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Thermophysical Properties of Pure Ionic Liquids: Review of Present Situation Santiago Aparicio,*,† Mert Atilhan,‡ and Ferdi Karadas‡ Department of Chemistry, UniVersity of Burgos, 09001 Burgos, Spain, and Department of Chemical Engineering, Qatar UniVersity, 2713 Doha, Qatar
The large collection of thermophysical properties data for liquids available in the open literature is analyzed, describing its importance for industrial purposes. Although there has been a boom of thermophysical properties measurements for these fluids in the past decade, the reported analysis shows that studies have been centered on a limited number of fluids, whereas data collection for the new family ionic liquids is lacking. Measurements have been performed for limited temperature ranges, and pressure effects on the properties are extremely scarce in the literature; remarkable differences among data from different literature sources are reported. The need of sample purity quantification is analyzed together with the strong effect of these impurities on thermophysical properties. Available predictive models for the studied properties are analyzed together with the quality of their predictions. The main conclusion of this review is to point out the need of further systematic thermophysical studies, including for new environmentally friendly ionic liquids, carried out with interlaboratory comparisons, which allow the development of reference data sets. 1. Introduction The interest in ionic liquids has been boosted in recent years in both industry and academia due to increased need for environmentally friendly fluids to meet regional and international environmental regulations. A recent literature survey using the SCOPUS tool1 led to 7585 articles when the keyword ionic liquid was searched in the article title field for the 2005-2010 period, with a remarkable increase in the number of published articles from 1088 in 2005 to almost double that in 2009 (1955 published references). The interest on these fluids is not only academic but also industrial; a search in the USPTO Web Patent Database2 led to 550 references when the term ionic liquid was searched for the 2005-2010 period. The attention that ionic liquids have attracted from the scientific community has also produced a remarkable number of review works (247 up to 2010 year) in which the main aspects of this technology are studied for different fields of interest. One of the most remarkable fields of study within the ionic liquids area is the measurement of relevant physical properties. Accurate knowledge of thermophysical properties is valuable as it is required to decide whether the use of ionic liquids could be extended from the laboratory level to large-scale industrial applications. The development of reliable and economical process design for industrial applications of ionic liquids relies on the knowledge of their most relevant thermophysical properties such as density, viscosity, and heat capacity. The number of published thermophysical studies for ionic liquids has increased remarkably in the past few years with a large number of research groups doing measurements around the world. Therefore, in our opinion, it is necessary to analyze this large amount of information according to the following aspects: (i) What are the industrial needs for thermophysical properties? Are we measuring what is really required? (ii) Which ionic liquids have been measured? How can we extend the studies to new families of ionic liquids? (iii)What is the accuracy of available measurements? Which are the most suitable techniques for these fluids? Do we * To whom correspondence should be addressed. E-mail: sapar@ ubu.es. † University of Burgos. ‡ Qatar University.
have enough information on the pressure-temperature effects on these properties? The existence of the IUPAC ionic liquids database (IL Thermo)3 allows analyzing the present status of the available data for pure or mixed ionic liquids. Some excellent reviews have been published4-7 describing some of the main aspects of thermophysical properties for ionic liquids. In this work, we aim to analyze the most recent thermophysical literature on ionic liquids according to the three criteria mentioned in the previous paragraph. 2. Relevance of Thermophysical Properties The central role of thermodynamics for chemical and related industries has been shown by many authors for common fluids,8-10 and for ionic liquids.11,12 The review works by Zhao13 and Plechkova and Seddon14 pointed to a remarkable use and interest for ionic liquids in industry, despite the consideration that this technology is still in its infancy. Nevertheless, one of the main barriers for the development of ionic liquids for industrial applications is the scarce knowledge of their thermophysical properties,15 both for pure and mixed fluids, in the wide pressure and temperature ranges required for process design purposes. Some recent works have shown the most relevant aspects of thermophysical properties for ionic liquids.6,7 The possible number of ionic liquids may be as large as 1018 (including binary and ternary ionic liquids) according to some authors,16,17 but a detailed analysis of the literature shows that basic properties have been measured only for a very limited number of ionic liquids. The IL Thermo database contains information for just 162 pure ionic liquids,3 with many of these studies reported at atmospheric pressure and ambient temperature conditions. Therefore, there is a clear need for systematic thermodynamic and thermophysical measurements for ionic liquids to point out their availability for use at the industrial processes level. Several international studies under the auspices of IUPAC have been developed in the past few years18 to advance the knowledge of thermophysical properties of ionic liquids for industrial purposes. Moreover, the development of reference ionic liquids19,20 would allow improving the quality, reliability, and accuracy of future thermophysical studies.
10.1021/ie101441s 2010 American Chemical Society Published on Web 09/24/2010
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A remarkable problem rising from the available literature data for ionic liquids is that results from different laboratories or from different methods frequently do not agree within their stated uncertainties due to problems produced by the applied methodology or the purity of the samples used in experiments.12 Chemical engineers require several thermophysical properties for process design purposes. Uncertainties of the data used are critically important for process design calculations. A common practice for design purposes is to over-design processes and over-size equipment to take into account uncertainties, thus avoiding plants that do not meet specifications. This procedure leads to an increase of costs of plant building and products production, and thus, lower uncertainties in available data would lead to a decrease in plant and production costs. Nevertheless, not all the thermophysical properties have the same relevance for industrial purposes and their impact on process design is different. The main thermophysical properties required for ionic liquids, considering their industrial applications and the available data are detailed below.
Table 1. Normal Melting Temperature, Tm, of Selected Ionic Liquids Tm/K uncertainty/Ka
purity
reference
method
1-butyl-3-methylimidazolium hexafluorophosphate 283.1 276.43 277.15 284.1 283.1 280.03
2.0 1.0 3.0 1.0 1.0 0.77
247.1 287.57 286.1
110 1.1 1.0
0.059 water mass % 99.2 mass % 98 mass % not stated 0.004 water mass % 99.8%
28 29 30 31 32 33
DSC/DTA DSC/DTA visual DSC/DTA DSC/DTA DSC/DTA
1-ethyl-3-methylimidazolium tetrafluoroborate 97% 99% 0.005 water mass %
21 25 34
DSC/DTA DSC/DTA DSC/DTA
1-butyl-3-methylimidazolium 1,1,1-trifluoro-N-[(trifluoromethyl)sulfonyl]methanesulfonamide 271.1 270.1 267.61 272.1 270.1 270.35
1.0 1.0 0.77 4.0 1.0 0.050
314.1 341.94 340.1 340 340.1
4.0 0.0 2.0 2.0 2.0
3. Melting Point and Decomposition Temperatures Melting and decomposition temperatures, are one of the most remarkable properties for ionic liquids, especially for their application as alternative solvents, because they determine the liquid range of the fluids. Most ionic liquids show wide liquid ranges, as large as 200-300 °C, which is in contrast with the reduced liquid range for many common organic fluids, and thus, their use as solvents may be extended over wide temperature ranges. Moreover, the high decomposition temperature of many ionic liquids allows their use as thermal storage fluids leading to very efficient energy storage capacities including their use in solar thermal electric power systems.21-25 The melting point of ionic liquids rises from a subtle balance of cation and anion symmetry, flexibility of chains in the ions, and charge accessibility. Increasing length of alkyl chains in the cations leads to remarkable decreases in the melting points;26 for example, melting point of 1-alkyl-3-methylimidazolium hexafluorophosphate changes within 333-334 K for ethyl chains and 276-284 K for butyl chains.3 Besides the cation, the anion also influences the melting point of IL, noting an increase in the size of the anion with the same charge results in a decrease of melting point for a fixed cation.27 The IL Thermo3 database contains normal melting temperature data for 84 pure ionic liquids; we show in Table 1 the available data for some selected relevant ionic liquids. Reported results show that the common measurement method is differential scanning calorimetry (DSC) although visual observation is also frequently used in the literature. The available data show remarkable differences between the melting point values for each ionic liquid reported in different literature sources. These disagreements between different sources are very remarkable for some common ionic liquids such as 1-butyl-3-methylimidazolium chloride, Table 1, for which a melting point in the 340-342 K range is reported using a visual observation approach, whereas a 314.1 K value is reported using DSC/DTA. Moreover, for 60 of the 84 ionic liquids found in the IL Thermo3 database a single melting point data is available, and thus, they should be taken with caution. 1-Hexyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]amide ionic liquid was proposed by IUPAC as a reference ionic liquid;20 available melting point data are in the 266-272 K range.3 Shimizu et al.41 reported two transitions: the one at around 265 K should be the fusion of a metastable phase, whereas the one at around 271 K should be the fusion of the stable crystalline phase; crystal to liquid
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0.046 water 0.004 water 99.8% 0.001 water 0.011 water
mass % mass %
mass % mass %
31 32 33 35 36 37
DSC/DTA DSC/DTA DSC/DTA DSC/DTA DSC/DTA adiabatic calorimetry
1-butyl-3-methylimidazolium chloride
a
0.22 water mass % 99% -
28 30 38 39 40
DSC/DTA visual visual visual visual
Uncertainties assigned in IL Thermo database.3
transition. Therefore, the procedures to measure melting point temperatures should consider the appearance of complex solid-liquid transition features to obtain reliable data. Scurto et al.42,43 reported a remarkable effect of dissolved gases on the melting points of several families of ionic liquids. The addition of CO2 leads to large melting point depressions, even for small amounts of CO2. These depressions are remarkably large for ionic liquids containing fluorinated anions, because of the strong interaction of CO2 with the anion,44 and thus, leading to depressions as large as 120 K42 obtained for tetrabutylammonium tetrafluoroborate. Depression of ionic liquids melting points is also obtained through confinement in nanospaces; Kanakubo et al.45 reported a decrease proportional to the inverse of the pore size, leading to depressions as large as 30-40 K, which is remarkably larger than the depressions obtained for common solvents. The effect of pressure on the melting point of ionic liquids was studied by Domanska and Moravski46 showing a remarkable increase in the melting temperature with increasing pressure. Balaban et al.47 explained this behavior using the topological defects model. Melting temperatures are known for a small number of ionic liquids, and thus, several methods have been developed to predict them from their molecular structure. Trohalaki and Pachter48 predicted melting points using quantitative structureproperty relationships (QSPRs) for imidazolium ionic liquids reporting results with acceptable quality despite the simplicity of the descriptors used in the predictions. A QSPR approach was also used by Varnek et al.49 for a large collection of ionic liquids developing 16 different types of structure-melting point models, the error of predictions being in the 26-49 K range; this quality of predictions is clearly too low for any industrial application. Carrera and Aires-de-Sousa used a neural networks approach50 to predict melting points of pyridinium bromide ionic liquids with moderate success. Bini et al.51 used a recursive neural network method for the prediction of melting points for pyridinium-based ionic liquids leading to predictions with
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deviations usually lower than 20 K. Naoto et al.52 used molecular dynamics simulations for the prediction of melting points showing good agreements with experimental results, nevertheless, this approach is computationally very costly, and thus, its use for industrial applications would be impractical. The range of thermal stability of ionic liquids is very large, with decomposition temperature values reported for some ionic liquids as large as 400 °C. This property is usually measured by thermogravimetric analysis (TGA). The onset of thermal decomposition for the common ionic liquids based in the alkylimidazolium cation reported by Huddleston et al.53 showed that the nature of the anion mainly determines this property, whereas the effect of the cation is much lower. Increasing the cation size does not lead to a large change in the decomposition temperature; for example, for 1-alkyl-3-methyl imidazolium tetrafluoroborate ionic liquids, the onset value is 676.15 K for butyl chains and 633.15 K for octadecyl chains.53 For common anions, the onset of thermal decompositions is Tf2N (bis(trifluoromethylsulfonyl)imide) > BF4 >PF6 > halides,53 with a large effect on going to ionic liquids containing halides anions (100-150 K lower decomposition temperatures than for the other anions). Large onsets for the decomposition temperatures were also reported for pyridinium54 or tetraalkylphosphonium55 based ionic liquids. Reported decomposition temperatures (commonly reported as the onset for the decomposition from thermogravimetric analysis) should be handled with caution when required for industrial purposes. Ngo et al.21 reported the strong effect of the sample pan composition used for TGA experiments for certain ionic liquids, for example a difference up to 100 K is obtained if aluminum pans are used instead of alumina pans for PF6- containing ionic liquids because of the catalyzer effect of aluminum. Moreover, the endothermic character of the decomposition process in alumina sample pans is changed to exothermic behavior for aluminum pans.21 It should be also noted that the decomposition of the ionic liquid starts at lower temperatures that the commonly reported onset values, being this start decomposition temperatures even 100 K lower54 than the onset values. Kosmulski et al.56 showed that thermal stability of some ionic liquids is overrated, especially whether long-term thermal stability is required, some alkylimidazolium based ionic liquids show appreciable mass losses when maintained at temperatures around 473 K; which is well below the TGA decomposition temperatures. Moreover, these decompositions are accelerated by the presence of substances such as silica.56 Seeberg et al.57 proposed a new method to analyze long-term thermal stability of ionic liquids using a model to analyze the kinetics of mass loss from isothermal and nonisothermal TGA experiments. Prediction of ionic liquids decomposition temperatures using theoretical tools is a complex task. Kroon et al.58 used quantum chemical calculations to analyze the cation and anion effect on the decomposition mechanism; the authors correlated the activation energies of the decomposition processes with the experimental decomposition temperatures showing good correlations, and thus, pointing to a possible method for the prediction of decomposition temperatures. Hao et al.59 also used a quantum chemical approach to predict the degradation mechanism of 1-allyl-3-methylimidazolium chloride ionic liquids. 4. Density and PVT Behavior Density, and its pressure-temperature evolution (PVT behavior), is one of the most important properties for any fluid. PVT data can be considered as fundamental data for developing
equations of state, which are the main tool used for thermophysical properties prediction for process design purposes, and solution theories for ionic liquids. It is also required for many relevant industrial problems such as liquid metering applications or for the design of different types of equipment such as condensers, reboilers, separation trains, or even storage vessels. For ambient pressure and temperature conditions, density for most ionic liquids is in the 1.05-1.35 g cm-3 range. An analysis of the IL Thermo database3 shows that this is the property most commonly measured in the literature, data for 88 different pure fluids may be found. Nevertheless, the analysis of the data shows two main problems. Measurements have been centered on a reduced number of compounds, for example density data for 1-butyl-3-methylimizazolium hexafluorophosphate may be found in 43 different literature sources (in spite of the well-known decomposition problems of this fluid, which would hinder many of its possible industrial applications)60 or in 50 references for 1-butyl-3-methylimizazolium tetrafluoroborate, however, the data for new or environmentally friendly ionic liquids are almost absent. Moreover, most of the density data are reported at atmospheric pressure conditions and for temperatures close to 25 °C, and thus, PVT studies for wide pressure-temperature ranges are very scarce as seen in Table 2. As reported in Table 2, PVT data are available for only 37 pure ionic liquids, and with the exception of a small number of fluids (mainly 1-butyl-3-methylimidazolium hexafluorophosphate and 1-butyl-3-methylimidazolium tetrafluoroborate), a single literature source is available for most ionic liquids. Valderrama et al.87 reported a study in which deviations up to 17% between atmospheric pressure densities were reported by different authors. This comparison was limited to a reduced number of ionic liquids and to ambient pressure conditions. We report in Figure 1 a comparison for the available data for the ubiquitous 1-butyl-3-methylimidazolium hexafluorophosphate and 1-butyl-3-methylimidazolium tetrafluoroborate ionic liquids. These two ionic liquids were selected because they have been widely studied and a large collection of experimental density data may be found in the literature. Reported results show that although most of the available data are in good agreement within their uncertainty levels, other data may be clearly considered as outliers with remarkable differences with remaining data. Comparison for PVT data for 1-butyl-3-methylimidazolium hexafluorophosphate is reported in Figure 2a from two different literature sources using two different experimental methods: Tekin et al.63 using vibrating tube densimeters (VTD) and Gu and Brennecke66 using a high-pressure densimeter; results show remarkable differences between both data sets in the pressuretemperature ranges studied. These differences reported in Figures 1 and 2 may be mainly justified considering the different purities of the samples used and because of problems with the measuring procedures. Therefore, intercomparison among available data is necessary to ensure the accuracy levels required for industrial purposes, and thus, considering that for many ionic liquids density data may be obtained only from a single literature source, or even for a single temperature, further systematic studies are required. The analysis of Table 2 information shows that most of the PVT measurements were performed using VTD, mainly because of the well-known advantages of this technique: (i) possibility of obtaining large collections of density data for wide pressure and temperature ranges in a short period at moderate economical cost, and (ii) high accuracy of the reported measurements if calibration and measurement procedures are performed carefully. Nevertheless, VTD density measurements have to be performed
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Table 2. Ionic Liquids for Which PVT Data Are Available in the Open Literature ionic liquid 1-ethyl-3-methylimidazolium hexafluorophosphate 1-butyl-3-methylimidazolium hexafluorophosphate
1-hexyl-3-methylimidazolium hexafluorophosphate 1-octyl-3-methylimidazolium hexafluorophosphate 1-butyl-2,3-dimethylimidazolium hexafluorophosphate 1-ethyl-3-methylimidazolium tetrafluoroborate 1-butyl-3-methylimidazolium tetrafluoroborate
1-hexyl-3-methylimidazolium tetrafluoroborate 1-octyl-3-methylimidazolium tetrafluoroborate N-butylpyridinium tetrafluoroborate 1-ethyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide 1-propyl-3-methyllimidazolium bis[(trifluoromethyl)sulfonyl]imide 1-butyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide 1-pentyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide 1-hexyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide 1-heptyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide 1-octyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide 1-decyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide 1,3-diethylimidazolium bis[(trifluoromethyl)sulfonyl]imide 1-methyl-1-propylpyrrolidinium bis[(trifluoromethyl)sulfonyl]imide 1-butyl-1-methylpyrrolidinium bis[(trifluoromethyl)sulfonyl]imide 3-methyl-1-propylpyridinium bis[(trifluoromethyl)sulfonyl]imide 1-methyl-1-propylpiperidinium bis[(trifluoromethyl)sulfonyl]imide trihexyltetradecylphosphonium bis(trifluoromethylsulfonyl)imide trihexyl(tetradecyl)phosphonium bis[(trifluoromethyl)sulfonyl]imide butyl(trimethyl)ammonium bis[(trifluoromethyl)sulfonyl]imide trihexyltetradecylphosphonium chloride trihexyl(tetradecyl)phosphonium acetate 1-ethyl-3-methylimidazolium triflate trihexyltetradecylphosphonium acetate 1-butyl-3-methylimidazolium trifluoromethanesulfonate 1,3-dimethylimidazolium methylsulfate 1-ethyl-3-methylimidazolium ethyl sulfate
1-butyl-3-methylimidazolium octylsulfate 1-butyl-3-methylimidazolium tricyanomethane 1-n-butyl-3-methyl-imidazolium dicyanamide methyltrialkylammonium dicyanamide l-ethyl-3-methylimidazolium tosylate
considering the effect of samples’ viscosity on the results (damping effects on the vibrating tube). Many ionic liquids show moderate to high viscosity data, which should be considered to correct VTD measurements (which is not always done).121
temperature range/K
pressure range/MPa
method
ref
352.7-472.4 293.15-353.15 298.15-398.15 293.00-415.00 312.80-472.30 298.2-323.2 312.9-472.3 293.15-353.15 293.15-393.15 312.8-472.3 298.2-323.2 293.15-393.15 293.15-393.15 313-473 293.15-393.15 283.15-323.15 293.15-353.15 298.15-398.15 293.50-414.92 313.1-472.2 293.15-393.15 283.15-323.15 298.34-332.73 273.15-348.15 313-473 283.15-323.15 283.15-373.15 293.15-393.15 298.2-323.2 283.15-323.15 298.2-323.2 293.5-414.92 293.15-393.15 298.15-333.15 293.49-414.91 298.20-328.20 293.15-473.15 298.15-333.15 298-333 298.15-373.15 293.15-338.15 293.15-393.15 293.15-393.15 293.15-393.15 293.15-393.15 293.15-393.15 293.15-393.15 293.15-423.15 293.15-393.15 293.15-393.15 298.14-333.14 298.15-333.43 293.15-343.15 298.14-333.14 298.15-334.11 293.15-393.15 298.14-333.14 293.15-393.15 318.15-428.15 293.46-414.95 293.15-473.15 293.15-393.15 283.15-343.15 312.9-472.6 318.15-428.15 298.15-398.15 293.15-393.15 293.15-473.15 293.15-473.15 318.15-428.15
0.10-200 0.10-20 0.10-40 0.10-40 0.10-200 0.10-202.11 0.10-200 0.10-20 0.10-10 10-200 0.10-204.18 0.10-10 0.10-10 0.10-200 0.10-30 0.10-60 0.10-20 0.21-39.85 0.10-40 0.10-200 0.10-10 0.10-60 0.10-59.92 0.10-300 0.10-200 0.10-60 0.10-100 0.10-206.94 0.10-10 0.10-60 0.10-204.18 0.10-40 0.10-30 0.10-59.59 0.10-40 0.10-59.10 0.10-60 0.10-59.59 0.10-60 0.10-40 0.10-65.02 0.10-30 0.10-30 0.10-35 0.10-35 0.10-35 0.10-35 0.10-40 0.10-35 0.10-35 0.10-65 0.21-65.01 0.10-40 0.10-65 0.21-65.01 0.10-35 0.10-65 0.10-10 0.10-60 1.00-40 0.10-60 0.10-35 0.10-35 0.10-200 0.10-60 0.10-40 0.10-30 0.10-60 0.10-60 0.10-60
bellows piezometric vibrating tube vibrating tube bellows HP densitom. bellows piezometric vibrating tube bellows HP densitom. vibrating tube vibrating tube bellows vibrating tube vibrating tube piezometric vibrating tube vibrating tube bellows vibrating tube vibrating tube vibrating tube vibrating tube bellows vibrating tube vibrating tube HP densitom. vibrating tube vibrating tube HP densitom. vibrating tube vibrating tube vibrating tube vibrating tube vibrating tube vibrating tube vibrating tube vibrating tube vibrating tube vibrating tube vibrating tube vibrating tube vibrating tube vibrating tube vibrating tube vibrating tube vibrating tube vibrating tube vibrating tube vibrating tube vibrating tube vibrating tube vibrating tube vibrating tube vibrating tube vibrating tube vibrating tube vibrating tube vibrating tube vibrating tube vibrating tube vibrating tube bellows vibrating tube vibrating tube vibrating tube vibrating tube vibrating tube vibrating tube
61 62 63 64 65 66 61 67 68 61 66 68 68 61 69 70 62 63 64 65 68 70 71 72 61 70 73 66 68 70 66 64 69 74 64 75 76 74 75 77 78 69 69 79 79 80 80 81 80 80 82 82 81 82 82 69 82 68 83 64 76 79 84 65 85 86 69 76 76 83
Moreover, viscosity effects on VTD measurements depends not only on the fluid’s viscosity but also on the vibrating tube design.122 Errors rising from viscosity effects on VTD may be as high as 7.5 × 10-4 g cm-3 for the most viscous ionic liquids
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Figure 1. Comparison among available literature experimental density, F, data for the reported ionic liquids as a function of temperature for 0.1 MPa. Data from (b) refs 32, 33, 66, 88-108 for 1-butyl-3-methylimidazolium hexafluorophosphate and from (9) refs 31, 32, 72, 89, 95, 97, 98, 102, 104, 105, 107, 109-120 for 1-butyl-3-methylimidazolium tetrafluoroborate. Outliers: (a) x,88 gray filled circle,66 +;105 (b) !,97 gray filled square,102 ×,104 ~.105
Figure 2. Comparison among available literature experimental density, F, and isothermal compressibility, kT, data for 1-butyl-3-methylimidazolium hexafluorophosphate. Symbols: circles, data for 298.2 K, squares, data for 323.2 K; filled symbols, data from Tekin et al.,63 empty symbols, data from Gu and Brennecke.66
(>289 mPa s).122 Therefore, several correction equations have been proposed in the literature,121-123 together with the available studies on accuracy and calibration methods for VTD measurements.122,124 The measured PVT data would allow the calculation of relevant derived thermodynamic properties such as isobaric thermal expansivity, βP, isothermal compressibility, κT, and internal pressure, Pi, through straightforward relationships. Nevertheless, available data of these derived mechanical coefficients are extremely scarce, βP data may be found for only 11 ionic liquids in the IL Thermo database.3 Unusual behaviors are reported in the literature for some ionic liquids, such as the decreasing value of βP with increasing temperature for isobaric conditions.83,85,121,125,126 Nevertheless, reported values of these derived properties calculated from PVT data should be taken with caution because they are very sensitive to the selected mathematical functions used to describe the pressure and temperature effects on density data.126 Results reported in Figure 2b for 1-butyl-3-methylimidazolium hexafluorophosphate show remarkable differences among the different data sets for isothermal compressibility. Therefore, several solutions have been proposed to overcome this problem such as the use of extensive fitting to describe the whole PVT behavior of the fluids,83,85 or the determination of these derived properties not from PVT data but from isobaric heat capacity measurements.126 The relevance of PVT data for ionic liquids, together with the extremely large number of possible ionic liquids, has led to a remarkable interest in developing models to predict this property. One of the first approaches was the use of group
contribution methods. Gardas and Coutinho127 developed an extended method based on the Ye and Shreeve128 group contribution approach for imidazolium (mean percent deviation, MPD, 0.45%), phosphonium (MPD 1.49%), pyridinium (MPD 0.41%) and pyrrolidinium (MPD 1.57%) based ionic liquids for 273.15-393.15 K and 0.10-100 MPa pressure and temperature ranges, respectively. The main disadvantage of this model is that mechanical coefficients, βP and κT, are required for density predictions, and thus, concern that accurate values of these coefficients are difficult to obtain. Jacquemin et al.81 developed an alternative group contribution approach considering that the temperature effect on density is proportional to the effective molar volume fraction of ions constituting the ionic liquid leading to predictions for density within 0.36% and for mechanical coefficients within 20%. Alternative group contribution methods have been developed by Lazzu´s129 (with 0.73% average deviations) and Wang et al.130 based on the electrolyte perturbation theory, leading to 0.63% average deviations for the studied fluids. Lazzu´s131 developed a QSPR approach for density predictions involving 11 molecular descriptors, which applied to 163 ionic liquids led to predictions within 2%. Valderrama et al.132 used a hybrid group contribution and neural networks approach for the prediction of ionic liquids density; the predictions are in excellent agreement with experimental data for the 24 studied ionic liquids leading to 0.26% average deviations, although pressure effects on density predictions were not analyzed. Lazzu´s133 also used a hybrid group contribution and neural networks approach; the training of the model was performed using data sets from 131 different ionic liquids that
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allowed developing predictions for 33 imidazolium ionic liquids leading to 0.49% average deviations with experimental results. Therefore, many of the reported models lead to predictions that are claimed to be accurate enough for engineering calculation purposes; nevertheless, most of these models have been tested for a limited number of ionic liquids, whereas their capabilities for new families of ionic liquids have not been studied. Moreover, the application of these models requires new experimental PVT data for new families of ionic liquids to develop and analyze their predictive abilities. Another approach to predict PVT behavior of ionic liquids is to carry out molecular modeling studies, the application of molecular dynamics of Monte Carlo methods leads to values of density for different pressures and temperatures whether the NPT ensemble is used. This computational approach has been applied successfully for different types of ionic liquids83,134-139 leading to deviations usually in the 1-5% range, which are obviously too large for process design purposes. Moreover, the predictions of derived mechanical coefficients may lead to errors as high as 20-30%.83,134 Likewise, the application of computational studies for the prediction of thermophysical properties such as PVT behavior requires very large time-consuming computational studies with remarkable economical costs, which, in our opinion, would hinder, in their present status, their application for property prediction purposes for engineering applications. Density predictions for ionic liquids were performed by Palomar et al. using the COSMO-RS approach140 leading to 1.7% average deviations for the studied ionic liquids. This approach requires very low computational costs, and thus, it would be very attractive for industrial purposes. Nevertheless, the results were reported for a reduced number of fluids, a single temperature, and for atmospheric pressure conditions, and thus, conclusions on the performance of the approach for PVT predictions could not be inferred at this moment. Moreover, the available PVT data would allow the development of equations of state (EOS) for ionic liquids, which may lead to a pivotal role for engineering purposes, considering that EOS have been the preferred choice in the industry for process design purposes for decades because of their simplicity, accuracy, and reliability. The use of traditional cubic type EOS for ionic liquids has been proposed by some authors,141,142 mainly for phase equilibria prediction purposes; the critical properties and acentric factor for ionic liquids required for the use of these EOS are commonly predicted using empirical methods. Wang et al.143 used the square-well chain-fluid EOS to describe the PVT behavior of pure ionic liquids leading to deviations lower than 0.2%. Xu et al.144 developed a lattice-fluid equation of state for modeling of PVT and phase equilibria properties of ionic liquids systems; model parameters were fitted to experimental PVT properties leading to correlations with deviations lower than 0.5%. Tsiotsias et al.145 have recently reported a nonelectrolyte equation of state using the nonrandom hydrogen bonding approach. They also fitted the model parameters to experimental density values, leading to excellent correlations, and used these parameters for phase equilibria for pure and mixed fluids. Karakatsani et al.146 parameterized the tPC-PSAFT equation of state using available experimental data for imidazolium based ionic liquids. Tome et al.147 correlated the PVT experimental data for imidazolium ionic liquids using the Sanchez-Lacombe EOS leading to deviations lower than 0.53% for the studied ionic liquids.
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5. Transport Properties In this section, we will analyze the available information on viscosity, self-diffusion coefficient, electric conductivity, and thermal conductivity for pure ionic liquids. Transport properties are among the most relevant ones required for chemical process design and development.148 Viscosity is the key transport property for industrial purposes, it is required for the design of processing units, to analyze the fluids’ efficacy for applications such as lubrication, to study heat and mass transfer processes, to design equipment such as pumping systems, and flow meter device equations such as orifice meters. One of the main characteristics of some ionic liquids is their high viscosity, which is frequently remarkably larger than those for the organic fluids commonly used in the chemical industry.149 This large viscosity could be considered a disadvantage for some industrial applications because it would negatively affect processes such as pumping, mixing, stirring, combined heat, and mass transfer operations. On the contrary, the large viscosities would be favorable for the use of ionic liquids in other applications such as lubrication. Nevertheless, viscosity may be fine-tuned through an adequate combination of cation and anion leading to the range of viscosity desired for each application, although this process would require wide collection of accurate viscosity data for different families of fluids and understanding of the relationships between this property and the ionic liquid intermolecular forces. Viscosity data for ionic liquids are still scarce in the literature. Values for only 80 fluids may be found in the IL Thermo database,3 with most of the data being reported at atmospheric pressure and for a limited temperature range. The knowledge of pressure and temperature effect on viscosity (PηT behavior) is very important because viscosity values are very sensitive to temperature and because ionic liquids have been proposed to be used frequently under high pressure conditions, and thus, the knowledge of the viscosity under these conditions is crucial for applications such as lubrication. Another important issue for viscosity measurements for ionic liquids is the purity of the samples because of the well-known strong effect of impurities such as water on the reported values.95,150 In Table 3 PηT data available were extracted from the open literature. To our knowledge data for only 12 pure ionic liquids may be found in the literature. We report in Figure 3 a comparison for available viscosity data for 1-butyl-3-methylimidazolium hexafluorophosphate, for which data are available from a large collection of literature sources using different experimental approaches. Results reported in Figure 3 show extremely large deviations among data reported in the different literature sources with deviations up to 30%. These results cannot be justified according to the stated purities of the used samples in each study, all the sources (with the exception of Baker et al.155 in which purity analysis were not performed) reported water contents measured through Karl Fischer titration lower than 0.08%. Likewise, results reported in Figure 3 were obtained using four different experimental approaches as cone and plate, capillary, concentric cylinders, and rotational viscometers, and results obtained with the same experimental approach do not agree between them (i.e., cone and plate viscometers were used by Seddon et al.,89 Baker et al.,155 and Tokuda et al.32 with deviations in the 10-30% range). Hence, the presence of nonquantified impurities could be responsible for the huge differences among different sources of data. Therefore, it may be concluded that available viscosity data should be handle with caution when used for process design purposes, and, in our opinion, interlaboratory studies using common samples for relevant ionic liquids would be very advisible. This was done in the literature for the 1-hexyl-
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Table 3. Ionic Liquids for Which PηT Data Are Available in the Open Literature ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate
1-hexyl-3-methylimidazolium hexafluorophosphate 1-octyl-3-methylimidazolium hexafluorophosphate 1-butyl-3-methylimidazolium tetrafluoroborate 1-hexyl-3-methylimidazolium tetrafluoroborate 1-octyl-3-methylimidazolium tetrafluoroborate 1-ethyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide 1-butyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide 1-hexyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide 1-decyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide l-ethyl-3-methylimidazolium tosylate 1,3-dimethylimidazolium methylsulfate
temperature range/K
pressure range/MPa
method
ref
293.15-353.15 273.15-353.15 293.15-353.15 298.15 293.15-353.15 298.15-343.15 273.15-353.15 293.15-353.15 273.15-343.15 293.15-353.15 273.15-348.15 298.15-343.15 273.15-353.15 298.15-343.15 273.15-353.15 293.15-433.15 298.15-343.15 298.15-343.15 318.15-438.15 318.15-438.15
0.10-20 0.10-249.30 15-20 0.10-50.60 0.10-20 0.10-126 0.10-238.50 0.10-20 0.10-175.90 0.10-20 0.10-250.14 0.10-121.81 0.10-200 0.10-125.53 0.10-298.90 0.10-40 0.10-124 0.10-122.57 0.10-70 0.10-70
rolling ball falling body rolling ball moving piston rolling ball moving piston falling body rolling ball falling body rolling ball falling body moving piston falling body moving piston falling body vibrating wire viscometry moving piston moving piston moving piston moving piston
62 93 151 152 67 152 153 67 154 62 72 152 154 152 153 77 152 152 83 83
3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide ionic liquid used in the IUPAC project as reference,20 with results analogous to those reported in this work in Figure 3 for 1-butyl3-methylimidazolium hexafluorophosphate. The pressure effect on viscosity has been scarcely studied in open literature as shown in Table 3. The comparison among the different literature sources is difficult due to the scarcity in availability of data at the different pressure-temperature ranges studied. Nevertheless, we report in Figure 4 a comparison among three literature sources for 1-hexyl-3-methylimidazolium hexafluorophosphate; in this case a good agreement between available data may be inferred although extracting more general conclusions is not possible at this moment. The relevance of viscosity data for industrial purposes led to the development of predictive models for ionic liquids. Nevertheless, these models should be used with caution because of the scarcity of available experimental data, especially for high pressure conditions or for new ionic liquids for which viscosity data are not available. Gardas and Coutinho149 developed a group contribution method for viscosity estimation of ionic
Figure 3. Percentage deviations of viscosity data for 1-butyl-3-methylimidazolium hexafluorophosphate measured at 0.1 MPa from different literature sources compared with data from Seddon et al.89 (ηref, zero line, 0.0076 water mass %, cone and plate viscometer). Symbols: (b) Baker et al. (purity not stated, cone and plate viscometer),155 (9) Jacquemin et al. (0.019 water mass %, 0.01% halides, concentric cylinders viscometer),95 (2) Tokuda et al. (0.04 water mass %, cone and plate viscometer),32 (f) Pereiro et al. (0.03 water mass %, capillary tube viscometer),99 ([) Jiqin et al. (0.043 water mass %, rotational viscometer),156 and ( ×) Fan et al. (0.01 water mass %, capillary tube viscometer).106
liquids using the Orrick-Erbar-type approach.157 The model was applied to 29 different ionic liquids in the 20-120 °C and 4.1-20883 mPa s ranges (for atmospheric pressure conditions) leading to a mean percentage deviation of 7.78%. A drawback of this model is that density data are required for viscosity predictions. The authors claim that available experimental viscosity data show large discrepancies among different sources, as aforementioned in this work; in our opinion the model deviations are too large for industrial purposes and its predictive ability should be checked against larger databases including new reliable experimental data. Gardas and Coutinho158,159 also developed a group contribution model based on the VogelTammann-Fulcher equation that applied to 25 ionic liquids leading to a mean percentage deviation of 7.50%. The QSPR approach was also used for ionic liquids viscosity estimation. Matsuda et al.160,161 used a combined QSPR and group contribution approach, which led to acceptable correlation results, but with moderate accuracy for predictive purposes, nevertheless, the authors proposed this model for reverse design of ionic liquids, that is to say, to find an ionic liquid with a required viscosity. Bini et al.162 developed a QSPR correlation of viscosities of ionic liquids containing the bis(trifluoromethylsulfonyl)imide anion, and the authors show the difficulties in describing ionic liquids’ viscosity with a QSPR approach and conclude that further improvements have to be developed. Atomistic simulations have also been used for the prediction
Figure 4. Comparison of available literature experimental viscosity, η, data for 1-hexyl-3-methylimidazolium hexafluorophosphate. Symbols: (O) Tomida et al.,67 (0) Ahosseini et al.,152 (]) Harris et al.153
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of viscosity of ionic liquids, but this approach has several problems. Hu and Margulis163 showed that computing viscosity for highly viscous ionic liquids is very challenging because to reach the hydrodynamic limit, where experimental data are measured, using simulations is far beyond current computational capabilities. These authors also showed the existence of nonNewtonian behavior on a nanoscale and of locally heterogeneous environments, which are extremely difficult to reproduce using current computational approaches.163 Nevertheless, several authors have used atomistic simulations to predict the viscosity of ionic liquids via different computational approaches. Maginn164 used reverse nonequilibrium molecular dynamics for the prediction of viscosity leading to 15% deviation between simulation and experiment for [1-methyl-ethyl-3-methylimidazolium][bis(trifluoromethane)sulfonamide], which may be considered a very good agreement considering the purely predictive character of the used approach. Borodin et al.165 analyzed predictions using both equilibrium and nonequilibrium molecular dynamics simulations for 1-methyl-3-ethyl-imidazolium bis(trifluoromethane)sulfonimide reporting an adequate agreement between the apparent time-dependent viscosity extracted from equilibrium MD simulations and the shear-rate-dependent viscosity extracted from NEMD simulations. Zhao et al.166 used nonequilibrium molecular dynamics for the prediction of 1-n-butyl 3-methylimidazolium hexafluorophosphate viscosity using reverse nonequilibrium molecular dynamics leading to an underestimation of values compared with experimental ones, which is attributed by the authors to a faster ion dynamics described in the used model. Equilibrium simulations using the Einstein167 or Green-Kubo168 methods have been used in several works. Rey-Castro and Vega169 used the Green-Kubo approach for the prediction of 1-ethyl-3-methylimidazolium chloride viscosity, leading to an overestimation of the property of almost 1 order of magnitude. Borodin and Smith170 used the Einstein approach to predict N-methyl-Npropylpyrrolidinium bis(trifluoromethanesulfonyl)imide viscosity leading to an acceptable agreement with experimental values. Ko¨ddermann et al.171 using Green-Kubo approach, obtained an excellent agreement with experimental data for 1-ethyl-3methylimidazolium bis[(trifluoromethyl)sulfonyl]amide ionic liquid in the wide temperature range studied. Van Oanh et al.172 compared equilibrium (using Green-Kubo) and nonequilibrium simulations for 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide leading to the same viscosity values with both approaches but with an overestimation with respect to experiments. Therefore, atomistic simulations of viscosity are extremely computationally demanding and poorly accurate for industrial purposes, and thus, in our opinion, in the current situation they are not an alternative for experimental measurements. Additional transport properties are also relevant for industrial purposes. The large viscosity values for many ionic liquids make some other transport properties coupled to viscosity, such as the diffusion coefficients and ionic electrical conductivity, so low that the use of many ionic liquids for applications such as electrolyte materials are precluded up to now. Moreover, for mass transfer operations, the mass transfer coefficient must be high to take place at a sufficiently high rate, and for heat transfer purposes the thermal conductivity coefficients must also be large enough for the rate of heat transfer to be high. Experimental information on additional transport properties is extremely scarce, as data are available for only 36, 17, and 13 ionic liquids for electrical conductivity, thermal conductivity, and selfdiffusion coefficients, respectively.
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Noda et al. measured self-diffusion coefficients of several 1-alkyl-3-methylimidazolium liquids using a NMR spin echo approach showing that they obey the Vogel-Fulcher-Tamman equation and analyzing their relationship with other transport properties. Umecky et al.174 measured self-diffusion coefficients of 1-butyl-3-methylimidazolium hexafluorophosphate using pulsed-field gradient spin-echo NMR technique showing that this property is very sensitive to impurities. Tokuda et al.36 measured self-diffusion coefficients of several 1-alkyl-3methylimidazolium 1,1,1-trifluoro-N-[(trifluoromethyl)sulfonyl]methanesulfonamide ionic liquids using NMR spin echo technique. Kanakubo et al.175-177 measured the pressuretemperature effect on electrical conductivity and self-diffusion coefficients of several 1-alkyl-3-methylimidazolium hexafluorophosphate and tetrafluoroborate ionic liquids; they used an NMR spin echo approach for self-diffusion measurements. Selfdiffusion coefficients are frequently predicted in parallel to viscosity as explained in the previous paragraph. Atomistic simulations frequently report calculated values of self-diffusion coefficients,83,85,178,179 nevertheless, the almost absence of experimental data for most ionic liquids frequently hinders the analysis of predictions’ accuracy. Moreover, the calculation of self-diffusion coefficients using Einstein’s relationship from the mean square displacement obtained from atomistic simulations, as commonly done in the literature, requires very long simulations to obtain reliable D values, due to the sluggish behavior of most ionic liquids.180,181 Thermal conductivity is commonly measured using the hot wire method, and the available data are almost limited to 1-alkyl3-methylimidazolium containing ionic liquids, with most of the studied fluids showing a weak dependence of the property with temperature.182 Ge et al.182 showed that low mole fractions of water (up to 0.01) or chloride (up to 0.05) have little effect on the thermal conductivity, however, larger mole fractions caused thermal conductivity to increase. Electrical conductivity of pure ionic liquids is low at room temperature, which would hinder some of their possible applications for electrochemical purposes, but a remarkable increase with temperature has been reported for many ionic liquids.183,184 Imidazolium based ionic liquids were measured in wide temperature ranges by Leys et al.184 showing that most of the studied liquids are glass-formers and electrical conductivity obeys the Vogel-Fulcher-Tamman equation. The glass-forming behavior of ionic liquids was extensively studied by Xu et al.185 using experimental viscosity, electrical conductivity, and glass transition temperature data, together with the so-called Walden plots (a plot of fluidity and equivalent conductivity in log scale). The available studies show that electrical conductivity values are strongly related to ionic mobility, which is determined by the interionic interactions rising from Coulombic and van der Waals interactions between the involved ions. Leys et al.184 showed that for 1-alkyl-3methylimidazolium ionic liquids, electrical conductivity decreases with increasing cation alkyl chain, which is related to lower ion mobility because of the stronger van der Waals forces for larger alkylic chains; the effect of anion on electric conductivity is weaker, except for strongly hydrogen bonding anions. 6. Heat Capacity Heat capacity at constant pressure, Cp, has been measured for the most common ionic liquids. Values for 58 pure ionic liquids may be found in the open literature.3 Most of the available data were measured using DSC. Pressure effect on heat capacity has been scarcely studied. Some authors used
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Figure 5. Comparison among available literature experimental isobaric heat capacity, Cp, data for the reported ionic liquids. All data for 0.1 MPa. Symbols: (b) Holbrey et al.,188 (9) Kabo et al.,90 (2) Fredlake et al.,31 (O) Troncoso et al.,33 (0) Yu et al.,189 (4) Nieto de Castro et al.,190 (() Rebelo et al.,191 (1) Kim et al.,192 (f) Waliszewski et al.,193 (]) van Valkenburg et al.,25 (+) Garcı´a-Miaja et al.,112 and (×) Sanmamed et al.187
experimental PVT data together with Cp measured at a single pressure (usually for ambient pressure conditions) to calculate the whole Cp-P-T behavior using well-known thermodynamics relationships;68,71,83,85 nevertheless, other authors have showed some problems related with this approach and recommended carrying out experimental Cp-P-T studies.186,187 The available Cp data show frequently remarkable disagreements among different literature sources. We report in Figure 5 a comparison for 1-butyl-3-methylimidazolium hexafluorophosphate and 1-butyl-3-methylimidazolium tetrafluoroborate, for which a large collection of data is available. The reported data show very large differences among literature sources (up to 20% in some cases), for both fluids, that are far beyond the uncertainty limits of every source. The most probable reason of these divergences may rise from the experimental methodology used, particularly for the calibration procedures used for DSC methodology,190 which was used in all literature sources reported in Figure 5 (with the exception of Kabo et al.90). The effects of these uncertainties on chemical process design were recently analyzed by Franca et al.12 showing the effects of heat capacity uncertainties on the heat storage capacity of ionic liquids to replace current fluids. It is well-known that many ionic liquids have large heat storage capacity (up to 50% larger than current heat transfer fluids).12,25 The uncertainties rising from relevant thermophysical properties such as Cp lead to an over dimensioning of heat exchangers, that, together with the current ionic liquids pricing, could hinder their industrial application. Moreover, ionic liquids have been proposed as alternative thermal storage media for solar thermal power systems,194,195 and the thermal storage density (E) may be calculated from density and Cp values, together with the temperature range used.194 If 1-butyl-3-methylimidazolium hexafluorophosphate were proposed as a possible alternative thermal storage media, E would be strongly dependent on density and heat capacity values used in the calculations. We report in Table 4 these calculations using average properties at 25 °C together with the values for (10% and (1% uncertainties for heat capacity and density, respectively. For a 50 MW solar plant, with ten hours of storage, the required volume of ionic liquid is reported in Table 4 for each case, at a moderate $3/ liter cost of the fluid (which is an optimistic estimation at the current situation),14 the remarkable effect of density/heat capacity uncertainties on the final costs may be inferred from Table 4 results. We should remark that in the Table 4 analysis we have included only the raw material’s cost, additional operational costs should be added. Impurities will have also a strong effect on thermophysical properties, and thus, they should be consid-
Table 4. Effect of Density/Heat Capacity Uncertainties on the Cost of Using 1-Butyl-3-Methyl Imidazolium Hexafluorophosphate As Thermal Storage Media for a 50 MW Solar Power Plant with Ten Hours of Storage at a $ 3/L Cost of Fluida -3
density/kg m Cp/J mol-1 K-1 cp/J kg-1 K-1 E/MJ m-3 Q/m3 total cost/M$
average
average + u %
average - u %
1360 360 1266.8 172.3 10448.0 31.3
1373.6 396 1393.5 191.4 9404.1 28.2
1346.4 324 1140.1 153.5 11726.1 35.2
a A 100 °C temperature interval was used for the calculation of thermal storage density (E). u stands for density (1%) or heat capacity (10%) uncertainties; Q stands for required volume of ionic liquid.
ered for costs analysis; the most suitable option would be to measure the properties of the fluid with the exact characteristics that is going to be used.12 Several methods for Cp estimation of ionic liquids have been proposed. Gardas and Coutinho196 proposed a second-order group contribution method leading to a mean percentage deviation of 0.36% for 19 studied pure ionic liquids in wide temperature ranges for ambient pressure data. The same authors also proposed a correlation between heat capacity and molar volume leading to a 1.85% mean percentage deviation for 21 ionic liquids at 25 °C. Ge et al.197 extended the Joback method to ionic liquids, which when applied to 53 ionic liquids led to a 2.9% mean deviation. Strechan et al.198 reported a predictive method based on the correlation of Cp with intramolecular vibrational contribution, leading to 0.9% mean deviation for the 6 studied ionic liquids. This method required computing intramolecular vibrational contribution from quantum-chemical methods for each temperature, which is computationally very demanding. Cadena et al.199 used atomistic molecular dynamics simulations to predict isobaric heat capacity of pyridinium-based ionic liquids leading to 1.4% of average deviation with experimental values for the three studied ionic liquids, which can be considered excellent accuracy; nevertheless, the use of this approach is computationally very demanding, and thus, its use for industrial purposes would be very limited. 7. Surface Tension Surface properties of ionic liquids have been scarcely studied in the literature. Measurements at the ionic liquid and air interface for only 43 pure ionic liquids can be found in the IL Thermo database, corresponding an 84% of the available data for imidazolium based ionic liquids.3 Interfacial properties are
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underestimate experimental values in around 20% in the whole temperature range studied, although the slope of the surface tension decreasing with increasing temperature is accurately reproduced. Molecular dynamics simulations for 1-hexyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide developed by Sanmartı´n et al.210 led to an excellent agreement with experimental results. Nevertheless, accurate predictions of surface tension from atomistic simulations is quite difficult.208,211 8. Volatility, Vapor Pressure, and Vaporization Enthalpy
Figure 6. Comparison among available literature experimental surface tension, σ, data for 1-butyl-3-methylimidazolium hexafluorophosphate. All data for 0.1 MPa. Symbols: (b) Kilaru et al. (ring tensiometer),200 (9) Pereiro et al. (ring tensiometer),99 (2) Gathee et al. (capillary rise),201 and (() Klomfar et al. (ring tensiometer).202
very important for many industrial applications, such as control of the mass transfer efficiency for gas-liquid or liquid-liquid extraction operations or for multiphasic homogeneous catalysis. Most of the reported data for surface tension are for very narrow temperature ranges and many of them are for a single temperature. Moreover, remarkable differences among data from different literature sources may be found for several ionic liquids. Available data reported in Figure 6 for 1-butyl-3methylimidazolium hexafluorophosphate show divergences up to 10% for some literature sources. Impurities have a large effect on surface tension of ionic liquids. For hydrophilic ionic liquids the effect is more remarkable leading up to 6% differences when compared with surface tension of dried fluids, whereas the effect on hydrophobic ionic liquids is less important leading to differences with dried liquids usually within the uncertainty levels of the measurements.203 The qualitative effect of water content is not fully clarified. Whereas some authors have reported a decrease and a further increase with increasing water content,203 others have shown an almost linear increase with increasing water content.204 Thus, it seems that the water effect is strongly dependent on the nature of the ionic liquid and the possible intermolecular interactions that may be developed with the water molecules. The development of predictive methods for surface tension of ionic liquids has been done in a few studies. Gardas and Coutinho205 developed a QSPR approach to predict surface tension of ionic liquids using the parachor concept206 (which may be calculated from density/surface tension data). They developed a density correlation method together with the QSPR correlation for parachors leading to a 5.75% mean percent deviation for 38 imidazolium based ionic liquids. Gardas and Countinho205 also developed a correlation for surface tension with molecular volumes leading to 4.50% mean percent deviation, and a correlation of parachors with molecular volume leading to 6.03% mean percent deviation. Gardas et al.207 developed a QSPR correlation between the molar volume, which was calculated with a density correlation, and parachors that for 41 imidazolium based ionic liquids leaded to a 2% mean percent deviation. Gonza´lezMelchor et al.208 reported molecular dynamics simulations of the surface tension of ionic liquids showing the decreasing values of this property with ion size asymmetry. Heggen et al.209 also used molecular dynamics simulation to predict surface tension of 1-butyl-3-methylimidazolium hexafluorophosphate as a function of temperature; the predicted results
It was claimed for a long time that one of the main characteristics of ionic liquids is that they are not volatile fluids, and thus, they would not show measurable vapor pressure, hence, they could not be distilled. Nevertheless, the possible volatility of ionic liquids was first explored by Rebelo et al.212 and Paulechska et al.,213 although the pioneering work of Earle et al.214 showed for the first time that ionic liquids could be distilled under reduced pressure for high temperatures, although for temperatures close to ambient conditions their vapor pressure remains almost negligible. Moreover, the measurement of vapor-liquid equilibria properties was shown to be extremely difficult. The competing effects of fluids decomposition leads to the impossibility of measuring properties such as critical values. Likewise, the ionic character of these fluids leads to large vaporization enthalpies, ∆HVap, (in the 120-200 kJ mol-1 range,215,216 but not as large as 300 kJ mol-1 as reported in some studies212), which are remarkably larger than for common organic fluids. A recent review by Esperanca et al.217 analyzed in detail the available information on the volatility of aprotic ionic liquids, the main conclusions of the work were (i) it is not possible to measure the vapor pressures and enthalpies of vaporization of these ionic liquids, and thus most of the reported results should present systematic errors, and (ii) the accurate measurement of these properties is essential both for theoretical and practical purposes. Prediction of vaporization enthalpy was done by Emel’yanenko et al.218 through a combination of combustion calorimetry with high-level ab initio calculations. Armstrong et al.219 measured the vaporization enthalpy of eight imidazolium based ionic liquids and developed a model relating these heats of vaporization with molar volume. Verevkin220 developed an empirical group contribution equation for vaporization enthalpy, which in spite of its simplicity led to differences with experimental results not exceeding 5 kJ mol-1 for the studied fluids. Most of the theoretical studies on ionic liquids vaporization have been developed using atomistic simulations217 for two main reasons: (i) ∆HVap is probably the most suitable property for developing and validating new force fields parametrizations,181 and (ii) atomistic simulations allow analysis of the different contributions, Coulombic and van der Waals types, to the overall heats of vaporization. Nevertheless, although predictions of vaporization enthalpies using atomistic simulations215,217 frequently lead to accurate results, available studies are limited to a very reduced number of ionic liquids, and thus, atomistic simulations predictions for new families of ionic liquids may not be validated with experimental data.83,139 Therefore, the need for new experimental data is required to continue with the advancement of this computational approach. 9. Conclusions The available open literature for the most relevant thermophysical properties for ionic liquids was analyzed in this work.
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Thermophysical data available in the literature are limited to a reduced number of ionic liquids, with most of the data centered on imidazolium based ionic liquids and including anions such as hexafluorophosphate or tetrafluoroborate. Data for new families of ionic liquids, with more suitable properties for industrial purposes or with truly adequate environmental profiles, are almost absent. The properties of some ionic liquids have been studied repeatedly in many literature sources, therefore, reporting results for systems that were previously studied without reporting any additional relevant information to those previously reported. Many of the available data have been reported for samples with purity not fully clarified. This clearly hinders intercomparison among different data sets and leads to the remarkable differences reported in this work for all the studied properties. Most of the available data are measured for very narrow temperature ranges close to ambient conditions, and the pressure effect on the most relevant properties is limited to a reduced number of fluids. This could hinder high pressure industrial applications of ionic liquids, and does not allow extraction of information on properties important for fluids’ characterization such as PVT behavior. It is observed that the qualitative and quantitative effects of impurities on thermophysical properties have not been fully clarified. It is obvious that for many industrial applications ultrapure samples are not going to be used, and thus, it is required to quantify how thermophysical properties change with impurity levels. Predictive models have been developed for most of the reported properties. Nevertheless, the quality of these models’ predictions has been studied for a very limited number of ionic liquids, because of the absence of reliable and accurate data, and thus, their application for industrial purposes is, in our opinion, limited to the conditions for which they were tested and thus they can not be used with predictive purposes for new families of ionic liquids for process design purposes. Because of the enormous amount of possible ionic liquids, thermophysical studies should be developed in a systematic way, analyzing the cation/anion effects on the most relevant thermophysical properties, including new families of ionic liquids, and those that are environmentally friendly and nontoxic, and not limiting most of the studies to commercially available fluids. Interlaboratory studies, using common welldefined samples, for selected reference fluids, including the most relevant families of cations/anions, should be developed, thus allowing the development of a reference database for these fluids. In our opinion, unfortunately, some of the available thermophysical literature should be handled with caution if used for industrial process design purposes, because data accuracy is frequently not clearly clarified, and thus could lead to design errors or would require over- or under-sizing of equipment and design conditions. Literature Cited (1) Scopus database. Elsevier, 2010. Searched May 20, 2010. (2) Patent Full-Text and Full-Page Image Databases. United States Patent and Trademark Office. Searched May 20, 2010. (3) Ionic Liquids Database - (IL Thermo). NIST Standard Reference Database #47. National Institute of Standards and Technology: Gaithersburg, MD, 2006. (4) Marsh, K. N.; Boxall, J. A.; Lichtenthaler, R. Room temperature ionic liquids and their mixtures - a review. Fluid Phase Equilib. 2004, 219, 93. (5) Heintz, A. Recent developments in thermodynamics and thermophysics of non-aqueous mixtures containing ionic liquids. A review. J. Chem. Thermodyn. 2005, 37, 525.
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ReceiVed for reView July 6, 2010 ReVised manuscript receiVed September 7, 2010 Accepted September 13, 2010 IE101441S
