ionic liquids with water, a large number of papers are available in the opened literature [6–29]. Most of the work concerns the measurement of density, viscosity, heat capacity and refractive index. As regards to the measurement of phase equilibria for binary mixtures of ionic liquid and water [30–37], a significant amount of publications come from our laboratory.
∗ Corresponding author. Tel.: +48 22 234 56 40; fax: +48 22 628 27 41. E-mail address: [email protected] 0378-3812/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.fluid.2013.11.003
This work is a continuation of our studies on ionic liquids for use in absorption refrigeration. It is clear that knowledge of the interactions with water and specifically phase equilibria is the key to a successful adaptation of (IL + water) binary mixtures in this technology. Absorption refrigeration technology as environmentally friendly arousing growing interest around the world. Technological development causes a continuous increase of demand for energy, thus it is necessary to re-use waste energy from other processes. This energy may be used for heating or cooling. Absorption refrigeration technology allows to use the waste heat that is created in a variety of industrial processes and normally lost and thrown into the atmosphere. The aim is therefore to improve the efficiency of production and development of previously wasted energy. A good alternative may be applied in development of absorption refrigeration compressor, whose main advantage is the use of heat, the absorption refrigeration. Currently, mainly due to the increasing interest in environmental aspects of that type of equipment, there are a lot of research in progress. One of the most important factors determining the efficiency of absorption chillers are the properties of working fluids. In this
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technology the volatile substance is used as a refrigerant, while the less volatile substance is used as a absorbent. Currently the synthetic refrigerants are widely used. However, due to the protection of the environment (problems with the ozone layer and the greenhouse effect) the requirements have been tightened. Therefore, works are carried out to introduce new synthetic working fluids, as well as the return to the old natural systems such as: (H2 O + LiBr) and (NH3 + H2 O) [38]. However, natural systems present disadvantages that is corrosion and crystallization of the (LiBr–H2 O) system, or toxicity and explosibility of the (NH3 + H2 O) [39]. This creates a new opportunities for the (ionic liquid + water) binary systems. There are many investigation on about the use of (IL + water) binary systems as an alternative working pairs for absorption refrigeration [19,20,40–45]. In this work the thermal characterization of pure ionic liquids and the mutual solubility with water are presented. The main aim of this study is to assess the first step of suitability of the investigated ionic liquids for absorption cooling.
phase transition temperature (Ttr ) and enthalpy of phase transition (tr H), temperature (Tfus ) and enthalpy of melting (fus H). The experiments were performed with DSC 1 STARe System (Mettler Toledo) calorimeter equipped with liquid nitrogen cooling system and operating in a heat-flux mode. The sample cell was constantly fluxed with high purity nitrogen at constant flow rate of 20 mL min−1 . The instrument was calibrated with the 99.9999 mol% purity indium sample and with high purity heptane, octane, decane and water. The sample was sealed in ambient air in hermetic aluminum pans having mass of about 50 mg. An empty hermetic aluminum pan was used as a reference. Sample size of about 10 mg was used throughout this study and the heat flow was normalized by the actual weight of each sample. The experiments were carried out using 10 K min−1 heating rate. The calorimetric accuracy from calibration was 1%. The experimental data were analysed using STAR software. The experimental results are presented in Table 3.
2. Experimental
Simultaneous TG/DTA experiments were performed using a MOM Derivatograph—PC (Hungary). The experiments were carried out using matched labyrinth platinic crucibles with Al2 O3 in the reference pan. The crucible design hampered the migration of volatile decomposition products, reducing the rate of gas evolution and, in turn, increasing the contact time of the reactants. All TG/DTA curves were obtained at a 5 K min−1 heating rate with a nitrogen dynamic atmosphere (flow rate of 20 dm3 h−1 ). The temperatures of decomposition are presented in Table 3.
2.1. Chemicals The (solid + liquid) and (liquid + liquid) phase equilibria with water have been determined for the following ionic liquids: 1-ethyl-3-methylimidazolium tricyanomethanide, [EMIM] [TCM] (Io-li-tec, >0.980), 1-butyl-3-methylimidazolium tricyanomethanide, [BMIM][TCM] (Io-li-tec, >0.980), 1-butyl3-methylimidazolium dicyanamide, [BMIM][DCA] (Io-li-tec, >0.980), 1-butyl-1-methylpyrrolidinium tricyanomethanide, [BMPYR][TCM] (Io-li-tec, >0.980), 1-butyl-1-methylmorpholinium tricyanomethanide, [BMMOR][TCM] (Io-li-tec, >0.970), trihexyltetradecylphosphonium tricyanomethanide, [P6,6,6,14 ][TCM] (Merck, >0.980), 1-butyl-1-methylpyrrolidinium tetracyanoborate, [BMPYR][TCB] (Merck, >0.980) and 1-butyl-1-methylpyrrolidinium trifluoromethanesulfonate, [BMPYR][CF3 SO3 ] (Merck, >0.990). The structures of cations and anions of the investigated ionic liquids are presented in Table 1. Before the experiment, each ionic liquid was purified under vacuum at temperature T = 353 K for approximately 24 h in order to reduce the volatile chemicals and water. The specification of the IL including purities and purification methods are collected in Table 2. Karl Fischer titration technique (model SCHOTT Instruments TitroLine KF) was used to determine the water content. In this method a small sample of IL was firstly dissolved in dry methanol and then titrated with steps of 2.5 L. For volumetric titration the titrant CombiTitrant 5 No. 1.8805.1000 Merck onecomponent reagent was used. The lower determination limit of the technique used in this work is approximately 50–100 ppm H2 O. The analysis showed that the water content was as follows: 551 ppm for [EMIM][TCM], 1510 ppm for [BMIM][TCM], 867 ppm for [BMIM][DCA], 654 ppm for [BMPYR][TCM], 281 ppm for [BMMOR][TCM], 497 ppm for [P6,6,6,14 ][TCM], 436 ppm for [BMPYR][TCB] and 427 ppm for [BMPYR][CF3 SO3 ], The high-purity water used for the experiment was deionized by a reverse osmosis unit with an ion-exchange system (Cobrabid-Aqua, Poland), and next degassed in an ELMA Germany ultrasonic bath at about 320 K before each measurement. 2.2. Differential scanning microcalorimetry, (DSC) Differential scanning microcalorimetry technique (DSC) was used to determine the basic thermal characteristics of the investigated ionic liquids, that is: the glass transition temperature (Tg,1 ), the heat capacity at the glass-transition temperature (Cp(g),1 ),
2.3. Thermograwimetry, (TG/DTA)
2.4. Phase equilibria measurements (Solid + liquid) and (liquid + liquid) phase equilibria have been determined using a dynamic method. More detailed description of the apparatus and procedure have been presented previously [46]. The (IL + water) mixtures were prepared by weighing the pure components on Mettler Toledo XA105 balance with an uncertainty of 1 × 10−4 g. Mixtures were prepared by mass, and errors did not exceed 1 × 10−4 in IL mole fraction. The sample was heated very slow (<2 K h−1 ) with continuous stirring inside of a Pyrex glass cell placed in a thermostat. The temperature of crystal disappearance, detected visually, were measured with an electronic thermometer P 550 (DOSTMANN electronic GmbH) with the probe totally immersed in the thermostatting liquid. The uncertainties of the temperature measurements were judged to be 0.05 K. The repeatability of the SLE/LLE experimental points was 0.05 K. The results of the solubility measurements are presented in Table 4. The experimental data have been correlated using NRTL, Wilson and UNIQUAC equations. The parameters and root-mean-square deviations are collected in Tables 5 and 6.
3. Modeling For the tested binary systems with immiscibility gap the parameters of the LLE correlation have been derived using the NRTL equation, based on the excess Gibbs energy [47]. The description of this equation and the activity coefficient formula was presented by us earlier [32]. In this correlation the adjustable parameters (g12 − g22 ) and (g21 − g11 ) were found. The parameter ˛12 is a constant (˛12 = ˛21 ) and was taken into account by choosing the value that gave the lowest deviation. The calculated values of the NRTL parameters and the corresponding root mean-square deviations are presented in Table 5. For the correlation of the (solid + liquid) phase equilibria three equations, that is: NRTL [xlvii], UNIQUAC [48] and Wilson [49] were used. The results of the correlation as well as
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275
Table 1 Structures, abbreviations and names for the ions studied in this work. Structure
N
N
N
N
Abbreviation
Name
[EMIM]+
1-Ethyl-3-methylimidazolium
[BMIM]+
1-Butyl-3-methylimidazolium
[BMPYR]+
1-Butyl-1-methylpyrrolidinium
[BMMOR]+
1-Butyl-1-methylmorpholinium
[P6,6,6,14 ]+
Trihexyltetradecylphosphonium
[TCM]−
Tricyanomethanide
[DCA]−
Dicyanamide
[CF3 SO3 ]−
Trifluoromethanesulfonate
[TCB]−
Tetracyanoborate
+
N
+
+
N
+
O
+
P
N
CN
N
N
-
N
N F
F
O S
F
O-
O N
N
-
B
N
N
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Table 2 The sources and mass fraction purities of materials. Chemical name
Source
Initial mass fraction purity
Purification method
Final mass fraction purity
Analysis method
1-Ethyl-3-methylimidazolium tricyanomethanide, [EMIM][TCM] 1-Butyl-3-methylimidazolium tricyanomethanide, [BMIM][TCM] 1-Butyl-3-methylimidazolium dicyanamide, [BMIM][DCA] 1-Butyl-1-methylpyrrolidinium tricyanomethanide, [BMPYR][TCM] 1-Butyl-1-methylpyrrolidinium tetracyanoborate, [BMPYR][TCB] 1-Butyl-1-methylpyrrolidinium trifluoromethanesulfonate, [BMPYR][CF3 SO3 ] 1-Butyl-1-methylmorpholinium tricyanomethanide, [BMMOR][TCM] Trihexyltetradecylphosphonium tricyanomethanide, [P6,6,6,14 ][TCM] Water
Io-li-tec
>0.980
Vacuum heating
0.9998
Karl–Fischer
Io-li-tec
≥0.980
Vacuum heating
0.9997
Karl–Fischer
Io-li-tec
>0.980
Vacuum heating
0.9997
Karl–Fischer
Io-li-tec
≥ 0.980
Vacuum heating
0.9997
Karl - Fischer
Merck
≥0.980
Vacuum heating
0.9996
Karl–Fischer
Merck
≥0.990
Vacuum heating
0.9996
Karl - Fischer
Io-li-tec
≥0.970
Vacuum heating
0.9997
Karl–Fischer
Merck
≥ 0.980
Vacuum heating
0.9996
Karl–Fischer
Own source
–
Distillation, filtration
≥0.999
Density
Table 3 Thermal properties of the investigated ionic liquids: glass transition temperature (Tg,1 ) and heat capacity change at glass transition temperature (Cp(g),1 ), melting temperature (Tm ) and enthalpy (Hm ), phase transition temperature (Ttr ) and enthalpy (Htr ) and decomposition temperature (Td ) at pressure p = 0.1 MPaa .
a
Ionic liquid
Tg,1 (K)
Cp(g),1 (J mol−1 K−1 )
Ttr (K)
tr H (kJ mol−1 )
Tm (K)
[EMIM][TCM] [EMIM][TCM] [50] [BMIM][TCM] [BMIM][DCA] [17] [BMPYR][TCM] [BMPYR][TCB] [BMPYR][CF3 SO3 ] [51] [BMMOR][TCM] [P6,6,6,14 ][TCM]
181.51 179.55 187.61 205.15 178.01
103.6 93.85 135.2
237.8 236.97
1.04 1.62
263.85 257.15
m H (kJ mol−1 ) 9.02 6.48
Td (K) 628 622 632
267.15 152.7
210.3
292.77 277.56
20.58
604 656
203.66
8.48
590 701
123.9
Standard uncertainties u are as follows: u(Tg,1 ) = 0.1 K; u(Cp(g),1 ) = 5 J mol−1 K−1 , u(Tfus ) = 0.1 K; u(fus H) = 0.5 kJ mol−1 , u(Td ) = 0.1 K.
the values of the parameters and standard deviations are collected in Table 6 and in Figs. 1–8. 4. Results and discussion The main goal of this work is to determine the basic thermal characterization of pure ionic liquids and to measure the phase equilibria for (IL + water) binary mixtures. The compounds are selected in such a way that it was possible to determine the impact of ionic liquids modifications, such as: the cation and anion family, or the alkyl chain length in the cation substituent on mutual
solubility with water. The knowledge of thermophysical properties and phase equilibria is necessary to determine the applicability of ionic liquids as an absorbent in absorption refrigeration. The thermal characterization of pure ionic liquids have been done using DSC and TG/DTA technique. The glass transition temperature, heat capacity at glass transition temperature, temperature and enthalpy of phase transition, temperature and enthalpy of melting and decomposition temperature of compounds have been determined and collected in Table 3. The phase transition was observed only for [EMIM][TCM]. Most of the ILs tested in this work were liquid at room temperature. In the case of [BMPYR][TCM] 340
280
330
270
320
T/ K
T/ K
260
250
310 300 290
240
230 0.0
280
0.1
0.2
x1
0.3
0.4
0.5
Fig. 1. Experimental and calculated (solid + liquid) phase equilibria of {[EMIM][TCM] (1) + water (2)} binary system: , experimental points; (-) Wilson equation; (-) NRTL equation; (-) UNIQUAC equation.
270 0.0
0.1
0.2
x1
0.3
0.4
0.5
Fig. 2. Experimental and calculated (liquid + liquid) phase equilibria of {[BMIM][TCM] (1) + water (2)} binary system: , experimental points. Solid line has been calculated using the NRTL equation.
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Table 4 Experimental (solid + liquid) and (liquid + liquid) phase equilibria for {ionic liquid (1) + water (2)} binary system at pressure p = 0.1 MPaa . x1
TSLE (K)
[EMIM][TCM] (1) + water (2) 0.3338 241.48 0.3127 245.66 0.2919 249.22 252.17 0.2739 254.29 0.2556 256.72 0.2365 0.2143 259.29 0.1941 261.57 0.1707 264.09 [BMIM][TCM] (1) + water (2) 0.1541 0.1475 0.1371 0.1277 0.1191 0.1087 0.1010 0.0956 0.0890 0.0759 0.0649 0.0565 [BMIM][DCA] (1) + water (2) 0.2183 245.03 253.03 0.1912 0.1698 257.65 260.87 0.1499 0.1260 264.23 265.10 0.1169 266.98 0.1003 267.80 0.0894 268.24 0.0818 [BMPYR][TCM] (1) + water (2) 266.52 1.0000 0.8576 261.92 0.7552 259.48 256.67 0.6591 0.5934 254.87 251.93 0.5435 0.4813 246.64 0.1225 0.1212 0.1181 0.1142 0.1104 0.1063 0.1021 [BMPYR][TCB] (1) + water (2) 292.85 1.0000 288.22 0.8811 287.09 0.8521 285.48 0.8105 284.36 0.7757 283.21 0.7330 282.45 0.7020 281.49 0.6705 280.68 0.6477 280.09 0.6242 279.93 0.5807 0.5515 0.5248 0.4957 [BMPYR][CF3 SO3 ] (1) + water (2) 277.78 1.0000 0.9236 275.32 271.78 0.8447 269.09 0.7966 266.81 0.7573 264.35 0.7111 0.6591 261.24 257.97 0.6041 254.33 0.5411 249.78 0.4738 247.73 0.4397 245.42 0.4067
TLLE (K)
286.20 291.50 297.42 302.93 307.64 312.17 315.53 317.16 319.27 322.78 324.97 325.74
x1
TSLE (K)
0.1444 0.1194 0.0950 0.0615 0.0424 0.0212 0.0113 0.0068 0.0000
266.62 268.56 269.86 271.06 271.77 272.41 272.81 272.92 273.15
0.0462 0.0420 0.0328 0.0240 0.0181 0.0148 0.0119 0.0097 0.0080 0.0068 0.0061
0.0672 0.0655 0.0544 0.0433 0.0418 0.0291 0.0204 0.0148 0.0000
TLLE (K)
326.73 326.81 326.76 325.97 323.88 321.51 317.66 313.20 306.91 301.49 293.64
269.33 269.44 270.01 270.51 270.53 271.28 271.80 272.09 273.15
287.44 288.77 294.4 300.28 305.09 310.12 315.33
0.0967 0.0891 0.0825 0.0707 0.0385 0.0245 0.0162 0.0125 0.0108 0.0091 0.0079 0.0064 0.0055 0.0049
321.27 326.89 330.84 338.04 343.06 341.87 336.94 334.19 331.56 327.07 322.33 313.24 305.76 300.75
287.77 296.07 302.67 309.96
0.4807 0.4691 0.4527 0.4409 0.4189 0.3978 0.3703 0.3461 0.3252 0.2880 0.0017 0.0014 0.0010 0.0009
314.56 317.69 320.70 323.23 328.33 332.65 338.52 344.29 349.54 355.86 352.79 339.15 316.70 307.75
0.3151 0.2947 0.2750 0.2552 0.2377 0.2191 0.2027 0.1826 0.1627 0.1453 0.1257 0.1077
256.55 258.61 260.80 262.29 263.58 264.58 265.62 266.76 267.34 267.98 268.60 269.27
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M. Królikowska / Fluid Phase Equilibria 361 (2014) 273–281
Table 4 (Continued)
a
x1
TSLE (K)
0.3732 0.3544 0.3345 [BMMOR][TCM] (1) + water (2) 0.4620 0.4007 0.3570 0.3021 0.3210 0.2864 0.2777 0.2548 0.2490 0.2264 0.2147 0.1867 0.1631 0.1564 0.1378 0.1275 [P6,6,6,14 ][TCM] (1) + water (2) 0.7601 0.7072 0.6514
TLLE (K)
x1
TSLE (K)
249.00 252.20 254.42
0.0766 0.0391 0.0000
270.26 271.57 273.15
237.72 248.81 255.76 262.29 260.14 264.02 264.69 266.55 266.85 268.48 269.15 270.13 271.26 271.80 272.16 272.29
0.1155 0.1147 0.0614 0.0836 0.0810 0.0712 0.0607 0.0520 0.0472 0.0411 0.0327 0.0263 0.0192 0.0139 0.0000
272.43 272.83 272.94 273.02 273.02 273.02 273.02 273.02 273.02 273.02 273.02 273.02 273.02 273.02 273.15
288.15 309.20 326.72
TLLE (K)
276.52 278.04 281.56 284.04 285.61 286.46 286.68 286.12 284.64 282.54 278.97
0.6367 0.5967 0.5597
332.32 343.80 354.49
Standard uncertainties u are as follows: u(x1 ) = 0.0001 and u(T) = 0.05 K.
Table 5 Correlation of the (liquid + liquid) phase equilibrium data by means of the NRTL equation: parameters (g12 − g22 = a12 + b12 T), (g21 − g11 = a21 + b21 T) and deviations x .
(g12 − g22 ) (J mol−1 )
[BMIM][TCM] [BMPYR][TCM] [BMPYR][TCB] [BMMOR][TCM] [P6,6,6,14 ][TCM]
(g21 − g11 ) (J mol−1 )
a12
b12
a21
b21
19333.9 2083.011 27016.11 −1835.34 18014.33
−125.016 −73.0128 −81.074 −15.1838 −81.47422
−8621.12 15568.73 1866.52 28610.34 −16885.5
154.0846 84.83004 44.56813 −45.9824 178.8447
we were unable to determine the temperature and enthalpy of melting from DSC technique due to lack of crystallization. The internal stirring elements in the dynamic method allowed for the sample crystallization and made possible to determine the temperature of melting. The highest decomposition temperature was determined for [P6,6,6,14 ][TCM] (701 K) and the lowest for [BMMOR][TCM] (590 K). For the family of TCM-based ionic liquids apart from [BMMOR][TCM] similar decomposition temperature were observed. The experimental (solid + liquid) and (liquid + liquid) phase equilibrium data (temperature vs. ionic liquid mole fraction) for the (IL + water) binary mixtures are presented in Table 4 and in Figs. 1–8. The parameters of the correlation equations are collected in Tables 5 and 6. The strong interactions between ionic liquid and water cause complete miscibility in the liquids phase for [EMIM][TCM], [BMIM][DCA] and [BMPYR][CF3 SO3 ]. For these systems it was possible to measure only the liquidus curves at low
˛
103 × x (K)
NRTL parameters
Ionic liquid
8.1 6.3 5.1 1.0 7.7
0.10 0.10 0.36 0.30 0.10
temperature. These mixtures reveal the simple eutectic systems. For the ([BMPYR][CF3 SO3 ] + water) it was possible to detect the characteristic eutectic point at T = 245.42 K for x1 = 0.4067. For the other mentioned systems it was impossible to measure this points due to the very low temperature and problem with crystallization. As was presented in our latest paper on phase equilibria, the complete miscibility in the liquid phase over whole range of ionic liquid mole fraction were observed for other dicyanamide-based ionic liquids with pyrrolidinium, pyridinium and piperidinium cations [36]. The comparison of phase equilibria for dicyanamidebased ionic liquids with water was presented in Fig. 9. Phase equilibria with upper critical solution temperature (UCST) were observed for five binary mixtures of water and [BMIM][TCM], [BMPYR][TCM], [BMPYR][TCB], [BMMOR][TCM] and [P6,6,6,14 ][TCM]. It was possible to detect the maximum of the curves for [BMIM][TCM], [BMPYR][TCM] and for [BMMOR][TCM]. The UCST increase in the following
Table 6 Correlation of the experimental (solid + liquid) phase equilibrium data of {IL (1) + water (2)} binary systems by means of UNIQUAC, Wilson and NRTL equation along with obtained values of parameters and root-mean-square deviations (RMSD) of temperature ( T ) as a measure of goodness of the correlation. Ionic liquid
RMSD
Parameters UNIQUAC u12 (J mol
[EMIM][TCM] [BMIM][DCA] [BMPYR][CF3 SO3 ]
2758.77 2773.25 2447.21
Wilson −1
)
u21 (J mol 1790.57 465.54 7382.03
−1
)
˛12 (J mol 2577.51 1853.30 4274.09
NRTL −1
)
˛21 (J mol 91614.38 13569.27 4952.85
−1
)
UNIQUAC −1
g12 (J mol 6615.47 12414.21 22654.76
)
−1
g21 (J mol −1184.80 −5814.38 −18421.90
)
Wilson
NRTL
1.31 1.58 0.85
0.36 0.19 1.09
T (K) 0.72 1.18 1.47
M. Królikowska / Fluid Phase Equilibria 361 (2014) 273–281
290
270
280
260
270
T/ K
T/ K
280
279
250
260
240
250
230 0.0
0.1
0.2
240 0.0
0.3
0.2
0.4
0.8
1.0
Fig. 6. Experimental and calculated (solid + liquid) phase equilibria of {[BMPYR] [CF3 SO3 ] (1) + water (2)} binary systems: , experimental points. Solid lines have been calculated using the NRTL equation.
360
300
340
290
320
280
T/ K
T/ K
Fig. 3. Experimental and calculated (solid + liquid) phase equilibria of {[BMIM][DCA] (1) + water (2)} binary systems: , experimental points. Solid lines have been calculated using the NRTL equation.
300
270 260
280
250
260 240 0.0
0.6
x1
x1
0.2
0.4
0.6
0.8
240 0.0
1.0
0.1
0.2
x1 Fig. 4. Experimental and calculated (solid + liquid) and (liquid + liquid) phase equilibria of {[BMPYR][TCM] (1) + water (2)} binary systems: , experimental points. Solid lines have been calculated using the NRTL equation.
order: [BMMOR][TCM] (T = 286.7 K, x1 = 0.0411) < [BMIM][TCM] (T = 326.81 K, x1 = 0.042) < [BMPYR][TCM] (T = 343.1 K, x1 = 0.0385). For the binary mixtures of water and [BMPYR][TCB] and [P6,6,6,14 ][TCM] the maximum of the immiscibility curves
0.3
0.4
0.5
x1 Fig. 7. Experimental and calculated (solid + liquid) and (liquid + liquid) phase equilibria of {[BMMOR][TCM] (1) + water (2)} binary systems: , experimental points. Solid lines have been calculated using the NRTL equation.
were not detected because the boiling point of water was lower. On the basis of the comparison of phase diagram for ([cation][TCM] + water) binary system presented in Fig. 10, the following trends can be noticed; the solubility of water in the ionic liquids increases in the following 400
370
350
360
T/ K
T/ K
330
320
310
280
290
270 0.0
0.2
0.4
x1
0.6
0.8
1.0
Fig. 5. Experimental and calculated (solid + liquid) and (liquid + liquid) phase equilibria of {[BMPYR][TCB] (1) + water (2)} binary systems: , experimental points. Solid lines have been calculated using the NRTL equation.
240 0.0
0.2
0.4
x1
0.6
0.8
1.0
Fig. 8. Experimental and calculated (liquid + liquid) phase equilibria of {[P6,6,6,14 ][TCM] (1) + water (2)} binary systems: , experimental points. Solid lines have been calculated using the NRTL equation.
280
M. Królikowska / Fluid Phase Equilibria 361 (2014) 273–281
310
410 380
290
270
T/ K
T/ K
350 320 290
250
260 230
0.0
0.2
0.4
0.6
0.8
1.0
x1 Fig. 9. Experimental and literature (solid + liquid) phase equilibria of {dicyanamidebased IL (1) + water (2)} binary systems: , [BMIM][DCA]; ♦, [BMPYR][DCA] [36]; 䊉, [BMPy] [DCA] [36]; , [BMPIP][DCA] [36].
order: [P6,6,6,14 ][TCM] < [BMIM][TCM] < [BMPYR][TCM] < [BMMOR] [TCM]. The best solubility was observed for [BMMOR][TCM] due to the more polar nature of ionic liquid resulting from the addition of oxygen atom into a cation structure. Due to the presentence of four aliphatic group and more hydrophobic nature of [P6,6,6,14 ][TCM] high immiscibility gap (from x1 = 0.76) was observed. Apart from that it was observed that the mutual solubility decreases with increase of the alkyl chain length in the imidazolium ring substituent. For [BMIM][TCM] the immiscibility gap started from x1 = 0.15 while for [EMIM][TCM] complete miscibility in the liquid phase was observed for whole range of IL mole fraction. Close agreement between experimental work and the data reported by Freire et al. was observed for [BMIM][TCM] [31]. In Fig. 11 the comparison of the experimental and literature (solid + liquid) and (liquid + liquid) for ([BMPYR][anion] + water) binary mixtures is presented. It is worth to point out that the deviation from ideal solution becomes positive in the following order: [DCA]− < [TCM]− < [TCB]− . In the case of [DCA] anion the negative deviations from ideal solution were observed. Addition of cyano-group in [TCM] anion results in stronger delocalization of negative charge from the core atom, and increases the steric hindrence. As a result H-bonds in the binary mixtures of ([BMPYR][TCM] + water) are less favorable in comparison to 360 340
T/ K
320 300 280
230 0.0
0.2
0.4
x1
0.6
0.8
1.0
Fig. 11. Experimental and literature (solid + liquid) and (liquid + liquid) phase equilibria of {1-butyl-1-methylpyrrolidinium-based IL (1) + water (2)} binary systems: , [BMPYR] [TCM]; , [BMPYR][TCB]; , [BMPYR][CF3 SO3 ]; , [BMPYR][NTf2 ] [30,31]; , [BMPYR] [FAP] [35]; ♦, [BMPYR][SCN] [33]; *, [BMPYR][DCA] [36].
([BMPYR][DCA] + water) systems. Similar behavior was observed in the case of ([BMPYR][TCM] + water) binary systems. 5. Conclusion In this work, new data of (solid + liquid) and (liquid + liquid) phase equilibria for eight (ionic liquid + water) binary mixtures were presented. Additionally, the basic thermal characterization of pure ionic liquids have been done using DSC and TG/DTA technique. Complete miscibility in the liquid phase was with water observed for [EMIM][TCM], [BMIM][DCA], or [BMPYR][CF3 SO3 ]. The experimental data of (liquid + liquid) phase equilibria with UCST were observed for water and [BMIM][TCM], or [BMPYR][TCM], or [BMPYR][TCB], or [BMMOR][TCM] and [P6,6,6,14 ][TCM]. The highest immiscibility gap in the tested binary systems was observed for [P6,6,6,14 ][TCM]. Apart from that, the immiscibility gap increases with the increase of the alkyl chain length in the imidazolium ring. As regards to the LLE in binary systems of tricyanomethanide-based ionic liquids and water the solubility increases in the following order: [P6,6,6,14 ][TCM] < [BMIM][TCM] < [BMPYR][TCM] < [BMMOR][TCM]. From the comparison of the mutual solubility of water and ionic liquids with [BMPYR] cation, it was observed that the solubility increases as follows: [DCA]− < [TCM]− < [TCB]− . The knowledge of effects of ionic liquid structure on the thermal characterization and phase equilibria with water is useful for developing IL as an absorbent in absorption refrigeration. The experimental data presented in this work, may give new informations about the possibility of using ionic liquids in this technology. Acknowledgment
260 240 220
0.0
0.2
0.4
x1
0.6
0.8
1.0
Fig. 10. Experimental and calculated (solid + liquid) and (liquid + liquid) phase equilibria of {tricyanomethanide-based IL (1) + water (2)} binary systems: , [EMIM][TCM]; 䊉, [BMIM][TCM]; , [BMIM][TCM] [31]; , [BMPYR][TCM]; , [BMMOR][TCM]; , [P6,6,6,14 ][TCM]. Solid lines have been calculated using the NRTL equation.
Funding for this research was provided by the National Science Centre in years 2011–2014 (Grant No. 2011/01/D/ST5/02760). This work has been supported by the European Union in the framework of European Social Fund through the Warsaw University of Technology Development Programme. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.fluid.2013.11.003.
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Fluid Phase Equilibria journal homepage: www.elsevier.com/locate/fluid
(Solid + liquid) and (liquid + liquid) phase equilibria of (IL + water) binary systems. The influence of the ionic liquid structure on mutual solubility Marta Królikowska ∗ Department of Physical Chemistry, Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland
a r t i c l e
i n f o
Article history: Received 6 September 2013 Received in revised form 4 November 2013 Accepted 5 November 2013 Available online 11 November 2013 Keywords: Ionic liquid Water Phase diagram UCST NRTL
a b s t r a c t In this work binary (solid + liquid) and (liquid + liquid) phase equilibria measurements for the new ionic liquids: 1-ethyl-3-methylimidazolium tricyanomethanide, [EMIM][TCM], 1-butyl-3-methylimidazolium tricyanomethanide, [BMIM][TCM], 1-butyl-1-methyl-pyrrolidinium tricyanomethanide, [BMPYR][TCM], 1-butyl-1-methylmorpholinium tricyanomethanide, [BMMOR][TCM], trihexyltertadecylphosphonium tricyanomethanide, [P6,6,6,14 ][TCM] with water over a wide temperature range and at atmospheric pressure have been determined using dynamic method. In order to discuss the influence of the ionic liquid structure on mutual miscibility with water the phase equilibria for 1-butyl1-methyl-pyrrolidinium trifluoromethanesulfonate, [BMPYR][CF3 SO3 ], 1-butyl-1-methylpyrrolidinium tetracyanoborate, [BMPYR][TCB] and 1-butyl-3-methylimidazolium dicyanamide, [BMIM] [DCA] were detected. For the tested binary systems with immiscibility gap the parameters of the LLE correlation have been derived using the NRTL equation. For the binary mixtures of water and [EMIM][TCM], [BMIM][DCA] and [BMPYR][CF3 SO3 ] the complete miscibility in the liquid phase over whole composition range was observed. For these systems the (solid + liquid) phase equilibria have been correlated by means of NRTL, UNIQUAC and Wilson equations. The influence of the ionic liquid structure on water solubility was discussed. Additionally the basic thermal characterization of pure ionic liquids that is: the glass transition temperature, the change of heat capacity at the glass-transition temperature, temperature and enthalpy of phase transition and temperature and enthalpy of melting were determined using DSC technique. Decomposition temperature was detected by thermograwimetry. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Due to the fascinating properties, room temperature ionic liquids (RTILs) have gained an increasing attention over recent decade. A numerous number of combinations of cations and anions allows of modification of the thermodynamic properties and design of ionic liquids for a clearly defined purpose [1–5]. In order to design any technological process with ionic liquids the knowledge of physical and thermodynamic properties, either for pure ILs or mixed with other solvents are required. Regarding to the study of physical properties for binary mixtures of ionic liquids with water, a large number of papers are available in the opened literature [6–29]. Most of the work concerns the measurement of density, viscosity, heat capacity and refractive index. As regards to the measurement of phase equilibria for binary mixtures of ionic liquid and water [30–37], a significant amount of publications come from our laboratory.
∗ Corresponding author. Tel.: +48 22 234 56 40; fax: +48 22 628 27 41. E-mail address: [email protected] 0378-3812/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.fluid.2013.11.003
This work is a continuation of our studies on ionic liquids for use in absorption refrigeration. It is clear that knowledge of the interactions with water and specifically phase equilibria is the key to a successful adaptation of (IL + water) binary mixtures in this technology. Absorption refrigeration technology as environmentally friendly arousing growing interest around the world. Technological development causes a continuous increase of demand for energy, thus it is necessary to re-use waste energy from other processes. This energy may be used for heating or cooling. Absorption refrigeration technology allows to use the waste heat that is created in a variety of industrial processes and normally lost and thrown into the atmosphere. The aim is therefore to improve the efficiency of production and development of previously wasted energy. A good alternative may be applied in development of absorption refrigeration compressor, whose main advantage is the use of heat, the absorption refrigeration. Currently, mainly due to the increasing interest in environmental aspects of that type of equipment, there are a lot of research in progress. One of the most important factors determining the efficiency of absorption chillers are the properties of working fluids. In this
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technology the volatile substance is used as a refrigerant, while the less volatile substance is used as a absorbent. Currently the synthetic refrigerants are widely used. However, due to the protection of the environment (problems with the ozone layer and the greenhouse effect) the requirements have been tightened. Therefore, works are carried out to introduce new synthetic working fluids, as well as the return to the old natural systems such as: (H2 O + LiBr) and (NH3 + H2 O) [38]. However, natural systems present disadvantages that is corrosion and crystallization of the (LiBr–H2 O) system, or toxicity and explosibility of the (NH3 + H2 O) [39]. This creates a new opportunities for the (ionic liquid + water) binary systems. There are many investigation on about the use of (IL + water) binary systems as an alternative working pairs for absorption refrigeration [19,20,40–45]. In this work the thermal characterization of pure ionic liquids and the mutual solubility with water are presented. The main aim of this study is to assess the first step of suitability of the investigated ionic liquids for absorption cooling.
phase transition temperature (Ttr ) and enthalpy of phase transition (tr H), temperature (Tfus ) and enthalpy of melting (fus H). The experiments were performed with DSC 1 STARe System (Mettler Toledo) calorimeter equipped with liquid nitrogen cooling system and operating in a heat-flux mode. The sample cell was constantly fluxed with high purity nitrogen at constant flow rate of 20 mL min−1 . The instrument was calibrated with the 99.9999 mol% purity indium sample and with high purity heptane, octane, decane and water. The sample was sealed in ambient air in hermetic aluminum pans having mass of about 50 mg. An empty hermetic aluminum pan was used as a reference. Sample size of about 10 mg was used throughout this study and the heat flow was normalized by the actual weight of each sample. The experiments were carried out using 10 K min−1 heating rate. The calorimetric accuracy from calibration was 1%. The experimental data were analysed using STAR software. The experimental results are presented in Table 3.
2. Experimental
Simultaneous TG/DTA experiments were performed using a MOM Derivatograph—PC (Hungary). The experiments were carried out using matched labyrinth platinic crucibles with Al2 O3 in the reference pan. The crucible design hampered the migration of volatile decomposition products, reducing the rate of gas evolution and, in turn, increasing the contact time of the reactants. All TG/DTA curves were obtained at a 5 K min−1 heating rate with a nitrogen dynamic atmosphere (flow rate of 20 dm3 h−1 ). The temperatures of decomposition are presented in Table 3.
2.1. Chemicals The (solid + liquid) and (liquid + liquid) phase equilibria with water have been determined for the following ionic liquids: 1-ethyl-3-methylimidazolium tricyanomethanide, [EMIM] [TCM] (Io-li-tec, >0.980), 1-butyl-3-methylimidazolium tricyanomethanide, [BMIM][TCM] (Io-li-tec, >0.980), 1-butyl3-methylimidazolium dicyanamide, [BMIM][DCA] (Io-li-tec, >0.980), 1-butyl-1-methylpyrrolidinium tricyanomethanide, [BMPYR][TCM] (Io-li-tec, >0.980), 1-butyl-1-methylmorpholinium tricyanomethanide, [BMMOR][TCM] (Io-li-tec, >0.970), trihexyltetradecylphosphonium tricyanomethanide, [P6,6,6,14 ][TCM] (Merck, >0.980), 1-butyl-1-methylpyrrolidinium tetracyanoborate, [BMPYR][TCB] (Merck, >0.980) and 1-butyl-1-methylpyrrolidinium trifluoromethanesulfonate, [BMPYR][CF3 SO3 ] (Merck, >0.990). The structures of cations and anions of the investigated ionic liquids are presented in Table 1. Before the experiment, each ionic liquid was purified under vacuum at temperature T = 353 K for approximately 24 h in order to reduce the volatile chemicals and water. The specification of the IL including purities and purification methods are collected in Table 2. Karl Fischer titration technique (model SCHOTT Instruments TitroLine KF) was used to determine the water content. In this method a small sample of IL was firstly dissolved in dry methanol and then titrated with steps of 2.5 L. For volumetric titration the titrant CombiTitrant 5 No. 1.8805.1000 Merck onecomponent reagent was used. The lower determination limit of the technique used in this work is approximately 50–100 ppm H2 O. The analysis showed that the water content was as follows: 551 ppm for [EMIM][TCM], 1510 ppm for [BMIM][TCM], 867 ppm for [BMIM][DCA], 654 ppm for [BMPYR][TCM], 281 ppm for [BMMOR][TCM], 497 ppm for [P6,6,6,14 ][TCM], 436 ppm for [BMPYR][TCB] and 427 ppm for [BMPYR][CF3 SO3 ], The high-purity water used for the experiment was deionized by a reverse osmosis unit with an ion-exchange system (Cobrabid-Aqua, Poland), and next degassed in an ELMA Germany ultrasonic bath at about 320 K before each measurement. 2.2. Differential scanning microcalorimetry, (DSC) Differential scanning microcalorimetry technique (DSC) was used to determine the basic thermal characteristics of the investigated ionic liquids, that is: the glass transition temperature (Tg,1 ), the heat capacity at the glass-transition temperature (Cp(g),1 ),
2.3. Thermograwimetry, (TG/DTA)
2.4. Phase equilibria measurements (Solid + liquid) and (liquid + liquid) phase equilibria have been determined using a dynamic method. More detailed description of the apparatus and procedure have been presented previously [46]. The (IL + water) mixtures were prepared by weighing the pure components on Mettler Toledo XA105 balance with an uncertainty of 1 × 10−4 g. Mixtures were prepared by mass, and errors did not exceed 1 × 10−4 in IL mole fraction. The sample was heated very slow (<2 K h−1 ) with continuous stirring inside of a Pyrex glass cell placed in a thermostat. The temperature of crystal disappearance, detected visually, were measured with an electronic thermometer P 550 (DOSTMANN electronic GmbH) with the probe totally immersed in the thermostatting liquid. The uncertainties of the temperature measurements were judged to be 0.05 K. The repeatability of the SLE/LLE experimental points was 0.05 K. The results of the solubility measurements are presented in Table 4. The experimental data have been correlated using NRTL, Wilson and UNIQUAC equations. The parameters and root-mean-square deviations are collected in Tables 5 and 6.
3. Modeling For the tested binary systems with immiscibility gap the parameters of the LLE correlation have been derived using the NRTL equation, based on the excess Gibbs energy [47]. The description of this equation and the activity coefficient formula was presented by us earlier [32]. In this correlation the adjustable parameters (g12 − g22 ) and (g21 − g11 ) were found. The parameter ˛12 is a constant (˛12 = ˛21 ) and was taken into account by choosing the value that gave the lowest deviation. The calculated values of the NRTL parameters and the corresponding root mean-square deviations are presented in Table 5. For the correlation of the (solid + liquid) phase equilibria three equations, that is: NRTL [xlvii], UNIQUAC [48] and Wilson [49] were used. The results of the correlation as well as
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275
Table 1 Structures, abbreviations and names for the ions studied in this work. Structure
N
N
N
N
Abbreviation
Name
[EMIM]+
1-Ethyl-3-methylimidazolium
[BMIM]+
1-Butyl-3-methylimidazolium
[BMPYR]+
1-Butyl-1-methylpyrrolidinium
[BMMOR]+
1-Butyl-1-methylmorpholinium
[P6,6,6,14 ]+
Trihexyltetradecylphosphonium
[TCM]−
Tricyanomethanide
[DCA]−
Dicyanamide
[CF3 SO3 ]−
Trifluoromethanesulfonate
[TCB]−
Tetracyanoborate
+
N
+
+
N
+
O
+
P
N
CN
N
N
-
N
N F
F
O S
F
O-
O N
N
-
B
N
N
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Table 2 The sources and mass fraction purities of materials. Chemical name
Source
Initial mass fraction purity
Purification method
Final mass fraction purity
Analysis method
1-Ethyl-3-methylimidazolium tricyanomethanide, [EMIM][TCM] 1-Butyl-3-methylimidazolium tricyanomethanide, [BMIM][TCM] 1-Butyl-3-methylimidazolium dicyanamide, [BMIM][DCA] 1-Butyl-1-methylpyrrolidinium tricyanomethanide, [BMPYR][TCM] 1-Butyl-1-methylpyrrolidinium tetracyanoborate, [BMPYR][TCB] 1-Butyl-1-methylpyrrolidinium trifluoromethanesulfonate, [BMPYR][CF3 SO3 ] 1-Butyl-1-methylmorpholinium tricyanomethanide, [BMMOR][TCM] Trihexyltetradecylphosphonium tricyanomethanide, [P6,6,6,14 ][TCM] Water
Io-li-tec
>0.980
Vacuum heating
0.9998
Karl–Fischer
Io-li-tec
≥0.980
Vacuum heating
0.9997
Karl–Fischer
Io-li-tec
>0.980
Vacuum heating
0.9997
Karl–Fischer
Io-li-tec
≥ 0.980
Vacuum heating
0.9997
Karl - Fischer
Merck
≥0.980
Vacuum heating
0.9996
Karl–Fischer
Merck
≥0.990
Vacuum heating
0.9996
Karl - Fischer
Io-li-tec
≥0.970
Vacuum heating
0.9997
Karl–Fischer
Merck
≥ 0.980
Vacuum heating
0.9996
Karl–Fischer
Own source
–
Distillation, filtration
≥0.999
Density
Table 3 Thermal properties of the investigated ionic liquids: glass transition temperature (Tg,1 ) and heat capacity change at glass transition temperature (Cp(g),1 ), melting temperature (Tm ) and enthalpy (Hm ), phase transition temperature (Ttr ) and enthalpy (Htr ) and decomposition temperature (Td ) at pressure p = 0.1 MPaa .
a
Ionic liquid
Tg,1 (K)
Cp(g),1 (J mol−1 K−1 )
Ttr (K)
tr H (kJ mol−1 )
Tm (K)
[EMIM][TCM] [EMIM][TCM] [50] [BMIM][TCM] [BMIM][DCA] [17] [BMPYR][TCM] [BMPYR][TCB] [BMPYR][CF3 SO3 ] [51] [BMMOR][TCM] [P6,6,6,14 ][TCM]
181.51 179.55 187.61 205.15 178.01
103.6 93.85 135.2
237.8 236.97
1.04 1.62
263.85 257.15
m H (kJ mol−1 ) 9.02 6.48
Td (K) 628 622 632
267.15 152.7
210.3
292.77 277.56
20.58
604 656
203.66
8.48
590 701
123.9
Standard uncertainties u are as follows: u(Tg,1 ) = 0.1 K; u(Cp(g),1 ) = 5 J mol−1 K−1 , u(Tfus ) = 0.1 K; u(fus H) = 0.5 kJ mol−1 , u(Td ) = 0.1 K.
the values of the parameters and standard deviations are collected in Table 6 and in Figs. 1–8. 4. Results and discussion The main goal of this work is to determine the basic thermal characterization of pure ionic liquids and to measure the phase equilibria for (IL + water) binary mixtures. The compounds are selected in such a way that it was possible to determine the impact of ionic liquids modifications, such as: the cation and anion family, or the alkyl chain length in the cation substituent on mutual
solubility with water. The knowledge of thermophysical properties and phase equilibria is necessary to determine the applicability of ionic liquids as an absorbent in absorption refrigeration. The thermal characterization of pure ionic liquids have been done using DSC and TG/DTA technique. The glass transition temperature, heat capacity at glass transition temperature, temperature and enthalpy of phase transition, temperature and enthalpy of melting and decomposition temperature of compounds have been determined and collected in Table 3. The phase transition was observed only for [EMIM][TCM]. Most of the ILs tested in this work were liquid at room temperature. In the case of [BMPYR][TCM] 340
280
330
270
320
T/ K
T/ K
260
250
310 300 290
240
230 0.0
280
0.1
0.2
x1
0.3
0.4
0.5
Fig. 1. Experimental and calculated (solid + liquid) phase equilibria of {[EMIM][TCM] (1) + water (2)} binary system: , experimental points; (-) Wilson equation; (-) NRTL equation; (-) UNIQUAC equation.
270 0.0
0.1
0.2
x1
0.3
0.4
0.5
Fig. 2. Experimental and calculated (liquid + liquid) phase equilibria of {[BMIM][TCM] (1) + water (2)} binary system: , experimental points. Solid line has been calculated using the NRTL equation.
M. Królikowska / Fluid Phase Equilibria 361 (2014) 273–281
277
Table 4 Experimental (solid + liquid) and (liquid + liquid) phase equilibria for {ionic liquid (1) + water (2)} binary system at pressure p = 0.1 MPaa . x1
TSLE (K)
[EMIM][TCM] (1) + water (2) 0.3338 241.48 0.3127 245.66 0.2919 249.22 252.17 0.2739 254.29 0.2556 256.72 0.2365 0.2143 259.29 0.1941 261.57 0.1707 264.09 [BMIM][TCM] (1) + water (2) 0.1541 0.1475 0.1371 0.1277 0.1191 0.1087 0.1010 0.0956 0.0890 0.0759 0.0649 0.0565 [BMIM][DCA] (1) + water (2) 0.2183 245.03 253.03 0.1912 0.1698 257.65 260.87 0.1499 0.1260 264.23 265.10 0.1169 266.98 0.1003 267.80 0.0894 268.24 0.0818 [BMPYR][TCM] (1) + water (2) 266.52 1.0000 0.8576 261.92 0.7552 259.48 256.67 0.6591 0.5934 254.87 251.93 0.5435 0.4813 246.64 0.1225 0.1212 0.1181 0.1142 0.1104 0.1063 0.1021 [BMPYR][TCB] (1) + water (2) 292.85 1.0000 288.22 0.8811 287.09 0.8521 285.48 0.8105 284.36 0.7757 283.21 0.7330 282.45 0.7020 281.49 0.6705 280.68 0.6477 280.09 0.6242 279.93 0.5807 0.5515 0.5248 0.4957 [BMPYR][CF3 SO3 ] (1) + water (2) 277.78 1.0000 0.9236 275.32 271.78 0.8447 269.09 0.7966 266.81 0.7573 264.35 0.7111 0.6591 261.24 257.97 0.6041 254.33 0.5411 249.78 0.4738 247.73 0.4397 245.42 0.4067
TLLE (K)
286.20 291.50 297.42 302.93 307.64 312.17 315.53 317.16 319.27 322.78 324.97 325.74
x1
TSLE (K)
0.1444 0.1194 0.0950 0.0615 0.0424 0.0212 0.0113 0.0068 0.0000
266.62 268.56 269.86 271.06 271.77 272.41 272.81 272.92 273.15
0.0462 0.0420 0.0328 0.0240 0.0181 0.0148 0.0119 0.0097 0.0080 0.0068 0.0061
0.0672 0.0655 0.0544 0.0433 0.0418 0.0291 0.0204 0.0148 0.0000
TLLE (K)
326.73 326.81 326.76 325.97 323.88 321.51 317.66 313.20 306.91 301.49 293.64
269.33 269.44 270.01 270.51 270.53 271.28 271.80 272.09 273.15
287.44 288.77 294.4 300.28 305.09 310.12 315.33
0.0967 0.0891 0.0825 0.0707 0.0385 0.0245 0.0162 0.0125 0.0108 0.0091 0.0079 0.0064 0.0055 0.0049
321.27 326.89 330.84 338.04 343.06 341.87 336.94 334.19 331.56 327.07 322.33 313.24 305.76 300.75
287.77 296.07 302.67 309.96
0.4807 0.4691 0.4527 0.4409 0.4189 0.3978 0.3703 0.3461 0.3252 0.2880 0.0017 0.0014 0.0010 0.0009
314.56 317.69 320.70 323.23 328.33 332.65 338.52 344.29 349.54 355.86 352.79 339.15 316.70 307.75
0.3151 0.2947 0.2750 0.2552 0.2377 0.2191 0.2027 0.1826 0.1627 0.1453 0.1257 0.1077
256.55 258.61 260.80 262.29 263.58 264.58 265.62 266.76 267.34 267.98 268.60 269.27
278
M. Królikowska / Fluid Phase Equilibria 361 (2014) 273–281
Table 4 (Continued)
a
x1
TSLE (K)
0.3732 0.3544 0.3345 [BMMOR][TCM] (1) + water (2) 0.4620 0.4007 0.3570 0.3021 0.3210 0.2864 0.2777 0.2548 0.2490 0.2264 0.2147 0.1867 0.1631 0.1564 0.1378 0.1275 [P6,6,6,14 ][TCM] (1) + water (2) 0.7601 0.7072 0.6514
TLLE (K)
x1
TSLE (K)
249.00 252.20 254.42
0.0766 0.0391 0.0000
270.26 271.57 273.15
237.72 248.81 255.76 262.29 260.14 264.02 264.69 266.55 266.85 268.48 269.15 270.13 271.26 271.80 272.16 272.29
0.1155 0.1147 0.0614 0.0836 0.0810 0.0712 0.0607 0.0520 0.0472 0.0411 0.0327 0.0263 0.0192 0.0139 0.0000
272.43 272.83 272.94 273.02 273.02 273.02 273.02 273.02 273.02 273.02 273.02 273.02 273.02 273.02 273.15
288.15 309.20 326.72
TLLE (K)
276.52 278.04 281.56 284.04 285.61 286.46 286.68 286.12 284.64 282.54 278.97
0.6367 0.5967 0.5597
332.32 343.80 354.49
Standard uncertainties u are as follows: u(x1 ) = 0.0001 and u(T) = 0.05 K.
Table 5 Correlation of the (liquid + liquid) phase equilibrium data by means of the NRTL equation: parameters (g12 − g22 = a12 + b12 T), (g21 − g11 = a21 + b21 T) and deviations x .
(g12 − g22 ) (J mol−1 )
[BMIM][TCM] [BMPYR][TCM] [BMPYR][TCB] [BMMOR][TCM] [P6,6,6,14 ][TCM]
(g21 − g11 ) (J mol−1 )
a12
b12
a21
b21
19333.9 2083.011 27016.11 −1835.34 18014.33
−125.016 −73.0128 −81.074 −15.1838 −81.47422
−8621.12 15568.73 1866.52 28610.34 −16885.5
154.0846 84.83004 44.56813 −45.9824 178.8447
we were unable to determine the temperature and enthalpy of melting from DSC technique due to lack of crystallization. The internal stirring elements in the dynamic method allowed for the sample crystallization and made possible to determine the temperature of melting. The highest decomposition temperature was determined for [P6,6,6,14 ][TCM] (701 K) and the lowest for [BMMOR][TCM] (590 K). For the family of TCM-based ionic liquids apart from [BMMOR][TCM] similar decomposition temperature were observed. The experimental (solid + liquid) and (liquid + liquid) phase equilibrium data (temperature vs. ionic liquid mole fraction) for the (IL + water) binary mixtures are presented in Table 4 and in Figs. 1–8. The parameters of the correlation equations are collected in Tables 5 and 6. The strong interactions between ionic liquid and water cause complete miscibility in the liquids phase for [EMIM][TCM], [BMIM][DCA] and [BMPYR][CF3 SO3 ]. For these systems it was possible to measure only the liquidus curves at low
˛
103 × x (K)
NRTL parameters
Ionic liquid
8.1 6.3 5.1 1.0 7.7
0.10 0.10 0.36 0.30 0.10
temperature. These mixtures reveal the simple eutectic systems. For the ([BMPYR][CF3 SO3 ] + water) it was possible to detect the characteristic eutectic point at T = 245.42 K for x1 = 0.4067. For the other mentioned systems it was impossible to measure this points due to the very low temperature and problem with crystallization. As was presented in our latest paper on phase equilibria, the complete miscibility in the liquid phase over whole range of ionic liquid mole fraction were observed for other dicyanamide-based ionic liquids with pyrrolidinium, pyridinium and piperidinium cations [36]. The comparison of phase equilibria for dicyanamidebased ionic liquids with water was presented in Fig. 9. Phase equilibria with upper critical solution temperature (UCST) were observed for five binary mixtures of water and [BMIM][TCM], [BMPYR][TCM], [BMPYR][TCB], [BMMOR][TCM] and [P6,6,6,14 ][TCM]. It was possible to detect the maximum of the curves for [BMIM][TCM], [BMPYR][TCM] and for [BMMOR][TCM]. The UCST increase in the following
Table 6 Correlation of the experimental (solid + liquid) phase equilibrium data of {IL (1) + water (2)} binary systems by means of UNIQUAC, Wilson and NRTL equation along with obtained values of parameters and root-mean-square deviations (RMSD) of temperature ( T ) as a measure of goodness of the correlation. Ionic liquid
RMSD
Parameters UNIQUAC u12 (J mol
[EMIM][TCM] [BMIM][DCA] [BMPYR][CF3 SO3 ]
2758.77 2773.25 2447.21
Wilson −1
)
u21 (J mol 1790.57 465.54 7382.03
−1
)
˛12 (J mol 2577.51 1853.30 4274.09
NRTL −1
)
˛21 (J mol 91614.38 13569.27 4952.85
−1
)
UNIQUAC −1
g12 (J mol 6615.47 12414.21 22654.76
)
−1
g21 (J mol −1184.80 −5814.38 −18421.90
)
Wilson
NRTL
1.31 1.58 0.85
0.36 0.19 1.09
T (K) 0.72 1.18 1.47
M. Królikowska / Fluid Phase Equilibria 361 (2014) 273–281
290
270
280
260
270
T/ K
T/ K
280
279
250
260
240
250
230 0.0
0.1
0.2
240 0.0
0.3
0.2
0.4
0.8
1.0
Fig. 6. Experimental and calculated (solid + liquid) phase equilibria of {[BMPYR] [CF3 SO3 ] (1) + water (2)} binary systems: , experimental points. Solid lines have been calculated using the NRTL equation.
360
300
340
290
320
280
T/ K
T/ K
Fig. 3. Experimental and calculated (solid + liquid) phase equilibria of {[BMIM][DCA] (1) + water (2)} binary systems: , experimental points. Solid lines have been calculated using the NRTL equation.
300
270 260
280
250
260 240 0.0
0.6
x1
x1
0.2
0.4
0.6
0.8
240 0.0
1.0
0.1
0.2
x1 Fig. 4. Experimental and calculated (solid + liquid) and (liquid + liquid) phase equilibria of {[BMPYR][TCM] (1) + water (2)} binary systems: , experimental points. Solid lines have been calculated using the NRTL equation.
order: [BMMOR][TCM] (T = 286.7 K, x1 = 0.0411) < [BMIM][TCM] (T = 326.81 K, x1 = 0.042) < [BMPYR][TCM] (T = 343.1 K, x1 = 0.0385). For the binary mixtures of water and [BMPYR][TCB] and [P6,6,6,14 ][TCM] the maximum of the immiscibility curves
0.3
0.4
0.5
x1 Fig. 7. Experimental and calculated (solid + liquid) and (liquid + liquid) phase equilibria of {[BMMOR][TCM] (1) + water (2)} binary systems: , experimental points. Solid lines have been calculated using the NRTL equation.
were not detected because the boiling point of water was lower. On the basis of the comparison of phase diagram for ([cation][TCM] + water) binary system presented in Fig. 10, the following trends can be noticed; the solubility of water in the ionic liquids increases in the following 400
370
350
360
T/ K
T/ K
330
320
310
280
290
270 0.0
0.2
0.4
x1
0.6
0.8
1.0
Fig. 5. Experimental and calculated (solid + liquid) and (liquid + liquid) phase equilibria of {[BMPYR][TCB] (1) + water (2)} binary systems: , experimental points. Solid lines have been calculated using the NRTL equation.
240 0.0
0.2
0.4
x1
0.6
0.8
1.0
Fig. 8. Experimental and calculated (liquid + liquid) phase equilibria of {[P6,6,6,14 ][TCM] (1) + water (2)} binary systems: , experimental points. Solid lines have been calculated using the NRTL equation.
280
M. Królikowska / Fluid Phase Equilibria 361 (2014) 273–281
310
410 380
290
270
T/ K
T/ K
350 320 290
250
260 230
0.0
0.2
0.4
0.6
0.8
1.0
x1 Fig. 9. Experimental and literature (solid + liquid) phase equilibria of {dicyanamidebased IL (1) + water (2)} binary systems: , [BMIM][DCA]; ♦, [BMPYR][DCA] [36]; 䊉, [BMPy] [DCA] [36]; , [BMPIP][DCA] [36].
order: [P6,6,6,14 ][TCM] < [BMIM][TCM] < [BMPYR][TCM] < [BMMOR] [TCM]. The best solubility was observed for [BMMOR][TCM] due to the more polar nature of ionic liquid resulting from the addition of oxygen atom into a cation structure. Due to the presentence of four aliphatic group and more hydrophobic nature of [P6,6,6,14 ][TCM] high immiscibility gap (from x1 = 0.76) was observed. Apart from that it was observed that the mutual solubility decreases with increase of the alkyl chain length in the imidazolium ring substituent. For [BMIM][TCM] the immiscibility gap started from x1 = 0.15 while for [EMIM][TCM] complete miscibility in the liquid phase was observed for whole range of IL mole fraction. Close agreement between experimental work and the data reported by Freire et al. was observed for [BMIM][TCM] [31]. In Fig. 11 the comparison of the experimental and literature (solid + liquid) and (liquid + liquid) for ([BMPYR][anion] + water) binary mixtures is presented. It is worth to point out that the deviation from ideal solution becomes positive in the following order: [DCA]− < [TCM]− < [TCB]− . In the case of [DCA] anion the negative deviations from ideal solution were observed. Addition of cyano-group in [TCM] anion results in stronger delocalization of negative charge from the core atom, and increases the steric hindrence. As a result H-bonds in the binary mixtures of ([BMPYR][TCM] + water) are less favorable in comparison to 360 340
T/ K
320 300 280
230 0.0
0.2
0.4
x1
0.6
0.8
1.0
Fig. 11. Experimental and literature (solid + liquid) and (liquid + liquid) phase equilibria of {1-butyl-1-methylpyrrolidinium-based IL (1) + water (2)} binary systems: , [BMPYR] [TCM]; , [BMPYR][TCB]; , [BMPYR][CF3 SO3 ]; , [BMPYR][NTf2 ] [30,31]; , [BMPYR] [FAP] [35]; ♦, [BMPYR][SCN] [33]; *, [BMPYR][DCA] [36].
([BMPYR][DCA] + water) systems. Similar behavior was observed in the case of ([BMPYR][TCM] + water) binary systems. 5. Conclusion In this work, new data of (solid + liquid) and (liquid + liquid) phase equilibria for eight (ionic liquid + water) binary mixtures were presented. Additionally, the basic thermal characterization of pure ionic liquids have been done using DSC and TG/DTA technique. Complete miscibility in the liquid phase was with water observed for [EMIM][TCM], [BMIM][DCA], or [BMPYR][CF3 SO3 ]. The experimental data of (liquid + liquid) phase equilibria with UCST were observed for water and [BMIM][TCM], or [BMPYR][TCM], or [BMPYR][TCB], or [BMMOR][TCM] and [P6,6,6,14 ][TCM]. The highest immiscibility gap in the tested binary systems was observed for [P6,6,6,14 ][TCM]. Apart from that, the immiscibility gap increases with the increase of the alkyl chain length in the imidazolium ring. As regards to the LLE in binary systems of tricyanomethanide-based ionic liquids and water the solubility increases in the following order: [P6,6,6,14 ][TCM] < [BMIM][TCM] < [BMPYR][TCM] < [BMMOR][TCM]. From the comparison of the mutual solubility of water and ionic liquids with [BMPYR] cation, it was observed that the solubility increases as follows: [DCA]− < [TCM]− < [TCB]− . The knowledge of effects of ionic liquid structure on the thermal characterization and phase equilibria with water is useful for developing IL as an absorbent in absorption refrigeration. The experimental data presented in this work, may give new informations about the possibility of using ionic liquids in this technology. Acknowledgment
260 240 220
0.0
0.2
0.4
x1
0.6
0.8
1.0
Fig. 10. Experimental and calculated (solid + liquid) and (liquid + liquid) phase equilibria of {tricyanomethanide-based IL (1) + water (2)} binary systems: , [EMIM][TCM]; 䊉, [BMIM][TCM]; , [BMIM][TCM] [31]; , [BMPYR][TCM]; , [BMMOR][TCM]; , [P6,6,6,14 ][TCM]. Solid lines have been calculated using the NRTL equation.
Funding for this research was provided by the National Science Centre in years 2011–2014 (Grant No. 2011/01/D/ST5/02760). This work has been supported by the European Union in the framework of European Social Fund through the Warsaw University of Technology Development Programme. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.fluid.2013.11.003.
M. Królikowska / Fluid Phase Equilibria 361 (2014) 273–281
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