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Abstract
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The shear viscosity of room-temperature ionic liquid (IL) 1-n-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)
imide [bmim][Tf2N] is calculated over a temperature range 298–353 K, using the reverse nonequilibrium
molecular dynamics simulation technique. The results of this work show that while the use of equilibrium
molecular dynamics simulation techniques might be inefficient for viscosity calculations of ILs, the reverse
nonequilibrium molecular dynamics technique is an efficient tool for this purpose. Our findings indicate
that the shear rate for crossover from the Newtonian plateau to the shear thinning regime, corresponds to
the relaxation time for the slowest microscopic scale motions, i.e., exchange of counterions in an ion’s
solvation shell (ion-pair relaxation time). The closeness of the time scales and activation energies for zeroshear-
rate viscosities to the relaxation times and the corresponding activation energies for ion-pair
formation/rupture, connects macroscopic dynamic properties with local atomic-level motions of the IL.
The calculated viscosity coefficients and relaxation times for reorientations of the cation and anion as well
as their corresponding activation energies are in very good agreement with experimental data.
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