J. Phys. Chem. B 2008, 112, 7499–7505
7499
Study on Physicochemical Properties of Ionic Liquids Based on Alanine [Cnmim][Ala] (n ) 2,3,4,5,6) Da-Wei Fang,†,‡ Wei Guan,† Jing Tong,† Zhen-Wei Wang,† and Jia-Zhen Yang*,† College of Chemistry, Laboratory of Green Chemistry, Liaoning UniVersity, Shenyang 110036, P. R. China, and The Institute of Salt Lakes, Chinese Academy of Science, Xining 810008, P. R. China ReceiVed: February 12, 2008; ReVised Manuscript ReceiVed: March 28, 2008
According to Fukumoto’s method, a new series of ionic liquids (ILs) based on alanine, [Cnmim][Ala] (n ) 2,3,4,5,6), which comprise 1-alkyl-3-methylimidazolium cation ([Cnmim]+) and alanine anions ([Ala]-), were prepared and characterized. In terms of standard addition method, the density and surface tension of amino acid ILs [Cnmim][Ala] (1-alkyl-3-methylimidazolium R-aminopropionic acid salt) were measured in the temperature range 293.15-343.15 ( 0.05 K. The volume and surface properties of the ILs [Cnmim][Ala] were discussed. A new method of determining parachor of ionic compound was proposed and was applied to estimate the physicochemical properties of amino acid ionic liquids (AAILs): molecular volume, surface tension, molar enthalpy of vaporization, and thermal expansion coefficient. In comparison with Deetlefs’s method of using neutral parachor contribution, the method proposed in this work makes smaller error in estimating properties of AAILs. 1. Introduction Recently, amino acid ionic liquids (AAILs) have become one of the most rapidly growing new research areas of ionic liquid (IL) and have attracted considerable attention from industry and the academic community because they are derived from natural ions and are heralded as new natural ILs or bio-ILs.1–4 AAILs can be expected to find application in all of the biological, medical, and pharmaceutical sciences. However, the fundamental physical properties of an AAIL are extremely important in determining whether a particular AAIL is appropriate for a given application. Recently, there is a developing trend in the literature toward estimation of thermodynamic properties for ILs, which is to be commended because it provides valuable insight into the origins of the behavior of ILs.5 As a continuation of our previous investigation,6,7 this paper reports that a new series of ILs based on alanine, [Cnmim][Ala] (n ) 2,3,4,5,6; 1-alkyl-3-methylimidazolium R-aminopropionic acid salt), were prepared and characterized. The density and surface tension of the AAILs were measured at 293.15-343.15 ( 0.05 K. Because trace water is a problematic impurity in the AAILs, the standard addition method (SAM) was applied in these measurements.8,9 The volume and surface properties of the ILs are discussed in terms of Glasser’s theory.10 Recently, Deetlefs et al.11 predicted physical properties of ILs by using parachor contribution data for neutral organics which do not take Coulombic interactions into account. Thus, a new method to determine parachor of the ILs was proposed on the basis of homologous properties of ILs [Cnmim][Ala] and was used to estimate the properties of other AAILs. By using estimated values of the molecular volume and surface tension, we also predicted the standard molar entropy, the lattice energy, the molar enthalpy of vaporization, the saturated vapor pressure at various temperatures, and the thermal expansion coefficient of AAILs [Cnmim][Ala] and [Cnmim][Gly]. * Corresponding author. E-mail:
[email protected]. † Liaoning University. ‡ Chinese Academy of Science.
Figure 1. Preparation of ILs [Cnmim][Ala] by the neutralization method. 1, [Cnmim]Br (n ) 2,3,4,5,6); 2, [Cnmim][OH] (n ) 2,3,4,5,6); 3, [Cnmim][Ala] (n ) 2,3,4,5,6).
2. Experimental Section 2.1. Chemicals. Deionized water was distilled in a quartz still, and its conductance was 0.8-1.2 × 10-4 S · m-1. Alanine was recrystallized twice from water8 and was dried under reduced pressure. N-methylimidazole (AR grade reagent) was vacuum-distilled prior to use. All alkyl halides (AR grade reagent) were distilled before use. Ethyl acetate and acetonitrile were distilled and then stored over molecular sieves in tightly sealed glass bottles. Anion-exchange resin (type 717) was purchased from Shanghai Chemical Reagent Co. Ltd. and activated by regular method before use. 2.2. Preparation of ILs [Cnmim][Ala]. The ILs based on alanine were prepared by a neutralization method (see Figure 1) according to Fukumoto.3 First, a series of [Cnmim]Br (n ) 2,3,4,5,6) were synthesized according to literature.12,13 Then, aqueous 1-alkyl-3-methylimidazolium hydroxide ([Cnmim][OH] (n ) 2,3,4,5,6)) was prepared from [Cnmim]Br (n ) 2,3,4,5,6) by using anion-exchange resin over a 100 cm column. However, these [Cnmim][OH] are not particularly stable, and they should be used immediately after preparation. These onium hydroxide aqueous solutions were added dropwise to a slightly excess alanine aqueous solution. The mixture was stirred under cooling for 12 h. Then, water was evaporated under reduced pressure. To this reaction mixture, the mixed solvents (volumetric ratio: acetonitrile/methanol ) 9/1) were added, and the mixture was stirred vigorously. The mixture was then filtered to remove excess alanine. Filtrate was evaporated to remove solvents. The products of [Cnmim][Ala] (n ) 2,3,4,5,6) were dried in vacuo for 2 days at 80 °C. Structure of the resulting ILs based on alanine was confirmed by 1H NMR spectroscopy (Varian XL-
10.1021/jp801269u CCC: $40.75 2008 American Chemical Society Published on Web 06/03/2008
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TABLE 1: Values of Density (G, g · cm-3) of ILs [Cn mim][Ala] Containing Various Amounts of Water, w2, at 298.15 K 103w2 [C2mim][Ala] [C3mim][Ala] [C4mim][Ala] [C5mim][Ala] [C6mim][Ala]
7.90
9.40
11.0
12.5
13.5
0
r
104s
1.1093 7.30 1.0886 7.00 1.0703 7.10 1.0520 7.40 1.0334
1.1070 9.20 1.0863 9.30 1.0680 9.00 1.0497 9.20 1.0311
1.1046 11.0 1.0839 11.2 1.0656 10.9 1.0473 10.9 1.0287
1.1024 12.5 1.0817 12.6 1.0634 12.4 1.0451 12.9 1.0265
1.1011 13.3 1.0804 13.6 1.0621 13.7 1.0438 13.9 1.0252
1.1209 0 1.0978 0 1.0794 0 1.0610 0 1.0426
0.999
0.9
0.999
1.9
0.996
3.2
0.999
1.5
0.999
1.5
TABLE 2: Values of Surface Tension (γ, mJ · m-2) of Ils [Cnmim][Ala] Containing Various Amounts of Water, w2, at 298.15 K 103w2 [C2mim][Ala] [C3mim][Ala] [C4mim][Ala] [C5mim][Ala] [C6mim][Ala]
7.10
10.1
12.1
14.7
18.2
0
r
102s
53.1 6.80 50.5 5.60 48.1 4.90 46.4 5.50 43.8
52.7 9.30 50.1 7.90 47.7 8.40 46.0 8.80 43.4
52.1 10.5 49.5 10.6 47.2 11.7 45.4 11.9 42.8
51.8 16.1 49.3 13.7 46.9 14.4 45.2 13.0 42.6
51.3 18.3 48.8 15.9 46.4 18.7 44.7 16.4 42.1
51.1 0 48.5 0 46.1 0 44.4 0 41.8
0.998
7.4
0.996
9.0
0.999
5.2
0.995
1.3
0.997
9.6
Figure 2. Plot of density versus water content, w2, in the ILs at 298.15 K. 9 [C2mim][Ala], F ) 1.1209 - 1.471 × 10-3w2; b [C3mim][Ala], F ) 1.0987 - 1.364 × 10-3w2; 2 [C4mim][Ala], F ) 1.0794 - 1.259 × 10-3w2; 1 [[C5mim][Ala], F ) 1.0610 - 1.265 × 10-3w2; ( [C6mim][Ala], F ) 1.0426 - 1.257 × 10-3w2.
Figure 3. Plot of surface tension versus water content, w2, in the ILs at 298.15 K. 9 [C2mim][Ala], γ ) 44.8 + 0.1972w2; b [C3mim][Ala], γ ) 42.6 + 0.1619w2; 2 [C4mim][Ala], γ ) 40.6 + 0.1815w2; 1 [[C5mim][Ala], γ ) 39.1 + 0.1785w2; ( [C6mim][Ala], γ ) 36.9 + 0.2021w2.
300), and all spectra of 1H NMR are listed in Section A of the Supporting Information. Analysis of [C2mim][Ala] by 1H NMR resulting in a spectrum is in good agreement with the literature.3 Differential scanning calorimetric (DSC) measurements showed that ILs [Cnmim][Ala] had no melting point but a glass-transition
temperature (Tg) ranging from -63 to -69 °C. All traces of DSC and Tg are listed in section B of the Supporting Information. Also, thermal gravimetric analysis revealed that all ILs [Cnmim][Ala] were stable at temperatures of at least 200 °C. The water content (w2) of the AAILs, determined by a Karl Fischer moisture titrator (ZSD-2 type), was less than 0.7 wt%. In comparison with the literature,3 the glass-transition temperature of [C2mim][Ala] in this work, -68.45 °C, was lower because of higher water content. 2.3. Measurement of Density and Surface Tension. Because the ILs [Cnmim][Ala] have a strong hydrogen-bonding ability, the small amounts of water in the ILs are difficult to be removed by common method, so that trace water becomes the most problematic impurity. In order to eliminate the effect of the impurity water, the SAM was applied to the measurement of densities and surface tensions.8,9 According to SAM, a series of samples of water-containing [Cnmim][Ala] were prepared. The densities of degassed water were measured by a Westphal balance and were in good agreement with the literature14 within experimental error ((0.0002 g · cm-3). Then, the densities of thesamplesweremeasuredinthetemperaturerange293.15-343.15 K. The sample was placed in a cell with a jacket and was thermostatted at each temperature with an accuracy (0.05 K. By using the tensiometer of the forced bubble method (DPAW type produced by Sang Li Electronic Co.), the surface tension of water was measured and was in good agreement with the literature14 within experimental error ((0.1 mJ · m-2). Then, the surface tension of samples was measured by the same method in the temperature range 293.15-343.15 ( 0.05 K. As an example, the values of density and surface tension of the samples of the ILs containing various content of water at 298.15 K are listed in Tables 1 and 2. The data at other temperatures are listed in Section C of the Supporting Information. Each value of density and surface tension in these tables is the average of triple measurements. According to SAM, the values of density and surface tension at given temperatures were plotted against the water content, w2, of samples of given ILs so that straight lines were obtained, and the slopes were the values of density and surface tension of pure ILs [Cnmim][Ala].
Physicochemical Properties of ILs Based on Alanine [Cnmim][Ala]
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TABLE 3: Values of Density (G, g · cm-3) of Pure ILs [Cnmim][Ala] at 293.15-343.15 ( 0.05 K T (K)
[C2mim][Ala]
[C3mim][Ala]
[C4mim][Ala]
[C5mim][Ala]
[C6mim][Ala]
293.15 298.15 303.15 308.15 313.15 318.15 323.15 328.15 333.15 338.15 343.15
1.1240 1.1209 1.1180 1.1156 1.1124 1.1096 1.1059 1.1026 1.1001 1.0980 1.0942
1.1010 1.0978 1.0947 1.0922 1.0890 1.0865 1.0828 1.0795 1.0770 1.0748 1.0710
1.0827 1.0794 1.0764 1.0739 1.0707 1.0682 1.0648 1.0612 1.0587 1.0567 1.0530
1.0642 1.0610 1.0581 1.0556 1.0524 1.0499 1.0461 1.0428 1.0403 1.0382 1.0344
1.0457 1.0426 1.0398 1.0374 1.0341 1.0318 1.0278 1.0247 1.0220 1.0200 1.0163
TABLE 4: Values of Surface Tension (γ, mJ · m-2) of Pure ILs [Cnmim][Ala] at 293.15-343.15 ( 0.05 K T (K)
[C2mim][Ala]
[C3mim][Ala]
[C4mim][Ala]
[C5mim][Ala]
[C6mim][Ala]
293.15 298.15 303.15 308.15 313.15 318.15 323.15 328.15 333.15 338.15 343.15
53.1 52.7 52.1 51.8 51.3 51.1 50.7 50.3 50.0 49.9 49.6
50.5 50.1 49.5 49.3 48.8 48.5 48.1 47.7 47.5 47.3 47.0
48.1 47.7 47.2 46.9 46.4 46.1 45.7 45.3 45.1 44.9 44.7
46.4 46.0 45.4 45.2 44.7 44.4 44.0 43.6 43.3 43.1 42.9
43.8 43.4 42.8 42.6 42.1 41.8 41.4 41.0 40.7 40.5 40.3
TABLE 5: Values of Volume Properties and Surface Properties of ILs [Cnmim][Ala] and [Cnmim][Gly] (n ) 2, 3, 4, 5, 6) at 298.15 K
a
IL
Vm (nm3)
S0 (J · K-1 · mol-1)
103Sa (mJ · K-1 · m-2)
Ea (mJ · m-2)
UPOT (kJ · mol-1)
[C2mim][Ala] [C3mim][Ala] [C4mim][Ala] [C5mim][Ala] [C6mim][Ala] [C2mim][Gly]a [C3mim][Gly] [C4mim][Gly] [C5mim][Gly] [C6mim][Gly]
0.2948 0.3222 0.3492 0.3772 0.4062 0.2653 0.2931 0.3209 0.3487 0.3765
396.9 431.1 464.8 499.7 535.8 360.2 394.8 429.5 464.1 498.8
70.3 70.5 70.7 70.8 71.0
73.7 71.1 68.8 67.1 64.6
456 446 437 428 421 469 457 446 437 429
Reference 8.
As an example, Figures 2 and 3 show the density and surface tension at 298.15 K against w2. The figures at other temperatures are listed in Section C of the Supporting Information. The correlation coefficients, r, of all linear regressions are larger
Figure 4. Plot of ln F versus T for ILs [Cnmim][Ala]. 9 [C2mim][Ala], ln F ) 0.2741 - 5.36 × 10-4T; b [C3mim][Ala], ln F ) 0.2565 5.47 × 10-4T; 2 [C3mim][Ala], ln F ) 0.2409 - 5.51 × 10-4T; 1 [C4mim][Ala], ln F ) 0.2276 - 5.64 × 10-4T; ( [C5mim][Ala], ln F ) 0.2115 - 5.69 × 10-4 T.
than 0.99, and the standard deviation, s, is within experimental error. These facts show that the SAM is suitable for ILs [Cnmim][Ala].
Figure 5. Plot of Vm versus the number of carbons (n) in the alkyl chains of the [Cnmim][Ala]. 0, Vm ) 0.23876+ 0.02779n with s ) 6.8 × 104 and r ) 0.9999; b S0 ) 327.1 + 34.64n with a s ) 0.8 and r ) 0.9999.
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TABLE 6: Values of Parachor and Surface Tension of ILs at 298.15 Ka IL
M+ (g · mol-1)
P+
Pexp
Pest
γexp (mJ · m-2)
γest (mJ · m-2)
[C2mim][Ala] [C3mim][Ala] [C4mim][Ala] [C5mim][Ala] [C6mim][Ala] [C2mim][Gly] [C3mim][Gly] [C4mim][Gly] [C5mim][Gly] [C6mim][Gly]
110.91 124.93 138.95 152.96 166.98 110.91 124.93 138.95 152.96 166.98
296.3 334.2 370.9 409.6 445.9 296.3 334.2 370.9 409.6 445.9
478.3 516.2 552.9 591.6 627.9 423.3
478.3 515.6 553.0 590.3 627.7 423.3 461.2 497.9 536.6 572.9
52.7 50.1 47.7 46.0 43.4 49.2
52.7 49.8 47.8 45.6 43.3 49.3 46.6 44.0 42.6 40.7
527.0
a P+, parachor of cations; Pexp, experimental parachor; Pest, estimated parachor; γexp: experimental surface tension; γest, estimatedsurface tension.
The molecular volume, Vm, of ILs [Cnmim][Ala] is the sum of the cation volume and anion volume. The value of Vm was calculated by using the following equation
Vm ) M ⁄ (NF)
Figure 6. Plot of parachor P versus M+. Fitting equation: P ) 182.0 + 2.673M+.
where M is the molar mass of ILs and N is Avogadro’s constant. The values of Vm calculated by using eq 2 are listed in Table 5. When plotting Vm against the number (n) of carbons in the alkyl chain of ILs [Cnmim][Ala], a good straight line is obtained (see Figure 5), and its slope, 0.0278 nm3, is a mean contribution of methylene (-CH2-) to the molecular volume and is in good agreement with the value of 0.0275 nm3 from the ILs [Cnmim][BF4] and [Cnmim][NTf2].10 The value 0.0278 nm3 and the value of the density of [C2mim][Gly]8 were used to estimate the molecular volume of ILs [Cnmim][Gly], and the estimated values are listed in Table 5. According to Glasser’s theory,10 the values of the standard entropy, S0(298), for ILs extimated by using eq 3
S0(298) ) 1246.5Vm + 29.5
Figure 7. Comparison of measured and estimated surface tension (y ) 1.05692x - 2.76373).
3. Results and Discussion The values of density and surface tension of pure ILs [Cnmim][Ala] (n ) 2, 3, 4, 5, 6) at various temperatures were obtained by using the SAM and are listed in Tables 3 and 4, respectively. 3.1. Volumetric and Surface Properties of the ILs [Cnmim][Ala]. By plotting the values of ln F against T, a straight line was obtained (see Figure 4) for a given IL, and its empirical linear equation is:
ln F ) b - RT
(1)
where b is an empirical constant and the negative value of the slope, R ) -(∂ ln F/∂T)p, is the thermal expansion coefficient of the IL [Cnmim][Ala]. Values of R are listed in Table 9. Relativity coefficients of all linear fittings of ln F versus T are larger than 0.99, and standard deviations are within experimental error.
(2)
(3)
are listed in Table 5. By the least-squares method, the slope of the linear regression of S0(298) for ILs [Cnmim][Ala] against n is 34.6 J · K-1 · mol-1 (see Figure 5) which is the contribution of methylene group to the standard entropy of the ILs. This value is in agreement with the value of 33.9 J · K-1 · mol-1 from [Cnmim][BF4].10 The experimental values of γ for a given [Cnmim][Ala] have been fitted against T by the least-squares method to a linear equation, and a straight line was obtained, with all correlation coefficients of the fitting larger than 0.99 and the standard deviations within experimental error. All plots of γ versus T are listed in section C of the Supporting Information. From the slopes of the fitting lines, values of the surface excess entropy, Sa, were obtained and are listed in Table 5. In addition, the values of the surface excess energy, Ea, likewise may be obtained from the surface tension, Ea ) γ - T(∂γ/∂T)p at 298.15 K and are also listed in Table 5. In comparison with fused salts, for example, Ea ) 146 mJ · m-2 for fused NaNO3, the values of Ea for [Cnmim][Ala] are much lower and are close to those of organic liquids, for example, 67 mJ · m-2 for benzene and 51.1 mJ · m-2 for n-octane.15 This fact shows that the interaction energy between ions in the ILs [Cnmim][Ala] is much lower than that in inorganic fused salts because the surface excess energy is dependent on interaction energy between ions; that is, the crystal energy of ILs [Cnmim][Ala] is much lower than that of inorganic fused salts. The crystal energy, UPOT, of ILs may be estimated with Glasser’s empirical equation10
Physicochemical Properties of ILs Based on Alanine [Cnmim][Ala]
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0 0 TABLE 7: Molar Enthalpy of Vaporization, ∆gl Hm (298) and ∆gl Hm (Tb) for ILs [Cnmim][Ala] and [Cn mim][Gly]
IL
107k (J · K-1)
Tc (K)
Tb (K)
0 ∆lgHm (Tb) (kJ · mol-1)
0 ∆lgHm (298) (kJ · mol-1)
[C2mim][Ala] [C3mim][Ala] [C4mim][Ala] [C5mim][Ala] [C6mim][Ala]
1.655 1.782 1.912 2.027 2.170
1301 1238 1179 1140 1078
780 743 708 684 647
70.2 66.9 63.7 61.6 58.2
159.9 161.4 162.3 164.5 163.1
IL
0 ∆lgHm (298) (kJ · mol-1)
[C2mim][Gly] [C3mim][Gly] [C4mim][Gly] [C5mim][Gly] [C6mim][Gly]
139.7 141.2 141.8 145.0 145.9
0 TABLE 8: Values of ∆gl Hm and p of [C2mim][Ala] at Various Temperatures
T/K (kJ · mol ) p (kPa) -1
0 ∆lgHm
750
700
650
600
550
500
450
400
350
300
75.8 64.6
85.1 25.7
94.4 7.85
103.7 1.70
113.0 2.37 × 10-1
122.3 1.81 × 10-2
131.6 6.06 × 10-4
140.9 6.39 × 10-6
150.2 1.23 × 10-8
159.5 1.73 × 10-12
TABLE 9: Parameters of Interstice Model for ILs, [Cnmim][Ala] and [Cnmim][Gly], at 298.15 K IL
10-24V (cm3)
∑V (cm3)
V (cm3 · mol-1)
102∑V/V
104R (K-1, Calc)
104R (K-1, exp.)
[C2mim][Ala] [C3mim][Ala] [C4mim][Ala] [C5mim][Ala] [C6mim][Ala] [C2mim][Gly] [C3mim][Gly] [C4mim][Gly] [C5mim][Gly] [C6mim][Gly]
14.83 15.99 17.19 18.19 19.84 16.40 17.82 19.39 20.37 21.79
17.86 19.26 20.70 21.90 23.89 19.76 21.47 23.36 24.54 26.25
177.5 194.0 210.3 227.2 244.6 159.8 176.5 193.3 210.0 226.7
10.06 9.92 9.84 9.64 9.76 12.36 12.16 12.09 11.68 11.58
5.06 5.00 4.95 4.85 4.91 6.22 6.12 6.08 5.88 5.82
5.36 5.47 5.51 5.64 5.69 5.20a
a
Reference 8.
Figure 8. Plot of vapor pressure, p, and ∆gl H0m of the IL [C2mim][Ala] versus temperature T.
UPOT ) 1981.2(F ⁄ M)1/3 + 103.8
(4)
The estimated values of UPOT are listed in Table 5. From Table 5, it is shown that the crystal energies of [Cnmim][Ala] and [Cnmim][Gly] are much lower than those of inorganic fused salts, for example, UPOT ) 613 kJ · mol-1 for fused CsI,14 which is the lowest crystal energy among alkali-chlorides. As pointed out by Krossing,16 the low crystal energy is the underlying reason for forming AAILs at room temperature. 3.2. Predicting Surface Tension of AAILs by Using Parachors. According to the definition of parachor, P,11
P ) (Mγ1/4) ⁄ F
(5)
the experimental parachor of ILs [Cnmim][Ala] at various
temperatures were calculated and are listed in section D of the Supporting Information. The plots of experimental parachors versus temperature, which are listed in section D of Supporting Information, shows that parachors increase slightly with the temperature for a given IL. As an example, the values of the experimental parachors at 298.15 K are listed in Table 6. When plotting the experimental values of P at 298.15 K against the molar mass of cations, M+, of ILs [Cnmim][Ala], a straight line was obtained (see Figure 6), and its empirical equation is P ) 182.0 + 2.673M+, with standard deviation s ) 0.60 and relativity coefficient r ) 0.9999. The value of the intercept, 182.0, corresponds to the contribution of alanine anion [Ala]-. Accordingly, the parachors of all cations, P+, of the ILs [Cnmim][Ala] were obtained and are listed in Table 6. The plot of cation parachors versus n (n is number of carbons in the alkyl chains of the [Cnmim][Ala]) is a straight line, and the slope, 37.5, is the mean contribution of methylene (-CH2-). By using the parachor contribution of methylene and alanine anion, the parachors of other AAILs may be estimated. In terms of the data in the literature,8 the experimental value of parachor for ILs [C2mim][Gly], 423.3, was determined from eq 5. The contribution of the anion [Gly]- to the parachor is 127 according to 296.3 of [C2mim]+. Thus, all parachors of [Cnmim][Gly] (n ) 2, 3, 4, 5, 6) were estimated and are listed in Table 6. In order to prove the method of determining the parachor contribution for ions, the densities and surface tensions at 298.15 K of [C5mim][Gly] were measured (these data are listed in section E of the Supporting Information). The experimental values of the parachor were calculated with eq 5 and are listed in Table 6. When comparing the estimated parachor of [C5mim][Gly] with the experimental one, a good
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agreement was found, and the relative deviation (E% ) [experimental value - estimated value]/experimental value) is 1.8%. In the absence of sufficient data of parachor-contribution values for ions, Deetlefs et al.11 considered that the parachor of ILs may be calculated by using neutral parachor-contribution values,17 so that parachor may become a tool to predict physical properties of ILs. By using values calculated with Deetlefs’s method, the parachor of [C5mim][Gly] was calculated, 555.5. Comparing predicted parachor values [C5mim][Gly] obtained by using the neutral parachor-contribution values with experimental values, the mean relative deviations are larger than 5.4% (E% ) [experimental value - neutral calculated value]/ experimental value). A larger error implies that the parachors calculated by using neutral contribution data do not account for Coulombic interaction in the AAILs. Because the parachor is available as a link between the structure, density, and surface tension, it may become a tool to estimate the properties of ILs. Therefore, the values of the surface tension for the ILs [Cnmim][Gly] and [Cnmim][Ala] were estimated by using eq 5 and are listed in Table 6. As illustrated by Figure 7, the estimated surface tensions of the ILs correlate quite well with their matching experimental values (relativity coefficient r ) 0.993). Subsequently, the estimated values of the surface tension were used for predicting vaporization enthalpies and thermal expansion coefficients for AAILs. 3.3. Estimation of Vaporization Enthalpies for AAILs. Kabo and his co-workers18 proposed an empirical equation for the estimation of the enthalpy of vaporization, ∆gl H0m (298 K), of ILs
∆g1Hm0 (298) ) 0.01121(γV2⁄3N1⁄3) + 2.4
(6)
where V is molar volume of ILs and N is Avogadro’s constant. By using eq 6 and the above estimated values of molar volume and surface tension, the values of molar enthalpy of vaporization, ∆gl H0m (298), for the ILs [Cnmim][Ala] and [Cnmim][Gly] were predicted and are listed in Table 7. From the values in Table 7, ∆gl H0m([Cn+1mim][Ala]) is larger than ∆gl H0m([Cnmim][Ala]), and ∆gl H0m([Cn+1mim][Gly]) is larger than ∆gl H0m([Cnmim][Gly]). This implies that the estimated enthalpy of vaporization of ILs increases with an increase of the length of the aliphatic chains in the 1-alkyl-3-methylimidazoliun cation and is in a great agreement with the values of [Cnmim][NTf2] in Table 4 of ref 18. Rebelo et al.19 proposed a method to estimate the hypothetical temperature of normal boiling point (NBP) of ILs, Tb, in terms of the critical temperature, Tc. They thought that the relationship between Tb and Tc is Tb ≈ 0.6Tc for ILs. The molar enthalpy of vaporization for the ILs at NBP, ∆gl H0m(Tb), can be estimated by Trouton’s constant (∼90 J · mol-1 · K-1). The critical temperature, Tc, of the ILs was estimated by using Eo¨tvo¨s equation15
γV2/3 ) k(Tc - T)
(7)
where V is molar volume of the ILs, Tc is the critical temperature, and k is an empirical constant. The linear regressions of the product of γ and V2/3 for [Cnmim][Ala] against the absolute temperature T were made, and straight lines were obtained. From the slopes and the intercepts of the straight lines, the values of k and Tc were determined and are listed in Table 7. According to Rebelo’s method, the predicted values of ∆gl H0m(Tb) of the ILs [Cnmim][Ala] are also listed in Table 7. From Table 7, the difference between ∆gl H0m(Tb) estimated by using Rebelo’s method and ∆gl H0m(298) estimated by using Kabo’s method is very large. This is because of the heat-capacity
difference between the liquid and gas phases at different temperatures. We suppose a linear change of ∆gl H0m with temperature in the range between 298 K and Tb; the vapor pressure, p, of the ILs at various temperatures may be estimated by using the Clapeyron-Clausius equation. As an example, the estimated values of ∆gl H0m and p of IL [C2mim][Ala] at various temperatures are listed in Table 8. Figure 8 is the plot of the vapor pressure, p, and ∆gl H0m of the IL against the temperature, T. It shows that the vapor pressure was very small, only 10-4 Pa at 298 K. That was consistent with our experience on ILs. 3.4. Interstice Model for ILs. According to the interstice model,20,21 the values of the average volume of the interstices of ILs [Cnmim][Ala] and [Cnmim][Gly], V, were calculated and are listed in Table 9. Then, the molar volume of the interstice, ∑ν ) 2Nν, and the volume fractions of the interstice, ∑ν/V, are in the range 9.64-12.36%. This is in good agreement with the values of the majority of materials which exhibit 10-15% volume expansion in the process from the solid to the liquid state. This result means that the interstice model is reasonable. The molar volume of ILs, V, consists of the inherent volume, Vi, and the total volume of the all interstices; that is,
V ) Vi + 2Nν
(9)
If the expansion of the IL volume only results from the expansion of the interstices when temperature increases, the expression of the thermal expansion coefficients, R, can be derived from the interstice model
R ) (1 ⁄ V)(∂V ⁄ ∂T)p ) 3Nν ⁄ VT
(10)
The values of R calculated by using eq 10 and of the corresponding experimental values for ILs [Cnmim][Ala] and [Cnmim][Gly] at 298.15 K are listed in Table 9. From Table 9, the calculated R are in good agreement with the experimental R. This result means that the interstice model is reasonable and may be used to estimate thermal expansion coefficient of the AAILs. At the same time, Abbott22,23 modified the hole theory for ILs and tested it on the measured and calculated viscosities or conductivities which showed a good consistency. That is also reasonable. But in fact, there is a great difference between the interstice theory and the hole theory. The hole theory is designed to describe spontaneous density fluctuations of molecular extent that occur in liquids as the constituent particles move about under thermal agitation. In the unmelted crystal, one important density fluctuation that appears in equilibrium at high temperatures is the unoccupied lattice site or missing particle. On account of the rigid geometrical structure of the crystal, such a small low-density region can be equal in size to only one characteristic elementary volume, determined by the crystal structure. Furthermore, motion of these empty regions can proceed only by discrete jumps, produced by a shift of a particle in the crystal into a neighboring unoccupied site. However, the situation in the liquid is much less restrictive, because extra freedom of particle movement attending the melting of the rigid crystal implies not only a continuum of possible sizes and shapes for the low-density regions or holes, but movement of these holes may occur by a relatively continuous drift rather than by discrete jumps. However, in the interstice theory, the inherent interstices exist in ILs on account of the large size and great asymmetry of ions and cannot vanish during thermal movement. Conclusion A new series of ILs based on alanine, [Cnmim][Ala] (n ) 2,3,4,5,6), were prepared and characterized. In terms of SAM,
Physicochemical Properties of ILs Based on Alanine [Cnmim][Ala] the density and surface tension of [Cnmim][Ala] were measured in the temperature range 293.15-343.15 ( 0.05 K. The volume and surface properties of the ILs [Cnmim][Ala] were discussed. A new method of determining parachor of ionic compound was proposed and applied to estimate the physicochemical properties of AAILs: molecular volume, surface tension, molar enthalpy of vaporization, and thermal expansion coefficient. In comparison with Deetlefs’s method using neutral parachor contribution, the method proposed in this work makes smaller error in estimating properties of AAILs. Acknowledgment. This project was supported by NSFC (20773056) and Bureau of Liaoning Province (20060359) P. R. China. We thank Mr. Donghui Guo at Mettler-Toledo Co. Ltd. Supporting Information Available: This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Tao, G. H.; He, L.; Liu, W.-S.; Xu, L.; Xiong, W.; Wang, T.; Kou, Y. Green Chem. 2006, 8, 639–646. (2) Fukumoto, K.; Ohno, H. Chem. Commun. 2006, 3081–3083. (3) Fukumoto, K.; Yoshizawa, M.; Ohno, H. J. Am. Chem. Soc. 2005, 127, 2398–2399. (4) Ohno, H.; Fukumoto, K. Acc. Chem. Res. 2007, 40, 1122–1129. (5) Krossing, I.; Slattery, J. M. Z. Phys. Chem. 2006, 220 (10-11), 1343-1359. (6) Yang, J.-Z.; Li, J.-G.; Fang, D.-W.; Zhang, Q.-G.; Feng, R.-K.; Tao, C. Chem. J. Chin. UniV. 2007, 28, 492–495.
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