Functionalized ionic liquids for thermal energy storage: validating a COSMO-RS based prediction of isobaric heat capacity by a comparison with experimental data

Ionic liquids are sometimes considered to show almost magical properties in a wide range of applications. However, many studies show that even if the variability of the available structures may rather be an advantage as to the... [ view full abstract ]

Ionic liquids are sometimes considered to show almost magical properties in a wide range of applications. However, many studies show that even if the variability of the available structures may rather be an advantage as to the fine-tuning of the desired properties, ionic liquids are far from being the green panacea it was believed to be two decades ago when research in this field was gaining momentum. This of course is no reason for discontinuing to study properties and applications of ionic liquids. Instead, such studies should be rationalized in terms of finding reliable structure-property relationships that will enable us to identify the most promising structures for a given application [1]. 

To contribute to this endeavour we decided to focus on a study of isobaric heat capacity. A sufficiently high thermal energy density of some ionic liquids may lead to their use as thermal fluids [2], however, experimental heat capacity data are still scarce. In light of the wide range of structures available and of the time-consuming and demanding calorimetric experiments, a reliable prediction of heat capacity seems to be an option to facilitate the task of selecting ionic liquids for thermal energy storage applications. 

In this contribution we have therefore selected a homologous series of functionalized 3-alkyl-1-[(1R,2S,5R)-(–)-menthoxymethyl]imidazolium bis(trifluoromethylsulfonyl)imide ionic liquids for which existing experimental isobaric heat capacity data are compared with a prediction by means of a COSMO-RS based method proposed by Preiss et al. [1] Even though this in silico prediction is an indirect one, taking as a starting point the molar volumes obtained from COSMO files and making use of an empirical relationships between the molar volume and heat capacity, a validation using a large range of data may bring a deeper insight into this prediction methodology and contribute to its refinement and consequently to a more efficient design of ionic-liquid-based thermal storage processes.

References

1. Deetlefs, M.;  Fanselow, M.;  Seddon, K. R. Ionic liquids: the view from Mount Improbable. RSC Adv. 2016, 6, 4280-4288.
2. Holbrey, J. D.;  Reichert, W. M.;  Reddy, R. G.;  Rogers, R. D. Heat Capacities of Ionic Liquids and Their Applications as Thermal Fluids. In Ionic Liquids as Green Solvents, Vol. 856; Rogers, R. D.;   Seddon, K.,  Eds.; American Chemical Society 856: 2003.
3. Preiss, U. P. R. M.;  Slattery, J. M.;  Krossing, I. In Silico Prediction of Molecular Volumes, Heat Capacities, and Temperature-Dependent Densities of Ionic Liquids. Ind. Eng. Chem. Res. 2009, 48, 2290-2296. 

Acknowledgments: Financial support of the Czech Science Foundation under Grant No.17-08218S.