J. Phys. Chem. B 2007, 111, 3197-3200
3197
Estimation of Physicochemical Properties of Ionic Liquid C6MIGaCl4 Using Surface Tension and Density Jing Tong,†,‡ Qing-Shan Liu,† Wei Guan,† and Jia-Zhen Yang*,† Department of Chemistry, Liaoning UniVersity, Shenyang, 110036, People’s Republic of China, and The Institute of Salt Lakes, Chinese Academy of Science, Xining, 810008, People’s Republic of China ReceiVed: December 21, 2006; In Final Form: February 4, 2007
An ionic liquid (IL) C6MIGaCl4 (1-methyl-3-hexylimidazolium chlorogallate) was prepared by directly mixing GaCl3 and 1-methyl-3-hexylimidazolium chloride with molar ratio of 1/1 under dry argon. The density and surface tension of the IL were determined in the temperature range of 283.15-338.15 K. The ionic volume and surface entropy of the IL were estimated by extrapolation, respectively. In terms of Glasser’s theory, the standard molar entropy and lattice energy of the IL were estimated, respectively. By use of Kabo’s method, the molar enthalpy of vaporization of the IL, ∆lgHm0 (298 K), at 298 K was estimated. According to the interstice model, the thermal expansion coefficient of IL C6MIGaCl4, R, was calculated and in comparison with experimental value; their magnitude order is in good agreement.
1. Introduction Ionic liquids (ILs) are a class of compounds that are being extensively studied because of their unusual properties.1-5 In particular, these liquids have very low vapor pressures. Thus, they have great potential as “green” solvents for industrial processes. This potential is being explored vigorously as recent publications hint at their use in many typical chemical reactions.6,7 To expand our knowledge of IL chemistry and because of our interest in the chemistry of gallium,8-12 we synthesized a new IL C6MIGaCl4 (1-methyl-3-hexylimidazolium chlorogallate) and measured the density and the surface tension under dry argon in the temperature range of 283.15-338.15 K. Recently, there is a developing trend in the literature toward estimation of physicochemical properties of the IL,13 which is to be commended because it provides. For this reasons, in this paper the ionic volume and surface properties of the IL were estimated by extrapolation. In terms of Glasser’s theory,14 the lattice energy and standard entropy of the IL were estimated. By use of Kabo’s method,15 the molar enthalpy of vaporization, ∆lgHm0 (298 K), at 298 K was estimated. An interstice model was applied to calculate the thermal expansion coefficient of IL C6MIGaCl4, R, and the magnitude order of its value calculated was the same as experimental one. 2. Experimental 2.1. Chemicals. Anhydrous GaCl3 (99.999%-Ga) was obtained from Alfa Aesar Co. It was opened in the glovebox filled with dry argon and used without further purification. 1-Methylimidazole AR grade reagent was obtained from ACROS and was vacuum distilled prior to use. Chlorohexane AR grade reagent was obtained from Beijing Chemicals Co. and was distilled before use. Ethyl acetate and acetonitrile were distilled and then stored over molecular sieves in tightly sealed glass bottles, respectively. 2.2. Preparation of ILs. According to ref 16, 1-methyl-3hexylimidazolium chloride (C6MIC) was synthesized. The * To whom correspondence should be addressed. † Liaoning University. ‡ Chinese Academy of Sciences.
TABLE 1: Values of Density, G, and Surface Tension, γ, of C6MIGaCl4 at 283.15-338.15 K T/K F/g‚cm-3 γ/mJ‚m-2 T/K F/g‚cm-3 γ/mJ‚m-2
283.15 1.35881 41.2 313.15 1.33331 39.5
288.15 1.35452 40.9 318.15 1.32914 39.2
293.15 1.35020 40.6 323.15 1.32497 38.8
298.15 1.34596 40.4 328.15 1.32084 38.6
303.15 1.34174 40.0 333.15 1.31671 38.4
308.15 1.33752 39.7 338.15 1.31260 38.2
product is a slightly yellow liquid. The yield is approximately 80%. Analysis of C6MIC by 1H NMR resulting in a spectrum is good agreement with the literature.16 GaCl3 was added slowly with stirring to a small glass vial containing the equal molar C6MIC in a glovebox filled with dry argon, and then the slightly yellow and transparent IL compound C6MIGaCl4 was obtained. Analysis of the product by H NMR gave a spectrum identical to that for C6MIC. The thermal decomposition temperatures, Td ) 702.7 K, for the IL was determined by thermogravimetric analysis using a TA Instruments (SDT) model Q600 thermogravimetric analyzer. 2.3. Measurement of Density and Surface Tension. The density of the sample was measured with an Anton Paar DMA 4500 oscillating U-tube densitometer, provided with automatic viscosity correction under dry argon from 283.15 to 338.15 K. The temperature in the cell was regulated to (0.01 K with a solid state thermostat. The apparatus was calibrated once a day with dry air and double-distilled freshly degassed water. By use of the tensiometer of the forced bubble method (DPAW type produced by Sang Li Electronic Co.), the surface tension of water was measured from 283.15 to 338.15 ( 0.05 K and was in good agreement with that in literature17 within experimental error (0.1 × 10-3 N‚m-1. The surface tension of IL C6MIGaCl4 was measured by the same method under dry argon in the same temperature range. 3. Results and Discussion The values of density and surface tension of IL C6MIGaCl4 are listed in Table 1, respectively. Each value in Table 1 is average of three determinations. 3.1. Estimation of Volumetric Properties for the IL. The experimental values of ln F against (T - 298.15) were fitted
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Tong et al.
Figure 3. Plot of γV2/3 vs T. The correlation coefficient is 0.996. Figure 1. Plot of ln F vs T.
volume of anion GaCl4-, which is 0.1548 ( 0.0025 nm3, which is close to 0.145 nm3 obtained by Jenkins,19 so that the volume, V+, and the radii, r+, of the cations were estimated and are listed in Table 2. In comparison with the value of volume of AlCl4reported previously,10 which is 0.1390 nm3, the volume of anion GaCl4- is much larger. According to Glasser’s theory,14 the empirical equation of calculation of the standard entropy for IL is
S0(298)/J‚K‚mol-1 ) 1246.5(Vm/nm3) + 29.5
Figure 2. Plot of Vm vs M+. The correlation coefficient is 0.999.
by the method of the least square, and an empirical equation ln F ) 0.29713 - 6.29 × 10-4 (T - 298.15) was obtained (see Figure 1). The correlation coefficient is 0.9999, standard deviation s ) 2.8 × 10-5. The coefficient of thermal expansion of C6MIGaCl4 R is defined by following equation
R ≡ (1/V)(∂V/∂T)p ) -(∂ ln F/∂T)p
(1)
then R ) (6.29 × 10-4 K-1 was calculated. In eq 1, V is molar volume of IL C6MIGaCl4. From the experimental density, the molecular volume, Vm, of C6MIGaCl4 was calculated using following equation
Vm ) M/(NF)
(2)
where M is molar mass (378.8 g‚mol-1), N is Avogadro constant, and Vm ) 0.4675 nm3 for C6MIGaCl4 and Vm ) 0.4405 nm3 for C5MIGaCl4,10 and Vm ) 0.4109 nm3 for C4MIGaCl4,12 and Vm ) 0.3634 nm3 for C2MIGaCl418 at 298.15 K. The data of IL-based on gallium are listed in Table 2. From Table 2, the mean contribution to molecular volume per methylene (-CH2-) group is 0.0278 nm3. This agrees well with methylene contributions of 0.0280 nm3 from n-alcohols, 0.0272 nm3 from n-amines, and 0.0267 nm3 from n-paraffins.14 As may be seen in Figure 2, there is linear relationship between molecular volume Vm/nm3 and molar mass of cations, M+/g‚mol-1. Then the linear regression of Vm against M+ was carried out, and the correlation coefficient is 0.999. The intercept of the linear regression may be approximately regarded the
(3)
so that S0(298)/J‚K-1‚mol-1 ) 612.3 for C6MIGaCl4 and S0(298)/J‚K-1‚mol-1 ) 482.4 for C2MIGaCl4. This implies entropy contribution per methylene group to standard entropy is 32.5 J‚K-1‚mol-1. This value is in excellent agreement with the value of 32.2 J‚K-1‚mol-1 from earlier literature for an extended group of organic compound.20 The value is closer than the value of 33.9 J‚K-1‚mol-1 from [Cn-mim][BF4].14 3.2. Estimation of Surface Properties for the IL. In general, surface tension, γ, of many liquid almost linearly decreases while temperature elevates and the relationship is expressed in Eo¨tvo¨s equation21
γV2/3 ) k(Tc - T)
(4)
where V is molar volume of the liquid, Tc is critical temperature, and k is an empirical constant. The linear regression of product of γ and V2/3 obtained from this experiment against absolute temperature T was made, and a good straight line was obtained (see Figure 3). The correlation coefficient of the linear regression is 0.996. From the slope of the straight line, the value of k ) 1.678 × 10-7 J‚K-1 and from the intercept Tc ) 1274 K were obtained, respectively. For majority of organic liquids k is about 2.1 × 10-7 J‚K-1, but for fused salts with large polarity is rather small, for example, k ) 0.4 × 10-7 J‚K-1 for fused NaCl;21 therefore, the magnitude of k can represent the polarity of the IL. The value of k ) 1.678 × 10-7 J‚K-1 implies that C6MIGaCl4 has medium polarity between organic liquid and fused salt. The values of γ obtained at different temperature have been fitted against T by the least-squares to a linear equation (see Figure 4). The correlation coefficient and the standard deviation of the fitting are 0.998 and 7.2 × 10-5, respectively. From the slope of the fitted line, the surface excess entropy, Sa, could be
TABLE 2: Volumetric Properties of IL Based on Gallium (Vm/nm3) at 298.15 K IL
M/g‚mol-1
M+/g‚mol-1
F/g‚cm-3
Vm/nm3
V+/nm3
r+/nm
S0/J‚K-1‚mol-1
C2MIGaCl4 C4MIGaCl4 C5MIGaCl4 C6MIGaCl4
322.8 350.8 364.8 378.8
111.2 139.2 153.2 167.2
1.4745 1.4174 1.3756 1.34596
0.3634 0.4109 0.4405 0.4675
0.2086 0.2561 0.2857 0.3127
0.368 0.394 0.409 0.421
482.4 541.6 578.6 612.3
Physicochemical Properties of IL C6MIGaCl4
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Figure 4. Plot of γ vs T. The correlation coefficient is 0.998.
TABLE 3: Estimated Values for the Vaporization Enthalpy of ILs IL
γ/mJ‚m-2
V (×106)/ m3‚mol-1
C5MIGaCl4a C6MIGaCl4b C4MIAlCl4c C5MIAlCl4a C5MIInCl4a
43.1 40.4 45.6 42.6 43.4
256.2 281.4 248.8 265.4 271.3
a
∆lgHm0(298 K)/ kJ‚mol-1 170.8 166.6 173.2 168.9 174.6
Reference 10. b This work. c Reference 22.
obtained, that is Sa ) - (∂γ/∂T)p ) 55.49 × 10-6 J‚K-1‚m-2. In addition, the surface excess energy likewise may be obtained from the surface tension measured in this work: Ea ) γ - T(∂γ/ ∂T)p ) 56.92 mJ‚m-2. In comparison with fused salts, for example, Ea ) 146 mJ‚m-2 (for fused NaNO3), the value of Ea for C6MIGaCl4 is much lower and is close to organic liquid, for example, 67 (for benzene) and 51.1 mJ‚m-2 (for n-octane).21 This fact shows that interaction energy between ions in C6MIGaCl4 is less than that in fused salts. According to Glasser’s theory,14 the crystal energy, UPOT, may be estimated using following equation
UPOT/kJ‚mol-1 ) 1981.2(F/M)1/3 + 103.8
ν ) 0.6791(kbT/γ)3/2
(7)
where kb is Boltzmann constant, T the thermodynamic temperature, and γ the surface tension of the IL. According to eq 7, the values of average volume of the interstices of ILs at different temperatures are obtained. From Table 1, the surface tension of C6MIGaCl4 γ ) 40.4 mJ‚m-2 at 298.15 K and the average volume of interstice V ) 22.11 × 10-24 cm3, then total volume of the interstice is ∑V ) 26.62 cm3 (formula unit)-1. The volume fraction of interstice, ∑V/V, is about 9.5% for IL C6MIGaCl4. They are closest approach to that of majority of materials, which exhibit ∼10-15% volume expansion in the transformation from the solid to the liquid state. The volume of IL, V, consists of the inherent volume, Vi, and total volume of the all interstices, ∑V ) 2N V, that is
V ) Vi + 2Nν
(8)
If the expansion of IL volume only results from the expansion of the interstices when temperature increases, then calculation expression of R was derived from the interstice model
(5)
so that UPOT ) 406.1 kJ‚mol-1 for C6MIGaCl4 was obtained and the value is much less than that of fused salts, for example, UPOT ) 613 kJ‚mol-1 for fused CsI,17 which is the lowest crystal energy among alkali chlorides. The low crystal energy is the underlying reason for forming IL at room temperature. 3.3. Estimation of Vaporization Enthalpies for the IL. Kabo and colleagues15 put forward an empirical equation for estimation of the enthalpy of vaporization, ∆lgHm0 (298 K), of ILs
∆lgHm0 (298 K) ) A(γV2/3N1/3) + B
3.4. Interstice Model for IL. For pure IL a new theoretic model4 is put forward on the basis of the following assumptions: (1) Because of the large size and the asymmetric shape, the ions may not be closely packed and lots of interstices between ions come into existence. (2) To calculate the volume easily, the interstice is regarded as a bubble. (3) There are 2N interstices for 1 mol 1-1 IL, where N is Avogadro’s constant. (4) The interstice in C6MIGaCl4 can move about like an ion or another particle; in the movement the interstice does not vanish but can be compressed and expanded, which has an extra feature of motion of an interstice called the breathing motion. According to the same procedure of the hole model of molten salt, the expression of calculation of interstice volume, V, was obtained on the classical statistical mechanics
(6)
R ) (1/V)(∂V/∂T)p ) 3Nν/VT
(9)
The values of R (calculated) ) 4.76 × 10-4 K-1 at 298.15 K and R (experimental) ) 6.29 × 10-4 K-1. The magnitude order of thermal expansion coefficient R (calculated) is in good agreement with R (experimental) so that this result means that the interstice model can estimate thermal expansion coefficient of ILs. Acknowledgment. This project was supported by NSFC (20473036) and Bureau of Liaoning Province (2004066C) People’s Republic of China. References and Notes
where N is Avogadro’s constant, A and B are empirical parameters, their values are A ) 0.01121 and B ) 2.4 kJ‚mol-1, respectively. The molar enthalpy of vaporization for IL C6MIGaCl4 calculated from eq 6 was found to be 166.6 kJ‚mol-1 at 298 K. With application of Kabo’s method to our previous work, the results are listed in Table 3. From the values in Table 3, ∆lgHm0 (C5MIGaCl4) is larger than ∆lgHm0 (C6MIGaCl4) and ∆lgHm0 (C4MIAlCl4) is larger than ∆lgHm0 (C5MIAlCl4). This implies that the estimated enthalpy of vaporization of ILs decreases with length of aliphatic chains in the 1-alkyl-3methylimidazoliun cation, and it can be interpreted considering that longer side chains decrease the relative importance of Coulomb forces leading to smaller values of ∆lgHm0. This is in a great agreement with Table 5 in ref 15.
(1) Rogers, R. D.; Seddon, K. S. Ionic liquids Industrial applications for green chemistry, ACS Symposium series 818, ACS, Washington DC, 2002. (2) Yang, J.-Z.; Zhang, Q.-G.; Wang, B.; Tong, J. J. Phys. Chem. B 2006, 110, 22521-22524. (3) Wang, J.; Pei, Y.; Zhao, Y.; Hu, Z. Green Chem. 2005, 7, 144154. (4) Yang, J.-Z.; Lu, X.-M.; Gui, J.-S.; Xu, W.-G. Green Chem. 2004, 6, 541-543. (5) Seddon, K. S. J. Chem. Tech. Biotechol. 1997, 68, 351-356. (6) Endure, F. CHEMPHYSCHEM 2002, 3, 144-154. (7) Dupout, J; Souza, R. F. de; Suarez, P. A. Z. Chem. ReV. 2002, 102, 3667-3692. (8) Yang, J.-Z.; Jin, Y.; Xu, W.-G.; Zhang, Q.-G.; Zang, S.-L. Fluid Phase Equilib. 2005, 227, 41-46. (9) Yang, J.-Z.; Zhang, R.-B.; Xue, H.; Tian P. J. Chem. Thermodyn. 2002, 34, 402-407. (10) Tong, J.; Hong, M.; Guan, W.; Yang, J.-Z. J. Chem. Thermodyn. 2006, 38, 1416-1421.
3200 J. Phys. Chem. B, Vol. 111, No. 12, 2007 (11) Yang, J. Z.; Tian, P; He, L. L. Z.; Xu, W. G. Fluid Phase Equilib. 2003, 204, 295-302. (12) Xu, W. G.; Lu, X.-M.; Gui, J.-S.; Zhang, Q.-G.; Yang, J. Z. Chin. J. Chem. 2006, 24, 331-335. (13) Krossing, I.; Slattery, J. M. Z. Phys. Chem. 2006, 220, 1343-1359. (14) Glasser, L. Thermochim. Acta. 2004, 421, 87-93. (15) Zaitsau, D. H.; Kabo, G. J.; Strechan, A. A.; Paulechka, Y. U.; Tschersich, A.; Verevkin, S. P.; Heintz, A. J. Phys. Chem. A 2006, 110, 7303. (16) Huddleston, J. G.; Visser, A. E.; Reichert, W. M.; Willauer H. D. Green Chem. 2001, 3, 156-164.
Tong et al. (17) Lide, D. R. Handbook of Chemistry and Physics, 82nd ed.; CRC Press: Boca Raton, 2001-2002. (18) Yang, J.-Z.; Xue, F.; Zhang, Q.-G. J. Mol. Liq. 2006, 128, 81-84. (19) Jenkins, H. D. B.; Roobottom, H. K.; Passmore, J.; Glasser, L. Inorg. Chem. 1999, 38, 3609-3620. (20) Janz, G. J. Thermodynamic Properties of Organic Compounds; Academic Press: New York, 1967. (21) Adamson, A. W. Physical chemistry of surfaces, 3rd ed.; JohnWiley: New York, 1976. (22) Tong, J.; Zhang, Q.-G.; Hong, M.; Yang, J.-Z. Acta Phis-Chim. Sin. 2006, 22 (1), 71-75.
