Ion Pairing and Dynamics of the Ionic Liquid 1-Hexyl-3-methylimidazolium Bis(irifluoromethylsulfonyl)amide ([C6mim] [NTf2]) in the Low Dielectric Solvent Chloroform

The structural and dynamic behavior of the ionic liquid 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([C(6)mim][NTf2]) in chloroform has been studied by experimental measurements of H-1 and F-19 self-diffusion coefficients, viscosity, and excess molar volume in the concentration range of 0.001-1.0 mol.kg(-1) and temperatures ranging from 15 to 45 degrees C. Within measurement uncertainty, the H-1 and F-19 self-diffusion coefficients are identical at the same experimental conditions of concentration and temperature, indicating that even to the lowest measured concentrations the cation and anion are not completely dissociated. The combined experimental data indicates a progression from ion pairing to aggregate formation as concentration increases where at concentrations near 0.1 mol.kg(-1) aggregate formation becomes dominant. Concurrently with the formation of the IL aggregates at higher concentrations, we also observe an apparent breakdown of the validity of the Stokes-Einstein equation, which we explain by translational motion to become dominated by individual ion pairs moving rapidly between IL aggregates.