Article

pubs.acs.org/jced

Density, Surface Tension, and Refractive Index of Ionic Liquids

Homologue of 1-Alkyl-3-methylimidazolium Tetrafluoroborate
[Cnmim][BF4] (n = 2,3,4,5,6)
Wei-Guo Xu, Long Li, Xiao-Xue Ma, Jie Wei, Wen-Bin Duan, Wei Guan,* and Jia-Zhen Yang
Key Laboratory of Green Synthesis and Preparative Chemistry of Advanced Materials, Liaoning University, Shenyang 110036, China

ABSTRACT: The values of density, surface tension, and refractive index were measured for a
classic series of air and water stable ionic liquids (ILs) based on tetrafluoroborate, [Cnmim][BF4](n
= 2,3,4,5,6)(1-alkyl-3-methylimidazolium tetrafluoroborate) in the temperature range of (298.15 to
338.15 ± 0.02) K. In terms of Glasser’s theory, the standard molar entropy and lattice energy of the
ILs were calculated. Using Kabo’s method, the molar enthalpy of vaporization of the IL, ΔlgHm0
(298 K), was estimated. According to the interstice model, the thermal expansion coefficient of ILs
[Cnmim][BF4] (n = 2,3,4,5,6), α, were calculated, and in comparison with experimental value, their
magnitude order is in good agreement. To test this new concept of ionic parachor, [BF4]− was
chosen as a reference ion and its individual value of ionic parachor was determined in terms of extrathermodynamic assumption.
Then, using ionic parachor of [BF4]−, the ionic parachors for all corresponding imidazolium cations, [Cnmim]+, were calculated
and in good agreement with those obtained by Guan’s estimation, and the 17 anion ionic parachors were also obtained. In terms
of ionic parachor, the surface tension and refractive index of the investigated ionic liquids were estimated, and the estimated
values correlate quite well with their matching experimental values.

1. INTRODUCTION values of ionic parachors for [BF4]− and [Cnmim]+, the

As the most common of the ionic liquid [Cnmim][BF4] (n = parachor and surface tension of some ionic liquids were
2,3,4,5,6), they are promising compounds for being used for estimated. (4) Using the above estimated surface tension and
batteries, organic synthesis, extractions, and alloy electro- parachor, the values of refractive index, nD, could be estimated
deposition.1−6 In recent years, there has been a developing and compared to experimental values. (5) The interstice model
trend in the literature toward estimation of the physicochemical was applied to calculate the thermal expansion coefficient, α, of
properties for compounds by semiempirical methods, in ILs [Cnmim][BF4] (n = 2,3,4,5,6) and the magnitude order of
particular, for ILs.7−9 Although the estimated result cannot be its value calculated was the same as the experimental one.
regarded as accurate physicochemical data, it is to be
commended because it provides valuable insight into the 2. EXPERIMENTAL SECTION
origins of the behavior of materials. Among all the semi- 2.1. Chemicals. Pure ILs [Cnmim][BF4] (n = 2,3,4,5,6)
empirical methods, parachor is the simplest.10−12 Not long ago, were purchased from Lanzhou Institute of Chemical Physics,
Guan et al.13 proposed a new concept, ionic parachor, and the purity is more than 99%. The purity of ILs and the mass
determined the average value of the ionic parachor for reference fraction contribution of the impurities are listed in Table 1.
ion [OAc]− in terms of the extrathermodynamic assumptions
so that the values of the ionic parachor for all corresponding Table 1. Purity of ILs and Mass Fraction Contribution of the
imidazolium cations, [Cnmim]+, were obtained. Using the new Impurities
concept, ionic parachor, estimations of the properties of ionic mass fraction purity of 1-
liquids become easier. purity ILs methylimidazole halogen water
As a continuation of our previous investigation,14,15 in this [C2mim][BF4] ≥0.99 <0.001 <0.001 <0.001
article, the following contents were reported: (1) the values of [C3mim][BF4] ≥0.99 <0.001 <0.001 <0.002
density, surface tension, and refractive index for [Cnmim][BF4] [C4mim][BF4] ≥0.99 <0.001 <0.0008 <0.001
(n = 2,3,4,5,6) were measured at (298.15 ± 0.02) K. (2) In [C5mim][BF4] ≥0.99 <0.002 <0.001 <0.002
terms of Glasser’s theory,16 the lattice energy and standard [C6mim][BF4] ≥0.99 <0.001 <0.001 <0.002
entropy of the ILs were estimated. Using Kabo’s method,17 the
molar enthalpy of vaporization, ΔlgHm0 (298 K), was estimated. 2.2. Measurements of Density, Surface Tension, and
(3) If [BF4]− was chosen as a reference ion, its particular value
of ionic parachor could be determined in terms of the Refractive Index. The density of degassed water was
extrathermodynamic assumption. In order to compare with
the previous article, the values of ionic parachor for the Received: January 10, 2012
corresponding cations, [Cnmim]+, in the [Cnmim][BF4] were Accepted: June 20, 2012
calculated with the reference value of [BF4]−; then, using the Published: July 3, 2012

© 2012 American Chemical Society 2177 dx.doi.org/10.1021/je3000348 | J. Chem. Eng. Data 2012, 57, 2177−2184
Journal of Chemical & Engineering Data Article

measured by a Westphal balance at (293.15 ± 0.02) K and was Table 3. Values of Surface Tension, γ/mJ·m−2, of Pure ILs
in good agreement with the literature18 within experimental [Cnmim][BF4] (n = 2,3,4,5,6)
error ± 0.0002 g·cm−3. Then, the densities of the samples were
[C2mim] [C3mim] [C4mim] [C5mim] [C6mim]
measured in the temperature range of (298.15 to 338.15 ± T/K [BF4] [BF4] [BF4] [BF4] [BF4]
0.02) K. The sample was placed in a cell with a jacket and was
298.15 50.1 47.0 44.7 42.9 41.0
thermostatted at each temperature. Using the tensiometer of
53.9725 44.528 42.819 40.928
the forced bubble method (DPAW type produced by Sang Li
54.0125 45.7429
Electronic Co.), the surface tension of water was measured at
54.419 50.3127 44.530 39.519
(298.15 to 338.15 ± 0.02) K and was in good agreement with
54.426 51.119 44.4131
the literature18 within experimental error ± 0.1 mJ·m−2. Then,
52.586 44.3727 39.232
the values of surface tension of the samples were measured by
303.15 49.7 44.3 42.5 40.7
the same method in the same temperature range of (298.15 to 308.15 49.3 46.4 44.0 42.1 40.3
338.15 ± 0.02) K. 313.15 49.0 46.1 43.6 41.7 39.9
The refractive index, nD, of the [Cnmim][BF4] (n = 2,3,4,5,6) 318.15 48.7 45.8 43.3 41.3 39.4
was measured by an Abbe refractometer at (298.15 to 338.15 ± 323.15 48.4 45.4 42.9 40.9 39.0
0.02) K. First, the refractive index of degassed water was 328.15 48.0 45.0 42.5 40.5 38.6
measured by the instrument at (298.15 to 338.15 ± 0.02) K 333.15 47.6 44.5 42.1 40.2 38.3
and were in good agreement with the literature18 within 338.15 47.3 44.1 41.7 39.9 38.0
experimental error ± 0.0001.

3. RESULTS AND DISCUSSION Table 4. Values of Refractive Index, nD, of Pure ILs
[Cnmim][BF4] (n = 2,3,4,5,6)
The values of density, surface tension, and refractive index for
the samples of [Cnmim][BF4] (n = 2,3,4,5,6) are listed in [C2mim] [C3mim] [C4mim] [C5mim] [C6mim]
T/K [BF4] [BF4] [BF4] [BF4] [BF4]

Table 2. Values of Density, ρ/g·cm−3, of Pure ILs 298.15 1.4121 1.4165 1.4211 1.4238 1.4270
1.412333 1.421834
[Cnmim][BF4] (n = 2,3,4,5,6)
1.4235
[C2mim] [C3mim] [C4mim] [C5mim] [C6mim] 1.4215036
T/K [BF4] [BF4] [BF4] [BF4] [BF4] 1.421823
298.15 1.2798 1.2361 1.2015 1.1719 1.1463 1.421921
1.23719 1.201420 1.17319 1.1459624 303.15 1.4109 1.4153 1.4200 1.4228 1.4260
1.2013421 308.15 1.4099 1.4142 1.4192 1.4216 1.4249
1.27996 1.20126,a 313.15 1.4090 1.4132 1.4182 1.4206 1.4237
1.201122 318.15 1.4080 1.4120 1.4172 1.4197 1.4226
1.27919 1.201023 323.15 1.4069 1.4110 1.4162 1.4185 1.4215
1.20119 328.15 1.4057 1.4098 1.4151 1.4175 1.4204
303.15 1.2777 1.2339 1.1991 1.1696 1.1441 333.15 1.4047 1.4086 1.4140 1.4164 1.4191
308.15 1.2752 1.2312 1.1977 1.1675 1.1420 338.15 1.4036 1.4075 1.4130 1.4153 1.4179
313.15 1.2724 1.2288 1.1955 1.1657 1.1403
318.15 1.2700 1.2268 1.1932 1.1633 1.1383
323.15 1.2675 1.2241 1.1912 1.1613 1.1361
328.15 1.2649 1.2218 1.1885 1.1592 1.1345
333.15 1.2620 1.2193 1.1865 1.1572 1.1322
338.15 1.2593 1.2167 1.1840 1.1551 1.1302
a
Calculated from ρ = 1.4135 − 7.0925·10−4 (T/K).

Tables 2−4, respectively. Each value in the tables is the average

of triple measurements.
Since [Cnmim][BF4] (n = 2,3,4,5,6) are most common ILs,
many researchers have previously reported one or more of
these three properties (density, surface tension, and refractive
index) in similar temperature range. In order to compare with
this work, the experimental data of these three properties in
literature were collected and are listed in Tables 2−4. From Figure 1. Plot of ln ρ vs T for ILs [Cnmim][BF4]. Square,
these tables, it can be seen that the experimental data for the [C2mim][BF4] ln ρ = 0.3680 − 4.0567·10−4 T, s = 2.1·10−4, r =
ionic liquids (ILs) obtained by different authors differ often 0.99; circle, [C3mim][BF4] ln ρ = 0.3294 − 3.9367·10−4 T, s =
considerably from each other, for example, the values of surface 1.3·10−4, r = 0.99; triangle [C4mim][BF4] ln ρ = 0.2927 − 3.6533·10−4
tension differ by up to several millinewton per meter, data of T, s = 2.8·10−4, r = 0.99; downward facing triangle, [C5mim][BF4] ln ρ
density by up to 0.0197 g/cm3, and values of refractive index by = 0.2657 − 3.5933·10−4 T, s = 0.7·10−4, r = 0.99; left facing triangle,
up to 0.0127. The majority of data in this work are close to the [C6mim][BF4] ln ρ = 0.2403 − 3.4833·10−4 T, s = 1.3·10−4, r = 0.99.
mean of the experimental data published in literature. This fact
shows that data in this work are more reliable.
2178 dx.doi.org/10.1021/je3000348 | J. Chem. Eng. Data 2012, 57, 2177−2184
Journal of Chemical & Engineering Data Article

Table 5. Values of Volume Properties and Surface Properties of ILs [Cnmim][BF4] (n = 2, 3,4, 5, 6), at 298.15 K
M Vm S0 103Sa Ea UPOT
−1 −1 −1
ionic liquid g·mol nm 3
J·K ·mol mJ·K−1·m−2 mJ·m−2 kJ·mol−1
[C2mim][BF4] 197.978 0.2569 349.7 69.0 70.7 473
[C3mim][BF4] 212.004 0.2848 384.5 72.3 68.5 460
[C4mim][BF4] 226.030 0.3124 418.9 74.3 66.8 450
[C5mim][BF4] 240.056 0.3402 453.5 76.3 65.6 440
[C6mim][BF4] 254.082 0.3681 488.3 78.3 64.3 431

Figure 4. Plot of Ea s number of carbons (n) in the alkyl chains of the

[Cnmim][BF4] (square, Ea = 73.46 − 1.57n, with s = 0.374 and r =
Figure 2. Plot of Vm vs number of carbons (n) in the alkyl chains of 0.99).
the [Cnmim][BF4] (square, Vm = 0.2014 + 0.0278n, with s = 0.8·10−3
and r = 0.99). Table 6. Molar Enthalpy of Vaporization, ΔlgHm0 (298 K),
and ΔlgHm0 (Tb) for ILs [Cnmim][BF4]
107 k ΔlgHm0(Tb) ΔlgHm0(298 K)
−1 −1
ionic liquid J·K Tc/K Tb/K kJ·mol kJ·mol−1
[C2mim][BF4] 1.989 1024 614 55.26 139.08
[C3mim][BF4] 2.233 948 569 51.21 139.74
[C4mim][BF4] 2.440 900 540 48.60 141.33
[C5mim][BF4] 2.653 860 516 46.44 143.53
[C6mim][BF4] 2.869 822 493 44.37 144.56

Table 7. Values of Ionic Parachor for the ILs According to

the Extrathermodynamic Assumption
V+(crystal) V−(crystal)
ionic liquid P(exp) nm3a nm3a P+ P−
[C2mim][BF4] 411.6 0.156 0.073 280.4 131.2
[C3mim][BF4] 449.1 0.178 0.073 318.5 130.6
[C4mim][BF4] 486.4 0.196 0.073 354.4 132.0
Figure 3. Plot of surface tension vs T for ILs [Cnmim][BF4]. Square, [C5mim][BF4] 524.2 0.219 0.073 393.2 131.1
[C2mim][BF4] γ = 70.6 − 0.069T, s = 0.045, r = 0.99; circle, [C6mim][BF4] 560.9 0.242 0.073 430.9 130.0
[C3mim][BF4] γ = 68.7 − 0.072T, s = 0.103, r = 0.99; triangle, a
Reference 40.
[C4mim][BF4] γ = 66.9 − 0.074T, s = 0.041, r = 0.99; downward
facing triangle, [C5mim][BF4] γ = 65.6 − 0.076T, s = 0.053, r = 0.99;
left facing triangle, [C6mim][BF4] γ = 64.4 − 0.078T, s = 0.072, r = where b is an empirical constant, the negative value of slope, α
0.99. = −(∂ ln ρ/∂T)p, is thermal expansion coefficient of the IL
[Cnmim][BF4], and values of α are listed in Table 11 as
experimental ones. Correlation coefficients of all linear fitting of
ln ρ vs T are larger than 0.99, and standard deviations are
3.1. Estimation of Volumetric and Surface Properties within experimental error.
for the ILs. Plotting of values of ln ρ against T, a straight line The molecular volume, Vm, of ILs [Cnmim][BF4] is the sum
was obtained (see Figure 1) for given IL, and its empirical of the cation volume and anion volume. The value of Vm was
linear equation is calculated using the following equation:
ln ρ = b − αT (1) Vm = M /(Nρ) (2)

2179 dx.doi.org/10.1021/je3000348 | J. Chem. Eng. Data 2012, 57, 2177−2184

Journal of Chemical & Engineering Data Article

Table 8. Values of Ionic Parachor of ILs at 298.15 K

[C2mim]+ [C3mim]+ [C4mim]+ [C5mim]+ [C6mim]+
cation parachor 280.6 318.1 355.4 393.2 429.9
[OAc]−,13 [Ala]−,14 [NTf2]−,41 [GaCl4]−,42 [Lact]−,43
anion parachor 85.4 198.6 337.4 284.3 189.1
[EtSO4]−,44 [PF6]−,45 [FeCl4]−,46 [InCl4]−,47 [ZnCl3]−,48
anion parachor 227.3 196.9 287.3 323.7 242.6
[SCN]−,49 [CH3SO4]−,50 [Val]−,51 [Glu]−,52 [Gly]−,53
anion parachor 129.7 192.7 102.0 298.2 133.9
[B(CN)4]−,54 [AlCl4]−,55
anion parachor 292.9 301.3

Table 9. Parachors and Surface Tensions of the Investigated Ionic Liquids

[Cnmim][NTf2] [Cnmim][Lact]
[Cnmim]X Pex Pest ΔP γex γest Δγ Pex Pest ΔP γex γest Δγ
[C2mim]X 628.8 618.0 10.8 35.8 33.4 2.4 446.0 446.2 −0.2 48.7 48.8 −0.1
[C3mim]X 657.0 655.5 1.5 32.9 32.6 0.3 483.5 483.7 −0.2 46.1 46.2 −0.1
[C4mim]X 692.9 692.8 0.1 32.0 32.0 0.0 521.0 521.0 0.0 44.0 44.0 0.0
[C5mim]X 722.5 730.6 −8.1 30.1 31.5 −1.4 558.5 558.8 −0.3 42.3 42.4 −0.1
[C6mim]X 762.8 767.3 −4.5 30.2 30.9 −0.7 596.0 595.5 0.5 40.8 40.7 0.1
[Cnmim][OAc] [Cnmim][Val]
Pex Pest ΔP γex γest Δγ Pex Pest ΔP γex γest Δγ
[C2mim]X 369.1 366.0 3.1 38.1 36.6 1.5 510.9 511.1 −0.2 39.1 39.1 0.0
[C3mim]X 405.0 403.5 1.5 36.8 36.1 0.7 548.4 548.6 −0.2 37.8 37.8 0.0
[C4mim]X 441.1 440.8 0.3 35.2 35.4 −0.2 585.9 585.9 0.0 36.9 36.9 0.0
[C5mim]X 476.9 478.6 −1.7 34.1 34.8 −0.7 623.4 623.7 −0.3 36.0 36.1 −0.1
[C6mim]X 512.1 515.3 −3.2 33.0 34.0 −1.0 660.9 660.4 0.5 35.2 35.1 0.1

Figure 5. Plot of the estimated parachor for ILs [Cnmim] ([NTf2], Figure 6. Plot of the estimated surface tension for ILs [Cnmim]-
[Lact], [OAc], and [Val]) vs their experimental values. Pest = −4.487 + ([NTf2], [Lact], [OAc], and [Val]) vs their experimental values. γest =
1.008Pex; s = 3.47; r = 0.99. Square, [Cnmim][NTf2] (n = 2−6); circle, 1.172 + 0.968γEx; s = 0.82; r = 0.99. Square, [Cnmim][NTf2] (n = 2−
[Cnmim][Lact] (n = 2−6); triangle [Cnmim][OAc] (n = 2−6); 6); circle, [Cnmim][Lact] (n = 2−6); triangle, [Cnmim][OAc] (n =
downward facing triangle, [Cnmim][Val] (n = 2−6). 2−6); downward facing triangle, [Cnmim][Val] (n = 2−6).

where M is molar mass of ILs, N is Avogadro's constant. The

calculated values of Vm using eq 2 are listed in Table 5. Plotting the slope of the linear regression of S0(298)/J·K·mol−1 for ILs
Vm against the number (n) of carbons in the alkyl chain of ILs [Cnmim][BF4] against n is 34.6 J·K−1·mol−1, which is the
[Cnmim][BF4], a good straight line was obtained (see Figure contribution of per methylene group to the standard entropy of
2), and its slope, 0.0278 nm3, is a mean contribution of per the ILs. This value is in agreement with the value of 33.9
methylene (−CH2−) to the molecular volume and is in good J·K−1·mol−1 from [Cnmim][BF4].16
agreement with 0.0275 nm3 from the ionic liquids [Cn− S 0(298)/(J ·K−1·mol−1) = 1246.5(Vm/nm 3) + 29.5 (3)
mim][NTf2].16According to Glasser’s theory,16 the estimated
values of the standard entropy, S0 (298)/J·K−1·mol−1, for ILs The lattice energy, UPOT, of ILs may be estimated in terms of
using eq 3 are listed in Table 5. By the least-squares method, Glasser’s empirical equation:16
2180 dx.doi.org/10.1021/je3000348 | J. Chem. Eng. Data 2012, 57, 2177−2184
Journal of Chemical & Engineering Data Article

UPOT/kJ·mol−1 = 1981.2(ρ /M )1/3 + 103.8 (4)

the values of UPOT are obtained and listed in Table 5. From

Table 5, it is shown that lattice energies of [Cnmim][BF4] are
much less than that of inorganic fused salts, for example, UPOT
= 613 kJ·mol−1 for fused Cs,I18 which is the least lattice energy
among alkali halides. The low lattice energy is the underlying
reason for forming ILs [Cnmim][BF4] at room temperature.
From Table 5, it can be seen that the mean contribution of per
methylene (−CH2−) to the lattice energy is approxximately
equal to −10 kJ·mol−1.
The experimental values of γ for given [Cnmim][BF4] have
been fitted against T by the least-squares method to a linear
equation, and several good straight lines were obtained with all
correlation coefficients of the fitting are larger than 0.99, and
the standard deviations are within the experimental error (see
Figure 3).
Figure 7. Plot of refractive index, nD, vs T for ILs [Cnmim][BF4]. The surface excess entropy, Sa = −(∂γ/∂T)p may be obtained
Square, [C2mim][BF4] nD = 1.4748 − 2.1033·10−4T, s = 9.2·10−5, r =
0.99; circle, [C3mim][BF4] nD = 1.4832 − 2.2367·10−4T, s = 6.4·10−5,
from the slopes of fitting lines and are listed in Table 5. In
r = 0.99; triangle, [C4mim][BF4] nD = 1.4814 − 2.0200·10−4T, s = addition, the values of the surface excess energy Ea likewise may
9.5·10−5, r = 0.99; downward facing triangle, [C5mim][BF4] nD = be obtained from the surface tension: Ea = γ − T(∂γ/∂T)p at
1.4869 − 2.1167·10−4T, s = 6.7·10−5, r = 0.99; left facing triangle, 298.15 K and are also listed in Table 5. In comparison with
[C6mim][BF4] nD = 1.4950 − 2.2767·10−4T, s = 8.6·10−5, r = 0.99. fused salts, for example, Ea = 146 mJ·m−2 for fused NaNO3, the
values of Ea for [Cnmim][BF4] are much lower and are close to
Table 10. Experimental Values of Rm, nD, 1024αp at 298.15 K that of organic liquids, for example, 72.1 mJ·m−2 for benzene
and 51.1 mJ·m−2 for n-octane.18 By plotting Ea against the
ionic liquid Rm nD(ex) nD(est) 1024αp number (n) of carbons in an alkyl chain of ILs [Cnmim][BF4], a
[C2mim][BF4] 38.50 1.4121 1.4213 15.26 good straight line was obtained (see Figure 4) and its slope,
[C3mim][BF4] 43.09 1.4165 1.4244 17.08 −1.57 kJ·mol−1, is a mean contribution of per methylene
[C4mim][BF4] 47.72 1.4211 1.4299 18.92 (−CH2−) to the surface excess energy.
[C5mim][BF4] 52.25 1.4238 1.4311 20.71 This fact shows that interaction energy between ions in the
[C6mim][BF4] 56.91 1.4270 1.4329 22.56 ILs [Cnmim][BF4] is much less than that in inorganic fused
salts because the surface excess energy is dependent on
interaction energy between ions, that is, the lattice energy of ILs
[Cnmim][BF4] is much less than that of inorganic fused salts.
In general, surface tension, γ, of many liquids almost linearly
decreases, while temperature elevates, and the relationship is
expressed in Eötvös equation37

γV 2/3 = k(Tc − T ) (5)

where V is molar volume of the liquid, Tc is critical temperature,

and k is an empirical constant. The linear regression of the
product of γ and V2/3 obtained from this experiment against
absolute temperature T was made, and a good straight line was
obtained. The correlation coefficients of the linear regression
were all over 0.99. From the intercept of the fitted straight line,
Tc (critical temperature) were obtained. The molar enthalpy of
vaporization for the ILs, ΔlgHm0(Tb), could be estimated by the
hypothetical temperature of normal boiling point (NBP) of
ionic liquids, Tb, and Trouton constant (∼90 J·mol−1·K−1),
Figure 8. Plot of the estimated refractive index for ILs [Cnmim][BF4] ΔlgHm0(Tb) = 90Tb, according to Rebelo et al.38 The
vs their experimental values: nD(est) = 0.2681 + 0.817nD(ex); s =
8.47·10−4; r = 0.99.
relationship between Tb and Tc is Tb ≈ 0.6Tc for ionic liquids.
From Tc, NBP of ionic liquid [Cnmim][BF4], Tb, were obtained

Table 11. Parameters of Interstice Model for ILs, [Cnmim][BF4], at 298.15 K

ionic liquid 10−24 v/cm3 ∑v/cm3 V/cm3·mol−1 102 ∑v/V 104α/K−1 (calcd) 104α/K−1 (exptl) 104α/K−1 (est)
[C2mim][BF4] 16.0 19.25 173.95 11.1 5.57 4.06 5.57
[C3mim][BF4] 17.6 21.19 192.70 11.0 5.53 3.94 5.52
[C4mim][BF4] 19.0 22.84 210.96 10.8 5.45 3.65 5.45
[C5mim][BF4] 20.2 24.29 229.13 10.6 5.33 3.59 5.31
[C6mim][BF4] 21.6 26.00 247.67 10.4 5.28 3.48 5.26

2181 dx.doi.org/10.1021/je3000348 | J. Chem. Eng. Data 2012, 57, 2177−2184

Journal of Chemical & Engineering Data Article

and are listed in Table 6. Then, the predicted values of ΔlgHm0 respectively; Vm−(IL) is the ionic volume of the corresponding
(Tb) were also calculated and are listed in Table 6. anion in an IL, Vm is molecular volume of the IL, P−(IL) is
According to Kabo’s empirical equation,39 the molar enthalpy ionic parachor of the corresponding anion, and P is
of vaporization, ΔlgHm0 (298 K), of ionic liquids can be experimental value of parachor for the IL. If the values of
estimated by V+(crystal) and V−(crystal) may be obtained, the ionic parachor
of cation and anion can be determined with eq 10.
Δl g Hm 0(298 K) = 0.01121(γV 2/3N1/3) + 2.4 kJ ·mol−1 The values of V+(crystal) and V−(crystal) for [Cnmim][BF4]
(6) obtained from literature40 are listed in Table 7. According to
where γ is surface tension, V is molar volume, N is Avogadro’s the extrathermodynamic assumption, the values of ionic
constant. The values of the molar enthalpy of vaporization, parachor of cation and anion for the ILs can be calculated by
ΔlgHm0 (298 K), for [Cnmim][BF4] were calculated by the eq 10 and also listed in Table 7. From Table 7, the average of
estimated values of V and γ from eq 6, and the results are listed ionic parachor, P−, that can be seen for anion [BF4]− is 131.0 ±
in Table 6. 0.7.
From Table 6, ΔlgHm0 (298 K) estimated by Kabo’s empirical If [BF4]− was used as the reference ion, reference value of P−
equation is very different from ΔlgHm0 (Tb) estimated by = 131.0, values of the ionic parachor for all corresponding
Rebelo. This is because the heat capacity is different between imidazolium cations, [Cnmim]+, were obtained and are listed in
the liquid and the gas phases at different temperatures. Table 8. From Table 8, it can be seen that the values of the
3.2. Ionic Parachor. The parachor, P, is a relatively old ionic parachor for all corresponding imidazolium cations,
concept that is available as a link between the structure, density, [Cnmim]+, are in good agreement with the values obtained by
and surface tension of liquids.10,11 The parachor as a tool to Guan’s estimation.13 Then, using data in literature and P+ of
predict physical properties of substances was developed in 1924 [Cnmim]+, the 17 anion ionic parachors were obtained and are
by Sugden12 and was defined using the following equation: also listed in Table 8.
3.3. Predicting Parachor and Surface Tension of ILs
P = (Mγ 1/4)/ρ (7) with the Ionic Parachor. In order to verify the reliability of
where γ is surface tension, M is molar mass, and ρ is the density ionic parachors for [Cnmim]+ and the anions in Table 8, the
of a substance. However, the vast majority of parachor studies parachors for the ILs ([Cnmim][NTf2], [Cnmim][Lact],
have focused on uncharged compounds. The data of individual [Cnmim][OAc], and [Cnmim][Val]) were estimated. The
group contribution of parachor determined by Rowley and co- estimated values, Pest, and the experimental values, Pex,
workers11 are from the neutral molecule and difficult in the determined by eq 12 are listed in Table 9, where ΔP means
application of an ionic liquid because no consideration of the the difference between the experimental value and correspond-
Coulomb force between ions. Although a number of early ing estimated value of parachor for the ILs, that is, ΔP = Pex −
studies attempted to determine parachor for ions, these studies Pest. As can be seen from Table 9, only ΔP for [C2mim][NTf2]
were hampered by the experimental difficulties encountered in is relatively large (relative deviation is close to 2%). This may
determining the surface tensions and densities of high melting be due to the use of a different author’s data in calculation of
salts and no related investigations followed. However, since the experimental parachor for [Cnmim][NTf2]. For the
numerous ionic liquids are fluid at room temperature, they offer remaining ILs, ΔP are very small, that is, the experimental
a solution to determine experimental parachor of ionic values of parachor are in accordance with the estimated one.
compounds. This implies that data of ionic parachors for [Cnmim]+ and the
Therefore, ionic parachor was proposed as a new concept,13 anions are reliable. Figure 5 is a comparative plot of estimated
that is, ions composed of ionic liquids can be seen as an parachor values as a function of corresponding experimental
independent descriptors of parachor. Ionic parachor, Pi, can be values for the investigated ionic liquids and shows that
defined with the following equation: estimated parachor and the experimental values are highly
correlated (correlation coefficient, r = 0.99; standard deviation,
Pi = γ 1/4Vi (8) s = 3.47) and extremely similar (gradient = 1.00; intercept =
−4.49).
where Vi is molar volume of an ion in IL. According to the
In terms of ionic parachor values in Table 8, the predicted
additivity, the parachor value of an ionic liquid is equal to the
values of surface tension for the ILs using eq 7 and their
sum of ionic parachors of the corresponding cation and anion
corresponding experimental values are listed in Table 9. In
P = P+ + P − (9) Table 9, Δγ means the difference between the experimental
determined and corresponding estimated surface tension for
where P+ and P− are ionic parachor of cation and anion,
the ILs, that is, Δγ = γex − γest. As can be seen from the Table 9,
respectively. Now, the key question is how the experimental
only the values of Δγ for [C2mim][NTf2] is relatively larger
value of parachor for an ionic liquid is divided into ionic
(relative deviation is close to 7%). As illustrated by Figure 6, the
parachors of the corresponding cation and anion. The
predicted surface tensions of the ILs correlate quite well with
extrathermodynamic assumption was recommended for the
their matching experimental values (correlation coefficient, r =
evaluation of individual ionic parachor:
0.99; standard deviation, s = 0.97).
V −(crystal)/V+(crystal) 3.4. Estimation of Refractive Index with Ionic Para-
chor for the ILs. The experimental values of nD for given
= Vm −(IL)/[Vm − Vm −(IL)] [Cnmim][BF4] have been fitted against T by the least-squares
= P −(IL)/[P − P −(IL)] method to a linear equation and several good straight lines were
(10)
obtained, with all correlation coefficients of the fitting larger
where V+(crystal) and V−(crystal) are the corresponding than 0.99 and the standard deviations within experimental error
cationic and anionic volume derived from crystal structures, (see Figure 7).
2182 dx.doi.org/10.1021/je3000348 | J. Chem. Eng. Data 2012, 57, 2177−2184
Journal of Chemical & Engineering Data

■
Article

The Lorentz−Lorenz relationship between the refractive REFERENCES

index and the mean molecular polarizability, αp, leads to the (1) Wasserscheid, P.; Welton, T., Eds. Ionic Liquids in Synthesis;
definition of molar refraction Rm56 Wiley-Verlag: Weinheim, Germany. 2003.
(2) Welton, T. Room-Temperature Ionic Liquids: Solvents for
R m = [(nD2 − 1)/(nD2 + 2)](M /ρ) = (4πN /3)αp Synthesis and Catalysis. Chem. Rev. 1999, 99, 2070−2084.
(11) (3) Huddleston, J. G.; Willauer, H. D.; Swatloski, R. P.; Visser, A. E.;
Rogers, R. D. Room-Temperature Ionic Liquids as Novel Media for
According to eq 11, the values of Rm and αp were calculated ‘Clean’ Liquid-Liquid Extraction. Chem. Commun. 1998, 1765−1766.
from nD values of ILs without water and are listed in Table 10. (4) Brennecke, J. F.; Maginn, E. J. Ionic Liquids: Innovative Fluids for
From Table 10, the average difference of Rm ([Cnmim][BF4] (n Chemical Processing. AIChE J. 2001, 47, 2384−2389.
= 2,3,4,5,6)) is 4.60. In other words, the contribution value per (5) Plechkova, N. V.; Seddon, K. R. Applications of Ionic Liquids in
methylene (−CH2−) group is 4.6. the Chemical Industry. Chem. Soc. Rev. 2008, 37, 123−150.
Substitution of eq 7 into eq 11 and rearrangement yields10 (6) Rilo, E.; Pico, J.; García-Garabal, S.; Varela, L. M.; Cabeza, O.
Density and Surface Tension in Binary Mixtures of CnMIM-BF4 Ionic
γ 1/4 = (P /R m)[(nD2 − 1)/(nD2 + 2)] (12) Liquids with Water and Ethanol. Fluid Phase Equilib. 2009, 285, 83−
89.
In terms of eq 12, the refractive index of IL homologue (7) Krossing, I.; Slattery, J. Semi-Empirical Methods to Predict the
[Cnmim][BF4] (n = 2, 3, 4, 5, 6) could be predicted. The Physical Properties of Ionic Liquids: An Overview of Recent
Developments. Z. Phys. Chem. 2006, 220 (10−11), 1343−1359.
estimated values of nD are also shown in Table 10. Figure 8 is a
(8) Hu, Y.-F.; Liu, Z.-C.; Xu, C.-M.; Zhang, X.-M. The Molecular
plot of the estimated refractive index vs their experimental Characteristics Dominating the Solubility of Gases in Ionic Liquids.
values. A good straight line was obtained with correlation Chem. Soc. Rev. 2011, 40, 3802−3823.
coefficients of the fitting larger than 0.99 and the standard (9) Preiss, U.; Bulut, S.; Krossing, I. In Silico Prediction of the
deviations within experimental error. Melting Points of Ionic Liquids from Thermodynamic Considerations:
3.5. Estimation of Thermal Expansion Coefficients A Case Study on 67 Salts with a Melting Point Range of 337 °C. J.
with Ionic Parachor for the ILs. For pure ionic liquids, a new Phys. Chem. B 2010, 114, 11133−11140.
theoretic model,44 the expression of calculation of interstice (10) Deetlefs, M.; Seddon, K. R.; Shara, M. Predicting Physical
volume, v, was obtained on the classical statistical mechanics Properties of Ionic Liquids. Phys. Chem. Chem. Phys. 2006, 8, 642−
649.
ν = 0.6791(k bT /γ )3/2 (13) (11) Knotts, T. A.; Wilding, W. V.; Oscarson, J. L.; Rowley, R. L. Use
of the DIPPR Database for Development of QSPR Correlations:
where kb is Boltzmann constant, T the thermodynamic Surface Tension. J. Chem. Eng. Data 2001, 46 (5), 1007−1012.
temperature, and γ the surface tension of ionic liquid. (12) Sugden, S. J. The Variation of Surface Tension with
Temperature and Some Related Functions. J. Chem. Soc 1924, 125, 32.
According to eq 13, the values of average volume of the (13) Guan, W.; Ma, X.-X.; Li, L.; Tong, J.; Fang, D.-W.; Yang, J.-Z.
interstices of ionic liquids at different temperatures are Ionic Parachor and Its Application in Acetic Acid Ionic Liquid
obtained, which were listed in Table 11. Homologue 1-Alkyl-3-methylimidazolium Acetate {[Cnmim][OAc](n
The volume of ionic liquids, V, consists of the inherent = 2,3,4,5,6)}. J. Phys. Chem. B 2011, 115, 12915−12920.
volume, Vi, and total volume of the all interstices, that is (14) Fang, D.-W.; Guan, W.; Tong, J.; Wang, Z.-W.; Yang, J.-Z. Study
on Physicochemical Properties of Ionic Liquids Based on Alanine
V = Vi + 2Nν (14) [Cnmim][Ala] (n = 2,3,4,5,6). J. Phys. Chem. B 2008, 112, 7499−7505.
(15) Yang, J.-Z.; Zhang, Q.-G.; Wang, B.; Tong, J. Study on the
If the expansion of IL volume only results from the expansion Properties of Amino Acid Ionic Liquid EMIGly. J. Phys. Chem. B 2006,
of the interstices when temperature increases, then calculation 110, 22521−22524.
expression of thermal expansion coefficients, α, was derived (16) Glasser, L. Lattice and Phase Transition Thermodynamics of
from the interstice model Ionic Liquids. Thermochim. Acta 2004, 421, 87−93.
(17) Zaitsau, D. H.; Kabo, G. J.; Strechan, A. A.; Paulechka, Y. U.;
α = (1/V )(∂V /∂T )p = 3Nν /VT (15) Tschersich, A.; Verevkin, S. P.; Heintz, A. Experimental Vapor
Pressures of 1-Alkyl-3-methylimidazolium Bis-
The values of α(calculated) calculated using eq 15 and of (trifluoromethylsulfonyl)imides and a Correlation Scheme for
corresponding experimental values, α(experimental), for ILs Estimation of Vaporization Enthalpies of Ionic Liquids. J. Phys.
[Cnmim][BF4] at 298.15 K are listed in Table 11. From Table Chem. A 2006, 110, 7303−7306.
11, the magnitude order of α(calculated) are in good agreement (18) Lide, D. R., Ed. Handbook of Chemistry and Physics, 82nd ed.;
with α(experimental) and α(estimated), so that this result CRC Press: Boca Raton, FL, 2002.
(19) Shirota, H.; Mandai, T.; Fukazawa, H.; Kato, T. Comparison
means that the interstice model is reasonable.

■
between Dicationic and Monocationic Ionic Liquids: Liquid Density,
Thermal Properties, Surface Tension, and Shear Viscosity. J. Chem.
AUTHOR INFORMATION Eng. Data 2011, 56, 2453−2459.
Corresponding Author (20) Navia, P.; Troncoso, J.; Romaní, L. Excess Magnitudes for Ionic
Liquid Binary Mixtures with a Common Ion. J. Chem. Eng. Data 2007,
*Tel: +86 24 62207797. Fax: +86 24 62207797. E-mail:
52, 1369−1374.
guanweilnu@yahoo.com.cn. (21) Iglesias-Otero, M. A.; Troncoso, J.; Carballo, E.; Romani, L.
Funding Density and Refractive Index in Mixtures of Ionic Liquids and Organic
This project was supported by NSFC (21173107), Education Solvents: Correlations and Predictions. J. Chem. Thermodyn. 2008, 40,
Bureau, Science and Technology of Liaoning Province 949−956.
(LS2010068 and 201102078), People's Republic of China. (22) Jacquemin, J.; Ge, R.; Nancarrow, P.; Rooney, D. W.; Gomes,
M. F. C.; Pádua, A. A. H.; Hardacre, C. Prediction of Ionic Liquid
Notes Properties. I. Volumetric Properties As a Function of Temperature at
The authors declare no competing financial interest. 0.1 MPa. J. Chem. Eng. Data 2008, 53, 716−726.

2183 dx.doi.org/10.1021/je3000348 | J. Chem. Eng. Data 2012, 57, 2177−2184

Journal of Chemical & Engineering Data Article

(23) Soriano, A. N.; Doma, B. T., Jr.; Li, M.-H. Solubility of Carbon (42) Tong, J.; Liu, Q.-S.; Guan, W.; Yang, J.-Z. Estimation of
Dioxide in 1-Ethyl-3-methylimidazolium 2-(2-Methoxyethoxy) Ethyl- Physicochemical Properties of Ionic Liquid C6MIGaCl4 Using Surface
sulfate. J. Chem. Thermodyn. 2008, 40, 1654−1660. Tension and Density. J. Phys. Chem. B 2007, 111 (12), 3197−3120.
(24) Klomfar, J.; Souckova, M.; Patek, J. Temperature Dependence (43) Wang, J.-Y.; Jiang, H.-C.; Liu, Y.-M.; Hua, Y.-Q. Density and
Measurements of the Density at 0.1 MPa for 1-Alkyl-3-methylimida- Surface Tension of Pure 1-Ethyl-3-methylimidazolium L-Lactate Ionic
zolium-Based Ionic Liquids with the Trifluoromethanesulfonate and Liquid and Its Binary Mixtures with Water. J. Chem. Thermodyn. 2011,
Tetrafluoroborate Anion. J. Chem. Eng. Data 2010, 55, 4054−4057. 43, 800−804.
(25) Souckova, M.; Klomfar, J.; Patek, J. Surface Tension of 1-Alkyl- (44) Yang, J.-Z.; Lu, X.-M.; Gui, J.-S.; Xu, W.-G. A New Theory for
3-methylimidazolium Based Ionic Liquids with Trifluoromethanesul- Ionic Liquids: The Interstice Model Part 1. The Density and Surface
fonate and Tetrafluoroborate Anion. Fluid Phase Equilib. 2011, 30, Tension of Ionic Liquid EMISE. Green Chem. 2004, 6, 541−543.
3184−3190. (45) Huddleston, J. G.; Visser, A. E.; Reichert, W. M.; Willauer, H.
(26) Zhou, Z.-B.; Matsumoto, H.; Tatsumi, K. Structure and D.; Broker, G. A.; Rogers, R. D. Characterization and Comparison of
Hydrophilic and Hydrophobic Room Temperature Ionic Liquids
Properties of New Ionic Liquids Based on Alkyl- and Alkenyltri-
Incorporating the Imidazolium Cation. Green Chem. 2001, 3, 156−
fluoroborates. ChemPhysChem 2005, 6, 1324−1332.
164.
(27) Wang, J.-Y.; Zhang, X.-J.; Liu, Y.-M.; Hu, Y.-Q. Interfacial
(46) Zhang, Q.-G.; Yang, J.-Z.; Lu, X.-M.; Gui, J.-S.; Huang, M.
Tensions of Imidazolium-Based Ionic Liquids with N-Alkanes and Studies on an Ionic Liquid Based on FeCl3 and Its Properties. Fluid
Cyclohexane. J. Chem. Eng. Data 2011, 56, 3734−3737. Phase Equilib. 2004, 226, 207−211.
(28) Freire, M. G.; Carvalho, P. J.; Fernandes, A. M.; Marrucho, I. (47) Zang, S.-L.; Zhang, Q.-G.; Huang, M.; Wang, B.; Yang, J.-Z.
M.; Queimada, A. J.; Coutinho, J. A. P. Surface tensions of Studies on the Properties of Ionic Liquid EMIInCl4. Fluid Phase
imidazolium based ionic liquids: Anion, cation, temperature and Equilib. 2005, 230, 192−196.
water effect. J. Colloid Interface Sci. 2007, 314, 621−630. (48) Zhang, Q.-G.; Wei, Y. Study on Properties of Ionic Liquid Based
(29) Liu, W.; Zhao, T.; Zhang, Y.; Wang, H.; Yu, M. The Physical on ZnCl2 with 1-Butyl-3-methylimidazolium Chloride. J. Chem.
Properties of Aqueous Solutions of the Ionic Liquid [BMIM][BF4]. J. Thermodyn. 2008, 40, 640−644.
Sol. Chem. 2006, 35, 1337−1346. (49) Domanska, U.; Krolikowska, M. Effect of Temperature and
(30) Yi, F.-P.; Li, J.-Z.; Chen, B. Study of Ionic Liquid As Surface Composition on the Surface Tension and Thermodynamic Properties
Active Agent. Acta Chim. Sin. 2008, 66, 239−244. of Binary Mixtures of 1-Butyl-3-methylimidazolium Thiocyanate with
(31) Klomfar, J.; Souckova, M.; Patek, J. Surface Tension Alcohols. J. Colloid Interface Sci. 2010, 348, 661−667.
Measurements with Validated Accuracy for Four 1-Alkyl-3-methyl- (50) Wang, J.-Y.; Zhao, F.-Y.; Liu, Y.-M.; Wang, X.-L.; Hu, Y.-Q.
imidazolium Based Ionic Liquids. J. Chem. Thermodyn. 2010, 42, 323− Thermophysical Properties of Pure 1-Ethyl-3-methylimidazolium
329. Methylsulphate and Its Binary Mixtures with Alcohols. Fluid Phase
(32) Ghatee, M. H.; Zolghadr, A. R. Surface Tension Measurements Equilib. 2011, 305, 114−120.
of Imidazolium-Based Ionic Liquids at Liquid−Vapor Equilibrium. (51) Tong, J.; Song, B.; Wang, C.-X.; Li, L.; Guan, W.; Fang, D.-W.;
Fluid Phase Equilib. 2008, 263, 168−175. Yang, J.-Z. Prediction of the Physicochemical Properties of Valine
(33) Soriano, A. N.; Doma, B. T., Jr.; Li, M.-H. Density and Ionic Liquids [Cnmim][Val] (n = 2,3,4,5,6) by Semiempirical
Refractive Index Measurements of 1-Ethyl-3-methylimidazolium-based Methods. Ind. Eng. Chem. Res. 2011, 50, 2418−2423.
Ionic Liquids. J. Taiwan Inst. Chem. E 2010, 41, 115−121. (52) Guan, W.; Tong, J.; Chen, S.-P.; Liu, Q.-S.; Gao, S.-L. Density
(34) Soriano, A. N.; Doma, B. T., Jr.; Li, M.-H. Measurements of the and Surface Tension of Amino Acid Ionic Liquid 1-Alkyl-3-
Density and Refractive Index for 1-n-Butyl-3-methylimidazolium-based methylimidazolium Glutamate. J. Chem. Eng. Data 2010, 55, 4075−
4079.
Ionic Liquids. J.Chem. Thermodyn. 2009, 41, 301−307.
(53) Fang, D.-W.; Tong, J.; Guan, W.; Wang, H.; Yang, J.-Z.
(35) Kim, K.-S.; Shin, B.-K.; Lee, H.; Ziegler, F. Refractive Index and
Predicting Properties of Amino Acid Ionic Liquid Homologue of 1-
Heat Capacity of 1-Butyl-3-methylimidazolium Bromide and 1-Butyl-
Alkyl-3-methylimidazolium Glycine. J. Phys. Chem. B 2010, 114,
3-methylimidazolium Tetrafluoroborate, and Vapor Pressure of Binary 13808−13814.
Systems for 1-Butyl-3-methylimidazolium Bromide + Trifluoroethanol (54) Tong, J.; Liu, Q.-S.; Kong, Y.-X.; Fang, D.-W.; Welz-Biermann,
and 1-Butyl-3-methylimidazolium Tetrafluoroborate + Trifluoroetha- U.; Yang, J.-Z. Physicochemical Properties of an Ionic Liquid
nol. Fluid Phase Equilib. 2004, 218, 215−220. [C2mim][B(CN)4]. J. Chem. Eng. Data 2010, 55, 3693−3696.
(36) Tariq, M.; Forte, P. A. S.; Costa Gomes, M. F.; Canongia Lopes, (55) Tong, J.; Liu, Q.-S.; Xu, W.-G.; Fang, D.-W.; Yang, J.-Z.
J. N.; Rebelo, L. P. N. Densities and Refractive Indices of Imidazolium- Estimation of Physicochemical Properties of Ionic Liquids 1-Alkyl-3-
and Phosphonium-Based Ionic Liquids: Effect of Temperature, Alkyl methylimidazolium Chloroaluminate. J. Phys. Chem. B 2008, 112,
Chain Length, and Anion. J. Chem. Thermodyn. 2009, 41, 790−798. 4381−4386.
(37) Adamson, A. W. Physical Chemistry of Surfaces, 3rd ed.; John- (56) Ersfeld, B.; Felderhof, B. U. Retardation Correction to the
Wiley: New York, 1976; translated by Gu, T. R. Science Press: Beijing, Lorentz−Lorentz Formula for the Refractive Index of a Disordered
China, 1986. System of Polarizable Point Dipoles. Phys. Rev. 1998, E57, 1118−1126.
(38) Rebelo, L. P. N.; Lopes, J. N. C.; Esperança, J. M. S. S.; Filipe, E.
On the Critical Temperature, Normal Boiling Point, and Vapor
Pressure of Ionic Liquids. J. Phys. Chem. B 2005, 109, 6040−6043.
(39) Zaitsau, D. H.; Kabo, G. J.; Strechan, A. A.; Paulechka, Y. U.
Experimental Vapor Pressures of 1-Alkyl-3-methylimidazolium Bis-
(trifluoromethylsulfonyl)imides and a Correlation Scheme for
Estimation of Vaporization Enthalpies of Ionic Liquids. J. Phys.
Chem. A 2006, 110, 7303−7306.
(40) Slattery, J. M.; Daguenet, C.; Dyson, P. J.; Schubert, T. J. S.;
Krossing, I. How to Predict the Physical Properties of Ionic Liquids: A
Volume-Based Approach. Angew. Chem., Int. Ed. 2007, 46, 5384−5388.
(41) Tariq, M.; Serro, A. P.; Mata, J. L.; Saramago, B.; Esperanca, J.
M. S. S.; Lopes, J. N. C.; Rebelo, L. P. N. High-Temperature Surface
Tension and Density Measurements of 1-Alkyl-3-methylimidazolium
Bistriflamide Ionic Liquids. Fluid Phase Equilib. 2010, 294, 131−138.

2184 dx.doi.org/10.1021/je3000348 | J. Chem. Eng. Data 2012, 57, 2177−2184