Study on enthalpy and molar heat capacity of solution for the ionic liquid [C2mim][OAc] (1-Ethyl-3-methylimidazolium acetate)

Article abstract of DOI:10.1021/je300768j

An acetic acid ionic liquid (AcAIL) [C₂mim][OAc] (1-ethyl-3-methylimidazolium acetate) was prepared by the neutralization method. Using the solution-reaction isoperibol calorimeter, molar enthalpies of solution, δ_solH_m, for the ionic liquid [C ₂mim][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, δ_solH_m^θ, for [C₂mim] [OAc] were obtained in the temperature range of (288.15 to 308.15 ± 0.01) K. Plotting δ_solH_m^θ 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, δC_p,m ^θ = 411 J·K^-1·mol^-1, for [C₂mim][OAc], and the specific heat capacity of solution, δC_p^θ = 2.4 J·g ^-1·K^-1, was also obtained.

Full text of DOI:10.1021/je300768j

Article
pubs.acs.org/jced
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-3-
methylimidazolium 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, ΔCpθ = 2.4 J·g−1·K−1, was also obtained.
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.
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 synthe-
sis 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 applica-
tion of AcAIL. As a continuation of our previous investiga-
tion,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.
Table 1. Source and Purity of the Materials
purity
source
ethyl acetate (AR)
acetone (AR)
Shenyang Reagent Co. LTD
Shenyang Reagent Co. LTD
Shenyang Reagent Co. LTD
Shenyang Reagent Co. LTD
ACROS
acetonitrile (AR)
acetic acid (AR)
1-methylimidazole (AR)
bromoethane (AR)
Shenyang Reagent Co. LTD
Shanghai Reagent Co. LTD
anion-exchange resin (type 717)
THAM (Tris-(hydroxymethyl)aminomethane) Shenyang Reagent Co. LTD
(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 [C2mim][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
Received: July 11, 2012
Accepted: October 17, 2012
Published: October 31, 2012
© 2012 American Chemical Society
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Table 2. Values of Molar Solution Enthalpies for IL [C2mim][OAc], ΔsolHm/kJ·mol−1, and Extrapolation Function, Y/kJ·mol−1,
a
at 288.15 K, Pressure p = 0.1 MPa
W/g
m/mol·kg−1
−ΔsHm/kJ·mol−1
ΔT/K
−Y/kJ·mol−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
51.77 0.25
51.59 0.21
51.26 0.18
51.10 0.11
50.80 0.14
50.59 0.15
50.40 0.28
50.21 0.17
49.94 0.34
49.73 0.27
49.46 0.19
49.29 0.14
0.3149 0.0001
0.3585 0.0001
0.4052 0.0001
0.4504 0.0001
0.4932 0.0001
0.5405 0.0001
0.5843 0.0001
0.6282 0.0001
0.6718 0.0001
0.7182 0.0001
0.7640 0.0001
0.8057 0.0001
52.03 0.25
51.87 0.21
51.56 0.18
51.41 0.11
51.13 0.14
50.93 0.15
50.76 0.28
50.58 0.17
50.32 0.34
50.12 0.27
49.86 0.19
49.70 0.14
a
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
a
293.15 K, Pressure p = 0.1 MPa
W/g
m/mol·kg−1
−ΔsHm/kJ·mol−1
ΔT/K
−Y/kJ·mol−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
49.99 0.23
49.78 0.16
49.52 0.34
49.35 0.21
49.16 0.27
48.94 0.13
48.70 0.36
48.51 0.13
48.31 0.26
48.09 0.11
47.92 0.13
47.76 0.18
0.2894 0.0001
0.3341 0.0001
0.3772 0.0001
0.4088 0.0001
0.4670 0.0001
0.5093 0.0001
0.5541 0.0001
0.5999 0.0001
0.6430 0.0001
0.6897 0.0001
0.7310 0.0001
0.7742 0.0001
50.28 0.23
50.08 0.16
49.85 0.34
49.71 0.21
49.53 0.27
49.32 0.13
49.09 0.36
48.90 0.13
48.73 0.26
48.50 0.11
48.36 0.13
48.21 0.18
a
W is the sample mass, and ΔT is the experimental temperature difference.
Table 4. Values of Molar Solution Enthalpy for IL [C2mim][OAc], ΔsolHm/kJ·mol−1, and Extrapolation Function, Y/kJ·mol−1, at
a
298.15 K, Pressure p = 0.1 MPa
W/g
m/mol·kg−1
−ΔsHm/kJ·mol−1
ΔT/K
−-Y/kJ·mol−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
48.15 0.21
47.97 0.23
47.87 0.34
47.72 0.14
47.54 0.16
47.40 0.17
47.30 0.27
47.16 0.23
46.97 0.24
46.84 0.31
46.67 0.30
46.58 0.18
0.2591 0.0001
0.2981 0.0001
0.3351 0.0001
0.3736 0.0001
0.4098 0.0001
0.4491 0.0001
0.4872 0.0001
0.5243 0.0001
0.5628 0.0001
0.6014 0.0001
0.6389 0.0001
0.7218 0.0001
48.46 0.21
48.30 0.23
48.23 0.34
48.11 0.14
47.94 0.16
47.81 0.17
47.73 0.27
47.60 0.23
47.43 0.24
47.31 0.31
47.15 0.30
47.08 0.18
a
W is the sample mass, and ΔT is the experimental temperature difference.
1
([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
confirmed by H NMR (Varian XL-300,DMSO, δ/ppm rela-
tive 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
1
(s, 1H). By estimating from the H NMR results, the sample
purity of [C2mim][OAc] is more than 99 % (mass fraction). Dif-
ferential 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.
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Table 5. Values of Molar Solution Enthalpy for IL [C2mim][OAc], ΔsolHm/kJ·mol−1, and Extrapolation Function, Y/kJ·mol−1, at
a
303.15 K, Pressure p = 0.1 MPa
W/g
m/mol·kg−1
−ΔsHm/kJ·mol−1
ΔT/K
−Y/kJ·mol−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
46.18 0.24
46.02 0.21
45.91 0.23
45.75 0.26
45.61 0.34
45.50 0.31
45.35 0.28
45.21 0.17
45.11 0.18
44.95 0.15
44.71 0.13
44.67 0.16
0.2267 0.0001
0.2614 0.0001
0.3038 0.0001
0.3443 0.0001
0.3801 0.0001
0.4205 0.0001
0.4604 0.0001
0.4952 0.0001
0.5343 0.0001
0.5744 0.0001
0.6085 0.0001
0.6481 0.0001
46.52 0.24
46.38 0.21
46.28 0.23
46.16 0.26
46.03 0.34
45.95 0.31
45.81 0.28
45.69 0.17
45.61 0.18
45.46 0.15
45.32 0.13
45.21 0.16
a
W is the sample mass, and ΔT is the experimental temperature difference.
Table 6. Values of Molar Solution Enthalpy for IL [C2mim][OAc], ΔsolHm/kJ·mol−1, and Extrapolation Function, Y/kJ·mol−1, at
a
308.15 K, Pressure p = 0.1 MPa
W/g
m/mol·kg−1
−ΔsHm/kJ·mol−1
ΔT/K
−Y/kJ·mol−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
43.97 0.23
43.81 0.31
43.66 0.14
43.51 0.16
43.41 0.17
43.24 0.23
43.06 0.18
42.95 0.19
42.77 0.26
42.60 0.27
42.51 0.21
42.35 0.26
0.1891 0.0001
0.2191 0.0001
0.2542 0.0001
0.2895 0.0001
0.3243 0.0001
0.3624 0.0001
0.3928 0.0001
0.4281 0.0001
0.4659 0.0001
0.5016 0.0001
0.5281 0.0001
0.5623 0.0001
44.34 0.23
44.21 0.31
44.08 0.14
43.95 0.16
43.87 0.17
43.73 0.23
43.56 0.18
43.47 0.19
43.31 0.26
43.16 0.27
43.07 0.21
42.94 0.26
a
W is the sample mass, and ΔT is the experimental temperature difference.
■
,
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.
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θ
θ
■
Figure 2. Plot of the standard molar enthalpy of solution, ΔsolHm, against (T − 298.15) K for [C2mim][OAc]. , ΔsolHm = −49.27 + 0.4114(T − 298.15).
2.3. Determination of the Enthalpy of Solution. On the
basis of other calorimetric apparatus,14−16 an online solution-
reaction 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
The values of ΔsolHθm and β obtained from the intercepts and
the slopes of linear regressions are listed in Table 7, respec-
Table 7. Values of Standard Molar Solution Enthalpy for IL
[C2mim][OAc], ΔsolHmθ /kJ·mol−1 in Temperature Range of
a
(288.15 to 308.15) K
T/K
0.01) K. The mean molar solution enthalpies are ΔsolH
=
m
(17542 31) J·mol−1 for KCl and (−29794 28) J·mol−1
288.15
293.15
298.15
303.15
308.15
−ΔsolHθm/kJ·mol−1
β
53.32
42.61
0.02
51.42
38.09
0.02
49.24
25.51
0.03
47.23
23.53
0.03
45.13
25.62
0.03
for THAM, which are in good agreement with the corre-
sponding 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.
s
r
0.999
0.999
0.998
0.999
0.999
a
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. RESULTS AND DISCUSSION
3.2. Standard Molar Heat Capacity of Solution, ΔCpθ,m
.
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 mol-
alities 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.
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.
According to Archer’s method,11 in terms of a Debye−
Huckel limiting term, the values of the standard molar enthalpy
̈
of solution for [C2mim][OAc], ΔsolHθm, can be obtained using
the following equation:
Y = ΔsolHm − (AH/b) ln(1 + bI1/2) = ΔsolHmθ + βm
ASSOCIATED CONTENT
■
(1)
S
* Supporting Information
1H 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−Huckel 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 cal-
culated from experimental data and listed in Tables 2−6, respec-
tively. 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:
guanweilnu@yahoo.com.cn.
■
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Funding
This project was supported by NSFC (21173107), Education
Bureau, Science and Technology of Liaoning Province
(LS2010068, 201102078), People's Republic of China.
Article
(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.
Notes
The authors declare no competing financial interest.
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