Viscous Behavior of Imidazolium-Based Ionic Liquids - Industrial

Article pubs.acs.org/IECR

Viscous Behavior of Imidazolium-Based Ionic Liquids Mert Atilhan,*,† Johan Jacquemin,‡ David Rooney,‡ Majeda Khraisheh,† and Santiago Aparicio*,§ †

Department of Chemical Engineering, Qatar University, Doha 2173, Qatar School of Chemistry and Chemical Engineering, Queen’s University Belfast, Belfast, BT7 1NN, Northern Ireland, U.K. § Department of Chemistry, University of Burgos, Burgos 09001, Spain ‡

S Supporting Information *

ABSTRACT: The viscosity of four imidazolium-based ionic liquids is analyzed as a function of pressure and temperature. Experimental measurements were carried out using an electromagnetic moving piston viscometer in the 303−353 K and 0.1−70 MPa ranges on synthesized ultrapure samples, and compared with available literature data. Molecular dynamics simulations were used to analyze the fluids’ dynamic properties from a nanoscopic viewpoint, with special attention paid to self-diffusion coefficients and dynamic viscosity. Simulated properties are in excellent agreement with experimental results in spite of the glasslike dynamics of some of the studied fluids.

1. INTRODUCTION The viscous behavior of fluids is governed by intermolecular forces and factors such as the size and/or shape of the involved molecules.1 Furthermore, it is one of the most important physical properties for the design of industrial processes involving heat, mass, or momentum transfer, or dissolution or absorption of compounds in fluids. As such processes are strongly dependent on the viscosity of the used fluids, this property should be accurately known as a function of pressure and temperature over the full range of expected process conditions in order to correctly size the equipment, and subsequent process economics and optimization.2−4 Ionic liquids, ILs, are among the most attractive alternatives to traditional fluids for many engineering applications such as toxic gas removal, gas sweetening, and industrial hydraulic applications.5 Nevertheless, the highly viscous behavior of some ILs has been considered to be a serious drawback for large-scale use of these fluids particularly as this tends to increase the capital and operational costs of such processes.6 However, for specific processes, such as lubrication, it may be an advantage.7 One of the most important characteristics of ILs is the large number of possible anion−cation combinations leading to low melting temperature compounds,8 producing fluids with very different viscous behaviors. This ranges from low-viscosity fluids, similar to common organic solvents, to glasslike extremely viscous fluids.9 The possibility of controlling the viscous behavior of ILs and at the same time other desired properties through a judicious combination of the involved ions10,11 yields tailor-made fluids. While functional modification is not specific to ILs, these materials are more suited to such design approaches and thus have a remarkable advantage when compared to traditional organic solvents for many engineering applications.12 However, such a molecular design requires a deep knowledge, at the nanoscopic level, of the relationship between the molecular structures of the involved ions,13,14 their intermolecular forces, and the macroscopic viscosity as a function both of pressure and temperature.15 Viscosity is probably the most important physical property for the © 2013 American Chemical Society

industrial application of ILs and the scale-up from the laboratory to the industrial level of a desired process,9 and thus, an accurate knowledge of this property is required. Such tools if available would play a pivotal role in the advancement of IL technologies. The experimental measurement of the viscosity for any fluid with the required uncertainty is extremely important and one where serious problems can arise from the absence of suitable reference data as well as other metrological problems.16 Viscosity metrological problems are even more remarkable for ILs, for which extremely large deviations between the available data sets are inferred.17,18 These discrepancies rise from the applied experimental methods and from the purity of used samples. Available studies have showed the strong effect of impurities (e.g., unreacted starting materials, water, halide, and/or lithium) on the viscous behavior of ILs,19,20 and thus available experimental studies using highly impure or poorly characterized samples and/or methods lead to unreliable viscosity data, which in turn result in errors when used for process design purposes. Another serious problem in the industrial use of ILs is the scarcity of viscosity data over wide pressure and temperature ranges. Unfortunately, most of the available literature data are measured at ambient pressure, which hinders development utilization of high pressure in relevant processes. The available studies on high-pressure viscosity of some ILs were reviewed elsewhere,18 and these studies were all devoted to imidazolium-based ILs. Moreover, there are other reported systematic experimental and computational studies on the high-pressure viscosity specifically for pyridium-based ILs, which analyze the effects of anion and cation properties on viscous behavior.21 Classic molecular dynamics simulations are extremely useful for inferring the most relevant features controlling dynamic properties of ILs at the nanoscopic level.21−24 Although Received: Revised: Accepted: Published: 16774

September 16, 2013 October 21, 2013 October 22, 2013 October 22, 2013 dx.doi.org/10.1021/ie403065u | Ind. Eng. Chem. Res. 2013, 52, 16774−16785

Industrial & Engineering Chemistry Research

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Table 1. Molar Mass (M), Water Content in Mass Fraction (ww), Bromide Mass Fraction Content (wBr−), and Lithium Mass Fraction Content (wLi+) for the Samples Used in This Work

a

ionic liquid

M/g· mol−1

ww × 10−6 a

wBr− × 10−6 b

wLi+ × 10−6c

[EMIM][Tf2N] [BMIM][Tf2N] [BMIM][BF4] [BMIM][PF6]

391.31 419.37 0.02 284.18

12 55 250 135

[BMIM][Tf2N] ≈ [EMIM][Tf2N] > [BMIM][BF4], decreasing with increasing pressure under isothermal conditions, with the exception of [BMIM]-

Figure 3. (a) Pressure−viscosity coefficients, αη, at 333.15 K and (b) temperature−viscosity coefficients, βη, at 40 MPa, for the studied ILs. All values calculated from eqs 1 and 2 and parameters reported in Table 3.

decreasing nonlinearly with increasing temperature under isobaric conditions (Figure 3b). 3.2. Molecular Dynamics Simulations. Simulations were carried out to investigate the performance of the proposed methods to predict the viscous behavior of the studied liquids and to analyze the structural properties at the nanoscopic level in relation to the dynamics of these fluids. To check the performance of the proposed force field parametrizations, the density was calculated and is compared in Table 4 with experimental data obtained during this work and from the literature.78,79 Density data obtained from the simulations 16778

dx.doi.org/10.1021/ie403065u | Ind. Eng. Chem. Res. 2013, 52, 16774−16785


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Table 4. Densities, ρ, of Studied Ionic Liquids Obtained from Molecular Dynamics Simulations in Comparison with Experimental Data as a Function of Pressure and Temperaturea ρ/g·cm−3 P/T

[EMIM][Tf2N]

[BMIM][Tf2N]

[BMIM][BF4]

[BMIM][PF6]

303 K/0.1 MPa

1.5139b (0.01) 1.513 81c 1.526d 1.5681b 1.4423b (−1.48) 1.464 01c 1.461d 1.4904b

1.4169b (−1.06) 1.432 05c 1.431 86e 1.4604b 1.3601b (−1.80) 1.385 07c 1.384 88e 1.4148b

1.1824b (−1.34) 1.198 50c 1.1976f 1.2219b 1.1469b (−1.43) 1.163 50c 1.1636f 1.1765b 1.195g,h

1.3603b (−0.25) 1.363 76c 1.3601h 1.4030b 1.3155b (−0.58) 1.323 20c 1.3211h 1.3551b 1.359h

303 K/70 MPa 353 K/0.1 MPa

353 K/70 MPa

%RD is shown in parentheses, where %RD = 100(ρMD − ρexp,this work)/ρexp,this work). bMolecular dynamics simulations from this work. cExperimental data from this work. dGardas et al.78 eHarris et al.46 fSoriano et al.51 gData at 352.6 K. hMachida et al.79

a

Figure 4. Site−site radial distribution functions, g(r), for relevant pairs as a function of pressure and temperature. Atom code: H1, hydrogen atom in imidazolium ring bonded to carbon atom placed between the N atoms; H2, hydrogen atoms in imidazolium ring opposite H1; O, oxygen atoms in [Tf2N]− anion; F, fluorine atoms in [BF4]− or [PF6]− anions. Δg(r) stands for the differential radial distribution functions with respect to those at 303 K and 0.1 MPa.

al.27 Similarly, Andreussi et al.23 have also reported remarkable changes in the RDFs corresponding to tail−tail alkyl sites in the imidazolium cation with increasing temperature. The analysis of the pressure effects on RDFs in [BMIM][PF6] was reported by Zhao et al.,27 who reported remarkable conformational changes for the imidazolium cation on going to pressures as high as 6000 bar. RDFs for [alkylmethylimidazolium][Tf2N] were analyzed by Bodo et al.80 through a comparison of experimental X-ray diffraction data and molecular dynamics simulations; these authors showed the strong hydrogen bonding of H1 imidazolium atoms with oxygen atoms of the sulfonyl anion group. In the case of [BMIM][BF4], Prado et al.81 also showed strong anion−cation interactions through the cationic H1 acidic site. Site−site RDFs between H1 and H2 imidazolium cation

showed a trend of an average of 2% deviation from the experimental density data. Structural information was analyzed using calculated radial distribution functions, RDFs, as shown in Figure 4. Several recent literature works have analyzed RDFs for the ILs studied in this work. Pal et al.24 analyzed site−site RDFs in [BMIM][PF6] showing the strong interaction between the imidazolium hydrogen atom placed between the two nitrogen atoms, which will be named H1 in this work, and fluorine atoms in the [PF6]− anion, because of the most acidic character of this atom in comparison with the two remaining imidazolium hydrogens (named H2 in this work). These H1−F and H2−F RDFs decrease remarkably with temperature. Analogous conclusions were inferred by Andreussi et al.23 and Zhao et 16779



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atoms and sulfonyl oxygen in [Tf2N]− anion or fluorine atoms in [BF4]− or [PF6]− anions are reported in Figure 4. Results reported for the four studied ILs show prevailing anion−cation interactions through the H1 cation site in comparison with weaker interactions through the H2 cation site. On going from [EMIM][Tf2N] to [BMIM][Tf2N] interaction through the H1 site is strengthened, and interaction with anion fluorine atoms is stronger for [PF6]− than for [BF4]− anion. The effect of pressure and temperature on RDFs is also reported in Figure 4, in which differential RDFs, with respect to the data collected at 303 K and 0.1 MPa. For [EMIM][Tf2N] and [BMIM][Tf2N], RDFs corresponding to the H1/O sites is weakened with both increasing pressure and temperature, especially for [BMIM][Tf2N]. The effects of pressure and temperature on H2containing RDFs are less remarkable, although they are also weakened upon heating and/or compression. Similar results are obtained for [BMIM][BF4] and [BMIM][PF6]. Because of the changes in imidazolium cation alkylic tail−tail correlations with increasing temperature reported by Andreussi et al.,23 RDFs for imidazolium tail carbon atoms are reported in Figure 5. In the case of short alkyl chains, [EMIM][Tf2N], a

Figure 6. N2−C2−C3−C4 torsional angle in alkylimidazolium cations as a function of pressure and temperature.

tion is calculated from the reported torsional distribution and reported in Table 5. The percentage of gauche conformation Table 5. Percentage of Gauche Conformation in [BMIM]+ Cation, %xG, Obtained from Molecular Dynamics Simulations in Comparison with Experimental Data as a Function of Pressure and Temperaturea %xG P/T 303 303 353 353

K/0.1 MPa K/70 MPa K/0.1 MPa K/70 MPa

[BMIM][Tf2N]

[BMIM][BF4]

[BMIM][PF6]

30.0 32.0 33.5 34.5

27.5 29.4 31.3 32.7

37.9 38.9 39.4 40.0

a

Values calculated from N2−C2−C3−C4 torsional angle distributions as reported in Figure 6.

calculated in this work for [BMIM][PF6] at 303 K and 0.1 MPa (37.9%) is in very good agreement with the values reported by Canongia et al. (38%)82 and Zhao et al. (37.4%).27 The percentage of gauche form increases with increasing pressure and temperature, for all the studied ILs (Table 5). Zhao et al.27 showed that the gauche form for [BMIM][PF6] increases roughly by 5.5% on going from 0.1 to 600 MPa, in agreement with experimental results by Russina et al.83 In this work, the gauche percentage increases 1−2% on going from ambient pressure to 70 MPa. Likewise, the gauche percentage follows the order [BMIM][PF6] > [BMIM][Tf2N] > [BMIM][BF4] (Figure 6b and Table 5), with differences of up to 10% between [BMIM][PF6] and [BMIM][BF4], and thus showing that the anion has a remarkable effect on cation conformational changes. The analysis of dynamic properties was done initially through the calculation of self-diffusion coefficients from the mean square displacements (msd’s) using Einstein’s equation. The studied ILs are very viscous fluids, especially [BMIM][PF6], and thus, they are characterized by low diffusion rates. Andreussi et al.23 reported a detailed study on the simulation times required to fulfill the requirement of reaching fully diffusive regime in the molecular dynamics simulations for [BMIM][PF6]. The diffusion regime is characterized by the β parameter, which is defined as the slope of log−log plots of msd’s as a function of simulation time. The fully diffusive regime is characterized by β = 1. Results reported by Andreussi et al.23 showed, for their [BMIM][PF6] force field para-

Figure 5. Site−site radial distribution functions, g(r), and the corresponding running integrals, N, between tail carbon atoms in alkylimidazolium cations, as a function of pressure and temperature.

first peak at 9.1 Å is obtained, with a shoulder at lower distances, and is almost constant with increasing pressure. The situation changes on going to [BMIM][Tf2N]: the first peak shifts to 5.3 Å and is obtained at 303 K, which is even reinforced with increasing pressure. Therefore, a correlation between alkylic groups in imidazolium cations is inferred, tending to self-aggregation and improving cation−anion interactions through H1/O sites, as may be concluded from the comparison of RDFs in Figure 4a,b. Another remarkable feature for the studied ILs is the conformational changes in the imidazolium cation, which were also analyzed using molecular dynamics by Canongia et al.82 and Zhao et al.27 In the case of systems containing the [BMIM] cation, the presence of anti and gauche forms in the butyl chain was described at room temperature.27,82 Torsional distribution with respect to the N2−C2−C3−C4 angle is reported in Figure 6, as a function of temperature, pressure, and anion type. The peak appearing around ±60° corresponds to the gauche conformation, and the maxima around ±180° correspond to the anti form. The temperature and pressure effect on the N2−C2−C3−C4 torsional distribution is reported in Figure 6a; the anti form prevails over the gauche one for all the studied temperatures and pressures. Moreover, the percentage of gauche conforma16780



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Table 6. Diffusion Times, t, Required To Reach Diffusive Regime (β ≈ 1)a t/ns P/T 303 303 353 353

K/0.1 MPa K/70 MPa K/0.1 MPa K/70 MPa

[EMIM][Tf2N] 2.3 2.7 0.6 1.2

[BMIM][Tf2N]

(1.7) (2.4) (0.5) (0.9)

2.6 3.8 0.9 1.7

(2.4) (3.5) (0.7) (1.4)

[BMIM][BF4] 3.4 7.0 1.0 1.6

(3.1) (6.5) (1.0) (1.5)

[BMIM][PF6] 6.9 (6.2) b 1.5 (1.3) 2.5 (2.2)

a Anion values are not in parentheses; cation values are in parentheses. bFully diffusive regime not reached for the considered 10 ns simulation time; at t = 10 ns, β = 0.87 and 0.91, for anion and cation, respectively, is obtained.

metrization, simulations times up to 108 ps to reach β = 1 at 300 K. Although force field parametrization by Andreussi et al.23 led to [BMIM][PF6] viscosity values an order of magnitude larger than experimental ones, and to self-diffusion coefficients an order of magnitude lower than experimental data, their results showed the importance of using simulation times long enough to obtain reliable results. The viscosities, and thus the self-diffusion coefficients, for the ILs studied in this work follow the order [BMIM][PF6] (493−25.5 mPa·s) > [BMIM][BF4] (165−13.9 mPa·s) > [BMIM][Tf2N] (87.0−9.5 mPa·s) > [EMIM][Tf2N] (51.1−7.81 mPa·s), where the values in parentheses show the experimental maxima and minima viscosity data in the 303−353 K and 0.1−70 MPa ranges (Table S1, Supporting Information). Therefore, the most remarkable issues to reach fully diffusive regimes in the considered simulation times (10 ns) would be for [BMIM][PF6], and for low temperatures (especially 303 K) and higher pressures. To analyze the quality of the self-diffusion data reported in this work, the simulation time required to reach β = 1 is calculated for all the simulations and reported in Table 6. Results in Table 6 show that, for all the studied ILs, pressures, and temperatures, fully diffusive regimes were reached during the simulation times, with the exception of [BMIM][PF6] at 303 K and 70 MPa, for which β around 0.9 is obtained for both the anion and the cation. It should be remarked that the time required to reach β = 1 for [BMIM][PF6] at 303 K and 0.1 MPa in this work is significantly lower than the value reported by Andreussi et al.,23 but it could be justified considering that the calculated self-diffusion coefficients in their work are remarkably lower than those obtained in this work. The behavior of the msd is reported in Figure 7, from which, on one side, the faster cation than anion self-diffusion is inferred for all the studied temperatures and pressures (Figure 7a) and, on the

other side, the poor diffusive behavior of [BMIM][PF6] in the studied simulation time in comparison with the remaining ILs is concluded. Calculated ion self-diffusion coefficients are reported in Table 7.84,85 Calculated values are in very good agreement with experimental available data, with calculated data larger than experimental ones, but always predicting self-diffusion coefficients of the same order of magnitude as experimental values. Cation self-diffusion coefficients are larger than anion ones, with the largest differences for [EMIM][Tf2N]. Dynamic viscosity was also calculated from simulations using the Green− Kubo method, as tabulated in Table 8. Viscosity predictions are in excellent agreement with experimental data, with the largest deviations obtained for [BMIM][PF6] because of the nonfulfillment of the condition of fully diffusive regime explained in the previous sections. Interionic interaction energy increases both from Coulombic and Lennard-Jones type contributions, although considering the ionic character of the involved compounds the Coulombic contribution clearly dominates their structuring and properties.27 Therefore, the interionic interaction energy was calculated in this work. This property was analyzed with respect to viscosity data of the studied ILs, and thus, the relationship between the interionic interaction energy density (EintVm−1) and viscosity is plotted in Figure 8. First, from Figure 8, it may be concluded that [BMIM][Tf2N] has an anomalous behavior in comparison with the remaining ILs. As a rule, the more viscous fluids have the largest EintVm−1 values, for all the studied pressures and temperatures, with the exception of [BMIM][Tf2N], which is more viscous than [EMIM][Tf2N], but it has a slightly lower EintVm−1 value than [EMIM][Tf2N]. For any ionic liquid in particular, there is a clear trend between viscosity and EintVm−1 (vertical black, blue, green, and red dashed arrows in Figure 8), which shows increasing viscosity by increasing pressure is coupled with increasing EintVm−1, and the reverse effect for increasing pressure. For fixed temperature and pressure, and changing IL type (horizontal gray dashed arrows in Figure 8), there is also an almost linear relationship between viscosity and EintVm−1 with the exception of [BMIM][Tf2N], which behaves like an outlier on the plot. Therefore, a strong linear correlation between the interionic interaction energy density and viscosity may be concluded from the results in Figure 8.

4. CONCLUSION A combined high-pressure experimental and computational study on the viscosity of four selected imidazolium-based ILs is reported in this work. Experimental measurements reported using noncommercial high quality samples and an electromagnetic piston viscometer, with ±2% uncertainty, are in very good agreement, at both ambient and high pressure conditions, with high quality available literature data. Experimental viscosity

Figure 7. Ion center-of-mass mean square displacements, msd’s, obtained from molecular dynamics simulations and Einstein’s relationship as a function of pressure and temperature. 16781



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Table 7. Center-of-Mass Self-Diffusion Coefficients, D, Calculated from Molecular Dynamics Simulations, in Comparison with Experimental and Simulated Data from the Literature, at Reported Temperatures and Pressuresa 1011D/m2·s−1 303 K/0.1 MPa

[EMIM][Tf2N] [BMIM][Tf2N]

303 K/70 MPa

353 K/0.1 MPa

353 K/70 MPa

anion

cation

anion

cation

anion

cation

anion

cation

5.83 ± 0.61b 3.70c 3.72 ± 0.42b 2.70d 0.16 (300 K)f

10.44 ± 0.91b 5.88c 4.79 ± 0.42b 3.37d 3.2 (298 K)e 0.25 (300 K)f 3.67 ± 0.30b 1.85d 1.6 (298 K)e 0.13 (300 K)f 1.97 ± 0.16b 0.95d 0.8 (298 K)e 0.050 (300 K)f

3.29 ± 0.35b

4.67 ± 0.39b

20.32 ± 2.27b

2.14 ± 0.27b

33.78 ± 3.87b 20.95c 20.59 ± 2.13b 14.76d 2.3 (350 K)f

13.43 ± 1.54b

1.62 ± 0.18b

22.79 ± 2.25b 14.04c 16.74 ± 1.47b 12.53d 1.5 (350 K)f

9.18 ± 0.77b

11.96 ± 1.40b

1.35 ± 0.15b

1.46 ± 0.18b

18.24 ± 1.55b 10.99d 1.4 (350 K)f

18.72 ± 1.94b 10.48d 1.5 (350 K)f

10.38 ± 1.11b

9.15 ± 1.04b

0.63 ± 0.08b

0.78 ± 0.11b

11.45 ± 0.92b 5.64d 0.23 (350 K)f

14.40 ± 1.52b 7.02d 0.42 (350 K)f

5.64 ± 0.52b

7.34 ± 0.69b

[BMIM][BF4]

3.25 ± 0.38b 1.74d 0.092 (300 K)f

[BMIM][PF6]

1.48 ± 0.16b 0.70d 0.021 (300 K)f 0.45 (298 K)g

0.71 (298 K)g a

Simulated values in this work calculated using Einstein’s equation from mean square displacements. bValues from molecular dynamics simulations obtained in this work. cExperimental values from Tokuda et al.41 dExperimental values from Tokuda et al.40 eExperimental values from Jin et al.84 f Values from molecular dynamics simulations by Andreussi et al.23 gValues from molecular dynamics simulations by Pal et al.85

Table 8. Dynamic Viscosity, η, Calculated from Molecular Dynamics Simulations, in Comparison with Experimental and Simulated Data from the Literature, at the Reported Temperatures and Pressuresa η/mPa·s 303 K/0.1 MPa [EMIM][Tf2N]

[BMIM][Tf2N]

[BMIM][BF4]

[BMIM][PF6]

303 K/70 MPa

31 ± 3b 26.4c 26.9d 28.5e 50 ± 5b 41.2c 40.2f 41.5g 72 ± 8b 84.0c 77.2f 79.0h 145 ± 13b 209c 189.2f 206.7h

353 K/0.1 MPa

353 K/70 MPa

61 ± 6b 50.1c

10 ± 1b 7.87c 7.8d

17 ± 2b 13.9c

114 ± 12b 89.7c

13 ± 1b 9.44c 9.3f 9.53g 16 ± 2b 13.7c 13.3f 13.5h 23 ± 3b 24.1c 24.1f 25.8h

24 ± 3b 17.0c

178 ± 16b 169c

353 ± 32b 532c

30 ± 3b 22.4c

45 ± 5b 47.3c

a Simulated values were calculated using Green−Kubo method. bValues from molecular dynamics simulations obtained in this work. cValues obtained in this work from eqs 1 and 2 and parameters in Table 3. dExperimental values from Tokuda et al.41 eExperimental values from Ahosseini et al.30 fExperimental values from Tokuda et al.40 gExperimental values from Harris et al.46 hExperimental values from Tomida et al.70

up to 10% larger for ILs containing [PF6]− anion than for those containing [BF4]−. The dynamic behavior was analyzed from calculated msd’s, self-diffusion coefficients, and viscosity. The time required to reach fully diffusive conditions is reached for all the studied systems and conditions, with the exception of [BMIM][PF6]. Predicted self-diffusion coefficients and viscosity data are in very good agreement with available experimental information. Finally, a strong correlation between IL viscosity and interionic interaction energy density is inferred, which justifies the changes of viscosity with temperature, pressure, and type of ILs, with the exception of [BMIM][Tf2N].

data were successfully correlated with a seven-parameter Taittype fitting equation, leading to deviations lower than 2%. Pressure−viscosity and temperature−viscosity coefficients were calculated from the correlation of experimental data. Molecular dynamics simulations were performed as a function of pressure and temperature. Structural properties were analyzed showing strong anion−cation interactions through the H1 acidic hydrogen in the imidazolium ring. Imidazolium alkyl tails aggregate for all the systems containing [BMIM] cation, which is reinforced by increasing pressure. The gauche form for the [BMIM]+ alkyl group increases with increasing pressure and temperature, and its percentage in comparison with anti form is 16782



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(8) Rogers, R. D.; Seddon, K. R. Ionic LiquidsSolvents of the Future? Science 2003, 302, 792. (9) Rooney, D.; Jacquemin, J.; Gardas, R. Thermophysical properties of ionic liquids. Top. Curr. Chem. 2010, 290, 185. (10) Gurkan, B.; Goodrich, B. F.; Mindrup, E. M.; Ficke, E.; Massel, M.; Seo, S.; Senftle, T. P.; Wu, H.; Glaser, M. F.; Shah, J. K.; Maginn, E. J.; Brennecke, J. F.; Schneider, W. F. Molecular Design of High Capacity, Low Viscosity, Chemically Tunable Ionic Liquids for CO2 Capture. J. Phys. Chem. Lett. 2010, 1, 3494. (11) Butler, S. D.; Müller-Plathe, F. A Molecular Dynamics Study of Viscosity in Ionic Liquids Directed by Quantitative Structure− Property Relationships. ChemPhysChem 2012, 13, 1791. (12) Werner, S.; Haumann, M.; Wassercheid, P. Ionic Liquids in Chemical Engineering. Annu. Rev. Chem. Biomol Eng. 2010, 1, 203. (13) Gharagheizi, F.; Ilani-Kaskkouli, P.; Mohammadi, A. H.; Rmajgernath, D.; Richon, D. Development of a Group Contribution Method for Determination of Viscosity of Ionic Liquids at Atmospheric Pressure. Chem. Eng. Sci. 2012, 80, 326. (14) Chen, D. B. K.; Liang, M. J.; Wu, T. Y.; Wang, H. P. A High Correlate and Simplified QSPR for Viscosity of Imidazolium-based Ionic Liquids. Fluid Phase Equilib. 2013, 350, 37. (15) Yu, G.; Zhao, D.; Wen, L.; Yang, S.; Chen, X. Viscosity of Ionic Liquids: Database, Observation, and Quantitative Structure-Property Relationship Analysis. AIChE J. 2012, 58, 2885. (16) Nieto de Castro, C. A.; Santos, F. J. V.; Fareleira, J. M. N. A.; Wakeham, W. A. Metrology of Viscosity: Have We Learned Enough? J. Chem. Eng. Data 2009, 54, 171. (17) Nieto de Castro, C. A. Thermophysical properties of ionic liquids: Do We Know How to Measure them Accurately? J. Mol. Liq. 2010, 156, 10. (18) Aparicio, S.; Atilhan, M.; Karadas, F. Thermophysical Properties of Pure Ionic Liquids: Review of Present Situation. Ind. Eng. Chem. Res. 2010, 49, 9580. (19) Seddon, K. R.; Stark, A.; Torres, M. J. Influence of chloride, water, and organic solvents on the physical properties of ionic liquids. Pure Appl. Chem. 2000, 72, 225. (20) Jacquemin, J.; Husson, P.; Padua, A. A. H.; Majer, V. Density and viscosity of several pure and water-saturated ionic liquids. Green Chem. 2006, 8, 172. (21) Bandrés, I.; Alcalde, R.; Lafuente, C.; Atilhan, M.; Aparicio, S. On the Viscosity of Pyridinium Based Ionic Liquids: An Experimental and Computational Study. J. Phys. Chem. B 2011, 115, 12499. (22) Bedrov, D.; Borodin, O.; Li, Z.; Smith, G. D. Influence of Polarization on Structural, Thermodynamic, and Dynamic Properties of Ionic Liquids Obtained from Molecular Dynamics Simulations. J. Phys. Chem. B 2010, 114, 4984. (23) Andreussi, O.; Marzari, N. Transport properties of roomtemperature ionic liquids from classical molecular dynamics. J. Chem. Phys. 2012, 137, 044508. (24) Pal, T.; Biswas, R. Rank-dependent orientational relaxation in an ionic liquid: an all-atom simulation study. Theor. Chem. Acc. 2013, 132, 1348. (25) Aparicio, S.; Alcalde, R.; García, B.; Leal, J. M. High-Pressure Study of the Methylsulfate and Tosylate Imidazolium Ionic Liquids. J. Phys. Chem. B 2009, 113, 5593. (26) Shah, J. K.; Maginn, E. J. Molecular Dynamics Investigation of Biomimetic Ionic Liquids. Fluid Phase Equilib. 2010, 294, 197. (27) Zhao, Y.; Liu, X.; Lu, X.; Zhang, S.; Wang, J.; Wang, H.; Gurau, G.; Rogers, R. D.; Su, L.; Li, H. The Behavior of Ionic Liquids under High Pressure: A Molecular Dynamics Simulation. J. Phys. Chem. B 2012, 116, 10876. (28) Karadas, F.; Köz, B.; Jacquemin, J.; Deniz, E.; Rooney, D.; Thompson, J.; Yavuz, C. T.; Kraished, M.; Aparicio, S.; Atilhan, M. High Pressure CO2 Absorption Studies on Imidazolium-based Ionic Liquids: Experimental and Simulation Approaches. Fluid Phase Equilib. 2013, 351, 74. (29) Aparicio, S.; Atilhan, M.; Khraisheh, M.; Alcalde, R. Study on Hydroxylammonium-Based Ionic Liquids. I. Characterization. J. Phys. Chem. B 2011, 115, 12473.

Figure 8. Relationship between viscosity obtained from molecular dynamics simulations and EintVm−1, where Eint and Vm are the total intermolecular interaction energy (Coulombic + Lennard-Jones) and molar volume, both obtained from molecular dynamics simulations.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental dynamic viscosity (Table S1) and pressure− viscosity and temperature−viscosity coefficients obtained from the fit of experimental viscosity data (Table S2). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This publication was made possible by NPRP Grant 09-739-2284 from the Qatar National Research Fund (a member of the Qatar Foundation). The statements made herein are solely the responsibility of the authors.

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REFERENCES

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Viscous Behavior of Imidazolium-Based Ionic Liquids - Industrial

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