Density, speed of sound, and electrical conductance of ionic liquid 1-hexyl-3-methyl-imidazolium bromide in water at different temperatures

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J. Chem. Thermodynamics 40 (2008) 852–859

Density, speed of sound, and electrical conductanceof ionic liquid 1-hexyl-3-methyl-imidazolium bromide

in water at different temperatures

Hemayat Shekaari a,*, Yagoub Mansoori a, Rahmat Sadeghi b

a Department of Chemistry, Faculty of Science, University of Mohaghegh Ardabili, Ardabil, Iranb Department of Chemistry, Faculty of Science, University of Kurdistan, Sanandaj, Iran

Received 2 November 2007; received in revised form 25 December 2007; accepted 7 January 2008Available online 12 January 2008

Abstract

Densities, speeds of sound, and electrical conductances in the aqueous solutions of room temperature ionic liquid, 1-hexyl-3-methyl-imidazolium bromide, [HMIm]Br were measured at temperatures T = (283.15 to 308.15) K and atmospheric pressure. Apparent molarvolumes V/, isentropic compressibilities jS, apparent molar isentropic compressibilities j/, and molar conductivities K were determined.The corresponding limiting apparent molar quantities were found by extrapolation to infinite dilution with using Redlich–Mayer equa-tion. The obtained limiting apparent molar quantities at different temperatures indicate that (ion + solvent) interactions in the studiedsystem. Limiting apparent molar expansibility E�/ values for aqueous solutions of this ionic liquid have a positive values. The value ofð@2V �/=@T 2ÞP was found small and negative which indicating the ionic liquid under study is predominantly a structure breaker. Isentropiccompressibility isotherms intersect approximately at a concentration m = 1.5825 mol � kg�1, which may be an indication of the structuralinteractions and clathrate formation. Calculated ion association constant Ka values increase with increasing temperature show that theextent of ion association increases with increasing temperature.� 2008 Elsevier Ltd. All rights reserved.

Keywords: Ionic liquids; 1-Hexyl-3-methyl-imidazolium bromide; Apparent molar volume; Isentropic compressibility; Molar conductivity

1. Introduction

In the recent years, it has been increased attention to afamily of compounds with very unique properties: Room-temperature ionic liquids or briefly ionic liquids. Theystand for a class of chemicals composed entirely of ionswith very low melting points below T = 373.15 K. Becauseof their negligible vapor pressure at normal temperature,ionic liquids have been used as replacement for volatileorganic solvents (VOC). One of the most extensively stud-ied classes of ionic liquids is based upon the imidazoliumcation and these types of ionic liquids have been used inmany fields such as synthesis, catalysis, electrochemistry,and industrial applications [1–6].

0021-9614/$ - see front matter � 2008 Elsevier Ltd. All rights reserved.

doi:10.1016/j.jct.2008.01.003

* Corresponding author. Tel.: +98 451 5514702; fax: +98 451 5514701.E-mail address: [email protected] (H. Shekaari).

Aqueous solutions of ionic liquids have been used inmany processes mainly for synthesis of other ionic liquidssuch as [RMIm]PF6 or [RMIm]BF4 and extractive pro-cesses involving Ionic liquids on an industrial scale. Anexcellent example for the application of such ionic liquidsmixtures in electrochemistry is the generation of hydrogen[7]. In spite of extensive applications of ionic liquids andtheir aqueous solutions, there is limited information onthe thermodynamic properties of ionic liquids. Industrialdevelopment demands on reliable reference data on thethermodynamic properties of pure ionic liquids and theirmixtures with other compounds. On the other hand,thermodynamic properties of such solutions reveal(ion + solvent), (ion + ion) and (solvent + solvent) interac-tions. To date, a number of papers have measured some ofphysical properties such as density, viscosity, and speed ofsound of these type of solutions and calculated volumetric,

H. Shekaari et al. / J. Chem. Thermodynamics 40 (2008) 852–859 853

and compressibility properties. These papers mainly reportthese quantities for whole composition range of ionic liq-uids and only some other works have been performed fordilute solution of ionic liquids [8–17].

This work is related to study of thermodynamic proper-ties of aqueous solutions 1-hexyl-3-methyl-imidazoliumbromide [HMIm]Br and their dependency with the temper-ature. With this consideration, in the present work, theprocedure to synthesize the ionic liquid is described andmeasured density, speed of sound, and electrical conduc-tance data in its aqueous solutions. Apparent molar vol-umes and apparent molar isentropic compressibilities ofstudied mixtures were calculated at temperaturesT = (283.15 to 308.15) K and at atmospheric pressure.The corresponding limiting apparent molar quantities werefound by extrapolation to infinite dilution with using Red-lich–Mayer equation. Molar conductances of this systemhas been correlated with low concentration chemical model(lcCM) of conductance and ion association constants havebeen obtained at different temperatures.

2. Experimental

2.1. Materials

Reagents used for the synthesis of [HMIm]Br wereN-methylimidazole (>99%), 1-bromohexane (>99%), Ace-tonitrile (GR, >99.8%), toluene (>99%), and purchasedfrom Merck. These reagents were used without furtherpurification. The double distilled water was used for prep-aration of solutions. The pure water had a conductivityapproximately 1.3 lS � cm�1.

2.2. Synthesis of ionic liquid

[HMIm]Br was prepared and purified by using the proce-dure described in the literature [8,9]. Briefly, [HMIm]Br wassynthesized by direct alkylation of N-methylimidazole withan excess of 1-bromohexane in a round bottom flask atT = 353.15 K for 48 h under a nitrogen atmosphere. Thecrude product was dissolved in acetonitrile and crystallizedby adding dropwise to toluene in ice bath. The productwas dried in high vacuum at T = 333.15 K using a rotaryevaporator for at least 4 h in 0.7 kPa. The obtained ionicliquid has purity greater than mass fraction 0.98, whichwas used after vacuum desiccated for at least 48 h to removetrace amount of moisture. Water contents found by KarlFischer method in the [HMIm]Br was less than mass fraction0.05. Ionic liquid was analysed by 1H NMR (Brucker Av-300)and IR (Buck Scientific) spectra to confirm the absence ofany major impurities and they were found to be in goodagreement with those reported in the literature [18,19].

2.3. Method of measurement

Aqueous solutions of [HMIm]Br for measurements wereprepared by weighing corresponding ionic liquid in sealed

vials and dilution by water. Each vial was weighted witha precision (10�4 g) analytical balance (Sartrious). All thesolutions were kept tightly sealed to minimize evaporation.Measurements were performed immediately after prepara-tion of solutions.

Density and speed of sound data were continuouslymeasured using a commercial density and speed of soundmeasurement apparatus (Anton Paar DSA 5000). Both ofdensity and speed of sound are extremely sensitive to tem-perature, so it was kept constant within ±1.0 � 10�3 Kusing the built in Peltier Method. The reproducibility ofdensity and speed of sound measurements was better than±3.0 � 10�6 g � cm�3 and ±0.5 m � s�1, respectively. Theapparatus was calibrated with double distilled deionized,and degassed water, and dry air at atmospheric pressure.

Conductance measurements were carried out on a digi-tal conductivity meter (Metrohm model 712) with a sensi-tivity of 0.1% and a dipping-type conductivity cell withplatinized electrodes with a cell constant of 0.824 cm�1

under nitrogen atmosphere and at a frequency of 1 MHz.The cell constant was calibrated with aqueous KC1 solu-tion. About 50 ml of pure solvent was placed in the con-ductivity cell and the cell was closed. Weighed pure ILwas added with a syringe to the cell containing solvent. Cir-culating water from a thermostatically regulated batharound the sample holder with double wall to maintainthe temperature with a precision of 0.02 K. The conductiv-ity cell was purged with nitrogen during each run. The con-ductivities of aqueous solutions of the ionic liquid solutionswere always corrected for the contribution of the purewater.

3. Results and discussion

3.1. Volumetric properties

The measured density values of aqueous solutions of[HMIm]Br versus molality of [HMIm]Br at temperaturesT = (283.15 to 308.15) K in interval 5 K are given in table1. As it can be observed, the density decreases as both tem-perature and water compositions.

The apparent molar volumes V/ of [HMIm]Br werederived from the densities of the solutions using thefollowing

V / ¼Md� ðd � d0Þ

mdd0

; ð1Þ

where d and d0 are densities of aqueous solutions[HMIm]Br and pure water, respectively and m is molalityof [HMIm]Br; M (in kg � mol�1) is molar mass of[HMIm]Br. The apparent molar volumes are also listedin table 1.

By assuming that the aqueous solutions of [HMIm]Brbehave like those of 1:1 aqueous electrolyte in the diluteregion, the concentration dependence of V/ can bedescribed using the following Redlich–Mayer equation inthe dilute region as [20]

TABLE 1Experimental values of density d, speed of sound u, and apparent molarvolume V/, isentropic compressibility js and apparent molar isentropiccompressibility j/ for aqueous solutions of [HMIm]Br at differenttemperatures

m/(mol �kg�1)

10�3 � d/(kg � m�3)

u/(m � s�1)

106 �V//(m3� mol�1)

1012 �js/Pa�1

1014 � j//(m3 � mol�1�Pa�1)

T = 283.15 K

0.0042 0.999919 1448.70 195.20 476.52 �10.460.0048 0.999941 1448.79 196.66 476.45 �9.540.0067 1.000033 1449.21 197.34 476.13 �8.890.0081 1.000096 1449.29 197.85 476.04 �6.830.0139 1.000379 1450.30 198.00 475.25 �5.790.0295 1.001151 1452.94 197.72 473.16 �4.870.0369 1.001520 1453.35 197.55 472.72 �3.220.0487 1.002104 1455.35 197.35 471.14 �3.460.0640 1.002842 1457.93 197.51 469.13 �3.580.0730 1.003271 1459.38 197.61 468.00 �3.560.0940 1.004288 1462.85 197.52 465.31 �3.620.1183 1.005446 1466.89 197.49 462.22 �3.670.1423 1.006600 1470.86 197.35 459.20 �3.700.1639 1.007599 1474.04 197.46 456.77 �3.550.2206 1.010253 1482.68 197.30 450.27 �3.400.2581 1.011963 1488.08 197.28 446.25 �3.250.3034 1.014040 1494.64 197.13 441.44 �3.140.3543 1.016301 1501.62 197.09 436.37 �2.970.3874 1.017818 1506.11 196.89 433.13 �2.890.4492 1.020542 1513.65 196.73 427.68 �2.650.6269 1.028050 1535.23 196.43 412.70 �2.210.8614 1.037256 1557.70 196.25 397.33 �1.501.0007 1.042275 1568.46 196.32 390.01 �1.071.3210 1.053013 1586.34 196.40 377.38 �0.161.7388 1.065229 1600.03 196.65 366.69 0.852.0867 1.074156 1607.38 196.89 360.33 1.48

T = 288.15 K

0.0042 0.999312 1467.02 196.95 464.97 �4.740.0048 0.999334 1467.07 198.22 464.93 �3.940.0067 0.999423 1467.40 198.92 464.68 �3.900.0081 0.999485 1467.43 199.31 464.63 �2.270.0139 0.999762 1468.41 199.32 463.88 �2.870.0295 1.000518 1470.81 198.94 462.02 �2.810.0369 1.000880 1471.09 198.73 461.68 �1.340.0487 1.001447 1472.96 198.62 460.25 �1.780.0640 1.002170 1475.26 198.74 458.48 �1.950.0730 1.002592 1476.58 198.80 457.47 �1.990.0940 1.003585 1479.77 198.72 455.05 �2.150.1183 1.004717 1483.49 198.70 452.26 �2.270.1423 1.005846 1487.10 198.55 449.56 �2.330.1639 1.006823 1490.04 198.65 447.35 �2.230.2206 1.009416 1497.86 198.50 441.56 �2.130.2581 1.011087 1502.72 198.48 437.98 �2.000.3034 1.013114 1508.68 198.34 433.66 �1.920.3543 1.015322 1515.02 198.31 429.1 �1.790.3874 1.016804 1519.09 198.11 426.18 �1.730.4492 1.019461 1525.87 197.96 421.3 �1.520.6269 1.026781 1545.27 197.69 407.86 �1.150.8614 1.035745 1565.11 197.54 394.15 �0.511.0007 1.040636 1574.56 197.6 387.60 �0.141.3210 1.051103 1590.30 197.68 376.18 0.661.7388 1.063056 1602.46 197.89 366.33 1.542.0867 1.071818 1608.87 198.07 360.44 2.10

T = 293.15 K

0.0042 0.998411 1483.21 198.27 455.29 �1.220.0048 0.998432 1483.24 199.61 455.26 �0.550.0067 0.998518 1483.54 200.41 455.04 �1.10

TABLE 1 (continued)

m/(mol �kg�1)

10�3 � d/(kg � m�3)

u/(m � s�1)

106 �V//(m3�mol�1)

1012 �js/Pa�1

1014 � j//(m3 � mol�1�Pa�1)

0.0081 0.998579 1483.57 200.69 454.99 0.050.0139 0.99885 1484.47 200.62 454.31 �1.050.0295 0.999592 1486.67 200.1 452.63 �1.420.0369 0.999946 1486.81 199.91 452.39 0.010.0487 1.000504 1488.54 199.73 451.09 �0.520.0640 1.001211 1490.65 199.87 449.49 �0.760.0730 1.001625 1491.86 199.92 448.58 �0.830.0940 1.002596 1494.78 199.86 446.40 �1.010.1183 1.003707 1498.15 199.81 443.90 �1.150.1423 1.004812 1501.44 199.68 441.47 �1.220.1639 1.005768 1504.11 199.79 439.48 �1.140.2206 1.008306 1511.19 199.64 434.28 �1.070.2581 1.009943 1515.58 199.62 431.07 �0.960.3034 1.011926 1521.01 199.49 427.16 �0.910.3543 1.014084 1526.74 199.47 423.05 �0.800.3874 1.015533 1530.40 199.27 420.43 �0.750.4492 1.018130 1536.42 199.13 416.08 �0.550.6269 1.025278 1553.85 198.89 403.96 �0.240.8614 1.034026 1571.30 198.75 391.70 0.341.0007 1.038798 1579.58 198.83 385.82 0.671.3210 1.049027 1593.40 198.89 375.46 1.381.7388 1.060744 1604.06 199.06 366.39 2.152.0867 1.069356 1609.56 199.20 360.96 2.64

T = 298.15 K

0.0042 0.997246 1497.46 199.63 447.19 0.260.0048 0.997266 1497.48 201.05 447.16 0.890.0067 0.997351 1497.73 201.64 446.98 0.420.0081 0.997413 1497.74 201.62 446.94 1.440.0139 0.997679 1498.58 201.60 446.32 0.110.0295 0.998405 1500.55 201.20 444.83 �0.310.0369 0.998754 1500.60 200.96 444.64 1.040.0487 0.999300 1502.18 200.83 443.47 0.490.064 0.999994 1504.08 200.95 442.04 0.250.0730 1.000398 1505.16 201.03 441.23 0.190.0940 1.001354 1507.83 200.93 439.25 �0.030.1183 1.002444 1510.92 200.88 436.98 �0.190.1423 1.003527 1513.91 200.76 434.78 �0.270.1639 1.004465 1516.32 200.86 433.00 �0.210.2206 1.006953 1522.75 200.73 428.29 �0.160.2581 1.008555 1526.73 200.72 425.38 �0.080.3034 1.010498 1531.62 200.59 421.85 �0.030.3543 1.012613 1536.76 200.57 418.16 0.070.3874 1.014031 1540.06 200.39 415.79 0.110.4492 1.016573 1545.40 200.26 411.89 0.290.6269 1.023564 1561.01 200.04 400.93 0.560.8614 1.032118 1576.29 199.92 389.94 1.091.0007 1.036783 1583.54 199.99 384.64 1.391.3210 1.046799 1595.58 200.04 375.23 2.021.7388 1.058306 1604.80 200.17 366.90 2.692.0867 1.066779 1609.39 200.29 361.91 3.13

T = 303.15 K

0.0042 0.995843 1509.84 201.03 440.50 1.240.0048 0.995864 1509.82 202.11 440.50 2.220.0067 0.995948 1510.06 202.61 440.33 1.490.0081 0.996006 1510.09 202.97 440.28 2.210.0139 0.996267 1510.84 202.84 439.73 1.010.0295 0.996983 1512.62 202.24 438.38 0.560.0369 0.997324 1512.54 202.06 438.28 1.940.0487 0.997863 1514.00 201.86 437.2 1.350.0640 0.998545 1515.72 201.98 435.91 1.090.0730 0.998942 1516.73 202.06 435.15 1.00

(continued on next page)

854 H. Shekaari et al. / J. Chem. Thermodynamics 40 (2008) 852–859

TABLE 1 (continued)

m/(mol �kg�1)

10�3 � d/(kg � m�3)

u/(m � s�1) 106 �V//(m3�mol�1)

1012 �js/Pa�1

1014 � j//(m3 � mol�1�Pa�1)

0.0940 0.999878 1519.17 201.99 433.35 0.770.1183 1.000950 1522.00 201.93 431.28 0.600.1423 1.002014 1524.70 201.81 429.30 0.530.1639 1.002936 1526.91 201.91 427.66 0.570.2206 1.005377 1532.67 201.78 423.42 0.620.2581 1.006949 1536.25 201.78 420.79 0.700.3034 1.008855 1540.65 201.66 417.60 0.730.3543 1.010929 1545.26 201.65 414.26 0.820.3874 1.012318 1548.20 201.47 412.12 0.860.4492 1.014808 1552.93 201.35 408.61 1.030.6269 1.021657 1566.84 201.15 398.70 1.270.8614 1.030031 1580.17 201.05 388.81 1.751.0007 1.034602 1586.48 201.12 384.02 2.021.3210 1.044425 1596.90 201.16 375.46 2.581.7388 1.055742 1604.73 201.25 367.82 3.192.0867 1.064092 1608.43 201.33 363.26 3.58

196

198

200

202

0 0.05 0.1 0.15 0.20 .25

m / (mol . kg-1)

Vφ

- A

v . m

1/2 /c

m3 . m

ol-1

FIGURE 1. The dependence of apparent molar volume (V/) as functionof molality of [HMIm]Br at different temperatures: (�) T = 283.15 K; (N)T = 288.15 K; (�) T = 293.15 K; (s) T = 298.15 K; (j) T = 303.15 K.

H. Shekaari et al. / J. Chem. Thermodynamics 40 (2008) 852–859 855

V / ¼ V �/ þ AV m1=2 þ BV m; ð2Þ

where V �/ is the limiting apparent molar volume (equal tothe partial molar volume at infinite dilution, V �2) and BV isan empirical parameter. The value of AV is the Pitzer–De-bye–Huckel limiting slope for apparent molar volumes inwater at different temperatures, which were calculated byAnanthaswamy and Atkinson [21], which are shown in ta-ble 2. The values of V �/ and BV values were obtained by aleast-squares analysis of equation (2) and their values foreach mixture at the studied experimental temperatures to-gether with standard deviations are listed in table 2. The cal-culated V/ values of [HMIm]Br in aqueous solutions at theexperimental temperatures were also plotted in the form ofV / � Av

ffiffiffiffimp

against molality of ionic liquid (figure 1). It isobserved that V �/ value at 298.15 K obtained in this workhas good agreement with that of by Wang (200.9cm3 � mol�1) [22]. At infinite dilution, each ion is sur-rounded only by the solvent molecules and being infinitelydistant with other ions. It follows, therefore, that V �/ is unaf-fected by (ion + ion) interaction and it is a measure only ofthe (ion + solvent) interaction [23]. As it can be seen It fol-lows from table 2, the values of V �/ for aqueous solutions of[HMIm]Br are greater than the corresponding values foraqueous solutions of [BMIm]Br as studied in our previouspaper [24]. This is due to the larger intrinsic volume of[HMIm]+ cation versus [BMIm]+ cation and difference in(ion + solvent) interactions.

TABLE 2The values of V �/, Bv, Av, Bj, j�/, and Ak obtained for each mixture at the exp

T/K 106 � V �/=ðm3 �mol�1Þ

106 � BV/(m3 � mol�2 � kg)

Av/(cm3 �mol�3/2 � kg1/2)

106 � r(

283.15 197.05 �1.38 1.6420 0.61288.15 198.47 �3.22 1.7150 0.51293.15 199.77 �4.59 1.7922 0.52298.15 200.91 �5.33 1.8743 0.45303.15 202.06 �6.25 1.9616 0.43

Table 2 shows BV values decrease with increasing tem-perature indicating the increased non-electrostatic interac-tions of [HMIm]Br at high temperatures [25]. This trendwas also observed for 1-propyl-3-methyl-imidazolium tet-rafluoroborate [26].

The limiting apparent molar volumes V �/ at the experi-mental temperatures were fitted to second order polyno-mial of the following type in terms of absolute temperature

V �/ ¼ Aþ BðT =KÞ þ CðT=KÞ2; ð3Þ

where A, B, and C are empirical parameters and T is theabsolute temperature. Values of coefficients A, B, and C

of the above equation obtained by least square analysisare 45.175 ± 3.128, 1.422 ± 0.213, and �0.002 ±3.641.10�4, respectively.

The parameter that measures the variation of volumewith temperature is the apparent molar expansibility,which was defined by equation (4). The limiting apparentmolar expansibility E�/ can be obtained by the followingequation:

E�/ ¼ ð@V �/=@T ÞP ¼ Bþ 2CT : ð4Þ

The values of E�/ for different solutions of the studied ionicliquid at different temperatures are given in table 3. Wenote that at each temperature, E�/ values for aqueous solu-tion of ionic liquid have positive values and decrease withrising temperature. Positive expansibility (i.e. increasing

erimental temperatures

V/) 1014 � j�/=ðm3 �mol�2�kg � Pa�1Þ

1014 � Bj/(m3 � mol�1�Pa�1)

104 � Ak/(cm3 � mol�3/2�kg1/2 � atm�1)

1014 � r(j/)

�7.16 19.12 �2.7258 1.79�3.33 0.51 �3.0689 0.73�1.05 �8.89 �3.4173 0.39

0.22 �12.13 �3.7784 0.431.15 �14.13 �4.1573 0.47

TABLE 3The infinite dilution apparent molar expansibility, E�/ values derived

T/K 283.15 288.15 293.15 298.15 303.15

E�/=ðcm3 �mol�1 �K�1Þ 0.2892 0.2692 0.2492 0.2292 0.2092

856 H. Shekaari et al. / J. Chem. Thermodynamics 40 (2008) 852–859

volume with increasing temperature) is a characteristicproperty of aqueous solutions of hydrophobic hydration.This result shows that on heating, some water moleculesmay be released from the hydration layers. This wouldincrease the solution volume a little more rapidly than thatof the pure water and so E�/ would be positive. The isobaricexpansivity of water is �0.25 � 10�3 K�1 under ambientconditions [27].

It has been shown that the sign of ð@2V �/=@T 2ÞP is abetter criterion in characterizing the long-range structure-making and breaking capacity of the electrolytes in solu-tion [28]. The general thermodynamic expression is asfollows:

ð@CP=@PÞT ¼ �T ð@2V �/=@T 2ÞP ¼ �2CT ð5Þ

If the sign of ð@2V �/=@T 2ÞP is small or negative, the elec-trolyte is a structure breaker; otherwise, it is a structuremaker. The value of ð@2V �/=@T 2ÞP for aqueous solution ofionic liquid has small and negative values (�0.004) forstudied systems. Therefore, ionic liquid under study arepredominantly a structure breaker. This is attributed tothe absence of caging or packing effects at a lowerconcentration.

3.2. Compressibility properties

Figure 2 shows the variations of experimental speeds ofsound parameter (Du = usolution � uwater) for the studied

1

51

101

151

0 0.5 11 .5 22 .5

m /(mol . kg-1)

u sol

utio

n-u

wat

er /m

. s-1

FIGURE 2. Variation of speed of sound parameter, Du as a function ofmolality of [HMIm]Br at different temperatures: (e) T = 283.15 K; (j)T = 288.15 K; (N) T = 293.15 K; (s) T = 298.15 K; (d) T = 303.15 K.

solutions, as function of temperature, for differentcompositions.

The isentropic compressibility js was evaluated for theinvestigated systems from Laplace–Newton’s equation

js ¼1

du2; ð6Þ

where u and d are speed of sound and density of the solu-tion, respectively.

Figure 3 shows the plot of the isentropic compressibili-ties of the aqueous solutions of [HMIm]Br versus molalityof ionic liquids at each working temperature. Decreasing ofjs values versus ionic liquids molality is due to the com-bined effect of hydration of ions and breaking of three-dimensional network structure of water. It can be seenthe isentropic compressibility isotherms intersect approxi-mately at a concentration m = 1.5825 mol � kg�1 and itsvalues for the five temperatures become identical. Thismeans that js has no further temperature dependence. Thisis indicating that all the water in the solution is now in thehydrated sphere of ions and these as Onori thought [29],would have a js with no temperature dependence and theywould be held tight by the ion and be little dependent onthe solvent temperature. This phenomenon has also beenobserved in aqueous electrolyte and non-electrolytesolutions [30,31]. The intersection point for the aqueoussolution of [BMIm]Br has been found close to 1.2772mol � kg�1 in our previous work. Glinski [32] has shownthat intersection point of the isentropic compressibility inaqueous electrolyte and non-electrolyte solutions is an indi-cator for formation of a clathrate-like structure in aqueoussystems. Because of being an ionic liquid is organic electro-lyte there is possible the formation of clathrate-like struc-

360

390

420

450

480

0 0.5 1 1.5 2

m /(mol . kg-1)

1012

. κs

/Pa-1

FIGURE 3. Variation of isentropic compressibility, js, versus molality of[HMIm]Br at different temperatures: (e) T = 283.15 K; (j) T = 288.15 K;(N) T = 293.15 K; (s) T = 298.15 K; (d) T = 303.15 K.

H. Shekaari et al. / J. Chem. Thermodynamics 40 (2008) 852–859 857

ture in aqueous solutions of ionic liquid systems. It hasbeen proved that in pure form, imidazolium based ionic liq-uids have a polymeric supramolecular structure, which isascribed to the hydrogen bonding between the imidazoliumcations and their respective anions and therefore, imidazo-lium based ionic (liquid + solvent) mixtures are highlycomplex. It is shown that sometimes ionic liquids formaggregates in water at low concentrations [33–36].

The apparent molar isentropic compressibility j/ of thestudied solutions was determined from the relation

TABLE 4Molar concentrations c, molar conductivities K of aqueous solution of [HMIm

T = 283.15 K T = 288.15 K T = 293.15 K

c/(mol � m�3)

K/(S � cm2 � mol�1)

c/(mol � m�3)

K/(S � cm2 � mol�1)

c/(mol � m�3)

K/(S �

0.71 97.26 0.89 103.05 0.60 106.1.38 96.91 2.42 101.88 1.37 105.2.41 96.09 2.95 101.47 2.18 105.3.23 96.09 3.75 101.25 2.82 104.4.08 95.46 5.44 100.47 4.05 103.4.96 95.41 6.89 100.18 5.93 103.5.96 94.62 9.37 99.00 7.59 102.6.83 94.60 11.85 97.97 9.37 101.7.54 94.50 14.44 97.00 10.25 100.9.33 94.14 17.59 95.69 12.60 99.11.66 93.16 20.53 94.65 14.68 97.14.10 92.63 24.47 93.22 17.10 96.16.70 91.89 28.82 91.63 19.49 95.21.00 90.58 32.68 90.36 21.85 94.24.46 89.56 36.91 89.06 24.61 93.28.21 88.51 41.83 87.66 27.44 92.33.35 86.93 46.40 86.45 30.87 91.37.71 85.74 52.14 84.91 34.66 90.43.69 84.15 58.97 83.22 38.02 89.45.83 83.64 41.20 88.

45.50 86.51.39 85.

-5.5

-4

-2.5

-1

0.5

0 0.05 0.1 0.15 0.2 0.25

m / mol . kg-1

1014

(kφ

-Ak.m

1/2

)/ m

3 . P

a-1. m

ol-1

FIGURE 4. The dependence of apparent molar isentropic compressibil-ity, j/, as function of concentration of [HMIm]Br at different tempera-tures: (r) T = 283.15 K; (N) T = 288.15 K; (�) T = 293.15 K; (s)T = 298.15 K; (j) T = 303.15 K.

j/ ¼ðjsd0 � js0dÞ

mdd0

þ jsMd

; ð7Þ

where js0 and js are the isentropic compressibility of puresolvent and mixture, respectively. An equation of the Red-lich–Mayer type

j/ ¼ j�/ þ Ajm1=2 þ Bjm; ð8Þ

was used for correlating of the experimental apparent mo-lar isentropic compressibility data. Here j�/ is the limitingapparent molar isentropic compressibility, Bj is an empir-ical parameter and Aj is the Debye–Huckel slope for theapparent molar compressibility and taken from the ref[11]. The values of j�/, and Bj obtained for aqueous solu-tion of ionic liquids at the experimental temperatures arelisted in table 2. Figure 4 shows the calculated j/ valuesof [HMIm]Br in aqueous solutions in the form ofj/ � Aj

ffiffiffiffimp

against molality of ionic liquid at the experi-mental temperatures. It can be seen limiting apparent mo-lar isentropic compressibilities values increase withincreasing temperature indicate that the solvent moleculessurrounding the [HMIm]Br would present greater resis-tance to compression than the bulk [37].

3.3. Conductometry measurements

Conductometry is an electrochemical technique, whichprovides us with most precise data for ions in solution,information concerning association of ions and characterof the ion-pair. Conductivity measurements yield boththe association constant as well as information about therelative solvating ability of solvents for the various ions.

]Br at different temperatures

T = 298.15 K T = 303.15 K

cm2 � mol�1)c/(mol � m�3)

K/(S � cm2 � mol�1)

c/(mol � m�3)

K/(S � cm2 � mol�1)

27 0.76 106.63 0.64 109.1763 1.57 106.28 1.45 107.7558 2.39 106.20 2.28 107.0886 3.32 105.73 3.14 106.4771 3.94 105.36 4.11 105.6224 4.62 105.19 5.65 105.2538 5.24 104.65 7.37 104.4313 6.68 103.56 9.98 103.3053 8.02 102.70 12.35 102.4710 9.95 101.66 15.04 101.3084 12.15 100.36 18.88 99.9992 15.12 98.89 22.32 98.7286 17.97 97.65 26.57 97.0096 21.82 95.90 31.69 95.3898 25.91 94.26 36.29 93.6388 30.63 92.33 40.92 92.4562 35.36 90.65 44.95 91.2841 39.61 89.16 50.10 89.8821 44.40 87.67 63.84 85.3828 49.44 86.1496 59.14 83.3928

858 H. Shekaari et al. / J. Chem. Thermodynamics 40 (2008) 852–859

There are many different equations, which are preferred byauthors for the treatment of conductance data [38]. Thelow concentration chemical model (LcCM) of conductivityequation is widely recently applied for the correlation ofconductance data in aqueous and non-aqueous electrolytesolution [39]. The conductivity equation of the lcCM isthe Fuoss–Onsager type equation.

The molar concentrations c were determined fromc = md/(1 + M2d), where m, d, and M2 are the molalityof ionic liquid, the corresponding solution densities andmolar mass of ionic liquid, respectively.

The calculated molar conductivities K using the mea-sured specific conductivities j in the aqueous solutions of[HMIm]Br are given in table 4. Figure 5 shows the depen-dence of the molar conductivity K on the molar concentra-tion c for the investigated system at different experimentaltemperatures. It is shown that the molar conductivity Kdecreases with the increasing amount of ionic liquid.Increasing in ionic liquid concentration causes the forma-tion of ion-pair in dilute region and the stronger ion asso-ciation in the studied mixtures.

Analysis of conductivity data in the framework of thelow-concentration chemical model (LcCM) uses the set ofequations

K ¼ a½K0 � SðcaÞ1=2 þ Eca lnðcaÞ þ J 1 caþ J 2ðcaÞ3=2�; ð9Þ

KA ¼1� aa2cc2

�; ð10Þ

ln c� ¼ �jq

1þ jR; ð11Þ

where

80.00

90.00

100.00

110.00

0.00 20.00 40.00 60.00 80.00

c /mol.m-3

Λ/S

.cm

2 .mol

-1

FIGURE 5. Molar conductivity of aqueous solutions of [HMIm]Br atdifferent temperatures (j) T = 283.15 K; (+) T = 288.15 K; (M)T = 293.15 K; () T = 298.15 K; (d) T = 303.15 K as function of molarconcentration of ionic liquid.

j2 ¼ 2 � 103NAz2e2ace0ekBT

; ð12Þ

q ¼ z2e2

8pe0ekBT; ð13Þ

where K and K0 are the molar conductivities at molarity c

and infinite dilution, a is the fraction of oppositely chargedions acting as ion pairs, Ka is the ion association constantof [HMIm]Br, R distance parameter, c± is the correspond-ing mean activity coefficient of the free ions. The coeffi-cients of equation (9) reflect the relaxation (rel) andelectrophoretic (el) effect. The rest parameters have theusual meaning.

Three-parameter fits of molar conductivity data yieldsthe association constant Ka, the limiting molar conductivityK0 and distance parameter R by non-linear least-squaresiteration. Ka, K0, and R parameters are summarized intable 5.

The standard deviation r(F) of the measured quantitiesF(exp.) and the calculated one F(calc.) was computed asfollows:

rðF Þ ¼X½F ðexp:Þ � F ðcalc:Þ�2=ðn� pÞ

� �1=2

; ð14Þ

where F represents j/ or V/ or K, and n and p show thenumber of the experimental data and parameters,respectively.

Increasing Ka values with increasing temperature intable 5 show that the extent of ion association in[HMIm]Br + H2O solutions increases with increasingtemperature and (ion + solvent) interactions decreases.Therefore, the strong ion solvation was observed for[HMIm]Br + H2O solutions at low temperature. Theseobservations also support our earlier view [24]. Hydrationof this ionic liquid is similar to alkylammonium halide[40], which occurs by the occupation of intermolecular cav-ities of an ice-like water structure by organic chains of theseions. This pattern of ion hydration is called ‘‘hydrophobic-hydration” or ‘‘cage-association”. These ions are incapableof donor–acceptor interaction with solvent molecules andion pairs in such solutions can be mainly stabilized byintrinsic ice-like water structure and interaction of hydro-carbon portion of ionic liquid is considerable importanceto the phenomenon of their ion pair formation in aqueoussolutions.

TABLE 5Ionic association constant Ka, limiting molar conductance K0, distanceparameter R, and standard deviation r(K) in aqueous solutions of[HMIm]Br in low concentrations <102 mol � m�3 at different temperatures

T/K Ka/(dm3 � mol�1) K0/(S � cm2 � mol�1) 1010 � R/m r(K)

283.15 2.94 97.98 6.00 0.04288.15 3.73 103.90 6.00 0.03293.15 4.84 107.27 10.02 0.08298.15 4.90 108.36 5.14 0.20303.15 4.93 109.46 9.89 0.17

H. Shekaari et al. / J. Chem. Thermodynamics 40 (2008) 852–859 859

R values reflect the most probable separation betweenthe ions in the ion-pair. As seen from table 5, the R valuesfor the studied systems have not good order based on theother obtained parameters are not very close to the sumof the ionic radii in the crystal.

4. Conclusions

Volumetric and electrical conductance properties ofionic liquid, 1-butyl-3-methyl-imidazolium bromide([HMIm]Br) in water have been reported at temperaturesT = (283.15 to 308.15) K. Using density and speed ofsound data, apparent molar volume, and apparent molarisentropic compressibility data for the studied solutionsat each temperature have been evaluated and correlatedvia the Redlich–Mayer type equations for obtaining limit-ing apparent molar volume and apparent molar isentropiccompressibility. These parameters show that the interac-tions between [HMIm]Br and water decrease with increas-ing temperature. The obtained limiting apparent molarexpansibility values also confirm this conclusion. Isentropiccompressibility isotherms intersect approximately at a con-centration m = 1.5825 mol � kg�1, which may be an indica-tion of the structural interactions and clathrate formation.The value of ð@2V �/=@T 2ÞP is small and negative and there-fore, the ionic liquid under study is predominantly a struc-ture breaker. Ion association constant Ka values increasewith increasing temperature show that the extent of ionassociation increases with increasing temperature and(ion + solvent) interactions decreases.

Acknowledgement

The Research Council of The University of MohagheghArdabili (Iran) is gratefully acknowledged for their finan-cial support.

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JCT 07-353