Origin of the Low-Viscosity of [emim][(FSO2)2N] Ionic Liquid and Its Lithium Salt Mixture: Experimental and Theoretical Study of Self-Diffusion Coefficients, Conductivities, and Intermolecular Interactions

The temperature-dependent viscosity, ionic conductivity, and self-diffusion coefficients of an ionic liquid, 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)amide ([emim][FSA]), and its Li salt mixture were studied with reference to emim bis(trifluoromethyl-sulfonyl)amide ([emim][TFSA]) systems. The stabilization energies for the formation of the FSA− complexes with emim+ and Li+ were calculated by the MP2/6-311G** level ab initio method. The stabilization energies calculated for the FSA− complexes with emim+ and Li+ (−77.0 and −134.3 kcal/mol) were smaller than those for the corresponding TFSA− complexes (−78.8 and −137.2 kcal/mol). The weaker electrostatic and induction interactions are the causes of the smaller interaction energies for the FSA− complexes. The weaker interaction between the FSA− and emim+ can be one of the causes of the lower viscosity of the [emim][FSA] ionic liquid compared with that of the [emim][TFSA] ionic liquid. The weaker interaction between the FSA− and Li+ compared with that between the TFSA− and Li+ explains the fact that the addition of Li salt to the [emim][FSA] ionic liquid induces a little increase of the viscosity and a little decrease of the ionic conductivity and self-diffusion coefficients of ions. The FSA− in the Li[FSA] complex prefers the cis form due to the stronger attraction and smaller deformation energy of the cis-FSA− compared with the trans-FSA−.