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(-).