Heat Capacities of Ionic Liquids and Their Applications as Thermal

Chapter 11

Heat Capacities of Ionic Liquids and Their Applications as Thermal Fluids 1

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John D. Holbrey , W. Matthew Reichert , Ramana G. Reddy , and Robin D. Rogers

Downloaded by MONASH UNIV on September 14, 2013 | http://pubs.acs.org Publication Date: August 26, 2003 | doi: 10.1021/bk-2003-0856.ch011

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Center for Green Manufacturing and Departments of Chemistry, and Metallurgical and Materials Engineering, The University of Alabama, Tuscaloosa, A L 35487

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The specific heat capacity of five common ILs containing 1-alkyl-3-methylimidazolium cations have been determined using modulated DSC. In each case, the specific heat capacities for the ILs were between 1.17-1.80 J g K at 100 °C, and increased linearly with temperature in the liquid region studied. The heat capacity was also determined for the related, higher melting organic salt, 1-butyl-3methylimidazolium tetraphenylborate, in both the crystalline state and in the melt (above 140 °C), and it is significantly higher than for the ionic liquids. The results are compared with those of common organic thermalfluidsand indicate that ILs could be considered as candidate thermal fluids for heat transfer applications. -1

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© 2003 American Chemical Society

In Ionic Liquids as Green Solvents; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

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Introduction While the bulk of current research on room temperature ionic liquids (ILs) is focused on their use as solvents for chemical reactions (/), separations (2), and electrochemistry (J), ILs have properties which make them potentially excellent performance fluids for use in a wide range of engineering and materials applications. For example, the negligible vapor pressure exhibited by many ILs over a wide temperature range provides liquid stability under low pressure-high vacuum conditions, that may enable the use of IL lubricants (4) in space applications and as liquid substrates for mass spectroscopy (5) and electron microscopy. A number of engineering parameters need to be determined for the ILs in order to assess their applicability to materials applications, and for process design. In particular, the general absence of specific heat capacity (cp) data is a significant hurdle for the design of chemical reactors and heat transfer systems, required if any IL processes are to be developed beyond the laboratory scale. Heat capacities of pure substances are employed in many thermodynamic calculations, from thermochemistry to solution chemistry and are also required for the evaluation of other basic thermodynamic properties (6). The heat capacities of ILs and their mixtures are of importance in engineering work associated with the design and operation of reactors and heat pumps, required for scale-up, pilot-plant, and commercialization of IL technologies. Not only does the design of plant equipment require a knowledge of heat capacities over a wide range of operating temperatures, but this data is also helpful when storage or low temperature operation are considered. The heat capacities (cp) for a range of l-alkyl-3-methylimidazolium ([C mim] ) ILs containing either a common cation, or common anion, have been determined by modulated DSC. The magnitude of the heat capacities of ILs (with relevance to engineering and thermalfluids)and the effect of differing structural group-contributions on the results have been examined. +

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Experimental 1 -Butyl-3-methylimidazolium (trifluoromethanesulfonyl)imide ([C mim][NTf ], Covalent Associates, MA, solvent grade 98%) was used as received. All other ILs (I-butyM-methylimidazolium chloride ([C mim]Cl), l-butyl-3-methylimidazolium hexafluorophosphate ([C mim][PF ]), 1-ethyl-3methylimidazolium hexafluorophosphate ([C mim][PF ]), l-hexyl-3-methylimidazolium hexafluorophosphate ([C mim][PF ]), and l-butyl-3-methylimidazolium tetraphenylborate ([C mim][BPh ])) were, prepared using literature procedures (7, 8) and were dried in vacuo. The water content (determined by 4

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Karl-Fischer titration) of the ILs was < 0.2 wt %, except for the hexafluorophosphate, [C mim][PF ] which contained 0.9 wt % H 0. Modulated differential scanning calorimetry (MDSC) was performed using a TA Instruments model 2920 Modulated DSC (New Castle, DE) cooled with a liquid nitrogen cryostat. The calorimeter was calibrated for temperature and cell constants using indium (melting point 156.61 °C, ΔΗ 28.71 Jg" ), and for heat capacity using a standard sapphire sample. Data was collected at constant atmospheric pressure, using samples between 10-40 mg in aluminum sample pans sealed using pin-hole caps. Experiments were performed heating at 3 °C min' with a modulation amplitude of ±3 °C, thé modulation period was fixed at 60 s. The DSC was adjusted so that zero heat flow was between 0 and -0.5 mW, and the baseline drift was less than 0.1 mW over the temperature range 0-180 °C. An empty sample pan was used as reference; matched sample and reference pans (within ± 0.20 mg) were used. The heat capacity (reversing flow component) was taken directly from the instrument. Data was collected for three runs with different samples of each IL, then collated and averaged. Samples were loaded at ambient temperature and equilibrated at 25 °C for 5 min with selected modulation period and temperature amplitude. Data were collected on a heating ramp at 3 °C min" from 25 °C to 180 °C. For fC mim][NTf ], the sample was loaded at ambient temperature, then cooled to -80 °C in order to ensure initial crystallization of the sample, and data was collected on heating from -80 °C to 180 °C at 3 °C min . 6

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Results Heat capacities of a series of [C mim]X ILs with varying anion type and [C mim][PF ] ILs with varying cation substitution were determined between 0 and 180 °C by modulated DSC. The ILs studied, experimental temperature range, and measured cp are shown in Table 1. The final results of the measurements in the liquid phase are plotted in Figures 1-3 and given for selected temperatures in Table 1. The data followed a linear increase in heat capacity with temperature, and were fitted to a linear equation Cp = k + xT. Values for the parameters are shown in Table 1 along with the experimental temperature ranges. Solid-liquid transitions, in [C mim][BPh ], [C mim][PF ], [C mim]Cl, and [C mim][NTf ] are characterized by peaks in the heat capacity, and in increase in the underlying heat capacity on melting. However, the change in Cp on melting is pronounced only for [C mim][BPh ]. Thermal degradation of the commercial sample of [C mim][NTf ] was indicated above 145 °C by deviations from linearity in the Cp profile. All the ILs were essentially anhydrous (< 0.2 wt % water), with the exception of [C mim][PF ] which contained 0.9 wt % water by Karl-Fisher 4

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124 titration, and was observed in the DSC heat flow trace as a peak corresponding to vaporization (onset temperature 137 °C, ΔΗ 1.376 J g", equivalent to 1 wt % free water). It is notable that the cp profile either side of this transition are comparable, indicating that the change in heat capacity response of the IL is essentially insensitive to low wt % impurities. 1

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Table 1. Specific Heat Capacity (c , J g Κ* ) of the ILs, Determined at Selected Temperatures in the Liquid Region p

Τ (°C) [C mim]Cl [C mim][PF ] [C mim][PF ] [C mim][PF ] [C mim][NTf ] 1.04 20 1.06 30 1.36 1.15 1.08 40 1.38 1.17 1.41 1.11 50 1.21 1.13 60 1.44 1.23 1.15 1.47 70 1.71 1.25 1.18 1.50 80 1.74 1.13 1.28 1.20 90 1.52 1.77 1.15 1.31 1.22 100 1.80 1.55 1.17 1.33 1.25 110 1.83 1.58 1.20 1.35 1.27 120 1.60 1.86 1.22 1.38 1.29 130 1.89 1.63 1.24 1.40 1.32 140 1.92 1.66 1.27 1.43 1.32 150 1.95 1.29 1.69 1.45 1.34 160 1.98 1.72 1.31 1.48 1.35 170 2.01 1.74 1.33 1.49 1.37 180 1.77 2.03 1.52 1.35 k 0.99 1.50 1.27 0.95 1.08 2.34 xx 10 2.95 2.76 2.21 2.49 20-145 25-180 AT(°C) 60-180 70-180 25-180


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Data is fitted with a linear regression y = k + xT.

The salts that were solid at room temperature ([C mim][PF ], [C mim]Cl, and [C mim][BPh ]) all melt within the sampling temperature range. A marked increase in slope occurs around the melting transition as an influence of the endothermic event on the heat capacity signal. For [C mim][BPh ], shown in Figure 1, an initial solid-solid transition at 95 °C was observed followed by melting at 130 °C, in agreement with the published melting point (8). The heat capacity in the liquid region was greater than in the solid. On melting, the specific heat capacity was 3.84 J g" K" and increased, linearly to 4.0 J g" K~ at 180 °C. On cooling rapidly from the molten state, the salt forms a glass which 2

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125 has a cp profile identical to the initial liquid phase (dashed line in Figure 1). The heat capacity in the liquid phase for [C mim][BPh ] is remarkably high compared to most liquids, including the other ILs measured here (see later), with a value in the range 3.8-4.0 J g" K" , compared to 4.184 J g K for H 0; however, the liquidus range is only ca. 70 °C (130-200 °C). 4

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Temperature /°C Figure 1. Specific heat capacity as a function of temperature for [C4mim][BPh4] showing solid-solid and solid-liquid transformations. Data for the ionic liquids [C mim][PF ], [C mim][PF ], [C mim][PF ], [C mim]Cl, and [C mim][NTf ] are shown in Figures 2 and 3, separated into two sets containing either a common cation (Figure 2), or common anion (Figure 3). Data is presented between 25-180 °C; a linear increase in c with Τ in the liquid region for each IL, and the melting transitions for [C mim]Cl (57 °C) and [C mim][PF ] (59 °C) are shown, initial melting of solid [C mim][NTf ] at -4 °C is not shown. The variation in Cp with changes in IL become apparent comparing the two sets of data. The heat capacity increases linearly with an increase in temperature. Differences in heat capacity for the different ILs are due to changes in the anions and increasing alkyl-chain substitution in the cation. All the ILs screened displayed a linear increase in heat capacity with temperature in the liquid region. For ILs with a common cation, the specific heat capacity varies with anion, and decreases following the order [BPh ]" > Cl" > [BF ]~ > [PF ]* « [NTf ]\ The heat capacities also increase with alkyl-chain substitution on the cation within the series of ILs containing a common anion. The two factors, chain length and anion type, appear to contribute independently to the Cp values of the ILs. 2

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Temperature /°C Figure 2. Specific heat capacity (cp) as a function of temperature for [C mim]XILs; [C^imJCl (o), [C mim][PFe](u) and [C4mim][NTf2] (V)· 4

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Temperature /°C Figure 3. Specific heat capacity (cp) as a function of temperature for the hexafluorophosphate-containing ILs; [C2mim]fPF^J (V), [C4mim][PF^](n) and [Cemim][PFe] (o). t


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For the three hexafluorophosphate ILs, the heat capacities increase with increasing alkyl-chain length, [C mim][PF ] < [C mim][PF ] < [C mim][PF ]. After conversion to molar heat capacities (Figure 4), the increment in heat capacity for each methylene group was 41 J mol" per methylene group. This is greater than the increase anticipated based on changes in Cp with A for linear alkanes (9), possibly indicative of changes in liquid structuring interactions through the series of ILs that are more than purely van der Waals interactions observed for hydrocarbons. 2

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Temperature /°C Figure 4. Molar heat capacity (cm) as a function of temperature for the three hexafluorophosphate ILs; [C2mimJ[PF^J (v), [C4mim][PF$](u), and [C6mim][PFg] (°). Anions with high hydrogen bond acceptor characteristics (i.e., the halide salts) allow strong and extensive hydrogen bonding networks to exist within the liquid structure (10). Increased alkyl-chain length provides additional rotational and vibrational modes within the cation that can also absorb energy. The results obtained here then, are related to the supramolecular liquid structure of these solvents and the molecular structure of the component ions. The data correlate in some aspects with viscosity, Tg, or melting point, and with crystalstructure/mesophase structure packing (//). It has been suggested that variations in layer spacing in the 2D-sheets structures of long-chain amphiphilic ILs correlates with the degree of hydrogen-bonding interactions between anions and cations (//), and so to the structuredness' of the phase (both liquid and crystalline). Here, from the Cp data, we see another clear indication of the changes in structure-controlling interactions from the changes in cp with anion 4


128 type for the l-butyl-3-methylimidazolium-containing ILs with the common cation.

Discussion Liquids typically display specific heat capacities in the range 1.6 < Cp < 2.1 J g'K" . Exceptions are strongly hydrogen-bonding liquids, such as water, liquid ammonia, liquid HF, and H S0 . The results determined here indicate that ILs respond to temperature gradients much more like organic molecular solvents, than strongly hydrogen bonding liquids or high temperature molten salts and liquid metals. This is not an unreasonable observation, when the molecular composition of ILs is considered. One of the most important and widely used applications of liquids as materials in industrial processes is as heat transfer fluids. Many industrial processes require a heat transfer fluid with an operational temperature range up to 300-370 °C. Materials for this range are either liquids (with low vapor pressure and heat transfer in the liquid phase) or liquid/vapor systems. The most widely used synthetic heat transfer fluids are: 73 % diphenyloxide-27 % biphenyl mixtures, hydrogenated terphenyl/quaterphenyl mixtures, and dibenzyltoluene. Choice depends on consideration of the thermal fluid characteristics and operating requirements. Common, representative thermal fluids and operating temperature ranges are shown in Table 2. 1


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Table 2. Composition and Operating Temperature Ranges for Some Common, Commercial Thermal Fluids Commercial Name Dowfrost Dowtherm MX Dowtherm G Syltherm XLT Fluorinert FC70 Therminol 55 Therminol 59 Therminol 66 Therminol 72 Therminol 75 Therminol D12 Therminol VP1 Therminol XP

Composition propylene glycol/water alkylaromatic bi/terphenyl polysiloxane perfluorocarbon hydrocarbon alkylaromatic terphenyl aromatic ter/quatphenyl hydrocarbon biphenyl/phenyloxide mineral oil

operating Γ range /°C -45 to 120 -25 to 330 -6 to 360 -100 to 260 -25 to 215 -25 to 290 -46 to 316 0 to 345 -10 to 380 80 to 385 -85 to 230 12 to 400 -20 to 315


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The properties, characteristics, and requirements of different thermal storage media, with reference to applications in solar energy power plants are described by Geyer (12). If the potential application of ILs is considered to be as an alternative to liquid thermal storage media, with applicability from ambient temperature and above, then an artificial, but desirable property set for an 'ideal' thermal fluid are: thermal storage density of 1.9Jcm" K" , upper temperature of 430 °C, no vapor pressure below maximum operating temperature, and materials compatibility with copper and stainless steel used in heat exchangers and piping. These criteria would define liquids with performance properties that surpass those of current mineral oil, aromatic, and siloxane thermal fluids. How do ILs compare? The favorable materials properties of ILs that can be utilized to advantage include: lack of a measurable vapor pressure (advantage in high temperature applications, and in reducing hazards associated with flash points, flammability, etc.), wide thermal range (potentially from below ambient temperature to above 300 °C), linear thermal expansion characteristics, and reasonable thermal characteristics which may allow them to be substituted directly for hot-oil and synthetic aromatic thermal fluids. The long-term, high temperature thermal stability of some imidazolium-based ILs has been reported (13). Decomposition temperatures of ILs can vary, depending on the environment and the nature of the anion present, but in general, imidazolium salts decompose around 350-450 °C (14), via dealkylation of the imidazolium cation (75). Notably ILs with halide anions have lower decomposition temperatures. A comparison of some potential thermal storage parameters and properties for ILs with common thermal heat transfer fluids is shown in Table 3. Calculated volumetric heat capacities (in J cm" K" ) at 25 °C are also shown in Table 3. Potential thermal storage densities (E) for a standardized 100 °C temperature differential were calculated for the ILs studied here from the specific heat capacities calculated and published density data. In each case, the potential storage density of the liquids is in the range 150-200 MJ m". This is comparable with oil/aromatic thermal fluids. Combining the heat capacity data collected for [C mim][PF ], [C mim][PF ], and [C mim][NTf ] with published density data (16) as a function of temperature from the literature, heat capacity characteristics of the ILs can be extrapolated and compared with available data for commercial heat transfer fluids. Figure 5 shows the volumetric heat capacities for [C mim][PF ], [C mim][PF ], and [C mim][NTf ] from ambient to 180 °C based on a linear extrapolation of the IL thermal expansion between 2595 °C, compared with representative alkylaromatic, terphenyl, polysiloxane, and propylene glycol/water heat transfer fluids. The data indicate that the ILs have comparable heat adsorption characteristics to current thermal fluids. It is worth noting that Cp and density of the ILs appear to vary in opposite senses between the ILs, so that the least dense IL ([C mim]Cl) has the highest


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value of c and the densest IL ([C mim][NTf ]) exhibited the lowest specific heat capacity. A direct result of this, is that the volumetric heat capacity, Cp (and subsequently, potential thermal storage density of the liquids) are approximately equal for all the l-alkyl-3-methylimidazolium ILs investigated here. This may allow cost/stability issues to be treated without incurring other design and operating penalties. p

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Table 3. Calculated Volumetric Heat Capacity for ILs at 25 °C


Liquid Dowtherm HT Thermal oil CH NH CI C H NH Br (CH ) NCI (CH ) NBr (C H ) NBr (C H ) NBr [C mim]Cl [C mim][BF ] [C mim][BF ] [C mim][PF ] [C mim][PF ] [C mim][PF ] [C mim][NTf ] a

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Density Viscosity /cPs /g cm 953 1.01 0.89 1.9

Mp /°C

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225-230 107-108 >300 >300 285 102-106 57.1 ~1.20 5.8 1.20 -71 (T ) 1.20 60.5 1.10 6.5 1.37 -80 (T ) 1.30 -5.1 1.44

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Data from NIST for solid salts at 25 °C (17); extrapolated for liquid from crystal structure density (1.39 g em" ); c extrapolated to 25 °C from linear regression of higher temperature data; d Data from Ref. 18. 3

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Within limits, the properties of ILs can be varied by changes in the structure and nature of the components present. High temperature molten salt and formulation science, and liquid crystal technologies teach us that significant improvements to the properties of the IL systems can be made by the incorporation of additives/blends of ILs (19). From the data, the characteristics and thermal range of ILs are comparable with synthetic thermal fluids currently commercially available. Thus, the energy storage characteristics should also allow direct substitution in applications.


131 3.0 Dowtherm MX Syltherm XLT Dowtherm G

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Temperature /°C Figure 5. A comparison of volumetric heat capacity performance characteristics with temperature for the ILs [C^imJfPF^J, [CfimimJfPFfi], and [C4mim][NTf2], and commercial heat transferfluids:Dowtherm MX (alkylated aromatics), Dowtherm G (di- and tri-aryl compounds), and Syltherm XLT (polysiloxane).

Conclusions Heat capacity data for the ILs, [C mim]Cl, [C mim][PF ], [C mim][PF ], [C mim][PF ], and [C mim][NTf ] have been determined; the values are comparable to those for other organic liquids, and are lower than those of strongly hydrogen-bonding liquids such as water, HF, or H S0 . The Cp profile determined for the higher melting salt, [C mim][BPh ] was greater, however this salt has only a limited, high temperature liquidus range. The ILs investigated have volumetric heat capacity profiles that are comparable to those of conventional synthetic thermal fluids, used in heat transfer operations and it may be concluded that ILs have potential for use as heat transferfluids(20). 4

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Since many ILs show typical decomposition temperatures of 400+ °C (recorded dynamically by TGA), an operating range from ambient to 350 °C may be feasible. However, long term stability under high temperature operating conditions needs further evaluation. Lower temperature operation can be achieved with a suitable formulation with desirable temperature/viscosity characteristics. The operational temperature range for the ILs may depend on the limitations for turbulent pumping of these relatively viscous liquids; at higher temperatures, the viscosity decreases significantly, thus using


132 [C mim][NTf ] as an example, the viscosity varies from 30 cPs at 25 °C to 6 cPs at 100 °C. Similarly, data on the vapor pressure of ILs at high temperatures is needed. If ILs do, genericaily, prove to have exceptionally low to nonmeasurable vapor pressures then these liquids can offer some exceptional advantages over conventional thermal oils as heat transfer fluids, eliminating the hazards associated with vapor above the liquids. Among the unresolved issues with an 1L system is the assessment of economics. Many ILs currently known are intrinsically complex salts, containing expensive groups, for example, the bis(trifluoromethane sulfonyl)imide anion. Attempts to address this by identifying simpler, lower cost cations and anions, or manufacturing routes are required to make these materials economically realizable, particularly on the scales that could be envisaged for this sort of application.


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Acknowledgments This research has been supported by the U.S. Environmental Protection Agency STAR program through grant number R-82825701-0 (Although the research described in this article has been funded in part by EPA, it has not been subjected to the Agency's required peer and policy review and therefore does not necessarily reflect the views of the Agency and no official endorsement should be inferred). Additional support was provided to the Center for Green Manufacturing from the National Science Foundation Grant EPS-9977239 and the Environmental Management Science Program of the Office of Environmental Management, U.S. Department of Energy, grant DE-FG0701ER63296.

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Huddleston, J. G.; Willauer, H. D.; Swatloski, R. P.; Visser, A. E.; Rogers, R. D. Chem. Commun. 1998, 1765; Visser, A. E.; Swatloski, R. P.; Reichert, W. M.; Griffin S. T.; Rogers, R. D. Ind. Eng. Chem. Res. 2000, 39, 3596: Visser, A. E.; Swatloski, R. P.; Griffin S. T.; Hartman D. H.; Rogers, R. D.

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4 Ye, C.; Lui, W.; Chen, Y.; Yu, L. Chem. Commun. 2001, 2244. 5 Gannon, T. J.; Law, G.; Watson, P. R.; Carmichael, A. J.; Seddon, K. R. Langmuir 1999, 15, 8429. 6 Gokcen Ν. Α.; Reddy, R. G. Thermodynamics, Plenum, New York, 1996; Honig, J. M. Thermodynamics, Academic Press, San Diego, 1999. 7 Huddleston, J. G.; Visser, A. E.; Reichert, W. M.; Willauer, H. D., Broker, G. Α.; Rogers, R. D. Green Chem. 2001, 3, 156. 8 Dupont, J.; Suarez, P. A. Z.; De Souza, R. F.; Burrow, RF.; Kintzinger, J.-P. Chem. Eur. J. 2000, 6, 2377. 9 CRC Handbook of Chemistry and Physics, Lide, D. R., Ed; 71 edition, CRC, Boston, 1991. 10 Elaiwi, Α.; Hitchcock, P. B.; Seddon, K. R.; Srinivasan, N.; Tan, Y. M.; Welton, T.; Zora, J. A. J. Chem. Soc., Dalton Trans. 1995, 3467. 11 Bradley, A. E.; Hardacre, C.; Holbrey, J. D.; Johnston, S.; McMath, S. E. J.; Nieuwenhuyzen, M. Chem. Mater. 2002, 14, 629. 12 Geyer, M. A. in Solar Power Plants, Winter, C.-J.; Sizmann, R. L.; VantHull, L. L., Eds.; Springer-Verlag Berlin, 1991;p199. 13 Zhang, Z.; Reddy, R. G. in EPD Congress 2002, Taylor, P. R.ed.;TMS, Warrendale PA, 2002;p199. 14 Ngo, H. L.; LeCompte, K.; Hargens, L.; McEwan, A. B. Thermochim. Acta. 2000, 357-358, 97: Bonhôte, P.; Dias, A. P.; Papageorgiou, N.; Kalyanasundaram, K.; Grätzel, M. Inorg. Chem. 1996, 35, 1168: Holbrey, J. D.; Seddon, K. R. J. Chem. Soc., Dalton Trans. 1999, 2133: Takahashi, S.; Koura, N.; Kohara, S.; Saboungi, M.-L.; Curtiss, L. A. Plasmas & Ions 1999, 2, 91. 15 Chan, Β. Κ. M.; Chang, N.-H.; Grimmett, M. R. Aust. J. Chem. 1977, 30, 2005. 16 Seddon, K. R.; Stark, Α.; Torres, M.-J. in Clean Solvents: Alternative Media for Chemical Reactions and Processing, Abraham, Μ. Α.; Moens, L., eds.; ACS Symposium Series 819, American Chemical Society, Washington DC, 2002;p34. 17 Linstrom, P. J; Mallard, W. G. Eds., NIST Chemistry WebBook, NIST Standard Reference Database Number 69, July 2001, National Institute of Standards and Technology, Gaithersburg MD, 20899 (http://webbook.nist.gov). 18 Wilkes, J. S. in Proceedings of the 13th International Molten Salt Symposium, The Electrochemical Society, Philadelphia, PA, 2003, in press. 19 Sun, J.; Forsyth, M.; MacFarlane, D. R. J. Phys. Chem. Β 1998, 102, 8858. 20 Wu, B.; Reddy, R. G.; Rogers, R. D. in Proceedings of Solar Forum 2001, Solar Energy: The Power to Choose, ASME, Washington, DC, April 21-25, 2001.


Heat Capacities of Ionic Liquids and Their Applications as Thermal

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