Measurement of heat capacities of ionic liquids by differential scanning calorimetry

doi:10.1016/j.fluid.2006.03.015

Fluid Phase Equilibria

Volume 244, Issue 1, 5 June 2006, Pages 68-77

https://doi.org/10.1016/j.fluid.2006.03.015 Get rights and content

Abstract

Heat capacities for nine ionic liquids (IL) have been determined with the “three-step” method using two different differential scanning calorimeters (DSC). In addition, the heat capacities of these ionic liquids have been studied by the modulated-temperature differential scanning calorimetry (MDSC™). The measurements cover a temperature range from 315 to 425 K.

Introduction

Ionic liquids are salts with melting points below 373 K and convenient viscosities. They also show thermal stability at high temperatures and high solubility for both polar and non-polar organic as well as inorganic substances. Their most remarkable characteristic is their low vapor pressure, which is nearly impossible to measure. For this reason they are non-flammable. Consequently, ionic liquids could be in a position to replace flammable and volatile organic solvents in chemical processes.

Ionic liquids normally consist of a large organic cation and an inorganic anion. Due to a large number of possible combinations of various cations and anions, the number of imaginable ionic liquids seems to be unlimited. That is why they are discussed as “designer solvents” for chemical reactions or separation processes, i.e. for any problem the adequate ionic liquid could be created. Detailed information about the synthesis and application of ionic liquids is described elsewhere [1].

Because of their interesting properties they could play a significant role in the future, so that a large number of research groups have started with a systematic measurement and collection of the thermophysical data of ionic liquids [2], [3], [4], [5], [6]. So far more than 13,000 data points for ionic liquids are available [7].

The heat capacity is one of the basic pure component properties for any material and its knowledge is necessary for many engineering calculations. For most ionic liquids these values are still missing. Therefore heat capacities for nine ionic liquids have been measured. The ionic liquids which have been investigated are listed in Table 1. In the column on the right hand side the abbreviations for the ionic liquids used in this work are given. The structures of the cations and anions of the ionic liquids are shown in Fig. 1.

In order to check the accuracy of the used calorimeters, heat capacities of standard materials like water, benzene and tetrahydrofuran have been measured too.

Section snippets

Materials

All nine ionic liquids were received from Merck. Before the measurements were started, they were purified by vacuum evaporation to remove the last traces of impurities. Water, benzene and tetrahydrofuran were purified by distillation (in case of water by bi-distillation). Their resulting purities higher than 99.9% were determined by gas chromatography.

Measurement of heat capacities

Heat capacity and transition enthalpies are the basic quantities available through calorimetry. The measurements reported here were performed

Conclusion

If ionic liquids shall be applied in industrial processes in the future, their thermodynamic properties (e.g. heat capacities as f(T)) are needed for the design of a chemical facility. Heat capacities of nine ionic liquids have been measured between 315 and 425 K with an accuracy of ±5%. For the determination, two different types of measuring techniques have been applied: the differential scanning calorimetry and the modulated-temperature differential scanning calorimetry, whereby differential

Acknowledgement

The authors thank the Deutsche Forschungsgemeinschaft (DFG: GM 10/19-2) for the financial support of this work.

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Measurement of heat capacities of ionic liquids by differential scanning calorimetry

Abstract

Introduction

Section snippets

Materials

Measurement of heat capacities

Conclusion

Acknowledgement

Fluid Phase Equilibr.

Fluid Phase Equilibr.

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Fluid Phase Equilibr.

Thermochim. Acta

Thermochim. Acta

Thermochim. Acta

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Ionic Liquids in Synthesis

J. Chem. Eng. Data