Density and surface tension of ionic liquids [Cnpy][NTf 2] (n = 2, 4, 5)

Article abstract of DOI:10.1021/je100507n

Density and surface tension of the air- and water-stable hydrophobic ionic liquids N-alkylpyridinium bis(trifluoromethylsulfonyl)imide ([C _npy][NTf₂], n = 2, 4, 5) were measured in the temperature range T = (283.15 to 338.15) K. The melting temperatures of the samples were determined by differential scanning calorimetry (DSC). Decomposition temperatures are higher than 600 K as determined by thermogravimetric analysis (TG).

Full text of DOI:10.1021/je100507n

4928
J. Chem. Eng. Data 2010, 55, 4928–4930
Density and Surface Tension of Ionic Liquids [Cnpy][NTf2] (n ) 2, 4, 5)
Qing-Shan Liu,† Miao Yang,† Pei-Fang Yan,† Xiu-Mei Liu,† Zhi-Cheng Tan,*,†,‡ and Urs Welz-Biermann*,†
China Ionic Liquid Laboratory, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China, and
Thermochemistry Laboratory, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China
Density and surface tension of the air- and water-stable hydrophobic ionic liquids N-alkylpyridinium
bis(trifluoromethylsulfonyl)imide ([Cnpy][NTf2], n ) 2, 4, 5) were measured in the temperature range T )
(283.15 to 338.15) K. The melting temperatures of the samples were determined by differential scanning
calorimetry (DSC). Decomposition temperatures are higher than 600 K as determined by thermogravimetric
analysis (TG).
Introduction
Ionic liquids (ILs) as organic salts have exhibited low melting
temperature, good solvation, nonvolatility properties, and higher
thermodynamic stability, so both industrial and scientific com-
munities are advertising them for broad ranges of applications
as green solvents. Actually, most ILs are hydrophilic; therefore,
the anions for hexafluorophosphate (PF6-) and bis(trifluoro-
methylsulfonyl)imide (NTf2-) ILs attracted much concern as
hydrophobic compounds when they appeared,1-4 and the
properties were reporting in succession when the hydrophobic
compounds were synthesized.5-11 In this paper, the authors
a temperature range of (153 to 413) K. The data of the thermal
synthesized three pyridinium ILs, which are also the air- and
properties of the three compounds are listed in Table 1.
water-stable hydrophobic compounds. The study of the proper-
The decomposition temperature of the ILs was determined
ties of these three ILs is significant in many concerned fields.
on a Netzsch TG 209 at a heating rate of 10 K·min-1 under a
nitrogen atmosphere over a temperature range of (313 to 823)
K (see Figure S9 of DSC and TG). The percent of the mass
loss at different temperatures is listed in Table 2.
Experimental Section
Preparation of ILs ([Cnpy][NTf2], n ) 2, 4, 5). N-Alkylpy-
ridinium bromide ([Cnpy][Br] n ) 2, 4, 5) was synthesized
according to the procedure described in an earlier paper.12
[C2py][NTf2], [C4py][NTf2], and [C5py][NTf2] were synthesized
according to the ion exchange reaction, where 80 g of [Cnpy][Br]
(n ) 2, 4, 5) was placed in a 500 mL round-bottomed flask and
dissolved in 30 mL of distilled water. Then, the equivalent
amount of bis(trifluoromethanesulfonyl)imide (HN(SO2CF3)2
donated by Rhodia Co.) was added dropwise into the flask at
room temperature and stirred for 3 h. The purity of
HN(SO2CF3)2 is 99.9 % by mass which was dissolved in the
distilled water, and the mass fraction of the aqueous solution is
70 %. The lower liquid was washed several times with 30 mL
of distilled water until no Br- was indicated by the solution of
AgNO3/HNO3. The final product was dried under the vacuum
at 353 K for 24 h. 1H and 13C NMR spectra were recorded (see
Figures S1, S2, S3, S4, S5, S6, S7, and S8 and Tables S1, S2,
and S3 in the Supporting Information) except those for
[C2py][Br].12 From the NMR, impurity peaks were not found.
The reaction equations are following:
Measurement of Density and Surface Tension. The density
(F/g·cm-3) and surface tension (γ/mJ·m-2) were measured at
atmospheric pressure. Since the trace water still exists in the
ILs after drying by common methods, the presence of water
becomes the most problematic impurity and needs to be
confirmed before and after measurement. The mass fraction of
water is the average of three measurements carried out by a
Cou-Lo Aquamax Karl Fischer Moisture Meter (v.10.06) and
is listed in Table 3.
The densities of degassed water were measured by a Westphal
balance. The results are in good agreement with the literature
values,13 and the uncertainty is ( 0.0002 g ·cm-3. Then, the
densities of the samples were measured in the temperature range
of T ) (283.15 to 338.15) K. The sample was placed in a cell
with a jacket. The density values were recorded at per 5 K
intervals. The temperature was controlled by a thermostat with
an uncertainty of ( 0.05 K. Each data point of the density is
the average value of three measurements. The results are listed
in Table 4.
Measurement of DSC and TG. Differential scanning calo-
rimetry (DSC) was carried out on a Q2000 V24.4 Build 116 at
a heating rate of 10 K ·min-1 under a nitrogen atmosphere over
Using the tensiometer (DP-AW type produced by Sang Li
Electronic Co.) of the forced bubble method, the surface tension
of the double-distilled freshly degassed water was measured in
the range of T ) (283.15 to 338.15) K, which is in good
agreement with the literature,13 and the uncertainty is ( 0.1
mJ·m-2. The temperature was controlled by a thermostat with
an uncertainty of ( 0.05 K. The surface tension of the sample
* Corresponding authors. E-mail: tzc@dicp.ac.cn (Zhi-Cheng Tan) and
uwb@dicp.ac.cn (Urs Welz-Biermann).
† China Ionic Liquid Laboratory.
‡ Thermochemistry Laboratory.
10.1021/je100507n  2010 American Chemical Society
Published on Web 09/09/2010

Journal of Chemical & Engineering Data, Vol. 55, No. 11, 2010 4929
Table 1. Molecular Weight and Thermal Properties from the DSC
MW
Tg
Tm
∆Hm
∆Sm
g · mol-1
K
K
kJ· mol-1 J · K-1 · mol-1
[C2py][NTf2] 388.30 235.24 ( 0.01 303.65 ( 0.01 18.9 ( 0.2 62.2 ( 0.2
[C4py][NTf2] 416.35
299.05 ( 0.01 27.9 ( 0.3 93.2 ( 0.3
[C5py][NTf2] 430.38 196.22 ( 0.01 272.84 ( 0.01 22.8 ( 0.2 83.6 ( 0.2
Table 2. Mass Loss of the ILs [C2py][NTf2], [C4py][NTf2], and
[C5py][NTf2] Following the Temperature Recorded by TG
T/K
100 δm/m
[C2py][NTf2]
[C4py][NTf2]
[C5py][NTf2]
611.45 ( 0.1a
1.93
3.04
3.49
5
10
20
50
80
90
611.85 ( 0.1a
585.55 ( 0.1a
608.65 ( 0.1
635.65 ( 0.1
656.65 ( 0.1
678.65 ( 0.1
695.65 ( 0.1
713.65 ( 0.1
723.05 ( 0.1b
632.35 ( 0.1
651.35 ( 0.1
666.35 ( 0.1
685.35 ( 0.1
700.35 ( 0.1
707.35 ( 0.1
636.75 ( 0.1
650.75 ( 0.1
664.75 ( 0.1
683.75 ( 0.1
697.75 ( 0.1
702.75 ( 0.1
Figure 1. Plot of F vs (T/K - 298.15): 2, [C2py][NTf2]; 9, [C4py][NTf2];
b, [C5py][NTf2].
7.88
1.57
0.70
710.85 ( 0.1b
721.35 ( 0.1b
a The onset temperature. b The terminal temperature, respectively.
Table 3. Mass Fraction of Water before and after the Measurement
of Density and Surface Tension
density
before
surface tension
104 w
after
before
after
[C2py][NTf2]
[C4py][NTf2]
[C5py][NTf2]
3.32
6.11
1.66
7.70
8.91
4.20
5.03
8.82
6.14
6.32
9.05
6.83
was measured with the same method. Each data point of the
surface tension is the average value of five measurements.
The results of the surface tension of the samples are listed
in Table 4.
Figure 2. Plot of γ vs (T/K - 298.15): 2, [C2py][NTf2]; 9, [C4py][NTf2];
b, [C5py][NTf2].
Result and Discussion
The molecular weights of the three compounds are listed in
Table 1. The glass temperature (Tg), crystallization temperature
(Tc), and melting temperature (Tm) can be obtained from the
DSC thermograms during the programmed heat/cool/heat step.
From the DSC, the melting temperature of the three samples
decreases with the increase of methylene. There are two
endothermic peaks for [C2py][NTf2], which indicated that it
probably has the transition from solid to solid at 292.80 K and
melting at 303.65 K. There are two exothermic peaks for
[C5py][NTf2] at (234.98 and 256.36) K, which indicated that it
has two crystal forms.
From the TG, the decomposition temperature of the three ILs
is higher than 600.0 K. The samples exhibited excellent thermal
stability up to 600.0 K at the scanning rate of 10 K ·min-1; the
mass loss of the three ILs is more than 90 % from the onset
temperature to the terminal temperature. The onset temperature
of mass loss decreased with increasing methylene.
The values of F and γ for the three ILs decreased with
increasing temperature as shown in Figures 1 and 2. The
experimental data were correlated with the following equation
Table 4. Experimental Values of Density, G, and Surface Tension, γ, of ILs [C2py][NTf2], [C4py][NTf2], and [C5py][NTf2]
[C2py][NTf2] [C4py][NTf2] [C5py][NTf2]
T
F
γ
F
γ
F
γ
K
g·cm-3
mJ·m-2
g·cm-3
mJ·m-2
g·cm-3
mJ·m-2
283.15
288.15
293.15
298.15
303.15
308.15
313.15
318.15
323.15
328.15
333.15
338.15
1.4331 ( 0.0002
1.4296 ( 0.0002
1.4259 ( 0.0002
1.4214 ( 0.0002
1.4169 ( 0.0002
1.4128 ( 0.0001
1.4083 ( 0.0001
1.4038 ( 0.0001
1.3989 ( 0.0001
1.3942 ( 0.0001
1.3893 ( 0.0001
1.3851 ( 0.0001
33.1 ( 0.1
32.8 ( 0.1
32.7 ( 0.1
32.5 ( 0.1
32.2 ( 0.1
32.0 ( 0.1
31.8 ( 0.1
31.5 ( 0.1
31.3 ( 0.1
31.1 ( 0.1
30.8 ( 0.1
30.5 ( 0.1
1.5457 ( 0.0002
1.5414 ( 0.0002
1.5375 ( 0.0002
1.5332 ( 0.0002
1.5291 ( 0.0001
1.5249 ( 0.0001
1.5205 ( 0.0001
1.5164 ( 0.0001
1.5122 ( 0.0001
1.5078 ( 0.0001
1.5037 ( 0.0001
37.7 ( 0.1
37.6 ( 0.1
37.4 ( 0.1
37.1 ( 0.1
36.9 ( 0.1
36.7 ( 0.1
36.6 ( 0.1
36.4 ( 0.1
36.1 ( 0.1
35.9 ( 0.1
35.6 ( 0.1
1.4547 ( 0.0002
1.4506 ( 0.0002
1.4462 ( 0.0001
1.4417 ( 0.0001
1.4372 ( 0.0001
1.4332 ( 0.0001
1.4291 ( 0.0001
1.4245 ( 0.0001
1.4205 ( 0.0001
33.4 ( 0.1
33.2 ( 0.1
32.9 ( 0.1
32.8 ( 0.1
32.4 ( 0.1
32.1 ( 0.1
32.0 ( 0.1
31.8 ( 0.1
31.5 ( 0.1

4930 Journal of Chemical & Engineering Data, Vol. 55, No. 11, 2010
Table 5. Fitting Parameters and Standard Deviations for the G and
γ of ILs [C2py][NTf2], [C4py][NTf2], and [C5py][NTf2]
Literature Cited
(1) Fuller, J.; Carlin, R. T.; Long, H. C. D.; Haworth, D. Structure of
1-ethyl-3-methylimidazolium hexafluorophosphate: model for room
temperature molten salts. J. Chem. Soc., Chem. Commun. 1994, 299–
300.
F/g·cm-3
γ/mJ·m-2
ILs
104A
B
s
103A
B
s
[C2py][NTf2]
[C4py][NTf2]
[C5py][NTf2]
8.40
8.59
8.87
1.5374
1.4547
1.4210
0.0001
0.0002
0.0006
41.6
47.7
46.2
37.4
33.4
32.4
0.0619
0.0698
0.0554
(2) Gordon, C. M.; Holbrey, J. D.; Kennedy, R.; Seddon, K. R. Ionic
liquid crystals: hexafluoroposphate salts. J. Mater. Chem. 1998, 8,
2627–2636.
(3) Bonhoˆte, P.; Dias, A. P.; Papageorgiou, N.; Kalyanasundaram, K.;
Gra¨tzel, M. Hydrophobic, highly comductive ambient-temperature
molten salts. Inorg. Chem. 1996, 35, 1168–1178.
Y ) B - A·(T/K - 298.15)
(1)
where Y are F/g·cm-3 and γ/mJ·m-2; A and B are adjustable
parameters. The fitting parameters and standard deviations are
listed in Table 5.
(4) Cammarata, L.; Kazarian, S. G.; Salter, P. A.; Welton, T. Molecular
states of water in room temperature ionic liquids. Phys. Chem. Chem.
Phys. 2001, 3, 5192–5200.
(5) Restolho, J.; Serro, A. P.; Mata, J. L.; Saramago, B. Viscosity and
surface tension of 1-ethanol-3-methylimidazolium tetrafluoroborate and
1-methyl-3-octylimidazolium tetrafluoroborate over a wide temperature
range. J. Chem. Eng. Data 2009, 54, 950–955.
(6) kabo, G. J.; blokhin, A. V.; Paulechka, Y. U.; Kabo, A. G.;
Shymanovich, M. P.; Magee, J. W. Thermodynamic properties of
1-butyl-3-methylimidazolium hexafluorophosphate in the condensed
state. J. Chem. Eng. Data 2004, 49, 453–461.
According to the Auerbach relation,14 the speed of sound can
be calculated from the density and surface tension by the
following equation
u/m·s-1 ) (γ/(0.00063F))2/3
(2)
(7) Fredlake, C. P.; Crosthwaite, J. M.; Hert, D. G.; Aki, S. N. V. K.;
Brennecke, J. F. Thermophysical properties of imicazolium-based ionic
liquids. J. Chem. Eng. Data 2004, 49, 954–961.
(8) Harris, K. R.; Woolf, L. A.; Kanakubo, M. Temperature and pressure
dependence of the viscosity of the ionic liquid 1-butyl-3-methylimi-
dazolium hexafluorophosphate. J. Chem. Eng. Data 2005, 50, 1777–
1782.
(9) Anthony, J. L.; Maginn, E. J.; Brennecke, J. F. Solubilities and
thermodynamic properties of gases in the ionic liquid 1-n-butyl-3-
methylimizazolium hexafluorophosphate. J. Phys. Chem. B 2002, 106,
7315–7320.
(10) Tokuda, H.; Hayamizu, K.; Ishii, K.; Susan, M. A. B. H.; Watanabe,
M. Physicochemical properties and structures of room temperature
ionic liquids. 1. Variation of anionic species. J. Phys. Chem. B 2004,
108, 16593–16600.
(11) Tokuda, H.; Hayamizu, K.; Ishii, K.; Susan, M. A. B. H.; Watanabe,
M. Physicochemical properties and structures of room temperature
ionic liquids. 2. Variation of alkyl chain length in imidazolium cation.
J. Phys. Chem. B 2005, 109, 6103–6110.
(12) Tong, B.; Liu, Q. S.; Tan, Z. C.; Welz-Biermann, U. Thermochemistry
of alkyl pyridinium bromide ionic liquids: Calorimetric measurements
and calculations. J. Phys. Chem. A 2010, 114, 3782–3787.
(13) Lide, D. R. Handbook of Chemistry and Physics, 82nd ed.; CRC Press:
Boca Raton, 2001-2002.
(14) Bandre´s, I.; Giner, B.; Artigas, H.; Royo, F. M.; Lafuente, C.
Thermophysic comparative study of two isomeric pyridinium-based
ionic liquids. J. Phys. Chem. B 2008, 112, 3077–3084.
(15) Tong, J.; Liu, Q. S.; Xu, W. G.; Fang, D. W.; Yang, J. Z. Estimation
of physicochemical properties of ionic liquids 1-alkyl-3-methylimi-
dazolium chloroaluminate. J. Phys. Chem. B 2008, 112, 4381–4386.
(16) Tong, J.; Liu, Q. S.; Guan, W.; Yang, J. Z. Estimation of physico-
chemical properties of ionic liquid C6MIGaCl4 using surface tension
and density. J. Phys. Chem. B 2007, 111, 3197–3200.
(17) Liu, Q. S.; Tong, J.; Tan, Z. C.; Welz-Biermann, U.; Yang, J. Z.
Density and surface tension of ionic liquid [C2mim][PF3(CF2CF3)3]
and prediction of properties [Cnmim][PF3(CF2CF3)3] (n ) 1, 3, 4, 5,
6). J. Chem. Eng. Data 2010, 55, 2586–2589.
where γ is surface tension; F is density; and u is speed of sound.
The estimated values at 298.15 K for the three compounds are
listed in Table S4 (Supporting Information).
The physicochemical properties (molecular volume, Vm,
parachor, P, thermal expansion coefficient, R, standard entropy,
S0, lattice energy, Upot, and molar enthalpy of vaporization,
0
∆lgHm ) were estimated by using the experimental data of density
and surface tension. The process of the estimation has been
described in previous papers.15-17 These estimated values are
listed in Table S4 (Supporting Information).
From Table S4 (Supporting Information), the molecular
volume, Vm, standard entropy, S0, and parachor, P, are plotted
against the number of the carbons, n, in the alkyl chain of the
samples ([Cnpy][NTf2], n ) 2, 4, 5) and shown in Figures S10
and S11 (Supporting Information).
From the Figures S10 and S11 (Supporting Information), the
contribution per methylene (-CH2-) group to the molecular
volume, standard entropy, and parachor was found to be 0.0278
nm3, 34.7 J ·K-1 ·mol-1, and 32.7. The values of 0.0278 nm3
and 34.7 J ·K-1 ·mol-1 are in good agreement with the literature
values of 0.0272 nm3 and 33.9 J ·K-1 ·mol-1 for [Cnmim][BF4],18
0.0282 nm3 and 35.1 J ·K-1 ·mol-1 for [Cnmim][NTf2],18 0.0270
nm3 and 34.6 J ·K-1 ·mol-1 for [Cnmim][AlCl4],15 and 0.0278
nm3 and 33.7 J ·K-1 ·mol-1 for [Cnmim][Ala].19 The value of
32.7 is similar to the value of 31.1 for [Cnmim][AlCl4]15 but
much less than the value of 37.5 for [Cnmim][Ala]19 and 39.9
for the neutral parachor contribution values of methylene.20
Conclusions
(18) Glasser, L. Lattice and phase transition thermodynamics of ionic
liquids. Thermochim. Acta 2004, 421, 87–93.
Density and surface tension of [Cnpy][NTf2] (n ) 2, 4, 5)
were experimentally measured. The order of the density and
surface tension is [C2py][NTf2] > [C4py][NTf2] > [C5py][NTf2],
at 298.15 K. The order of melting point is [C2py][NTf2] (303.65
K) > [C4py][NTf2] (299.05 K) > [C5py][NTf2] (273.44 K),
according to differential scanning calorimetry. They exhibited
higher thermal stability up to 600.0 K from the thermogravi-
metric analysis.
(19) Fang, D. W.; Guan, W.; Tong, J.; Wang, Z. W.; Yang, J. Z. Study on
physicochemical properties of ionic liquids based on alanine
[Cnmim][Ala] (n ) 2, 3, 4, 5, 6). J. Phys. Chem. B 2008, 112 (25),
7499–7505.
(20) Knotts, T. A.; Wilding, W. V.; Oscarson, J. L.; Rowley, R. L. Use of
the DIPPR database for development of QSPR correlations: Surface
Tension. J. Chem. Eng. Data 2001, 46, 1007–1012.
Received for review May 14, 2010. Accepted August 18, 2010. Funding
for this research was provided by the National Natural Science
Foundation of China (NSFC No. 20901076) and Knowledge Innovation
Program of the Chinese Academy of Science, DICP (Grant K2009D03).
Supporting Information Available:
Additional experimental details. This material is available free
of charge via the Internet at http://pubs.acs.org.
JE100507N