Density, Viscosity, and Vapor–Liquid Equilibrium of the Ternary

Jan 11, 2017 - In this work, a ternary system consisting of water, 1-ethyl-3-methylimidazolium acetate ([EMIm][OAc], high reduction of vapor pressure)...
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Density, Viscosity, and Vapor−Liquid Equilibrium of the Ternary System Water−1-Ethyl-3-methylimidazolium Acetate−1-Ethyl-3Methylimidazolium Trifluoromethanesulfonate Markus Bücherl,*,+ Fabian Hardock,+ Elisa Kaiser,+ Andreas Netsch,+ and Karlheinz Schaber+ +

Institute of Technical Thermodynamics and Refrigeration, Karlsruhe Institute of Technology (KIT), Engler - Bunte - Ring 21, 76131 Karlsruhe, Germany ABSTRACT: Mixtures of ionic liquids and water offer an interesting opportunity to replace corrosive and/or partly miscible working pairs like lithium bromide−water in absorption chillers. Such an alternative must feature both a high depression of the water vapor pressure and a low viscosity to be feasible. However, hitherto published literature data of binary systems of ionic liquids and water fulfill only one of the two criteria. A mixture of two different ionic liquids may exhibit a solution for this dilemma. In this work, a ternary system consisting of water, 1-ethyl-3methylimidazolium acetate ([EMIm][OAc], high reduction of vapor pressure), and 1-ethyl-3-methylimidazolium trifluoromethanesulfonate ([EMIm][OTf], low viscosity) was investigated. Experimental data for density, dynamic viscosity, and vapor−liquid equilibrium in the temperature range from 293.15 to 353.15 K at water mass fractions between 5% and 20% and different mass fractions of [EMIm][OTf] are presented. The viscosity of the ternary mixture measured with a falling ball viscometer range between 90.9 and 1.9 mPa·s at a relative mean deviation below 3.6%. The density measured with a glass pycnometer range between 1087 and 1285 kg·m−3 at a relative mean deviation of 0.33%. The water vapor pressure measured by infrared spectroscopy range between 293 and 15164 Pa. The relative mean deviation is below 15.1%. The water vapor pressure data are correlated using a nonrandom two liquid model. The thermodynamic properties show that the considered ternary system can indeed be used as a suitable working mixture in absorption refrigerators.

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INTRODUCTION Ionic liquids (ILs) are molten salts with a melting temperature below 373.15 K at atmospheric pressure. They consist of an organic cation and an inorganic or organic anion. Because of the large number of possible cation/anion combinations, the thermophysical properties of the ILs cover a wide range which makes them adaptable for different applications.1,2 Today ionic liquids are in use as an alternative for classical organic solvents, as lubricants or as electrolytes.2 The extremely low volatility of many ILs also makes them ideal candidates for solvents in absorption cycles. This work aims at the application in absorption refrigerators. Water−lithium bromide often used in absorption refrigerators is highly corrosive and the limited solubility of the salt causes a risk of crystallization during operation. The substitution of lithium bromide by certain ionic liquids may help to overcome those disadvantages, because they are fully miscible with water and feature a negligible vapor pressure as well. Recent simulation results show that absorption refrigerators using ionic liquids are indeed feasible.3−5 Pilot plant realizations proved that the performance quantified by the coefficient of performance is in the same range like that in absorption chillers with the working pair water−lithium bromide. 3,5 However, the generated cooling power is considerably lower for the working pair water−ionic liquid, compared to operation with water−lithium bromide.5 The © 2017 American Chemical Society

main reason for this reduced performance can be explained by the low diffusion coefficients of water in the hitherto used ionic liquids, which limit the mass transfer in the absorber and desorber. The diffusion coefficient is inversely proportional to the dynamic viscosity of the ionic liquid.6 Hence, to overcome this problem an alternative ionic liquid with a low viscosity and a high reduction of water vapor pressure has to be found. But this proved as a very difficult task. As an alternative, a binary mixture of two ionic liquids, one of which with a low viscosity and the other with a strong water vapor depression, may be used. So the mixture may exhibit both advantages of a high reduction of the vapor pressure and a low viscosity. The ionic liquid 1-ethyl-3-methylimidazolium acetate ([EMIm][OAc]) shows a high depression of the water vapor pressure that leads to favorably low circulation rates of the solvent in the absorption refrigeration process.7 Literature data for the viscosity of [EMIm][OAc] at 293.15 K vary between 179.18 and 202.3 mPa·s.9 The reason for this discrepancy are the impurities of the ionic liquids. A potential low-viscosity alternative IL is 1-ethyl-3-methylimidazolium triflate ([EMIm][OTf]). Its viscosity at 293.15 K is around 50 mPa·s.9,10 Unfortunately, [EMIm][OTf] exhibits a lower reduction of the Received: September 13, 2016 Accepted: December 20, 2016 Published: January 11, 2017 720

DOI: 10.1021/acs.jced.6b00801 J. Chem. Eng. Data 2017, 62, 720−728

Journal of Chemical & Engineering Data

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Table 1. Specification of the Chemical Samples chemical name

source

mass fraction purity

CAS

analysis method

1-ethyl-3-methylimidazolium acetate 1-ethyl-3-methylimidazolium triflate water

IoLiTec IoLiTec deionization setup (Pulse Purge 2 from ELGA)

99.2 99.1 1

143314-17-4 145022-44-2 7732-18-5

electrical conductivity

Figure 1. Dynamic VLE measurement cell: TH, water thermostat; EC, equilibrium cell; D, demister; TC, thermocouple; TIC, temperature control of the six different sections; PT, platinum resistance thermometer; CP, circulation pump; FTIR, Fourier transform infrared spectrometer.

water vapor pressure,14 which would cause an inefficiently high solvent circulation rate. Our aim is to combine the advantages of the two ionic liquids by using appropriate mixtures. In this paper, the results of the measurements of the vapor−liquid equilibria (VLE), densities, and viscosities of the ternary systems water (component 1), [EMIm][OAc] (component 2), and [EMIm][OTf] (component 3) are presented.

Line KF). The uncertainty of this measurement is 0.01 of the water mass fraction. The density was measured using a calibrated glass pycnometer with a volume of 9.994 cm3 ± 0.004 cm3. The weight of the glass pycnometer was determined using the aforementioned scale. The density was measured at 293.15, 313.15, 333.15, and 353.15 K under atmospheric pressure. The temperatures of the samples were kept constant using a temperature control unit from (Julabo, F 32) with an accuracy of 0.01 K. In addition to that, the temperature was controlled with a mercury thermometer with an accuracy of 0.5 K. The water mass fraction of the ternary mixture was varied among 0.05, 0.10, 0.15, and 0.20. Each measurement was repeated six times. The viscosity was measured with a falling ball viscometer (Thermo Electron, model Haake Type C). For these measurements, the samples from the density measurements were reused. The viscosity was measured at 293.15, 313.15, 333.15, and 353.15 K. The temperature was controlled using a platinum resistance temperature sensor (Pt 100). The uncertainty was estimated to be 0.5 K due to temperature fluctuations. In order to obtain thermal equilibrium for the measurement, each sample was kept for 30 min under isothermal conditions at each temperature. For the calculation of the viscosities, the density values obtained from the pycnometer measurements were used. Each measurement was repeated six times, also. The vapor−liquid equilibrium measurements were focused on ternary samples with a low mass fraction of [EMIm][OTf] because these mixtures show a strong depression of the water

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EXPERIMENTAL SECTION Materials and Purity. The ionic liquids were produced by Ionic Liquids Technologies GmbH, Heilbronn, Germany (IoLiTec). Both ionic liquids contain small amounts of water. The [EMIm][OAc] used had a minimal purity of 99.2%. The fraction of halides is below 1%. The [EMIm][OTf] had a minimal purity of 99.1%. The fraction of halides is below 2500 ppm. The quality of water, which was used for the mixture, had an electrical conductivity of 1.6 μS·cm−1 which was measured with the hand-held conductivity meter LF 340-A by WTW (Wissenschaftlich Technische Werkstatt). The chemical samples are specified in Table 1. Apparatus and Procedure. The samples of the mixtures were prepared gravimetrically using an electronic scale by Mettler Toledo model PR 2003 Delta Range. The stated uncertainty is 0.001 g, which results in an uncertainty of the mass fraction of the components in the ternary samples of lower than 0.0001. The purity and water content of the ionic liquids as stated in the certificate of analysis provided by the manufacturer were taken into account for the calculation of the mass fractions. The water content of the final ternary samples was verified by volumetric Karl Fischer titration (Schott, Titro 721

DOI: 10.1021/acs.jced.6b00801 J. Chem. Eng. Data 2017, 62, 720−728

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Table 2. Experimental Values of the Density ρ of the Ternary Mixture Water (1)−[EMIm][OAc] (2)−[EMIm][OTf] (3) at Different Mole Fractions xi (Mass Fraction ξi), Temperatures T and Pressure 0.1 MPaa T/K

x1

x2

ξ1

ξ2

ρ/kg·m−3

T/K

x1

x2

ξ1

ξ2

ρ/kg·m−3

293.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15

0.337 0.341 0.348 0.359 0.371 0.373 0.385 0.395 0.406 0.419 0.426 0.518 0.522 0.533 0.543 0.554 0.564 0.585 0.590 0.602 0.609 0.630 0.635 0.645 0.655 0.666 0.671 0.683 0.694 0.706 0.712 0.707 0.711 0.721 0.731 0.741 0.751 0.762 0.773 0.778 0.337 0.341 0.348 0.359 0.371 0.373 0.385 0.395 0.406 0.419 0.426 0.518 0.522 0.533 0.543 0.554 0.564 0.585 0.590 0.602

0.640 0.612 0.555 0.492 0.426 0.395 0.324 0.250 0.172 0.089 0.045 0.465 0.442 0.393 0.344 0.292 0.240 0.180 0.125 0.064 0.032 0.355 0.335 0.295 0.255 0.211 0.189 0.144 0.098 0.050 0.025 0.281 0.264 0.229 0.193 0.157 0.119 0.080 0.041 0.021 0.640 0.612 0.555 0.492 0.426 0.395 0.324 0.250 0.172 0.089 0.045 0.465 0.442 0.393 0.344 0.292 0.240 0.180 0.125 0.064

0.050 0.050 0.050 0.050 0.051 0.050 0.050 0.050 0.050 0.050 0.050 0.100 0.100 0.100 0.100 0.100 0.099 0.103 0.100 0.100 0.100 0.150 0.150 0.150 0.150 0.150 0.150 0.150 0.150 0.150 0.150 0.200 0.200 0.200 0.200 0.200 0.200 0.200 0.201 0.200 0.050 0.050 0.050 0.050 0.051 0.050 0.050 0.050 0.050 0.050 0.050 0.100 0.100 0.100 0.100 0.100 0.099 0.103 0.100 0.100

0.900 0.850 0.750 0.650 0.549 0.500 0.400 0.300 0.200 0.100 0.050 0.850 0.800 0.700 0.600 0.499 0.400 0.300 0.200 0.100 0.050 0.800 0.750 0.650 0.551 0.450 0.400 0.300 0.200 0.100 0.050 0.750 0.700 0.600 0.500 0.401 0.300 0.200 0.100 0.050 0.900 0.850 0.750 0.650 0.549 0.500 0.400 0.300 0.200 0.100 0.050 0.850 0.800 0.700 0.600 0.499 0.400 0.300 0.200 0.100

1116.69 1127.62 1153.34 1174 1183.79 1215.88 1244.8 1273.45 1301.77 1327.17 1343.71 1120.74 1131.07 1154.78 1177.54 1201.4 1226.03 1251.31 1281.78 1303.65 1321.2 1120.74 1131.57 1153.42 1177.22 1198.35 1208.62 1233.85 1259.41 1282.51 1296.01 1119.62 1129.71 1151.08 1172.87 1192.86 1217.09 1240.27 1261.77 1275.39 1106.91 1117.07 1140.91 1165.49 1173.24 1202.99 1232.1 1259.84 1288.27 1316.08 1329.24 1110.29 1119.66 1143.03 1166.61 1189.25 1214.8 1239.06 1266.56 1292.56

333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 353.15 353.15 353.15 353.15 353.15 353.15 353.15 353.15 353.15 353.15 353.15 353.15 353.15 353.15 353.15 353.15 353.15 353.15 353.15 353.15

0.337 0.341 0.348 0.359 0.371 0.373 0.385 0.395 0.406 0.419 0.426 0.518 0.522 0.533 0.543 0.554 0.564 0.585 0.590 0.602 0.609 0.630 0.635 0.645 0.655 0.671 0.671 0.683 0.694 0.706 0.712 0.707 0.711 0.721 0.731 0.741 0.751 0.762 0.773 0.778 0.337 0.341 0.348 0.359 0.371 0.373 0.385 0.395 0.406 0.419 0.426 0.518 0.522 0.533 0.543 0.554 0.564 0.585 0.590 0.602

0.640 0.612 0.555 0.492 0.426 0.395 0.324 0.250 0.172 0.089 0.045 0.465 0.442 0.393 0.344 0.292 0.240 0.180 0.125 0.064 0.032 0.355 0.335 0.295 0.255 0.189 0.189 0.144 0.098 0.050 0.025 0.281 0.264 0.229 0.193 0.157 0.119 0.080 0.041 0.021 0.640 0.612 0.555 0.492 0.426 0.395 0.324 0.250 0.172 0.089 0.045 0.465 0.442 0.393 0.344 0.292 0.240 0.180 0.125 0.064

0.050 0.050 0.050 0.050 0.051 0.050 0.050 0.050 0.050 0.050 0.050 0.100 0.100 0.100 0.100 0.100 0.099 0.103 0.100 0.100 0.100 0.150 0.150 0.150 0.150 0.150 0.150 0.150 0.150 0.150 0.150 0.200 0.200 0.200 0.200 0.200 0.200 0.200 0.201 0.200 0.050 0.050 0.050 0.050 0.051 0.050 0.050 0.050 0.050 0.050 0.050 0.100 0.100 0.100 0.100 0.100 0.099 0.103 0.100 0.100

0.900 0.850 0.750 0.650 0.549 0.500 0.400 0.300 0.200 0.100 0.050 0.850 0.800 0.700 0.600 0.499 0.400 0.300 0.200 0.100 0.050 0.800 0.750 0.650 0.551 0.400 0.400 0.300 0.200 0.100 0.050 0.750 0.700 0.600 0.500 0.401 0.300 0.200 0.100 0.050 0.900 0.850 0.750 0.650 0.549 0.500 0.400 0.300 0.200 0.100 0.050 0.850 0.800 0.700 0.600 0.499 0.400 0.300 0.200 0.100

1097.24 1107.03 1128.84 1154.09 1163.82 1190.71 1217.27 1246.08 1273.27 1300.76 1315.9 1098.64 1108.23 1130.58 1153 1176.75 1202.2 1224.86 1252.26 1277.72 1290.31 1099.38 1108.59 1128.72 1150.33 1184.48 1172.98 1207.74 1232.16 1254.78 1267.19 1097.68 1105.48 1125.06 1145.86 1166.93 1188.46 1210.19 1233.52 1244.4 1087.35 1096.92 1117.86 1142.73 1149.42 1180.63 1204.49 1233.52 1261.14 1280.49 1293.72 1090.7 1098.62 1118.65 1141.92 1165.36 1182.28 1205.11 1239.32 1258.54

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DOI: 10.1021/acs.jced.6b00801 J. Chem. Eng. Data 2017, 62, 720−728

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Table 2. continued T/K

x1

x2

ξ1

ξ2

ρ/kg·m−3

T/K

x1

x2

ξ1

ξ2

ρ/kg·m−3

313.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15

0.609 0.630 0.635 0.645 0.655 0.671 0.671 0.683 0.694 0.706 0.712 0.707 0.711 0.721 0.731 0.741 0.751 0.762 0.773 0.778

0.032 0.355 0.335 0.295 0.255 0.189 0.189 0.144 0.098 0.050 0.025 0.281 0.264 0.229 0.193 0.157 0.119 0.080 0.041 0.021

0.100 0.150 0.150 0.150 0.150 0.150 0.150 0.150 0.150 0.150 0.150 0.200 0.200 0.200 0.200 0.200 0.200 0.200 0.201 0.200

0.050 0.800 0.750 0.650 0.551 0.400 0.400 0.300 0.200 0.100 0.050 0.750 0.700 0.600 0.500 0.401 0.300 0.200 0.100 0.050

1305.6 1109.97 1120.3 1141.13 1163.57 1197.95 1186.02 1221.69 1246.68 1270.9 1282.89 1108.55 1117.39 1141.21 1159.77 1180.36 1204.31 1225.94 1248.16 1260.37

353.15 353.15 353.15 353.15 353.15 353.15 353.15 353.15 353.15 353.15 353.15 353.15 353.15 353.15 353.15 353.15 353.15 353.15 353.15 353.15

0.609 0.630 0.635 0.645 0.655 0.671 0.671 0.683 0.694 0.706 0.712 0.707 0.711 0.721 0.731 0.741 0.751 0.762 0.773 0.778

0.032 0.355 0.335 0.295 0.255 0.189 0.189 0.144 0.098 0.050 0.025 0.281 0.264 0.229 0.193 0.157 0.119 0.080 0.041 0.021

0.100 0.150 0.150 0.150 0.150 0.150 0.150 0.150 0.150 0.150 0.150 0.200 0.200 0.200 0.200 0.200 0.200 0.200 0.201 0.200

0.050 0.800 0.750 0.650 0.551 0.400 0.400 0.300 0.200 0.100 0.050 0.750 0.700 0.600 0.500 0.401 0.300 0.200 0.100 0.050

1275.59 1088.07 1097.06 1116.06 1138.87 1166.4 1160.71 1188.53 1210.65 1235.36 1247.07 1087.09 1094.33 1112.55 1132.3 1153.97 1173.18 1195.6 1218.62 1230.23

a

The expanded uncertainty U is U(T) = 0.5 K with a 0.95 level of confidence. The relative standard uncertainty ur is ur(ρ) = 0.002314 and the standard uncertainties (u) are u(x1) = 0.002, u(x2) = 0.002, u(ξ1) = 0.001, u(ξ2) = 0.001, u(p) = 5 kPa.

vapor pressure. For the VLE measurements, a dynamic setup was used in which the composition of the gas phase is analyzed by Fourier-Transform Infrared Spectroscopy (FTIR) at ambient pressure. This setup has already been described in previous publications.3,6,7 A schematic flowsheet of the VLE measurement setup is shown in Figure 1. A liquid sample of 210 g is contained in a glass flask (500 mL) and immersed in a water thermostat (Lauda, E200). The vapor phase is continuously drawn from the head via a demister, passing a heated hose, and is fed into the FTIR spectrometer (BOMEM, Hartmann & Braun, model 9100) for the measurement of the water vapor concentration. After analysis, the vapor passes a circulation pump and is returned to the flask through a glass frit. The total pressure inside the flask matches the ambient pressure due to a connection with a long hose. The temperature in the liquid phase is measured with a Pt 100 sensor. Prior to measurement, the temperature remains constant for 30 min with a maximal variation of 0.1 K. For the evaluation of the vapor phase water concentration from the FTIR spectrum, the spectrum was integrated between 2112.863 and 2117.200 cm−1. This peak area was correlated to the water vapor partial pressure using a calibration procedure described by Merkel.11 The mean deviation of the calibration is 6.1%.

Figure 2. Density of the ternary mixture versus mole fraction x3 of [EMIm][OTf] with a constant water mass fraction of 0.1 at different temperatures (■, 293,15 K; ●, 313.15 K; ▲, 333.15 K; ▼, 353.15 K) at atmospheric pressure.

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[EMIm][OAc] (around 1100 kg/m3 at 293.15 K [2, 1]) and water (998.25 kg·m−3 at 293.15 K [3]). The experimental values of the density show a small positive deviation from the ideal mixture. The influence of temperature is very low. With an increase of the temperature from 293.15 to 353.15 K, the density reduces by about 30 kg·m−3. The results of dynamic viscosity measurements are shown in Table 3. The standard deviation for each sample at each temperature is lower than 0.9 mPa·s or 3.6% of the measured value in all cases. The average of the standard deviation is 0.06 mPa·s. It can be seen that the viscosity of the mixture decreases with increasing water content. This behavior is consistent to previous investigations of water−ionic liquid mixtures.6,7 At constant water mass fraction, the viscosity of the samples decreases also with higher mass fraction of [EMIm][OTf] as is

RESULTS AND DISCUSSION The results of the density measurements are listed in Table 2. The standard deviation of the five measuring repetitions is smaller than 3.8 kg·m−3, or 0.33% of the measured value. The mean deviation of the density over all is 0.32 kg·m−3. The strongest influence is the mass fraction of [EMIm][OTf] in the mixture. As is shown in Figure 2, the densities of the mixture increase with a higher mass fraction of [EMIm][OTf] at constant water mass fraction. The influence of the ratio of the mass fractions of water and [EMIm][OAc] at a constant mass fraction of [EMIm][OTf] is low. This behavior was to be expected considering the higher density of pure [EMIm][OTf] (1390.2 kg·m−3 at 293.15 K [1]) as compared to pure 723

DOI: 10.1021/acs.jced.6b00801 J. Chem. Eng. Data 2017, 62, 720−728

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Table 3. Experimental Values of the Dynamic Viscosity η of the Ternary Mixture Water (1)−[EMIm][OAc] (2)−[EMIm][OTf] (3) at Different Mole Fractions xi (Mass Fraction ξi), Temperatures T, and Pressure 0.1 MPaa T/K

x1

x2

ξ1

ξ2

η/mPa·s

T/K

x1

x2

ξ1

ξ2

η/mPa·s

293.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15

0.337 0.341 0.348 0.359 0.371 0.373 0.385 0.395 0.406 0.419 0.426 0.518 0.522 0.533 0.543 0.554 0.564 0.585 0.590 0.602 0.609 0.630 0.635 0.645 0.655 0.666 0.671 0.683 0.694 0.706 0.712 0.707 0.711 0.721 0.731 0.741 0.751 0.762 0.773 0.778 0.337 0.341 0.348 0.359 0.371 0.373 0.385 0.395 0.406 0.419 0.426 0.518 0.522 0.533 0.543 0.554 0.564 0.585 0.590 0.602

0.640 0.612 0.555 0.492 0.426 0.395 0.324 0.250 0.172 0.089 0.045 0.465 0.442 0.393 0.344 0.292 0.240 0.180 0.125 0.064 0.032 0.355 0.335 0.295 0.255 0.211 0.189 0.144 0.098 0.050 0.025 0.281 0.264 0.229 0.193 0.157 0.119 0.080 0.041 0.021 0.640 0.612 0.555 0.492 0.426 0.395 0.324 0.250 0.172 0.089 0.045 0.465 0.442 0.393 0.344 0.292 0.240 0.180 0.125 0.064

0.050 0.050 0.050 0.050 0.051 0.050 0.050 0.050 0.050 0.050 0.050 0.100 0.100 0.100 0.100 0.100 0.099 0.103 0.100 0.100 0.100 0.150 0.150 0.150 0.150 0.150 0.150 0.150 0.150 0.150 0.150 0.200 0.200 0.200 0.200 0.200 0.200 0.200 0.201 0.200 0.050 0.050 0.050 0.050 0.051 0.050 0.050 0.050 0.050 0.050 0.050 0.100 0.100 0.100 0.100 0.100 0.099 0.103 0.100 0.100

0.900 0.850 0.750 0.650 0.549 0.500 0.400 0.300 0.200 0.100 0.050 0.850 0.800 0.700 0.600 0.499 0.400 0.300 0.200 0.100 0.050 0.800 0.750 0.650 0.551 0.450 0.400 0.300 0.200 0.100 0.050 0.750 0.700 0.600 0.500 0.401 0.300 0.200 0.100 0.050 0.900 0.850 0.750 0.650 0.549 0.500 0.400 0.300 0.200 0.100 0.050 0.850 0.800 0.700 0.600 0.499 0.400 0.300 0.200 0.100

90.9 87.1 79.7 72.1 62.3 58.5 50.0 41.9 34.1 28.2 23.9 57.5 54.9 48.7 42.6 36.6 32.4 26.1 20.4 16.6 13.3 40.6 38.4 33.3 28.9 24.5 23.0 18.9 15.0 11.4 9.7 30.7 28.4 24.9 24.0 17.2 13.9 11.0 8.4 7.2 34.3 33.3 31.4 28.9 25.8 25.0 22.1 19.5 16.7 14.4 12.9 23.9 22.8 20.7 18.7 16.7 15.1 12.6 10.5 8.8

333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 353.15 353.15 353.15 353.15 353.15 353.15 353.15 353.15 353.15 353.15 353.15 353.15 353.15 353.15 353.15 353.15 353.15 353.15 353.15 353.15

0.337 0.341 0.348 0.359 0.371 0.373 0.385 0.395 0.406 0.419 0.426 0.518 0.522 0.533 0.543 0.554 0.564 0.585 0.590 0.602 0.609 0.630 0.635 0.645 0.655 0.671 0.671 0.683 0.694 0.706 0.712 0.707 0.711 0.721 0.731 0.741 0.751 0.762 0.773 0.778 0.337 0.341 0.348 0.359 0.371 0.373 0.385 0.395 0.406 0.419 0.426 0.518 0.522 0.533 0.543 0.554 0.564 0.585 0.590 0.602

0.640 0.612 0.555 0.492 0.426 0.395 0.324 0.250 0.172 0.089 0.045 0.465 0.442 0.393 0.344 0.292 0.240 0.180 0.125 0.064 0.032 0.355 0.335 0.295 0.255 0.189 0.189 0.144 0.098 0.050 0.025 0.281 0.264 0.229 0.193 0.157 0.119 0.080 0.041 0.021 0.640 0.612 0.555 0.492 0.426 0.395 0.324 0.250 0.172 0.089 0.045 0.465 0.442 0.393 0.344 0.292 0.240 0.180 0.125 0.064

0.050 0.050 0.050 0.050 0.051 0.050 0.050 0.050 0.050 0.050 0.050 0.100 0.100 0.100 0.100 0.100 0.099 0.103 0.100 0.100 0.100 0.150 0.150 0.150 0.150 0.150 0.150 0.150 0.150 0.150 0.150 0.200 0.200 0.200 0.200 0.200 0.200 0.200 0.201 0.200 0.050 0.050 0.050 0.050 0.051 0.050 0.050 0.050 0.050 0.050 0.050 0.100 0.100 0.100 0.100 0.100 0.099 0.103 0.100 0.100

0.900 0.850 0.750 0.650 0.549 0.500 0.400 0.300 0.200 0.100 0.050 0.850 0.800 0.700 0.600 0.499 0.400 0.300 0.200 0.100 0.050 0.800 0.750 0.650 0.551 0.400 0.400 0.300 0.200 0.100 0.050 0.750 0.700 0.600 0.500 0.401 0.300 0.200 0.100 0.050 0.900 0.850 0.750 0.650 0.549 0.500 0.400 0.300 0.200 0.100 0.050 0.850 0.800 0.700 0.600 0.499 0.400 0.300 0.200 0.100

16.6 16.3 15.7 14.6 13.2 13.2 12.0 10.8 9.6 8.6 7.8 12.1 11.7 10.9 10.0 9.1 8.4 7.2 6.2 5.4 4.7 9.0 8.7 8.0 7.2 6.2 6.5 5.4 4.7 3.9 3.5 7.1 6.7 6.0 5.4 4.8 4.1 3.5 4.6 4.1 9.5 9.4 9.0 8.6 7.9 7.9 7.3 6.8 6.1 5.7 5.3 7.1 6.9 6.5 6.0 5.6 5.3 4.7 4.1 3.7

724

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Table 3. continued T/K

x1

x2

ξ1

ξ2

η/mPa·s

T/K

x1

x2

ξ1

ξ2

η/mPa·s

313.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15

0.609 0.630 0.635 0.645 0.655 0.671 0.671 0.683 0.694 0.706 0.712 0.707 0.711 0.721 0.731 0.741 0.751 0.762 0.773 0.778

0.032 0.355 0.335 0.295 0.255 0.189 0.189 0.144 0.098 0.050 0.025 0.281 0.264 0.229 0.193 0.157 0.119 0.080 0.041 0.021

0.100 0.150 0.150 0.150 0.150 0.150 0.150 0.150 0.150 0.150 0.150 0.200 0.200 0.200 0.200 0.200 0.200 0.200 0.201 0.200

0.050 0.800 0.750 0.650 0.551 0.400 0.400 0.300 0.200 0.100 0.050 0.750 0.700 0.600 0.500 0.401 0.300 0.200 0.100 0.050

7.4 17.3 16.6 14.8 13.2 11.0 11.5 9.3 7.8 6.2 5.5 13.5 12.5 11.0 9.6 8.3 6.9 5.8 4.6 4.1

353.15 353.15 353.15 353.15 353.15 353.15 353.15 353.15 353.15 353.15 353.15 353.15 353.15 353.15 353.15 353.15 353.15 353.15 353.15 353.15

0.609 0.630 0.635 0.645 0.655 0.671 0.671 0.683 0.694 0.706 0.712 0.707 0.711 0.721 0.731 0.741 0.751 0.762 0.773 0.778

0.032 0.355 0.335 0.295 0.255 0.189 0.189 0.144 0.098 0.050 0.025 0.281 0.264 0.229 0.193 0.157 0.119 0.080 0.041 0.021

0.100 0.150 0.150 0.150 0.150 0.150 0.150 0.150 0.150 0.150 0.150 0.200 0.200 0.200 0.200 0.200 0.200 0.200 0.201 0.200

0.050 0.800 0.750 0.650 0.551 0.400 0.400 0.300 0.200 0.100 0.050 0.750 0.700 0.600 0.500 0.401 0.300 0.200 0.100 0.050

3.3 5.4 5.3 4.9 4.5 4.0 4.1 3.6 3.1 2.7 2.5 4.3 4.1 3.8 3.4 3.1 2.7 2.4 2.1 1.9

a

Expanded uncertainty U is U(T) = 0.08 K with 0.95 level of confidence. The relative standard uncertainty ur is ur(η) = 0.06916 and the standard uncertainties u are u(x1) = 0.002, u(x2) = 0.002, u(ξ1) = 0.001, u(ξ2) = 0.001, u(p) = 5 kPa.

[OTf] shows a viscosity reduction from 90.9 mPa·s at 293.15 K to 9.5 mPa·s at 353.15 K. Compared to data for the binary mixture water−[EMIm][OAc] with a water mass fraction of 5%, the viscosity drops from 93.7 to 9.51 mPa·s.7 Thus, our values seem plausible and the effect of the [EMIm][OTf] addition is small but significantly larger than the measuring uncertainty. Difficulties arose during the viscosity measurements of the mixtures with a water mass fraction of 5%, high mass fractions of [EMIm][OAc] and at temperatures higher than 333.15 K. In these cases, dark cords were observed in the liquid, likely created by incompatibility of the gaskets of the falling ball viscometer with the sample liquid. Because the effect of this contamination on the viscosity is not known, an increased uncertainty of the given values should be assumed. These cords were already detected by Römich et al.7 during experiments with the binary system water−[EMIm][OAc] for mass fractions of water lower than 0.15. The isothermal vapor−liquid equilibrium measurements were conducted at temperatures ranging from 293.15 to 353.15 K in steps of 10 K with different compositions. The volatile impurities of the [EMIm][OAc], which were detected in the vapor phase, did not influence the spectrum inside our integration boundaries. The results of the vapor−liquid equilibrium measurements are shown in Table 4. The mean relative deviation is 15.1% of the water partial pressure. The aim is to find a ternary mixture with a similar water vapor pressure like the binary system water−[EMIm][OAc] but with a considerably lower viscosity than the binary mixture. This is especially important for mixtures with a low water mass fraction. The water partial pressure of a binary mixture water− [EMIm][OAc] with a water mass fraction of 5% at 293.15 K is lower as 140 Pa.7 For a ternary mixture with a water mass fraction of 5% the water vapor pressure increases only slightly from 293 to 321 Pa by increasing the [EMIm][OTf] mass fraction from 5% to 45%, which is within the uncertainty range of about 60 Pa in this case. In contrast, the viscosity of the

shown in Figure 3. However, the influence of the water fraction is much stronger than the variation of the mass ratio of the

Figure 3. Viscosity of the ternary mixture versus mole fraction of [EMIm][OTf] with constant water mass fraction of 0.05 at different temperatures (■, 293.15 K; ●, 313.15 K; ▲, 333.15 K; ▼, 353.15 K) under atmospheric pressure.

ionic liquids. Analogous to the density, the effect can be explained by the viscosity of the pure components: 51.9 mPa·s for [EMIm][OTf],9 179.1 mPa·s for [EMIMm][OAc],8 and 1.0016 mPa·s for water,12 all values at 293.15 K. The values of the viscosity also show a strong influence of the temperature. With an increase of the temperature, the viscosity decreases. The strongest temperature dependency is evident in the samples with a high [EMIm][OAc] mass fraction. The following consideration demonstrates the compatibility of ternary and binary viscosity data. A ternary mixture consisting of a water mass fraction of 5%, an [EMIm][OAc] mass fraction of 90%, and an [EMIm][OTf] mass fraction of 5% [EMIm]725

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Table 4. Experimental Values of the Vapor−Liquid Equilibrium (p1 is the partial pressure of water) of the ternary mixture water (1)−[EMIm][OAc] (2)−[EMIm][OTf] (3) at Different Temperatures T, and Mole Fractions xia T/K

x1

x2

p1/Pa

T/K

x1

x2

p1/Pa

293.13 303.12 313.13 323.19 333.17 343.18 353.15 293.17 303.12 313.13 323.15 333.12 343.15 353.15 293.12 303.11 313.16 323.14 333.16 343.17 353.16 293.11 303.12 313.19 323.18 333.20 343.11 353.14 293.13 303.18 313.14 323.16 333.16 343.17 353.17

0.337 0.337 0.337 0.337 0.337 0.336 0.336 0.630 0.630 0.630 0.629 0.628 0.627 0.626 0.517 0.517 0.517 0.517 0.516 0.515 0.514 0.707 0.707 0.706 0.706 0.705 0.704 0.702 0.399 0.399 0.399 0.399 0.398 0.397 0.396

0.640 0.640 0.640 0.640 0.640 0.640 0.641 0.355 0.355 0.356 0.356 0.356 0.357 0.357 0.465 0.465 0.465 0.465 0.466 0.466 0.467 0.281 0.281 0.281 0.281 0.282 0.282 0.283 0.512 0.512 0.512 0.512 0.512 0.513 0.513

293 349 395 449 534 1238 2025 359 464 826 1934 3278 5038 7479 319 414 516 1165 2024 3228 4728 371 578 1613 3266 5171 7862 11619 313 377 506 627 1045 2191 3094

293.13 303.16 313.19 323.16 333.17 343.12 353.15 293.15 303.15 313.17 323.18 333.11 343.11 353.15 293.12 303.13 313.13 323.12 333.19 343.15 353.15 293.12 303.19 313.18 323.11 333.11 343.11 353.13 293.13 303.11 313.15 323.15 333.16 343.15 353.19

0.532 0.532 0.532 0.531 0.531 0.530 0.529 0.645 0.645 0.644 0.644 0.643 0.641 0.640 0.720 0.720 0.719 0.719 0.718 0.716 0.713 0.399 0.399 0.399 0.398 0.398 0.396 0.395 0.552 0.552 0.551 0.550 0.549 0.548 0.546

0.394 0.394 0.394 0.395 0.395 0.395 0.396 0.296 0.296 0.296 0.296 0.297 0.297 0.298 0.230 0.230 0.230 0.230 0.230 0.231 0.232 0.379 0.379 0.379 0.380 0.380 0.381 0.381 0.292 0.292 0.292 0.292 0.293 0.293 0.294

374 504 723 1369 2120 3569 5064 368 523 1257 2453 4190 6245 9500 438 834 1942 3321 5139 8979 15164 321 418 508 1290 2136 3421 4845 411 534 1614 2609 4050 6117 9051

a Expanded uncertainty U is U(T) = 0.15 K with a 0.95 level of confidence. The relative standard uncertainty ur is ur(p1 > 1800 Pa) = 0.1 and ur(p1 ≤ 1800 Pa) = 0.2 and the standard uncertainties are u(x1) = 0.002, u(x2) = 0.002.

ternary mixture decreases by about 40 mPa·s from 93.7 mPa·s to 58.5 mPa·s by increasing the [EMIm][OTf] mass fraction up to 45%. This example demonstrates the possibility to reduce the viscosity of working fluids of absorption refrigeration cycles without notably affecting the water vapor pressure by applying ternary mixtures. The experimental data were correlated using the non-random two liquid (NRTL) model. The logarithmic activity coefficients were calculated with eq 113 ln γi =

3 ∑ j = 1 xjGjiτji 3 ∑k = 1 xkGki

3

+

∑ j=1

Gji = exp( −αijτij)

R is the universal gas constant. For the fitting of the NRTL parameters, we used a least-squares method minimizing the following objective function (eq 4) n

F=

RT

=

aij + bij(T − 273.15 K) RT

(4)

Binary vapor−liquid equilibrium data of the systems water− [EMIm][OAc]7 and water−[EMIm][OTf]14 were included during the regression. The resulting values of the parameters are listed in Table 5. The fit has a root-mean-square deviation of 2.68. As it can be seen in Figures 4 and 5, the NRTL model is able to represent the experimental values. The ternary model also matches the data of the binary systems water−[EMIm][OAc],7 which is shown in Figure 6, and water−[EMIm][OTf],14 which can be seen in Figure 7. However, the deviation is slightly larger compared with the binary models.7,14

For a better representation of the temperature dependency, a linear approach was used inside the expression for τij (eq 2). The equations for the NRTL (see eq 2 and eq 3) parameters are gji − gii

∑ (γ1,exp − γ1,cal)2 i=1

3 xjGji ⎛ ∑ xG τ ⎞ ⎜τij − l = 1 l li li ⎟ 3 3 ∑k = 1 xkGkj ⎜⎝ ∑k = 1 xkGkj ⎟⎠

(1)

τji =

(3)

(2) 726

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Table 5. NRTL Parameters for the Ternary System Water (1)−[EMIm][OAc] (2)−[EMIm][OTf] (3) α12 α23 a12/J·mol−1 a13/J·mol−1 a21/J·mol−1 a23/J·mol−1 a31/J·mol−1 a32/J·mol−1

0.11030659 0.4 10771.16 5723.880 −28285.55 −17174.58 −7397.447 −3692.591

α13 b12/J·mol−1·K−1 b13/J·mol−1·K−1 b21/J·mol−1·K−1 b23/J·mol−1·K−1 b31/J·mol−1·K−1 b32/J·mol−1·K−1

0.1 −74.39081 −17.04970 83.01991 792.9089 13.10278 −95.71636

Figure 6. Water vapor pressure versus mole fraction of water of the binary system water−[EMIm][OAc] [7] and the calculated water vapor pressure with the NRTL parameters of the ternary system water−[EMIm][OAc]−[EMIm][OTf] at different temperatures (■, 293,15 K; ●, 313.15 K; ▲, 333.15 K; ▼, 353.15 K).

Figure 4. Water vapor pressure versus mole fraction water of the ternary system water−[EMIm][OAc]−[EMIm][OTf] with constant [EMIm][OTf] mass fraction of 0.05 and the calculated water vapor pressure with the NRTL parameters at different temperatures (■, 293.15 K; ●, 313.15 K; ▲, 333.15 K; ▼, 353.15 K).

Figure 7. Water vapor pressure versus mole fraction of water of the binary system water−[EMIm][OTf] [6] and the calculated water vapor pressure with the NRTL parameters of the ternary system water−[EMIm][OAc]−[EMIm][OTf] at different temperatures (■, 293,15 K; ●, 313.15 K; ▲, 333.15 K ▼, 353.15 K).

composition. The obtained data are consistent with literature data for the bordering binary systems of water−[EMIm][OAc] and water−[EMIm][OTf]. The mean relative deviations of density, viscosity, and vapor pressure are 0.027%, 0.38%, and 15.1%, respectively. The NRTL model is able to reproduce the available ternary and binary experimental vapor−liquid equilibrium data. The new experimental data attest to the suitability of ternary mixtures as an alternative for absorption refrigerators. Especially promising seem to be mixtures with a mass fraction of [EMIm][OTf] between 0.2 and 0.45, because they exhibit a significant reduction of the viscosity at nearly similar reduction of the water vapor pressure compared to the binary mixture water−[EMIm][OAc]. For example, the ternary mixture with a water mass fraction of 5% and a [EMIM][OTf] mass fraction of 5% has a water vapor pressure of about 300 Pa at 20 °C, which

Figure 5. Water vapor pressure versus mole fraction of water of the ternary system water−[EMIm][OAc]−[EMIm][OTf] at a constant [EMIm][OTf] mass fraction of 0.2 and the calculated water vapor pressure with the NRTL parameters at different temperatures (■, 293.15 K; ●, 313.15 K; ▲, 333.15 K; ▼, 353.15 K).

■

CONCLUSION The density, viscosity, and the vapor−liquid equilibrium of the system water−[EMIm][OAc]−[EMIM][OTf] was measured in the temperature range from 293.15 to 353.15 K with varying 727

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(12) Huber, M. L.; Perkins, R. A.; Laesecke, A.; Friend, D. G.; Sengers, J. V.; Assael, M. J.; Metaxa, I. M.; Vogel, E.; Mares, R.; Miyagawa, K. New International Formulation for the viscosity of water. J. Phys. Chem. Ref. Data 2009, 38, 101−125. (13) Renon, H. Local compositions in thermodynamics excess functions for liquid mixtures. AIChE J. 1968, 14, 135−144. (14) Merkel, N.; Weber, C.; Faust, M.; Schaber, K. Influence of anion and cation on the vapor pressure of binary mixtures of water + ionic liquid and on the thermal stability of the ionic liquid. Fluid Phase Equilib. 2015, 394, 29−37.

is nearly the same like that of a ternary mixture with the same water mass fraction of 5% and an [EMIm][OTf] mass fraction of 45%. In contrast, the viscosity of the ternary mixture decreases by about 40 to 58.5 mPa·s by increasing the [EMIm][OTf] mass fraction from 5% up to 45% .

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +49 721 608 42731. Fax: +49 721 60842335. ORCID

Markus Bücherl: 0000-0003-4686-0260 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding

The authors thanks the State of Baden-Württemberg for the financial support. Notes

The authors declare no competing financial interest.

■

REFERENCES

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