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Study on Enthalpy and Molar Heat Capacity of Solution for the Ionic Liquid [C2mim][OAc] (1-Ethyl-3-methylimidazolium acetate) Xiao-Xue Ma, Long Li, Jie Wei, Wen-Bin Duan, Wei Guan,* and Jia-Zhen Yang College of Chemistry, Liaoning University, Shenyang 110036, People's Republic of China S Supporting Information *

ABSTRACT: An acetic acid ionic liquid (AcAIL) [C2mim][OAc] (1-ethyl-3methylimidazolium acetate) was prepared by the neutralization method. Using the solution-reaction isoperibol calorimeter, molar enthalpies of solution, ΔsolHm, for the ionic liquid [C2mim][OAc] with different molalities were measured in the temperature range from (288.15 to 308.15 ± 0.01) K with an interval of 5 K. In terms of Archer’s method, the standard molar enthalpies of solution, ΔsolHθm, for [C2mim][OAc] were obtained in the temperature range of (288.15 to 308.15 ± 0.01) K. Plotting ΔsolHθm against (T − 298.15) K, a good straight line was obtained, with the slope of the line being the standard molar heat capacity of solution, ΔCθp,m = 411 J·K−1·mol−1, for [C2mim][OAc], and the specific heat capacity of solution, ΔCθp = 2.4 J·g−1·K−1, was also obtained.

1. INTRODUCTION Because of their minuscule vapor pressure, nonflammability, and dual natural polarity, ionic liquids (ILs) have been expected to be applied in many physical and chemical fields.1 As a new-generation of “greener ionic liquids”, acetic acid ionic liquids (AcAILs) have attracted considerable attention from industry and the academic community, because they have several unique properties including strong solubilities and good catalytic properties, which are useful for an enzyme-”friendly” cosolvents for resolution of amino acids, ultrasonic irradiation toward synthesis of trisubstituted imidazoles,3 assisted transdermal delivery of sparingly soluble drugs,4 and some catalytic reactions.5 It is known that enthalpy, molar heat capacity, and specific heat capacity of a solution are the basic thermodynamic datum and of great importance for any industrial process for application of AcAIL. As a continuation of our previous investigation,6−9 this paper reports the following. (1) An acetic acid ionic liquid [C2mim][OAc] (1-ethyl-3-methylimidazolium acetate) was prepared by the neutralization method according to Fukumoto.10 (2) The molar enthalpies of solution for IL [C2mim][OAc] were measured in the temperature range of (288.15 to 308.15 ± 0.01) K with an interval of 5 K. (3) The value of the standard molar enthalpy of solution, ΔsolHθm, for [C2mim][OAc] was obtained in terms of Archer’s method.11 The standard molar heat capacity of solution, ΔCθp,m, for [C2mim][OAc] was obtained from the slope of the straight line of ΔsolHθm vs (T − 298.15) K, and the specific heat capacity of solution, ΔCθp, for [C2mim][OAc] was calculated.

reduced pressure. 1-Methylimidazole was distilled under reduced pressure before use. Bromoethane, ethyl acetate, and acetonitrile were distilled and then stored over molecular sieves in tightly sealed glass bottles. KCl was dried in a vacuum oven at 408 K for 6 h, and THAM (Tris-(hydroxymethyl)aminomethane), GR grade reagent, was dried in a vacuum oven before use. The source and purity of the materials are also listed in Table 1. Table 1. Source and Purity of the Materials purity ethyl acetate (AR) acetone (AR) acetonitrile (AR) acetic acid (AR) 1-methylimidazole (AR) bromoethane (AR) anion-exchange resin (type 717) THAM (Tris-(hydroxymethyl)aminomethane) (GR) KCl (99.99 %)

Shenyang Reagent Co. LTD

Anion-exchange resin (type 717) was purchased from Shanghai Chemical Reagent Co. Ltd. and activated by regular method before use. 2.2. Preparation of [C 2 mim][OAc]. According to Fukumoto’s method,10 [C2mim][OAc] was synthesized. At first, [C2mim]Br was synthesized according to the literature.12,13 Then an aqueous solution of 1-ethyl-3-methylimidazolium hydroxide

2. EXPERIMENTAL SECTION 2.1. Chemicals. Deionized water was distilled in a quartz still, and its conductance was (0.8 to 1.2)·10−4 S·m−1. The AR grade acetic acid was recrystallized twice and was dried under © 2012 American Chemical Society

source Shenyang Reagent Co. LTD Shenyang Reagent Co. LTD Shenyang Reagent Co. LTD Shenyang Reagent Co. LTD ACROS Shenyang Reagent Co. LTD Shanghai Reagent Co. LTD Shenyang Reagent Co. LTD

Received: July 11, 2012 Accepted: October 17, 2012 Published: October 31, 2012 3171

dx.doi.org/10.1021/je300768j | J. Chem. Eng. Data 2012, 57, 3171−3175

Journal of Chemical & Engineering Data

Article

Table 2. Values of Molar Solution Enthalpies for IL [C2mim][OAc], ΔsolHm/kJ·mol−1, and Extrapolation Function, Y/kJ·mol−1, at 288.15 K, Pressure p = 0.1 MPaa

a

W/g

m/mol·kg−1

0.5124 0.5955 0.6843 0.7704 0.8519 0.9420 1.0254 1.1089 1.1920 1.2803 1.3675 1.4470

0.03014 0.03503 0.04025 0.04532 0.05011 0.05541 0.06032 0.06523 0.07012 0.07531 0.08044 0.08512

−ΔsHm/kJ·mol−1 51.77 51.59 51.26 51.10 50.80 50.59 50.40 50.21 49.94 49.73 49.46 49.29

± ± ± ± ± ± ± ± ± ± ± ±

0.25 0.21 0.18 0.11 0.14 0.15 0.28 0.17 0.34 0.27 0.19 0.14

ΔT/K 0.3149 0.3585 0.4052 0.4504 0.4932 0.5405 0.5843 0.6282 0.6718 0.7182 0.7640 0.8057

± ± ± ± ± ± ± ± ± ± ± ±

0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001

−Y/kJ·mol−1 52.03 51.87 51.56 51.41 51.13 50.93 50.76 50.58 50.32 50.12 49.86 49.70

± ± ± ± ± ± ± ± ± ± ± ±

0.25 0.21 0.18 0.11 0.14 0.15 0.28 0.17 0.34 0.27 0.19 0.14

W is the sample mass, and ΔT is the experimental temperature difference.

Table 3. Values of Molar Solution Enthalpy for IL [C2mim][OAc], ΔsolHm/kJ·mol−1, and Extrapolation Function, Y/kJ·mol−1, at 293.15 K, Pressure p = 0.1 MPaa

a

W/g

m/mol·kg−1

0.5121 0.5984 0.6817 0.7735 0.8551 0.9367 1.0234 1.1118 1.1951 1.2852 1.3651 1.4484

0.03001 0.03523 0.04012 0.04551 0.05036 0.05514 0.06027 0.06544 0.07033 0.07566 0.08034 0.08522

−ΔsHm/kJ·mol−1 49.99 49.78 49.52 49.35 49.16 48.94 48.70 48.51 48.31 48.09 47.92 47.76

± ± ± ± ± ± ± ± ± ± ± ±

0.23 0.16 0.34 0.21 0.27 0.13 0.36 0.13 0.26 0.11 0.13 0.18

ΔT/K 0.2894 0.3341 0.3772 0.4088 0.4670 0.5093 0.5541 0.5999 0.6430 0.6897 0.7310 0.7742

± ± ± ± ± ± ± ± ± ± ± ±

0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001

−Y/kJ·mol−1 50.28 50.08 49.85 49.71 49.53 49.32 49.09 48.90 48.73 48.50 48.36 48.21

± ± ± ± ± ± ± ± ± ± ± ±

0.23 0.16 0.34 0.21 0.27 0.13 0.36 0.13 0.26 0.11 0.13 0.18



a

W/g

m/mol·kg−1

0.5139 0.6025 0.6888 0.7745 0.8522 0.9408 1.0331 1.1086 1.1988 1.2827 1.3622 1.4521

0.03023 0.03544 0.04052 0.04556 0.05013 0.05534 0.06077 0.06521 0.07052 0.07545 0.08013 0.08542

−ΔsHm/kJ·mol−1 48.15 47.97 47.87 47.72 47.54 47.40 47.30 47.16 46.97 46.84 46.67 46.58

± ± ± ± ± ± ± ± ± ± ± ±

0.21 0.23 0.34 0.14 0.16 0.17 0.27 0.23 0.24 0.31 0.30 0.18

ΔT/K 0.2591 0.2981 0.3351 0.3736 0.4098 0.4491 0.4872 0.5243 0.5628 0.6014 0.6389 0.7218

± ± ± ± ± ± ± ± ± ± ± ±

0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001

−-Y/kJ·mol−1 48.46 48.30 48.23 48.11 47.94 47.81 47.73 47.60 47.43 47.31 47.15 47.08

± ± ± ± ± ± ± ± ± ± ± ±

0.21 0.23 0.34 0.14 0.16 0.17 0.27 0.23 0.24 0.31 0.30 0.18


confirmed by 1H NMR (Varian XL-300,DMSO, δ/ppm relative to TMS. See Figure A of the Supporting Information): 1.471−1.525 (t, 3H), 1.912 (s, 3H), 3.886 (s, 3H), 4.264−4.236 (q, 2H), 7.416−7.422 (d, 1H), 7.484−7.490 (d, 1H), 8.715 (s, 1H). By estimating from the 1H NMR results, the sample purity of [C2mim][OAc] is more than 99 % (mass fraction). Differential scanning calorimetric (DSC) measurements showed that AcAIL [C2mim][OAc] had no obvious melting point, but its glass transition temperature was Tg = −75.5 °C. The traces of DSC are listed in Figure B of the Supporting Information.

([C2mim][OH]) was prepared from [C2mim]Br using anion exchange resin over a 100 cm column. The aqueous solution was added dropwise to a slightly excess equimolar acetic acid aqueous solution. After the mixture was stirred under cooling for 12 h, the water was evaporated at (40 to 50) °C under reduced pressure. Finally, under vigorous stirring, the mixed solvent of acetonitrile/methanol (volumetric ratio 9/1) was added and filtered to remove excess acetic acid. The filtrate was evaporated to remove solvents. The product was dried in vacuo for 2 days at 80 °C. The structure of [C2mim][OAc] was 3172



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a

W/g

m/mol·kg−1

0.5173 0.5970 0.6877 0.7771 0.8561 0.9455 1.0338 1.1108 1.1971 1.2857 1.3612 1.4487

0.03043 0.03512 0.04045 0.04571 0.05036 0.05562 0.06081 0.06534 0.07042 0.07563 0.08007 0.08522

−ΔsHm/kJ·mol−1 46.18 46.02 45.91 45.75 45.61 45.50 45.35 45.21 45.11 44.95 44.71 44.67

± ± ± ± ± ± ± ± ± ± ± ±

0.24 0.21 0.23 0.26 0.34 0.31 0.28 0.17 0.18 0.15 0.13 0.16

ΔT/K 0.2267 0.2614 0.3038 0.3443 0.3801 0.4205 0.4604 0.4952 0.5343 0.5744 0.6085 0.6481

± ± ± ± ± ± ± ± ± ± ± ±

0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001

−Y/kJ·mol−1 46.52 46.38 46.28 46.16 46.03 45.95 45.81 45.69 45.61 45.46 45.32 45.21

± ± ± ± ± ± ± ± ± ± ± ±

0.24 0.21 0.23 0.26 0.34 0.31 0.28 0.17 0.18 0.15 0.13 0.16



a

W/g

m/mol·kg−1

0.5117 0.6001 0.6902 0.7718 0.8636 0.9469 1.0285 1.1118 1.1951 1.2818 1.3634 1.4535

0.03012 0.03533 0.04061 0.04545 0.05082 0.05574 0.06052 0.06541 0.07036 0.07548 0.08025 0.08553

−ΔsHm/kJ·mol−1 43.97 43.81 43.66 43.51 43.41 43.24 43.06 42.95 42.77 42.60 42.51 42.35

± ± ± ± ± ± ± ± ± ± ± ±

0.23 0.31 0.14 0.16 0.17 0.23 0.18 0.19 0.26 0.27 0.21 0.26

ΔT/K 0.1891 0.2191 0.2542 0.2895 0.3243 0.3624 0.3928 0.4281 0.4659 0.5016 0.5281 0.5623

± ± ± ± ± ± ± ± ± ± ± ±

0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001

−Y/kJ·mol−1 44.34 44.21 44.08 43.95 43.87 43.73 43.56 43.47 43.31 43.16 43.07 42.94

± ± ± ± ± ± ± ± ± ± ± ±

0.23 0.31 0.14 0.16 0.17 0.23 0.18 0.19 0.26 0.27 0.21 0.26


Figure 1. Plot of Y (extrapolation function calculated from eq 1) against m for [C2mim][OAc] in the temperature range of (288.15 to 308.15) K. ■, 308.15 K: Y = −45.12 + 25.62m, s = 0.02, r = 0.999. Red circle, 303.15 K: Y = −47.23 + 23.53m, s = 0.02, r = 0.999. Blue triangle, 298.15 K: Y = −49.24 + 25.51m, s = 0.03, r = 0.998. Green triangle, 293.15 K: Y = −51.42 +38.09m, s = 0.03, r = 0.999. Pink triangle, 288.15 K: Y = −53.32 + 42.61m, s = 0.03, r = 0.999. 3173



Article

Figure 2. Plot of the standard molar enthalpy of solution, ΔsolHθm, against (T − 298.15) K for [C2mim][OAc]. ■, ΔsolHθm = −49.27 + 0.4114(T − 298.15).

The values of ΔsolHθm and β obtained from the intercepts and the slopes of linear regressions are listed in Table 7, respec-

2.3. Determination of the Enthalpy of Solution. On the basis of other calorimetric apparatus,14−16 an online solutionreaction isoperibol calorimeter was constructed, and the detailed experimental procedure has been described elsewhere.8 The performance and accuracy of the calorimetric system were tested by measuring the molar enthalpy of solution of KCl in water and THAM in 0.1 mol·dm−3 HCl (aq) at (298.15 ± 0.01) K. The mean molar solution enthalpies are ΔsolH m = (17542 ± 31) J·mol−1 for KCl and (−29794 ± 28) J·mol−1 for THAM, which are in good agreement with the corresponding published data: (17536 ± 9) J·mol−1 for KCl17,18 and (- 29739 ± 10) J·mol−1 for THAM.18 These results show that calorimeters can be applied in our research work. The molar enthalpies of a solution of [C2mim][OAc] with different molalities, ΔsolHm, were measured at the temperature ranging of (288.15 to 308.15 ± 0.01) K with an interval of 5 K.

Table 7. Values of Standard Molar Solution Enthalpy for IL [C2mim][OAc], ΔsolHθm/kJ·mol−1 in Temperature Range of (288.15 to 308.15) Ka T/K −ΔsolHθm/kJ·mol−1 β s r a

288.15

293.15

298.15

303.15

308.15

53.32 42.61 0.02 0.999

51.42 38.09 0.02 0.999

49.24 25.51 0.03 0.998

47.23 23.53 0.03 0.999

45.13 25.62 0.03 0.999

s is the standard deviation. r is correlation coefficient.

tively. The values of the correlation coefficients of all linear regressions, r, are larger than 0.99. 3.2. Standard Molar Heat Capacity of Solution, ΔCθp,m. When the experimental values of ΔsolHθm for AcAIL [C2mim][OAc] were plotted against (T − 298.15) K, a good straight line was obtained (see Figure 2). The linear regression equation is ΔsolHθm = −49.27 + 0.4114(T − 298.15) with a correlation coefficient, r = 0.999, and standard deviation, s = 0.07. The slope of the linear regression means the standard molar heat capacity of solution, ΔCθp,m = 411 J·K−1·mol−1, for [C2mim][OAc]. According to the ΔCθp,m, the specific heat capacity of solution, ΔCθp = 2.4 J·g−1·K−1, for [C2mim][OAc] was also calculated.

3. RESULTS AND DISCUSSION 3.1. Standard Molar Enthalpy of Solution, ΔsolHθm, for AcAIL [C2mim][OAc]. The measured values of molar solution enthalpies, ΔsolHθm, of [C2mim][OAc] and with various molalities at (288.15 to 308.15 ± 0.01) K are listed in Tables 2−6, respectively. As can be seen from these tables, the dissolution process of the AcAIL is a typical exothermal. According to Archer’s method,11 in terms of a Debye− Hückel limiting term, the values of the standard molar enthalpy of solution for [C2mim][OAc], ΔsolHθm, can be obtained using the following equation:

■

Y = Δsol Hm − (AH /b) ln(1 + bI1/2) = Δsol Hmθ + βm

ASSOCIATED CONTENT

S Supporting Information *

(1)

1

H NMR spectra (Figure A and Table 1) and thermal analysis (Figure B). This material is available free of charge via the Internet at http://pubs.acs.org.

where m is the molality, I is the ionic strength (I = m for the 1:1 electrolyte [C2mim][OAc]), b is a constant that is 1.2,19 AH is the Debye−Hückel parameter for enthalpy and its value at different temperature was taken from the literature,19 β is the empirical constant, and Y is the extrapolation function calculated from experimental data and listed in Tables 2−6, respectively. According to eq 1, plotting the values of Y against various molarities, good straight lines were obtained (see Figure 1).

■

AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 24 62207797. Fax: +86 24 62202380. E-mail: [email protected]. 3174



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Funding

(18) Montgomery, R. L.; Melaugh, R. A.; Lau, C.-C.; Meier, G. H.; Chan, H. H.; Rossini, F. D. Determination of the Energy Equivalent of A Water Solution Calorimeter with a Standard Substance. J. Chem. Thermodyn. 1977, 9, 915−936. (19) Pitzer, K. S. In Activity Coefficients in Electrolyte Solution; Pitzer, K. S., Ed.; CRC Press: Boca Raton, FL, 1991; Chapter 3.

This project was supported by NSFC (21173107), Education Bureau, Science and Technology of Liaoning Province (LS2010068, 201102078), People's Republic of China. Notes

The authors declare no competing financial interest.

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REFERENCES

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