Ion-Pairing and Electrical Conductivity in the Ionic Liquid 1-n-Butyl-3-methylimidazolium Methylsulfate...

Molecular dynamics simulations are performed on ionic liquid (IL) 1-n-butyl-3-methylimidazolium methylsulfate. The calculated densities, diffusion coefficients, and electrical conductivities are in very good agreement with the experiment. While the Nernst-Einstein equation largely overestimates the electrical conductivities compared to experiment, its correction for the joint translation of counterions substantially improves its accuracy. Taking into account the role of correlated motions of ions, expressed in terms of the collective mean-squared-displacement, predicts electrical conductivities in close agreement with the experiment. An examination of different dynamical processes shows that three types of motions occur in this IL. Short relaxation times (<1 ps) and low activation energies (≈4 kJ mol^-1) are associated with local structural properties such as hydrogen bonding. The second class, such as structural relaxation of hydrogen bonds, has longer relaxation times (a few ten picoseconds) and higher activation energies (≈10 kJ mol^-1). The slowest motions (relaxation times of a few ten nanoseconds and activation energies of ≈35 kJ mol^-1) belong to the correlated motions of counterions. Ion-pairing, diffusion, and electrical conductivity belong to this third class. Closeness of the relaxation times and activation energies of the third class processes reveal that exchange of counterions in an ion's solvation shell governs the macroscopic dynamic properties of IL.