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Calorimetric Study on the Ion Pairing and Aggregation of 1-Ethyl-3-Methylimidazolium bis(trifluoromethylsulfonyl)amide ([C2mim][NTf2]) and Related Ionic Liquids in the Low-Dielectric Constant Solvent Chloroform

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Abstract

A calorimetric study of dissolution of the ionic liquids (ILs) 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([C2mim][NTf2]), 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([C6mim][NTf2]), and 1-hexyl-3-methylimidazolium tris(trifluoromethylsulfonyl)methide ([C6mim][CTf3]) into chloroform (CHCl3) is presented with particular focus on [C2mim][NTf2]. The interpretation of the calorimetric data for [C2mim][NTf2] was aided by additional NMR self-diffusion measurements and viscosity measurements that through the Stokes–Einstein equation provided information about the average size of the species present. It is evident that the main equilibrium species are ion pairs and aggregates. An estimate for the enthalpy contribution from aggregate formation for [C2mim][NTf2] was found to be −2.09 kJ per mol of added IL at 288.2 K and slightly decreasing in magnitude to −1.11 kJ·mol−1 at 318.2 K. While all three ILs release heat upon dissolution into CHCl3, different temperature trends are observed demonstrating the fine balance of competing contributions from breaking IL interactions, cavity formation for the solutes to reside in, and the establishment of new solute–solvent interactions.

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Acknowledgments

This report is based upon work supported by the National Science Foundation under RUI-Grant No. 0842960 (to. M. M. H) and DFG STA1027/2-3 (to A. S.). We thank the Chemnitz University of Technology, Chair of Technical Chemistry, for providing the facilities and access to their RC1e calorimeter with particular thanks to Dr. Enrico Dietzsch for his invaluable assistance, and Merck (M. Watermann) for the donation of the ionic liquid samples. We thank Mrs. Petra Weiss from the Institute for Physical Chemistry at the Friedrich-Schiller-University in Jena for the DSC measurements.

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Scharf, N.T., Stark, A. & Hoffmann, M.M. Calorimetric Study on the Ion Pairing and Aggregation of 1-Ethyl-3-Methylimidazolium bis(trifluoromethylsulfonyl)amide ([C2mim][NTf2]) and Related Ionic Liquids in the Low-Dielectric Constant Solvent Chloroform. J Solution Chem 42, 2034–2056 (2013). https://doi.org/10.1007/s10953-013-0082-y

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