Abstract
Using classical molecular dynamics simulations, we investigate the mobility of ions in [Bmim][TFSI], a typical room temperature ionic liquid. Analyzing the trajectories of individual cations and anions, we estimate the time that ions spend in bound, clustered states, and when the ions move quasi-freely. Using this information, we evaluate the average portion of free ions that dominate conductivity. The amount of thus defined free ions comprises 15-25%, monotonically increasing with temperature in the range of 300-600 K, with the rest of the ions being temporarily bound, moving rather in local potentials. The conductivities as a function of temperature, calculated from electric current autocorrelation functions, reproduce reported experimental data well. Interestingly, for free ions the Nernst-Einstein relationship between the mobility and diffusion coefficient holds fairly well. In analogy with electronic semiconductors, one can speak about an ionic semiconductor model for ionic liquids with valence (or excitonic) and conduction band states for ions, separated by an energy gap. The obtained band gap for the ionic liquid is, however, very small, about 0.026 eV, allowing for easy interchanges between the two dynamic states.
Highlights
IntroductionThe rediscovery of room-temperature ionic liquids (RTILs) was a revolution in chemistry [1]
In order to test the validity of the two-state concept, we address these questions as well as a few accompanied ones related to the mechanisms of ionic diffusion and conductance in room-temperature ionic liquids (RTILs)
There may be two alternative definitions of the free and bound states—a kinetic one and a static one. The former definition refers to the ability of ions to participate in electrical direct-current conductivity of RTILs
Summary
The rediscovery of room-temperature ionic liquids (RTILs) was a revolution in chemistry [1]. RTILs, solvent-free electrolytes, are almost universal solvents for synthesis and catalysis [1]. We could “count” solvents, but for RTILs there are practically an unlimited.
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