Research Note pubs.acs.org/IECR
Prediction of Physicochemical Properties of Valine Ionic Liquids [Cnmim][Val] (n = 2, 3, 4, 5, 6) by Empirical Methods: 2 Wei-Guo Xu, Xiao-Xue Ma, Long Li, Jing Tong, and Wei Guan* College of Chemistry, Liaoning University, Shenyang 110036, People’s Republic of China ABSTRACT: A new ionic liquid [C3mim][Val] (1-propyl-3-methylimidazolium valine) was synthesized and characterized by 1H NMR. To avoid the influence of trace water in the ionic liquid (IL), the standard addition method (SAM) was employed; the density and surface tension of the [C3mim][Val] were determined over the temperature range of 303.15−333.15 K (±0.05 K). The experimental density, surface tension, and parachor values at 298.15 K are in good agreement with that predicted by Tong et al. [J. Tong et al., Ind. Eng. Chem. Res. 2011, 50, 2418−2423]. Based on the experimental data for [C3mim][Val], a series of physicochemical propertiesthe molecular volume (Vm), surface tension (γ), refractive index (nD), molar enthalpy of vaporization (ΔlgHm0), the thermal expansion coefficient (α) values, the standard molar entropy (S0), and the molar polarization (Rm)for the homologue of [Cnmim][Val] (n = 2, 4, 5, 6) were estimated using the empirical methods. The estimated results are consistent with one of Tong’s estimations, in terms of the experimental data for [C2mim][Val].
1. INTRODUCTION Room-temperature ionic liquids (RTILs), which consist solely of ions, are a class of compounds that are being extensively studied, because of their unusual properties. Thus, they have great potential as “green” solvents for industrial processes.1,2 Among all these ionic liquids, amino acid ionic liquids (AAILs) have some special properties, such as strong hydrogen bonding ability, which is useful for dissolving biomaterials such as DNA, cellulose, and other carbohydrates. Thus, AAILs have attracted more attention from biological, medical, and pharmaceutical sciences as new functional materials.3−5 Recently, there has been a developing trend in the literature toward estimation of the physicochemical properties for compounds by semiempirical methods, in particular, for ionic liquids (ILs).6−13 Even if the estimated results cannot be deemed to be accurate physicochemical data, it still has significance, because it provides valuable information for the sources of the synthesis of ILs.14−19 Not long ago, Tong et al.19 synthesized an IL [C2mim][Val] and then, with experimental data of [C2mim][Val], the density, the surface tension, and other physical and chemical properties for the homologue of [Cnmim][Val] (Including [C3mim][Val]) were estimated in terms of semiempirical methods. Although Tong’s estimates in ref 19 would certainly be very interesting and worthwhile, without some sort of confirmation, unsupported estimation of this type cannot be regarded as reliable. Therefore, to test the reliability of Tong’s estimates, this paper reports the following: (1) A new IL [C3mim][Val] (1-propyl-3-methylimidazolium valine) has been synthesized and characterized. (2) The density, surface tension, and refractive index values of the IL [C3mim][Val] were measured in the temperature range of 303.15−333.15 K (±0.05 K). Since AAILs can strongly form hydrogen bonds with water, which is the problematic impurity, the standard addition method (SAM)15 was applied in these measurements. (3) Based on the experimental data for [C3mim][Val], the molecular volume (Vm), the surface tension (γ), the © 2012 American Chemical Society
refractive index (nD), the molar enthalpy of vaporization (ΔlgHm0), the thermal expansion coefficient (α), the standard molar entropy (S0), and the molar polarization (Rm) for the homologue of [Cnmim][Val] (n = 2, 4, 5, 6) were estimated using the empirical methods. In comparison to our previous work,19 the estimated results are proven to be reasonable.
2. EXPERIMENTAL SECTION AND RESULTS 2.1. Chemicals and Instruments. Deionized water was distilled, and its conductance was (0.8−1.2) × 10−4 S m−1. Nmethylimidazole (analytical reagent (AR)-grade reagent), 1bromopropane (AR-grade reagent), ethyl acetate, and acetonitrile were distilled prior to use. Valine was purchased from Hefeibomei Biotechnology Co., Ltd. with 99% purity. Anionexchange resin (type 717) was purchased from Shanghai Chemical Reagent Co., Ltd. and activated via the regular method before use. The densities were measured by a DMA 4500 Density Meter (Anton Paar Co.). The surface tension was measured by a tensiometer of the forced bubble method (DPAW type, produced by Sang Li Electronic Co.). The refractive indices were measured by a WZS-1 Abbe refractometer (Shanghai Optics Instrument Co.). All instruments were calibrated by standard methods before measurements. 2.2. Synthesis of IL [C3mim][Val]. [C3mim]Br (1-propyl3-methyl-imidazolium bromide) was synthesized according to the literature,20 and IL [C3mim][Val] was prepared via a neutralization method, according to Fukumoto et al.5 The sample then was characterized by 1H NMR, and the content of water was also determined using a Karl Fischer moisture titrator (ZSD-2 type). The 1H NMR spectrum confirmed the structure of [C3mim][Val]. Received: Revised: Accepted: Published: 4105
July 14, 2011 November 15, 2011 February 23, 2012 February 23, 2012 dx.doi.org/10.1021/ie201530b | Ind. Eng. Chem. Res. 2012, 51, 4105−4111
Industrial & Engineering Chemistry Research
Research Note
Table 1. Density Values of IL [C3mim][Val] Containing Various Known Amounts of Water in the Temperature Range of 303.15−333.15 Ka Density, ρ (g cm−3) temp, T (K) 298.15 303.15 308.15 313.15 318.15 323.15 328.15 333.15 a
8.00 × 10 w2 3
1.09421 1.09090 1.08781 1.08512 1.08181 1.07909 1.07631
10.37 × 10 w2 3
1.09559 1.09260 1.08929 1.08639 1.08329 1.08012 1.07729
12.90 × 103 w2 1.09710 1.09426 1.09091 1.08788 1.08458 1.08158 1.07892
15.14 × 103 w2 1.09859 1.09545 1.09229 1.08942 1.08612 1.08302 1.08019
17.42 × 103 w2 1.10021 1.09693 1.09370 1.09081 1.08712 1.08421 1.08143
0 × 103 w2 1.09174 1.08904 1.08596 1.08281 1.08014 1.07731 1.07450 1.07172
corr coeff, r
stand dev, s (× 104)
0.999 0.998 0.999 0.999 0.998 0.998 0.998
1.19 1.29 0.18 1.10 1.44 1.42 1.44
b
w2 is the water content in mass fraction. bEstimated value.
2.3. Measurement of Density of [C3mim][Val]. For the AAIL, [C3mim][Val] has a strong hydrogen bonding ability, water is the most likely impurity for ILs, and the small amounts of water in ILs may cause large changes of their physicochemical properties. However, trace amounts of water are difficult to remove from ILs using common methods, especially for AAILs. Therefore, in order to eliminate the effect of the impurity water, the SAM was employed to the measurements. According to SAM, a series of samples of water-contained [C3mim][Val] were prepared. The densities for the samples, which were prepared by SAM, were measured in the temperature range of 303.15−333.15 K with an accuracy of ±0.05 K, and the results are listed in Table 1. Each value of density in Table 1 was the average of three measurements. A set of good straight line was obtained (see Figure 1) by plotting the values of the densities against the water
content in the IL. This shows that the SAM is appropriate. From the intercepts of these straight lines, the values of densities for IL [C3mim][Val] without water were obtained at different temperatures, and the results are listed in Table 1. The values of correlation coefficient and standard deviation for the fitting are also presented in Table 1. Table 1 shows that the experimental value of density (ρ = 1.09174 g cm−3) obtained by extrapolation for [C3mim][Val] at 298.15 K is in good agreement with value obtained by Tong’s estimation19 (1.09167 g cm−3). 2.4. Measurement of Surface Tension of [C3mim][Val]. The surface tension values for a series of samples were measured in the temperature range of 303.15−333.15 K with an accuracy of ±0.05 K. The results are listed in Table 2. Each value in Table 2 is the average of three measurements. A set of good straight lines was obtained (see Figure 2) by plotting surface tension against water content. This shows that the SAM is suitable. From the intercepts of these straight lines, the values of surface tension for [C3mim][Val] without water were obtained, and the results are listed in Table 2. The values of correlation coefficient (r) and standard deviation (s) for the fitting are also listed in Table 2. 2.5. Measurement of Refractive Index of [C3mim][Val]. The values of the refractive index of samples for [C3mim][Val] with known different water contents were determined in the temperature range of 298.15−333.15 (±0.05 K) (see Table 3). A good straight line was obtained (see Figure 3) by plotting the values of the refractive index against the water content, and it is clear that the standard addition method has good applicability. The values of the refractive index for [C3mim][Val] without water were obtained from the intercepts of these straight lines, and the results are listed in Table 3. The values of the correlation coefficient r and standard deviation s for the fitting are shown in Table 3.
3. DISCUSSION 3.1. Estimation of Volumetric Properties of [Cnmim][Val]. First, the experimental values of ln ρ of IL [C3mim][Val] without water against (T − 298.15) were fitted and an empirical equation was obtained.
Figure 1. Plot of density versus the amount of water in [C3mim][Val] in the temperature range of 303.15−333.15 K: (black square, ■) 303.15 K: ρ = 1.08904 + 6.34812 × 10−4 w2, s = 1.18 × 10−4, r = 0.999; (red circle, ●) 308.15 K: ρ = 1.08596 + 6.31933 × 10−4 w2, s = 1.29 × 10−4, r = 0.998; (blue triangle, ▲) 313.15 K: ρ = 1.08281 + 6.26097 × 10−4 w2, s = 1.84 × 10−4, r = 0.999; (teal green inverted triangle, ▼) 318.15 K: ρ = 1.08014 + 6.09950 × 10−4 w2, s = 1.10 × 10−4, r = 0.999; (pink left-pointing triangle, ◀) 323.15 K: ρ = 1.07731 + 5.69666 × 10−4 w2, s = 1.44 × 10−4, r = 0.998; (olive green right-pointing triangle, ▶) 328.15 K: ρ = 1.07450 + 5.56295 × 10−4 w2, s = 1.42 × 10−4, r = 0.998; and (dark purple tilted square, ◆) 333.15 K: ρ = 1.07172 + 5.56603 × 10−4 w2, s = 1.44 × 10−4, r = 0.998.
ln ρ = 0.0878 − 5.31 × 10−4(T − 298.15)
(1)
The correlation coefficient of the fitting is 0.999. The intercepts of the empirical equation are the density values for IL [C3mim][Val] at 298.15 K. According to the definition of the thermal expansion coefficient, α = −(∂(ln ρ)/∂T)p, where V is the molar volume, ρ is the density, and the negative slope of the empirical equation is the coefficient of thermal expansion (that is, α = 5.3 × 10−4 K−1 for [C3mim][Val]). 4106
dx.doi.org/10.1021/ie201530b | Ind. Eng. Chem. Res. 2012, 51, 4105−4111
Industrial & Engineering Chemistry Research
Research Note
Table 2. Values of Surface Tension of IL [C3mim][Val] Containing Various Known Amounts of Water in the Temperature Range of 303.15−333.15 Ka Surface Tension, γ (mJ m2) temp, T (K) 298.15 303.15 308.15 313.15 318.15 323.15 328.15 333.15 a
7.97 × 10 w2 3
41.2 40.7 40.3 39.9 39.6 39.2 38.9
10.29 × 10 w2 3
42.1 41.8 41.5 41.1 40.5 40.3 39.8
12.82 × 103w2
15.11 × 103w2
43.2 42.7 42.4 42.1 41.7 41.2 40.9
17.30 × 103w2
44.1 43.8 43.5 42.9 42.5 42.1 41.7
0 × 103w2 38.1 37.8 37.3 36.9 36.6 36.2 35.8 35.5
45.2 44.7 44.4 44.0 43.6 43.3 42.9
corr coeff, r
stand dev, s (× 102)
0.99 0.99 0.99 0.99 0.99 0.99 0.99
7.0 7.8 9.5 10.6 6.7 11.9 10.8
b
w2 is the water content in mass fraction. bEstimated value.
Figure 3. Plot of refractive index versus the amount of water in [C3mim][Val] in the temperature range of 303.15−338.15 K: (black square, ■) 303.15 K: nD = 1.4908 − 2.83913 × 10−4w2, s = 2.8 × 10−5, r = 0.99; (red circle, ●) 308.15 K: nD = 1.4895 − 2.92416 × 10−4 w2, s = 4.3 × 10−5, r = 0.99; (blue triangle, ▲) 313.15 K: nD = 1.4881 − 2.92416 × 10−4w2, s = 4.3 × 10−5, r = 0.99; (teal green inverted triangle, ▼) 318.15 K: nD = 1.4868 − 3.00534 × 10−4 w2, s = 5.3 × 10−5, r = 0.99; (pink left-pointing triangle, ◀) 323.15 K: nD = 1.4855 − 3.09525 × 10−4 w2, s = 4.7 × 10−5, r = 0.99; (olive green right-pointing triangle, ▶) 328.15 K: nD = 1.4843 − 3.18354 × 10−4 w2, s = 3.5 × 10−5, r = 0.99; (dark purple titled square, ◆) 333.15 K: nD = 1.4829 − 3.18354 × 10−4 w2, s = 3.5 × 10−5, r = 0.99.
Figure 2. Plot of surface tension versus the amount of water in [C2mim][Val] in the temperature range of 303.15−338.15 K: (black square, ■) 303.15 K: γ = 37.8 + 0.4256 × 10−4w 2, s = 0.078, r = 0.99; (red circle, ●) 308.15 K: γ = 37.3 + 0.4257 × 10−4 w 2, s = 0.076, r = 0.99; (blue triangle, ▲) 313.15 K: γ = 36.9 + 0.4342 × 10−4w 2, s = 0.095, r = 0.99; (teal green inverted triangle, ▼) 318.15 K: γ = 36.6 + 0.4257 × 10−4 w 2, s = 0.120, r = 0.99; (pink left-pointing triangle, ◀) 323.15 K: γ = 36.2 + 0.4259 × 10−4 w 2, s = 0. 089, r = 0.99; (olive green right-pointing triangle, ▶) 328.15 K: γ = 35.8 + 0.4253 × 10−4 w2, s = 0.132, r = 0.99; (dark purple titled square, ◆) 333.15 K: γ = 35.5 + 0.4213 × 10−4 w 2, s = 0.119, r = 0.99.
Table 3. Values of the Refractive Index of IL [C3mim][Val] Containing Various Amounts of Water in the Temperature Range of 308.15−333.15 Ka Refractive Index, n temp, T (K) 298.15 303.15 308.15 313.15 318.15 323.15 328.15 333.15 a
7.89 × 103w2 1.4886 1.4872 1.4858 1.4845 1.4831 1.4818 1.4804
8.92 × 103w2 1.4883 1.4868 1.4854 1.4841 1.4828 1.4814 1.4800
10.09 × 103w2
11.23 × 103w2
1.4880 1.4865 1.4851 1.4838 1.4824 1.4811 1.4797
1.4876 1.4862 1.4848 1.4834 1.4820 1.4807 1.4793
12.54 × 103w2 1.4873 1.4858 1.4844 1.4831 1.4817 1.4803 1.4789
0 × 103w2 1.4921 1.4908 1.4895 1.4881 1.4868 1.4855 1.4843 1.4829
corr coeff, r
stand dev, s (× 105)
Rm
3.5 4.3 4.3 5.3 4.7 3.5 3.5
64.10b 64.11 64.15 64.18 64.19 64.21 64.24 64.25
b
0.99 0.99 0.99 0.99 0.99 0.99 0.99
w2 is the water content in mass fraction. bEstimated value. 4107
dx.doi.org/10.1021/ie201530b | Ind. Eng. Chem. Res. 2012, 51, 4105−4111
Industrial & Engineering Chemistry Research
Research Note
Table 4. Predicted Volumetric Properties of the Homologous Series [Cnmim][Val] (n = 2, 3, 4, 5, 6) at 298.15 K ionic liquid, IL a
[C2mim][Val] [C2mim][Val] [C3mim][Val]b [C4mim][Val] [C5mim][Val] [C6mim][Val] a
ρ (g cm−3)
Vm (nm3)
S0 (J K−1mol−1)
V (cm3 mol−1)
UPOT (kJ mol−1)
1.11254 1.11239 1.09174 1.07383 1.05835 1.04456
0.3393 0.3395 0.3673 0.3951 0.4229 0.4507
452.4 452.7 487.3 522.0 566.6 591.3
204.3265 204.3790 221.1146 237.8502 254.5858 271.3214
440.2 440.1 431.4 423.6 416.4 409.8
Data taken from ref 19. bExperimental value.
Table 5. Predicted Values of the Surface Tension (γ), the Parachor (P), the Molar Enthalpy of Vaporization (ΔlgHm0), the Refractive Index (nD), the Molar Polarization (Rm), the Thermal Expansion Coefficient (α), and the Polarization Coefficient (αp) of the Homologous Series [Cnmim][Val] (n = 2, 3, 4, 5, 6) at (298.15 K) ionic liquid, IL [C2mim] [C2mim] [C3mim] [C4mim] [C5mim] [C6mim] a
a
[Val] [Val] [Val]b [Val] [Val] [Val]
P
P′(1%)
−ΔP
γ (× 103 N m−1)
ΔlgHm0 (kJ mol−1)
α (× 104 K−1) (calc)
nD
Rm
αp (× 1024)
510.9 511.3 548.8 586.3 623.8 661.3
535.54 535.54 575.44 615.34 651.24 695.14
24.64 24.24 26.64 29.04 27.44 33.84
39.1 39.3 38.1 37.1 36.2 35.4
130.8 131.5 134.3 137.2 140.0 142.8
6.05 6.00 5.84 5.67 5.51 5.37
1.4941 1.4945 1.4922 1.4900 1.4883 1.4865
59.49 59.50 64.10 68.70 73.30 77.90
23.60 23.59 25.41 27.24 29.06 30.88
Data taken from ref 19. bExperimental value.
where M is the molar mass, γ the surface tension, and ρ the density. As a first approximation, the parachor amounts to the sum of the contributions of the various components of a molecule. Despite the fact that the estimated results cannot be regarded as accurate data, they are still helpful for the synthesis of ILs with special properties. According to the surface tension (γ) and density (ρ), the experimental parachor [C3mim][Val] was obtained, P = 548.8, from eq 5, which is in good agreement with the value obtained using Tong’s estimation (548.4).19 In our previous paper,16 we have found that the contribution per methylene to the parachor is 37.5. As a first approximation, the parachor is equal to the sum of the contribution of the various components of the molecule. Given that each methylene (−CH2−) group has almost the same chemical environment in the alkyl chains of the imidazolum-based ILs, we proposed a semiempirical method of predicting P of the IL homologue,16 which is the sum of an experimental value of [C3mim][Val] and contributions of methylene (−CH2−). Meanwhile, we predicted a parachor P′(1%) using the neutral contribution to the parachor, where (1%) means that the allowable error of training set is set at
