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

A calorimetric study of dissolution of the ionic liquids (ILs) 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([C(2)mim][NTf2]), 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([C(6)mim][NTf2]), and 1-hexyl-3-methylimidazolium tris(trifluoromethylsulfonyl)methide ([C(6)mim][CTf3]) into chloroform (CHCl3) is presented with particular focus on [C(2)mim][NTf2]. The interpretation of the calorimetric data for [C(2)mim][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 [C(2)mim][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 center dot 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.