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Thermophysical Properties of the Binary Mixture of Water + Diethylmethylammonium Trifluoromethanesulfonate and the Ternary Mixture of Water + Diethylmethylammonium Trifluoromethanesulfonate + Diethylmethylammonium Methanesulfonate Nina C. Merkel,*,† Christiane Römich,† Richard Bernewitz,‡ Hannes Künemund,† Marco Gleiß,† Sven Sauer,§ Thomas J. S. Schubert,§ Gisela Guthausen,‡ and Karlheinz Schaber† †

Institute for Technical Thermodynamics and Refrigeration, Karlsruhe Institute of Technology (KIT), 76131 Karlsruhe, Germany SRG10-2, Institute for Mechanical Engineering and Mechanics, KIT, 76131 Karlsruhe, Germany § IoLiTec Ionic Liquids Technologies GmbH, 74076 Heilbronn, Germany ‡

ABSTRACT: New working pairs of room-temperature ionic liquids (RTILs) and water offer an opportunity to replace the highly corrosive and partly immiscible working pair of lithium bromide (LiBr) and water in absorption cycles like the absorption chiller. To estimate the suitability of these working pairs, the knowledge of thermophysical properties is inevitable. Due to the lack of literature data the following properties of two RTIL−water mixtures will be presented in this paper. Vapor−liquid equilibria (VLE) of the binary mixture of water + diethylmethylammonium trifluoromethanesulfonate ([DEMA][OTf]) and the ternary mixture of water + [DEMA][OTf] + diethylmethylammonium methanesulfonate ([DEMA][OMs]) were measured in the temperature range T = (293.15 to 353.15) K. The VLE measurements were carried out by Fourier transform infrared (FTIR) spectroscopy in a dynamic cell. The experimental VLE data were fitted with the nonrandom two-liquid (NRTL) model. Heat capacities, densities, and viscosities of the binary mixtures water + [DEMA][OTf] and [DEMA][OTf] + [DEMA][OMs] were measured. The measurements of the heat capacity were conducted via differential scanning calorimetry (DSC) in the temperature range T = (293.15 to 363.15) K. The density was measured with a pycnometer and the viscosity with a falling sphere viscometer. The temperature range for both was T = (293.15 to 353.15) K. Diffusion coefficients of water in the RTILs were determined by pulsed field gradient-nuclear magnetic resonance spectroscopy (PFG-NMR) in the temperature range of T = (288 to 313) K.

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water.7,8 The absorption chiller offers the opportunity for ambient cooling by the usage of waste heat or solar energy. A description of the process can be found in literature.9 The temperature range of the solution cycle in the process is set by the cooling water temperature in the absorber (around 298 K) and the temperature of the heat source used in the desorber (around 353 K). For a good efficiency of this process a high water vapor pressure reduction by the solvent is desired.15−18 To find a suitable RTIL for the application in an absorption chiller and to dimension this process, experimental data of the vapor−liquid equilibrium (VLE) and the fit to existing models are necessary. In this work the nonrandom two-liquid (NRTL) model presented by Renon and Prausnitz was used.19 Kato et al. showed that the fit of the VLE data of systems containing RTILs in a temperature range from (303.15 to 353.15) K results in lower overall average errors using the NRTL model in

INTRODUCTION Room-temperature ionic liquids (RTILs) are salts which are liquid at room temperature and atmospheric pressure. They are composed of an organic cation and an inorganic or organic anion. Due to the numerous possible combinations of anions and cations, physical and chemical properties can be adapted over a wide range.1,2 The most preferable properties for industrial applications are a low melting point, a wide liquid range, a negligible vapor pressure, and nonflammability.3 This leads to a large number of possible applications in synthetic, analytical, and engineering processes. Today RTILs are used for example as extraction agents in separation processes, and they are also suited to replace conventional organic solvents due to their negligible vapor pressure.3,4 Other fields of application such as the use as lubricants, in electrochemistry, and in bioscience are also being considered.5,6 This led to an enormous increase of scientific investigations over the past years.1−18 Our group is investigating new working pairs for the absorption chiller to replace the highly corrosive and partly immiscible but still commonly used working pair LiBr and © 2014 American Chemical Society

Received: January 30, 2013 Accepted: February 11, 2014 Published: February 20, 2014 560

dx.doi.org/10.1021/je400097b | J. Chem. Eng. Data 2014, 59, 560−570

Journal of Chemical & Engineering Data

Article

Table 1. Experimental VLE Data for Temperature T, Partial Pressure p1 with Combined Expanded Uncertainty uc(p1), and Mole Fraction x1 with Standard Uncertainty u(x1) for the System Water (1) + [DEMA][OTf] (2)a T/K

x1

u(x1)

p1/Pa

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

0.128 0.150 0.172 0.240 0.327 0.399 0.449 0.506 0.557 0.623 0.694 0.753 0.804 0.808 0.858 0.920 1.000

0.0001 0.0012 0.0012 0.0007 0.0007 0.0017 0.0005 0.0018 0.0018 0.0025 0.0060 0.0095 0.0131 0.0218 0.0053 0.0232 5·10−7 b

186 241 302 425 632 835 994 1128 1254 1459 1670 1820 1963 1893 2044 2108 2374

96 96 96 96 96 96 96 96 96 97 97 97 97 97 97 98 98

303.15 303.15 303.15 303.15 303.15 303.15 303.15 303.15 303.15 303.15 303.15 303.15 303.15 303.15 303.15 303.15

0.150 0.172 0.240 0.326 0.398 0.448 0.506 0.556 0.622 0.693 0.752 0.804 0.808 0.858 0.920 1.000

0.0012 0.0012 0.0007 0.0007 0.0017 0.0005 0.0018 0.0018 0.0025 0.0060 0.0095 0.0131 0.0218 0.0053 0.0232 5·10−7 b

468 560 843 1123 1459 1677 1920 2132 2463 2776 3068 3369 3243 3600 3823 4214

96 96 96 96 97 97 97 98 98 99 282 300 293 314 328 353

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

0.127 0.150 0.172 0.239 0.326 0.398 0.448 0.505 0.555 0.621 0.692 0.751 0.803 0.807 0.857 0.919 1.000

0.0001 0.0012 0.0012 0.0007 0.0007 0.0017 0.0005 0.0018 0.0018 0.0025 0.0060 0.0095 0.0131 0.0218 0.0053 0.0232 5·10−7 b

691 861 995 1384 1876 2356 2675 3142 3670 4468 5164 5604 6160 6145 6516 6931 7501

96 96 96 96 97 98 99 287 318 369 415 444 483 481 507 536 576

323.15 323.15 323.15 323.15 323.15

0.127 0.149 0.171 0.238 0.325

0.0001 0.0012 0.0012 0.0007 0.0007

1161 1362 1581 2154 2875

96 96 97 98 271

Table 1. continued

uc(p1)/Pa

561

T/K

x1

u(x1)

p1/Pa

uc(p1)/Pa

323.15 323.15 323.15 323.15 323.15 323.15 323.15 323.15 323.15 323.15 323.15 323.15

0.396 0.446 0.503 0.554 0.619 0.691 0.750 0.802 0.806 0.856 0.918 1.000

0.0017 0.0005 0.0018 0.0018 0.0025 0.0060 0.0095 0.0131 0.0218 0.0053 0.0232 5·10−7 b

3906 4722 5571 6360 7566 8691 9428 10300 10213 10873 11502 12429

333 386 442 496 581 661 714 777 771 819 866 934

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

0.126 0.148 0.171 0.237 0.323 0.394 0.444 0.501 0.551 0.619 0.689 0.748 0.800 0.804 0.855 0.917 1.000

0.0001 0.0012 0.0012 0.0007 0.0007 0.0017 0.0005 0.0018 0.0018 0.0025 0.0060 0.0095 0.0131 0.0218 0.0053 0.0232 5·10−7 b

1746 1997 2342 3332 5002 6559 7698 8940 10175 12010 13676 15051 16445 16378 17346 18369 19791

97 97 98 298 404 510 590 679 768 903 1027 1130 1236 1230 1304 1382 1492

343.15 343.15 343.15 343.15 343.15 343.15 343.15 343.15 343.15 343.15 343.15 343.15 343.15 343.15 343.15 343.15 343.15

0.126 0.148 0.170 0.235 0.321 0.392 0.441 0.498 0.548 0.614 0.686 0.745 0.797 0.801 0.853 0.916 1.000

0.0001 0.0012 0.0012 0.0007 0.0007 0.0017 0.0005 0.0018 0.0018 0.0025 0.0060 0.0095 0.0131 0.0218 0.0053 0.0232 5·10−7 b

2538 2830 3568 5428 7890 10119 11857 13838 15611 18446 21066 23205 25327 25109 26848 28660 30914

98 99 312 433 604 764 892 1039 1172 1388 1591 1758 1925 1908 2046 2192 2374

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

0.125 0.146 0.168 0.233 0.318 0.388 0.437 0.494 0.544 0.610 0.681 0.741

0.0001 0.0012 0.0012 0.0007 0.0007 0.0017 0.0005 0.0018 0.0018 0.0025 0.0060 0.0095

3869 5576 4562 8152 11740 15031 17682 20601 23352 27207 31382 34902

331 443 375 622 883 1129 1330 1554 1769 2075 2413 2703



Article

to a solution of trifluoromethanesulfonic acid (382 mL, 4.3 mol) in 1 L of water keeping the reaction mixture at room temperature. After the addition of the amine was completed, the resulting mixture was stirred overnight and filtered via a glass fiber filter. Finally the water was removed under reduced pressure at 323 K. The requested RTIL is obtained as colorless to yellowish oil in quantitative yield (> 97 % in mass). The following commercially available materials were used: diethylmethylamine (Acros, 98.0 %) and trifluoromethanesulfonic acid (Sigma-Aldrich; in water; w = 70 %) where w is the mass fraction of the substance in the solution in weight %. A Metrohm modular setup (709 IC Pump, 732 IC Detector, 733 IC Separation Center, 753 Suppressor Module, 762 IC Interface, 812 Valve Unit) with an accuracy of ± 10 ppm was used to detect impurities and confirmed that the RTIL was halide free. The water content of the RTIL was determined by Karl Fischer titration (Metrohm, 795 KFT Titrino) with an accuracy of ± 50 ppm. These measurements were carried out before each series of measurements and taken into account for the calculation of the composition of the respective mixtures. The preparation of [DEMA][OMs] has already been described (> 97 % in mass).14

Table 1. continued T/K

x1

u(x1)

p1/Pa

uc(p1)/Pa

353.15 353.15 353.15 353.15 353.15

0.794 0.798 0.850 0.913 1.000

0.0131 0.0218 0.0053 0.0232 5·10−7 b

37866 38171 40715 43571 47166

2951 2977 3193 3439 3755

a

The expanded uncertainty U is U(T) = 0.15 K with a 0.95 level of confidence. bDetermined from the electrical conductivity κ of pure water κ = 3 μS·cm−1.

comparison to the universal quasichemical (UNIQUAC) model.5 Therefore this model is regarded adequate and is commonly used for systems containing RTILs.20 In this paper we present the thermodynamic properties VLE, specific heat capacity, density, and viscosity of a mixture of water and the hygroscopic RTIL diethylmethylammonium trifluoromethanesulfonate ([DEMA][OTf]). Diffusion coefficients of water dissolved in the RTIL were measured as a function of temperature and concentration. As it is not easy to design an RTIL which possesses all desired properties for a certain application, it is also possible to mix two or more RTILs to improve the properties.8 Here the RTIL diethylmethylammonium methanesulfonate ([DEMA][OMs]) was added to [DEMA][OTf] to investigate the influence on the thermophysical properties. The thermophysical properties of the binary mixture of water and [DEMA][OMs] have already been presented in an earlier publication.14 The specific heat capacity, density, and viscosity of [DEMA][OTf] + [DEMA][OMs] over the whole concentration range between pure [DEMA][OTf] and pure [DEMA][OMs] (both containing a residual water content) are presented. Finally the VLE data of water and a [DEMA][OTf]/[DEMA][OMs] mixture with a mass ratio of 7:3 is presented. Both investigated systems show a reduction of the water vapor pressure compared to the vapor pressure of pure water. As explained before, this makes them applicable in absorption cycles.

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EXPERIMENTAL SECTION Materials and Purities. [DEMA][OTf] was prepared by slowly adding one equivalent of diethylmethylamine (520 mL, 4.3 mol)

Figure 2. Vapor pressure versus mole fraction for water (1) + [DEMA][OTf] (2) at three temperatures.

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. 562



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Table 4. Experimental Values of Dynamic Viscosity η with Standard Uncertainty u(η) at Temperature T and Mole Fraction x1 with Standard Uncertainty u(x1) for the System Water (1) + [DEMA][OTf] (2) at Pressure p = 0.1 MPaa

To prepare a mixture of [DEMA][OTf]/[DEMA][OMs] with a mass ratio of 7:3, the respective amounts of [DEMA][OTf] and [DEMA][OMs] were weighed with a microbalance (Mettler Toledo, PR 200 3DR) with an accuracy of ± 10−6 kg. The resulting mixture contained 29.85 wt % of [DEMA][OMs]. As the RTILs are not completely anhydrous, the water content of the mixture had to be determined by Karl Fischer titration (Schott, TitroLine KF) with an accuracy of ± 1 %. The quality of pure water was determined by measuring the electrical conductivity κ and gave a result of κ = 3 μS·cm−1. Table 2. Binary NRTL Parameters Fitted to Experimental VLE Data of Water (1) + [DEMA][OTf] (2) (Table 1) with the Root Mean Square Deviation (RMSD) Δg12/J·mol−1 Δg21/J·mol−1

system water (1) + [DEMA][OTf] (2)

−1261.40

64.92

α12

rmsd

0.3

3.52

Table 3. Experimental Values of Density ρ with Standard Uncertainty u(ρ) at Temperature T and Mole Fraction x1 with Standard Uncertainty u(x1) for the System Water (1) + [DEMA][OTf] (2) at Pressure p = 0.1 MPaa T/K

x1

u(x1)

ρ/kg·m−3

293.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15

0.037 0.434 0.586 0.759 0.844 0.925 0.972 1.000

0.0004 0.0094 0.0043 0.0088 0.0078 0.0029 0.0303 5·10−7 b

1294.1 1278.0 1259.2 1226.7 1195.9 1135.0 1062.8 997.1

313.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15

0.037 0.434 0.586 0.759 0.844 0.925 0.972 1.000

0.0004 0.0094 0.0043 0.0088 0.0078 0.0029 0.0303 5·10−7 b

1277.4 1261.5 1243.3 1211.9 1180.8 1122.0 1053.0 990.4

333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15

0.037 0.434 0.586 0.759 0.844 0.925 0.972 1.000

0.0004 0.0094 0.0043 0.0088 0.0078 0.0029 0.0303 5·10−7 b

1261.4 1245.6 1227.3 1195.6 1165.3 1107.7 1041.1 980.9

353.15 353.15 353.15 353.15 353.15 353.15 353.15 353.15

0.037 0.434 0.586 0.759 0.844 0.925 0.972 1.000

0.0004 0.0094 0.0043 0.0088 0.0078 0.0029 0.0303 5·10−7 b

1245.5 1229.4 1210.6 1177.0 1147.3 1091.0 1027.3 970.1

T/K

x1

u(x1)

η/mPa·s

293.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15

0.037 0.434 0.586 0.759 0.844 0.925 0.972 1.000

0.0004 0.0094 0.0043 0.0088 0.0078 0.0029 0.0303 5·10−7 b

53.28 22.31 14.49 7.65 4.99 2.86 1.69 1.20

313.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15

0.037 0.434 0.586 0.759 0.844 0.925 0.972 1.000

0.0004 0.0094 0.0043 0.0088 0.0078 0.0029 0.0303 5·10−7 b

25.83 12.10 7.98 4.38 2.89 1.75 1.20 0.98

333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15

0.037 0.434 0.586 0.759 0.844 0.925 0.972 1.000

0.0004 0.0094 0.0043 0.0088 0.0078 0.0029 0.0303 5·10−7 b

14.62 7.33 4.97 2.81 1.94 1.28 1.01 0.88

353.15 353.15 353.15 353.15 353.15 353.15 353.15 353.15

0.037 0.434 0.586 0.759 0.844 0.925 0.972 1.000

0.0004 0.0094 0.0043 0.0088 0.0078 0.0029 0.0303 5·10−7 b

9.18 4.89 3.38 2.01 1.45 1.08 0.91 0.82

a

The expanded uncertainty U is U(T) = 0.08 K with a 0.95 level of confidence, and the relative expanded uncertainty Ur is Ur(η) = 0.069. bDetermined from the electrical conductivity κ of pure water κ = 3 μS·cm−1.

Apparatus and Procedure. The VLE measurements were conducted in the same dynamic measurement apparatus as the measurements of water + [DEMA][OMs] by Römich et al.14 For the convenience of the reader, the apparatus is again shown in Figure 1 and briefly explained in the following section. The temperature of the liquid phase in the glass flask was measured with a platinum resistance thermometer (PT 100 sensor) with an accuracy of ± 0.01 K and held constant with a water thermostat (Lauda, E200). The gas phase was pumped through the Fourier transform infrared (FTIR) spectrometer (BOMEM Hartmann & Braun, model 9100) where the IR absorbance of the water vapor in the gas phase was measured. The area of the water peak at (2112.863 to 2117.2) cm−1 was used to determine the partial pressure of the water vapor, employing the calibration described by Römich et al.14 Based on the calibration used, water vapor pressures which are higher than the resolution limit of 100 Pa can be determined with a mean deviation of ± 2.6 % and a

a

The expanded uncertainty U is U(T) = 0.5 K with a 0.95 level of confidence, and the relative expanded uncertainty Ur is Ur(ρ) = 0.0023. bDetermined from the electrical conductivity κ of pure water κ = 3 μS·cm−1. 563



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Table 5. Experimental Values of Specific Heat Capacity c at Temperature T and Mole Fraction x1 with Standard Uncertainty u(x1) for the System Water (1) + [DEMA][OTf] (2) at Pressure p = 0.1 MPaa T/K

x1

u(x1)

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

0.026 0.289 0.400 0.588 0.684 0.757 0.808 0.843 0.893 0.924 0.948 0.971 0.990 1.000

0.001 0.001 0.001 0.001 0.007 0.005 0.024 0.026 0.035 0.090 0.027 0.049 0.021 5·10−7

303.15 303.15 303.15 303.15 303.15 303.15 303.15 303.15 303.15 303.15 303.15 303.15 303.15 303.15

0.026 0.289 0.400 0.588 0.684 0.757 0.808 0.843 0.893 0.924 0.948 0.971 0.990 1.000

0.001 0.001 0.001 0.001 0.007 0.005 0.024 0.026 0.035 0.090 0.027 0.049 0.021 5·10−7

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

0.026 0.289 0.400 0.588 0.684 0.757 0.808 0.843 0.893 0.924 0.948 0.971 0.990 1.000

0.001 0.001 0.001 0.001 0.007 0.005 0.024 0.026 0.035 0.090 0.027 0.049 0.021 5·10−7

323.15 323.15 323.15 323.15 323.15 323.15 323.15 323.15 323.15 323.15 323.15 323.15 323.15 323.15

0.026 0.289 0.400 0.588 0.684 0.757 0.808 0.843 0.893 0.924 0.948 0.971 0.990 1.000

0.001 0.001 0.001 0.001 0.007 0.005 0.024 0.026 0.035 0.090 0.027 0.049 0.021 5·10−7

b

b

b

b

c/J·kg−1·K−1

T/K

x1

1520 1590 1650 1800 1930 2090 2250 2390 2640 2990 3200 3660 4010 4170

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

0.026 0.289 0.400 0.588 0.684 0.757 0.808 0.843 0.893 0.924 0.948 0.971 0.990 1.000

0.001 0.001 0.001 0.001 0.007 0.005 0.024 0.026 0.035 0.090 0.027 0.049 0.021 5·10−7

1530 1610 1670 1830 1960 2120 2280 2420 2670 3010 3220 3660 4010 4160

343.15 343.15 343.15 343.15 343.15 343.15 343.15 343.15 343.15 343.15 343.15 343.15 343.15 343.15

0.026 0.289 0.400 0.588 0.684 0.757 0.808 0.843 0.893 0.924 0.948 0.971 0.990 1.000

0.001 0.001 0.001 0.001 0.007 0.005 0.024 0.026 0.035 0.090 0.027 0.049 0.021 5·10−7

1550 1630 1690 1850 1990 2150 2310 2450 2690 3030 3230 3670 4010 4160

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

0.026 0.289 0.400 0.588 0.684 0.757 0.808 0.843 0.893 0.924 0.948 0.971 0.990 1.000

0.001 0.001 0.001 0.001 0.007 0.005 0.024 0.026 0.035 0.090 0.027 0.049 0.021 5·10−7

1570 1650 1710 1880 2020 2180 2340 2480 2710 3050 3250 3670 4010 4160

363.15 363.15 363.15 363.15 363.15 363.15 363.15 363.15 363.15 363.15 363.15 363.15 363.15 363.15

0.026 0.289 0.400 0.588 0.684 0.757 0.808 0.843 0.893 0.924 0.948 0.971 0.990 1.000

0.001 0.001 0.001 0.001 0.007 0.005 0.024 0.026 0.035 0.090 0.027 0.049 0.021 5·10−7

c/J·kg−1·K−1

u(x1)

b

b

b

b

1580 1670 1740 1900 2040 2210 2370 2500 2730 3060 3260 3680 4020 4160 1600 1690 1750 1930 2070 2230 2390 2530 2750 3080 3270 3680 4020 4160 1610 1710 1770 1950 2090 2250 2410 2540 2770 3090 3270 3680 4020 4170 1630 1730 1790 1970 2110 2270 2430 2560 2780 3100 3280 3690 4020 4170

a Expanded uncertainties U are U(T) = 0.16 K and U(c) = 40 J·kg−1·K−1 with 0.95 level of confidence. bDetermined from the electrical conductivity κ of pure water κ = 3 μS·cm−1.

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Table 6. Diffusion Coefficients D of Water (1) in [DEMA][OTf] (2) at Temperature T, Mass Fraction ξ1, and Mole Fraction x1 at Pressure p = 0.1 MPaa T/K

a

ξ1

x1

D/m2·s−1 −10

T/K

ξ1

x1

D/m2·s−1

288 288 288

0.118 0.182 0.285

0.638 0.745 0.840

2.4·10 3.6·10−10 4.0·10−10

303 303 303

0.118 0.182 0.285

0.638 0.745 0.840

2.8·10−10 5.3·10−10 7.4·10−10

293 293 293

0.118 0.182 0.285

0.638 0.745 0.840

2.5·10−10 4.0·10−10 5.8·10−10

308

0.118

0.638

2.7·10−10

298 298 298

0.118 0.182 0.285

0.638 0.745 0.840

2.7·10−10 4.5·10−10 6.6·10−10

313 313 313

0.118 0.182 0.285

0.638 0.745 0.840

2.9·10−10 9.2·10−10 2.0·10−9

Standard uncertainties u are u(T) = 0.5 K and u(x1) = 0.02. The standard uncertainty for the determination of the diffusion coefficient was 5 %.

Table 7. Diffusion Coefficients D of Water (1) in [DEMA][OMs] (2) at Temperature T, Mass Fraction ξ1, and Mole Fraction x1 at Pressure p = 0.1 MPaa T/K

a

ξ1

x1

D/m2·s−1 −10

T/K

ξ1

x1

D/m2·s−1

288 288

0.180 0.298

0.690 0.812

1.3·10 2.7·10−10

303 303

0.180 0.298

0.690 0.812

2.3·10−10 4.3·10−10

293 293

0.180 0.298

0.690 0.812

1.6·10−10 3.2·10−10

313 313

0.180 0.298

0.690 0.812

3.4·10−10 6.3·10−10

298 298

0.180 0.298

0.690 0.812

1.9·10−10 3.6·10−10

Standard uncertainties u are u(T) = 0.5 K and u(x1) = 0.008. The standard uncertainty for the determination of the diffusion coefficient was 5 %.

Figure 4. Vapor pressure versus mole fraction for water (1) + [DEMA][OTf]/[DEMA][OMs] (2) (mass ratio: 7:3) at three temperatures.

Figure 3. Dynamic viscosity versus mass fraction of [DEMA][OTf] + [DEMA][OMs] at four temperatures.

maximum measurement deviation of ± 9.5 %. The vapor phase was recirculated through the liquid phase. This enhances the heat and mass transfer and accelerates the approach of the vapor−liquid equilibrium. For each mixture the water concentration of the liquid phase was determined by volumetric Karl Fischer titration (Schott, TitroLine KF) with an accuracy of ± 1 % of the measured mass fraction of water. Densities, viscosities, and specific heat capacities of the mixtures were measured with the same devices described by Römich et al.14 The water concentration of each sample was again verified by Karl Fischer titration. The density was measured by a glass-pycnometer with a capacity of V = 9.994 mL ± 0.004 mL. The dynamic viscosity was determined using a Haake falling ball viscometer (Thermo Electron GmbH, Type C). The density and viscosity measurements were

conducted over the whole concentration range and a temperature range of T = (293.15 to 353.15) K in 20 K steps at atmospheric pressure. The specific heat capacities were measured in the range of T = (293.15 to 363.15) K in 10 K steps by using a differential scanning calorimeter (Setaram, Micro-DSC VII). The measurement method was validated with pure water (electrical conductivity κ = 3 μS·cm−1) based on literature data,23,24 resulting in a maximum measurement deviation of ± 1.1 %. For the determination of diffusion coefficients via pulsed field gradient-nuclear magnetic resonance (PFG-NMR) spectroscopy, the mixtures of water + RTIL were filled into NMR glass tubes with an outer diameter of 5 mm to a filling level of about 10 mm. Samples were measured in a spectrometer (Bruker, DRX-200) equipped with a Diff30 probe (Bruker) in a 565



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Table 8. Experimental Values of Density ρ with Standard Uncertainty u(ρ) at Temperature T and Mole Fractions x1 and x2 for the System Water (1) + [DEMA][OMs] (2) + [DEMA][OTf] (3) at Pressure p = 0.1 MPaa T/K

x1

x2

ρ/kg·m−3

293.15 293.15 293.15 293.15 293.15 293.15

0.027 0.050 0.070 0.089 0.107 0.114

0.000 0.233 0.432 0.602 0.750 0.817

1294.1 1258.2 1224.8 1194.3 1164.7 1151.4

313.15 313.15 313.15 313.15 313.15 313.15 313.15

0.027 0.050 0.070 0.089 0.107 0.114 0.121

0.000 0.233 0.432 0.602 0.750 0.817 0.879

1277.4 1242.3 1209.5 1178.9 1150.5 1137.2 1124.2

333.15 333.15 333.15 333.15 333.15 333.15 333.15

0.027 0.050 0.070 0.089 0.107 0.114 0.121

0.000 0.233 0.432 0.602 0.750 0.817 0.879

1261.4 1226.8 1194.8 1165.9 1136.8 1124.1 1110.7

353.15 353.15 353.15 353.15 353.15 353.15 353.15

0.027 0.050 0.070 0.089 0.107 0.114 0.121

0.000 0.233 0.432 0.602 0.750 0.817 0.879

1245.5 1211.9 1180.2 1151.5 1123.6 1110.6 1097.6

Table 9. Experimental Values of Dynamic Viscosity η with Standard Uncertainty u(η) at Temperature T and Mole Fractions x1 and x2 for the System Water (1) + [DEMA][OMs] (2) + [DEMA][OTf] (3) at Pressure p = 0.1 MPaa

a

The expanded uncertainty U is U(T) = 0.5 K with a 0.95 level of confidence, standard uncertainties u are u(x1) = 0.001 and u(x2) = 0.001, and the relative expanded uncertainty Ur is Ur(ρ) = 0.0023.

T/K

x1

x2

η/mPa·s

293.15 293.15 293.15 293.15 293.15 293.15

0.027 0.050 0.070 0.089 0.107 0.114

0.000 0.233 0.432 0.602 0.750 0.817

53.28 78.07 101.71 116.75 125.17 128.31

313.15 313.15 313.15 313.15 313.15 313.15 313.15

0.027 0.050 0.070 0.089 0.107 0.114 0.121

0.000 0.233 0.432 0.602 0.750 0.817 0.879

25.83 34.01 40.86 44.36 45.78 46.13 46.52

333.15 333.15 333.15 333.15 333.15 333.15 333.15

0.027 0.050 0.070 0.089 0.107 0.114 0.121

0.000 0.233 0.432 0.602 0.750 0.817 0.879

14.62 17.87 20.35 21.41 21.56 21.66 21.64

353.15 353.15 353.15 353.15 353.15 353.15 353.15

0.027 0.050 0.070 0.089 0.107 0.114 0.121

0.000 0.233 0.432 0.602 0.750 0.817 0.879

9.18 10.70 11.74 12.07 12.03 12.06 12.05

a

Expanded uncertainty U is U(T) = 0.05 K with 0.95 level of confidence, and standard uncertainties u are u(x1) = 0.001 and u(x2) = 0.001, and the relative expanded uncertainty Ur is Ur(ρ) = 0.069.

temperature range of T = (288 to 313) K. A PFG-stimulated echo (STE) sequence was applied, which is described in literature.25 Measurement parameters were chosen as follows: the diffusion time was Δ = 60 ms. The maximum gradient amplitude was adjusted according to the sample between g ∈ [0.8, 2] T·m−1 and the gradient amplitude was varied linearly in 32 steps. The gradient duration was set to δ = 2 ms. The spoiler gradient during the longitudinal storage period amounted 0.1 T·m−1 for 2.2 ms. The 90° pulse was adjusted to 4.3 μs, and the delay between the first two pulses was τ = 6 ms. For data processing, fit routines were used in the Bruker Topspin Software V1.5 allowing the processing of the data via the Stejskal Tanner relation.26

Δg21 while the nonrandomness factor α12 was held at a constant value of 0.3. This value is recommended by Renon and Prausnitz for systems with only small deviations from ideality.19 In this model the logarithmic activity coefficient of water γ1 is given as ⎛ exp( − 2α12τ21) ln γ1 = (1 − x1)2 ⎜τ21 2 ⎝ [x1 + (1 − x1) exp(−α12τ21)] + τ12

■

⎞ ⎟ [(1 − x1) + x1 exp(−α12τ12)]2 ⎠ exp( − 2α12τ12)

(1)

with

RESULTS AND DISCUSSION The isothermal VLE measurements were conducted at T = (293.15 to 353.15) K in 10 K steps and at different mole fractions of water (x1). The water vapor pressure data were measured with a mean deviation of ± 2.6 %. No volatile component beside water was detected in the gas phase. The experimental data of water + [DEMA][OTf] are listed in Table 1. The experimental data was fitted using the excess Gibbs free energy model NRTL to extract the energy coefficients Δg12 and

τ12 =

Δg12 RT

(2)

and

τ21 =

Δg21

(3) RT −1 −1 In eqs 2 and 3 R = (8.314) J·mol ·K is the molar gas constant. 566



Article

Table 10. Experimental VLE Data for Temperature T, Partial Pressure p1 with Combined Expanded Uncertainty uc(p1) and Mole Fraction x1 with Standard Uncertainty u(x1) for the System Water (1) + [DEMA][OTf]/[DEMA][OMs] (2); Mass Ratio 7:3a

a

T/K

x1

u(x1)

p1/Pa

uc(p1)/Pa

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

0.151 0.184 0.245 0.326 0.409 0.489 0.564 0.649 0.729 0.808 0.887 0.946 1.000

0.0003 0.0015 0.0022 0.0007 0.0009 0.0013 0.0009 0.0002 0.0021 0.0017 0.0087 0.0058 5·10−7 b

106 186 270 360 567 718 925 1163 1420 1694 1927 2255 2221

96 96 96 96 96 96 96 96 97 97 97 98 98

303.15 303.15 303.15 303.15 303.15 303.15 303.15 303.15 303.15 303.15 303.15 303.15 303.15

0.151 0.184 0.245 0.326 0.409 0.488 0.564 0.649 0.729 0.807 0.887 0.946 1.000

0.0003 0.0015 0.0022 0.0007 0.0009 0.0013 0.0009 0.0002 0.0021 0.0017 0.0087 0.0058 5·10−7 b

266 374 523 726 1050 1315 1670 2035 2425 2820 3402 3817 4069

96 96 96 96 96 96 97 97 98 99 302 328 343

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

0.151 0.184 0.244 0.325 0.408 0.488 0.564 0.648 0.728 0.807 0.886 0.945 1.000

0.0003 0.0015 0.0022 0.0007 0.0009 0.0013 0.0009 0.0002 0.0021 0.0017 0.0087 0.0058 5·10−7 b

498 657 954 1295 1783 2181 2658 3462 4403 5326 6311 6981 7217

96 96 96 96 97 98 99 306 365 426 493 540 556

323.15 323.15 323.15 323.15 323.15 323.15

0.151 0.184 0.244 0.325 0.408 0.487

0.0003 0.0015 0.0022 0.0007 0.0009 0.0013

809 1099 1506 2063 2756 3648

96 96 97 98 99 317

T/K

x1

u(x1)

p1/Pa

uc(p1)/Pa

323.15 323.15 323.15 323.15 323.15 323.15 323.15

0.563 0.647 0.727 0.806 0.885 0.945 1.000

0.0009 0.0002 0.0021 0.0017 0.0087 0.0058 5·10−7 b

4790 6160 7656 9010 10466 11558 12114

390 483 587 684 790 870 911

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

0.150 0.183 0.243 0.324 0.406 0.486 0.561 0.646 0.725 0.804 0.884 0.944 1.000

0.0003 0.0015 0.0022 0.0007 0.0009 0.0013 0.0009 0.0002 0.0021 0.0017 0.0087 0.0058 5·10−7 b

1269 1729 2302 3217 4836 6246 8018 9999 12228 14409 16778 18416 19347

96 97 98 291 393 488 613 756 919 1082 1261 1386 1458

343.15 343.15 343.15 343.15 343.15 343.15 343.15 343.15 343.15 343.15 343.15 343.15 343.15

0.150 0.183 0.243 0.323 0.405 0.484 0.559 0.643 0.723 0.802 0.883 0.943 1.000

0.0003 0.0015 0.0022 0.0007 0.0009 0.0013 0.0009 0.0002 0.0021 0.0017 0.0087 0.0058 5·10−7 b

1916 2516 3570 5348 7666 9873 12682 15695 18948 22510 26029 28442 29809

97 98 312 427 588 746 953 1179 1427 1703 1981 2174 2285

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

0.182 0.241 0.321 0.402 0.481 0.556 0.641 0.720 0.800 0.880 0.941 1.000

0.0015 0.0022 0.0007 0.0009 0.0013 0.0009 0.0002 0.0021 0.0017 0.0087 0.0058 5·10−7 b

3918 5660 8242 11724 14846 19438 23688 28717 33905 39504 42392 46000

334 448 629 882 1115 1465 1796 2196 2620 3090 3337 3652

The expanded uncertainty U is U(T) = 0.09 with a 0.95 level of confidence. bDetermined from the electrical conductivity κ of pure water κ = 3 μS·cm−1.

To calculate the NRTL coefficients Δg12 and Δg21 the following function was minimized:

The density of water + [DEMA][OTf] was measured with a mean measurement deviation of ± 0.17 % and a maximum measurement deviation of ± 0.23 % which was determined by validation with pure water (electrical conductivity κ = 3 μS·cm−1). As the density is only used for calculating the dynamic viscosity, this accuracy is adequate for the purpose of investigating the suitability of a new working pair for an absorption chiller. Each experiment was repeated two times. The results and the

i=1

F=

∑ (ln γ1.cal − ln γ1.exp)2 n

(4)

The fit and the experimental results in Figure 2 show a good agreement with each other. The energy coefficients and the rootmean-square deviation are shown in Table 2. 567



Article

efficiency.15 Nevertheless the viscosity is important for the estimation of the heat and mass transfer in the apparatuses which are both enhanced by a low viscosity. In comparison with water + [DEMA][OMs],14 water + [DEMA][OTf] has a lower dynamic viscosity and caused by the higher density an even smaller kinematic viscosity which results in better heat and mass transfer. The results of the specific heat capacities of water + [DEMA][OTf] are shown in Table 5. For water + [DEMA][OTf] a positive deviation from an ideal mixture was found over the whole concentration and temperature range. Compared to water + [DEMA][OMs]14 it possesses a lower specific heat capacity which is favorable in absorption cycles to realize higher temperature shifts in the apparatuses. The via PFG-NMR determined diffusion coefficients (D) of water in [DEMA][OTf] and [DEMA][OMs] at different mass fractions (ξ) of water are shown in Tables 6 and 7, respectively. It can be seen that the diffusion coefficients of water in [DEMA][OTf] are higher than the ones in [DEMA][OMs] due to the lower viscosity. Therefore [DEMA][OTf] is favorable for better heat and mass transfer in absorption cycles to enhance the efficiency. Though comparing the VLE data of water + [DEMA][OTf] and water + [DEMA][OMs]14 under the aspect of using them as working pairs in absorption chillers, the higher water vapor pressure reduction of [DEMA][OMs] leads to a better thermodynamic suitability for the process. As the lowering of the vapor pressure is a colligative property, we assumed that a mixture of the two RTILs may combine the favorable properties of both. To determine a reasonable mass

Table 11. Binary NRTL Parameters Fitted to Experimental VLE Data Water (1) + [DEMA][OTf]/[DEMA][OMs] (2) (Table 10) with the Root Mean Square Deviation (RMSD) Δg12/J·mol−1 Δg21/J·mol−1

system water (1) + [DEMA][OTf]/ [DEMA][OMs] (2); mass ratio: 7:3

−3604.6

1054.5

α12

RMSD

0.3

1.88

uncertainties are given in Table 3. The measurement data show a decrease of density with increasing temperature and increasing water concentration. In regard to the water concentration it shows a slightly negative deviation from the ideal solution. Compared to the density of water + [DEMA][OMs]14 the one of water + [DEMA][OTf] is slightly higher. The dynamic viscosity measurements were repeated four times at each concentration and temperature. The viscosity data and the standard deviations (SD) of the measurements are given in Table 4. The data show, as expected, that the dynamic viscosity decreases with increasing temperature. At low water concentration a slight addition of water causes a sharp decrease in the viscosity. Therefore, processes which allow water concentrations around 15 wt % and higher do not have to deal with viscosities over 10 mPa·s. The measurement accuracy was again validated with pure water (electrical conductivity κ = 3 μS·cm−1). The mean measurement deviation is ± 3.1 %, and the maximum measurement deviation is ± 6.9 %. Below 2 mPa·s the maximum deviation is ± 0.47 mPa·s. This accuracy is sufficient for our purpose because the viscosity has no direct impact on the thermodynamic evaluation of the process and the

Table 12. Experimental Values of Specific Heat Capacity c at Temperature T and Mole Fractions x1 and x2 for the System Water (1) + [DEMA][OMs] (2) + [DEMA][OTf] (3) at Pressure p = 0.1 MPaa

a

T/K

x1

x2

c/J·kg−1·K−1

T/K

x1

x2

c/J·kg−1·K−1

293.15 293.15 293.15 293.15 293.15 293.15

0.035 0.049 0.081 0.088 0.101 0.071

0.000 0.220 0.416 0.570 0.748 0.929

1518 1597 1680 1742 1825 1889

333.15 333.15 333.15 333.15 333.15 333.15

0.035 0.049 0.081 0.088 0.101 0.071

0.000 0.220 0.416 0.570 0.748 0.929

1583 1661 1744 1805 1889 1953

303.15 303.15 303.15 303.15 303.15 303.15

0.035 0.049 0.081 0.088 0.101 0.071

0.000 0.220 0.416 0.570 0.748 0.929

1534 1611 1695 1756 1840 1903

343.15 343.15 343.15 343.15 343.15 343.15

0.035 0.049 0.081 0.088 0.101 0.071

0.000 0.220 0.416 0.570 0.748 0.929

1599 1677 1759 1821 1905 1970

313.15 313.15 313.15 313.15 313.15 313.15

0.035 0.049 0.081 0.088 0.101 0.071

0.000 0.220 0.416 0.570 0.748 0.929

1550 1628 1711 1772 1856 1919

353.15 353.15 353.15 353.15 353.15 353.15

0.035 0.049 0.081 0.088 0.101 0.071

0.000 0.220 0.416 0.570 0.748 0.929

1613 1692 1774 1836 1921 1986

323.15 323.15 323.15 323.15 323.15 323.15

0.035 0.049 0.081 0.088 0.101 0.071

0.000 0.220 0.416 0.570 0.748 0.929

1567 1645 1728 1789 1873 1936

363.15 363.15 363.15 363.15 363.15 363.15

0.035 0.049 0.081 0.088 0.101 0.071

0.000 0.220 0.416 0.570 0.748 0.929

1630 1708 1790 1853 1938 2004

Expanded uncertainties U are U(T) = 0.16 K and U(c) = 40 J·kg−1·K−1 with a 0.95 level of confidence, and standard uncertainties u are u(x1) = 0.001, u(x2) = 0.001. 568



■

ratio of the RTILs first density and viscosity were measured over the whole concentration range from pure [DEMA][OTf] to pure [DEMA][OMs] (both containing a residual water content). The measurement data are presented in Tables 8 and 9, respectively. The residual water contents of the mixtures were again determined by Karl Fischer titration and are also shown in Tables 8 and 9. As can be seen in Figure 3, the viscosity of [DEMA][OTf] + [DEMA][OMs] shows a nonideal behavior. To obtain a possible absorption fluid for absorption chillers, a mixture with a significantly lower viscosity than [DEMA][OMs] is desired for better heat and mass transfer. The viscosity results of [DEMA][OTf] + [DEMA][OMs] led to the conclusion that a mixture with a mass ratio of 7:3 possesses a significantly lower viscosity than water + [DEMA][OMs]. Due to the colligative behavior of the water vapor reduction, this mixture was also expected to have a more favorable VLE behavior than water + [DEMA][OTf]. The VLE data of water + [DEMA][OTf]/[DEMA][OMs] (mass ratio: 7:3) were measured as described before and are listed in Table 10. In comparison to water + [DEMA][OTf] a slightly higher water vapor pressure reduction was detected due to the [DEMA][OMs] content in the mixture. The corresponding NRTL coefficients are shown in Table 11. The good agreement of the experimental results and the fit can be seen in Figure 4. Concerning the heat capacity and regarding the measurement accuracy of 1.1 % we observed an ideal behavior over the whole concentration range of the binary mixture [DEMA][OTf] + [DEMA][OMs]. The measurement data and the residual water contents are shown in Table 12.

Article

AUTHOR INFORMATION

Corresponding Author

*Tel.: +49 721 60842733. Fax: +49 721 60842335. E-mail: [email protected]. Funding

The authors thank the German Federal Ministry of Education and Research (BMBF) for financial support. Notes

The authors declare no competing financial interest.

■

REFERENCES

(1) Holbrey, J. D.; Seddon, K. R. Ionic Liquids. Clean Products Processes 1999, 1, 223−236. (2) Wasserscheid, P.; Stark, A. Handbook of Green Chemistry, Vol. 6, Ionic Liquids; Wiley-VCH-Verlag: Weinheim, Germany, 2010. (3) Kato, R.; Krummen, M.; Gmehling, J. Measurement and correlation of vapor-liquid equilibria and excess enthalpies of binary systems containing ionic liquids and hydrocarbons. Fluid Phase Equilib. 2004, 224, 47−54. (4) Ren, J.; Zhao, Z.; Zhang, X. Vapor pressures, excess enthalpies, and specific heat capacities of the binary working pairs containing the ionic liquid 1-ethyl-2-methylimidazolium dimethylphosphate. J. Chem. Thermodyn. 2011, 43, 576−583. (5) Predel, T.; Pohrer, B.; Schlücker, E. Ionic liquids as alternative lubricants for special applications. Chem. Eng. Technol. 2010, 33 (1), 132−136. (6) Armand, M.; Endres, F.; MacFarlane, D. R.; Ohno, H.; Scrosati, B. Ionic-liquid materials for the electrochemical challenges of the future. Nat. Mater. 2009, 8, 621−629. (7) Kim, Y. J.; Kim, S.; Joshi, Y. K.; Fedorov, A. G.; Kohl, P. A. Thermodynamic analysis of an absorption refrigeration system with ionic-liquid/refrigerat mixture as a working fluid. Energy 2012, 44, 1005−1016. (8) Schneider, M.-C.; Seiler, M. Nachhaltige Bereitstellung von Absorptionskälte und -wärme durch multifunktionale Arbeitsstoffpaare. Chem. Ing. Techn. 2011, 83 (9), 1502−1509. (9) Ziegler, F. Recent developments and future prospects of sorption heat pump systems. Int. J. Therm. Sci. 1999, 38, 191−208. (10) Constantinescu, D.; Schaber, K.; Agel, F.; Klingele, M. H.; Schubert, T. J. S. Viscosities, vapor pressures and excess enthalpies of choline lactate + water, choline glycolate + water and choline methanesulfonate + water systems. J. Chem. Eng. Data 2007, 52 (4), 1280−1285. (11) Jork, C.; Kristen, C.; Piaraccini, D.; Stark, A.; Chiappe, C.; Beste, Y. A.; Arlt, W. Tailor-made ionic liquids. J. Chem. Themodyn. 2005, 37, 537−558. (12) Rogers, R. D.; Seddon, K. R. Ionic LiquidsSolvents of the future? Science 2003, 302, 792−793. (13) Brennecke, J. F.; Maginn, E. Ionic Liquids: Innovative Fluids for Chemical Processing. AIChE J. 2001, 47, 2384−2389. (14) Römich, Ch.; Merkel, N. C.; Valbonesi, A.; Schaber, K.; Sauer, S.; Schubert, T. J. S. Thermodynamic Properties of Binary mixtures of Water and Room-Temperature Ionic Liquids: Vapor Pressures, Heat Capacities, Densities, and Viscosities of Water + 1-Ethyl-3methylimidazolium Acetate and Water + Diethylmethylammonium Methane Sulfonate. J. Chem. Eng. Data 2012, 57 (8), 2258−2264. (15) Römich, Ch.; Merkel, N.; Schaber, K.; Schubert, T. J. S.; Koch, M. A Comparison Between Lithium bromide−water and Ionic Liquid−Water as Working Solution for Refrigeration Cycles. Proc. 23rd IIR Int. Congr. Refrig. 2011, Paper 287. (16) Wasserscheid, P.; Seiler, M. Leveraging Gigawatt Potentials by Smart Heat-Pump Technologies Using Ionic Liquids. ChemSusChem 2011, 4, 459−463. (17) Yokozeki, A.; Shiflett, M. B. Water Solubility of Ionic Liquids and Application to Absorption Cycles. Ind. Eng. Chem. Res. 2010, 49, 9496−9503.

■

CONCLUSION In this work, VLE data were measured for the binary mixture water + [DEMA][OTf] and for the ternary mixture water + [DEMA][OTf]/[DEMA][OMs] with a mass ratio of 7:3. They were conducted via FTIR spectroscopy and fitted with the NRTL model. Density and viscosity were measured for the systems water + [DEMA][OTf] and [DEMA][OTf] + [DEMA][OMs], and the diffusion coefficients were determined for water in [DEMA][OTf] and in [DEMA][OMs]. Concluding the work, it can be said that [DEMA][OTf] has a lower water vapor pressure reduction capability than [DEMA][OMs] and is therefore inferior regarding the thermodynamic efficiency of an absorption chiller. However concerning the viscosity and diffusion coefficient [DEMA][OTf] is favorable because the enhanced heat and mass transfer have a positive impact on the apparatus design. To combine these preferable properties we investigated a mixture of both RTILs. The determination of the mass ratio of the RTILs in a possible working solution was based on the reduction of the viscosity of [DEMA][OMs] by [DEMA][OTf]. A 7:3 ratio of [DEMA][OTf]/[DEMA][OMs] was regarded suitable. Due to the content of [DEMA][OMs] in the RTIL mixture, the water vapor pressure reduction was enhanced in comparison to [DEMA][OTf]. Both working pairs [DEMA][OMs]−water and [DEMA][OMs]/[DEMA][OTf] (mass ratio 7:3)−water are considered as possible working pairs for absorption chillers. The impact of the different properties on the efficiency of the real process has to be analyzed by future measurements in a test plant. 569



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