Thermophysical Properties of the Binary Mixture 1-Propylpyridinium

Article pubs.acs.org/jced

Thermophysical Properties of the Binary Mixture 1‑Propylpyridinium Tetrafluoroborate with Methanol Mónica García-Mardones, Santiago Martín, Ignacio Gascón, and Carlos Lafuente* Departamento de Química Física, Facultad de Ciencias, Universidad de Zaragoza, 50009 Zaragoza, Spain

ABSTRACT: In this contribution we report a complete thermophysical characterization of the binary mixture 1propylpyridinium tetrafluoroborate + methanol. We have obtained the following properties at p = 1 atm and T = (293.15, 303.15, 313.15, and 323.15) K: densities, refractive indices, speeds of sound, kinematic and absolute viscosities, and ionic conductivities. Moreover, the isothermal vapor−liquid equilibrium at T = (303.15 and 323.15) K has also been determined. From experimental data, excess molar volumes, excess isentropic compressibilities, refractive index deviations, viscosity deviations, and excess Gibbs functions have been calculated. The properties obtained have been analyzed considering structural and energetic contributions in the mixture.

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INTRODUCTION One of the most relevant properties of ionic liquids (ILs) is that they possess low melting temperatures (below 373 K). These compounds have received considerable interest in the past decade, as revealed by the exponential growth of the number of publications related with this field. ILs are mainly investigated as substitutes of traditional volatile organic solvents (VOCs) since they show negligible vapor pressure, high thermal stability, good solvation, and elevated ionic conductivity. Moreover, ILs are promising candidates to be used in different areas like synthesis, catalysis, electrochemistry, or engineering, between other possible applications.1−5 However, their use is still scarce, being one of the reasons the limited knowledge of their physicochemical properties. In this sense, studies that report accurate values of these properties could be very useful to acquire a deeper understanding about the structure of the ILs and the interactions at molecular level. Another important problem for industrial application of ILs is their high viscosity. To solve this difficulty, ILs can be mixed with different solvents. Binary mixtures present lower viscosity than pure ILs and can be useful for a variety of applications, although it is necessary to characterize their physicochemical properties to evaluate more specifically their potential uses. The most commonly used IL contains the imidazolium ring. There are several publications related with the characterization of the thermophysical properties of these ILs also pure and mixed with different solvents.6−9 In this framework, we have © 2014 American Chemical Society

carried out a deep study about the thermophysical properties of the binary mixture 1-propylpyridinium tetrafluoroborate, [ppy][BF4], plus methanol. The following properties were determined: densities, refractive indices, speeds of sound, kinematic and absolute viscosities, and ionic conductivity at p = 1 atm and T = (293.15, 303.15, 313.15, and 323.15) K and isothermal vapor−liquid equilibrium at T = (303.15 and 323.15) K. The corresponding excess or deviation properties were also obtained. It can be mentioned that this ionic liquid is not miscible with higher alkanols. We also have compared the behavior of the mixture under study with the properties previously reported for the system 1-butylpyridinium tetrafluoroborate [bpy][BF4] + methanol.10−13 This comparison allows evaluating the influence of the cation alkyl chain length in the properties of the mixture. As far as we know, there has not been published any previous paper related with the thermophysical characterization of the system under study.

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EXPERIMENTAL SECTION In Table 1 the information about the ionic liquid and alkanol used in this work is summarized. To eliminate the maximum quantity of water from the ionic liquid, it was dried in vacuum Received: December 31, 2013 Accepted: March 18, 2014 Published: March 25, 2014 1564

dx.doi.org/10.1021/je401120a | J. Chem. Eng. Data 2014, 59, 1564−1573

Journal of Chemical & Engineering Data

Article

measured using a thermometer F250 (Automatic Systems Laboratories) and controlled within ± 0.01 K by means of a Lauda E-200 thermostat. As recommended, the cell calibration is carried out using aqueous KCl solutions of different concentrations supplied by CRISON. The estimated uncertainty of conductivity measurements is ± 1 %. The exact composition of the liquid mixtures was determined by mass difference, measured using a Sartorius semimicro balance CP225-D within ± 1·10−5 g. This gives an uncertainty in the mole fraction of ± 1·10−4. All of the mixtures were prepared in glass vials just before property measurements to prevent composition modification. The vapor−liquid equilibrium was determined using a commercial Labodest unit by Fischer. This is a dynamic recirculating type still made in glass and equipped with a Cotrell pump. The temperature of equilibrium was measured with a F25 thermometer from Automatic Systems Laboratories, equipped with a PT100 probe. To measure pressure in the still we used a Digiquartz 735-215A-102 pressure transducer (Paroscientific) equipped with a Digiquartz 735 display unit. The uncertainty in the temperature and pressure equilibrium measurement in the still is ± 0.02 K and ± 0.01 kPa, respectively. The composition of the phases in equilibrium was determined measuring their density. From these analyses, the error in the mole fraction was estimated to be ± 0.0005. The properties of the pure liquids at the three temperatures are collected in Table 2.

Table 1. Provenance and Purity of the Liquid Components chemical name 1-propylpyridinium tetrafluoroborate methanol

source

initial purity (mass fraction)

purification method

IoLiTec

0.98

none

Aldrich

0.998

none

(0.05 kPa) stirring continuously during 24 h before its use and stored in desiccators. After this, the water content of [ppy][BF4] was less than 100 ppm, as determined using the Karl Fischer method with an automatic titrator Crison KF 1S2B; on the other hand, the halide content was checked by 19F NMR, the content being less than 100 ppm. Densities, ρ, and speeds of sound, u, of all of the samples were measured at the same time using an Anton Paar DSA 5000 densimeter and sound analyzer with temperature control within ± 0.005 K. During density measurement, this apparatus makes an automatic correction based on the viscosity of the sample. Densimeter calibration was made using dry air and ultrapure water provided by SH Calibration service GmbH. Previous studies have shown that the uncertainty of ρ and u measurements is ± 5·10−2 kg·m−3 and ± 0.5 m·s−1, respectively. The refractive indices, nD, were measured at 589.3 nm sodium wavelength with an automatic refractometer AbbematHP DR. Kernchen, that allows to obtain an uncertainty in nD measurements of ± 5·10−6, after proper calibration using deionized double-distilled water. The sample temperature during measurement is controlled by a Peltier device in an intervals of ± 0.02 K. Simultaneously, an additional Peltier thermostat controls the internal temperature of the refractometer. Kinematic viscosities, ν, were measured using three models of Ubbelohde viscosimeters together with a Schoot-Geräte AVS-440 automatic measuring unit. A Schoot-Geräte CT 1150/ 2 thermostat controls the temperature of the samples within ± 0.01 K. The viscosimeter constants, provided by the supplier, were k = 0.3213 mm2·s−2, 0.0209 mm2·s−2, and 0.00929 mm2· s−2. The expression used to calculate the kinematic viscosities includes the correction due to kinetic energy. Once density and kinematic viscosity of the samples are known, the absolute viscosity, η, can be obtained using: η = ρ·ν. It has been estimated that the uncertainty in the absolute viscosity is ± 0.5 %. A Crison conductimeter, model GLP31, was used to measure ionic conductivities, κ. The temperature of the samples was

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RESULTS AND DISCUSSION The values of all of the properties experimentally determined can be found in Tables 3 to 7 together with calculated properties. Excess volumes, VE, were obtained from the density of the mixture, ρ, the mole fraction of the components in the mixture, xi, and the molar masses, Mi, and densities, ρi, of the pure compounds, using the well-known equation: VE =

⎛1

∑ xiMi⎜⎜ i

⎝ρ

−

1⎞ ⎟⎟ ρi ⎠

(1)

Under the experimental conditions used ultrasonic absorption is negligible, consequently isentropic compressibility, κS, and excess isentropic compressibility, κES , can be obtained using experimental density, ρ, and speed of sound, u, values:

Table 2. Thermophysical Properties of Pure Components at Several Temperaturesa T K

ρ kg·m

u −3

αp

Cp −1

m·s

293.15 303.15 313.15 323.15

1256.673 1249.537 1242.427 1235.309

1653.87 1629.76 1606.32 1583.31

293.15 303.15 313.15 323.15

791.243 781.813 772.287 762.628

1118.83 1085.99 1053.55 1021.53

−1

J·mol ·K 361 366 372 377 80.19 82.14 84.31 86.73

−1

kK

η

−1

nD

1-Propylpyridinium Tetrafluoroborate 0.584 1.445883 0.579 1.443220 0.574 1.440566 0.568 1.437925 Methanol 1.1873 1.328426 1.2109 1.324333 1.2405 1.320152 1.2765 1.316700

κ −1

mPa·s

mS·cm

160.00 90.750 56.021 36.989

3.02 5.16 7.99 11.54

0.5833 0.5070 0.4438 0.3873

p

B·106

kPa

m3·mol−1

21.930

−1828

55.785

−1167

Standard uncertainties u are u(T) = 0.01 K and u(p) = 0.01 kPa, and the combined expanded uncertainties Uc are Uc(ρ) = 5·10−2 kg·m−3, Uc(u) = 0.5 m·s−1, Uc(nD) = 5·10−6, Uc (η) = 0.5 %, and Uc(κ) = 1 % with a 0.95 level of confidence (k ≈ 2). a

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Table 3. Experimental Densities, ρ, Speeds of Sound, u, and Isentropic Compressibilities, κS, and Calculated Excess Volumes, VE, and Excess Isentropic Compressibilities, κES , for the Binary Mixture 1-Propylpyridinium Tetrafluoroborate (1) + Methanol (2) at Atmospheric Pressure and at Several Temperaturesa T/K = 293.15 ρ x1

kg·m

κS

u −3

−1

m·s

0.0000 0.0280 0.0577 0.0967 0.1459 0.1952 0.3213 0.4002 0.4965 0.5988 0.6904 0.8011 0.8824 0.9400 1.0000

791.243 846.441 893.625 944.389 995.788 1036.487 1110.622 1143.048 1173.266 1198.326 1216.195 1233.621 1244.232 1250.692 1256.673

1118.83 1164.91 1208.52 1257.23 1311.09 1362.87 1458.01 1496.48 1541.51 1575.71 1598.38 1621.48 1637.09 1646.39 1653.87

0.0000 0.0280 0.0577 0.0967 0.1459

772.287 829.037 876.892 927.831 979.474

1053.55 1104.32 1151.45 1204.80 1261.31

0.1952 0.3213 0.4002 0.4965 0.5988 0.6904 0.8011 0.8824 0.9400 1.0000

1020.192 1094.597 1127.344 1157.839 1183.217 1201.318 1218.896 1229.831 1236.333 1242.427

1314.30 1409.27 1453.16 1492.67 1527.33 1550.31 1573.60 1589.41 1598.55 1606.32

TPa

T/K = 303.15 V ·10 E

−1

1009.63 870.60 766.19 669.91 584.21 519.43 423.56 390.66 358.68 336.10 321.84 308.32 299.88 294.97 290.92 T/K = 313.15 1166.57 989.09 860.13 742.51 641.75 T/K = 313.15 567.45 460.00 420.07 387.64 362.30 346.34 331.32 321.87 316.53 311.94

m ·mol 3

κES

6 −1

TPa

−1

−0.3100 −0.4734 −0.6141 −0.7477 −0.8151 −0.8768 −0.8597 −0.7690 −0.6704 −0.5593 −0.3988 −0.2665 −0.1450

−69.50 −110.00 −134.55 −145.30 −148.15 −126.17 −105.17 −84.79 −63.40 −45.48 −27.00 −15.54 −7.90

−0.4120 −0.6178 −0.7675 −0.9071

−96.00 −149.81 −182.89 −195.00

−0.9612 −0.9923 −0.9655 −0.8547 −0.7405 −0.6133 −0.4222 −0.2950 −0.1550

−195.81 −162.80 −138.32 −108.06 −80.47 −57.80 −34.21 −19.66 −9.85

ρ kg·m

κS

u −3

−1

m·s

781.813 837.960 885.479 936.347 987.600 1028.349 1102.548 1135.039 1165.530 1190.700 1208.714 1226.215 1236.991 1243.502 1249.537

1085.99 1134.45 1179.75 1230.94 1286.16 1339.40 1433.17 1477.19 1516.74 1550.87 1573.96 1597.31 1612.65 1622.37 1629.76

762.828 820.035 867.868 918.920 970.736

1021.53 1073.80 1123.48 1178.72 1236.95

1011.680 1086.600 1119.585 1150.298 1175.762 1193.836 1211.543 1222.551 1229.151 1235.309

1290.96 1385.42 1429.49 1467.34 1503.92 1527.08 1550.54 1566.45 1574.96 1583.31

TPa

VE·106 −1

1084.54 927.27 811.41 704.84 612.11 542.05 441.58 403.75 372.95 349.18 333.96 319.64 310.85 305.53 301.30 T/K = 323.15 1256.57 1057.60 912.88 783.25 673.28 T/K = 323.15 593.10 479.48 437.10 403.76 376.04 359.20 343.32 333.35 327.99 322.92

m ·mol 3

−1

κES TPa−1

−0.3690 −0.5535 −0.7001 −0.8217 −0.8852 −0.9273 −0.8978 −0.8082 −0.6975 −0.5815 −0.4059 −0.2766 −0.1500

−82.01 −128.57 −157.34 −168.52 −171.20 −143.10 −122.01 −95.57 −71.15 −51.19 −30.35 −17.35 −8.90

−0.4600 −0.6679 −0.8222 −0.9656

−110.75 −173.87 −211.66 −225.08

−1.0242 −1.0627 −1.0363 −0.9216 −0.7922 −0.6407 −0.4378 −0.3009 −0.1601

−225.03 −185.12 −156.80 −121.38 −91.01 −65.28 −38.69 −22.18 −10.85

a Standard uncertainties u are u(T) = 0.005 K, and u(x1) = 0.0001, and the combined expanded uncertainties Uc are Uc(ρ) = 5·10−2 kg·m−3 and Uc(u) = 0.5 m·s−1 with a 0.95 level of confidence (k ≈ 2).

1 ρu 2

(2)

κSE = κS − κSid

(3)

κS =

measured in our laboratory, while literature values15,16 were used for molar heat capacities. We have plotted excess volumes and excess isentropic compressibilities in Figures 1 and 2. From refractive indices of the mixture the corresponding refractive index deviations in mole fraction can be calculated by means of the following equation:

where:

and the ideal isentropic compressibility, κidS , is given by the following expression obtained by Benson and Kiyohara:14 κSid =

⎡

∑ ϕi⎢κS ,i + i

⎢⎣

(∑i ϕα )2 TVi αi2 ⎤ i i ⎥ − T (∑ xiVi ) Cp , i ⎥⎦ (∑i xiCp , i) i

ΔnD = nD −

∑ xinD,i i

(5)

(4)

where nD and nD,i are refractive index of the mixture and component i, respectively. Refractive index deviations are represented in Figure 3. The four-body McAllister equation17 has been used to correlate kinematic viscosities. This equation was chosen because it allows correlation of systems that present great differences in size between the components:

in this equation ϕi and xi are, respectively, the volume fraction and the mole fraction of component i in the mixture, T is the temperature in K, and Vi, αp,i, Cp,i, and κS,i are, respectively, the following properties of the pure component i: molar volume, isobaric expansibility molar heat capacity, and constant pressure and isentropic compressibility. All of the properties used in this equation are gathered in Table 2. Isobaric expansibilities and molar volumes have been obtained using density values 1566



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Table 4. Experimental Refractive Indices, nD, and Calculated Refractive Index Deviations, ΔnD, for the Binary Mixture 1Propylpyridinium Tetrafluoroborate (1) + Methanol (2) at Atmospheric Pressure and at Several Temperaturesa T/K = 293.15 x1

nD

0.0000 0.0279 0.0576 0.0965 0.1457 0.1949 0.2548 0.3209 0.3998 0.4960 0.5983 0.6901 0.8007 0.8821 1.0000

1.328426 1.343770 1.356573 1.369876 1.382831 1.393009 1.402804 1.411212 1.418896 1.425954 1.431760 1.436013 1.440140 1.442654 1.445883

T/K = 303.15 ΔnD

nD

0.012067 0.021381 0.030115 0.037292 0.041691 0.044450 0.045094 0.043511 0.039269 0.033059 0.026530 0.017666 0.010619

1.324333 1.339898 1.353187 1.366933 1.380061 1.390301 1.400012 1.408391 1.415916 1.423155 1.428902 1.433205 1.437377 1.439966 1.443220

T/K = 313.15 ΔnD

nD

0.012248 0.022006 0.031116 0.038406 0.042797 0.045387 0.045895 0.044064 0.039854 0.033427 0.026840 0.017839 0.010727

1.320152 1.336099 1.349941 1.364087 1.377593 1.388060 1.397549 1.405780 1.413040 1.420265 1.426092 1.430340 1.434607 1.437371 1.440566

T/K = 323.15 ΔnD

nD

ΔnD

0.012587 0.022853 0.032303 0.039897 0.044439 0.046716 0.046975 0.044759 0.040388 0.033884 0.027102 0.018027 0.010966

1.316700 1.333043 1.347535 1.362152 1.375970 1.386333 1.395767 1.403733 1.410938 1.417700 1.423406 1.427715 1.431926 1.434640 1.437925

0.012961 0.023852 0.033742 0.041608 0.046006 0.048179 0.048120 0.045784 0.040872 0.034165 0.027370 0.018149 0.010971

Standard uncertainties u are u(T) = 0.01 K, and u(x1) = 0.0001, and the combined expanded uncertainty Uc is Uc(nD) = 5·10−6 with 0.95 level of confidence (k ≈ 2).

a

Table 5. Experimental Kinematic Viscosities, ν, and Calculated Absolute Viscosities, η, and Viscosity Deviation, Δη, for the Binary Mixture 1-Propylpyridinium Tetrafluoroborate (1) + Methanol (2) at Atmospheric Pressure and at Several Temperaturesa x1

T/K = 293.15

0.0000 0.0505 0.0993 0.1910 0.2910 0.3918 0.4929 0.6117 0.7331 0.7923 0.8425 0.8956 0.9447 1.0000

T/K = 303.15

T/K = 313.15

T/K = 323.15

ν

η

Δη

ν

η

Δη

ν

η

Δη

ν

η

Δη

mm2·s−1

mPa·s

mPa·s

mm2·s−1

mPa·s

mPa·s

mm2·s−1

mPa·s

mPa·s

mm2·s−1

mPa·s

mPa·s

0.7372 1.4350 2.660 5.5450 8.8852 12.627 17.280 24.375 37.780 49.806 62.872 80.207 100.38 127.32

0.5833 1.2671 2.5204 5.7303 9.7370 14.394 20.258 29.276 46.219 61.380 77.913 99.919 125.55 160.00

−7.367 −13.893 −25.302 −37.236 −48.649 −58.902 −68.823 −71.233 −65.509 −56.978 −43.438 −25.629

0.6485 1.0750 1.8767 3.5876 5.7707 7.9110 10.869 15.614 24.214 31.016 38.816 48.401 59.073 72.627

0.5070 0.9388 1.7602 3.6743 6.2771 8.9544 12.658 18.635 29.442 37.996 47.818 59.946 73.487 90.750

−4.125 −7.708 −14.069 −20.491 −26.910 −32.330 −37.074 −37.222 −34.011 −28.719 −21.383 −12.273

0.5747 0.8750 1.3674 2.4808 3.9848 5.3838 7.3480 10.520 16.547 20.776 25.933 31.566 37.712 45.090

0.4438 0.7578 1.2731 2.5233 4.3028 6.0520 8.5010 12.476 19.999 25.299 31.756 38.868 46.645 56.021

−2.493 −4.690 −8.536 −12.314 −16.167 −19.337 −21.964 −21.188 −19.178 −15.510 −11.351 −6.303

0.5078 0.7850 1.0944 1.8987 3.0367 4.0088 5.3340 7.5609 11.879 14.973 18.394 22.006 25.793 29.943

0.3873 0.6730 1.0093 1.9153 3.2545 4.4752 6.1305 8.9104 14.269 18.123 22.393 26.936 31.717 36.989

−1.563 −3.013 −5.463 −7.784 −10.253 −12.298 −13.866 −12.951 −11.263 −8.832 −6.231 −3.248

a Standard uncertainties u are u(T) = 0.01 K and u (x1) = 0.0001, and the combined expanded uncertainty Uc is Uc(η) = 0.5 % with a 0.95 level of confidence (k ≈ 2).

the molar masses of the pure compounds, and ν1112, ν1122, and ν2221 are the adjustable parameters of the model. The values of these parameters and the standard deviation at each temperature can be found in Table 8. The overall standard deviation is 0.14 mm2·s−1, the deviation at the lower temperature being slightly higher. Taking into account that viscosity correlation in a mixture becomes more difficult when the viscosities of the components are so different, we can see that eq 7 is adequate to correlate viscosity data of mixtures formed by ionic liquids and alkanols. Experimental kinematic viscosities together with correlated values have been represented in Figure 4. The viscosity deviation is a magnitude widely used to describe the variation of the viscosity of a liquid mixture with composition,18−21 viscosity deviations can be calculated by using the following equation:

ln ν = x14 ln ν1 + 4x13x 2 ln ν1112 + 6x12x 22 ln ν1122 ⎡ M ⎤ + 4x1x 23 ln ν2221 + x 24 ln ν2 − ln⎢x1 + x 2 2 ⎥ M1 ⎦ ⎣ ⎡ (3 + M 2 /M1) ⎤ 2 + 4x13x 2 ln⎢ ⎥ + 6x1 ⎣ ⎦ 4 ⎡ (1 + 3M 2 /M1) ⎤ ⎡ (1 + M 2 /M1) ⎤ 3 x 22 ln⎢ ⎥ + 4x1x 2 ln⎢ ⎥ ⎦ ⎣ ⎦ ⎣ 2 4 ⎡M ⎤ + x 24 ln⎢ 2 ⎥ ⎣ M1 ⎦ (6)

where ν, ν1, and ν2 are kinematic viscosities of the mixture, component 1, and component 2, respectively, M1 and M2 are 1567



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Table 6. Experimental Conductivities, κ, for the Binary Mixture 1-Propylpyridinium Tetrafluoroborate (1) + Methanol (2) at Atmospheric Pressure and at Several Temperaturesa

In relation with the isothermal vapor−liquid equilibrium, it should be underlined that ionic liquids have very low vapor pressures. For this reason, the partial pressure of the IL can be considered zero, and the total vapor pressure of the mixture is the partial pressure of methanol.23 The activity coefficients of the components in the liquid mixture were fitted using the Wilson equation:24

κ/mS·cm−1 x1

T/K = 293.15

T/K = 303.15

T/K = 313.15

T/K = 323.15

0.0313 0.0365 0.0422 0.0658 0.0775 0.0894 0.1294 0.1409 0.1703 0.2002 0.2456 0.2819 0.3507 0.3937 0.4334 0.4640 0.5398 0.6350 0.7918 0.8940 0.9384 1.0000

25.8 28.4 30.7 36.6 38.6 40.5 43.7 44.0 44.8 44.1 42.2 39.3 34.6 31.3 27.7 25.5 19.83 14.58 6.39 4.22 3.55 3.02

29.9 32.8 35.9 42.9 45.6 48.4 51.5 52.5 53.8 53.5 51.6 48.0 43.1 39.4 35.2 31.6 25.9 20.1 10.38 7.02 6.25 5.16

34.1 37.7 41.3 49.5 52.7 55.9 60.3 61.2 63.0 62.6 60.5 56.8 51.5 47.6 43.2 39.1 32.6 25.0 15.01 10.51 9.40 7.99

38.7 42.5 46.8 56.2 60.0 63.7 69.8 70.5 72.3 72.0 70.0 66.4 60.5 56.3 51.2 47.8 39.7 31.5 19.60 14.82 13.36 11.54

ln γi = −ln(∑ xj Λij) + 1 − j

Λij =

i

A + Bx1 + Cx12 + Dx13 1 + Ex1

⎛ (V 0 − B ) · (p − p 0 ) ⎞ 2 22 2 ⎟ pcal = x 2γ2p20 exp⎜⎜ ⎟ RT ⎠ ⎝

where Q is the excess or deviation property, Ai adjustable parameters, and x1 and x2 are the mole ionic liquid and alkanol, respectively. All of parameters and resultant standard deviations, shown in Table 8.

(12)

(13)

where x2 is the methanol mole fraction in the liquid mixture, p is the total pressure, and p02, V02, and B22, are, respectively, the following properties of methanol: vapor pressure, molar volume, and second virial coefficient. These data are collected in Table 2. B22 values are given in TRC tables.25 Wilson parameters and average deviations in pressure, Δp, obtained in the correlations can be found in Table 8. The p−x1 diagrams and the excess Gibbs functions are graphically represented in Figures 7 and 8. Taking account all of the properties obtained, we have carried out a detailed comparison between the systems [ppy][BF4] or [bpy][BF4] + methanol, to analyze the effect of the length of the alkyl chain attached to the pyridine nitrogen in the behavior of the mixtures containing ILs. As reflected in Figures 1 and 2, excess volumes and excess isentropic compressibilities show negative values in the complete composition range and become more negative when temperature rises. These excess properties are relatively high in absolute value, besides all of the curves obtained are remarkably asymmetric and their minimum values are shifted toward the region rich in methanol. When this behavior is compared with the mixture containing [bpy][BF4], it can be remarked that both systems present similar values for these properties, being excess volumes slightly more negative in the mixture with [bpy][BF4] and excess isentropic compressibilities are a little more negative in the system containing [ppy][BF4]. Both excess properties depend on structural and energetic effects in the mixture. Negative values can be explained by the accommodation of methanol molecules in the interstices of the IL network26,27 and the attractive interaction between unlike

(7)

(8)

∑i = 0 Ai (x1 − x 2)i 1 + ∑j = 1 Bj (x1 − x 2) j

(11)

where p is the calculated pressure, which were obtained using the following equation that reflects the lack of ideality of the vapor phase and the variation of the Gibbs function with pressure:

where A, B, C, D, and E are adjustable parameters and x1 is the mole fraction of the ionic liquid in the mixture. These parameters along with conductivity standard deviations are given in Table 8. The ionic conductivities are plotted in Figure 6. Excess volumes, excess isentropic compressibilities, refractive index deviations, and viscosity deviations have been fitted using the following equation:22 Q = x1x 2

(10)

cal

where η and ηi are absolute viscosities of the mixture and component i, respectively. Viscosity deviations are represented in Figure 5. Ionic conductivity, κ, has been correlated using a classical polynomial equation modified to fit the asymmetric behavior of the mixture:12 κ=

⎛ λij − λii ⎞ V ◦j ⎟ ◦ exp⎜ − Vi RT ⎠ ⎝

⎛ pexp − pcal ⎞ F = ∑ ⎜⎜ i exp i ⎟⎟ pi ⎠ i ⎝

Standard uncertainties u are u(T) = 0.01 K and u(x1) = 0.0001, and the combined expanded uncertainty Uc is Uc(κ) = 1 % with a 0.95 level of confidence (k ≈ 2).

∑ xiηi

k

xk Λki ∑j xj Λkj

where γi, V°i , and xi represents, respectively, activity coefficients, molar volumes, and mole fractions of component i in the liquid mixture; (λij − λii) are Wilson parameters, R is the gas constant, and T is the absolute temperature. To obtain the adjustable parameters of the Wilson equation, the following objective function was minimized in the whole range of experimental data:

a

Δη = η −

∑

(9)

and Bj are fractions of the fitting σ(Q), are 1568



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Table 7. Isothermal VLE Data for Binary Mixture 1-Propylpyridinium Tetrafluoroborate (1) + Methanol (2) at Several Temperatures: Experimental Pressures, p, and Liquid-Phase Compositions, x1, and Correlated Activity Coefficients, γi, and Excess Gibbs Functions, GEa T/K = 303.15

a

x1

p/kPa

0.0000 0.0278 0.0388 0.0433 0.0529 0.0614 0.0744 0.0810 0.0978 0.1145 0.1295 0.1481 0.1752 0.2321 0.2618 0.3065 0.3678 0.4196 0.4715 0.5029 0.5301

21.930 21.430 21.405 21.385 21.295 21.175 21.095 21.210 21.180 20.978 20.865 20.580 20.120 19.580 19.130 18.360 17.070 15.950 14.705 13.750 13.185

0.5695 0.6210 0.6789 0.7101

12.150 10.855 9.100 8.375

T/K = 323.15

γ1

γ2

4.742 3.778 3.500 3.037 2.734 2.393 2.258 1.993 1.805 1.677 1.555 1.425 1.257 1.201 1.142 1.088 1.060 1.040 1.031 1.025 T/K = 303.15 1.018 1.011 1.006 1.005

1.013 1.021 1.024 1.032 1.038 1.048 1.053 1.066 1.079 1.090 1.103 1.122 1.158 1.175 1.199 1.229 1.250 1.269 1.279 1.287 1.298 1.311 1.322 1.328

E

G /J·mol

−1

x1

p/kPa

141 180 195 223 244 274 287 316 340 358 376 396 418 422 420 407 388 364 347 332

0.0000 0.0281 0.0375 0.0438 0.0541 0.0602 0.0671 0.0841 0.0994 0.1014 0.1322 0.1452 0.1641 0.2229 0.2589 0.3003 0.3606 0.4091 0.4600 0.4935 0.5295

55.785 54.595 54.250 53.930 53.780 53.575 53.550 52.890 52.565 52.530 51.725 51.570 50.905 48.715 47.020 45.270 42.150 39.050 36.235 33.995 31.910

309 276 237 215

0.5805 0.6581 0.6803 0.7120

28.675 23.575 22.025 19.985

γ1

γ2

GE/J·mol−1

3.489 3.036 2.803 2.502 2.357 2.218 1.955 1.783 1.764 1.537 1.468 1.387 1.225 1.165 1.117 1.071 1.048 1.031 1.023 1.017 T/K = 323.15 1.010 1.004 1.003 1.002

1.008 1.013 1.016 1.022 1.026 1.030 1.041 1.051 1.052 1.071 1.079 1.090 1.123 1.141 1.159 1.183 1.200 1.215 1.223 1.231

116 145 163 189 203 218 250 274 277 313 324 338 363 368 367 356 340 320 304 286

1.241 1.253 1.255 1.259

259 214 201 182

Standard uncertainties u are u(T) = 0.02 K, u(p) = 0.01 kPa, and u(x1) = 0.0005 with a 0.95 level of confidence (k ≈ 2).

Figure 2. Excess isentropic compressibilities, κES , for 1-propylpyridinium tetrafluoroborate (1) + methanol (2) as a function of mole fraction, x1: ■, T = 293.15 K; □, T = 303.15 K; ●, T = 313.15 K; ○, T = 323.15 K; , eq 10.

Figure 1. Excess volumes, VE, for 1-propylpyridinium tetrafluoroborate (1) + methanol (2) as a function of mole fraction, x1: ■, T = 293.15 K; □, T = 303.15 K; ●, T = 313.15 K; ○, T = 323.15 K; , eq 10.

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Figure 4. Kinematic viscosities, ν, for 1-propylpyridinium tetrafluoroborate (1) + methanol (2) as a function of mole fraction, x1: ■, T = 293.15 K; □, T = 303.15 K; ●, T = 313.15 K; ○, T = 323.15 K; , four-body McAllister equation.

Figure 3. Refractive index deviations, ΔnD, for 1-propylpyridinium tetrafluoroborate (1) + methanol (2) as a function of mole fraction, x1: ■, T = 293.15 K; □, T = 303.15 K; ●, T = 313.15 K; ○, T = 323.15 K; , eq 10.

Table 8. Adjusted Parameters and Standard Deviations, σ(Q), or Deviations in Pressure, Δp, for Fitting Equations function

T/K

A0

A1

A2

A3

B1

σ(Q)

VE·106/m3·mol−1

293.15 303.15 313.15 323.15 293.15 303.15 313.15 323.15 293.15 303.15 313.15 323.15 293.15 303.15 313.15 323.15

−3.0917 −3.2294 −3.4319 −3.6829 −335.28 −380.23 −429.93 −484.97 0.156495 0.158396 0.160360 0.162500 −241.384 −132.432 −79.396 −50.651

−1.0671 −1.0250 −0.9880 −1.0487 169.02 200.50 228.42 265.10 0.011048 0.001281 −0.011781 −0.023520 −201.635 −104.14 −59.194 −36.878

0.0303 −0.0456 −0.0720 0.2074 −56.57 −68.71 −90.65 −128.30 −0.012748 −0.002378 0.012313 0.023781 −113.936 −43.396 −13.975 −0.325 ν1122

−1.1545 −1.0003 −1.0477 −1.0039 −18.70 −23.20 −6.03 19.05 0.011451 0.001688 −0.006274 −0.014171 9.682 21.986 25.031 26.026

0.9583 0.9453 0.9418 0.9552 0.84 0.84 0.84 0.83 0.760846 0.700027 0.632873 0.583290

0.0054 0.0047 0.0097 0.0098 1.13 0.94 0.79 1.02 0.000054 0.000058 0.000077 0.000090 0.698 0.441 0.308 0.218 σ(ν)

κES /TPa−1

ΔnD

Δη/mPa·s

function

T/K

ν1112

ν/mm2·s−1

293.15 303.15 313.15 323.15

48.4143 32.0141 22.9560 17.9017

function

T/K

κ/mS·cm−1

293.15 303.15 313.15 323.15

function GE/J·mol−1

A 8.375 8.713 9.388 10.109 T/K 303.15 323.15

B

6.6173 4.7647 3.4208 2.3496 C

835.00 995.72 1158.43 1322.74

−1653.44 −1886.79 −2112.13 −2326.72

ν2221 49.6722 23.6935 13.2050 9.5185 D

E 8.219 8.211 8.269 8.275

λ12 − λ11

839.8 933.1 1022.0 1104.7 λ21 − λ22

5639.01 4100.25

1854.22 1836.44

1570

0.2200 0.1179 0.1205 0.0969 σ(κ) 0.32 0.59 0.54 0.44 Δp/kPa 0.095 0.124



Article

Figure 5. Viscosity deviations, Δη, for 1-propylpyridinium tetrafluoroborate (1) + methanol (2) as a function of mole fraction, x1: ■, T = 293.15 K; □, T = 303.15 K; ●, T = 313.15 K; ○, T = 323.15 K; , eq 10

Figure 7. p−x1 diagram for 1-propylpyridinium tetrafluoroborate (1) + methanol (2): □, T = 303.15 K; ○, T = 323.15 K; , Wilson equation.

Figure 8. Excess Gibbs functions, GE, for 1-propylpyridinium tetrafluoroborate (1) + methanol (2) as a function of mole fraction, x1: , T = 303.15 K; ----, T = 323.15 K.

Figure 6. Conductivities, κ, for 1-propylpyridinium tetrafluoroborate (1) + methanol (2) as a function of mole fraction, x1: ■, T = 293.15 K; □, T = 303.15 K; ●, T = 313.15 K; ○, T = 323.15 K; , eq 9.

smaller velocity in these systems, and the refractive index is bigger than in an ideal mixture.29 Viscosity deviations shown in Figure 5 are negative for any composition of the mixture and become less negative when temperature rises. This behavior is characteristic of mixtures without strong specific interactions between their components.30 The curves obtained are clearly asymmetric, and the minimum Δη values are shifted toward the rich region in ionic

molecules that contributes to produce a volume contraction.28 Finally, the asymmetry of the curves is due to the high differences that exists between the molar volumes of the pure compounds. Figure 3 shows that refractive index deviation values are positive and the curves obtained are also asymmetric; moreover ΔnD increases when temperature rises. The light propagates at 1571



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liquid, xIL ≈ 0.7. Comparing these results with those reported for the binary system containing [bpy][BF4], it can been remarked that both systems behave quite similarly but Δη values are more negatives in the mixture containing [bpy][BF4]. This property depends on the size and shape of the compounds and the interaction between the components of the mixture.28,29,31 Negative values of Δη denote that the Coulombic forces between ions decrease by the presence of methanol in the mixture. Additionally, the bigger size of the cation [bpy] compared to [ppy] is responsible of the more negative values of Δη in the mixture containing [bpy][BF4]. Figure 6 shows the ionic conductivities of the mixture, which increase when temperature rises due to the increment of the mobility of the ions. Conductivity curves are similar to those obtained for other binary systems that contain ionic liquids based on pyridinium and methanol: starting from the pure IL, ionic conductivity continuously rises when methanol is added, because viscosity of the mixture diminishes and the mobility of charge carriers rises,32 until the maximum of the curve is reached at xIL ≈ 0.17. In the region with low IL concentration the dominant effect is aggregation; consequently the number of charge carriers is smaller and conductivity diminishes.33,34 The system containing [ppy][BF4] presents higher κ values than the mixture with [bpy][BF4] as it could be expected, since ionic conductivity, for ILs containing the same anion, depends on the size of the cation; that is, the smaller the cation is, the higher the ionic conductivity of the IL is. The vapor pressures of the liquid mixtures are given in Figure 7. As it can be observed, vapor pressure diminishes when the mole fraction of the IL in the mixture increases. From experimental data, activity coefficients and excess Gibbs function values have been calculated: activity coefficients are greater than unity over the entire temperature and composition ranges studied therein; thus the system shows positive deviation from the ideal behavior (positive GE values as can be observed in Figure 8) confirming that the interactions of IL−methanol cannot compensate the weakening of the Coulombic interactions. GE curves are slightly asymmetric, with the maximum displaced toward the region poor in IL; additionally GE values rise when temperature decreases. Also it should be commented that GE values are bigger in the system containing [bpy][BF4] than in the mixture with [ppy][BF4] due to the bigger size of the butylpyridinium cation.

Article

AUTHOR INFORMATION

Corresponding Author

*Tel.: +34976762295. Fax: +34976761202. E-mail: celadi@ unizar.es. Funding

We are grateful for financial assistance from Gobierno de Aragón and Fondo Social Europeo “Construyendo Europa desde Aragón”. Notes

The authors declare no competing financial interest.

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REFERENCES

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CONCLUSIONS This paper reports a complete thermophysical characterization of the binary mixture formed by an ionic liquid, 1propylpyridinium tetrafluoroborate, and a short chain alkanol, methanol. The following properties have been determined: density, speed of sound, refractive index, absolute viscosity, ionic conductivity, and vapor pressure. From experimental data we have obtained excess volume, excess isentropic compressibility, refractive index deviation, viscosity deviation, and excess Gibbs function. Moreover, we have compared the behavior of this system with the mixture 1-butylpyridinium tetrafluoroborate + methanol, previously characterized. In both systems the Coulombic forces between ions decrease with the addition of methanol. This effect is more marked in the mixture containing [bpy][BF4 ] due to the bigger size of the cation 1butylpyridinium. Moreover the system with [bpy][BF4] presents lower ionic conductivity and higher viscosity. 1572



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Thermophysical Properties of the Binary Mixture 1-Propylpyridinium

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