Article pubs.acs.org/jced
Thermodynamic and Topological Studies of 1‑Ethyl-3-methylimidazolium Tetrafluoroborate + Pyrrolidin-2-one and 1‑Methyl-pyrrolidin-2-one Mixtures Dimple Sharma,† Soniya Bhagour,‡ and V. K. Sharma*,‡ †
Prabhu Dayal Memorial (P. D. M.) College of Engineering for Women, Bahadurgarh, Haryana, India Department of Chemistry, Maharshi Dayanand (M. D.) University, Rohtak, Haryana, India
‡
ABSTRACT: Excess molar enthalpies, HE of 1-ethyl-3-methylimidazolium tetrafluoroborate (1) + pyrrolidin-2-one or 1-methyl pyrrolidin-2-one (2) binary mixtures at 298.15 K, densities, ρ, and speeds of sound, u, data of the same mixtures at (293.15, 298.15, 303.15, and 308.15) K have been measured over entire mole fraction range. The observed densities and speeds of sound data have been utilized to determine their excess molar volumes, VE, and excess isentropic compressibilities, κES . The topology of the constituents of mixtures (graph theory) has been employed to calculate the VE, HE, and κES of ionic liquid mixtures. The analysis of the measured data in terms of graph theory suggests that investigated (1 + 2) mixtures are characterized by interactions forming a 1:1 molecular complex, and VE, HE, and κES values calculated by graph theory are close to the experimental ones.
1. INTRODUCTION Ionic liquids are air- and water-stable room temperature liquids and are comprised of ions. These compounds present unique physical and chemical properties such as an extremely low vapor pressure, a broad liquid temperature range, high ionic conductivity, nonflammability, and chemical and thermal stability.1−5 They have recently been utilized as green solvents,6 as adsorption media for gas separation,7 as the separating agent in extractive distillation,8 as electrolyte in lithium ion batteries9 and dye-sensitized solar cells,10 solvent for reactions in chemical industries11,12 and as a solvent to absorb carbon dioxide.13 Ionic liquids or their mixtures with organic solvents therefore can be used as working fluids to enhance the efficiency of the chemical equipment such as batteries, photoelectric cells, and solar cells, and so forth. Heat transfer fluids are found in a large number of industrial and consumer applications. All of these properties make ionic liquids potential environmentally friendly substitutes to the traditional organic solvents in most of the chemical industry processes. Further, the properties of ionic liquids can be tuned by changing the structure of component ions. The use of mixtures of ionic liquids with organic solvents allows change and control of the properties of the mixtures to suit a given situation. 1-Ethyl 3-methylimidazolium tetrafluoroborate is air- and water-stable ionic liquid6,14−17 and is also used as thermal fluid. The structure of lactams is of great interest as they are related to many structural problems in molecular biology.18 Self-association of lactams serves as a model for hydrogen bonding in nucleic acid amides. Liquid mixtures of 1-ethyl-3-methylimidazolium tetrafluoroborate or pyrrolidin-2-one or 1-methyl pyrrolidin-2-one may, therefore, comprise a class of mixtures of great interest from both fundamental and applied research points of view. In the present studies we © 2012 American Chemical Society
report densities, speeds of sound data, and excess molar enthalpies data of 1-ethyl-3-methylimidazolium tetrafluoroborate (1) + pyrrolidin-2-one or 1-methyl pyrrolidin-2-one (2) binary mixtures.
2. EXPERIMENTAL SECTION 1-Ethyl-3-methylimidazolium tetrafluoroborate [emim][BF4] (Fluka, 0.98 GC) was used without further purification. The content of water in ionic liquid was regularly checked using Karl Fischer titration,19 and the content of water was less than 336 ppm. Pyrrolidin-2-one (2-Py; Fluka, 0.99 GC) was purified by vacuum distillation over calcium oxide,20 and 1-methyl pyrrolidin2-one (NMP) [Fluka, 0.99 GC] was purified by fractionally distilled under reduced pressure.21 The density and speed of sound values for the purified liquids are recorded in Table 1 and are also compared with literature values.22−32 Densities, ρ, and speeds of sound, u, values of the pure liquids and their binary mixtures were measured using a commercial density and sound analyzer apparatus (Anton Paar DSA 5000) in the manner as described elsewhere.33,34 The calibration of the equipment was carried out with the doubly distilled, deionized water. The mole fraction of each mixture (made by mixing the two components in an airtight glass bottle) was obtained from the measured apparent masses of the components with uncertainty of 1·10−4. All of the measurements were performed on an electric balance. The uncertainties in the density and speed of sound measurements are 0.5 kg·m−3 and 0.1 m·s−1, respectively. The uncertainty in VE values calculated Received: June 11, 2012 Accepted: October 29, 2012 Published: November 13, 2012 3488
dx.doi.org/10.1021/je300542s | J. Chem. Eng. Data 2012, 57, 3488−3497
Journal of Chemical & Engineering Data
Article
Table 1. Comparison of Densities, ρ, and Speeds of Sound, u, Pure Liquids with Their Literature Values at T/K = 293.15, 298.15, 303.15, and 308.15 ρ/kg·m−3 T/K
liquid 1-ethyl-3-methylimidazolium tetrafluoroborate
pyrrolidin-2-one
1-methyl-pyrrolidin-2-one
exptl
293.15 298.15
1283.89 1279.91
303.15
1276.26
308.15
1272.07
293.15 298.15
1111.28 1107.15
303.15
1103.02
308.15 293.15
1098.90 1033.23
298.15
1028.23
303.15
1023.46
308.15
1018.66
u/m·s−1 lit.
exptl a
1284.3 1279.6b 1280.07c 1276.5a 1275.7b 1271.9b 1272.48c 1107.01d 1107.20e 1102.0f 1103.37g 1098.63g 1033.23g 1032.87i 1028.23g 1023.47j 1023.40k
lit.
α(·103)/K−1
1631.05 1619.40
0.615 0.596
1607.63
0.614
1596.31
0.648
1650.13 1633.92 1617.14 1601.85 1565.52 1546.02 1527.24 1507.41
1633.2h 1633.95g 1617.61g 1601.87g 1565.30g 1565.50i 1545.10g 1546.06h 1526.5g 1527.21i 1507.38g
0.743 0.746 0.748 0.749 0.951 0.950 0.934 0.942
a
Reference 22. bReference 23. cReference 24. dReference 25. eReference 26. fReference 27. gReference 28. hReference 29. iReference 30. jReference 31. k Reference 32.
where ϕi is the volume fraction of component (i) in the mixed state. κS,i, vi, αi, and Cp,i are the isentropic compressibility, molar volume, thermal expansion coefficient, and molar heat capacity, respectively, of the pure component (1). The Cp values of investigated liquids were taken from literature.27,28,37,38 The α values for [emim][BF4], 2-PY, and NMP were calculated by using experimental density data in the manner as described elsewhere39 and are recorded in Table 1. Such κES values for the various mixtures are recorded in Table 2. The VE, HE, and κES data of the studied mixtures at 298.15 K are plotted in Figures 1 to 3. The observed VE, HE, and κES data were expressed by eq 5.
from density results is 0.1 %. Further, the uncertainty in the temperature measurement is ± 0.01 K. Excess molar enthalpies, HE, for the studied mixtures were measured by a two-drop calorimeter (model 4600) supplied by the Calorimeter Sciences Corporation (CSC, Lindon, UT, USA) at 298.15 K in a manner described elsewhere.35 The uncertainties in the measured HE values are 1 %.
3. RESULTS Densities, ρ, and speeds of sound, u, of [emim][BF4] (1) + 2Py or NMP (2) binary mixtures at (293.15, 298.15, 303.15, and 308.15) K and excess molar enthalpies, HE, of the same mixtures at 298.15 K are listed in Tables 2 and 3. The densities and speed of sound values of mixtures were employed to calculate excess molar volumes, VE, and isentropic compressibilities, κS, using 2
VE =
X E(X = V or H or κS) = x1x 2[X (0) + X (1)(2x1 − 1) + X (2)(2x 2 − 1)2 ] (5)
2
∑ xiMi(ρ)−1 − ∑ xiMi(ρi )−1 i=1
where X(n)(X = V or H or κS) (n = 0 to 2), and so forth, are the parameters characteristic of (1 + 2) mixtures. These parameters were determined by fitting XE(X = V or H or κS) data to eq 5 using least-squares methods and are recorded along with standard deviations, σ(XE) (X = V or H or κS) defined by
(1)
i=1
κS = (ρu 2)−1
(2)
where ρ is the density of mixture and xi, Mi, and ρi are the mole fraction, molar mass, and density of component 1. The excess isentropic compressibilities, κES , values were determined using eq 3. κSE = κS − κSid
E E σ(X E) = [ ∑ (Xexptl − Xcalc,eq5 )2 /(m − n)]0.5
where m is the number of data points and n is the number of adjustable parameters in eq 6 in Tables 2 and 3.
(3)
The κidS values were obtained in the manner suggested by Benson and Kiyohara36
4. DISCUSSION The densities, speeds of sound, excess molar volumes, excess isentropic compressibilities, and excess molar enthalpies data of the studied mixtures are not available in the literature with which the observed data can be compared. The densities and speed of sound values of the investigated mixtures increase with the decrease in 2-Py or NMP and an increase in temperature which in turn suggest that physical properties of [emim][BF4]
2
κSid =
2
⎡
i=1
⎢⎣
∑ φi⎢κS ,i +
Tviαi2 ⎤ ⎥ − T (∑ xivi) Cp , i ⎥⎦ i=1 2
(∑ φα )2 i i i=1 2
(∑ xiCp , i) i=1
(6)
(4) 3489
dx.doi.org/10.1021/je300542s | J. Chem. Eng. Data 2012, 57, 3488−3497
Journal of Chemical & Engineering Data
Article
Table 2. Measured Densities, ρ; Excess Molar Volumes, VE; Speeds of Sound, u; Isentropic Compressibilities, κS; and Excess Isentropic Compressibilities, κES , Data for the Various (1 + 2) Mixtures as a Function of Mole Fraction, x1, of Component (1) from T/K = 293.15, 298.15, 303.15, and 308.15a x1
ρmix/kg·m3
VE/cm3·mol−1
u/m·s−1
κS/TPa−1
κES /TPa−1
x1
ρmix/kg·m3
VE/cm3·mol−1
u/m·s−1
κS/TPa−1
κES /TPa−1
e
T/K = 308.15 0.1099 1137.87 −0.3399 1608.29 339.77 −9.81 0.1704 1155.81 −0.4918 1610.96 333.38 −13.40 0.2402 1173.98 −0.6342 1613.33 327.26 −16.30 0.2698 1180.96 −0.6845 1614.10 325.02 −17.18 0.3397 1195.95 −0.7793 1615.33 320.45 −18.52 0.3717 1202.18 −0.8116 1615.86 318.58 −18.91 0.4289 1212.40 −0.8500 1616.15 315.78 −19.07 0.4719 1219.44 −0.8675 1615.90 314.06 −18.81 0.5305 1228.15 −0.8684 1615.71 311.91 −18.26 0.5717 1233.71 −0.8529 1615.02 310.76 −17.50 0.6316 1241.13 −0.8143 1614.05 309.28 −16.22 0.6708 1245.54 −0.7743 1613.19 308.51 −15.18 0.7314 1251.76 −0.6895 1611.58 307.59 −13.30 0.7706 1255.43 −0.6220 1610.34 307.17 −11.92 0.8289 1260.40 −0.5009 1607.90 306.89 −9.51 0.8722 1263.75 −0.3960 1605.57 306.96 −7.44 0.9088 1266.36 −0.2953 1603.39 307.16 −5.55 0.9214 1267.21 −0.2575 1602.55 307.28 −4.85 1-Ethyl-3-methylimidazolium Tetrafluoroborate (1) + 1-Methyl-pyrrolidin-2one (2) T/K = 293.15f 0.1113 1083.27 −0.7593 1577.90 370.77 −12.76 0.1812 1110.82 −1.1332 1583.98 358.80 −17.59 0.2289 1128.09 −1.3455 1587.69 351.66 −19.86 0.2709 1142.36 −1.5062 1590.06 346.23 −21.00 0.3297 1160.85 −1.6805 1593.04 339.45 −21.78 0.3714 1172.96 −1.7677 1594.97 335.13 −21.84 0.4315 1189.17 −1.8515 1596.90 329.76 −21.07 0.4773 1200.58 −1.8820 1597.84 326.24 −19.91 0.5256 1211.67 −1.8715 1599.19 322.71 −18.51 0.5709 1221.40 −1.8374 1600.00 319.82 −16.78 0.6317 1233.32 −1.7380 1601.74 316.04 −14.35 0.6772 1241.45 −1.6245 1603.24 313.38 −12.36 0.7311 1250.38 −1.4584 1605.30 310.35 −9.89 0.7342 1250.84 −1.4442 1605.56 310.13 −9.79 0.8324 1264.96 −1.0245 1611.55 304.39 −5.50 0.8727 1270.09 −0.8152 1615.12 301.83 −3.95 0.9129 1274.80 −0.5799 1619.24 299.18 −2.49 0.9304 1276.75 −0.4716 1621.27 297.97 −1.91 T/K = 298.15g 0.1113 1078.98 −0.8233 1558.39 381.62 −13.14 0.1812 1107.06 −1.2438 1564.71 368.95 −18.20 0.2289 1124.60 −1.4799 1568.58 361.40 −20.55 0.2709 1139.07 −1.6581 1571.17 355.63 −21.74 0.3297 1157.88 −1.8602 1574.48 348.39 −22.58 0.3714 1170.19 −1.9642 1576.73 343.74 −22.68 0.4315 1186.50 −2.0554 1579.04 338.02 −21.85 0.4773 1197.97 −2.0897 1580.98 333.96 −20.92 0.5256 1209.07 −2.0768 1582.56 330.24 −19.38 0.5709 1218.76 −2.0361 1584.20 326.93 −17.75 0.6317 1230.53 −1.9163 1586.89 322.71 −15.35 0.6772 1238.61 −1.7930 1588.98 319.76 −13.34 0.7311 1247.29 −1.5953 1591.99 316.34 −10.89 0.7342 1247.77 −1.5828 1592.03 316.20 −10.69 0.8324 1261.53 −1.1124 1599.46 309.85 −6.34 0.8727 1266.47 −0.8758 1603.22 307.2 −4.60 0.9129 1271.06 −0.6206 1607.61 304.42 −3.00 0.9304 1272.96 −0.5043 1609.79 303.14 −2.37
1-Ethyl-3-methylimidazolium Tetrafluoroborate (1) + Pyrrolidin-2-one (2) T/K = 293.15b 0.1099 1149.23 −0.2619 1655.09 317.65 −8.68 0.1704 1167.01 −0.3976 1656.39 312.32 −11.73 0.2402 1185.20 −0.5393 1657.01 307.30 −14.12 0.2698 1192.24 −0.5941 1656.73 305.58 −14.72 0.3397 1207.39 −0.7008 1655.97 302.03 −15.64 0.3717 1213.68 −0.7378 1655.21 300.74 −15.72 0.4289 1224.05 −0.7909 1653.72 298.73 −15.58 0.4719 1231.12 −0.8119 1652.36 297.50 −15.18 0.5305 1239.88 −0.8202 1650.25 296.16 −14.32 0.5717 1245.49 −0.8117 1648.65 295.39 −13.53 0.6316 1252.87 −0.7730 1646.16 294.54 −12.12 0.6708 1257.25 −0.7308 1644.60 294.08 −11.11 0.7314 1263.41 −0.6457 1642.29 293.46 −9.44 0.7706 1267.04 −0.5767 1640.60 293.22 −8.20 0.8289 1271.98 −0.4568 1638.39 292.88 −6.35 0.8722 1275.32 −0.3538 1636.61 292.75 −4.85 0.9088 1277.96 −0.2591 1635.03 292.71 −3.51 0.9214 1278.82 −0.2245 1634.52 292.69 −3.05 T/K = 298.15c 0.1099 1145.44 −0.2879 1638.37 325.24 −8.64 0.1704 1163.28 −0.4296 1640.10 319.58 −11.86 0.2402 1181.48 −0.5736 1641.45 314.14 −14.48 0.2698 1188.52 −0.6283 1641.77 312.15 −15.27 0.3397 1203.62 −0.7319 1642.08 308.12 −16.48 0.3717 1209.93 −0.7712 1641.89 306.59 −16.72 0.4289 1220.26 −0.8205 1641.12 304.28 −16.72 0.4719 1227.33 −0.8419 1640.02 302.93 −16.33 0.5305 1236.07 −0.8487 1638.56 301.32 −15.57 0.5717 1241.64 −0.8358 1637.06 300.52 −14.71 0.6316 1249.02 −0.7963 1634.83 299.56 −13.25 0.6708 1253.43 −0.7574 1633.27 299.08 −12.15 0.7314 1259.60 −0.6717 1630.79 298.52 −10.26 0.7706 1263.22 −0.6006 1629.09 298.28 −8.91 0.8289 1268.14 −0.4776 1626.51 298.07 −6.77 0.8722 1271.48 −0.3730 1624.62 297.98 −5.11 0.9088 1274.08 −0.2741 1623.01 297.96 −3.65 0.9214 1274.95 −0.2394 1622.43 297.97 −3.13 T/K = 303.15d 0.1099 1141.73 −0.3169 1622.27 332.8 −9.09 0.1704 1159.64 −0.4627 1624.91 326.60 −12.66 0.2402 1177.82 −0.6035 1627.13 320.68 −15.54 0.2698 1184.83 −0.6554 1628.05 318.43 −16.51 0.3397 1200.05 −0.7670 1629.17 313.96 −17.94 0.3717 1206.30 −0.8008 1629.50 312.20 −18.30 0.4289 1216.55 −0.8411 1629.41 309.61 −18.41 0.4719 1223.58 −0.8574 1629.15 307.93 −18.22 0.5305 1232.31 −0.8603 1628.35 306.05 −17.55 0.5717 1237.88 −0.8460 1627.66 304.92 −16.88 0.6316 1245.24 −0.8015 1626.01 303.74 −15.46 0.6708 1249.64 −0.7599 1624.55 303.21 −14.28 0.7314 1255.84 −0.6739 1622.32 302.55 −12.31 0.7706 1259.49 −0.6048 1620.49 302.35 −10.80 0.8289 1264.37 −0.4730 1617.81 302.18 −8.43 0.8722 1267.75 −0.3710 1615.39 302.28 −6.45 0.9088 1270.39 −0.2746 1613.37 302.41 −4.73 0.9214 1271.26 −0.2395 1612.57 302.50 −4.09 3490
dx.doi.org/10.1021/je300542s | J. Chem. Eng. Data 2012, 57, 3488−3497
Journal of Chemical & Engineering Data
Article
Table 2. continued x1
ρmix/kg·m3
VE/cm3·mol−1
u/m·s−1
κS/TPa−1
κES /TPa−1
x1
ρmix/kg·m3
h
0.1113 0.1812 0.2289 0.2709 0.3297 0.3714 0.4315 0.4773 0.5256 0.5709 0.6317 0.6772 0.7311 0.7342 0.8324 0.8727 0.9129 0.9304 a
1074.90 1103.19 1120.90 1135.50 1154.38 1166.71 1183.14 1194.66 1205.85 1215.50 1227.39 1235.46 1244.14 1244.62 1258.31 1263.22 1267.71 1269.55
T/K = 303.15 −0.8843 1539.67 −1.3217 1546.41 −1.5703 1550.12 −1.7585 1553.37 −1.9629 1557.14 −2.0650 1559.33 −2.1627 1562.44 −2.1979 1564.86 −2.1893 1566.94 −2.1391 1569.38 −2.0254 1572.35 −1.8952 1575.21 −1.6917 1578.84 −1.6787 1579.05 −1.1871 1587.17 −0.9427 1591.26 −0.6697 1595.92 −0.5442 1598.03
VE/cm3·mol−1
u/m·s−1
κS/TPa−1
κES /TPa−1
404.15 390.11 381.89 375.31 367.00 361.88 355.02 350.46 345.82 341.92 336.80 333.28 329.06 328.83 321.46 318.36 315.30 313.97
−14.12 −19.53 −21.86 −23.25 −24.30 −24.26 −23.70 −22.60 −21.28 −19.58 −17.19 −15.09 −12.65 −12.50 −7.74 −5.86 −3.96 −3.13
i
392.44 379.05 371.28 364.97 357.27 352.50 346.23 341.82 337.75 334.03 329.55 326.21 322.44 322.23 315.48 312.63 309.71 308.45
−13.58 −18.88 −21.13 −22.58 −23.48 −23.42 −22.74 −21.84 −20.32 −18.80 −16.25 −14.32 −11.85 −11.70 −7.09 −5.27 −3.54 −2.78
0.1113 0.1812 0.2289 0.2709 0.3297 0.3714 0.4315 0.4773 0.5256 0.5709 0.6317 0.6772 0.7311 0.7342 0.8324 0.8727 0.9129 0.9304
1070.58 1099.07 1116.88 1131.56 1150.48 1162.94 1179.43 1190.89 1202.10 1211.81 1223.71 1231.77 1240.43 1240.90 1254.52 1259.35 1263.76 1265.58
T/K = 308.15 −0.9326 1520.26 −1.3895 1527.20 −1.6480 1531.19 −1.8429 1534.50 −2.0497 1538.97 −2.1641 1541.47 −2.2649 1545.39 −2.2917 1547.90 −2.2831 1550.98 −2.2364 1553.53 −2.1198 1557.66 −1.9858 1560.74 −1.7757 1565.21 −1.7622 1565.47 −1.2543 1574.69 −0.9961 1579.30 −0.7095 1584.19 −0.5784 1586.40
The footnote letters indicate the (1 + 2) mixtures at T/K = 293.15, 298.15, 303.15, and 308.15, respectively. The uncertainty in mole fraction values is 1·10−4. The uncertainty in temperature is ± 0.01 K. The uncertainty in density values is 0.5 kg·.m−3. The uncertainty in speed of sound values is 0.1 m·s−1. The uncertainty in VE values is 0.1 %. Also included are various Vn and κnS (n = 0 to 2) parameters of eq 5 along with standard (1) (2) deviations, σ(VE) and σ(κES ). bV(0) = −3.2805; V(1) = −0.3034; V(2) = 0.6086; σ(VE) = 0.0008 cm3·mol−1. κ(0) S = −59.12; κS = 29.20; κS = −10.87; E (1) (2) −1 c (0) (1) (2) E 3 −1 (0) σ(κS ) = 0.03 TPa . V = −3.3899; V = −0.2493; V = 0.4210; σ(V ) = 0.0008 cm ·mol . κS = −64.08; κS = 27.64; κS = −4.0; σ(κES ) = (1) (2) E 0.03 TPa−1. dV(0) = −3.4131; V(1) = −0.0471; V(2) = 0.1629; σ(VE) = 0.0008 cm3·mol−1. κ(0) S = −71.97; κS = 22.86; κS = −5.53; σ(κS ) = 0.03 (1) (2) E −1 e (0) (1) (2) E 3 −1 (0) TPa . V = −3.4794; V = −0.0471; V = −0.0600; σ(V ) = 0.0008 cm ·mol . κS = −74.39; κS = 21.23; κS = −15.02; σ(κS ) = 0.03 TPa−1. f (0) (1) (2) E −1 g (0) V = −7.5255; V(1) = 0.2249; V(2) = 0.0541; σ(VE) = 0.0017 cm3·mol−1. κ(0) = S = −77.18; κS = 61.04; κS = −6.78; σ(κS ) = 0.04 TPa . V (1) (2) E (1) (2) E 3 −1 (0) −1 h (0) −8.3544; V = 0.3030; V = 0.4251; σ(V ) = 0.0019 cm ·mol . κS = −80.92; κS = 59.87; κS = −9.43; σ(κS ) = 0.04 TPa . V = −8.7869; (1) (2) E −1 i (0) = −9.1815; V(1) = V(1) = 0.3030; V(2) = 0.1525; σ(VE) = 0.0020 cm3·mol−1. κ(0) S = −84.65; κS = 58.69; κS = −12.08; σ(κS ) = 0.04 TPa . V (1) (2) E (2) E 3 −1 (0) −1 0.3030; V = −0.0017; σ(V ) = 0.0021 cm ·mol . κS = −88.11; κS = 58.51; κS = −14.58; σ(κS ) = 0.04 TPa .
disruption of the 2-PY−2-PY interactions upon mixing is larger than that related to the breaking of the NMP−NMP interactions (process ii). Consequently, the HE values for [emim][BF4] (1) + NMP (2) mixtures are more negative than those of [emim][BF4] (1) + 2-Py (2). Thus assumptions i−iii are the only ones that explain the HE behavior of these mixtures. The negative values of VE and κES data of the various (1 + 2) mixtures suggest that 2-Py or NMP molecules gives relatively more packed arrangement in [emim][BF4] as compared to the pure state. This may be due to (i) smaller molar volumes of 2-Py (76.77 cm3·mol−1) and NMP (96.41 cm3·mol−1) as compared to [emim][BF4] (154.70 cm3·mol−1) which allows the organic molecular liquids to fit in to the interstices of the ionic liquid in mixed state; and (ii) ion−dipole interactions between 2-Py or NMP with [emim][BF4]. The ∂VE/∂T and ∂κES /∂T values for various (1 + 2) mixtures are negative. The decrease in speed of sound values with the increase in temperature for the investigated mixture also supports this viewpoint. 4.1. Graph Theory. 4.1.1. Excess Molar Volume. An important aspect of ionic liquid [emim][BF4] is the nature of interactions operating among the ions present. The question of interest involves the specific point on the [emim]+ cation or [BF4]− anion where interactions with counterion or 2-Py or NMP exist in the mixed state. The addition of 2-Py or NMP to [emim][BF4] or vice versa would influence the interactions in the mixed state which in turn reflects in the topology of one or two constituent molecules. The excess molar volume, VE, reflects the packing effect, so it was worthwhile to analyze the observed VE data of (1 + 2) mixtures in terms of graph theory
can be adjusted to meet the need of applications of [emim][BF4] by adding 2-Py or NMP or changing temperature. The VE, HE, and κES values of the investigated mixtures are negative over entire composition range and for an equimolar composition vary in the order: NMP > 2-Py. The excess molar enthalpy, HE, data of liquid mixtures is related to the net destruction and creation of interactions among the constituents of mixtures in the mixing process. The sign and magnitude of HE data roughly reflects the balance between the interactions among like and unlike species existing in mixtures. The HE values of the investigated mixtures may be explained by assuming that the processes (i) [emim][BF4] is characterized by ionic interactions and exist as monomer. (ii) The addition of 2-Py or NMP to [emim][BF4] ruptures the ionic interactions in [emim][BF4] as well as their associations and forms monomers of 2-Py or NMP. (iii) There are ion− dipole interactions between (the N atom of the [emim] ring and the fluorine atom of the [BF4]− anion) with the oxygen atom of 2-Py and proton of the −CH3 group attached to the nitrogen atom of NMP to form a 1:1 molecular complex. The positive HE values indicate that interactions between unlike molecules are weaker than between like ones, whereas negative values point to the opposite. The HE values for the studied mixture are exothermic which means that contributions to HE due to processes i and iii far out weigh the contribution to HE due to process ii. HE values for [emim][BF4] (1) + 2-PY (2) mixtures are higher than those of [emim][BF4] (1) + NMP (2). As amide−amide interactions become weaker in the sequence primary > secondary > tertiary, so the contribution to HE from the 3491
dx.doi.org/10.1021/je300542s | J. Chem. Eng. Data 2012, 57, 3488−3497
Journal of Chemical & Engineering Data
Article
Table 3. Measured Excess Molar Enthalpies, HE, Values for the Various (1 + 2) Mixtures as a Function of Mole Fraction, x1, of Component (1) at T/K = 298.15a x1
HE/J·mol−1
x1
HE/J·mol−1
1-Ethyl-3-methylimidazolium Tetrafluoroborate (1) + Pyrrolidin-2-one (2)b 0.1024 −111.0 0.5869 −300.0 0.1789 −181.9 0.6158 −289.0 0.2154 −211.2 0.6716 −262.8 0.2825 −259.3 0.7257 −230.1 0.3279 −282.8 0.7982 −175.2 0.3912 −301.3 0.8391 −144.4 0.4456 −311.1 0.8824 −104.7 0.4932 −316.8 0.9011 −88.3 0.5487 −309.7 0.9203 −72.6 1-Ethyl-3-methylimidazolium Tetrafluoroborate (1) + 1-Methyl-pyrrolidin-2one (2)c 0.0915 −133.4 0.5102 −478.7 0.1456 −207.3 0.5874 −473.7 0.1998 −275.2 0.6324 −455.1 0.2349 −320.2 0.6984 −405.9 0.2879 −368.3 0.7458 −363.5 0.3247 −405.6 0.7988 −311.1 0.3724 −439.3 0.8252 −272.9 0.4085 −458.9 0.8865 −190.8 0.4754 −475.0 0.9145 −147.1
Figure 2. Excess isentropic compressibilities, κSE, for 1-ethyl-3methylimidazolium tetrafluoroborate (1) + pyrrolidin-2-one (2) at ■, 293.15 K; ●, 298.15 K; ▲, 303.15 K; ▼, 308.15 K; 1-ethyl-3methylimidazolium tetrafluoroborate (1) + 1-methyl-pyrrolidin-2-one (2) at ◆, 293.15 K; ★, 298.15 K; ▶, 303.15 K; ⬢, 308.15 K.
The uncertainty in mole fraction values is 1·10−4. The uncertainty in temperature is ± 0.01 K. The uncertainty in HE values is 1 %. Also included are various Hn (n = 0 to 2) parameters along with standard deviations, σ(HE). bH(0) = −1256.5; H(1) = 134.2; H(2) = 231.2; σ(HE) = 1.9 J·mol−1. cH(0) = −1924.5; H(1) = −151.7; H(2) = 269.7; σ(HE) = 2.8 J·mol−1. a
Figure 3. Excess molar enthalpies, HE, for 1-ethyl-3-methylimidazolium tetrafluoroborate (1) + 2-pyrrolidinone (2) at ★, 298.15 K; 1-ethyl-3-methylimidazolium tetrafluoroborate (1) + 1-methyl-pyrrolidin2-one (2) at ●, 298.15 K.
calculate VE of the studied mixtures. According to graph theory, VE is given by40 V E = α12{[∑ x1(3ξ1)m ]−1 − Figure 1. Excess molar volumes, VE, for 1-ethyl-3-methylimidazolium tetrafluoroborate (1) + pyrrolidin-2-one (2) at ■, 293.15 K; ●, 298.15 K; ▲, 303.15 K; ▼, 308.15 K; 1-ethyl-3-methylimidazolium tetrafluoroborate (1) + 1-methyl-pyrrolidin-2-one (2) at ◆, 293.15 K; ★, 298.15 K; ▶, 303.15 K; ⬢, 308.15 K.
∑ x1(3ξ1)−1}
(7)
where α12 is a constant characteristic of the (1 + 2) mixture. The (3ξi), (3ξi) m (i = 1 or 2) are the connectivity parameters of the third degree of the components (1) and (2) in pure and mixed states and are defined41 by 3
(δmνδnνδoνδpν)−0.5
∑
ξ=
m
